An iridium-based electrocatalyst and method of forming the same

Abstract

There is provided an iridium oxide based electrocatalyst comprising manganese (Mn)- doped iridium oxide particles, wherein each of the Mn-doped iridium oxide particles comprises 1-25 wt % Mn based on the total mass of the manganese-doped iridium oxide particle, and wherein the electrocatalyst has a thermodynamic oxygen evolution reaction (OER) overpotential of ≤ 0.78 V. There is also provided an electrode assembly comprising the electrocatalyst, and a method of forming the electrocatalyst.

Description

[0001] An iridium-based electrocatalyst and method of forming the same

[0002] Technical Field

[0003] The present invention relates to an improved iridium-based electrocatalyst, and method of forming the electrocatalyst.

[0004] Background

[0005] Proton exchange membrane (PEM) water electrolysis is an important technological aspect for sustainable hydrogen production. However, conventional anodes used in PEM water electrolysis set ups usually face low oxygen evolution reaction (OER) which leads to substantial energy losses and limits overall efficiency of water splitting for hydrogen generation.

[0006] There is therefore a need for an improved electrocatalyst for PEM water electrolysis.

[0007] Summary of the invention

[0008] The present invention seeks to address these problems, and / or to provide an improved electrocatalyst, particularly for use in, but not limited to, water electrolysis such as acidic water electrolysis.

[0009] According to a first aspect, there is provided an iridium oxide based electrocatalyst, the electrocatalyst comprising manganese (Mn)-doped iridium oxide particles, wherein each of the Mn-doped iridium oxide particles comprises 1-25 wt % Mn based on the total mass of the manganese-doped iridium oxide particle, wherein the electrocatalyst has a thermodynamic oxygen evolution reaction (OER) overpotential of < 0.78 V.

[0010] According to a particular aspect, each of the Mn-doped iridium oxide particles comprises 1-10 wt % Mn based on the total mass of the manganese-doped iridium oxide particle

[0011] The electrocatalyst may have a suitable oxygen vacancy formation energy. For example, the oxygen vacancy formation energy of the electrocatalyst may be, but not limited to > 3 eV.

[0012] The electrocatalyst may have a suitable overpotential. According to a particular aspect, the electrocatalyst may have an overpotential lower than overpotential of iridium oxide. In particular, the electrocatalyst may have an overpotential of < 260 mV at 10 mA / cm2. The electrocatalyst may have a suitable structure. According to a particular aspect, the Mn-doped iridium oxide particles may have a nanostructure. In particular, the nanostructure may have an average size of 1-2.5 nm.

[0013] The Mn-doped iridium oxide particles may comprise a suitable crystalline structure. For example, the Mn-doped iridium oxide particles may comprise rutile-phase Mn-doped iridium oxide.

[0014] According to a particular aspect, the oxidation state of manganese in the Mn-doped iridium oxide particles may be between MnO (II) and MnC>2 (IV).

[0015] The electrocatalyst may be stable for a prolonged period of time. According to a particular aspect, the electrocatalyst may be stable for at least 400 hours under 100 mA / cm2.

[0016] The electrocatalyst may have a suitable charge transfer resistance. According to a particular aspect, the electrocatalyst may have a charge transfer resistance of 36-40 Q at 1.5 V.

[0017] The electrocatalyst may be for any suitable use. For example, the electrocatalyst may be comprised in a proton exchange membrane (PEM). The PEM may be for use in water electrolysis.

[0018] According to a second aspect, there is provided an electrode assembly comprising the electrocatalyst according to the first aspect. The electrode assembly may be any suitable electrode assembly. According to a particular aspect, the assembly may comprise a proton exchange membrane wherein a surface of the proton exchange membrane may be coated with a layer comprising the electrocatalyst.

[0019] The electrode assembly may operate at a driving voltage of 1.64-1.77 V at 2 A / cm2.

[0020] According to a third aspect, there is provided a method of forming an electrocatalyst according to the first aspect, the method comprising: mixing a manganese salt and an iridium salt in a solvent to form a mixture, wherein the mixing comprises mixing the manganese salt and the iridium salt in a molar ratio of 0.5: 1-3:1; evaporating the solvent from the mixture to obtain a dried mixture; mixing the dried mixture with a molten salt solution to form a molten mixture; heating the molten mixture for a pre-determined period of time; and freeze quenching the molten mixture to form the electrocatalyst.

[0021] According to a particular aspect, the pre-determined period of time may be 3- 6 hours. The heating may comprise annealing.

[0022] The method may further comprise grinding the dried mixture into a powder prior to the mixing the dried mixture.

[0023] Brief Description of the Drawings

[0024] In order that the invention may be fully understood and readily put into practical effect there shall now be described by way of non-limitative example only exemplary embodiments, the description being with reference to the accompanying illustrative drawings. In the drawings:

[0025] Figure 1 shows the electrochemical active surface area (ECSA) normalised polarisation curve of one embodiment of the invention (Mno.isIro.ssCh-^) as compared to quench-lrC>2 and commercial IrC ; and

[0026] Figure 2 shows the chronopotentiometry durability test of one embodiment of the invention (Mno.isIro.ssO^c) at different currents;

[0027] Figure 3 shows the schematic representation of an electrode assembly according to one embodiment of the invention;

[0028] Figure 4 shows the current-voltage polarization of an electrode assembly according to one embodiment at different operating temperatures:

[0029] Figure 5 shows the current-voltage polarization of an electrode assembly according to one embodiment with different catalyst-coated membranes; and

[0030] Figure 6 shows the chronopotentiometry curves of an electrode assembly according to one embodiment of the invention at current density of 2.0 A / cm2.

