Synergistic performance comparison of MnO₂-loaded activated carbon from cotton seed hulls for sustainable supercapacitor electrodes

This study presents a comparative investigation of MnO₂-loaded activated carbon derived from cotton seed hulls for supercapacitor applications, focusing on the influence of synthesis strategy on structural and electrochemical properties. Two fabrication routes such as hydrothermal treatment and incipient wet impregnation were employed to tailor pore architecture and MnO₂ distribution. The hydrothermal method involved K₂CO₃ activation and pyrolysis at 600 °C followed by MnO₂ deposition at 180 °C, whereas the impregnation route utilized KMnO₄-assisted loading with KOH activation at 800 °C. Physicochemical characterization using SEM with EDAX, XRD, BET, FTIR, and XPS confirmed successful integration of MnO₂ within the carbon matrix. The hydrothermally synthesized composite exhibited a mesoporous structure (average pore size ~17.76 nm) with moderate surface area (50.2 m²g⁻¹), while the impregnation-derived sample showed a higher surface area (424 m²g⁻¹) dominated by microporosity (~2.36 nm). Despite its lower surface area, the hydrothermal sample demonstrated superior electrochemical performance due to improved ion-accessible porosity and more effective utilization of electroactive sites. Electrochemical measurements in a three-electrode configuration revealed a higher specific capacitance of 97.63 Fg⁻¹ for the hydrothermal composite compared to 62.19 Fg⁻¹ for the impregnated sample at 1 Ag⁻¹, along with better rate capability. These results highlight that pore connectivity and MnO₂ distribution play a more critical role than surface area alone in determining capacitive performance. Electrochemical impedance analysis indicates that the hydrothermal composite possesses lower charge transfer resistance and enhanced ion diffusion, as reflected by its Nyquist response. Furthermore, it demonstrates superior cycling stability with higher Coulombic efficiency over 5000 cycles, confirming improved reversibility and structural integrity compared to the impregnated sample. Overall, this work establishes the hydrothermal approach as a more effective synthesis route for optimizing MnO₂–activated carbon composites and provides insights into the design of sustainable biomass-derived electrode materials.