Meanwhile the cations strengthen the filed in the Stern layer and stabilizes key intermediates in CO2 reduction. The activity and selectivity of the CO2 reduction reaction are sensitive to alkali metal cations in near neutral bicarbonate solutions. Several hypotheses such as local pH effect 17 and electrostatic interaction have been made to explain these cation effects. Inspired by these studies, we decided to explore a possible influence of alkali cations for CO2 reduction visà-vis the HER in strongly acidic medium. We first probed the effect of K + for CO2 reduction on SnO2/C at pH = 1. We used a three-electrode flow cell with a gas diffusion electrode (GDE) as working electrode (fig. ) for electrocatalytic tests. Fig. shows the cyclic voltammetry (CV) curves of SnO2/C in two different electrolyte solutions: an aqueous solution of 0.1 M trifluoromethanesulfonic acid (HOTf) and a solution containing 0.1 M HOTf + 0.4 M potassium trifluoromethanesulfonate (KOTf). HOTf is a strong acid and the addition of K + ions does not change the pH of the solution. The pHs of both solutions are 1. HER was the only reaction under a N2 atmosphere. The addition of K + greatly suppressed the HER as the potential was more negative than -0.8 V vs reversible hydrogen electrode (RHE) (blue and black curves in fig. ). In 0.1 M HOTf, the CV curves collected under N2 and CO2 were nearly identical, indicating the dominance of HER and the lack of CO2 reduction in the presence of CO2. Indeed, H2 was the only reduction product when CO2 was present (fig. ). In the solution containing 0.1 M HOTf plus 0.4 M KOTf, a higher current density was observed in the CV curve with CO2 at potentials more negative of -1.0 V vs RHE (red curve in fig. ). Formic acid and CO were detected as products in this potential range (fig. ).