Valorization of lignin waste: high electrochemical capacitance of lignin-derived carbons in aqueous and ionic liquid electrolytes

Abstract

This report describes the utilization of waste lignin-derived activated carbons (LACs) as high-energy/high-power electrode materials for electric double layer capacitors (EDLCs). The influence of the carbon-pore structure on the capacitance of the LACs in two aqueous (H₂SO₄ and KCl) and two ionic liquid (1-ethyl-3-methylimidazolium ethylsulfate, [EMIm][EtSO₄], and 1-butyl-3-methylimidazolium tetrafluoroborate, [BMIm][BF₄]) electrolytes is evaluated. In EDLCs containing aqueous H₂SO₄ as the electrolyte, the LACs exhibit specific capacitances of up to 223 F g⁻¹ and good cycling stability, with an energy density of 5.0 W h kg⁻¹ at a power density of 200 W kg⁻¹. EDLCs containing KCl achieved a specific capacitance of 203 F g⁻¹, and an energy density of 7.1 W h kg⁻¹ at a power density of 510 W kg⁻¹. The specific capacitances of the LACs in [EMIm][EtSO₄] and [BMIm][BF₄] were up to 147 F g⁻¹ and 175 F g⁻¹, respectively. The energy density in the IL electrolytes is up to 25 W h kg⁻¹ at a power density of 500 W kg⁻¹ and 16.4 W h kg⁻¹ at 15 kW kg⁻¹. We demonstrate that the electrochemical performance of the LACs depends not only the surface area and pore size, but also on the pore-wall thickness.