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.

Conducting electrochemical CO2 reduction with acidic electrolyte is a promising strategy to achieve high utilization efficiency of CO2, which is an essential prerequisite for industrializable CO2 electroreduction technique. Recent progress of CO2 electroreduction in acidic electrolyte has validated that alkali cations in the electrolyte play a vital role to suppress hydrogen evolution and promote CO2 reduction. However, the addition of alkali cations causes precipitation of bicarbonate on gas diffusion electrode (GDE), flooding of electrolyte through GDE, and drifting of the pH of the electrolyte during electrolysis. In this work, we realized the electroreduction of CO2 in metal cation-free acidic electrolyte by covering the catalyst with cross-linked poly-diallyldimethylammonium chloride. This polyelectrolyte provides high density of cationic sites immobilized on the surface of catalyst, which suppresses the mass transport of H+ and modulates the interfacial field strength. By adopting this strategy, the Faradaic efficiency (FE) of CO reached 92% with Ag catalyst and the FE of formic acid reached 74% with In catalyst. More importantly, with metal cation-free acidic electrolyte, the amount of electrolyte flooding through the GDE decreased to 1% of that with alkali cation-containing acidic electrolyte, and the pH values of both catholyte and anolyte kept constant. Thanks to these features, the stability of CO2 reduction performance was greatly improved.