Examination of Near-Electrode Concentration Gradients and Kinetic Impacts on the Electrochemical Reduction of CO2 using Surface-Enhanced Infrared Spectroscopy

Localized concentration gradients within the electrochemical double layer during various electrochemical processes can have wide-ranging impacts; however, experimental investigation to quantitatively correlate the rate of surface-mediated electrochemical reaction with the interfacial species concentrations has historically been lacking. In this work, we demonstrate a spectroscopic method for the in situ determination of the surface pH using the CO, reduction reaction as a model system. Attenuated total reflectance surface-enhanced infrared absorption spectroscopy is employed to monitor the ratio of vibrational bands of carbonate and bicarbonate as a function of electrode potential. Integrated areas of vibrational bands are then compared with those obtained from calibration spectra collected in electrolytes with known pH values to determine near-electrode proton concentrations. Experimentally determined interfacial proton concentrations are then related to the resultant concentration overpotentials to examine their impact on electrokinetics. We show that, in CO2-saturated sodium bicarbonate solutions, a concentration overpotential of over 150 mV can be induced during electrolysis at -1.0 V vs RHE, leading to substantial losses in energy efficiency. We also show that increases in both convection and buffering capacity of the electrolyte can mitigate interfacial concentration gradients. On the basis of these results, we further discuss how increases in concentration overpotential affect the mechanistic interpretations of the CO, reduction electrocatalysis, particularly in terms of Tafel slopes and reaction orders.