An In Operando Probe of CO ₂ Diffusion by Microfluidic Spectroelectrochemistry

CO 2 diffusion is the rate-determining step for electrochemical CO 2 reduction at large current densities. Herein, we develop a microfluidic spectroelectrochemical approach to measure the diffusion coefficients of CO 2 in bicarbonates in operando for the first time. Our results establish the critical role of cation-affected electrolyte chemistry on the measured CO 2 concentration profiles, which leads to erroneous measurements of CO 2 diffusion coefficients in electrolytes during the reaction. After isolating the effect of cations using crown ethers, we find that CO 2 diffusion follows the classical theory. With our approach, we quantify that the HCO 3 – /CO 2(aq) equilibrium constant can be modulated by more than 3 orders of magnitude under electrolysis conditions. From these results, we further generalize the potential-dependent mechanisms of CO 2 supply to the electrode. The local CO 2 supply is dominated by CO 2 diffusion in the weak steric effect regime and by the HCO 3 – /CO 2(aq) equilibrium reaction in the strong steric effect regime.