CONDUCTIVE ALL-POLYMER GAS DIFFUSION LAYERS FOR ELECTROCHEMICAL DEVICES

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to and claims the priority benefit of U.S. Provisional Application No. 63/696,331, entitled “Conductive All-Polymer Gas Diffusion Layers for Electrochemical Devices” filed Sep. 18, 2024, the contents of which are hereby incorporated by reference in their entirety into the present disclosure.

GOVERNMENT SUPPORT CLAUSE

This invention was made with government support under contract number DE-SC0023257 awarded by the Department of Energy. The government has certain rights in the invention.

TECHNICAL FIELD

The present application relates to electrochemical devices, and specifically to conductive, all-polymer gas diffusion layers for use in electrochemical reactors.

BACKGROUND

This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, these statements are to be read in this light and are not to be understood as admissions about what is or is not prior art.

Gas diffusion layers are specialized components used in various electrochemical systems where gas-phase reactants and products are involved. These layers are designed to facilitate the interaction between gas, an electrolyte, and a catalyst for carrying out efficient electrochemical reactions.

Gas diffusion electrodes are commonly used in a range of applications, including fuel cells, metal-air batteries, and electrolyzers. In fuel cells, for instance, the electrodes are essential for the oxidation of hydrogen at the anode and the reduction of oxygen at the cathode, processes that generate electricity. In metal-air batteries, the electrodes facilitate the reduction of oxygen, which is a step in the battery's discharge process. In electrolyzers, the electrodes are used to convert electrical energy into chemical energy by driving reactions like the splitting of water into hydrogen and oxygen.

Gas diffusion electrodes typically have multiple layers, each with a specific function. The gas diffusion layer (GDL) is often made of a porous, hydrophobic material, usually based on carbon, such as carbon paper or carbon cloth. This layer allows gases like oxygen or hydrogen to diffuse through the electrode while preventing the liquid electrolyte from flooding the pores, which could otherwise block the gas pathways. On top of the GDL is the catalyst layer, which is often composed of finely dispersed precious metals like platinum or palladium. This layer is where the actual electrochemical reaction occurs, converting the gas into ions or other products that participate in the overall reaction.

The choice of materials and the design of the electrodes are important for optimizing performance, as they balance the need for efficient gas transport, electrical conductivity, catalytic activity, and durability under operating conditions.

SUMMARY

Various embodiments of the systems and methods described herein provide for improved gas diffusion layers for use in electrochemical reactors systems. In one embodiment, a method of forming a gas diffusion layer can include depositing a metallic layer over a porous polytetrafluoroethylene (PTFE) layer, oxidizing 3,4-ethylenedioxythiophene (EDOT) over the metallic layer, and forming a porous poly(3,4-ethylenedioxythiophene) (PEDOT) layer over the porous PTFE layer. The porous PEDOT layer can directly contact the porous PTFE layer. In some embodiments of the method, the depositing of the metallic layer onto the porous PTFE layer can include initiating a thermal evaporation process. In other embodiments, the oxidizing of the EDOT over the metallic layer can include initiating electrochemical oxidation to initiate polymerization. In still other embodiments, the forming of the porous PEDOT layer over the porous PTFE layer can include dissolving the metallic layer. In still other embodiments, the dissolving of the metallic layer can include applying one or more chemical agents to the metallic layer. In still other embodiments, the metallic layer can include gold. In some embodiments, the forming of the porous PEDOT layer over the porous PTFE layer can include initiating an aqua regia process to dissolve the gold. In still other embodiments, the porous PEDOT layer can comprise PEDOT doped with one or more of PF₆⁻, BF₄⁻, ClO₄⁻, and CF₃SO₃⁻. In still other embodiments, the EDOT can be polymerized in an electrolyte comprising 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF₆), LiBF₄, LiClO₄ or LiCF₃SO₃ in acetonitrile. In some embodiments, the polymerization can include applying a three-electrode configuration consisting of a working electrode, a counter electrode and a reference electrode, wherein the working electrode is the porous PTFE layer coated with the metallic layer, the counter electrode is a carbon paper, and the reference electrode is Ag/AgCl in saturated KCl. In some embodiments, the polymerization can include immersing the electrodes in an electrolyte comprising the EDOT and BMIMPF6, LiBF₄, LiClO₄ or LiCF₃SO₃ in acetonitrile. In some embodiments, the polymerization can include applying a potential of about 2.1 V versus Ag/AgCl or a current of about 2 mA for a certain time. In still other embodiments, the metallic layer can have a thickness of about 150 nm. In still other embodiments, the porous PEDOT layer can have a thickness of about 1 μm. In still other embodiments, fibers of the PEDOT layer can have an average width of about 20 nm.

In another embodiment, a method of electrochemically reducing CO₂ can include providing an electrode comprising a PEDOT-PTFE gas diffusion layer, introducing CO₂ to the electrode, and applying a current density of at least about 100 mA/cm² to reduce CO₂ to CO. In this method embodiment, the PEDOT-PTFE gas diffusion layer can comprise a porous PEDOT layer formed over a porous PTFE substrate, wherein the porous PEDOT layer directly contacts the porous PTFE layer. In some embodiments, this method can be performed in an alkaline, neutral, or acidic electrolyte. In some other embodiments of this method, the electrode can comprise silver nanoparticles supported on the PEDOT-PTFE gas diffusion layer.

In another embodiment, an electrochemical system can comprise an electrochemical reactor, an electrode comprising a PEDOT-PTFE gas diffusion layer, wherein the PEDOT-PTFE gas diffusion layer comprises a porous PEDOT layer formed over a porous PTFE substrate, wherein the porous PEDOT layer directly contacts the porous PTFE layer, and a catalyst layer supported on the PEDOT-PTFE gas diffusion layer. In some embodiments of the system, the catalyst layer can comprise silver nanoparticles, and the reactor is configured for CO₂ reduction at current densities of at least about 100 mA/cm².

This summary is provided to introduce a selection of the concepts that are described in further detail in the detailed description and drawings contained herein. This summary is not intended to identify any primary or essential features of the claimed subject matter. Some or all of the described features may be present in the corresponding independent or dependent claims, but should not be construed to be a limitation unless expressly recited in a particular claim. Each embodiment described herein does not necessarily address every object described herein, and each embodiment does not necessarily include each feature described. Other forms, embodiments, objects, advantages, benefits, features, and aspects of the present disclosure will become apparent to one of skill in the art from the detailed description and drawings contained herein. Moreover, the various apparatuses and methods described in this summary section, as well as elsewhere in this application, can be expressed as a large number of different combinations and subcombinations. All such useful, novel, and inventive combinations and subcombinations are contemplated herein, it being recognized that the explicit expression of each of these combinations is unnecessary.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims which particularly point out and distinctly claim this technology, it is believed this technology will be better understood from the following description of certain examples taken in conjunction with the accompanying drawings, in which like reference numerals identify the same elements and in which:

FIG. 1 depicts a schematic diagram showing one exemplary method of forming a PEDOT-PTFE-based gas diffusion layer;

FIG. 2 depicts a schematic diagram showing one example cell configuration for electrochemical synthesis of PEDOT on PTFE membrane;

FIG. 3A depicts a schematic diagram showing a PEDOT-PTFE-based gas diffusion layer material, showing an upper magnified portion including an SEM image of the PEDOT-PTFE gas diffusion layer from top-view and a lower magnified portion including an SEM image of the PEDOT-PTFE electrode from cross-sectional view using a focused ion beam (FIB) imaging device;

