Catalyst-coated ion-conducting membrane

Abstract

According to the invention there is provided a catalyst-coated ion-conducting membrane comprising an anode catalyst layer, a cathode catalyst layer, and an ion-conducting membrane layer disposed between the anode catalyst layer and the cathode catalyst layer. The anode catalyst layer comprises an anode electrocatalyst and an ion-conducting polymer. The anode electrocatalyst comprises particles of a platinum group metal or a platinum group metal alloy. The anode electrocatalyst is present in the anode catalyst layer at a loading of less than 0.20 mg of the platinum group metal per cm2 of the anode catalyst layer. The cathode catalyst layer comprises an ion-conducting polymer and a cathode electrocatalyst comprising platinum-containing particles and a carbon-based support. The carbon-based support comprises individual primary particles or an aggregate of primary particles, the primary particles comprising pores. Some of the platinum-containing particles are located within the pores and some of the platinum-containing particles are located on an external surface of the carbon-based support. The platinum-containing particles on the external surface of the carbon-based support have a mean average particle size of ≤3.0 nm. Less than 50% of the surface area of the platinum-containing particles is in contact with the ion-conducting polymer of the cathode catalyst layer. There is also provided a method of manufacturing a catalyst- coated ion-conducting membrane and a fuel cell comprising the ion-conducting membrane.

Description

[0001] Catalyst-coated ion-conducting membrane

[0002] Field of the Invention

[0003] This invention relates to a catalyst-coated ion-conducting membrane for an electrochemical device such as a proton exchange membrane (PEM) fuel cell or PEM electrolyser. The invention also relates to an associated electrochemical device comprising the catalyst-coated ion-conducting membrane; and to associated methods of producing the catalyst-coated ion-conducting membrane.

[0004] Background of the Invention

[0005] 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.

[0006] 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.

[0007] 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.

[0008] The electrocatalyst layers also generally comprise a proton conducting material, such as a proton conducting polymer, to aid transfer of protons from the anode electrocatalyst to the ion-conducting membrane and / or from the ion-conducting membrane to the cathode electrocatalyst.

[0009] Conventionally, the membrane electrode assembly can be constructed by a number of methods. Typically, the methods involve the application of one or both of the electrocatalyst layers to an ion-conducting membrane to form a catalyst coated ion-conducting membrane. Subsequently, a gas diffusion layer is applied to the electrocatalyst layer. Alternatively, an electrocatalyst layer is applied to a gas diffusion layer to form a gas diffusion electrode, which is then combined with the ion-conducting membrane. A membrane electrode assembly can be prepared by a combination of these methods e.g. one electrocatalyst layer is applied to the ion-conducting membrane to form a catalyst coated ion-conducting membrane, and the other electrocatalyst layer is applied as a gas diffusion electrode. The electrocatalyst layers are applied using an electrocatalyst ink which conventionally comprises an electrocatalyst material, an ion-conducting polymer, solvents and / or diluents, and any agents desired to be included in the electrocatalyst layer.

[0010] 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. The platinum or platinum alloy electrocatalyst can be in the form of unsupported nanometre sized particles (for example metal blacks) or can be deposited as discrete nanoparticles onto a support material (a supported electrocatalyst). Electrocatalysts can also be in the form of coatings or extended films deposited onto a support material.

[0011] PEMFCs have a number of applications, including automotive. In particular, PEMFCs can be beneficial for use in heavy-duty, long life automotive application such as trucks. Here, there is a desire to reduce cost of ownership over the long life required of the PEMFC which, in some circumstances, is more desirable than lowering upfront cost.

[0012] Summary of the Invention

[0013] The present invention seeks to address the above problems, desires and needs. In particular, the present inventors have found that providing a supported cathode electrocatalyst comprising a relatively small particle size wherein a majority of the platinum-containing particles are located within the pores of the catalyst support material can enable both high performance and good durability. As such, the cathode catalyst layer can comprise a lower metal loading (e.g. lower platinum loading) while still achieving acceptable performance and durability, thereby reducing cost.

[0014] Accordingly, in a first aspect there is provided a catalyst-coated ion-conducting membrane comprising an anode catalyst layer, a cathode catalyst layer, and an ionconducting membrane layer disposed between the anode catalyst layer and the cathode catalyst layer, wherein: the anode catalyst layer comprises an anode electrocatalyst and an ion-conducting polymer, wherein the anode electrocatalyst comprises a platinum group metal, and the anode electrocatalyst is present in the anode catalyst layer at a loading of less than 0.20 mg of the platinum group metal per cm2of the anode catalyst layer; and the cathode catalyst layer comprises an ion-conducting polymer and a cathode electrocatalyst comprising platinum-containing particles and a carbon-based support, wherein the carbon-based support comprises individual primary particles or an aggregate of primary particles, the primary particles comprising pores, wherein some of the platinum- containing particles are located within the pores and some of the platinum-containing particles are located on an external surface of the carbon-based support; wherein the platinum-containing particles on the external surface of the carbon-based support have a mean average particle size of <3.0 nm; and wherein less than 50% of the surface area of the platinum-containing particles is in contact with the ion-conducting polymer of the cathode catalyst layer.

[0015] According to a second aspect there is provided a method of manufacturing a catalyst- coated ion-conducting membrane, the method comprising the steps of: preparing a cathode electrocatalyst comprising platinum-containing particles supported on a carbon-based support, wherein the carbon-based support comprises individual primary particles or an aggregate of primary particles, the primary particles comprising pores, wherein some of the platinum-containing particles are located within the pores and some of the platinum-containing particles located on an external surface of the carbon-based support, the platinum-containing particles on the external surface of the carbon-based support have a mean average particle size of <3.0 nm; forming a cathode catalyst ink comprising the cathode electrocatalyst, an ionconducting polymer and a solvent, forming a layer of the cathode catalyst ink on a first side of an ion-conducting membrane; and drying the layer of cathode catalyst ink to form a cathode catalyst layer in which less than 50% of the surface area of the platinum-containing particles is in contact with the ionconducting polymer of the cathode catalyst layer; wherein the temperature of the cathode electrocatalyst during any step of the method of manufacturing the catalyst-coated ion-conducting membrane does not exceed 300 °C.

