pH Variation in the Acidic Electrochemical CO₂ Reduction Process

To address the carbonate problem in the alkaline electrochemical CO₂ reduction reaction (CO₂RR), more attention has been paid to the CO₂RR 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 CO₂RR is reliable. Herein, based on reported model electrocatalysts for acidic CO₂RR, by monitoring the varying of pH and alkali cation (K⁺) concentration along with the CO₂RR performance in initially acidic electrolyte solution (K₂SO₄ with pH = 3.5), we unveil their remarkable CO₂RR 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 CO₂RR performance in a constant acid solution. This means that some previously reported "remarkable acidic CO₂RR 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 CO₂RR 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 CO₂RR.