Green-synthesised biodegradable, recyclable Zn(II) complex-based polymer electrolytes for electrochemical capacitor applications

The adoption of resilient energy storage systems demands electrolytes that maintain a balance between superior properties and ecological friendliness. Petroleum products-based polymer electrolytes deliver high conductivity but are barely recyclable or naturally biodegradable. This prompts persistent uncertainty regarding resource demand and disposal issues. Although biopolymer electrolytes are widely employed as sustainable alternatives, their reusability and natural degradation remain insufficiently addressed. In this study, transition metal complex-integrated composite biopolymer electrolytes (CBEs) were formulated from methylcellulose, pectin, potassium phosphate (K 3 PO 4 ), glycerol, and a zinc (Zn(II)) complex. Colloidal zinc complex was synthesized from black tea leaves employing a green remediation procedure. Structural and thermal analyses revealed that incorporating Zn(II) complex promotes ionic mobility by boosting amorphous content and suppressing polymer relaxation temperature. The optimum CBE delivered an ambient temperature ionic conductivity of 2.21 × 10 −3 Scm −1 , cation transference number of 0.96 and appreciable electrochemical stability window of 2.79 V . By recycling via dissolution and recasting, the CBE preserved about 86% of its initial ionic conductivity after five cycles. Also, full decomposition of the CBE occurred within 71 days of soil surface exposure. When applied in an electrochemical double-layer capacitor, the optimum CBE realized a specific capacitance of 34.22 F g −1 , average energy density of 3.34 Whkg −1 , and power density of 263 Wkg −1 with stable cycling and Coulombic efficiency above 94%. These results establish recyclable, biodegradable CBEs as a credible route toward sustainable solid polymer electrolytes for energy storage.