WO2025068691A1

WO2025068691A1 - De-alloyed electrocatalyst

Info

Publication number: WO2025068691A1
Application number: PCT/GB2024/052467
Authority: WO
Inventors: Alejandro Martinez Bonastre; Jay Simon Clarkson; Jibreel HADDAD; Enrico Angelo PETRUCCO; Geoffrey Hugh SPIKES
Original assignee: Johnson Matthey Hydrogen Technologies Ltd
Current assignee: Johnson Matthey Hydrogen Technologies Ltd
Priority date: 2023-09-25
Filing date: 2024-09-25
Publication date: 2025-04-03
Anticipated expiration: 2026-03-25

Abstract

The present invention provides a process for preparing solid, non-porous, de-alloyed electrocatalyst particles, the process comprising the steps of: providing solid, non-porous, platinum group metal alloy precursor particles PMn in which P is a platinum group metal and M is at least one alloying metal; in a de-alloying step to provide the solid, non-porous, de- alloyed electrocatalyst particles; supplying carbon monoxide to the precursor particles under conditions which remove at least some of the metal M from the surface of the precursor particles; wherein the de-alloyed electrocatalyst particles are particles of a platinum group metal alloy PMX in which P is a platinum group metal and M is at least one alloying metal, wherein the total atomic composition relative to P of M at the surface of the de-alloyed electrocatalyst is lower than the total atomic composition relative to P of M in the bulk of the de-alloyed electrocatalyst, and wherein x is less than n.

Description

De-alloyed Electrocatalyst

Field of the Invention

The present disclosure relates to de-alloyed electrocatalysts and methods for their preparation. In particular, the electrocatalysts may be suitable for use in a fuel cell or electrolyser and the method is a gas phase method.

Background of the Invention

A fuel cell is an electrochemical cell comprising two electrodes separated by an electrolyte. A fuel, e.g. hydrogen, an alcohol 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, and the chemical energy of the fuel and the oxidant is converted to electrical energy and heat. Electrocatalysts are used to promote the electrochemical oxidation of the fuel at the anode and the electrochemical reduction of oxygen at the cathode.

Fuel cells are usually classified according to the nature of the electrolyte employed. Often the electrolyte is a solid polymeric membrane, in which the membrane is electronically insulating but ionically conducting. In the proton exchange membrane fuel cell (PEMFC) the ion-conducting membrane is proton conducting, and protons, produced at the anode, are transported across the ion-conducting membrane to the cathode, where they combine with oxygen to form water.

A principal component of the PEMFC is the membrane electrode assembly, which is essentially composed of five layers. The central layer is the polymer ion-conducting membrane. On either face of the ion-conducting membrane there is an electrocatalyst layer, containing an electrocatalyst designed for the specific electrolytic reaction. Finally, adjacent to each electrocatalyst layer there is a gas diffusion layer. The gas diffusion layer must allow the reactants to reach the electrocatalyst layer and must conduct the electric current that is generated by the electrochemical reactions. Therefore, the gas diffusion layer must be porous and electrically conducting.

The electrocatalyst layers generally comprise an electrocatalyst material comprising a metal or metal alloy suitable for the fuel oxidation or oxygen reduction reaction, depending on whether the layer is to be used at the anode or cathode. Electrocatalysts for fuel oxidation and oxygen reduction are typically based on platinum or platinum alloyed with one or more other metals. It is known to increase the activity of platinum alloy electrocatalysts by a process of de-alloying in which alloying metal is selectively removed from the surface of the electrocatalyst to provide a platinum rich surface. This is conventionally caried out by an acidleaching process. It is desirable to provide processes for preparing de-alloyed electrocatalysts which are safe and reliable, produce a consistently high performing electrocatalyst, and are amenable to industrial application.