[0016] The method can be a method of manufacturing a catalyst-coated ion-conducting membrane according to the first aspect.

[0017] According to a further aspect, there is provided a catalyst-coated ion-conducting membrane comprising an anode catalyst layer, a cathode catalyst layer, and an ionconducting membrane layer disposed between the anode catalyst layer and the cathode catalyst layer, wherein: the anode catalyst layer comprises an anode electrocatalyst and an ion-conducting polymer, wherein the anode electrocatalyst comprises a platinum group metal, and the anode electrocatalyst is present in the anode catalyst layer at a loading of less than 0.20 mg of the platinum group metal per cm2of the anode catalyst layer; and the cathode catalyst layer comprises an ion-conducting polymer and a cathode electrocatalyst comprising platinum-containing particles and a carbon-based support, wherein the cathode catalyst layer comprises pores, and the pores of the cathode catalyst layer that have a pore diameter of <50 nm have a pore volume of at least 50%, preferably at least 55% and more preferably at least 60%, of the total pore volume of the pores of the cathode catalyst layer, as measured by mercury intrusion porosimetry.

[0018] Preferably, the carbon-based support comprises individual primary particles or an aggregate of primary particles, wherein the primary particles comprise pores. Preferably, some of the platinum-containing particles are located within the pores and some of the platinum-containing particles are located on an external surface of the carbon-based support. Preferably, the platinum-containing particles on the external surface of the carbonbased support have a mean average particle size of <3.0 nm. Preferably, less than 50% of the surface area of the platinum-containing particles is in contact with the ion-conducting polymer of the cathode catalyst layer.

[0019] Brief Description of the Drawings

[0020] Figure 1 is a graph showing cell voltage as a function of cycle number at a current density of 0.1 A / cm2; and

[0021] Figure 2 is a graph showing cell voltage as a function of cycle number at a current density of 1.0 A / cm2.

[0022] Detailed Description of the Invention

[0023] Preferred and / or optional features of the invention will now be set out. Any aspect of the invention may be combined with any other aspect of the invention, unless the context demands otherwise. Any of the preferred or optional features of any aspect may be combined, singly or in combination, with any aspect of the invention, unless the context demands otherwise.

[0024] The invention provides a catalyst-coated ion-conducting membrane for an electrochemical device, such as a fuel cell and preferably a proton exchange membrane fuel cell. Preferably, the catalyst-coated ion-conducting membrane is a catalyst-coated protonconducting membrane. The catalyst-coated ion-conducting membrane comprises an anode catalyst layer, a cathode catalyst layer, and an ion-conducting membrane layer disposed between the anode catalyst layer and the cathode catalyst layer. Anode catalyst layer

[0025] The anode catalyst layer comprises an anode electrocatalyst and an ion-conducting polymer. The anode electrocatalyst comprises a platinum group metal. Preferably, the anode electrocatalyst comprises particles of the platinum group metal or an alloy of the platinum group metal, optionally supported on an electrically conductive support. That is, the anode electrocatalyst can be unsupported particles of the platinum group metal or the alloy of the platinum group metal (e.g. finely divided unsupported metal powder) or may be a supported electrocatalyst wherein the particles (e.g. nanoparticles) are dispersed on an electrically conductive support, such as an electrically conducting particulate carbon support. The platinum group metals are platinum, iridium, palladium, ruthenium, rhodium or osmium. Most preferably, the anode electrocatalyst is platinum, which may be alloyed with an alloying metal such as other precious metals (e.g. a different platinum group metal) or base metals. A base metal is tin or a transition metal which is not a noble metal. A noble metal is a platinum group metal, silver or gold. Preferred base metals are copper, cobalt, nickel, zinc, iron, titanium, molybdenum, vanadium, manganese, niobium, tantalum, chromium and tin. In some embodiments, the alloying metal can be aluminium, yttrium, scandium, gadolinium, lanthanum, tungsten, zirconium, or hafnium.

[0026] If the anode electrocatalyst is a supported electrocatalyst, the loading of platinum group metal particles on the electrically conductive support material is suitably in the range 10 wt% to 90 wt% or 10 wt% to 80wt%, such as 10wt% to 75wt%, preferably 10 wt% to 60wt%, and most preferably 20 wt% to 40 wt% based on the weight of the electrocatalyst. The loading of the platinum group metal particles can be determined using inductively coupled plasma mass spectrometry (ICPMS).

[0027] Preferably, the anode electrocatalyst comprises an electrically conductive support and particles supported on the electrically conductive support. The term “supported” will be readily understood by a skilled person. For example, it will be understood that the term “supported” includes the particles of the anode electrocatalyst being dispersed on (and / or in the pores of) the support material and bound or fixed to the support material by physical or chemical bonds. For instance, the anode 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.

[0028] The electrically conductive 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 electrically conductive carbon support can be prepared by the method disclosed in WO20 13 / 045894. Alternatively, the electrically conductive support can be a metal oxide or a mixed oxide, in particular a conductive mixed oxide such as niobia-doped titania, phosphorus- doped tin oxide and mixed platinum group metal oxides or mixed metal oxides (as disclosed in WO2012 / 080726), a carbide (e.g. tungsten carbide, molybdenum carbide or titanium carbide, suitably tungsten carbide or titanium carbide), a nitride, in particular a conductive nitride (e.g. titanium nitride or titanium aluminium nitride).

[0029] The anode electrocatalyst is present in the anode catalyst layer in a loading of less than 0.20 mg of the platinum group metal per cm2of the geometric area of the anode catalyst layer (“mgPGM / cm2”). Preferably, the anode electrocatalyst is present in the anode catalyst layer in a loading of 0.10 mgPGM / cm2or less.

