Impact of Electrolyte Anions on the Adsorption of CO on Cu Electrodes

The electrocatalytic reduction of carbon dioxide to hydrocarbons and oxygenates on Cu electrodes proceeds through surface-adsorbed CO. The adsorption and desorption of this intermediate play a key role in determining the product selectivity of this electrocatalytic process. It is therefore critical to understand the molecular factors that determine the adsorption of CO on Cu electrodes. In prior studies, it was suggested that specifically adsorbing anions of the supporting electrolyte compete with CO for surface sites at low overpotentials. However, prior infrared (IR) spectroscopy of CO adsorption on Cu electrodes did not compare the relative CO coverages in the presence of different anions and was restricted to a narrow range of electrolyte concentrations (0.1 to 0.2 M). Therefore, the impact of anions on the adsorption of CO on Cu is not fully understood to date. Herein we systematically explored the adsorption and desorption of CO on polycrystalline Cu electrodes in the presence of specifically and nonspecifically adsorbing anions (Cl-, SO42-, ClO4-) at two different concentrations (10 mM and 1 M) of the corresponding sodium salts. With surface-enhanced IR absorption spectroscopy (SEIRAS), we monitored the C O stretch band of atop-bound CO (COatop) and an infrared band of the hydration shells of interfacial anions as a function of electrode potential. We found that at an electrolyte concentration of 10 mM, the adsorption and desorption of COatop are virtually independent of the identity of the anions. In contrast, at an electrolyte concentration of 1 M, the COatop coverage is significantly impacted by the electrolyte anions. The saturation coverages of COatop are lower in the 1 M electrolytes compared with those in the 10 mM electrolytes. The magnitude and mechanism of the modulation depend on the identity of the anions. Weakly and nonspecifically adsorbing anions (SO42-, ClO4-) limit the COatop saturation coverage by blocking a fraction of CO adsorption sites. However, their site-blocking ability depends on the CO coverage, as evidenced by the hysteresis in the CO adsorption/desorption profiles, which likely originates from a reversible CO-induced surface reconstruction. Chloride ions, which can specifically adsorb on Cu electrodes, lower the CO coverage by modulating the CO adsorption energy. This modulation manifests itself in (1) a distinctly higher C O stretch frequency in the presence of this anion relative to that measured in the other electrolytes and (2) the absence of the hysteresis in the adsorption/desorption profiles. Our study highlights the intricate interplay between anions and surface-adsorbed CO at the Cu electrode/electrolyte interface.