Summary of the Invention

The present inventors advantageously found that solid, non-porous, de-alloyed electrocatalyst particles could be provided by using gaseous carbon monoxide in the dealloying step. This has the benefit of not requiring acidic solutions which are difficult to manage in an industrial environment. Surprisingly, the electrocatalyst particles prepared by this process have a higher mass activity than those prepared by conventional, acid-based, leaching methods. Additionally, more uniform de-alloying of a powder sample of particles can be achieved as compared with liquid-acid based methods due to the gas being able to more readily access all of the particles in the powder.

Accordingly, the present disclosure provides a process for preparing solid, non- porous, de-alloyed electrocatalyst particles, the process comprising the steps of: providing solid, non-porous, platinum group metal alloy precursor particles PM_n in which P is a platinum group metal and M is at least one alloying metal; in a de-alloying step to provide the solid, non-porous, de-alloyed electrocatalyst particles; supplying carbon monoxide to the precursor particles under conditions which remove at least some of the metal M from the surface of the precursor particles; wherein the de-alloyed electrocatalyst particles are particles of a platinum group metal alloy PM_X in which P is a platinum group metal and M is at least one alloying metal, wherein the total atomic composition relative to P of M at the surface of the de-alloyed electrocatalyst is lower than the total atomic composition relative to P of M in the bulk of the de-alloyed electrocatalyst, and wherein x is less than n.

The disclosure also provides de-alloyed electrocatalyst particles obtainable by the process of the disclosure. In particular, the electrocatalyst particles are oxygen reduction reaction electrocatalyst particles. The disclosure also provides a catalyst layer comprising the electrocatalyst particles. Additionally, the disclosure provides a gas diffusion electrode comprising the catalyst layer and a gas diffusion layer, as well as a catalyst-coated membrane for a fuel cell or a water electrolyser comprising an ion-conducting membrane with a cathode catalyst layer applied to a first face of the membrane and I or an anode catalyst layer applied to a second face of the membrane, wherein the cathode catalyst layer is a catalyst layer according to the disclosure. Also provided is a membrane-electrode assembly for a fuel cell or a water electrolyser comprising a catalyst-coated membrane according to the disclosure and at least one of a gas diffusion layer or a porous transport layer, or comprising a gas diffusion electrode according to the disclosure and an ion-conducting membrane. Additionally, a water electrolyser or a fuel cell comprising a catalyst-coated membrane according to the disclosure or a membraneelectrode assembly according to the disclosure.

Brief Description of the Drawings

Figure 1 is a transmission electron microscope (TEM) linescan of a de-alloyed electrocatalyst prepared by a process of the invention.

Figure 2 is a TEM linescan of another de-alloyed electrocatalyst prepared by a process of the invention.

Figure 3 is a chart providing mass activity data for electrocatalysts prepared by processes of the invention, along with comparative electrocatalysts.

Detailed Description

The de-alloyed electrocatalyst particles prepared in the present disclosure are solid and non-porous. That is to say, they are not nanoframes or hollow nanostructures and they do not comprise networks of pores which interconnect throughout the catalyst particles. The de-alloyed electrocatalyst particles comprise a solid volume which extends from the centre of the particles to a radial point at which at least some metal M has been removed. Whether or not an electrocatalyst particle is solid and non-porous can be confirmed, for example, by analysing linescan transmission electron microscope data obtained by a process such as the protocol discussed herein. When looking, for example, at a PtNi de-alloyed electrocatalyst particle, the fact that the particle is solid and non-porous is confirmed by there being an unbroken metal trace across the particle. If the particle is, for example, a nanoframe with networks of pores which interconnect throughout the particles then there will not be an unbroken metal trace across the particle. The precursor particles and the de-alloyed electrocatalyst particles may suitably have a particle size of less than or equal to about 15 nm, which is the number average particle size as determined by transmission electron microscope analysis, suitably less than or equal to about 10 nm, for example less than or equal to about 5 nm. The precursor particles and the de-alloyed electrocatalyst particles may suitably have a particle size of at least about 2 nm. Electrocatalyst particles within such particle size ranges are particularly suitable as electrocatalysts for the oxygen evolution reaction, and are typically selective for such a reaction when other reactions such as the hydrogen evolution reaction may occur.

