An oxygen evolution reaction electrocatalyst and methods for preparation and use thereof

Abstract

An oxygen evolution reaction electrocatalyst and a method for preparing said oxygen evolution reaction electrocatalyst are disclosed. The method comprises the steps of contacting a porous zeolite with a solution containing a noble metal precursor to form a noble metal doped zeolite, pyrolyzing the noble metal doped zeolite to form nanocrystals comprising at least the noble metal and a second metal, said nanocrystals being confined within a layer of carbon-based material, wherein the second metal and the carbon-based material are derived from the pyrolyzed zeolite, and annealing said layer of carbon-based material confined nanocrystals to form isolated noble metal atoms singly dispersed within a metal oxide lattice of the second metal as an oxygen evolution reaction electrocatalyst.

Description

AN OXYGEN EVOLUTION REACTION ELECTROCATALYST AND METHODS FOR PREPARATION AND USE THEREOF Field of the Invention

[0001] The present invention relates to an oxygen evolution reaction electrocatalyst, and methods for the preparation and use thereof. Whilst the invention has been developed for use in water electrolysis, it will be appreciated that the invention is not limited to this particular field of use. Background of the Invention

[0002] The following discussion of the prior art is provided to place the invention in an appropriate technical context and enable the advantages of it to be more fully understood. It should be appreciated, however, that any discussion of the prior art throughout the specification should not be considered as an express or implied admission that such prior art is widely known or forms part of the common general knowledge in the field.

[0003] Oxygen evolution reaction (OER) is used as a half-reaction in a range of important renewable energy conversion and storage systems, such as electrochemical water splitting, CO2 / N2 reduction, and reversible fuel cells. For example, Proton exchange membrane water electrolysis (PEMWE) possesses distinct advantages for hydrogen production compared to conventional alkaline water electrolysis, including high current density, safe operation, and high energy efficiency. Electrochemical CO2 reduction in acid can efficiently avoid carbonate formation and increase carbon utilization efficiency. Nevertheless, the anodic OER is a known sluggish process due to its four-electron transfer along with multi-intermediate species, resulting in large overpotential and low energy conversion efficiency. Acidic OER further suffers from additional steps and kinetics barrier for H2O adsorption / dissociation at active sites, in comparison to OER in alkaline media where abundant OH- are available for the direct deprotonation process. Another challenge for OER in acids is stability due to the severe dissolution and degradation of catalysts under highly corrosive and oxidative acidic conditions.

[0004] To date, the promising acidic OER electrocatalysts are still noble metals including Ruthenium (Ru)- and Iridium (Ir)-based compounds. In particular, Ru-basedmaterials are more active due to their weak binding strength with oxygenated intermediates, however, the accompanied over-oxidation of Ru into RuO4- moieties result in the severe dissolution and loss of active sites in acidic electrolyte with harsh oxidizing conditions. Although considerable efforts have been devoted to improving the OER activity and stability of Ru-based materials (e.g., foreign atoms doping, coordination environment, strain), the trade-off between activity and stability has not yet been well resolved and its high cost and scarcity still impede the large-scale application.

[0005] Single-atom catalysts (SACs) have recently attracted much attention due to their many advantages in electrochemical reactions, such as fully exposed active sites, high atomic efficiency, and reduced usage of precious metals. However, the widely reported metal-nitrogen-carbon (M-N-C) derived SACs, typically prepared by immobilizing transition metal and nitrogen atoms onto high surface area carbon supports, are intrinsically limited by the simplicity of their active sites, whereby the N- doped carbon support rarely exhibits catalytic performance. Moreover, the unavoidable carbon corrosion under oxidation conditions seriously limits their electrochemical applicability. In addition, the interactions between the single active sites and the carbon support play a tremendous role in determining the stability and activity of catalysts. In this regard, the current thinking is that to maximize the catalytic activity and stability of metal atoms, anchoring single atoms on supports is essential for the rational design of active and stable SACs.

[0006] The present invention seeks to provide an oxygen evolution reaction electrocatalyst, and methods for the preparation and use thereof, which will overcome or substantially ameliorate at least some of the deficiencies of the prior art, or to at least provide an alternative. Summary of the Invention

[0007] According to a first aspect, the present invention provides a method for preparing an oxygen evolution reaction electrocatalyst, comprising: - contacting a porous zeolite with a solution containing a noble metal precursor to form a noble metal doped zeolite;- pyrolyzing the noble metal doped zeolite to form nanocrystals comprising at least the noble metal and a second metal, said nanocrystals being confined within a carbon-based material, wherein the second metal and the carbon layer are derived from the pyrolyzed zeolite; and - annealing said layer of carbon-based material confined nanocrystals to form isolated noble metal atoms singly dispersed within a metal oxide lattice of the second metal as an oxygen evolution reaction electrocatalyst.

[0008] In one embodiment, the porous zeolite is a zeolitic imidazolate framework (ZIF) comprising atoms of the second metal and an organic bridging linker.

[0009] Suitably, the second metal is a transition metal selected from the group consisting of cobalt (Co) and zinc (Zn).

[0010] Preferably, the second metal is cobalt (Co) and the organic bridging linker is 2-methylimidazole (2-MIM).

[0011] Suitably, the noble metal precursor comprises a precursor of a noble metal selected from the group consisting of ruthenium (Ru), rhodium (Rh), palladium (Pd), Osmium (Os), iridium (Ir), platinum (Pt) and gold (Au).

[0012] Preferably, the noble metal precursor is ruthenium (III) chloride (RuCl3) or a salt or solvate thereof.

[0013] In one embodiment, pyrolysis of the noble metal doped zeolite is carried out at a temperature that falls within a range of about 500° C to about 700° C. For instance, in some embodiments, pyrolysis of the noble metal doped zeolite occurs at a temperature that falls within a range of about 500° C to about 690° C, about 510° C to about 680° C, about 520° C to about 670° C, about 530° C to about 660° C, about 540° C to about 650° C, about 550° C to about 640° C, about 560° C to about 630° C, about 570° C to about 620° C, about 580° C to about 610° C, about 590° C to about 605° C, and about 595° C to about 605° C.

[0014] In one embodiment, pyrolysis of the noble metal doped zeolite is conducted at a temperature of about 600° C under an inert atmosphere.

[0015] Suitably, the inert atmosphere is argon or nitrogen.

