Walnut shell derived N, S co-doped activated carbon for solid-state symmetry supercapacitor device

The kernel of the walnut constitutes a nutrient-dense food source. However, the walnut shell is useless and was thrown away. Therefore, as a small initiative towards waste-to-wealth, herein we developed walnut shell-derived highly porous heteroatom (nitrogen and sulfur) doped activated carbon (NS-WAC) via KOH activation and post-processing treatment by thioacetamide (TAA). The NS-WAC exhibits a high specific surface area (SSA) and various particle size distribution (PSD) involving micropores, mesopores, and macropores, conferring substantial supercapacitor advantages. The NS-WAC possesses a significant proportion of mesopores, which can decrease the ion diffusion path length and improve ion transport efficiency. Hence, NS-WAC demonstrates promising electrochemical performance achieving a high gravimetric specific capacitance of 271.4 Fg−1 at 0.5 Ag−1 in 3 M KOH aqueous electrolyte. Furthermore, the solid-state symmetric supercapacitor (SSC) device exhibited a gravimetric specific capacitance of 52 Fg−1 and volumetric specific capacitance of 74.36F cm−3 at a current density of 0.5 Ag−1. This SSC device also exhibited a gravimetric energy density of 23.4 Wh Kg−1 at a power density of 250 W Kg−1 at a current density of 0.5 Ag−1. It showed superior cycle stability of 91.8% after 10,000 cycles at 5 Ag−1.