Multiscale Modeling Reveals Mass Transport-Controlled Product Selectivity in Electrochemical CO 2 Reduction on Cu

Electrochemical CO 2 reduction is one of the most promising processes for a sustainable closure of the artificial carbon cycle. A severe limitation for wide-scale industrial applicability has been the absence of an efficient and selective electrocatalyst for the reduction of CO 2 to higher-reduced valuable chemicals and fuels. Cu is the only catalyst found to produce considerable amount of multicarbon products, albeit at high overpotentials. The mechanism of the conversion process is unclear, with different active sites and rate-determining steps being proposed. In addition, the mass transport of CO 2 has been suggested to have a significant impact on the product selectivity 1,2 . Gas-diffusion-layer(GDL)-based electrolyzers have become a state-of-the-art solution to provide a high concentration of CO 2 to the active sites to circumvent these mass transport limitations. In this work, we present a new multi-scale model based on first-principles kinetics 3 , and a modification of a recently reported gas diffusion electrode model 4 . We use this to generate a digital twin of experimental electrode systems and show that even in GDL systems, mass transport is the limiting factor governing all experimentally observed trends in product selectivity, irrespective of reaction mechanism or product pathway. We further find indications of C 2 products being predominantly formed in wider, accessible pores, while C 1 products are generated in deeper and thinner pores with less access to CO 2 . This work provides strong evidence for the importance of mass transport in designing CO 2 electrolyzers. References: Watkins, N. B. et al. Hydrodynamics Change Tafel Slopes in Electrochemical CO 2 Reduction on Copper. ACS Energy Lett. 8 , 2185–2192 (2023). Ringe, S. et al. Double layer charging driven carbon dioxide adsorption limits the rate of electrochemical carbon dioxide reduction on Gold. Nat. Commun. 11 , 33 (2020). Liu, X. et al. pH effects on the electrochemical reduction of CO (2) towards C 2 products on stepped copper. Nat. Commun. 10 , 1–10 (2019). Weng, L.-C., Bell, A. T. & Weber, A. Z. Modeling gas-diffusion electrodes for CO 2 reduction. Phys. Chem. Chem. Phys. 20 , 16973–16984 (2018).