[0031] Detailed Description

[0032] As explained above, there is a need for an improved electrocatalyst, particularly for protein exchange membrane (PEM) electrolysis.

[0033] PEM water electrolysis is significantly hindered by the sluggish oxygen evolution reaction (OER) kinetics at the anode. Iridium (Ir) and its derived oxides have been recognized as the only viable catalysts to meet stringent stability requirements for PEM electrolyser, while their intrinsic activity still fail to meet the high expectation originated from the large-scale commercialization. The conventional OER mechanism of lrC>2 involves four proton-coupled electron transfer (PCET) steps, where protons are desorbed from oxygen intermediates and directly released into the electrolyte. Most methods of improving OER is primarily focused on accelerating electron transfer to optimize the absorption energy of oxidation intermediates.

[0034] An alternative method of improving OER is by improving proton transfer process. Proton transfer is influenced by surface charge accumulation, which can be introduced by applied bias in the pre-OER voltage region. The bias-induced surface charge accumulation affects the bond cleavage and formation, with activation energy linearly decreasing with the storage amount of oxidation charges.

[0035] In general terms, the invention relates to an improved electrocatalyst for improving intrinsic catalytic activity of the electrocatalyst. In particular, the electrocatalyst may enhance catalytic performance and durability while decreasing iridium usage to reduce costs. The electrocatalyst may exhibit an oxygen overpotential of only 53% compared to commercialized lrC>2 at a current density of 20 mA cm'2. Upon adjustment for the electrochemically active surface area, the intrinsic activity's overpotential may amount to a reduction of almost 100mV.

[0036] In the electrocatalyst of the present invention, Mn with pseudocapacitive behaviour incorporated into the lrC>2 lattice can effectively regulate the adsorption behaviour and local electronic structure of Ir-O-Mn. Under bias, the dynamic redox induces a noticeable pseudo capacitance, triggering localized hole accumulation, and promoting subsequent electron transfer kinetics on Ir sites, thereby improving catalytic activity and efficiency. The significantly increased formation energy of oxygen vacancies suppresses lattice oxygen migration during the OER process, effectively improving the material stability.

[0037] In particular, use of the electrocatalyst of the present invention for applications, such as but not limited to water electrolysis, provides a viable alternative that may reduce the carbon footprint associated with hydrogen production.

[0038] According to a first aspect, there is provided an iridium oxide based electrocatalyst, the electrocatalyst comprising manganese (Mn)-doped iridium oxide particles, wherein each of the Mn-doped iridium oxide particles comprises 1-25 wt % Mn based on the total mass of the manganese-doped iridium oxide particle, wherein the electrocatalyst has a thermodynamic oxygen evolution reaction (OER) overpotential of < 0.78 V.

[0039] The Mn may be distributed within the iridium oxide matrix. In particular, the Mn may be uniformly distributed within the iridium oxide matrix.

[0040] In particular, each of the Mn-doped iridium oxide particles comprises 1-25 wt % Mn based on the total mass of the manganese-doped iridium oxide particle. For example, each of the Mn-doped iridium oxide particles comprises 1-25 wt %, 5-20 wt %, 7-18 wt %, 10-15 wt %, 12-14 wt %, 12.5-13 wt% Mn based on the total mass of the manganese-doped iridium oxide particle. Even more in particular, each of the Mn- doped iridium oxide particles comprises 1-10 wt %, preferably about 4 wt % Mn based on the total mass of the manganese-doped iridium oxide particle.

[0041] According to a particular aspect, the electrocatalyst may have a suitable chemical formula. In particular, the electrocatalyst may have a chemical formula of Mni.xlrxO2-n, wherein x may be 0.70-0.95 and o may be < 0.1. For example, x may be 0.75-0.90, 0.80-0.88, 0.82-0.87, 0.85-0.86. In particular, the electrocatalyst may have a chemical formula Mno.isIro.ssO^.

[0042] The thermodynamic OER overpotential may be < 0.78 V. In particular, the thermodynamic OER overpotential may be 0.48-0.78 V, 0.50-0.75 V, 0.55-0.70 V, 0.60- 0.65 V. Even more in particular, the thermodynamic OER overpotential may be about 0.48 V. The substitution of Mn for Ir reduces the thermodynamic OER overpotential and therefore, the thermodynamic OER overpotential of the electrocatalyst is less than the thermodynamic OER overpotential of iridium oxide without the presence of Mn. Accordingly, the Mn substitution optimises the OER process on Ir active sites.