FIG. 3B depicts a HAADF-STEM image of cross-section of the PEDOT-PTFE gas diffusion layer of FIG. 3A;

FIG. 3C depicts an overlapped EDX mapping image of C and S of the PEDOT-PTFE gas diffusion layer of FIG. 3A;

FIG. 4A depicts a graphical representation of Raman spectra of PTFE and PEDOT-PTFE;

FIG. 4B depicts a graphical representation of an FTIR spectrum of PEDOT-PTFE, the inset representing the spectrum of pristine PTFE;

FIG. 4C depicts a graphical representation of a survey scan XPS spectrum of PEDOT-PTFE;

FIG. 5A depicts a graphical representation of CO2RR performance in alkaline (0.8 M KOH) electrolyte at constant current densities;

FIG. 5B depicts a graphical representation of CO2RR performance in neutral (0.8 M KHCO₃) electrolyte at constant current densities;

FIG. 5C depicts a graphical representation of CO2RR performance in acidic (0.01 M H₂SO₄ with 0.4 M K₂SO₄) electrolyte at constant current densities;

FIG. 5D depicts a graphical representation of CO2RR performance in alkaline (0.8 M KOH) electrolyte at constant current densities;

FIG. 5E depicts a graphical representation of CO2RR performance in neutral (0.8 M KHCO₃) electrolyte at constant current densities;

FIG. 5F depicts a graphical representation of CO2RR performance in acidic (0.01 M H₂SO₄ with 0.4 M K₂SO₄) electrolyte at constant current densities;

FIG. 6A depicts a series of graphical representations of CO2RR stability tests of AgNP 1 mg·cm⁻² on PEDOT-PTFE and Sigracet® 22bb at constant current densities of −100 mA·cm⁻² in alkaline (0.8 M KOH), neutral (0.8 M KHCO₃), and acidic (0.01 M H₂SO₄ with 0.4 M K₂SO₄) electrolytes, along with a series of graphical representations of CO₂ flow rates showing improved resistance against electrolyte flooding; and

FIG. 6B depicts a series of graphical representations of CO2RR stability tests of AgNP 1 mg·cm⁻² on PEDOT-PTFE and Sigracet® 22bb at constant current densities-200 mA·cm⁻² in alkaline (0.8 M KOH), neutral (0.8 M KHCO₃), and acidic (0.01 M H₂SO₄ with 0.4 M K₂SO₄) electrolytes, along with a series of graphical representations of CO₂ flow rates showing improved resistance against electrolyte flooding.

The drawings are not intended to be limiting in any way, and it is contemplated that various embodiments of the technology may be carried out in a variety of other ways, including those not necessarily depicted in the drawings. The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present technology, and together with the description serve to explain the principles of the technology; it being understood, however, that this technology is not limited to the precise arrangements shown, or the precise experimental arrangements used to arrive at the various graphical results shown in the drawings.

DETAILED DESCRIPTION

The following description of certain examples of the technology should not be used to limit its scope. Other examples, features, aspects, embodiments, and advantages of the technology will become apparent to those skilled in the art from the following description, which is by way of illustration, one of the best modes contemplated for carrying out the technology. As will be realized, the technology described herein is capable of other different and obvious aspects, all without departing from the technology. Accordingly, the drawings and descriptions should be regarded as illustrative in nature and not restrictive.

It is further understood that any one or more of the teachings, expressions, embodiments, examples, etc. described herein may be combined with any one or more of the other teachings, expressions, embodiments, examples, etc. that are described herein. The following-described teachings, expressions, embodiments, examples, etc. should therefore not be viewed in isolation relative to each other. Various suitable ways in which the teachings herein may be combined will be readily apparent to those of ordinary skill in the art in view of the teachings herein. Such modifications and variations are intended to be included within the scope of the claims.

As used herein, the term “about” means that the amount or value in question may be the value designated or some other value about the same. The phrases are intended to convey that similar values within a range of ±5% of the indicated value promote equivalent results or effects.

Reference systems that may be used herein can refer generally to various directions (for example, upper, lower, forward and rearward), which are merely offered to assist the reader in understanding the various embodiments of the disclosure and are not to be interpreted as limiting. Other reference systems may be used to describe various embodiments, such as those where directions are referenced to the portions of the device, for example, toward or away from a particular element, or in relations to the structure generally (for example, inwardly or outwardly).

I. Exemplary Electrode Configurations

The demand for sustainable chemical production is rapidly increasing as carbon-emission regulations tighten. At the same time, the rapid global influx of solar and wind energy facilitates decarbonized chemical synthesis routes via electrification. In particular, electrochemical manufacturing is a promising way to produce valuable chemical commodities, powered by renewable energy. A substantial fraction of these envisioned decarbonized electrochemical pathways involves gaseous reactants, which require electrodes that incorporate GDLs in continuous flowing electrolyzers to mitigate low solubility and the slow diffusion of reactants. The most prominent example of this technology in recent work is for the electrochemical CO₂ reduction reaction (CO2RR), where GDLs enable conversion of CO₂ into high-value products at much greater current densities, compared to standard electrodes in batch cells. This is achieved by porous channels in the GDL transporting reactants from a gas stream to continuously replenish CO₂ in the aqueous phase near the electrode-electrolyte interface. The GDL also acts as a conductive current collector to facilitate electron transfer through the external circuit, making porous carbon paper the traditional GDL material of choice. The major challenge for this technology, however, is to prevent the liquid electrolyte from filling the gas channels and increasing mass transport resistance of gaseous reactant to the electrocatalyst surface—this is known as “flooding” in the GDL. This often causes undesired side reactions, and it decreases energy efficiency. In traditional carbon-based GDLs, flooding tends to occur at high reaction rates after several hours of operation, posing a barrier to desired performance for industrial chemicals manufacturing.

Recently, porous polytetrafluoroethylene (PTFE) membranes were used to replace carbon-based GDLs, taking advantage of the chemical stability and inherent hydrophobicity of PTFE. Due to the non-conductive nature of PTFE, magnetron sputtering of metal layers was required in previous work to impart conductivity, and this strategy shows enhanced catalytic performance and improved stability in CO2RR when the metal layer is catalytically suitable for the desired reaction. Despite these advances, the typical preparation method of sputtering metal with a thickness of 300-500 nanometers (nm) results in complications and drawbacks for general use in electrochemical devices. For example, these thin metal films coated on porous PTFE still suffer from low electrical conductivity, requiring additional processing steps to improve performance, like adding additional current collecting layers composed of carbon, thick copper, polymer-coated grids, or aluminum. These methods require several additional steps to make current collecting layers, and the metal or carbon layers can catalyze reactions, possibly resulting in unwanted side products. The drawback of PTFE GDLs that impedes progress for CO2RR and other decarbonized electrochemical reactions, is its severe limitation on electrocatalyst composition and morphology. Rational design of electrocatalysts, relying on structure-function relationships, has resulted in improved performances that employ various metal and alloy particles with a range of sizes, compositions, morphologies, and preferentially exposed facets. But exploiting these concepts in the current configuration of porous PTFE GDL is essentially impossible when conductivity is established by a sputtered contiguous metal film. Thus, enabling the best catalyst performance in combination with the robust and hydrophobic porous PTFE GDL requires a non-metal conductive layer, which will not contribute to the reaction, is compatible with any electrocatalyst motif, and maintains desired gas diffusion properties. To that end, described herein is an improved GDL structure composed of a porous conductive polymer layer assembled on a microporous PTFE layer in the first demonstration of a self-conductive PTFE-based all-polymer GDL.

