WATER ELECTROLYZER WITH CATION EXCHANGE MEMBRANE

Description

RELATED APPLICATIONS

This patent application claims the benefit under 35 U.S.C. § 119 (e) of U.S. Provisional Patent App. No. 63/735,540, filed Dec. 18, 2024, entitled “WATER ELECTROLYZER WITH CATION EXCHANGE MEMBRANE,” incorporated herein by reference in entirety.

BACKGROUND

Worldwide electricity generation is expected to double by 2050, in large part due to rapidly increasing demand from economically emerging nations. It is further expected that by 2050, two-thirds of the power generation will be from variable renewable electricity (VRE) generators, e.g., solar and wind, replacing the conventional power plants based on fossil fuels, i.e., coal, and natural gas. Temporally responsive electrical energy storage, including batteries and water electrolyzers, would be a key enabler in this transition, since solar and wind power are unpredictable and, unlike fossil-fueled power plants, cannot be turned on-or-off to meet demand. Thus, there is a need for storing excess renewable power generated during periods of low demand.

Hydrogen sources, and more specifically green hydrogen (H₂) from electrolysis of water, is expected to serve as the central link between a variable renewable electric (VRE) grid and hard to abate energy sectors, storing excess energy from VRE generators, solar and wind, when the electricity demand is low. Hydrogen may also be employed as a fuel in transportation and/or as a feedstock in the chemical industry, where when combined with captured or recycled CO₂, it can replace the conventional fossil feedstocks, petroleum and natural gas, or can provide green ammonia when combined with N₂ as a hydrogen carrier or as a fertilizer. It can also potentially help to decarbonize other large-scale industries including steel and cement manufacture. Candidate technologies poised to be most widely employed in such green H₂ generation include low-temperature electrolysis (LTE) of water, because of its technological maturity and high efficiency, and its ability to potentially directly use DC from solar and wind generators. Commercially mature LTE technologies include: 1) the liquid alkaline-water electrolyzer (L-AWE) involving a porous diaphragm (PD) and an alkaline water electrolyte feed; and 2) the proton-exchange membrane water electrolyzer (PEM-WE) with a pure deionized (DI)-water feed.

SUMMARY

An alkaline water electrolyzer (AWE) that incorporates a cation-exchange membrane (CEM) instead of a conventional porous diaphragm or an anion-exchange membrane used in the conventional AWE. The corresponding change in the nature of the charge carrier from the hydroxyl anion (OH⁻) in the conventional AWE to an alkali cation (A⁺) has a substantial effect on the electrochemistry and improved performance of the resulting CEM-alkaline water electrolyzer (CEM-AWE). The CEM-alkaline water electrolysis device combines advantages of: 1) non-PGM (precious group metal) catalysts involved in L-AWE (liquid alkaline water electrolyzer) and in AEM-AWE (anion exchange membrane-AWE), and 2) higher efficiency, differential pressure operation, responsiveness, and long-life of a PEM-WE (proton-exchange membrane water electrolyzer). The novel water electrolyzer combines the advantages of the two water-electrolysis technologies in the CEM-AWE involving non-PGM catalysts, graphite/SS PTLs and bipolar plates. Conventional approaches to electrolysis based hydrogen generation have not employed a CEM in an AWE.

The water electrolysis device includes an anode, a cathode, and an electrolysis cell housing the anode and cathode. A cation exchange membrane (CEM) is disposed in the electrolysis cell between the anode and cathode, and is configured for cationic transport to generate hydrogen. In an example configuration, the CEM is configured for sodium ion transport from an anode side of the electrolysis cell to a cathode side of the electrolysis cell, and generates hydrogen gas and hydroxides from water on the cathode side. Both the anode and cathode may be formed from non PGM (precious-group metal) materials, alleviating the need and cost of platinum and/or iridium. The CEM may be formed from Nafion® and employs a non-PGM catalyst such as a Ni alloy.

Configurations herein are based, in part, on the observation that low carbon emission hydrogen, or “green” hydrogen, presents a viable energy source due to the relative abundance of hydrogen. Green hydrogen (H₂) via low-temperature electrolysis (LTE) powered by excess variable renewable electricity (VRE) is a key part of the plans to help decarbonize the transportation and the manufacturing sectors and to help stabilize the electricity grid. Unfortunately, conventional approaches to hydrogen electrolysis suffer from the shortcomings of the high cost and potential supply-chain constraints of the proton-exchange membrane water electrolyzer (PEM-WE), the inflexibility of the liquid-alkaline water electrolyzer (L-AWE) for direct VRE coupling or for off-hour grid operation, and the low durability of anion-exchange membrane alkaline water electrolyzer (AEM-AWE) technologies.

In further detail, configurations herein provide a water electrolysis device, comprising a cathode and an anode in electrical communication in a containment, and a cation-exchange membrane (CEM) disposed between the cathode and the anode. An electrolyte is in fluidic communication with the anode, cathode and CEM, and a voltage source is connected across the anode and cathode, such that the CEM is configured to transport cations across the CEM for generation of hydrogen gas via a hydrogen evolution reaction.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of the invention will be apparent from the following description of particular embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

FIGS. 1A-1D show a schematic comparison of L-AWE (1A), PEM-WE (1B), AEM-WE (1C) and AEM-AWE (1D) water electrolyzer approaches;

FIG. 2 shows the disclosed Cation Exchange Membrane-Alkaline Water Electrolyzer (CEM-AWE);

FIG. 3A shows the structure and the electrochemical mechanism of the PEM-WE of FIG. 1B;

