An electrode for the hydrogen evolution reaction

Technical Field

[1 ] The invention relates to electrodes for the hydrogen evolution reaction, and in particular to an electrode comprising a conductive substrate and an electrocatalytic composition comprising a mixed metal oxide selected from a cuprous oxomolybdate and a cuprous oxotungstate. The invention also relates to a method of producing such electrodes, a system for electrolysis of water including such electrodes as a cathode, and a method of producing hydrogen using such electrodes.

Background of Invention

[2] Increasing environmental concern around the use of fossil fuels has triggered an urgent demand for clean and sustainable alternative energy sources. Hydrogen - particularly when produced from low carbon, renewable resources - is often considered an ideal replacement for fossil fuels. Hydrogen can be produced by electrolysis of water, according to overall equation (1 ) for water splitting.

2H ₂ 0 2H ₂ + 0 ₂ (1 )

[3] In an electrolysis system, the water splitting reaction proceeds via half reactions at the cathode and anode: the hydrogen evolution reaction (HER) is the cathodic reduction half-reaction that produces H ₂ , while the oxygen evolution reaction (OER) is the anodic oxidation half-reaction that produces 0 ₂ . In acidic media, HER and OER reactions according to equations (2) and (3) are believed to predominate. In basic media, reactions according to equations (4) and (5) predominate.

4H ⁺ + 4e 2 H ₂ (2)

2H ₂ 0 0 ₂ + 4H ⁺ + 4e (3)

4H ₂ 0 + 4e 2H ₂ + 4 OH (4)

40H 0 ₂ + 2 H ₂ 0 + 4e (5)

[4] The thermodynamic equilibrium voltage of the water splitting reaction is

1.23 V at 25°C and 1 atm. To produce sufficient current density in a practical electrolysis system, however, a significant overpotential is needed to overcome activation barriers and resistances within the electrodes and aqueous medium. Effective electrocatalysts are thus needed to reduce the activation barriers for the HER and OER at the cathode and anode, respectively.

[5] The development of molecular HER catalysts that functionally mimic the elegant water reduction chemistry of hydrogenase enzymes has attracted broad interest, but creating efficient and active systems that operate stably at low overpotential remains a challenge. Solid-state electrocatalysts are thus generally preferred. Platinum-based heterogeneous electrocatalysts offer excellent HER performance, but the high cost is an obstacle to wide-spread implementation.

[6] Various alternative electrocatalytic compositions, based on earth-abundant elements, have thus been proposed as platinum replacements for HER catalysis, including metal alloys, nitrides, borides, carbides, chalcogenides, and phosphides. Many of these materials, however, are limited to use in strongly acidic or strongly alkaline aqueous media. This is disadvantageous for many potential applications, such as artificial photosynthesis and inorganic-biological systems, and the corrosivity of strong acids and bases increases capital costs, operational complexity and safety concerns. In other cases, one or more intrinsic properties of the electrocatalytic compositions, such as activity, faradaic efficiency and long-term stability, is unsatisfactory, particularly when operated at the high current densities needed for a commercially practical electrolysis system.

[7] Furthermore, some reported systems suffer from disadvantages related to the integration of the HER electrocatalytic composition into the cathode, even where the material has intrinsically favourable catalytic properties. Electrodes for water electrolysis systems should ideally, as an integrated unit, have low internal resistance, and be amenable to low-cost and simple production techniques. For some electrolysis reactor designs, excellent mechanical strength and/or physical flexibility of the electrode may also be important design considerations.

[8] There is therefore an ongoing need for new electrodes for the hydrogen evolution reaction, which at least partially address one or more of the above- mentioned short-comings, or provide a useful alternative.

[9] A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that the document or matter was known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.

Summary of Invention

[10] In accordance with a first aspect, the invention provides an electrode for the hydrogen evolution reaction, comprising: a conductive substrate, and an electrocatalytic composition on the conductive substrate at an electrode surface, wherein the electrocatalytic composition comprises a mixed metal oxide selected from a cuprous oxomolybdate, a cuprous oxotungstate and mixtures thereof.

[1 1 ] In some embodiments, the mixed metal oxide is a cuprous oxomolybdate. In some such embodiments, the cuprous oxomolybdate comprises CU ₄ MO ₅ O _{I 7} .

[12] In some embodiments, the conductive substrate comprises metallic copper, wherein the metallic copper is at least proximate to the electrode surface. In some such embodiments, the metallic copper proximate to the electrode surface is present in a porous copper foam layer. In other such embodiments, the metallic copper proximate to the electrode surface is a copper alloy, such as a copper nickel alloy.

[13] In some embodiments, the electrocatalytic composition comprises nanoparticulates of the mixed metal oxide formed on the conductive substrate. In some such embodiments, the nanoparticulates have a primary nanoparticle size of less than 10 nm. In some embodiments, the nanoparticulates form a densely packed, continuous layer on the conductive substrate. The continuous layer may have a thickness of less than about 100 nm.

[14] In accordance with a second aspect, the invention provides a method of producing an electrode for the hydrogen evolution reaction, the method comprising: providing an electrocatalytic composition on a surface of a conductive substrate, wherein the electrocatalytic composition comprises a mixed metal oxide selected from a cuprous oxomolybdate, a cuprous oxotungstate and mixtures thereof.

[15] In some embodiments, providing the electrocatalytic composition comprises reacting metallic copper at the surface of the conductive substrate with at least one salt selected from a molybdate and a tungstate to form the mixed metal oxide.

[16] In some embodiments, the metallic copper at the surface of the conductive substrate is present in a porous copper foam layer. In other embodiments, the metallic copper proximate to the electrode surface is a copper alloy, such as a copper nickel alloy.

[17] In some embodiments, the metallic copper is reacted with the at least one salt in the presence of an oxidant. Suitable oxidants include O2 and H2O2 _. The oxidant may also be a component of the at least one salt.

[18] In some embodiments, reacting the metallic copper comprises contacting the surface of the conductive substrate with an aqueous solution of the salt. In some such embodiments, the aqueous solution further comprises an organic ligand or precursor thereof, such as an amide or amine. Suitable ligands include formamide and ammonia. In some embodiments, a volumetric ratio of the organic ligand or precursor to water in the aqueous solution is less than about 1.5, such as less than about 1.1.

[19] In some embodiments, the surface of the conductive substrate is contacted with the at least one salt for a time sufficient to form a continuous layer comprising densely packed nanoparticulates of the mixed metal oxide formed on the conductive substrate. In some such embodiments, the time is less than about 48 hours.

[20] In some embodiments, the al least one salt comprises a molybdate salt, such as a water-soluble molybdate salt. A suitable molybdate salt is Na ₂ Mo0 ₄ .

[21 ] In some embodiments, the mixed metal oxide comprises Cu ₄ MosOi7.

[22] In accordance with a third aspect, the invention provides an electrode for the hydrogen evolution reaction, produced according to any of the embodiments disclosed herein.

[23] In accordance with a fourth aspect, the invention provides an electrode for the hydrogen evolution reaction, comprising: a conductive substrate comprising metallic copper, an electrocatalytic composition on the metallic copper at a surface of the electrode, wherein the electrocatalytic composition comprises a reaction product of the metallic copper, at least one salt selected from a molybdate and a tungstate, and an oxidant.