Poly(3,4-ethylenedioxythiophene) (PEDOT) was utilized as the conductive layer due to its high electrical conductivity, mechanical durability, and successful implementation for diverse applications. Moreover, PEDOT is low-cost and has wide availability. Recent work also employs PEDOT for electrocatalytic reactions in multiple ways, for example, as an anchoring structure or coordination modifier, but the usage of PEDOT as a GDL component on PTFE has not been explored. PEDOT can also be fabricated in any form factor, making it a suitable choice to add electrical conductivity, while maintaining the original properties of the PTFE GDL.

As will be discussed below with the reference to the supplementary drawings in respect of the present disclosure, PEDOT doped with different dopants including hexafluorophosphate (PF₆⁻) (PEDOT:PF₆), perchlorate (ClO₄⁻) (PEDOT:ClO₄), trifluoromethanesulfonate (CF₃SO₃⁻) (PEDOT:CF₃SO₃), and tetrafluoroborate (BF₄⁻) (PEDOT:BF₄), was first synthesized on commercial PTFE membranes to form a thin, porous, and electrically conductive layer via electropolymerization, resulting in a PEDOT-coated PTFE (PEDOT-PTFE) GDL. The self-conductive GDL was then evaluated in a continuous flowing electrolyzer, with CO2RR as a probe reaction. The CO2RR performance of nanoparticle electrocatalysts assembled in this configuration was evaluated in acidic, neutral, and alkaline electrolytes. The PEDOT-PTFE GDLs show comparable results with those of the commercial carbon-based GDL, Sigracet® 22bb, during short-term testing. Lastly, CO2RR stability tests were executed at industrially relevant current densities (i.e., −100 mA·cm⁻² and −200 mA·cm⁻²), in which PEDOT-PTFE GDLs exhibited high resistance to electrolyte flooding compared to the carbon-based GDL in all electrolytes tested. These results highlight that the PEDOT layer imparts sufficient electrical conductivity to enable reaction on electrocatalyst nanoparticles, while maintaining the best properties of robust porous PTFE. Thus, PEDOT-PTFE can be widely used for any gas-fed electrocatalytic reactions requiring the application of a GDL, while providing better stability than carbon-based GDLs.

FIG. 1 illustrates a schematic diagram of an exemplary method 100 for producing a PEDOT-PTFE GDL 200 comprising an electrochemically synthesized PEDOT layer 210 on a PTFE substrate 220. As shown in FIG. 1, the electrochemically synthesized PEDOT 210 on the PTFE substrate 220 (PEDOT-PTFE GDL 200) can be fabricated through a multi-step process. A corresponding photograph of the resulting structure at each step is shown in FIG. 1.

At step S110 of the method 100, a porous PTFE substrate 220 is provided as the foundational hydrophobic support layer. In some embodiments, the porous PTFE substrate 220 can be a microporous membrane. In some embodiments, the PTFE substrate 220 may comprise the microporous PTFE with a pore size between about 0.1 μm and about 5.0 μm, providing inherent hydrophobicity and robust gas diffusion characteristics.

After providing the porous PTFE substrate 220, at step S120, a thin metallic layer 230 is deposited onto the PTFE substrate 220. In some embodiments, the metallic layer 230 is deposited onto the PTFE substrate 220 via thermal evaporation, using thermal evaporation equipment, to provide a metal-coated PTFE structure 240. In possible alternative embodiments, the deposition of the metallic layer can be achieved by sputtering, electron-beam evaporation, chemical vapor deposition, or other thin-film deposition techniques.

In some embodiments, the metallic layer 230 deposited on the porous PTFE substrate 220 may include conductive metals selected from gold, platinum, palladium, copper, nickel, or combinations thereof. The choice of metal can be selected based on deposition compatibility, ease of removal, or availability. Preferably, the metallic layer 230 is a layer of gold.

In some embodiments, the thickness of the thin metallic layer 230 may range from about 50 nm to about 300 nm. Thinner films may be advantageous for rapid removal by etching, while thicker films may provide improved conductivity during the polymerization step. In preferable embodiments, the metallic layer 230 has a thickness from about 100 nm to about 200 nm, and more preferably of about 150 nm, which ensures efficient conductivity for initiating electropolymerization.

In some particular embodiments, for implementing the step S120, the porous PTFE membrane 220 with polypropylene backer (such as, e.g., QL822, Sterlitech®) can be first placed in a thermal evaporator installed in an N₂ atmosphere glovebox, and then, a gold layer with a thickness of about 150 nm can be deposited on the PTFE side at a pressure of about 10⁻⁶ bar, with the deposition rate of 1.5 Å·s⁻¹.

Next, at step S130, 3,4-ethylenedioxythiophene (EDOT) is electrochemically oxidized on the metal-coated PTFE membrane structure 240 to initiate polymerization, thus forming a thin and porous PEDOT layer 210 over top of the porous PTFE layer 220. Particularly, in some embodiments of step S130, an electrolyte solution containing the monomer EDOT and a dopant can be introduced into an electrochemical cell. In some embodiments, the electrolyte composition can contain the dopant selected from PF₆⁻, ClO₄⁻, CF₃SO₃⁻, and BF₄⁻. In some preferable embodiments, the dopant is PF₆⁻, ClO₄⁻ or BF₄⁻. In some more preferable embodiments, the dopant is ClO₄⁻. In some other embodiments, the electrolyte composition can be varied to include additional one or more dopant ions, preferably selected from PF₆⁻, ClO₄⁻, CF₃SO₃⁻, and BF₄⁻, in order to tune the electrical conductivity, morphology, and/or hydrophobicity of the PEDOT layer 210.

The metal-coated PTFE structure 240 can serve as the working electrode, while a counter electrode and a reference electrode can be provided to complete the three-electrode configuration. A constant potential can be applied to oxidize EDOT monomers at the metallic layer 230, initiating polymerization and forming a porous PEDOT network 210. During electropolymerization, the PEDOT chains interconnect, forming a porous conductive layer 210 that covers the PTFE substrate 220. The interconnected morphology creates void spaces that preserve gas permeability while imparting electrical conductivity. One exemplary implementation of the electrochemical cell setup and a corresponding electropolymerization process is discussed below with reference to FIG. 2.

In some alternative embodiments, the electropolymerization step S130 can be carried out using a potentiostatic, galvanostatic, or pulsed electrochemical regime. For example, pulsed potential waveforms may be applied to tailor the morphology, porosity, or conductivity of the resulting PEDOT layer 210.

Subsequently, at step S140, the metallic layer 230 may be selectively dissolved by exposure to a suitable chemical agent (chemical etchant) while keeping the PEDOT layer 210 attached to the top of the PTFE membrane 220. In some preferable embodiments, the chemical agent used can be aqua regia. In some alternative embodiments, the metal layer removal step may be carried out with etchants other than aqua regia, such as iodine-based solutions, thiourea solutions, or other halogen-based etchants, or simple base and/or acid. Multiple sequential etching steps may be employed to ensure thorough removal while preserving the PEDOT structure.

