Self-reconstruction of highly active NiCo₂O₄ with triple-continuous transfer of electrons, ions, and oxygen for Zn-air batteries

The prohibitive cost and scarcity of the noble-metal catalysts needed for catalyzing the oxygen reduction/evolution reaction (ORR/OER) in the Zn–air battery (ZAB) limit the commercialization of this clean energy technology. Herein, a noble-metal-free spinel-type NiCo₂O₄ electrocatalyst with high activity and 3D porous structure is rationally designed by duplexing interface engineering concept through an adaptive combination of surfactant assistance and electrochemical reconstruction processes, resulting in co-enhanced behaviors of overall oxygen electrochemistry in ZABs. As expected, the conventional three-phase electrochemical interface could be expanded into 3D networks in whole air electrode, which allows the 3D triple-continuous passage of electrons, ions, and oxygen, thus boosting the oxygen electrochemistry of OER (1.62 V at E_j = 10) and ORR (0.79 V at E_1/2) in 0.1 M KOH electrolyte. Moreover, the ZAB with the as-prepared electrocatalyst demonstrates a low potential gap of 0.77 V and impressive long-term reversibility over 600 h at 5 mA cm⁻², surpassing most previous reports. Such superiority is ascribed, in part, to the self-reconstruction of NiOOH-NiCo₂O₄ interfacial layer with 3D networks during the OER process as observed by in-situ Raman, ex-situ XPS, and SEM, which contributes additionally to the high activity and durability of the air electrode by enhancing the charge and mass transfer in ZABs.