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In 2015, the United Nations (UN) has proposed a roadmap to achieve clean, cheap, and renewable energy in their sustainable development goals. As well as the modern technological advancement with zero-emission has been emphasized to upgrade the quality of lifestyles especially in developed countries . As a consequence of the necessity, nowadays smart and heavy-duty electric vehicles have been introduced in front of modern society. Those electrical vehicles are accelerated by a heavy-duty electrical energy storage device (hybrid batteries, supercapacitors) which can be easily changed within a short period and supply required power with efficient mileage too . In the context of energy storage, supercapacitors especially electric double-layer capacitors (EDLC) are good owing to their robust lifetime, longer cycle life, rapid charging-discharging, low maintenance cost, and high power energy storage for multipurpose applications . Usually, carbon-based materials viz. graphene, reduced graphene oxide (rGO), CNT, biomass-derived porous carbon materials, etc. are the most popular candidates of EDLC . Among them, rGO is special for their excellent electrochemical performance attributed to their morphological excellence including the expanded d-spacing and abundant surface area. But their highly agglomerate properties reduce their performance (conductivity, accessible surface area) thus limits their applications .
To address this issue, rGO can be synthesized by mixing with other transitional metal or metal oxide which will contribute to the expansion of the rGO layer, and enhance the supercapacitor performance by induces pseudocapacitance thus increasing the charging-discharging rate . Due to the various oxidation states and high theoretical capacitance (1370 Fg-1) manganese oxides are highly recommended as a high-performance supercapacitor electrode material . Moreover, the natural abundance of manganese is greater than most of the other transitional metals. Hence, the incorporation of MnxOy also attributes to rGO electrode performance by enhancing the surface area, interlayer distance, and charge transfer rate due to the fast-reversible redox reaction . The total electrochemical performance of the MnxOy-rGO electrode highly depends on the morphology and amount of incorporated MnxOy . Interestingly, the presence of high content of MnxOy results in poor performance of the rGO-MnxOy electrode. This might be occurred because of the higher agglomeration of MnxOy in and on between the rGO surface and blocking of rGO pores. As a result, the charge transfer resistance of the electrode material increases. So, it is very crucial to optimize the amount of MnxOy during the preparation of the MnxOy-rGO composite during the synthesis. In this work, the rGO-MnxOy composite was synthesized in a facile way by optimizing the amount of KMnO4. In brief, different concentration of KMnO4 was mixed with 0.2 g of GO suspension under vigorous stirring and sonication. 5 mL of glycerin was further added to form precursors for the preparation of MnxOy-rGO. Finally, the mixture was reduced by a one-step hydrothermal reduction process (180°C for 24 h) followed by washing and drying. The electrochemical capacitance performance of all compositions was evaluated using cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) in a two-electrode system with 0.5 M Na2SO4 aqueous electrolyte solution. The maximum specific capacitance was found at 280 Fg-1 at 1 Ag-1 current density for the rGO-MnxOy composite at the concentration of 0.05mM KMnO4. The summarized electrochemical data are represented in Table 1. The total work is expected to contribute to the next generation of hybrid supercapacitor applications.
Conc. of KMnO4
Potential window, (V)
Specific capacitance, Csp(Fg-1)
Energy density, E
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