Highly active air electrode catalysts for Zn-air batteries: Catalytic mechanism and active center from obfuscation to clearness

Carbon-based materials have been found to accelerate the sluggish kinetic reaction and are largely subject to the overall Zn-air batteries (ZABs) property, while their full catalytic mechanism is still not excavated because of the indistinct internal structure and immature in-situ technology. Up to now, systematic methods have been utilized to study and design promising high-performance carbon-based catalysts. To resolve the real active units and catalytic mechanism, developing molecular catalyst is a significant strategy. Herein, the review will initiate to briefly introduce the working principle and composition of ZABs. An important statement is correspondingly provided about the typical structure and catalytic mechanisms for the air cathode material. It also presents the tremendous endeavors on the catalytic performance and stability of carbon-based material. Furthermore, combined with theoretical calculation, the self-defined active sites are analyzed to understand the catalytic character, where the molecular catalyst is subsequently summarized and discussed through highlighting the unambiguous and controllable structure, in the hope of surfacing the optimum catalyst. Building on the fundamental understanding of carbon-based and molecular catalysts, this review is expected to provide guidance and direction toward designing future mechanistic studies and ORR electrocatalysts.