Catalyst layer

Abstract

A catalyst layer including an electrocatalyst and an oxygen evolution catalyst, wherein the oxygen evolution catalyst includes a crystalline metal oxide including: (i) one of more first metals selected from the group consisting of yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, magnesium, calcium, strontium, barium, sodium, potassium, indium, thallium, tin, lead, antimony and bismuth; (ii) one or more second metals selected from the group consisting of Ru, Ir, Os and Rh; and (iii) oxygen
characterized in that: (a) the atomic ratio of first metal(s):second metal(s) is from 1:1.5 to 1.5:1 (b) the atomic ratio of (first metal(s)+second metal(s)):oxygen is from 1:1 to 1:2 is disclosed.

Claims

1. A catalyst layer comprising: an electrocatalyst comprising platinum; and an oxygen evolution catalyst, wherein the oxygen evolution catalyst comprises a crystalline metal oxide of formula (AA).sub.a(BB).sub.bO.sub.c, the crystalline metal oxide having a pyrochlore type crystalline structure, wherein A and A are the same or different and are selected from the group consisting of yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, magnesium, calcium, strontium, barium, sodium, potassium, indium, thallium, tin, lead, antimony and bismuth; B is selected from the group consisting of Ru, Ir, Os, and Rh; B is selected from the group consisting of Ru, Ir, Os, Rh, Ca, Mg or a rare earth metal; a is 1.33 to 3; b is 2; c is from 3 to 10; the atomic ratio of (a+b) : c is from 1:1 to 1:2; and the atomic ratio of a:b is from 1:1.5 to 1.5:1, and, wherein the electrocatalyst and the oxygen evolution catalyst are present in said layer as a mixture.

2. An electrode comprising a gas diffusion layer and the catalyst layer of claim 1.

3. A catalysed membrane comprising a proton conducting membrane and the catalyst layer of claim 1.

4. A catalysed transfer substrate comprising the catalyst layer of claim 1.

5. A membrane electrode assembly comprising the catalyst layer of claim 1.

6. The catalyst layer of claim 1, wherein A and A are the same or different and are selected from the group consisting of sodium, potassium, calcium, strontium, barium lead, and cerium.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) In one specific embodiment, crystalline metal oxides of the formula (AA).sub.a(BB)O.sub.c. are used. In this formula: A, A, B and B are as hereinbefore defined; a is 0.66 to 1.5, b is 1 and c is 3 to 5. These crystalline metal oxides have a perovskite type crystalline structure, as described in Structural Inorganic Chemistry: Fifth Edition, Wells, A. F., Oxford University Press, 1984 (1991 reprint). Specific examples of crystalline metal oxides with a perovskite type crystalline structure include, but are not limited to, RERuO.sub.3; SrRuO.sub.3; PbRuO.sub.3; REIrO.sub.3; CaIrO.sub.3; BaIrO.sub.3; PbIrO.sub.3; SrIrO.sub.3; KIrO.sub.3; SrM.sub.0.5IrO.sub.0.5O.sub.3 (wherein RE and M are as hereinbefore defined).

(2) In a second specific embodiment, crystalline metal oxides of the formula (AA).sub.a(BB).sub.2O.sub.c. are used. In this formula: A, A, B and B are as hereinbefore defined; a is 1.33 to 3, b is 2 and c is 3 to 10, preferably 6-7. These crystalline metal oxides have a pyrochlore type crystalline structure, as described in Structural Inorganic Chemistry: Fifth Edition, Wells, A. F., Oxford University Press, 1984 (1991 reprint). Specific examples of crystalline metal oxides with a pyrochlore type crystalline structure include, but are not limited to, RE.sub.2Ru.sub.2O.sub.7; RE.sub.2Ir.sub.2O.sub.7; Bi.sub.2Ir.sub.2O.sub.7; Pb.sub.2Ir.sub.2O.sub.7; Ca.sub.2Ir.sub.2O.sub.7 (wherein RE is as hereinbefore defined).

