ELECTRODES USEFUL FOR MEMBRANE-FREE ELECTRODE ASSEMBLIES, MEMBRANE-FREE ELECTRODE ASSEMBLIES, METHODS, AND USES THEREOF

Description

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to United States Provisional Patent Application No. U.S. 63/419,062, filed Oct. 25, 2022 the entire contents of which are hereby incorporated by reference.

FIELD

The present disclosure relates generally to electrodes useful for membrane-free electrode assemblies, membrane-free electrode assemblies, methods of making, and uses thereof.

BACKGROUND

Advances have been made in CO₂ electroreduction (CO₂R) and CO electroreduction (COR) technology to produce various feedstock chemicals and fuel.^1,2 Inspired by the commercial water electrolyzer, low-temperature CO₂ electrolysis has transitioned from H-cell type configuration to flow-cell electrolyzers wherein the use of gas diffusion layer (GDL) has enabled industrially relevant current density.³ SUMMARY

In an embodiment of the present disclosure, there is provided:

• • 1. An electrode useful for membrane-free electrode assemblies, the electrode comprising: a catalyst layer; and a solid polymer electrolyte layer for ion-conduction supported on the catalyst layer.
  • 2. The electrode of embodiment 1, wherein the solid polymer electrolyte layer supported on the catalyst layer comprises the solid polymer electrolyte layer deposited on the catalyst layer.

In one or more embodiments, the solid polymer electrolyte layer is deposited directly on the catalyst layer. In one or more embodiments, the solid polymer electrolyte layer is deposited directly on the catalyst layer such that is it is not a standalone membrane.

• • 3. The electrode of embodiment 1 or 2, wherein the solid polymer electrolyte layer has a thickness of ≤25 μm, ≤20 μm, or ≤15 μm, or ≤10 μm, or ≤5 μm, or about 3 μm, or ≤1 μm. The electrode of embodiment 1 or 2, wherein the solid polymer electrolyte layer has a thickness of <25 μm, ≤20 μm, or ≤15 μm, or ≤10 μm, or ≤5 μm, or about 3 μm, or ≤1 μm.

In one or more embodiments, the solid polymer electrolyte layer has a thickness that is sufficient to support the standard and/or expected operation of an electrode assembly.

In one or more embodiments, the solid polymer electrolyte layer has a thickness that is sufficient to reduce, inhibit, or prevent short-circuiting of any electrode assembly is it used with. In one or more embodiments, the solid polymer electrolyte layer has a thickness that is on the order of micrometers (μm). In one or more embodiments, the solid polymer electrolyte layer has a thickness that is not on the order of nanometers (nm), as such a thickness may prevent an assembly from functioning; such as, by short circuiting.

• • 4. The electrode of any preceding embodiment, wherein the solid polymer electrolyte layer comprises an ionomer with integrated alkali metal cations. The electrode of any preceding embodiment, wherein the solid polymer electrolyte layer comprises an ionomer with integrated alkali metal cations; or comprises an ionomer free of integrated alkali metal cations. The electrode of any preceding embodiment, wherein the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations; or comprises an ionomer free of integrated transition metal cations.

In one or more embodiments, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the solid polymer electrolyte layer is prepared by the methods as herein.

In one or more embodiments, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the solid polymer electrolyte layer is directly deposited on an electrode or catalyst layer by the methods described herein.

In one or more embodiments, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the integrated transition metal cations act as a catalyst.

In one or more embodiments, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the integrated transition metal cations act as an ionomer-dispersed catalyst layer.

In one or more embodiments, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the resultant electrode is suitable for use in one or more of the electroreduction reactions as described herein.

In one or more embodiments, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the integrated transition metal cations facilitate catalysis of one or more of the electroreduction reactions as described herein.

In one or more examples, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the integrated transition metal cations facilitate catalysis of one or more of the gaseous components introduced into the one or more electroreduction reactions as described herein.

In one or more embodiments, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the resultant electrode provides C₂+ selectivity in one or more of the electroreduction reactions as described herein.

• • 5. The electrode of any preceding embodiment, wherein the solid polymer electrolyte layer comprises an ionomer loading of about 10 μL/cm² to about 150 μL/cm², or about 25 μL/cm² to about 100 μL/cm²; and/or an alkali metal cations concentration of about 0.1 M to about 3 M, or about 0.15 M to about 1 M. The electrode of any preceding embodiment, wherein the solid polymer electrolyte layer comprises an ionomer loading of about 5 μL/cm² to about 150 μL/cm², or about 10 μL/cm² to about 150 μL/cm², or about 25 μL/cm² to about 100 μL/cm²; and/or a metal cations concentration of about 0 M to about 10 M, or about 0 M to about 3 M, or about 0.1 M to about 3 M, or about 0.15 M to about 1 M.
  • 6. The electrode of any preceding embodiment, wherein the ionomer comprises Nafion, Sustainion XA9™, AEMION™ Sustainion™, Aquivion, or a combination thereof. The electrode of any preceding embodiment, wherein the ionomer comprises perfluorinated sulfonic acid; sulfonated polyphenylene; polystyrene vinyl benzyl methyl imidazolium chloride; or a combination thereof.

An ionomer is a polymer where at least some of the monomer units comprises an ionic functionality. In one or more embodiments, the ionomer is any ionomer acceptable for use in an ion exchange membrane. In one or more embodiments, the ionomer is any ionomer acceptable for use in a cation exchange membrane. In one or more embodiments, the ionomer is any ionomer acceptable for use in an anion exchange membrane. In one or more embodiments, the ionomer comprises anion exchange ionomer; cation exchange ionomer; or a combination thereof.

In one or more embodiments, the ionomer comprises a perfluorosulfonic acid polymer, a sulfonated tetrafluoroethylene based fluoropolymer-copolymer, or a combination thereof (e.g., Nafion.)

In one or more embodiments, the ionomer comprises a 1H-Imidazole, 1,2,4,5-tetramethyl-, compound with 1-(chloromethyl)-4-ethenylbenzene polymer with ethenylbenzene (e.g., Sustainion XA9™). In one or more embodiments, the ionomer is an alkaline ionomer comprising hydrophilic poly (4-vinylbenzyl alkyl-imidazolium chloride) (e.g., Sustainion XA9™)

In one or more embodiments, the ionomer comprises a hydrocarbon backbone (e.g., AEMION™) In one or more embodiments, the ionomer is an alkaline ionomercomprising hydrophilic poly (4-vinylbenzyl alkyl-imidazolium chloride) (e.g., AEMION™)

• • 7. The electrode of any preceding embodiment, wherein the alkali metal cation comprises Li⁺, Na⁺, K⁺, Cs⁺, or a combination thereof. The electrode of any preceding embodiment, wherein the alkali metal cation comprises Li, Na, K, Cs, Rb, Fr, or a combination thereof. The electrode of any preceding embodiment, wherein the transition metal cation comprises Cu, Ni, Ag, Au, Pt, Al, Zn, Sn, Bi, Pd, or a combination thereof.
  • 8. The electrode of any preceding embodiment, wherein the catalyst layer comprises Cu, Ni, Ag, Au, Pt, or a combination thereof. The electrode of any preceding embodiment, wherein the catalyst layer comprises Cu, Ni, Ag, Au, Pt, Al, Zn, Sn, Bi, Pd, or a combination thereof.
  • 9. The electrode of any preceding embodiment, wherein the catalyst layer further comprises a gas diffusion layer.
  • 10. The electrode of any preceding embodiment, wherein the electrode further comprises a gas diffusion layer, the catalyst layer being supported on the gas-diffusion layer.
  • 11. The electrode of any preceding embodiment, wherein the gas-diffusion layer comprises polytetrafluoroethylene, Sigracet 22 BB™, Carbon Paper Electrode, Carbon Cloth Electrode, or a combination thereof. The electrode of any preceding embodiment, wherein the gas-diffusion layer comprises polytetrafluoroethylene, Carbon Paper Electrode, Carbon Cloth Electrode, a porous metal, a metal foam, or a combination thereof. The electrode of any preceding embodiment, wherein the gas-diffusion layer comprises a non-woven carbon paper with a microporous Layer (MPL) comprising polytetrafluoroethylene (e.g., Sigracet 22 BB™). The electrode of any preceding embodiment, wherein the gas-diffusion layer comprises a porous metal comprising Ni, Cu, Ag, Zn, or a combination thereof. The electrode of any preceding embodiment, wherein the gas-diffusion layer comprises a metal foam comprising Ni, Cu, Ag, Zn, or a combination thereof.
  • 12. The electrode of any preceding embodiment, wherein the electrode is a cathode.
  • 13. The electrode of any preceding embodiment, wherein the electrode is an anode.
  • 14. The electrode of any preceding embodiment, wherein the solid polymer electrolyte layer bi-directionally conducts cations and anions. The electrode of any preceding embodiment, wherein the solid polymer electrolyte layer single-directionally conducts cations and/or anions.

In one or more embodiments, the solid polymer electrolyte layer single-directionally conducts cations and/or anions when the solid polymer electrolyte layer has a thickness approaching that of a standalone membrane. In one or more embodiments, the solid polymer electrolyte layer single-directionally conducts cations and/or anions when the solid polymer electrolyte layer has a thickness approaching that of a standalone membrane, where a standalone membrane has a thickness of about 25 μm to about 200 μm.

In one or more embodiments, the solid polymer electrolyte layer single-directionally conducts cations and anions when the solid polymer electrolyte layer has a loading on the catalyst layer that is ≥100 μL/cm².

In another embodiment of the present disclosure, there is provided:

• • 15. A membrane-free electrode assembly, the assembly comprising: an anode; and a cathode, the cathode comprising a catalyst layer; and a solid polymer electrolyte layer deposited on the catalyst layer for conducting ions.
  • 16. The assembly of embodiment 15, wherein the solid polymer electrolyte layer has a thickness of ≤25 μm, ≤20 μm, or ≤15 μm, or ≤10 μm, or ≤5 μm, or about 3 μm, or about ≤1 μm.
  • 17. The assembly of embodiment 15 or 16, wherein the solid polymer electrolyte layer comprises an ionomer with integrated alkali metal cations. The assembly of embodiment 15 or 16, wherein the solid polymer electrolyte layer comprises an ionomer with integrated alkali metal cations; or comprises an ionomer free of integrated alkali metal cations. The assembly of embodiment 15 or 16, wherein the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations; or comprises an ionomer free of integrated transition metal cations.
  • 18. The assembly of any preceding embodiment, wherein the solid polymer electrolyte layer comprises an ionomer loading of about 5 μL/cm² to about 150 μL/cm², or about 10 μL/cm² to about 150 μL/cm², or about 25 μL/cm² to about 100 μL/cm²; and/or an alkali metal cations concentration of about 0.1 M to about 3 M, or about 0.15 M to about 1 M.
  • 19. The assembly of any preceding embodiment, wherein the solid polymer electrolyte layer bi-directionally conducts cations and anions. The assembly of any preceding embodiment, wherein the solid polymer electrolyte layer single-directionally conducts cations and anions.
  • 20. The assembly of any preceding embodiment, wherein the ionomer comprises Nafion, Sustainion XA9™, AEMION™ Sustainion™, Aquivion, or a combination thereof. The assembly of any preceding embodiment, wherein the ionomer comprises perfluorinated sulfonic acid; sulfonated polyphenylene; polystyrene vinyl benzyl methyl imidazolium chloride; or a combination thereof.
  • 21. The assembly of any preceding embodiment, wherein the alkali metal cation comprises Li⁺, Na⁺, K⁺, Cs⁺, or a combination thereof. The assembly of any preceding embodiment, wherein the alkali metal cation comprises Li, Na, K, Cs, Rb, Fr, or a combination thereof. The assembly of any preceding embodiment, wherein the transition metal cation comprises Cu, Ni, Ag, Au, Pt, Al, Zn, Sn, Bi, Pd, or a combination thereof.
  • 22. The assembly of any preceding embodiment, wherein the catalyst layer comprises Cu, Ni, Ag, Au, Pt, or a combination thereof. The assembly of any preceding embodiment, wherein the catalyst layer comprises Cu, Ni, Ag, Au, Pt, Al, Zn, Sn, Bi, Pd, or a combination thereof.
  • 23. The assembly of any preceding embodiment, wherein the catalyst layer further comprises a gas diffusion layer.
  • 24. The assembly of any preceding embodiment, wherein the cathode further comprises a gas diffusion layer, the catalyst layer being supported on the gas-diffusion layer.
  • 25. The assembly of any preceding embodiment, wherein the gas-diffusion layer comprises polytetrafluoroethylene, Sigracet 22 BB™, Carbon Paper Electrode, Carbon Cloth Electrode, or a combination thereof. The assembly of any preceding embodiment, wherein the gas-diffusion layer comprises polytetrafluoroethylene, Carbon Paper Electrode, Carbon Cloth Electrode, a porous metal, a metal foam, or a combination thereof. The assembly of any preceding embodiment, wherein the gas-diffusion layer comprises a non-woven carbon paper with a microporous Layer (MPL) comprising polytetrafluoroethylene (e.g., Sigracet 22 BB™). The assembly of any preceding embodiment, wherein the gas-diffusion layer comprises a porous metal comprising Ni, Cu, Ag, Zn, or a combination thereof. The assembly of any preceding embodiment, wherein the gas-diffusion layer comprises a metal foam comprising Ni, Cu, Ag, Zn, or a combination thereof.
  • 26. The assembly of any preceding embodiment, wherein the anode comprises Ni, iridium oxide, or a combination thereof. The assembly of any preceding embodiment, wherein the anode comprises Ni, Pt, Pd, Ir, Fe, oxides thereof, alloys thereof, or a combination thereof. The assembly of any preceding embodiment, wherein the anode comprises Ni, or a NiFe layered double hydroxide catalyst.

In one or more embodiments, the anode comprises any metal or metal catalyst suitable for an electrode assembly. In one or more embodiments, the anode comprises any metal or metal catalyst suitable and/or stable for use in alkaline conditions. In one or more embodiments, the anode comprises any metal or metal catalyst suitable for minimizing overpotential. In one or more embodiments, the anode comprises any metal or metal catalyst suitable for and/or stable at higher current densities. In one or more embodiments, the anode comprises a transition metal catalyst. In one or more embodiments, the anode comprises a water oxidation catalyst; an organic oxidation catalyst; an oxidation catalyst, or a combination thereof.

In one or more embodiments, the assembly as described herein can operate and is stable at higher current densities, and thus exhibits better performance relative to assemblies that cannot operate or are not stable at higher current densities.

In another embodiment of the present disclosure, there is provided:

• • 27. Use of the electrode of any one of embodiments 1 to 14, or use of the membrane-free electrode assembly of any one of embodiments 15 to 26 for an electrolysis reaction, an electro-reduction reaction, or a combination thereof.
  • 28. The use of embodiment 27, wherein the reaction comprises CO₂ electrolysis, CO₂ electro-reduction, water electrolysis, CO electro-reduction, CO electrolysis, or a combination thereof.

In another embodiment of the present disclosure, there is provided:

• • 29. A method of making an electrode useful for membrane-free electrode assemblies, the method comprising: providing a catalyst layer; providing a solution comprising an ionomer and an alkali metal cation; depositing the solution on the catalyst layer; and forming the electrode. A method of making an electrode useful for membrane-free electrode assemblies, the method comprising: providing a catalyst layer; providing a solution comprising an ionomer and optionally comprising a metal cation; depositing the solution on the catalyst layer; and forming the electrode.
  • 30. The method of embodiment 29, wherein providing a solution comprising an ionomer and an alkali metal cation comprises: forming a ionomer solution comprising ionomer resin and solvent, optionally at a ratio of about 1:5 by volume of ionomer resin to solvent; forming a cation solution comprising alkali metal cation; and mixing the solutions together, optionally at a ratio of about 1:9 by volume of ionomer solution to cation solution. The method of embodiment 29, wherein providing a solution comprising an ionomer and optionally comprising a metal cation comprises: forming a ionomer solution comprising ionomer resin and solvent; and optionally forming a cation solution comprising metal cation and mixing the solutions together. The method of embodiment 29, wherein providing a solution comprising an ionomer and optionally comprising a metal cation comprises: forming a ionomer solution comprising ionomer resin and solvent at a resin:solvent ratio of about 1:1 to about 1:20; and optionally forming a cation solution comprising metal cation and mixing the solutions together at a cation solution:ionomer solution of about 1:1 to about 1:10.

In one or more embodiments, the solvent is used for dispersing the ionomer, otherwise referred to as ionomer resin. In one or more embodiments, the solvent comprises organic solvents, volatile organic solvents, water, aqueous solutions, or a combination thereof. In one or more embodiments, the solvent comprise methanol, ethanol, isopropanol, acetone, dichloromethane, THF, water, or a combination thereof.

• • 31. The method of embodiment 29 or 30, wherein providing a solution comprising an ionomer and an alkali metal cation comprises forming a ionomer solution having ionomer loading of about 25 μL/cm² to about 100 μL/cm²; and/or forming a cation solution having an alkali metal cations concentration of about 0.15 M to about 1 M. The method of embodiment 29 or 30, wherein providing a solution comprising an ionomer and optionally a metal cation comprises forming a ionomer solution having ionomer loading of about 25 μL/cm² to about 100 μL/cm²; and optionally forming a cation solution having metal cations concentration of about 0.15 M to about 1 M.
  • 32. The method of any preceding embodiment, wherein depositing the solution on the catalyst layer further comprises drying the solution deposited on the catalyst layer.
  • 33. The method of any preceding embodiment, wherein depositing the solution on the catalyst layer comprises depositing the solution on the catalyst layer by coating, spray coating, dipping, rolling, drop casting, or a combination thereof. The method of any preceding embodiment, wherein depositing the solution on the catalyst layer comprises depositing the solution on the catalyst layer by physical vapor deposition, chemical vapor deposition, physical vapor transport, electrochemical deposition, spray coating, dipping, rolling, drop casting, or a combination thereof.
  • 34. The method of any preceding embodiment, wherein providing a catalyst layer comprises providing a catalyst layer coupled to a gas diffusion layer.

In another embodiment of the present disclosure, there is provided:

• • 35. Use of the electrode made by the method of any one of embodiments 29 to 34 for an electrolysis reaction, an electro-reduction reaction, or a combination thereof.
  • 36. The use of embodiment 35, wherein the reaction comprises CO₂ electrolysis, CO₂ electro-reduction, water electrolysis, CO electro-reduction, CO electrolysis, or a combination thereof.

BRIEF DESCRIPTION OF THE FIGURES

Embodiments of the present disclosure will now be described, by way of example only, with reference to the attached Figures.

FIG. 1 depicts (a) illustration of AEM incorporated MEA for CO₂ electrolysis, (b) Faradaic efficiency of electrolysis products and cell voltage at different current density with AEM (Sustainion® X37-50 Grade RT) and 1 M KOH as anolyte, (c) concentration of OH⁻, CO₃²⁻ and HCO₃⁻ in the anolyte at 50 mA/cm² (initial volume 20 mL), (d) Faradaic efficiency of CO₂ electrolysis products and cell voltage at 50 mA/cm² over time.

FIG. 2 depicts (a) illustration of CEM incorporated MEA for CO₂ electrolysis, (b) Faradaic efficiency of electrolysis products and cell voltage at different current density with cation exchange membrane (Nafion 117) using 1 M KOH as anolyte, (c) Faradaic efficiency of electrolysis products and cell voltage at 50 mA/cm² over time, and (d) salt blockage in the serpentine channel and collected salt slurry draining from cathode outlet after 10 mins of CO₂R.

FIG. 3 depicts (a) schematic of MEA using USPE, (b) Faradaic efficiency of electrolysis products and cell voltage at different current density with USPE-100 using 1 M KOH as anolyte, (c) cross-sectional SEM image of USPE-50, Faradaic efficiency of electrolysis products and cell voltage at different current density with (d) USPE-50 using 1 M KOH as anolyte, and (e) USPE-50 using 3 M KOH as anolyte, (f) Faradaic efficiency of electrolysis products and cell voltage with USPE-50 at 50 mA/cm² over time (initial anolyte was 20 mL of 1 M KOH).

FIG. 4 depicts (a) SEM image of CISPE-50-0.15M, (b) Faradaic efficiency of electrolysis products and cell voltage at different current density with CISPE-50-0.15 M, (c) CISPE infused with different K⁺ concentration at 100 mA/cm², (d) CISPE-50-1M at different current density. The anolyte was 1 M KOH.

FIG. 5 depicts (a) Cell voltage and faradaic efficiency using CISPE-50-0.15M at 100 mA/cm² during reaction for 110 hours. Flowrate of humidified CO₂ was c.a. 6 sccm. (b) Summary of two performance metrics (stability and energy consumption for C₂H₄ production) of one-step CO₂ electrolysis in MEA setup. For depicted Ref [8, 9, 12, 14, 15, 17, 19, 21, 24, 63, 64 and 65] wherein a total product analysis was not reported and a high CO₂ flowrate was used, an achievable value of cathode separation was assumed to calculate the total energy consumption. Details of the performance metrics are provided in Table S.3 and the calculation is provided at Example 2, Section H.

FIG. 6 depicts illustration of anolyte titration using a strong acid as a titrant. Phenolphthalein and methyl orange were used as pH indicators.²

FIG. 7 depicts a schematic diagram of ion transport in (a) CEM and (b) AEM.

FIG. 8 depicts electrochemical impedance results taken at OCP in a three-electrode assembly. Nyquist plots of (a) pristine PTFE, (b) Cu sputtered PTFE (Cu/PTFE) and (c) USPE-50 respectively.

FIG. 9 depicts (a) Electrochemical impedance experimental setup taken at OCP in a Flow cell system. Air was circulated in the cathode, while 1 M KOH was circulated in the anode. (b) Nyquist plots of USPE-50 and CEM (Nafion™ 117). (c) The comparison of corresponding R_s values of the catalytic systems.

FIG. 10 depicts (a) Schematic diagram of proton conductivity experiment. The proton conductivity measurements for (b) the CEM and (c) USPE-50.

FIG. 11 depicts schematic charge and mass flow: (a) Anion exchange membrane (Sustainion® X37-50 Grade RT), (b) Cation exchange membrane (Nafion™ 117), (c) USPE-50 (d) CISPE-50-0.15M. Numbers are in millimole (mM) unit for 60 minutes basis at 50 mA/cm². Initial anolyte was 100 mL of 1 M KOH. The arrows indicate mass diffusion of compounds, or migration of ions. Diffusion mechanism is related to water uptake. KOH_(aq) diffusion indicates transport of hydrated K⁺ and its associated OH⁻. Similar explanation also stands for KHCO₃(a_q) transport. In case of commercial thick AEM and CEM, the charge balance is primarily governed by unidirectional ion migration (a and b). In contrast, the charge balance is governed by bidirectional ion migration mechanism (c and d) in case of directly deposited USPE and CISPE. The anion such as OH⁻, HCO₃⁻ and CO₃²⁻ are possible to migrate in AEM. To simplify the quantification, ion migration is written as CO₃²⁻ (final product in the anolyte). The aqueous K₂CO_3(aq) and KHCO_3(aq) are possibly diffuse in AEM and CEM. For simplicity, the carbonate diffuse is assumed as KHCO_3(aq)³.

FIG. 12 depicts the concentration of OH⁻ and CO₃ as measured and calculated. The measured OH⁻ concentration was lower than the concentration would be if the OH⁻ drop was only due to OH⁻ consumption for OER reaction. The extra OH⁻ drop can be related to OH⁻ diffusion and OH⁻ reaction with KHCO₃ from the anode. The measured CO₃²⁻ concentration was higher than the CO₃²⁻ concentration would be if the increase of CO₃²⁻ concentration was only due to the CO₃²⁻ migration. The extra CO₃²⁻ concentration can be related to the diffusion of KHCO_3(aq)³.

FIG. 13 depicts the faradaic efficiency and cell voltage of Nafion™ 211 using 1 M KOH as anolyte.

FIG. 14 depicts the salt precipitated in the serpentine channel after 1 hour using USPE-100 at 20 mA/cm².

FIG. 15 depicts SEM images of the USPE-50 surface. USPE-50 contains 50 mL/cm² of Nafion coating on sputtered Cu catalyst. The top-view SEM with high magnification (left) and low magnification (right) indicated the conformal coating of the gas diffusion layer by the USPE-50.

FIG. 16 depicts optical profilometry on the USPE coated catalyst surface. The studied area was 1100 μm×1000 μm. The maximum height (thickness) was 3.15 μm with the arithmetic mean height of 0.1284 μm and the root mean square height of 0.2048 (ISO 25178).

FIG. 17 depicts the faradaic efficiency and cell voltage of USPE-50 using 0.5 M K₂CO₃ as anolyte.

FIG. 18 depicts (a) FTIR spectra of USPE-50, CISPE-50-0.15M, and CISPE-50-1M. (b) Relative ratio of integrated area of —SO₃H⁺ to C—F as the representation of proton exchange with cation in the sulfonic acid groups in the Nafion structure.

FIG. 19 depicts the SEM (left) and EDX elemental mapping (right) of the cross-section of as-prepared CISPE-50-0.15M.

FIG. 20 depicts the faradaic efficiency of CO₂R using CISPE-50-0.15M and 1 M KOH anolyte at current density of 100 and different CO₂ flowrate.

FIG. 21 depicts the faradaic efficiency and cell voltage of (a) CISPE-50 with 0.15 M LiOH and (b) CISPE-50 with 0.15 M CsOH with 1 M KOH as anolyte.

FIG. 22 depicts the faradaic efficiency of CO₂R using CISPE-50-1M and 1 M KOH anolyte at current density of 100, 150 and 200 mA/cm².

FIG. 23 depicts schematic mass transport in different system as estimated in Section 1F: (a) Nafion™ 117 membrane with 3 M KOH as anolyte, (b) Nafion™ 211 membrane with 1 M KOH as anolyte, (c) USPE-25 with 1 M KOH as anolyte, and (d) USPE-100 with 1 M KOH as anolyte. The anion such as OH⁻, HCO₃⁻ and CO₃²⁻ are possible to transport in AEM. To simplify the quantification, ion migration was written as CO₃²⁻ (final product in the anolyte).

FIG. 24 depicts the concentration of hydroxide ion and carbonate ion in the anolyte at 50 mA/cm²: Initial volume of KOH anolyte was 30 mL.

FIG. 25 depicts the effect of current density and energy consumption on the levelized cost of C₂H₄. The methodology of the economic evaluation was adapted from Jouny et al.¹¹ with addition on carbonate recovery system. All parameter except CO₂ conversion (27%), C₂H₄ selectivity (65%) and electricity price ($0.02/kWh) was considered for the base scenario from Jouny et al.¹¹.

FIG. 26 depicts the summary of two performance matrices (current density and electrolyzer efficiency for C₂H₄) of one-step CO₂ electrolysis in MEA setup.

FIG. 27 depicts (a) a schematic diagram of AEM-based MEA configuration. (b) Faradaic efficiency and cell voltage of CO electrolysis using commercial AEM (Sustainion® X37-50 Grade RT Membrane) at different current densities and (c) over time at 50 mA/cm² using 1 M KOH. (d) The ion transport within the AEM is in mmol basis after 1 hour of operation at 50 mA/cm² using 1 M KOH. Bottom arrow and top arrow represent the electromigration (bottom) and diffusion (top), respectively. See Example 4—section G for the details of the ion transport experiment.

FIG. 28 depicts (a) schematic diagram of USPE. Faradaic efficiency and cell voltage of CO electrolysis at different current densities using (b) USPE-50; (c) USPE-12 with 1 M KOH anolyte; (d) cross-sectional SEM image of USPE-12. The dotted lines indicate the thickness of the ionomer layer. (e) Faradaic efficiency and cell voltage using USPE-12 at 50 mA/cm² with 1 M KOH anolyte for 120 minutes. (f) The electromigration of K⁺ and Faradaic efficiency of C₂H₄ using different loading of USPE with 1 M KOH as anolyte at 50 mA/cm².

FIG. 29 depicts (a) SEM image of pristine CISPE-12-0.15MKOH. The dotted lines indicate the thickness of the ionomer layer. (b) The anolyte flux to the cathode for Nafion™-117, USPE-50, USPE-12, and CISPE-12-0.15MCsOH under open circuit potential. The product distribution and cell voltage of COR with (c) CISPE-12-0.15M CsOH using different concentrations of anolyte (Ni foam as anode catalyst) at 150 mA/cm² and (d) CISPE-12-0.15 M CsOH with 0.5 M KOH anolyte and NiFe-LDH anode catalyst.

FIG. 30 depicts (a) The cell voltage and Faradaic efficiency using CISPE-12-0.15M CsOH at 100 mA cm⁻² during the reaction for 206 hours. (b) Summary of two performance matrices (current density and electrolyzer efficiency for C₂H₅OH production) of COR in MEA setup using alkaline anolyte.^{14, 16, 24, 40-44}

FIG. 31 depicts an illustration of anolyte titration using a strong acid as a titrant. Phenolphthalein and methyl orange were used as pH indicators 4.

FIG. 32 depicts a schematic diagram of ion transport in (a) CEM and (b) AEM based MEA.

FIG. 33 depicts OER studies at 1 M KOH. a) LSV at a scan rate of 5 mV/sec, b) Corresponding Tafel slopes, c) EIS at 1.6 V vs RHE, d-e) LSV and the overpotential difference at a defined current densities, f) GSTAT at 50 mA cm⁻² for 16 hr.

FIG. 34 depicts EIS studies in a flow-cell system at 1 M KOH. a) a typical flow-cell setup used for EIS study, b) Corresponding Nyquist plots at OCP (square=Cu/PTFE; circle=Cu/PTFE/Nafion-111; triangle=USPE-12), c) The observed R_s and R_ct values from the EIS results of USPE-12, Cu/PTFE, and Cu/PTFE/Nafion™-117(CEM).

FIG. 35 depicts the surface SEM image and the EDS analysis of the used sample for COR using AEM with 1 M KOH for 2 hours.

FIG. 36 depicts the selectivity and cell voltage using AEM MEA at 50 mA/cm² over time using different anolytes: (a) 0.2 M KOH anolyte (C₂H₄ FE drop=0.14%/minute, cell voltage drop=1.0 mV/minute) and (b) 1 M KOH+1 M KHCO₃ anolyte (C₂H₄ FE drop=0.22%/minute, cell voltage drop=3.2 mV/minute).

FIG. 37 depicts the product distribution and cell voltage of CO electrolysis using commercial AEM with different anolytes: (a) 0.2 M KOH and (b) 2 M KOH.

FIG. 38 depicts the effect of cation size on (a) CO electrolysis and (b) hydrogen evolution reaction. The electrolyte was 1 M KOH for 1 M [K⁺], and 1 K KOH+1 M K₂CO₃ for 3 M [K⁺].

FIG. 39 depicts (a) The schematic diagram of CEM MEA. (b) The Faradaic efficiency and cell voltage of CO electrolysis using commercial CEM (Nafion™-117) at different current densities and (c) over time at 50 mA/cm² using 1 M KOH (d) The ion transport within the CEM in mmol basis for 1 hour operation at 50 mA/cm² using 1 M KOH. Top arrow and bottom arrow represent the electromigration and diffusion, respectively. See Example 4—S1G for the details of ion transport experiment.

FIG. 40 depicts the surface SEM image and the EDX analysis of the used sample for COR using CEM with 1 M KOH for 2 hours.

