An advanced three‒dimensionally ordered macroporous NiCo2O4 spinel as bifunctional electrocatalyst for rechargeable Zn‒Air batteries

In this work, highly active NiCo2O4 spinels with three-dimensionally ordered macroporous (3DOM) structures were obtained using poly (methyl methacrylate) PMMA spheres as template, for which the high surface area of the 3DOM spinel, the presence of multi‒valences of Co and Ni species and the high Ni content improve the activity for the oxygen reactions, presenting a current density value similar to that obtained for benchmarked Pt/C. Also, the spinel shows only 10 mV of overpotential compared with benchmarked IrO2/C for oxygen evolution reaction (OER). The 3DOM spinel shows superior stability for both reactions, maintaining 80.5 (for ORR) and 100 % (for OER) of initial current after 10 h, respectively. This 3DOM spinel is used as an air electrode in a rechargeable Zn‒air battery, enabling an open-circuit potential of 1.44 V, which is similar to that obtained for Pt/C (1.46 V), while also displaying a similar performance (109 vs. 101 mW cm−2). Moreover, because of its excellent activity as a bifunctional electrocatalyst, the 3DOM spinel displays a superior charge‒discharge capability after being operated for 21 h, while it provides successfully the energy requirements of a multi-sensor, which is operated for more than 2 h.