This step S140 removes most of the metallic underlayer 230 while preserving the PEDOT layer 210, which remains adhered to the PTFE substrate 220. Residual metal (such as gold) species may remain in trace amounts, but their presence is minimal and does not interfere with the conductivity or chemical stability of the resulting structure 200.

In some embodiments, after electropolymerization and removal of the metallic layer, the resulting PEDOT-PTFE structure 200 can be subjected to thermal annealing, plasma treatment, or chemical post-treatments to enhance adhesion, conductivity, or long-term stability in electrochemical environments.

FIG. 2 illustrates an exemplary embodiment of an electrochemical cell configuration setup 300 which can be utilized for implementing step S130 of electrochemically oxidizing the EDOT on the metal-coated PTFE membrane structure 240 to initiate polymerization, and which enables synthesis of the PEDOT 210 directly on the PTFE membrane 220.

In one particular use of the setup 300 PEDOT:PF₆ is produced, and prior to the electropolymerization step S130, the Au-coated PTFE membrane structure 240 is cut into small rectangular pieces having dimensions of about 2.0×3.3 cm². To minimize potential drop through the Au-coated PTFE structure 240 for the electropolymerization step S130, an edge 242 of each side of each piece is painted with a conductive silver (Ag) paint (such as provided by SPI Supplies®), which serves to reduce resistance and ensure uniform potential distribution. A copper (Cu) tape strip 244 is put on the top side of the Au-coated PTFE structure 240, functioning as the current collector. Then, to confine polymerization only to the desired central region of the electrode, a masking layer 246, such as Kapton® tape, is used to mask the Ag-covered edge areas 242, thereby exposing only a defined active window 248, which is limited to an area with dimensions of about 1.2×2.5 cm². This approach prevents undesired deposition of PEDOT:PF₆ in non-functional regions of the metal-coated PTFE structure 240. In some embodiments, the masking of the metal-coated PTFE structure 240 with Kapton® tape can be replaced or supplemented by other masking approaches, such as photolithographic patterning, polymer resist coatings, or stencil-based masking, to create customized electrode geometries.

Then, a three-electrode configuration may be applied for the electropolymerization of PEDOT:PF₆⁻, which is composed of a working electrode 310, a counter electrode 320, and a reference electrode 330. In this exemplary configuration, the Au-coated PTFE membrane structure 240 serves as the working electrode 310, a sheet of a conductive carbon paper serves as the counter electrode, and Ag/AgCl immersed in saturated KCl serves as the reference electrode 330.

All electrodes 310, 320 and 330 can be immersed in an electrolyte solution 340. In the exemplary embodiment of FIG. 2, the electrolyte solution 340 consists of about 100 mL of acetonitrile (HPLC grade, Fisher Scientific) containing 0.01 M 3,4-ethylenedioxythiophene (EDOT) and 0.01 M 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF₆). The reaction vessel can be sealed with parafilm to minimize, preferably prevent, evaporation of acetonitrile.

In this exemplary embodiment, for the electropolymerization of PEDOT:PF₆ layer on the Au-coated PTFE membrane structure 240, a potentiostat such as VSP potentiostat (e.g., available from BioLogic®), is utilized to apply constant potential at 2.1 V versus Ag/AgCl. The process is continued until a total charge of about 200 mC is reached, which yields a thin and porous PEDOT:PF₆ layer 210 uniformly deposited over the exposed surface of the PTFE substrate 220. Though reaching of the total charge of about 200 mC is preferable, in some embodiments, the total charge passed during the polymerization may still be varied from about 50 mC to about 1000 mC, thereby allowing producing PEDOT films 210 of different thicknesses.

In some embodiments of the PEDOT layer 210 may have a thickness ranging from about 0.3 μm to 2 μm, preferably from 0.8 μm to 1.5 μm, depending on the desired balance between electrical conductivity and gas permeability. One of the most preferable thickness values for the PEDOT layer 210 is about 1 μm.

Following the electropolymerization, the resulting electrode can then be thoroughly washed with acetonitrile and dried in a chemical hood to remove residual solvent (i.e., monomer and electrolyte). After drying, the silver-painted edges 242 of all sides of the rectangular pieces can be trimmed to remove the Ag-covered areas. Subsequently, the electrodes 310, 320 and 330 can be immersed in the etchant solution, which can be aqua regia, for about 10 minutes to selectively dissolve the underlying metal (such as gold) layer 230, and repeated. This etching process may be repeated at least one (preferably two, or more) additional times to ensure effective removal of the metallic underlayer, while leaving the PEDOT layer 210 intact and adhered to the PTFE membrane 220.

The final products can then be washed with water (such as deionized water), dried in an oven, in some embodiments, at about 60° C. for about 30 minutes, and then placed in a fume hood. The resulting PEDOT-PTFE structure 200 comprises a robust, porous PTFE substrate 220 directly bonded to a conductive PEDOT network 210, which together form an all-polymer gas diffusion layer exhibiting both hydrophobicity and electrical conductivity.

In some alternative embodiments, the electropolymerization step S130 using the setup 300 can be applied to production of PEDOT:ClO₄. For this purpose, BMIMPF₆ can be replaced by LiClO₄ in the electrolyte solution 340.

In some alternative embodiments, the electropolymerization step S130 using the setup 300 can be applied to production of PEDOT:BF₄⁻. For this purpose, BMIMPF₆ can be replaced by LiBF₄ in the electrolyte solution 340.

In some alternative embodiments, the electropolymerization step S130 using the setup 300 can be applied to production of PEDOT:CF₃SO₃⁻. For this purpose, BMIMPF₆ can be replaced by LiCF₃SO₃ in the electrolyte solution 340.

A schematic illustration and microscopic characterization of the completed structure of a layered PEDOT-PTFE GDL structure 200, which may be formed into a gas diffusion electrode, is shown in FIG. 3A. In this exemplary embodiment, the PEDOT-PTFE GDL structure 200 is based on PEDOT doped with PF₆⁻, wherein a gold (Au) layer was used as the metallic layer 230 for electrochemical oxidation, and wherein the setup 300 of FIG. 2 was used for production of the PEDOT-PTFE GDL structure 200.

Particularly, two different coupled layers, PEDOT layer 210 and PTFE layer 220, are shown in FIG. 3A. The right upper and lower enlarged images in FIG. 3A represent respectively, a scanning electron microscopy (SEM) top-view image of the PEDOT-PTFE surface 250, and a SEM image of the PEDOT-PTFE cross-section 260 obtained by focused ion beam (FIB) milling.

After the electropolymerization, the morphology of the top (exterior-facing) GDL surface 250 was altered, as shown in the right upper, magnified SEM enlarged image of FIG. 3A. The image shows a porous PEDOT layer 210 comprising thin PEDOT chains (fibers) 252 interconnected to each other to form a contiguous polymeric conductive network. The PEDOT fibers 252 are shown to interconnect into a web-like morphology that creates void spaces 254. The void spaces 254 maintain gas permeability while simultaneously enabling electrical conductivity (e.g., 1590±140 S·cm⁻¹) through the PEDOT network. Such structure is likely characteristic of the electropolymerized PEDOT with PF₆⁻ dopant, as similarly reported for PEDOT:PF₆. Thus, electropolymerization creates a conductive porous PEDOT layer 210 on top of inherently hydrophobic PTFE 220, thus forming an improved GDL 200.