[0016] In one embodiment, annealing of the carbon-layer confined nanocrystals is conducted at a temperature that falls within a range of about 150° C to about 250° C. For instance, in some embodiments, annealing of the carbon-layer confined nanocrystals is conducted at a temperature that falls within a range of about 155° C to about 245° C, about 160° C to about 240° C, about 165° C to about 235° C, about 170° C to about 230° C, about 175° C to about 225° C, about 180° C to about 220° C, about 185° C to about 215° C, about 190° C to about 210° C, about 195° C to about 205° C.

[0017] In one embodiment, annealing of the carbon-layer confined nanocrystals is conducted in air at a temperature of about 200° C.

[0018] In one embodiment, the carbon-based material is selected from the group consisting of graphene, graphene oxide, fullerene, carbon nanotubes, activated carbon and graphite.

[0019] Preferably, the carbon-based material is graphene.

[0020] Preferably, the nanocrystals comprise a ruthenium-cobalt (RuCo) nanoalloy.

[0021] In one embodiment, the porous zeolite is immersed in the noble metal solution for about 30 minutes.

[0022] In one embodiment, the concentration of the noble metal solution ranges from about 0.5 to about 2 mg equivalent of noble metal per ml of solution. For instance, in some embodiments, the concentration of the noble metal solution ranges from about 0.6 to about 1.9 mg, about 0.7 to about 1.8 mg, about 0.8 to about 1.7 mg, about 0.9 to about 1.6 mg, about 1.0 to about 1.5 mg, about 1.1 to about 1.4 mg, about 1.2 to about 1.3 mg, equivalent of noble metal per ml of solution.

[0023] In a preferred embodiment, the oxygen evolution reaction electrocatalyst is RuSACs / Co3O4.

[0024] According to a second aspect, the present invention provides an oxygen evolution reaction electrocatalyst produced by the method according to the first aspect.

[0025] According to a third aspect, the present invention provides an oxygen evolution reaction electrode comprising:- an ion conducting membrane having first and second generally opposed major surfaces; - the oxygen evolution reaction electrocatalyst of the second aspect coated on the first major surface of the ion conducting membrane; and - a first fluid diffusion layer comprising an electrically conductive substrate adjacent to the first major surface of the ion conducting membrane.

[0026] In one embodiment, the oxygen evolution reaction electrocatalyst comprises not greater than 0.001 atomic percent Ru, based on the total collective cation and elemental metal content of the respective layer.

[0027] In one embodiment, the electrically conductive substrate of the first fluid diffusion layer comprises a porous metal screen or mesh selected from the group consisting of titanium (Ti), platinum (Pt), gold (Au), tantalum (Ta), niobium (Nb), zirconium (Zr), hafnium (Hf), or a metal alloy thereof.

[0028] Preferably, the ion conducting membrane comprises a proton exchange membrane.

[0029] According to a fourth aspect, the present invention provides an electrochemical device for water electrolysis comprising the oxygen evolution reaction electrode according to the third aspect.

[0030] Preferably, the electrochemical device further comprises a hydrogen evolution reaction catalyst coated on the second major surface of the ion conducting membrane and a second fluid diffusion layer comprising an electrically conductive substrate in electrical contact with the hydrogen evolution reaction catalyst.

[0031] In one embodiment, the electrically conductive substrate of the second fluid diffusion layer comprises activated carbon paper.

[0032] Preferably, the hydrogen evolution reaction catalyst comprises platinum (Pt).

[0033] In one embodiment, the device is a water electrolyzer.

[0034] In one embodiment, the device is a CO2 electrolyzer.

[0035] According to a fifth aspect, the present invention provides method of generating hydrogen gas (H2) and oxygen gas (O2) from water (H2O), the method comprising: - providing an oxygen evolution reaction electrode membrane electrode assembly (MEA) comprising an anode and a cathode catalyst, the anode being an oxygen evolution reaction electrode according to the third aspect, and having the cathode catalyst coated on the second major surface of the ion conducting membrane; - providing water in contact with the anode; and - providing an electrical potential with sufficient electrical current across the membrane electrode assembly (MEA) to convert at least a portion of the water (H2O) to hydrogen gas (H2) and oxygen gas (O2) at the cathode and anode, respectively.

[0036] Preferably, the cathode catalyst is a hydrogen evolution reaction catalyst comprising platinum (Pt).

[0037] In a preferred embodiment, the electrical current across the MEA is of a value that falls within a range of about 1 A to about 10 A. For instance, in some embodiments, the electrical current is of a value that falls within a range of about 1.0 A to about 1.5 A, about 1.5 A to about 2.0 A, about 2.0 A to about 2.5 A, about 2.5 A to about 3.0 A, about 3.0 A to about 3.5 A, about 3.5 A to about 4.0 A, about 4.0 A to about 4.5 A, about 4.5 A to about 5.0 A, about 5.0 A to about 5.5 A, about 5.5 A to about 6.0 A, about 6.5 A to about 7.0 A, about 7.0 A to about 7.5 A, about 7.5 A to about 8.0 A, about 8.0 A to about 8.5 A, about 8.5 A to about 9.0 A, about 9.0 A to about 9.5 A, and about 9.5 A to about 10.0 A.

[0038] In one embodiment, the oxygen evolution reaction electrode exhibits an overpotential below 0.5 V versus a reversible hydrogen electrode (RHE) at a current density of 10 mA / cm2during electrolysis of water in an acidic environment at room temperature.

[0039] In one embodiment, the oxygen evolution reaction electrode exhibits an overpotential below 0.1 V versus a reversible hydrogen electrode (RHE) at a currentdensity of 10 mA / cm2during electrolysis of water in an acidic environment at room temperature.

[0040] Other aspects of the invention are also disclosed.

[0041] Brief Description of the Drawings

[0042] Notwithstanding any other forms which may fall within the scope of the present invention, preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

[0043] Figure 1 shows a scanning electron microscopy (SEM) image of of Ru-doped ZIF-67; (Scale bar: 30 µm);

[0044] Figure 2 shows a) Co 2p XPS spectra of RuSACs / Co3O4 and Co3O4. b) Ru 3p XPS spectra of RuSACs / Co3O4 and RuO2. c) O 1s XPS spectra of RuSACs / Co3O4 and Co3O4. d) XANES and e) EXAFS spectra of Ru k-edge of RuSACs / Co3O4, RuO2, and Ru foil, and f) corresponding wavelet-transformed k3-weighted EXAFS spectra of Ru k-edge of RuSACs / Co3O4;