(3) In a third specific embodiment, crystalline metal oxides of the formula (AA).sub.a(BB).sub.3O.sub.c. are used. In this formula: A, A, B and B are as hereinbefore defined; a is 2 to 4.5, b is 3 and c is 10 to 11. These crystalline metal oxides have a KSbO.sub.3 type crystalline structure, as described as a cubic form with space group Pn3 in Structural Inorganic Chemistry: Fifth Edition, Wells, A. F., Oxford University Press, 1984 (1991 reprint). Specific examples of crystalline metal oxides with a KSbO.sub.3 type crystalline structure include, but are not limited to, K.sub.3Ir.sub.3O.sub.9; Sr.sub.2Ir.sub.3O.sub.9; Ba.sub.2Ir.sub.3O.sub.9; La.sub.3Ir.sub.3O.sub.11.

(4) In some of these compositions listed above, there may be oxygen vacancies which will reduce the oxygen stoichiometry in the crystalline structure. Similarly, some of the one or more first metal sites (or A, A sites) may be left vacant, reducing the stoichiometry of the first metal (or A, A metal) in the crystalline structure. Furthermore, in some instances, water molecules are known to occupy some vacant sites to provide a hydrated or partially hydrated crystalline metal oxide.

(5) Preferably, the specific surface area (BET) of the crystalline metal oxide is greater than 20 m.sup.2/g, preferably greater than 50 m.sup.2/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/[V.sub.a((P.sub.0/P)1)] vs P/P.sub.0 is then constructed for P/P.sub.0 values in the range 0.05 to 0.3 (or sometimes as low as 0.2), where V.sub.a is the quantity of gas adsorbed at pressure P, and P.sub.0 is the saturation pressure of the gas. A straight line is fitted to the plot to yield the monolayer volume (V.sub.m), from the intercept 1/V.sub.mC and slope (C1)/V.sub.mC, 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.

(6) The crystalline metal oxide can be made by a variety of routes, including solid state synthesis, hydrothermal synthesis, spray pyrolysis and in some cases co-precipitation. The direct solid state synthesis route involves heating stoichiometric mixtures of oxides and/or carbonates in air to high temperature, typically >800 C. Hydrothermal synthesis involves heating mixtures of appropriate starting salts and if necessary an oxidising agent at a more modest temperature (typically 200-250 C.) in a suitable sealed vessel. This method generally gives materials with much higher surface area (i.e. smaller crystallite size) than those prepared by solid state routes.

(7) The electrocatalyst comprises a metal (the primary metal), which is suitably selected from (i) the platinum group metals (PGM) (platinum, palladium, rhodium, ruthenium, iridium and osmium), or (ii) gold or silver, or (iii) a base metal or an oxide thereof.

(8) The primary metal may be alloyed or mixed with one or more other precious metals, or base metals or an oxide of a precious metal or base metal. The metal, alloy or mixture of metals may be unsupported or supported on a suitable inert support. In one embodiment, if the electrocatalyst is supported, the support is non-carbonaceous. Examples of such a support include titania, niobia, tantala, tungsten carbide, hafnium oxide or tungsten oxide. Such oxides and carbides may also be doped with other metals to increase their electrical conductivity, for example niobium doped titania.

(9) The electrocatalyst and oxygen evolution catalyst may be present in the catalyst layer either as separate layers or as a mixed layer or as a combination of the two. If present as separate layers, the layers are suitably arranged such that the oxygen evolution catalyst layer is next to the membrane in the MEA. In a preferred embodiment, the electrocatalyst and the oxygen evolution catalyst are present in the catalyst layer as a mixed layer.

(10) In an alternative embodiment of the invention, the electrocatalyst and the oxygen evolution catalyst are present in the catalyst layer as a mixed layer and the oxygen evolution catalyst acts as the support material for the electrocatalyst.