FIG. 41 depicts the selectivity and cell voltage using Nafion-117 at 50 mA/cm² over time using 1 M LiOH.

FIG. 42 depicts ion transport within the membrane at 50 mA/cm² for 1 hours in different configuration: (a) USPE-50, (b) USPE-25, (c) CISPE-12-0.15MKOH, and (d) CISPE-12-0.15MCsOH.

FIG. 43 depicts the top view SEM image of fresh sample of USPE-12, after Cu sputtering and direct deposition of ionomer; and before any experiments were conducted.

FIG. 44 depicts (a) The FTIR spectra of USPE-12, CISPE-12-0.15MKOH, and CISPE-12-0.15MCsOH. (b) The relative ratio of integrated area of —SO₃H to C—F as the representation of proton exchange with cation in the sulfonic acid groups in the Nafion structure.

FIG. 45 depicts the SEM and EDX images of fresh CISPE-12-0.15MKOH.

FIG. 46 depicts the selectivity and cell voltage using (a) CISPE-12-0.15 M KOH and (b) CISPE-12-0.15 M CsOH at different current densities using 1 M KOH.

FIG. 47 depicts the product distribution and cell voltage of CO electrolysis using CISPE-12-0.15MCsOH at different current densities with (a) 3 M KOH anolyte, (b) 0.5 M KOH anolyte, and (c) 0.2 M KOH anolyte using bare Ni anode catalyst.

FIG. 48 depicts the product distribution of CO electrolysis using CISPE-12-0.15M CsOH with 0.5 M KOH anolyte at different current densities. Anode catalyst:NiFe LDH. CO flowrate was 50 standard mL/min.

FIG. 49 depicts the product distribution of CO electrolysis using CISPE-12-0.15M CsOH at 100 mA/cm² with different CO flowrates. Anolyte: 0.5 M KOH; anode catalyst:NiFe LDH. For 1.2 standard mL/min, the actual flowrate was fluctuating from 0.8 to 1.4 standard mL/min.

FIG. 50 depicts the CO conversion of CISPE-12-0.15MCsOH using 0.5 M KOH anolyte with NiFe LDH at different CO flowrate.

FIG. 51 depicts the electrolyzer energy efficiency for COR-to-C₂H₄ at different current densities using pristine Ni foam (bottom curve) and NiFe LDH (top curve) as anode catalysts using 0.5 M KOH anolyte. CO flowrate was 50 standard mL min⁻¹.

FIG. 52 depicts the product distribution and cell voltage of CO electrolysis using CISPE-12-0.15MCsOH with 0.2 M KOH anolyte. Anode catalyst:NiFe LDH.

FIG. 53 depicts the product distribution of CO electrolysis using CISPE-12-0.15M CsOH at 100 mA/cm² with different anolyte concentrations. Anode catalyst:NiFe LDH.

FIG. 54 depicts a summary of performance matrices (a) current density, electrolyzer efficiency for C₂H₄ production and stability of CO electrolysis in MEA setup; (b) current density and energy efficiency for C₂H₄ production; (c) stability and energy efficiency for C₂H₄ production; (d) current density and energy efficiency for C₂H₄ and C₂H₅OH production. Data for our work was obtained using CISPE-12-0.15MCsOH with 0.5 M KOH anolyte.

FIG. 55 depicts (a) Top view appearance. SEM images of CISPE-12-0.15MCsOH after 206 hours of reaction (b) top-view and (c) cross section.

FIG. 56 depicts the SEM and EDX images of CISPE-12-0.15MCsOH after 206 hours of reaction.

FIG. 57 depicts the effect of current density and energy consumption on the levelized cost of C₂H₄.

FIG. 58 depicts (a) the schematic diagram of the MEA system; (b) partial current density of products C₂H₄ and H₂ and cell voltage at different current density; (c) schematic mass and ion transport through the membrane at 50 mA/cm². The dotted arrow and the solid arrow represent the diffusion and the electromigration of ion, respectively; and (d) change in faradaic efficiency of gas products and cell voltage at 150 mA/cm² overtime; using Sustainion® X37-50 Grade RT membrane and using 100 mL of 1 M KOH as anolyte in b-d.

FIG. 59 depicts cross sectional SEM image of 25 μL/cm² of Sustainion XA9/Cu; using solvent (a) methanol and (b) combination mixture of isopropyl alcohol and DI water (1:1 by mass). Faradaic efficiency of electrolysis gas products and cell voltage at different current density using 25 μL/cm² of Sustainion XA9/Cu; using solvent (c) methanol and (d) combination mixture of isopropyl alcohol and DI water (1:1 by mass); using 100 mL of 1 M KOH as anolyte in c and d.

FIG. 60 depicts (a) partial current density of products C₂H₄ and H₂ and cell voltage at different current density and (b) mass transport using 12.5 μL/cm² of Sustainion XA9/Cu, (c) partial current density of products C₂+ and H₂ and cell voltage at different current density and (d) mass transport using 25 μL/cm² of Sustainion XA9/Cu and (e) partial current density of products C₂H₄ and H₂ and cell voltage at different current density and (f) mass transport using 50 μL/cm² of Sustainion XA9/Cu; using 100 mL of 1 M KOH as anolyte. The dotted arrow and the solid arrow represent the diffusion and the electromigration of ion, respectively in b, d and f.

FIG. 61 depicts (a) Schematic diagram of the MEA system using Sustainion XA-9 ionomer coating on Cu catalyst; (b) cross sectional SEM image of 25 μL/cm² of Sustainion XA9/Cu; (c) partial current density towards C₂H₄ and H₂ and cell voltage at three different loadings of ionomer at 350 mA/cm²; (d) total products distribution and cell voltage at different current density using 25 μL/cm² of Sustainion XA9/Cu; using 100 mL of 1 M KOH as anolyte in c and d.

FIG. 62 depicts (a) Schematic diagram of the MEA system using bilayer ionomer coating on Cu catalyst; (b) cross sectional SEM image of 25 μL Sustainion XA9/25 μL Nafion/Cu; (c) partial current density towards C₂H₄ and H₂ and cell voltage at different current density using 25 μL/cm² of Sustainion XA9/25 μL/cm² Nafion/Cu; (d) change in faradaic efficiency of gas products and cell voltage over time at 150 mA/cm²; using 100 mL of 1 M KOH as anolyte in c and d.

FIG. 63 depicts (a) cross sectional SEM image of 25 μL/cm² of Sustainion XA9/25 μL/cm² Nafion+1 M KOH/Cu; (b) gas products distribution and cell voltage at 250 mA/cm² using different concentration of KOH infusion; (c) partial current density towards C₂₊ and H₂ products at different current density; using 100 mL of 1 M KOH as anolyte in b and c; (d) change in cell voltage and faradic efficiency of C₂₊ and H₂ at 350 mA/cm² current density over 24 hours period of time.

FIG. 64 depicts mass transport in bilayer ionomers with different cation concentration infused in Nafion layer; (a) 0 M, (b) 0.15 M, (c) 0.5 M and (d) 1 M. The dotted arrow and the solid arrow represent the diffusion and the electromigration of ion, respectively.

FIG. 65 depicts water diffusion through different ionomer samples from anode to cathode without any applied potential.

FIG. 66 depicts CO₂ mass transfer flux through different ionomer samples from cathode to anode.

DETAILED DESCRIPTION

Unless defined otherwise, all technical and scientific terms used herein have the meaning as commonly understood in the art.

As used in the specification and claims, the singular forms “a”, “an” and “the” include plural references unless the context dictates otherwise.

Carbon Dioxide Electroreduction (CO₂R)

Economic viability of carbon dioxide electroreduction (CO₂R) relies on improved performance accompanied with scalable system design. Membranes are commonly used for the separation of reduction and oxidation products, as well as to provide a suitable micro-environment for CO₂R. Commercial membranes often address only one of the key challenges in CO₂R: either they offer suitable micro-environment for CO₂R (e.g., anion exchange membrane) or suppress carbonate cross-over (e.g., cation exchange membrane and bipolar membrane).

Inspired by the proton exchange membrane (PEM) water electrolyzers, CO₂ electrolysis in a membrane electrode assembly (MEA) has gained momentum.⁴ The use of solid polymer electrolyte in a MEA offers several advantages over liquid catholyte in a flow-type cell, including significant reduction in ohmic overpotential due to thin polymer electrolyte, protection of the catalysts from delamination and poisoning, avoidance of cell leakage, as well as ease of scale up and handling.^4,5

Three types of membranes have been reported for CO₂R: namely (i) anion exchange membrane (AEM), (ii) cation exchange membrane (CEM) and (iii) bipolar membrane (BPM).⁶ When AEM is used, the charge balance is primarily governed by the transport of anionic species (e.g., carbonate (CO₃²⁻), hydroxide (OH⁻) and bicarbonate (HCO₃⁻))^{4, 7}, and the high pH local environment at the cathode surface improves the activity of CO₂R^8-10. Nevertheless, significant loss of CO₂, due to crossover in the form of anionic species across the AEM, has been recognized as a key challenge of AEM-based CO₂ electrolyzers^7,11,12. This carbonate crossover leads to the neutralization of the alkaline anolyte over the course of reaction, which results in dissolution of Ni-based low-cost catalyst.¹³ Moreover, this sets a requirement to find active and stable catalyst for the oxygen evolution reaction (OER) in near-neutral electrolyte. Consequently, most of the long-term CO₂ electrolysis experiments in MEA cell are performed in neutral electrolyte with precious Ir-based catalyst as anode (see Table S.3)^{8, 9, 12, 14-24}.

Alternatively, a bipolar membrane (BPM) can be operated in reverse bias mode (i.e., cation exchange layer facing the cathode) to suppress undesired CO₃²⁻ crossover with the benefit of using non-noble metal catalyst (e.g., Ni) in the alkaline anolyte.^25,26 Despite these advantages and encouraging results, BPM-based configurations have their own limitations, including excessive hydrogen evolution reaction (HER) due to high H⁺ flux at the cathode, delamination of cationic and anionic layers, and high operating voltage associated with water dissociation (thermodynamic voltage ˜0.83 V) at the bipolar interface.^27,28 There are additional challenges with BPM, including the voltage drop associated with ohmic losses and low hydration of the membrane.^26,29

Another MEA cell configuration to suppress CO₃²⁻ crossover is to use a cation exchange membrane (CEM) with neutral or acidic anolyte.^12,30,31 Perfluorinated sulfonic acid (PFSA) ionomer such as Nafion-based CEM can be used in CO₂ electrolysis wherein H⁺ as well as alkali metal cations electro-migrate via ion exchange mechanism, and negatively charged sulfonic acid groups suppress anion transport (i.e., OH⁻, CO₃²⁻).³² The high mobility of H⁺ together with well-developed and robust CEM offers lowest cell resistance. The use of CEM with acidic or neutral anolyte, however, creates a highly acidic reaction environment at the cathode which favors the HER¹². It has been demonstrated that with proper interface engineering, CO₂ regeneration can be achieved at the cathode by the incoming H⁺ instead of undergoing HER.¹² However, the stability of this approach has not been demonstrated for extended experimental period (>8 hours).¹²

In all these MEA configurations for CO₂ electrolysis, the use of alkaline anolyte offers multiple benefits. Apart from permitting the use of Ni-based catalyst (as opposed to precious Ir-based ones in neutral anolyte), it supports higher conductivity, faster OER kinetics and low cell resistance, resulting in low cell voltage^{30, 33}. Additionally, alkali metal cations from the anolyte can electro-migrate to the cathode instead of H⁺ under the operating conditions. It has been demonstrated that excessive cation migration towards the cathode leads to precipitation of carbonate salts which impedes cell performance^{19, 31}. However, slow crossing of cation towards the cathode as well as the presence of a small amount of cation is necessary to achieve high CO₂ activity^34-38.

Combining the benefits of the CEM and alkaline anolyte, it was thought that alkali cation migration from the anode could be tuned by reducing the thickness of the CEM, wherein charge balance can be governed by a bidirectional flow of cations and anions. Thus, if slower migration of alkali metal cation can be achieved simultaneously with slow CO₃²⁻ cross-over through the CEM, high stability and CO₂ utilization efficiencies may result.

Described herein is an electrode useful for membrane-free electrode assemblies, the electrode comprising: a catalyst layer; and a solid polymer electrolyte layer for ion-conduction supported on the catalyst layer. As described herein, the solid polymer electrolyte layer for ion-conduction supported on the catalyst layer is directly deposited on the catalyst layer, such that is it not a standalone membrane. As used herein, ‘membrane-free’ refers to a lack of a stand-alone membrane that is introduced as a separate component into electrode assemblies. A standalone membrane is one that is not directly deposited onto an electrode, or the catalyst layer of an electrode; and/or a standalone membrane is a pre-made component that is then used in an electrode assembly.

In an example, it is described and/or demonstrated that substituting commonly used thick and standalone AEM or CEM with a directly-deposited ultrathin (˜3 μm) PFSA ionomer (e.g., nafion) enabled a high electrolysis efficiency of 27% at 100 mA/cm². This translated into lower energy costs (including CO₂ conversion and carbonate regeneration) in a one-step CO₂ electrolysis to ethylene (e.g., about 296, or about 290 GJ/ton) with 110 hours of stable operation in a MEA CO₂R with alkaline anolyte. Detailed analysis indicated that such performance may have been achieved due to effects of i) bidirectional ion transport through the ultrathin ionomer layer limiting salt precipitation and carbonate cross-over, ii) low water uptake of the ultrathin ionomer layer limiting cathode flooding and improving CO₂R selectivity and stability, and iii) low cell voltage due to use of ultrathin ionomer layer and alkaline anolyte with Ni based anode. By infusing K⁺ into the ultrathin ionomer layer, further demonstrated was a ˜90% selectivity towards CO₂R products at 200 mA/cm² with a C₂₊ partial current density of 144 mA/cm².

Carbon Monoxide Electroreduction (COR)

Power-to-chemical routes have received growing attention in the recent past to generate multi-carbon (C₂₊) products. Carbonate formation has been a challenge on direct CO₂ electroreduction (CO₂R) to C₂₊ products for deployment to the commercial scale.¹ Fortunately, high selectivity and stability have been demonstrated for CO₂ electroreduction (CO₂R) to CO using high temperature electrolyzers (solid oxide systems).^{2, 3} The high system performance of CO₂R-to-CO amplifies the opportunity of CO electroreduction (COR) to be a downstream step of a two-step decarbonization strategy (i.e., CO₂→CO→C₂₊) with higher energy efficiency, thanks to the use of CO feed in COR which can avoid carbonate formation and consequently enable the use of alkaline electrolyte. Considering the low solubility of CO (i.e., approximately 2.89×10−5 gram CO/gram H₂O at 25° C.) in aqueous electrolytes,⁴ COR has transitioned from H-cell configuration to membrane electrode assembly (MEA) electrolyzers enabling the use of gas diffusion layer (GDL) to achieve industrially applicable current density (>300 mA/cm²).⁵ The presence of solid polymer electrolyte (SPE) in MEA (as opposed to liquid electrolyte in flow-cell configuration) leads to multiple benefits, such as less tendency to cell flooding, lower ohmic overpotential and catalyst protection from degradation.^{6, 7}

The membrane governs ion transport (e.g., conductivity and selectivity) and water transport (e.g., diffusion) in MEA.³ Membranes which primarily facilitate anion transport, namely anion exchange membrane (AEM) has been recently studied for COR.^5,8 COR can be operated using AEM in MEA using alkaline anolyte without sacrificing anolyte pH.⁸ In this configuration, the charge is mainly balanced by the electromigration of hydroxide ion (OH⁻) from the cathode to the anode which recovers the consumed OH⁻ during oxygen evolution reaction (OER). Water transport also plays a role in COR. When alkaline anolyte (e.g., potassium hydroxide, KOH) is used, the concentration gradient between the anode and the cathode promotes water diffusion across the AEM along with the hydrated cations and its corresponding anion (it is called aqueous KOH (KOH_(aq))). Wheeler et al. quantified the water transport across the AEM in MEA using 1 M KOH anolyte.⁹ The same group synthesized thin (˜25 μm) AEM and reported that thinner membrane suppresses water flux to the cathode due to the water repulsion by the hydrophobic cathode (back convection).¹⁰

It is widely accepted that OH⁻ promotes selectivity for C₂₊ by enhancing the dimerization of adsorbed CO at high pH.^{11, 12} One can exploit the benefits of OH⁻ by using a cation exchange membrane (CEM) which primarily allows cation transport from the anode and suppresses the transport of locally generated OH⁻ towards the anode. This in turn maintains high local pH at the cathode and intercepts the protons before it can be consumed for hydrogen evolution reaction (HER).¹³ However, in practice, the use of CEM for COR is challenging. Apart from water diffusion, the electroosmotic drag due to the migration of hydrated cation tends to magnify the water transport to the cathode. The excessive water transport to the cathode (cathode flooding) causes pore blockage by water.¹⁰ As a result, a high-pressure flow of CO (>4 bar) tends to be required to minimize the water accumulation to maintain the COR selectivity.¹⁴

Alkaline anolyte offers lower cell voltage due to lower overpotential for OER, resulting in high electrolyzer energy efficiency, and enables the use of non-noble anode catalyst (e.g., nickel).¹⁵ It has been reported that the use of alkaline anolyte (e.g., 1 M KOH) can exhibit a C₂₊ selectivity of up to ˜91%.¹⁶ However, a study by Ozden et al. showed that excessive concentration of alkaline anolyte (>3 M KOH) also leads to lower C₂₊ selectivity due to low CO availability at the catalyst layer, which is caused by a poisoning effect of incoming cations from the anolyte.¹⁶ By using a covalent organic framework (COF) that can offer controlled cation diffusion functionalities, Ozden et al. reported high energy efficiency (towards C₂₊) and single pass conversion efficiency of 41% and 91%, respectively. Another strategy to overcome this challenge is adding a thin coating of ionomer (e.g., Nafion at a thickness of about 10 nm) on top of the catalyst layer to act as a bridge between a standalone ion exchange membrane and an electrode (e.g., to improve dispersion of gaseous components between the electrode and standalone membrane). It is reported that the Nafion layer enhances CO availability at the catalyst interface.¹⁷ Nafion is a perfluorosulfonic acid (PFSA) polymer which consists of the sulfonic acid group (SO₃⁻) as hydrophilic side-chains over the hydrophobic polytetrafluoroethylene (PTFE) backbone. The SO₃⁻ facilitates electromigration of cation, while the PTFE backbone makes Nafion a mechanically robust and chemically inert cation transport ionomer.^{18, 19} The cation conducting nature (or ionic selectivity) of Nafion can be tuned into anion conducting by modifying the SO₃⁻ with proazaphosphatranium²⁰ and dimethylpiperazinium.^{21, 22} As described herein, another strategy to adjust the ionic conductivity of an ionomer such as Nafion is by controlling the thickness using a direct deposition method.²³ Previous studies reported that controlling the thickness enables the adjustment of the charge balance²³ and lessened the cathode flooding.¹⁰ As described herein, the ionomer is deposited directly onto a catalyst layer of an electrode to act as a solid polymer electrolyte, thereby replacing and negating the need of a standalone membrane. As described herein, the ionomer is deposited at a thickness on a scale that is greater than nanometers, such as on the scale of μm. Thicknesses on the scale of nanometer could cause an electrode assembly, such as a MEA, to short-circuit, and thus would not be suitable to replace a standalone membrane.

Combining these benefits, it was hypothesized that the electromigration of cation, as well as the water transport towards the cathode, may be optimized by changing the thickness of a directly deposited PFSA layer. If the charge balance is governed by electromigration of OH⁻, water transport can also be suppressed, thereby a stable and high selectivity COR would be possible as a result.

Described herein is an electrode useful for membrane-free electrode assemblies, the electrode comprising: a catalyst layer; and a solid polymer electrolyte layer for ion-conduction supported on the catalyst layer. In an example, use of a directly-deposited cation-infused ultrathin Nafion ionomer, which substituted the need for a standalone membrane, is described and/or demonstrated for enhanced CO electrolysis as indicated by high electrolysis energy efficiency (21%) toward ethylene (C₂H₄) at 100 mA/cm² for over 200 hours of operation in MEA configuration. In a broader perspective, this performance may enable a lower energy cost of a two-step CO₂-to-C₂H₄ reaction (e.g., about 218 GJ/ton-C₂H₄) using an alkaline anolyte. This performance was accomplished using an ultrathin Nafion layer which could suppress electromigration of K⁺ and water diffusion. Selectivity was further enhanced towards COR products at higher current densities by implanting Cs⁺ cation in the directly deposited Nafion layer. In contrast to the cation that electromigrates from the anode and accumulates at the cathode which subsequently impedes CO availability, the infused cation in the directly deposited solid polymer electrolyte (SPE) was found to maintain just the stoichiometric amount to enhance C₂₊ selectivity. In an example, C₂₊ selectivity refers to selectively forming carbon products comprising at least two carbons (e.g., C₂CH₄, C₂H₅OH, etc).

In an example, the present disclosure generally provides:

• • 1. An electrode useful for membrane-free electrode assemblies, the electrode comprising: a catalyst layer; and a solid polymer electrolyte layer for ion-conduction supported on the catalyst layer.
  • 2. The electrode of example 1, wherein the solid polymer electrolyte layer supported on the catalyst layer comprises the solid polymer electrolyte layer deposited on the catalyst layer.

In one or more examples, the solid polymer electrolyte layer is deposited directly on the catalyst layer.

In one or more examples, the solid polymer electrolyte layer is deposited directly on the catalyst layer such that is it is not a standalone membrane.

• • 3. The electrode of example 1 or 2, wherein the solid polymer electrolyte layer has a thickness of ≤25 μm, ≤20 μm, or ≤15 μm, or ≤10 μm, or ≤5 μm, or about 3 μm, or 51 μm. The electrode of example 1 or 2, wherein the solid polymer electrolyte layer has a thickness of <25 μm, ≤20 μm, or ≤15 μm, or ≤10 μm, or ≤5 μm, or about 3 μm, or ≤1 μm.

In one or more examples, the solid polymer electrolyte layer has a thickness that is sufficient to support the standard and/or expected operation of an electrode assembly. In one or more embodiments, the solid polymer electrolyte layer has a thickness that is sufficient to reduce, inhibit, or prevent short-circuiting of any electrode assembly is it used with.

In one or more examples, the solid polymer electrolyte layer has a thickness that is on the order of micrometers (μm).

In one or more examples, the solid polymer electrolyte layer has a thickness that is not on the order of nanometers (nm), as such a thickness may prevent an assembly from functioning; such as, by short circuiting.

• • 4. The electrode of any one of examples 1 to 3, wherein the solid polymer electrolyte layer comprises an ionomer with integrated alkali metal cations. The electrode of any one of examples 1 to 3, wherein the solid polymer electrolyte layer comprises an ionomer with integrated alkali metal cations; or comprises an ionomer free of integrated alkali metal cations. The electrode of any one of examples 1 to 3, wherein the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations; or comprises an ionomer free of integrated transition metal cations.

In one or more examples, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the solid polymer electrolyte layer is prepared by the methods as herein.

In one or more examples, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the solid polymer electrolyte layer is directly deposited on an electrode or catalyst layer by the methods described herein.

In one or more examples, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the integrated transition metal cations act as a catalyst.

In one or more examples, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the integrated transition metal cations act as an ionomer-dispersed catalyst layer.

In one or more examples, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the resultant electrode is suitable for use in one or more of the electroreduction reactions as described herein.

In one or more examples, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the integrated transition metal cations facilitate catalysis of one or more of the electroreduction reactions as described herein.

In one or more examples, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the integrated transition metal cations facilitate catalysis of one or more of the gaseous components introduced into the one or more electroreduction reactions as described herein.

In one or more examples, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the resultant electrode provides C₂+ selectivity in one or more of the electroreduction reactions as described herein.

• • 5. The electrode of any one of examples 1 to 4, wherein the solid polymer electrolyte layer comprises an ionomer loading of about 10 μL/cm² to about 150 μL/cm², or about 25 μL/cm² to about 100 μL/cm²; and/or an alkali metal cations concentration of about 0.1 M to about 3 M, or about 0.15 M to about 1 M. The electrode of any one of examples 1 to 4, wherein the solid polymer electrolyte layer comprises an ionomer loading of about 5 μL/cm² to about 150 μL/cm², or about 10 μL/cm² to about 150 μL/cm², or about 25 μL/cm² to about 100 μL/cm²; and/or a metal cations concentration of about 0 M to about 10 M, or about 0 M to about 3 M, or about 0.1 M to about 3 M, or about 0.15 M to about 1 M.
  • 6. The electrode of any one of examples 1 to 5, wherein the ionomer comprises Nafion, Sustainion XA9™, AEMION™ Sustainion™, Aquivion, or a combination thereof. The electrode of any one of examples 1 to 5 wherein the ionomer comprises perfluorinated sulfonic acid; sulfonated polyphenylene; polystyrene vinyl benzyl methyl imidazolium chloride; or a combination thereof.

An ionomer is a polymer where at least some of the monomer units comprises an ionic functionality. In one or more examples, the ionomer is any ionomer acceptable for use in an ion exchange membrane.

In one or more examples, the ionomer is any ionomer acceptable for use in a cation exchange membrane. In one or more examples, the ionomer is any ionomer acceptable for use in an anion exchange membrane. In one or more embodiments, the ionomer comprises anion exchange ionomer; cation exchange ionomer; or a combination thereof.

In one or more examples, the ionomer comprises a perfluorosulfonic acid polymer, a sulfonated tetrafluoroethylene based fluoropolymer-copolymer, or a combination thereof (e.g., Nafion.)

In one or more examples, the ionomer comprises a 1H-Imidazole, 1,2,4,5-tetramethyl-, compound with 1-(chloromethyl)-4-ethenylbenzene polymer with ethenylbenzene (e.g., Sustainion XA9™) In one or more embodiments, the ionomer is an alkaline ionomer comprising hydrophilic poly (4-vinylbenzyl alkyl-imidazolium chloride) (e.g., Sustainion XA9™)

In one or more examples, the ionomer comprises a hydrocarbon backbone (e.g., AEMION™). In one or more embodiments, the ionomer is an alkaline ionomer comprising hydrophilic poly (4-vinylbenzyl alkyl-imidazolium chloride) (e.g., AEMION™).

• • 7. The electrode of any one of examples 1 to 6, wherein the alkali metal cation comprises Li⁺, Na⁺, K⁺, Cs⁺, or a combination thereof. The electrode of any one of examples 1 to 6, wherein the alkali metal cation comprises Li, Na, K, Cs, Rb, Fr, or a combination thereof. The electrode of any one of examples 1 to 6, wherein the transition metal cation comprises Cu, Ni, Ag, Au, Pt, Al, Zn, Sn, Bi, Pd, or a combination thereof.
  • 8. The electrode of any one of examples 1 to 7, wherein the catalyst layer comprises Cu, Ni, Ag, Au, Pt, or a combination thereof. The electrode of any one of examples 1 to 7, wherein the catalyst layer comprises Cu, Ni, Ag, Au, Pt, Al, Zn, Sn, Bi, Pd, or a combination thereof.
  • 9. The electrode of any one of examples 1 to 8, wherein the catalyst layer further comprises a gas diffusion layer.
  • 10. The electrode of any one of examples 1 to 8, wherein the electrode further comprises a gas diffusion layer, the catalyst layer being supported on the gas-diffusion layer.
  • 11. The electrode of example 9 or 10, wherein the gas-diffusion layer comprises polytetrafluoroethylene, Sigracet 22 BB™, Carbon Paper Electrode, Carbon Cloth Electrode, or a combination thereof. The electrode of example 9 or 10, wherein the gas-diffusion layer comprises polytetrafluoroethylene, Carbon Paper Electrode, Carbon Cloth Electrode, a porous metal, a metal foam, or a combination thereof. The electrode of example 9 or 10, wherein the gas-diffusion layer comprises a non-woven carbon paper with a microporous Layer (MPL) comprising polytetrafluoroethylene (e.g., Sigracet 22 BB™). The electrode of example 9 or 10, wherein the gas-diffusion layer comprises a porous metal comprising Ni, Cu, Ag, Zn, or a combination thereof. The electrode of any preceding embodiment, wherein the gas-diffusion layer comprises a metal foam comprising Ni, Cu, Ag, Zn, or a combination thereof.
  • 12. The electrode of any one of examples 1 to 11, wherein the electrode is a cathode.
  • 13. The electrode of any one of examples 1 to 11, wherein the electrode is an anode.
  • 14. The electrode of any one of example 1 to 13, wherein the solid polymer electrolyte layer bi-directionally conducts cations and anions. The electrode of any one of example 1 to 13, wherein the solid polymer electrolyte layer single-directionally conducts cations and/or anions.

In one or more examples, the solid polymer electrolyte layer single-directionally conducts cations and/or anions when the solid polymer electrolyte layer has a thickness approaching that of a standalone membrane. In one or more examples, the solid polymer electrolyte layer single-directionally conducts cations and/or anions when the solid polymer electrolyte layer has a thickness approaching that of a standalone membrane, where a standalone membrane has a thickness of about 25 μm to about 200 μm.

In one or more examples, the solid polymer electrolyte layer single-directionally conducts cations and anions when the solid polymer electrolyte layer has a loading on the catalyst layer that is ≥100 μL/cm².

In another example, the present disclosure generally provides:

• • 15. A membrane-free electrode assembly, the assembly comprising: an anode; and a cathode, the cathode comprising a catalyst layer; and a solid polymer electrolyte layer deposited on the catalyst layer for conducting ions.
  • 16. The assembly of example 15, wherein the solid polymer electrolyte layer has a thickness of ≤25 μm, ≤20 μm, or ≤15 μm, or ≤10 μm, or ≤5 μm, or about 3 μm, or about ≤1 μm.
  • 17. The assembly of example 15 or 16, wherein the solid polymer electrolyte layer comprises an ionomer with integrated alkali metal cations. The assembly of example 15 or 16, wherein the solid polymer electrolyte layer comprises an ionomer with integrated alkali metal cations; or comprises an ionomer free of integrated alkali metal cations. The assembly of example 15 or 16, wherein the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations; or comprises an ionomer free of integrated transition metal cations.
  • 18. The assembly of any one of examples 15 to 17, wherein the solid polymer electrolyte layer comprises an ionomer loading of about 5 μL/cm² to about 150 μL/cm², or about 10 μL/cm² to about 150 μL/cm², or about 25 μL/cm² to about 100 μL/cm²; and/or an alkali metal cations concentration of about 0.1 M to about 3 M, or about 0.15 M to about 1 M.
  • 19. The assembly of any one of example 15 to 18, wherein the solid polymer electrolyte layer bi-directionally conducts cations and anions. The assembly of any one of example 15 to 18, wherein the solid polymer electrolyte layer single-directionally conducts cations and anions.
  • 20. The assembly of any one of examples 15 to 19, wherein the ionomer comprises Nafion, Sustainion XA9™, AEMION™ Sustainion™, Aquivion, or a combination thereof. The assembly of any one of examples 15 to 19, wherein the ionomer comprises perfluorinated sulfonic acid; sulfonated polyphenylene; polystyrene vinyl benzyl methyl imidazolium chloride; or a combination thereof.
  • 21. The assembly of any one of examples 15 to 20, wherein the alkali metal cation comprises Li⁺, Na⁺, K⁺, Cs⁺, or a combination thereof. The assembly of any one of examples 15 to 20, wherein the alkali metal cation comprises Li, Na, K, Cs, Rb, Fr, or a combination thereof. The assembly of any one of examples 15 to 20, wherein the transition metal cation comprises Cu, Ni, Ag, Au, Pt, Al, Zn, Sn, Bi, Pd, or a combination thereof.
  • 22. The assembly of any one of examples 15 to 21, wherein the catalyst layer comprises Cu, Ni, Ag, Au, Pt, or a combination thereof. The assembly of any one of examples 15 to 21, wherein the catalyst layer comprises Cu, Ni, Ag, Au, Pt, Al, Zn, Sn, Bi, Pd, or a combination thereof.
  • 23. The assembly of any one of examples 15 to 22, wherein the catalyst layer further comprises a gas diffusion layer.
  • 24. The assembly of any one of examples 15 to 22, wherein the cathode further comprises a gas diffusion layer, the catalyst layer being supported on the gas-diffusion layer.
  • 25. The assembly of example 23 or 24, wherein the gas-diffusion layer comprises polytetrafluoroethylene, Sigracet 22 BB™, Carbon Paper Electrode, Carbon Cloth Electrode, or a combination thereof. The assembly of example 23 or 24, wherein the gas-diffusion layer comprises polytetrafluoroethylene, Carbon Paper Electrode, Carbon Cloth Electrode, a porous metal, a metal foam, or a combination thereof. The assembly of example 23 or 24, wherein the gas-diffusion layer comprises a non-woven carbon paper with a microporous Layer (MPL) comprising polytetrafluoroethylene (e.g., Sigracet 22 BB™). The assembly of example 23 or 24, wherein the gas-diffusion layer comprises a porous metal comprising Ni, Cu, Ag, Zn, or a combination thereof. The assembly of example 23 or 24, wherein the gas-diffusion layer comprises a metal foam comprising Ni, Cu, Ag, Zn, or a combination thereof.
  • 26. The assembly of any one of examples 15 to 25, wherein the anode comprises Ni, iridium oxide, or a combination thereof. The assembly of any one of examples 15 to 25, wherein the anode comprises Ni, Pt, Pd, Ir, Fe, oxides thereof, alloys thereof, or a combination thereof. The assembly of any one of examples 15 to 25, wherein the anode comprises Ni, or a NiFe layered double hydroxide catalyst.