[0045] Figure 3 shows an electrocatalytic OER performance of RuSACs / Co3O4 with control samples. a) OER Polarization curves were obtained at 5 mV s-1in O2-saturated 0.5 M H2SO4. b) Comparison of overpotential at 10 mA cmgeo-2and mass activity at 1.53 V vs RHE. c) Chronopotentiometry of Ru SACs / Co3O4 and c-RuO2 at 10 mA cm-2in 0.5 M H2SO4. d) The dissolved Ru (up) and Co (down) ion concentrations measured for RuSACs / Co3O4, Co3O4, and c-RuO2 in electrolyte by ICP-MS. e) Comparison of overpotential and stability at 10 mA cmgeo-2with previously reported OER catalysts in acid;

[0046] Figure 4 shows in-situ Raman spectroscopy images of Ru SACs / Co3O4 at (a) different reaction times and (b) different applied potentials from OCP to 1.7 V during the oxygen evolution reaction (OER) process;

[0047] Figure 5 shows in-situ Raman spectroscopy images of c-RuO2 (a) different reaction times and (b) different applied potentials from the open circuit potential (OCP) to 1.7 V during the oxygen evolution reaction (OER) process; and

[0048] Figure 6 is a schematic representation of an exemplary water electrolyzer comprising an oxygen evolution reaction (OER) electrode bearing the RuSACs / Co3O4 electrocatalyst.Description of Preferred Embodiments of the Invention

[0049] It should be noted in the following description that like or the same reference numerals in different embodiments denote the same or similar features.

[0050] The present invention is predicated on a simple yet robust technology in which a metal organic framework is reacted with a noble metal precursor to form an electrocatalyst in which atoms of the noble metal are confined in an atomically distributed manner within a metal oxide lattice.

[0051] Metal organic frameworks (MOFs) are porous hybrid materials containing organic and inorganic components. They include inorganic metal ion secondary building blocks (SBUs), oxo-clusters, and bridging organic linkers.

[0052] In contrast to traditional porous materials, MOFs possess a crystalline nature with a large surface area and ultrahigh porosity, mixed with an exceptionally high level of organic and inorganic composition, which enables them to be used for various applications, including membranes, catalysis, gas capture and storage, adsorption, and energy Each MOF family, including ZIF (zeolitic imidazolate framework), is distinguished by a different metal node or organic linker used in their synthesis.

[0053] Specifically, ZIF materials contain a zeolite framework synthesized using metal sources such as Zn and Co and organic ligands such as imidazole or imidazole derivatives acting as bridging linkers. An important ZIF material is ZIF-67, which consists of a tetrahedral coordination of cobalt cations (Co2+) in combination with an imidazolate linker, 2-methylimidazole (2-MIM), affording a cubic crystal structure.

[0054] With this in mind, the inventors have developed a facile two-step strategy to confine atomically distributed atoms of a noble metal within a metal oxide lattice in a single-atom catalyst (SAC) type arrangement by pyrolyzing a noble metal-doped zeolitic imidazolate framework (ZIF-67).

[0055] This atomic distribution of noble metal atoms provides several advantages over conventional noble metal-based heterogenous catalysts, including:

[0056] 1. High activity

[0057] 2. High stability

[0058] 3. Low cost.

[0059] A more detailed explanation of the advantages of the as-prepared noble metal-based electrocatalyst will be discussed in the following section.

[0060] Method

[0061] A method for preparing an oxygen evolution reaction electrocatalyst according to a preferred embodiment of the present invention is now described.

[0062] According to a first step, the method comprises the step of contacting a porous zeolite with a solution containing a noble metal precursor to form a noble metal doped zeolite.

[0063] In a preferred embodiment, the porous zeolite is the zeolitic imidazolate framework (ZIF-67), which consists of a tetrahedral coordination of transition metal cations, in this case, cobalt cations (Co2+), in combination with the organic bridging imidazolate linker, 2-methylimidazole (2-MIM), affording a cubic crystal structure.

[0064] The noble metal precursor may comprise a precursor of a noble metal selected from the group consisting of ruthenium (Ru), rhodium (Rh), palladium (Pd), Osmium (Os), iridium (Ir), platinum (Pt) and gold (Au).

[0065] Preferably, the noble metal precursor is a Ruthenium (Ru)-based precursor or an Iridium (Ir)-based precursor. The Ruthenium (Ru)-based precursor may be selected from the group consisting of RuCl3 and Ru(acac)3.

[0066] Good results have been obtained by immersing ZIF-67 into a solution of RuCl3 comprising about 0.5 to 2 mg equivalent of Ru per ml of solution for 30 minutes in air at a temperature that falls within a range of about 20° C to about 30° C. By virtue of this step, the Ru-based precursor is in-situ trapped within the pores of ZIF-67 during crystallization to form Ru-doped ZIF-67.

[0067] It will be appreciated by persons skilled in the relevant art that the porous zeolite is not limited to ZIF-67, and that other zeolites may be employed. For instance, in one example, the porous zeolite may be ZIF-8, in which the transition metal cations are zinc (Zn) cations, and the organic bridging linker is 2-methylimidazole (2-MIM).

[0068] According to a second step, the method comprises the step of pyrolyzing the Ru-doped ZIF-67 at a temperature that falls within a range of about 500° C to about 700° C under an inert atmosphere of argon or nitrogen to form nanocrystals of a ruthenium-cobalt (RuCo) nanoalloy, confined within a layer of carbon-based material.The Co and carbon-based material are both derived from the pyrolysis of the ZIF-67 framework.

[0069] In this reaction, the carbon-based material is observed to be a graphene layer.

[0070] Thus, according to a third step, the method comprises the step of annealing said graphene-layer confined nanocrystals in air at a temperature that falls within a range of about 150° C to about 250° C to form isolated atoms of Ru singly dispersed within a lattice of cobalt oxide (Co3O4) as the oxygen evolution reaction (OER) electrocatalyst, RuSACs / Co3O4.

[0071] It will be appreciated by persons of skill in the relevant art that other forms of carbon-based material can be produced during the pyrolysis stage which may also serve the purpose of confining the nanocrystals of the RuCo nanoalloy prior to annealing. For example, the carbon-based material may also take the form of graphene oxide, fullerene, carbon nanotubes, activated carbon and / or graphite.

[0072] Electrochemical Device for Water Electrolysis

[0073] The present invention provides an electrochemical device for water electrolysis according to another preferred embodiment of the present invention.

[0074] As schematically represented in Figure 6, the water electrolyzer cell 10 takes the form of an oxygen evolution reaction electrode membrane electrode assembly (MEA).

[0075] Specifically, the water electrolyzer cell 10 comprises an ion conducting membrane (ICM) 40 having first and second generally opposed major surfaces, an anode portion adjacent to the first major surface of the ICM 40, and a cathode portion adjacent to the second major surface of the ICM 40.