(11) Suitably, the ratio (by weight) of the oxygen evolution catalyst to total electrocatalyst in the catalyst layer is from 20:1 to 1:20, preferably from 1:1 to 1:10. The actual ratio will depend on whether the catalyst layer is employed at the anode or cathode and whether the oxygen evolution catalyst is used as a support for the electrocatalyst.

(12) Suitably, the loading of the primary metal of the electrocatalyst in the catalyst layer is less than 0.4 mg/cm.sup.2, and is preferably from 0.01 mg/cm.sup.2 to 0.35 mg/cm.sup.2, most preferably 0.02 mg/cm.sup.2 to 0.25 mg/cm.sup.2.

(13) The catalyst layer may comprise additional components, for example a polymer binder, such as an ionomer, suitably a proton conducting ionomer. Examples of suitable proton conducting ionomers will be known to those skilled in the art, but include perfluorosulphonic acid ionomers, such as Nafion and ionomers made from hydrocarbon polymers.

(14) The catalyst layer of the invention has utility in electrochemical cells, and in particular in PEM fuel cells. Accordingly, a further aspect of the invention provides an electrode comprising a gas diffusion layer (GDL) and a catalyst layer according to the invention. In one embodiment, the electrode is an anode of a conventional fuel cell. In a second embodiment, the electrode is a cathode of a conventional fuel cell.

(15) The catalyst layer can be deposited onto a GDL using well known techniques, such as those disclosed in EP 0 731 520. The catalyst layer components may be formulated into an ink, comprising an aqueous and/or organic solvent, optional polymeric binders and optional proton-conducting polymer. The ink may be deposited onto an electronically conducting GDL using techniques such as spraying, printing and doctor blade methods. The anode and cathode gas diffusion layers are suitably based on conventional non-woven carbon fibre gas diffusion substrates such as rigid sheet carbon fibre papers (e.g. the TGP-H series of carbon fibre papers available from Toray Industries Inc., Japan) or roll-good carbon fibre papers (e.g. the H2315 based series available from Freudenberg FCCT KG, Germany; the Sigracet series available from SGL Technologies GmbH, Germany; the AvCarb series available from Ballard Material Products, United States of America; or the NOS series available from CeTech Co., Ltd. Taiwan), or on woven carbon fibre cloth substrates (e.g. the SCCG series of carbon cloths available from the SAATI Group, S.p.A., Italy; or the WOS series available from CeTech Co., Ltd, Taiwan). For many PEMFC and DMFC applications the non-woven carbon fibre paper, or woven carbon fibre cloth substrates are typically modified with a hydrophobic polymer treatment and/or application of a microporous layer comprising particulate material either embedded within the substrate or coated onto the planar faces, or a combination of both to form the gas diffusion layer. The particulate material is typically a mixture of carbon black and a polymer such as polytetrafluoroethylene (PTFE). Suitably the gas diffusion layers are between 100 and 300 m thick. Preferably there is a layer of particulate material such as carbon black and PTFE on the faces of the gas diffusion layers that contact the electrocatalyst layers.

(16) In PEM fuel cells, the electrolyte is a proton conducting membrane. The catalyst layer of the invention may be deposited onto one or both faces of the proton conducting membrane to form a catalysed membrane. In a further aspect the present invention provides a catalysed membrane comprising a proton conducting membrane and a catalyst layer of the invention. The catalyst layer can be deposited onto the membrane using well-known techniques. The catalyst layer components may be formulated into an ink and deposited onto the membrane either directly or indirectly via a transfer substrate.

(17) The membrane may be any membrane suitable for use in a PEM fuel cell, for example the membrane may be based on a perfluorinated sulphonic acid material such as Nafion (DuPont), Flemion (Asahi Glass) and Aciplex (Asahi Kasei); these membranes may be used unmodified, or may be modified to improve the high temperature performance, for example by incorporating an additive. 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, JSR Corporation, Toyobo Corporation, and others. The membrane may be a composite membrane, containing the proton-conducting material and other materials that confer properties such as mechanical strength. For example, the membrane may comprise an expanded PTFE substrate. Alternatively, the membrane may be based on polybenzimidazole doped with phosphoric acid and include membranes from developers such as BASF Fuel Cell GmbH, for example the Celtec-P membrane which will operate in the range 120 C. to 180 C.