P is a single platinum group metal, i.e. ruthenium, rhodium, palladium, osmium, iridium or platinum. Suitably, P is selected from ruthenium, rhodium, palladium and platinum, typically platinum and palladium. Most suitably, P is platinum.

M is suitably at least one base metal, typically no more than two base metals (i.e. ternary and binary alloys), most typically it is one base metal (i.e. a binary alloy). A base metal is tin or a transition metal which is not a noble metal. Suitable base metals include copper, cobalt, nickel, zinc, iron, titanium, molybdenum, vanadium, manganese, niobium, tantalum, chromium and tin. More suitably, the base metal is selected from nickel, copper, cobalt, and chromium. Most suitably, the base metal is nickel.

The de-alloying process reduces the amount of metal M in the particle. The metal M is removed from the surface of the particle to a radial point at which the solid volume begins. Accordingly x is less than n. The value of n is suitably greater than or equal to 1 , more suitably greater than or equal to 1.5, typically greater than or equal to 2.0. The value of n is typically less than or equal to 5. The value of x is suitably less than 1 , more suitably less than or equal to 0.7, more suitably less than or equal to 0.5. The value of x may suitably be greater than or equal to 0.1. That the de-alloyed particles have a total atomic composition relative to P of M at the surface of the de-alloyed electrocatalyst which is lower than the total atomic composition relative to P of M in the bulk of the de-alloyed electrocatalyst can be confirmed, for example, by analysing linescan transmission electron microscope data obtained by a process such as the protocol discussed herein. The “bulk” means the entire electrocatalyst particle. When looking, for example, at a PtNi alloy electrocatalyst particle, the fact that the particle is such a de-alloyed particle is confirmed by the trace for nickel starting to rise in counts after the trace for platinum starts to rise in counts, and/or the traces rising at different rates, as the traces continue across the particles. This can be seen in Figs. 1 and 2, including the circled part of Fig. 2. The de-alloyed electrocatalyst particles PtM_x may have a total atomic composition relative to P of M at the surface of the de-alloyed electrocatalyst which is between 20 to 99 %, suitably 30 to 99 %, lower than the total atomic composition relative to P of M in the bulk of the de-alloyed electrocatalyst. By “20 to 99 % lower” it is meant that the value obtained by the calculation:

(M atom%)bulk - M( atom%)surface x 100

(M atom%)bulk is between 20 and 99 inclusive. The bulk atomic percentage and can be confirmed by ICP- MS. The surface atomic percentage can be determined using x-ray photoelectron spectroscopy (XPS). The surface atomic percentage can also be confirmed using linescan data as obtained by the procedure set out in the examples and shown, for example, in Fig. 2. The ratio of counts for P and M can be converted to an atomic ratio by converting the counts using the scattering parameter of the metal in question. For example, such a ratio can be taken from the counts corresponding to about 0.8 nanometres depth of the particle, which corresponds with three atomic layers. This corresponds to the value of nanometres on the x- axis which is 0.8 nm from where the P trace begins to rise in the linescan plot.

The precursor particles, and thus also the de-alloyed electrocatalyst, may be supported or unsupported, typically supported, typically on an electrically conductive support. Supported takes its conventional definition in the art. For example, it will be understood that the term “supported” includes the electrocatalyst being dispersed on the support material and bound or fixed to the support material by physical or chemical bonds. For instance, the electrocatalyst may be bound or fixed to the support material by way of ionic or covalent bonds, or non-specific interactions such as van der Waals forces. The support itself is typically in a particulate form. The support may be an electrically conductive carbon support material. Suitably, the electrically conductive carbon support material is a carbon powder which may be, for example, a carbon black or graphitised carbon black for example a commercially available carbon black (such as from Cabot Corp. (Vulcan® XC72R) or Akzo Nobel (the Ketjen® black series)). Another suitable carbon support material is an acetylene black (e.g. those available from Chevron Phillips (Shawinigan Black®) or Denka).