The PEDOT structure 210 and its interface with PTFE 220 were experimentally probed with cross-sectional electron microscopy. The SEM images of the GDL cross-section 260 obtained by the focused ion beam (FIB) are shown in the right lower magnified and enlarged portion of FIG. 3A. The two distinct PEDOT and PTFE layers 210 and 220 are easily distinguished. In the cross-section of the PEDOT-PTFE structure 200, the PEDOT layer 210 is clearly seen to be deposited directly on top of the PTFE substrate 210. The top layer shows the interconnected PEDOT network with the void spaces 254 distributed evenly throughout the cross-section thickness of the PEDOT layer 210. The bottom layer exhibits image distortion and contrast irregularity, general characteristics of non-conductive material, consistent with the non-conductive nature of PTFE 220 and its insulating properties. Namely, the PTFE substrate 220 appears as a fibrous, porous layer with reduced contrast typical of the non-conductive materials. The thickness of the PEDOT layer 210 is about 1 μm as measured based on the cross-section SEM image. The layered arrangement of the PEDOT 210 and PTFE 220 illustrated in FIG. 3A demonstrates achievement of an all-polymer bilayer gas diffusion electrode 200 as a result of the production method 100 described above.

In some embodiments, the PEDOT fibers 252 forming the conductive layer may have diameters between about 10 nm and about 30 nm, with average values of about 15-25 nm, preferably about 20 nm. Smaller fiber diameters can enhance surface area and porosity, while larger fibers may improve mechanical robustness.

In some embodiments, the porous morphology of the PEDOT layer 210 may include the interconnected void spaces 254 having characteristic dimensions between about 10 nm and about 500 nm, facilitating efficient gas transport while maintaining electronic conduction pathways.

To further probe the structure of the PEDOT fibers 252, a cross-sectional high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of the PEDOT layer 210 was obtained as shown in FIG. 3B for the PEDOT-PTFE GDL 200 shown in FIG. 3A. This high-resolution image shows characteristic diameters of the PEDOT fibers 252, which were measured to be about 20 nm, and are distributed randomly throughout the network. Inter-fiber voids 254 are clearly visible, confirming the porous morphology that facilitates gas transport while maintaining electrical conductivity across the electrode.

FIG. 3C presents an overlapped energy-dispersive X-ray (EDX) elemental mapping image of the PEDOT-PTFE structure 200. The map highlights the distribution of carbon atoms 256 (shown with darker grey in the image) and sulfur atoms 258 (shown with lighter grey in the image) within the PEDOT fibers 252. The overlapped energy dispersive X-ray (EDX) elemental mapping image of carbon and sulfur shown in FIG. 3C indicates that the fiber composition is consistent with that of sulfur-containing PEDOT. The co-localization of carbon and sulfur signals confirms the presence of PEDOT polymer chains, with sulfur being characteristic of the thiophene units. The absence of signals in the PTFE substrate region further corroborates the distinction between the conductive PEDOT layer 210 and the non-conductive PTFE layer 220. In some embodiments, the adhesion between the PEDOT layer 210 and the PTFE substrate 220 may be enhanced by controlling polymerization time, potential, current, or electrolyte composition, thereby producing a mechanically stable bilayer electrode resistant to delamination during long-term electrochemical operation.

In some embodiments, cross-sectional imaging techniques such as FIB-SEM or HAADF-STEM may be used to monitor and control the morphology of the PEDOT layer 210 during the PEDOT-PTFE GDL production process, ensuring uniform thickness and continuous coverage across the PTFE substrate.

In some embodiments, the PEDOT layer 210 may exhibit an electrical conductivity of at least 500 S/cm, more preferably at least 1000 S/cm, and in some cases about 1500 S/cm or greater, thereby enabling efficient current collection in electrochemical devices.

II. Experimental Spectroscopic Characterizations of PF₆⁻-Doped PEDOT-PTFE

The results of Raman, Fourier transform infrared (FTIR), and X-ray photoemission spectroscopy (XPS) are shown in FIGS. 4A-C to elucidate structural features of PEDOT:PF₆ and confirm its formation on the PTFE membrane.

As depicted in FIG. 4A, the Raman spectrum shows three bands consistent with a PTFE signature, likely due to laser penetration through the thin and porous PEDOT layer 210. The spectrum mainly depicts some bands originating from oxyethylene and thiophene rings and characteristic vibrational bands of PEDOT, particularly within the range from 1250 to 1560 cm⁻¹, which is associated with the stretching between C_α and C_β stretching modes of the thiophene backbone. Additional peaks correspond to oxyethylene vibrations, confirming incorporation of EDOT units into the polymer chain. The observed spectral shifts and broadening are consistent with conjugation and doping effects within the PEDOT structure 210, thereby verifying that PEDOT has been successfully polymerized on the PTFE layer 220. The persistence of some PTFE peaks indicates partial laser penetration through the thin PEDOT layer 210.

Moreover, as shown in FIG. 4B, the FTIR spectrum further corroborates the formation of PEDOT 210 on the PTFE membrane 220. Multiple peaks are observed corresponding to C—C (carbon-carbon) bond stretching, C—S (carbon-sulfur) bond stretching, and vibrations within the thiophene and oxyethylene rings characteristic of PEDOT. Notably, characteristic peaks for the C—O—C stretching of the ethylenedioxy group appear at approximately 1100 cm⁻¹, confirming the incorporation of EDOT monomer units into the PEDOT backbone. The inset shows the spectrum of pristine PTFE, with dominant fluorocarbon-related vibrations. Comparison between the spectra indicates that the PEDOT layer 210 effectively covers the PTFE 220, introducing new vibrational modes while retaining underlying fluorocarbon features of PTFE. These results suggest a characteristic interfacial contact between PEDOT and PTFE layers 210, 220, which is advantageous for mechanical stability and charge transport.

Accordingly, both Raman and FTIR spectra illustrated in respective FIGS. 4A and 4B suggest the successful formation of the thin PEDOT layer 210 effectively covering the PTFE membrane 220.

In some embodiments, the FTIR spectra may be used to estimate the thickness of the PEDOT layer 210 by analyzing relative intensities of PEDOT versus PTFE peaks, thereby enabling non-destructive quality control.

The XPS survey spectrum shown in FIG. 4C provides the elemental composition of PEDOT layer 210 formed at the surface of the PTFE layer 220. The spectrum indicates the existence of six components (carbon (C), fluorine (F), oxygen (O), sulfur(S), and trace amounts of chlorine (Cl) and gold (Au)). The sulfur signal arises from thiophene groups within PEDOT 210, confirming its chemical identity. The presence of fluorine confirms the underlying PTFE scaffold 220. Minor residual gold indicates that small quantities of Au species remain embedded after etching, but these do not interfere with the functionality of the PEDOT-PTFE electrode 200. The atomic percentages of these trace elements are minimal and do not adversely affect electrode performance. In addition, nitrogen (N) species were observed, but this is not expected to alter the physical properties of PEDOT. The combination of the spectroscopic results indicates that PEDOT maintained its expected chemical and structural composition after aqua regia treatment.

In some embodiments, high-resolution XPS scans may be performed to quantify the oxidation state of sulfur within the PEDOT layer 210, which can provide information about polymer chain doping level and long-term stability.

