Oxygen evolution reaction catalyst

Abstract

The present invention provides an oxygen generation reaction catalyst, the oxygen generation reaction catalyst being an oxide material including iridium and tantalum, the oxygen generation reaction catalyst comprising a crystalline oxide phase having a rutile crystal structure, the crystalline oxide phase having a lattice constant a greater than 4.510 Å, the oxygen generation reaction catalyst having a lattice constant a of at least 50 m 2 The present invention provides an oxygen evolution reaction catalyst having a BET surface area of ​​1.0 to 1.5 nm / g.

Description

[Technical field]

[0001] The present invention relates to oxygen evolution reaction catalysts, and particularly, but not exclusively, to high surface area catalytic materials suitable for use in electrochemical fuel cells. [Background technology]

[0002] A fuel cell is an electrochemical cell that includes two electrodes separated by an electrolyte. A fuel, e.g., hydrogen, alcohols such as methanol or ethanol, or formic acid, is supplied to the anode, and an oxidant, e.g., oxygen or air, is supplied to the cathode. Electrochemical reactions occur at the electrodes, converting the chemical energy of the fuel and oxidant into electrical energy and heat. Electrocatalysts are used to facilitate the electrochemical oxidation of the fuel at the anode and the electrochemical reduction of oxygen at the cathode.

[0003] Fuel cells are usually classified according to the nature of the electrolyte used. In most cases, the electrolyte is a solid polymer membrane that is electronically insulating but ionically conducting. In proton exchange membrane fuel cells, the membrane is proton conducting, and protons generated at the anode are transported through the membrane to the cathode where they combine with oxygen to form water.

[0004] The main component of a proton exchange membrane fuel cell is the membrane electrode assembly, which is essentially composed of five layers. The central layer is a polymeric ion-conducting membrane. On either side of the ion-conducting membrane are electrocatalyst layers that contain electrocatalysts designed for a particular electrochemical reaction. Finally, adjacent to each electrocatalyst layer is a gas diffusion layer. The gas diffusion layers must allow reactants to reach the electrocatalyst layers and must conduct the electrical current produced by the electrochemical reaction. Thus, the gas diffusion layers must be porous and electrically conductive. The membrane electrode assembly can be constructed in several ways. An electrocatalyst layer may be applied to the gas diffusion layer to form a gas diffusion electrode. Two gas diffusion electrodes can be placed on either side of an ion-conducting membrane and stacked together to form a five-layer membrane electrode assembly. Alternatively, an electrocatalyst layer may be applied to both sides of the ion-conducting membrane to form a catalyst-coated ion-conducting membrane. Gas diffusion layers are then applied to both sides of the catalyst-coated ion-conducting membrane. Alternatively, a membrane electrode assembly can be formed from an ion conductive membrane coated on one side with an electrode catalyst layer, a gas diffusion layer adjacent to the electrode catalyst layer, and a gas diffusion electrode on the opposite side of the ion conductive membrane.

[0005] Typically, tens or hundreds of membrane electrode assemblies are required to provide enough power for most applications, and multiple membrane electrode assemblies are assembled to create a fuel cell stack. Flow field plates are used to separate the membrane electrode assemblies. The plates perform several functions: delivering reactants to the membrane electrode assemblies, removing products, providing electrical connections, and providing physical support.

[0006] It is known that there are many circumstances in which incorporating a water electrolysis catalyst into either the anode or cathode of a fuel cell can prove beneficial. For example, WO 01 / 15247 describes how the incorporation of an additional or second catalyst composition into the anode for the purpose of electrolyzing water can improve the resistance of a fuel cell to cell voltage reversal. Cell voltage reversal can occur when the cell is not adequately supplied with fuel (e.g., as a result of fuel access to a portion of the cell being blocked). When this occurs, the potential of the fuel cell anode rises to a high value, so that reactions other than fuel oxidation can occur at the anode, including water electrolysis and oxidation of the anode structural components. Oxidation of the anode structural components is undesirable as it can result in significant anode degradation. Incorporation of a catalyst composition that promotes the oxygen evolution reaction into the anode can reduce or avoid anode degradation by promoting water electrolysis over oxidation of the anode structural components.

[0007] Another example of a situation where it may be beneficial to promote water electrolysis is in a fuel cell where it is not practical or economical to purge hydrogen from the anode gas space with an inert gas such as nitrogen during shutdown, or when restarting the cell after being idle for some time. Both of these situations may result in a mixed composition of hydrogen and air on the anode while air is present on the cathode. Under these circumstances, as explained by Tang et al. (Journal of Power Sources 158 (2006) 1306-1312), an internal cell may exist, which results in a high potential on the cathode. If the catalyst layer contains carbon, the high potential may cause oxidation of the carbon, which causes great damage to the structure of the catalyst layer. However, if the cathode layer can sustain oxygen evolution, the high potential can be utilized for water electrolysis rather than carbon corrosion.

[0008] Finally, in regenerative fuel cells, the electrodes are bifunctional, with both the anode and cathode supporting two electrochemical reaction types at different times. When operating as a fuel cell, the cathode will reduce oxygen and the anode will oxidize hydrogen. When operating as an electrolyser, the cathode will generate hydrogen and the anode will generate oxygen. It may therefore be beneficial to incorporate both conventional hydrogen oxidation and oxygen evolution reaction catalysts in the anode of such fuel cells, since in such an arrangement the anode can effectively carry out both the hydrogen oxidation and oxygen evolution reactions.

[0009] Various electrocatalysts for oxygen evolution reaction are known in the art. For example, WO 2005 / 049199 discloses a catalyst for the electrolysis of water. For example, WO 11 / 021034 discloses a catalyst layer comprising an electrocatalyst and an oxygen evolution reaction catalyst, the oxygen evolution reaction catalyst comprising iridium or iridium oxide and one or more metals M or oxides thereof, M being selected from the group consisting of transition metals excluding ruthenium and Sn. EP 2475034 describes a catalyst layer comprising an iridium oxide component in combination with at least one other inorganic oxide component. WO 2016 / 038349 describes a method for producing a catalyst comprising iridium oxide and a metal oxide by flame spray pyrolysis. However, it is desirable to provide improved oxygen evolution reaction catalysts, in particular catalysts that can improve the stability of membrane electrode assemblies during repeated reversal events.

