Room temperature and rapid synthesis of ZnMn2O4 nanostructured spinel using deep eutectic solvent for high energy asymmetric supercapacitors

Abstract

Recently, binary metal oxides have drawn significant attention due to their interesting physico-chemical characteristics, makes them suitable for supercapacitor application. In the present work, high surface area spinel ZnMn2O4 is prepared using a reaction of deep eutectic solvent (DES) containing zinc chloride, choline chloride and glycerol with manganese precursor (KMnO4). The DES serves as both precursor and template for the growth of ZnMn2O4. Interestingly, the spinel structure of ZnMn2O4 is prepared within one-minute time at room temperature using DES route makes this process cost-effective. The reaction between DES and manganese precursors can be halted at any stage with the addition of water to the reaction mixture as it reduces existing bonding interaction between the reactants. With this advantage, we have varied reaction time between DES and KMnO4 and further studied the effect of reaction time on surface area and electrochemical properties of the obtained ZnMn2O4. The ZnMn2O4 sample, where DES and KMnO4 reacted for 10 min (ZnMnO-10) showed highest surface area of 118.5 m2/g with a pore volume of 0.19 cm3/g. Further, the ZnMnO-10 sample showed a good specific capacitance of 499.58 F/g at 0.1 A/g in a neutral electrolyte (1 M Na2SO4). The symmetric supercapacitor device fabricated using ZnMnO-10//ZnMnO-10 with energy and power density of 39.40 Wh/kg and 7.06 kW/kg, respectively. Further, the asymmetric supercapacitor device is also assembled using ZnMn2O4 and agro waste-derived carbon (HBC-800) showed an excellent Csp of 331.24 F/g at 0.2 A/g with energy density and power density of 74.50 Wh/kg and 5.4 kW/kg, respectively. The current work proposes the rapid and facile synthesis of spinel ZnMn2O4 utilizing green DES and the obtained ZnMn2O4 showed good specific capacitance and energy density for both symmetric and asymmetric supercapacitor devices.