[24] In some embodiments, the oxidant is O2 or H2O2 _. The oxidant may also be a component of the at least one salt.

[25] In some embodiments, the metallic copper at the surface of the conductive substrate is present in a porous copper foam layer. In other embodiments, the metallic copper is present in a copper alloy.

[26] In some embodiments, the salt is a molybdate salt, such as Na ₂ Mo0 _4. In some such embodiments, the reaction product comprises a cuprous oxomolybdate. In some such embodiments, the cuprous oxomolybdate comprises CU ₄ MO ₅ O _{I 7} .

[27] In some other embodiments, the salt is a tungstate salt, such as Na ₂ W0 _4. The reaction product may thus be a copper tungstate, such as a cuprous oxotungstate.

[28] In some embodiments, the reaction product comprises nanoparticulates formed on the metallic copper. In some such embodiments, the nanoparticulates have a primary nanoparticle size of less than 10 nm. In some embodiments, the nanoparticulates form a densely packed, continuous layer on the metallic copper. The continuous layer may have a thickness of less than about 100 nm.

[29] In accordance with a fifth aspect, the invention provides a system for electrolysis, comprising: a cathode for the hydrogen evolution reaction, wherein the cathode is an electrode according to any of the embodiments disclosed herein; an anode for an oxidation half-reaction; and a power supply connected to the cathode and the anode capable of providing a potential between the cathode sufficient to induce electrolysis of an aqueous composition.

[30] In some embodiments, the power supply is a photovoltaic solar cell.

[31 ] In accordance with a sixth aspect, the invention provides a method of producing hydrogen, the method comprising: contacting an electrode according to any of the embodiments disclosed herein with an aqueous composition; and applying a potential at the electrode sufficient to reduce a species in the aqueous composition to hydrogen.

[32] In some embodiments, the aqueous composition has a pH of greater than 4, such as greater than 6. In some embodiments, the aqueous composition has a pH of between 4 and 10, such as between 6 and 9. In some embodiments, the aqueous composition comprises sea water.

[33] In some embodiments, the potential at the electrode is less than 500 mV, for example less than or equal to 300 mV.

[34] Where the terms“comprise”,“comprises” and“comprising” are used in the specification (including the claims) they are to be interpreted as specifying the stated features, integers, steps or components, but not precluding the presence of one or more other features, integers, steps or components, or group thereof.

[35] Further aspects of the invention appear below in the detailed description of the invention.

Brief Description of Drawings

[36] Embodiments of the invention will herein be illustrated by way of example only with reference to the accompanying drawings in which:

[37] Figure 1 depicts Scanning electron microscopy (SEM) images of a copper foil substrate having a continuous layer of Cu ₄ MosOi7 nanoparticulates formed thereon, as prepared in example 2.

[38] Figure 2 depicts SEM images of an electrode comprising Cu ₄ MosOi7 formed on copper foam on copper plate substrate, as prepared in example 2.

[39] Figure 3 depicts Cu 2p, Cu LMM, and Mo 3d X-ray photoelectron spectroscopy (XPS) spectra of a Cu ₄ MosO-i7 layer formed on copper plate substrate, as prepared in example 2.

[40] Figure 4 is a graph of current density as a function of applied potential for electrodes produced in example 2, in a pH 7 phosphate buffer. [41 ] Figure 5 is a graph of current density as a function of applied potential for electrodes produced in example 2, in a pH 14 KOH solution.

[42] Figure 6 is a graph of current density as a function of applied potential for electrodes produced in example 2, in a pH 0.3 H ₂ S0 ₄ solution.

[43] Figure 7 depicts tafel plots for an electrode comprising CU ₄ MO ₅ O _{I 7} formed on copper foam on copper plate substrate, as produced in example 2, with comparison against a platinum wire electrode, in aqueous compositions having pH values of 7, 14, and 0.3.

[44] Figure 8 depicts SEM images of copper plates immersed for 24 hours in solutions with different ratios of formamide/0.2M Na ₂ Mo0 ₄ aqueous solution, as produced in example 3.

[45] Figure 9 is a graph of current density as a function of applied potential for electrodes produced in example 3 by immersing copper plates for 24 hours in solutions with different ratios of formamide/0.2M Na ₂ Mo0 ₄ , in a pH 7 buffer.

[46] Figure 10 depicts SEM images of copper plates immersed for different contact times in a 1 :1 formamide/0.2M Na ₂ Mo0 ₄ aqueous solution, as produced in example 4.

[47] Figure 1 1 is a graph of current density as a function of applied potential for electrodes produced in example 4 by immersing copper plates for different contact times in a 1 :1 formamide/0.2M Na ₂ Mo0 ₄ aqueous solution, in a pH 7 buffer.

[48] Figure 12 is a graph of current density at 300 mV overpotential for an electrode comprising CU ₄ MO ₅ O _I7 formed on copper plate substrate, during electrolysis over a 48 hour period in various aqueous compositions in example 5.

[49] Figure 13 is a graph of current density at 300 mV overpotential for an electrode comprising CU ₄ MO _S O _I7 formed on a copper plate substrate, during electrolysis over a 14 day period in pH 7 buffer in example 5. [50] Figure 14 is a graph of current density as a function of applied potential for an electrode comprising CuMo0 ₄ on glassy carbon produced in example 6, in pH 7 phosphate buffer, with comparison against cuprous oxomolybdate on copper electrodes.

[51 ] Figure 15 is a graph of current density as a function of applied potential for an electrode comprising copper tungstate formed on a copper plate substrate, as produced in example 7, with comparison against a cuprous oxomolybdate on copper electrode.

[52] Figure 16 is a graph of current density as a function of applied potential for electrodes comprising cuprous oxomolybdate formed on a copper-nickel alloy mesh substrate, as produced in example 10, with comparison against a Pt electrode and a cuprous oxomolybdate on copper plate electrode.

[53] Figure 17 depicts SEM images of the surface composition formed by immersing copper-nickel alloy mesh in a solution of (tetrabutyl ammonium) ₄ [S ₂ Moi ₈ 0 ₆₂ ] (POM) in acetonitrile, as done in example 1 1.

[54] Figure 18 is a graph of current density as a function of applied potential for electrodes comprising cuprous oxomolybdate formed on a copper-nickel alloy mesh substrate using (tetrabutyl ammonium) ₄ [S ₂ Mois0 ₆₂ ] (POM) and molybdic acid salts, as produced in example 1 1.

Detailed Description

[55] The invention relates to an electrode for the hydrogen evolution reaction (HER). The electrode comprises a conductive substrate and an electrocatalytic composition on the substrate at a surface of the electrode available for contact with an aqueous medium when performing the HER. The electrocatalytic composition is in electronic communication with the substrate, such that electrons may be transferred from the conductive substrate to the composition to form hydrogen via HER on the composition. The electrocatalytic composition preferably comprises a mixed metal oxide selected from a cuprous oxomolybdate, a cuprous oxotungstate and mixtures thereof. [56] As used herein, an electrode surface is an external or internal surface of the electrode available for contact with an aqueous medium when performing the hydrogen evolution reaction, such as during electrolysis.

