REDUCTION OF CO2 ON FLUORINE-DOPED SNO2 THIN-FILM ELECTRODES

The reduction process of fluorine-doped SnO2 (SnO2:F) electrode in the presence of CO2 was investigated by x-ray diffraction, x-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), colorimetry, and gas chromatography. It has been found that a SnO2:F with (200) preferred orientation undergoes the following five reduction stages: 1. Oxygen-defect formation and constant hydrogen evolution. 2. Acceleration of oxygen-defect formation and hydrogen evolution. At this stage, formyl -HCO- and methoxy -CH3O- are formed on the surface. 3. Dissolution of (200) surface and exposure of (110) surface. Dissolution of SnO2:F accompanies CO evolution. The CO formation rate increases, and H-2 formation rate decreases as the reduction proceeds. 4. Electrodeposition of dissolved Sn on an exposed (110) surface. The CO formation rate decreases and finally stops as the deposition proceeds. 5. The surface is covered with elemental Sn, and the oxide electrode acts as a metal electrode. Cyclic voltammetry cycled between -0.1 V and -1.4 V vs. SCE showed nearly the same current-voltage characteristics for SnO2 and Sn electrodes in a CO2 saturated solution, implying that a tin metal is oxidized by CO2 at less cathodic potentials (>-0.5 V vs. SCE).