Carbon-Supported Manganese Dioxide as Electrode Material For Asymmetric Electrochemical Capacitors

Abstract

Composites of carbon nanotubes and activated carbon with manganese dioxide (MN MnO2, DN MnO2, AC2 MnO2) were chosen as positive electrodes and activated carbon (AC1) with well-developed surface area as negative electrode. C/MnO2 composites were prepared with various C:MnO2 ratio (c.a. 30, 50 and 60% of MnO2). The electrochemical performance of symmetric and asymmetric capacitors was studied in two and three electrode configuration, using 1M Na2SO4 aqueous solutions as electrolyte. The capacitance properties were studied by cyclic voltammetry, galvanostatic charging/discharging and electrochemical impedance spectroscopy. The asymmetric cell composed of composite AC2 MnO2 3 gave the most preferred capacitance in the extended voltage range of 1.7 V at the level 142 F g-1, however, the capacitor with MN MnO2 3/ AC1 operating at a voltage of 1.7 V reached energy density above 20 Wh kg-1. Very good stability during 5000 cycles was obtained for composites charged to 1.1 V (MN MnO2 3) and 1.2 V (MN MnO2 2). The capacitance of these systems has been retarded at around 110 F g-1 at a load current density 2 A g-1. Increase of the operating voltage of the capacitor MN MnO2 3/ AC1 from 1.2 V to 1.7 V, has initially contributed to increase in capacitance, although after 5000 cycles significant capacitance fade, i.e. from ca. 150 F g-1 to ca. 70 F g-1 was observed.