[0076] The ICM 40 is a proton exchange membrane, sometimes referred to as a polymer-electrolyte membrane. A proton-exchange membrane (PEM) is a semipermeable membrane designed to conduct protons while acting as an electronic insulator and reactant barrier, for example, to oxygen gas (O2) and hydrogen gas (H2). Thus, the essential function of a PEM when incorporated into a membrane electrode assembly (MEA) of a PEM fuel cell or a PEM electrolyzer, is the separation of reactants and transport of protons, while blocking a direct electronicpathway through the membrane. PEMs can be made from either pure polymer membranes or from composite membranes, where other materials are embedded in a polymer matrix. One of the most common and commercially available PEM materials is the fluoropolymer, perfluorosulfonic acid (PFSA) (sold commercially under the name, Nafion™, by DuPont Chemicals, Wilmington, DE).

[0077] The anode portion takes the form of an oxygen evolution reaction (OER) electrode, comprising an anode 30 combined with a first fluid diffusion layer 20.

[0078] The anode 30 comprises an electrically conductive substrate positioned adjacent to the first major surface of the ICM 40. The electrically conductive substrate is typically in the form of a metal selected from the group consisting of titanium (Ti), gold (Au), or a combination thereof.

[0079] The (OER) electrocatalyst, RuSACs / Co3O4 (indicated in Figure 6 as reference numeral 35) is coated on the first major surface of the ICM 40.

[0080] The cathode portion takes the form of a hydrogen evolution reaction (HER) electrode, comprising a cathode 50 comprising an electrically conductive combined with a second fluid diffusion layer 60. The electrically conductive substrate is also typically in the form of a metal selected from the group consisting of titanium (Ti), gold (Au), or a combination thereof.

[0081] A hydrogen evolution reaction (HER) catalyst 55 is coated on the second major surface of the ICM 40.

[0082] In one embodiment, the HER catalyst comprises platinum (Pt).

[0083] The first and second fluid diffusion layers 20, 60 are an important channel for gases (hydrogen and oxygen) and electron transfer. The fluid diffusion layers 20, 60 confluence the hydrogen produced at the cathode 50 and the oxygen produced by the anode 30 and output them. Therefore, the fluid diffusion layers 20, 60 need to have high mechanical and chemical stability. The electrons generated at the anode 30 enter the external circuit through the anode fluid diffusion layer 20 and then enter the cathode catalytic layer through the cathode fluid diffusion layer 60. Therefore, the fluid diffusion layers 20, 60 also need to have high conductivity.

[0084] Typically, the first fluid diffusion layer 20 is manufactured from a metal foam or porous metal screen or mesh comprised, for example, a porous metal screen ormesh selected from the group consisting of titanium (Ti), platinum (Pt), gold (Au), tantalum (Ta), niobium (Nb), zirconium (Zr), hafnium (Hf), or a metal alloy thereof, that will not corrode, and yet will have adequate electrical conductivity.

[0085] For instance, in one embodiment, the first fluid diffusion layer 20 is manufactured from porous titanium (Ti) sputter coated or electroplated with a layer of platinum (Pt) or gold (Au) on the surface.

[0086] The second fluid diffusion layer 60 is typically manufactured from a carbon- based substrate such as activated carbon paper.

[0087] The electrochemical cell further comprises a power source 70 electrically connected to the anode 30 and the cathode 50 and configured to apply an electrical current across the water electrolyzer cell 10.

[0088] In an alternative embodiment, the electrochemical device may be a CO2 electrolyzer.

[0089] Method for Generating Hydrogen gas (H2) and Oxygen gas (O2) from Water (H2O)

[0090] Using the water electrolyzer cell 10 described above, water (H2O) is introduced into the anode portion of the cell 10, where it passes through the first fluid transport layer 20 and over the anode 30. An electrical potential is then applied across the MEA to generate an electrical current sufficient to convert at least a portion of the water (H2O) to hydrogen gas (H2) and oxygen gas (O2) at the cathode and anode, respectively.

[0091] As previously indicated, the ICM 40 permits only the passage of hydrogen ions (H+) through the ICM 40 to the cathode portion of the water electrolyzer cell 10. Since electrons cannot pass through the ICM 40, they instead, flow through an external electrical circuit in the form of an electrical current, as indicated by the curved arrow in Figure 6.

[0092] After passing through the ICM 40, the hydrogen ions (H+) combine with the electrons at the cathode 50, resulting in the formation of hydrogen gas (H2), which is collected via the second fluid transport layer 60. Meanwhile, oxygen gas (O2) is collected at the anode 30 via the first fluid transport layer 20.

[0093] The inventors have observed good results when the electrical current applied across the water electrolyzer cell 10 is of a value that falls within a range of about 1 A to about 10 A.

[0094] Under these conditions, the oxygen evolution reaction electrode exhibits an overpotential below 0.5 V, and more particularly below 0.1 V when versus a reversible hydrogen electrode (RHE) at a current density of 10 mA / cm2during electrolysis of water in an acidic environment at room temperature.

[0095] Results and Discussion

[0096] Electrocatalyst synthesis and characterization

[0097] For the fabrication of the RuSACs / Co3O4 electrocatalyst, the porous zeolitic imidazolate framework (ZIF-67) is employed as the host to in-situ trap a RuCl3 precursor within its pores during the crystallization process. Thereafter, Ru-doped ZIF- 67 is pyrolyzed in Ar at 600 °C to form solid RuCo nanocrystals confined in a graphene layer (denoted hereinafter as RuCo@G). Then, the final RuSACs / Co3O4 electrocatalyst is formed by annealing RuCo@G in air, whereby the metallic Co is oxidized, and the linker-derived graphene is selectively etched.

[0098] As shown in Figure 1, the scanning electron microscopy (SEM) image of Ru- doped ZIF-67 maintains the cubic morphology with rough surface. The X-ray- diffraction (XRD) pattern of Ru-doped ZIF-67 was well matched with that of ZIF-8, indicating that RuCl3 did not disturb the intrinsic framework structure of ZIF-67. After pyrolysis in N2, the XRD patterns match well with the hexagonal Co (JCPDS No.05- 0727) phase, along with a broad and weak peak located at 23° for carbon as evidenced by Raman spectra. RuCo@G was further oxidized under air to yield the atomically dispersed Ru sites embedded in the lattice of Co3O4. The synchrotron based XRD patterns revealed that the main phase can be indexed to cubic Fd3m Co3O4 phase (JCPDS No. 76-1802). Interestingly, Ru-doped Co3O4 shows almost identical diffraction peak to that of pure Co3O4, suggesting that the Ru dopant is trapped in-situ in the lattice of Co3O4 with loading.