[0010] The present invention has been completed in light of the above considerations. Summary of the Invention

[0011] In a first aspect, the present invention provides an oxygen evolution reaction catalyst, the oxygen evolution reaction catalyst being an oxide material comprising iridium and tantalum; The oxygen evolution catalyst comprises a crystalline oxide phase having a rutile crystal structure; The crystalline oxide phase has a lattice parameter a greater than 4.510 Å; The oxygen generation reaction catalyst is at least 50m 2 The present invention provides an oxygen evolution reaction catalyst having a BET surface area of ​​1.0 to 1.5 nm / g.

[0012] The BET surface area must be at least 55 m 2 / g, preferably at least 60m 2 / g, more preferably at least 70m 2 The upper limit of the surface area is not particularly limited, and may be, for example, 200 m 2 / g. The inventors have found that providing a higher surface area material improves the activity of fuel cell membrane electrode assemblies, particularly those containing an oxygen evolution reaction catalyst in the catalyst layer, and improves the stability during repeated reversal events. The BET surface area can be determined based on the N2 adsorption isotherm at 77K according to ISO standard 9277:2010(en).

[0013] Preferably, the oxygen evolution reaction catalyst is iridium present in an amount ranging from 60 atomic % to 95 atomic % based on the total atomic composition of iridium species and tantalum species in the oxygen generation reaction catalyst; and tantalum present in an amount ranging from 5 atomic % to 40 atomic % based on the total atomic composition of the iridium species and the tantalum species in the oxygen evolution reaction catalyst.

[0014] The amount of iridium and tantalum is determined by the molar amounts of iridium and tantalum included in the material preparation and can be confirmed using inductively coupled plasma mass spectrometry (ICPMS).

[0015] Iridium may be suitably present in an amount of at least 65 atomic %, preferably at least 70 atomic %, more preferably at least 75 atomic %, based on the total atomic composition of the iridium species and tantalum species in the oxygen generation reaction catalyst. Iridium may be suitably present in an amount of up to 85 atomic %, based on the total atomic composition of the iridium species and tantalum species in the oxygen generation reaction catalyst. In a preferred composition, iridium is present in an amount ranging from 70 atomic % to 85 atomic %, based on the total atomic composition of the iridium species and tantalum species in the oxygen generation reaction catalyst. In a more preferred composition, iridium is present in an amount ranging from 75 atomic % to 85 atomic %, based on the total atomic composition of the iridium species and tantalum species in the oxygen generation reaction catalyst.

[0016] Tantalum may be present in an amount of at least 15 atomic %, based on the total atomic composition of the iridium and tantalum species in the oxygen generation reaction catalyst. Tantalum may be present in an amount of up to 35 atomic %, preferably up to 30 atomic %, more preferably up to 25 atomic %, based on the total atomic composition of the iridium and tantalum species in the oxygen generation reaction catalyst. In a preferred composition, tantalum is present in an amount ranging from 15 atomic % to 30 atomic %, more preferably from 15 atomic % to 25 atomic %, based on the total atomic composition of the iridium and tantalum species in the oxygen generation reaction catalyst.

[0017] The oxygen generation reaction catalyst may optionally contain metal species other than iridium and tantalum, for example in an amount of 5 atomic % or less based on the total atomic composition of the metal species in the oxygen generation reaction catalyst, preferably in an amount of 1 atomic % or less based on the total atomic composition of the metal species in the oxygen generation reaction catalyst. Preferably, iridium and tantalum constitute substantially all of the metal species present in the oxygen generation reaction catalyst. In other words, the metal species in the oxygen generation reaction catalyst consists essentially of iridium and tantalum, preferably iridium and tantalum. The total iridium and tantalum to oxygen atomic ratio is typically about 1:2. The oxygen generation reaction catalyst may optionally contain metal species in metallic form, for example in an amount of 5 atomic % or less based on the total atomic composition of the metal species in the oxygen generation reaction catalyst, preferably in an amount of 1 atomic % or less based on the total atomic composition of the metal species in the oxygen generation reaction catalyst. Such metal species in metallic form may include iridium and / or tantalum.

[0018] The crystalline oxide phase has a rutile MO2 (M is the metal species) structure. Phase identification is performed by comparing the X-ray diffraction pattern with the PDF-4+ database, release 2021. The material contains a single crystalline oxide phase with relative reflection intensities consistent with a rutile MO2 phase, but with reflection positions inconsistent with pure IrO2. Stated differently, the crystalline oxide phase may be a single oxide structure containing iridium and tantalum, i.e., a mixed metal oxide (not simply a mixture of iridium oxide and tantalum oxide). The crystalline oxide phase may be a single oxide structure containing Ir x Ta y O2, where x+y=1. For example, x may be 0.60 to 0.80, and y may be 0.20 to 0.40. In particular, x may be 0.65 to 0.75, and y may be 0.25 to 0.35.

[0019] The lattice constant a of the crystalline oxide phase is preferably greater than 4.550 Å. The upper limit of the lattice constant a is not particularly limited, but a is typically less than 4.800 Å, preferably less than 4.750 Å, and more preferably less than 4.650 Å. For example, the lattice constant a may be about 4.580 Å. The lattice constant c of the crystalline oxide phase may be greater than 3.160 Å, preferably greater than 3.170 Å. The upper limit of the lattice constant c is not particularly limited, but c is typically less than 3.200 Å, and preferably less than 3.180 Å. For example, the lattice constant c may be about 3.177 Å. The values ​​given for the lattice constants a and c are obtained at ambient temperature, i.e., about 25° C. Powder X-ray diffraction (PXRD) data were collected in reflection geometry using a Bruker AXS D8 diffractometer with CuKα radiation (λ=1.5406+1.54439Å) over the range 10<2θ<130° with 0.04° steps. To extract the lattice parameters, reflection profiles were modeled using a fundamental parameter approach [2] using reference data collected from NIST660 LaB6 and subjected to Pawley refinement using Topas [1]. The data were fitted from 20 to 77°2Θ using a Pawley model in P42 / mnm (same space group as IrO2) to extract the lattice parameters. For comparison, rutile IrO2 has a crystal phase parameter a of 0.4498 nm.