The precursor particles can be prepared by any conventional technique. For example using a method analogous to the general method of preparation of carbon supported catalysts described in WO2013/045894. For example, by firstly preparing a dispersion of a preformed platinum group metal catalyst, e.g. Pt/C, in a suitable solvent, e.g. water, and to this adding a salt of the metal M dissolved in a suitable solvent, e.g. water, with a base and with appropriate mixing. Once deposition of the metal M onto the Pt/C catalyst is complete, the formed material is isolated, e.g. by filtration, died, and annealed.

The de-alloying step is carried out by supplying carbon monoxide to the precursor particles under conditions which remove at least some of the alloying metal M from the surface of the precursor particles. As will be evident to a skilled person the process is akin to the Mond process in which nickel is extracted by exposure to carbon monoxide which acts as both a reducing agent and a complexing agent, removing the nickel as a complex such as Ni(CO)4. In the present process, carbon monoxide is typically supplied in gaseous form. Inert gases such as nitrogen may also be present, but gas to which the precursor particles are exposed will be predominantly carbon monoxide, typically greater than about 99% by total volume, for example about 100% by total volume carbon monoxide. The temperature at which the precursor particles are exposed to carbon monoxide will typically be elevated, i.e. greater than ambient temperature (about 25°C). This can be achieved, for example, by exposing the precursor particles to carbon monoxide in a furnace, such as an electrically heated furnace, by flowing carbon monoxide through the furnace. The precursor particles can suitably be exposed to carbon monoxide at a temperature of at least about 100°C, suitably at least about 150°C. The precursor particles can suitably be exposed to carbon monoxide at a temperature of at most about 300°C, suitably at most about 250°C. Such a range is advantageous in terms of balancing effectiveness of the de-alloying step, with not damaging the catalyst. The time for which the precursor particles are exposed to carbon monoxide will depend on the nature and amount of the particles. A skilled person can determine when the exposure should end, for example, by monitoring nickel carbonyl content in the gaseous output which can be measured, for example, by chemical absorption and nickel analysis by ICP-OES. Generally, a suitable time for exposure to carbon monoxide is less than our equal to about 24 hours, suitably less than or equal to about 12 hours, for example less than or equal to about 5 hours. A suitable time is typically at least about 0.25 hours. The process can be carried out at ambient pressure, but elevated pressure can enable lower temperatures to be used as well as quicker process times. The precursor particles may be exposed to carbon monoxide at a pressure of less than or equal to about 50 bara, suitably less than or equal to about 30 bara, suitably less than or equal to about 15 bara, for example less than or equal to about 5 bara. The precursor particles may typically be exposed to carbon monoxide at a pressure of greater than or equal to about 1 bara. The carbon monoxide may typically be supplied at a gas hourly space velocity (GHSV) of about 5000 Nl/l/hr or less, suitably about 3000 Nl/l/hr or less, suitably about 2000 Nl/l/hr or less. The carbon monoxide may typically be supplied at a GHSV of at least about 500 Nl/l/hr. The de-alloying step can additionally comprise a separate step of contacting the precursor particles with an acidic solution to remove some of the alloying metal from the surface of the precursor particles, either before or after supplying carbon monoxide, typically after. However, the de-alloying step suitably may not comprise a step of contacting the precursor particles with an acidic solution.