[0099] The Ru content determined by inductively coupled plasma mass spectroscopy (ICP-MS) is 2.7 wt.%. In the Raman spectrum, three characteristic peaks of Co3O4 located at 191, 471, and 671 cm-1are corresponding to F1g(1), Eg, and A1g modes of Co-O bond, respectively. Generally, A1g band shows a strong dependenceupon the octahedral cations and the F2g(1) band is the feature of tetrahedral cations. Compared to pure Co3O4, the redshift and decreased intensity of the A1g peak for Co- O (683 to 678 cm-1) vibration in CoO6 octahedra can be attributed to the distortion of CoO6 octahedra after introducing the atomically dispersed Ru sites. In contrast, the unchanged wavenumber of F2g(1) band indicates that Ru atoms are high possibly located at the position of Co3+sites in octahedral units, which is due to the preference of cations with larger radii for locating at the octahedral sites (Ru3+: 68 pm, Co2+: 65 pm).

[0100] The surface compositions and electronic structure are investigated by X-ray photoelectron spectroscopy (XPS). The presence of Ru, Co, and O elements in XPS survey spectra is consistent with the EDX mapping results. The Ru 3p peaks of Ru SACs / Co3O4 shift negatively toward a lower binding energy compared with c-RuO2, suggesting an increased electron density of Ru sites in RuSACs / Co3O4 (Figure 2a). Simultaneously, the Co 2p peaks for RuSACs / Co3O4 shifted positively toward a higher binding energy compared with that of Co3O4, suggesting a lower oxidation state of Co after incorporating Ru atoms (Figure 2b). Besides, the higher Metal-O peak in Ru SAC / Co3O4 indicates that the O atoms are positively charged (Figure 2c). This suggests that some electrons were transferred from Co to Ru through the bridged O in RuSAC / Co3O4, confirming the strong electronic interactions between Ru species and Co3O4, and the establishment of a synergistic effect.

[0101] The X-ray absorption spectroscopy (XAS) was further performed to provide a detailed understanding of the chemical state and coordination configurations of Ru atoms. The near-edge absorption energy of the Ru K-edge in Ru SACs / Co3O4 situated between the Ru foil and c-RuO2 references, indicating the average valence state of Ru in RuSACs / Co3O4 is lower than +4, which is in line with the XPS results (Figure 2d). The first derivative of EXAFS and corresponding fitted results show that the average valence state of Ru is +3.4 in RuSACs / Co3O4. Notably, compared with c- RuO2 with highly symmetrical octahedral structures, a pre-edge peak of RuSACs / Co3O4 near 22114 eV is related to the transition of Ru 1s to the unoccupied Ru 4d level. The higher pre-edge peak intensity in RuSACs / Co3O4 suggests a more disordered Ru coordination geometry than that in c-RuO2.

[0102] As shown in the Fourier transform of the EXAFS (FT-EXAFS) spectra of Ru K-edge (Figure 2e). the dominant peak at ≈1.48 Å (without phase correction) for RuSACs / Co3O4 can be assigned to shell coordination of the nearest Ru-O bond, without the Ru-Ru or Ru-Co peaks at ≈2.3 Å, suggesting the Ru atoms are coordinated with O atoms in Co3O4. Importantly, compared with c-RuO2, RuSACs / Co3O4 shows a significant decreased length of Ru-O interatomic distance, presumably owing to its long-range structural disorder.

[0103] From the fitted EXAFS results, the average Ru-O coordination number of RuSACs / Co3O4 was determined to be 4.8, which is much lower than c-RuO2 of 6.0. This further confirmed the disordered structure of Ru atoms in RuSACs / Co3O4. The large amount of availably unsaturated Ru species might serve as active centres to enhance reactants' adsorption and possess improved intrinsic OER activity.

[0104] The corresponding wavelet-transforms (WT) of the k3-weighted EXAFS spectra of Ru k-edge of RuSACs / Co3O4 further confirm the above structure (Figure 2f), where the dominant intensity at 7.5 Å-1is ascribed to the coordinated configuration of the Ru-O structure. Similarly, the WT contour maximum at around 9.4 Å-1is ascribed to the coordinated configuration of the Ru-Co structure.

[0105] Electrochemical performance

[0106] The electrochemical performance of the as-synthesized electrocatalyst was evaluated by a three-electrode system in O2-saturated 0.5 M H2SO4 with 85% iR- compensation, with RuO2NPs / Co3O4, Co3O4, and commercial RuO2 (denoted as c- RuO2) as references.

[0107] As shown in the linear-scan-voltammograms (LSV) curves, RuSACs / Co3O4 demonstrated superior catalytic activity with the lowest onset potential of 1.35 V vs a reversible hydrogen electrode (RHE) among the tested samples (Figure 3a). The overpotential to deliver current densities of 10 / 100 mA cm-2is only 186 / 223 mV, which is much lower than that of each of RuO2NPs / Co3O4 (207 / 256 mV), and c-RuO2 (286 / 370 mV) (Figure 3b).

[0108] The Ru SACs / Co3O4 electrocatalyst exhibited an ultrahigh mass activity of 28066 ARu-1, which is about 527-times higher than that of c-RuO2 (53 ARu-1), suggesting that the Ru SACs / Co3O4 electrocatalyst has a superior intrinsic activity. The BET- normalized current density was adopted to eliminate the effect of particle size and surface area and compare OER activity among the tested catalysts, which shows the same activity trend with mass specific activity. The turnover frequency (TOF) wascalculated to understand the intrinsic activity of the electrocatalyst based on the reported protocols.

[0109] At an overpotential of 300 mV, the Ru SACs / Co3O4 electrocatalyst yields an ultrahigh TOF value of 9.20 s-1in 0.5 M H2SO4, which is 1840 times higher than c- RuO2 (0.005 s-1), suggesting its superior intrinsic activity and high atom utilization efficiency (Figure 3b).