FIG. 3B shows the transport and electrochemical reaction mechanism of the AEM-AWE of FIG. 1D;

FIG. 4 shows the transport and electrochemical reaction mechanism of the CEM-AWE of FIG. 2;

FIG. 5 shows a Pourbaix diagram for the H₂/O₂ system of electrolyzers at 298 K;

FIGS. 6A-6B show the Mars-van Krevelen redox mechanism for oxygen evolution reaction (OER) at the anode;

FIG. 7A shows hydrogen evolution reaction (HER) exchange current density as a ratio of limiting current density as a function of pH;

FIG. 7B shows HER current-potential polarization curves at 1600 rpm and sweep rate of 50 mV/s for Pt(111) at various pH values;

FIG. 8 shows the bifunctional HER mechanism on Ni(OH)₂ clusters decorating the substrate metal M in alkaline electrolyte, aided by the alkali metal ion A⁺;

FIG. 9 is a schematic diagram of the CEM-AWE shown for the case when the cation A⁺ is a sodium (Na⁺) cation;

FIG. 10 shows the beneficial effect of using a bifunctional catalyst at the negative electrode of the CEM-AWE;

FIG. 11. shows the effect of alkali electrolyte concentration on the CEM-AWE performance; and

FIG. 12 shows a beneficial effect of a pH differential on the CEM-AWE performance.

DETAILED DESCRIPTION

Configurations discussed below describe membrane-electrode assembly (MEA) structures, catalyst configurations, alkaline electrolyte environments, cation transport membranes, and the proton-coupled electron transfer (PCET) pathways responsible for enhanced hydrogen evolution. Configurations herein further relates to alkaline water electrolyzer designs that incorporate non-precious-metal catalysts, stainless steel, nickel or carbon-based porous transport layers, and MEA architectures enabling low cost, high current density, long-term stability, compatibility with intermittent renewable electricity, and generation of pressurized hydrogen.

Low-temperature electrolysis (LTE), generally defined as operating at <100° C., is beneficial for producing low-carbon hydrogen using electricity generated from variable renewable energy (VRE) sources such as wind and solar. The generated hydrogen can then supply any suitable downstream energy/feedstock consumers. LTE configurations may include: 1) liquid alkaline water electrolyzers (L-AWE) with a porous diaphragm (PD) as a separator, 2) anion-exchange-membrane water electrolyzers (AEM-WE), 3) AEM-alkaline water electrolyzer (AEM-AWE), and 4) proton-exchange-membrane water electrolyzers (PEM-WE). FIGS. 1A-1D show a schematic comparison of L-AWE 10 (1A), PEM-WE 20 (1B), AEM-WE 30 (1C) and AEM-AWE 40 (1D) water electrolyzer approaches. Collectively, each of these technologies introduces various factors addressing cost reduction, dynamic operability, differential pressure operation, or large-scale deployment. The two most commercially advanced are the L-AWE and the PEM-WE technologies, while others are at various stages of development.

FIGS. 1A-1D contextualize the improvements provided by the disclosed CEM approach, and delineate the key differences from incumbent technologies. Thus, the two commercially developed technologies are: 1) the liquid alkaline-water electrolyzer (L-AWE) involving a porous diaphragm (PD) and a liquid alkaline water feed; and 2) the proton-exchange membrane water electrolyzer (PEM-WE) requiring a pure DI-water feed.

The L-AWE systems employ a concentrated (20-40 wt %, or 5-9 M) alkaline solution such as NaOH or KOH as the electrolyte, with nickel- and/or iron-based transition metal catalysts coated on the current collector/porous transport layer (catalyst-coated substrate, CCS) at both electrodes. These non-precious group metal (non-PGM) catalysts are stable in the AWE as a result of the low potentials of the hydrogen and the oxygen electrodes at high pH, as described by:

Φ H ⁢ E ⁢ R = Φ H ⁢ E ⁢ R o - 2 . 3 ⁢ 0 ⁢ 2 ⁢ 6 ⁢ ( R ⁢ T F ) ⁢ pH -

for the Nernst potential of the hydrogen evolution reaction (HER), and

Φ O ⁢ E ⁢ R = Φ O ⁢ E ⁢ R o - 2 . 3 ⁢ 0 ⁢ 2 ⁢ 6 ⁢ ( R ⁢ T F ) ⁢ pH +

for that of the oxygen evolution reaction (OER). Here R is gas constant, Fis Faraday's constant, Tis temperature, and pH₋ and pH₊ are the pH of the negative and positive electrodes/electrolytes.

It may be noted that the cell voltage, i.e., the difference between the anode and the cathode potentials:

V o = Φ O ⁢ E ⁢ R - Φ H ⁢ E ⁢ R = V 0 o - 2 . 3 ⁢ 0 ⁢ 2 ⁢ 6 ⁢ ( R ⁢ T F ) ⁢ ( pH + - pH - )

remains unchanged at

V 0 o = 1 . 2 ⁢ 29 ⁢ V ,

as shown below in the Pourbaix diagram in FIG. 5, unless there is a differential of pH between the two electrodes; here the standard cell Nernst potential is

V 0 o = Φ O ⁢ E ⁢ R o - Φ H ⁢ E ⁢ R o ,

and the pH differential ΔpH=pH₊−pH₋.

