EP4448843A2

EP4448843A2 - Passivated electrodes in electrolyzers and fuel cells

Info

Publication number: EP4448843A2
Application number: EP22851200.0A
Authority: EP
Inventors: Shannon Boettcher; Minkyoung KWAK; Grace LINDQUIST; Kasinath OJHA
Original assignee: University of Oregon
Current assignee: University of Oregon
Priority date: 2021-12-17
Filing date: 2022-12-16
Publication date: 2024-10-23

Abstract

A construct including an anode, a cathode, at least one electrolyte, a catalyst in contact with at least one of the anode or the cathode, and a passivation coating disposed on a surface of the catalyst, and the passivation coating comprises a passivation material selected from a metal oxide, a metal hydroxide, a metal oxyhydroxide, or a metal carbonate, or a mixture thereof, wherein the metal is a transition metal, an alkaline earth metal, a lanthanide series metal, aluminum or silicon.

Description

PASSIVATED ELECTRODES IN ELECTROLYZERS AND FUEL CELLS

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of the earlier filing date of U.S. Provisional Application No. 63/291,295, filed on December 17, 2021, which is incorporated herein by reference in its entirety.

ACKNOWLEDGMENT OF GOVERNMENT SUPPORT

This invention was made with government support under grant number DE-EE0008841 awarded by U.S. Department of Energy. The government has certain rights in the invention.

BACKGROUND

Electrochemical energy storage and production technologies are essential for renewable energy implementation. They are used currently for applications such as electrolysis, electrodialysis, water desalination, wastewater treatment, chemical production, and the food and beverage industry.

These devices use similar components and operating principles. A voltage is applied to the device and ions in an electrolyte are selectively transported from one electrode to another. In this regard, before further explaining the embodiments of the invention here, it is to be understood that the invention is given in the context of water electrolyzers typically used to produce hydrogen and oxygen but may be embodied in other applications which utilize a catalyst and polymer or solid-electrolyte interface.

Specifically for water electrolysis, proton exchange membrane water electrolysis (PEMWE) and alkaline water electrolysis (AWE) systems are leading technologies and are in commercial use. Anion exchange membrane water electrolysis (AEMWE) is an emerging technology that in principle combines advantages for both PEMWE (zero-gap configuration with pure-water feed and no soluble electrolyte) and AWE systems (use of inexpensive non-platinum group metal catalysts). Still, fast degradation of anodes in the system limits the commercialization of AEM water electrolyzers.

The dominant source of degradation in AEMWE systems is the ionomer polymer used in the catalyst layer (CL). This can be prone to both mechanical and chemical degradation. Mechanical degradation includes, but is not limited to, catalyst or electrode detachment, catalyst layer morphology changes, catalyst layer cracking or peeling or other changes caused by polymer swelling or dehydration. Chemical degradation includes, but is not limited to, direct electrochemical oxidation of the ionomeric polymer, nucleophilic attack of hydroxide ions, radical reactivity with oxygen radical intermediates, and attack by other electrogenerated species. While not as rapid, these ionomer degradation pathways are also present in other membrane-based devices that include a catalyst and ionomer interface at the anode.

Another factor limits the durability of AEMWE systems is degradation of catalysts themselves, for example, dissolution of catalysts as soluble species, detachment of catalyst particles from the porous substrate and surface reconstruction of catalysts, which all deactivate the electrodes. SUMMARY

One embodiment disclosed herein is a construct comprising: an anode; a cathode; at least one electrolyte; a catalyst in contact with at least one of the anode or the cathode; and a passivation coating disposed on a surface of the catalyst, and the passivation coating comprises a passivation material selected from a metal oxide, a metal hydroxide, a metal oxyhydroxide, or a metal carbonate, or a mixture thereof, wherein the metal is a transition metal, an alkaline earth metal, a lanthanide series metal, aluminum or silicon.

Another embodiment disclosed herein is a construct comprising: an anode; a cathode; an ion exchange membrane positioned between the anode and the cathode; and a catalyst layer deposited on either the anode or a side of the ion exchange membrane that is in contact with the anode, wherein the catalyst layer comprises an ionomer and a passivation-coated catalyst.

