A membrane electrode assembly for an electrochemical hydrogen compressor

Abstract

The invention provides a membrane electrode assembly for an electrochemical hydrogen compressor, the membrane electrode assembly comprising a proton exchange membrane arranged between an anode and a cathode, wherein the anode comprises an electrocatalyst for dihydrogen oxidation and the cathode comprises an electrocatalyst for proton reduction, and wherein the proton exchange membrane comprises a semicrystalline polymeric matrix comprising a hydrophilic polymer and particles of an inorganic metal compound dispersed in the semicrystalline polymeric matrix.

Description

[0001] A membrane electrode assembly for an electrochemical hydrogen compressor

[0002] Technical Field

[0003] [1] The invention relates to a membrane electrode assembly for an electrochemical hydrogen compressor. The membrane electrode assembly includes a proton exchange membrane which comprises a semicrystalline polymeric matrix comprising a hydrophilic polymer and particles of an inorganic metal compound dispersed in the semicrystalline polymeric matrix. The invention further relates to a method of producing a membrane electrode assembly, an electrochemical hydrogen compressor, and a method of compressing dihydrogen.

[0004] Background of Invention

[0005] [2] Green dihydrogen (H2) is important in the transition to a clean-energy economy as a carrier for renewable energy and to replace H2 or other chemical reductants currently derived from fossil fuel sources. However, H2 produced by electrolysis of water must be compressed to high pressures, preferably above 70 bar, for storage or transport. Mechanical compressors are conventionally used for commercial-scale compression of H2 and other gases, but are particularly unsatisfactory for H2 service due to the low efficiency and limited compression ratios achievable with such a low density gas.

[0006] [3] Electrochemical hydrogen compressors provide a potential advancement in compression technology for green or fossil-derived H2 due to the high efficiencies and compression ratios that can theoretically be achieved, as well as the lack of moving parts. An electrochemical hydrogen compressor includes at least one membrane electrode assembly comprising a proton exchange membrane (PEM) arranged between an anode and a cathode. Protons produced by electrochemical oxidation of gaseous H2 at the anode are driven across the PEM, due to an applied potential, and recombined by electrochemical reduction on the cathode to form gaseous H2 at a higher pressure. The electrochemical hydrogen compressor thus converts electrical power to chemical potential in the form of high pressure H2 gas. If necessary, multiple membrane electrode assemblies can be arranged in series so that a desired operating pressure can be achieved by multi-stage compression.

[0007] [4] The overall electrochemical process is represented by the following equations (1) to (3):

[0008] Anode: H22H++ 2e’ (1)

[0009] Cathode: 2H++2e_— > H2 (2) Overall: H2 (Low pressure - Anode) — > H2 (High pressure - Cathode) (3)

[0010] [5] Significant research effort in the area of electrochemical hydrogen compressor has focused on the performance and cost of the anode and cathode, for example by reducing the required loading of noble metal electrocatalysts such as platinum.

[0011] [6] Ionomeric PEMs, for example perfluorosulfonic acid (PFSA) polymer membranes such as Nation, have been widely used in PEM-based electrochemical systems such as fuel cells, electrolysers and electrochemical hydrogen compressors due to their high proton conductivity as well as good chemical and mechanical stability. However, the solid Nation matrix includes significant amorphous domains and a relatively open structure which allows gaseous H2 to permeate with high flux. The compression ratio and efficiency of Nation-based electrochemical hydrogen compressors is thus undesirably affected by back-diffusion of gaseous H2 from the high-pressure cathode side to the low-pressure anode side. H2 permeability of the PEM is less of a concern in other applications such as fuel cells and electrolysers where no adverse H2 pressure gradient exists across the membrane or the anode compartment is liquid-full.

[0012] [7] There is therefore an ongoing need for new membrane electrode assemblies, and electrochemical hydrogen compressors based on such assemblies, which at least partially address one or more of the above-mentioned short-comings, or provide a useful alternative.

[0013] [8] 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.

[0014] Summary of Invention

[0015] [9] The invention relies on the finding that highly crystalline hydrophilic polymers such as polyvinyl alcohol may be used to provide a PEM for an electrochemical hydrogen compressor provided the PEM polymeric matrix is doped with an inorganic metal compound such as particulate metal oxide. The high crystallinity of the polymeric matrix, which may be only slightly disrupted by the particulate additive, provides a H2 permeability which is substantially lower than a Nation PEM, thus limiting the back-diffusion of gaseous H2 from the high-pressure cathode side to the low-pressure anode side. The proton conductivity of the doped PEM is sufficient to facilitate compression of H2 with good efficiency and excellent compression ratios, despite the lack of ionic functionality on the polymeric matrix. Without wishing to be limited by any theory, it is proposed that the inorganic metal compound particles facilitate proton transfer through the otherwise low-conductivity polymeric matrix by providing proton-reactive surface functionalities which support facile proton exchange between neighbouring dopant particles dispersed in the matrix.

[0016]

[0010] In accordance with a first aspect the invention provides a membrane electrode assembly for an electrochemical hydrogen compressor, the membrane electrode assembly comprising a proton exchange membrane arranged between an anode and a cathode, wherein the anode comprises an electrocatalyst for dihydrogen oxidation and the cathode comprises an electrocatalyst for proton reduction, and wherein the proton exchange membrane comprises a semicrystalline polymeric matrix comprising a hydrophilic polymer and particles of an inorganic metal compound dispersed in the semicrystalline polymeric matrix.

[0017]

[0011] In some embodiments, the hydrophilic polymer is a polyvinyl alcohol, for example an un-crosslinked polyvinyl alcohol.

[0018]

[0012] In some embodiments, the semicrystalline polymeric matrix has a degree of crystallinity of greater than 70%, such as greater than 75%, for example greater than 80%.

[0019]

[0013] In some embodiments, the inorganic metal compound is a metal oxide, such as a transition metal oxide.

[0020]

[0014] In some embodiments, the metal oxide comprises an oxide of a metal selected from the period 4, group 3-12 metal elements, such as from the period 4, group 3-10 metal elements, for example from the period 4, group 4-8 metal elements. In some embodiments, the metal oxide comprises vanadium(V) oxide (V2O5).

[0021]

[0015] In some embodiments, the particles of an inorganic metal compound have a D50 particle size of less than 500 nm, such as less than 350 nm, for example between 50 nm and 250 nm.

[0022]

[0016] In some embodiments, the proton exchange membrane comprises the particles of an inorganic metal compound in an amount of less than 1 wt.%, or less than 0.6 wt.%, such as in the range of 0.05 wt.% to 0.6 wt.%, for example in the range of 0.05 wt.% to 0.2 wt.%.

[0023]

[0017] In some embodiments, the proton exchange membrane has an H2 permeability of less than 0.5 Barrer, such as less than 0.1 Barrer, when measured at 35°C and 9 bar pressure differential using dry H2 gas feed.

[0024]

[0018] In some embodiments, the proton exchange membrane has an H2 permeability of less than 0.07 Barrer, such as less than 0.05 Barrer, when measured at 35°C and 9 bar pressure differential using dry H2 gas feed.

[0025]

[0019] In some embodiments, the proton exchange membrane has a proton conductivity of at least 1.0x1 O'4S / cm, or at least 1.5x1 O'4S / cm, such as at least 3x1 O'4S / cm, for example at least 7x1 O'4S / cm, when measured at 22°C with a four-point probe conductivity meter.

[0020] In some embodiments, the anode and the cathode comprise continuous layers adjacent to opposite surfaces of the proton exchange membrane.

