Research progress in transition metal oxide based bifunctional electrocatalysts for aqueous electrically rechargeable zinc-air batteries

Zinc-air batteries (ZABs) are rising contenders for future applications in the power sources sector, due to their intrinsically elevated energy capacity coexisting with eco-friendly characteristics. Although rechargeable alkaline aqueous ZABs hold a considerable position in current research reports, their practical large-scale adoption is still obstructed by poor resistivity and cyclability. The bifunctional electrocatalyst in the air electrode appears to be the controlling factor of the efficiency of the air-cell, thus special effort has been invested into discovering effective and low-cost alternatives. Transition metal oxides have invariably been considered as competent ORR and OER electrocatalysts for other energy applications. However, a vast progress in testing of such nano structured catalysts in experimental aqueous alkaline ZABs has been detected in the last years, hence the scope of this article is to give a detailed summary of the electrochemical properties and performance of these transition metal oxide - based materials, in order to provide fertile ground for further performance-oriented electrocatalysts’ exploration. In the following sections, after defining certain fundamental operational aspects and challenges of the zinc air cell, special emphasis is devoted to broadened examination of recent advances in simple metal oxide, spinel oxide and perovskite oxide - based nanomaterials and nanohybrids applied as bifunctional electrocatalysts in rechargeable liquid alkaline ZAB configurations.