[57] As used herein, an electrocatalytic composition refers to a material which, under suitable reaction conditions, catalyses reactions involving the transfer of electrons to or from a reagent. In the context of heterogeneous electrocatalytic compositions, it will be appreciated that the structural configuration of the active catalytic sites is often uncertain, and it is conventional that electrocatalytic compositions are defined with reference to a bulk composition capable of providing active catalytic sites at a surface thereof.

[58] As used herein, a mixed metal oxide is a solid metal oxide including at least two different metallic elements in its crystalline structure.

[59] As used herein, the term “cuprous” refers to compositions comprising copper in the 1 + oxidation state. The term“cupric” refers to compositions comprising copper in the 2+ oxidation state.

Conductive substrates

[60] The electrode comprises a conductive substrate, on which the electrocatalytic composition is disposed. Without wishing to be bound by theory, the primary role of the substrate is to transfer electrons to the electrocatalytic composition, thereby to catalyse the HER. As such, the conductive substrate may include both electrical conductors, such as a metal, and semiconductors.

[61 ] Electrical conductors are suitable for externally powered electrolysis systems, in which an electrical current supplied by an external power source is transmitted via the conductive substrate to the electrocatalytic composition. A semiconductor substrate may be suitable, as an example, for an electrode in an “artificial photosynthesis” electrolysis system. In such systems, the conductive substrate comprises a light-absorbing semiconductor with a bandgap in excess of 1.23eV (typically above 1.6eV). In use, light falls incident on the semiconductor and photoelectrons with sufficient energy to electrolyse water are transferred to the electrocatalytic composition, where the HER takes place. A variety of suitable semiconductor substrates for artificial photosynthesis systems have been reported, including silicon-based materials.

[62] The conductive substrate may be a metal or other suitable electrical conductor, such as glassy carbon. In some embodiments, the substrate comprises metallic copper. Copper substrates are particularly advantageous in some embodiments, as the mixed metal oxide may be conveniently formed directly on the substrate by reaction with molybdate or tungstate salts, as will be described in greater detail hereafter. Moreover, copper is a relatively cost-effective and abundant material with good chemical and electrochemical stability and excellent mechanical properties. The entire substrate may be metallic copper, or the metallic copper may be present proximate to the electrode surface. For example, a metallic copper layer may be produced on another substance, such as another metal, by well-known techniques such as electroplating.

[63] In some embodiments, the conductive substrate comprises metallic copper in a porous copper layer proximate to the electrode surface. Such layers, also known as copper foam layers, may be produced by electrodepositing dendritic copper onto a conductive material, for example metallic copper plate or wire, from a copper salt solution. A substrate with a copper foam layer advantageously provides a high surface area of metallic copper at the electrode surface on which the electrocatalytic composition may be formed. Copper foam fabricated by other methods is also commercially available.

[64] In some embodiments, the conductive substrate comprises metallic copper as part of a copper alloy. Certain copper alloys may offer the advantage of reduced susceptibility to corrosion in long term use. An example of such an alloy is a copper- nickel alloy. The inventors have demonstrated that an electrocatalytic composition according to the invention may be formed on a copper-nickel alloy substrate by reaction of the metallic copper therein with certain molybdate salts, as will be described in greater detail hereafter.

Electrocatalytic compositions

[65] The electrocatalytic composition comprises a mixed metal oxide selected from a cuprous oxomolybdate, a cuprous oxotungstate and mixtures thereof. The mixed metal oxide is on the conductive substrate at a surface of the electrode, and is thus available for contact with an aqueous medium. The electrocatalytic composition is also in electronic communication with the substrate, such that electrons may be transferred from the conductive substrate to electrocatalytically produce hydrogen, i.e. by reducing H ⁺ or H ₂ 0 according to equations (2) or (4).

[66] In some embodiments, the electrocatalytic composition consists of the mixed metal oxide. The composition may thus consist essentially of Cu, Mo and O elements only in the case of cuprous oxomolybdates, or of Cu, W and O elements only in the case of cuprous oxotungstates.

[67] The mixed metal oxide may be formed as a layer on, and directly chemically bonded to, the conductive substrate at a surface thereof. Such a configuration is advantageous due to the intimate contact, and thus electrochemical communication, provided between the electrocatalytic composition and substrate, and due to the simplicity of fabrication, as will be described in greater detail hereafter. However, it is not excluded that other components may be present in the electrocatalytic composition, including other catalyst components, polymeric components, for example an ionomer such as Nafion, or conductive additives. It is envisaged that a powder of the mixed metal oxide may be coated onto the substrate, optionally together with other additives, to produce the electrode.

[68] In some embodiments, the mixed metal oxide is a cuprous oxomolybdate. An example of a suitable cuprous oxomolybdate is CU ₄ MO ₅ O _I7 . This mixed metal oxide has a triclinic P-1 crystalline structure (unit cell parameters a = 9.573, b = 10.958 and c = 6.782) in which Cu ^{1 +} and Mo ⁶⁺ atoms are connected by oxygen bridges

[69] Without wishing to be bound by any theory, the inventors consider that the active catalytic sites may comprise reduced copper atoms, e.g. Cu° atoms, adjacent to (or functionalised by) molybdenum atoms, since - at the potentials where the hydrogen reduction reaction occurs - Cu° is the thermodynamically favoured species. The CU ^{1 +} -0-MO ⁶⁺ structural motif is believed to be a likely reducible precursor of these active HER catalytic sites, and thus an important element of the bulk and/or as formed electrocatalytic composition. By contrast, the cupric molybdate CuMo0 ₄ which lacks such a structural element was found to provide poor activity for the electrocatalytic HER. Suitable electrocatalytic mixed metal oxides having the Cu ^{1 +} -0-Mo ⁶⁺ motif may include other cuprous oxomolybdates of the general formula Cu _x Mo _y O _z , wherein at least some of the Cu atoms are in the +1 oxidation state, and wherein x, y and z denote the relative atomic abundance of the elements in the oxide. Previously reported examples of such compounds include Cu ₂ Mo0 ₄ , CU ₂ MO ₃ O _{I O,} Cu ₆ Mo ₅ Oi ₈ , CU ₆ MO ₄ O _{I 5} , CU ₄ MO ₆ O ₂₀ and CU _X MO ₃ 0 _I2 , wherein x ^« 3.85 (the latter two being mixed cuprous cupric molybdates).

[70] The mixed metal oxide may be present as a nanoparticulate in the electrocatalytic composition. The nanoparticulate mixed metal oxide may have a primary nanoparticle size of less than 100 nm, such as less than 10 nm. A nanoparticulate morphology is believed to contribute to increased current density (and thus hydrogen production) of the electrode at a given potential, since a high surface area of catalytic phase is available for contact with the aqueous medium. The nanoparticulate mixed metal oxide is preferably formed in a densely packed, continuous layer on the surface of the conductive substrate. The layer may have a thickness of less than 100 nm, for example about 60 nm, which permits suitable electronic communication between the conductive substrate and the catalytic sites.

[71 ] As described elsewhere herein, the conductive substrate may comprise metallic copper at least proximate to the electrode surface. In such embodiments, the cuprous oxomolybdate (or tungstate) mixed metal oxide may be formed directly on the copper substrate surface by methods which will be described hereafter. The mixed metal oxide is believed to be chemically bonded to the underlying metallic copper, as a result of the synthetic methodology. This intimate contact provides excellent electronic communication between the conductive substrate and the electrocatalytic composition, resulting in low internal resistance and increased current density through the electrode at a given potential.

