非貴金属酸化物ベース酸素還元触媒

抄録

Reducing the overpotential for the oxygen reduction reaction (ORR) is necessary to increase the energy transformation efficiency of polymer electrolyte fuel cells (PEFCs). We have focused on oxides of group 4 (or 5) elements as alternatives to the current Pt catalysts: Those oxides are stable under the PEFC environment and can exhibit activity for the ORR, and recently, ideal active sites were found in such oxides by simulation. This clearly points out the strong potentiality of oxide-based materials to reach the equilibrium potential for the ORR. In more details, first-principle calculation indicated that two potential active sites which surpass Pt catalysts can be formed at oxide surfaces by (i) substitutional dopants (e.g., Pd and Rh) at the titania surface, and (ii) formation of stable oxygen vacancies at the surface of zirconium oxides. Being different from the simulation, the actual catalysts need electron transfer to the catalyst active sites and those oxides above mentioned are semiconductors with large band-gaps, hence carbon additives are often needed for the catalysts to form an appropriate electron conduction path from electric conductive supports to the active sites on the oxide surface. Careful analyses of such catalysts with carbon additives, we found that the crystalline structures of the titanium oxides (or atomic arrangements) and oxygen vacancies of the zirconium oxides were of great importance for the ORR activity. We also report the investigation of adsorbed oxygen species by the temperature programmed desorption.