[0043] The electrocatalyst leverages capacitive non-noble metal modifiers such as Mn to regulate the local electronic structure and adsorption behaviour of lrO2. Under applied bias, these modifications dynamically trigger redox reactions at active sites, inducing localized charge accumulation on the electrocatalyst surface. This mechanism enhances the OER kinetics at iridium sites, resulting in superior catalytic activity. In this way, the overall electrolyser overpotential is reduced which in turn improves energy conversion efficiency.

[0044] The electrocatalyst may have a suitable oxygen vacancy formation energy. For example, the oxygen vacancy formation energy of the electrocatalyst may be, but not limited to > 3 eV. In particular, the oxygen vacancy formation energy of the electrocatalyst may be 3.0-4.0 eV, 3.2-3.8 eV, 3.25-3.75 eV, 3.35-3.65 eV, 3.56-3.60 eV. Even more in particular, the oxygen vacancy formation energy of the electrocatalyst may be about 3.56 eV, which may be greater than the oxygen vacancy formation energy of iridium oxide without the presence of Mn.

[0045] The electrocatalyst may have a suitable overpotential. According to a particular aspect, the electrocatalyst may have an overpotential lower than overpotential of iridium oxide. In particular, the electrocatalyst may have an overpotential of < 260 mV at current density of 10 mA / cm2. For example, the overpotential of iridium oxide without the presence of Mn is about 400 mV at current density of 10 mA / cm2. Thus, the incorporation of Mn into the electrocatalyst significantly enhances the OER activity of iridium oxide. The electrocatalyst may have an overpotential of < 260 mV, particularly about 258 mV at current density of 10 mA / cm2. The overpotential difference between the electrocatalyst with and without Mn may increase with higher current density.

[0046] According to a particular aspect, the electrochemical double-layer capacitance (Cdi) may be used for activity normalization. In particular, the electrocatalyst may have a higher Cdi and electrochemical active surface area (ECSA) compared to iridium oxide, as can be seen in Figure 1. This may show that the electrocatalyst has an increase in the density of active sites compared to the iridium oxide electrocatalyst without Mn.

[0047] The electrocatalyst may have a suitable structure. According to a particular aspect, the Mn-doped iridium oxide particles may have a nanostructure. The nanostructures may comprise a structure or an arrangement having at least one dimension on the nanoscale. The nanostructures may have one dimension in the nanoscale, two dimensions in the nanoscale, or three dimensions in the nanoscale. For example, nanoscale may refer to a range of < 1000 nm. According to a particular aspect, the nanostructure may have a suitable size. In particular, the nanostructure may have an average size of 1-2.5 nm. For example, the nanostructure may have an average size of 1.2-2.2 nm, 1.4-2.0 nm, 1.5-1.8 nm, 1.6-1.7 nm. Even more in particular, the nanostructure may have an average size of 1-2 nm. The unique sub-nanocluster structure maximizes the exposure of active sites, significantly improving the efficiency of the catalytic process.

[0048] The Mn-doped iridium oxide particles may comprise a suitable crystalline structure. For example, the Mn-doped iridium oxide particles may comprise rutile-phase Mn-doped iridium oxide. In particular, The Mn doping does not alter the crystalline structure of the iridium oxide.

[0049] According to a particular aspect, the oxidation state of manganese in the Mn-doped iridium oxide particles may be between MnO (II) and MnC>2 (IV).

[0050] The electrocatalyst may be stable for a prolonged period of time. According to a particular aspect, the electrocatalyst may be stable for at least 400 hours under 100 mA / cm2, as can be seen in Figure 2. In particular, the electrocatalyst may be stable for at least 100-900 hours, 150-875 hours, 200-850 hours, 250-800 hours, 300-750 hours, 350-700 hours, 400-650 hours, 450-600 hours, 500-550 hours.

[0051] The electrocatalyst may have a suitable charge transfer resistance. According to a particular aspect, the electrocatalyst may have a charge transfer resistance of 36-40 Q at 1.5 V. In particular, the charge transfer resistance may vary depending on the working voltage. This is very much lower than the charge transfer resistance of iridium oxide. The lowered resistance advantageously leads to decrease in energy loss and increased power output.

[0052] The electrocatalyst may be for any suitable use. For example, the electrocatalyst may be comprised in a proton exchange membrane (PEM). The PEM may be for use in water electrolysis.

[0053] According to a particular aspect, the electrocatalyst may be coated on a surface of the PEM. In particular, the electrocatalyst may be coated on a surface of an anode layer of the PEM.

[0054] According to a second aspect, there is provided an electrode assembly comprising the electrocatalyst according to the first aspect. The electrode assembly may be any suitable electrode assembly. According to a particular aspect, the assembly may comprise a proton exchange membrane wherein a surface of the proton exchange membrane may be coated with a layer comprising the electrocatalyst. The membrane may be any suitable proton exchange membrane.

[0055] In particular, the electrode assembly may comprise an anode layer and a cathode layer, and a membrane between the anode layer and the cathode layer. The anode layer may comprise the electrocatalyst according to the first aspect. The cathode layer may comprise a coating with a suitable catalyst. For example, the cathode layer may comprise a coating comprising platinum on carbon catalyst.

