Low-Cost Rapid Template-Free Synthesis of Nanoscale Zinc Spinets 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 spinet 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.