The positive and negative electrode (+ve and −ve) and overall cell chemistry of an alkaline water electrolyzer, e.g., based on an AEM/PD for hydroxyl anion (OH⁻), may for the case of a generic alkali cation A⁺ (e.g., A⁺=Na⁺, K⁺, or Li⁺) in an alkaline (AOH) electrolyte be represented as shown in Table I:

TABLE I
	+ ve ⁢ Electrode : 2 ⁢ ( A + ⁢ OH - ) ⇄ 2 ⁢ A + + 1 2 ⁢ O 2 ( g ) + H 2 ⁢ O ⁢ ( l ) + 2 ⁢ e - ; Φ + , 0 o = + 0.401 ⁢ V
	AEM/PD Conduction: 2OH− ⇄ 2OH−
	- ve ⁢ Electrode : 2 ⁢ H 2 ⁢ O ⁢ ( l ) + 2 ⁢ A + + 2 ⁢ e - ⇄ H 2 ( g ) + 2 ⁢ ( A + ⁢ OH - ) ; Φ - , 0 o = - 0.828 ⁢ V
	Cell ⁢ Overall : H 2 ⁢ O ⁢ ( l ) ⇄ H 2 + 1 2 ⁢ O 2 ; V 0 o = Φ + , 0 o - Φ - , 0 o = 1.229 V

The L-AWE electrolyzer 10 typically operates at a voltage V well above

V 0 o = Φ + , 0 o - Φ - , 0 o = 1.229 V

owing to the Faradaic and Ohmic overpotentials, often with a cell voltage V≥2 V at current densities of about i≈300 mA cm⁻² and with a voltage efficiency of about 70%. Due to the high stability of Ni-based catalysts/electrodes in alkaline electrolytes, the system lifetime usually exceeds 20 years. However, the main drawback of L-AWE is the limited current density because of the large Ohmic loss resulting from the use of relatively thick porous diaphragms that create significant ionic resistance hydroxyl anion (OH⁻) and only partially prevent gas crossover. Diaphragm thickness limits performance, current density is modest, and operation is slow to respond to rapid changes in power input. These structural and operating characteristics reduce compatibility with VRE sources that require dynamic load following.

The PEM-water electrolyzer 20 typically incorporates perfluorosulfonic acid (PFSA) (e.g., Nafion®) proton-exchange membrane (PEM) as the solid polymeric electrolyte, and precious-group metal (PGM) catalysts coated directly on the membrane (catalyst-coated membrane, CCM), which improves contact and performance.

Table II shows the electrode and the overall cell chemistry of a PEM-water electrolyzer 20:

TABLE II
	+ ve ⁢ Electrode : H 2 ⁢ O ⁢ ( l ) ⇄ 1 2 ⁢ O 2 + 2 ⁢ H + + 2 ⁢ e - ; Φ + , 0 o = 1.229 V
	PEM Conduction: 2H+ ⇄ 2H+
	- ve ⁢ Electrode : 2 ⁢ H + + 2 ⁢ e - ⇄ H 2 ; Φ - , 0 o = 0. V
	Cell : H 2 ⁢ O ⁢ ( l ) ⇄ H 2 + 1 2 ⁢ O 2 ⁢ V 0 o = Φ + , 0 o - Φ - , 0 o = 1.229 V

The PEM electrolyzer can deliver a rather high current density of above 1000 mA cm⁻² at moderate cell voltages (V˜1.7 V), while maintaining high voltage efficiency. The low gas crossover rate of polymer electrolyte membrane provides hydrogen with a higher purity than the L-AWE. The thin MEA further allows the design of more compact device and lower Ohmic resistance.

In short, PEM-WEs 20 provide high current densities, superior gas separation, differential pressure, and efficient zero-gap operation. However, they depend heavily on precious-group-metal (PGM) catalysts—including iridium for oxygen evolution and platinum for hydrogen evolution—and require Au- or Pt-coated titanium porous transport layers (PTLs) and titanium hardware to resist corrosion in high potential acidic operating conditions.

These material requirements impose substantial cost and supply-chain constraints, making PEM-WE challenging to scale to the multi-gigawatt range envisioned for global decarbonization. In this respect, alkaline water electrolysis is advantageous, as the scale of implementation is not limited by the availability of scarce and expensive raw materials. On the other hand, even though L-AWE is a more commercially mature technology, the new technological context of the renewable energy system demands more from the electrolyzer systems in terms of higher energy efficiency, enhanced rate capability, as well as dynamic, part-load, and differential pressure operation capability.

The AEM-WE 30 and AEM-AWE 40 technologies are somewhat less mature, and aim to combine the advantages of PEM-WE architecture with alkaline non-PGM catalysis by enabling OH-transport through an anion-selective dense polymer membrane. However, AEMs are not yet sufficiently chemically stable under strongly alkaline conditions. Hydroxide (OH⁻) attack on quaternary ammonium functional groups and polymer backbones leads to rapid degradation, particularly at elevated temperatures and high voltage bias. As a result, despite significant ongoing research, long-term durability remains inadequate for commercial deployment.

A longstanding assumption in the LTE field is that cation-exchange membranes (CEMs), such as PFSA or Nafion®, are unsuitable under alkaline conditions due to incompatibility with presumed electrode reaction mechanisms described above, which are based on the assumption of the hydroxyl ion (OH⁻) serving as the charge carrier between the two electrodes in both the L-AWE or AEM-AWE configurations, along with potential susceptibility to nucleophilic attack by OH⁻. Consequently, alkaline electrolysis using a CEM has not been embraced in conventional approaches.

An electrolysis device employing a cation exchange membrane (CEM) transports cations of a positively charged alkali metal such as sodium (Na⁺) from the anode side to the cathode side, instead of passing anions such as negatively charged anions such as hydroxide (OH⁻) ions from the cathode side to the anode side. FIG. 2 shows a schematic of the disclosed Cation Exchange Membrane Water Electrolyzer (CEM-WE) 100.

