HYBRID SUPERCAPACITORS BASED ON α-MnO2/CARBON NANOTUBES COMPOSITES

Abstract

In recent years, attention has been focused on use of various metal oxides as supercapacitor electrodes for high-power applications 1-4 . The energy storage mechanism with these materials is based mainly on fast faradaic redox reactions, which occur at the interface between the oxide and the electrolyte 4-5 , giving rise to the so-called “pseudo-capacitance”. The great advantage in this case is that the specific capacitance which can be obtained is higher than for electrochemical capacitors where the charge is only stored in the double layer. The characteristics required for the use of a metal oxide as capacitor electrode are a pseudo-capacitive behavior, a large specific surface area, a high conductivity and electrochemical stability. Among the various metal oxides studied, amorphous and hydrated manganese dioxide (a-MnO2·nH2O) is the most promising owing to the low cost of the raw material. However, due to the low electrical conductivity of a-MnO2·nH2O, a conducting additive is required to realize a composite electrode for supercapacitors. In this study, taking into account that carbon nanotubes (CNTs) are strongly entangled, forming an interconnected conducting network, we propose to use them as alternative agent to carbon black. Since the electrochemical behavior of manganese oxide has been previously studied only with three-electrode cell systems and in the positive range of potential 6-12 , we also propose to test real supercapacitors, i.e. two electrode cells, based on the a-MnO2/CNTs composites. Therefore, for the first time, the problems of using manganese oxide in symmetric capacitors will be extensively discussed. Finally, the asymmetric construction, previously proposed by Hong et al. 13 , will be optimized in order to allow both