Electrode properties

[72] In some embodiments, the electrodes require an overpotential of less than 0.4 V, or less than 0.3 V, such as less than 0.2 V, to produce a current density of 10 mA/cm ² in aqueous compositions having a pH in the range of 0 to 14, such as neutral and alkaline pH values. The inventors have surprisingly found that electrodes according to embodiments of the invention produce a comparable, or even superior, performance to Pt wire electrodes at pH values of 7 and 14.

[73] In some embodiments, the electrodes are capable of sustaining a current density greater than about 15 mA/cm ² , and/or faradaic efficiencies of great than 95% (or substantially 100%), at an overpotential of 300 mV for periods in excess of 2 days, or 14 days, in aqueous compositions having a pH in the range of 0 to 14, such as neutral and alkaline pH values.

[74] In some embodiments, the electrodes are flexible or bendable. The electrodes may advantageously exhibit excellent adhesion of the electrocatalytic composition to the substrate, such that the electrode may be flexed or bent without substantially degrading its electrochemical performance or mechanical integrity.

[75] It will be appreciated that the electrodes of the invention may be provided in a wide variety of configurations and sizes, depending on the intended purpose. The electrodes may further include other well-known components as required for a particular implementation, such as in an apparatus for electrolysis of water. Such components may include an electrical contact for electrically connecting the conductive substrate electrode to a power supply, and a casing around part of the electrode.

Methods of producing the electrodes

[76] The invention also relates to a method of producing an electrode for the hydrogen evolution reaction. The method comprises providing an electrocatalytic composition on a surface of a conductive substrate, thus placing the electrocatalytic composition in electronic communication with the substrate such that electrons may be transferred from the conductive substrate to the composition when the electrode is used to electrocatalyse the HER. The electrocatalytic composition comprises a mixed metal oxide selected from a cuprous oxomolybdate, a cuprous oxotungstate and mixtures thereof. [77] In some embodiments, the electrode is prepared by reacting a metallic copper substrate with a molybdate or tungstate salt, as will be described in greater detail hereafter. The inventors consider that this approach is particularly advantageous due to the simplicity of fabrication and the excellent electrochemical contact thus provided between the electrocatalytic mixed metal oxide and the conductive substrate. However, it is not excluded that the electrocatalytic composition may be provided on the substrate by other means, such as by coating the substrate with a powder of the mixed metal oxide, optionally together with other additives in a coating composition. A wide variety of different cuprous oxomolybdates or tungstates may be provided on a conductive substrate in this way.

Reaction of metallic copper with molybdate or tungstate salts

[78] In some embodiments, the electrocatalytic composition is provided by reacting metallic copper at the surface of the conductive substrate with at least one salt selected from a molybdate and a tungstate, thereby forming the mixed metal oxide. The metallic copper may be present in copper metal or a copper alloy. As described herein, the substrate may be formed of metallic copper, or may comprise metallic copper at the surface, for example in a plating layer or porous copper foam layer. Preferably, the copper surface to be reacted should be relatively free of oxidation. Polishing of a copper plate to produce a pristine metallic surface prior to reaction has thus been found to provide satisfactory formation of electrocatalytic layers thereon.

[79] The cuprous oxomolybdate or tungstate mixed metal oxide is typically prepared by immersing the copper-containing substrate in a solution, for example an aqueous solution, of the salt at ambient temperatures (such as between 15 and 30°C). The concentration of the salt in the solution may be between about 0.05 and about 0.2 M, but higher or lower concentrations are also envisaged.

[80] An oxidant is generally required to oxidise the metallic copper to Cu ⁺¹ . Oxygen (O2) and hydrogen peroxide (H2O2) have been found to be suitable extraneous oxidants for this purpose. For example, oxygen naturally dissolved in an aqueous solution by exposure of the solution to the atmosphere was found sufficient to facilitate the required oxidative transformation. However, it is envisaged that a broad range of oxidants capable of oxidising copper metal should also be effective. The inventors have also found that a polyoxomolybdate precursor salt (namely, (tetrabutyl ammonium) ₄ [S ₂ Moi8062]) is effective for forming an electrocatalytic reaction product layer on a copper surface in the absence of an extraneous oxidant, and consider that a component of the salt itself (e.g. a Mo ⁶⁺ atom) may serve as the necessary oxidant. Thus, in some embodiments, the oxidant is a component of the salt selected from a molybdate and a tungstate.

[81 ] The formation of the cuprous oxomolybdate or tungstate may be facilitated by the presence of an organic ligand or precursor thereof. The ligand may be a water-soluble amide or amine, such as formamide or ammonia. Without wishing to be bound by any theory, the inventors believe that the ligand may facilitate the reaction by ligating and stabilizing Cu ⁺¹ in an intermediate species during the formation of the cuprous oxomolybdate or tungstate. In the case of formamide, the volumetric ratio of the formamide to water in an aqueous solution of the precursor salt may be less than about 1.5:1 , or less than about 1.1 :1 , such as between about 0.2:1 and about 1.1. Excessive amounts of formamide appear to result in corrosion of metallic copper substrate without concomitant formation of the target mixed metal oxide.

[82] In some embodiments, the salt is a molybdate salt, for example a water- soluble molybdate salt such as Na ₂ Mo0 ₄ . The molybdate salts may be selected from a MO0 ₄ ² molybdate salt (such as of alkali or alkali earth metals, e.g. Na ₂ Mo0 ₄ ), a polyoxomolybdate salt (such as (tetrabutyl ammonium) ₄ [S ₂ Moi8062]) or molybdic acid.

[83] In some such embodiments, the cuprous oxomolybdate formed is Cu ₄ Mo ₅ Oi7. Without wishing to be bound by any theory, it is proposed that the formation of Cu ₄ MosOi7 proceeds according to overall reaction (6):

[84] The formation of a cuprous oxomolybdate such as CU ₄ MO ₅ O _{I 7} by reaction of metallic copper with a molybdate salt is considered surprising. It has previously been reported (Xu et al., J. Phys. Chem. B 2006, 110, 17400) that reaction of metallic copper with molybdate salts in the presence of formamide under oxidative conditions produces the cupric salt ammonium copper molybdate, i.e. (NH ₄ ) ₂ Cu(Mo0 ₄ ) ₂ . It was proposed in that report that the metallic Cu is oxidised directly to Cu ²⁺ . [85] The contact time for producing CU ₄ MO ₅ O _I7 on the copper substrate may be less than about 48 hours, such as between about 12 and 24 hours. The inventors have found that this provides a continuous layer of densely packed nanoparticulates of Cu ₄ Mo ₅ Oi7 on the substrate, while avoiding over-oxidation of the copper. However, it will be appreciated that suitable contact times will depend on various parameters such as the contact temperature and the concentration of the salt and any other components in the bath.

[86] In some embodiments, the salt is a tungstate salt, for example a water- soluble tungstate salt such as Na ₂ W0 ₄ . The inventors have found that longer contact times are needed to form an electrocatalytic copper tungstate layer than to form a Cu ₄ Mo ₅ Oi7 layer. Suitable contact times in an aqueous Na^O formamide bath may be greater than 24 hours, for example about 5 days.