[0020] The crystalline oxide phase may have a crystallite size calculated from the (110) hkl reflection in the range of 4.0 nm to 10.0 nm, preferably 6 nm to 8 nm. Suitably, the ratio of the crystallite size calculated from the (110) hkl reflection to the crystallite size calculated from the (002) hkl reflection is in the range of 1.0 to 6.0, suitably 1.2 to 5.5, preferably 2.5 to 4.0, for example 2.5 to 3.5. Powder X-ray diffraction (PXRD) data were collected in reflection geometry using a Bruker AXS D8 diffractometer with CuKα radiation (λ=1.5406+1.54439 Å) over the range 10<2θ<130° in 0.04° steps. Peak phase refinement was performed using Topas [1] with the reflection profile modeled using a fundamental parameter approach [2] using reference data collected from NIST660 LaB6. To obtain the crystallite size along the crystal planes, reflections from the rutile phase are fitted using a set of peaks with independent sample broadening. The crystallite size is calculated using the volume-weighted column height LVol-IB method. [3]

[0021] The oxygen evolution reaction catalyst may also include an amorphous phase, preferably an amorphous oxide phase. The amorphous phase may include an oxide of tantalum, typically tantalum pentoxide Ta2O5. The crystallinity of the oxide material, i.e. the area ratio between the crystalline phase and the amorphous phase, is at most 90%, typically at most 80%. Preferably, the crystallinity of the oxide material is at least 60%. Preferably, the crystalline and amorphous phases constitute at least 90% by weight, typically at least 95% by weight, for example at least 99% by weight, of the oxide material. The oxide material may essentially consist of a crystalline oxide phase and an amorphous phase, preferably a crystalline oxide phase and an amorphous phase. Powder X-ray diffraction (PXRD) data were collected in reflection geometry using a Bruker AXS D8 diffractometer with CuKα radiation (λ=1.5406+1.54439Å) over the range 10<2θ<130° in 0.04° steps. For the determination of crystallinity, the X-ray scattering data of the crystalline oxide phase and the amorphous phase were fitted in the range of 20-77°2Θ. The total area from peaks that model the contributions from the crystalline phase and from the total X-ray scattering was used to calculate the scattering percentage.

[0022] In a second aspect, the present invention provides a method for the synthesis of an oxygen evolution reaction catalyst according to the invention, the method comprising: Providing an aqueous solution of iridium and tantalum compounds; spray drying the solution to form a dry powder; and calcining the powder at a temperature less than 500° C. to form an oxygen evolution reaction catalyst.

[0023] The present inventors have surprisingly found that by controlling the calcination temperature during the preparation of the oxygen evolution reaction catalyst, it is possible to prepare an oxygen evolution reaction catalyst having improved properties in the catalyst layer.

[0024] The step of preparing an aqueous solution of compounds of iridium and tantalum may include the sub-steps of preparing an aqueous solution of a compound of iridium and mixing this aqueous solution with an aqueous solution of a compound of tantalum.

[0025] The iridium and tantalum compounds may be provided in suitable amounts to provide a desired ratio of iridium to tantalum, for example an Ir:Ta ratio of 6-8Ir:2-4Ta, such as 7:3.

[0026] Aqueous solutions of iridium and tantalum compounds may be prepared by dissolving a suitable iridium or tantalum compound, respectively, and adding a solubilizer to facilitate the dissolution of the compound in water, if necessary. A suitable iridium compound is IrCl3. A suitable tantalum compound is TaCl5, which requires the addition of hydrochloric acid as a solubilizer.

[0027] The step of spray drying the resulting mixture to form a dry powder may be carried out, for example, at an inlet temperature of about 200 to about 400° C., an atomizer pressure of about 2 bar, and an air flow rate of, for example, about 5 to about 15 kg / hr.

[0028] A calcination step is used in the preparation of the oxygen generation reaction catalyst. The calcination step may be a one-step calcination step, in which the powder is calcined at less than 500°C, preferably ≦450°C, and preferably ≧400°C for a specified time to form the oxygen generation reaction catalyst. Alternatively, a two-step calcination step may be used, in which the powder is calcined at less than 500°C, preferably ≦450°C, and preferably ≧400°C for a first specified time, and then further calcined at less than 500°C, preferably ≦450°C, and preferably ≧400°C for a second specified time. The first and second calcination steps may be performed at the same temperature or at different temperatures. The first and second times may be the same or different. The length of the one-step calcination step, the first and second specified times for calcination may suitably be 1 hour or more, typically 3 hours or more. The length of the single firing step, the first defined time period and the second defined time period for firing may suitably be up to 10 hours.

[0029] Optionally, additional processing steps may be carried out between the first and second firing steps, for example, the powder may be stirred or milled. In this case, the advantage is that agglomerates can be broken down before the second firing step, allowing for a more uniform firing. However, the first and second firing steps may be carried out consecutively, without any additional processing steps between the first and second firing steps. Firing may also be carried out with continuous stirring of the powder, as in rotary firing with or without beads or balls to promote the breaking up of agglomerates.

[0030] Calcination may be carried out in a suitable gas atmosphere, such as, for example, air, N2, Ar, He, CO2, CO, O2, H2, and mixtures thereof. Preferably, calcination is carried out in an air atmosphere.

[0031] The oxygen evolution reaction catalyst of the present invention may be used in a variety of electrochemical applications, however one particularly preferred application is in electrochemical fuel cells.

[0032] In a third aspect, the present invention provides a catalyst layer comprising an oxygen evolution reaction catalyst according to the present invention and a second electrocatalyst material, in which the oxygen evolution reaction catalyst of the present invention is the first electrocatalyst.

