Deciphering the true active phases of bifunctional oxygen electrocatalyst in rechargeable zinc-air batteries: A case study of CoMoO₄

Efficient bifunctional oxygen electrocatalysis is essential for rechargeable metal-air batteries; however, their real active phases under operational conditions remain largely unexplored. In this study, using CoMoO₄ as a model electrode, the surface reconstructions during the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are elucidated through in situ Raman spectroscopy and electrochemical analyses. Our results reveal that the in situ generated CoO₂ acts as the primary active phase for OER, while β-CoOOH dominates the ORR process. Density functional theory calculations further confirm that the formation of these phases optimizes the electronic structure and reduces reaction energy barriers. An assembled zinc-air battery delivers a maximum power density of 138.3 mW cm⁻² with an excellent long-period cycling test for 320 h. This work offers valuable insights for the design of efficient oxygen electrocatalysts.