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Abstract: 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 0.5molL−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 0.75V versus Hg/Hg2SO4 at a sweep rate of 2mVs−1. Rectangular and symmetric image was observed between −0.5 and 0.5V 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 0.5V versus Hg/Hg2SO4 in Li2SO4, Na2SO4 and K2SO4 aqueous solutions with different salt concentrations ranging from 0.05 to 1molL−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. [Copyright &y& Elsevier]

Abstract: Six samples of α-MnO2 with different K ion contents are synthesized by using KMnO4 and Mn(CH3COO)2·4H2O with different pH values at room temperature. The samples are labeled as Mx where x denotes the pH value. The anomalous effect of K ion content on the electrochemical capacitance properties of amorphous MnO2 is investigated systematically. It is found that with the increase of the K ion content, the MnO2 nanospheres form a more dense aggregates, the surface area dramatically decreases from 273 (M2) to 14 m2 g−1 (M12), and pore volume decreases from 0.349 (M2) to 0.014 cm3 g−1 (M12). The specific capacitance (SC) is decreased by 17.6% from 157 (M2) to 131 F g−1 (M12). In addition, the SC values are greatly influenced by the molar ratio of K to Mn in the range of 0–0.4. And it is found that part of K ions in the MnO2 tunnels can be replaced by Na ions during electrochemical reaction. Finally, an appropriate explanation of the effect of K ion content on the SC values is provided. [Copyright &y& Elsevier]

Abstract: Effect of the electrolyte particularities on manganese dioxide in the neutral electrolytes containing alkaline (Li+, Na+, or K+) and alkaline-earth (Mg2+, Ca2+, or Ba2+) metal cations for electrochemical capacitor is investigated by cyclic voltammetry and electrochemical impedance spectroscopy. A very high capacitance of 357.9Fg−1 is measured when Ca2+ ion is used as electrolyte cation. The results show that cations of the electrolyte instead of proton or anions are responsible for the pseudo-capacitive behavior of manganese dioxide. The capacitance of manganese dioxide is found to depend strongly on the electrolyte particularities, for example, pH value, cation and anion species, and salt concentrations. A logarithmic dependence of capacitance of MnO2 on cation activity is obtained for all alkaline and alkaline-earth metal cations. The capacitance of MnO2 can be doubled by replacing univalent cation with bivalent cation at a close cation activity. [Copyright &y& Elsevier]