Synergistic enhancement of coconut shell biochar for energy storage via KOH activation and electrooxidation

Biomass-derived biochar is a promising supercapacitor (SC) electrode material, but its performance is often limited by inadequate porosity and insufficient oxygen-containing functional groups when prepared via single-step pyrolysis. Herein, we propose a synergistic strategy combining KOH activation with electrooxidation to fabricate functional coconut shell biochar. KOH activation at 800 °C creates highly porous biochar, which is subsequently electrooxidized under ambient conditions. The biochar oxidation reaction (BOR) exhibits a 150 mV lower overpotential at 10 mA cm−2 compared to the oxygen evolution reaction. When coupled with hydrogen evolution reaction, the system achieves a hydrogen production rate more than four times that of conventional water electrolysis. Prolonged electrooxidation facilitates the formation of micropores and their transformation into mesopores. Comprehensive characterization shows that the combined pyrolysis–electrooxidation approach endows the biochar with an exceptionally high specific surface area of up to 2067.92 m2 g−1, along with abundant oxygen-containing functional groups. When employed as an SC electrode, the electrochemically-modified biochar delivers a maximum specific capacitance of 394.1 F g−1 at 1 A g−1. Theoretical calculations further confirm enhanced K+ adsorption capacity and improved charge transfer kinetics in the modified biochar. This work offers a novel avenue for designing high-performance biochar-based materials for energy storage.