Biowaste-based porous carbon for supercapacitor: The influence of preparation processes on structure and performance

Here, a series of porous carbon based supercapacitor electrode materials have been synthesized by means of pyrolysis and hydrothermal methods combining with KOH activation using the biomass wastes mung bean husks as resources. The influence of synthesis process on the morphology, structure and supercapacitor performance of mung bean husks derived porous carbons has been investigated systematically. Especially, it is found that these oxygen-containing groups on the biochar play a crucial role in fabricating the three-dimensional (3D) hierarchical porous structure carbon. The original bio-structured porous carbon (PC_3-600), the 3D architecture porous carbon (HPC_2-700) and the porous carbon block (HPPC_2-700) have a high specific surface area, and the former mainly contains micropores and the latter two possess multistage pores. The specific capacitance of PC_3-600, HPC_2-700 and HPPC_2-700 is respectively up to 390 F g^-1, 353 F g^-1, 304 F g^-1 at 1 A g^-1, and still maintains as high as 287 F g^-1, 270 F g^-1 and 235 F g^-1 with corresponding retention ratio of 73.5%, 76.48%, 77.3% even at a high current density of 50 A g^-1. HPC_2-700//HPC_2-700 symmetric supercapacitor achieves a high energy density of 20.4 Wh kg^-1 at 872 W kg^-1 in 1 M Na₂SO₄ electrolyte.