Electrochemical CO₂ Reduction in Acidic Electrolytes: Spectroscopic Evidence for Local pH Gradients.

Area of Science:

• Electrochemistry
• Surface Science
• Catalysis

Background:

• Electrochemical CO2 reduction (CO2R) is crucial for carbon utilization.
• Operating CO2R under acidic conditions presents challenges due to competing hydrogen evolution.
• Understanding interfacial phenomena at the electrode surface is key to optimizing CO2R.

Purpose of the Study:

• To investigate the role of alkali cations (M+) in electrochemical CO2 reduction (CO2R) at low pH.
• To elucidate the local electrode surface pH and its influence on CO2R selectivity.
• To explore strategies for enhancing CO2R performance under acidic conditions using in situ techniques.

Main Methods:

• Utilized attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) for in situ analysis.
• Employed fluorescent confocal laser scanning microscopy for surface imaging.
• Performed in situ local pH measurements and Stark tuning experiments.

Main Results:

• Demonstrated that CO2R can occur in acidic electrolytes without metal cations at appropriate current densities.
• Showed that alkali cations (M+) increase local pH, promoting CO2R by suppressing proton (H+) mass transport and altering interfacial water structure.
• An organic film formed via in situ electrodeposition enabled selective CO2R (FE_CO ~ 65%) over hydrogen evolution in strong acid (pH 1) with low cation concentrations.

Conclusions:

• Local surface pH and proton transport dynamics are critical factors for efficient CO2R in acidic media.
• Alkali cations can enhance CO2R by modifying the interfacial environment, but their effectiveness is limited by proton consumption rates.
• Employing an organic film is a viable strategy to achieve high CO2R selectivity under challenging acidic conditions.