[0056] The cathode layer may have any suitable catalyst loading. For example, the cathode layer may have a catalyst loading of 0.2-0.4 mg cm-2. The anode layer may have any suitable electrocatalyst loading. For example, the anode layer may have an electrocatalyst loading of 0.8-1.0 mg cm'2.

[0057] The cathode layer may be formed by dispersing a cathode catalyst in a solution to form a cathode dispersion, and coating the cathode dispersion on a polymer substrate. The cathode catalyst may be any suitable catalyst, such as, but is not limited to, a platinum on carbon catalyst. The solution may comprise any suitable solvent. For example, the solution may comprise an alcohol and an ionomer. In particular, the solution may comprise ethanol, isopropanol, sulfonated tetrafluoroethylene-based fluoropolymercopolymer, n-propanol, deionized water, or any combination thereof. According to a particular aspect, the cathode dispersion may comprise an ionomer-to- catalyst mass ratio of 10-25%. The coating may be blade coating, dip coating, spin coating, spray coating, or any combination thereof. The polymer substrate may be, but is not limited to, a fluoropolymer substrate. For example, the polymer substrate may be a polytetrafluoroethylene (PTFE) substrate.

[0058] The anode layer may be formed by dispersing the electrocatalyst of the first aspect, in a solution to form an anode dispersion, and coating the anode dispersion on a polymer substrate. The solution may be as described above. According to a particular aspect, the anode dispersion may comprise an ionomer-to-electrocatalyst mass ratio of 10- 25%. The coating may be as described above. The polymer substrate may be as described above.

[0059] Each of the anode and cathode may further comprise a porous transport layer. The transport layer may comprise platinum coated with porous titanium. The porosity of the titanium may be about 50% and the thickness may be 1 mm. The transport layer may further comprise carbon fiber paper with a suitable thickness. For example, the thickness may be 370 ± 10 pm.

[0060] The electrode assembly may operate at a driving voltage of 1.5-1.8 V at 2 A / cm2. In particular, the driving voltage may be 1.55-1.75 V, 1.60-1.70V at 2 A / cm2. Even more in particular, the driving voltage may be 1.64-1.77 V at 2 A / cm2. In particular, the electrode assembly may operate at continuous operation for 800 hours at a very low driving voltage.

[0061] The introduction of capacitive non-noble metal modifiers increases the oxygen vacancy formation energy, thereby suppressing lattice oxygen activation and enhancing catalyst stability. The incorporation of Mn with pseudocapacitive behaviour into the lrO2lattice effectively regulates the adsorption behaviour and local electronic structure of Ir-O-Mn sites. In particular, the electrocatalyst described above refines the electronic configuration of the Ir-O-Mn active sites, promoting the deprotonation of oxygen intermediaries and thereby boosting the native catalytic efficiency of iridium dioxide. The inclusion of manganese serves to balance charge compensation, warding off severe autolysis, the dissolution of iridium, and the undue loss of lattice oxygen, which in turn bolsters the material's durability. Even more in particular, under applied bias, dynamic redox reactions induce notable pseudocapacitive effects, triggering localized hole accumulation and enhancing electron transfer kinetics at Ir active sites, thereby significantly improving catalytic activity. Simultaneously, the substantial increase in oxygen vacancy formation energy suppresses lattice oxygen migration during the OER process, effectively enhancing material stability.

[0062] According to a third aspect, there is provided a method of forming an electrocatalyst according to the first aspect, the method comprising: mixing a manganese salt and an iridium salt in a solvent to form a mixture, wherein the mixing comprises mixing the manganese salt and the iridium salt in a molar ratio of 0.5: 1-3:1; evaporating the solvent from the mixture to obtain a dried mixture; mixing the dried mixture with a molten salt solution to form a molten mixture; heating the molten mixture for a pre-determined period of time; and freeze quenching the molten mixture to form the electrocatalyst.

[0063] The manganese salt may be any suitable salt. For example, the manganese salt may comprise, but is not limited to a halide, an acetate, and / or a sulfate. In particular, the manganese salt may comprise a chloride.

[0064] The iridium salt may be any suitable salt. For example, the iridium salt may comprise, but is not limited to a halide and / or a chloroiridic acid. In particular, the iridium salt may comprise a chloride. According to a particular aspect, the solvent may be any suitable solvent. For example, the solvent may be an organic solvent. In particular, the solvent may comprise, but is not limited to, ethanol. Even more in particular, the solvent may comprise ethanol and optionally water. The water may be deionized water. According to a particular aspect, the solvent may comprise ethanol and deionized water in a volume ratio of 1 :1.

[0065] The mixing may be followed by an ultrasound treatment. The ultrasound treatment may be for a pre-determined period of time. For example, the pre-determined period of time may be 1 hour. The ultrasound treatment may result in the formation of a homogeneous suspension.

[0066] The evaporating may comprise subjecting the mixture to elevated temperature to form a dried mixture. For example, the evaporating may comprise heating the mixture to a suitable temperature. In particular, the temperature may be 60-100°C. For example, the temperature may be 75-90°C, 80-85°C. Even more in particular, the temperature may be about 60°C. The evaporating may be performed in any suitable manner. For example, the evaporating may be in an oven. The dried mixture may be formed into a powder. For example, the dried mixture may be ground into a powder.