A further embodiment disclosed herein is a construct comprising: an anode; a cathode; at least one electrolyte; and at least one of an anode catalyst layer or a cathode catalyst layer, wherein the anode catalyst layer or the cathode catalyst layer comprises an electrode catalyst, a passivation material additive, and at least one polymer.

An additional embodiment disclosed herein is a method comprising: forming a passivation coating on a surface of an electrode catalyst material resulting in a passivated electrode catalyst material, and contacting the passivated electrode catalyst material with at least one electrolyte, wherein the passivation coating comprises a passivation material selected from a metal oxide, a metal hydroxide, a metal oxyhydroxide, or a metal carbonate, or a mixture thereof, wherein the metal is a transition metal, an alkaline earth metal, a lanthanide series metal, aluminum or silicon.

A further embodiment disclosed herein is a method comprising: mixing a passivation material and at least one polymer; contacting the resulting mixture with an electrode catalyst material resulting in a passivated electrode catalyst material; and contacting the passivated electrode catalyst material with at least one electrolyte.

Another embodiment disclosed herein is a method comprising: mixing a passivation material, at least one polymer, and an electrode catalyst material resulting in a passivated electrode catalyst material, wherein the resulting mixture forms a passivation layer between an electrode and an electrolyte, or between an electrode and an ion exchange membrane.

The foregoing and other features will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B. Schemes of (FIG. 1A) a membrane electrode assembly (MEA) and (FIG. IB) an electrochemical device.

FIGS. 2A-2B. Schemes of a membrane-electrode interface of a conventional MEA and its degradation modes.

FIGS. 3A-3B. Schemes of MEAs with a passivation coating on either anode catalyst layer or cathode catalyst layer.

FIGS. 4A-4B. Schemes of a membrane-electrode interface with a passivation coating deposited on a catalyst layer and its working mechanisms.

FIG. 5. Schematic configuration of passivation coatings; i. a continuous inorganic passivation coating, ii. a continuous organic passivation coating and iii. a mixed layer of passivation additives and organic polymers (ionomers).

FIGS. 6A-6B. Schemes of MEAs where passivation additives are incorporated in either anode catalyst layer or cathode catalyst layer.

FIGS. 7A-7B. Schemes of a membrane-electrode interface with a passivation additive incorporated in the catalyst layer and its working mechanisms.

FIGS. 8A-8C. (FIG. 8A) SEM cross-sectional image of an exemplified passivated anode. (FIG. 8B) Alkaline-membrane electrolyzer data showing thin layer of HfOz passivation coatings coupled with Ir-based water oxidation catalysts can be used with a pure water-fed electrolyzer. (FIG. 8C) Durability data showing improved final voltage and voltage degradation rate with passivation coatings.

FIGS. 9A-9B. (FIG. 9 A) Alkaline membrane electrolyzer data showing HfOz passivation coatings coupled with CoO_x-based water oxidation catalysts can be used with a pure water-fed electrolyzer. (FIG. 9B) Durability data of passivated anodes operated at 1 A/cm². Passivated anodes have smaller initial voltage loss due to less ionomer oxidation. FIGS. 10A-10B. (FIG. 10A) Alkaline membrane electrolyzer data of passivated anode with a coating of Mg(OH)2 passivation additive and ionomer mixture on Ir catalyst coated anode and un-passivated anode with a pure ionomer layer deposited on Ir catalyst coated anode. (FIG. 10B) Durability data of passivated anodes operated at 500 A/cm².

FIG. 11. Sample XPS analysis of ionomers on un-passivated and passivated anodes showing suppression of ionomer oxidation and degradation with passivation coatings.

DETAILED DESCRIPTION

Disclosed herein is a passivation coating or additive for application in any electrochemical device wherein an incoming reactant is transformed to an outgoing product. This includes, but is not limited to, electrolyzers, fuel cells, and electrodialysis systems. Electrolyzer systems may include, but are not limited to, water electrolysis for hydrogen production, CO2 electrolysis to any carbon-containing product, and nitrogen or nitrate electrolysis to ammonia or other nitrogen-containing products. Fuel cells include cells that consume H2, methanol, hydrocarbons, phosphoric acid or any other chemical fuel. Electrodialysis systems include, but are not limited to, the transport of any ions across any number of separators in which a faradaic reaction occurs at the electrodes.