Carbon monoxide may be supplied to the precursor particles in a closed-loop process in which the metal M may be removed from the process by chemical vapour transport. The closed-loop process may comprise monitoring transport of volatile metal carbonyl species such that the species decompose and metal M is deposited at a decomposition site remote from the precursor particles. Put another way, the deposition rate and total amount of pure nickel deposited at the decomposition zone may be monitored by at least one of various well- known methods to determine de-alloying process completion, for example, by chemical absorption and nickel analysis by ICP-OES. At such a decomposition site, the metal is sequestered from the metal complex, e.g. Ni(CO)4, such that it can be effectively transported away from the process whilst the carbon monoxide can re-enter the process for maximum efficiency. The metal may be sequestered by application of any suitable conditions, for example decreased carbon monoxide pressure, addition of oxygen, or heat treatment. The ideal decomposition temperature is below the temperature the Boudouard reaction occurs. For example, the volatile metal carbonyl species is exposed to a temperature of at least about 200°C, suitably at least about 250°C at the decomposition site, suitably no more than about 450°C. When not at the decomposition site, the gas is suitably maintained at a temperature of less than about 200°C, suitably less than about 180°C.

The de-alloyed electrocatalyst particles prepared by the process are suitable for use as an electrocatalyst in a fuel cell or electrolyser, typically proton exchange membrane fuel cell or electrolyser. For example, the electrocatalyst may be an oxygen reduction reaction catalyst which typically is used in a proton exchange membrane fuel cell cathode catalyst layer. The present disclosure accordingly also provides a de-alloyed electrocatalyst, suitably a de-alloyed oxygen evolution reaction electrocatalyst obtainable by, suitably obtained by, the process disclosed herein.

The de-alloyed electrocatalyst prepared by the present process may be incorporated into a catalyst-coated membrane for a fuel cell or a water electrolyser which comprises an ion-conducting membrane with a cathode catalyst layer applied to a first face of the membrane and I or an anode catalyst layer applied to a second face of the membrane, typically for a proton exchange membrane fuel cell in which the cathode catalyst layer comprises a dealloyed electrocatalyst obtainable by, suitable obtained by, the process disclosed herein. To prepare the catalyst layer, the electrocatalyst and any additional components are dispersed in an aqueous and/or organic solvent to prepare a catalyst ink. If required, particle break-up is carried out by methods known in the art, such as high shear mixing, milling, ball milling, passing through a microfluidiser etc. or a combination thereof, to achieve a suitable particle size distribution. After preparation of the catalyst ink, the ink is deposited onto a substrate (e.g. gas diffusion layer, ion-conducting membrane or a carrier/transfer substrate) to form the catalyst layer. The ink may be deposited by any suitable technique known to those in the art, including but not limited to gravure coating, slot die (slot, extrusion) coating, screen printing, rotary screen printing, inkjet printing, spraying, painting, bar coating, pad coating, gap coating techniques such as knife or doctor blade over roll, and metering rod application. In the catalyst- coated membrane, the catalyst layer is deposited onto an ion-conducting membrane either by direct coating of a catalyst ink onto the membrane, or indirectly by transfer from a decal transfer substrate, to form a catalyst-coated membrane. The catalyst-coated membrane of the invention may comprise a second catalyst layer on its opposite face, which may be in accordance with the invention or otherwise. The ion-conducting membrane may suitably be any membrane suitable for use in a proton exchange membrane fuel cell, for example the membrane may be based on a perfluorinated sulphonic acid material such as Nation™ (Chemours Company), Aquivion® (Solvay Specialty Polymers), Flemion® (Asahi Glass Group) and Aciplex™ (Asahi Kasei Chemicals Corp.) and perfluorosulphonic acid ionomer material supplied by 3M®. Alternatively, the membrane may be based on a sulphonated hydrocarbon membrane such as those available from FuMA-Tech GmbH as the fumapem® P, E or K series of products, or from JSR Corporation, Toyobo Corporation, and others.