[0067] The molten salt solution for the mixing the dried mixture may be any suitable molten salt solution. For example, the molten salt solution may comprise, but is not limited to, sodium nitrate. The molten salt solution may be formed by any suitable method. For example, the molten salt solution may be formed by calcining the molten salt. Even more in particular, the molten salt solution may be formed by calcining 4-8 g of salt at a temperature of 380-480°C. In particular, the calcining temperature may be 390-450°C, 400-440°C, 410-430°C, 415-420°C.

[0068] The heating may comprise any suitable type of heating. For example, the heating may comprise, but not limited to, annealing the molten mixture.

[0069] The pre-determined period of time for the heating may be any suitable period of time. For example, the pre-determined period of time may be 3-6 hours. In particular, the pre-determined period of time may be 3.5-5.5 hours, 4-5 hours.

[0070] The freeze quenching may be by any suitable means. For example, the free quenching may be, but not limited to, by directly pouring the molten mixture into ice water for rapid cooling. The method may further comprise washing the quenched product. The washing may comprise washing the quenched product with deionized water, ethanol, or a mixture thereof. The washed product may be centrifuged to collect the solid. The solid may be mixed in a further solvent to form a mixture, such as but not limited to perchloric acid solution, and optionally washed again. The washed product after quenching may be dried, for example dried overnight, and / or in a vacuum oven at 80 °C.

[0071] Having now generally described the invention, the same will be more readily understood through reference to the following embodiment which is provided by way of illustration, and is not intended to be limiting.

[0072] Examples

[0073] Materials and methods

[0074] Synthesis of Mn-doped iridium oxide electrocatalyst and control samples

[0075] For synthesizing Mni.xlrxO2-a, manganese chloride (0.25 mmol) and iridium chloride (0.25 mmol) were first dissolved in 10 mL of a mixed solution of deionized water and ethanol (volume ratio 1 :1). After vigorous ultrasound for 1 h, the homogenous suspension was evaporated in an oven at 60°C. The dried product was ground into fine powder for further use.

[0076] 8 g of sodium nitrate was calcined to 400°C to form a clear molten salt solution. The strongly alkaline Lux-Flood base, NaNOs, acts as both solvent and reaction medium, aiding in the precipitation of metal cations into oxides. Additionally, it accelerates nucleation kinetics, promoting the concurrent formation of solid solution nanoparticles. The previously prepared powder was uniformly and rapidly sprinkled into the molten salt solution, and the mixture was further annealed for 5 hours. Before cooling, the crucible was directly removed from the muffle furnace, and the molten salt solution was poured directly into ice water for quenching. The product was first washed with deionized water and ethanol and then collected by centrifugation. The collected sample was placed in 0.1 M HCIO4 solution and stirred at 80°C for 30 min, followed by a second wash. The resulting sample obtained after centrifugation was dried in a vacuum oven overnight at 80°C.

[0077] The quenched lrC>2 was prepared using the same process as Mni-xlrxC>2-a, except that only the sole iridium source was used during the annealing treatment. Samples with different Mn contents were obtained by adjusting the initial amount of Mn salt added, while other processing methods remained consistent with the standard sample.

[0078] Characterisation and results

[0079] The x-ray diffraction (XRD) patterns (Rigaku Ultima IV, JPN) confirm that both Mm. xlrxO2-a and lrC>2 exhibit the same rutile phase, indicating that the Mn decoration does not alter the crystalline phase of I rC>2-

[0080] Both transmission electron microscopy (TEM) and high resolution aberration-corrected scanning transmission electron microscopy (HAADF-STEM) images reveal the clear lattice fringes attributing to the {101} facet of I rC>2 with an average size of 2 nm.

[0081] Multiple etching X-ray photoelectron spectroscopy (XPS) was performed to investigate the bonding situation of Ir and Mn more accurately. The survey spectra revealed the ratio of Mn to Ir initially decreased and then increased with increasing plasma etching depth, but the overall change was not substantial, demonstrating the highly homogeneous steric distribution of Mn over the I rC>2.