In these systems, ion-selective-membranes, including proton-selective, anion-selective, and bipolar membranes, are used (e.g. OH- or H⁺ in alkaline or acidic membrane water electrolysis and fuel cell systems, carbonate or bicarbonate in neutral pH CO2 reduction systems, Na⁺, Cl", SO4^2-, Ca²⁺ Mg²⁺ in electrodialysis systems, and NOs- in nitrate reduction systems). The membrane is assembled with an anode and cathode (FIG. 1A), and at the electrode for the above-described devices in which a reactant is transformed to a product, faradaic reactions occur to achieve charge balance in the system (FIG. IB). The electrode where these reactions occur may utilize a catalyst to lower the energetic requirement for the reaction. Often this catalyst is coated on either the electrode or membrane in a matrix that also includes a polymer binder, which is electrically insulating and may be either ionically conductive or non-conductive. The matrix of the catalyst coating may also include a mixture of both ionically conductive and non-ionically conductive polymer. A non-conductive polymer includes any plastic material stable at the operating temperature range of the device and may include, but is not limited to, polyfluorinated materials (e.g. polytetrafluoroethylene or Teflon®), polyimide materials (e.g. Vespel®), polysulfones and functionalized varieties (e.g. poly ethersulfone), polyphenylenes (e.g. polyphenylene oxide, polyphenylene ether, and Noryl®), poly-(benzimidazole)-type materials, polystyrenes and functionalized varieties, polyinyls and functionalized varieties (e.g. polyvinyl chloride), acrylics and functionalized varieties, and polyesters and functionalized varieties (e.g. polyethylene terephthalate). The passivation material, forming either passivation coating or passivation additive in the electrode, functions as an interfacial physical and mechanical barrier to prevent unwanted degradation processes in the electrochemical processes. The passivation coating or passivation additive improves the durability of the catalyst layer in electrolyzer and fuel cell systems using interfacial engineering techniques. In addition to protecting the polymer from oxidative damage, the passivation materials could be used to prevent unwanted reactivity of reactants or products at the anode or cathode by size exclusion through the passivation materials. Further, the passivation materials could be used to prevent degradation modes other than at the ionomer, for example catalyst dissolution, detachment and reconstruction at the cathode or anode.

The passivation materials disclosed herein could also be utilized in the above-described cells or other electrochemical devices, for example pseudocapacitive devices, medical devices such as neural stimulators, and sensor systems, where the passivation coating or passivation additive has the same function, i.e. to protect sensitive cell components such as the electrolyte from deleterious faradaic reactions or dissolution primarily due to oxidation.

We have identified that a limiting factor of anion exchange membrane (AEM) water electrolysis (WE) systems is the stability of ionomers at the anode catalyst layer. At anode potentials used for oxygen generation, ionomers are also oxidized, leading to performance loss, and severely limiting durability. Disclosed herein are anodes that selectively and efficiency generate Oz and specifically prevent the deleterious ionomer oxidation charge-transport pathways. In particular, disclosed herein are passivation coatings or additives for AEMWE systems that improve the durability of AEM electrolyzers and that can enable competitive lifetime, performance, and cost. These passivation coatings or additives have further applications beyond AEMWE anode systems as previously described.

A conventional anode or cathode is comprised of an electrode and a mixture of catalyst nanoparticles and ionomers as a catalyst layer. The electrode may be any electronically conductive material used to transport liquid or gaseous reactants and products to and from the catalyst commonly referred to as may be a porous transport or gas diffusion layer. The catalyst layer may be deposited on the electrode material or membrane. At the anode in AEMWE, hydroxide ions transport toward catalysts through ionomer network, and oxygen evolution reaction occurs at the catalyst and ionomer interface. The catalysts and ionomers are electrically contacted, and thus ionomers must be stable under the highly oxidizing environment, which is possible for expensive fluorocarbon Nation ionomers under acidic conditions but so far has been unattainable for alkaline ionomers (for example, functionalized poly(aryl piperidinium) anion exchange polymers, poly-(benzimidazole)-based anion exchange ionomers, and imidazolium-functionalized styrene and vinylbenzyl chloride based anion-exchange polymers). Also affecting the durability of AEMWE is catalyst dissolution and/or reconstruction at the electrode surface that decrease the catalytic activity of the electrodes over time by various mechanisms, leading to the cell voltage degradation (FIGS. 2A-B).

