Hydrothermal Synthesis of MnO ₂ and Hemp‐Derived Activated Carbon Composites for Tailored Electrochemical Performance

This study investigates hydrothermal synthesis of manganese dioxide composited with hemp‐derived activated carbon (MnO ₂ /AC) as electrodes for supercapacitors. The effects of key hydrothermal parameters, including carbon ratio, reaction temperature, and reaction time were systematically examined. Phase‐pure α‐MnO ₂ was uniformly anchored on carbon framework. Morphology of MnO ₂ evolved from nanowalls to well‐defined nanorods with increasing reaction temperature and time. Meanwhile, the specific surface area of MnO ₂ /AC decreased from 1712 to 1538 m ² g ⁻¹ due to partial pore blocking, while a predominantly mesoporous structure was retained. Electrochemical measurements in 1 M Na ₂ SO ₄ demonstrate that the optimized MnO ₂ /AC composites achieve a specific capacitance of 216.8 F g ⁻¹ at 1 A g ⁻¹ . Charge‐storage mechanism analysis reveals a balanced contribution between surface‐controlled capacitive processes and diffusion‐controlled pseudocapacitance, which is directly correlated with preserved mesoporosity and moderate MnO ₂ coverage. When the MnO ₂ /AC composites were assembled into an asymmetric supercapacitor using AC as the negative electrode, the device operates stably up to 2.4 V and delivers outstanding cycling stability over 95% after 22,000 charge–discharge cycles at 5 A g ⁻¹ . These results demonstrate that controlled growth of MnO ₂ , rather than maximum oxide loading, is essential for optimizing charge‐storage mechanisms and achieving high‐performance biomass‐derived supercapacitor electrodes.