Nanoarchitectonics with valorization of lignin-rich walnut shell into sulfur and nitrogen doped porous carbon functionalized with manganese oxide as an advanced electrode material for supercapacitor applications

The conversion of lignocellulosic biomass into functional carbon materials through pyrolysis offers a sustainable strategy for developing advanced electrode materials for energy storage. In this study, lignin-rich walnut shell was employed as a renewable precursor for synthesizing manganese oxide-functionalized sulfur and nitrogen co-doped porous carbon. The composite was prepared through KOH-assisted pyrolysis, followed by heteroatom doping and potassium permanganate-mediated manganese oxide incorporation. The high lignin content promoted aromatic carbon condensation, resulting in a conductive carbon framework with interconnected micro/ mesoporous structures. Sulfur and nitrogen co-doping, together with manganese oxide functionalization, enhanced the surface chemistry, electrical conductivity, and electrochemical activity of the porous carbon. Structural and electrochemical characterizations confirmed improved ion transport and charge-storage behavior. Electrochemical performance was investigated in acidic (1 M H2SO4), neutral (1 M Na2SO4), and alkaline (1 M KOH) electrolytes. The electrode exhibited the best performance in 1 M KOH owing to its higher ionic conductivity and faster ion diffusion. It delivered a specific capacitance of 278 F g-1 at 5 mV s-1 and retained 96 F g-1 at 100 mV s-1 from cyclic voltammetry measurements, while galvanostatic charge-discharge analysis yielded a maximum specific capacitance of 300 F g-1 at 2.0 A g-1. Furthermore, the electrode retained 93.6% of its initial capacitance after 10,000 charge-discharge cycles at 6.0 A g-1. These findings demonstrate the potential of lignin-rich walnut shell as a sustainable precursor for producing high-performance porous carbon electrodes for next-generation supercapacitors.