[0082] For high-resolution Mn 2p XPS spectra, two sets of peaks located between 640 and 655 eV were attributed to the peaks of Mn3+(641.5 and 652.9 eV) and Mn4+(643.6 and 654.7 eV). The X-ray absorption near-edge structure (XANES) and corresponding extended X-ray absorption fine-structure (EXAFS) spectra of Mni-xlrxO2-a and lrC>2 at the Ir L3 edge were investigated. The amplified XANES spectra reveal that the Ir L3- edge absorption energy and white line peak of Mn1-xlrxO2-o were slightly higher than those of lrC>2, suggesting a relatively higher oxidation state of Ir in Mni.xlrxC>2-a, consistent with the XPS analysis results. The nearly identical Ir L3-edge EXAFS spectra of Mni.xlrxC>2-a and lrC>2 indicated that they possess the same coordination structure before and after Mn incorporation. The prominent peak of the lr-0 bond in Mni-xlrxC>2-a was located at approximately 1.56 A, which was smaller than that of lrC>2 (1.61 A). The XANES spectrum at the Mn K-edge also suggested that the oxidation state of Mn in Mni.xlrxC>2-a fell between MnO (II) and MnC>2 (IV). The corresponding EXAFS spectrum revealed a prominent peak around 1.36 A, indicative of Mn-0 coordination. The EXAFS spectrum of Mni.xlrxC>2-a exhibited a peak shape similar to that of MnC>2, and no distinct formation of Mn metal bonds, indicating the substitution of Mn for Ir within the I rC>2 lattice, thereby inducing an increase in the oxidation state of Ir. Additionally, the introduction of Mn-0 bonds with shorter bond lengths caused lattice contraction in the material, which was further investigated by high-resolution synchrotron X-ray diffraction (HE-XRD). The complementary HE-XRD experiments aimed to investigate distinct atomic pair distribution functions (PDFs) in as-synthesized Mni.xlrxO2-a nanoparticles. The PDFs of Mni.xlrxO2-a and I rC>2 fit well with models based on the tetragonal (P42 / mnm) structure. The first peak in the PDF for Mni.xlrxC>2-a was positioned at 1.97 A, which was shorter than in rutile-type lrC>2 (approximately 1.99 A). The slight contraction in the structure of Mni.xlrxC>2-a may be attributed to the smaller radius of Mn substituting Ir within the lattice.

[0083] Electrochemical oxygen evolution performance and mechanism

[0084] Following OER standard measurements, the OER activities of the Mni-xlrxC>2-a catalyst and control samples were evaluated in 0.1 M HCIO4 electrolyte. The control samples included quench-lrC>2 and RuOxprepared via the same method, Mni-xlrxC>2-a nanoparticles with varying Mn content, Mni-xlrxC>2-a nanoparticles synthesized by natural cooling method and commercial-lrO2 nanoparticles (com-lrO2). The linear sweep voltammetry (LSV) curves showed that quench-lrC>2 exhibited better OER activity than com-lrO2. The overpotential of com-lrO2 for achieving current density of 10 mA cm'2was approximately 400 mV. The introduction of Mn significantly enhanced the OER activity of lrO2 - Mni-xlrxO2-a achieved an overpotential of 258 mV to reach 10 mA cm'2, lower than the overpotential of lrO2 by 142 mV. This overpotential difference increased rapidly with higher current densities, owing to the significantly improved Tafel slope of Mni-xlrxO2-a (78.1mV dec1) compared to both quench-lrO2 (102.5 mV dec1) and com-lrO2 (153.1 mV dec1).

[0085] Electrochemical double-layer capacitance was calculated for activity normalization. Mni-xlrxC>2-a and quench-lrC>2 exhibited significantly higher Cdi and electrochemical active surface area (ECSA) compared to com-lrC>2. Particularly, Mni-xlrxC>2-a exhibited the highest Cdi, nearly five times that of com-lrC>2, suggesting a significant increase in the density of active sites. Considering ECSA-normalized OER activity, the specific activity of Mni-xlrxO2-a still surpassed that of com-lrO2 (Figure 1). While maintaining the solid solution state, the content of Mn in Mni-xlrxO2-a was further optimized. Upon comparing the electrochemical performance of various samples, Mno.15lro.85Ox demonstrated superior OER activity, characterized by the lowest overpotential and the smallest Tafel slope. Mno.15lro.s5Ox did not exhibit the largest electrochemical active surface area. The normalized LSV curves demonstrated that Mno.15lro.s5Ox exhibited intrinsic activity superior to samples with different Mn contents and quench-lrO2. It was evident that the incorporation of Mn fundamentally promoted the intrinsic OER activity. Furthermore, the quenched Mno.isIro.ssO^c demonstrated superior OER activity compared to the naturally cooled Mno.isIro.ssO^c and RuOx. Since both samples had similar electrochemical surface areas, the quenched Mno.isIro.ssO^c possessed higher intrinsic catalytic activity.

[0086] While achieving high OER activity was essential, ensuring good stability was even more important for practical applications. Based on the investigation into the intrinsic OER activity and initial stability of Mni.xlrxO2-a and control samples, lrO2 failed to maintain low degradation rates during stability tests. Mn incorporation decelerated above degradation rates. The optimized Mni-xlrxO2-a demonstrated continuous stepped stability at various current densities, maintaining stable operation for 400 hours under conditions of 100 mA cm'2(Figure 2).

[0087] Following the chronopotentiometry durability test of Mni-xlrxC>2-a, XPS spectra showed that the Mn / lr ratio remained at 12.2:87.8, with an increase in the proportion of Mn(IV). Despite a slight aggregation of nanoparticles observed after the reaction, STEM mapping confirmed the uniform distribution of Mn. Inductively coupled plasma mass spectrometry (ICP-MS) analysis of the electrolyte after durability testing revealed the limited metal leaching (approximately 20 ppb of Mn and 2 ppb of Ir), verifying the structure stability of catalyst under operating conditions.

