Sheet-like ZnCo2O4 microspheres and pomelo peel waste-derived activated carbon for high performance solid state asymmetric supercapacitors

A novel asymmetric supercapacitor based on zinc cobalt oxide (ZnCo ₂O ₄) as a positive electrode and pomelo peel-based activated carbon (PPAC-4) as a negative electrode was fabricated. ZnCo ₂O ₄ was synthesized using hydrothermal method and shows a distinct sheet-like structure clubbed together to form microspheres. PPAC-4 was synthesized via the chemical activation of pomelo peel hydrochar using KHCO ₃ as the activation agent. The as-prepared ZnCo ₂O ₄ microspheres displayed maximum specific capacitance of 422 F g ⁻¹ at an applied current density of 1 A g ⁻¹, whereas PPAC-4 demonstrated a capacitance of 356 F g ⁻¹ at a current density of 1 A g ⁻¹. Moreover, the novel ZnCo ₂O ₄//PPAC-4 asymmetric solid-state supercapacitor with hydroxyethyl cellulose/potassium hydroxide (HEC/KOH) gel electrolyte displayed maximum operating voltage of 1.6 V with a superior energy density of 29.8 W h kg ⁻¹ at a power density of 796.3 W kg ⁻¹. Besides, a long-term cycling stability of 83% was achieved after 10 000 charge/discharge cycles, indicating its promise for future energy storage applications.