The catalyst layer may be deposited onto a gas diffusion layer to form a gas diffusion electrode of the disclosure which comprises a catalyst layer of the disclosure and a gas diffusion layer. The gas diffusion layer comprises a gas diffusion substrate and, preferably, a microporous layer. When a microporous layer is present, the catalyst layer is deposited onto the microporous layer. Typical gas diffusion substrates include non-woven papers or webs comprising a network of carbon fibres and a thermoset resin binder (e.g. the TGP-H series of carbon fibre paper available from Toray Industries Inc., Japan or the H2315 series available from Freudenberg FCCT KG, Germany, or the Sigracet® series available from SGL Technologies GmbH, Germany or AvCarb® series from Ballard Power Systems Inc.), orwoven carbon cloths. The carbon paper, web or cloth may be provided with a pre-treatment prior to fabrication of the electrode and being incorporated into a membrane electrode assembly either to make it more wettable (hydrophilic) or more wet-proofed (hydrophobic). The nature of any treatments will depend on the type of fuel cell or electrolyser and the operating conditions that will be used. The substrate can be made more wettable by incorporation of materials such as amorphous carbon blacks via impregnation from liquid suspensions, or can be 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. Typical microporous layers comprise a mixture of a carbon black and a polymer such as polytetrafluoroethylene (PTFE).

Also provided is a membrane-electrode assembly for a fuel cell or a water electrolyser comprising a catalyst-coated membrane according to the disclosure; and at least one of a gas diffusion layer or a porous transport layer, or comprising a gas diffusion electrode according to the disclosure and an ion-conducting membrane. Also provided is a water electrolyser or a fuel cell, suitably a proton exchange membrane fuel cell or electrolyser comprising a catalyst- coated membrane disclosed herein or a membrane-electrode assembly disclosed herein.

Examples

Comparative Examples

Comparative Example 1 was prepared using a method analogous to the general method of preparation of carbon supported catalysts described in WO2013/045894 and then annealed.

To prepare Comparative Examples 2 and 3, 1 kg of a particulate carbon black supported platinum (30w% Pt/C, 300 g Pt, 1.538 moles) catalyst material precursor was prepared using a method analogous to the general method of preparation of carbon supported catalysts described in WO2013/045894. This precursor was dispersed in water 75.7 L in a reaction vessel. Solid NaHCCh (577g, 6.869 moles) was added. Subsequently, a 6.1 L solution of nickel nitrate (907.1 g Ni(NOs)2.6H2O, 183 g Ni, 3.11 moles) in demineralised water was added. When deposition of the nickel was complete the catalyst was recovered by filtration and washed on the filter bed with demineralised water until free of soluble ions. The material was dried and annealed.

942g of the alloyed PtNi₂/C was slurried in 18 L 0.5 M H2SO4 and heated to 80°C for 24 h before being filtered and washed on the filter bed with demineralised water until free of soluble ions before being dried to provide 822 g of Comparative Example 2.

Subsequently, 260g of Comparative Example 2 was re-slurried in a mixture of 4 L 0.625 M H2SO4 and 1 L n-propanol and heated to 80°C for 24 h before being filtered and washed on the filter bed with demineralised water until free of soluble ions before being dried to provide 251 g of Comparative Example 3.

Examples of the Invention

To prepare Example 1 , 11.9g of Comparative Example 2 was added to a %” stainless steel reactor. The reactor was placed in an electrically heated furnace with temperatures measuring; furnace heaters, reactor wall and process. Reactor was connected to a gas feed line that could supply; hydrogen, nitrogen and carbon monoxide by mass flow controllers. Pressure was controlled by a gas pressure regulator that would be set manually. Nitrogen was introduced at a GHSV of 2000 Nl/l/hr at ambient pressure to sweep contents of oxygen. Hydrogen was then introduced at a GHSV of 2000 Nl/l/hr and the reactor heated to 180°C and held for 30 minutes. The reactor was then swept with nitrogen and the pressure increased to 30 barg and the process temperature increased to 160°C. Nitrogen flow was stopped and then carbon monoxide was introduced at a GHSV of 3000 Nl/l/hr. Once carbon monoxide was detected at the reactor exit the test was started and contacted with carbon monoxide for 105 minutes. The vent gas from the reactor was passed through an absorption train to extract the nickel carbonyl formed in the reactor and perform nickel analysis by ICP-OES. Nickel analysis performed on the absorption train liquids was measured at 102000 micrograms/litre. After the 105 minutes the carbon monoxide flow was stopped, and nitrogen introduced into the reactor at a GHSV of 2000 Nl/l/hr. The reactor was cooled to ambient temperature and the pressure reduced to ambient. The product was then discharged onto carbon dioxide and collected after the carbon dioxide had evaporated.

