Electrochemically Self-Doped TiO2 Nanotube Arrays for Supercapacitors

The application of highly ordered TiO2 nanotube arrays (NTAs) for energy storage devices such as supercapacitors has been attractive and of great interest owing to their large surface area and greatly improved charge-transfer pathways compared to those of nonoriented structures. Modification of the semiconductor nature of TiO2 is important for its application in constructing high-performance supercapacitors. Hence, the present study demonstrates a novel method involving fabrication of self-doped TiO2 NTAs by a simple cathodic polarization treatment on the pristine TiO2 NTAs to achieve improved conductivity and capacitive properties of TiO2. The self-doped TiO2 NTAs at -1.4 V (vs SCE) exhibited 5 orders of magnitude improvement on carrier density and 39 times enhancement in capacitance compared to those of the pristine TiO2 NTAs. Impedance analysis based on a proposed simplified transmission line model proved that the enhanced capacitive behavior of the self-doped TiO2 NTAs was due to a decrease of charge-transport resistance through the solid material. Moreover, the MnO2 species was introduced onto the TiO2 NTAs by an impregnation-electrodeposition method, and the optimal specific capacitance achieved (1232 F g(-1)) clearly confirmed the suitability of self-doped TiO2 NTAs as effective current collector materials for supercapacitors.