[0110] To gain insight into the reasons for the catalytic performance of each electrocatalyst, the inventors investigated the electrochemically active surface area (ECSA) based on the double-layer capacitance (Cdl) calculated by cyclic voltammetry method. The largest Cdl value for RuSACs / Co3O4 implies more accessible active sites to the reactants during OER process, which possibly accounts for its remarkable catalytic activity. The smallest charge transfer resistance in Nyquist plots and Tafel slopes for Ru SACs / Co3O4 indicate its more favourable reaction kinetics and charge transfer efficiency, further supporting its excellent performance towards acidic OER. The Faradaic efficiencies (FE) of Ru SACs / Co3O4 was determined to be 96%, indicating most of the charge transfer process was dominated by OER electrocatalysis. The effect of post-oxidation temperatures and catalyst loading are also investigated.

[0111] This result suggests that the strong synergistic effect between Ru atoms and Co3O4 in RuSACs / Co3O4 efficiently suppresses the dissolution of Ru and Co ions in the electrolytes during the acidic OER process, thereby achieving an excellent stability. To the best of our knowledge, the superior performance of RuSACs / Co3O4 outperforms most of the previously reported electrocatalysts for the OER process in acidic media (Figure 3e, Table 1). More strikingly, the stability is still well-maintained at 1 A cm-2in proton exchange membrane (PEM) water electrolysers by employing RuSACs / Co3O4 and Pt / C as anode and cathode electrodes, respectively.Table 1. Comparison of the electrocatalytic properties of RuSACs / Co3O4 with reported catalysts for OER in acidic media at 10 mA cmgeo-2. Catalysts Electrolyte Overpotential@10 Stability@10 mA cmgeo-2 / mV mA cmgeo-2 / h RuSAC / Co3O4 0.5 M H2SO4 185 200 RuIr@CoNC 0.5 M H2SO4 223 40 Ir−NiCo2O40.5 M H2SO4240 70 Y1.7Sr0.3Ru2O7 0.05 M H2SO4 264 28 IrW 0.5 M H2SO4 291 120 RuB2 0.5 M H2SO4 223 10 Sn-RuO2 0.5 M H2SO4 178 150 a / c-RuO2 0.1 M HClO4 220 60 Cr0.6Ru0.4O2 0.5 M H2SO4 178 10 Ru3MoCeOx 0.5 M H2SO4 164 100 Ru / RuS2 0.5 M H2SO4 201 24 6H-SrIrO3 0.5 M H2SO4 248 30 RuFeOx 0.1 M HClO4 187 10 Co0.11Ru0.89O2-δ 0.5 M H2SO4 169 50 Ufd-RuO2 / CC 0.5 M H2SO4 179 20 Al‐Ni‐Co‐Ir‐Mo 0.5 M H2SO4 233 45 Ru-SA / Ti3C2Tx 0.1 HClO4 290 32

[0112] The durability of RuSACs / Co3O4 was assessed by chronopotentiometry method in H-cell with c-RuO2 and Co3O4 as references. RuSACs / Co3O4 exhibited remarkable stability for 200 h at 10 mA cm-2along with only 6 mV increase in overpotential (Figure 3c).

[0113] In contrast, c-RuO2 suffered from a fast activity degradation within 4 h. The average increase rate of overpotential for RuSACs / Co3O4 was 0.03 mV h-1, which is 211 times lower than that of the c-RuO2 (6.33 mV h-1). The ex-situ ICP-MS results show that the concentration of dissolved Ru ions in RuSACs / Co3O4 slightly increased during the first few hours and stabilized at a low level of 6.4 ppb afterward. Importantly, the dissolved Co ion concentration is much lower than that for pure Co3O4 after the OER process (Figure 3d).

[0114] Discussion

[0115] To investigate the electronic structure of RuSACs / Co3O4 under OER conditions, the inventors performed in-situ XAS characterization. The slight increasedpre-edge peak intensity suggests that a few electrons were transferred from Ru 4d to the nearby Co / O atoms, resulting in an increased oxidation state of Ru in RuSACs / Co3O4 with increasing OER potentials. Interestingly, the calculated average oxidation state of Ru is still lower than +4 at different potentials and after the stability test. The almost identical maximum peak indicates a much low degree of structural distortion of RuSACs / Co3O4. These results suggest that RuSACs / Co3O4 is flexible with respect to structural distortions and the redox reaction of Ru, which effectively prevent the dissolution and over-oxidation of Ru during the OER and thus achieves excellent stability.

[0116] In-situ Raman and in-situ synchrotron-based XRD experiments were also carried out to investigate the structural stability of the RuSACs / Co3O4 electrocatalyst during the OER process.

[0117] As shown in Figure 4, the in-situ Raman spectroscopy images for the RuSACs / Co3O4 electrocatalyst taken during the OER process shows no obvious surface phase evolution throughout the whole process of OER at (a) different applied potentials and (b) extended reaction times.

[0118] In contrast, as shown in Figure 5, the in-situ Raman spectroscopy images for c-RuO2takenduring the OER process, the three major bands observed at 523, 642, and 710 cm-1correspond to the E1g, A1g, and B2g vibration modes, respectively, which suggest that c-RuO2 undergoes a severe loss in terms of its original structure with increasing (a) applied potential and (b) extended reaction times. The observed Ru atoms and uniformly distributed Ru, Co and O atoms as a whole, clearly demonstrate the robust properties and excellent stability of the RuSACs / Co3O4 electrocatalyst for acidic OER.

[0119] According to the acidic OER mechanism, it is assumed that the water molecules dissociation to generate oxygenated intermediate species, that is OH*, is the prerequisite for the forward steps. The inventors, therefore, experimentally probed the adsorption energy of OH* species according to the determination of the onset potential of methanol oxidation reaction (MOR) since the electrophilic OH* is prone to react with nucleophiles such as alcohol molecules. The lowest onset potential (1.24 V vs RHE) and highest current density of MOR for the RuSACs / Co3O4 electrocatalyst suggests a high concentration of OH* intermediates, which could be originated fromthe more accessible low-coordinated Ru species and the synergistic interaction between Ru and Co3O4. The higher surface coverage of OH* intermediates is supposed to accelerate the formation of electrophilic O(II-δ)-species, they are more susceptible to nucleophilic attack from water molecule.

[0120] Impressively, the OER performance and bonding strength of OH* for the catalysts share the same trend:

[0121] RuSACs / Co3O4> RuO2NPs / Co3O4> c-RuO2.

[0122] Besides, the largest current density-enhancement on the RuSACs / Co3O4 electrocatalyst (1.78 times) compared to c-RuO2 (1.49 times) after adding methanol implies that the RuSACs / Co3O4 electrocatalyst displays a weaker bonding of OH*, thus allowing the subsequent steps to occur at a fast rate, agreeing well with the high surface reactivity of the RuSACs / Co3O4 electrocatalyst. The similar varying trend for the concentration of OH* species and OER performance suggests that the concentration of oxygen species plays an important role in promoting OER performance.