To prepare Example 2, 12.71g of a PtNi2/C material prepared by a procedure analogous to the procedure used to prepare Comparative Example 1 and was added to a %” stainless steel reactor. The reactor was placed in an electrically heated furnace with temperatures measuring, furnace heaters, reactor wall and process. Reactor was connected to a gas feed line that could supply; hydrogen, nitrogen and carbon monoxide by mass flow controllers. Pressure was controlled by a gas regulator that would be set manually. Nitrogen was introduced at a GHSV of 2000 Nl/l/hr at ambient pressure to sweep contents of oxygen. Hydrogen was then introduced at a GHSV of 2000 Nl/l/hr and the reactor heated to 180°C and held for 30 minutes. The reactor was then swept with nitrogen and the pressure increased to 30 barg and the process temperature increased to 214°C. Nitrogen flow was stopped and then carbon monoxide was introduced at a GHSV of 1000 Nl/l/hr. Once carbon monoxide was detected at the reactor exit the test was started and contacted with carbon monoxide for 199 minutes. The vent gas from the reactor was passed through an absorption train to extract the nickel carbonyl formed in the reactor and perform nickel analysis by ICP-OES. Nickel analysis performed on the absorption train liquids was measured at 934000 micrograms/litre. After the 199 minutes the carbon monoxide flow was stopped and nitrogen introduced into the reactor at a GHsV of 2000 NL/l/hr. The reactor was cooled to ambient temperature and the pressure reduced to ambient. The product was then discharged onto carbon dioxide and collected after the carbon dioxide had evaporated.

Transmission Electron Microscope (TEM) analysis

To obtain TEM data, samples were ground between two glass slides and dusted onto a holey carbon coated Cu TEM grid. The samples were examined in the JEM 2800 (Scanning) Transmission Electron Microscope using the following instrumental conditions: Voltage (kV) 200; C2 aperture (urn) 70 and 40; Dark-field (Z-contrast) imaging in scanning mode using an off-axis annular detector. The SE signal was acquired simultaneously with the other STEM images providing topological information of the sample. Compositional analysis by X-ray emission detection in the scanning mode.

Figs. 1 and 2 show both that the de-alloyed electrocatalyst particles prepared above are solid, and that the surface predominantly comprises platinum. The blue trace shows the concentration of platinum as a scan is carried out across the three particles shown in the TEM image. The red trace shows the concentration of nickel as a scan is carried out across the three particles shown in the TEM image. The fact that the surface comprises predominantly platinum is evident from the trace for nickel starting to rise in counts after the trace for platinum starts to rise in counts and the traces rising at different rates as the trace continues across the particles. The fact that the electrocatalyst particles are solid and not, for example, nanoframes, is evident from the unbroken traces for both metals.

Mass Activity

Catalyst-coated membranes (CCMs) of 50 cm² active area were prepared by depositing anode and cathode catalyst layers onto a PTFE sheet and transferring the appropriate layers to either side of a perfluorosulphonic acid (PFSA) reinforced membrane (15 pm thickness) at a temperature of between 150°C to 200°C. A gas diffusion layer was applied to each face of each CCM to form complete membrane electrode assemblies (MEAs). The gas diffusion layer used was a carbon fibre paper with a hydrophobic microporous layer containing carbon and PTFE applied to the face in contact with the CCM.