[0030] The ion-conducting polymer of the anode catalyst layer is suitably a proton-conducting polymer. Preferred ion-conducting polymers are partially- or fully-fluorinated sulphonic acid polymers e.g. perfluorinated sulphonic acid polymers. For example, the ion-conducting polymer 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.). Alternatively, the ion-conducting materials may be based on a sulphonated hydrocarbon polymer, such as those available from FuMA-Tech GmbH as the fumapem® P, E or K series of products, and others. The ion-conducting polymer in the anode catalyst layer suitably has an equivalent weight in a range of 650 to 900 g / mol, preferably 700 to 850 g / mol, and preferably 700 to 800 g / mol.

[0031] The anode catalyst layer can further comprise an oxygen evolution reaction (OER) catalyst. The OER catalyst catalyses the oxygen evolution reaction. The OER catalyst suitably does not comprise platinum. In some embodiments, the OER catalyst can comprise ruthenium, ruthenium oxide, iridium, iridium oxide, or mixtures thereof. The OER catalyst can further comprise one or more metals M or an oxide of M. For example, the OER catalyst can comprise iridium or iridium oxide and one or more metals M or an oxide thereof. As a further example, the OER catalyst can comprise ruthenium or ruthenium oxide and one or more metals M or an oxide thereof. In some embodiments, the OER catalyst can comprise iridium, ruthenium and one or more metals M (or oxides thereof). M is a transition metal (other than iridium or ruthenium) or tin. M can be a Group 4 metal, such as titanium, zirconium or hafnium. M can be a Group 5 metal, such as vanadium, niobium or tantalum. M can be a Group 6 metal, such as chromium, molybdenum or tungsten. M can be tin. M can be selected from the group consisting of tantalum, titanium, zirconium, hafnium, niobium and tin, preferably tantalum, titanium and tin. The iridium or oxide thereof and the one or more metals (M) or oxide thereof may either exist as mixed metals or oxides or as partly or wholly alloyed materials or as a combination thereof. Preferably, the OER catalyst can be unsupported. Ion-conducting membrane layer

[0032] The ion-conducting membrane layer preferably has a thickness of 15 pm or less, and more preferably 12 pm or less. The thickness of the ion-conducting membrane layer may be measured by scanning electron microscopy (SEM). SEM analysis is carried out on cross sections of the ion-conducting membrane and / or catalyst-coated ion-conducting membrane measured at multiple (for example 10) points. The thickness values are then determined by calculating the arithmetic mean of the measured values. Alternatively, the thickness of the ionconducting membrane layer may be measured using a low force high precision gauge instrument (e.g. VL-50B Litematic™ available from Mitutoyo (UK) Ltd.), which may give a direct reading of the membrane thickness. A motorised spindle is used to take measurement readings with a measuring force of 0.01 N. At least three readings are taken from different locations on the ion-conducting membrane layer (prior to adding catalyst layers) at a temperature of 20 °C ± 3 °C, a relative humidity (RH) of 30-50%.

[0033] The ion-conducting membrane layer suitably comprises an ion-conducting polymer. The ion-conducting polymer is suitably a proton-conducting polymer. Preferred ion-conducting polymers are partially- or fully-fluorinated sulphonic acid polymers e.g. perfluorinated sulphonic acid polymers. For example, the ion-conducting polymer 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.). Alternatively, the ion-conducting materials may be based on a sulphonated hydrocarbon polymer, such as those available from FuMA-Tech GmbH as the fumapem® P, E or K series of products, and others.

[0034] The ion-conducting polymer in the ion-conducting membrane layer suitably has an equivalent weight in a range of 650 to 900 g / mol, preferably 700 to 800 g / mol.

[0035] The ion-conducting membrane layer can comprise at least one reinforcing component, or optionally two reinforcing components. The reinforcing component is a porous material having pores extending through the thickness of the material in the through-plane direction. Suitably, the reinforcing component is a porous polymer material. Suitably, the porous polymer material is a porous fluoropolymer material, such as a microporous web or fibres of a polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), ethylene tetrafluoroethylene (ETFE), perfluoroalkoxy alkane (PFA), or fluorinated ethylene propylene (FEP). For example, the planar reinforcing component may comprise electrospun PVDF or forcespun PVDF. In a preferred embodiment, the porous polymer material is expanded PTFE (ePTFE), for example, such as the microporous web structures of ePTFE supplied by Donaldson Company, Inc., known as Tetratex®, or supplied by other manufacturers. In other preferred embodiments, the reinforcing component can comprise a network of fibres (e.g. nanofibres), such as a network comprising polybenzimidazole (PBI) fibres. The network of fibres can be a non-woven mat of fibres (e.g. nanofibres), such as an electrospun mat of fibres or nanofibres.

[0036] Cathode catalyst layer

[0037] The cathode catalyst layer comprises an ion-conducting polymer and a cathode electrocatalyst. The cathode electrocatalyst comprises a platinum-containing particles (e.g. nanoparticles) and a carbon-based support. The platinum-containing particles are supported on the carbon-based support. The carbon-based support comprises individual primary particles or an aggregate of primary particles. Aggregates are particles that have associated into a cluster composed of two or more primary particles which are permanently bound to each other; the total specific surface area of the aggregate is less than the sum of the surface areas of the primary particles before they were aggregated. In contrast, agglomerates can be readily broken down into the individual support particles or aggregates under imposition of low energy agitation. The primary particles comprise pores. Some (and preferably a majority, i.e. more than half, and more preferably at least 55%) of the platinum-containing particles are located within the pores. Some (and preferably a minority, i.e. less than half, and more preferably less than 45%) of the platinum-containing particles are located on an external surface of the carbon-based support. The proportion of platinum-containing particles located within the pores and those located on the external surface of the carbon-based support can be determined using (scanning) transmission electron microscopy (TEM) by comparing and analysing dark field (Z-contrast) imaging in scanning mode using an off-axis annular detector, and secondary electron images.

