In-Situ Estimation of Near Electrode pH during the Electrochemical Reduction of CO2 Using Attenuated Total Reflectance Surface Enhanced Infrared Absorption Spectroscopy

Abstract

Localized changes in pH within the electrochemical double layer during various electrochemical processes has been a well-established phenomenon for decades; however, experimental quantification of surface pH changes is difficult. In this work, we demonstrate a new method for the in­-situ­ determination of pH near the electrode surface using the CO2 reduction reaction as an example system. Briefly, attenuated total reflectance surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) is employed to monitor the ratio of carbonate and bicarbonate peaks as a function of electrode potential. Peak areas are compared with those in obtained from calibration spectra at various pH adjusted using potassium hydroxide. In CO2 saturated 1.0 M sodium bicarbonate solution, it is found that the pH near the electrode changes by over 1.3 units during electrolysis at -0.9 V vs. RHE, leading to an underestimation of the applied overpotential by over 0.08 V. The effect of buffer concentration was investigated by repeating the same procedure in 0.25 and 0.5 M sodium bicarbonate solutions. It is observed that despite a significant decrease in current, larger pH gradients arise during electrolysis with decreasing buffer capacity of the electrolyte. In addition to the effect of buffer capacity, the effect of stirring the electrolyte on near-electrode pH is studied using a novel spectroelectrochemical cell for ATR-SEIRAS designed to allow for stirring of the electrolyte during spectroscopic measurements.