An In Operando Probe of CO₂ Diffusion by Microfluidic Spectroelectrochemistry

Area of Science:

• Electrochemistry
• Chemical Engineering
• Physical Chemistry

Background:

• Carbon dioxide (CO2) diffusion is crucial for efficient electrochemical CO2 reduction, especially at high current densities.
• Accurate measurement of CO2 diffusion coefficients in electrolytes is essential for optimizing CO2 reduction reactions.
• Electrolyte chemistry, particularly cation effects, can significantly influence CO2 concentration profiles and lead to inaccurate diffusion measurements.

Purpose of the Study:

• To develop and apply a microfluidic spectroelectrochemical approach for in operando measurement of CO2 diffusion coefficients.
• To investigate and correct for the impact of cation-affected electrolyte chemistry on CO2 diffusion measurements.
• To quantify the modulation of the bicarbonate/aqueous CO2 equilibrium constant under electrolysis conditions.

Main Methods:

• Development of a microfluidic spectroelectrochemical cell for real-time measurements.
• Utilizing crown ethers to isolate and mitigate cation effects on electrolyte chemistry.
• Analyzing CO2 concentration profiles to determine diffusion coefficients and equilibrium constants.

Main Results:

• Demonstrated that cation-affected electrolyte chemistry can lead to erroneous CO2 diffusion coefficient measurements.
• Showed that CO2 diffusion adheres to classical theory when cation effects are isolated.
• Quantified a modulation of the HCO3-/CO2(aq) equilibrium constant by over three orders of magnitude during electrolysis.

Conclusions:

• The developed microfluidic spectroelectrochemical method provides accurate in operando measurements of CO2 diffusion.
• Understanding cation effects and HCO3-/CO2(aq) equilibrium is critical for controlling CO2 supply to the electrode.
• CO2 supply mechanisms are potential-dependent, dominated by diffusion or equilibrium reactions based on steric effects.