Mesoporous TiO2 and Co-doped TiO2 Nanotubes/Reduced Graphene Oxide Composites as Electrodes for Supercapacitors

A series of mesoporous TiO2 nanotubes/reduced graphene oxide (TiO2NT/RGO) and Co-doped TiO2NT/RGO composites were synthesized via a hydrothermal method. In this method, graphene oxide (GO) was reduced to RGO, whereas TiO2NT and Co-doped TiO2NT were prepared using poly(ethylene-glycol)-block-poly(propy glycol)-block-poly(ethylene-glycol) as a structure-directing agent through a simple solvothermal route. Their morphology and properties were determined using X-ray diffraction, field emission scanning electron microscopy, UV-vis diffuse reflectance spectra, and Raman measurement. Results indicated that TiO2NT displayed mesoporous structures with lengths ranging from about 3 mu m to 10 mu m, tube diameters ranging from about 400 nm to 600 nm, wall thickness of about 250 nm, and mesoporous walls of about 6 nm. The electrochemical performance of the TiO2-based electrodes for supercapacitor was confirmed by cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge-discharge in 0.5 mol L-1 Na2SO4 aqueous electrolyte solution. The Co-doped TiO2NT/RGO composite electrode with 10.00 wt.% GO exhibited a specific capacitance of 27.5 F g(-1) at a current density of 0.2 A g(-1) in a symmetrical two-electrode cell, whereas the specific capacitance of TiO2 was 0.7 F g(-1). The Co-doped TiO2NT/RGO composite electrode exhibited high rate capability and excellent long-term cycle stability. The outstanding performance of the Co-doped TiO2NT/RGO composite electrode was attributed to the synergistic effects on capacitive behavior of Co-doped TiO2NT combined with high electronic conductivity of the RGO sheets. (C) 2016 Elsevier Ltd. All rights reserved.