[0026]

[0021] In some embodiments, at least one of and optionally both of the electrocatalyst for dihydrogen oxidation and the electrocatalyst for proton reduction comprises a noble metal, for example platinum. In some embodiments, the noble metal is dispersed on a conductive support, such as a carbonaceous support.

[0027]

[0022] In accordance with a second aspect the invention provides a membrane electrode assembly for an electrochemical hydrogen compressor, the membrane electrode assembly comprising a proton exchange membrane arranged between an anode and a cathode, wherein the anode comprises an electrocatalyst for dihydrogen oxidation and the cathode comprises an electrocatalyst for proton reduction, and wherein the proton exchange membrane comprises a semicrystalline polymeric matrix comprising polyvinyl alcohol and particles of a metal oxide dispersed in the semicrystalline polymeric matrix.

[0028]

[0023] Various embodiments of the second aspect may generally be as disclosed herein in the context of the first aspect.

[0029]

[0024] In accordance with a third aspect the invention provides a method of producing a membrane electrode assembly, the method comprising: (i) dispersing particles of an inorganic metal compound in a membrane precursor liquid comprising a hydrophilic polymer or precursor thereof; (ii) producing a proton exchange membrane from the membrane precursor liquid, wherein the proton exchange membrane comprises a semicrystalline polymeric matrix comprising the hydrophilic polymer and particles of the inorganic metal compound dispersed in the semicrystalline polymeric matrix; and (iii) arranging the proton exchange membrane between an anode and a cathode, wherein the anode comprises an electrocatalyst for dihydrogen oxidation and the cathode comprises an electrocatalyst for proton reduction.

[0030]

[0025] In some embodiments, the membrane precursor liquid is a solution of the hydrophilic polymer, for example an aqueous solution of the hydrophilic polymer. Producing the proton exchange membrane may comprise comprises casting and drying the solution.

[0031]

[0026] In accordance with a fourth aspect the invention provides an electrochemical hydrogen compressor, comprising: a membrane electrode assembly according to any embodiment of the first or second aspect; anode and cathode compartments separated by the proton exchange membrane of the membrane electrode assembly; and a power supply to apply a potential between the anode and cathode of the membrane electrode assembly, wherein the anode compartment is configured to receive a gas comprising dihydrogen for oxidation of the dihydrogen on the anode to form protons, and wherein the cathode compartment is configured to retain compressed dihydrogen gas formed by reduction of the protons on the cathode.

[0027] In some embodiments, the electrochemical hydrogen compressor comprises a plurality of membrane electrode assemblies according to any embodiment of the first or second aspect arranged in series. The cathode compartment of at least one membrane electrode assembly may form the anode compartment of a subsequent membrane electrode assembly.

[0032]

[0028] In accordance with a fifth aspect the invention provides a method of compressing dihydrogen, the method comprising: contacting a gas comprising dihydrogen with the anode of a membrane electrode assembly according to any embodiment of the first or second aspect; and applying a potential between the anode and cathode of the membrane electrode assembly, wherein the dihydrogen is oxidised on the anode to form protons and wherein the protons permeate through the proton exchange membrane and are reduced on the cathode to form compressed dihydrogen.

[0033]

[0029] In some embodiments, the dihydrogen is compressed with a single-stage compression ratio of at least 10, such as at least 12, for example at least 16.

[0034]

[0030] In some embodiments, the efficiency of compression across the proton exchange membrane is greater than 30%, such as greater than 40%, for example greater than 50%.

[0035]

[0031] In some embodiments, the method comprises further compressing the compressed dihydrogen with another membrane electrode assembly according to any embodiment of the first or second aspect. In this manner, dihydrogen may be compressed in multiple stages.

[0036]

[0032] 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.

[0037]

[0033] Further aspects of the invention appear below in the detailed description of the invention.

[0038] Brief Description of Drawings

[0039]

[0034] Embodiments of the invention will herein be illustrated by way of example only with reference to the accompanying drawings in which:

[0040]

[0035] Figure 1 schematically depicts a membrane electrode assembly according to some embodiments of the disclosure.

[0041]

[0036] Figure 2 schematically depicts an electrochemical hydrogen compressor according to some embodiments of the disclosure.

[0042]

[0037] Figure 3 shows XRD diffraction patterns for a polyvinyl alcohol membrane (not particulate additive) and a Nation 115 membrane, as determined in Example 2.

[0038] Figure 4 is a graph showing the water uptake (%) of membranes soaked in deionized water for 24 h, as determined in Example 3.

[0043]

[0039] Figure 5 depicts a perspective view of the electrochemical hydrogen compressor as used in Example 6, when assembled.

[0044]

[0040] Figure 6 depicts an exploded view of an electrochemical hydrogen compressor as used in Example 6.

[0045]

[0041] Figure 7 depicts a channel plate of the electrochemical hydrogen compressor as used in Example 6.

[0046]

[0042] Figure 8 is a graph showing the current density and cathode pressure over time when H2 was compressed at a potential of 0.05V in an electrochemical hydrogen compressor comprising (i) a PVA-based PEM doped with 0.1 wt.% V2O5 and (ii) a Nation PEM, in Example 6.

[0047]

[0043] Figure 9 is a graph showing the current density and cathode pressure over time when H2 was compressed at a potential of 0.1V in an electrochemical hydrogen compressor comprising (i) a PVA-based PEM doped with 0.1 wt.% V2O5 and (ii) a Nation PEM, in Example 6.

[0048]

[0044] Figure 10 is a graph showing the calculated rate of H2 back-diffusion from cathode compartment to anode compartment when H2 was compressed at a potential of 0.05V in electrochemical hydrogen compressors comprising (i) PVA-based PEMs doped with various metal oxides and (ii) a Nation PEM, in Example 6.

[0049]

[0045] Figure 11 is a graph showing the calculated rate of H2 back-diffusion from cathode compartment to anode compartment when H2 was compressed at a potential of 0.1V in electrochemical hydrogen compressors comprising (i) PVA-based PEMs doped with various metal oxides and (ii) a Nation PEM, in Example 6.

[0050]

[0046] Figure 12 is a graph showing the calculated compression efficiency when H2 was compressed at a potential of 0.05V in electrochemical hydrogen compressors comprising (i) PVA-based PEMs doped with various metal oxides and (ii) a Nation PEM, in Example 6.

[0051]

[0047] Figure 13 is a graph showing the calculated compression efficiency when H2 was compressed at a potential of 0.1V in electrochemical hydrogen compressors comprising (i) PVA-based PEMs doped with various metal oxides and (ii) a Nation PEM, in Example 6. Detailed Description

[0052] Membrane electrode assembly

[0053]

[0048] The present invention relates to a membrane electrode assembly for an electrochemical hydrogen compressor. The membrane electrode assembly comprises a proton exchange membrane arranged between an anode and a cathode. The anode comprises an electrocatalyst for dihydrogen oxidation and the cathode comprises an electrocatalyst for proton reduction. The proton exchange membrane comprises a semicrystalline polymeric matrix comprising a hydrophilic polymer and particles of an inorganic metal compound dispersed in the semicrystalline polymeric matrix.

[0054]

[0049] Figure 1 schematically depicts a membrane electrode assembly (MEA) 100 according to some embodiments. MEA 100 comprises proton exchange membrane (PEM) 102 arranged between anode 104 and cathode 106. PEM 102 comprises a semicrystalline polymeric matrix comprising a hydrophilic polymer and particles of an inorganic metal compound dispersed in the semicrystalline polymeric matrix. Anode 104 comprises an electrocatalyst for dihydrogen oxidation and cathode 106 comprises an electrocatalyst for proton reduction.