Systems for electrolysis

[87] The invention also relates to a system, or apparatus, for electrolysis. The system includes a cathode for the hydrogen evolution reaction, where the cathode is an electrode according to the invention. The system further includes an anode for an oxidation half-reaction; and a power supply connected to the cathode and the anode. The power supply is capable of providing a potential between the cathode and the anode sufficient to induce electrolysis of an aqueous composition.

[88] The anode may be a conventional anode for the OER. Such anodes are known to the skilled person, and include nickel foam electrodes and electrodes modified with iridium oxide, ruthenium oxide or nickel/iron oxide. It will be appreciated that the anode may also be an anode for other oxidation half-reactions expected in electrolysis applications, for example oxidation of Cl to Cl ₂ .

[89] The power supply may be a conventional power supply for a water electrolysis system, such as a direct current power source. Optionally, the power supply may be a photovoltaic solar cell.

[90] The system for electrolysis may include additional conventional components of such devices, such as wires, casings, membrane separators between the cathode and the anode, and ports for withdrawing evolved hydrogen and other product gases, e.g. oxygen.

Method of producing hydrogen

[91 ] The invention also relates to a method of producing hydrogen, for example in a system as described herein. The method comprises contacting an electrode according to the invention with an aqueous composition, and applying a potential at the electrode sufficient to reduce a species in the aqueous composition to hydrogen. The species is generally H ⁺ and/or H ₂ 0, consistent with equations (2) and (4).

[92] The inventors have demonstrated that the methods of the invention can advantageously be carried out in a wide variety of aqueous compositions, including acidic, neutral, alkaline and high salinity compositions. Accordingly, in its broadest form, the invention is not considered to be limited to aqueous compositions with specific solutes or pH ranges.

[93] In some embodiments, the aqueous composition comprises a mineral acid, such as sulfuric acid (H ₂ S0 ₄ ). Suitable mineral acid solutions may range from highly dilute to concentrated, for example from greater than 0 mol/litre to about 10 mol/litre. In some embodiments, the aqueous composition comprises a strong base, such as potassium or sodium hydroxide (KOH or NaOH). Suitable strong base solutions may range from highly dilute to concentrated, for example from greater than 0 mol/litre to about 10 mol/litre.

[94] In some embodiments, the aqueous composition has a pH of greater than 4, such as greater than 6. Unlike many other electrodes with earth-abundant electrocatalytic compositions, the electrodes according to embodiments of the invention do not require strongly acidic conditions to operate effectively. In some embodiments, the aqueous composition has a pH of between 4 and 10, such as between 6 and 8. At least some electrodes of the invention are advantageously effective in aqueous media with neutral pH values, in contrast with many electrodes for HER which require strongly acidic or strongly alkaline media.

[95] In some embodiments, the aqueous composition may be sea water. Since chloride ions are most readily oxidised at the anode, the overall electrolysis of seawater involves the reaction according to equation (6). Good electrocatalytic performance is thus required at neutral to basic conditions, since OH is produced as a co-product. The electrodes according to the present invention, which are active and stable under such conditions, may thus be useful for electrolysing cheap and abundant sea water.

2H ₂ 0 + 2CI H ₂ + 20H + Cl ₂ (6)

[96] The aqueous compositions of the invention may include other conventional components for electrolysis applications, such as a water soluble electrolyte to improve the conductivity of the aqueous composition. The inventors have found that the electrodes of the invention are satisfactorily active and stable even in 1 M NaCI.

[97] In some embodiments, hydrogen is produced by applying an overpotential of less than about 500 mV, such as about 300 mV. In some such embodiments, hydrogen is produced in aqueous compositions at a current density on the electrode of greater than about 15 mA/cm ² , for example in aqueous compositions having a pH in the range of 0 to 14. In some such embodiments, the current density is greater than about 20 mA/cm ² in acidic compositions, for example at a pH of less than 4. The hydrogen may be produced at faradaic efficiencies of great than 95%, or substantially 100%. The hydrogen production capability, as measured by current density produced at 300 mV, may be substantially stable for periods in excess of 2 days in aqueous compositions, for example compositions having a pH in the range of 0 to 14.

EXAMPLES

[98] The present invention is described with reference to the following examples. It is to be understood that the examples are illustrative of and not limiting to the invention described herein.

Materials and techniques

[99] Copper plate and copper foil (99.99%) were obtained from ZNXC (China). CuNi mesh (Ni 65%, Cu 33%, Fe 2%) was obtained from Goodfellow Cambridge Limited. Na ₂ Mo0 ₄ -2H ₂ 0 (99.0%), CuS0 ₄ -5H ₂ 0 (99.0%) and H ₂ Mo0 ₄ was purchased from Sigma-Aldrich. NaH ₂ P0 ₄ (99.0%), Na ₂ HP0 ₄ (99.0%), NaOH (99.0%), H ₂ S0 ₄ (98%), and formamide (99.0%) were purchased from Merck. (Tetrabutyl ammonium) ₄ [S ₂ Moi ₈ 0 ₆₂ ] (hereafter POM) was synthesized according to literature procedures (Inorg Chem. 2001 , 40(4):703-9). All chemicals were used as supplied by the manufacturer. All aqueous solutions were prepared using high purity water obtained from a Milli Q water (18.2 MW cm) purification system.

[100] X-ray Diffraction (XRD) data were collected with a Bruker D8 ADVANCE powder diffractometer (Cu Ka radiation, l = 0.15406 nm). Scanning electron microscopy (SEM) images were collected on JEOL JSM-7001 field emission SEM. Transmission electron microscopic (TEM) images were collected on a FEI Tecnai G2 T20 TWIN TEM. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images and energy-dispersive X- ray spectra (EDS) were acquired with a FEI Tecnai G2 F20 S-TWIN FEGTEM operated at 200 kV. Gas chromatography (GC) was performed with an Agilent 7820 A gas chromatography system equipped with a HP— plot molesieve (5 A) column and a thermal conductivity detector (TCD). The carrier gas was nitrogen (99.99%) for H ₂ analysis. The retention time was compared with that obtained with authentic sample of Fl ₂ gas. X-ray photoelectron spectroscopy (XPS) analysis was performed using an AXIS Ultra DLD spectrometer (Kratos Analytical, Manchester, UK) with a monochromated 180 W Al Ka source (15 kV, 12 mA), a hemispherical analyzer operating in the fixed analyzer transmission mode and standard aperture (analysis area: 0.3 mm c 0.7 mm).

[101] All electrochemical experiments were performed at room temperature (typically 22 °C) using a standard three-electrode cell configuration with a CFII700D electrochemical workstation (CHI Instruments, Austin, Texas, USA). Objects of study were used as the working electrodes (cathode) and graphite rod as the counter electrode (anode). Reference electrodes were chosen based on the pH of the electrolyte solution: saturated calomel electrode (SCE) for acidic solution, Ag/AgCI (3M KCI) for neutral solution, and FlgO reference for alkaline solution. The scan rate of all linear sweep voltammetry (LSV) scans was 10 mV s ¹ . Potentials when measured vs references are converted to the reversible hydrogen electrode (RFIE) scale using the formula: E(vs. RFIE) = E(vs. ref.) + E ⁰ _ref + 0.0586 c pH.

