Rice straw-derived activated carbon/ZnO nanocomposite as a high-performance electrode for asymmetric supercapacitors

The synthesis of electrode materials from biomass waste has attracted considerable attention due to their low cost and high electrochemical performance in energy devices. Therefore, in this study, activated carbon was prepared from rice straw waste and composited with zinc oxide (ZnO) nanoparticles to enhance the electrochemical performance of an asymmetric supercapacitor. AC, ZnO, and AC-ZnO exhibit specific surface areas of 201, 255, and 578 m² g^-1, respectively, with average pore diameters of 1.37 nm, 1.73 nm, and 2.45 nm, respectively. The AC-ZnO composite exhibited a higher specific capacitance of 244 F g^-1, compared with 206 F g^-1 for ZnO and 137 F g^-1 for AC at a current density of 0.5 A g^-1. The charge-transfer resistance (R _ct) values for AC, ZnO, and AC-ZnO were 0.050, 0.061, and 0.035 Ω, respectively. After 10 000 charge-discharge cycles, the corresponding capacitance retentions were 83.5%, 75.3%, and 89.9%, respectively. The AC-ZnO composite exhibits a hybrid capacitive behavior due to the electrical double-layer capacitor behavior of AC and the pseudocapacitive behavior of ZnO. The specific capacitance, energy density, and power density of the assembled device are 149 F g^-1, 20.66 Wh kg^-1, and 500 W kg^-1, at 1 A g^-1. Moreover, the assembled device exhibited 97% capacitance retention and 100% coulombic efficiency after 5000 charge-discharge cycles.