In one or more examples, the anode comprises any metal or metal catalyst suitable for an electrode assembly.

In one or more examples, the anode comprises any metal or metal catalyst suitable and/or stable for use in alkaline conditions.

In one or more examples, the anode comprises any metal or metal catalyst suitable for minimizing overpotential.

In one or more examples, the anode comprises any metal or metal catalyst suitable for and/or stable at higher current densities.

In one or more examples, the anode comprises a transition metal catalyst. In one or more examples, the anode comprises a water oxidation catalyst; an organic oxidation catalyst; an oxidation catalyst, or a combination thereof.

In one or more examples, the assembly as described herein can operate and is stable at higher current densities, and thus exhibits better performance relative to assemblies that cannot operate or are not stable at higher current densities.

In another example, the present disclosure generally provides:

• • 27. Use of the electrode of any one of examples 1 to 14, or use of the membrane-free electrode assembly of any one of examples 15 to 26 for an electrolysis reaction, an electro-reduction reaction, or a combination thereof.
  • 28. The use of example 27, wherein the reaction comprises CO₂ electrolysis, CO₂ electro-reduction, water electrolysis, CO electro-reduction, CO electrolysis, or a combination thereof.

In another example, the present disclosure generally provides:

• • 29. A method of making an electrode useful for membrane-free electrode assemblies, the method comprising: providing a catalyst layer; providing a solution comprising an ionomer and an alkali metal cation; depositing the solution on the catalyst layer; and forming the electrode. A method of making an electrode useful for membrane-free electrode assemblies, the method comprising: providing a catalyst layer; providing a solution comprising an ionomer and optionally comprising a metal cation; depositing the solution on the catalyst layer; and forming the electrode.
  • 30. The method of example 29, wherein providing a solution comprising an ionomer and an alkali metal cation comprises: forming a ionomer solution comprising ionomer resin and solvent, optionally at a ratio of about 1:5 by volume of ionomer resin to solvent; forming a cation solution comprising alkali metal cation; and mixing the solutions together, optionally at a ratio of about 1:9 by volume of ionomer solution to cation solution. The method of example 29, wherein providing a solution comprising an ionomer and optionally comprising a metal cation comprises: forming a ionomer solution comprising ionomer resin and solvent; and optionally forming a cation solution comprising metal cation and mixing the solutions together. The method of example 29, wherein providing a solution comprising an ionomer and optionally comprising a metal cation comprises: forming a ionomer solution comprising ionomer resin and solvent at a resin:solvent ratio of about 1:1 to about 1:20; and optionally forming a cation solution comprising metal cation and mixing the solutions together at a cation solution:ionomer solution of about 1:1 to about 1:10.

In one or more examples, the solvent is used for dispersing the ionomer, otherwise referred to as ionomer resin. In one or more examples, the solvent comprises organic solvents, volatile organic solvents, water, aqueous solutions, or a combination thereof. In one or more examples, the solvent comprise methanol, ethanol, isopropanol, acetone, dichloromethane, THF, water, or a combination thereof.

• • 31. The method of example 29 or 30, wherein providing a solution comprising an ionomer and an alkali metal cation comprises forming a ionomer solution having ionomer loading of about 25 μL/cm² to about 100 μL/cm²; and/or forming a cation solution having an alkali metal cations concentration of about 0.15 M to about 1 M. The method of example 29 or 30, wherein providing a solution comprising an ionomer and optionally a metal cation comprises forming a ionomer solution having ionomer loading of about 25 μL/cm² to about 100 μL/cm²; and optionally forming a cation solution having metal cations at a concentration of about 0.15 M to about 1 M.
  • 32. The method of any one of examples 29 to 31, wherein depositing the solution on the catalyst layer further comprises drying the solution deposited on the catalyst layer.
  • 33. The method of any one of examples 29 to 32, wherein depositing the solution on the catalyst layer comprises depositing the solution on the catalyst layer by coating, spray coating, dipping, rolling, drop casting, or a combination thereof. The method of any one of examples 29 to 32, wherein depositing the solution on the catalyst layer comprises depositing the solution on the catalyst layer by physical vapor deposition, chemical vapor deposition, physical vapor transport, electrochemical deposition, spray coating, dipping, rolling, drop casting, or a combination thereof.
  • 34. The method of any one of examples 29 to 33, wherein providing a catalyst layer comprises providing a catalyst layer coupled to a gas diffusion layer.

In another example, the present disclosure generally provides:

• • 35. Use of the electrode made by the method of any one of examples 29 to 34 for an electrolysis reaction, an electro-reduction reaction, or a combination thereof.
  • 36. The use of example 35, wherein the reaction comprises CO₂ electrolysis, CO₂ electro-reduction, water electrolysis, CO electro-reduction, CO electrolysis, or a combination thereof.

In one or more embodiments, the present disclosure also generally provides:

• • 1. An electrode useful for membrane-free electrode assemblies, the electrode comprising: a catalyst layer; and a solid polymer electrolyte layer for ion-conduction supported on the catalyst layer.
  • 2. The electrode of embodiment 1, wherein the solid polymer electrolyte layer supported on the catalyst layer comprises the solid polymer electrolyte layer deposited on the catalyst layer.
  • 3. The electrode of embodiment 1 or 2, wherein the solid polymer electrolyte layer has a thickness of ≤25 μm, ≤20 μm, or ≤15 μm, or ≤10 μm, or ≤5 μm, or about 3 μm, or ≤1 μm.
  • 4. The electrode of any one of embodiments 1 to 3, wherein the solid polymer electrolyte layer comprises an ionomer with integrated metal cations.
  • 5. The electrode of any one of embodiments 1 to 4, wherein the solid polymer electrolyte layer comprises an ionomer free of integrated metal cations.
  • 6. The electrode of any one of embodiments 1 to 5, wherein the solid polymer electrolyte layer comprises an ionomer loading of about 5 μL/cm² to about 150 μL/cm², or about 10 μL/cm² to about 150 μL/cm², or about 25 μL/cm² to about 100 μL/cm².
  • 7. The electrode of any one of embodiments 1 to 6, wherein the solid polymer electrolyte layer comprises metal cations at a concentration of about 0 M to about 10 M, or about 0.1 M to about 3 M, or about 0.15 M to about 1 M.
  • 8. The electrode of any one of embodiments 1 to 7, wherein the ionomer comprises an anion exchange ionomer, a cation exchange ionomer, or a combination thereof; and/or a perfluorinated sulfonic acid, a sulfonated polyphenylene; a polystyrene vinyl benzyl methyl imidazolium chloride; or a combination thereof.
  • 9. The electrode of any one of embodiments 1 to 8, wherein the metal cations are absent; or comprise Li, Na, K, Cs, Rb, Fr, Cu, Ni, Ag, Au, Pt, Al, Zn, Sn, Bi, Pd, or a combination thereof.
  • 10. The electrode of any one of embodiments 1 to 9, wherein the catalyst layer comprises Cu, Ni, Ag, Au, Pt, Al, Zn, Sn, Bi, Pd, or a combination thereof.
  • 11. The electrode of any one of embodiments 1 to 10, wherein the catalyst layer further comprises a gas diffusion layer.
  • 12. The electrode of any one of embodiments 1 to 11, wherein the electrode further comprises a gas diffusion layer, the catalyst layer being supported on the gas-diffusion layer.
  • 13. The electrode of embodiment 11 or 12 wherein the gas-diffusion layer comprises polytetrafluoroethylene, carbon paper electrode, carbon cloth electrode, a porous metal, a metal foam, or a combination thereof.
  • 14. The electrode of any one of embodiments 1 to 13, wherein the electrode is a cathode.
  • 15. The electrode of any one of embodiments 1 to 14, wherein the electrode is an anode.
  • 16. The electrode of any one of embodiment 1 to 15, wherein the solid polymer electrolyte layer bi-directionally conducts cations and/or anions.
  • 17. The electrode of any one of embodiment 1 to 16, wherein the solid polymer electrolyte layer single-directionally conducts cations and/or anions.
  • 18. The electrode of any one of embodiments 1 to 17, wherein the solid polymer electrolyte layer comprises a first ionomer layer deposited on the catalyst layer, and a second ionomer layer supported on the first ionomer layer.
  • 19. The electrode of embodiment 18, wherein the first ionomer layer and/or the second ionomer layer has a thickness of ≤25 μm, ≤20 μm, or ≤15 μm, or ≤10 μm, or ≤5 μm, or about 3 μm, or ≤1 μm.
  • 20. The electrode of embodiment 18 or 19, wherein the first ionomer layer and/or the second ionomer layer comprises an ionomer with integrated metal cations.
  • 21. The electrode of any one of embodiments 18 to 20, wherein the first ionomer layer and/or the second ionomer layer comprises an ionomer loading of about 5 μL/cm² to about 150 μL/cm², or about 10 μL/cm² to about 150 μL/cm², or about 25 μL/cm² to about 100 μL/cm².
  • 22. The electrode of any one of embodiments 18 to 21, wherein the first ionomer layer and/or the second ionomer layer comprises metal cations at a concentration of about 0 M to about 10 M, or about 0.1 M to about 3 M, or about 0.15 M to about 1 M.
  • 23. The electrode of any one of embodiments 18 to 22, wherein the first ionomer layer and/or the second ionomer layer comprises an anion exchange ionomer, a cation exchange ionomer, or a combination thereof; and/or a perfluorinated sulfonic acid, a sulfonated polyphenylene; a polystyrene vinyl benzyl methyl imidazolium chloride; or a combination thereof.
  • 24. The electrode of any one of embodiments 18 to 23, wherein the metal cations are absent; or comprise Li, Na, K, Cs, Rb, Fr, Cu, Ni, Ag, Au, Pt, Al, Zn, Sn, Bi, Pd, or a combination thereof.

In one or more embodiments, the solid polymer electrolyte layer is deposited directly on the catalyst layer. In one or more embodiments, the solid polymer electrolyte layer is deposited directly on the catalyst layer such that is it is not a standalone membrane. In one or more embodiments, the solid polymer electrolyte layer has a thickness that is sufficient to support the standard and/or expected operation of an electrode assembly. In one or more embodiments, the solid polymer electrolyte layer has a thickness that is sufficient to reduce, inhibit, or prevent short-circuiting of any electrode assembly is it used with. In one or more embodiments, the solid polymer electrolyte layer has a thickness that is on the order of micrometers (μm). In one or more embodiments, the solid polymer electrolyte layer has a thickness that is not on the order of nanometers (nm), as such a thickness may prevent an assembly from functioning; such as, by short circuiting.

In one or more embodiments, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the solid polymer electrolyte layer is prepared by the methods as herein. In one or more embodiments, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the solid polymer electrolyte layer is directly deposited on an electrode or catalyst layer by the methods described herein. In one or more embodiments, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the integrated transition metal cations act as a catalyst. In one or more embodiments, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the integrated transition metal cations act as an ionomer-dispersed catalyst layer. In one or more embodiments, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the resultant electrode is suitable for use in one or more of the electroreduction reactions as described herein. In one or more embodiments, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the integrated transition metal cations facilitate catalysis of one or more of the electroreduction reactions as described herein. In one or more examples, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the integrated transition metal cations facilitate catalysis of one or more of the gaseous components introduced into the one or more electroreduction reactions as described herein. In one or more embodiments, when the solid polymer electrolyte layer comprises an ionomer with integrated transition metal cations, the resultant electrode provides C₂₊ selectivity in one or more of the electroreduction reactions as described herein.

An ionomer is a polymer where at least some of the monomer units comprises an ionic functionality. In one or more embodiments, the ionomer is any ionomer acceptable for use in an ion exchange membrane. In one or more embodiments, the ionomer is any ionomer acceptable for use in a cation exchange membrane. In one or more embodiments, the ionomer is any ionomer acceptable for use in an anion exchange membrane. In one or more embodiments, the ionomer comprises anion exchange ionomer; cation exchange ionomer; or a combination thereof. In one or more embodiments, the ionomer comprises a perfluorosulfonic acid polymer, a sulfonated tetrafluoroethylene based fluoropolymer-copolymer, or a combination thereof (e.g., Nafion.) In one or more embodiments, the ionomer comprises a 1H-Imidazole, 1,2,4,5-tetramethyl-, compound with 1-(chloromethyl)-4-ethenylbenzene polymer with ethenylbenzene (e.g., Sustainion XA9™). In one or more embodiments, the ionomer is an alkaline ionomer comprising hydrophilic poly (4-vinylbenzyl alkyl-imidazolium chloride) (e.g., Sustainion XA9™). In one or more embodiments, the ionomer comprises a hydrocarbon backbone (e.g., AEMION™). In one or more embodiments, the ionomer is an alkaline ionomer comprising hydrophilic poly (4-vinylbenzyl alkyl-imidazolium chloride) (e.g., AEMION™).

In one or more embodiments, the solid polymer electrolyte layer single-directionally conducts cations and/or anions when the solid polymer electrolyte layer has a thickness approaching that of a standalone membrane. In one or more embodiments, the solid polymer electrolyte layer single-directionally conducts cations and/or anions when the solid polymer electrolyte layer has a thickness approaching that of a standalone membrane, where a standalone membrane has a thickness of about 25 μm to about 200 μm. In one or more embodiments, the solid polymer electrolyte layer single-directionally conducts cations and anions when the solid polymer electrolyte layer has a loading on the catalyst layer that is 100 μL/cm².

In one or more embodiments, the present disclosure also generally provides:

• • 25. A membrane-free electrode assembly, the assembly comprising: an anode; and a cathode, the cathode comprising a catalyst layer; and a solid polymer electrolyte layer deposited on the catalyst layer for conducting ions.
  • 26. The assembly of embodiment 25, wherein the solid polymer electrolyte layer has a thickness of ≤25 μm, ≤20 μm, or ≤15 μm, or ≤10 μm, or ≤5 μm, or about 3 μm, or about ≤1 μm.
  • 27. The assembly of embodiment 25 or 26, wherein the solid polymer electrolyte layer comprises an ionomer with integrated metal cations.
  • 28. The assembly of embodiment 25 or 26, wherein the solid polymer electrolyte layer comprises an ionomer free of integrated metal cations.
  • 29. The assembly of any one of embodiments 25 to 28, wherein the solid polymer electrolyte layer comprises an ionomer loading of about 5 μL/cm² to about 150 μL/cm², or about 10 μL/cm² to about 150 μL/cm², or about 25 μL/cm² to about 100 μL/cm².
  • 30. The assembly of any one of embodiments 25 to 29, wherein the solid polymer electrolyte layer comprises metal cations at a concentration of about 0 M to about 10 M, or about 0.1 M to about 3 M, or about 0.15 M to about 1 M.
  • 31. The assembly of any one of embodiment 25 to 30, wherein the solid polymer electrolyte layer bi-directionally conducts cations and/or anions.
  • 32. The assembly of any one of embodiment 25 to 31, wherein the solid polymer electrolyte layer single-directionally conducts cations and/or anions.
  • 33. The assembly of any one of embodiments 25 to 32, wherein the ionomer comprises an anion exchange ionomer, a cation exchange ionomer, or a combination thereof; and/or a perfluorinated sulfonic acid, a sulfonated polyphenylene; a polystyrene vinyl benzyl methyl imidazolium chloride; or a combination thereof.
  • 34. The assembly of any one of embodiments 25 to 33, wherein the metal cations are absent; or comprise Li, Na, K, Cs, Rb, Fr, Cu, Ni, Ag, Au, Pt, Al, Zn, Sn, Bi, Pd, or a combination thereof.
  • 35. The assembly of any one of embodiments 25 to 34, wherein the catalyst layer comprises Cu, Ni, Ag, Au, Pt, Al, Zn, Sn, Bi, Pd, or a combination thereof.
  • 36. The assembly of any one of embodiments 25 to 35, wherein the catalyst layer further comprises a gas diffusion layer.
  • 37. The assembly of any one of embodiments 25 to 36, wherein the cathode further comprises a gas diffusion layer, the catalyst layer being supported on the gas-diffusion layer.
  • 38. The assembly of embodiment 36 or 37, wherein the gas-diffusion layer comprises polytetrafluoroethylene, carbon paper electrode, carbon cloth electrode, a porous metal, a metal foam, or a combination thereof.
  • 39. The assembly of any one of embodiments 25 to 38, wherein the anode comprises Ni, Pt, Pd, Ir, Fe, oxides thereof, alloys thereof, or a combination thereof.
  • 40. The assembly of any one of embodiments 25 to 38, wherein the anode comprises Ni, or a NiFe layered double hydroxide catalyst.
  • 41. The assembly of any one of embodiments 25 to 40, wherein the assembly is configured for use with an electrolyte.
  • 42. The assembly of any one of embodiments 25 to 40, further comprising a reference electrode.
  • 43. The assembly of any one of embodiments 25 to 42, wherein the solid polymer electrolyte layer comprises a first ionomer layer deposited on the catalyst layer, and a second ionomer layer supported on the first ionomer layer.
  • 44. The assembly of embodiment 43, wherein the first ionomer layer and/or the second ionomer layer has a thickness of ≤25 μm, ≤20 μm, or ≤15 μm, or ≤10 μm, or ≤5 μm, or about 3 μm, or ≤1 μm.
  • 45. The assembly of embodiment 43 or 44, wherein the first ionomer layer and/or the second ionomer layer comprises an ionomer with integrated metal cations.
  • 46. The assembly of any one of embodiments 43 to 45, wherein the first ionomer layer and/or the second ionomer layer comprises an ionomer loading of about 5 μL/cm² to about 150 μL/cm², or about 10 μL/cm² to about 150 μL/cm², or about 25 μL/cm² to about 100 μL/cm².
  • 47. The assembly of any one of embodiments 43 to 46, wherein the first ionomer layer and/or the second ionomer layer comprises metal cations at a concentration of about 0 M to about 10 M, or about 0.1 M to about 3 M, or about 0.15 M to about 1 M.

In one or more embodiments, the anode comprises any metal or metal catalyst suitable for an electrode assembly. In one or more embodiments, the anode comprises any metal or metal catalyst suitable and/or stable for use in alkaline conditions. In one or more embodiments, the anode comprises any metal or metal catalyst suitable for minimizing overpotential. In one or more embodiments, the anode comprises any metal or metal catalyst suitable for and/or stable at higher current densities. In one or more embodiments, the anode comprises a transition metal catalyst. In one or more embodiments, the anode comprises a water oxidation catalyst; an organic oxidation catalyst; an oxidation catalyst, or a combination thereof.

In one or more embodiments, the assembly as described herein can operate and is stable at higher current densities, and thus exhibits better performance relative to assemblies that cannot operate or are not stable at higher current densities.

In one or more embodiments, the present disclosure also generally provides:

• • 48. Use of the embodiments of any one of embodiments 1 to 24, or use of the membrane-free electrode assembly of any one of embodiments 25 to 47 for an electrolysis reaction, an electro-reduction reaction, or a combination thereof.
  • 49. The use of embodiment 27, wherein the reaction comprises CO₂ electrolysis, CO₂ electro-reduction, water electrolysis, CO electrolysis, CO electro-reduction, N₂ electrolysis, N₂ electro-reduction, O₂ electrolysis, O₂ electro-reduction, or a combination thereof.

In one or more embodiments, the present disclosure also generally provides:

• • 50. A method of making an electrode useful for membrane-free electrode assemblies, the method comprising: providing a catalyst layer; providing a solution comprising an ionomer and optionally comprising a metal cation; depositing the solution on the catalyst layer; and forming the electrode.
  • 51. The method of embodiment 50, wherein providing a solution comprising an ionomer and optionally comprising a metal cation comprises: forming a ionomer solution comprising ionomer resin and solvent; and optionally forming a cation solution comprising a metal cation and mixing the solutions together.
  • 52. The method of embodiment 50 or 51, wherein providing a solution comprising an ionomer and optionally comprising a metal cation comprises: forming a ionomer solution comprising ionomer resin and solvent at a resin:solvent ratio of about 1:1 to about 1:20; and optionally forming a cation solution comprising a metal cation and mixing the solutions together at a cation solution:ionomer solution ratio of about 1:1 to about 1:10.
  • 53. The method of any one of embodiments 50 to 52, wherein providing a solution comprising an ionomer and optionally comprising a metal cation comprises forming a ionomer solution having an ionomer loading of about 5 μL/cm² to about 150 μL/cm².
  • 54. The method of any one of embodiments 50 to 52, wherein providing a solution comprising an ionomer and optionally comprising a metal cation comprises forming a cation solution having metal cations at a concentration of about 0.1 M to about 10 M.
  • 55. The method of any one of embodiments 50 to 54, wherein depositing the solution on the catalyst layer further comprises drying the solution deposited on the catalyst layer.
  • 56. The method of any one of embodiments 50 to 55, wherein depositing the solution on the catalyst layer comprises depositing the solution on the catalyst layer by physical vapor deposition, chemical vapor deposition, physical vapor transport, electrochemical deposition, spray coating, dipping, rolling, drop casting, or a combination thereof.
  • 57. The method of any one of embodiments 50 to 56, wherein providing a catalyst layer comprises providing a catalyst layer coupled to a gas diffusion layer.
  • 58. The method of any one of embodiments 50 to 57, wherein providing a catalyst layer comprises applying a catalytic metal onto a support; and forming the catalyst layer.
  • 59. The method of any one of embodiments 50 to 58, wherein applying a catalytic metal on a support comprises depositing the catalytic metal on the support by physical vapor deposition, chemical vapor deposition, physical vapor transport, electrochemical deposition, spray coating, dipping, rolling, drop casting, or a combination thereof.
  • 60. The method of any one of embodiments 50 to 59, wherein the catalytic metal comprises Cu, Ni, Ag, Au, Pt, Al, Zn, Sn, Bi, Pd, or a combination thereof.
  • 61. The method of any one of embodiments 50 to 60, wherein the support comprises a gas diffusion layer.
  • 62. The method of any one of embodiments 50 to 61, wherein the gas-diffusion layer comprises polytetrafluoroethylene, carbon paper electrode, carbon cloth electrode, a porous metal, a metal foam, or a combination thereof.
  • 63. The method of any one of embodiments 50 to 62, wherein providing a solution comprising an ionomer and optionally comprising a metal cation comprises:
    • forming a first ionomer solution comprising a first ionomer resin and solvent, and optionally forming a first cation solution comprising a first metal cation and mixing the solutions together; and
    • forming a second ionomer solution comprising a second ionomer resin and solvent, and optionally forming a second cation solution comprising a second metal cation and mixing the solutions together.
  • 64. The method of any one of embodiments 50 to 63, wherein providing a solution comprising an ionomer and optionally comprising a metal cation comprises:
    • forming a first ionomer solution comprising a first ionomer resin and solvent at a resin:solvent ratio of about 1:1 to about 1:20, and optionally forming a first cation solution comprising a first metal cation and mixing the solutions together at a cation solution:ionomer solution ratio of about 1:1 to about 1:10; and
    • forming a second ionomer solution comprising a second ionomer resin and solvent at a resin:solvent ratio of about 1:1 to about 1:20, and optionally forming a second cation solution comprising a second metal cation and mixing the solutions together at a cation solution:ionomer solution ratio of about 1:1 to about 1:10.
  • 65. The method of any one of embodiments 50 to 64, wherein providing a solution comprising an ionomer and optionally comprising a metal cation comprises forming a first ionomer solution and/or a second ionomer solution having an ionomer loading of about 5 μL/cm² to about 150 μL/cm².
  • 66. The method of any one of embodiments 50 to 65, wherein providing a solution comprising an ionomer and optionally comprising a metal cation comprises forming a cation solution having metal cations at a concentration of about 0.1 M to about 10 M.
  • 67. The method of any one of embodiments 50 to 66, wherein depositing the solution on the catalyst layer comprises depositing the first ionomer solution on the catalyst layer; and drying the solution deposited on the catalyst layer to form a first ionomer layer.
  • 68. The method of any one of embodiments 50 to 67, wherein depositing the solution on the catalyst layer further comprises depositing the second ionomer solution on the first ionomer layer; and drying the solution deposited on the first ionomer layer.
  • 69. The method of any one of embodiments 50 to 68, wherein depositing the solution on the catalyst layer comprises depositing the solution on the catalyst layer by physical vapor deposition, chemical vapor deposition, physical vapor transport, electrochemical deposition, spray coating, dipping, rolling, drop casting, or a combination thereof.

In one or more embodiments, the solvent is used for dispersing the ionomer, otherwise referred to as ionomer resin. In one or more embodiments, the solvent comprises organic solvents, volatile organic solvents, water, aqueous solutions, or a combination thereof. In one or more embodiments, the solvent comprise methanol, ethanol, isopropanol, acetone, dichloromethane, THF, water, or a combination thereof.

In one or more embodiments, the present disclosure also generally provides:

• • 70. Use of the electrode made by the method of any one of embodiments 50 to 69 for an electrolysis reaction, an electro-reduction reaction, or a combination thereof.
  • 71. The use of embodiments 70, wherein the reaction comprises CO₂ electrolysis, CO₂ electro-reduction, water electrolysis, CO electrolysis, CO electro-reduction, N₂ electrolysis, N₂ electro-reduction, O₂ electrolysis, O₂ electro-reduction, or a combination thereof.

To gain a better understanding of the invention described herein, the following examples are set forth. It should be understood that these examples are for illustrative purposes only. Therefore, they should not limit the scope of this invention in anyway.

EXAMPLES

Example 1—Directly-Deposited Ultrathin Solid Polymer Electrolyte for CO₂ Electrolysis, Such as Enhanced CO₂ Electrolysis

Commercial membranes often address only one of the challenges in CO₂R: either they offer suitable micro-environment for carbon dioxide electroreduction (CO₂R) (e.g., anion exchange membrane) or suppress carbonate cross-over (e.g., cation exchange membrane and bipolar membrane). Herein described is a cation-infused bifunctional ultrathin (˜3 μm) solid polymer electrolyte (CISPE) that, in one or more embodiments or examples, may address both of these challenges via bidirectional ion transport mechanism and suppressed anolyte diffusion or cathode flooding. An example of this directly-deposited ultrathin CISPE (that substitutes commonly used pre-made membrane), as prepared and tested, offered 110 hours of stable operation with relatively low energy cost (e.g., about 296, or about 290 GJ/ton) and/or full-cell energy efficiency of 27% for a one-step CO₂ electrolysis to C₂H₄ at 100 mA/cm² (e.g., for the end-to-end process of CO₂ capture and electro-reduction, carbonate regeneration, CO₂ separation from anode and cathode streams in a membrane electrode assembly (MEA) CO₂R cell.) When compared to commercial anion exchange membrane, the CISPE demonstrated 40% reduction of carbonate crossover. By applying this design strategy, a 90% selectivity was achieved towards CO₂R products at 200 mA/cm² with C₂+ partial current density of 142 mA/cm². The herein described CISPE may offer stability, and efficient electrolysis of high-value feedstock chemicals and fuels using low-cost catalysts.

Performance was evaluated relative to a reference AEM in a MEA cell with a cathode consisting of a porous polytetrafluoroetheylene (PTFE) gas diffusion layer (GDL) sputtered with a ˜300 nm Cu catalyst layer (FIG. 1a) (see Example 2 for more details). The Faradaic efficiency (FE) of CO₂R in this configuration in 1 M potassium hydroxide (KOH) anolyte and at different current densities is demonstrated in FIG. 1b. A continuous drop of the selectivity for H₂ and CO with an increase of the C₂H₄ selectivity was noted as the current density was increased from 25 mA/cm² to 100 mA/cm²; this was found to be consistent with literature reports.³³ The highest partial current density towards C₂H₄ using AEM was 45 mA/cm².

The concentration of OH⁻, HCO₃⁻ and CO₃²⁻ in the anolyte was measured over time at 50 mA/cm² to assess the CO₃²⁻ and HCO₃⁻ cross-over through the AEM (FIG. 1c) (see Example 2, section F). Initially, the oxygen evolution reaction (OER, R.1 below) occurred in the anode given that plenty of OH⁻ were available in the anolyte as indicated by high concentration of OH⁻. As the OH⁻ was continuously consumed in OER, the OH⁻ concentration decreased with time (FIG. 1c). In addition, higher water uptake of the thick AEM contributed to the decline of OH⁻ concentration (OH⁻ was hydrated)³⁹. In the cathode, CO₂ electrolysis along with hydrogen evolution reaction (HER) occurred with OH⁻ generation (see reactions R.6 and R.7 below). The OH⁻ instantaneously reacted with unreacted CO₂ and HCO leading to CO₃ formation via reactions R.8 and R.9 below. The formed CO₃²⁻ also possibly reacted with excess CO₂ and H₂O to form HCO₃⁻ (R.10 below). This enabled anions (OH⁻, HCO₃⁻, CO₃²⁻) migration and increase of CO₃²⁻ concentration (R.5 below) in the anolyte (FIG. 1c). This observation was consistent with previous studies using AEMs, wherein the high pH local environment during CO₂R lead to formation of carbonates (CO₃²⁻ and HCO₃⁻ with OH⁻) which then crossed-over to the anolyte^7,40. The present analysis suggested that when using an AEM, the charge was balanced by the CO₃²⁻ migration along with possible OH⁻ and KHCO₃ migration to the anode (FIG. 11a), which was consistent with previous computational and experimental studies^{4, 7, 41, 42}. Due to the higher water uptake of the thick membrane, the diffusion of KOH and KHCO₃ or K₂CO₃ through the AEM was also observed in the present experiments, which was consistent with earlier reports^39,43. Both KHCO₃ and K₂CO₃ diffusion as well as CO₃²⁻ and HCO₃⁻ migration were responsible for the increase of CO₃²⁻ concentration in the anolyte as shown in FIG. 12. After 90 minutes of reaction, the OER (R.2) occurred in the anode due to the absence of OH⁻. The proton (H⁺) from the OER reacted with CO₃²⁻ to form HCO ₃⁻, CO₂ and H₂O via the reactions R.3 and R.4. This explained the increase of HCO₃⁻ and the decrease of CO₃²⁻ after 120 minutes as shown in FIG. 1c.