To eliminate ionomer degradation and loss as well as protect catalyst materials, disclosed herein, as one example, is a new electrode (anode or cathode) with an additional thin coating of a passivating material at the catalyst/electrolyte (e.g, ionomer) interface (FIGS. 3A-B). For example, in an electrolyzer this additional coating on the anode allows the transport of hydroxide ions but insulates against electron transfer between catalysts and ionomers. Consequently, this passivated anode is expected to suppress the degradation of ionomers but maintain the high catalytic activity for the oxygen evolution reaction. Along with suppressed ionomer degradation, this passivation coating serves as a physical barrier at the catalyst-ionomer interface where dissolution of catalysts toward the electrolytes and surface reconstruction of catalysts are minimized (FIGS. 4A-B).

Materials used to passivate the electrode catalyst and prevent electron transfer but allow ion transfer (e.g. OH- in alkaline membrane systems, or H⁺ in acidic systems, or carbonate or bicarbonate in neutral pH CO2 reduction systems), include HfO_x, TiO_x, ZrO_x, TaO_x, ScO_x, A1O_X, LaO_x, SiO_x, Mg(OH)_x, Ca(OH)_x, Sr(OH)_x, Ba(OH) _x, mixtures or alloys of these oxides or hydroxides, where x is typically between 1 and 3. In certain embodiments, x is very close to 2. In some embodiments, the materials are amorphous materials with O and OH in different ratios. The coatings could be amorphous or poly crystalline, either with effectively no porosity and small ions moving through the molecular framework of the materials or pores ranging in size from 1 angstrom to 50 nm in diameter. The thicknesses of the passivation coating over the electrode catalyst could range from 0.1 nm to 100 nm. The coatings could be deposited onto the electrode material by atomic layer deposition, sol gel chemistry (using dip coating, spin coating, and spray coating), electrodeposition, chemical layer by layer deposition, or other suitable thin film deposition techniques (FIG. 5 (i)). The passivation coatings may be further treated by subsequent thermal, chemical, or physical processes to control properties, for example by heating in air, nitrogen gas, argon gas, oxygen gas, hydrogen gas, mixed gases, or plasma treatment at temperatures ranging from 50 °C to 1000 °C, with typical ranges from 200 °C to 350 °C. The passivation material could be deposited on the catalyst layer after coating the gas diffusion electrode by these methods, incorporated into the electrode catalyst material during synthesis/production, or formed in situ during operation from liquid or soluble passivation material precursors, which give passivation materials as products, added to an electrochemical cell. The passivation materials can be metal salts, where the metal could be a transition metal, an alkaline earth metal, a lanthanide series metal, or mixed metals of the forementioned metals, wherein halide, nitrate, sulfate, carbonate, or other common anions are used as a counterion. The passivation coating may have self-healing properties whereby dissolutionredeposition equilibria continually repair defects and pinholes in the passivation coating. Other examples of passivation coatings could be chemically stable organic polymers deposited by the above method or by layer-by-layer assembly. These polymers include thin-films or porous networks of the previously described conductive ionomers and non-conductive polymers (FIG. 5 (ii)).

The passivation coating could be a mixture of ionomer (ion conducting polymers), catalyst binder (non-ion conducting polymer or other material used to improve catalyst adhesion to the electrode as previously described) and additives composed of oxidatively stable organics or inorganic particles. An example of an inorganic additive would be metal oxides or hydroxide nanoparticles, as described above, synthesized using bottom-up sol-gel synthesis, hydrothermal or solvothermal methods, or top-down synthesis, for example, via the liquid-phase exfoliation from bulk inorganic powders. The inorganic additives could have different shapes and dimensionalities, such as nanospheres, nanoclusters, onedimensional nanowires, rods, tubes, two-dimensional nanosheets, platelets, three-dimensional nanostructures, or mixture of those with compositions mentioned above. A mixture of ionomer and inorganic additive could be prepared with and without catalyst binder by mechanical shaking, stirring or sonication in water, polar solvent, non-polar solvent, or any of mixed solvent. A mixed layer of organic polymers and inorganic particles with different weight ratios would form nano- or micron scale films over the catalyst layer (FIG. 5 (iii)).

