Remarkable supercapacitive performance of TiO2 nanotube arrays by introduction of oxygen vacancies

Highly ordered TiO2 nanotube arrays (TNAs) with large specific surface exhibit excellent electrochemical performance but their poor electrical conductivity limits the practical applications in supercapacitor. Here, we present a simple strategy for improving the electrical conductivity of TNAs by introduction of oxygen vacancies in NaBH4 aqueous solution. The reduced TNAs (rTNAs) electrodes exhibit significant improvement on electrical conductivity and carrier density compared to those of the pristine TNAs. Interestingly, the rTNAs electrode obtained from 5 M NaBH4 aqueous solution exhibits the highest specific capacitance of 23.24 mF cm−2 at a scan rate of 2 mV s−1 with outstanding long term cycling stability. Moreover, the reduced MnCo2O4/TNAs electrodes are also prepared by a facile chemical bath deposition method and then reduced in NaBH4 aqueous solution. The reduced MnCo2O4/TNAs electrode prepared in 5 M NaBH4 aqueous solution achieves a high specific capacitance of 484.35 mF cm−2 at a scan rate of 2 mV s−1. The enhancement of capacitance and cycling stability of reduced MnCo2O4/TNAs electrodes can be attributed to the improved electrical conductivity of MnCo2O4/TNAs by introduction of oxygen vacancies and the tubular structure of TNAs that accelerate the electron and ions transport.