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

Conducting electrochemical reduction of CO 2 in acidic media can effectively increase the utilization efficiency of CO 2 . Alkali cations (M + ) are indispensable for CO 2 reduction in acidic media, while the high concentration of M + results in bicarbonate precipitation in a gas diffusion electrode. To develop selective and sustainable CO 2 reduction techniques in acidic media, quantitative understandings of cation effects on reduction rates of CO 2 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 CO 2 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 CO 2 . Only in M + -containing solutions, the electric field strength within the Stern layer increases as the potential moves negatively, and CO 2 reduction can be accelerated by applying a larger overpotential. These aspects together determine the selectivity of CO 2 reduction in acidic solution.