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.