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

A series of mesoporous TiO 2 nanotubes/reduced graphene oxide (TiO 2 NT/RGO) and Co-doped TiO 2 NT/RGO composites were synthesized via a hydrothermal method. In this method, graphene oxide (GO) was reduced to RGO, whereas TiO 2 NT and Co-doped TiO 2 NT 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 TiO 2 NT displayed mesoporous structures with lengths ranging from about 3   μm to 10   μm, 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 TiO 2 -based electrodes for supercapacitor was confirmed by cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge–discharge in 0.5   mol   L −1 Na 2 SO 4 aqueous electrolyte solution. The Co-doped TiO 2 NT/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 TiO 2 was 0.7   F   g −1 . The Co-doped TiO 2 NT/RGO composite electrode exhibited high rate capability and excellent long-term cycle stability. The outstanding performance of the Co-doped TiO 2 NT/RGO composite electrode was attributed to the synergistic effects on capacitive behavior of Co-doped TiO 2 NT combined with high electronic conductivity of the RGO sheets.