Effect of Solvents on Aprotic CO<sub>2</sub> Reduction: A Study on the Role of CO<sub>2</sub> Mass Transport in the Product Selectivity between Oxalate and Carbon Monoxide
Maximilian König, Jan Vaes, Deepak Pant, Elias Klemm
Abstract
A Koutecký–Levich study was performed at a Pb rotating disc electrode to determine the diffusion coefficients of CO 2 in four solvents within a temperature range of 5–50 °C. Acetonitrile (AcN), dimethyl sulfoxide (DMSO), dimethylformamide (DMF), and propylene carbonate (PC) were investigated as solvents. The diffusion coefficients D CO2 at 20 °C were determined to be D CO2 (AcN) = 2.89 ± 0.1 × 10 –9 m 2 ·s –1, D CO2 (DMF) = 3.37 ± 0.10 × 10 –9 m 2 ·s –1, D CO2 (PC) = 1.18 ± 0.06 × 10 –9 m 2 ·s –1, and D CO2 (DMSO) = 0.38 ± 0.10 × 10 –9 m 2 ·s –1 . Additionally, galvanostatic experiments were conducted at a Pb cathode to study the role of the mass transport in the CO 2 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 CO 2 concentration, with low c (CO 2 ) favoring the dimerization reaction. At constant c (CO 2 ), solvents with low CO 2 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 CO 2 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 CO 2 reduction reaction in ionic liquids.