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. HIGHLIGHTS Sustainable Electrode Material: Utilizes waste banana peel activated carbon (BPAC) functionalized enzymatically (using papain) as a core material for supercapacitor electrodes, emphasizing a Green Chemistry approach. Enzyme Ratio Optimization: Surface functionalization was systematically studied by varying the BPAC-to-papain enzyme ratio (1:1, 1:2, 2:1), with electrochemical performance evaluated via CV, GCD and EIS. Optimal High Performance: The BPAC-EP (2:1) ratio demonstrated superior performance: Highest specific capacitance (67.54 F g−1), high power density (146.03 Wh kg−1), lowest internal resistance (IR drop 0.091 V), reduced charge transfer resistance (Rct) and increased double-layer capacitance (Cdl). Critical Enzyme Loading: Performance is highly sensitive to enzyme amount. Adding too little enzyme (BPAC 1:2 ratio) drastically reduced specific capacitance to only 8 Fg−1, highlighting the importance of optimal functionalization. Mechanism & Confirmation: Enhanced stability and efficiency are attributed to active polar groups (-OH, -COOH, -NH2) introduced by enzymatic modification, confirming successful functionalization and offering a sustainable path for biomass-based supercapacitors. GRAPHICAL ABSTRACT