[0038] Suitably, the carbon-based support 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-based support is an acetylene black (e.g. those available from Chevron Phillips (Shawinigan Black®) or Denka). Most preferably, the carbon-based support is carbon black.

[0039] The platinum-containing particles can be platinum particles or platinum alloy particles. For example, the platinum-containing particles can be platinum alloyed with at least one an alloying metal such as another precious metal (e.g. a different platinum group metal) or base metals. A base metal is tin or a transition metal which is not a noble metal. A noble metal is a platinum group metal, silver or gold. Preferred base metals are copper, cobalt, nickel, zinc, iron, titanium, molybdenum, vanadium, manganese, niobium, tantalum, chromium and tin. In some embodiments, the alloying metal can be aluminium, yttrium, scandium, gadolinium, lanthanum, tungsten, zirconium, or hafnium. Most preferably, the platinum-containing particles are platinum particles. The platinum-containing particles located on the external surface of the carbon-based support have a mean average particle size of <3.0 nm, preferably <2.8 nm, and preferably <2.6 nm. The mean average particle size of the platinum-containing particles located on the external surface of the carbon-based support can be determined using TEM, and directly measuring the particle size across a representative area of the cathode catalyst layer.

[0040] The total platinum-containing particles (i.e. located within the pores of the primary particle and on an external surface of the carbon-based support) can have a mean average particle size which is similar to (e.g. within 10% of) the mean average particle size of the platinum-containing particles located on the external surface of the carbon-based support. For example, the difference between the mean average particle size of all platinum-containing particles and the mean average particle size of the platinum-containing particles located on an external surface of the carbon-based support can be ±0.5 nm or less, preferably ±0.3 nm or less.

[0041] Less than 50% of the surface area of the platinum-containing particles is in contact with the ion-conducting polymer of the cathode catalyst layer. Preferably, less than 45% and more preferably less than 40%, of the surface area of the platinum-containing particles is in contact with the ion-conducting polymer. Preferably, at least 25%, and more preferably at least 30%, of the surface area of the platinum-containing particles is in contact with the ion-conducting polymer. The amount of surface area in contact with the ion-conducting polymer can be in a range comprising any combination of the aforementioned upper and lower limits. The amount of the surface area in contact with the ion-conducting polymer can be determined using electrochemical methods, for example cyclic voltammetry under wet (100%RH) and dry (30%RH) conditions, using the methods described below. In particular, a ratio ECAdry / ECAwet of an electrochemically active surface area of the cathode electrocatalyst in the cathode catalyst layer at 30 %RH (ECAdry) to an electrochemically active surface area of the cathode electrocatalyst in the cathode catalyst layer at 100 %RH (ECAwet), as measured by cyclic voltammetry, is less than 0.50, preferably less than 0.45. The ECAdry / ECAwet ratio can be at least 0.25 and more preferably 0.30. The ECAdry / ECAwet ratio can be in a range comprising any combination of the aforementioned upper and lower limits, for example, the ECAdry / ECAwet ratio can be in a range of 0.30 to 0.45.

[0042] The cathode catalyst layer can have a loading of the platinum-containing particles of less than 0.55 mg of platinum per cm2of geometric area of the cathode catalyst layer (“mgPt / cm2”). Preferably, the cathode catalyst layer has a loading of the platinum-containing particles of less than 0.50 mgPt / cm2. The cathode catalyst layer can have a loading of the platinum-containing particles of at least 0.25 mg, and preferably at least 0.30 mg, of platinum- containing particle per cm2of geometric area of the cathode catalyst layer. The loading of platinum-containing particles in the cathode catalyst layer can be in a range comprising any combination of the aforementioned upper and lower limits.

[0043] The specific surface area (BET) of the cathode catalyst layer can be at least 200 m2per gram of carbon-based support in the catalyst layer (m2 / gC), preferably at least 225 m2 / gC, and more preferably at least 260 m2 / gC. The specific surface area (BET) of the cathode catalyst layer can be in a range of from 200 m2 / gC to 500 m2 / gC, preferably 225 m2 / gC to 450 m2 / gC, and more preferably 260 m2 / gC to 400 m2 / gC. The specific surface area (BET) of the carbonbased support (prior to forming the cathode electrocatalyst) can be at least 500 m2 / g, preferably at least 700 m2 / g. The determination of the specific surface area by the BET method is carried out by the following process: after degassing to form a clean, solid surface, a nitrogen adsorption isotherm is obtained, whereby the quantity of gas adsorbed is measured as a function of gas pressure, at a constant temperature (usually that of liquid nitrogen at its boiling point at one atmosphere pressure). A plot of 1 / [Va((Po / P)-1)] vs P / Po is then constructed for P / Po values in the range 0.05 to 0.3 (or sometimes as low as 0.2), where Vais the quantity of gas adsorbed at pressure P, and Po is the saturation pressure of the gas. A straight line is fitted to the plot to yield the monolayer volume (Vm), from the intercept 1 / VmC and slope (C- 1) / VmC, where C is a constant. The surface area of the sample can be determined from the monolayer volume by correcting for the area occupied by a single adsorbate molecule. More details can be found in ‘Analytical Methods in Fine Particle Technology’, by Paul A. Webb and Clyde Orr, Micromeritics Instruments Corporation 1997.

[0044] The cathode electrocatalyst can have a modal pore size of at least 30 nm, preferably at least 40 nm, and more preferably at least 45 nm, when measured in the cathode catalyst layer. The cathode electrocatalyst can have a modal pore size of 80 nm or less, 70 nm or less, and preferably 65 nm or less, when measured in the cathode catalyst layer. The cathode electrocatalyst can have a modal pore size in a range comprising any of the aforementioned upper and lower limits, when measured in the cathode catalyst layer. For example, the cathode electrocatalyst can have a modal pore size in a range of 30 nm to 80 nm, preferably 40 nm to 65 nm, and more preferably 45 nm to 60 nm, when measured in the cathode catalyst layer. The modal pore size can be determined using mercury intrusion porosimetry.