(18) In a further embodiment of the invention, the substrate onto which the catalyst of the invention is applied is a transfer substrate. Accordingly, a further aspect of the present invention provides a catalysed transfer substrate comprising a catalyst layer of the invention. The transfer substrate may be any suitable transfer substrate known to those skilled in the art but is preferably a polymeric material such as polytetrafluoroethylene (PTFE), polyimide, polyvinylidene difluoride (PVDF), or polypropylene (especially biaxially-oriented polypropylene, BOPP) or a polymer-coated paper such as polyurethane coated paper. The transfer substrate could also be a silicone release paper or a metal foil such as aluminium foil. The catalyst layer of the invention may then be transferred to a GDL or membrane by techniques known to those skilled in the art.

(19) A yet further aspect of the invention provides a membrane electrode assembly comprising a catalyst layer, electrode or catalysed membrane according to the invention. The MEA may be made up in a number of ways including, but not limited to:

(20) (i) a proton conducting membrane may be sandwiched between two electrodes (one anode and one cathode), at least one of which is an electrode according to the present invention;

(21) (ii) a catalysed membrane coated on one side only by a catalyst layer may be sandwiched between (a) a gas diffusion layer and an electrode, the gas diffusion layer contacting the side of the membrane coated with the catalyst layer, or (b) two electrodes, and wherein at least one of the catalyst layer and the electrode(s) is according to the present invention;

(22) (iii) a catalysed membrane coated on both sides with a catalyst layer may be sandwiched between (a) two gas diffusion layers, (b) a gas diffusion layer and an electrode or (c) two electrodes, and wherein at least one of the catalyst layer and the electrode(s) is according to the present invention.

(23) The MEA may further comprise components that seal and/or reinforce the edge regions of the MEA for example as described in WO2005/020356. The MEA is assembled by conventional methods known to those skilled in the art.

(24) Electrochemical devices in which the catalyst layer, electrode, catalysed membrane and MEA of the invention may be used include fuel cells, in particular proton exchange membrane (PEM) fuel cells. The PEM fuel cell could be operating on hydrogen or a hydrogen-rich fuel at the anode or could be fuelled with a hydrocarbon fuel such as methanol. The catalyst layer, electrode, catalysed membrane and MEA of the invention may also be used in fuel cells in which the membranes use charge carriers other than protons, for example OH.sup. conducting membranes such as those available from Solvay Solexis S.p.A., FuMA-Tech GmbH. The catalyst layer and electrode of the invention may also be used in other low temperature fuel cells that employ liquid ion conducting electrolytes, such as aqueous acids and alkaline solutions or concentrated phosphoric acid. Other electrochemical devices in which the catalyst layer, electrode, catalysed membrane and MEA of the invention may be used are as the oxygen electrode of regenerative fuel cells, and as the anode of an electrolyser where oxygen evolution is performed by the water electrolysis catalyst and contaminant hydrogen is recombined with oxygen by the electrocatalyst.

(25) Accordingly, a further aspect of the invention provides a fuel cell, preferably a proton exchange membrane fuel cell, comprising a catalyst layer, an electrode, a catalysed membrane or an MEA of the invention.

(26) The present invention will now be described further with reference to the following examples which are illustrative, but not limiting, of the invention.