[0088] Potential-dependent in-situ XAFS experiments were conducted to gain deeper insights into the alterations in the electronic structure of Mni-xlrxC>2-a. The XANES spectra of the Ir Ls-edge showed highly similar patterns at different potentials. Ir oxidation state remained almost unchanged in the pre-OER potential region (open circuit potential, ocp~1.23V). As the applied potential increased, the peak position shifted to higher energy, indicating a slight increase in the oxidation state during the OER process. Fourier transform of the Ir Ls-edge EXAFS spectra revealed a primary coordination peak corresponding to lr-0 coordination, fitted at 1.61 A. At different potentials, there were no significant changes observed in the position of the lr-0 coordination peak. Quantitative EXAFS fitting had also been performed to obtain information about the coordination numbers. The coordination number of lr-0 at ocp was 6.4, and it remained stable in both the pre-OER and OER potential region.

[0089] The Mn K-edge shifted to higher energy with increasing applied bias, accompanied by a progressively increasing trend in its oxidation state. Interestingly, Mn-0 bond length oscillation was observed. This phenomenon was attributed to structural relaxation induced by Mn pre-oxidation upon applied bias, followed by structural contraction resulting from Ir participating in the oxidation reaction, and subsequently, the oxidation of the metal upon OER potential region. The findings above demonstrate that Ir remains largely unchanged, whereas Mn undergoes pre-oxidation. Meanwhile, upon the onset of applied voltage, its coordination number rapidly increases, influenced by its oxidation state changes.

[0090] Kinetic investigations had been conducted to elucidate the correlation between the OER catalytic behavior, and the structural information revealed by in-situ EXAFS analysis. To understand the redox characteristics and the POET dynamics in lrO2 and Mni-xlrxO2-a, a series of cyclic voltammetry (CV) and differential pulse voltammetry (DPV) experiments were performed in buffer-free HCIO4 solutions across a pH range from 0.4 to 1.6. CV measurements were utilized to assess the capacitive properties within the pre-OER potential region, whereas DPV measurements were employed to identify the oxidation contribution from the overall capacitance. The CV profiles for lrC>2 remained consistent across all pH conditions, lacking any discernible redox peaks in both CV and DPV. In contrast, Mno.isIro.ssO^a exhibited pronounced double-layer capacitance in CV analyses alongside obvious oxidation peaks within the pre-OER region as identified by DPV, which is attributed to the oxidation of surface Mn species. Remarkably, although Mno.i5lro.8502-a demonstrated superior intrinsic catalytic activity, the derived pH-dependence slope at the onset of OER current density was similar to that of lrO2, thus having a comparable PCET kinetics. Therefore, the promotion in intrinsic activity was mainly attributed to the pseudocapacitive effects of Mn species rather than the shift in the OER reaction pathway. Conventionally, oxidation at active sites within the pre-OER window was considered to be related to reactant adsorption. However, given the unlikely direct occurrence of reactions at Mn sites, the pseudocapacitive behavior of Mn species promoted surface charge accumulation (in the form of holes), thereby potentially facilitating adsorption and deprotonation processes for subsequent OER steps at adjacent Ir sites.

[0091] Further operando EIS was undertaken to reveal the electrode kinetics and the interfacial dynamics between the electrode and electrolyte. The potential range was set from 1.00 to 1.60 V versus the RHE, aligning with the CV and DPV parameters. Bode plots of lrO2 indicated there was no significant OER activity until the applied potential exceeded 1.50 V, as evidenced by a decrease in phase angle within the low-frequency region. A parallel trend was observed for Mno.i5lro.8502-a, along with a reduced onset potential for OER to 1.45 V. Notably, Mno.isIro.ssO^a demonstrated a distinct frequency response relative to lrC>2, indicating differences in the time constants associated with electrochemical reactions. By applying the distribution of relaxation times (DRT) analysis to the EIS spectra, particularly between 1.45 to 1.60 V, different electrode processes based on a variation on time-constant T was deconvoluted. Generally speaking, from high to low frequency (negatively correlated to T), one emerging peak can be ascribed to either electrode contact, charge transfer, mass transport, and gas evolution, or other complicated electrochemical processes. Without specifically interpretation, it was concluded that the peaks associated with Mno.isIro.ssO^a consistently appeared at higher frequencies with smaller equivalent resistance compared to the corresponding peaks of lrC>2, indicating relatively accelerated kinetics. Thus, the peaks belonging to Mno.isIro.ssO^a were consistently present at higher frequencies compared to corresponding peaks of lrC>2, indicating relatively accelerated kinetics. Moreover, the Nyquist plots of Mno.isIro.ssC -a exhibited a significantly smaller semi-circle, demonstrating a favorable charge transfer kinetics. In detail, the charge transfer resistance for Mno.isIro.ssC -a stood at a mere 36.7 Q, even significantly lower than the 133.6 Q for lrC>2 at 1.55 V, confirming its enhanced electrochemical efficiency for acidic OER electrocatalysis.

[0092] Proton exchange membrane cell performance

[0093] The ultimate test of OER catalysts is their performance in operating PEMWE. In contrast to the traditional three-electrode system, fundamental properties such as porosity, stability, and conductivity become more crucial for the catalyst's performance under the working conditions.

