Unraveling the Impact of Common-Ion Effect on Acidic CO₂ Electroreduction via Exploring Local pH Variation

Understanding the determinants of local pH in acidic CO₂ electroreduction (CO₂R) is crucial for suppressing the competing hydrogen evolution reaction (HER). Here we reveal that although the interplay between the formation rate of current-induced OH^- and the transport of H⁺ from the bulk electrolyte determines the local pH in acidic media, the construction of local alkalinity that mitigates HER from proton reduction is also closely linked to the presence of alkali cations owing to the conservation of local electroneutrality. Notably, we found the influence of cations on catalytic performance in acid (pH ≤ 1.2) strongly depends on the anion species by comparing the commonly used K₂SO₄ and KCl electrolytes in the acidic CO₂R field. Under identical bulk pH adjusted with sulfuric acid, we found that the common-ion effect of the anion leads to a substantially larger amount of HSO₄^- when using K₂SO₄ electrolytes compared to the utilization of KCl electrolytes. The extra HSO₄^- in commonly used sulfate electrolytes not only offers an additional proton source for HER but also buffers the local pH during acidic CO₂R. In situ Raman spectroscopy was employed to determine the trend of local pH variation, proving the role of the common-ion effect in the local pH.