pH Variation in the Acidic Electrochemical CO Reduction Process.

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