High temperature monolithic biochar supercapacitor using ionic liquid electrolyte

A supercapacitor comprising of two binder-free biochar monolith electrodes and 1-butyl-3-methylimidazolium tetrafluoroborate based ionic liquid electrolyte was studied at room temperature and 140°C by cyclic voltammetry, constant-current charge-discharge, and electrochemical impedance spectroscopy. The supercapacitor exhibits an operating voltage window of approximately 6 V. It is found that increasing temperature from room temperature to 140°C considerably increases its specific mass capacity and its charge-discharge rate by a factor of approximately 10. The specific capacity of the supercapacitor calculated from the voltammetric measurements depended on scan rates. At 140°C, a capacity of 21 F g⁻¹ was obtained at 5 mV s⁻¹ and this value decreases to around 10 F g⁻¹ at 100 mV s⁻¹; the constant-current charge-discharge profiles exhibit pseudo-linear voltage-time responses during the discharges. The supercapacitor shows good stability characteristics of no obvious performance decay after 1000 cycles within a voltage window of 6 V. Electrochemical impedance spectra of the supercapacitor display a wide linear region corresponding to diffusion control. The energy densities of the supercapacitor that are normalized to the total active electrode materials are higher than 20 Wh kg⁻¹ when its power density is lower than 2000 W kg⁻¹. These facts suggest that the high-temperature biochar supercapacitor would be a promising energy-storage device with high energy and power density.