Referring to FIG. 2, configurations disclosed and claimed herein depict that not only are CEMs entirely compatible with alkaline-water electrolysis (AWE), albeit with altered electrode reaction mechanisms due to the transport of alkali cation (A⁺), such as sodium 101, instead of the conventional hydroxyl ion (OH⁻), accruing the advantages of AWE such as low-cost catalysts and materials and domestic supply-chain, but provide better performance than the L-AWE, while also providing the advantages of the PEM-WE such as dynamic and differential pressure operation.

There is an unmet need for such a cost-effective, high-performance low-temperature electrolyzer that supports intermittent renewable operation and pressurized hydrogen production without relying on scarce and expensive materials involving supply-chain issues, and involving commercially available ion-exchange membranes that are durable. In contrast to the anion or proton exchange membrane approaches of the AWE and PEM-WE, respectively, the cation-exchange membrane approach in AWE overcomes these shortcomings.

FIG. 3A shows the electrochemical mechanism of the PEM-WE of FIG. 1B, employing hydrogen ions, or protons (H⁺) to pass across the membrane from the anode side 22 to the cathode side 24. Similarly, FIG. 3B shows the electrochemical mechanism of the AEM-AWE electrolyzer 40 of FIG. 1D, where negatively charged hydroxide ions pass across the membrane from the cathode side 42 to the anode side 44.

FIG. 3A therefore shows the structure of a conventional PEM-WE 20. The strongly acidic conditions in the PEM-WE and the corresponding high electrode potentials at the anode (FIG. 5) require IrO₂-black catalysts for adequate stability. These are encased in PFSA-gel for proton transport. For the same reasons, the anode porous transport layer (PTL) is based on Pt- or Au-coated titanium. On the cathode side, the lower potentials FIG. 5 allow the use of C-supported Pt as HER catalysts along with the use of carbon-based-gas diffusion layers (GDLs) as the PTL. Thus, the main issue with PEM-WE is its high cost, because of the precious-group metal (PGM)-catalyzed membrane, along with the Pt- or Au-coated Ti PTL and bipolar plates. The PEM-WE provides excellent performance, is responsive and amenable to start-stop operation and thus VRE integration.

The L-AWE 10, on the other hand, is well-established, cost-effective, mature technology for up to GW range, based on low-cost and readily available materials. However, it provides a low current density and hence a large footprint, low efficiency, and lack of differential pressure or dynamic operability as needed for renewable power. The PEM-WE 20 overcomes some disadvantages of the L-AWE 10 with high current densities, long-life, and differential pressure and dynamic operability, but involves expensive materials, noble metal catalysts with tenuous supply-chains, and ultra-pure water as feed. Thus, AEM-AWE are being developed to combine the advantages of PEM-WE and the L-AWE, but the AEMs available so far are not durable.

Providing a novel combination of selected features, the disclosed approach proposes a hybrid structure that combines the two approaches in a cation-exchange membrane-alkaline water electrolyzer (CEM-AWE) 100 as shown in FIG. 2, involving catalysts and materials more commonly used in L-AWE, but with a higher performance and operational characteristics of the PEM-WE that make it suitable for direct integration with VRE power sources. Thus, the PD of a L-AWE 10, or the AEM of an AEM-AWE 40 is replaced by the CEM 110 e.g., Nafion®. The corresponding change in the nature of the charge carrier from the hydroxyl anion (OH⁻) in the conventional AWE to an alkali cation (A⁺) has a significant effect on the chemistry and performance of the resulting CEM-alkaline water electrolyzer (CEM-AWE).

The transport and reaction mechanism of an AEM-AWE 30 is shown schematically in FIG. 3B, and may be compared with that of the proposed CEM-AWE in FIG. 4, where the changes in the chemistry and transport caused by the switch of the charge carrier from OH⁻ in the conventional AWE to the alkali cation A⁺ in CEM-AWE can be readily appreciated. As shown in FIG. 3B, the AEM largely precludes cations such as Na⁺, allowing only anions such as OH⁻ to go through from the negative to the positive electrode chamber. Here, the OH⁻ anions readily undergo OER even on non-PGM catalysts such as Ni(OH)₂:

On the other hand, the water that may be fed to the cathode or crosses over from the anode, undergoes water dissociation (WD):

where the hydroxyl ions formed cross over to the anode, while the protons undergo PCET to form H₂:

In contrast, as shown in FIG. 4, the CEM 110 as disclosed precludes transfer of the OH⁻ ions across it, but readily allows cations such as Na⁺ ions resulting from ionization of NaOH:

to crossover from the anode to the cathode. The remaining OH⁻ anions readily undergo OER at the anode as described above for the conventional AWE.

The Na⁺ ions that crossover from the anode to the cathode promote water dissociation

forming NaOH, which may be transferred to the anode for another electrolysis cycle, while the remaining protons undergo PCET to form H₂ as above.