Anode:

Cathode:

The change of the OH⁻ concentration hampered the steady electrolysis operation (FIG. 1d). The decrease of OH⁻ lead to a rise of the overall cell voltage, which was attributed to the sluggish OER kinetics and higher ohmic resistance due to lower ion conductivity of HCO₃⁻ and CO₃²⁻ compared to OH⁻.^{33, 44} A gradual drop of the CO₂R product selectivity and increased HER was noted, as shown in FIG. 1d. This observation was consistent with a report by Endrödi et al. who demonstrated that the use of alkaline anolyte with an AEM leads to salt precipitation at the cathode, which erodes the selectivity towards CO₂R products.³¹ These results indicated that continuous operation in this mode would therefore lead to a decline of the CO₂ product selectivity as well as near-neutral anolyte (due to CO₃²⁻ crossover) with consequent Ni catalyst degradation. In other words, the AEM in the MEA cell with alkaline anolyte and Ni based OER catalyst was not stable for extended operation.

Use of a CEM was considered to be a possible solution to suppress CO₃²⁻ crossover to the anode. However, Colin et al. demonstrated that the CEM incorporated in a MEA cell with deionized (DI) water as anolyte primarily drives the HER at the cathode (˜100% selectivity).¹² To avoid such high proton flux from OER, 1 M KOH as the anolyte was used and CO₂R performance measured. A MEA was assembled using a CEM (Nafion 117 membrane), 300 nm sputtered Cu catalyst on PTFE GDL (FIG. 2a) (see Example 2 for more details). The faradaic efficiency (FE) of CO₂R in this configuration is shown in FIG. 2b at different current densities. A maximum selectivity of 40% towards C₂H₄ was obtained at 50 mA/cm². Further increase of the current shifted the selectivity primarily towards the HER. The CO₂ electrolysis activity at low current densities can be explained by the basic local environment of the cathode for CO₂ electrolysis due to OH⁻ diffusion from anode. However, the selectivity and the cell voltage was not stable even at 50 mA/cm² (FIG. 2c). A salt slurry was observed at the cathode outlet within few minutes (FIG. 2d). In contrast to stable operation using the CEM in the MEA with neutral anolyte as reported in literature¹², the charge was mainly balanced by alkali cation (K⁺) migration to the cathode when KOH was used as anolyte. Additionally, CO₃²⁻ was not detected in the anolyte when using a CEM in the titration experiment (FIG. 11b). The combination of K⁺ migration and negligible CO₃²⁻ crossover to the anolyte resulted in an accumulation of K⁺ and CO₃²⁻ in the cathode. This then lead to salt formation at the cathode, loss in selectivity, and higher cell voltage¹⁹.

The above results using the AEM and CEM in a MEA cell with alkaline anolyte signify some challenges and benefits. The ease of anion transport through the AEM compromises the stability of the cell, as demonstrated by the loss of selectivity and anolyte alkalinity. In the case of the CEM, charge balance by the K⁺ migration to the cathode resulted in failure of the cell, as evident from CO₂R selectivity drop, salt accumulation, and rapid increase in cell voltage. Despite these challenges, one of the key benefits of CEM that was desirable to exploit was the ability to migrate K⁺ from the anode to cathode and to block CO₃²⁻ transport across the membrane. Studies have demonstrated a 3-fold increase in CO₂ reduction selectivity due to crossover of K⁺ from the anode to the cathode in a MEA with BPM³⁸. However, the results described above suggested that excessive K⁺ migration was detrimental to stable cell operation when commercial thick AEM and CEM (FIG. 1d and FIG. 2c) were used. Based on these observations, it was hypothesized that the anion transport may be controlled by tuning the total negative charge of sulfonic acid (e.g., fixed —SO₃⁻) sites within a PFSA; which may be achieved by reducing the thickness of the PFSA. Charge balance may be achieved via bidirectional migration of K⁺ and CO₃²⁻ ions. As a result, suppressed migration of K⁺ to the cathode may enhance CO₂R selectivity and reduce CO₃²⁻ crossover (as compared to the AEM).

To investigate this, CO₂R was carried out using a CEM (Nafion 211 membrane, 25 μm; CEM-211), which was substantially thinner than Nafion 117 (183 μm; CEM-117) in 1 M KOH as anolyte. While ˜0.5 V lower cell voltage was observed at similar current densities with Nafion 211 due to the lower resistance, no noticeable difference was observed of the CO₂ electrolysis selectivity compared to the Nafion 117 membrane (FIG. 2b and FIG. 13). Similar to the MEA with Nafion 117 system, a reduction of the CO₂ product selectivity and a rise in cell voltage was observed when using Nafion 211 due to salt accumulation at the cathode GDL. This result indicated that the electro-migration of K⁺ through the 25 μm thick Nafion 211 membrane was similar to that of the 183 μm thick Nafion 117 membrane.

Limited by the availability of Nafion membrane with thickness below 25 μm, PFSA ionomer such as Nafion was spray coated to act as an ultrathin solid polymer electrolyte (USPE) on the sputtered Cu surface (see Example 2 for details) (FIG. 3a). The use of USPE may substitute the need for commonly used thick pre-made commercial membranes; which may lead to a reduction of the cell voltage. It was hypothesized that apart from the ease and flexibility of tuning the thickness of the USPE, the direct coating approach^46-49 may protect the catalyst from commonly reported degradation mechanism (e.g., dissolution, aggregation etc.) due to the intimate contact between the catalyst and ionomer.^50-53 Additionally, intimate contact between the catalyst and ionomer may allow enhanced CO₂ transport across the catalyst surface⁹ and it may also minimize anolyte diffusion.

The USPE was developed by spraying ionomer with a loading of 100 μL/cm² (denoted as USPE-100) on the surface of Cu catalyst and performed CO₂R in 1 M KOH as anolyte (see Example 2 for more details). While no noticeable improvement in selectivity was observed, better cell stability (e.g., the salt accumulation was significantly lower) was observed over commercially available Nafion 117 and Nafion 211 (FIG. 3b and FIG. 14). This result provided preliminary evidence that salt precipitation due to K⁺ transport towards cathode may be suppressed by using a USPE. Next, a USPE was developed with ionomer loading of 50 μL/cm² (denoted as USPE-50) and CO₂R was performed in 1 M KOH as anolyte. Cross sectional SEM indicates a USPE-50 layer with ˜3 μm thickness (FIG. 3c). The uniformity and conformal coating of the USPE-50 was further confirmed using top-view SEM and optical profilometry analysis (FIG. 15 and FIG. 16). With 1 M KOH anolyte, a reduction of the HER selectivity (˜9%) and enhanced selectivity towards C₂H₄(˜38%) was found at the current densities of 50 and 75 mA/cm² (FIG. 3d) as compared to USPE-100 (FIG. 3b). The highest C₂H₄ partial current density that was observed and measured was 28 mA/cm². At current densities beyond 75 mA/cm², a drop in C₂H₄ selectivity was measured, with enhanced selectivity towards the HER and methane.

It was believed that the use of 1 M KOH anolyte would limit the CO₂ availability at the cathode at higher current densities due to OH⁻ diffusion and consequent carbonate formation.³⁹ To investigate this, the concentration difference was tripled by using 3 M KOH as anolyte and CO₂R performance was measured. As shown in FIG. 3e, the CO₂R selectivity decreased when using 3 M KOH as compared to 1 M KOH, which supported the hypothesis that OH⁻ diffusion is faster and consequently limits CO₂ availability at the cathode. To further investigate this, 0.5 M K₂CO₃ anolyte was used. The absence of OH⁻ diffusion (since replaced with carbonate) deteriorated CO₂R activity (see Example 2, FIG. 17). This was attributed to the absence of OH⁻ in the anolyte (e.g., 0.5 M K₂CO₃): the OER lead to the generation of H⁺ at the anode (2H₂O→O₂+4H⁺+4e⁻), which in turn acidified the cathode and promoted HER.

It was hypothesized that the improved selectivity of USPE-50 as compared to CEM-117, CEM-211, and USPE-100 may be attributed to the reduced migration and deposition of K⁺ at the cathode. The presence of K⁺ in the cathode may suppress HER and enhance CO₂R. To investigate this, the stability of CO₂R using USPE-50 (FIG. 3f) was tested. In contrast to CEM-117, CEM-211, and USPE-100, no salt was observed on the cathode and in the serpentine channel during 5 hours run when using USPE-50 (FIG. 3f, inset). The C₂H₄ and H₂ selectivity gradually improved and declined over the first 3 hours, respectively; which indicated that the reduced migration of K⁺ plays an important role to improve the selectivity over time. On the other hand, the direct deposition of USPE may provide intimate contact between PFSA ionomer and the catalyst surface, as opposed to the possibility of having an air-gap between the pre-made thick membrane (Nafion 117 and Nafion 211) and catalyst surface. Despite the ultrathin ionomer layer, the intimate contact may play an important role to suppress KOH diffusion induced salt formation at the cathode; leading to longer effective CO₂R. KOH loss from the anolyte without applied potential was compared for Nafion-117 and USPE-50, which should be primarily governed by KOH diffusion (see Example 2, Section G). An estimate of nearly twice the mass loss when using Nafion-117 as compared to USPE-50 was made. Despite the ultrathin layer of USPE, the intimate contact was important to suppress KOH diffusion and salt precipitation. Due to the gradual loss of OH⁻ (due to OER induced loss and HCO₃⁻ diffusion), a gradual increase of the cell voltage over time was noted (FIG. 3f). It was considered that the lower cell voltage of USPE-50 (FIG. 3d) versus Nafion™ 117 (FIG. 2b) may be attributed to the ultrathin layer with enhanced ionic conductivity. Electrochemical impedance spectroscopy (EIS) measurements (see Example 2, Section I) revealed lower resistance with facile ion transport in the USPE as compared to Nafion™ 117 (FIG. 9).

The presence of alkali metal cations such as Cs⁺, K⁺, Na⁺, Li⁺ in the liquid catholyte have been reported to have positive effect on the CO₂ electrolysis activity and selectivity in a flow-type CO₂ electrolysis cell.^{31, 37, 55-58} When using a solid polymer electrolyte (SPE) such as in a MEA cell, the metal cations can be transported from alkaline anolyte³¹, which can have a promotional effect on CO₂R at low current density (˜50 mA/cm²) (FIG. 3d). However, at higher current density, the electro-migrated K⁺ was not sufficient to suppress HER. Increasing the concentration of KOH to enhance K⁺ transport did not suppress HER, instead it had a detrimental effect on CO₂R (FIG. 3e). It was considered that the presence of cations in the local environment may be important to improve CO₂R selectivity at higher current density. It was considered that by infusing alkali metal cation into the USPE during the spray coating, a cation-infused USPE (denoted as CISPE) may be developed to maintain low activity of HER and enhance partial current density towards C₂H₄.

First, a Nafion solution was mixed with 0.15 M KOH solution and then the mixture was sprayed on the electrode with a loading of 50 μL/cm² (see Example 2, Section C) (this configuration is noted as CISPE-50-0.15M). Initially, 0.15 M KOH was selected to be infused to the Nafion since the H⁺ of the —SO₃⁻ group can be stoichiometrically exchanged with K⁺ at ˜0.13 M KOH (see Example Section E). The ion exchange between H⁺ in the —SO₃⁻ group with K⁺ was further indicated by using FTIR (FIG. 18). The thickness of CISPE-50-0.15M was similar to that of USPE-50 (FIG. 4a). The presence of K⁺ in the Nafion was further indicated by elemental mapping using Energy Dispersive X-Rays (FIG. 19).

It was noted that CISPE-50-0.15M coating suppressed HER activity at higher current density as compared to USPE-50 (FIG. 4b, FIG. 20, and FIG. 3d, respectively). The partial current density towards C₂H₄ reached 52 mA/cm² when using CISPE-50-0.15M as compared to 28 mA/cm² for USPE-50. These results indicated a positive role of alkali metal cation in promoting the activity of CO₂ electrolysis when infused/implanted in the polymer metrics of Nafion. Experiments were carried out by infusing smaller cation (Li⁺) and larger cation (e.g., Cs⁺) in CISPE (FIG. 21). Cs⁺ infusion led to a higher partial current density of 60 mA/cm². This indicated that the promotion effect of larger cations in liquid electrolyte on CO₂ electrolysis performance can be extended towards solid polymer electrolytes.^{37, 55, 59, 60} The implanted cations (that is the case with CISPE-50-0.15M) were found to be more effective than the ones transported from the anolyte (i.e., USPE-50) to improve the CO₂R partial current density. In another words, increasing the concentration of cation in the local environment of CO₂R is an important factor to improve C₂₊ selectivity. To test this, experiments using the CISPE-50 with higher loading of K⁺ were carried out (FIG. 4c). At current density of 100 mA/cm², increasing the K⁺ loading in the Nafion (from 0.15 M to 1 M) increased the faradaic efficiency of gas products (e.g., CO and C₂H₄ combined) from 47% to 57%, while the faradaic efficiency of H₂ remained the same. At 200 mA/cm², 90% selectivity towards CO₂R products was achieved using CISPE-50-1.0M and 1 M KOH anolyte, with 100 mA/cm² partial current density towards C₂H₄; the latter is >2-fold and >5-fold higher than that of commercial AEM and Nafion-117 in the same testing condition, respectively (FIG. 1b, FIG. 2b, and FIG. 4d). The partial current density towards total C₂₊ products was estimated to be 144 mA/cm² (FIG. 22).

To investigate the underlining cause of enhanced CO₂R performance of CISPE as opposed to the AEM (Sustainion® X37-50 Grade RT), CEM (Nafion 117), the mass transport (both charged and neutral) through ultrathin solid polymer electrolytes were investigated (see Example 2 Section F). In the case of USPE-50, the charge was balanced by bidirectional ion migration including the migration of K⁺ to the cathode and migration of CO₃²⁻ to the anode (FIG. 11). The bidirectional ion migration provided: (1) limited crossover of CO₃²⁻, its measured concentration was 40% lower than the case with AEM (FIG. 11a); and (2) limited K⁺ migration, 74% lower than the case with CEM, which resulted in minimum salt formation (FIG. 11a and FIG. 24). With further control experiments using thinner USPE-25 (25 μL/cm² Nafion) and thicker USPE-100 (100 μL/cm² Nafion) (FIG. 23c and FIG. 23d, respectively), it was considered that the thickness of the USPE can be fine-tuned to control the bidirectional ion transport mechanism between anode and cathode. On the other hand, the present mass balance analysis indicated that the water flux (as observed in case of AEM and Nafion™ 117 as in FIG. 11a and FIG. 11b) to the cathode (and associated salt diffusion) was suppressed in the presence of USPE (FIG. 11c, FIG. 23c and FIG. 23d). This further supported the hypothesis that direct deposition of USPE ensured lower water uptake and intimate contact at the catalyst/ionomer interface, which suppressed cathode flooding, and salt precipitation (see Example 2, Section G).

Ion transport in CISPE-50-0.15M was further investigated (FIG. 11d). The transport properties of Nafion ionomer depend on its water content, which is governed by the interaction between the cation and fixed —SO₃⁻ sites. Shouwen et al. reported that the water uptake (A) capacity of Nafion membrane decreased upon replacing H⁺ with monovalent cations in the order of increasing ionic radius: λ(H⁺)>λ(Li⁺)>λ(Na⁺)>λ(K⁺)>λ(Cs⁺).⁶¹ Lower water flux also resulted in lower diffusion of KHCO₃ from the cathode to the anode in CISPE-50-0.15M as compared to the case of USPE-50.

At 100 mA/cm², further demonstrated was over 110 hours of stable operation using CISPE-50-0.15 (FIG. 5a) with 18% single pass conversion efficiency. It was noted that such high stability was achieved while using alkaline anolyte to reduce the cell voltage, which led to a demonstrated record high one-step CO₂-to-C₂H₄ electrolysis efficiency of 27% at 100 mA/cm² (Example 2, Section H, and FIG. 26). This demonstrated that use of a directly-deposited ionomer (e.g., that substituted a pre-made commercial membrane) MEA wherein ˜3 μm coating of cation-infused USPE enhanced CO₂ electrolysis performance. An energy calculation for the end-to-end process (including CO₂ capture, conversion and carbonate regeneration, see Example 2, Section H) indicated that the herein described CISPE enables low energy cost (e.g., about 296, or about 290 GJ/ton) for one-step CO₂ electrolysis to C₂H₄as compared to prior reports (FIG. 5b, Table S.3, FIG. 26). While the energy for carbonate regeneration from an alkaline anolyte was comparable to that of the energy required for CO₂ separation from produced O₂ in a neutral anolyte system (Example 2, section H); in an example, the overall low energy cost of the herein described system appeared to be from the use of alkaline anolyte which offers higher electrolysis efficiency.

Relative to prior systems, the herein described system appears to offer: 1) use of ultrathin ionomer with bidirectional ion transport as well as the use of alkaline anolyte, leading to lower cell voltage that can compensate the energy requirement for carbonate and CO₂ regeneration, 2) lower water flux by the ultrathin ionomer that limits cathode flooding with improved selectivity and stability, 3) direct deposition method that offers intimate contact between the catalyst and ionomer, resulting in better cathode stability by protecting the copper from dissolution, physical detachment and possible poisoning⁶², and/or 4) infused K⁺ in the solid polymer electrolyte that suppresses water flux and HER. Technoeconomic analysis suggests that lowering the energy cost may be important to driving down the economics of ethylene electrosynthesis for practical applications (FIG. 25).

The development of an ultrathin solid polymer electrolyte (USPE) using a facile spray coating approach to improve the stability and reduce the crossover in CO₂ electrolysis using MEA with alkaline anolyte is described herein. The use of commercial thick AEM and CEM with alkaline anolyte lead to high crossover and low stability in a MEA, respectively. The use of an ultrathin (˜3 μm) USPE directly coated onto a polycrystalline Cu cathode suppressed K+(74% less as compared to commercial CEM) or CO₃²⁻ crossover (40% less as compared to commercial AEM) and cathode flooding, which enabled enhanced MEA stability over commercial thick CEM and AEM. This improved performance was achieved by the directly-deposited ultrathin (˜3 μm) ionomer layer, which suppressed water uptake and cathode flooding as well as allowed good migration of K⁺ and CO₃²⁻ between the electrodes.

It was further demonstrated that by implanting K⁺ within the USPE (e.g., CISPE) during the spray-coating process, the water flux or cathode flooding could be suppressed with good K⁺ migration to cathode. This then enabled the demonstration of a high electrolyzer energy efficiency of 27% in one-step CO₂ conversion to C₂H₄, along with low energy cost of about 296, or about 290 GJ/ton C₂H₄, as well as 110 hours of stable operation at 100 mA/cm². This facile, low-cost and scalable approach to directly coat the cathode with an ultrathin polymer eliminated the need for a commonly used pre-made membrane (and associated cost of production, pre-treatment with acid for purity, hot pressing and assembling). Additionally, the use of a cation-infused USPE (CISPE) eliminated the need of commonly used precious metal anodes (e.g., IrO₂) and allowed CO₂ electrolysis with alkaline anolyte.

Example 2—Directly-Deposited Ultrathin Solid Polymer Electrolyte for CO₂ Electrolysis—Supplementary Information

S1. Experimental Information

A. Materials

Potassium hydroxide (KOH), lithium hydroxide (LiOH), cesium hydroxide (CsOH), and methanol were purchased from Sigma Aldrich (ACS reagent). Sustainion® XA-9 solution (5% in ethanol) and Nafion™ perfluorinated resin solution (5 wt. % in mixture of lower aliphatic alcohol and water) were received from Dioxide Materials and Sigma Aldrich, respectively. The membranes such as Sustainion® X37-50 Grade RT, Nafion 117 and Nafion 211 membranes were received from Fumasep.

B. Experimental Setup

A membrane electrode assembly (MEA), consisting of anode (grade 2 titanium) and cathode (904L stainless steel) was made by Dioxide materials. A humidified CO₂ and 1 M KOH (unless mentioned otherwise) with a flowrate of ˜60 sccm was fed to the cathode and anode sides, respectively using a flow meter (Cole-parmer 39067) and peristaltic pump (Fisherbrand™ Variable-Flow Peristaltic Pumps), respectively. BioLogic potentiostats with 10 A booster was used to obtain the electrochemical response without iR correction. Parsche Airbrush set was used to spray the solution on the target sheet. The gas products were analyzed using Perkin Elmer Gas Chromatography with flame ionization detector (FID) and thermal conductivity detector (TCD). The liquid products were identified using BrukerAVANCE III 600 MHz nuclear magnetic resonance spectroscopy equipped with pulsed-field gradient probes.

C. Cathode Preparation

The cathode catalyst was approximately 300 nm sputtered copper on polytetrafluoroethylene (PTFE). The Nafion spray solution was prepared by diluting Nafion™ perfluorinated resin solution (Sigma Aldrich) in methanol (Sigma Aldrich) with a ratio of 1:5 by volume. The ultra-thin solid polymer electrolyte (USPE) was fabricated by spray-coating (Paasche airbrush) of the desired quantity of Nafion spray solution on top of the cathode catalyst and dried for overnight under atmospheric condition. For the sample with the area of ˜4 cm², USPE-50 (contains 50 μL/cm² of Nafion™ perfluorinated resin solution) was synthesized by spray coating 1200 μL of the Nafion spray solution stock.

The cation-infused solid polymer electrolyte (CISPE) was prepared by spray-coating of the desired quantity of a Nafion-cation spray solution and dried overnight. The Nafion-cation spray solution was prepared by mixing Nafion spray solution stock with cation solution at the desired concentration with a ratio of 1:9 by volume. Cation solution could be LiOH, KOH, or CsOH. For example, to synthesize ˜4 cm² of CISPE-50-0.15M (contains 50 μm/cm² of Nafion™ perfluorinated resin solution with 0.15 M KOH), the Nafion-cation spray solution was prepared by adding 0.15 M KOH solution to the Nafion spray solution. Then, 1333 μL of the Nafion-KOH spray solution was spray coated to the cathode catalyst and dried overnight.

On the anode side, a Ni-foam sheet (0.08 mm, MTI Corporation) was used as catalyst to avoid damage on the Nafion layer which may result in short circuit.

D. Characterization

The morphology and structure of USPE and CISPE were characterized by scanning electron microscopy (SEM). SEM observation was performed using a FEI Quanta 250 FEG field emission scanning electron microscope which was equipped with EDS analysis.

Fourier Transform Infrared (FTIR) spectra were recorded by Perkin Elmer Frontier FT-IR spectrometer in the range of 4000 to 400 cm- to study the chemical structure of ionomer before and after adding KOH.

E. Mass of Nafion in the Solution

Properties of Nafion™ perfluorinated resin (5 wt. % in solution)

• • Chemical formula=(C₇HF₁₃O₅S·C₂F₄)_x
  • Molecular weight (MW_naf)=554 gram/mol
  • Density (ρ_naf)=0.93 g/mL

The density of the diluted Nafion solution (ρ_sol-A) was calculated as follow:

ρ sol · A = V naf × ρ naf × x naf V naf × V meoh = 1 ⁢ mL × 0.93 g sol mL × 5 100 ⁢ g naf g sol 1 ⁢ mL + 5 ⁢ mL = 0 . 0 ⁢ 465 ⁢ g naf 6 ⁢ mL = 0.00775 g naf / mL

V_naf, V_meoh and x_naf are the volume of Nafion™ perfluorinated resin, volume of methanol and mass fraction of Nafion in the Nafion™ perfluorinated resin, respectively.

The mass of Nafion in the cathode with Nafion loading of 50 μL/cm² was estimated as follows:

m n ⁢ a ⁢ f = 0.05 mL cm 2 × 0.93 g sol mL × 5 1 ⁢ 0 ⁢ 0 ⁢ g n ⁢ a ⁢ f g sol = 2.325 mg / cm 2

The modified Nafion was synthesized by adding a certain concentration of cation hydroxide solution in the Nafion solution with a volumetric ratio of Nafion solution: cation hydroxide solution of 9:1. Firstly, the mass of Nafion in 9 mL of Nafion solution was calculated.

m naf = 9 ⁢ mL × 0.00775 g naf / mL = 0.0698 g

The mass fraction of sulfonic acid group (x_sa) in the Nafion was estimated as follows (assume 1 mol Nafion as a basis):

x s ⁢ a = 81 ⁢ g 554 ⁢ g = 0.15

The mole of sulfonic acid was calculated as follow:

n s ⁢ a = 0 . 0 ⁢ 698 ⁢ g × 0.15 81 ⁢ g / mol = 0.000129 mol

The required concentration of 1 mL cation hydroxide solution to stoichiometrically neutralize 1.29×10⁻⁴ mol of sulfonic acid was 0.129 M.

F. Measurement of Hydroxide and Carbonate Concentrations Using Total Alkalinity Method

The mass flow for a constant charge transfer was measured by operating at constant current density (50 mA/cm²) for 60 minutes in 1 M KOH anolyte. A 50 mA/cm² current density was selected because at this current density was observed considerable CO₂R activity on all studied samples. The experiments were carried out for 60 minutes to ensure the change of the anolyte was observable and the system was still under alkaline condition to avoid proton generation in the anode. Measured was the concentration of OH⁻ and CO₃²⁻ in the anolyte before and after the reaction using total alkalinity method to estimate molar mass of ion transport via migration and diffusion mechanism.

The concentrations of hydroxide and carbonate was measured by a practical method from industry.^{1, 2} On each measurement, 1 mL of the sample was taken and poured in a transparent beaker. Then, one drop of phenolphthalein (ACS reagent, Sigma Aldrich) was added to the beaker. 0.1 M HCl (ACS reagent, Sigma Aldrich) was gradually added to the beaker using 20 μL-scale pipets. The total volume of HCl was measured until the color of phenolphthalein indicator changed from pink to transparent (phenolphthalein alkalinity/PA). The phenolphthalein alkalinity represented the titration of OH⁻ and 1/2 of CO₃²⁻ present in the electrolyte. Then, one drop of methyl orange (ACS reagent, Sigma Aldrich) was added to the beaker. The HCl was again added and its total volume was measured until the color of methyl orange indicator changed from yellow to light orange (total alkalinity/TA). This titration step represented the neutralization of the other half of CO₃²⁻ present in the electrolyte (FIG. 6). The results of titration were calculated using Table S1. Control samples were analyzed and verified the accuracy of the method.

From the titration data, the following information was estimated: charge transferred from anode to the cathode, ion migration and ion diffusion. To interpret the data (material balance), four basic rules were assumed: (1) cation exchange membrane facilitates cation migration, (2) anion exchange membrane facilitates anion migration, (3) ion movement disobey rule (#1) and (#2) was assumed as diffusion of hydrated cation and its corresponding anion, (4) remaining materials after applying rule (#1) to (#3) was assumed as non-ideality of the membrane (e.g., cation exchange membrane allows anion to move). The ion transport in cation exchange membrane (CEM) includes migration of K⁺ (charge difference) and diffusion of KOH_(aq) and KHCO_3(aq) (concentration difference), as shown in FIG. 7a. The later transport mechanism was a result of water uptake properties of Nafion™ 117. In AEM, the ion transport consisted of migration of CO₃²⁻ and diffusion of KOH_(aq) and KHCO_3(aq) (concentration difference), as shown in FIG. 7b. Salt was the bicarbonate which resided in the cathode (Table S.2).

For instance, on the experiment using the Nafion 117 membrane using 1 M KOH as anolyte for ˜1 hour operation:

q = - 7 ⁢ 41 ⁢ sA F = - 96 ⁢ 485 ⁢ sA / mol n e = - 7 ⁢ 31 ⁢ sA 96 ⁢ 485 ⁢ sA mol × 1000 ⁢ mmol 1 ⁢ mol = - 7.7 ⁢ mmol

Where q, F and n_e are the total charge (from the potentiostat), Faraday constant and number of moles of charge, respectively. The minus sign indicates the negative charge flows from the anode to the cathode.

In the anolyte:

c OH , i ⁢ n - = 0.989 M ❘ "\[LeftBracketingBar]" v , i ⁢ n = 12 ⁢ mL n OH , i ⁢ n - = 0.989 mol L ⁢ 1 ⁢ L 1000 ⁢ mL ⁢ 102 ⁢ mL × 1000 ⁢ mmol 1 ⁢ mol = 100.8 mmol c CO 3 2 - , i ⁢ n = 0.007 M n CO 3 2 - , i ⁢ n = 0.007 mol L ⁢ 1 ⁢ L 1000 ⁢ mL ⁢ 102 ⁢ mL × 1000 ⁢ mmol 1 ⁢ mol = 0.7 mmol

Where c_{OH-, in}; v, _in, and n_OH⁻, in; c_CO3²⁻_{, in}; and n_CO3²⁻, in are the initial concentration, initial volume, and initial number of moles of OH⁻; initial concentration and initial number of moles of carbonate, respectively. After reaction:

c OH , fn - = 0.926 M v , f ⁢ n = 98 ⁢ mL n OH , f ⁢ n - = 0.926 mol L ⁢ 1 ⁢ L 1000 ⁢ mL ⁢ 98 ⁢ mL × 1000 ⁢ mmol 1 ⁢ mol = 90.7 mmol c CO 3 2 - , fn = 0.014 M n CO 3 2 - , fn = 0.014 mol L ⁢ 1 ⁢ L 1000 ⁢ mL ⁢ 98 ⁢ mL × 1000 ⁢ mmol 1 ⁢ mol = 1.2 mmol

Where C_OH⁻_{, fn}; v_{, fn}; and n_OH⁻_{, fn}; c_CO3²⁻_{, fn}; and n_CO3²⁻_{, fn} are the final concentration, final volume, and final number of moles of OH⁻; final concentration and final number of moles of carbonate, respectively.