[0033] The second electrocatalyst material is preferably (i) the platinum group metals (platinum, palladium, rhodium, ruthenium, iridium, and osmium); (ii) gold or silver; (iii) base metals; or an alloy or mixture containing one or more of these metals or their oxides. The base metal is tin or a transition metal that is not a noble metal. The noble metal is a platinum group metal (platinum, palladium, rhodium, ruthenium, iridium, or osmium), silver, or gold. Preferred base metals are copper, cobalt, nickel, zinc, iron, titanium, molybdenum, vanadium, manganese, niobium, tantalum, chromium, and tin.

[0034] The second electrocatalyst material preferably does not include iridium or tantalum. Preferably, the second electrocatalyst material is an electrocatalyst material for the anode or cathode, preferably the anode, of a fuel cell. Typically, the second electrocatalyst material comprises a platinum group metal other than iridium, or an alloy of a platinum group metal other than iridium with, preferably, a base metal, preferably a base metal as defined above. In particular, the second electrocatalyst material comprises platinum, or an alloy of platinum with a base metal, preferably a base metal as defined above, more preferably titanium, vanadium, chromium, niobium, or tantalum. Alternatively, preferably, the second electrocatalyst material may comprise an alloy of platinum with another platinum group metal, preferably rhodium or ruthenium.

[0035] Preferably, the catalyst layer is an anode catalyst layer, preferably an anode catalyst layer for a proton exchange membrane fuel cell.

[0036] The loading of the base metal, e.g., a platinum group metal as defined herein, of the second electrocatalyst material in the catalyst layer may be selected based on the intended use of the catalyst layer, in particular whether the catalyst layer is intended for use in an anode or a cathode. Preferably, when used as an anode for a proton exchange membrane fuel cell, the loading of the base metal, e.g., a platinum group metal as defined herein, in the catalyst layer is suitably from 0.02 to 0.2 mg / cm. 2 , typically 0.02 to 0.15 mg / cm 2 , preferably 0.02 to 0.1 mg / cm 2 may be also possible.

[0037] The second electrocatalyst material is preferably in the form of particles, which may or may not be supported. The term "supported" will be readily understood by those skilled in the art. For example, the term "supported" will be understood to mean that the electrocatalyst particles are dispersed on a support material and are bonded or fixed to the support material by physical or chemical bonds. For example, the electrocatalyst may be bonded or fixed to the support material by ionic or covalent bonds, or non-specific interactions such as van der Waals forces.

[0038] The oxygen evolution reaction catalyst of the present invention is preferably in the form of particles which may be supported or unsupported, preferably unsupported. The oxygen evolution reaction catalyst of the present invention is preferably in the form of particles dispersed in a catalyst layer.

[0039] In the case where the particles are supported, the support material may be a conductive carbon support material. Suitably, the support material may be a carbon powder, such as carbon black or graphitized carbon black, such as commercially available carbon black (such as Cabot Corp. (Vulcan® XC72R) or Akzo Nobel (Ketjen® Black series)). Another suitable carbon support material is acetylene black (such as that available from Chevron Phillips (Shawinigan Black®) or Denka). The support material may also be a conductive carbon support material specifically designed for use in fuel cells, such as those described in WO 2013 / 045894. Alternatively, the support material may be a non-carbonaceous material. Examples of such support materials include titania, niobia, tantala, tungsten carbide, hafnium oxide, or tungsten oxide. Such oxides and carbides may also be doped with other metals to enhance electrical conductivity, such as, for example, niobium-doped titania.

[0040] The oxygen evolution reaction catalyst of the present invention and the second electrocatalyst material may be supported on the same support material or on different support materials.

[0041] The weight ratio of the oxygen evolution reaction catalyst of the present invention to the second electrode catalyst material in the catalyst layer may be 10:1 to 1:10. The weight ratio may be selected depending on whether the catalyst layer is intended for use in an anode or a cathode. In the case of an anode catalyst layer, preferably for a proton exchange membrane fuel cell, the weight ratio is preferably at least 0.5:1, preferably at least 0.75:1. The weight ratio is preferably at most 10:1, preferably at most 5:1, more preferably at most 2:1, even more preferably at most 1:1. In the case of a cathode catalyst layer, preferably for a proton exchange membrane fuel cell, the weight ratio is preferably 1:1 to 1:10, preferably 1:2 to 1:5.

[0042] The catalyst layer may include additional components in addition to the oxygen evolution reaction catalyst according to the present invention and the second electrode catalyst material. Such components may include, but are not limited to, ion-conducting polymers, such as proton-conducting polymers, included to improve ionic conductivity within the layer, hydrogen peroxide decomposition catalysts, hydrophobic additives (e.g., polymers such as polytetrafluoroethylene (PTFE) or inorganic solids, with or without surface treatment) or hydrophilic additives (e.g., polymers or inorganic solids such as oxides) to control reactant and water transport properties. The selection of additional components depends on whether the catalyst layer is used in the anode or cathode, and it is within the ability of a person skilled in the art to determine which additional components are appropriate.

[0043] To prepare the catalyst layer, the oxygen evolution reaction catalyst of the present invention, supported or unsupported, the second electrocatalyst material, supported or unsupported, and any additional components can be dispersed in an aqueous and / or organic solvent to prepare the catalyst ink. If necessary, particle crushing can be performed by methods known in the art, such as high shear mixing, milling, ball milling, passing through a microfluidizer, and the like, or combinations thereof, to achieve a suitable particle size distribution. After preparation of the catalyst ink, the ink can be deposited on a substrate (e.g., a gas diffusion layer, an ion-conducting membrane, or a support material / transfer substrate) to form the catalyst layer. The ink can be deposited by any suitable technique known in the art, including, but not limited to, gravure coating, slot die (slot, extrusion) coating, screen printing, rotary screen printing, inkjet printing, spraying, painting, gap coating techniques such as bar coating, pad coating, knife or doctor blade over roll, and application of a metering rod.