[0055]

[0050] MEA 100 may be configured as a stack of layers comprising PEM 102 sandwiched between anode 104 and cathode 106. Both electrodes may be continuous gas-permeable layers covering one side of PEM 102 on at least the portion of PEM 102 intended to provide proton transfer when MEA 100 is used in an electrochemical hydrogen compressor.

[0056] Anode and cathode

[0057]

[0051] When the membrane electrode assembly is used in an electrochemical hydrogen compressor, dihydrogen is oxidised on the anode to form protons which permeate through the proton exchange membrane to the cathode where they are reduced back to dihydrogen. The proton exchange membrane is thus arranged between the anode and cathode so as to allow facile transfer of protons from the anode to the PEM, and from the PEM to the cathode. Preferably, the anode and cathode are configured such that protons passing from anode to cathode permeate through the entire proton-exchange area of the PEM, i.e. the entire portion of the PEM area which separates cathode and anode compartments and is thus available for proton transfer. In some embodiments, therefore, the anode and the cathode are both configured as continuous and typically gas-permeable layers adjacent to opposite surfaces of the proton exchange membrane. Typically, the continuous layers of the electrodes are formed directly on the PEM surfaces.

[0052] The anode and cathode both comprise an electrocatalyst, which may be the same or different. The electrocatalyst on the anode is used to catalyse dihydrogen oxidation, as represented by equation (1), and the electrocatalyst on the cathode is used to catalyse proton reduction, as represented by equation (2). Equations (2) and (1) are, respectively, the hydrogen evolution reaction (HER) and its reverse reaction. Accordingly, the electrocatalysts of the anode and cathode may generally comprise a wider range of electrocatalysts known for the HER, particularly those suitable for the HER under acidic and neutral conditions. Such electrocatalysts include noble metal electrocatalysts, non-noble metal electrocatalysts, and metal-free electrocatalysts. In some embodiments, the electrocatalysts of the anode and cathode are particulate electrocatalysts.

[0058]

[0053] In some embodiments, the electrocatalyst for dihydrogen oxidation (the anodic electrocatalyst) comprises a noble metal. In some embodiments, the electrocatalyst for proton reduction (the cathodic electrocatalyst) comprises a noble metal. The noble metal of the anodic and / or cathodic electrocatalysts may independently be selected from platinum, palladium, iridium, rhodium, and ruthenium, preferably from platinum, iridium and ruthenium. In some embodiments the noble metal is platinum. Noble metal electrocatalysts are commonly dispersed on a conductive support to enhance the catalytic surface area and conductivity. Suitable conductive supports include carbonaceous supports such as the carbon black. In some exemplary embodiments, the anodic and cathodic electrocatalysts both comprise platinum on carbon black (Pt / C).

[0059]

[0054] The anode and cathode may comprise one or more additives to enhance electrical conductivity, proton conductivity and / or adhesion. In some embodiments, the anode and / or the cathode comprise a binder, for example a polymeric binder. The binder may bind particles of electrocatalyst together and adhere the electrocatalyst to the PEM surface, while still allowing access of gaseous H2 to and from the electrocatalyst surface. In some embodiments, the binder is a proton-conductive polymeric binder, which may thus facilitate transfer of protons from the anodic electrocatalyst to the PEM, and from the PEM to the cathodic electrocatalyst. Suitable proton-conductive polymeric binders may include ionomers, for example perfluorosulfonic acid (PFSA) functionalized polymers such as Nation.

[0060] Proton exchange membrane

[0061]

[0055] The proton exchange membrane (PEM) comprises a semicrystalline polymeric matrix which comprises (and in some embodiments consists of) a hydrophilic polymer. Particles of an inorganic metal compound are dispersed in the semicrystalline polymeric matrix.

[0062]

[0056] As used herein, a hydrophilic polymer refers to polymers which can absorb water, and encompasses both water soluble polymers and water swellable polymers.

[0057] Without wishing to be bound by any theory, the PEM does not rely on ionizable functional groups covalently bonded to the hydrophilic polymer to transport protons in use. The hydrophilic polymer is thus typically not an ionomer. In some embodiments, the hydrophilic polymer is a non-ionic hydrophilic polymer. As used herein, a non-ionic polymer refers to a polymer which is substantially neutral, i.e. it does not include ionized functional groups covalently bonded to the polymer, when in contact with, or dissolved in, water at neutral pH (pH 7). Typically, the hydrophilic polymer is a non-ionic polymer which is substantially neutral, i.e. it does not include ionized functional groups covalently bonded to the polymer, when in contact with, or dissolved in, water at neutral and mildly acidic pH (e.g. in the range of pH 2 to 7).

[0063]

[0058] The hydrophilic polymer is typically a poor proton conductor in the absence of the particles of inorganic metal compound, in contrast to proton-conductive ionomers such as Nation. Typically, the hydrophilic polymer does not include perfluorosulfonic acid groups as are present in Nation or other PFSA ionomers. In some embodiments, therefore, the hydrophilic polymer does not include a plurality of pendant perfluoroalkylsulfonic acid groups. In some embodiments, the hydrophilic polymer does not include a plurality of pendant sulfonic acid groups (-SO2OH). In some embodiments, the hydrophilic polymer does not include a plurality of pendant acid groups, such as sulfonic acid and carboxylic acid groups (-COOH). The hydrophilic polymer may be substantially free of perfluorosulfonic acid groups and is typically substantially free of sulfonic acid groups and / or substantially free of acid groups including both sulfonic acid and carboxylic acid groups. In some embodiments, the hydrophilic polymer is a non-fluorinated polymer.

[0064]

[0059] The hydrophilic polymer is preferably a semicrystalline polymer such that the polymeric matrix has a high degree of crystallinity. A high degree of crystallinity may be desirable as this is associated with lower permeability to gaseous H2. In some embodiments, the semicrystalline polymeric matrix has a degree of crystallinity of greater than 50%, or greater than 60%, or greater than 70%, such as greater than 75%, for example greater than 80%, as determined from X-Ray Diffraction (XRD) data. The degree of crystallinity may be calculated as the area of all crystalline peaks as a percentage of the combined areas of all crystalline and amorphous peaks from the XRD diffraction pattern, measured at room temperature using XRD analytical methods routinely used by those skilled in polymer characterisation.

[0065]

[0060] In some embodiments, the hydrophilic polymer is a polyvinyl alcohol. As used herein, a polyvinyl alcohol is a polymer comprising the repeating unit -(CH2-CH[OH])-, and includes random or block polyvinyl alcohol copolymers, and cross-linked or uncross-linked polymeric structures. In some embodiments, the polyvinyl alcohol comprises the repeating unit -(CH2-CH[OH])- in an amount of at least 50 wt.%, such as at least 80 wt.%, or at least 90 wt.%. Polyvinyl alcohol is typically produced by hydrolysis of polyvinyl acetate (or other vinyl ester polymer), and the polyvinyl alcohol may thus include a residual fraction of pendant acetate (or other ester) groups.

[0066]

[0061] In some embodiments, the polyvinyl alcohol is a non-ionic polymer as defined herein. Such polymers are also expected to be neutral under the conditions of use in an electrochemical hydrogen compressor (i.e. neutral to acidic pH as protons are generated). In some embodiments, the hydrophilic polymer is an un-crosslinked polyvinyl alcohol, for example an un-crosslinked polyvinyl alcohol homopolymer, i.e. containing only the repeating unit -(CH2-CH[OH])- optionally with a minor fraction of un-hydrolysed repeating units comprising acetate or other ester. The polyvinyl alcohol may be at least 85% hydrolysed, such as at least 98%, or at least 99% hydrolysed (meaning that this fraction of the precursor acetate or other ester groups are hydrolysed to hydroxy groups).