Under the alkaline condition, the oxygen evolution reaction (OER) obeys the following reaction:

The generation of one mole of electron consumes one mole of OH⁻:

n OH - , cons = n e = - 7.7 ⁢ mmol

The measured OH⁻ loss was calculated as follows:

n OH , m ⁢ s - = n OH , fn - - n OH , i ⁢ n - = 90.7 mmol - 100.8 mmol = - 10.1 ⁢ mmol

Then, the difference between the OH⁻ for OER and the hydroxide ions loss (n_OH⁻_{, dif}) was calculated:

n OH - , dif = n OH - , m ⁢ s - n OH - , cons = - 10.1 ⁢ mmol - ( - 7.7 ⁢ mmol ) = - 2.4 ⁢ mmol

The above value indicated that the actual OH⁻ consumption was 2.4 mmol larger than the one that was used for OER. The extra OH⁻ consumption on top of OER could be due to the water uptake (KOH_(aq) diffusion) or carbonation reaction to convert bicarbonate (HCO₃⁻) into carbonate (CO₃²⁻) via thefollowing reaction:

The nature of CEM does not allow migration of anion from the anode to the cathode. However, from the experiment, the increase of CO₃²⁻ concentration was observed, as an indication of HCO₃⁻/CO₃²⁻ crossover via diffusion mechanism. For simplicity, it was assumed that KHCO_3(aq) (hydrated K⁺ and its corresponding HCO₃⁻) transported from the cathode to the anode, as proposed earlier³. Please note that one mole of CO₃²⁻ is equal to one mole of HCO₃⁻ according to the carbonation reaction of HCO₃⁻. The mole of HCO₃⁻ crossover

( n H ⁢ C ⁢ O 3 2 - , xover )

can be calculated as follow:

n H ⁢ C ⁢ O 3 2 - , xover = n C ⁢ O 3 2 - , gen

Where the generation of

CO 3 2 - ( n C ⁢ O 3 2 - , gen )

in the anolyte can be calculated as follow:

n CO 3 2 - , gen = n CO 3 2 - , fn - n CO 3 2 - , i ⁢ n = 1.2 mmol - 0.7 mmol = 0.5 mmol

Recalling the HCO₃⁻ crossover equation:

n H ⁢ C ⁢ O 3 2 - , xover = n C ⁢ O 3 2 - , gen n H ⁢ C ⁢ O 3 2 - , xover = 0.5 mmol

The occurrence of KHCO_3(aq) transport can be explained by the reaction between CO₂ and KOH in the cathode

The CO₂ was supplied to the cathode as the feed for CO₂R, while the presence of KOH in the cathode was due to the water uptake properties of CEM which allowed KOH_(aq) (hydrated K⁺ and its corresponding OH⁻) to transport from the anode to the cathode via diffusion mechanism⁴. The rate of OH⁻ that diffuse from anode to cathode (n_OH⁻_{, d-carb}) can be estimated as follow:

n OH - , d - carb = n H ⁢ C ⁢ O 3 2 - , xover = 0.5 mmol

Recalling that KHCO_3(aq) diffuse from the cathode to the anode

( n H ⁢ C ⁢ O 3 2 - , xover = 0.5 mmol ) .

Once HCO₃⁻ reaches the anode, it reacts with OH⁻ to form CO₃²⁻ and H₂O:

From the above reaction, it is known that to one mole of OH⁻ is required to convert HCO₃⁻ into CO₃²⁻. As the reactions occur in the anolyte, the amount of OH⁻ that was consumed in the anolyte (n_OH⁻_{, carb}) is estimated as follow:

n OH - , carb = - ( 1 × n H ⁢ C ⁢ O 3 2 - , xover ) = - ( 1 × 0.5 mmol ) = - 0.5 ⁢ mmol

Summarizing the OH⁻ consumption due to carbonate transport:

• • OH⁻ diffusion which results in HCO₃⁻ formation (n_OH⁻_{, d-carb})=−0.5 mmol
  • OH⁻ Consumption for carbonate reaction (n_OH⁻_{, carb})=−0.5 mmol

Recall that there are an extra 2.4 mmol of OH⁻ consumption on top of OH⁻ consumption for OER. It was considered that the remaining OH⁻ loss in the anode was due to OH⁻ diffusion from the anode to the cathode.

n OH - , d - lost = n O ⁢ H - , dif - n O ⁢ H - , d - carb - n O ⁢ H - , carb = - 2.4 ⁢ mmol - ( - 0.5 ⁢ mmol ) - ( - 0.5 ⁢ mmol ) = 1.4 mmol

The transport mechanism can be migration of ion and diffusion. The migration mechanism is the transport of ion due to the charge difference between two spots. The diffusion mechanism is the transport of ion due to the concentration difference. The diffusion of anion species is to be followed by the diffusion of the corresponding cation. For example, the diffusion of OH⁻ is followed by the diffusion of K⁺ as the corresponding counterion of KOH_(aq)⁴. Similarly, the diffusion of HCO₃⁻ is followed by the diffusion of K⁺. Hence, the material balance of K⁺ can be calculated.

Begin with the amount of K⁺ before reaction (n_{K+, in}):

n K + , in = n OH - , in + ( 2 × n CO 3 2 - , in ) = 100.8 mmol + 0.7 mmol = 102.2 mmol

Then, estimate the amount of K⁺ after reaction:

n K + , fn = n O ⁢ H - , fn + ( 2 × n C ⁢ O 3 2 - , xover ) = 90.7 mmol + 1.2 mmol = 91.9 mmol

The mole of K⁺ consumption can be calculated as follow:

n K + , djff = n K + , fn - n K + , in = 102.2 mmol - 91.9 mmol = - 9.1 ⁢ mmol

In the CEM, K⁺ migrated from the anode to the cathode to balance the charge. The amount of K⁺ migration (n_K+mgr) can be estimated as follows:

n K + , mgr = n e = - 7.7 ⁢ mmol

The amount of K⁺ consumption was higher than its counterpart of K⁺ migration.

n K + , djf = n K + , mgr - n K + , m ⁢ s = - 7.7 ⁢ mmol - ( - 9.1 ⁢ mmol ) = - 1.4 ⁢ mmol

From the above calculation, it is shown that an extra 1.4 mmol of K⁺ is consumed on top of the K⁺ consumption due to migration mechanism. Thus, the extra K⁺ loss can be related to the K⁺ transport via diffusion mechanism. Remember that diffusion of K⁺ is strongly related to the diffusion of its corresponding anion. For example, the K⁺ diffusion along with OH⁻ diffusion from the anode to the cathode which results in the HCO₃⁻ formation (n_{K+, d-carb}) can be calculated as follow:

n K + , d - carb - ct = n OH - , d - carb = - 0.5 ⁢ mmol

The K⁺ diffusion along with HCO₃⁻ from the cathode to the anode can be calculated as follow:

n K + , d - carb - an = n H ⁢ C ⁢ O 3 2 - , xover = 0.5 mmol

The K⁺ loss after considering the diffusion of KOH_(aq) for KHCO_3(aq) formation and the diffusion of KHCO_3(aq) can be considered as diffusion of KOH which lost (remain) in the cathode.

n K + , d - lost = n K + , djf - ( n K + , m ⁢ g ⁢ r + n K + , d - c ⁢ a ⁢ r ⁢ b - c ⁢ t + n K + , d - c ⁢ a ⁢ r ⁢ b - a ⁢ n ) = - 9.1 ⁢ mmol - ( - 7.7 ⁢ mmol + ( - 0.5 ⁢ mmol ) + 0.5 mmol ) = - 1.4 ⁢ mmol

The experiments were conducted for 1.1 hours. The value as calculated above is normalized for 1 hour experiment. The final value is presented in Figure S.5. The solubility of KOH, K₂CO₃ and KHCO₃ in water is summarized in Table S.2.

G. Measurement of Mass Diffusion

The mass transport experiments (diffusion experiment) were carried out using a similar setup with CO₂ electrolysis using Nafion-117 or CISPE in MEA without applied potential. For instance, the transport flux of the Nafion-117 system was executed with sputtered copper in the cathode, Ni foam in the anode and Nafion-117 between the anode and the cathode. Similarly, the transport flux of the CISPE system was carried out with CISPE in the cathode and Ni foam in the anode. No standalone membrane was used in CISPE system. The humidified N₂ at 60 sccm was directed to the cathode to mimic the cathode condition in CO₂ electrolysis and to avoid salt formation. 1 M KOH solution at 60 sccm was circulated in the anode. The convection effect due to pressure difference that is generated by the peristaltic pump can be neglected due to the dihedral angle of 90°. The experiment was terminated when the change of anolyte level become observable.

Nafion - 117 Initial ⁢ anolyte ⁢ volume ⁢ ( V in ) = 20. mL Final ⁢ anolyte ⁢ volume ⁢ ( V fn ) = 19.1 mL Volume ⁢ difference ⁢ ( V diff ) = V in - V fn = 20. mL - 19.1 mL = 0.9 mL Density ⁢ of ⁢ 1 ⁢ M ⁢ KOH ⁢ ( ρ a ⁢ n ) = 1.05 g / mL Mass ⁢ difference ⁢ ( m diff ) = 0.9 mL × 1.05 g / mL = 0.945 g Duration ⁢ ( t ) = 69 , TagBox[",", "NumberComma", Rule[SyntaxForm, "0"]] 780 ⁢ s Area ⁢ ( A ) = 3.99 cm 2 Flux ⁢ ( N ) = m diff A × t = 0 . 9 ⁢ 45 ⁢ g 3.99 cm 2 × 69 , TagBox[",", "NumberComma", Rule[SyntaxForm, "0"]] 780 ⁢ s = 3.39 × 1 ⁢ 0 - 6 ⁢ g c ⁢ m 2 ⁢ s USPE - 50 Initial ⁢ anolyte ⁢ volume ⁢ ( V in ) = 20. mL Final ⁢ anolyte ⁢ volume ⁢ ( V fn ) = 19.5 mL Volume ⁢ difference ⁢ ( V diff ) = V in - V fn = 20. mL - 19.5 mL = 0.5 mL Density ⁢ of ⁢ 1 ⁢ M ⁢ KOH ⁢ ( ρ a ⁢ n ) = 1.05 g / mL Mass ⁢ difference ⁢ ( m diff ) = 0.5 mL × 1.05 g / mL = 0.525 g Duration ⁢ ( t ) = 77 , TagBox[",", "NumberComma", Rule[SyntaxForm, "0"]] 100 ⁢ s Area ⁢ ( A ) = 3.96 cm 2 Flux ⁢ ( N ) = m diff A × t = 0.525 g 3.96 cm 2 × 77 , TagBox[",", "NumberComma", Rule[SyntaxForm, "0"]] 100 ⁢ s = 1.72 × 10 - 6 ⁢ g c ⁢ m 2 ⁢ s

H. Performance Calculation

The CO₂R experiment was carried out at current density (j) of 100 mA/cm² using with a flowrate humidified CO₂ inlet (v) of 6 sccm (standard cm³ per minute).

v = 6 ⁢ cm 3 / min R = 82. 0 ⁢ 6 ⁢ atm ⁢ cm 3 mol ⁢ K T = 298.15 K ⁢ ( 25 ⁢ ° ⁢ C . ) P = 1 ⁢ atm n CO ⁢ 2 = V ⁢ P RT = 6 ⁢ cm 3 min ⁢ 1 ⁢ atm 8 ⁢ 2 . 0 ⁢ 6 ⁢ atm ⁢ cm 3 mol ⁢ K 298.15 K ⁢ 60 ⁢ min 1 ⁢ hour = 1.47 × 1 ⁢ 0 - 2 ⁢ mol hour

Where R, T, P and n represent gas constant, temperature, pressure and mole flow, respectively.

i =  100 ⁢ mA / cm 2 A = 4 ⁢ cm 2 j = i × A = 100 ⁢ mA cm 2 × 4 ⁢ cm 2 × 1 ⁢ A 1000 ⁢ mA = 0.4 A

Where i, A and j are current density, electrode area and total current, respectively. Then, the energy supplied to the electrolyzer (E_el) was calculated for 1 hour basis.

E el = j × V × 1 ⁢ hour = 0.4 A × 2.8 V × 1 ⁢ hour 1 ⁢ hour = 1.14 Whour hour × 3.6 kJ 1 ⁢ Whour = 4.1 kJ hour × 1 ⁢ GJ 10 6 ⁢ kJ = 4.1 × 10 - 6 ⁢ GJ hour

From the Faradaic efficiency (FE), the flowrate of products (n_prod) was calculated:

n prod = FE × j n e × F

F is the Faraday constant 96485 sA/mol. n_e is the number of electrons involved in the reaction.

The flowrate of CO₂ converted into product in the electrolyzer can be estimated from the product flowrate.

n m - CO ⁢ 2 = n m - ⁢ prod

• • n_m-CO2 and n_m-prod indicate the molar flowrate (mol/hour) of CO₂ that is consumed to produce m-carbon atom product and the generated m-carbon atom product, respectively. For example, CO and C₂H₄ are considered as one-atom and two-atom products from CO₂R.

The single pass conversion (X) can be calculated as follow:

X = ∑ n n - CO 2 n CO ⁢ 2

The energy efficiency of electrolyzer for C₂H₄ production (E_el-C2H4) can be calculated as follow [8]:

E el - C 2 ⁢ H 4 = V C 2 ⁢ H 4 × j V cell × j

Where

V C 2 ⁢ H 4 o

is defined as follow:

V C 2 ⁢ H 4 = V C 2 ⁢ H 4 O × FE C 2 ⁢ H 4

and FE_C2H4 are the standard potential and Faradaic efficiency of C₂H₄, respectively.

The energy required for CO₂ gas separation (E_CO2) in the cathode is estimated as follow:

E CO ⁢ 2 = n CO ⁢ 2 - r × E am

• • where n_CO2-r is the flowrate of CO₂ captured, while E_am is the energy required to capture CO₂ based on amine-based CO₂ capture (3.6 GJ/ton CO₂) [9].

The energy required for carbonate recovery (E_carb) in the anode is estimated as follow:

E carb = n carb - r × E calc

• • where n_carb-r is the flowrate of carbonate, while E_calc is the energy required to regenerate carbonate (1.41 GJ/ton carbonate or equivalent with 4.43 GJ/ton CO₂) [10].

The flowrate of carbonate was calculated based on the experimental data in FIG. 11 which consider both migration and diffusion of carbonate ions (CO₃²⁻ and HCO₃⁻). For example, the flowrate of carbonate ion in the case of CISPE-50-0.15M with 1 M KOH anolyte was 1.03 times the carbonate migration. Similarly, for the AEM with 1 M KOH anolyte, the flowrate of carbonate was 1.74 times the carbonate migration.

The calculation and the data for calculation is shown above in the Example 2, Section H.

I. Electrochemical Impedance Spectroscopy (EIS)

EIS Study in Three-Electrode Cell and Flow Cell Systems

The observed CO₂R studies suggested that the USPE (3 mm), devoid of the standalone CEM (180 mm), endorsed high selective CO₂ reduction at the electrode/electrolyte interface (FIG. 3d). To further inquire about the nature of ionic conductivity, EIS were carried out in a three-electrode set up for the (a) PTFE, (b) Cu sputtered PTFE (Cu/PTFE) and (c) USPE-50 at open circuit potential (OCP) in 1 M KOH from 10 kHz to 0.1 Hz with an amplitude of 5 mV. From FIG. 8a-c, the PTFE delivered high R_s (solution resistance, 17,025Ω) due to hydrophobic behaviour from polymer backbones. Once Cu was sputtered, R_s was decreased up to 6 folds (3,045Ω). Further addition of directly deposited ultrathin ionomer layer (USPE-50) resulted in reduced R_s to 30Ω indicating facile ion transport. This facile ion transport resulted in lower cell voltage as was observed in performing CO₂ electrolysis experiment.

To specifically unveil the ionic resistivity in a flow cell system, it was then opted to perform EIS study in a representative cell as shown in FIG. 9a. From the corresponding R_s values examined from Nyquist plots given in FIGS. 9b and c, mainly when comparing USPE-50 and CEM, the resistance at the interface was higher in the latter one with 6-fold increment (32.4Ω). The presence of directly deposited ultrathin Nafion coating that strongly adhered to the Cu sputtered surface supports facile ion transport, which resulted in lower cell voltage. In case of CEM, because of high thickness (180 μm), resistance was increased for the ionic transport.

Proton Conductivity Studies

Also performed were proton conductivity study for directly deposited ultrathin ionomer, that was compared with the commercial thick stand-alone Nafion™ 117 membrane. Please note that the experimental setup for CO₂R was different than the setup for proton conductivity experiment. In CO₂R experiment 1 M KOH was used as cation source, while in the proton conductivity experiment humidified environment was used as proton source. Nafion consists of hydrophobic polymeric backbone and hydrophilic sulfonic acid regions. These hydrophilic groups allow proton transport from the hydrated ionic clusters. This nature of proton transport is affected via many factors such as temperature, humidity, and thickness of the membrane. Based on this, proton conductivity measurements were carried out in a conductivity cell that consisted of Pt electrodes connected via Au wire, and the temperature was around 25° C. with the relative humidity of 3%. The studied samples were CEM and the USPE-50. The results of EIS measurement at open circuit potential are given in FIG. 10.

From the R_s values obtained, conductivity(s) was calculated. A conductivity of 0.01035 S/m² was calculated for CEM, and 0.01951 S/m² for USPE-50, indicating that the latter offers improved proton conductivity. These results suggested that the presence of directly deposited ultrathin Nafion improved the proton conductivity. This could be attributed to more active sites to occupy hydrated ionic clusters with the modified microstructural features in USPE-50. As shown from SEM and optical profilometry (FIG. 3c, FIG. 15, and FIG. 12), USPE-50 had Nafion sprayed over the surface of sputtered copper that helps to keep the migrated ions in close vicinity of CO₂. However, as the membrane environment was different from CO₂R in MEA assembly, where USPE-50 offered good migration of (K⁺), this may be correlated to more available SO₃ groups to ensure transport of ions. These findings suggested that the directly deposited ultrathin Nafion offered improved performance versus commercially available thick Nafion membrane with improved ion conductivities, low cell voltage and stable CO₂ electroreduction.

TABLE S.1
Hydroxide, carbonate and bicarbonate alkalinities as a function
of total alkalinity and phenolphthalein alkalinity.

	Result of titration	Hydroxide	Carbonate	Bicarbonate
	PA > 0.5TA	2PA − TA	2(TA − PA)	0
	PA < 0.5TA	0	2PA	TA − 2PA
	PA = TA	T	0	0
	PA = 0.5TA	0	2P	0
	PA = 0	0	0	TA

TABLE S.2
Solubility in water at 25° C. (298.15 K)

	Name	Solubility (g/100 mL of water)	Ref.

	K2CO3	111	[5]
	KHCO3	22.8	[6]
	KOH	121	[7]

TABLE S.3
Reports on one-step CO2 electrolysis to produce ethylene (C2H4) in MEA setup (A and B).
A -

Cell voltage, V	2.85	3.65	3.75	3.8	3.9	3.75	3.75	3.7	3.8	3.9	4.55	3.4	3.5
Faradaic	65%	60%	40%	63%	35%	35%	42%	60%	57%	53%	50%	68%	27%
efficiency
Current Density,	100	120	120	220	100	250	100	300	138	1100	400	100	300
mA/cm2
Single Pass	18%	2%	2%	4%	36%	9%	29%	5%	2%	6%	15%	2%	12%
Conversion*
CO2 Capture,	18	20	33	23	37	42	30	11	22	27	25	15	68
GJ/ton C2H4
Electrolyzer	181	251	387	249	460	442	369	255	275	304	376	206	535
electricity, GJ/ton
C2H4
Cathode CO2 gas	30	30	30	30	30	30	30	30	30	30	30	30	30
separation,
GJ/ton C2H4**
Anode CO2 gas	0	56	85	54	0	97	0	56	59	64	68	0	125
separation,
GJ/ton C2H4
Carbonate	67	0	0	0	0	0	0	0	0	0	0	106	0
regeneration,
GJ/ton C2H4
Stability, hours	110	195	100	100	9	4	200	70	157	60	100	8	20
Total energy	296	357	535	356	526	611	429	352	387	424	498	357	758
cost, GJ/ton C2H4
Electrolyzer	27%	20%	13%	20%	11%	11%	13%	19%	18%	16%	13%	24%	9%
energy efficiency
for C2H4
Anolyte	1M	0.1M	0.1M	0.1M	0.01M	0.1M	0.01	0.1M	0.1M	0.1M	0.1M	1M	0.1M
	KOH	KHCO3	KHCO3	KHCO3	H2SO4	KHCO3	H2SO4	KHCO3	KHCO3	KHCO3	KHCO3	KOH	KHCO3
Reference	Present	[12]	[13]	[14]	[15]	[16]	[17]	[18]	[19]	[20]	[21]	[22]	[23]
	work

B -

Cell voltage, V	2.85	3.65	3.75	3.8	3.9	3.75	3.75	3.7	3.8	3.9	4.55	3.4	3.5
Faradaic efficiency	65%	60%	40%	63%	35%	35%	42%	60%	57%	53%	50%	68%	27%
Current Density,	100	120	120	220	100	250	100	300	138	1100	400	100	300
mA/cm2
Single Pass	18%	2%	2%	4%	36%	9%	29%	5%	2%	6%	15%	2%	12%
Conversion*
CO2 Capture,	18	20	33	23	37	42	30	11	22	27	25	15	68
GJ/ton C2H4
Electrolyzer	181	251	387	249	460	442	369	255	275	304	376	206	535
electricity, GJ/ton
C2H4
Cathode CO2 gas	30	30	30	30	30	30	30	30	30	30	30	30	30
separation, GJ/ton
C2H4**
Anode CO2 gas	0	56	85	54	0	97	0	56	59	64	68	0	125
separation, GJ/ton
C2H4
Carbonate	67	0	0	0	0	0	0	0	0	0	0	100	0
regeneration,
GJ/ton C2H4
Stability, hours	110	195	100	100	9	4	200	70	157	60	100	8	20
Total energy cost,	290	357	535	356	526	611	429	352	387	424	498	357	758
GJ/ton C2H4
Electrolyzer energy	28%	20%	13%	20%	11%	11%	13%	19%	18%	16%	13%	24%	9%
efficiency for C2H4
Anolyte	1M	0.1M	0.1M	0.1M	0.01M	0.1M	0.01	0.1M	0.1M	0.1M	0.1M	1M	0.1M
	KOH	KHCO3	KHCO3	KHCO3	H2SO4	KHCO3	H2SO4	KHCO3	KHCO3	KHCO3	KHCO3	KOH	KHCO3
Reference	Our	[12]	[13]	[14]	[15]	[16]	[17]	[18]	[19]	[20]	[21]	[22]	[23]
	work
*The value is based on actual flowrate used in those reports. Assumptions were made to calculate the single pass conversion. See Example 2, Section H for details.
**The cathode CO2 gas separation energy is assumed to be constant at 30 GJ/ton C2H4 for consistent comparison across the literature reports wherein various CO2 flow rate was used.

Background, Detailed Description—Carbon Dioxide Electroreduction (CO₂R), and Example 1 References

• 1. R. I. Masel, Z. Liu, H. Yang, J. J. Kaczur, D. Carrillo, S. Ren, D. Salvatore and C. P. Berlinguette, Nature Nanotechnology, 2021, 16, 118-128.
• 2. H. Shin, K. U. Hansen and F. Jiao, Nature Sustainability, 2021, 4, 911-919.
• 3. D. Higgins, C. Hahn, C. Xiang, T. F. Jaramillo and A. Z. Weber, ACS Energy Letters, 2019, 4, 317-324.
• 4. L.-C. Weng, A. T. Bell and A. Z. Weber, Energy & Environmental Science, 2019, 12, 1950-1968.
• 5. N. Vassal, E. Salmon and J. F. Fauvarque, Electrochimica Acta, 2000, 45, 1527-1532.
• 6. M. G. Kibria, J. P. Edwards, C. M. Gabardo, C.-T. Dinh, A. Seifitokaldani, D. Sinton and E. H. Sargent, Advanced Materials, 2019, 31, 1807166.
• 7. G. O. Larrazebal, P. Strøm-Hansen, J. P. Heli, K. Zeiter, K. T. Therkildsen, I. Chorkendorff and B. Seger, ACS Applied Materials & Interfaces, 2019, 11, 41281-41288.
• 8. Y. Wang, Z. Wang, C.-T. Dinh, J. Li, A. Ozden, M. Golam Kibria, A. Seifitokaldani, C.-S. Tan, C. M. Gabardo, M. Luo, H. Zhou, F. Li, Y. Lum, C. McCallum, Y. Xu, M. Liu, A. Proppe, A. Johnston, P. Todorovic, T.-T. Zhuang, D. Sinton, S. O. Kelley and E. H. Sargent, Nature Catalysis, 2020, 3, 98-106.
• 9. F. P. Garcia de Arquer, C.-T. Dinh, A. Ozden, J. Wicks, C. McCallum, A. R. Kirmani, D.-H. Nam, C. Gabardo, A. Seifitokaldani, X. Wang, Y. C. Li, F. Li, J. Edwards, L. J. Richter, S. J. Thorpe, D. Sinton and E. H. Sargent, Science, 2020, 367, 661-666.
• 10. Z. Liu, H. Yang, R. Kutz and R. I. Masel, Journal of The Electrochemical Society, 2018, 165, J3371-J3377.
• 11. J. A. Rabinowitz and M. W. Kanan, Nature Communications, 2020, 11, 5231.
• 12. C. P. O'Brien, R. K. Miao, S. Liu, Y. Xu, G. Lee, A. Robb, J. E. Huang, K. Xie, K. Bertens, C. M. Gabardo, J. P. Edwards, C.-T. Dinh, E. H. Sargent and D. Sinton, ACS Energy Letters, 2021, 6, 2952-2959.
• 13. Á. Vass, B. Endrödi, G. F. Samu, Á. Balog, A. Kormányos, S. Cherevko and C. Janáky, ACS Energy Letters, 2021, 6, 3801-3808.
• 14. A. Ozden, F. Li, F. P. García de Arquer, A. Rosas-Hernández, A. Thevenon, Y. Wang, S.-F. Hung, X. Wang, B. Chen, J. Li, J. Wicks, M. Luo, Z. Wang, T. Agapie, J. C. Peters, E. H. Sargent and D. Sinton, ACS Energy Letters, 2020, 5, 2811-2818.
• 15. F. Li, A. Thevenon, A. Rosas-Hernández, Z. Wang, Y. Li, C. M. Gabardo, A. Ozden, C. T. Dinh, J. Li, Y. Wang, J. P. Edwards, Y. Xu, C. McCallum, L. Tao, Z.-Q. Liang, M. Luo, X. Wang, H. Li, C. P. O'Brien, C.-S. Tan, D.-H. Nam, R. Quintero-Bermudez, T.-T. Zhuang, Y. C. Li, Z. Han, R. D. Britt, D. Sinton, T. Agapie, J. C. Peters and E. H. Sargent, Nature, 2020, 577, 509-513.
• 16. Y. Xu, F. Li, A. Xu, J. P. Edwards, S.-F. Hung, C. M. Gabardo, C. P. O'Brien, S. Liu, X. Wang, Y. Li, J. Wicks, R. K. Miao, Y. Liu, J. Li, J. E. Huang, J. Abed, Y. Wang, E. H. Sargent and D. Sinton, Nature Communications, 2021, 12, 2932.
• 17. C. M. Gabardo, C. P. O'Brien, J. P. Edwards, C. McCallum, Y. Xu, C.-T. Dinh, J. Li, E. H. Sargent and D. Sinton, Joule, 2019, 3, 2777-2791.
• 18. G. Zhang, Z.-J. Zhao, D. Cheng, H. Li, J. Yu, Q. Wang, H. Gao, J. Guo, H. Wang, G. A. Ozin, T. Wang and J. Gong, Nature Communications, 2021, 12, 5745.
• 19. Y. Xu, J. P. Edwards, S. Liu, R. K. Miao, J. E. Huang, C. M. Gabardo, C. P. O'Brien, J. Li, E. H. Sargent and D. Sinton, ACS Energy Letters, 2021, 6, 809-815.
• 20. A. Robb, A. Ozden, R. K. Miao, C. P. O'Brien, Y. Xu, C. M. Gabardo, X. Wang, N. Zhao, F. P. Garcia de Arquer, E. H. Sargent and D. Sinton, ACS Applied Materials & Interfaces, 2022, 14, 4155-4162.
• 21. G. O. Larrazábal, V. Okatenko, I. Chorkendorff, R. Buonsanti and B. Seger, ACS Applied Materials & Interfaces, 2022, DOI: 10.1021/acsami.1c18856.
• 22. X. Wang, Z. Wang, F. P. Garcia de Arquer, C.-T. Dinh, A. Ozden, Y. C. Li, D.-H. Nam, J. Li, Y.-S. Liu, J. Wicks, Z. Chen, M. Chi, B. Chen, Y. Wang, J. Tam, J. Y. Howe, A. Proppe, P. Todorovid, F. Li, T.-T. Zhuang, C. M. Gabardo, A. R. Kirmani, C. McCallum, S.-F. Hung, Y. Lum, M. Luo, Y. Min, A. Xu, C. P. O'Brien, B. Stephen, B. Sun, A. H. Ip, L. J. Richter, S. O. Kelley, D. Sinton and E. H. Sargent, Nature Energy, 2020, 5, 478-486.
• 23. Z. Yin, H. Peng, X. Wei, H. Zhou, J. Gong, M. Huai, L. Xiao, G. Wang, J. Lu and L. Zhuang, Energy & Environmental Science, 2019, 12, 2455-2462.
• 24. H. Wu, J. Li, K. Qi, Y. Zhang, E. Petit, W. Wang, V. Flaud, N. Onofrio, B. Rebiere, L. Huang, C. Salameh, L. Lajaunie, P. Miele and D. Voiry, Nature Communications, 2021, 12, 7210.
• 25. Z. Yan, J. L. Hitt, Z. Zeng, M. A. Hickner and T. E. Mallouk, Nature Chemistry, 2021, 13, 33-40.
• 26. D. A. Salvatore, D. M. Weekes, J. He, K. E. Dettelbach, Y. C. Li, T. E. Mallouk and C. P. Berlinguette, ACS Energy Letters, 2018, 3, 149-154.
• 27. S. Z. Oener, M. J. Foster and S. W. Boettcher, Science, 2020, 369, 1099-1103.
• 28. Y. Chen, A. Vise, W. E. Klein, F. C. Cetinbas, D. J. Myers, W. A. Smith, T. G. Deutsch and K. C. Neyerlin, ACS Energy Letters, 2020, 5, 1825-1833.
• 29. S. Z. Oener, L. P. Twight, G. A. Lindquist and S. W. Boettcher, ACS Energy Letters, 2021, 6, 1-8.
• 30. Á. Vass, A. Kormányos, Z. Kószó, B. Endrödi and C. Janáky, ACS Catalysis, 2022, 12, 1037-1051.
• 31. B. Endrödi, A. Samu, E. Kecsenovity, T. Halmágyi, D. Sebök and C. Janáky, Nature Energy, 2021, 6, 439-448.
• 32. A. Kusoglu and A. Z. Weber, Chemical Reviews, 2017, 117, 987-1104.
• 33. P. Mardle, S. Cassegrain, F. Habibzadeh, Z. Shi and S. Holdcroft, The Journal of Physical Chemistry C, 2021, 125, 25446-25454.
• 34. J. Resasco, L. D. Chen, E. Clark, C. Tsai, C. Hahn, T. F. Jaramillo, K. Chan and A. T. Bell, Journal of the American Chemical Society, 2017, 139, 11277-11287.
• 35. S. Ringe, E. L. Clark, J. Resasco, A. Walton, B. Seger, A. T. Bell and K. Chan, Energy & Environmental Science, 2019, 12, 3001-3014.
• 36. A. G. Fink, E. W. Lees, Z. Zhang, S. Ren, R. S. Delima and C. P. Berlinguette, ChemElectroChem, 2021, 8, 2094-2100.
• 37. M. C. O. Monteiro, F. Dattila, B. Hagedoorn, R. Garcia-Muelas, N. López and M. T. M. Koper, Nature Catalysis, 2021, 4, 654-662.
• 38. K. Yang, M. Li, S. Subramanian, M. A. Blommaert, W. A. Smith and T. Burdyny, ACS Energy Letters, 2021, DOI: 10.1021/acsenergylett.1c02058, 4291-4298.
• 39. D. G. Wheeler, B. A. W. Mowbray, A. Reyes, F. Habibzadeh, J. He and C. P. Berlinguette, Energy & Environmental Science, 2020, 13, 5126-5134.
• 40. D. A. Salvatore, C. M. Gabardo, A. Reyes, C. P. O'Brien, S. Holdcroft, P. Pintauro, B. Bahar, M. Hickner, C. Bae, D. Sinton, E. H. Sargent and C. P. Berlinguette, Nature Energy, 2021, 6, 339-348.
• 41. C. McCallum, C. M. Gabardo, C. P. O'Brien, J. P. Edwards, J. Wicks, Y. Xu, E. H. Sargent and D. Sinton, Cell Reports Physical Science, 2021, 2, 100522.
• 42. A. Pǎtru, T. Binninger, B. Pribyl and T. J. Schmidt, Journal of The Electrochemical Society, 2019, 166, F34-F43.
• 43. M. A. Blommaert, R. Sharifian, N. U. Shah, N. T. Nesbitt, W. A. Smith and D. A. Vermaas, Journal of Materials Chemistry A, 2021, 9, 11179-11186.
• 44. L. Giordano, B. Han, M. Risch, W. T. Hong, R. R. Rao, K. A. Stoerzinger and Y. Shao-Horn, Catalysis Today, 2016, 262, 2-10.
• 45. A. Reyes, R. P. Jansonius, B. A. W. Mowbray, Y. Cao, D. G. Wheeler, J. Chau, D. J. Dvorak and C. P. Berlinguette, ACS Energy Letters, 2020, 5, 1612-1618.
• 46. M. Klingele, B. Britton, M. Breitwieser, S. Vierrath, R. Zengerle, S. Holdcroft and S. Thiele, Electrochemistry Communications, 2016, 70, 65-68.
• 47. M. Klingele, M. Breitwieser, R. Zengerle and S. Thiele, Journal of Materials Chemistry A, 2015, 3, 11239-11245.
• 48. P. Holzapfel, M. Bühler, C. Van Pham, F. Hegge, T. Böhm, D. McLaughlin, M. Breitwieser and S. Thiele, Electrochemistry Communications, 2020, 110, 106640.
• 49. P. Veh, B. Britton, S. Holdcroft, R. Zengerle, S. Vierrath and M. Breitwieser, R S C Advances, 2020, 10, 8645-8652.
• 50. C. A. Moreno-Camacho, J. R. Montoya-Torres, A. Jaegler and N. Gondran, Journal of Cleaner Production, 2019, 231, 600-618.
• 51. S. Hui, N. Shaigan, V. Neburchilov, L. Zhang, K. Malek, M. Eikerling and P. D. Luna, Nanomaterials, 2020, 10, 1884.
• 52. S. Popović, M. Smiljanić, P. Jovanovič, J. Vavra, R. Buonsanti and N. Hodnik, Angewandte Chemie International Edition, 2020, 59, 14736-14746.
• 53. J. Huang, N. Hörmann, E. Oveisi, A. Loiudice, G. L. De Gregorio, O. Andreussi, N. Marzari and R. Buonsanti, Nature Communications, 2018, 9, 3117.
• 54. Y. S. Li, T. S. Zhao and R. Chen, Journal of Power Sources, 2011, 196, 133-139.
• 55. M. Akira and H. Yoshio, Bulletin of the Chemical Society of Japan, 1991, 64, 123-127.
• 56. M. R. Thorson, K. I. Siil and P. J. A. Kenis, Journal of The Electrochemical Society, 2012, 160, F69-F74.
• 57. J. E. Huang, F. Li, A. Ozden, A. S. Rasouli, F. P. G. d. Arquer, S. Liu, S. Zhang, M. Luo, X. Wang, Y. Lum, Y. Xu, K. Bertens, R. K. Miao, C.-T. Dinh, D. Sinton and E. H. Sargent, Science, 2021, 372, 1074-1078.
• 58. M. C. O. Monteiro, F. Dattila, N. López and M. T. M. Koper, Journal of the American Chemical Society, 2021, DOI: 10.1021/jacs.1c10171.
• 59. S. Banerjee, Z.-Q. Zhang, A. S. Hall and V. S. Thoi, ACS Catalysis, 2020, 10, 9907-9914.
• 60. D. Gao, I. T. McCrum, S. Deo, Y.-W. Choi, F. Scholten, W. Wan, J. G. Chen, M. J. Janik and B. Roldan Cuenya, ACS Catalysis, 2018, 8, 10012-10020.
• 61. S. Shi, A. Z. Weber and A. Kusoglu, Electrochimica Acta, 2016, 220, 517-528.
• 62. D. V. Esposito, ACS Catalysis, 2018, 8, 457-465.
• 63. Y. Xu, R. K. Miao, J. P. Edwards, S. Liu, C. P. O'Brien, C. M. Gabardo, M. Fan, J. E. Huang, A. Robb, E. H. Sargent and D. Sinton, Joule, 2022, 6, 1333-1343.
• 64. N.-H. Tran, H. P. Duong, G. Rousse, S. Zanna, M. W. Schreiber and M. Fontecave, ACS Applied Materials & Interfaces, 2022, 14, 31933-31941.
• 65. W. Choi, Y. Choi, E. Choi, H. Yun, W. Jung, W. H. Lee, H.-S. Oh, D. H. Won, J. Na and Y. J. Hwang, Journal of Materials Chemistry A, 2022, 10, 10363-10372.

