pH Variation in the Acidic Electrochemical CO 2 Reduction Process

To address the carbonate problem in the alkaline electrochemical CO reduction reaction (CO RR), more attention has been paid to the CO RR conducted in acidic electrolytes. The pH stability of such an acidic electrolyte is vital to make sure that the conclusion made in the so-called acidic CO RR is reliable. Herein, based on reported model electrocatalysts for acidic CO RR, by monitoring the varying of pH and alkali cation (K ) concentration along with the CO RR performance in initially acidic electrolyte solution (K SO with pH = 3.5), we unveil their remarkable CO RR performance along with the rapid pH increase up to 9.5 in the cathode chamber and decrease down to 2.4 in the anode chamber due to the diffusion of K along with protons through the proton exchange membrane from the anode to the cathode chamber. We further reveal the rapid collapse of their CO RR performance in a constant acid solution. This means that some previously reported "remarkable acidic CO RR performances" actually originate from the alkaline rather than acidic electrolyte, and the conclusions made in such work need to be reconsidered. We also summarize the actual relationship between the CO RR performance and catholyte pH in widely used Bi- and Sn-based catalysts. This work provides deeper insights into the stability of acidity and the pH effect on electrocatalysts for the CO RR.