[0067]

[0062] The hydrophilic polymer may have a suitable molecular weight which may be sufficiently high to form a robust film. In some embodiments, the hydrophilic polymer such as un-crosslinked polyvinyl alcohol may have an average molecular weight (Mw) in the range of 10 to 1000 kDa, such as in the range of 50 to 750 kDa, or in the range of 100 to 500 kDa, or in the range of 100 to 200 kDa. Mwmay be as determined by size exclusion chromatography.

[0068]

[0063] Hydrophilic polymer as disclosed herein is typically the main or only polymeric component present in the semicrystalline polymeric matrix of the PEM. In some embodiments, the semicrystalline polymeric matrix comprises the hydrophilic polymer in an amount of at least 80 wt.%, or at least 90 wt.%, or at least 95 wt.%, or at least 95 wt.%, such as 100 wt.% of the polymeric components in the semicrystalline polymeric matrix. As already noted, the PEM does not rely on ionic polymer functionality to provide proton conductivity. The semicrystalline polymeric matrix of the PEM may therefore not contain a polymer, such as Nation, which comprises a plurality of pendant sulfonic acid groups (-SO2OH). The semicrystalline polymeric matrix typically does not contain an ionomer.

[0069]

[0064] The PEM may have a suitable thickness for use in an electrochemical hydrogen compressor. In particular, it should be sufficiently thick to withstand the pressure differential between the anode and cathode compartments. Within the thickness range where the pressure differential can be accommodated without mechanical failure, thinner membranes are expected to provide increased proton conductivity but also increased back-diffusion of H2. The PEM thickness can thus be selected to achieve a desired balance of properties for a particular implementation. In some embodiments, the PEM has a thickness in the range of 40 pm to 500 pm, or in the range of 50 pm to 200 pm, such as in the range of 80 pm to 200 pm, for example in the range of 100 pm to 160 pm, or in the range of 115 pm to 145 pm.

[0065] The PEM comprises particles of an inorganic metal compound dispersed in the semicrystalline polymeric matrix. Preferably, the particles are dispersed substantially uniformly throughout the polymeric matrix, as this arrangement is expected to best support proton transport through the PEM.

[0070]

[0066] Without wishing to be limited by any theory, it is believed that the particles of inorganic metal compound facilitate proton transport through the PEM due to the presence of proton-reactive surface functional groups present on the particle surfaces, particularly surface functional groups which can react by protonation-deprotonation reaction mechanisms. Metal- bonded heteroatoms, such as O, N or S atoms, may provide suitable sites for protonation and deprotonation. Accordingly, a variety of inorganic metal compounds having such proton- reactive surface functional groups may be used.

[0071]

[0067] The particles of inorganic metal compound may include particles selected from metal oxides, metal hydroxides and combinations thereof. Oxygen atoms at the surface of such inorganic metal compounds, for example bridging oxygen atoms between metal atoms, may be susceptible to protonation and deprotonation, and it is proposed that this capability facilitates proton transfer in the PEM. Particles of other inorganic metal compounds, such as metal nitrides and metal sulfides, are also expected to include proton-reactive surface functionality and the particles of inorganic metal compound may therefore also comprise such compounds.

[0072]

[0068] In some embodiments, the inorganic metal compound is a metal oxide. Suitable metal oxides may include oxides of the transition metals, such as an oxide of a metal selected from the period 4, group 3-12 metal elements, or from the period 4, group 3-10 metal elements, for example an oxide of a metal from the period 4, group 4-8 metal elements, i.e. titanium, vanadium, chromium, manganese and iron. In some embodiments, the metal oxide comprises vanadium(V) oxide (V2O5), iron (II, III) oxide (FesC ), or manganese (IV) dioxide (MnC>2).

[0073]

[0069] The particles of inorganic metal compound may have a particle size small enough to allow dispersion within the PEM, preferably without unacceptably compromising the physical integrity of the membrane or excessively disrupting the crystallinity of the polymeric matrix. In some embodiments, the particles have a D50 particle size of less than 500 nm, such as less than 350 nm. In some embodiment, the particles have a D50 of between 50 nm and 250 nm. As used herein, the D50 particle size refers to the particle size selected such that 50% of the inorganic metal compound (by number) is present in particles smaller than the D50. The D50 particle size may be as measured by dynamic light scattering.

[0074]

[0070] The PEM includes a suitable amount of the particles of inorganic metal compound to facilitate proton transfer, preferably without unacceptably increasing the permeability of the PEM to gaseous H2. In some embodiments, the proton exchange membrane comprises the particles of an inorganic metal compound in an amount of less than 1 wt.%, such as less than 0.6 wt.%, for example in the range of 0.02 wt.% to 0.6 wt.%, or in the range of 0.05 wt.% to 0.2 wt.%. The inventors have demonstrated particularly high H2 compression ratios when using 0.1 wt.% metal oxide particles.

[0075]

[0071] The PEM preferably has a low permeability to gaseous hydrogen (H2 permeability), thus limiting back-diffusion of compressed H2 from the cathode compartment to the anode compartment in the electrochemical hydrogen compressor. The H2 permeability is preferably substantially less than that of a Nation membrane of the same thickness. While H2 compressed by an electrochemical hydrogen compressor may in practice be moist, the observed permeability of the PEM to dry H2 is considered a reliable direction indicator of H2 back-diffusion under realistic working conditions. In some embodiments, the PEM has an H2 permeability of less than 0.5 Barrer, preferably less than 0.1 Barrer, such as less than 0.07 Barrer, for example less than 0.05 Barrer, when measured at 35°C and 9 bar pressure differential using a dry H2 gas feed (9 bar on retentate side, high vacuum = 0 bar initially on the permeate side).

[0076]

[0072] The PEM preferably has a good proton conductivity, thus ensuring that the performance of the electrochemical hydrogen compressor is not unacceptably impeded by high resistance to the flow of protons from anode to cathode. The proton conductivity is typically orders of magnitude greater than a comparative PVA membrane (lacking inorganic metal compound) of the same thickness, but may be lower than that of a Nation membrane of the same thickness. Nevertheless, the performance in an electrochemical hydrogen compressor may in some embodiments be similar or superior to a Nation PEM due to the reduced H2 back- diffusion. While the PEM may be partially hydrated when used to compress H2 (due to moisture in the H2 feed), the proton conductivity of the PEM as determined when the PEM is dry is considered a reliable directional indicator of proton conductivity under realistic working conditions. In some embodiments, the dry PEM (e.g. after drying at 70°C for 24 hours in a vacuum oven) has a proton conductivity of at least 1.0x10-4S / cm, such as at least 1.5x10-4S / cm, or at least 3x1 O'4S / cm, or at least 5x1 O'4S / cm, for example at least 7x1 O'4S / cm, as determined when measured at 22°C with a four-point probe conductivity meter.

[0077] Method of producing the membrane electrode assembly

[0078]

[0073] The membrane electrode assembly may be produced by the following steps: (i) dispersing particles of an inorganic metal compound in a membrane precursor liquid comprising a hydrophilic polymer or precursor thereof; (ii) producing a proton exchange membrane from the membrane precursor liquid, wherein the proton exchange membrane comprises a semicrystalline polymeric matrix comprising the hydrophilic polymer and particles of the inorganic metal compound dispersed in the semicrystalline polymeric matrix; and (iii) arranging the proton exchange membrane between an anode and a cathode, wherein the anode comprises an electrocatalyst for dihydrogen oxidation and the cathode comprises an electrocatalyst for proton reduction.