EXAMPLE 2 AND FIGS. 6-26 REFERENCES

• 1. How do the alkalinity methods measure hydroxides, carbonates, and bicarbonates?2021 [cited 2021; Available from: https://support.hach.com/app/answers/answer_view/a_id/1000041/-/how-do-the-alkalinity-methods-measure-hydroxides %2C-carbonates %2C-and-bicarbonates %3F.
• 2. Michatowski, T. and A. G. Asuero, New Approaches in Modeling Carbonate Alkalinity and Total Alkalinity. Critical Reviews in Analytical Chemistry, 2012. 42(3): p. 220-244.
• 3. Blommaert, M. A., et al., Orientation of a bipolar membrane determines the dominant ion and carbonic species transport in membrane electrode assemblies for CO₂ reduction. Journal of Materials Chemistry A, 2021. 9(18): p. 11179-11186.
• 4. Hu, J., et al., Mechanism and transfer behavior of ions in Nafion membranes under alkaline media. Journal of Membrane Science, 2018. 566: p. 8-14.
• 5. PubChem Compound Summary for CID 11430, Potassium carbonate. [cited 2022; Available from: https://pubchem.ncbi.nlm.nih.gov/compound/Potassium-carbonate.
• 6. Trypuć, M., U. Kietkowska, and D. Stefanowicz, Solubility Investigations in the KHCO3+NH4HCO3+H2O System. Journal of Chemical & Engineering Data, 2001. 46(4): p. 800-804.
• 7. PubChem Compound Summary for CID 14797, Potassium hydroxide. [cited 2022; Available from: https://pubchem.ncbi.nlm.nih.gov/compound/Potassium-hydroxide #section=Solubility.
• 8. Sisler, J., et al., Ethylene Electrosynthesis: A Comparative Techno-economic Analysis of Alkaline vs Membrane Electrode Assembly vs CO₂-CO-C2H4 Tandems. ACS Energy Letters, 2021. 6(3): p. 997-1002.
• 9. Romeo, L. M., et al., Comparative Analysis of the Efficiency Penalty in Power Plants of Different Amine-Based Solvents for CO2 Capture. Industrial & Engineering Chemistry Research, 2020. 59(21): p. 10082-10092.
• 10. Keith, D. W., et al., A Process for Capturing CO₂ from the Atmosphere. Joule, 2018. 2(8): p. 1573-1594.
• 11. Jouny, M., W. Luc, and F. Jiao, General Techno-Economic Analysis of CO₂ Electrolysis Systems. Industrial & Engineering Chemistry Research, 2018. 57(6): p. 2165-2177.
• 12. Li, F., et al., Molecular tuning of CO₂-to-ethylene conversion. Nature, 2020. 577(7791): p. 509-513.
• 13. Gabardo, C. M., et al., Continuous Carbon Dioxide Electroreduction to Concentrated Multi-carbon Products Using a Membrane Electrode Assembly. Joule, 2019. 3(11): p. 2777-2791.
• 14. Ozden, A., et al., High-Rate and Efficient Ethylene Electrosynthesis Using a Catalyst/Promoter/Transport Layer. ACS Energy Letters, 2020. 5(9): p. 2811-2818.
• 15. O'Brien, C. P., et al., Single Pass CO₂ Conversion Exceeding 85% in the Electrosynthesis of Multicarbon Products via Local CO₂ Regeneration. ACS Energy Letters, 2021. 6(8): p. 2952-2959.
• 16. Larrazábal, G. O., et al., Investigation of Ethylene and Propylene Production from CO₂ Reduction over Copper Nanocubes in an MEA-Type Electrolyzer. ACS Applied Materials & Interfaces, 2022.
• 17. Xu, Y., et al., A microchanneled solid electrolyte for carbon-efficient CO₂ electrolysis. Joule, 2022. 6(6): p. 1333-1343.
• 18. Wang, Y., et al., Catalyst synthesis under CO₂ electroreduction favours faceting and promotes renewable fuels electrosynthesis. Nature Catalysis, 2020. 3(2): p. 98-106.
• 19. Xu, Y., et al., Self-Cleaning CO₂ Reduction Systems: Unsteady Electrochemical Forcing Enables Stability. ACS Energy Letters, 2021. 6(2): p. 809-815.
• 20. García de Arquer, F. P., et al., CO₂ electrolysis to multicarbon products at activities greater than 1 A cm⁻². Science, 2020. 367(6478): p. 661-666.
• 21. Wu, H., et al., Improved electrochemical conversion of CO₂ to multicarbon products by using molecular doping. Nature Communications, 2021. 12(1): p. 7210.
• 22. Tran, N.-H., et al., Selective Ethylene Production from CO₂ and CO Reduction via Engineering Membrane Electrode Assembly with Porous Dendritic Copper Oxide. ACS Applied Materials & Interfaces, 2022. 14(28): p. 31933-31941.
• 23. Choi, W., et al., Microenvironments of Cu catalysts in zero-gap membrane electrode assembly for efficient CO2 electrolysis to C2+ products. Journal of Materials Chemistry A, 2022. 10(19): p. 10363-10372.

Example 3—Modulating Cation and Water Transports for CO Electrolysis, Such as Enhanced CO Electrolysis, Via Lonomer Coating

Electrification of the chemical industry has been considered an enabler for energy transition on a massive scale. In this context, carbon monoxide electroreduction (COR) to produce multi-carbon (C₂₊) products is considered one of the forefront of emerging technologies. A challenge in COR comes from cation crossover to the cathode via electromigration and water diffusion, which limits CO availability and impedes product selectivity. Commercial anion exchange membrane (AEM) suppress the electromigration of cations, however, tends to suffer from water diffusion which facilitates cation crossover. As described herein, these challenged due to cation crossover and water diffusion may be addressed by directly depositing an ultrathin nafion ionomer on a cathode (sputtered Cu) surface. The approach may enable full-cell energy efficiency of 21% for converting CO into ethylene (C₂H₄) at 100 mA/cm² with over 200 hours of stable operation. Also observed and exhibited were high energy efficiency for ethanol (C₂H₅OH) production with CO-to-C₂H₅OH electrolysis efficiency of 17%. The herein described approach to directly deposit ultrathin ionomer on a cathode to enhance system performance may benefit other electrochemical systems to overcome challenges associated with scalability, stability, and efficiency to produce high-value chemicals.

Results and Discussion

Performance of commercial AEM (Sustainion® X37-50 Grade RT Membrane) in MEA was first investigated. The cathode used was ˜300 nm copper catalyst sputtered on a PTFE gas diffusion layer (GDL) (FIG. 27a). Cell voltage and Faradaic efficiency (FE) of COR using 1 M KOH as anolyte at various current densities are depicted in FIG. 27b. Observed was a continuous drop of C₂H₄ selectivity when current density was increased from 25 to 200 mA/cm², which is in line with the literature.²⁴ While using AEM, the highest partial current density for C₂H₄ was found to be only 24 mA/cm².

Next investigated was the selectivity and cell voltage over time at 50 mA/cm² (FIG. 27c). A continuous increase of HER activity was observed, and consequently, a continuous decrease of COR selectivity during 2 hours of reaction. This was associated with the accumulation of cation (e.g., K⁺) which blocks the catalyst surface and thus limits the CO availability.¹⁶ Accumulation of K⁺ was confirmed using energy-dispersive X-ray (EDX) analysis (FIG. 35). A possible mechanism of K⁺ crossover is water transport that brings KOH_(aq) from the anolyte. To investigate this, COR experiments were carried out using different anolyte. It was observed that the COR selectivity and stability varied with the change in anolyte concentrations which indicated water transport within AEM (FIGS. 36 and 37). An ion transport experiment (Example 4—Section S1G) further confirmed this, elucidating the diffusion of water (with KOH_(aq)) to the cathode, as well as the electromigration of OH⁻ to the anode (FIG. 27d). This observation was consistent with literature, wherein the AEM facilitated OH⁻ transport to the anode as well as water diffusion to the cathode.^{8, 9, 23} Another possible reason for the depletion of COR selectivity was the promotion effect of cation on HER.²⁵ To investigate this, linear sweep voltammetry (LSV) was carried out using different anolytes with different K⁺ concentrations. Observed was lower COR and HER activities on higher K⁺ concentrations when using Cu catalysts (FIG. 38). This ruled out the possibility of the promotion effect of cation on HER activity. Instead, the negative impact of cations on COR selectivity could be attributed to catalyst poisoning due to cation accumulation.¹⁶

Looking at the adverse effect of K⁺ crossover via water diffusion on CO availability in AEM, it was considered that the degradation (gradual loss of selectivity) for COR in CEM would be faster than in AEM due to the electromigration of cation to the cathode. To investigate this a commercial CEM (Nafion™ 117 membrane) in MEA using 1 M KOH anolyte (FIG. 39a) was assembled. It was noticed that the highest partial current density for C₂H₄ was 11 mA/cm², which was lower than its counterpart using AEM (FIG. 39b and FIG. 27b). A stability experiment at 50 mA/cm² showed that the selectivity for C₂H₄ drops within 5 minutes of reaction, while the cell voltage increased by ˜350 mV due to cation crossover (FIG. 39c). K⁺ accumulation was observed on the used sample surface using EDX analysis (FIG. 40). To further confirm the cathode poisoning due to cation accumulation, similar experiments were performed using 1 M LiOH anolyte in which Li⁺ has a smaller size than K⁺. The degradation of C₂H₄ selectivity was lower when using 1 M LiOH anolyte as compared to its counterpart when using 1 M KOH, because with the same rate of diffusion, the small cation covers less cathode area (reduced cathode poisoning) as compared to the large cation (FIG. 41 and FIG. 39c). From the ion transport experiment (Example 4—section S1G), it was observed that the charge was majorly balanced by the electromigration of K⁺ to the cathode, while electromigration of OH⁻ was not observed (FIG. 39d). All these observations suggested that cation accumulation and subsequent loss of CO in the cathode were responsible for the lower C₂H₄ selectivity, which was consistent with literature.¹⁶

Limited by the availability of commercial membrane that is relatively thinner (<30 μm), it was sought to directly deposit hydrophobic PFSA ionomer (e.g., Nafion) on a cathode catalyst (Example 4—section S1C). This ultrathin solid polymer electrolyte (USPE) served as a separator that modulated ion transport as well as water transport between anode and cathode in a MEA configuration and substituted use of a standalone membrane. It was considered that a direct deposition method could protect a cathode catalyst from physical degradation (e.g., dissolution, detachment, poisoning) due to the direct contact between ionomer and catalyst.^{14, 26, 27} Furthermore, direct contact between the ionomer (e.g., PFSA) and catalyst may enhance CO availability on the catalyst surface.¹⁷ The thin layer of the membrane may also minimize the water transport from the anode.^{10, 23}

Based on this, 50 μL/cm² of Nafion was spray coated onto a cathode catalyst surface (denoted as USPE-50, FIG. 28a) and carried out COR experiments with 1 M KOH anolyte (Example 4—section S1B). Although lower COR selectivity was observed at lower current densities (e.g., 25 mA/cm² and 50 mA/cm²), an improved selectivity was observed at higher current densities (e.g., 75 mA/cm² and 100 mA/cm²) as compared to Nafion™ 117 CEM (FIG. 28b and FIG. 39b). This finding provided initial support that electromigration of K+ could be adjusted by changing the thickness of a SPE. To measure the K⁺ transport, an ion transport experiment was carried out using USPE-50 at 50 mA/cm² (Example 4—section S1G). It was found that the charge balance was governed by bidirectional electromigration of K⁺ and OH⁻ (FIG. 42a). While water transport (e.g., KOH_(aq) diffusion) to the cathode was not observed in a 1-hour ion transport experiment, it was considered that water transport occurred during the reaction. To investigate this, a diffusion experiment was performed for 18 hours with 1 M KOH anolyte under open circuit potential (OCP) in a MEA configuration (Example 4—section S1H). Without applying potential, it was observed that the anolyte transported to the cathode, which was considered primarily due to the concentration gradient of water between the anode and the cathode. It was found that the water flux in USPE-50 was half of the water flux compared to Nafion™-117 (Example 4), which supported that the directly deposited USPE suppressed water transport and consequently provided higher CO availability.

To further reduce the thickness of the USPE, 12 μL/cm² PFSA was spray coated onto a sputtered Cu (USPE-12), then performed COR with 1 M KOH anolyte. Enhanced C₂H₄ selectivity was observed at all the studied current densities using USPE-12 (FIG. 28c). When using USPE-12, the calculated highest partial current density towards C₂H₄ was 11 mA/cm² at 50 mA/cm², which was higher than the partial current density towards C₂H₄ using USPE-50 (4 mA/cm² at 50 mA/cm²). USPE-12 delivered a minimum K⁺ electromigration and the charge was mainly balanced by the electromigration of OH⁻ to the anode (FIG. 42b). It was observed that the cell voltage of USPE-12 was lower than the cell voltage of the Nafion™_117 membrane. This could be attributed to the lower resistance in USPE-12 as compared to the Nafion™_117 (FIG. 34). It was found that the thickness of USPE-12 was ˜0.7 μm as measured using cross-sectional SEM (FIG. 28d). Uniformity and consistency of the PFSA coating was investigated using SEM (FIG. 43). COR experiments were carried using USPE-12 at 50 mA/cm² for 2 hours (FIG. 28e). The stable cell voltage and C₂H₄ selectivity may be attributed to the direct coating of the ionomer layer which can protect the catalyst surface from degradation (i.e., surface poisoning by the cation accumulation).^{14, 26, 27}

From the previous observations using different USPE, it was noticed that C₂H₄ selectivity can be increased by suppressing the electromigration of K⁺ to the cathode (FIG. 28f). One strategy to further suppress the electromigration of K⁺ may be done by replacing the H⁺ with larger cations (e.g., Li⁺, Na⁺, Cs⁺) within PFSA to suppress ionic conductivity.^{28, 29} A recent study reported that K⁺ crossover has a detrimental impact on the COR selectivity due to limited CO availability.¹⁶ However, there are also studies which report that to some extent alkali metal cations enhance the activity of COR by stabilizing the dimer intermediates.^30-33 While the positive effect of hydrated cations (from catholyte) affects the selectivity in the flow cell and H-cell configurations, ^30-32 a similar effect in MEA might not be consistently present due to the thin and stagnant interfacial layer between the membrane and cathode surface.^{16, 34} Examples 1 and 2 herein described that implanted cations in a Nafion layer enhance CO₂R selectivity, despite the fact that bulk cations crossing over from an anode offered a detrimental effect on CO₂R selectivity.²³ This suggested that the role of mobilized cation from the anode is different than the role of an immobilized implanted cation. Therefore, it was considered that the presence of cation in the USPE could also enhance CO availability due to lower electromigration of K⁺ to the cathode. This was explored by adding 0.15 M KOH to the PFSA solution and then depositing the mixed KOH and PFSA solution with a loading of 12 μL/cm² to the sputtered copper by spray coating (Example 4—Section S1C). This was denoted as CISPE-12-0.15MKOH. 0.15 M KOH solution was selected to be added to the PFSA solution because H⁺ of the SO₃⁻ group can be stoichiometrically and fully exchanged with K⁺ at ˜0.13 M KOH. The transformation of H⁺-form into K⁺-form within PFSA is confirmed using Fourier-transformed infrared spectroscopy (FTIR) (FIG. 44). The SEM analysis showed the thickness of CISPE-12-0.15MKOH to be ˜0.7 μm (FIG. 29a). The presence of implanted K⁺ was also confirmed by the EDX analysis (FIG. 45).

K⁺ infusion within the PFSA (CISPE-12-0.15MKOH) was observed to suppress the activity of HER as indicated by the lower Faradaic efficiency toward H₂ as compared to USPE-12 (FIG. 46a and FIG. 28c). The highest partial current density toward C₂H₄ on CISPE-12-0.15MKOH was 17 mA/cm² (at a current density of 75 mA/cm²) which was higher than 11 mA/cm² (at a current density of 50 mA/cm²) on USPE-12. This result supported that the presence of implanted K⁺ in the PFSA (as opposed to bulk cation which tends to excessively accumulate at the catalyst surface) has a beneficial effect on overall COR activity. The ion transport experiment also indicated that the K⁺ infusion into the Nafion layer suppressed electromigration of K⁺ to the cathode (FIG. 42c).

COR experiments were then carried out using a larger cation by introducing 0.15 M CsOH to the PFSA solution and spray-coating the mixed solution to the sputtered copper (denoted as CISPE-12-0.15MCsOH). A larger cation on the catalyst surface has been reported to stabilize the dimer intermediate as well as minimize water at the surface, which can be beneficial for enhanced C₂₊ formation and COR selectivity.^{23, 33, 35} The highest partial current density towards C₂H₄ reached 40 mA/cm² when using CISPE-12-0.15MCsOH at 100 mA/cm², which was two folds higher compared to CISPE-12-0.15MKOH (FIGS. 46a and 46b). From the ion transport experiment, it was observed that in both cases (CISPE-12-0.15M KOH and CISPE-12-0.15McsOH), the charge was majorly balanced by the electromigration of OH⁻ to the anode, while electromigration of K⁺ to the cathode was not observed (FIGS. 42c and 42d).

This finding indicated that the enhanced COR activity and C₂H₄ selectivity could be attributed to the larger cations in the PFSA structure which stabilizes dimer intermediates.^{23, 33, 35} FIG. 29b summarizes the anolyte flux to the cathode under open circuit potential when using Nafion™-117, USPE-50, USPE-12, and CISPE-12-0.15MCsOH from the anolyte diffusion experiments. The low anolyte flux on both CISPE-12-0.15MKOH and CISPE-12-0.15MCsOH as compared to the Nafion™-117 also provides further evidence that the directly deposited SPE with implanted cation plays a role in COR selectivity by suppressing water transport to the cathode, thus minimizing the GDL pore blockage.¹⁰

It was sought to optimize COR selectivity using CISPE-12-0.15MCsOH by changing the anolyte concentrations and measuring the COR selectivity (FIGS. 29c, 47a, 47b, and 47c). Lower HER selectivity was found on lower anolyte concentration, which may be attributed to the lower K⁺ crossover due to water diffusion. Although 0.2 M KOH anolyte exhibits lower HER activity, higher C₂H₄ selectivity was observed when using 0.5 M KOH anolyte, with the highest partial current density towards C₂H₄ of 47 mA/cm² at 150 mA/cm², which can be attributed to the local pH at the cathode.^{12, 36} While high C₂₊ selectivity was achieved using 0.5 M or 0.2 M KOH anolytes, observed was a higher overpotential for OER as compared to 1 M or 3 M KOH anolytes. To suppress the anodic overpotential, nickel iron-layered double hydroxide (NiFe LDH) was synthesized as the anode catalyst via a chloride corrosion treatment as opposed to pristine nickel foam.³⁷ A a lower cell voltage was noticed (around 0.4 V) when using NiFe LDH as compared to pristine nickel foam, while the COR selectivity in both cases was identical (FIGS. 29d, 47b, 48 and 49). This enables high CO conversion (up to 90% CO single pass conversion) and more energetically efficient COR-to-C₂H₄(up to 21% full-cell energy efficiency) using 0.5 M KOH anolyte (FIGS. 50 and 51). Also observed was a lower cell voltage when using NiFe LDH with 0.2 M KOH anolyte, with liquid products like C₂H₅OH as a major product at 100 mA/cm² (FIGS. 47c, 52, and 53). To support the highly active nature of NiFe LDH, Tafel, Electrochemical Impedance Spectroscopy (EIS), and stability studies (FIG. 33) were conducted to showcase its improvement towards OER as compared to the bare Ni foam as an anode (Example 4—section S2A).^{38, 39}

A long-term COR experiment was carried out to understand the stability of CISPE-12-0.15MCsOH in a MEA configuration. The ultrathin PFSA layer suppressed K⁺ crossover to the cathode due to electromigration as well as water diffusion. The presence of implanted Cs⁺ further enhanced C₂H₄ selectivity at higher current densities, resulting in higher partial current densities toward C₂H₄. Over 200 hours of stable operation was demonstrated using CISPE-12-0.15MCsOH at the cathode and NiFe LDH at the anode; using 0.5 M KOH anolyte with the CO-to-C₂H₄ electrolyzer efficiency (EE) of 21% at 100 mA/cm² (FIGS. 30a, 54a, 54b, 54c and 54d). The image of the post-reaction catalyst after the stability experiment provides evidence of the stable nature of the Nafion layer (FIGS. 55 and 56). With the same configuration using 0.2 M KOH anolyte, a CO-to-C₂H₅OH electrolysis efficiency of 17% was exhibited (FIG. 30) using an ultrathin PFSA layer (0.7 μm) for enhanced CO-to-C₂H₅OH electrolysis (Table S6).

Considering the motivation of CO electrolysis as a part of the carbon utilization strategy, an energy calculation for two-step CO₂-to-C₂H₄(CO₂ capture, CO₂-to-CO conversion, CO-to-C₂H₄ conversion and separation, see Example 4, Section I) and found that the herein described USPE/CISPE in MEA configuration enables lower energy consumption (e.g., 218 GJ/ton-C₂H₄) in alkaline anolyte (Table S6). In regard to CO electrolysis, the herein described USPE/CISPE in MEA configurations may provide: 1) use of ultrathin PFSA layer (as opposed to the thick standalone membrane) that can suppress K⁺ crossover to the cathode by controlling electromigration and water transport, 2) direct contact between the PFSA layer and copper surface that may protect the catalyst surface from physical degradation,⁴⁵ 3) implanted Cs⁺ in the PFSA layer that may further suppress water flux and HER activity; 4) use of highly active NiFe LDH anode by replacing pristine Ni foam for improved energy efficiency. From the technoeconomic analysis, it was highlighted that with further development, COR may be a promising intermediate step for producing low-cost C₂H₄, particularly when the sustainable electricity price reach $0.02 per kWh (FIG. 57).

Non-Binding Conclusion

Herein described and/or developed is an ultrathin solid polymer electrolyte (USPE) to augment the energy efficiency of CO electrolysis in MEA. Investigations began with a standalone AEM and CEM using alkaline anolyte and it was observed that the challenge on both standalone membranes comes from K⁺ crossover to the cathode which results in the cathode flooding. Herein, K⁺ crossover was suppressed by using a cation infused USPE (CISPE) which was directly deposited to copper surface of a catalyst layer, resulting in stable and highly efficient CO electrolysis. The crossover of K⁺ was suppressed by reducing the thickness of the ionomer layer, which also enabled controlling of water transport. Further utilized was NiFe LDH to suppress anodic overpotential. Then demonstrated was a higher electrolysis energy efficiency of 21% and 17% in C₂H₄ and C₂H₅OH production, respectively, from CO using alkaline anolyte at 100 mA/cm² with over 200 hours of stable operation. From the CO₂-to-C₂H₄ perspective, this result can be translated into a lower energy cost of about 218 GJ/ton-C₂H₄. The direct ionomer coating on the cathode catalyst offered a facile and scalable approach to eliminate the use of a standalone membrane to enhance the selectivity, stability, and efficiency of electrochemical systems.