[0094] A PEMWE electrolyser cell with COM Mno.i5lro.8502-a serving as the anode for OER, commercial Pt / C as the cathode catalyst for HER, and a Nation 212 membrane was assembled (Figure 3). As the Ir mass loading increased, the performance of the PEMWE cell using COM Mno.isIro.ssO^a gradually improved, reaching its optimum with the Ir mass loading of 0.8 mgircm-2. The current-voltage characteristic curve of the electrolyser cell using Mni-xlrxO2-a (0.8 mgircm-2) (without iR compensation) demonstrated peak activity at an operating temperature of 80°C, with a driving voltage of only 1.637 V at 2 A cm-2(Figure 4). It displayed an almost linear slope and demonstrated unique mass efficiency characteristics at high current densities. The constructed PEMWE exhibited a small impedance of less than 0.1 Q cm-2, with a decrease in iR contribution of only -100 mV at 2 A cm-2. Meanwhile, with the same Ir loading on the membrane electrodes, the driving voltage of CCM Mno.isIro.ssO^a (1.637 V) was lower than that of CCM quench-lrC>2 (1.775 V) and CCM com-lrC>2 (1 .867 V) at 2 A cm-2(Figure 5). Within the broad cell voltage range of 1.5 to 1.8 V, CCM Mno.i5lr0.8502-a exhibited the best mass activity. Importantly, the PEMWE based on the CCM Mn0.i5lro.8502-a demonstrated superior stability compared to com-lrC>2, operating at a high current density of 2.0 A cm'2for 800 hours with a low cell voltage growth rate of 19.4 V h'1(Figure 6). After the durability test of the PEMWE, XPS analysis showed that the Ir / Mn ratio remained relatively stable compared to the fresh sample. The slight increase in Mn(IV) proportion also aligned with the mechanism of capacitance- promoted activity. While the catalytic material exhibited slight agglomeration, the basic morphology of nanoparticles and crystallinity of the Mno.isIro.ssCh-a remain preserved, confirming the excellent stability of the material and the assembled membrane electrode. Whilst the foregoing description has described exemplary embodiments, it will be understood by those skilled in the technology concerned that many variations may be made without departing from the present invention.

Claims

Claims1. An iridium oxide based electrocatalyst, the electrocatalyst comprising manganese (Mn)-doped iridium oxide particles, wherein each of the Mn-doped iridium oxide particles comprises 1-25 wt % Mn based on the total mass of the manganese- doped iridium oxide particle, wherein the electrocatalyst has a thermodynamic oxygen evolution reaction (OER) overpotential of < 0.78 V.

2. The electrocatalyst according to claim 1 , wherein each of the Mn-doped iridium oxide particles comprises 1-10 wt % Mn based on the total mass of the manganese- doped iridium oxide particle.

3. The electrocatalyst according to claim 1, wherein the oxygen vacancy formation energy of the electrocatalyst is > 3 eV.

4. The electrocatalyst according to claim 1 , wherein the electrocatalyst has an overpotential lower than overpotential of iridium oxide.

5. The electrocatalyst according to claim 4, wherein the electrocatalyst has an overpotential of < 260 mV at 10 mA / cm2.

6. The electrocatalyst according to claim 1 , wherein the Mn-doped iridium oxide particles have a nanostructure.

7. The electrocatalyst according to claim 6, wherein the nanostructure has an average size of 1-2.5 nm.

8. The electrocatalyst according to claim 1 , wherein the Mn-doped iridium oxide particles comprise rutile-phase Mn-doped iridium oxide.

9. The electrocatalyst according to claim 1, wherein oxidation state of manganese in the Mn-doped iridium oxide particles is between MnO (II) and MnC>2 (IV).

10. The electrocatalyst according to claim 1 , wherein the electrocatalyst is stable for at least 400 hours under 100 mA / cm2.

11. The electrocatalyst according to claim 1, wherein the electrocatalyst has a charge transfer resistance of 36-40 Q at 1.5 V.

12. The electrocatalyst according to claim 1, wherein the electrocatalyst is comprised in a proton exchange membrane.

13. An electrode assembly comprising the electrocatalyst according to claim 1.

14. The electrode assembly according to claim 13, wherein the assembly comprises a proton exchange membrane, and wherein a surface of the proton exchange membrane is coated with a layer comprising the electrocatalyst.

15. The electrode assembly according to claim 13, wherein the electrode assembly operates at a driving voltage of 1.64-1.77 V at 2 A / cm2.

16. A method of forming an electrocatalyst according to claim 1 , the method comprising:- mixing a manganese salt and an iridium salt in a solvent to form a mixture, wherein the mixing comprises mixing the manganese salt and the iridium salt in a molar ratio of 0.5:1-3:1 ;- evaporating the solvent from the mixture to obtain a dried mixture; mixing the dried mixture with a molten salt solution to form a molten mixture heating the molten mixture for a pre-determined period of time; and freeze quenching the molten mixture to form the electrocatalyst.

17. The method according to claim 16, wherein the method further comprises grinding the dried mixture into a powder prior to the mixing the dried mixture.

18. The method according to claim 16, wherein the pre-determined period of time is 3-6 hours.

19. The method according to claim 16, wherein the heating comprises annealing the molten mixture.

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