The disclosed approach of the CEM-AWE 100 overcomes the combined limitations of the L-AWE, the AEM-AWE, and the PEM-WE by employing a cation-exchange membrane (CEM) in an alkaline water electrolyzer instead of a porous diaphragm (PD) or an anion-exchange membrane (AEM). FIG. 4 shows the hence altered transport and electrochemical reaction mechanism of the CEM-AWE 100 of FIG. 2, and FIG. 9, discussed further below, and depicts a physical containment structure of an example configuration. The CEM electrolyzer 100 enables the transport of an alkali cation (A⁺), such as a sodium-ion (Na⁺) 101 or a potassium ion (K⁺), from the anode side 102 to the cathode side 104 as the central charge carrier, instead of the conventional hydroxyl anion (OH⁻) used as the charge carrier from the cathode to the anode in conventional alkaline water electrolyzers. While the anode chemistry in the resulting CEM-AWE still seeks hydrogen gas, as do the conventional L-AWE or the AEM-AWE, the change in the charge carrier alters the hydrogen evolution reaction (HER) mechanism involving cation-assisted water dissociation (Na⁺+H₂O⇄H⁺+Na⁺OH⁻) followed by proton-coupled electron transfer (PCET) reaction (2H⁺+2e⁻⇄H₂ (g)), requiring a distinctive design of the cathode catalyst. The replacement of a porous separator in an L-AWE by a thin solid-polymer CEM 110 that can further be directly catalyzed (catalyzed membrane), rather than the conventional catalyzed substrate, reduces Ohmic as well as Faradaic resistance and improves performance while allowing intermittent operation and generation of pressurized hydrogen.

In contrast to conventional approach involving AEMs, CEMs can operate efficiently and stably in an alkaline water electrolyzer environment, and the switch from the hydroxyl anion (OH⁻) in L-AWE and AEM- to the alkali cation (A⁺) as the charge carrier in CEM, while changing the cathode chemistry, does not impose additional kinetic limitations at the cathode with the appropriate catalysts. In fact, transported alkali cations—e.g., Na⁺—actively participate in lowering the activation barrier for the hydrogen evolution reaction (HER) under alkaline conditions by stabilizing transition states associated with water dissociation (FIG. 8), which is an integral step in the electrode mechanism.

The underlying cathodic reaction behind the electrolyzer is the hydrogen evolution reaction (HER), which is an electrochemical reaction in which hydrogen gas (H₂) is produced by the reduction of protons or water at an electrode. It is a core reaction in electrolysis, fuel cells (reverse reaction), corrosion, and electrocatalysis:

In most electrolyzers, as described above, hydroxide anions pass through the membrane between the anode and cathode. The disclosed approach, in contrast, passes cations of alkali metals through the CEM. In simplest terms, an alkali cation is a +1 charged ion formed when an alkali metal atom loses its single valence electron. This highly reactive alkali cations include Sodium (Na), Potassium (K), Lithium (Li), Rubidium (Rb), Cesium (Cs), and Francium (Fr), a rather rate and radioactive element. Electrochemical aspects of the CEM shown in FIG. 4 are shown in FIGS. 6A-8.

FIG. 5 shows a Pourbaix diagram for the H₂/O₂ system of electrolyzers at 298 K. The Pourbaix diagram for the H₂/O₂ system in FIG. 5 shows that while the thermodynamic Nernst potential for the water electrolyzer cell remains unchanged at 1.229 V regardless of the pH (pH=14 for L-AWE, and pH=0 for PEM-WE), the kinetic overpotential is a function of pH. Thus, the oxygen evolution reaction (OER) becomes somewhat less sluggish at higher pH while the hydrogen evolution reaction (HER) becomes substantially more sluggish in large part because of the paucity of protons at higher pH.

FIGS. 6A-6B show the Mars-van Krevelen redox mechanism for OER at the anode. The OER under alkaline conditions is known to proceed via the Mars-van Krevelen redox mechanism (Deng, B., Yu, G., Zhao, W., Long, Y., Yang, C., Du, P., . . . & Wu, H. (2023). A self-circulating pathway for the oxygen evolution reaction. Energy & Environmental Science, 16(11), 5210-5219), as shown in FIGS. 6A-6B for a Ni catalyst, where the Ni cycles between 2+ and 3+ oxidation states, i.e., as Ni²⁺(OH)₂ and Ni³⁺(O)(OH). In fact, Ni(OH)₂ is a common and effective OER catalyst, either on its own or in combination with others, that is commonly used in AWEs.

Such a mechanism applies, however, to other oxophilic transition metal catalysts M besides Ni as well, e.g., Fe, Co, etc. Thus, in general, the OER redox mechanism in alkaline electrolyte at the positive electrode involving the M(OH)₂ and M(O)OH intermediate is shown in Table III:

TABLE III
Redox: 2 {   (OH−)2 + (A+OH−) ⇄     (OH−)3 + A+ + e−}
Superoxo: 2 {   (OH−)3 ⇄     (O2−)(OH−) + H2O}
OER: 2   (O2−)(OH−) + 2(A+ + OH−) ⇄ 2   (OH−)2 + O2(g) + 2A+ + 2e−
+ve Electrode Overall: 4(A+OH−) ⇄ O2(g) + 2H2O + 4A+ + 4e−

FIG. 7A shows HER exchange current density as a ratio of limiting current density versus electrolyte pH, and FIG. 7B shows HER current-potential polarization curves at 1600 rpm and sweep rate of 50 mV/s for Pt(111) at various pH values. FIGS. 7A-7B show the dramatic effect of pH on the hydrogen evolution reaction. Under acidic conditions, as in PEM-WE, the abundance of protons allows the proton-coupled electron transfer (PCET) and the HER to proceed readily. On the other hand, at high pH, the source of protons for PCET changes to H₂O, i.e., H₂O⇄H⁺+OH⁻, which is a demanding reaction, and explains why most metal catalysts at high pH, including Pt, display 2-3 orders of magnitude lower activity than at low pH, as seen for Pt in FIG. 7A. Thus, the proton donor is hydronium ion (H₃O⁺) in an acidic electrolyte, while it is H₂O in alkaline electrolyte. The switch from the proton (H₃O⁺) branch to the water (H₂O) branch occurs at pH 2-4, as shown in FIG. 7B for Pt.

