TiO2 nanowire/nanotubes arrays on titanium substrate as a negative electrode for supercapacitor

Supercapacitors as storage devices have recently attended due to their high power density, long cycle life, fast charge-discharge rate. However, there is a limitation on the energy density of supercapacitors. According to the equation E=1/2 CV2 (where E, C and V are the energy density, the specific capacitance and the cell voltage) the energy density of supercapacitors can be enhanced by improving the specific capacitance and the cell voltage. Among these strategies, high cell voltage is more efficient and useful to improve the energy density (energy density is related to V2 in the equation). Therefore, fabricating novel positive electrodes and negative electrodes with higher positive operating potential and lower negative operating potential, respectively, can be an efficient strategy to enhance the energy density of asymmetric supercapacitors [1, 2]. In this work, TiO2 nanowires/nanotubes array electrode was synthesized on titanium (Ti) foil by anodization method. The Ti foil was anodized at 50 V over 2h in a two-electrode cell with an electrolyte, containing ethylene glycol, 0.5wt% NH4F, 2.5wt% H2O, a Ti foil as the anode, and another Ti foil as the cathode. The obtained TiO2 nanowires/nanotubes array electrode was annealed at 550 ⁰C for 1 h Based on FESEM images, the nanowires formed on top of the nanotubes array. In some regions, the top of nanotubes is exposed. The nanotubes arrays with nanowires formed on their top improve surface area for effective interface of the TiO2 nanowires/nanotubes array/Ti electrode and electrolyte. The average tube diameter of the TiO2 nanotubes arrays was at about 100 nm (Fig. 1). The TiO2 nanowires/nanotubes array/Ti electrode is used as the negative electrode for asymmetric 14th Annual Electrochemistry Seminar of Iran Materials and Energy Research Center (MERC), 12- 13 Dec, 2018 192 supercapacitor. Electrochemical measurements showed that the TiO2 nanowires/nanotubes array structure is suitable for the electronic conduct and ionic transport. The TiO2 nanowires/nanotubes array/Ti electrode as the negative electrode for supercapacitor at the ultra-negative potential region in 0.5 M Na2SO4 electrolyte, exhibited high capacitive performance and high rate-capacity. The CV curves at scan rate of 0.05 V/s and negative potential of -1, -1.2, -1.4 and -1.6 -0 V vs Ag/AgCl electrode for the TiO2 nanowires/nanotubes array/Ti electrode indicated quasi-symmetric CV curves, only slight distortion from hydrogen at the potential of -1.6 V. These behaviors were resulted from the electrochemical stability of TiO2 and inert for hydrogen evolution reaction.