Boosted Oxygen Reduction Reaction Activity by Ordering Cations in the A-Site of a Perovskite Catalyst

Abstract

As concerns related to climate change continue to grow, there has been an increasing number of studies on materials that can enhance the efficiency of energy conversion/storage systems. Since the oxygen reduction reaction (ORR) is considered as a rate-determining reaction for the O2 conversion system, it is essential to develop an efficient electrocatalyst that enables greater ORR activity. Perovskite has been explored as a promising electrocatalyst due to its excellent ORR activity at high and room temperatures. Recently, unique perovskite structures that can contain high oxygen vacancy ratios, such as brownmillerite, Ruddlesden–Popper, and layered perovskite, have been studied for exhibiting high ORR performance. However, few studies have dealt with the electrochemical properties of perovskite, which has different structures but the same cation stoichiometry, to know the exact effect of a specific structure. In this study, we synthesized two materials with different structures (cubic structure perovskite and layered structure perovskite) but with the same cation stoichiometry. The layered structure rather than the cubic structure showed cation ordering in the A-site of the perovskite structure with an enhanced oxygen vacancy ratio. As a result, the layered structure exhibits superior electrochemical performance than the cubic structure at high and room temperatures by a factor of ca. 2.7 at 500 °C in fuel cell mode, and ca. 5.5% decrease in overpotential (η) at −3 mA cm–2 for ORR catalysis. Moreover, perovskite hybridized with nitrogen doped reduced graphene oxide (N-rGO) could compensate for the intrinsic shortcoming of perovskite oxide and exert a catalytic synergy (27% decrease in η).