[0045] The cathode catalyst layer can comprise pores. The pores of the cathode catalyst layer (suitably having a pore diameter in a range of >3 nm to <1 pm) have a total pore volume, which can be determined using mercury intrusion porosimetry. The pores of the cathode catalyst layer that have a pore diameter of less than 50 nm (and suitably >3 nm) can have a pore volume of at least 50%, preferably at least 55%, and more preferably at least 60%, of the total pore volume of the cathode catalyst layer, as measured by mercury intrusion porosimetry. The pores of the cathode catalyst layer that have a pore diameter of >50 nm (and suitably <1 pm) can have a pore volume of less than 50%, preferably less than 45%, and more preferably less than 40%, of the total pore volume of the cathode catalyst layer, as measured by mercury porosimetry. The total pore volume of the catalyst layer suitably comprises the pore volume attributed to pores having a pore diameter in a range of >3 nm to <1 pm, as measured by mercury intrusion porosimetry.

[0046] The determination of the modal pore size and pore volume by mercury intrusion porosimetry can be carried out by the following process. The electrocatalyst layer to be measured was cut into strips which were stacked and rolled prior to loading into a specialised sample holder known as a penetrometer. The penetrometer containing the strips was mounted into a Micromeritics Autopore IV 9520 mercury porosimeter and the mercury pressure increased from ~3.0 to 60,000 psia in small steps, with accompanying measurements of the volume of mercury intruded into the sample, derived from capacitance changes measured along the stem of the penetrometer. The pore size distribution was then calculated from the Washburn equation, assuming a contact angle for Hg of 130°, which relates the applied pressure to the diameter of the pores into which mercury is intruded, thereby giving the amount of porosity in pores from ~60 pm to 3 nm in diameter. The intrusion curves were corrected for ion-conducting membrane compression by measuring samples of the bare ion-conducting membrane in the penetrometer over the same pressure range (~3.0 to 60,000 psia). The resulting apparent volume of intrusion, due to ion-conducting membrane compression, was subtracted from the data for the catalysed ion-conducting membrane supported layers to ensure that no apparent pore volume due to ion-conducting membrane compression was assigned to the electrocatalyst layer.

[0047] The porosity at <1 pm and >3 nm pore diameter is selected as the appropriate pore size range for calculating porosity characteristics in the layers of this invention (including total pore volume). This avoids misleading information from large, inhomogeneous features such as cracks or voids.

[0048] The ion-conducting polymer of the cathode catalyst layer can be different or the same as the ion-conducting polymer used in the anode catalyst layer and / or the ion-conducting membrane layer. The ion-conducting polymer of the cathode catalyst layer is suitably a protonconducting polymer. Preferred ion-conducting polymers are partially- or fully-fluorinated sulphonic acid polymers e.g. perfluorinated sulphonic acid polymers. For example, the ionconducting polymer 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.). Alternatively, the ion-conducting materials may be based on a sulphonated hydrocarbon polymer, such as those available from FuMA-Tech GmbH as the fumapem® P, E or K series of products, and others. The ion-conducting polymer in the cathode catalyst layer suitably has an equivalent weight in a range of 650 to 900 g / mol, preferably 700 to 800 g / mol. The cathode catalyst layer can have a weight ratio of the ion- conducting polymer to the carbon-based support of at least 0.8, preferably at least 0.9, and more preferably at least 1 .0. The cathode catalyst layer can have a weight ratio of the ionconducting polymer to the carbon-based support of 1.2 or less, preferably 1.1 or less. The cathode catalyst layer can have a weight ratio of the ion-conducting polymer to the carbonbased support in a range comprising any combination of the aforementioned upper and lower limits.

[0049] Method of manufacture

[0050] The invention also provides a method of manufacturing a catalyst-coated ion-conducting membrane, such as the catalyst-coated ion-conducting membrane described above. The method comprises preparing a cathode electrocatalyst comprising platinum-containing particles supported on a carbon-based support. The carbon-based support comprises individual primary particles or an aggregate of primary particles. The primary particles comprise pores. The step of preparing the cathode electrocatalyst includes depositing a proportion of the platinum-containing particles so they are located within the pores of the primary particles, and depositing some of the platinum-containing particles so they are located on an external surface of the carbon-based support. Suitable methods can include effecting a precipitation reaction of a Pt salt in the presence of a carbon-based support, such as carbon black. The maximum temperature of the step of preparing the cathode electrocatalyst does not exceed 300 °C. Keeping the temperature of the cathode electrocatalyst to below about 300 °C helps to prevent sintering and agglomeration of the platinum-containing particles. As such, the modal particle size can be kept small. In particular, the modal particle size of the platinum-containing particles that are located on the external surface of the carbon-based support, which are more susceptible to sintering and agglomeration, can be kept small. For example, the platinum-containing particles on the external surface of the carbon-based support have a mean average particle size of <3.0 nm, preferably <2.8 nm, and preferably <2.6 nm. The mean average particle size of the platinum-containing particles located on the external surface of the carbon-based support can be determined using TEM, as described above.

[0051] The method further comprises the step of forming a cathode catalyst ink comprising the cathode electrocatalyst, an ion-conducting polymer and a solvent. The cathode catalyst ink is a dispersion comprising the cathode electrocatalyst, the ion-conducting polymer and the solvent. In some embodiments, the cathode electrocatalyst can be combined with a dispersion of the ion-conducting polymer dispersed in the solvent, and mixed to form the cathode catalyst ink. The solvent can be a single solvent or a mixture of solvents. Preferably, the solvent can comprise at least one of water, ethanol, 1 -propanol and / or iso-propanol. The method further comprises the step of forming a layer of the cathode catalyst ink on a first side of an ion-conducting membrane. Suitable methods of forming a layer of cathode catalyst ink on an ion-conducting membrane can include slot-die (slot, extrusion) coating (whereby the dispersion is squeezed out by gravity or under pressure via a slot onto the ionconducting membrane), spray coating, knife coating, bar coating, inkjet printing, gravure printing, curtain coating, dip coating, or laser induced forward transfer (LIFT).

