Oxide based supercapacitors: I-manganese oxides

Manganese oxide is among those that have pseudocapacitive (Faradic) behavior in aqueous solutions, while charge storage mechanism of manganese oxide electrodes has been well established [1, 2]. Pseudocapacitive reactions have been observed both on the surface and in bulk of the electrodes. Surface Faradaic reaction is due to the surface adsorption of electrolyte cations (C+ = H+, Li+, Na+ and K+) onto surface of the manganese oxide, given by [3, 4]: (Formula Presented) Bulk Faradaic reaction is related to intercalation or deintercalation of the electrolyte cations inside the manganese oxide, expressed as [3, 4]: (Formula Presented) Both charge storage mechanisms involve the redox reaction between III and IV oxidation states of Mn. As compared with the RuO2, hydrated manganese oxides have lower specific capacitances, usually in the range of 100-200 F.g-1 in alkali salt solutions. Moreover, the currently available MnO2-based supercapacitors have encountered serious limitations, such as intrinsic problem of MnO2, low specific capacitance, low energy density, structural instability, weak long-term cyclability, and low rate-capacity. For practical applications, the electrode materials should have high reversible capacitance, structural flexibility, long-time stability, fast cation diffusion at high charge-discharge rates, cost-effectiveness and environmental friendliness [5].