Active site-engineered bifunctional electrocatalysts of ternary spinel oxides, M0.1Ni0.9Co2O4 (M: Mn, Fe, Cu, Zn) for the air electrode of rechargeable zinc-air batteries

Discovering efficient and cost-effective catalysts for rechargeable metal-air batteries is one of the major scientific challenges in future energy storage/conversion technologies because of the sluggish kinetics of the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Based on the octahedral site preference energy (OSPE) model, engineering the active sites of ternary M0.1Ni0.9Co2O4 for the OER and ORR in alkaline solutions is demonstrated in this work. From the X-ray photoelectron spectroscopy (XPS) and OSPE models, Fe-doped NiCo2O4 (Fe0.1Ni0.9Co2O4) provides the highest Co2+/Co3+ ratio and the lowest Ni2+/Ni3+ ratio, leading to enhanced electrocatalytic activities toward both the OER and ORR in alkaline electrolytes from rotating ring disk voltammograms. In addition, all ternary oxides are examined as bifunctional electrocatalysts for the air electrode of rechargeable zinc-air batteries using the polarization curves of the ORR and OER in 6 M KOH in ambient air. The full-cell configuration using a Fe0.1Ni0.9Co2O4-coated air electrode exhibits a maximum power density of 150 mW cm(-2) at a current density of 250 mA cm(-2) in ambient air and facilitates long-term cycling stability (over 66.7 h at 10 mA cm(-2)). These results confirm the excellent bifunctional electrocatalytic activity of Fe0.1Ni0.9Co2O4, which is considered to be a practical catalyst for the air electrode of rechargeable Zn-air batteries.