In some embodiments, spectroscopic characterization may be employed not only as a verification step but also as a process control measure during scale-up manufacturing, ensuring reproducibility of PEDOT layer deposition across multiple PTFE membranes 220.

III. Comparisons of CO2RR Performances Between PF₆⁻-Doped PEDOT-PTFE And Carbon-Based GDL

To demonstrate the utility of PEDOT-PTFE 200 as an effective gas diffusion layer, its performance was compared to that of commercial carbon-based Sigracet® 22bb in a continuous-flow electrochemical cell. CO2RR on Ag was selected as a representative test case, because Ag selectively produces CO as the only carbon product from the gaseous CO₂ reactant. Thus, tracking the faradaic efficiency in a flow cell with Ag electrocatalysts indicates the mass transfer performance-more CO means CO₂ mass transport is facilitated through the GDL, while more H₂ indicates gas channels may be flooded, resulting in reaction of the aqueous electrolyte. This simple system enables direct evaluation of GDL mass transport properties during a catalytic reaction. To test the CO2RR activity of Ag nanoparticles (Ag NPs) loaded on PEDOT-PTFE 200 (comprising PEDOT:PF₆ layer) and Sigracet® 22bb containing 5 wt. % PTFE in its microporous layer, the drop-casting method was used to introduce electrocatalyst to the GDLs, and the performance was measured in the lab-made custom flow cell, controlled by a SP-200 potentiostat (such as available by BioLogic®). Specifically, the catalyst ink, containing 3 mg of Ag NPs (20-40 nm, available by Fisher Scientific®), 350 μL of deionized (DI) water (18.2 MΩ), 150 μL of 2-propanol (HPLC grade, available by Fisher Scientific®), and 30 μL of 5 wt % Nafion dispersion (such as D520CS, available by Ion Power®), was first sonicated at room temperature (RT) for 30 min. Then, a small portion of the ink was cast on the exposed GDL area (1.2×2.5 cm²), masked by Kapton tape for both PEDOT-PTFE 200 and Sigracet® 22bb. GDLs were carefully moved to the oven at 70° C. for drying and repeated until 1.0 mg·cm⁻² of catalyst loading was achieved.

The GDL was then placed in the flow cell with a separator, counter, and reference electrodes. For all electrocatalytic measurements, Hg/Hg₂SO₄ in saturated K₂SO₄ was used as a reference, and the potential versus reversible hydrogen electrode (RHE) was calculated by the following equation:

E ⁡ ( V ⁢ vs . RHE ) = E ⁡ ( V ⁢ vs . Hg / Hg 2 ⁢ SO 4 ) + 0.62 V + 0.0591 V × pH .

The resistance between the reference and working electrode was estimated by constant potential electrochemical impedance spectroscopy (EIS) at open circuit potential, and 85% was compensated by the EC Lab® software. For the CO2RR activity tests, 1 mg·cm⁻² of IrO₂ (99.99%, Fisher Scientific®) on Sigracet® 22bb was utilized as a counter electrode.

Three types of electrolytes were prepared for acidic, neutral, and alkaline cases. The acidic electrolyte was prepared by diluting H₂SO₄ (ACS grade, Fisher Scientific®) with K₂SO₄ (99+%, Thermo Scientific®) to target concentrations. KHCO₃ was used as the neutral electrolyte and obtained by constant bubbling of CO₂ in K₂CO3 (99.997%, Thermo Scientific) solution at least overnight. The alkaline electrolyte was prepared by diluting KOH (99.99%, Sigma Aldrich®) to a target concentration. For separator, Fumasep® FAS-50 for neutral and alkaline electrolytes and Nafion® 212 for the acidic electrolyte were placed between the catholyte and anolyte chambers.

Chronopotentiometry (CP) at different current densities (−20, −50, −100, −150, and −200 mA·cm⁻²) was used for measuring the CO2RR performance of Ag NPs on GDLs. Each constant current step was held for 20 min. The electrolyte was circulated at a constant rate of 5 mL′min⁻¹ by a peristaltic pump for both catholyte and anolyte and the circulated volume for each channel was 15 mL. CO₂ (99.99%, Indiana Oxygen®) controlled by a mass flow controller (Alicat®) was continuously fed to the gas chamber with the mass flow rate at 20 standard cm³·min⁻¹ (sccm) during CO2RR measurements. The in-line gas chromatograph (GC, Agilent® 7890A) was equipped with a thermal conductivity detector (TCD) and flame ionization detector (FID) with Jetanizer® (Activated Research Company) for the detection of gas products. Faradaic efficiencies for gas products were calculated using the following equations:

FE ⁡ ( % ) = v ⁢ n ⁢ N e ⁢ P ⁢ F R ⁢ T i × 1 ⁢ 0 ⁢ 0

where v is the mass flow rate of CO₂ (20 sccm), n is the concentration obtained from GC, N_e is the number of electrons used for a certain product. P is atmospheric pressure, F is faradaic constant, R is gas constant, T is the temperature (in here, equal to 293 K), and i is the current density.

CP at two different current densities (−100 and −200 mA·cm⁻²) was carried out for evaluation of the stability of GDLs in different electrolytes. The general experimental setup was similar to the procedure mentioned above except for the counter electrode and the circulated volume of electrolytes. For stability tests, Pt foil (0.05 mm thick, 99.99%, Thermo Scientific®) was used instead of IrO₂ on Sigracet® 22bb, and the volume of electrolyte circulating the catholyte and anolyte chambers was increased to 30-50 mL.

The CO2RR performance of the PEDOT-PTFE 200 with Ag nanoparticle (AgNP) mass loading of 1.0 mg·cm⁻² in different electrolytes is shown in FIGS. 5A-5F, to examine its performance compared to a carbon-based GDL (Sigracet® 22bb). Three electrolytes were selected: 0.8 M KOH, 0.8 M KHCO₃, and 0.01 M H₂SO₄ with 0.4 M K₂SO₄, representing alkaline electrolyte (see FIG. 5D), neutral electrolyte (see FIG. 5E), and acidic electrolyte (see FIG. 5F), respectively, to show the versatility of the PEDOT-PTFE GDL 200 across a range of common aqueous electrolysis conditions. Sigracet® 22bb was also tested under identical conditions (see FIG. 5A for alkaline electrolyte, FIG. 5B for neutral electrolyte, and FIG. 5C for acidic electrolyte) and compared as a representative of a widely used carbon-based GDL. During constant current experiments, PEDOT-PTFE 200 showed comparable results to Sigracet® 22bb in terms of potential required to achieve a specific current and product selectivity for all electrolytes, as illustrated in FIGS. 5A-5F. This indicates that PEDOT-PTFE 200 imparts sufficient conductivity and gas permeability to perform efficient CO2RR without the need for a contiguous sputtered metal layer. Additionally, conducting the same experiment on bare PEDOT-PTFE 200 without catalyst present showed negligible electrocatalytic reaction, indicating the contribution from PEDOT-PTFE 200 and remaining Au species can be excluded from observed results. The faradaic efficiency (FE) of H₂ from Ag NP on PEDOT-PTFE 200 was less than 1.5% in acidic and alkaline electrolytes up to currents of −150 mA·cm⁻². whereas H₂ FE exceeded 1.7% in the case of Sigracet® 22bb in the same regime (as shown in FIGS. 5A, 5C, 5D, 5F). At the high current of −200 mA·cm⁻², PEDOT-PTFE 200 exhibited slightly increased H₂ FE (ca. 1.8%), which was still on-par with Sigracet® 22bb at that current. The H₂ FE was higher on PEDOT-PTFE 200 in the neutral electrolyte (between 1.9 and 4.2%), but it was still comparable to Sigracet® 22bb (as shown in FIGS. 5B, 5E). Other than H₂, CO was the only gas product detected in all current densities, and the CO faradaic efficiencies at each current density were comparable for both GDLs in all electrolytes. This confirms that the PEDOT-PTFE structure 200 can sustain selective CO₂ reduction across a broad pH window, thereby making it a versatile candidate for integration into different types of electrolyzer systems. In particular, the stable CO selectivity across conditions demonstrates that the PEDOT layer 210 does not interfere with catalytic activity of Ag nanoparticles but rather provides a robust conductive and permeable support.

