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

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 0.03–0.08 (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 0–1.5 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 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.