[0079]

[0074] The membrane precursor liquid may be a solution of the hydrophilic polymer or precursor thereof in a solvent, preferably a polar solvent such as water. In some embodiments, the membrane precursor liquid is a solution of the hydrophilic polymer, such as an aqueous solution of polyvinyl alcohol. The particles of an inorganic metal compound are preferably dispersed uniformly in the membrane precursor liquid, for example with agitation or sonication.

[0080]

[0075] The PEM may be produced from the membrane precursor liquid by conventional membrane-forming techniques, for example by casting the membrane precursor liquid and drying the cast layer to produce the PEM with desired thickness.

[0081]

[0076] The proton exchange membrane may be arranged between an anode and a cathode by applying anode and cathode compositions to opposite sides of the PEM, preferably thereby producing the anode and cathode as continuous and typically gas-permeable layers on the PEM surfaces. The anode and cathode compositions may be the same or different. The anode composition comprises an electrocatalyst for dihydrogen oxidation, or precursor thereof, and the cathode composition comprises an electrocatalyst for proton reduction, or precursor thereof. In some embodiments, the anode and cathode compositions are slurry compositions (inks) comprising particulate electrocatalysts (or precursors thereof) and other functional components such as a polymeric binder carried in a liquid carrier, preferably an aqueous liquid carrier. The inks may be applied to the PEM surfaces, to provide the required areal loading of electrocatalyst, and dried thereon to produce the electrodes, preferably as continuous gas-permeable layers.

[0082] Electrochemical hydrogen compressor

[0083]

[0077] The invention further relates to an electrochemical hydrogen compressor, comprising a membrane electrode assembly as disclosed herein, anode and cathode compartments separated by the proton exchange membrane of the membrane electrode assembly, and a power supply configured to apply a potential between the anode and cathode of the membrane electrode assembly. The anode compartment is configured to receive a gas comprising dihydrogen for oxidation of the dihydrogen on the anode to form protons, and the cathode compartment is configured to retain compressed dihydrogen gas formed by reduction of the protons on the cathode.

[0084]

[0078] Figure 2 schematically depicts an electrochemical hydrogen compressor 200 according to some embodiments. Compressor 200 comprises membrane electrode assembly 100 as disclosed herein with reference to Figure 1. Anode compartment 208, fitted with H2 inlet 209, and cathode compartment 210, fitted with H2 outlet 211 , are separated by proton exchange membrane 102. Compressor 200 comprises a power supply 212 configured to apply a potential between anode 104 and cathode 106. In some embodiments, as depicted, power supply 212 is electrically connected to anodic current collector 214 and cathodic current collector 216, and the potential is transmitted to anode 104 and cathode 106 by gas diffusion layers 218 and 220 present in anode compartment 208 and cathode compartment 210 respectively. Alternatively, the power supply may be directly connected to anode and cathode. The gas diffusion layers when used may be electrically and thermally conductive porous layers which electrically connect the metallic current collectors and electrodes while allowing the reactant and product H2 gases to flow to and from the electrodes. Suitable gas diffusion layers may comprise carbon materials such as non-woven carbon paper.

[0085]

[0079] In use, a potential is applied between anode 104 and cathode 106, and a gas 220 comprising H2 is flowed via inlet 209 into anode compartment 208. The H2 is thus oxidised on anode 104 to form protons, via equation (1), and the protons permeate through proton exchange membrane 102 and are reduced on cathode 106, via equation (2) to form compressed H2 in cathode compartment 210. The compressed H2 may then be flowed out of outlet 211 as compressed H2 product 222.

[0086]

[0080] Compressor 200 as depicted is a single-stage compressor. In other embodiments, however, the electrical hydrogen compressor is a multi-stage compressor and thus comprises a plurality of membrane electrode assemblies as disclosed herein arranged in series. Each membrane electrode assembly in series comprises anode and cathode compartments separated by the proton exchange membrane of that membrane electrode assembly. Optionally, the multi-stage compressor is configured, e.g. as a stack, such that the cathode compartment of at least one membrane electrode assembly forms the anode compartment of the next membrane electrode assembly in series.

[0087]

[0081] In use, the compressed H2 produced at the cathode of at least one membrane electrode passes freely to the anode of the next membrane electrode assembly in series. By applying a potential between the electrodes of each membrane electrode assembly in series, the H2feed gas can thus be compressed in a series of stages to desirably high total pressures.

[0088] Method of compressing dihydrogen

[0089]

[0082] The invention further relates to a method of compressing dihydrogen. The method comprises contacting a gas comprising dihydrogen with the anode of a membrane electrode assembly as disclosed herein, and applying a potential between the anode and cathode of the membrane electrode assembly. The dihydrogen is oxidised on the anode to form protons, and the protons permeate through the proton exchange membrane and are reduced on the cathode to form compressed dihydrogen.

[0090]

[0083] With reference again to Figure 2, gas 220 comprising H2 is flowed via inlet 209 into anode compartment 208, where it contacts anode 104. A potential is applied between anode 104 and cathode 106 such that the H2 is oxidised on anode 104 to form protons. The protons permeate through proton exchange membrane 102 and are reduced on cathode 106 to form compressed H2 in cathode compartment 210. The compressed H2 may then be flowed out of outlet 211 as compressed H2 product 222.

[0091]

[0084] The potential applied between the anode and cathode is sufficient to oxidise H2 at the cathode, drive the protons across the PEM, and reduce the protons at the cathode. It will be appreciated that higher potentials may increase the electrochemical reaction rate, and are thus one factor that may favourably increase the compression ratio. However, excessive potentials may reduce the efficiency of compression due to increased activation, ohmic and mass transport losses. In some embodiments, the potential is between 0.01 V and 1.5 V, or between 0.02 and 0.5 V, such as between 0.02 V and 0.2 V, for example between about 0.03 V and 0.15V.

[0092]

[0085] In some embodiments, the gas comprising H2 comprises water vapour, in an amount of up to 100% relative humidity. Without wishing to be bound by any theory, water may advantageously facilitate proton conductivity by maintaining the PEM in an at least partially hydrated state and / or by acting as a proton carrier in the PEM. Water may be added intentionally to the gas feed to achieve these desirable outcomes. If required, the water may subsequently be removed from the compressed H2 product by conventional methods, as for H2 produced by PEM electrolysers where the H2 product is typically water saturated.

[0093]

[0086] In some embodiments, the gas comprising H2 is substantially pure H2, aside from any water vapour as described above. The substantially pure H2 may be supplied at low pressure such as between about 1 bar(a) and 10 bar(a), or less than 2 bar(a), for example about 1 bar(a). The pure H2 feed is thus compressed to produce a compressed H2 product with a higher pressure than the pure H2 feed, depending on the compression ratio achieved.

[0094]

[0087] In other embodiments, the gas comprising dihydrogen may comprise other gases, for example as trace impurities or in more substantial amounts such that the partial pressure of H2 in the gas feed is significantly lower than the total feed pressure. Thus, for example, the partial pressure of H2 at the anode may be well below 1 bar if the total feed pressure is 1 bar(a). H2 present in the gas feed is selectively transported across the proton exchange membrane, according to the principles disclosed here, thus reducing the amount of H2 present in the feed gas and typically producing a high purity H2 product. The compressed H2 produced at the cathode will have a higher pressure than the H2 partial pressure at the anode, even if the absolute H2 pressure in the cathode compartment remains low. Such methods may be useful for simultaneously purifying and compressing H2. For example, the methods disclosed herein may be useful for removing impurities such as carbon monoxide from H2 to be used in a fuel cell.

