Design and synthesis of high performance flexible and green supercapacitors made of manganese‐dioxide‐decorated alkali lignin.

State‐of‐the‐art flexible supercapacitors suffer from electrochemical and environmental performance issues including manufacturing expense limitations. Therefore, a highly effective and low‐cost strategy to develop supercapacitors is needed. To address this, a plant‐based, high‐performance, lightweight, low‐cost, quasi‐solid state, and composite electrode for flexible supercapacitors has been synthesized using the hydrothermal method. The composite electrode is made of alkali lignin and is decorated with MnO2 particles including an Al substrate. An Al/lig/MnO2 based anode and an Al/AC (activated carbon) based cathode were sandwiched using an inorganic polymer gel‐type electrolyte made of PVA/H3PO4 in order to assemble the supercapacitor. Synchrotron tomography and SEM are employed to study the detailed electrode morphology. Electrochemical impedance spectroscopy (EIS), cyclic charge‐discharge (CCD), and cyclic voltammetry (CV) have been used to assess electrochemical performance. Optimization is carried out using a series of lignin:MnO2 samples with varying constituent ratios. After 3000 charge‐discharge cycles, the highest specific capacitance value achieved at 40 mA g−1 reached 379 mF cm−2, (900 times reported). The capacitance retention, maximum energy density, and maximum power density are 80%, 6 Wh kg−1, and 355 W kg−1 respectively. Due to the superior electrochemical performance, the supercapacitor shows exceptional potential for future sustainable and green electronics. [ABSTRACT FROM AUTHOR]