Heteroatom co-doped green pea peel-derived biochar for high-performance energy storage applications

Green pea peel (GPP) is a waste, and it is abundant and available to be used for biochar synthesis. GPP-derived biochar (GP) is used vastly in wastewater treatment. Moreover, heteroatom co-doping of GP could be better than its single-doped and undoped in enhancement of active sites and conductivity, and in developing electrodes for supercapacitor applications. However, uncontrolled heteroatom co-doping clogs the pores in the biochar and stops the electrolyte from penetrating the porous structure, which results in reduced capacitance and higher resistance in the biochar. This study presented the controlled synthesis of GP, nitrogen (N)-doped biochar (NGP), and N and sulfur (S) <i>ex situ</i> co-doped GP (NSGP) through carbonization and chemical activation. As revealed by the characterization techniques, the synthesized GP, NGP, and NSGP are nanosheets with amorphous structures and defective structures. The specific capacitance of the NSGP-based electrode material, as determined by electrochemical characterizations, is 257.01 F g^-1, more than the 230.22 F g^-1 and 208.78 F g^-1 of NGP and GP at 1 A g^-1, respectively. The assembled NSGP//NSGP supercapacitor device has an 80.25 F g^-1 specific capacitance at 1 A g^-1, an energy density of 13.87 W h kg^-1, and a 500 W kg^-1 power density with a 99.46% capacity retention after 5000 cycles at 5 A g^-1. It demonstrates that NSGP has better electrochemical performance than NGP and GP because of the improved active sites and conductivity.