Example 3, and Detailed Description—Carbon Monoxide Electroreduction (COR) References

• [1] J. A. Rabinowitz, M. W. Kanan Nature Communications. 2020, 11, 5231.
• [2] R. Küngas Journal of The Electrochemical Society. 2020, 167, 044508.
• [3] R. Küngas, P. Blennow, T. Heiredal-Clausen, T. Holt Nørby, J. Rass-Hansen, J. B. Hansen, P. G. Moses ECS Transactions. 2019, 91, 215.
• [4] H. C. Van Ness, M. Abbott, M. Swihart, J. M. Smith, Loose Leaf for Introduction to Chemical Engineering Thermodynamics, McGraw-Hill Education, 2021.
• [5] M. Jouny, W. Luc, F. Jiao Nature Catalysis. 2018, 1, 748-755.
• [6] L.-C. Weng, A. T. Bell, A. Z. Weber Energy & Environmental Science. 2019, 12,1950-1968.
• [7] N. Vassal, E. Salmon, J. F. Fauvarque Electrochimica Acta. 2000, 45, 1527-1532.
• [8] S. Garg, C. A. Giron Rodriguez, T. E. Rufford, J. R. Varcoe, B. Seger Energy & Environmental Science. 2022, 15, 4440-4469.
• [9] D. G. Wheeler, B. A. W. Mowbray, A. Reyes, F. Habibzadeh, J. He, C. P. Berlinguette Energy & Environmental Science. 2020, 13, 5126-5134.
• [10] A. Reyes, R. P. Jansonius, B. A. W. Mowbray, Y. Cao, D. G. Wheeler, J. Chau, D. J. Dvorak, C. P. Berlinguette ACS Energy Letters. 2020, 5, 1612-1618.
• [11] G. Iijima, T. Inomata, H. Yamaguchi, M. Ito, H. Masuda ACS Catalysis. 2019, 9, 6305-6319.
• [12] H. Xiao, T. Cheng, W. A. Goddard, III, R. Sundararaman Journal of the American Chemical Society. 2016, 138, 483-486.
• [13] C. J. Bondue, M. Graf, A. Goyal, M. T. M. Koper Journal of the American Chemical Society. 2021, 143, 279-285.
• [14] D. S. Ripatti, T. R. Veltman, M. W. Kanan Joule. 2019, 3, 240-256.
• [15] Á. Vass, A. Kormányos, Z. Kószó, B. Endrödi, C. Janáky ACS Catalysis. 2022, 12, 1037-1051.
• [16] A. Ozden, J. Li, S. Kandambeth, X.-Y. Li, S. Liu, O. Shekhah, P. Ou, Y. Zou Finfrock, Y.-K. Wang, T. Alkayyali, F. Pelayo Garcia de Arquer, V. S. Kale, P. M. Bhatt, A. H. Ip, M. Eddaoudi, E. H. Sargent, D. Sinton Nature Energy. 2023.
• [17] F. P. Garcia de Arquer, C.-T. Dinh, A. Ozden, J. Wicks, C. McCallum, A. R. Kirmani, D.-H. Nam, C. Gabardo, A. Seifitokaldani, X. Wang, Y. C. Li, F. Li, J. Edwards, L. J. Richter, S. J. Thorpe, D. Sinton, E. H. Sargent Science. 2020, 367, 661-666.
• [18] A. Kusoglu, A. Z. Weber Chemical Reviews. 2017, 117, 987-1104.
• [19] S. Maurya, S.-H. Shin, Y. Kim, S.-H. Moon RSC Advances. 2015, 5, 37206-37230.
• [20] X. Kong, K. Wadhwa, J. G. Verkade, K. Schmidt-Rohr Macromolecules. 2009, 42, 1659-1664.
• [21] H. L. S. Salerno, F. L. Beyer, Y. A. Elabd Journal of Polymer Science Part B: Polymer Physics. 2012, 50, 552-562.
• [22] M.-s. J. Jung, C. G. Arges, V. Ramani Journal of Materials Chemistry. 2011, 21, 6158-6160.
• [23] M. A. Adnan, A. Shayesteh Zeraati, S. K. Nabil, T. A. AI-Attas, K. Kannimuthu, C.-T. Dinh, I. D. Gates, M. G. Kibria Advanced Energy Materials. 2023, 13, 2203158.
• [24] N.-H. Tran, H. P. Duong, G. Rousse, S. Zanna, M. W. Schreiber, M. Fontecave ACS Applied Materials & Interfaces. 2022, 14, 31933-31941.
• [25] M. C. O. Monteiro, A. Goyal, P. Moerland, M. T. M. Koper ACS Catalysis. 2021, 11, 14328-14335.
• [26] M. Klingele, M. Breitwieser, R. Zengerle, S. Thiele Journal of Materials Chemistry A. 2015, 3,11239-11245.
• [27] M. Klingele, B. Britton, M. Breitwieser, S. Vierrath, R. Zengerle, S. Holdcroft, S. Thiele Electrochemistry Communications. 2016, 70, 65-68.
• [28] K. Hongsirikarn, J. G. Goodwin, S. Greenway, S. Creager Journal of Power Sources. 2010, 195, 7213-7220.
• [29] V. I. Volkov, A. V. Chernyak, O. I. Gnezdilov, V. D. Skirda Solid State Ionics. 2021, 364, 115627.
• [30] E. Pérez-Gallent, G. Marcandalli, M. C. Figueiredo, F. Calle-Vallejo, M. T. M. Koper Journal of the American Chemical Society. 2017, 139, 16412-16419.
• [31] M. Akira, H. Yoshio Bulletin of the Chemical Society of Japan. 1991, 64, 123-127.
• [32] J. Li, D. Wu, A. S. Malkani, X. Chang, M.-J. Cheng, B. Xu, Q. Lu Angewandte Chemie International Edition. 2020, 59, 4464-4469.
• [33] J. Resasco, L. D. Chen, E. Clark, C. Tsai, C. Hahn, T. F. Jaramillo, K. Chan, A. T. Bell Journal of the American Chemical Society. 2017, 139, 11277-11287.
• [34] W.-C. Liao, D.-H. Tsai, W.-Z. Hong, Y.-H. Huang, L.-C. Lin, Y.-T. Pan Chemical Engineering Journal. 2022, 434, 134765.
• [35] S. Ringe, E. L. Clark, J. Resasco, A. Walton, B. Seger, A. T. Bell, K. Chan Energy & Environmental Science. 2019, 12, 3001-3014.
• [36] H.-J. Peng, M. T. Tang, J. Halldin Stenlid, X. Liu, F. Abild-Pedersen Nature Communications. 2022, 13, 1399.
• [37] D. Tyndall, S. Jaskaniec, B. Shortall, A. Roy, L. Gannon, K. O'Neill, M. P. Browne, J. Coelho, C. McGuinness, G. S. Duesberg, V. Nicolosi npj2D Materials and Applications. 2021, 5, 73.
• [38] X. Li, C. Liu, Z. Fang, L. Xu, C. Lu, W. Hou Small. 2022, 18, 2104354.
• [39] S. Anantharaj, K. Karthick, S. Kundu Materials Today Energy. 2017, 6, 1-26.
• [40] X. Wang, P. Ou, A. Ozden, S.-F. Hung, J. Tam, C. M. Gabardo, J. Y. Howe, J. Sisler, K. Bertens, F. P. Garcia de Arquer, R. K. Miao, C. P. O'Brien, Z. Wang, J. Abed, A. S. Rasouli, M. Sun, A. H. Ip, D. Sinton, E. H. Sargent Nature Energy. 2022, 7, 170-176.
• [41] A. Ozden, Y. Wang, F. Li, M. Luo, J. Sisler, A. Thevenon, A. Rosas-Hernández, T. Burdyny, Y. Lum, H. Yadegari, T. Agapie, J. C. Peters, E. H. Sargent, D. Sinton Joule. 2021, 5, 706-719.
• [42] S. Overa, B. S. Crandall, B. Shrimant, D. Tian, B. H. Ko, H. Shin, C. Bae, F. Jiao Nature Catalysis. 2022, 5, 738-745.
• [43] J. Li, H. Xiong, X. Liu, D. Wu, D. Su, B. Xu, Q. Lu Nature Communications. 2023, 14, 698.
• [44] C. Zhao, G. Luo, X. Liu, W. Zhang, Z. Li, Q. Xu, Q. Zhang, H. Wang, D. Li, F. Zhou, Y. Qu, X. Han, Z. Zhu, G. Wu, J. Wang, J. Zhu, T. Yao, Y. Li, H. J. M. Bouwmeester, Y. Wu Advanced Materials. 2020, 32, 2002382.
• [45] D. V. Esposito ACS Catalysis. 2018, 8, 457-465.

Example 4—Modulating Ion and Water Transports for Enhanced CO Electrolysis Via Lonomer Coating—Supplementary Information

S1. Experimental Information

A. Materials

Potassium hydroxide (KOH), cesium hydroxide (CsOH), and methanol were received from Sigma Aldrich (ACS reagent). Nafion™ perfluorinated resin solution (5 wt. % in a mixture of lower aliphatic alcohol and water) was purchased from Sigma Aldrich. Sustainion® X37-50 Grade RT and Nafion™ 117 were procured from Fuel Cell store.

B. Experimental Setup

A membrane electrode assembly (MEA), consisting of anode side (grade 2 titanium) and cathode side (904L stainless steel) was fabricated by Dioxide materials with the serpentine channel area of 5 cm². Copper tape was used to place the cathode on the cathode side. A wider Kapton tape was put on top of copper tape on each side to avoid the short circuit, resulting in an active area of the cathode of ˜4 cm². The MEA experiments using standalone membrane were prepared by sandwiching the cathode (sputtered Cu), stand alone membrane (AEM or CEM) and the anode (with Ni foam). For MEA experiments with the ultra-thin solid polymer electrolyte (USPE) or the cation-infused solid polymer electrolyte (CISPE) without using a standalone membrane, the cell was prepared by sandwiching the cathode (with USPE or CISPE) and the anode (with Ni foam). A humidified CO (flowrate of ˜50 standard mL min⁻¹ unless mentioned otherwise) and 1 M KOH (50 mL min⁻¹ unless mentioned otherwise) were fed to the cathode and anode sides, respectively using a flow meter (Cole-parmer 39067) and peristaltic pump (Fisherbrand™ Variable-Flow Peristaltic Pumps), respectively. BioLogic potentiostats with 10 A booster was used to obtain the electrochemical response without iR correction. The gas products were analyzed using Perkin Elmer Gas Chromatography (Clarus 590) with flame ionization detector (FID) and thermal conductivity detector (TCD). The liquid products were identified using Bruker AVANCE III 600 MHz nuclear magnetic resonance spectroscopy equipped with pulsed-field gradient probes. The linear sweep voltammetry (LSV) was carried out in a flow cell with Ag/AgCl as the reference electrode (E(RHE)=E (Ag/AgCl)+E^o (Ag/AgCl)+0.059 pH). The LSV for HER or COR was measured by flowing air or CO in the cathode compartment, respectively.

C. Cathode Preparation

The cathode catalyst was made by sputtering 300 nm of copper on polytetrafluoroethylene (PTFE) using Angstrom sputtering system. The ultra-thin solid polymer electrolyte (USPE) was made by spray-coating (Paasche airbrush) the desired quantity of nafion spray solution on top of the cathode catalyst and dried overnight under atmospheric conditions. The Nafion spray solution (Solution A) was made of Nafion™ perfluorinated resin solution (Sigma Aldrich) and methanol (Sigma Aldrich) with a ratio of 1:5 by volume. For example, 288 μL of Solution A was used when fabricating ˜4 cm² of USPE-12 (12 μL/cm² of Nafion™ perfluorinated resin solution).

The cation-infused solid polymer electrolyte (CISPE) was synthesized by spray-coating of the desired quantity of cation-nafion spray solution and dried overnight. A cation-nafion spray solution was prepared by adding the desired concentration of cation solution into Nafion spray solution with a ratio of 1:9 by volume. Cation solution could be KOH or CsOH. For example, to synthesize ˜4 cm² of CISPE-12-0.15MKOH (containing 50 μL/cm² of Nafion™ perfluorinated resin solution with 0.15 M KOH), initially 9 mL of Solution A and 1 mL of 0.15 M KOH solution was taken to make Solution B. Then, 320 μL of Solution B was spray coated to the cathode catalyst and dried overnight.

D. Anode Preparation

Bare Ni foam and NiFe layered double hydroxide (LDH) was used as anodes. The Ni-foam sheet (0.08 mm, MTI Corporation) was always used as catalyst, except stated otherwise. The bare Ni foam was used without any treatment¹. In the case of NiFe LDH, there are many approaches to preparing it, such as wet-chemical, electrodeposition, hydrothermal and solvothermal, and herein was followed a simple solution immersion route². In brief, Ni-foam was taken with 2×2 cm² dimensions and soaked into the 10 ml solution of 0.25 M FeCl₃ for 4 min. After this, it was washed with DI water multiple times and dried at room temperature conditions. This concentration was selected because at high concentrations such as 0.5 M, due to the continuous Ni²⁺ leaching, Ni foam itself could become degraded. Initially, the FeCl₃ solution was brown and after the reaction with Ni foam, it displayed greenish color indicating the subsequent leaching of Ni²⁺ ions from Ni foam into the solution. The excess concentration of metal halides is used in industries for the corrosion of metals. Here, the used Cl⁻ ions, due to their high nucleophilicity, easily get into contact with the Ni foam and react with the Ni surface along with Fe³⁺ species. The synergistic presence of Ni²⁺ and Fe³⁺ along with OH⁻ and CO₃²⁻ generated in-situ, formed NiFe LDH structures over the Ni foam. The possible formation of NiFe LDH over Ni foam can be explained as follows,

Initially, there are Fe³⁺ and Cl⁻ ions in the solution, and with the higher standard reduction potentials of 0.77 V and 1.36 V respectively, they effortlessly oxidize the Ni surface (−0.25 V).

Other than these reactions, hydrolysis of Fe³⁺, O₂, and CO₂ consumption reactions appear near the surface to generate the OH⁻ and CO₃²⁻ ions to form a NiFe LDH over the Ni foam.

The produced protons couple with the electrons and form H₂ molecules.

From these observations, it appeared that the generated Ni²⁺ ions were inclined to react with Fe³⁺ along with OH⁻ ions to construct the NiFe LDH structures. After this, the dried electrodes are ready for the oxygen evolution reaction (OER) studies.

E. Characterization

The morphology and structure of USPE and CISPE were characterized by scanning electron microscopy (SEM). SEM observation was performed using an FEI Quanta 250 FEG field emission scanning electron microscope which was equipped with energy-dispersive X-ray spectroscopy (EDS) analysis.

Fourier Transform Infrared (FTIR) spectra were recorded by Perkin Elmer Frontier FT-IR spectrometer in the range of 4000 to 400 cm⁻¹ to study the chemical structure of ionomer before and after adding CsOH solution or KOH solution.

F. Mass of Nafion in the Solution

Properties of Nafion™ perfluorinated resin (5 wt. % in solution)

Chemical ⁢ formula = ( C 7 ⁢ HF 13 ⁢ O 5 ⁢ S · C 2 ⁢ F 4 ) x Molecular ⁢ weight ⁢ ( MW naf ) = 554 ⁢ gram / mol Density ⁢ ( ρ naf ) = 0.93 g / mL

The density of the diluted Nafion solution (ρ_sol-A) can be calculated as follow:

ρ sol - A = V naf × ρ naf × x naf V naf + V meoh = 1 ⁢ mL × 0.93 g sol mL × 5 100 ⁢ g naf g sol 1 ⁢ mL + 5 ⁢ mL = 0.0465 g naf 6 ⁢ mL = 0.00775 g naf / mL

V_naf, V_meoh and X_naf are the volume of Nafion™ perfluorinated resin, volume of methanol and mass fraction of Nafion in the Nafion™ perfluorinated resin, respectively.

The mass of Nafion in the cathode with Nafion loading of 50 μL/cm² can be estimated as follow:

m naf = 0.05 mL cm 2 × 0.93 g sol mL × 5 100 ⁢ g naf g sol = 2.325 mg / cm 2

The modified Nafion was synthesized by adding a certain concentration of cation hydroxide solution in the Nafion solution with a volumetric ratio of Nafion solution:cation hydroxide solution of 9:1. Firstly, mass of Nafion in 9 mL of Nafion solution was calculated.

m naf = 9 ⁢ mL × 0.00775 g naf / mL = 0.0698 g

The mass fraction of sulfonic acid group (x_sa) in the Nafion can be estimated as follows (assume 1 mol Nafion as a basis):

x sa = 81 ⁢ g 554 ⁢ g = 0.15

The mole of sulfonic acid was calculated as followed:

n sa = 0.0698 g × 0.15 81 ⁢ g / mol = 0.000129 mol

The 0.129 M of 1 mL cation hydroxide solution was required to stoichiometrically neutralize 1.29×10⁻⁴ mol of sulfonic acid.

G. Ion Transport Experiment

Mass flow was measured for a constant charge transfer by operating at constant current density (50 mA/cm²) for ˜1 hour in 1 M KOH anolyte. A 50 mA/cm² current density was selected because at this current density there was observed considerable COR activity in all the studied samples. The experiments were carried out for ˜1 hour to ensure the change of the anolyte was observable. Concentration of OH⁻ and CO₃²⁻ was measured in the anolyte before and after the reaction using the total alkalinity method to estimate the molar mass of ion transport via migration and diffusion mechanism.

The concentrations of hydroxide and carbonate were measured by a practical method from industry^{3, 4}. For instance, 1 mL of the sample before and after reaction each was taken and poured into a transparent beaker. Then, one drop of phenolphthalein (ACS reagent, Sigma Aldrich) was added to the beaker. 0.1 M HCl (ACS reagent, Sigma Aldrich) was gradually added to the beaker using 20 μL-scale pipets. The total volume of HCl was measured until the color of phenolphthalein indicator changed from pink to transparent (it is called as phenolphthalein alkalinity (PA)). The PA represents the titration of OH⁻ and 1/2 of CO₃²⁻ present in the electrolyte. Then, one drop of methyl orange (ACS reagent, Sigma Aldrich) was added to the beaker. The HCl was again added, and its total volume was measured until the color of methyl orange indicator changed from yellow to light orange (it is called as total alkalinity (TA)). This titration step represents the neutralization of the other half of CO₃²⁻ present in the electrolyte (FIG. 31). The results of titration were calculated using Table S4. Control samples were analyzed and verified the accuracy of the method.

TABLE S4
Hydroxide, carbonate, and bicarbonate alkalinities as a function
of total alkalinity and phenolphthalein alkalinity.

	Result of titration	Hydroxide	Carbonate	Bicarbonate
	PA > 0.5TA	2PA − TA	2(TA − PA)	0
	PA < 0.5TA	0	2PA	TA − 2PA
	PA = TA	T	0	0
	PA = 0.5TA	0	2P	0
	PA = 0	0	0	TA

From the titration data, the following information was estimated: charge transferred from anode to the cathode, ion migration and ion diffusion. To interpret the titration data (material balance), four fundamental rules were assumed: (1) cation exchange membrane facilitates cation migration, (2) anion exchange membrane facilitates anion migration, (3) ion movement disobey rule (#1) and (#2) was assumed as diffusion of hydrated cation and its corresponding anion, (4) the ion leftover after applying rule (#1) to (#3) was assumed as a change of the membrane functionalities (e.g., cation exchange membrane allows anion to move). The ion transport in cation exchange membrane (CEM) included migration of K+(charge difference) and diffusion of KOH_(aq) (concentration difference), as shown in FIG. 32a. The later transport mechanism was a result of water uptake properties of Nafion™ 117. In AEM, the ion transport consisted of migration of OH⁻ and diffusion of KOH_(aq) (concentration difference), as shown in FIG. 32b.

Here, AEM case was selected to give an example of ion balance calculation. The charge transport in the experiment using the Sustainion® X37-50 Grade RT membrane in 1 M KOH anolyte for ˜1.6 hour operation:

q = - 1093 ⁢ sA F = - 96 ⁢ 485 ⁢ sA / mol n e = - 1093 ⁢ sA 96 ⁢ 485 ⁢ sA mol × 1000 ⁢ mmol 1 ⁢ mol = - 11.3 ⁢ mmol

Where q, F and n_e are the total charge (acquired from the potentiostat), Faraday constant and number of moles of charge, respectively. The minus sign indicated the negative charge flowed from the anode to the cathode.

In the anolyte:

c OH - , in = 0.951 M v , in = 100 ⁢ mL n OH - , in = 0.951 mol L ⁢ 1 ⁢ L 1000 ⁢ mL ⁢ 100 ⁢ mL × 1000 ⁢ mmol 1 ⁢ mol = 95.1 mmol c CO 3 2 - , in = 0.01 M n CO 3 2 - , in = 0.01 mol L ⁢ 1 ⁢ L 1000 ⁢ mL ⁢ 100 ⁢ mL × 1000 ⁢ mmol 1 ⁢ mol = 1. mmol

Where C_OH⁻_{, in}, V_{, in}, and

n OH - , in ; c CO 3 2 - , in , and ⁢ n CO 3 2 - , in

were the initial concentration, initial volume, and initial number of moles of OH⁻; initial concentration and initial number of moles of carbonate, respectively. The presence of carbonate in the initial anolyte was considered to be due to the impurities of KOH or the reaction between KOH, which is hygroscopic, and the CO₂ in the air.

After reaction:

c OH - , fn = 0.932 M v , fn = 100 ⁢ mL n OH - , fn = 0.932 mol L ⁢ 1 ⁢ L 1000 ⁢ mL ⁢ 100 ⁢ mL × 1000 ⁢ mmol 1 ⁢ mol = 93.2 mmol c CO 3 2 - , fn = 0.01 M n CO 3 2 - , fn = 0.01 mol L ⁢ 1 ⁢ L 1000 ⁢ mL ⁢ 100 ⁢ mL × 1000 ⁢ mmol 1 ⁢ mol = 1. mmol

Where C_OH⁻_{, fn}, V_{, fn}, and

n OH - , fn ; c CO 3 2 - , fn , and ⁢ n CO 3 2 - , fn

are the final concentration, final volume, and final number of moles of OH⁻; final concentration and final number of moles of carbonate, respectively.

Under the alkaline condition, the oxygen evolution reaction (OER) obeys the following reaction:

The generation of one mole of electron consumes one mole of OH⁻:

n OH - , cons = n e = - 11.3 ⁢ mmol

The measured mole of OH⁻ loss was calculated as follows:

n OH - , ms = n OH - , fn - n OH - , in = 93.2 mmol - 95.1 mmol = - 1.9 ⁢ mmol

Given that AEM facilitated the anion transport, it was expect that the charge was balanced by the electromigration of OH⁻ to the anode. This means, when the OER consumed 11.3 mmol of OH⁻, then the electromigration of OH⁻ was also 11.3 mmol. However, from the above calculation, it was observed that 1.9 mmol of OH⁻ was lost A possible reason of such OH⁻ lost was considered to be that OH⁻ diffused to the cathode. When OH⁻ diffuses to the cathode, there is also a possibility for carbonate to diffuse to the cathode. However, a change of carbonate concentration after the reaction was not observed. This may be attributed to the low concentration of carbonate relative to the concentration of OH⁻. The concentration of carbonate was 95 times lower than the concentration of OH⁻. The possible diffusion of carbonate was beyond the precision limit of the current measurement method.

Next calculated was the material balance of K⁺, begining with the amount of K⁺ before reaction (n_{K+, in}):

n K + , in = n OH - , in + ( 2 × n CO 3 2 - , in ) = 95.1 mmol + 2. mmol = 97.1 mmol

• • Then, estimate was the amount of K⁺ after reaction:

n K + , fn = n OH - , fn + ( 2 × n CO 3 2 - , fn ) = 93.2 mmol + 2. mmol = 95.2 mmol

The mole of K⁺ difference was calculated as follow:

n K + , diff = n K + , fn - n K + , in = 95.2 mmol - 97.1 mmol = - 1.9 ⁢ mmol

From the above calculation, the mole of K⁺ decreased by 1.9 mmol after the reaction. Recalling the nature of AEM which facilitates anion transport, thus, a reason for the K⁺ loss was considered due to the K⁺ diffusion to the cathode.

The experiments were conducted for 1.6 hours. The value as calculated above was normalized for 1.6 hours experiment to obtain the value in hourly basis. For instance, the calculated electromigration of OH⁻ for 1.6 hours was 11.3 mmol. In hourly basis, the electromigration of OH⁻ was 7.6 mmol per hour. Similarly, the diffusion of KOH_(aq) for 1.6 hours was 1.9 mmol. In hourly basis. The diffusion of KOH_(aq) was 1.3 mmol per hour (1.9 mmol per 1.6 hours).

H. Measurement of Anolyte Diffusion

Mass transport experiments (diffusion experiment) were carried out using a similar setup with CO electrolysis using CISPE in MEA without applied potential. For instance, the transport flux of the transport flux of the CISPE system was carried out with CISPE in the cathode and Ni foam in the anode. No standalone membrane was used in CISPE system. The humidified CO at 50 standard mL min⁻¹ was directed to the cathode to mimic the cathode condition in CO electrolysis. 1 M KOH solution (anolyte) at 50 standard mL min⁻¹ was circulated in the anode. Before experiment, the initial weight of anolyte was measured. The experiments were performed for couple of hours to ensure a mass change of the anolyte. Then, measured again was the final weight of the anolyte. Mass transport calculation of USPE-50 was selected as an example.

Initial ⁢ anolyte ⁢ mass ⁢ ( m in ) = 15.6588 g Final ⁢ anolyte ⁢ mass ⁢ ( m fn ) = 15.1225 9 Mass ⁢ difference ⁢ ( m diff ) = m in - m fn = 15.6588 mL - 15.1225 mL = 0.5363 mL Duration ⁢ ( t ) = 66 , 600 ⁢ s Area ⁢ ( A ) = 4.2 cm 2 Flux ⁢ ( N ) = m diff A × t = 0.5363 g 4.2 cm 2 × 66 , 600 ⁢ s = 1.92 × 10 - 6 ⁢ g cm 2 ⁢ s

I. Performance Calculation

CO electrolysis (COR) experiment was carried out at a current density (i) of 100 mA/cm² using with a flowrate humidified CO inlet (v) of 6 standard mL min⁻¹.

v = 6 ⁢ cm 3 / min R = 82.06 atm ⁢ cm 3 mol ⁢ K T = 298.15 K ⁡ ( 25 ⁢ ° ⁢ C . ) P = 1 ⁢ atm n CO = V ⁢ P RT = 6 ⁢ cm 3 min ⁢ 1 ⁢ atm 82.06 atm ⁢ cm 3 mol ⁢ K 298.15 K ⁢ 60 ⁢ min 1 ⁢ hour = 1.47 × 10 - 2 ⁢ mol hour

Where R, T, P and n represent gas constant, temperature, pressure and mole flow, respectively.

i = 100 ⁢ mA / cm 2 A = 4 ⁢ cm 2 j = i × A = 100 ⁢ mA cm 2 × 4 ⁢ cm 2 × 1 ⁢ A 1000 ⁢ mA = 0.4 A

Where i, A and j are current density, electrode area and total current, respectively. Then, calculated was the energy supplied to the electrolyzer (E_el) for 1 hour basis.

E el = j × V × 1 ⁢ hour = 0.4 A × 2.8 V × 1 ⁢ hour 1 ⁢ hour = 1.14 Whour hour

Then converted was the unit from

W ⁢ hour hour

into

GJ hour

as follows:

E el = 1.14 W ⁢ hour hour × 3 ⁢ 6 × 1 ⁢ 0 - 6 ⁢ GJ 1 ⁢ Whour = 4.1 × 1 ⁢ 0 - 6 ⁢ G ⁢ J h ⁢ o ⁢ u ⁢ r

From the Faradaic efficiency (FE), one can calculate the flowrate of products (n_prod)

n prod = F ⁢ E × j n e × F

F is the Faraday constant 96485 sA/mol. n_e is the number of electrons involved in the reaction. For example, the flowrate of C₂H₄ production at 100 mA/cm² (FE_C2H4=48%) can be calculated as follow:

n C ⁢ 2 ⁢ H ⁢ 4 = 0.48 × 0.4 A 8 × 9 ⁢ 6 ⁢ 4 ⁢ 8 ⁢ 5 ⁢ sA mol = 2.49 × 1 ⁢ 0 - 7 ⁢ mol s

Then converted was the unit from

mol s

into

mol h

as follow:

n C ⁢ 2 ⁢ H ⁢ 4 = 2.49 × 1 ⁢ 0 - 7 ⁢ mol s × 3600 ⁢ s 1 ⁢ hour = 8.96 × 1 ⁢ 0 - 4 ⁢ mol hour

The same method was used to calculate the flowrate of the other products. The flowrate of CO converted into product in the electrolyzer can be estimated from the product flowrate.

n m - CO = m × n m - ⁢ p ⁢ r ⁢ o ⁢ d

n_m-CO indicate the molar flowrate of CO that is consumed to produce m-carbon atom product. n_m-prod represent the molar flowrate of the generated m-carbon atom product. For example, the flowrate of CO consumption for C₂H₄ production (two carbon product, m=2) was calculated as follow:

n CO ⁢ for ⁢ C ⁢ 2 ⁢ H ⁢ 4 = 2 × n C ⁢ 2 ⁢ H ⁢ 4 = 2 × 8.96 × 1 ⁢ 0 - 4 ⁢ mol hour = 1.79 × 1 ⁢ 0 - 3 ⁢ mol hour

The summary of product flowrate and the reactant flowrate is given in Table S5.

TABLE S5
Summary of product flowrate and the reactant flowrate

	Faradaic	Mole flowrate of	Mole flowrate of
Product	efficiency	product, mol/h	CO, mol/h

H2	23.0%	1.72 × 10−3	0.00
CH4	1.1%	2.64 × 10−5	2.64 × 10−5
C2H4	48.0%	8.96 × 10−4	1.79 × 10−3
C3H8	0.0%	0.00	0.00
CH3OH	0.4%	1.46 × 10−5	1.46 × 10−5
C2H5OH	24.6%	4.58 × 10−4	9.17 × 10−4
C3H8O	2.5%	3.13 × 10−5	9.40 × 10−5
C2H3O2—	0.2%	9.07 × 10−6	1.81 × 10−5
Total		3.15 × 10−3	2.86 × 10−3

The single pass conversion (χ) can be calculated as follow:

x = ∑ n m - C ⁢ O n C ⁢ O = 2 × 8.96 × 1 ⁢ 0 - 3 ⁢ mol hour 1.47 × 10 - 2 ⁢ mol hour = 19 ⁢ %

The energy efficiency of electrolyzer for C₂H₄ production (E_el-C2H4) can be calculated as follow ⁵:

E e / - C 2 ⁢ H 4 = V C 2 ⁢ H 4 × j V cell × j

Where V_C2H4 is defined as follow:

V C 2 ⁢ H 4 = V C 2 ⁢ H 4 o × FE C 2 ⁢ H 4

V C 2 ⁢ H 4 o

and FE_C2H4 are the standard potential and Faradaic efficiency of C₂H₄, respectively. Thus, the energy efficiency of the electrolyzer for CISPE-12-0.15M at 100 mA/cm² is calculated as follow:

E el - C 2 ⁢ H 4 = V C 2 ⁢ H 4 o × FE C 2 ⁢ H 4 × j V cell × j = - 1.06 ⁢ V × 0.48 × 0.4 A - 2.41 ⁢ V × 0.4 A = 21.1 %

The detail calculation and the required data for calculation is available in the spreadsheet.

S2. Supplementary Results

A. Electrocatalytic OER Activity of NiFe LDH in 1 M KOH

To assess the OER activity trends, initially carried out were OER studies in a 3-electrode assembly that consisted of a NiFe LDH working electrode, Pt counter electrode, and Ag/AgCl reference electrode in a 1 M KOH electrolyte. Linear sweep voltammetry (LSV) from the backward cyclic voltammetry (CV) was scanned at 5 mV/sec and the resultant curves are presented here as FIG. 33a. The overpotential at 50 mA cm⁻² was considered to correlate with the improved performance of the NiFe LDH and it displayed a 0.4 V reduction in overpotential compared to Ni foam. The extracted Tafel slope from the LSV at the log scale again proved the induced charge transport characteristics and the observed Tafel slope values were 56 and 150 mV/dec for NiFe LDH and Ni foam respectively (FIG. 33b). As another important factor, electrochemical impedance spectroscopy (EIS) was studied at a potential of 1.6 V vs RHE as seen in FIG. 33c. The noted intrinsic resistance (R_s) charge transfer resistance (R_ct) were compared and NiFe LDH shows low R_s (2.37Ω) and R_ct (0.817Ω) compared to Ni foam (2.52 and 40.29Ω), further affirming the superior nature of the NiFe LDH. The voltage reduction was further addressed at specified current densities from 100 to 400 mA cm⁻² from the LSV (FIG. 33d) and in all the cases NiFe LDH demonstrated around 0.32-0.35 V lesser requirement compared to bare Ni foam as shown in FIG. 33e. The Galvanostatic (GSTAT) study was checked for long-term stability at 50 mA cm⁻² current density with NiFe LDH and delivered superior stability for 16 h as mentioned in FIG. 33f. From the overall OER studies, it was considered that the NiFe LDH was a commendable OER electrocatalyst and was further explored in the CO electrolysis study as an anode.

B. EIS Study in a Flow-Cell System

The special role of the USPE over the standalone CEM (Nafion™-117) in the electrocatalytic CO reduction was verified with the EIS studies in a flow-cell system as depicted in FIG. 34a. EIS studies were performed at an open circuit potential (OCP) in a frequency region of 1 MHz to 0.1 Hz with an AC amplitude of 5 mV in 1 M KOH. The electrolyte behavior on the electrode surface and the nature of their interface could easily be judged with the EIS results in terms of the R_s and R_ct values of the working electrode. The Nyquist plot for the bare sputtered Cu/PTFE, USPE-12, and standalone CEM is provided in FIG. 34b. The USPE-12 showed higher ionic conductivity over others when considering the lower R_s and R_ct values. The observed R_s and R_ct trend was as follows, USPE-12<Cu/PTFE<Cu/PTFE/CEM which suggested that USPE-12 could deliver improved performance with less resistance in the system. The R_s and R_ct bar graph in FIG. 34c supported the betterment of USPE-12 electrodes towards CO electrolysis. A closer look into R_s values suggested that USPE-12 showed the lowest value of 4.721Ω compared to Cu/PTFE (6.286Ω) and CEM (33.62Ω). The R_ct values have also followed a similar trend with 51.24, 82.17, and 90.69 Ω for USPE-12, Cu/PTFE, and CEM respectively demonstrating the improved role of the USPE towards CO electrolysis applications.

