The Role of Protons in CO₂ Reduction on Gold under Acidic Conditions

Carbonate formation constitutes a major obstacle in the electrochemical CO₂ reduction reaction (CO₂RR), restricting the industrial implementation of this reaction. Even when adopting mild acidic electrolytes, carbonate formation is still observed. The fundamental reason lies in the inevitable OH^- generation when H₂O is the proton donor, leading to subsequent carbonate formation. Thus, exploring the reaction pathway of the CO₂RR in the acid, especially if a proton can directly participate in the reaction, is critical. Herein, we employed a rotating ring-disk electrode and surface interrogation scanning electrochemical microscopy to investigate the electrode process of the CO₂RR in acid. A pH-dependent behavior of CO₂RR is observed, indicating proton acting as the reactant in the RDS, originating from the early onset of CO₂ adsorption under locally acidic conditions.