Specifically Adsorbed Carbonate Ions and Copper Surface Reconstruction: The Effect of Double Layer Charging Revealed by Time-Resolved Sum Frequency Generation Spectroscopy

Understanding the surface interaction of adsorbed CO on Cu electrodes during electrochemical CO2R is crucial to understand the C2+ product selectivity unique to Cu. However, a significant overpotential is required to achieve the CO surface coverages on Cu necessary for CO dimerization, which is often attributed to site blocking by specific anion adsorption. Performing time-resolved vibrational sum frequency generation with spectral collection sufficiently fast to resolve electrochemical double layer (EDL) charging, the time-resolved frequency shift of CO is measured following a potential jump. Prior to CO desorption, frequency shifts specific to EDL reconstruction and the associated electric field change (Stark tuning response) are isolated from the competing effects of dipolar coupling and CO coverage-induced surface reconstruction. This enables the first measurement of a Stark tuning slope in the potential region where specific anion adsorption has been predicted to occur on Cu. The results reveal a surprising potential-dependent frequency profile with a 4-fold lower Stark tuning slope than expected for a clean Cu surface based on Gouy-Chapman-Stern theory. From this, we determine the absolute surface charge density of the Cu electrode as a function of potential and show that specific anion adsorption cannot account for CO site blocking. Rather, results reveal that Cu is at least 50% oxidized at potentials as negative as -0.2 V vs RHE, indicating that CO desorption occurs by Cu reconstruction to a malachite-like surface on the millisecond time scale mediated by EDL charging.