Bifunctional electrocatalysts for Zn-air batteries

Rechargeable metal-air batteries have recently attracted significant attention because of their high theoretical energy output and low cost. Among metal-air batteries, Zn-air batteries (ZABs) have garnered renewed interest as one of the most viable future options to power energy grids and electric vehicles. However, the slow oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) kinetics represent a limiting factor for the energy conversion efficiency of state-of-the-art ZABs. The focus of this review is the latest advances related to the development of non-precious metal catalysts. Four groups of bifunctional catalysts used in primary/secondary ZABs are reviewed. These include transition metal oxides (single/mixed-metal oxides, spinels and perovskites), transition metals, carbon-based materials and precious metals/alloys. ZAB electrochemistry and challenges originating from individual components of the system, such as the Zn electrodes, electrolytes and separators, are also outlined. In addition, the design and architecture of the air-electrode are discussed for future development of ZABs.