[0095]

[0088] The compression of H2 across the PEM may be characterised by a compression ratio, i.e. the ratio of the pressure of the compressed H2 produced at the cathode relative to the (partial) pressure of H2 present in the feed gas comprising H2. The methods disclosed herein may, in some embodiments, provide similar or higher compression ratios than are achievable with a membrane electrode assembly comprising a Nation PEM. In some embodiments, the H2 present in the gas comprising H2 is compressed across the PEM (i.e. in a single stage of compression) with a compression ratio of at least 10, or at least 12, such as at least 14, or at least 16, for example about 20 or greater.

[0096]

[0089] The compression of H2 across the PEM may be characterised by an efficiency, i.e. ratio of the actual power consumption to the ideal (theoretical) power consumption required to achieve the observed compression ratio. The methods disclosed herein may, in some embodiments, provide higher efficiencies than are achievable with a membrane electrode assembly comprising a Nation PEM. It will be appreciated, that the efficiency may be affected by the applied potential. In some embodiments, the H2 present in the gas comprising H2 is compressed across the PEM (i.e. in a single stage of compression) with an efficiency of greater than 30%, or greater than 40%, such as greater than 50%, for example about 60% or more.

[0097]

[0090] In some embodiments, the method comprises further compressing the compressed H2 with another membrane electrode assembly as disclosed herein, i.e. in a multistage compressor comprising a plurality of membrane electrode assemblies as disclosed herein arranged in series. The compressed H2 produced at the cathode of at least one membrane electrode passes freely to the anode of the next membrane electrode assembly in series. By applying a potential between the electrodes of each membrane electrode assembly in series, the H2feed gas can thus be compressed in a series of stages to desirably high total pressures.

[0098] EXAMPLES

[0099]

[0091] 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

[0100]

[0092] Polyvinyl alcohol (PVA, 146-186 kDa and 99+% hydrolysed), vanadium oxide (V2O5), Nation monomer (5% solution), 2-propanol (> 99.5%), and ammonia nitrate (NH4NO3) were purchased from Sigma Aldrich (St. Louis, USA). Manganese dioxide (MnC>2) and iron(l 1 , 111) oxide (FesC ) were obtained from Chem-Supply (Gillman, AU). Nation 115 film (135 pm thick) and Pt / C (platinum nominally 40% on carbon black) were purchased from Thermo Fisher Scientific (Waltham, USA). Gas diffusion layers (GDL, Sigracet 39 BB, electrical resistivity of < 13 mQ.cm2) was obtained from Fuel Cell Store (Texas, USA).

[0101]

[0093] The specific surface area (SSA) of the metal oxides was determined by gas sorption measurements (Micromeritics, US) carried out using the Brunauer-Emmelt-Teller (BET) method. Pore volume was computed employing density functional theory (DFT). Particle size was investigated by a Nano Sight NS300 (Malvern, UK). The analyzed particle size and BET data of the metal oxides is shown in Table 1.

[0102] Table 1.

[0103]

[0094] Membrane morphology was analyzed using a flex-scanning electron microscopy (Flex-SEM 1000, Hitachi, Japan). X-ray diffraction (XRD) patterns were obtained by a D8 Advance Bruker (Billerica, US), using X-ray generator at 40 kV and 40 mA. Example 1. Preparation of PVA membranes

[0104]

[0095] Clear PVA solutions were prepared by dissolving 2.0 g of PVA in 40 mL of deionized water at 150°C. For membranes containing dispersed metal oxide, 0.1 wt.% or 0.5 wt.% of the required metal oxide were added to the PVA solution with continuous stirring for 30 min. Following this, 10 wt.% of NH4NO3 was dispersed in the mixture to enhance ionic conductivity after cooling to ambient temperature. The homogeneous solutions were then cast onto a petri dish and dried at room temperature for a week to produce a membrane with a thickness of about 130 pm. Membranes were thus obtained as shown in Table 2.

[0105] Table 2.

[0106] Example 2. Crystallinity of PVA membranes

[0107]

[0096] The membranes were characterised by XRD. Figure 3 shows the XRD diffraction patterns for an undoped PVA membrane and a Nation 115 membrane. The Nation 115 film provides two diffraction peaks at 17.23° and 39.66° due to the semi-crystalline nature of the perfluorocarbon. The undoped PVA membrane provides a peak at 19.61° and small structure at 41.23° consistent with the semi-crystalline structure of PVA. This structural characteristic of PVA is provided by hydrogen bonds formed both within individual polymer chains (intramolecular) and between adjacent polymer chains (intermolecular).

[0108]

[0097] Loading the PVA membrane with the three metal oxides at 0.1 wt.% provided similar XRD diffraction patterns to undoped PVA though with a peak shift of the membranes towards wider angles with an increase in the concentration of metal oxides.

[0098] The calculated degree of crystallinity calculated using the Origin software package from the XRD data for the membranes is shown in T able 2. The degree of crystallinity is defined as the area of all crystalline peaks as a percentage of the combined areas of all crystalline and amorphous peaks. The undoped PVA membrane is substantially more crystalline than the Nation 115 membrane.

[0109]

[0099] The addition of metal oxides reduced the PVA’s crystallinity slightly due to particle disruption of polymer chain packing, with the loss of crystallinity dependent on the concentration of the metal oxide. A loading of 0.1 wt.% MnC>2 or V2O5 particles had only a very minor effect on membrane crystallinity. FesC particles had a slightly stronger effect on crystallinity than MnC>2 and V2O5 particles. Without wishing to be limited by any theory, this is not due to particle size effects (FesC particles are the smallest; see Table 1) but may be due to the relative affinity of the metal oxides for PVA.

[0110] Example 3. Wettability of PVA membranes

[0111]

[0100] The wettability properties of a membrane may affect the performance of an electrochemical hydrogen compressor both by impacting the permeability, and therefore back diffusion of H2, and the proton conductivity. Wettability of the membranes was investigated by determining water uptake based on the mass change of the membranes soaked in deionized water for 24 h, and the results are shown in Figure 4. Undoped PVA exhibits high hydrophilicity due to the numerous hydroxyl groups. The small decrease in water sorption for doped PVA membranes, likely due to the intrinsic hydrophobicity of the metal oxides, was dependent on the loading and distribution of metal oxides in the matrix. Significant agglomeration and non- uniform distribution of FesC in the membrane, evident from scanning electron microscopy imaging, may have contributed to higher water uptake that MnC>2 and V2O5 which were more homogeneously dispersed in the membrane matrix. Nation film was less hydrophilic with only about 10% water uptake.

[0112] Example 4. Permeability of PVA membranes

[0113]

[0101] Hydrogen gas permeation through the membranes was tested at an operating temperature of 35°C and a test pressure of 9 bar using an experimental apparatus as reported in Zhang et al, Energy & Fuels, 36 (2022) 14500-14511. Dry, high purity H2 at 9 bar(a) was thus applied on the retentate side, and the permeate side was initially at 0 bar (high vacuum). The permeability was calculated according to equation (4): where P is permeability (Barrer), L is membrane thickness (cm), V is volume of downstream chamber (cm3), A is the effective area of the film (cm2), T is temperature (K), pf is the pressure of feed (psia), dp / dt is the slope of pressure in the downstream chamber versus time (mmHg / s).

