  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.