[0044] When a catalyst layer is deposited on a support material / transfer substrate by coating the catalyst ink on the support material / transfer substrate, a catalyst support material / transfer substrate is formed. The support material / transfer substrate is intended to be removed from the layer in a subsequent step. For example, a catalyst layer may be transferred to a gas diffusion layer or ion conductive membrane by decal transfer, and the support material / transfer substrate is removed immediately after the transfer process or at some point thereafter.

[0045] Prior to removing the support material / transfer substrate, additional layers may be deposited on the exposed surface of the catalyst layer, for example, an ion-conducting ionomer layer may be applied from an ionomer dispersion using any suitable deposition technique known in the art as described above for depositing the catalyst layer. Further additional layers may be added as needed, for example, as described in International Application PCT / GB2015 / 050864. The support material / transfer substrate is removed from the catalyst layer at the appropriate time. The support material / transfer substrate may be formed from any suitable material that can be removed without damaging the catalyst layer. Examples of suitable materials include fluoropolymers, such as polytetrafluoroethylene (PTFE), ethylene tetrafluoroethylene (ETFE), perfluoroalkoxy polymer (PFA), fluorinated ethylene propylene (FEP - a copolymer of hexafluoropropylene and tetrafluoroethylene), and polyolefins, such as biaxially oriented polypropylene (BOPP).

[0046] The properties of the catalyst layer, such as thickness, electrocatalyst loading, porosity, pore size distribution, average pore size, and hydrophobicity, vary depending on whether it is used in the anode or cathode. The thickness of the catalyst layer may be preferably at least 1 μm, typically at least 5 μm. The thickness of the catalyst layer may be preferably 15 μm or less, typically 10 μm or less.

[0047] In a fourth aspect, the present invention provides a gas diffusion electrode comprising a gas diffusion layer and a catalyst layer according to the third aspect.

[0048] Preferably, the catalyst layer is directly adjacent to the gas diffusion layer. This may be achieved, for example, by depositing the catalyst layer directly on the gas diffusion layer. The gas diffusion layer may be based on or may include a conventional gas diffusion substrate. Typical substrates include nonwoven papers or webs comprising a network of carbon fibers and a thermosetting resin binder (e.g., TGP-H series of carbon fiber papers available from Toray Industries Inc., Japan, or H2315 series available from Freudenberg FCCT KG, Germany, or Sigracet series available from SGL Technologies GmbH, Germany, or AvCarb series from Ballard Power Systems Inc.), or carbon fiber cloth. The carbon paper, web, or cloth may be pretreated prior to the manufacture of the electrode and incorporated into the membrane electrode assembly to make it either more wettable (hydrophilic) or more resistant to wettability (hydrophobic). The nature of any treatments will depend on the type of fuel cell and the operating conditions used. The substrate can be made more wettable by incorporating materials such as amorphous carbon black by impregnation from a liquid suspension, or made more hydrophobic by impregnating the pore structure of the substrate with a colloidal suspension of a polymer such as PTFE or polyfluoroethylenepropylene (FEP), followed by drying and heating above the softening point of the polymer. In applications such as proton exchange membrane fuel cells, a microporous layer may be applied to the gas diffusion substrate on the side that contacts the catalyst layer. The microporous layer typically comprises a mixture of carbon black and a polymer such as polytetrafluoroethylene (PTFE).

[0049] In a fifth aspect, the present invention provides a catalytic membrane comprising an ion-conducting membrane and a catalytic layer according to the third aspect.

[0050] Here, a catalyst-coated ion-conducting membrane is formed by either directly coating a catalyst ink onto the membrane or indirectly depositing a catalyst layer onto the ion-conducting membrane by transfer from a carrier substrate or transfer substrate. The ion-conducting membrane may be any membrane suitable for use in a proton exchange membrane fuel cell, for example, the membrane may be based on perfluorinated sulfonic acid materials such as Nafion™ (Chemours Company), Aquivion™ (Solvay Specialty Polymers), Flemion™ (Asahi Glass Group), and Aciplex™ (Asahi Kasei Chemicals Corp.). Alternatively, the membrane may be based on sulfonated hydrocarbon membranes such as those available as fumapem™ P, E, or K series products from FuMA-Tech GmbH, JSR, Toyobo, and other companies. Alternatively, the membrane may be based on polybenzimidazole doped with phosphoric acid operating in the range of 120°C to 180°C.

[0051] The ion-conducting membrane component may include one or more materials that impart mechanical strength to the ion-conducting membrane component. For example, the ion-conducting membrane component may include a porous reinforcing material such as an expanded PTFE material or a nanofiber network, e.g., an electrospun fiber network.

[0052] The ion-conducting membrane may include one or more hydrogen peroxide decomposition catalysts, either as a layer on one or both sides of the membrane, or as a layer embedded within the membrane, uniformly dispersed throughout or within the layer. Examples of hydrogen peroxide decomposition catalysts suitable for use are known to those skilled in the art and include metal oxides such as cerium oxide, manganese oxide, titanium oxide, beryllium oxide, bismuth oxide, tantalum oxide, niobium oxide, hafnium oxide, vanadium oxide, and lanthanum oxide, suitably cerium oxide, manganese oxide or titanium oxide, preferably cerium dioxide (ceria).

[0053] The ionically conductive membrane component may optionally include a recombination catalyst, in particular a catalyst for the recombination of unreacted H2 and O2 that can diffuse into the membrane from the anode and cathode, respectively, to produce water. Suitable recombination catalysts include high surface area oxide support materials (such as silica, titania, zirconia) or metals (such as platinum, palladium) on carbon supports. Further examples of recombination catalysts are disclosed in EP 0631337 and WO 00 / 24074.

[0054] In a sixth aspect, the present invention provides a membrane electrode assembly comprising a catalyst layer according to the third aspect, a gas diffusion electrode according to the fourth aspect, or a catalyst membrane according to the fifth aspect.