[0052] The method further comprises drying the layer of cathode catalyst ink. Drying the layer of cathode catalyst ink removes all (or substantially all) of the solvent from the cathode catalyst ink to form the cathode catalyst layer.

[0053] The temperature of the cathode electrocatalyst during any step in the method of manufacturing the cathode catalyst layer does not exceed 300 °C, preferably does not exceed 280 °C, and more preferably does not exceed 260 °C. The temperature of the cathode electrocatalyst during any step in the method of manufacturing the catalyst-coated ionconducting membrane does not exceed 300 °C, preferably does not exceed 280 °C, and more preferably does not exceed 260 °C.

[0054] The method can further comprise the steps of forming a layer of an anode catalyst ink on a second side of the ion-conducting membrane, wherein the second side is opposite the first side; and drying the layer of the anode catalyst ink. The step of forming the layer of anode catalyst ink on the second side of the ion-conducting membrane can be performed prior to the formation of the cathode catalyst layer. However, it is preferable for the steps of forming the anode catalyst layer to be performed after the steps of forming the cathode catalyst layer. Suitable methods of forming a layer of anode catalyst ink can include slot-die (slot, extrusion) coating (whereby the dispersion is squeezed out by gravity or under pressure via a slot onto the ion-conducting membrane), spray coating, knife coating, bar coating, inkjet printing, gravure printing, curtain coating, dip coating, or laser induced forward transfer (LIFT).

[0055] Preferably, the method of manufacturing the catalyst-coated ion-conducting membrane is a roll-to-roll process. For example, an elongate strip of ion-conducting membrane can be supplied from an unwinding roller and the layer of cathode catalyst ink can be deposited onto the first side of the ion-conducting membrane. After the cathode and / or anode catalyst layers have been formed, the catalyst-coated ion-conducting membrane can be wound on a roller for storage, transport, and / or further processing.

[0056] Examples

[0057] Example 1

[0058] A cathode catalyst ink was prepared using the following method. First, a cathode electrocatalyst comprising platinum supported on a carbon black support was prepared. The carbon black support was dispersed in water. The carbon support material had a specific surface area (BET) of about 850 m2 / g. The dispersion was mixed using a shear mixer to form a slurry. A Pt salt (K2PtCk) was added to the slurry to give a nominal loading of 50 wt.% platinum with respect to the total weight of the electrocatalyst. The pH was maintained between 5.0 and 7.0 and the slurry was stirred. Formaldehyde was then added to effect a precipitation reaction and deposit platinum onto the carbon support material. Once the reaction was complete, the slurry was filtered to recover the catalyst material, which was then washed on the filter bed and dried overnight at a temperature of 100-120 °C. The catalyst was not annealed or subject to any other heat treatment in order to avoid growing the crystallite size of the platinum particles present on the external surface of the support material. The mean average particle size of the Pt nanoparticles on the external surface of the support material was 2.50 ±0.16 nm, as measured by TEM. The mean average particle size of all Pt nanoparticles (i.e. both in the pores of the primary particles and on the external surface of the support material) was 2.47 ±0.03 nm, as measured by TEM.

[0059] The electrocatalyst was mixed with a dispersion of an ion-conducting polymer (AGC, 800EW) in a water and ethanol mix. This mixture was mechanically agitated using a stirrer until all of the catalyst had been wetted and dispersed in the liquid medium. The ink was then milled using an Eiger mill to form a well-dispersed ink. The ion-conducting polymer content in the ink was 100-110 wt.% with respect to the weight of carbon.

[0060] The cathode catalyst ink was deposited directly onto a 12 pm thick proton conducting membrane using a slot die coating process to form a wet catalyst layer in a single pass. The wet catalyst layer was dried and annealed. The catalyst layer had a platinum loading of 0.4 mgPt / cm2of geometric area of the cathode catalyst layer. The ion-conducting polymer formed an ion-conducting network connecting the electrocatalyst particles in the cathode catalyst layer to each other and also to the proton conducting membrane. The cathode electrocatalyst has a modal pore size of about 47 nm, when measured by mercury intrusion porosimetry in the cathode catalyst layer. In the cathode catalyst layer, the pore volume of pores that have a pore diameter of <50 nm was 61% of the total pore volume of the cathode catalyst layer, as measured by mercury intrusion porosimetry.

[0061] An anode catalyst ink was prepared using the following method. A PFSA ion-conducting polymer (Solvay, 790EW) and an electrocatalyst material (20 wt.% Pt / C - HiSPEC® 3000 available from Johnson Matthey Hydrogen Technologies Limited) in a mix of water and n- propanol. This mixture was mechanically agitated using a stirrer until all of the catalyst had been wetted and dispersed in the liquid medium. The ink was processed through an Eiger mill to form a well-dispersed ink.

[0062] The anode catalyst ink was used to form an anode catalyst layer. The anode catalyst ink was coated directly onto the ion-conducting membrane on the opposite side to the cathode catalyst layer using a slot die coating process at a nominal platinum loading of 0.05 mgPt / cm2. The coating was dried to remove the solvent and form an anode catalyst layer.

[0063] A commercial gas diffusion layer was applied to each face of each catalyst coated ionconducting membrane to form a complete membrane electrode assembly. 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 catalyst coated ion-conducting membrane.

[0064] Example 2

[0065] A catalyst coated ion-conducting membrane was prepared in the same way as Example 1 , except for the following differences. The ion-conducting polymer was supplied by 3M (800EW) and the solvent was a water and n-propanol mix. Rather than depositing the cathode catalyst ink directly onto the ion-conducting membrane, the ink for the cathode catalyst layer was coated onto a skived PTFE sheet using a slot die coating process where the metal loading (in mgPt / cm2of the geometric area of the catalyst layer) was controlled to 0.4 mgPt / cm2. The coating was dried to remove the solvent and form a cathode catalyst layer.

