Sustainable activated carbon for aqueous and flexible zinc-ion energy storage

Carbons with high specific surface area (SSA), hierarchical porous structure, and abundant surface functionalities are highly desirable for electrodes in high-performance electrochemical energy storage systems. Here, we have explored bitter apple peel, a biomass waste, to derive carbons with such desirable traits and investigated its electrochemical performance as active material in aqueous supercapacitor (SC) and zinc-ion hybrid SC (ZIHSC). Benefited by its high SSA (1475.3 m 2 /g) and surface oxygen functionalities (C-O, C=O, etc), the optimised activated carbon (BAC-3) demonstrated excellent electrochemical performance. When tested as an aqueous SC cathode, the BAC-3 demonstrates a specific capacitance of 178.5 F/g at a high current density of 5 A/g, and no capacity degradation after 10,000 CV cycles. To improve its energy performance further, the zinc-ion storage traits of the synthesised materials are also investigated. As an aqueous ZIHSC cathode, the optimised BAC-3 achieved exceptional energy and power densities of 147.6 Wh/kg and 4.5 kW/kg, respectively, and showed ultra-stable capacitance over 100,000 cycles. Even the hydrogel-electrolyte based ZIHSC assembled with BAC-3 cathode and Zn anode reached an energy density as high as 135 Wh/kg, exhibited good flexibility and cycling stability, and successfully powered a red LED, suggesting promising practical application prospects. The work provides critical insights into biomass-resource utilization for value-added products for a sustainable environment and economy. Excellent energy storage performance of fruit peel derived activated carbon (BAC) in diverse energy storage systems: from aqueous supercapacitor and zinc ion hybrid supercapacitor (ZIHSC) to hydrogel-electrolyte based flexible ZIHSCs.