Electro-Deposition and Dissolution of MnO2on a Graphene Composite Electrode for Its Utilization in an Aqueous Based Hybrid Supercapacitor

The electro-deposition and dissolution of MnO2is studied for its potential utilization in hybrid supercapacitors. A commercially available graphene powder is used as high surface area substrate and capacitive type electrode. An aqueous electrolyte composed of 0.5 M H2SO4+ 0.5 M MnSO4is used. The deposition-dissolution reaction of MnO2is shown to provide the bulk of the charge storable in the battery type electrode. Furthermore, the potential window of stability of this electrode reaction is compatible with the aqueous electrolyte. Two complete systems were assembled and tested for their charge/discharge behavior. It is shown that energy and power densities of 25 Wh kg−1and 980 W kg−1, respectively, can be achieved for at least 5000 cycles. Limitations of the system in terms of cycle numbers and electrolyte volume are discussed in more detail. The electrolyte volume to electrode mass ratio is shown to be a critical factor in determining the charge/discharge cycle number of the device. This is believed to be due to the fact that the efficiency of the MnO2electro-dissolution/deposition process is less than 100%. This, in turn, could result in gradual pH changes and a lower Mn2+concentration in the electrolyte during continues charging and discharging. This eventually results in lowering the energy and power density of the system. However, it is shown that an energy density of 25 Wh kg−1is achievable, thus being comparable with similar hybrid systems.