Passivated electrodes are not limited to a configuration of additional coatings of passivation materials deposited on catalyst coated electrodes or membranes but includes an electrode design of passivation materials incorporated into conventional catalyst layers as passivation additives. One example of such electrode designs consists of well-dispersed catalyst nanoparticles with metal oxide/hydroxide (passivating) nanoparticle additives in an ink with ionomers (FIGS. 6A-B). Upon operation the passivating oxide/hydroxide additives may assemble at the catalyst ionomer interface, preventing substantial deleterious oxidation reactions from occurring (FIGS.7A-B).

One embodiment is where the passivation coatings or additives are applied by the methods above on any anode or cathode materials (sometimes referred to as electrocatalysts) such as metal oxides, metal sulfides, metal selenides, metal phosphides, carbon and carbon nitrides, and metals, or mixture of any of those materials. Illustrative electrode (electrocatalyst) materials include, but are not limited to, IrO_x, RuO_x, CoO_x, NiO_x, FeO_x, MoS_x, WS_X, MoSe_x, NiP, FeP, CoP, Pt, Pb, Bi, Au and alloys between these compositions of mixed cations and mixed anions, where x is typically between 1 and 3. In certain embodiments, x is very close to 2. In some embodiments, the materials are amorphous materials with O and OH in different ratios. These catalysts can be loaded on the electrode support (typically a gas diffusion layer or porous transport layer) or membrane at a mass loading of 1 pg/cnr to 100 mg/cm², more particularly from 1-5 mg/cm². Electrode support materials include, but are not limited to, Ti, Ni, steel, stainless steel, Cu, and Pt-coated Ti or stainless steel.

The electrolytes used in the electrochemical cell (e.g., a water electrolyzer) could be solid polymer electrolytes (ionomers) of basic or acidic character such as, but not limited to, perfluorosulfonic cation exchange polymers (e.g, Nation), poly-(benzimidazole)-based anion exchange ionomers (e.g., Aemion), imidazolium-functionalized styrene and vinylbenzyl chloride based anion exchange polymers (e.g., Sustainion), and functionalized poly(aryl piperidinium) anion exchange polymers (e.g, PiperlON-type materials), or related ion-conducting polymer systems.

In another embodiment, the electrolyte could be an aqueous electrolyte such as soluble acids, bases, or salts, including but not limited to, H2SO4, H3PO4, HCIO4, HC1, NaCl, KC1, phosphate containing buffers, carbonate containing buffers, borate containing buffers, K2SO4, KOH, NaOH or LiOH.

In another embodiments, the electrolytes could be a nonaqueous electrolyte such as tetraalkyl and Li tetrafluoroborate, hexafluorophosphate and carbonate salts in solvents including but not limited to acetonitrile, methanol, toluene, chloroform, dichloromethane, tetrahydrofuran and other ethers including ethylene or propylene carbonate.

The passivated electrode disclosed herein is applicable at the anode and/or cathode under a range of aqueous pH conditions (from pH -1 to 15) as well as non-aqueous conditions used in electrosynthesis or other applications of electrolyzers and fuel cells. In certain embodiments, the passivation material is typically applied to the reactive anode catalyst that operates under harsh alkaline conditions typically defined by hydroxide concentrations in excess of 0.1 M and oxidative potentials in excess of 1.23 V versus the reversible hydrogen electrode.

In certain embodiments the passivated electrodes disclosed herein do not undergo volume expansion of the electrode material in electrolyzers.

The porosity of the passivation coating and the catalyst/gas-diffusion-electrode architecture can be tuned for allowing transport of product and reactant gas and or soluble species to and from the electrode catalyst surface without interference by the passivation coating. For example, the porosity and crystallinity of the passivation coating can be optimized by changing deposition temperature and using annealing after film deposition, which may significantly affect hydroxide ion transport and mechanical and electrochemical stability of the films. Similarly, the porosity of passivation additives can be tuned using chemical or mechanical etching.