[102] Bulk electrolysis was conducted in a gas tight two-compartment FI-shaped electrolysis cell under a N ₂ atmosphere, with a porous glass frit separating the anode and cathode half-cells. High purity N ₂ gas was introduced to saturate the solution and remove oxygen prior to electrochemical measurements. After N ₂ bubbling for approximately 20 min, the electrolysis cell was sealed tightly with a rubber stopper. The controlled potentials used during bulk electrolysis were selected based on voltammetric data obtained under corresponding conditions.

[103] Gas chromatography was used to identify gaseous products in the headspace. Calibration curves for H ₂ were constructed by injecting a known amount of pure H ₂ and plotting the peak area against the amount injected. The gaseous product was identified by comparing the retention time with pure standard H ₂ gas and quantified from the relevant calibration curve.

Example 1

[104] A copper foam layer was prepared on copper plate by a hydrogen bubble templated electrodeposition method, as described in H.-C. Shin and M. Liu, Chemistry of Materials, 2004, 16, 5460-5464. SCE was used as reference electrode and a graphite rod was used as counter electrode. The electrolyte solution contained 0.4 M CuS0 ₄ and 1.2 M H ₂ S0 ₄ . A constant potential of -4.0 V vs. SCE was applied for 6 seconds to electrodeposit porous copper dendrite onto the copper substrate. The as- prepared copper foam was then thoroughly rinsed withe deionised (Dl) water and acetone and dried in air.

Example 2

[105] A sandpaper polished copper plate, a copper foil and a freshly prepared copper plate with copper foam surface layer (as prepared in Example 1 ) were separately immersed for 24 hours in solutions prepared by mixing formamide and 0.2 M aqueous Na ₂ Mo0 ₄ (1 :1 volumetric ratio) in an open beaker. At the end of this period, the colour of the copper substrates was visibly changed from reddish-orange to dark brown due to the presence of a new composition in a surface layer on the copper substrates (electrodes E1 -E3 in Table 1 ).

[106] An electrode E2 was bent to 180 degrees, yet the surface layer remained tightly adhered to the foil substrate, showing the excellent mechanical strength and physical flexibility of this material. [107] The surface layer on an electrode E2 was imaged with SEM, as depicted in Figure 1 . The SEM images reveal that the composition is composed of densely arranged nanoparticulates, with a primary nanoparticle size of less 10 nm: Figures 1 a and 1 b. A cross sectional view of the foil shows that the thickness of the layer is about 60 nm: Figure 1 c.

[108] An electrode E3 was also imaged with electron microscopy, as depicted in Figure 2. Figures 2a and 2b are SEM images of the copper foam after surface modification in the formamide/molybdate bath. A highly porous microstructure composed of copper dendrite is evident. When observed with TEM, it was evident that the smooth surface of the copper dendrite in the pristine copper foam had been transformed to an irregular surface, as seen in Figure 2c. A FIRTEM image, seen in Figure 2d, reveals three types of lattice spacing in the crystalline composition on the surface of the copper dendrite, having d-spacings of 5.20, 2.48, and 2.1 1 A. These correspond to the (1 ,2,0), (1 ,-1 , 2), and (-3,-1 ,3) lattice of Cu ₄ MosOi7 (JCPDS no. 81 - 1 106), respectively. EDS mapping images in STEM-HAADF mode also showed that Cu, Mo, and O elements are uniformly distributed on the surface of the copper dendrite, again consistent with the mixed metal oxide structure of Cu ₄ MosOi7.

[109] The surface of an electrode E1 was characterized by XPS, as depicted in Figure 3. It was analysed at an emission angle of 0°, as measured from the surface normal. Assuming typical values for the electron attenuation length of relevant photoelectrons, the XPS analysis depth (from which 95% of the detected signal originates) ranges between 5 and 10 nm for a flat surface. Since the actual emission angle is ill-defined in the case of rough surfaces (ranging from 0 ^Q to 90 ^Q ) the sampling depth may range from 0 nm to around 10 nm.

[1 10] Figure 3a shows the high-resolution Cu 2p spectra, while Figure 3b shows the high-resolution spectra of the Cu Auger region (Cu L ₂ M _4,5 M _4,5 and L ₃ M _4,5 M _4,5 ). No detection of L ₂ M _4,5 M _4,5 (565.4 eV) and L ₃ M _4,5 M _4,5 (568.2 eV) associated with Cu(0) was observed. The Cu 2p and Auger region spectra are consistent with Cu being in the state Cu ₂ 0 / Cu(l). The assignment of Cu ₂ 0 was confirmed using the calculated Auger parameter and comparing with literature values (M. C. Biesinger, Surface and Interface Analysis, 2017, 49, 1325-1334). Figure 3c shows the Mo 3d spectrum. Based on peak position and peak shape of the Mo 3d peaks, the Mo is assigned as M0O3 / Mo(VI), which is in accordance with literature (J. Baltrusaitis et al, Applied Surface Science, 2015, 326, 151 -161 ). The XPS results further confirm that the surface composition on the copper substrate is Cu ₄ MosOi7.

[1 1 1 ] Electrodes E1 and E3 were used as the working electrode for electrochemical tests without further modification. For comparison, the performances of Pt wire, unmodified Cu plate, and unmodified Cu foam (as prepared in Example 1 ) electrodes were also evaluated. Electrochemical tests were performed in three media: 0.5M phosphate buffer, 1 M KOH solution, and 0.5M H ₂ S0 ₄ solution, having a pH of 7, about 14, and about 0.3 respectively, while bubbling N ₂ through the solution.

[1 12] The LSV polarization curves of the electrodes at pH of 7, 14 and 0.3 are depicted in Figures 4, 5 and 6 respectively. At a neutral pH of 7, the E3 electrode, being CU ₄ MO ₅ OI ₇ formed on copper foam on copper plate, exhibited excellent performance that surpassed the Pt wire electrode (Figure 4). A current density of 10 mA cm ² was achieved at only 139 mV overpotential, compared with 170 mV for the Pt wire electrode. The onset potential (1 mA cm ² ) of electrode E3 is only 32 mV. At the alkaline pH of 14, electrode E3 shows comparable performance with Pt wire, with 10 mA cm ² achieved at 183 mV overpotential, 30 mV higher than that of Pt wire (Figure 5). At the acidic pH of 0.3, the performance of electrode E3 is not comparable with Pt wire, requiring an overpotential of 212 mV to reach 10 mA cm ² (Figure 6).

[1 13] The tafel plots for electrodes E3 and platinum wire are shown in Figure 7. The tafel slopes of electrode E3 in pH 7, pH 14, and pH 0.3 media are 70 mV dec ¹ , 1 19 mV dec ¹ , and 96 mV dec ¹ , respectively. By comparison, the tafel slopes of Pt wire in the three media are 73 mV dec ¹ , 121 mV dec ¹ , and 38 mV dec ¹ , respectively. For all the results reported here, no IR compensation was applied.

[1 14] The results demonstrate that a cuprous oxomolybdate mixed metal oxide composition, specifically Cu Mo ₅ 0 ₇ , on a conductive substrate such as metallic Cu is an excellent HER catalyst in both neutral and alkaline medium, and has at least comparable performance to other non-noble earth-abundant HER catalysts in acidic media. Table 1

a Volumetric ratio of formamide (I) to 0.2 M aqueous Na ₂ Mo0 ₄

Example 3

[1 15] The effect of oxygen (0 ₂ ) on the formation of the electrocatalytic surface layer was investigated by immersing a polished copper plate for 24 hours in a solution of formamide and 0.2 M aqueous Na ₂ Mo0 ₄ (1 :1 volumetric ratio) under N ₂ . No surface layer was formed when the reaction system was kept under N ₂ atmosphere, demonstrating that 0 ₂ is essential for the formation of the Cu ₄ MosOi7 layer.