FIG. 8 shows the bifunctional HER mechanism on Ni(OH)₂ clusters on substrate metal M in alkaline electrolyte, aided by alkali metal ion A⁺. In fact, various suitable substrate metals M, oxophilic oxyhydroxides, M(OH)₂, and cations A⁺ may be employed. In FIG. 8, the bifunctional alkaline HER mechanism on Ni(OH)₂ clusters decorating the substrate metal M is shown, and aided by alkali metal ion A⁺. Thus, HER in alkaline electrolytes also in fact proceeds via proton-coupled electron transfer, or PCET, 2H⁺+2e⁻⇄H₂ (g), just as in PEM-WE where protons are plentiful, but the protons are produced at the high pH via water dissociation (WD) as the step first, H₂O⇄H⁺+OH⁻, catalyzed by an oxophilic transition metal hydr(oxy)oxide, e.g., Fe(OH)₃, and Ni(OH)₂, via the Mars-van Krevelen redox mechanism for OER cycling between Ni²⁺(OH)₂ and Ni³⁺(O)(OH), and aided by an alkali metal ion A⁺, e.g., Na⁺/K⁺, in solution. Thus, as shown in FIGS. 7A and 7B, under strongly alkaline conditions, HER is suppressed both because there is a paucity of protons, and because WD is a sluggish reaction without an appropriate catalyst. Such a catalyst is bifunctional, where the Ni²⁺(OH)₂, or more generally, (OH)₂, catalyzes water dissociation while the metal M, e.g., Pt catalyzes the PCET.

FIG. 9 is a schematic diagram of the CEM-AWE as in FIG. 4, shown for the case when the cation A⁺ is a sodium (Na⁺) cation. Referring to FIG. 9, the disclosed water electrolysis device 100 includes an anode chamber and a cathode chamber separated by a catalyzed cation-exchange membrane sandwiched between two porous electrodes. Stainless-steel, Nickel, or carbon-based porous transport layers (PTLs) may be used and pressed directly against the catalyzed membrane, creating a zero-gap configuration that minimizes ionic resistance and improves efficiency. The membrane-electrode assembly (MEA) is mechanically compressed by cell frames or bipolar plates constructed from corrosion-resistant stainless steels.

The anode comprises a catalyst active for oxygen evolution in alkaline media. As indicated via a discussion of the mechanism above, the -hydr(oxy)oxide species are active for OER, where the oxophilic metal, =Ni, Fe, Co, Cu, Mn, Ru, etc. More generally O_xH_y catalysts may be employed. Suitable catalysts include nickel-iron oxyhydroxide, nickel-cobalt oxide, cobalt-iron spinel, and related mixed metal oxides. These catalysts exhibit high activity in alkaline conditions and are stable for extended operation. In fact, Fe-hydr(oxy)oxide is more active than Ni or Co species. However, Fe is not adequately stable. However, -hydr(oxy)oxide species, when decorated with Fe, are found to be active. Thus, there is a strong correlation between OER current density versus Fe surface coverage (ML) on metal hydr(oxy)oxides, Fe-O_xH_y, or Fe-O_xH_y (=Ni, =Mn, Cu, Co).

As shown in the schematic in FIG. 9, the CEM-AWE is a water electrolysis device 100 that includes the cation-exchange membrane (CEM) 110 interposed between an anode (positive electrode) 112 on the anode side 102 and a cathode (negative electrode) 114 on the cathode side 104. The cathode 114 and the anode 112 are in electrical communication in a containment 150 defining a layered structure. The anode 112 and cathode 114 are each defined by respective conductive plates 122 (anode) and 124 (cathode), such that the conductive plates 122, 124 are arranged flanking the CEM 110 on respective opposed sides of the containment 150. The containment 150 encapsulates the electrolyzer 100 with an alkaline electrolyte, typically an NaOH or KOH solution in a concentration between 0.01 M to 3.0 M, usually with a seal 151 or similar physical fluidic containment.

The CEM 110 is used for the transport of alkali metal cations, typically Na⁺ or K⁺, such that the CEM 110 is configured to selectively transport alkali cations cation (A⁺) from the anode 122 to the cathode 124, and may be catalyzed to promote the electrochemical activity. In the example configuration, the cation-exchange membrane 110 may be coated directly with the anode catalyst 132 on one side and the cathode catalyst 134 on the other to provide an intimate contact and a large electrocatalytic surface area (ECSA). The anode catalyst 132 is typically defined by an oxide layer adjacent the CEM 110 including oxides such as NiFe oxyhydroxide, NiCo oxide, CoFe spinel and mixed metal oxides. The cathode catalyst 134 resides on the other side between the CEM 110 and the cathode, and is typically defined by a bifunctional structure, e.g., a catalyst layer including nickel compounds such as Ni, NiMo, NiMoFe, Ni(OH)₂-modified nickel, and Ru(OH)₂-deposited on metal M such as nickel or Pt, or nickel foam.

The catalyzed membrane 111 is further sandwiched between a typically Nickel/Stainless Steel-based porous transport layer (Ni-PTL) 142 for the anode 112 and a carbon-gas-diffusion layer (C-GDL) 144 for the cathode 114 serving as current collectors that allow effective transport of water as well as the evolved gases (H₂). The resulting layered structure forms a membrane-electrode assembly (MEA) 155 assembled between Nickel/Stainless Steel bipolar plates 122, 124 with flow channels 123, 125 for the flow of electrolyte (negolyte for the negative electrode, and posolyte for the positive electrode) feed and for the effective removal of the evolved oxygen 152 at the anode 112 and hydrogen 154 at the cathode 114.

