Highly Conducting Spaced TiO$_2$ Nanotubes Enable Defined Conformal Coating with Nanocrystalline Nb$_2$O$_5$ and High Performance Supercapacitor Applications

In this work, we report on the electrochemical behavior of nitrided spaced TiO$_2$ nanotubes conformally coated with a nanocrystalline Nb$_2$O$_5$ layer and find for these hierarchical structures an excellent supercapacitor performance. Highly aligned conductive 1D electrodes were obtained by a three step process: i) growth of self-organized nanotubes with defined and adjustable intertube spacing, ii) conformal Nb$_2$O$_5$ decoration in the tube interspace (while providing full electrolyte access to the entire active area), and iii) high temperature nitridation. Key is the growth of a nanotube array with regular tube-to-tube interspacing that enables an optimized decoration with secondary materials such as Nb$_2$O$_5$. We observe an increase in electrode capacitance from 158 $\mu$F cm-2 for bare TiO$_2$ NTs, to 1536 $\mu$F cm-2 for TiO$_2$/Nb$_2$O$_5$ NTs, to finally 37 mF cm-2 for Nb$_2$O$_5$ decorated and then nitrided nanotubes. This drastic increase can be ascribed firstly to the defined spacing established between the tube arrays that then allows for a conformal coating with a secondary active coating. Secondly, nitridation causes a drastic increase of the electron conductivity of the entire scaffold and thus reduces resistive losses.