Pseudocapacitance of Amorphous TiO2 Thin Films Anchored to Graphene and Carbon Nanotubes Using Atomic Layer Deposition

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• 作者：Xiang Sun ; Ming Xie ; Jonathan J. Travis ; Gongkai Wang ; Hongtao Sun ; Jie Lian ; Steven M. George
• 刊名：Journal of Physical Chemistry C
• 出版年：2013
• 出版时间：November 7, 2013
• 年：2013
• 卷：117
• 期：44
• 页码：22497-22508
• 全文大小：726K
• 年卷期：v.117,no.44(November 7, 2013)
• ISSN：1932-7455

文摘

Amorphous TiO₂ thin films were conformally coated onto the surface of both graphene (G) and multiwalled carbon nanotube (CNT) samples using atomic layer deposition (ALD). An ultrathin Al₂O₃ adhesion layer was employed to obtain the conformal TiO₂ ALD films. Using 1 M KOH as the electrolyte, the electrochemical characteristics of TiO₂ ALD films grown using 25 and 50 TiO₂ ALD cycles were then determined using cyclic voltammetry, galvanostatic charge/discharge curves, and electrochemical impedance spectroscopy. Because the TiO₂ ALD films were ultrathin, the poor electrical conductivity and low ionic diffusivity of TiO₂ did not limit the ability of the TiO₂ ALD films to display high specific capacitance. The specific capacitances of the TiO₂ ALD-coated G and CNT samples after 50 TiO₂ ALD cycles were 97.5 and 135 F/g, respectively, at 1 A/g. The pseudocapacitance of the TiO₂ ALD films greatly exceeded the electric double layer capacitance of the uncoated G and CNT samples. The galvanostatic charge/discharge experiments also revealed that the charge storage was dependent on the thickness of the TiO₂ ALD film. This observation argues that the pseudocapacitance is derived largely from the TiO₂ bulk and is not limited to the TiO₂ surface. The molar ratio of stored charge to TiO₂ was estimated to be in the range of 0.03鈥?.08 (mol stored charge/mol TiO₂) for the various TiO₂ ALD-coated G and CNT samples. An optimized asymmetric cell was also developed based on TiO₂ ALD-coated CNT as the positive electrode and uncoated CNT as the negative electrode. This energy storage device could be reversibly operated over a wide voltage range of 0鈥?.5 V in the aqueous 1 M KOH electrolyte. An energy density of 4.47 W路h/kg was achieved on the basis of the total weight of both electrodes. This energy density was 4 times higher than the symmetric CNT cell. The TiO₂ ALD-coated G and CNT electrodes and the asymmetric cell based on the TiO₂ ALD-coated electrode exhibited excellent stability over >1000 cycles. The results of this study demonstrate that metal oxide ALD on high surface area conducting carbon substrates can be used to fabricate high energy storage supercapacitors.