[0066] Rather than depositing the anode catalyst ink directly onto the ion-conducting membrane, the ink for the anode catalyst layer was coated onto a skived PTFE sheet using a slot die coating process where the metal loading (in mgPt / cm2of the geometric area of the catalyst layer) was controlled to 0.05 mgPt / cm2. The coating was dried to remove the solvent and form a cathode catalyst layer.

[0067] Catalyst-coated ion-conducting membranes (with an active area of 50 cm2) were prepared by transferring the cathode and anode catalyst layers from their respective PTFE sheets to either side of an ion-conducting membrane (thickness of 12 pm) respectively using a decal transfer process using heat and pressure.

[0068] Comparative Example 1

[0069] A membrane electrode assembly was prepared in the same way as Example 1 , except for the following differences. The support for the electrocatalyst was a carbon-based support prepared as described in WO2013045894A1. The support had a specific surface area (BET) of about 450-550 m2 / g and had a micropore surface area of about 65-75 m2 / g. The mean average particle size of the Pt nanoparticles on the external surface of the support material was 3.7 nm, as measured by TEM. The mean average particle size of all Pt nanoparticles (i.e. both in the pores of the primary particles and on the external surface of the support material) was 3.0 nm, as measured by TEM. The cathode electrocatalyst had a modal pore size of ~68 nm, when measured by mercury porosimetry in the cathode catalyst layer. In the cathode catalyst layer, the pore volume of pores that have a pore diameter of <50 nm was 44% of the total pore volume of the cathode catalyst layer, as measured by mercury intrusion porosimetry. The nominal platinum loading was 50 wt.% with respect to the total weight of the electrocatalyst. The ion-conducting polymer content in the ink was 80 wt.% with respect to the weight of carbon. The cathode catalyst layer had a platinum loading of 0.6 mgPt / cm2of geometric area of the cathode catalyst layer.

[0070] Cell testing

[0071] Cell testing was conducted on a test stand available from Greenlight Innovation in a 50 cm2screener cell. The cells were initially conditioned prior to cell testing.

[0072] Polarisation curves were performed at 80 °C, 40 %RH, and 50 kPag. Polarisation curves were collected in air at a stoichiometry of 1.8.

[0073] The electrochemically active surface area (ECA) was determined under wet and dry conditions by CO stripping cyclic voltammetry. The CO stripping cyclic voltammetry was performed at 80 °C, 100%RH (wet) or 30%RH (dry), and 100 kPag, and the potential was cycled between +0.125 V and +1 .0 V (with respect to the anode potential) at a fixed scan rate of 20-25 mV / s. For the ECA measurement, hydrogen was flowed on the anode side of the MEA and nitrogen on the cathode. Once the cathode side was fully purged the flow was switched to 10% v / v CO in nitrogen for 10 minutes, before switching back to pure nitrogen for a further 10 minutes. With hydrogen still flowing on the anode side, 3 voltammetry cycles were performed. The charge was calculated based on the area under the CO stripping peak recorded on the first cycle, relative to the stable baseline from the second / third cycle. The calculated charge was converted to a surface area using a literature conversion factor (420 pC / cm2for platinum). The surface area measurement was normalised by the mass of the cathode electrocatalyst in the cathode catalyst layer to provide the electrochemical surface area (ECA) in m2 / g. The electrochemical surface area measured under wet conditions (ECAWet) corresponds to the total electrochemically active surface area of the platinum- containing particles, both inside the pores and on the external surface of the carbon support material. The electrochemical surface area measured under dry conditions (ECAdry) corresponds to the electrochemically active surface area of the platinum-containing particles that is in contact with the ion-conducting polymer. The ratio ECAdry / ECAwet is a measure of the proportion of surface area of the platinum-containing particles that is in contact with the ionconducting polymer, and may also be used as a measure of the proportion of the surface area of the platinum-containing particles located on an external surface of the electrocatalyst.

[0074] The cell was subjected to an accelerated stress test as follows. The cell was initially held at 80 °C, 100 %RH and an atmospheric pressure outlet condition with a flow of H2 gas on the anode and N2 gas on the cathode. The cathode potential was cycled using a square wave (3 second holds) at +0.6 V and +0.95 V (with respect to the anode potential) for 1000 cycles. The cell was then reconditioned at 80 °C, 40%RH and 50kPag for 4 hours. Polarisation curves and cyclic voltammetry were performed as previous described. This process was repeated for cumulative cycles of 5,000, 10,000, and 30,000 cycles.

[0075] Results and discussion

[0076] Properties of cathode catalyst layers for Example 1 and Comparative Example 1 are shown in Table 1 below.

[0077] Table 1 :

[0078] Figures 1 and 2 show performance at 0.1 A / cm2and 1.0 A / cm2respectively from polarisation curves performed during the accelerated stress test described above. Despite Examples 1 and 2 having lower platinum loading in the cathode catalyst layers compared to Comparative Example 1 , Examples 1 and 2 exhibit improved kinetic performance. Furthermore, Examples 1 and 2 exhibit less of a loss in cell performance compared to Comparative Example 1 after 30,000 Pt dissolution cycles. Without being bound by any theory or conjecture, it is believed that providing a cathode catalyst with relatively small platinum- containing particle size and a ECAdry / ECAwet ratio of less than 0.50 such that a majority of the surface area of the platinum-containing particles is not in contact with the ion-conducting polymer (and preferably a majority of the electrochemically active surface area of the platinum- containing particles resides in the pores of primary particles of the carbon-based support), a balance of good kinetic performance and durability can be achieved at lower platinum loadings.

