Influence of Temperature on ZnO/CoO Nanocomposites for High Energy Storage Supercapacitors.

We developed a two-step chemical bath deposition method followed by calcination for the production of ZnO/CoO nanocomposites. In aqueous reactions, ZnO nanotubes were first densely grown on Ni foam, and then flat nanosheets of CoO developed and formed a porous film. The aspect ratio and conductivity of the CoO nanosheets were improved by the existence of the ZnO nanotubes, while the bath deposition from a mixture of Zn/Co precursors (one-step method) resulted in a wrinkled plate of Zn/Co oxides. As a supercapacitor electrode, the ZnO/CoO nanosheets formed by the two-step method exhibited a high capacitance, and after being calcined at 450 °C, these nanosheets attained the highest specific capacitance (940 F g) at a scan rate of 5 mV s in the cyclic voltammetry analysis. This value was significantly higher than those of single-component electrodes, CoO (785 F g) and ZnO (200 F g); therefore, the presence of a synergistic effect was suggested. From the charge/discharge curves, the specific capacitance of ZnO/CoO calcined at 450 °C was calculated to be 740 F g at a current density of 0.75 A g, and 85.7% of the initial capacitance was retained after 1000 cycles. A symmetrical configuration exhibited a good cycling stability (Coulombic efficiency of 99.6% over 1000 cycles) and satisfied both the energy density (36.6 Wh kg) and the power density (356 W kg). Thus, the ZnO/CoO nanocomposite prepared by this simple two-step chemical bath deposition and subsequent calcination at 450 °C is a promising material for pseudocapacitors. Furthermore, this approach can be applied to other metal oxide nanocomposites with intricate structures to extend the design possibility of active materials for electrochemical devices.