[0055] As will be appreciated by those skilled in the art, the membrane electrode assembly can be constructed by many methods, provided that it includes at least one catalyst layer. For example, the membrane electrode assembly may include a catalyst coated ion conducting membrane, including two catalyst layers, at least one of which is the catalyst layer of the present invention, with a gas diffusion layer applied to each catalyst layer. Alternatively, the membrane electrode assembly may include an ion conducting membrane sandwiched between two gas diffusion electrodes, at least one of which is the gas diffusion electrode of the present invention. The membrane electrode assembly may also include a catalyst coated ion conducting membrane with one catalyst layer and a gas diffusion electrode on the opposite side of the ion conducting membrane, where either or both of the catalyst layer and the gas diffusion electrode are of the present invention.

[0056] The electrochemical device in which the catalyst layer, gas diffusion electrode, catalyst membrane and membrane electrode assembly of the present invention can be used includes a fuel cell, in particular a proton exchange membrane fuel cell.Accordingly, in a seventh aspect, the present invention provides a fuel cell comprising the catalyst layer according to the third aspect, the gas diffusion electrode according to the fourth aspect, the catalyst membrane according to the fifth aspect, or the membrane electrode assembly according to the sixth aspect.The fuel cell of the present invention is preferably a proton exchange membrane fuel cell.

[0057] Proton exchange membrane fuel cells may operate with hydrogen or hydrogen-rich fuels at the anode, or may be fueled with hydrocarbon fuels such as methanol. The catalyst layers, gas diffusion electrodes, catalyst membranes, and membrane electrode assemblies of the present invention may also be used in fuel cells where the membrane uses charge carriers other than protons, e.g., OH-MS available from Solvay Solexis SpA, FuMA-Tech GmbH. - It can also be used in conductive films.

[0058] The catalyst layers and gas diffusion electrodes of the present invention may also be used in other low temperature fuel cells that use liquid ionically conducting electrolytes such as aqueous acid and alkaline solutions or concentrated phosphoric acid. Another electrochemical device in which the catalyst layers, gas diffusion electrodes, catalyst membranes and membrane electrode assemblies of the present invention may be used is as the anode electrode of a regenerative fuel cell in which both the hydrogen oxidation reaction and the oxygen evolution reaction take place.

[0059] The oxygen evolution reaction catalyst of the present invention may also be used in the anode of a proton exchange membrane electrolyzer. Thus, in an eighth aspect, the present invention provides an anode catalyst layer for a proton exchange membrane electrolyzer, the anode catalyst layer comprising the oxygen evolution reaction catalyst of the present invention. In a ninth aspect, the present invention provides a proton exchange membrane electrolyzer comprising the anode catalyst layer of the eighth aspect of the present invention. Those skilled in the art will appreciate that there are similarities between such a catalyst layer and the catalyst layer of the third aspect of the present invention, and that all aspects suitable for the anode of a proton exchange membrane electrolyzer described above with respect to the catalyst layer of the third aspect of the present invention are intended to apply to the catalyst layer of the eighth aspect of the present invention.

[0060] The present invention includes combinations of the described aspects and preferred features, except where such combinations are clearly essential or explicitly avoided. In particular, any aspect of the invention may be combined with any other aspect of the invention, unless the context requires otherwise. Any preferred or optional feature of any aspect may be combined, singly or in any combination, with any aspect of the invention, unless the context requires otherwise. [Brief description of the drawings]

[0061] BRIEF DESCRIPTION OF THE DRAWINGS Embodiments and experiments illustrating the principles of the present invention are now described with reference to the accompanying drawings.

[0062] [Figure 1] FIG. 2 is an X-ray diffraction pattern of an oxygen evolution reaction catalyst according to the present invention. [Diagram 2] FIG. 2 is a plot of voltage and resistance versus reversal hold times at 200 mA / cm2 for a membrane electrode assembly according to the present invention and two other membrane electrode assemblies. [Diagram 3] FIG. 1 is a plot showing carbon corrosion versus number of reversal holds at a set current density for a membrane electrode assembly according to the present invention and two other membrane electrode assemblies. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0063] Aspects and embodiments of the present invention will now be described with reference to the accompanying figures and examples. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this specification are incorporated herein by reference.

[0064] Three different materials were prepared and characterized. The first of these ("Example 1") is a high surface area oxygen evolution reaction catalyst according to the present invention. The second material ("Comparative Example 1") is an iridium tantalum mixed oxide oxygen evolution reaction catalyst as disclosed in WO 11 / 021034. The third material ("Comparative Example 2") is also an iridium tantalum mixed oxide oxygen evolution reaction catalyst, similar to that disclosed in WO 11 / 021034, but with a different Ir:Ta ratio.

[0065] As shown and discussed below, membrane electrode assemblies having anodes containing oxygen evolution reaction catalysts according to the present invention have been found to exhibit improved activity towards OER and have been shown to exhibit improved stability towards cell reversal and reduced carbon loss after repeated reversal events.

[0066] Example 1: Synthesis of oxygen evolution reaction catalyst according to the present invention The synthesis method for the oxygen evolution reaction catalyst according to this embodiment can be divided into three main steps. 1) providing an aqueous solution of iridium and tantalum compounds; 2) spray drying the resulting mixture; 3) calcining the product to form an oxygen evolution reaction catalyst.

[0067] TaCl5 (Alfa Aesar) was received in sealed ampoules of approximately 100 g each. The ampoules were scored open and the TaCl5 powder was poured into a glass bottle. The mass of the TaCl5 was measured to an accuracy of ±0.01 g. 200 mL of concentrated HCl was measured out and poured into a separate glass bottle. The TaCl5 was slowly dissolved in the concentrated HCl under constant stirring.

[0068] Using the measured mass of TaCl5, IrCl3 was weighed out to give a final Ir:Ta ratio of 7:3. This was dissolved in a beaker containing 1400 mL of demineralized HO under constant stirring. This process was repeated four more times to yield five separate bottles containing approximately 100 g of TaCl5 in 200 mL of concentrated HCl, and five corresponding beakers containing IrCl3 dissolved in HO.

