An advanced three-dimensionally ordered macroporous NiCo2O4 spinel as a 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 a template, in which the high surface area of the 3DOM spinel, the presence of multiple 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 benchmark Pt/C. Also, the spinel needs only 10 mV more overpotential compared with benchmark IrO2/C to achieve 10 mA cm−2 during the oxygen evolution reaction (OER). The 3DOM spinel shows superior stability for both reactions, retaining 80.5 (for the ORR) and 100% (for the OER) of the 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 successfully supplying the energy requirements of a multi-sensor, which is operated for more than 2 h.