Beyond surface area: Tailoring ultramicropores, micropores, and surface functionalities of biomass carbon via one-step thermal process for enhanced capacitive performance

Biomass-derived carbon has shown significant potential as an active electrode material for supercapacitors, primarily due to its intrinsic porous architecture and the presence of heteroatom dopants. However, achieving hierarchical porous carbon through a simple, single-stage thermal process with minimal resources is a significant challenge. In this study, we propose molten base carbonization and activation (MBCA) with wet biomass feedstock as a viable strategy for converting abundant waste coffee grounds into high-performance porous carbon. This method utilizes wet biomass and KOH to enable the intercalation of potassium ions into the biomass components matrix. Through nucleophilic interactions at various reactive oxygen sites, carbon and hydrogen bonds are disrupted, with K + intercalation resulting in organic potassium compounds prior to drying. These potassium compounds facilitate carbonization and in-situ activation in a single-stage thermal process. The optimized carbon sample (2-WCW) had a specific surface area of 1187 m 2 g −1 , abundant ultramicropores, oxygen and nitrogen functionalities. It also displayed high specific capacitance of 351 F g −1 at 5 mV s −1 and 120 F g −1 at 10 A g −1 in 6 M KOH with cycling stability of over 50,000 cycles and 96 % capacitance retention. The assembled device using an organic electrolyte displayed a specific energy and power of 5.6 Wh kg −1 and 5000 W kg −1 respectively, and further exhibited good self-discharge after 100 h. This study offers insights into the importance of the state of the biomass before thermal treatment for tuning its porous structure and surface functionalities for enhanced electrochemical performance in supercapacitors.