The group of Koper showed that CO2 reduction to CO was feasible in an HClO4-NaClO4 mixed solution with a mild acidity (pH ≥ 3). They proposed that the HER was suppressed by OH - anions generated from CO2 reduction. During the preparation of our publication, Huang et al. reported efficient CO2 electroreduction on Cu catalysts in strongly acidic solutions (pH = 0.67) with H3PO4-KCl mixed electrolytes. In both studies, alkali cations are essential for CO2 reduction, but the mechanism of cation-promotion was unclear.