Quantitative Understanding of Cation Effects on the Electrochemical Reduction of CO2 and H+ in Acidic Solution

Conducting electrochemical reduction of CO2 in acidic media can effectively increase the utilization efficiency of CO2. Alkali cations (M+) are indispensable for CO2 reduction in acidic media, while the high concentration of M+ results in bicarbonate precipitation in a gas diffusion electrode. To develop selective and sustainable CO2 reduction techniques in acidic media, quantitative understandings of cation effects on reduction rates of CO2 and H+ are demanded. Previous study shows that M+ in acidic media can modulate the electric field distribution in a double layer, which suppresses H+ migration and stabilizes the intermediate of CO2 reduction. In this work, we conducted a more quantitative study through the combination of electrochemical experiments and generalized modified Poisson-Nernst-Planck (GMPNP) simulations. When the concentration of M+ is higher than that of H+, the migration of H+ is substantially suppressed. The diffusion rate of H+ is also influenced by the concentration of M+. Furthermore, the concentration and identity of M+ affect the electric field within the Stern layer, which is the driving force of the electron transfer from the cathode to CO2. Only in M+-containing solutions, the electric field strength within the Stern layer increases as the potential moves negatively, and CO2 reduction can be accelerated by applying a larger overpotential. These aspects together determine the selectivity of CO2 reduction in acidic solution.