Effect of alkaline and alkaline–earth cations on the supercapacitor performance of MnO2 with various crystallographic structures
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The electrochemical performances of the α-, γ-, and δ-MnO2 with different crystallographic structures were systematically investigated in 0.5 mol/L Li2SO4, 0.5 mol/L Na2SO4, 1 mol/L Ca(NO3)2, and 1 mol/L Mg(NO3)2 electrolytes. The results showed that the electrochemical performances of the manganese dioxides depended strongly on the crystallographic structures of MnO2 as well as the cation in the electrolytes. Because the δ-MnO2 consists with layers of structure and the interlayer separation is 7 Å, which is suitable for insertion/extraction of some alkaline and alkaline–earth cations, the δ-MnO2 electrode showed the higher specific capacitance than that of α-MnO2 and γ-MnO2. We also found that the α-, γ-, and δ-MnO2 electrodes in the Mg(NO3)2 electrolyte showed a higher specific capacitance, while all the α-, γ-, and δ-MnO2 electrodes in the Li2SO4 electrolyte exhibited a better cycle life. The reason for the different behavior of Li+ and Mg2+ during the charge/discharge process can be ascribed to the charge effect of the cations in the electrolytes. The ex situ X-ray diffraction (XRD) and long-time cyclic voltammogram measurements were used to systematically study the energy storage mechanism of MnO2-based electrodes. A progressive crystallinity loss of the materials is also observed upon potential cycling at the oxidized states. A reasonable charge/discharge mechanism is proposed in this work.
KeywordsAlkaline cations Alkaline–earth cations MnO2 Supercapacitor performance Various crystallographic structures
We acknowledge the financial support from National Natural Science Foundation of China (20963011, 21063014 and 21163021), Natural Science Foundation of Xinjiang (2011211A001), and Open Project Program of Xinjiang Laboratory of Advanced Functional Materials (XJDX0902-2010-09).
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