Electrochemically Self-Doped TiO₂ Nanotube Arrays for Supercapacitors

Abstract

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