Transition metal oxide-based oxygen reduction reaction electrocatalysts for energy conversion systems with aqueous electrolytes

In the past decades, there has been a strong incentive to develop electric vehicles by the introduction of batteries to reduce the dependence on petroleum oil and mitigate the tailpipe emissions. Lithium-ion batteries have dominated the electric vehicle market due to their high capacity and energy efficiency. However, the insufficient energy density of lithium-ion batteries is still a big problem for the development of electric vehicles. Metal-air batteries have been considered among the most promising power sources for electric vehicles due to some attractive advantages such as high energy density, low cost and environmental friendliness. One of the most important issues for metal-air batteries is developing oxygen reduction reaction catalysts with high catalytic activity and stability. Transition metal oxides are a series of important catalysts for the oxygen reduction reaction in alkaline solution. The purpose of this paper is to provide a comprehensive review of the recent progress in transition metal oxide-type catalysts for the ORR in aqueous media, including simple transition metal oxide-type catalysts, perovskite-type catalysts, spinel-type catalysts and other ternary transition metal oxides catalysts (such as double perovskite oxides, pyrochlore oxides, Ruddlesden-Popper oxides, LiCoO2-related oxides, and Mn-based mullite oxides). Moreover, we also discuss the factors influencing the transition metal oxide-type catalysts for the ORR in metal-air batteries with aqueous electrolytes.