Influence of Oxidative and Hydrothermal Pre-Treatments on KOH Activation of Coconut Fiber for Enhanced Supercapacitor Performance

The development of sustainable electrode materials for supercapacitors requires a deeper understanding of the relationship between precursor structure, processing, and electrochemical performance. In this work, coconut-fiber-derived activated carbons were synthesized via KOH activation, and the influence of oxidative and hydrothermal pre-treatments was systematically investigated. The materials were characterized by X-ray diffraction (XRD), Raman spectroscopy, and Fourier transform infrared spectroscopy (FTIR), while electrochemical performance was evaluated using cyclic voltammetry and galvanostatic charge–discharge measurements in a three-electrode system with 1 M H2SO4 electrolyte. The results show that hydrothermal pre-treatment leads to improved electrochemical performance, with CF-HTC-AC exhibiting a specific capacitance of ~332 F g−1 at 0.5 A g−1 and enhanced rate capability. In contrast, the oxidatively treated sample (CF-OC-AC) presents a higher diffusion-controlled contribution, indicating a stronger pseudocapacitive behavior associated with oxygen-containing functional groups. These findings demonstrate that electrochemical performance is governed by a balance between capacitive and diffusion-controlled processes rather than by a single structural parameter. The hydrothermal pre-treatment provides an effective strategy to optimize this balance, highlighting precursor conditioning as a key factor in the design of biomass-derived carbon electrodes for supercapacitor applications.