Pseudocapacitance of Amorphous TiO 2 Thin Films Anchored to Graphene and Carbon Nanotubes Using Atomic Layer Deposition

Amorphous TiO 2 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 Al 2 O 3 adhesion layer was employed to obtain the conformal TiO 2 ALD films. Using 1 M KOH as the electrolyte, the electrochemical characteristics of TiO 2 ALD films grown using 25 and 50 TiO 2 ALD cycles were then determined using cyclic voltammetry, galvanostatic charge/discharge curves, and electrochemical impedance spectroscopy. Because the TiO 2 ALD films were ultrathin, the poor electrical conductivity and low ionic diffusivity of TiO 2 did not limit the ability of the TiO 2 ALD films to display high specific capacitance. The specific capacitances of the TiO 2 ALD-coated G and CNT samples after 50 TiO 2 ALD cycles were 97.5 and 135 F/g, respectively, at 1 A/g. The pseudocapacitance of the TiO 2 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 TiO 2 ALD film. This observation argues that the pseudocapacitance is derived largely from the TiO 2 bulk and is not limited to the TiO 2 surface. The molar ratio of stored charge to TiO 2 was estimated to be in the range of 0.03–0.08 (mol stored charge/mol TiO 2 ) for the various TiO 2 ALD-coated G and CNT samples. An optimized asymmetric cell was also developed based on TiO 2 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 0–1.5 V in the aqueous 1 M KOH electrolyte. An energy density of 4.47 W·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 TiO 2 ALD-coated G and CNT electrodes and the asymmetric cell based on the TiO 2 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.