Claims

Claims1. A catalyst-coated ion-conducting membrane comprising an anode catalyst layer, a cathode catalyst layer, and an ion-conducting membrane layer disposed between the anode catalyst layer and the cathode catalyst layer, wherein: the anode catalyst layer comprises an anode electrocatalyst and an ion-conducting polymer, wherein the anode electrocatalyst comprises particles of a platinum group metal or a platinum group metal alloy, and the anode electrocatalyst is present in the anode catalyst layer at a loading of less than 0.20 mg of the platinum group metal per cm2of the anode catalyst layer; and the cathode catalyst layer comprises an ion-conducting polymer and a cathode electrocatalyst comprising platinum-containing particles and a carbon-based support, wherein the carbon-based support comprises individual primary particles or an aggregate of primary particles, the primary particles comprising pores, wherein some of the platinum- containing particles are located within the pores and some of the platinum-containing particles are located on an external surface of the carbon-based support; wherein the platinum-containing particles on the external surface of the carbon-based support have a mean average particle size of <3.0 nm; and wherein less than 50% of the surface area of the platinum-containing particles is in contact with the ion-conducting polymer of the cathode catalyst layer.

2. A catalyst-coated ion-conducting membrane according to claim 1, wherein less than 45% of the surface area of the platinum-containing particles is in contact with the ionconducting polymer.

3. A catalyst-coated ion-conducting membrane according to claim 1 or 2, wherein at least 25%, preferably at least 30% of the surface area of the platinum-containing particles is in contact with the ion-conducting polymer.

4. A catalyst-coated ion-conducting membrane according to any previous claim, wherein the cathode catalyst layer has a loading of the platinum-containing particles of less than 0.55 mg of platinum per cm2of geometric area of the cathode catalyst layer.

5. A catalyst-coated ion-conducting membrane according to any previous claim, wherein the cathode catalyst layer has a loading of the platinum-containing particles of at least 0.25 mg of platinum-containing particle per cm2of geometric area of the cathode catalyst layer.

6. A catalyst-coated ion-conducting membrane according to any previous claim, wherein the platinum-containing particles on the external surface of the carbon-based support have a mean average particle size of <2.8 nm.

7. A catalyst-coated ion-conducting membrane according to any previous claim, wherein the specific surface area (BET) of the cathode catalyst layer is at least 225 m2 / g, preferably at least 260 m2 / g.

8. A catalyst-coated ion-conducting membrane according to any previous claim, wherein the cathode catalyst layer comprises pores, and the pores of the cathode catalyst layer that have a pore diameter of less than 50 nm have a pore volume of at least 50% of the total pore volume of the cathode catalyst layer, as measured by mercury intrusion porosimetry.

9. A catalyst-coated ion-conducting membrane according to any previous claim, wherein the cathode electrocatalyst has a modal pore size in a range of 30 nm to 80 nm when measured in the cathode catalyst layer.

10. A catalyst-coated ion-conducting membrane according to any previous claim, wherein the ion-conducting polymer in the cathode catalyst layer has an equivalent weight in a range of 650 to 900 g / mol, preferably 700 to 800 g / mol.

11. A catalyst-coated ion-conducting membrane according to any previous claim, wherein the platinum-containing particles are platinum particles.

12. A catalyst-coated ion-conducting membrane according to any previous claim, wherein the carbon-based support is carbon black.

13. A catalyst-coated ion-conducting membrane according to any previous claim, wherein the cathode catalyst layer has a weight ratio of the ion-conducting polymer to carbon of at least 0.8.

14. A catalyst-coated ion-conducting membrane according to any previous claim, wherein the anode electrocatalyst is present in the anode catalyst layer in a loading of 0.10 mg or less of the platinum group metal per cm2of the anode catalyst layer.

15. A catalyst-coated ion-conducting membrane according to any previous claim, wherein the ion-conducting membrane layer has a thickness of 15 pm or less.

16. A catalyst-coated ion-conducting membrane comprising an anode catalyst layer, a cathode catalyst layer, and an ion-conducting membrane layer disposed between the anode catalyst layer and the cathode catalyst layer, wherein: the anode catalyst layer comprises an anode electrocatalyst and an ion-conducting polymer, wherein the anode electrocatalyst comprises a platinum group metal, and the anode electrocatalyst is present in the anode catalyst layer at a loading of less than 0.20 mg of the platinum group metal per cm2of the anode catalyst layer; and the cathode catalyst layer comprises an ion-conducting polymer and a cathode electrocatalyst comprising platinum-containing particles and a carbon-based support, wherein the cathode catalyst layer comprises pores, and the pores of the cathode catalyst layer that have a pore diameter of <50 nm have a pore volume of at least 50% of the total pore volume of the pores of the cathode catalyst layer, as measured by mercury intrusion porosimetry.

17. A fuel cell comprising the catalyst-coated ion-conducting membrane of any of claims 1 to 16.

18. A method of manufacturing a catalyst-coated ion-conducting membrane, the method comprising the steps of: preparing a cathode electrocatalyst comprising platinum-containing particles supported on a carbon-based support, wherein the carbon-based support comprises individual primary particles or an aggregate of primary particles, the primary particles comprising pores, wherein some of the platinum-containing particles are located within the pores and some of the platinum-containing particles located on an external surface of the carbon-based support, the platinum-containing particles on the external surface of the carbon-based support have a mean average particle size of <3.0 nm; forming a cathode catalyst ink comprising the cathode electrocatalyst, an ionconducting polymer and a solvent, forming a layer of the cathode catalyst ink on a first side of an ion-conducting membrane; and drying the layer of cathode catalyst ink to form a cathode catalyst layer in which less than 50% of the surface area of the platinum-containing particles is in contact with the ionconducting polymer of the cathode catalyst layer;wherein the temperature of the cathode electrocatalyst during any step of the method of manufacturing the catalyst-coated ion-conducting membrane does not exceed 300 °C.

19. A method according to claim 18, wherein the temperature of the cathode electrocatalyst during any step in the method of manufacturing the cathode catalyst layer does not exceed 280 °C, and preferably does not exceed 260 °C.

20. A method according to claim 18 or 19, wherein the method further comprises the steps of: forming a layer of an anode catalyst ink on a second side of the ion-conducting membrane opposite the first side; and drying the layer of the anode catalyst ink.

21. A method according to any of claims 18 to 20, wherein the method is a roll-to-roll process.

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