Preparation of Pt-Ni Alloy Thin Films with Various Compositions by Sputtering and their Activity for Oxygen Reduction Reaction

Abstract

In recent years, the depletion of fossil fuels and global environmental issues have become serious problems. Fuel cells have attracted significant attention as the energy source for the next generation. Fuel cells can obtain electrical energy from chemical energy by the chemical reaction of hydrogen and oxygen. Thus, there is no pollutant in the exhaust gas and the energy conversion efficiency is very high in the fuel cell system. Among them, the polymer electrolyte fuel cell (PEFC) is the most advanced type due to its low operating temperature and high output power. Although the household fuel cell and fuel cell vehicle have been put into practical use, widespread use of these systems is suppressed for the reasons of high cost and scarcity of Pt, which is used as the electrocatalyst [1,2]. In the PEFC, the hydrogen oxidation reaction occurs at the anode and the oxygen reduction reaction (ORR) occurs at the cathode. At the cathode, a large amount of Pt is required because the ORR overvoltage is very high. Therefore, decreasing the Pt use is strongly desired [3]. For the purpose of decreasing the Pt use, various efforts have been conducted. The first one was the development of a cathode catalyst without Pt. For example, Ota’s group reported the preparation and ORR activity of the oxynitride and the partially oxidized carbonitride of Zr or Ta [4-8]. These materials showed comparable setup potentials for the ORR to Pt and are electrochemically stable. Thus the oxynitride and partially oxidized carbonitride of Zr or Ta are promising materials as the cathode catalyst. However, the oxygen reduction current of these materials are dramatically lower than that of Pt. Cao et al. reported that cobalt molybdenum oxynitride showed a good ORR activity under both acidic and alkaline conditions. However, it still does not surpass Pt [9]. Nabae et al. applied the carbon alloy catalyst, which was prepared by pyrolysis of a polymer containing Fe, Co, and N, to the PEFC cathode [10-12]. Although these carbon alloys showed a comparable performance to Pt, their low stabilities under PEFC operating conditions were pointed out. Thus, realization of a non-platinum PEFC cathode is still difficult. On the other hand, the core-shell catalyst with low Pt content was investigated. The core-shell catalyst, on which a Pt monolayer shell was formed on the core particle of the nonplatinum metal, can possibly decrease the Pt use and increase the Pt utilization. For example, the Pt-Pd core-shell catalyst showed a higher ORR activity than the bulk Pt catalyst [13-15]. The simplest method to improve the catalytic activity and decrease the Pt use of the Pt catalyst is alloying. There are many reports about the ORR activity of the Pt series alloys. Toda et al. reported that Pt-M (M=Fe, Co, and Ni) showed a higher ORR activity than Pt [16]. For these catalysts, Pt rearranges and a Pt surface is formed after dissolution of M at the alloy surface. It has been clarified that the improvement in the ORR activity was caused by the modification of the electronic state of the Pt surface with the underlying alloy (Pt-M). Recently, the Pt-Co alloy has been considered to be a superior catalyst of the Pt series binary alloys and the Pt-Co alloy with various compositions and Pt series ternary alloys are being investigated [17-19]. In this study, the ORR activity of the Pt-Ni alloy was systematically investigated in order to minimize the Pt use in the PEFC cathode. Because Ni is a homologous element of Pt and shows a good catalytic activity for various reactions, it is expected to decrease the Pt use by alloying Pt with Ni. For that purpose, the Pt-Ni alloy thin films with various compositions were prepared by Preparation of Pt-Ni Alloy Thin Films with Various Compositions by Sputtering and their Activity for Oxygen Reduction Reaction