[1 16] The effect of the ratio of formamide to 0.2 M aqueous Na ₂ Mo0 ₄ in the bath was investigated, as shown in Table 1 (electrodes E5-E9 and E1 ). Figure 8 shows SEM images of copper plates immersed for 24 hours in solutions with different ratios of formamide/0.2M Na ₂ Mo0 ₄ aqueous solution (volumetric ratios of 0:1 , 1 :4, 1 :2, 1 :1 , 2:1 , 1 :0). Electrode E5, obtained in 0.2M Na ₂ Mo0 ₄ aqueous solution without the presence of formamide, seen in Figure 8a, contained isolated particles (hundreds of nanometers in size) formed on the copper surface. The copper surface of electrode E9, produced in pure formamide, seen in Figure 8f, showed corrosion but no growth of a discrete surface layer. Electrodes E6, E7, E1 and E8, obtained by immersion on solutions with different volumetric ratios of formamide/Na ₂ Mo0 ₄ aqueous solution, displayed correspondingly different morphological characteristics, as seen in Figures 8b-8e. Increasing the formamide content lead to more serious corrosion while a more aqueous solution favoured the formation of a continuous layer of Cu ₄ MosOi7.

[1 17] The LSV polarization curves of electrodes E5-E9 and E1 , in 0.5 M pH 7 phosphate buffer, are depicted in Figure 9. E9 unsurprisingly shows the highest HER overpotential due to the absence of any Mo content. Electrodes E1 , E7 and E6 gave the lowest HER overpotential (E1 was optimum), consistent with the formation of continuous layers of CU ₄ MO ₅ O _I7 on the copper substrate as seen in Figure 8.

[1 18] In view of the results obtained in example 3, but without wishing to be bound by any theory, it is believed that the synthesis of Cu ₄ MosOi7 on copper substrate proceeds according to the reaction (6) disclosed herein.

Example 4

[1 19] The effect of the exposure time of the copper substrate to the growth solution was investigated, as shown in Table 1 (electrodes E10-E12 and E1 ). Figure 10 shows SEM images of copper plates immersed in 1 :1 formamide/0.2M Na ₂ Mo0 ₄ aqueous solution for 6 hours (E10), 12 hours (E1 1 ), 24 hours (E1 ), and 48 hours (E12). After 6 hours, Figure 10a, no obvious surface layer can yet be observed. After 12 hours, Figure 10b, a discontinuous layer was formed on the copper surface. A continuous surface layer comprising the densely arranged nanoparticles was observed after 24 hours, Figure 10c. After 48 hours, Figure 10d, curved flakes in the micrometer size range were observed, which are believed to be (NH ₄ ) ₂ Cu(Mo0 ₄ ) ₂ .

[120] The LSV polarization curves of electrodes E10-E12 and E1 , in 0.5 M pH 7 phosphate buffer, are depicted in Figure 1 1. The best results were obtained with E1 and E1 1 (24 and 12 hours immersion, respectively). E10 gave the highest HER overpotential due to the low coverage of CU ₄ MO ₅ O _I7 (see Figure 10a). E12 also produced an inferior result due to the deleterious effect on the electrocatalytic composition caused by the prolonged immersion in the formamide/Na ₂ Mo0 ₄ bath, again consistent with the appearance of the electrode surface shown (see Figure 10d). Example 5

[121] Hydrogen peroxide was investigated as an alternative oxidant to 0 ₂ in the preparation of electrodes according to the invention. A sandpaper polished copper plate was immersed in a solution prepared by mixing formamide, 0.2 M aqueous Na ₂ Mo0 ₄ (1 :1 volumetric ratio) and 30% H ₂ 0 ₂ (300 pL per 3 ml_ total solution). After only four hours, the colour of the copper substrate was visibly changed from reddish- orange to dark brown due to the presence of the electrocatalytic composition. The copper plate was then removed from the solution and rinsed with water and ethanol three times to produce electrode E13 (Table 1 ).

[122] Electrode E13 was used as the working electrode in an LSV electrochemical test in pH 7 phosphate buffer without further modification. Excellent HER activity was obtained: a current density of 10 mA cm ² was achieved at c.a. 0.20

V overpotential (compared with c.a. 0.27 V for electrode E1 prepared over 24 hours using ambient 0 ₂ as the oxidant).

[123] Ammonia was investigated as an alternative organic ligand to formamide in the preparation of electrodes according to the invention. A sandpaper polished copper plate was immersed in a solution prepared by mixing ammonia and 0.2 M aqueous Na ₂ Mo0 ₄ (30 pL of 20% ammonia solution per 3 ml_ total solution) in an open beaker. After 24 hours, the colour of the copper substrate was visibly changed from reddish-orange to dark brown due to the presence of electrocatalytic composition. The copper plate was then removed from the solution and rinsed with water and ethanol three times to produce electrode E14 (Table 1 ).

[124] Electrode E14 was used as the working electrode in an LSV electrochemical test in pH 7 phosphate buffer without further modification. Excellent HER activity was obtained: a current density of 10 mA cm ² was achieved at c.a. 0.24

V overpotential (compared with c.a. 0.27 V for electrode E1 prepared using formamide as the ligand).

Example 6

[125] In order to investigate the stability of the electrodes comprising cuprous oxomolybdate on Cu substrate, electrodes E1 (CU ₄ MO ₅ O _I7 on Cu plate) were subjected to long-term bulk electrolysis at 300 mV overpotential in each of a) 0.5 M H ₂ S0 ₄ , b) 0.5 M pH 7 phosphate buffer, c) 1 M KOH, and d) 1 M NaCI solution. The corresponding i-t curves are shown in Figure 12. The electrocatalyst showed very high stability in in 0.5 M H ₂ S0 ₄ , 0.5 M pH 7 phosphate buffer, and 1 M KOH, with steady state catalytic current densities located around 25, 18, and 17 imA cm ² , respectively. A stable current with little decay was observed in NaCI medium during the 48 hours bulk electrolysis experiment.

[126] A 14 day stability test of an E1 electrode at 300 mV overpotential in phosphate buffer was also conducted, with the results shown in Figure 13. The electrocatalyst exhibited excellent stability in the testing period, with the current variation attributable primarily to temperature fluctuation. The electrode provided very stable current density in a 1 hour isothermal test period at the end of the 14 days test.

[127] The faradaic efficiency of the cuprous oxomolybdate HER electrocatalyst catalyst was investigated by conducting bulk electrolysis using an E3 electrode (Cu ₄ Mo ₅ Oi7 on Cu foam) in an air-tight H-type electrolysis cell separated by a glass frit. Gas product was detected by a GC and quantified by standard calibration curve. H ₂ faradaic efficiency (FE _H 2) is calculated according the equation (3).

