Mixed-valence cobalt oxides bifunctional electrocatalyst with rich oxygen vacancies for aqueous metal-air batteries

As the bifunctional oxygen electrocatalysts, the mixed-valence metal oxide with multiple oxidation state of metal ions usually gives analogous alloy effect. However, the irreversible surface reconstruction of the electrocatalyst during the electrochemical catalysis usually changes the oxidation state of metal ions in the near-face to higher oxidation state especially in the mixed-valence metal oxide. For the modulation equilibrium, we adopt a new approach to prepare the mixed-valence cobalt oxide with the regulatable ratio of Co 2+ /Co 3+ by inducing air at a certain temperature during the cooling procedure. The obtained Co/CoO/Co 3 O 4 /NCS with hollow nanosphere possesses multiple heterointerfaces accompanying rich oxygen vacancies. Together with the contribution of the mixed valence and oxygen vacancies, Co/CoO/Co 3 O 4 /NCS exhibits an exceptional ORR activity with a half-wave potential of 0.844 V and superb OER activity with a low overpotential of 285 mV at 10 mA cm −2 . The as-fabricated Zn-air batteries with Co/CoO/Co 3 O 4 /NCS catalyst exhibit a remarkable open-circuit voltage (1.47 V), a competitive power density of 146.5 mW cm −2 , and good durability without nearly no performance decay for over 400 h at 5.0 mA cm −2 . More attractively, when constructed into Mg-air batteries, Co/CoO/Co 3 O 4 /NCS cathode realizes an excellent stable discharge voltage over 90 h at 20 mA cm −2 .