Development of Highly Active and Durable Platinum Core-shell Catalysts for Polymer Electrolyte Fuel Cells

Abstract

They are clean and highly efficient energy sources without emission of global warming CO2 gas. PEFCs are operated at low temperatures in the range of room temperature to 80 °C, and are thereby suitable for electric power sources in small-scale stationary co-generation systems and in electric motor-driven vehicles (so-called the fuel cell vehicles (FCVs)). In Japan, 1 kW-class stationary co-generation systems (called ENE-FARM) have been already commercialized since 2009, and quite recently Japanese motor companies announced that they are planning to commercialize FCVs in FY2014. In PEFCs, platinum is used as catalysts for both the anode and the cathode. Though hydrogen oxidation reaction (HOR) at the anode is facile, oxygen reduction reaction (ORR) at the cathode is a slow reaction. Therefore a large amount of Pt is currently used at the cathode (typically, 0.5 mg cm2). The natural resource of Pt is extremely small (probable reserve: 70,000 tons) and the price of Pt is very expensive (>5000 Yen g1 in Apr. 2014). If we continue to use Pt catalyst at the present level, the scarcity of Pt as well as its high cost will disturb the world-wide spread of FCVs in the near future. Therefore a drastic reduction of Pt usage at the cathode by improving the ORR activity of the Pt catalyst is one of the most important issues in the development of FCVs1). There have been several methods to improve the ORR activity of Pt catalysts, which include Pt_M (M: 3d transition metals such as Co, Ni, Cu, etc.) alloy catalysts2)~5) and core-shell structured catalysts in which Pt monolayer (ML) is formed on non-Pt metal core nanoparticles6)~8). We have so far developed PtML core-shell catalysts using Au and Pd core materials in a research project supported by New Energy and Industrial Technology Development Organization (NEDO), Japan with 11 academic and industrial institutions since FY20089)~13). Here we first review the concept and characteristics of PtML core-shell catalysts. We overview a novel preparation method of PtML coreshell catalysts that is suitable for mass production developed by us, and the results on the activity and durability of the resulting core-shell catalysts. On the basis of these results, the potentials and difficulties of the 55 Journal of the Japan Petroleum Institute, 58, (2), 55-63 (2015)