The demand for energy storage devices made from biodegradable materials has increased significantly due to sustainability. Currently, such devices possess vital issues, such as high manufacturing costs and toxicity, low reliability, as well as poor electrochemical performance. In this research, microwave synthesis was conducted to fabricate a low-cost, high-performing, plant-based electroactive material. MnO2 microparticles fabricated via microwave irradiation were deposited on two plant-based materials as substrates made of Al/lignin and Al/AC/lignin. The quasi-solid-state supercapacitors were assembled using a polymeric gel electrolyte of PVA/H3PO4. Scanning electron microscopy was performed to examine the polydispersity, morphology, and porosity of the micro-MnO2 deposited materials. FTIR and UV-vis spectroscopy were performed to study the composition and verify deposition of micro-MnO2 on the lignin-based matrixes. Cyclic voltammetry (CV) was employed to study the polarization resistance of the system. The cyclic charge-discharge (CCD) and electrochemical impedance spectroscopy (EIS) were performed to observe cyclic performance and interfacial resistances. Electrochemical tests showed that after 700 cycles of charge-discharge, both the supercapacitors exhibited high capacitance retention above 90%. Compared to the existing technology, this method enables consistent material structurization with tunable properties due to the controlled heating time and exposure to radiation with minimal waste. This work provides an alternative approach to synthesize low-cost and scalable green composite electrodes for flexible supercapacitors.