FEH2 = 96485 ^* M _H2 ^* 2/(total charge) ^* 100% (3)

[128] The FEH2 distribution as a function of applied potential is shown in Table 2. Under all the potentials applied, FEH2 values are close to (and within estimated experimental error of) 100%. No other gaseous electrolysis products were detected by GC.

Table 2

Example 7

[1 29] CUMO0 ₄ powder was prepared by mixing 5 ml_ of a 0.1 M CuS0 ₄ aqueous solution with 5 mL of a 0.1 M Na ₂ Mo0 ₄ aqueous solution at room temperature with stirring for 5 minutes. Formation of a light blue precipitate was observed. The precipitate was separated by centrifuge, and washed with water and ethanol three times. The product was dried under vacuum to obtain the CuMo0 ₄ powder.

[1 30] A CUMO0 ₄ modified glassy carbon electrode was prepared by dispersing 5 mg of the CuMo0 ₄ powder and 20 pL of a 5% Nafion solution in a mixed solvent of 0.3 mL ethanol and 1.2 mL water. 5 pL of this solution was drop cast onto a 3 mm glassy carbon plate electrode and dried under infrared lamp to get the CuMo0 modified glassy carbon electrode.

[131] The LSV polarization curve of the electrode consisting of CuMo0 ₄ on glassy carbon, in 0.5 M pH 7 phosphate buffer is compared against an unmodified copper plate, electrode E1 (Cu ₄ MosO-i7 on Cu foam) and electrode E3 (Cu ₄ MosOi7 on Cu foam), in Figure 14. The electrode comprising a cupric molybdate electrocatalytic composition gave rather poor performance, with a current density of 10 mA cm ² only achieved at 627 mV overpotential.

Example 8

[132] A polished copper plate was immersed in a solution prepared by mixing formamide and 0.2 M aqueous Na ₂ W0 ₄ (1 :1 volumetric ratio) in an open beaker. No development of a surface layer was visibly apparent after 24 hours. However, after five days exposure, the colour of the copper substrate was visibly changed from reddish-orange to a dark colour due to the presence of a new copper tungstate composition in a surface layer (electrode E13 in Table 3). The inventors believe that the surface layer on the copper substrate of the E13 electrode comprises a copper tungstate such as a cuprous oxotungstate.

Table 3

[133] Electrode E13 was used as the working electrode in an electrochemical test without further modification. The LSV polarization curve of the electrodes in 0.5M phosphate buffer (pH of 7), with comparison against unmodified Cu plate and electrode E1 (Cu plate surface-modified with Cu ₄ MosOi7), is depicted in Figure 15.

[134] Although the performance of electrode E13 with copper tungstate electrocatalytic composition is not equivalent to that of E1 with CU ₄ MO ₅ O _{I 7} , yet is still superior to unmodified Cu plate. A current density of 10 mA cm ² was achieved at 469 mV overpotential. Moreover, at higher overpotentials, the current density produced with the E13 electrode approaches that of the E1 electrode.

[135] The inventors believe that the performance of the copper tungstate-based electrodes can be improved by optimisation of the growth time, the bath composition and the selection of a high surface area copper foam substrate.

Example 9

[136] A solar cell driven water splitting experiment was conducted using a copper plate with CU ₄ MO ₅ O _I7 layer as the cathode (electrode E1 ; Table 1 ) and a nickel foam as the anode. A commercial polycrystalline silicon solar cell (2 V, 130 mA) was used as the power supply. Both the cathode and anode were immersed in 1 M KOH solution in an electrochemical cell. When the system was exposed to direct sunlight, hydrogen started bubbling from the cathode and oxygen started bubbling form the anode.

Example 10

[137] Pieces of copper-nickel alloy (CuNi) mesh (3 cm by 0.5 cm) were immersed in acetone and ultrasonicated for 5 min. The CuNi mesh was then transferred to 1 M HCI solution and further ultrasonicated for 5 min. Finally, the cleaned CuNi mesh was rinsed with ethanol and dried under a N ₂ gas flow at room temperature.

[138] Cleaned mesh pieces were immersed, for 24 hours or 3 days, in solutions prepared by mixing formamide and 0.2 M aqueous Na ₂ Mo0 ₄ (1 :1 volumetric ratio) in an open beaker. After removal, the mesh was washed with water and acetone and dried in air, to give electrodes E14 and E15 per Table 4. SEM analysis revealed the presence of a new composition in a rough surface coating on the mesh. Table 4

[139] Electrodes E14 and E15 were used as the working electrode in an electrochemical test without further modification. The LSV polarization curve of these electrodes in 1 M KOH solution (pH of 14), with comparison against a Pt electrode and electrode E1 (CU ₄ MO ₅ O _I5 on Cu plate), is depicted in Figure 16. Excellent HER performance was obtained, with a current density of 10 mA cm ² achieved at only c.a. 260 mV overpotential. The performance of the E14 and E15 electrodes, with CuNi mesh substrate, is similar to that of electrode E1 with copper metal substrate, and the inventors consider that the electrocatalytic composition similarly comprises a cuprous oxymolybdate formed by reaction of Cu in the CuNi mesh with the molybdate salt. In further support of this, immersion of a pure metallic nickel substrate in the water/formamide solution of Na2Mo0 ₄ resulted in dissolution of the nickel, presumably as soluble nickel molybdate, without formation of a surface coating on the substrate.

[140] The results demonstrate that a cuprous oxomolybdate may be formed on a copper alloy substrate, by reaction of the metallic copper present in the alloy with a molybdate salt in the presence of an oxidant (in this case, air or POM when present).

Example 11

[141] Pieces of CuNi mesh, cleaned as described in Example 10, were immersed, for 24 hours, 2 days or 3 days, in a 10 mM solution of (tetrabutyl ammonium) ₄ [S ₂ Moi ₈ 0 ₆₂ ] (POM) in acetonitrile, under N ₂ atmosphere. A piece of cleaned CuNi mesh was also immersed for 24 hours in a 100 mM solution of molybdic acid (H ₂ Mo0 ₄ ) in formamide / water (1 :1 ratio), in an open beaker. After removal, the coated meshes were washed with water and acetone and dried in air, giving electrodes E16, E17, E18 and E19 per Table 4. SEM analysis of E18, as seen in Figure 17(a), revealed the presence of a new composition in a surface coating on the mesh. Higher resolution imaging of E18 seen in Figure 17(b) shows that the composition coats the mesh surface continuously, apart from some cracking. SEM analysis of E19 showed a similar cracked surface coating on the CuNi substrate.

[142] Electrodes E16-E19 were used as the working electrode in electrochemical tests without further modification. The LSV polarization curve of these electrodes in 1 M KOH solution (pH of 14), with comparison against electrode E1 (CU ₄ MO ₅ O _I5 on Cu plate), is depicted in Figure 18. Good HER performance was obtained for E16-E18, with a current density of 10 mA cm ² achieved at only c.a. 290 mV overpotential. Electrode E19 also gave reasonable HER performance, with a current density of 10 mA cm ² achieved at c.a. 363 mV overpotential.

[143] The results demonstrate that a electrocatalytic cuprous oxomolybdate salt may be formed on a metallic copper substrate using a variety of molybdate precursor salts. Furthermore, the POM salt was able to act as both the source of molybdate and the oxidant for oxidizing metallic Cu.

[144] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is understood that the invention includes all such variations and modifications which fall within the spirit and scope of the present invention.