Influence of Microstucture on the Charge Storage Properties of Chemically Synthesized Manganese Dioxide

Abstract

α-MnO 2 was synthesized by a very simple coprecipitation technique and tested as active electrode material for an electrochemical supercapacitor. The powder presents a poorly crystallized cryptomelane phase with a chemical composition of K 0.05 MnO 2 H 0.10 ·0.15H 2 O. Different aqueous electrolytes were tested including 0.1 M Na 2 SO 4, 0.5 M K 2 HPO 4 /KH 2 PO 4 buffer solution, 0.3 M H 2 SO 4, and 1 M NaOH, but interesting pseudocapacitance behavior was only observed in the case of 0.1 M Na 2 SO 4 . Further testing using this electrolyte showed that an average capacitance of 166 F/g can be reproducibly obtained within a voltage range −0.4/+0.5 V vs Hg/Hg 2 SO 4 using a sweep rate of 2 mV/s. This interesting value is mainly due to the chimisorption of Na + ions and/or protons at the surface of the α-MnO 2 electrode. Nearly all the Mn surface atoms are involved in the pseudocapacitive process. Therefore, the high specific capacitance seems to be related to the high surface area of the MnO 2 powder rather than intercalation of Na + ions and/or protons in the structure of α-MnO 2 . An optimum composition of 80% of active material in the composite electrode was determined. With such a composition, the α-MnO 2 electrode can withstand 1000 cycles with 100% capacitance retention.