A flexible solid-state asymmetric supercapacitor device comprising cobalt hydroxide and biomass-derived porous carbon

Development in the field of alternative and renewable energy sources is becoming necessary considering the current energy demands of the growing technologies. The main challenge associated with the produced energy is to store it for future use, such that it can be used when needed. Supercapacitors are among the electrochemical energy storage systems that provides higher power density, faster charging-discharging, high specific capacitance (C _S), and long cycling life. Herein, the fabrication of a flexible solid-state asymmetric supercapacitor (ASC) device is reported, where Co(OH)₂ hollow spheres and biomass-derived porous carbon (PC) are the cathode and anode, respectively. Co(OH)₂ is a highly redox active material, whereas PC is an electric double-layer capacitive (EDLC) material. In this device, aqueous KOH solution (electrolyte) encapsulated in PVA gel (separator) was used to bind the electrodes. This Co(OH)₂//PC ASC device exhibited a high C _S of 260 F g^-1 (at 2 A g^-1). It retained ∼91% of the initial C _S value (at 6 A g^-1) till ∼5000 cycles. Electrochemical impedance spectroscopy (EIS) study confirmed low internal resistance (0.95 Ω) and charge transfer resistance (1.41 Ω) values of Co(OH)₂//PC. These results indicate that the high electron transfer process in the electrode-electrolyte interface during the electrochemical reaction, which is responsible for the excellent performance of this ASC device. The high-performance Co(OH)₂//PC ASC device exhibited an energy density of 76.7 W h kg^-1 at a power density of 1416.9 W kg^-1. To demonstrate its practical use, LED lights were illuminated using this Co(OH)₂//PC ASC device.