EXAMPLE 1

Na0.54Ca1.18Ir2O6.0.66H2O

(27) To a 22 ml volume autoclave, 8 ml of 10 M NaOH solution, 0.5 ml of de-ionised water, 0.250 g (1.0610.sup.3 mole) Ca(NO.sub.3).sub.2 and 0.411 g (1.0610.sup.3 mole) IrCl.sub.3 was added and stirred for 1 hour. 0.174 g (2.2310.sup.3 mole) Na.sub.2O.sub.2 was added to the reaction solution and stirred for another 10 minutes; then the same weight Na.sub.2O.sub.2 was added again before closing the autoclave. The autoclave was heated at 240 C. for 96 hr in an oven. The autoclave was cooled to room temperature. The reaction mixture was transferred to a beaker and left to settle. The solution was decanted leaving the precipitate, rinsed with de-ionised water and repeated several times. The precipitate was then similarly washed with excess 1M H.sub.2SO.sub.4 then with de-ionised water and dried to yield a black powder.

(28) At 240 C., maximum pressure generated inside the autoclave was not more than 51 bar. (H.sub.2O vapour pressure of Water=34 bar+decomposition of all Na.sub.2O.sub.2=17 bar maximum).

(29) An alternative preparation may add concentrated H.sub.2O.sub.2 dropwise while stirring instead of Na.sub.2O.sub.2 addition and/or may use solid NaOH in place of prepared NaOH solution. Before any peroxide compound is added all other reagents are well-mixed.

(30) The following Examples were prepared by a similar method:

(31) TABLE-US-00001 Example Autoclave No. Composition Reagents conditions Example 1 Na.sub.0.54Ca.sub.1.18Ir.sub.2O.sub.60.66H.sub.2O 1Ca(NO.sub.3).sub.24H.sub.2O + 240 C. 1IrCl.sub.37H.sub.2O + 8 ml 10M 96 hours NaOH + 4.2Na.sub.2O.sub.2 + 0.5 ml H.sub.2O Example 2 Bi.sub.2Ir.sub.2O.sub.7 1NaBiO.sub.3 + 1.25IrCl.sub.37H.sub.2O + 240 C. 8 ml 5M NaOH + 8Na.sub.2O.sub.2 120 hours Example 3 Pb.sub.2Ir.sub.2O.sub.7 1Pb(NO.sub.3).sub.2 + 1IrCl.sub.37H.sub.2O + 6 ml 240 C. H.sub.2O + 2Na.sub.2O.sub.2 + NaOH 112 hours Example 4 Na.sub.0.8Sr.sub.2.2Ir.sub.3O.sub.10.1 0.75Sr(NO.sub.3).sub.2 + 1IrCl.sub.37H.sub.2O + 240 C. 100NaOH + 3 ml H.sub.2O + 10 l 72 hours conc HF + 6 ml conc H.sub.2O.sub.2 Example 5 Na.sub.0.66Ce.sub.1.34Ru.sub.2O.sub.7 0.66CeCl.sub.37H.sub.2O + 1RuCl.sub.3nH.sub.2O + 225 C. 5M NaOH + 10Na.sub.2O.sub.2 120 hours Example 6 Na.sub.0.66Ce.sub.1.34Ir.sub.2O.sub.7 0.66CeCl.sub.37H.sub.2O + 1 240 C. IrCl.sub.37H.sub.2O + 5M NaOH + 120 hours 10Na.sub.2O.sub.2 Example 7 Na.sub.0.66Ce.sub.1.34Ru.sub.0.6Ir.sub.1.4O.sub.7 0.66CeCl.sub.37H.sub.2O + 225 C. 0.3RuCl.sub.3nH.sub.2O + 0.7IrCl.sub.37H.sub.2O + 120 hours 5M NaOH + 10Na.sub.2O.sub.2 Example 8 Na.sub.0.54Ca.sub.1.18Ir.sub.2O.sub.60.66H.sub.2O 1Ca(NO.sub.3).sub.24H.sub.2O + 240 C. 1IrCl.sub.37H.sub.2O + 22 ml 10M 70 hours NaOH + 4.2Na.sub.2O.sub.2 + 0.5 ml H.sub.2O

COMPARATIVE EXAMPLE 1

(32) An unsupported RuO.sub.2/IrO.sub.2 mixed oxide with a nominal Ru:Ir atomic ratio of 90:10.

