pH Variation\nin the Acidic Electrochemical CO<sub>2</sub> Reduction Process

To address the carbonate problem in the alkaline electrochemical\nCO<sub>2</sub> reduction reaction (CO<sub>2</sub>RR), more attention\nhas been paid to the CO<sub>2</sub>RR conducted in acidic electrolytes.\nThe pH stability of such an acidic electrolyte is vital to make sure\nthat the conclusion made in the so-called acidic CO<sub>2</sub>RR\nis reliable. Herein, based on reported model electrocatalysts for\nacidic CO<sub>2</sub>RR, by monitoring the varying of pH and alkali\ncation (K<sup>+</sup>) concentration along with the CO<sub>2</sub>RR performance in initially acidic electrolyte solution (K<sub>2</sub>SO<sub>4</sub> with pH = 3.5), we unveil their remarkable CO<sub>2</sub>RR performance along with the rapid pH increase up to 9.5\nin the cathode chamber and decrease down to 2.4 in the anode chamber\ndue to the diffusion of K<sup>+</sup> along with protons through the\nproton exchange membrane from the anode to the cathode chamber. We\nfurther reveal the rapid collapse of their CO<sub>2</sub>RR performance\nin a constant acid solution. This means that some previously reported\n“remarkable acidic CO<sub>2</sub>RR performances” actually\noriginate from the alkaline rather than acidic electrolyte, and the\nconclusions made in such work need to be reconsidered. We also summarize\nthe actual relationship between the CO<sub>2</sub>RR performance and\ncatholyte pH in widely used Bi- and Sn-based catalysts. This work\nprovides deeper insights into the stability of acidity and the pH\neffect on electrocatalysts for the CO<sub>2</sub>RR.