• Energy Research

The current scenario of energy storage and generation is demanding higher energy density supercapacitors. We need economic and green systems to adapt to the recent climate policies as well. Supercapacitors are eco-friendly energy storage devices with high power density and long-life cycle.1 There are mainly three different configuration of supercapacitor, symmetric, asymmetric, and hybrid based on the electrode material. Symmetric cells are constructed using identical materials in both electrodes. The performance of symmetric capacitor is limited by their small voltage window and lower energy density. Hybrid supercapacitors with asymmetric configuration are new alternatives with relatively high energy density compared to their symmetric counter parts. Generally, different materials are used in each electrode to design both the asymmetric cells or hybrid cells.2,3 Manganese oxide (MnO2) is a well-known electrode material for supercapacitor, due to its high theoretical capacitance (1370 F/g), stability in aqueous electrolyte, low toxicity and very low cost. Charging and discharging occurs mainly by fast surface redox reactions occurring in manganese oxide materials electrode with the help of cations (e.g. K+, H+, Na+). Apart from its good electrochemical behavior, MnO2 suffers from a low electronic conductivity and in a limited potential range. The performance of MnO2 electrodes might be enhanced by combining the oxide with electrically conductive carbon materials.4,5 Herein, we report the synthesis of MnO2 by a simple co-precipitation technique and its use as the positive electrode of the supercapacitor. Hybrid supercapacitors with asymmetric configuration has been constructed with a commercial activated carbon as negative electrode, MnO2 as positive electrode and Na+ exchange Aquivion membrane that has the dual function of separator and electrolyte. The hybrid cell exhibited well rectangular voltammograms at different scan rates and exhibiting high specific capacitance of 124 F/g at 0.2 A/g and energy density of 11 Wh kg-1. In addition, the type of hybrid supercapacitor was able to withstand harsh cycling by combining galvanostatic charging and discharging and floating conditions (i.e. 140 hours at 1.6 V) for up to 10,000 cycles without affecting the capacitance stability. Self-discharge studies on the cell were carried out, after 10000 charge discharge cycles. The cell was charged at 1.6 V for 3 h, then during the self-discharge, it retained more than 1 V up to 400 min. Further, the well EDLC behavior of the cell was improved by using a combination of MnO2 - carbon as positive electrode and activated carbon as negative electrode. Well rectangular cyclic voltammograms was exhibited for the fast charge discharge rates. The modified cell showed a high specific capacitance of about 100 F/g at 0.2 A/g. The synergistic effect of MnO2 and carbon resulted in a perfectly reversible charge storage redox processes, which occurs at the positive electrode. A detailed electrochemical study of these cells was carried out and compared with the symmetric carbon/carbon supercapacitor. A comprehensive analysis will be given during presentation.