An alkaline electrolyte is typically employed, containing NaOH or KOH, supplied to one or both electrode compartments, and/or a pure water feed may be provided to the cathode, providing a pH differential between the two electrodes that may be used to reduce cell voltage and enhance performance.

A voltage source 105 connects to the anode 112 and cathode 114, such that the CEM 110 is configured to transport cations across the CEM for generation of hydrogen gas 154 from the hydrogen evolution, via a gas water separator 160. Similarly, an electrolyte storage 162 and pump maintains the flow and concentration of the electrolyte. A fluidic pathway 164 between the cathode side 104 and the anode side 102 of the containment 150 is configured for transporting sodium hydroxide generated at the cathode side to the anode side.

The full containment 150, including the MEA 155 in a layered arrangement with the voltage source and fluid connections forms a layered construction with minimal to no space between the layers. The containment 150 therefore forms a closed, zero-gap arrangement of the layers of the containment, including a sequential, adjacent arrangement of:

• • I: the anode conductive plate 122,
  • ii: the porous transport layer 142,
  • iii: the anode catalyst 132,
  • iv: the CEM 110,
  • v: the cathode catalyst 134
  • vi: the gaseous diffusion layer 144, and
  • vii: the cathode conductive plate 124.

During operation of the CEM electrolyzer 100, the alkali cation A⁺, e.g., Na⁺ migrates through the CEM 110, enhancing water dissociation and hydrogen evolution at the cathode side 1104. Newly formed hydroxide ions (OH⁻) associate with Na⁺ to form sodium hydroxide (NaOH), which is recirculated to the anode side 102, where OH participates directly in the oxygen evolution reaction (OER). This establishes a sodium shuttle cycle that maintains mass and charge balance and sustains HER by replenishing proton equivalents at the cathode 114 including, in general, the monovalent cationic charge transfer species,

A + = H + , L ⁢ i + , N ⁢ a + , K + , Rb + , C ⁢ s + ⁢ NH 4 + ,

etc. The resulting CEM-AWE configuration achieves high performance while enabling the use of non-PGM catalysts, stainless steel, Nickel or carbon porous transport layers, and low-cost hardware. The architecture also supports differential-pressure operation and dynamic load following, making it suitable for intermittent renewable power sources.

It is evident that a variety of structures and materials including various cation-exchange membranes, anode and cathode catalysts, porous-transport layers (PTLs), and membrane-electrode assemblies (MEAs) including catalyzed membranes and/or catalyzed substrates are defined by the disclosed approach. Conventional approaches do not exhibit cation transport as exhibited by the disclosed CEM-AWE 100.

Referring again to FIG. 4, the CEM 110 is flanked by the anode catalyst 132 and the cathode catalyst 134. For the selection of appropriate cathode catalysts, recall from above disclosure that following ingredients are needed in a good bifunctional catalyst for HER in an alkaline electrolyte:

• • 1) A substrate metal M with good electronic conductivity and an optimal H binding energy, e.g., Pd or Ag, decorated with an
  • 2) Oxophilic metal hydr(oxy)hydroxide, e.g., Ni, with optimal OH binding energy, in an
  • 3) Electrolyte with alkali metal ions, A⁺, e.g., Na⁺, to facilitate removal of OH from surface.

Examples of other oxophilic metal hydr(oxy)hydroxide potentially include, e.g., =Ni, Co, Fe, Mn, Mo, Ce, Zr, W, Ru, and Ir, for promoting water dissociation, while those for the metal M conceivable include M=Pt, Pd, Rh, Ag, Fe, Ni, etc., that promote PCET. An ideal, albeit a bit pricey, example of the bifunctional catalyst would be Pt—RuO₂. A cheaper alternative would be Pd—Ni(OH)₂. Other examples include Ni—Ni(OH)₂, Ag—Ni(OH)₂, Fe—Ni(OH)₂, Ni—MoPO_x, Ni—Zn(OH)₂, Ni—Co(OH)₂.

To assist in the selection of (OH)₂ for activating the water to evolve hydrogen in alkaline electrolytes, Pt surface was modified by depositing 3d-transition metal (Ni Co, Fe, Mn) hydroxide clusters. It was found that Ni(OH)₂ is unique in this regard. Upon testing a variety of substrate metals, M. it was found that Raney Ni/Ni(OH)₂ represents an excellent and cost-effective choice for a HER catalyst. Nickel metal, NiMo alloys, NiMoFe alloys, and Ni(OH)₂-decorated Ni surfaces are particularly suitable. Structured electrodes such as nickel foam or porous sintered nickel may be used to enhance mass transport and bubble release.

The cation-exchange membrane (CEM) may include perfluorosulfonic acid (PFSA) polymers, perfluoroimide acid (PFIA), short-side-chain PFSA, or composite membranes incorporating inorganic fillers to enhance mechanical or alkaline stability. The pendant ionomer units in cation-exchange membranes may include anionic functional groups including carboxylates, sulfonates, phosphonates, and the like. The pendant ionomer groups are charge-balanced by exchangeable cations such as protons, H⁺, and alkaline earth metals, such as Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, or

NH 4 + .

