Comprehensive utilization of lignocellulosic biomass for the electrode and electrolyte in zinc-ion hybrid supercapacitors

Abstract

Zinc-ion hybrid supercapacitors (ZHSs) with carbon cathodes and aqueous electrolytes are promising as they easily combine advantages such as high energy density, high power density, long cycle life and good safety. Herein, a novel strategy can simultaneously fabricate porous carbon as the cathode and a cellulose-based hydrogel as the electrolyte from biomass. Lignocellulosic biomass is hydrothermally treated with a concentrated ZnCl₂ solution to obtain a solid residue and a filtered liquid. From the solid residue, porous carbon can be directly prepared by carbonization and it shows a high specific capacitance of 263 F g⁻¹ for the storage of zinc salt ions. From the filtered liquid, the cellulose-based hydrogel is synthesized by the addition of an ionic crosslinking agent and it has a wide voltage window of 2.0 V as the electrolyte for ZHSs. The ZHSs assembled from the sustainable carbon cathode and hydrogel electrolyte show a high capacity of 229 mA h g⁻¹ at 0.5 A g⁻¹ and retain a capacity of 86 mA h g⁻¹ at 10 A g⁻¹, and the energy density reaches 226 W h kg⁻¹ with a power density of 492 W kg⁻¹. The ZHSs also have an excellent capacity retention of almost 100% after 30 000 cycles.