Supercapacitive behaviors of N/S co-doped biomass porous carbon-based supercapacitors in “water-in-salt” electrolyte

Abstract

The design of carbon electrode materials and electrolytes is distinct significance for improving the energy density of supercapacitors. Herein, N/S co-doped biomass porous carbon materials based on sustainable resource were prepared by in-situ expansion, heteroatom doping, and subsequent KOH activation strategy by using camellia seed shell as carbon source and ammonium persulfate as dopant and expander. The as-prepared N/S co-doped biomass porous carbon (N/S-CSAC-650) possesses the characteristics of high specific surface area, large pore volume, and suitable heteroatom content. The N/S-CSAC-650 electrode achieves a high specific capacitance of 335.9 F g−1 in the 3 M KOH electrolyte at 0.5 A g−1. The constructed symmetric supercapacitor demonstrates excellent cyclic stability and shows a high retention rate of 98.4 % after 30,000 charge and discharge processes at 2 A g−1. Especially, the energy density of supercapacitors is usually constrained by the narrow electrochemical stability window (ESW) of dilute aqueous solution (1.23 V), in order to effectively widen the ESW of water, herein the “water-in-salt” (WIS) electrolyte strategy is put forward and the ESW of water was extended to 2.94 V by using a 25 M KAc WIS electrolyte. The operating voltage window of the symmetric supercapacitor assembled by 25 M KAc WIS electrolyte and N/S-CSAC-650 electrodes can reach 2.0 V, and a high energy density of 48.86 Wh kg−1 is realized.