Papain-Enzyme Functionalized Banana Peel Carbon for High-Performance Supercapacitor Electrodes via a Green Chemistry Route

In this study, a novel and sustainable strategy is proposed for developing high-performance supercapacitor electrodes based on banana peel-derived activated carbon (BPAC) functionalized with the papain enzyme. Surface modification was carried out by covalently immobilizing papain onto BPAC at different BPAC-to-enzyme ratios (1:1, 1:2 and 2:1), followed by comprehensive electrochemical characterization using cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS). The BPAC-EP (2:1) electrode exhibited superior performance, achieving a specific capacitance of 67.546 F g⁻¹ and an energy density of 146.034 Wh kg⁻¹, with a low internal resistance (IR drop of 0.0915 V). EIS analysis confirmed a substantial decrease in charge transfer resistance (Rct) and an increase in double-layer capacitance (Cdl), indicating enhanced electrochemical kinetics. In contrast, the 1:2 composition showed a significant performance decline, with a specific capacitance of only 8 F g⁻¹, highlighting the critical role of enzyme loading. The enhancement is attributed to the introduction of polar functional groups (–OH, –COOH, – NH₂) through enzymatic treatment, which improves surface polarity and charge transport. This work demonstrates the novelty of employing enzymatic functionalization as a green chemistry route to upgrade biomass waste into efficient electrode materials for sustainable energy storage applications.