Tuning the Interfacial Reaction Environment for CO₂ Electroreduction to CO in Mildly Acidic Media

A considerable carbon loss of CO₂ electroreduction in neutral and alkaline media severely limits its industrial viability as a result of the homogeneous reaction of CO₂ and OH^- under interfacial alkalinity. Here, to mitigate homogeneous reactions, we conducted CO₂ electroreduction in mildly acidic media. By modulating the interfacial reaction environment via multiple electrolyte effects, the parasitic hydrogen evolution reaction is suppressed, leading to a faradaic efficiency of over 80% for CO on the planar Au electrode. Using the rotating ring-disk electrode technique, the Au ring constitutes an in situ CO collector and pH sensor, enabling the recording of the Faradaic efficiency and monitoring of interfacial reaction environment while CO₂ reduction takes place on the Au disk. The dominant branch of hydrogen evolution reaction switches from the proton reduction to the water reduction as the interfacial environment changes from acidic to alkaline. By comparison, CO₂ reduction starts within the proton reduction region as the interfacial environment approaches near-neutral conditions. Thereafter, proton reduction decays, while CO₂ reduction takes place, as the protons are increasingly consumed by the OH^- electrogenerated from CO₂ reduction. CO₂ reduction reaches its maximum Faradaic efficiency just before water reduction initiates. Slowing the mass transport lowers the proton reduction current, while CO₂ reduction is hardly influenced. In contrast, appropriate protic anion, e.g., HSO₄^- in our case, and weakly hydrated cations, e.g., K⁺, accelerate CO₂ reduction, with the former providing extra proton flux but higher local pH, and the latter stabilizing the *CO₂^- intermediate.