Electrodeposited manganese oxides on three-dimensional carbon nanotube substrate: Supercapacitive behaviour in aqueous and organic electrolytes

Thin amorphous manganese oxide layers with a thickness of 3-5 nm are electrodeposited on a carbon nanotube (CNT) film substrate that has a three-dimensional nanoporous structure (denoted as MnO₂/CNT electrode). For the purpose of comparison, manganese oxide films are also electrodeposited on a flat Pt-coated Si wafer substrate (denoted as MnO₂ film electrode). The pseudocapacitive properties of the MnO₂ film and MnO₂/CNT electrodes are examined in both aqueous electrolyte (1.0 M KCl) and non-aqueous organic electrolyte (1.0 M LiClO₄ in propylene carbonate). While both types of electrode show pseudocapacitive behaviour in the aqueous electrolyte, only the MnO₂/CNT electrode does so in the organic electrolyte, due to its high oxide/electrolyte interfacial area and improved electron conduction through the CNT substrate. Compared with the MnO₂ film electrode, the MnO₂/CNT electrode shows a much higher specific capacitance and better high-rate capability, regardless of the electrolyte used. Use of the organic electrolyte results in a ∼6 times higher specific energy compared with that obtained with the aqueous electrolyte, while maintaining a similar specific power. The construction of a three-dimensional nanoporous network structure consisting of a thin oxide layer on a CNT film substrate at the nm scale and the use of an organic electrolyte are promising approaches to improving the specific energy of supercapacitors.