The sum of faradaic efficiencies reveals the effectiveness of product mass transfer through the GDL. Assuming no liquid products are formed, FE should sum to 100% for each measurement. A lower summation means some gas products were not efficiently transported through the GDL and were instead trapped in the catholyte stream and not detected by GC. In all cases, the FE summation decreases at higher current densities, but this effect is more pronounced on PEDOT-PTFE 200. This result is expected, because a higher flux of gas products will experience greater mass transfer resistance through GDL pores. While the PEDOT-PTFE configuration 200 performs exceptionally well at low current densities, the pore size and structure of the PEDOT layer 210 has not yet been optimized, leading to greater diffusion barriers at high current density. Specifically, the structure of PEDOT-PTFE 200 consists of two microporous layers in series—PEDOT 210 and PTFE 220—to achieve both conductivity and hydrophobicity. This adds resistance to gas diffusion, but this effect could be mitigated by controlling the PEDOT pore size and structure. Such tuning may be achieved by modifying electropolymerization conditions or dopant identity. Even without such optimization, the PEDOT-PTFE 200 captures roughly 90% of gas products at high current density, proving its efficacy as a GDL. This finding suggests that future improvements in porosity engineering and layer thickness control could allow PEDOT-PTFE GDL 200 to surpass conventional carbon-based GDLs, particularly under industrially relevant high-current operation. Furthermore, the observed balance between hydrophobicity from the PTFE layer 220 and conductivity from the PEDOT layer 210 highlights a synergistic effect that can be leveraged for other gas-evolving electrochemical reactions beyond CO2RR, such as hydrogen evolution or oxygen reduction.

IV. Stability Comparisons Between PF₆⁻-Doped PEDOT-PTFE and Carbon-Based GDL

A benefit of a PTFE-based GDL is enhanced stability against electrolyte flooding, so 20 hour CO2RR stability tests were carried out to evaluate the durability of PEDOT-PTFE GDL 200 compared to Sigracet® 22bb in alkaline, neutral, and acidic electrolytes. The results in FIGS. 6A-B show CO faradaic efficiency (FE) and CO₂ mass flow rate at constant current over time. Current densities of −100 and −200 mA·cm⁻² were chosen for stability tests, because they represent industrially relevant operating conditions. Namely, FIG. 6A shows stability test data of CO2RR at −100 mA·cm⁻² using PEDOT-PTFE 200 and Sigracet®22bb. FIG. 6B shows similar stability test data at −200 mA·cm 2, again comparing PEDOT-PTFE 200 to Sigracet® 22bb. These conditions also create an environment conducive to electrolyte flooding phenomena observed for carbon-based GDL failure. Electrolyte flooding becomes severe at high current due to the decrease in breakthrough pressure, attributed to electrolyte carbonation and salt formation, leading to decrease of hydrophobicity of GDLs. In these tests, the CO FE provides an evaluation of the extent of gas channel blockage (either by flooding or some other mechanism), where it is assumed that decreased CO FE in favor of H₂ results from an increase in the diffusion path length of reactant CO₂ from the gas-liquid interface to the liquid-electrocatalyst interface. The flowrate of CO₂ through the gas channel, measured by the mass flow controller, was also used as a GDL flooding indicator. Observation of uncontrolled flow rate oscillations suggested severe flooding of the entire GDL thickness, resulting in liquid impeding CO₂ flow on the backside of the GDL.

In alkaline environment (0.8 M KOH), CO FE decreases continuously over time for both GDLs, but this is substantially mitigated by PEDOT-PTFE 200. The CO FE decreases below 60% on Sigracet® 22bb in less than 16 hours of operation at both −100 and −200 mA·cm⁻², at which point the experiment was stopped. On the other hand, the PEDOT-PTFE 200 enabled high CO FE for the duration of the 20-hour test, maintaining 93.9% at −100 mA·cm⁻² and 90.0% at −200 mA·cm⁻² at the end of testing. This clearly shows the improved resistance to flooding for the PEDOT-PTFE configuration 200 in alkaline electrolyte. It should be noted, however, that severe flooding affecting the gas flow rate was not observed for either GDL in these conditions.

The stability tests in neutral electrolyte (0.8 M KHCO₃) showed different trends compared to the alkaline case, but still demonstrated enhanced durability of PEDOT-PTFE 200 over Sigracet® 22bb. Notably, Sigracet® 22bb maintained very high CO FE for both currents (near 100%) in the first four hours, while PEDOT-PTFE 200 decreased moderately from 90% to 80% in the same time. Sigracet® 22bb held this performance until 16 hours at −100 mA·cm⁻², and then decreased continuously to the end of the 20-h test. This decrease was coincident with CO₂ flowrate oscillations, indicative of severe flooding. The same flowrate oscillations for Sigracet® 22bb were observed around 7 hours at −200 mA·cm⁻², which was followed by a precipitous drop in CO FE that is considered GDL failure. PEDOT-PTFE 200 avoided severe flooding under these conditions, showing stable CO FE for the duration of the 20-hour test at both currents. The performance enhancement of PEDOT-PTFE 200 is especially evident for-200 mA·cm⁻², where the Sigracet® 22bb flooded and failed at an early stage.

Stability tests in acidic electrolyte are of particular interest, because CO2RR in acidic conditions has much higher maximum theoretical energy efficiency than neutral or alkaline electrolytes. This is due to homogeneous consumption of CO₂ by hydroxide ions that occurs to a lesser extent in acids. FIGS. 6A-B show that performance with both GDLs exhibited lower CO FE over the stability tests in acid compared to the other electrolytes, but again, PEDOT-PTFE 200 outperformed the carbon-based GDL. Both GDLs showed similar performance under-100 mA·cm⁻², decreasing from 90% to 80% CO FE in the first 14 hours. Then, CO FE decreases rapidly for Sigracet® 22bb, while PEDOT-PTFE 200 continues a smooth but shallower decrease to the end of the 20-h test. The point at which Sigracet® 22bb performance rapidly decreases is accompanied by some moderate CO₂ flowrate oscillations, which may indicate severe flooding. These flowrate oscillations were not observed for PEDOT-PTFE 200. The same phenomenon was seen on Sigracet® 22bb at −200 mA·cm⁻², but it occurred after only 5 hours, and caused rapid GDL failure. This is in stark contrast to PEDOT-PTFE 200, which showed no signs of severe flooding for the 20-hour duration at the same current density. It is notable that the acidic condition did cause more and continuous reduction in CO FE on PEDOT-PTFE 200, compared to other electrolytes, and this may be related to low solubility of K₂SO₄ (12 g/100 mL H₂O) compared to the solubility of KHCO₃ (22.4 g/100 mL H₂O) and KOH (55 g/100 mL H₂O). High current densities could create high local ion concentrations near the electrode, resulting in salt precipitation that could impede CO₂ transport to the electrocatalyst. This problem should be addressable by judicious choice of supporting electrolyte as well as controlling PEDOT pore structures. Despite this, it remains clear that PEDOT-PTFE 200 is a more robust GDL option in the acidic electrolyte than Sigracet® 22bb.

