Status on electrodeposited manganese dioxide and biowaste carbon for hybrid capacitors: The case of high-quality oxide composites, mechanisms, and prospects

Energy is the driver of technology, life, and society. Renewable energy (RE) is sustainable and is expected to be part of the future energy mix. This has created the necessity for low-cost, safe, and reliable energy storage to secure a continuous energy supply by intermittent RE sources. The rich chemistry of manganese dioxide (MnO2) and its advantageous characteristics of being low cost, environmentally friendly, stable redox couple, and nontoxic have attracted wide applications. Among the different forms of MnO2, electrolytic manganese dioxide (EMD, γ-MnO2) is a well-known electrode material in the battery energy storage community. Depending on the synthesis method, the MnO2 material can have diverse crystallographic forms, and each form takes its own morphology, surface, and pore properties influencing the electrochemical properties. Therefore, in this review, the status of electrochemically synthesized MnO2 and its physical and electrochemical properties are comprehensively discussed along with the charge-storage mechanism of different Mn oxides and their aging factors. Here, the anodic oxidation technique to convert Mn2+ to its higher oxidation state, Mn4+, by electrolysis and revealing the reaction mechanisms has been extensively reviewed. However, the limitations, such as available lower capacitance, its retention, and cyclability of EMD in comparison with other transition metal oxides such as iron, cobalt, and nickel oxides, have hindered its practical applications in supercapacitors. Therefore, to obtain a high-quality bulk EMD material, we have extended our measures to map novel additives in the electrolytic bath, such as biopolymer alginates, and examined the effect of varying surfactants to improve the morphology of EMD powder along with optimization strategies on the synthesis methods are summarized. Recent material progress indicated the challenge and importance of the delicate tailoring of the EMD characteristics to make it an advanced electrode for supercapacitors. The pristine EMD modified by electrodepositing the MnO2 using surfactant or biopolymer-mediated electrolyte solutions led to a high-quality product with enhanced specific capacitance. The biopolymer interacted with the ions in the electrolyte solution, while the surfactants affected the nucleation at the anode surface, helping to improve the EMD properties. Subsequently, a review of the valorization of biomass waste was carried out by converting the waste into activated carbon (AC) for capacitor electrodes. Finally, the electrochemical performance of the hybrid capacitor fabricated using high-quality EMD cathode and biomass waste such as Mango Seed Husk and Winery Residues-derived AC anode in aqueous electrolyte showed excellent storage properties proving its potential development in commercial applications is discussed.