Capsule-shaped calcium and cobalt-doped ZnO electrodes for high electrochemical supercapacitor performance

Metal co-doping of metal oxide nanostructures is a promising approach for enhancing the electrochemical performance of supercapacitors. Herein, calcium (Ca) and cobalt (Co) co-doped ZnO capsules (Ca-Co@ZnO) were fabricated using a facile and single-step hydrothermal process. The physical, chemical, and morphological properties of the Ca-Co@ZnO were analyzed using a range of characterization techniques such as X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy. Ca-Co@ZnO showed a remarkable supercapacitor performance in a 3 M KOH aqueous electrolyte. The specific capacitance was 1020 F/g at a current density of 0.75 A/g, which was 3.2, 1.7, and 1.6 times higher than the pristine ZnO, Ca-ZnO, and Co-ZnO capsules, respectively. Ca-Co@ZnO showed more than 50% capacity retention at a higher current density and strong cycling stability up to 5000 cycles with only 8% capacity loss. The Ca-Co@ZnO//Ca-Co@ZnO symmetric performance was also investigated. This device showed a specific capacitance of 187 F/g at a current density of 1 A/g and an energy density of 25.9 Wh/kg at a power density of 556.6 W/kg. The superior performance was attributed to the fast electron accessibility, strong ion diffusion, and higher active sites. Overall, the superior electrochemical performance and novel structures could be beneficial for developing metal co-doped metal oxide electrodes for supercapacitor applications.