SWCNT/ZnO nanocomposite decorated with carbon dots for photoresponsive supercapacitor applications

In this work, we report the fabrication of an optically responsive hybrid supercapacitor. The hybrid electrode material for the supercapacitor was synthesized by attaching carbon dots (CDs) on the SWCNT/ZnO nano-composites. The optical properties of CDs and ZnO have been explored by operating the supercapacitor under illuminated conditions (UV light). It was observed that the areal capacitance of the fabricated supercapacitor got enhanced by ~ 41.38% at 50 mV/s scan rate under UV light. The photo-charging and galvanostatic discharging of the device were also examined. The maximum photo-charged areal capacitance value was calculated to be 1.53 mF/cm(2) at a current density of 1.25 mu A/cm(2), whereas the values of areal energy density and areal power density at this current density value were 19.85x10-3 mu Wh/cm(2), and 0.0953 mu W/cm(2), respectively. The working mechanism of the supercapacitive system has also been explored. It is observed that the overall capacitance of the hybrid electrode is a combination of both electric double layer capacitance (EDLC) and pseudocapacitance. The EDLC and pseudocapacitance contributions were confirmed by using the Dunn method, and the values of the individual capacitance contributions are 69.35 and 30.65% for EDLC and pseudocapacitance, respectively. Further, it was observed that the pseudocapacitance phenomenon was dominated by the diffusion of the ions in the electrode material. The diffusivity of the electrolytic ions was calculated to be 0.13 and 0.14 mm(2)/s for the oxidation peaks, whereas 1 and 0.07 mm(2)/s for the reduction peaks, respectively. Moreover, the reaction mechanism of the system has also been explored, and the occurrence of intercalation of K+ ions in the defects of ZnO has been confirmed with justified explanations.