Electrochemical Reduction of CO2 to Methanol at TiO2 Nanotube Electrodes

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 degrees C, the optimal TiO2NTs electrode is found to be the one calcined at 450 degrees C (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 center dot 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.