[0069] Prior to spray drying, TaCl5 (in concentrated HCl) was mixed with the corresponding IrCl3 solution (in demineralized H2O) and stirred. The mixture was fed through a 5 mm diameter silicone feed tube and a 1.0 mL nozzle and spray dried (GEA-Niro A / S mobile unit spray dryer) at an inlet temperature of 290°C, atomizer pressure of 2 bar, and air flow rate of 9 kg / hr ± 0.5 kg / hr. The resulting powder was collected in a powder bottle and the process was repeated for each TaCl5 / IrCl3 pair.

[0070] The powders from each powder bottle containing dried Ir / Ta chloride were placed in separate crucibles and fired at 450 °C for 6 h [the heating rate from room temperature was 10 °C min -1The powder was then crushed at 14,000 RPM using a sieve with 0.08 cm openings to break up agglomerates. The crushed powder was then calcined at 450°C for 6 hours [the heating rate from room temperature was 10°C min -1 ] This process was repeated for each crucible and each final product was blended on a roller.

[0071] Comparative Example 1: Synthesis of Comparative Oxygen Evolution Reaction Catalyst For comparison, an IrTa mixed oxide oxygen evolution reaction catalyst was prepared by the conventional method disclosed in WO 2011 / 021034. Calcination was carried out at a temperature of 500°C.

[0072] Comparative Example 2: Synthesis of comparative oxygen evolution reaction catalysts with different Ir:Ta ratios To evaluate the effect of varying the Ir:Ta ratio on the oxygen evolution reaction catalyst, further comparative examples were prepared using the same method as Comparative Example 1, but the amounts of TaCl5 and IrCl3 used were adjusted to obtain a final Ir:Ta weight ratio of 8:2, and calcination was performed at a temperature of 500°C.

[0073] BET surface area analysis of samples The BET surface area of ​​the prepared oxygen evolution reaction catalysts was determined based on N2 adsorption isotherms at 77 K according to ISO standard 9277:2010(en).

[0074] The following results were obtained.

[0075] [Table 1]

[0076] The oxygen generation reaction catalyst of Example 1 was found to exhibit a much higher surface area than the oxygen generation reaction catalysts of Comparative Example 1 and Comparative Example 2.

[0077] X-ray diffraction analysis FIG. 1 shows the X-ray diffraction pattern of the material prepared according to Example 1.

[0078] X-ray Data Collection Powder X-ray diffraction (PXRD) data were collected in reflection geometry using a Bruker AXS D8 diffractometer with CuKα radiation (λ=1.5406+1.54439Å) over the range 10<2θ<130° with 0.04° steps. Phase identification was performed using Bruker AXS Diffrac Eva V4.2 (2014) with reference to the PDF-4+ database, release 2021. The results showed that the material contained a single crystalline oxide phase with reflection intensities consistent with a rutile-type MO2 phase, but reflection positions inconsistent with IrO2.

[0079] Sample fitting Pawley refinement and peak phase refinement were performed using Topas [1], with reflection profiles modeled using a fundamental parameter approach [2] with reference data collected from NIST660 LaB6. The data were fitted to 20-77°2Θ using a Pawley model of P42 / mnm (same space group as IrO2) and lattice parameters were extracted. To obtain crystallite sizes along selected crystallographic planes, for the rutile phase the data were fitted separately using a set of peaks with independent sample-dependent broadening. In both cases, the contribution from any amorphous material was fitted using a separate set of peaks to allow calculation of the degree of crystallinity. All crystallite sizes were calculated using the volume-weighted column height LVol-IB method [3].

[0080] Table 2 shows the crystallinity and lattice constants a and c obtained from the data collected in Example 1.

[0081] [Table 2]

[0082] Table 3 shows the crystallite sizes obtained from the (110) and (002) hkl reflections.

[0083] [Table 3]

[0084] Preparation of membrane electrode assembly MEA1 contained the oxygen evolution reaction catalyst of Example 1 in the anode, MEA2 contained the oxygen evolution reaction catalyst of Comparative Example 1 in the anode, and MEA3 contained the oxygen evolution reaction catalyst of Comparative Example 2 in the anode.

[0085] The anode catalyst layer was prepared by forming an ink containing the PFSA ionomer, 20 wt% Pt / C electrocatalyst material, and oxygen evolution reaction catalyst dispersed in a water-propan-1-ol mixture. The mixture was mechanically stirred using an overhead stirrer until all of the catalyst was wetted and dispersed in the liquid. The ink was then processed through an Eiger ball mill to form a well-dispersed ink. The platinum loading was 0.08 mg Pt / cm. 2 The loading amount of the oxygen generating reaction catalyst of the present invention or the oxygen generating reaction catalyst of the comparative example is 0.067 mgIr / cm 2 and the ratio of platinum electrocatalyst to the oxygen evolution reaction catalyst of the present invention or the comparative oxygen evolution reaction catalyst was 1:0.86.

[0086] The cathode catalyst layer contained 50 wt% Pt / C electrocatalyst, and the carbon support was a carbon specifically designed for use in fuel cells, as described in WO 2013 / 045894. The cathode catalyst layer was prepared by forming an ink containing PFSA ionomer and 50 wt% Pt / C electrocatalyst dispersed in a water / propan-1-ol mixture. The mixture was mechanically stirred using an overhead stirrer until all of the catalyst was wetted and dispersed in the liquid. The ink was then processed through an Eiger ball mill to form a well-dispersed ink. The platinum loading was 0.4 mg Pt / cm. 2 It was.

[0087] The anode and cathode inks were deposited onto a PTFE sheet to form the catalyst layers, and the appropriate layers were transferred onto both sides of a PFSA-reinforced membrane (20 μm thick) to produce a 217 cm active area. 2 A catalyst-coated ion-conducting membrane was prepared.

[0088] A gas diffusion layer was applied to each side of each catalyst-coated ion-conducting membrane to form a complete membrane electrode assembly. The gas diffusion layers used were carbon fiber paper with a hydrophobic microporous layer containing carbon and PTFE applied to the side in contact with the catalyst-coated ion-conducting membrane.

