An In Operando Probe of CO₂ Diffusion by Microfluidic Spectroelectrochemistry

CO₂ diffusion is the rate-determining step for electrochemical CO₂ reduction at large current densities. Herein, we develop a microfluidic spectroelectrochemical approach to measure the diffusion coefficients of CO₂ in bicarbonates in operando for the first time. Our results establish the critical role of cation-affected electrolyte chemistry on the measured CO₂ concentration profiles, which leads to erroneous measurements of CO₂ diffusion coefficients in electrolytes during the reaction. After isolating the effect of cations using crown ethers, we find that CO₂ diffusion follows the classical theory. With our approach, we quantify that the HCO₃^-/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 CO₂ supply to the electrode. The local CO₂ supply is dominated by CO₂ diffusion in the weak steric effect regime and by the HCO₃^-/CO_2(aq) equilibrium reaction in the strong steric effect regime.