Effect of Solvents on Aprotic CO₂ Reduction: A Study on the Role of CO₂ Mass Transport in the Product Selectivity between Oxalate and Carbon Monoxide

A Koutecky '-Levich study was performed at a Pb rotating disc electrode to determine the diffusion coefficients of CO2 in four solvents within a temperature range of 5-50 degrees C. Acetonitrile (AcN), dimethyl sulfoxide (DMSO), dimethylformamide (DMF), and propylene carbonate (PC) were investigated as solvents. The diffusion coefficients D-CO2 at 20 degrees C were determined to be D-CO2(AcN) = 2.89 +/- 0.1 x 10(-9) m(2)center dot s(-1), D-CO2(DMF) = 3.37 +/- 0.10 x 10(-9) m(2)center dot s(-1), D-CO2(PC) = 1.18 +/- 0.06 x 10(-9) m(2)center dot s(-1), and D-CO2(DMSO) = 0.38 +/- 0.10 x 10(-9) m(2)center dot s(-1). Additionally, galvanostatic experiments were conducted at a Pb cathode to study the role of the mass transport in the CO2 reduction reaction and the resulting product selectivity between the dimerization of carbon dioxide anion radicals to form oxalate and the disproportion reaction to form carbon monoxide and carbonate. A main driver for the selectivity is the local CO2 concentration, with low c(CO2) favoring the dimerization reaction. At constant c(CO2), solvents with low CO2 diffusion coefficients seem to favor the oxalate formation. However, not all observed differences in selectivity can be attributed to mass transport properties, suggesting an additional solvent effect. Finally, a model was applied to decouple a factor describing the product distribution from the CO2 mass transport, showing a favorable oxalate formation for nucleophilic solvents DMF and DMSO. This suggests an additional favorable stabilization of reaction intermediates compared to AcN and PC. The results highlight the importance of considering mass transport effects when comparing electrolytes and solvents, which can also be relevant for CO2 reduction reaction in ionic liquids.