Low-Cost Rapid Template-Free Synthesis of Nanoscale Zinc Spinels for Energy Storage and Electrocatalytic Applications

Spinels form an interesting class of compounds, finding applications in metal-ion batteries and as catalysts for metal–air batteries and fuel cells. Here, we report a fast, template-free solution combustion method to synthesize nanoscale zinc spinels for applications as cathodes in low-cost aqueous zinc-ion batteries and oxygen reduction reaction (ORR) catalysts. It leads to the formation of phase-pure spinels with near spherical nanoscale morphology. Three spinels—ZnCo2O4 (ZCO), ZnMn2O4 (ZMO), and ZnMnCoO4 (ZMCO)—were investigated. ZMO and, for the first time, ZMCO were observed to show reversible Zn (de)insertion involving Mn4+/Mn3+ redox couple with a first discharge capacity of 109.4 mAh/g (i.e., >99% of theoretical capacity). Further, these Zn-based spinels showed appreciable oxygen reduction reaction (ORR) electrocatalytic activity. The ORR activity in alkaline solution was characterized by cyclic voltammetry (CV) and linear sweep voltammetry (LSV) using a rotating disk electrode (RDE) and was observed to be comparable to Pt/C with similar chronoamperometric stability. This enhanced ORR activity can be rooted to the optimal tuning of Co3+–O bond strength because of the presence of Mn3+. This work presents a robust synthesis route to prepare ZnMnCoO4 spinel acting as an economic cathode material for large-scale Zn-ion batteries for grid storage applications as well as an efficient and stable alternate ORR catalyst in alkaline solution.