P-Doped SnO₂ Powder as a Support for PEFC Cathode

Polymer electrolyte fuel cells (PEFCs) are used as power sources for residential and transportation use. In the present PEFCs, a large amount of platinum supported carbon is used as a catalyst in the cathode electrode. However, a platinum catalyst has some problems such as high cost, small resources, low durability, and large overvoltage for oxygen reduction reaction (ORR)considering the future widespread use of PEFC2. In addition, the durability of carbon support has become a serious problem especially under future driving conditions3 (high temperature and high voltage) for automobiles. In order to solve the problem, there is a strong demand for substitution of highly durable materials, that is, a non-platinum catalyst and a non-carbon support. Therefore, we have developed platinum substitution catalysts using group 4,5 metal oxides4. Regarding non-carbon supports, we have focused on Nb-doped TiO2 because of its high stability in acidic media. However, the specific surface area and the electro-conductivity of Nb-doped TiO2 are insufficient to support an oxide catalyst. Therefore, in this study, we focused on P-doped SnO2 (Mitsubishi Materials), which has excellent specific surface area, electronic conductivity, and stability under oxidative atmosphere, and examined its applicability as a cathode support. On the other hand, regarding the active sites, even carbon was used as a support, the oxygen reduction activities of group 4 and 5 oxide cathodes were much lower than that of platinum. This is because both quality and quantity of the active sites of oxide-based cathodes are lower than that of platinum. Recently, according to the first-principles calculation, Sugino et al, found that when Pd is doped into TiO2, the onset potential for ORR exceeds platinum (Because the adsorption Gibbs energies of the reaction intermediates of ORR are adjusted, the energy barrier along the reaction path become lower. The quality of active sites is improved by Pd doping into TiO26). Although no calculations have been performed for tin oxide, we focused on Pd doping into SnO2 to form the active sites in this study. Therefore, the purpose of this study was to modify P-doped SnO2 with Pd and investigate its catalytic activity. A 2 mM H2PdCl4 aqueous solution was mixed with ethanol, and P-doped SnO2 (Mitsubishi Materials) as a support was further added. An electron beam irradiation was carried out at room temperature. The conditions of the electron beam irradiation were as follows; electron beam energy of 2 MGy, absorbed dose to water of 500 kGy, and irradiation time of 50 min. Sample A was prepared by mixing 20 mL of a 2 mM H2PdCl4 aqueous solution and 1 mL of ethanol, that is, adjusting the volume ratio of (ethanol) / (H2PdCl4) = 0.05. Sample B was prepared by mixing 3.4 mL of 2 mM H2PdCl4 aqueous solution and 17 mL of ethanol, that is, adjusting the volume ratio of (ethanol) / (H2PdCl4) = 5. A loading Pd was adjusted to 1 wt% with respect to P-doped SnO2 in both samples. Figure 1 shows the comparison of potential-current curves for the ORR of Pd-modified P-doped SnO2. Sample A had higher catalytic activity than sample B. It was found that the ORR activity was improved by reducing the volume ratio of (ethanol) / (H2PdCl4) at the catalyst preparation. Acknowledgment This research is supported by the New Energy and Industrial Technology Development Organization (NEDO) and the Strategic International Joint Research Program (SICORP) of the Japan Science and Technology Agency (JST). In addition, the authors wish to thank the support of JSPS grants-in-aid for scientific research, Suzuki Foundation and Tonen General Sekiyu Research / Development Encouragement & Scholarship Foundation. References [1] M. Zhao et.al., Int. J. Hydrogen Energy, 39,13725 (2014) [2] NEDO, Technological development roadmap(FCV・Transportation) [https://www.nedo.go.jp/content/100871975.pdf] [3] T. Ioroi et.al., J. Power Sources, 223,183 (2013) [4] A. Ishihara, et al., J. Phys. Chem. C, 117, 18837 (2013). [5] Y. Inoue, et.al., IUMRS-ICA, B23 (2019). [6] Y. Yamamoto, et al., J. Phys. Chem. C, 123, 19486 (2019). Figure 1