[0123] The inventors further investigated the binding strength of oxygenated intermediate species (OH*) by cyclic voltammetry (CV) experiments in N2-saturated 0.5 M H2SO4 with a scan rate of 50 mV s-1. The positive shift of OH- desorption peak positions reveal that the binding strength between oxygenated intermediate species and active sites is significantly weakened, which is conducive to promote the proton- couple electron transfer step to form *O and result into a high reaction rate.

[0124] The above experimental results suggest that the superior OER performance of the Ru SACs / Co3O4 electrocatalyst with abundantly accessible unsaturated Ru3+species not only improves water adsorption on the surface of the electrocatalyst but also easily initiates the water dissociation, simultaneously displays a weaker binding strength between oxygenated intermediates and active sites, thereby achieving excellent OER performance in acidic media.

[0125] Conclusions

[0126] In summary, the inventors have constructed an efficient interface between Ru atoms and an Co3O4 support with only 2.7 wt.% Ru as a high-performance electrocatalyst for acidic OER.

[0127] The RuSACs / Co3O4 electrocatalyst exhibits superior electrocatalytic activity with a low overpotential of 185 mV to achieve a current density of 10 mA cm-2and long-term stability over 200 h with an ultralow degradation rate of 0.03 mV h-1in 0.5 M H2SO4, which shows a promising prospect for efficient and durable PEM water electrolysis techniques.

[0128] More strikingly, the RuSACs / Co3O4 electrocatalyst can operate steadily at a high current density of 1 A cm-2without obvious activity degradation in PEM electrolysers.

[0129] The excellent OER activity and stability of RuSACs / Co3O4 with abundant accessibly unsaturated Ru3+species is attributed, based on probing reaction and theoretical studies, to its enhanced water adsorption and successive water association, and optimized binding strength between oxygenated intermediates and active sites.

[0130] Experimental Section

[0131] Materials and Methods

[0132] Materials: cobalt nitrate (Co(NO3)2∙6H2O), 2-methylimidazole (2-MIM), ruthenium chloride (RuCl3), tannic acid (TA), cetyltrimethylammonium bromide (CTAB), Nafion (5 wt. % in a mixture of lower aliphatic alcohols and water), and commercial ruthenium oxide (c-RuO2) were purchased from Sigma-Aldrich.

[0133] Synthesis of Ru SACs / Co3O4: 584 mg of Co(NO3)2∙6H2O, 20.8 mg RuCl3, and 8 mg CTAB were dissolved in 20 mL water, and then rapidly injected into 140 mL solution with 9.08 g of 2-MIM, stirred for 30 min. The product was collected by centrifugation and washed with ethanol and dispersed into 40 mL ethanol. The obtained Ru-ZIF-67 was annealed at 600℃ for 1 h under N2 and cooled down to room temperature. Followed by the post-oxidation at 200℃ for 6 h in air to yield the final product (denoted as Ru SACs / Co3O4). Embodiments:

[0134] Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in anembodiment" in various places throughout this specification are not necessarily all referring to the same embodiment but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.

[0135] Similarly, it should be appreciated that in the above description of example embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the Detailed Description of Specific Embodiments are hereby expressly incorporated into this Detailed Description of Specific Embodiments, with each claim standing on its own as a separate embodiment of this invention.

[0136] Furthermore, while some embodiments described herein include some, but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination. Different Instances of Objects

[0137] As used herein, unless otherwise specified the use of the ordinal adjectives "first", "second", "third", etc., to describe a common object, merely indicate that different instances of like objects are being referred to, and are not intended to imply that the objects so described must be in a given sequence, either temporally, spatially, in ranking, or in any other manner. Specific Details

[0138] In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures andtechniques have not been shown in detail in order not to obscure an understanding of this description. Terminology

[0139] In describing the preferred embodiment of the invention illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, the invention is not intended to be limited to the specific terms so selected, and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar technical purpose. Terms such as "forward", "rearward", "radially", "peripherally", "upwardly", "downwardly", and the like are used as words of convenience to provide reference points and are not to be construed as limiting terms. Definitions

[0140] As used throughout, the singular forms “a,” “an,” and the include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “an electrocatalyst” includes a plurality of such electrocatalysts.

[0141] As used herein, the term “oxygen evolution reaction (OER)” refers to the anodic reaction 2H2O →O2+4H++4e- that accompanies, in aqueous electrolytes, cathodic processes such as hydrogen production via water electrolysis.

[0142] As used herein, the term “polymer electrolyte membrane (PEM)” refers to both cation exchange membranes, which generally comprise polymers having multiple covalently attached negatively charged groups, and anion exchange membranes, which generally comprise polymers having multiple covalently attached positively charged groups.

[0143] As used herein, the term “hydrogen evolution reaction (HER),” refers to the electrochemical reaction 4H++4e−→ 2H2.

[0144] As used herein, the term “membrane electrode assembly (MEA)” refers to a structure comprising a membrane that includes an electrolyte, typically a solid polymer electrolyte, and at least one but more typically two or more electrodes adjoining the membrane.

[0145] As used herein, the term “single-atom catalyst (SAC)” refers to a catalyst comprising a single atom of a noble metal supported on, or confined within, a metal oxide lattice.

[0146] As used herein, the terms “pyrolysis”, “pyrolyze”, “pyrolyzing” refer to the thermal decomposition of material in an oxygen-poor atmosphere (i.e., significantly less oxygen than required for complete combustion).

[0147] As used herein, the terms “anneal”, “annealing”, “annealed” refer to the process of heating and then cooling a material, usually for softening and making less brittle.

[0148] As used herein, the term “precious metal” or “noble metal”, used interchangeably, refers to one or more of Ru, Rh, Pd, Ag, Re, Os, Ir, Pt, and Au.

[0149] Ranges can be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another aspect includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another aspect. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint. Comprising and Including

[0150] In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" are used in an inclusive sense, i.e., to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

[0151] Any one of the terms: including or which includes or that includes as used herein is also an open term that also means including at least the elements / features that follow the term, but not excluding others. Thus, including is synonymous with and means comprising.Scope of Invention

[0152] Thus, while there has been described what are believed to be the preferred embodiments of the invention, those skilled in the art will recognize that other and further modifications may be made thereto without departing from the spirit of the invention, and it is intended to claim all such changes and modifications as fall within the scope of the invention. For example, any formulas given above are merely representative of procedures that may be used. Steps may be added or deleted to methods described within the scope of the present invention. Although the invention has been described with reference to specific examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms. Industrial Applicability

[0153] It is apparent from the above, that the arrangements described are applicable to industries relating to renewable energy conversion and storage systems.

