2, University of New South Wales, Sydney, New South Wales, Australia
Lithium ion (Li–ion) supercapacitors are a potential pathway to bridging the gap between high power and high energy electrochemical devices. Of the various materials studied as anodes for Li–ion batteries and supercapacitors, titanium dioxide nanotube arrays (TNTAs) are promising as they inherently have high surface area and fast, reversible Li–ion storage. Additionally, when grown via anodisation, the TNTA is amorphous – which tends to increase capacitance [1, 2] – and the morphology is readily tuneable through the anodisation conditions. However, there exists a trade-off when aiming to increase supercapacitor energy density through Li–ion storage, insomuch as the Li–ion storage processes have a tendency to reduce the power density and cyclability of the supercapacitor device. Interestingly, these storage mechanisms have also been shown to cause a self-improvement of the storage capabilities . Hence, there is great value in investigating the charge storage mechanisms in amorphous TNTA with the aim of balancing power density and cyclability with energy density.
This study investigates the charge storage mechanisms in amorphous TNTAs hierarchically grown on titanium mesh as a means to achieve high power supercapacitor anodes. TNTAs are grown via anodisation in a bath of glycerol, ammonium fluoride and water. It is proposed that the mesh macrostructure provides greater electrolyte access for increased power density and favourable adhesion between the TNTAs and the substrate, while the nanostructured TNTAs provide sites for Li–ion storage and increased surface area for double layer storage. The effects of the amorphous nanotube dimensions (wall thickness, diameter and length) on these charge storage mechanisms are also evaluated. These charge storage mechanisms are investigated through electrochemical techniques, as well as through SEM/TEM studies of the lithiated and delithiated materials, and FIB cross-sectional studies on the adhesion between TNTA film and the bulk titanium mesh before and after cycling.
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2. H. T. Fang, M. Liu, D. W. Wang, T. Sun, D. S. Guan, F. Li, J. Zhou, T. K. Sham and H. M. Cheng, Nanotechnology 20 (22), 225701 (2009).
3. H. Xiong, H. Yildirim, E. V. Shevchenko, V. B. Prakapenka, B. Koo, M. D. Slater, M. Balasubramanian, S. K. R. S. Sankaranarayanan, J. P. Greeley, S. Tepavcevic, N. M. Dimitrijevic, P. Podsiadlo, C. S. Johnson and T. Rajh, The Journal of Physical Chemistry C 116 (4). 3181-3187 (2012)