To measure electrocatalyst mass activity, the MEAs were conditioned by the application of a constant current density of 500 mA cm^-2 under H2/Air at 170 kPa_gauge-iniet, 100% RH, and 80 °C. The cell voltage was monitored until a stable value was observed. Afterward, the cathode catalyst layer was exposed to a series of cathode starvation steps followed by a 2 h current hold at 500 mA cm^-2 until a stable voltage was observed.

The catalyst kinetic mass activity was measured on the 50 cm² MEA with pure hydrogen and oxygen as the anode and cathode reactants respectively at 80°C, under fully humidified and pressurised anode and cathode (100%RH, 50 kPa_gauge-iniet) conditions. An oxygen polarization curve of current density holds from 0.05-0.5 A cm^-2 was performed, with the current density maintained for 3 min at each step, and the voltage recorded at the end of the hold. The electrocatalyst mass activity value at 0.9 V was obtained from the ascending polarization curve by extrapolation of the resistance-corrected (iR-corrected) current and normalising by the mass of platinum in the cathode catalyst layer. Fig. 3 illustrates that the electrocatalyst particles of Example 2 have far higher mass activity than Comparative Example 1. The electrocatalyst particles also have higher activity than Comparative Example 2 in which the de-alloyed electrocatalyst particles were prepared using a conventional acid leaching technique.

Claims

Claims:

1. A process for preparing solid, non-porous, de-alloyed electrocatalyst particles, the process comprising the steps of: providing solid, non-porous, platinum group metal alloy precursor particles PM_n in which P is a platinum group metal and M is at least one alloying metal; in a de-alloying step to provide the solid, non-porous, de-alloyed electrocatalyst particles; supplying carbon monoxide to the precursor particles under conditions which remove at least some of the metal M from the surface of the precursor particles; wherein the de-alloyed electrocatalyst particles are particles of a platinum group metal alloy PM_X in which P is a platinum group metal and M is at least one alloying metal, wherein the total atomic composition relative to P of M at the surface of the de-alloyed electrocatalyst is lower than the total atomic composition relative to P of M in the bulk of the de-alloyed electrocatalyst, and wherein x is less than n.

2. A process according to claim 1 , wherein P is platinum.

3. A process according to claim 1, wherein M is a single alloying metal, preferably nickel.

4. A process according to any preceding claim, wherein the de-alloyed electrocatalyst particles are supported.

5. A process according to any preceding claim, wherein the de-alloyed electrocatalyst particles have a particle size of less than or equal to about 15 nm.

6. A process according to any preceding claim, wherein the carbon monoxide is supplied in a closed-loop process.

7. A process according to claim 6, comprising monitoring transport of volatile metal carbonyl species such that the species decompose and metal M is deposited at a decomposition site remote from the precursor particles.

8. De-alloyed electrocatalyst particles obtainable by a process according to any preceding claim.

9. De-alloyed electrocatalyst particles according to claim 8, which are oxygen reduction reaction electrocatalyst particles.

10. A catalyst layer comprising electrocatalyst particles according to claim 8 or claim 9.

11. A gas diffusion electrode comprising a catalyst layer according to claim 10 and a gas diffusion layer.

12. A catalyst-coated membrane for a fuel cell or a water electrolyser comprising an ionconducting membrane with a cathode catalyst layer applied to a first face of the membrane and I or an anode catalyst layer applied to a second face of the membrane, wherein the cathode catalyst layer is a catalyst layer according to claim 10.

13. A membrane-electrode assembly for a fuel cell or a water electrolyser comprising a catalyst-coated membrane according to claim 12 and at least one of a gas diffusion layer or a porous transport layer, or comprising a gas diffusion electrode according to claim 10 and an ion-conducting membrane.

14. A water electrolyser or a fuel cell comprising a catalyst-coated membrane according to claim 12 or a membrane-electrode assembly according to claim 13.