N, P co-doped porous carbon/MnO₂ composites for zinc-ion hybrid capacitors

Environmental problems caused by the use of fossil fuels have become increasingly serious, and the development of clean energy devices and renewable energy has become an inevitable trend. In this paper, coffee grounds carbon/MnO₂ composite was prepared by a simple one-step activation method, and its application in zinc-ion hybrid capacitors (ZHSC) field was discussed. BET test showed that the specific surface area of coffee grounds carbon/MnO₂ composite was 550.25 m²/g, and the total pore volume was 0.6284 cm³/g. Electrochemical tests showed that electrode displayed excellent specific capacitance of 401.5 F/g at a current density of 0.5 A/g and 264 F/g at a current density of 20 A/g, which revealed good rate capability. Meanwhile, ZHSC reaches 74.2 mA·h/g, energy density is 39.1 W·h/kg and power density is 4264 W/kg at 0.5A/g current density. Furthermore, the capacitance retention rate is 98% and the Coulomb efficiency is 98.7% after 5000 charge-discharge cycles, at the current density of 10 A/g, which indicating it has good cyclic stability and reversibility. Therefore, coffee grounds carbon/MnO₂ composites provide a new idea for the exploration of biomass carbon and MnO₂ composites.