pH Variation in the Acidic Electrochemical CO₂ Reduction Process.

Area of Science:

• Electrochemistry
• Materials Science
• Chemical Engineering

Background:

• The electrochemical CO2 reduction reaction (CO2RR) is a promising technology for CO2 utilization.
• Conducting CO2RR in acidic electrolytes aims to avoid carbonate formation, a common issue in alkaline media.
• The stability of the acidic electrolyte's pH is crucial for reliable CO2RR performance evaluation.

Purpose of the Study:

• To investigate the pH stability of acidic electrolytes during CO2RR.
• To determine the influence of electrolyte pH on the performance of electrocatalysts for CO2RR.
• To re-evaluate previously reported CO2RR performances in acidic conditions.

Main Methods:

• Monitoring pH and K+ concentration in cathode and anode chambers during CO2RR in an initially acidic K2SO4 solution (pH 3.5).
• Evaluating CO2RR performance of model electrocatalysts under varying pH conditions.
• Testing catalyst performance in a constant acid solution to confirm findings.

Main Results:

• Electrocatalysts showed remarkable CO2RR performance, but the catholyte pH rapidly increased to 9.5 while the anolyte pH decreased to 2.4.
• This pH shift was attributed to K+ and proton diffusion through the proton exchange membrane.
• CO2RR performance drastically decreased in a constant acidic solution, indicating alkaline conditions were responsible for previous high performance.

Conclusions:

• Many reported high CO2RR performances in acidic electrolytes are artifacts of alkaline conditions created during the reaction.
• The pH of the electrolyte significantly affects CO2RR performance, and conclusions from studies with unstable pH require reconsideration.
• This work highlights the importance of monitoring and controlling electrolyte pH for accurate CO2RR catalyst assessment, particularly for Bi- and Sn-based catalysts.