The electrochemical reduction of aqueous carbon dioxide to methanol at molybdenum electrodes with low overpotentials

Electrolysis using molybdenum electrodes in a pH 4.2, 0.2 M Na2SO4 carbon dioxide saturated solution produces methanol as the major carbon-containing product. The reduction proceeds at room temperature and −0.7 to −0.8 V vs. SCE with faradaic efficiencies of ⩾ 50% (efficiencies of near 100% have been measured in some cases). These potentials represent electrolysis at only 160 mV negative of the standard potential corrected for pH. Methanol can also be obtained with good efficiency in 0.05 M H2SO4 at −0.57 to −0.67 V vs. SCE. Electrolysis also produces small amounts of carbon monoxide and traces of methane. No more than a trace of methane was produced even at 52°C. Extended electrolysis of molybdenum electrodes indicates that even over several days the production of methanol does not cease but the faradaic efficiency does increase. Molybdenum will reduce carbon dioxide to carbon monoxide at open circuit. Cyclic voltammetry implicates corrosion of molybdenum metal to molybdenum dioxide as the source of electrons for open circuit reduction of carbon dioxide. Analysis of the solution after electrolysis shows no molybdenum and cyclic voltammetry indicates no detectable molybdenum dioxide on the surface indicating that the electrodes are cathodically protected during electrolysis. If the electrode are cycled between −1.2 and +0.2 V vs. SCE before electrolysis, greater than 100% faradaic efficiencies are observed. The extent of cyclic is critical with yields reaching a maximum with cycling time and extended cycling leading to total passivation of the electrode. These efficiencies involve, at least in part, corrosion of the molybdenum metal. Cycled electrodes will also produce methanol at a low rate at open circuit.