Hybrid Energy Storage Devices Based on Monolithic Electrodes Containing Well-defined TiO2 Nanotube Size Gradients

Well-defined TiO2 nanotube size gradient thin films, manufactured using a bipolar electrochemistry approach, can be used as versatile monolithic hybrid electrodes for energy storage devices. The nanotube size distribution within the gradients can readily be controlled by altering the bipolar current and/or the length of the bipolar titanium sheet. As the electrochemical properties of the gradient electrodes can be carefully tailored by modifying the nanotube size gradient, this approach provides new possibilities for the manufacturing of hybrid electrodes with integrated energy and power density gradients. The freestanding anatase TiO2 nanotube size gradient electrodes provide unprecedented capacities at cycling rates from C/5 (i.e. 162 mAh cm(-2) or 169 mAh g(-1)) to 50C (i.e. 40 mAh cm(-2) or 42 mAh g(-1)). It is likewise shown that the size gradient electrodes facilitate fundamental studies of the charge/discharge process of TiO2 based electrodes. The results demonstrate that the different shapes of charge and discharge curves of TiO2 nanotube electrode can be explained by inherent differences between the lithiation and delithiation processes. (C) 2015 Elsevier Ltd. All rights reserved.