[0089] Membrane electrode assembly reversal test The MEA was tested in a single cell format. It was first conditioned using a cathode starvation protocol in which the MEA was held at 80°C, 100 kPa pressure, and 100% RH at both the anode and cathode. 500 mAcm was drawn from the cell. -2 A current of 500 mA cm was applied and the cathode stoichiometry was cycled between 2.0 and 0.0 while the anode stoichiometry was held constant at 1.5. After these 17 cathode starvation events, the MEA was recharged with a constant cathode stoichiometry of 2.0 and a current of 500 mA cm. -2 The MEA was then maintained at a current of 65° C. for 2 hours, after which a reconditioning protocol was performed in which the MEA was held at the experimental conditions of 65° C., ambient pressure, and 50% RH for 1 hour.

[0090] In the experimental conditions, OCV and 2000mAcm -2 The BOL polarization curves between were completed to evaluate the early life performance.

[0091] The MEA was then pumped to 200 mAcm -2 The anode gas flow was then switched to N2 for 5 min, after which the gas flow was switched back to H2 at 500 mA cm -2 The MEA was subjected to a cell inversion cycle in which a current of 1 A cm was applied for 15 minutes. This cell inversion cycle was repeated six times, after which the performance evaluation of the polarization curve was repeated. -2This sequence of six reversal cycles and polarization curve performance tests was repeated until the cell voltage was below 0.35 V or the cell voltage during reversal hold was below −1.2 V.

[0092] FIG. 2 shows the current density of 200 mA / cm for the membrane electrode assembly MEA1 according to the invention and two other membrane electrode assemblies MEA2 and MEA3. 2 1 is a plot of voltage and resistance versus number of reversal hold times for MEA1. At the start of the test, MEA1 has a small negative voltage value, indicating that the use of a catalyst layer containing the oxygen evolution reaction catalyst of the present invention results in an inherently more active membrane electrode assembly. Furthermore, this activity is maintained at a high level even after multiple cell reversals, indicating high resistance of the catalyst layer to cell reversal. MEA3 also retains better activity than MEA2, indicating improved cell reversal resistance of the catalyst layer containing the oxygen evolution reaction catalyst of Comparative Example 2 compared to the benchmark Comparative Example 1.

[0093] Carbon Corrosion Test During the described reversal cycle, the CO content of the anode and cathode exhaust gases was monitored using a Vaisala CARBOCAP® carbon dioxide probe GMP 343. The concentration of CO in the exhaust gas stream over the duration of the experiment is integrated to calculate the total C loss during the experiment.

[0094] 3 is a graph showing carbon loss with increasing number of inversion cycles. It can be seen that MEA1 experienced significantly less carbon corrosion compared to both MEA2 and MEA3. Thus, it can be seen that the oxygen generation reaction catalyst of the present invention provides better protection from carbon corrosion caused by cell inversion compared to the oxygen generation reaction catalyst of the comparative example.

[0095] References 1. Topas v4.2 / v5.0:General Profile and Structure Analysis Software for Powder Diffraction Data,Bruker AXS,Karlsruhe,Germany,(2003-2015). 2. R.W.Cheary and A.Coelho,J.Appl.Cryst.(1992),25,109-121 3. F.Bertaut and P.Blum(1949)C.R.Acad.Sci.Paris 229,666

Claims

1. An oxygen generation reaction catalyst, wherein the oxygen generation reaction catalyst is an oxide material containing iridium and tantalum, the oxygen generation reaction catalyst contains a crystalline oxide phase having a rutile-type crystal structure, the crystalline oxide phase has a lattice constant a larger than 4.510 Å, The oxygen generation reaction catalyst has a BET surface area of at least 50 m ２ / g and is an oxygen generation reaction catalyst.

2. Based on the total atomic composition of iridium species and tantalum species in the oxygen generation reaction catalyst, iridium present in an amount in the range of 60 atomic % or more and 95 atomic % or less, and based on the total atomic composition of iridium species and tantalum species in the oxygen generation reaction catalyst, tantalum present in an amount in the range of 5 atomic % or more and 40 atomic % or less. The oxygen generation reaction catalyst according to Claim 1, comprising

3. The oxygen generation reaction catalyst according to Claim 1, wherein the crystalline oxide phase contains a single-phase oxide structure containing iridium and tantalum.

4. A method for synthesizing the oxygen generation reaction catalyst according to Claim 1, wherein the method comprises a step of preparing an aqueous solution of a compound of iridium and tantalum, a step of spray-drying the solution to form a dry powder, and a step of forming the oxygen generation reaction catalyst by firing the powder at a temperature of less than 500 °C.

5. The step of preparing an aqueous solution of a compound of iridium and tantalum comprises a sub-step of preparing an aqueous solution of a compound of iridium and a sub-step of mixing the aqueous solution with an aqueous solution of a compound of tantalum. The method according to Claim 4.

6. The step of forming the oxygen generation reaction catalyst by firing the powder comprises a sub-step of firing the powder at less than 500 °C for a first specified time and then a sub-step of further firing the powder at less than 500 °C for a second specified time. The method according to Claim 4.

7. A catalyst layer comprising the oxygen generation reaction catalyst according to Claim 1 and a second electrode catalyst material.

8. The catalyst layer is an anode catalyst layer, and optionally, an anode catalyst layer for a proton exchange membrane fuel cell. The catalyst layer according to Claim 7.

9. The second electrode catalyst material is a platinum group metal (platinum, palladium, rhodium, ruthenium, iridium, and osmium), gold or silver, a base metal, or

10. The weight ratio of the oxygen generation reaction catalyst in the catalyst layer to the second electrode catalyst material is from 10:1 to 1:

10. The catalyst layer according to claim 7.

11. A gas diffusion electrode comprising a gas diffusion layer and the catalyst layer according to claim 7.

12. A catalyst membrane comprising an ion conductive membrane and the catalyst layer according to claim 7.

13. A membrane electrode assembly comprising the catalyst layer according to claim 7, the gas diffusion electrode according to claim 11, or the catalyst membrane according to claim 12.

14. A fuel cell comprising the catalyst layer according to claim 7, the gas diffusion electrode according to claim 11, or the catalyst membrane according to claim 12.

15. A fuel cell comprising the membrane electrode assembly according to claim 13.

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