C. CO Electrolysis Supplementary Results

Comments on FIGS. 36 and 37: COR in AEM MEA Using Different Anolytes.

To investigate that K⁺ transports to the cathode along with water transport, COR experiments were performed using AEM at 50 mA/cm² with different anolytes (0.2 M KOH and 1 M KOH+1 M KHCO₃) (FIGS. 36a and 36b). It was noticed that the decrease of COR selectivity was slower in lower anolyte concentrations, 0.2 M KOH<1 M KOH<1 M KOH+1 M KHCO₃. The COR experiments at different current densities using 0.2 M KOH anolyte and 2 M KOH anolyte also indicated that K⁺ accumulation limits CO availability (FIGS. 37a and 37b).

TABLE S6
List of earlier reports on CO electrolysis to produce ethylene
(C2H4) and ethanol (C2H5OH) in MEA setup using alkaline anolyte.

Cell voltage, V	2.32	2.64	3.5	2.73	2.4	2.22	2.15
Faradaic	39%	29%	70%	61%	41%	38%	30%
efficiency for
C2H4
Current	144	300	100	150	240	300	200
Density,
mA/cm2
Single Pass	94%	12%	3%	47%	93%	11%	55%
Conversion (%)
Partial current	56	87	70	92	98	114	60
density for
C2H4 mA/cm2
Partial current	108	270	80	123	218	225	180
density for C2+,
mA/cm2
CO make-up,	115	140	48	56	104	108	202
GJ/ton C2H4
Electrolyzer	164	251	138	123	161	161	197
electricity,
GJ/ton C2H4
CO recycle,	3	13	13	4	2	13	8
GJ/ton C2H4
Total energy	281	403	198	184	267	281	408
cost, GJ/ton
C2H4
Stability, hours	24	102	7.5	110	200	Not	120
						reported
Electrolyzer	17.8%	11.6%	21.2%	23.7%	18.1%	18.1%	14.8%
energy
efficiency to
C2H4 (%)
Electrolyzer	1.4%	6.8%	2.4%	8.1%	9.6%	4.3%	2.4%
energy
efficiency to
C2H5OH (%)
Electrolyzer	19.2%	18.4%	23.6%	31.8%	27.7%	22.4%	17.2%
energy
efficiency to
C2H4 + C2H5OH
Membrane	180	130	50	45	45	50	45
thickness μm
Reference	6	7	8	9	10	11	12

	Cell voltage, V	3	3.25	3.94	2.4	2.41	2.5
	Faradaic	15%	33%	39%	48%	43%	34%
	efficiency for
	C2H4
	Current	150	150	250	100	100	100
	Density,
	mA/cm2
	Single Pass	76%	76%	13%	19%	90%	20%
	Conversion (%)
	Partial current	23	50	98	48	43	34
	density for
	C2H4 mA/cm2
	Partial current	143	135	225	75	76	76
	density for C2+,
	mA/cm2
	CO make-up,	405	168	110	67	75	100
	GJ/ton C2H4
	Electrolyzer	550	250	278	138	155	205
	electricity,
	GJ/ton C2H4
	CO recycle,	7	4	13	13	3	18
	GJ/ton C2H4
	Total energy	962	421	400	218	233	322
	cost, GJ/ton
	C2H4
	Stability, hours	103	103	20	206	Not	Not
						reported	reported
	Electrolyzer	5.3%	11.6%	10%	21.1%	19%	14%
	energy
	efficiency to
	C2H4 (%)
	Electrolyzer	7.0%	3.5%	6.7%	10.7%	12.8%	17.5%
	energy
	efficiency to
	C2H5OH (%)
	Electrolyzer	12.3%	15.2%	17.2%	31.8%	33.1%	31.7%
	energy
	efficiency to
	C2H4 + C2H5OH
	Membrane	45	130	100	0.7	0.7	0.7
	thickness μm
	Reference	13	13	14	Present	Present	Present
					work	work	work

Comments on FIG. 57: Technoeconomic Analysis

The methodology of the economic evaluation was adapted from Jouny et al..¹⁵ with addition on the Solid Oxide CO₂ electrolysis for CO production (CO₂-to-CO electrolysis) (Faradaic efficiency=100%, current density=640 mA/cm², energy requirement=2 MWh/ton-CO, stack cost=$422/in^{2 16}). For estimating the capital and operating costs of Solid Oxide CO₂ electrolysis, the methodology by Ozden et al. was followed.⁹ The final desired product of the integrated CO₂ and CO electrolysis was ethylene (C₂H₄). The levelized cost of C₂H₄ was shown as a function of energy consumption and current density in FIG. 57. All parameters except CO₂ conversion in CO₂-to-CO electrolysis (50%), CO conversion in COR (90%), Faradaic efficiency of C₂H₄(48%) and electricity price ($0.02/kWh) was considered for the base scenario from Jouny et al.¹⁵

Example 4 References

• [1] S. Anantharaj, K. Karthick, S. Kundu Materials Today Energy. 2017, 6, 1-26.
• [2] X. Li, C. Liu, Z. Fang, L. Xu, C. Lu, W. Hou Small. 2022, 18, 2104354.
• [3] in How do the alkalinity methods measure hydroxides, carbonates, and bicarbonates?, Vol. 2021 (Ed.{circumflex over ( )}Eds.: Editor), HACH, City, 2021.
• [4] T. Michatowski, A. G. Asuero Critical Reviews in Analytical Chemistry. 2012, 42, 220-244.
• [5] J. Sisler, S. Khan, A. H. Ip, M. W. Schreiber, S. A. Jaffer, E. R. Bobicki, C.-T. Dinh, E. H. Sargent ACS Energy Letters. 2021, 6, 997-1002.
• [6] D. S. Ripatti, T. R. Veltman, M. W. Kanan Joule. 2019, 3, 240-256.
• [7] X. Wang, P. Ou, A. Ozden, S.-F. Hung, J. Tam, C. M. Gabardo, J. Y. Howe, J. Sisler, K. Bertens, F. P. Garcia de Arquer, R. K. Miao, C. P. O'Brien, Z. Wang, J. Abed, A. S. Rasouli, M. Sun, A. H. Ip, D. Sinton, E. H. Sargent Nature Energy. 2022, 7, 170-176.
• [8] N.-H. Tran, H. P. Duong, G. Rousse, S. Zanna, M. W. Schreiber, M. Fontecave ACS Applied Materials & Interfaces. 2022, 14, 31933-31941.
• [9] A. Ozden, Y. Wang, F. Li, M. Luo, J. Sisler, A. Thevenon, A. Rosas-Hernández, T. Burdyny, Y. Lum, H. Yadegari, T. Agapie, J. C. Peters, E. H. Sargent, D. Sinton Joule. 2021, 5, 706-719.
• [10] A. Ozden, J. Li, S. Kandambeth, X.-Y. Li, S. Liu, O. Shekhah, P. Ou, Y. Zou Finfrock, Y.-K. Wang, T. Alkayyali, F. Pelayo Garcia de Arquer, V. S. Kale, P. M. Bhatt, A. H. Ip, M. Eddaoudi, E. H. Sargent, D. Sinton Nature Energy. 2023.
• [11] B. Hasa, L. Cherniack, R. Xia, D. Tian, B. H. Ko, S. Overa, P. Dimitrakellis, C. Bae, F. Jiao Chem Catalysis. 2023, 3, 100450.
• [12] S. Overa, B. S. Crandall, B. Shrimant, D. Tian, B. H. Ko, H. Shin, C. Bae, F. Jiao Nature Catalysis. 2022, 5, 738-745.
• [13] J. Li, H. Xiong, X. Liu, D. Wu, D. Su, B. Xu, Q. Lu Nature Communications. 2023, 14, 698.
• [14] C. Zhao, G. Luo, X. Liu, W. Zhang, Z. Li, Q. Xu, Q. Zhang, H. Wang, D. Li, F. Zhou, Y. Qu, X. Han, Z. Zhu, G. Wu, J. Wang, J. Zhu, T. Yao, Y. Li, H. J. M. Bouwmeester, Y. Wu Advanced Materials. 2020, 32, 2002382.
• [15] M. Jouny, W. Luc, F. Jiao Industrial & Engineering Chemistry Research. 2018, 57, 2165-2177.
• [16] R. Anghilante, D. Colomar, A. Brisse, M. Marrony International Journal of Hydrogen Energy. 2018, 43, 20309-20322.

Example 5—Bilayer Ionomer Coatings Enabling Higher Partial Current Density Towards Multi Carbon Products in CO₂ Electrolysis

To perceive carbon recycling, electrochemical CO₂ reduction (eCO₂R) is a technology that can convert CO₂ into value-added products. Although progress has been made towards desired multi-carbon (C₂₊) products such as ethylene (C₂H₄); challenges remain to achieve this at industrial-scale current densities with high Faradaic efficiency (FE) or selectivity. The strategies to achieve this goal include exploring ways to enhance local CO₂ concentration and avoiding undesired and competing side reactions including carbonate formation and hydrogen evolution reaction (HER).

In this context, eCO₂R has transitioned from H-cell to membrane electrode assembly (MEA) where gas diffusion electrode (GDE) can overcome the solubility limit of aqueous CO₂. MEA is usually accompanied by anion exchange membrane (AEM) which can maintain high pH local environment at the cathode surface and improve the eCO₂R activity. As for the catalyst, copper (Cu) is a metal that favors C₂₊ products over single carbon ones owing to probable surface modification (reconstruction, oxidation etc.) that generates highly active local microenvironment^[1]. Combining this with MEA and operating under highly alkaline (i.e., high pH) electrolyte further enhances the selectivity towards C₂₊ products (C₂H₄). However, it has been shown that alkaline electrolyte in anode (anolyte) promotes electro-migration of excess metal cations towards cathode, resulting in carbonate salt precipitation, blocking the pores of gas diffusion layer, and thus hindering eCO₂R performance. To mitigate this, the herein described approach may be applied to directly-deposit an ultrathin layer of PFSA polymer (e.g., nafion)—which can replace commercial standalone membrane and may control cation migration towards cathode.

Ion conducting polymers (ionomers) are well known compounds with high CO₂ affinity within the organic layer. Use of ionomers can allow for tailoring of the local reaction environment by controlling the concentrations of CO₂, H₂O, OH⁻ and H⁺ due to the presence of charged hydrophilic side chains spread over a hydrophobic backbone. lonomers are exploited for these hydrophobic and hydrophilic functionalities, where the conformally differentiated domains can favor both gas and ion transport over catalyst surface. The hydrophobic moieties extend gas diffusion, while hydrophilic domains lead to better wettability and ion transport. As a result, the three-phase reaction interface involving these gas, ion and electron components, can be increased from the sub-micrometer regime to the several micrometer length scale^[3][4]. Nafion is widely used as ionomer for eCO₂R due to its robustness and proton conductivity. Using different ionomer layers with varying properties (acidity versus alkalinity, CO₂ availability, CO₂ permeability, water uptake, ion transport etc.) may impact eCO₂R by modulating the local microenvironment necessary for achieving high partial current density towards C₂₊ (j_C2+) products.

An alternative strategy to induce high j_C2+ is to employ sustainion using direct deposition approach. Sustainion is an alkaline ionomer with hydrophilic poly (4-vinylbenzyl alkyl-imidazolium chloride) unit within the nanopore network. It has been previously shown that sustainion can maintain ionic conductivity at lower specific resistance. Moreover, sustainion can also maintain high local pH with high CO₂ solubility (e.g., about 20 times higher than nafion) due to strong affinity by imidazolium groups^[3][5][6]. It was considered that as sustainion can provide higher CO₂/H₂O ratio at the catalyst vicinity, this may be a factor in regulating higher j_C2+ while maintaining selectivity due to alkaline nature. However, hydrophilic ionomers are presumed to be filled by electrolyte during operation as the gas phase CO₂ can only diffuse in dissolved form which increases the chance of carbonation. It was thus further considered that this loss of CO₂ may be avoided by changing the surface wettability that can control cation electromigration as well as keep high local pH, for example by implementing a hydrophilicity gradient (e.g., coating hydrophilic sustainion on top of bulk hydrophobic nafion).

Herein, it is described and/or demonstrated the effect of different ionomer layers to induce favorable microenvironments for selective C₂₊ production with high j_C2+ on Cu catalyst. Impact of a single layer and stacked layers of ionomer thin films was studied on efficient eCO₂R. First analyzed was the performance of sustainion ionomer layer yielding a partial current density of 280 mA/cm² towards C₂₊ products. However, it also contributed to high carbonate formation and thus high CO₂ loss because of elevated CO₂ permeability of this ionomer layer. To mitigate these issues, one additional layer of nafion ionomer was added with 1 M of K+ infusion which resulted in comparable partial current density (e.g., 280 mA/cm²) as single layer sustainion towards C₂₊ products, mostly C₂H₄ and C₂H₅OH at 350 mA/cm² current density. Hence, it was observed that the system described herein may offer a stable (24 hours) eCO₂R system with higher partial current density towards C₂₊ products, notably suppressing the HER and preserving the carbonate formations as AEM. Further analysis suggested that these results were achieved due to the interdependent effects of i) optimized water diffusion through ultrathin ionomer layers avoiding water accumulation and salt precipitation at the cathode; ii) high CO₂/H₂O availability at the cathode and iii) high local pH at the reaction environment because of direct contact of catalyst and ionomer layers.

Results and Discussions

Analysis began with using commercial AEM in a MEA system to check performance for eCO₂R. The MEA consisted of a porous polytetraflouroethylene (PTFE) as the gas diffusion layer (GDL), with sputtered copper (Cu) of 300 nm thickness as cathode catalyst and Nickel (Ni) foam as the anode. At the cathode, humidified CO₂ gas was fed and at anode, 1 M KOH was circulated as anolyte. The system schematic is shown in FIG. 58A-D.

The partial current density and the voltage at different current density (l) in this configuration have been demonstrated in FIG. 58(b). The maximum partial current density (j) obtained towards C₂H₄ (j_C2H4) and H₂ (j_H2) was 54 mA/cm² and 55 mA/cm² respectively at 150 mA/cm² current density, values which aligned with the previous reports. It was observed that FE_C2H4 started decreasing after that and that HERincreased. The total gas products distribution indicated that there were no significant liquid products at higher current and thus, liquid products were not analyzed with AEM. The mass transport in FIG. 58(c) indicated that excessive cations were coming with KOH from anode and sitting at the cathode, even at 50 mA/cm², which was considered due to a high-water uptake of the thick standalone AEM. The ion transport was measured at 50 mA/cm² throughout for a steady comparison. This can flood the serpentine channel and hinder gas diffusion. This excessive water transport can increase flooding and salt precipitation with increasing current density^[7] and thus can degrade eCO₂R performance at higher current density. Also, membranes with higher water uptake tend to experience poor mechanical strength and durability because of swelling^[8,9], which is consistent with results in FIG. 58(d). Following this, the C₂H₄ selectivity dropped by 46% and 70% in one hour and three hours of experiment respectively.

In this regard, it was considered that a thin membrane could suppress cathode flooding and thus CO₂ could diffuse well through the gas diffusion layer and therefore enhance performance of the system. As a membrane thinner than 50 μm was not commercially available, a thin layer of ionomer was deposited on top of catalyst using spray coating approach. In comparison to freestanding membranes, the herein described directly deposited membrane has demonstrated an enhancement in all three primary loss mechanisms, such as kinetic losses, ohmic losses, and/or mass transport losses which can translate into overall improved eCO₂R performance^[10].

Since an AEM with alkaline electrolyte has exhibited enhanced performance, as reported in the literature, particularly in alkaline environments with suppression of hydrogen (H₂) production characterized by higher pH levels^[4,6], a sustainion XA-9 ionomer was selected for direct deposition with alkaline medium. To spray the sustainion ionomer, three different solvents were tested (Deionized water or DI water, methanol or MeOH and a combination mixture of isopropyl alcohol or IPA and DI water) to investigate dispersion of the ionomer solution. With MeOH and IPA/DI water, it was observed that the coating was not homogenous due to poorer dispersion and hence, the electrochemical performance was not enhanced (FIG. 59A-D). Different loadings were tested (12.5, 25 and 50 μL) of sustainion ionomer to observe products selectivity and corresponding ion transports through the membrane (FIG. 60A-F).

A scanning electron microscopy (SEM) analysis of the 25 μL Sustainion XA-9 layer using DI water solvent is shown in FIG. 61(b) and thickness measured from the SEM shows˜2 μm. In this approach, there was intimate and uniform contact between the catalyst and the ionomer layer and thus the catalyst could be protected from the commonly reported degradation, and the CO₂ could transport well across the catalyst layer^[13][14]. Also, it appeared that the close proximity of the catalyst to the membrane or ionomer had a discernible impact on the localized pH at the catalyst surface^[11]. It was also considered that it may reduce the water flux coming from anode owing to back convention, and thus avoid water accumulation at cathode, thereby enhancing the CO₂ availability at the catalyst.

It was observed that the selectivity was not significantly different at the three different loadings, while there was improvement in partial current density of C₂₊ products and and decrease of HER with 25 μL (FIG. 61(c)). Thus, the 25 μL sustainion loading was focused on, and total products distribution and cell voltage at different current density was analyzed (FIG. 61(d)). ˜80% C₂₊ products (34% C₂H₄, 39% C₂H₅OH and 7% C₂H₃O⁻) and ˜14% H₂ at 350 mA/cm² current density which leads to record j_C2+ of 280 mA/cm² were observed.

The mass transport was measured (see example 6). With sustainion being an anion exchange ionomer, the charge transfer was governed by CO₃²⁻ which indicated that this configuration was acting as an AEM. It was found that the flooding as well as salt precipitation at cathode was suppressed reasonably compared to AEM, thus enhancing the local CO₂ availability. Also, the high OH⁻ diffusion flux from anode to cathode was considered to be playing a favourable role in improving the CO₂R selectivity owing to high local pH that can promote eCO₂R kinetics by suppressing HER. However, more KOH transport to the cathode can also yield to more KHCO₃, which in turn can cause CO₂ loss and also neutralizes the electrolyte as well as destabilizes the Ni based anode. This can be attributed to high CO₂ permeability through thinner sustainion layer. Reduced thickness of the directly deposited membrane can reduce water uptake and decrease the K⁺ deposition, suppressing HER and enhancing CO₂R selectivity. This supports the selectivity results in FIG. 61(d) and also the water uptake experiments discussed in Example 6.

As a next step, how to reduce gas permeability was investigated, and thereby how to mitigateCO₂ loss and KHCO₃ formation. In this regard, an experiment was designed to measure CO₂ flux across the directly deposited membrane (see Example 6). From there, the assumption of high CO₂ permeability was investigated with 25 μL/cm² Sustainion XA9 sample. It was discovered that CO₂ permeability was very high and thus a high amount of CO₂ was lost and a part of it was reacting chemically with KOH to form bicarbonate salts (KHCO₃). To address this, another layer of ionomer (nafion) was added underneath the sustainion layer to reduce CO₂ permeability and salt formation.

Also, considering the background charges, it was considered that sustainion and nafion could make a compact sandwich stack. Sustainion with positive background charge, may inhibit cations passing through—whereas negative background charge in nafion may exclude all anions and trap the eCO₂R generated OH⁻ to increase the local pH at the Cu surface^[3]. Thus, it the stacking order was set at Cu/nafion/sustainion XA-9. The SEM image in FIG. 62(b) shows two immiscible layer of ionomers with thickness of ˜1 μm and ˜2 μm of nafion and sustainion respectively. However, the addition of nafion layer underneath sustainion did not appear to improve the CO₂R activity (FIG. 62(c)).

To identify the issue, the corresponding ion transport was measured (see Example 6). It was found that the OH⁻ diffusion was too low to provide an alkaline microenvironment for the required eCO₂R reaction pathways. Though the slower OH⁻ diffusion was expected to eliminate CO₂ mass transport limitation at higher current^[7], it appeared the acidification of the local environment was ruling here over CO₂ availability—resulting in HER. Moreover, it was observed that K⁺ deposition was reduced, which suggests that the cathode is less flooded compared to AEM—making the system performance relatively more stable as is depicted in FIG. 62(d). From here, it was considered that the lower OH⁻ diffusion was helping to improve system stability, however because of the acidic nature of the nafion ionomer, the HER was relatively more dominant in this system.

As the reaction pathways were pH sensitive, to neutralize the acidic nature of the nafion, which was closest to the catalyst layer, KOH was incorporated into nafion matrix. The performance was then studied. The thickness under SEM was 1 μm and 2 μm respectively for the nafion and sustainion layers, which was the same as the system tested in the absence of integrated cation. Different concentrations of KOH infusion into nafion were tested and the gas products were evaluated (FIG. 63(b)). With the increase of KOH concentrations from 0 M to 1 M, the selectivity and the voltage were observed to improve gradually, with a maximum selectivity obtained with 1 M KOH.

Next investigated was Cu/25 μL/cm² Nafion+1 M KOH/25 μL/cm² Sustainion XA9 Selectivity and j were measured at different current density. The maximum j_C2+ achieved was 280 (FIG. 63(c)) mA/cm² at 350 mA/cm² with ˜76% C₂₊ products and ˜11% H₂. According to the literature^[15-19], alkali cations can help in cathode activation by promoting C—C bond formation and thus resulting to selective C₂₊ products formation. Also, the literature^[15-19] suggested that the presence of certain local electric fields induced by cations can have a stabilizing effect on adsorbed CO₂ molecules. These cations can cause a shift in the energy landscape, making it much easier for CO₂ to bind to the catalyst surface. It was considered that, for the selective CO₂R, an certain pH level is required at the reaction environment which requires higher OH diffusion to cathode. To investigate this, ion transport experiments were conducted. Ion transports from these results were close to that of AEM, while water accumulation and salt precipitates at the cathode were suppressed, resulting in higher CO₂ availability at the catalyst vicinity (FIG. 64A-D). As flooding and salt accumulation at the cathode were suppressed in this system, it offered a more stable electrochemical performance over 24 hours (FIG. 63(d)).

NON-BINDING CONCLUSIONS

This study advanced understanding of the role of ionomer layers in shaping microenvironments for selective C₂₊ production during eCO₂R. Demonstrated was the potential of sustainion and nafion ionomers in modulating the local reaction environment to achieve high j_C2+ while preserving selectivity and stability. The sustainion ionomer, owing to its high CO₂ solubility and alkaline nature, initially exhibited an j_C2+ of 280 mA/cm² towards C₂₊ products. However, it also posed challenges related to carbonate formation and CO₂ loss. By introducing an additional nafion ionomer layer infused with K⁺ ions, these issues could be addressed and/or managed, achieving a comparable j_C2+ of 280 mA/cm² towards C₂₊ products with enhanced stability.

Example 5 References

• [1] W. Li, Z. Yin, Z. Gao, G. Wang, Z. Li, F. Wei, X. Wei, H. Peng, X. Hu, L. Xiao, J. Lu, L. Zhuang, 2022, 7, 835.
• [2] M. A. Adnan, A. S. Zeraati, S. K. Nabil, T. A. AI-attas, K. Kannimuthu, C. Dinh, I. D. Gates, G. Kibria, 2023, 2203158, 1.
• [3] C. Kim, J. C. Bui, X. Luo, J. K. Cooper, A. Kusoglu, A. Z. Weber, A. T. Bell, Nat. Energy 2021, 6, 1026.
• [4] F. P. G. De Arquer, C. Dinh, A. Ozden, J. Wicks, C. Mccallum, A. R. Kirmani, D. Nam, C. Gabardo, A. Seifitokaldani, X. Wang, Y. C. Li, F. Li, J. Edwards, L. J. Richter, S. J. Thorpe, D. Sinton, E. H. Sargent, 2020, 2, 661.
• [5] K. J. Puring, D. Siegmund, J. Timm, F. Möllenbruck, S. Schemme, R. Marschall, U. Apfel, 2021, 2000088, 1.
• [6] C. O. Brien, C. Mccallum, 2020, DOI 10.1039/C9EE03077H.
• [7] D. G. Wheeler, B. A. W. Mowbray, A. Reyes, F. Habibzadeh, J. He, C. P. Berlinguette, Energy Environ. Sci. 2020, 13, 5126.
• [8] J. Han, L. Zhu, J. Pan, T. J. Zimudzi, Y. Wang, Y. Peng, M. A. Hickner, L. Zhuang, Macromolecules 2017, 50, 3323.
• [9] J. Pan, S. Lu, Y. Li, A. Huang, L. Zhuang, J. Lu, Adv. Funct. Mater. 2010, 20, 312.
• [10] P. Holzapfel, M. Bühler, C. Van Pham, F. Hegge, T. Böhm, D. McLaughlin, M. Breitwieser, S. Thiele, Electrochem. commun. 2020, 110, 106640.
• [11] S. Garg, C. A. Giron Rodriguez, T. E. Rufford, J. R. Varcoe, B. Seger, Energy Environ. Sci. 2022, DOI 10.1039/d2ee01818g.
• [12] A. R. Woldu, Z. Huang, P. Zhao, L. Hu, D. Astruc, Coord. Chem. Rev. 2022, 454, 214340.
• [13] C. A. Moreno-camacho, J. R. Montoya-torres, A. Jaegler, J. Clean. Prod. 2019, 231, 600.
• [14] S. R. Hui, N. Shaigan, V. Neburchilov, L. Zhang, K. Malek, 2020.
• [15] B. Endrödi, A. Samu, E. Kecsenovity, T. Halmágyi, D. Sebök, C. Janáky, 2021, 6, 439.
• [16] J. Resasco, L. D. Chen, E. Clark, C. Tsai, C. Hahn, T. F. Jaramillo, K. Chan, A. T. Bell, 2017, DOI 10.1021/jacs.7b06765.
• [17] J. H. Kim, H. Jang, G. Bak, W. Choi, H. Yun, E. Lee, D. Kim, J. Kim, Y. Lee, 2022, DOI 10.1039/D2EE01825J.
• [18] L. D. Chen, M. Urushihara, K. Chan, 2016, DOI 10.1021/acscatal.6b02299.
• [19] S. Ringe, K. Chan, S. Ringe, B. Seger, 2019, 12, DOI 10.1039/c9ee01341e.

Example 6—Bilayer Ionomer Coatings Enabling High Partial Current Density Towards

Multi Carbon Products in CO₂ Electrolysis—Supplementary Information

S1. Experimental Details

A. Materials

Potassium hydroxide (KOH), cesium hydroxide (CsOH), and methanol and isopropyl alcohol were purchased from Sigma Aldrich (ACS reagent). Sustainion® XA-9 solution (5% in ethanol) and Nafion™ perfluorinated resin solution (5 wt. % in mixture of lower aliphatic alcohol and water) were received from Dioxide Materials and Sigma Aldrich, respectively. The Sustainion® X37-50 Grade RT membrane was received from Dioxide materials. Deionized water was processed through Direct-Q® Water Purification System purchased from Sigma Aldrich.

B. Experimental Setup

A membrane electrode assembly (MEA), consisting of anode (grade 2 titanium) and cathode (904L stainless steel) was made by Dioxide materials. A humidified CO₂ and 1 M KOH (unless mentioned otherwise) with a flowrate of ˜50 sccm was fed to the cathode and anode sides, respectively using a flow meter (Cole-parmer 39067) and peristaltic pump (Fisherbrand™ Variable-Flow Peristaltic Pumps), respectively. BioLogic potentiostats with 10 A booster was used to obtain the electrochemical response without iR correction. Parsche Airbrush set was used to spray the solution on the target sheet. The gas products were analyzer using Perkin Elmer Gas Chromatography with flame ionization detector (FID) and thermal conductivity detector (TCD). The liquid products were identified using BrukerAVANCE III 600 MHz nuclear magnetic resonance spectroscopy equipped with pulsed-field gradient probes.

C. Cathode Preparation

The cathode catalyst was approximately 300 nm sputtered copper on polytetrafluoroethylene (PTFE). The bilayer ionomer coating (BUSPE) was fabricated by spray-coating of the desired quantity of Nafion solution first on top of the cathode catalyst and sustainion XA-9 solution was spray coated on nafion layer and each layer was dried for at least overnight under atmospheric condition. Nafion solution and was prepared by diluting Nafion™ perfluorinated resin solution (Sigma Aldrich) in methanol (Sigma Aldrich) with a ratio of 1:5 by volume. Different sustainion solutions were prepared by diluting sustainion XA-9 solution in deionized water with ratio 1:2 by volume, in methanol with a ratio of 1:5 by volume and with a combination mixture of isopropyl alcohol and deionized water with a ration of 1:1 by mass.

The cation infused nafion was prepared by spray-coating of the desired quantity of Nafion-cation solution on top of cathode catalyst and dried for overnight under atmospheric condition. A Nafion-cation solution was prepared by mixing Nafion solution with cation solution at the desired concentration. Cation solution could be KOH, or CsOH. The Nafion-cation solution was prepared by mixing Nafion solution and cation solution with a ratio of 1:9 by volume.

In the anode side, the Ni-foam sheet (0.08 mm, MTI Corporation) was used as catalysts to avoid damage on the Nafion layer that could result in short circuit.

D. Characterization

The morphology and structure of BUSPE were characterized by scanning electron microscopy (SEM). SEM observation was performed using a FEI Quanta 250 FEG field emission scanning electron microscope which was equipped with EDS analysis.

E. Mass of Nafion in the Solution

This has been done following the methods explained in Example 2.

F. Measurement of Hydroxide and Carbonate Concentrations Using Total Alkalinity Method

This has been done following the methods explained in Example 2. FIG. 64 using this total alkalinity method reflects the charge transfer, diffusion flux and the salt accumulation of different bilayer ionomer combination with different concentration of KOH starting from 0 M to 1 M. It is clear from the results that this direct deposition approach of bilayer ionomer coating reduces the cathode flooding by two-fold compared to AEM.

G. Measurement of Mass Diffusion

This has been done following the methods explained in Example 2. The measured diffusion flux for different ionomer combinations is presented in FIG. 65.

H. Measurement of CO₂ Permeability

To measure the CO₂ permeability, a separate experimental setup was prepared like section B except that there was no electrolyte flowing. Instead of liquid electrolyte, 99.99% pure nitrogen gas was circulated at the anode at 40 standard cm³ per minute flow rate. The electrodes were prepared as the methods mentioned in section C. The gas was collected from the anode outlet and analyzed in gas chromatography to measure the CO₂ mass transfer flux passing from the cathode to anode through different ionomer coatings. The measured CO₂ mass transfer flux for different samples is mentioned in FIG. 66.

The embodiments described herein are intended to be examples only. Alterations, modifications, and/or variations can be effected to the particular embodiments by those of skill in the art. The scope of the claims should not be limited by the particular embodiments set forth herein, but should be construed in a manner consistent with the specification as a whole.

The aspects, embodiments, and/or examples of the present disclosure being thus described, it should be recognized that said aspects, embodiments, and/or examples may be varied in ways that do not depart from the spirit and scope of the present disclosure, and that said variations are intended to be included within the scope of the following claims..

All publications, patents and patent applications mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication patent, or patent application was specifically and individually indicated to be incorporated by reference.