In addition to electrolyzers, the passivation coating and additive disclosed herein could also be utilized in fuel cells where the passivation coating or additive has the same function, i.e. to protect sensitive cell components such as the electrolyte from deleterious faradaic reactions primary due to oxidation. For example, but not limited to, application of the passivation coating or additive at the oxygen cathode of an alkaline membrane fuel cell where oxidative damage to the alkaline ionomer is also likely a problem. The passivation materials could be used to prevent unwanted reactivity of reactants or products at the anode or cathode by size exclusion through the passivation materials.

Certain embodiments are disclosed herein with reference to the following numbered clauses:

1. A construct comprising : an anode; a cathode; and at least one electrolyte, wherein at least one of the anode or the cathode comprises a passivation coating disposed on a surface of the anode or the cathode, and the passivation coating comprises a passivation material selected from HfO_x, TiO_x, ZrO_x, TaO_x, ScO_x, A1O_X, LaO_x, SiO_x, Mg(OH)_x, Ca(OH)x, Sr(OH)_x, Ba(OH)_x, a mixture thereof, or an alloy thereof, wherein x is 1 to 3.

2. A construct comprising: an anode; a cathode; and an anion exchange membrane positioned between the anode and the cathode, wherein the anode comprises an ionomer, and a passivation-coated anode catalyst. 3. The construct of clause 1 or 2, wherein the passivation coating has a thickness of 1 nm to

100 nm.

4. The construct of clause 1 or 2, wherein the passivation coating has pores having diameters of 1 angstrom to 20 nm.

5. The construct of clause 2, wherein the passivation coating comprises a passivation material selected from HfO_x, TiO_x, ZrO_x, TaO_x, ScO_x, A1O_X, LaO_x, SiO_x, Mg(OH)_x, Ca(OH)x, Sr(OH)_x, Ba(OH)_x, a mixture thereof, or an alloy thereof, wherein x is 1 to 3.

6. The construct of any one of clauses 1 to 5, wherein the passivation material is HfOj.

7. The construct of any one of clauses 1 to 5, wherein the passivation material is TiO .

8. The construct of any one of clauses 2 to 7, wherein the anode catalyst comprises IrOz, a nickel oxide or a cobalt oxide.

9. A water electrolyzer comprising the construct of any one of clauses 2 to 8, an inlet for water, an outlet for O2, and an outlet for H2.

10. A method comprising: forming a passivation coating on a surface of an electrode catalyst material resulting in a passivated electrode catalyst material, and contacting the passivated electrode catalyst material with at least one electrolyte, wherein the passivation material selected from HfO_x, TiO_x, ZrO_x, TaO_x, ScO_x, A1O_X, LaO_x, SiO_x, Mg(OH)_x, Ca(OH)x, Sr(OH)_x, Ba(OH)_x, a mixture thereof, or an alloy thereof, wherein x is 1 to 3.

11. The method of clause 10, wherein forming the passivation coating comprises atomic layer deposition of the passivation material, sol gel chemistry forming the passivation material, and/or chemical layer by layer deposition of the passivation material.

12. The method of clause 11, further comprising subsequently thermally and/or chemically treating the coating.

Examples

One type of passivated anodes was prepared with Ir -based oxygen evolution reaction catalyst layer on stainless steel with HfO2 passivation coating deposited by atomic layer deposition (ALD) with different numbers of deposition cycles to control thickness of HfOz coatings. Then a thin layer of a functionalized poly(aryl piperidinium) anion exchange ionomer (PiperlON) was spray-coated as a top layer. One sample SEM image of the passivated anodes is shown in FIG. 8A, consisting of stainless-steel substrate, Ir catalyst layer and HfOz thin film coating. The gas diffusion layers for a cathode were prepared similar to procedures in the literature and the assembled electrodes were tested in the AEM water electrolyzer at 56 ± 1 °C with pure-water feed. The polarization curves of the samples are shown in FIG.8B and individual curves are marked with the sample name as the number of HfOz AED cycles deposited to form the passivation coating. The coated anodes with thinner layers (fewer cycles) of HfOz show no significant reduction in the energy performance of the system, while the thicker layers of HfOz coatings lead to higher overall cell voltage as the coatings serve as a resistive film. Durability data operated at 500 mA/cm² shows that the voltage and voltage degradation rate are improved with thin layer of HfOz coatings (FIG. 8C).

