The Role of Ionic Electrolytes on Capacitive Performance of ZnO-Reduced Graphene Oxide Nanohybrids with Thermally Tunable Morphologies

In the present work, the role of the reaction temperatures on the morphologies of zinc oxide-reduced graphene oxide (ZnO-RGO) nanohybrids and their supercapacitive performance in two different aqueous electrolytes (1.0 M KCl and Na2SO4) were investigated. The ZnO-RGO nanohybrids were synthesized at two different temperatures (ca. 95 and 145 degrees C) by solvothermal method and labeled as ZnO-RGO-1 and ZnO-KGO-2, respectively. The structure and composition of ZnO-RGO nanohybrids were confirmed by means of X-ray diffraction, electron microscopes (scanning and transmission), X-ray photoelectron, photoluminescence, and Raman spectroscopy. These results show that the temperature allows a good control on loading and morphology of ZnO nanoassemblies in ZnO-RGO nanohybrids and at elevated temperature of 145 degrees C, ZnO nanoassemblies break and get completely embedded into RGO matrices. The electrochemical performance of ZnO RGO nanohybrids was examined by cyclic voltammograms (CVs), galvanostatic charge discharge (chronopotentiometry) and electrochemical impedance spectroscopy (EIS) in 1.0 M KCl and Na2SO4 aqueous electrolytes respectively. Combining the EIS and zeta potential behavior, a direct link between the charge transfer resistance and electrical double layers is established which is responsible for excellent capacitive performance of ZnO-RGO-2. The ZnO-KGO-2 displays high specific capacitance (107.9 F/g, scan rate = 50 mV s(-1)) in 1.0 M KCl and exhibits merely 4.2% decay in specific capacitance values over 200 cycles.