Polyol mediated synthesis of anisotropic ZnO nanomaterials and composite with rGO: Application towards hybrid supercapacitor

Abstract

Metal oxides nanomaterials are widely investigated for electrode materials of supercapacitor. However, the performance of supercapacitor strongly correlated to the variable oxidation state of metallic ions in metal oxides as well as morphology (shape and size) and porosity of the nanomaterials. Here, we have investigated the role of polyols such as ethylene glycol (EG), diethylene glycol (DEG) and polyethylene glycols (PEG, Mw = 400 and 1000) on the formation of ZnO nanostructures with anisotropicity in morphology. The phase of ZnO and coating with polyol was confirmed by X-ray diffraction and thermogravimetric (TG) analysis. FESEM image reveals nanosphere, nanoassemblies, nanorod, and hexagonal (cup shaped and rigid shaped) morphology of ZnO particles in different dimensions. The specific capacitance of the nanomaterials was measured by cyclic voltammetry method in a three electrodes system with 1 M KCl solution as electrolyte. Hexagonal rigid shaped ZnO gives better capacitance value of (6.42 F/g) followed by hexagonal cup shaped (6.4 F/g), nanoassemblies (5.75 F/g), nanorod (4.03 F/g), and nanosphere (3.67 F/g) at 10 mV/s. Further hexagonal shaped zinc oxide was made composite with reduced graphene oxide (rGO) and interestingly, the composite with 1:2 wt% of rGO to ZnO exhibits excellent specific capacitance of 251.16 F/g at 2 mV/s. Chronopotentiometry analysis indicated good charging and discharging behavior of the composite whereas electrochemical impedance spectroscopy revealed a good charge transfer characteristic.