Supercapacitive studies on amorphous MnO2 in mild solutions

Manganese dioxide has been synthesized by a self-reacting microemulsion method. The result of X-ray diffraction shows that lack of clear peaks and broadening of peaks indicate its amorphous nature. Typical cyclic voltammograms were measured in 0.5 mol L−1 Na2SO4 aqueous solution and the potential ranges were controlled from −0.5 to 0.5, from −0.65 to 0.65, and from −0.75 to 0.75 V versus Hg/Hg2SO4 at a sweep rate of 2 mV s−1. Rectangular and symmetric image was observed between −0.5 and 0.5 V indicating high electrochemical reversibility in this potential and sweep rate. The influence of cation species and concentration (activity) on electrochemical performances of MnO2 is also discussed in this work. Cyclic voltammetric experiments have been performed between −0.5 and 0.5 V versus Hg/Hg2SO4 in Li2SO4, Na2SO4 and K2SO4 aqueous solutions with different salt concentrations ranging from 0.05 to 1 mol L−1. The specific capacitance of MnO2 was found to depend strongly on the cation concentration and species, and specific capacitance value will increase with the increase of cation concentration for all electrolytes. The logarithmic dependence of capacitance of MnO2 on cation activity indicates that the charge storage for MnO2 is involved in a fast redox reaction through cation insertion.