An example of a membrane material with fluorinated or perfluorinated polymer backbone is Nafion®, manufactured by E. I. du Pont de Nemours and Company, Wilmington, Delaware. Non-fluorinated membranes include those with substantially aromatic backbones, e.g., polystyrene, polyphenylene, bi-phenyl sulfone (BPSH), or thermoplastics such as polyetherketones or polyethersulfones, and the like. Commercial manufacturers of AEMs include Fumatech (Germany), Solvay (Belgium), Tokuyama (Japan), Ionics (USA), Dioxide Materials (USA), and Asahi Glass (Japan). Alternative composite CEMs incorporating inorganic fillers, may also be employed.

In implementation, selection of the electrolyte and fluidic configuration may be guided by several factors. The CEM-alkaline water electrolyzer may be operated with an alkaline electrolyte supplied to the anode, the cathode, or both. The electrolyte may include sodium hydroxide and/or potassium hydroxide at concentrations between 0.01 M and 3 M. These concentrations are somewhat lower than those employed in L-AWE, and represent a balance of ionic conductivity and membrane durability, enhanced by higher water content and lower alkali content, and OER kinetics on the other hand, enhanced by higher alkali content. The electrolyte is continuously recirculated through each electrode compartment.

In some embodiments, the water feed may be directed to the cathode. Such a configuration can result in a pH differential across the electrolyzer, which can reduce cell potential as a well as HER overpotential, resulting in enhanced electrolyzer performance.

In other embodiments, a differential-pressure configuration is employed, in which hydrogen is produced at elevated pressure while oxygen is released at a lower pressure.

Flow-field variations, hydrophobic patterning, or plasma-treated electrodes may also be incorporated to optimize performance of the CEM-AWE. The alkaline environment and corrosion-resistant catalysts allow the device to avoid titanium hardware typically required in PEM systems.

Operating conditions for performance yield beneficial results in the CEM configuration. The electrolyzer may be operated at temperatures between 20° C. and 95° C. and at current densities ranging from 100 mA cm⁻² to more than 1,000 mA cm⁻². The zero-gap architecture reduces ionic losses, and the use of non-PGM catalysts lowers system cost. Gas crossover is minimized by the membrane, enabling efficient differential-pressure operation.

Dynamic responsiveness allows the system to follow rapidly varying loads, making it suitable for coupling with VRE sources. Start-up and shut-down may be performed at reduced current densities to minimize mechanical and chemical stress.

The device architecture facilitates manufacturing using processes analogous to PEM electrolyzer production, including roll-to-roll membrane handling, catalyst-coated electrode deposition, and MEA assembly. Stainless-steel hardware and bipolar plates reduce cost. The system may be scaled through stacking, with fluidic and electrical manifolding similar to existing electrolyzer systems.

Performance results of exemplary configurations are shown in FIGS. 10-12.

FIG. 10 shows the beneficial effect of using a bifunctional catalyst at the negative electrode of the CEM-AWE. Referring to FIG. 10, a dramatic improvement in performance of a CEM-AWE is shown when monofunctional water dissociation catalysts Ni(OH)₂ on the cathode is replaced by a bifunctional Pt-(O)(OH) catalyst where Pt catalyzes PCET while (O)(OH) catalyzes the water dissociation, as shown mechanistically in FIG. 8.

FIG. 11. shows the effect of alkali electrolyte concentration on the CEM-AWE performance. It is seen that increasing alkali concentration has diminishing returns, with little improvement beyond 3 M NaOH. This is expected to be due to the balance between water and alkali activity. The latter improves OER activity, while the former improves CEM hydration, reducing membrane transport resistance, and enhancing membrane durability.

FIG. 12 shows a beneficial effect of a pH differential on CEM-AWE performance, when the 2M NaOH feed at the cathode (negolyte) is replaced by DI water, resulting in a pH differential between the anode (pH=14) and the cathode (pH=9.5), ΔpH=14−9.5.

Therefore, enumerated features and advantage of the disclosed CEM approach include the following:

• • 1. Use of commercially available, durable and high performing CEMs 110 such as Nafion®;
  • 2. Use of non-PGM catalysts and Nickel/Stainless Steel/Graphitic cell component e.g., other than Platinum, Palladium, Rhodium, Ruthenium, or Iridium;
  • 3. Use of catalyst-coated membrane (CCM) instead of catalyst-coated substrate (CCS) as in conventional L-AWE;
  • 4. Use of ionomer within the catalyst coating to facilitate catalyst-layer bonding and cation transfer within the catalyst layer;
  • 5. Enhanced performance in comparison with convention L-AWE;
  • 6. Does not require ultrapure DI water feed, unlike the PEM-WE;
  • 7. Insensitivity of the alkaline electrolyte to atmospheric CO₂. In the conventional AWE, exposure of atmospheric CO₂ replaces the charge carrier from hydroxide ions to bicarbonate

( HCO 3 - )

or carbonate

( CO 3 2 - )

anions, significantly enhancing Ohmic resistance and reducing performance. There is no corresponding effect of atmospheric CO₂ on cation transport in CEM;

• • 8. Use of alkali electrolyte concentration that is not as high as that used in conventional L-AWE, which is 20-40 wt. %, or 5-9 M, and can cause significant corrosion, especially at higher temperatures;
  • 9. Use of water feed directed to the cathode provides a beneficial pH differential that can significantly reduce cell potential and enhance hydrogen evolution reaction (HER) and hence improved performance;
  • 10. Suitability for transient and intermittent operation unlike conventional L-AWE; and
  • 11. Suitability for supporting a differential pressure operation with generation of pressurized hydrogen.

While the system and methods defined herein have been particularly shown and described with references to embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.