[0114]

[0102] The membrane permeability was determined with the membrane as produced and after drying the membrane at 70°C for 24 hours in a vacuum oven. The results are shown in Table 3.

[0115] Table 3.

[0116]

[0103] All PVA membranes provided excellent hydrogen barrier properties compared to Nation, with H2 permeabilities lower by approximately two orders of magnitude. The lowest hydrogen permeability was observed for undoped PVA before drying the membrane. This may be because water present within the free volume regions of the membrane competes with H2 uptake. The very low H2 permeance for the undoped PVA membrane is consistent with its high crystallinity (see Table 2).

[0117]

[0104] Addition of the metal oxides to the membrane has only a minor effect on H2 permeability, with a general trend towards increasing permeability with increased metal oxide loading. The small increases in permeability may be attributed to the reduction in polymer crystallinity (see Table 2) and / or formation of voids around the encapsulated metal oxide particles through which H2 can permeate more freely. The permeation behavior of doped PVA membranes may also depend on the metal oxide particle size, with the smallest particles (Fe3O4) having less effect on permeability than the larger particles (MnC>2 and V2O5) even though the effect on crystallinity is opposite. Example 5. Conductivity of PVA membranes

[0118]

[0105] Conductivity measurements can be defined by various methods. Compared to the electrochemical impedance spectroscopy (EIS) technique, the measurement of the conductivity in the in-plane direction such as the four point probe method is much easier to carry out with higher stability, reproducibility and accuracy.

[0119]

[0106] The electrical resistivity of the dried membranes was measured at room temperature (22°C) using a four-point probe conductivity meter (Keithley, USA). The electrical conductivity is the reciprocal of electrical resistivity and was thus calculated using the equation (1): where oeis the electrical conductivity (S / cm), p is the resistivity (Q.cm).

[0120]

[0107] The resistance (R) and proton conductivity were calculated by equations (2) and (3): where R is the resistance (Q), p is the resistivity (Q.cm), L is membrane thickness (cm), and A is cross sectional area (cm2), and opis proton conductivity (S / cm).

[0121]

[0108] The results are shown in Table 4.

[0122] Table 4.

[0109] The proton conductivity of all PVA membranes was lower than Nation 115. This can be attributed to the insulating properties of PVA and the low specific surface area of the metal oxide particles. However, the presence of MnC>2, FesC or V2O5 particles improved the conductivity of PVA membranes by seven orders of magnitude, from 10'11S / cm to 10'4S / cm. The best results were obtained with V2O5 particles which provided high conductivity compared to MnC>2 and FesC .

[0123] Example 6. Electrochemical hydrogen compressor

[0124]

[0110] An electrocatalytic ink was formulated by mixing 67 wt.% Pt / C catalyst with 33 wt.% Nation solution (5 wt.% Nation) and sonicating the ink for 30 min before application. The ink was then coated over the membrane surface (36 cm2) at a loading of 0.5 mg / cm2and dried at 30°C. A membrane electrode assembly for each membrane in Table 2 was thus prepared by coating an electrode layer on each side of the membrane.

[0125]

[0111] As depicted in Figures 5 to 7, an electrochemical hydrogen compressor (300) was fabricated from a membrane electrode assembly (302), a set of channel plates (304), acting as aluminum current collectors, with serpentine carved flow channels (306) of 1 mm depth and 1 mm width and gas inlet / outlet 307, and supporting stainless-steel plates (308). Pairs of silicon rubber gaskets (310, 312) served as electrical insulators inserted between the membrane electrode assembly and the channel plates, and between the current collectors and the end plates, respectively. A pair of gas diffusion layers (313) was placed between channel plates (304) and the membrane electrode assembly (302) to create a gas diffusion pathway to the electrodes. The parts of the compressor were held together with screws (314), washers (316) and nuts (318). When applying a potential between the channel plates, current is transferred to the gas diffusion layers and then to the electrodes.

[0126]

[0112] The performance of the electrochemical hydrogen compressor was evaluated at ambient temperature (c.a. 22°C) by supplying hydrogen with 100% relative humidity (H2 feed passed through water bubbler) through inlet 307 to the anode compartment at an anode pressure of 1 bar. The H2 pressure of 1 bar was maintained in the anode compartment by static supply of H2.

[0127]

[0113] A potential was applied across the electrodes using a potentiostat (SP-150e Biologic, French) and the pressure of generated hydrogen in the cathode compartment was recorded by digital pressure gauge (DM 01 , Germany). Experiments were evaluated at two operating voltages: 0.05 V and 0.1 V, with a current limit of 1 A. Each experiment was performed for 48 h at ambient temperature to ensure steady state operation.

[0128]

[0114] The performance of electrochemical hydrogen compressors with a V2Os-doped PVA membrane (PVA / 0.1 wt.% V2O5) and a Nation 115 membrane are compared in Figure 8 (0.05 V potential) and Figure 9 (0.1 V potential), which show the current density and pressure change over time.

[0129]

[0115] The electrochemical hydrogen compressor model operates isothermally and is considered steady state when a constant pressure is maintained in the cathode compartment (Pc > Pa). This occurs when the forward electrochemical compression of hydrogen is equal to the back diffusion rate and represents the limitation in the compression ratio. As seen in Figures 8 and 9, the initial current density rapidly decreased as the electrochemical hydrogen compressor became charged and approached a steady-state value within the first 4 hours of operation. Thereafter the electrochemical hydrogen compressor maintained steady state behaviour throughout the 48 hours of testing.

[0130]

[0116] At steady state, the compressor with PVA / 0.1 wt.% V2O5 required a lower current density and produced a higher cathode compartment pressure than the compressor with Nation 115 membrane film. This is due to the significantly lower rate of H2 back-diffusion from cathode compartment to anode compartment because of the reduced H2 permeability of the PVA-based membrane.

[0131]

[0117] The performance of the compressors is voltage dependent (c.f. Figure 8 vs Figure 9), with an increased voltage resulting in a higher current density and thus a higher cathode pressure at which steady state is reached (i.e. where the forward electrochemical compression of hydrogen is equal to the back diffusion rate).

[0132]

[0118] The steady-state performance of electrochemical hydrogen compressors with all the metal-oxide doped PVA membranes (per Table 2) is shown in Table 5.

[0133] Table 5.

[0119] The metal oxide-doped PVA membranes all provided similar or better steady-state compression ratios, and significantly reduced current densities, than Nation 115 membrane. The lower current density can be attributed to the reduced back-diffusion of H2 through the membrane. Metal oxide doping of 0.1 wt.% provided a superior outcome to 0.5 wt.%, consistent with the permeability and proton conductivity measurements (Tables 3 and 4). This may be attributed in part to the morphology of the PVA membrane which is less disrupted at the lower metal oxide loading.

[0134]

[0120] The best compression ratio was obtained with V2Os-doped PVA membranes, with a compression ratio of nearly 20 achieved with a PVA / 0.1 wt.% V2O5 membrane and potential of 0.1 V. The superior effect of the V2O5 dopant compared to Fe3O4and MnC>2 dopants is consistent with the measured proton conductivities of the membranes (Table 4), rather than any difference in H2 permeability (Table 3).

