Abstract:

A series of highly ordered TiO₂ nanotube (TiO₂NTs) electrodes are prepared via potentiostatic anodization of Ti foil followed by calcining in air. X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), and potential steps determination are used to characterize the electrodes. The electrochemical reduction of CO₂ on these three TiO₂NTs electrodes is investigated by cyclic voltammetry and potentiostatic electrolysis in 0.1 mol·L^-1 KHCO₃ aqueous solution. Methanol is found to be the major product in electrochemical CO₂ reduction, while formic acid, formaldehyde, methane, and CO are formed as minor products. Compared with the electrodes sintered at 550 and 650℃, the optimal TiO₂NTs electrode is found to be the one calcined at 450℃ (TiO₂NTs-450). After 120 min of reaction, the Faradaic efficiency and partial current density of methanol is 85.8% and 0.2 mA·cm^-2 at -0.56 V vs. reversible hydrogen electrode (RHE), respectively. The trivalent titanium in TiO₂ serves as an efficient site for adsorption of CO₂ and stabilization of the adsorbed ·CO₂^- radical. Consequently, the reduction of CO₂ on TiO₂NTs electrodes involves a fast first electron and proton transfer followed by a slow second proton transfer as the rate-limiting step.

Key words: TiO₂ nanotube electrode, Electrochemical reduction, CO₂, Methanol, Reaction mechanism