The complicated mechanisms and sluggish kinetics of oxygen reduction reaction (ORR) and evolution reaction (OER) are important issues to be overcome for the popularizations of next-generation technologies such as fuel cell and water splitting. The noble-metal catalysts such as Pt, Pd (for ORR), Ir, Ru (for OER) are considered as promising materials to facilitate the oxygen-related catalysis, while their cost and scarcity can also prevent the devices from wide use. Therefore, non-noble metal catalysts have received much attention as candidates of an air-electrode material to realize the popularizations of next-generation energy devices. Among various materials investigated so far, perovskite-type mixed oxides are known as bifunctional catalysts which shows both of ORR and OER activities. In previous reports, several factors have been proposed as the descriptor of their activities (e.g. the occupancy of the e_g electronic states^1,2.)

In this work, we focused on the oxygen vacancy and non-stoichiometry of the metal-oxides. We chose LaNiO_3-δ as the model of perovskite-type mixed oxide, which is known as one of the most active bifunctional electrocatalysts among perovskite-type oxides, and investigated the influence of the amount of oxygen vacancies on ORR and OER activities. We prepared nonstoichiometric LaNiO_3-δ (δ=0.06, 0.11, 0.25, 0.48) by hydrogen reduction process, and compared their activities using linear sweep voltammetry (LSV).

The results showed their ORR activity decreased as the amount of oxygen vacancy sites increased, which agree with the previous report by Suntivich et al¹. that demonstrated the perovskite oxides with B site cation possessing one e_g electron showed the best ORR activity. We conducted further experiments using similar La-Ni mixed oxides such as Ruddlesden-Popper type La₂NiO₄ to clarify the role of oxygen vacancies, and the results indicated the oxygen vacancy sites could also contribute to ORR processes.

On the other hand, we found no relationship between non-stoichiometry and the OER activities of LaNiO_3-δ, suggesting other factors affecting the activity. From the results of CV and TEM analysis, we found that Ni hydroxide species existed on the surface of the catalysts. Ni hydroxide is a well-known electrochemical catalyst for OER, and these results indicated the amount of hydroxide species had a significant effect on OER activity of perovskite-type metal oxides.

1) J. Suntivich et al., Nat. Chem., 2011, 3, 546−550.

2) J, Suntivich. et al., Science, 2011, 334,1383-1385.

3) O. Diaz-Morales. et al., ACS. Catal., 2015, 5, 5380-5387