The other example is based on cobalt oxide electrode (oxygen evolution catalyst), where a high- surface-area cobalt oxide catalyst layer was prepared with electron beam deposition and thermal oxidation in air. The passivated anodes were similarly prepared using AED, and a thick layer of hydro xide-conducting polymer was deposited using dip coating. The un-passivated and passivated anodes were tested in the AEM water electrolyzer at 56 ± 1 °C with pure-water feed. The cell performance is shown in FIG. 9A, showing thinner HfOz coatings lead to small cell performance loss compared to un-passivated electrode. The assembled electrodes were tested at 1 A/cm² for about 100 hours (FIG. 9B). The calculated voltage degradation rates were calculated for last 50 hours as 1.1 mV/h for un-passivated anode and 0.5-0.6 mV/h for HfOz passivated anodes.

Another example is based on a passivation coating composed of ionomer and passivation additive dispersion/mixture that is deposited on Ir-based electrodes. In FIG. 10A, the polarization curves of Ir-based unprotected anode (marked as “ionomer”) and a nanoparticle Mg(OH)z and PiperlON ionomer mixed-layer- coated Ir anode were plotted (marked as “ionomer+LDH”). The Mg(OH)j addition to the passivation coating does not significantly affect the electrolyzer voltage efficiency, specifically, but not limited to, when the ionomer : Mg(OH)j weight ratio was 77 : 23. Furthermore, as shown in FIG. 10B, when Mg(OH)j was incorporated in the ionomer layer, the final voltage was lower (improved) relative to the electrode without the Mg(OH)j additive. This data shows that Mg(OH)j stabilizes the catalyst-ionomer interface.

The new passivation coatings improve durability through suppressing the oxidative degradation of polymer ionomers and binders and through modulating metal dissolution and redeposition. An exemplified XPS analysis showing ionomer oxidation protection is shown in FIG. 11 supporting the protection of ionomers by the HfOz films at the ionomer-catalyst interface. When compared to pristine ionomer structure deposited on the anode, after the electrolyzer operation, peaks appear at higher binding energies in C Is spectra which can be readily assigned to oxidized ionomers species containing C-O and O-C=O. The intensity of the peaks in N Is and F Is spectra also diminish as the loss of charged nitrogen and fluorinated carbon. Compared to the un-passivated control anode, the HfOz passivated anode shows suppressed oxidative damage as evidence in the C is spectra. Furthermore, the passivated anode has higher F/C and N⁺/C ratios compared to the un-passivated anode, meaning the HfOz protective passivation coating leads to retention of substantially more N and F functional moieties by suppression of ionomer degradation.

In view of the many possible embodiments to which the principles of the disclosed invention may be applied, it should be recognized that the illustrated embodiments are only preferred examples of the invention and should not be taken as limiting the scope of the invention.

Claims

What is claimed is:

1. A construct comprising : an anode; a cathode; at least one electrolyte; a catalyst in contact with at least one of the anode or the cathode; and a passivation coating disposed on a surface of the catalyst, and the passivation coating comprises a passivation material selected from a metal oxide, a metal hydroxide, a metal oxyhydroxide, or a metal carbonate, or a mixture thereof, wherein the metal is a transition metal, an alkaline earth metal, a lanthanide series metal, aluminum or silicon.

2. A construct comprising: an anode; a cathode; an ion exchange membrane positioned between the anode and the cathode; and a catalyst layer deposited on either the anode or a side of the ion exchange membrane that is in contact with the anode, wherein the catalyst layer comprises an ionomer and a passivation-coated catalyst.

3. A construct comprising: an anode; a cathode; at least one electrolyte; and at least one of an anode catalyst layer or a cathode catalyst layer, wherein the anode catalyst layer or the cathode catalyst layer comprises an electrode catalyst, a passivation material additive, and at least one polymer.

4. The construct of claim 1, wherein the passivation coating has a thickness of 0.1 nm to 100 nm.

5. The construct of claim 1, wherein the passivation coating has pores having diameters of 1 angstrom to 50 nm.

6. The construct of claim 1, wherein the passivation material is selected from HfO_x, TiO_x, ZrO_x, TaO_x, ScO_x, A1O_X, LaO_x, SiO_x, Mg(OH)_x, Ca(OH)x, Sr(OH)_x, Ba(OH)_x, a mixture thereof, or an alloy thereof, wherein x is 1 to 3.