Claims

Claims:

1. A method for preparing an oxygen evolution reaction electrocatalyst, comprising: - contacting a porous zeolite with a solution containing a noble metal precursor to form a noble metal doped zeolite; - pyrolyzing the noble metal doped zeolite to form nanocrystals comprising at least the noble metal and a second metal, said nanocrystals being confined within a layer of carbon-based material, wherein the second metal and the carbon layer are derived from the pyrolyzed zeolite; and - annealing said layer of carbon-based material confined nanocrystals to form isolated noble metal atoms singly dispersed within a metal oxide lattice of the second metal as an oxygen evolution reaction.

2. The method of claim 1, wherein the porous zeolite is a zeolitic imidazolate framework (ZIF) comprising atoms of the second metal and an organic bridging linker.

3. The method of claim 1, wherein the second metal is a transition metal selected from the group consisting of cobalt (Co) and zinc (Zn).

4. The method of claim 2, wherein the second metal is cobalt (Co), and the organic bridging linker is 2-methylimidazole (2-MIM).

5. The method of any one of claims 1 to 4, wherein the noble metal precursor comprises a precursor of a noble metal selected from the group consisting of ruthenium (Ru), rhodium (Rh), palladium (Pd), Osmium (Os), iridium (Ir), platinum (Pt) and gold (Au).

6. The method of claim 5, wherein the noble metal precursor is ruthenium (III) chloride (RuCl3) or a salt or solvate thereof.

7. The method of any one of claims 1 to 6, wherein pyrolysis of the noble metal doped zeolite is carried out at a temperature that falls within a range of about 500° C to about 700° C.

8. The method of claim 7, wherein pyrolysis of the noble metal doped zeolite is conducted at a temperature of 600° C under an inert atmosphere.

9. The method of claim 8, wherein the inert atmosphere is argon or nitrogen.

10. The method of any one of claims 1 to 9, wherein annealing of the carbon-layer confined nanocrystals is conducted at a temperature that falls within a range of about 150° C to about 250° C.

11. The method of claim 10, wherein annealing of the carbon-layer confined nanocrystals is conducted in air at a temperature of about 200° C.

12. The method of any one of claims 1 to 11, wherein the carbon-based material is selected from the group consisting of graphene, graphene oxide, fullerene, carbon nanotubes, activated carbon and graphite.

13. The method of claim 12, wherein the carbon-based material is graphene.

14. The method of any one of claims 3 to 13, wherein the nanocrystals comprise a ruthenium-cobalt (RuCo) nanoalloy.

15. The method of any one of claims 1 to 14, wherein the porous zeolite is immersed in the noble metal solution for about 30 minutes.

16. The method of any one of claims 1 to 15, wherein the concentration of the noble metal solution ranges from about 0.5 to about 2 mg equivalent of noble metal per ml of solution.

17. The method of any one of claims 1 to 16, wherein the oxygen evolution reaction electrocatalyst is RuSACs / Co3O4.

18. The method of claim 17, wherein the oxygen evolution reaction electrocatalyst comprises 2.7 wt.% Ru, based on the total percentage weight of the electrocatalyst.

19. An oxygen evolution reaction electrocatalyst produced by the method according to any one of claims 1 to 18.

20. An oxygen evolution reaction electrode comprising: - an ion conducting membrane having first and second generally opposed major surfaces; - the oxygen evolution reaction electrocatalyst of claim 19 coated on the first major surface of the ion conducting membrane; and - a first fluid diffusion layer comprising an electrically conductive substrate adjacent to the first major surface of the ion conducting membrane.

21. The oxygen evolution reaction electrode of claim 20, wherein the oxygen evolution reaction electrocatalyst comprises not greater than 0.001 atomic percent Ru, based on the total collective cation and elemental metal content of the respective layer.

22. The oxygen evolution reaction electrode of claim 21 or 20, wherein the electrically conductive substrate of the first fluid diffusion layer comprises a porous metal screen or mesh selected from the group consisting of titanium (Ti), platinum (Pt), gold (Au), tantalum (Ta), niobium (Nb), zirconium (Zr), hafnium (Hf), or a metal alloy thereof.

23. The oxygen evolution reaction electrode of any one of claims 20 to 22, wherein the ion conducting membrane comprises a proton exchange membrane.

24. An electrochemical device for water electrolysis comprising the oxygen evolution reaction electrode according to any one of claims 20 to 23.

25. The electrochemical device of claim 24, further comprising a hydrogen evolution reaction catalyst coated on the second major surface of the ion conducting membrane and a second fluid diffusion layer comprising an electrically conductive substrate in electrical contact with the hydrogen evolution reaction catalyst.

26. The electrochemical device of claim 25, wherein the electrically conductive substrate of the second fluid diffusion layer comprises activated carbon paper.

27. The electrochemical device of claim 25 or 26, wherein the hydrogen evolution reaction catalyst comprises platinum (Pt).

28. The electrochemical device of any one of claims 24 to 27, wherein the device is a water electrolyzer.

29. The electrochemical device of any one of claims 24 to 27, wherein the device is a CO2 electrolyzer.

30. A method of generating hydrogen gas (H2) and oxygen gas (O2) from water(H2O), the method comprising: - providing an oxygen evolution reaction electrode membrane electrode assembly (MEA) comprising an anode and a cathode catalyst, the anode being an oxygen evolution reaction electrode according to any one of claims 19 to 22, and having the cathode catalyst coated on the second major surface of the ion conducting membrane; - providing water in contact with the anode; and - providing an electrical potential with sufficient electrical current across the membrane electrode assembly (MEA) to convert at least a portion of the water (H2O) to hydrogen gas (H2) and oxygen gas (O2) at the cathode and anode, respectively.

31. The method of claim 30, wherein the cathode catalyst is a hydrogen evolution reaction catalyst comprising platinum (Pt). 32.The method of claim 30 or 31, wherein the oxygen evolution reaction electrode exhibits an overpotential below 0.5 V versus a reversible hydrogen electrode (RHE) at a current density of 10 mA / cm2during electrolysis of water in an acidic environment at room temperature.

33. The method of claim 30 or 31, wherein the oxygen evolution reaction electrode exhibits an overpotential below 0.1 V versus a reversible hydrogen electrode (RHE) at a current density of 10 mA / cm2during electrolysis of water in an acidic environment at room temperature.

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