CO 2 to CO Electrochemical Conversion in Molten Li 2 CO 3 Is Stable with Respect to Sulfur Contamination

The conversion of CO 2 to CO via the electrolysis of molten Li 2 CO 3 at 900°C has a number of advantages including close to 100% faradaic and thermodynamic efficiencies, the latter limited by heat loss. Here we report that when the incoming gas contains SO 2 , sulfur uptake by the melt occurs only in the presence of oxygen and leads to the formation of Li 2 SO 4 . The electrolysis of molten Li 2 CO 3 /Li 2 O containing <2 mol% Li 2 SO 4 produces CO, O 2 and elemental sulfur. Under these conditions, the decomposition potential of Li 2 SO 4 is measured to be only 0.15 V, which is much lower than the decomposition potential of Li 2 CO 3 (0.87 V). At sufficiently high cathode current densities (>1 A/cm 2 ), the sulfur reduction current is diffusion limited and a steady state can be reached in which the amount of sulfur entering the melt equals the amount of sulfur leaving the melt. Thus even with SO 2 -containing sources of CO 2 , electrochemical conversion of CO 2 to CO is able to proceed, but with reduced faradaic efficiency. Our findings suggest that the molten carbonate method for converting CO 2 to CO can use flue gas from power stations as an inexpensive and readily available source of CO 2 .