7. The construct of claim 2, wherein the passivation coating comprises a passivation material selected from HfO_x, TiO_x, ZrO_x, TaO_x, ScO_x, A1O_X, LaO_x, SiO_x, Mg(OH)_x, Ca(OH)x, Sr(OH)_x, Ba(OH)_x, a mixture thereof, or an alloy thereof, wherein x is 1 to 3.

8. The construct of claim 3, wherein the passivation material additive comprises HfO_x, TiO_x, ZrO_x, TaO_x, ScO_x, A1O_X, LaO_x, SiO_x, Mg(OH)_x, Ca(OH)x, Sr(OH)_x, Ba(OH)_x, a mixture thereof, or an alloy thereof, wherein x is 1 to 3.

9. The construct of claim 1, wherein the passivation material is HfOz.

10. The construct of claim 2, wherein the passivation coating comprises an organic polymer.

11. The construct of claim 3, wherein the passivation material comprises an organic polymer.

12. The construct of claim 1, wherein the catalyst comprises IrOz, a nickel oxide or a cobalt oxide.

13. The construct of claim 2, wherein the catalyst comprises IrOz, a nickel oxide or a cobalt oxide.

14. The construct of claim 3, wherein the electrode catalyst comprises IrOz, a nickel oxide or a cobalt oxide.

15. The construct of claim 3, wherein the at least one polymer comprises an ionomer.

16. A water electrolyzer comprising the construct of claim 1, an inlet for water, an outlet for Oz, and an outlet for Hz.

17. A method comprising: forming a passivation coating on a surface of an electrode catalyst material resulting in a passivated electrode catalyst material, and contacting the passivated electrode catalyst material with at least one electrolyte, wherein the passivation coating comprises a passivation material selected from a metal oxide, a metal hydroxide, a metal oxyhydroxide, or a metal carbonate, or a mixture thereof, wherein the metal is a transition metal, an alkaline earth metal, a lanthanide series metal, aluminum or silicon.

18. The method of claim 17, wherein the passivation material is selected from HfO_x, TiO_x, ZrO_x, TaO_x, ScO_x, A1O_X, LaO_x, SiO_x, Mg(OH)_x, Ca(OH)x, Sr(OH)_x, Ba(OH)_x, a mixture thereof, or an alloy thereof, wherein x is 1 to 3.

19. The method of claim 17, wherein forming the passivation coating comprises atomic layer deposition of the passivation material, sol gel chemistry forming the passivation material, and/or chemical layer by layer deposition of the passivation material.

20. The method of claim 19, further comprising subsequently thermally and/or chemically treating the coating.

21. A method comprising : mixing a passivation material and at least one polymer; contacting the resulting mixture with an electrode catalyst material resulting in a passivated electrode catalyst material; and contacting the passivated electrode catalyst material with at least one electrolyte.

22. The method of claim 21, wherein the passivation material is selected from HfO_x, TiO_x, ZrO_x, TaO_x, ScO_x, A1O_X, LaO_x, SiO_x, Mg(OH)_x, Ca(OH)x, Sr(OH)_x, Ba(OH)_x, a mixture thereof, or an alloy thereof, wherein x is 1 to 3.

23. The method of claim 22, wherein the at least one polymer comprises an ionomer.

24. A method comprising: mixing a passivation material, at least one polymer, and an electrode catalyst material resulting in a passivated electrode catalyst material, wherein the resulting mixture forms a passivation layer between an electrode and an electrolyte, or between an electrode and an ion exchange membrane.

25. The method of claim 24, wherein the passivation material is selected from HfO_x, TiO_x, ZrO_x, TaO_x, ScO_x, A1O_X, LaO_x, SiO_x, Mg(OH)_x, Ca(OH)x, Sr(OH)_x, Ba(OH)_x, a mixture thereof, or an alloy thereof, wherein x is 1 to 3.

26. The method of claim 25, wherein the at least one polymer comprises an ionomer.

- 14 -