Surface engineering of Zr[sbnd]Co embedded biochar for efficient and cost–effective energy storage application

Abstract

Advancement of sustainable and high–performance energy storage materials is critical for meeting growing global energy demands and enabling a low–carbon future. This study presents a novel, low–cost, and eco–friendly strategy for electrode fabrication using watermelon rind–derived biochar (B). Biochar was successively engineered through incorporation of zirconium (10 %, B[sbnd]Zr), embedded with cobalt (B–ZrCo), and surface functionalization with carboxylic acid groups (–COOH), resulting in development of activated (AcB–ZrCo) biochar. Optimized AcB–ZrCo electrode exhibited exceptional electrochemical performance in a three–electrode configuration, delivering a high specific capacitance of 1181 F/g at 1 A/g and 1110 F/g at 5 mV/s. These correspond to a remarkable energy density of 233 Wh/kg and a power density of 0.31 kW/kg. The Bayesian Ridge regression model was employed to optimize and validate capacitive and diffusive charge storage contributions, achieving reduced root mean square error (RMSE) and providing deeper insight into underlying storage mechanisms. In a practical two–electrode system, AcB–ZrCo retained a high capacitance of 936 F/g at 1 A/g, along with an energy density of 203.9 Wh/kg and a power density of 0.33 kW/kg. Moreover, electrode demonstrated outstanding long–term stability, maintaining its performance over 5000 charge–discharge cycles and sustaining high current operation for 50 h. These results underscore potential of AcB–ZrCo as a scalable, durable, and sustainable electrode material, offering a promising pathway toward next–generation energy storage technologies. © 2025 Elsevier B.V., All rights reserved.