[0135]

[0121] Permeability constants of the experimentally tested membranes were calculated according to the electrochemical reaction of the electrochemical hydrogen compressor, defined according to equation (4):

[0136] Permeabilit yy constant = - - 2.F.A.(pc,eq-Pa) (v4) ’ where leqis the equilibrium current (A), d is membrane thickness (m), F is Faraday’s constant (96485 C / mol), A is the active area of the membrane (m2), Pc,eq is the equilibrium pressure at cathode compartment (bar), and Pais the anode pressure (bar).

[0137]

[0122] The results are shown in Table 6. While similar permeability trends can be seen (permeability of Nation membrane much greater than metal-oxide doped PVA membrane), the calculated values of the permeabilities were significantly higher for the membranes under the high moisture conditions of the electrochemical hydrogen compressor experiments (Table 6) compared to the dry single gas experiments (Example 4, Table 2).

[0138] Table 6.

[0123] The calculated rate of back diffusion from cathode compartment to anode compartment is shown in Figure 10 (0.05 V potential) and Figure 11 (0.1 V potential). The backward flux is linearly related to the gaseous hydrogen concentration difference across the membrane and inversely proportional to the thickness of the membrane. The highest back diffusion is observed for Nation 115 membrane. The metal-oxide doped PVA membranes provided back diffusion rates about one quarter that of Nation at a metal oxide loading of 0.5 wt.% and about one half that of Nation at a metal oxide loading of 0.1 wt.%. Increased metal oxide concentration results in lower H2 permeability under realistic electrochemical hydrogen compressor conditions where the membrane is hydrated due to moisture in the gas feed, electrochemical reactions are taking place and there is a substantial pressure differential across the membrane. Less back diffusion occurs at the lower voltage due to the lower generated hydrogen pressures.

[0139]

[0124] The efficiency of the electrochemical hydrogen compressor is the ratio of the power consumption, W(actual)against the ideal W(ideal). The actual power is obtained by multiplying the applied cell voltage (E(cell)) by the total charge (I):

[0140] ^(actual)—E(cell) (5)

[0141]

[0125] The ideal power consumption is from the Nerst equation:

[0142]

[0126] The cathodic pressure rise with time can be defined as:

[0143]

[0127] Hence, the efficiency of the electrochemical hydrogen compressor (HEHC) becomes:

[0144] _ Wvv(iadeal) ■lEHC — w(actual) where R is universal gas constant (8.314 J / mol.K), T is the operating temperature (K), nnis the net moles transferred to the cathode (mol), pcand paare the cathodic and anodic pressure (bar).

[0145]

[0128] As calculated using this methodology, the efficiency of the investigated electrochemical hydrogen compressors is shown in Figure 12 (0.05 V potential) and Figure 13 (0.1 V potential). Notably, the metal oxide-doped PVA membranes operated at higher efficiencies compared to Nation despite the lower proton conductivity. The PVA membranes loaded with 0.1 wt.% metal oxides provided greater efficiency than those loaded with 0.5 wt.% metal oxides. The overall efficiency decreases with higher applied voltage due to increased activation, ohmic and mass transport losses.

[0129] 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.

Claims

Claims1. A membrane electrode assembly for an electrochemical hydrogen compressor, the membrane electrode assembly comprising a proton exchange membrane arranged between an anode and a cathode, wherein the anode comprises an electrocatalyst for dihydrogen oxidation and the cathode comprises an electrocatalyst for proton reduction, and wherein the proton exchange membrane comprises a semicrystalline polymeric matrix comprising a hydrophilic polymer and particles of an inorganic metal compound dispersed in the semicrystalline polymeric matrix.

2. The membrane electrode assembly according to claim 1, wherein the hydrophilic polymer is a polyvinyl alcohol.

3. The membrane electrode assembly according to claim 1 or claim 2, wherein the semicrystalline polymeric matrix has a degree of crystallinity of greater than 70%.

4. The membrane electrode assembly according to any one of claims 1 to 3, wherein the inorganic metal compound is a metal oxide.

5. The membrane electrode assembly according to claim 4, wherein the metal oxide comprises an oxide of a metal selected from the period 4, group 3-12 metal elements.

6. The membrane electrode assembly according to claim 4 or claim 5, wherein the metal oxide comprises vanadium(V) oxide (V2O5).

7. The membrane electrode assembly according to any one of claims 1 to 6, wherein the particles of an inorganic metal compound have a D50 particle size of less than 500 nm.

8. The membrane electrode assembly according to any one of claims 1 to 7, wherein the proton exchange membrane comprises the particles of an inorganic metal compound in an amount of less than 1 wt.%.

9. The membrane electrode assembly according to any one of claims 1 to 8, wherein the proton exchange membrane has an H2 permeability of less than 0.5 Barrer when measured at 35°C and 9 bar pressure differential using dry H2 gas feed.

10. The membrane electrode assembly according to any one of claims 1 to 9, wherein the proton exchange membrane has an H2 permeability of less than 0.07 Barrer when measured at 35°C and 9 bar pressure differential using dry H2 gas feed.

11. The membrane electrode assembly according to any one of claims 1 to 10, wherein the proton exchange membrane has a proton conductivity of at least 1.0x1 O'4S / cm when measured at 22°C with a four-point probe conductivity meter.

12. The membrane electrode assembly according to any one of claims 1 to 11, wherein the anode and the cathode comprise continuous layers adjacent to opposite surfaces of the proton exchange membrane.

13. The membrane electrode assembly according to any one of claims 1 to 12, wherein one or both of the electrocatalyst for dihydrogen oxidation and the electrocatalyst for proton reduction comprises a noble metal.

14. The membrane electrode assembly according to claim 13, wherein the noble metal is dispersed on a conductive support.

15. A method of producing a membrane electrode assembly, the method comprising:(i) dispersing particles of an inorganic metal compound in a membrane precursor liquid comprising a hydrophilic polymer or precursor thereof;(ii) producing a proton exchange membrane from the membrane precursor liquid, wherein the proton exchange membrane comprises a semicrystalline polymeric matrix comprising the hydrophilic polymer and particles of the inorganic metal compound dispersed in the semicrystalline polymeric matrix; and(iii) arranging the proton exchange membrane between an anode and a cathode, wherein the anode comprises an electrocatalyst for dihydrogen oxidation and the cathode comprises an electrocatalyst for proton reduction.

16. The method according to claim 15, wherein the membrane precursor liquid is a solution of the hydrophilic polymer, and wherein producing the proton exchange membrane comprises casting and drying the solution.

17. An electrochemical hydrogen compressor, comprising: a membrane electrode assembly according to any one of claims 1 to 14; anode and cathode compartments separated by the proton exchange membrane of the membrane electrode assembly; and a power supply to apply a potential between the anode and cathode of the membrane electrode assembly, wherein the anode compartment is configured to receive a gas comprising dihydrogen for oxidation of the dihydrogen on the anode to form protons, and wherein the cathode compartment is configured to retain compressed dihydrogen gas formed by reduction of the protons on the cathode.

18. The electrochemical hydrogen compressor according to claim 17, comprising a plurality of membrane electrode assemblies according to any one of claims 1 to 14 arranged in series.

19. A method of compressing dihydrogen, the method comprising: contacting a gas comprising dihydrogen with the anode of a membrane electrode assembly according to any one of claims 1 to 14; and applying a potential between the anode and cathode of the membrane electrode assembly, wherein the dihydrogen is oxidised on the anode to form protons and wherein the protons permeate through the proton exchange membrane and are reduced on the cathode to form compressed dihydrogen.

20. The method according to claim 19, wherein the dihydrogen is compressed with a single- stage compression ratio of at least 10 and / or wherein the efficiency of compression across the proton exchange membrane is greater than 30%.

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