All cases in FIGS. 6A-B exhibited CO FE decrease during operation, which can be attributed to three different phenomena: mild flooding, severe flooding, and pore blockage via solid precipitate (which can be exacerbated as a result of flooding). Considering precipitate formation should occur similarly on both GDLs in non-flooded conditions, this points to flooding as a differentiator of GDL stability. Severe flooding clearly afflicted Sigracet® 22bb's stability in neutral and acidic electrolytes, but the enhanced performance of PEDOT-PTFE 200 in alkaline electrolyte was not attributed to this same effect. Instead, mild flooding that does not fully penetrate the GDL thickness, could also reduce CO FE by increasing CO₂ mass transfer resistance. To analyze this effect, cross-section SEM/EDX was conducted after stability tests to detect traces of electrolyte or precipitates. The majority of K species was observed within remaining catalyst layer in PEDOT-PTFE 200, while K species were prominently distributed throughout the microporous layer of Sigracet® 22bb. Even though none of signals related to severe flooding were observed in some cases of Sigracet® 22bb, the results showed the presence of electrolyte penetration to GDL, considered mild flooding. This shows that PEDOT-PTFE 200 has higher resistance to any kind of flooding, whereas Sigracet® 22bb allows greater extent of flooding, leading to decreased GDL performance. This type of mild flooding can also cause compounding GDL degradation on carbon-based GDLs, which can also catalyze unwanted HER. Such reaction can decrease pressure needed for electrolyte breakthrough and can change wetting characteristics of the carbon, causing faster electrolyte flooding. PTFE, on the other hand, is highly hydrophobic and non-conductive, which prohibiting its participation in electrochemical reaction. This explains why PEDOT-PTFE GDL 200 did not show electrolyte flooding characteristics for all electrolytes regardless of current density whereas Sigracet® 22bb experienced extensive mild and severe electrolyte flooding. This makes PEDOT-PTFE 200 a suitable GDL candidate for extended operation.

As an example of its applicability in this regard, PEDOT-PTFE 200 was further tested at −100 mA·cm⁻² in the neutral electrolyte for much longer time (80 hours). PEDOT-PTFE 200 did not show any characteristics related to continuous electrolyte flooding for more than 3 days. The CO faradaic efficiency did decrease to 64.3% at 81 hours, with a corresponding increase in H₂ faradaic efficiency. This behavior suggests a decrease in catalyst detachment and/or gas diffusion ability, perhaps attributed to salt precipitation during the long-term operation, as opposed to the severe flooding that caused GDL failure for Sigracet® 22bb. Thus, PEDOT-PTFE 200 shows versatile application as a GDL across a range of operating conditions and electrolyte pH, making it a promising candidate to replace carbon-based GDLs. Its hydrophobic nature imparts enhanced stability against electrolyte flooding and the conductive polymer enables current collection without catalytic contributions from a sputtered metal layer.

Accordingly, as illustrated by the above exemplary and experimental data, PEDOT:PF₆ was synthesized as a conductive layer 210 on a PTFE membrane 220 through electropolymerization to create a self-conductive, hydrophobic, and robust GDL 200. The PEDOT layer 210 was composed of interconnected polymer chains 252, roughly 20 nm thick, that created a porous structure with about 1 μm thickness, suitable for efficient gas transport. With the addition of a catalyst layer, PEDOT-PTFE GDL 200 showed comparable results with carbon-based GDL for CO2RR in terms of faradaic efficiency and measured potential during short-term constant-current evaluation. In stability tests, PEDOT-PTFE GDL 200 exhibited superior stability of catalytic performance without electrolyte flooding across a wide range of electrolyte pH at industrially relevant currents. This is the first demonstration of a conductive. PTFE-based, all-polymer GDL, and it overcomes the most substantial challenges associated with previous iterations of carbon-based and PTFE-based GDLs.

IV. Performance of PEDOT-PTFE with Various Dopants

Similar performance tests were carried out in respect of PEDOT-PTFE doped with various other dopants (ClO₄⁻, CF₃SO₃⁻, and BF₄⁻). The CO2RR results of PEDOT-PTFE electropolymerized at 67 mC·cm⁻² with the said dopants revealed that PEDOT:ClO₄ achieved the highest CO FE across all current densities evaluated in the range from −50 to −400 mA·cm⁻², even maintaining a 73:9 selectivity for CO over H₂ at −400 mA·cm⁻². Among the other samples, CO FE of PEDOT:CF₃SO₃ was the lowest among the tested dopants. PEDOT:BF₄ (similarly to PEDOT:PF₆) maintained high CO FE to some extent until −150 mA·cm⁻², but a steep decrease in these values was observed at elevated current density. ClO₄⁻ dopant yielded the most porous structure allowing better gas transport and sustained high CO FE at elevated current regime up to −400 mA·cm⁻², namely, about 95% at −200 mA·cm⁻² and about 73% at −400 mA·cm⁻² using the commercial silver electrocatalyst. Therefore, ClO₄⁻ doped PEDOT exhibited superior performance due to the optimized physical structure, leading to the increased gas permeance and CO FE production during electrocatalytic CO₂ reduction.

Furthermore, the thickness and coverage of the PEDOT:ClO₄ layer were modified to optimize electrical conductivity, gas permeance, and hydrophobicity. The most promising condition was obtained at applied charge density of 33 mC·cm⁻² during electropolymerization of PEDOT with the ClO₄⁻ dopant. Finally, this best-performing candidate showed versatility with a wide-range of metal electrocatalysts and, through the stability tests, yielded more than 90% of CO FE for over 150 hours at −200 mA·cm⁻² and for more than 100 hours at −300 mA·cm⁻² during CO2RR on the commercially available Ag electrocatalysts. This emphasizes the importance of engineering the PEDOT structure for enabling efficient GDL performance and demonstrates the applicability of PEDOT-PTFE to enable stable electrocatalytic conversion of gas reactants in pursuit of electrified chemical manufacturing.

Further investigation for variation in PEDOT properties, for example, dopant modification and control of thickness and porosity, may lead to enhanced mass transport at high current with improved stability, enabling broad application of this configuration. The versatility of this design makes it suitable to improve the electrocatalytic performance of any device that involves gas-phase reactants and products, which is a step toward enabling decarbonized, electrified processes that are powered by the increasing availability of wind and solar energy.

While examples, one or more representative embodiments and specific forms of the disclosure have been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive or limiting. The description of particular features in one embodiment does not imply that those particular features are necessarily limited to that one embodiment. Some or all of the features of one embodiment can be used in combination with some or all of the features of other embodiments as would be understood by one of ordinary skill in the art, whether or not explicitly described as such. One or more exemplary embodiments have been shown and described, and all changes and modifications that come within the spirit of the disclosure are desired to be protected.