Fe-Doped ZnO/Activated Carbon Composite Derived from Oil Palm Empty Fruit Bunches for Asymmetric Supercapacitors

Abstract The rapid demand for sustainable, high-performance energy storage has accelerated the development of biomass-derived electrode materials for next-generation supercapacitors. However, the intrinsically low electrical conductivity and limited electrochemical activity of many metal oxide electrodes remain major challenges that restrict their practical application. In this work, a novel Fe-doped ZnO/activated carbon composite derived from oil palm empty fruit bunches (Fe-ZnO/AC) was successfully synthesized via a hydrothermal method and employed as an electrode material for asymmetric supercapacitors. ZnO was selected for its low-cost, abundance, nontoxicity, and facile synthesis, while Fe doping was introduced to enhance electrical conductivity, charge-transfer kinetics, and redox activity. Simultaneously, activated carbon derived from empty fruit bunch (EFB) waste provides a conductive, porous carbon matrix that facilitates ion diffusion and electron transport. This study highlights the synergistic integration of Fe-doped ZnO with activated carbon, enabling simultaneous enhancement of conductivity, electrochemical performance, structural stability, and sustainability within a single-electrode system, a strategy that remains rarely explored for asymmetric supercapacitor applications. The Fe-ZnO/AC electrode exhibited a high specific capacitance of 458.2 F g–1 at 0.1 A g–1 in a three-electrode configuration. In an asymmetric two-electrode configuration, the device delivered specific capacitances of 179.7 and 102.3 F g–1 for the coin cell and flexible cell, respectively. The corresponding energy densities reached 35.9 and 20.5 W h kg–1 with power densities of 239.7 and 240.4 W kg–1, respectively. These findings demonstrate the excellent electrochemical performance, stability, and energy storage capability of the Fe-ZnO/AC composite, highlighting its strong potential as a low-cost, sustainable, and high-performance electrode material for next-generation asymmetric supercapacitors.