Catalyst Layer

Abstract

A catalyst layer comprising an electrocatalyst and an oxygen evolution catalyst, wherein the oxygen evolution catalyst comprises a crystalline metal oxide comprising: (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
characterised 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 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. 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 RE (wherein RE is a rare earth metal), indium, thallium, tin, lead, antimony and bismuth; c is from 3-11; the atomic ratio of (a+b):c is from 1:1 to 1:2; the atomic ratio of a:b is from 1:1.5 to 1.5:1.

2. The catalyst layer according to claim 1, wherein a is 0.66 to 1.5; b is 1; and c is 3 to 5.

3. The catalyst layer according to claim 1, wherein a is 2 to 4.5; b is 3; and c is 10 to 11.

4. The catalyst layer according to claim 1, wherein the oxygen evolution catalyst and the electrocatalyst are in separate layers in the membrane electrode assembly.

5. The catalyst layer according to claim 1, wherein the oxygen evolution catalyst and the electrocatalyst are in a single layer in the membrane electrode assembly.

6. The catalyst layer according to claim 5, wherein the oxygen evolution catalyst acts as a support material for the electrocatalyst.

7. An electrode comprising a gas diffusion layer and a catalyst layer according to any one of claim 1.

8. A catalysed membrane comprising a proton conducting membrane and a catalyst layer according to claim 1.

9. A catalysed transfer substrate comprising a catalyst layer according to claim 1.

10. A membrane electrode assembly comprising a catalyst layer according to claim 1.

11. A fuel cell comprising a catalyst layer according to claim 1.

Description

EXAMPLE 1 (NA.SUB.0.54.CA.SUB.1.18.IR.SUB.2.O.SUB.6..0.66H.SUB.2.O)

[0069] To a 22 ml volume autoclave, 8 ml of 10M 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.

[0070] 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).

[0071] 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.

[0072] The following Examples were prepared by a similar method:

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

COMPARATIVE EXAMPLE 1

[0073] An unsupported RuO.sub.2/IrO.sub.2 mixed oxide with a nominal Ru:Ir atomic ratio of 90:10.

COMPARATIVE EXAMPLE 2

[0074] A TaIr mixed oxide was prepared in accordance with the preparation of Example 2 of WO2011/021034.

[0075] Typical Powder Characterisation/Analysis

[0076] 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.

[0077] 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.

[0078] Ink, Catalyst Layer and MEA Preparation

[0079] 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 3 W. 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.

[0080] 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.

[0081] 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).

[0082] Fuel Cell Testing

[0083] 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.

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

[0084] 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.