Oxygen Reduction Reaction Catalytic Activities of Pure Ni-Based Perovskite-Related Structure Oxides

Complex transition-metal oxides offer an interesting class of materials for use as oxygen reduction reaction (ORR) catalysts. A recent study proposed a volcano-shape relationship between ORR activity and eg electron number, but it was surprising that the relationship was applicable for many compounds with various transition-metal cations at the single site. Here we verified the relation for perovskite-related structure oxides containing Ni centers with valence state from 2+ to unusually high 4+, covering eg electron occupation in the range of 0 < eg ≤ 2. We found that Ni3+ with moderate eg-orbital-filling (eg ≈ 1) promotes the most efficient four-electron ORR reactions, giving the highest catalytic activity. Our results obtained from a series of compounds consisting of only NiO6 octahedra also suggest that band filling in the eg orbital of Ni is the key parameter affecting ORR catalytic activity and that the electronic structure primarily determines the catalytic property.