COMPARATIVE EXAMPLE 2

(33) A TaIr mixed oxide was prepared in accordance with the preparation of Example 2 of WO2011/021034.

(34) Typical Powder Characterisation/Analysis

(35) Samples were analysed by BET to determine surface area. A sample typically was degassed at 200 C. for 15 hrs under N.sub.2 flow before N.sub.2 adsorption BET surface area measurement was determined. Moisture content and thermal stability was determined by DSC. Elemental composition was determined via ICPES. Samples were analysed by XRD to identify crystallographic parameters.

(36) Sample chemical composition was refined based on modelling powder neutron diffraction data obtained using the POLARIS diffractometer at ISIS (R. Walton et al. Chem. Sci., 2011, 2, 1573). The XRD data was used to obtain a starting crystal structure from which peak intensities were matched in order to identify the fractional A and A content in an (AA).sub.a(BB).sub.bO.sub.c structure. A crystal structure with inclusive water accounted for evident moisture from DSC data. The refined chemical composition was compared against ICPES elemental data.

(37) Ink, Catalyst Layer and MEA Preparation

(38) 65 mg of the crystalline metal oxide example of the invention was added to a 5 ml vial with 1.7 mL H.sub.2O. The mixture was processed with a high intensity cm microtip ultrasonic probe for 2 minutes at 3W. The mixture was added to 0.65 g of HiSPEC 18600 (Johnson Matthey PLC) catalyst in a separate container. The vial was rinsed three times with 350 L de-ionised H.sub.2O and added to the container with the catalyst. The catalyst slurry was mixed manually with a spatula to wet all material, then mixed at 3000 rpm in a planetary mixer for 3 minutes. The mixed catalyst was dried at 80 C. in a fan-assisted oven.

(39) The dried catalyst was broken into a powder, aqueous Nafion solution (available from DuPont) was added to dried mixed-catalyst and the ink was shear-mixed in a planetary mixer using 5 mm YSZ ceramic beads. After having mixed for 3 minutes at 3000 rpm the ink was stirred manually with a spatula to break up any sediment. The ink was further milled for 5 minutes.

(40) The ink was screen-printed onto a PTFE sheet to give a layer having a targeted PGM loading of 0.1 mg/cm.sup.2. The layer was transferred from the PTFE (polytetrafluoroethylene) sheet onto a Nafion N112 membrane (available from DuPont) at 150 C. with pressure. A Pt/C layer was transferred to the opposite side of the N112 membrane simultaneously in order to produce a catalyst coated membrane (CCM).

(41) Fuel Cell Testing

(42) The CCM was assembled in the fuel cell hardware using Toray TGP-H-060 as the gas diffusion substrate, coated with a PTFE/carbon coating to form the gas diffusion layer. The fuel cell was tested at 80 C. and 10 psig with humidified H.sub.2/N.sub.2 gas reactants. The oxygen evolution mass activity of the mixed catalyst layer was determined at 1.5V vs RHE by scanning the potential from 20 mV to 1.6V at 5 mV/s. The results are shown in Table 1.

(43) TABLE-US-00002 TABLE 1 O.sub.2 Evolution Catalyst PGM Apparent Loading M.sub.act (1.5 V) BET Example No. g/cm.sup.2 A/g PGM m.sup.2/g Comparative Example 1 18.3 334 8 Comparative Example 2 13.9 291 45 Example 1 11.8 3051 68 Example 2 8.1 4717 42 Example 3 8.8 2500 18 Example 4 10.6 1322 38 Example 5 9.9 17458 50 Example 6 10.5 2279 87 Example 7 8.3 4530 91.5 Example 8 10.7 1186 28

(44) From the data it can be seen that the MEAs having the catalyst layers of the invention have a far higher oxygen evolution mass activity than the Comparative Examples.