Improved product selectivity of electrochemical reduction of carbon dioxide by tuning local carbon dioxide concentration with multiphysics models

Electrochemical reduction of carbon dioxide (CO₂) is promising to alleviate carbon emissions and produce fuels and materials in a circular way, yet effective tuning strategies and fundamental understanding are lacking. In particular, cell design is actually done by simplistic one- or two-dimensional models, which incorporate numerous assumptions, leading to potential errors and discrepancies. Here, we establish new two-dimensional multiphysics models that incorporate cell-specific geometry, gas–liquid two-phase flow, and electrochemical kinetics. We calculate the temporary and spatial variations of the local CO₂ concentration, electrochemical parameters, and products selectivity on the cathode surface, under different cell configurations and operating parameters. Products include dihydrogen (H₂), carbon monoxide (CO), formic acid (HCOOH), ethylene (C₂H₄), ethanol (C₂H₅OH), and propanol (C₃H₇OH). We further investigated the effect of local CO₂ concentration on CO₂ reduction performance. We find that high local CO₂ concentration, above 8.7 mM, enhances the selectivity for C₁ products and the cathode polarization, whereas extremely low local CO₂ concentration, of 0.75–5.5 mM, favors the selectivity for C₂₊ products, especially alcohols. C₂₊ selectivity ranges from 81.3 to 88.6% at 4.8–5.5 mM local CO₂ concentration, while alcohol products selectivity ranges between 54.9 and 65.4% at 0.75–1.9 mM CO₂ concentration. These findings are attributed to the reduced CO₂ diffusion layer thickness to less than 10 μm at the cathode. This enhances the CO₂ mass transfer efficiency to the cathode, and eliminates temporal and spatial variations of the local CO₂ distribution along the cathode surface.