Origin of Overpotential-Dependent Surface Dipole at CeO2-x/Gas Interface During Electrochemical Oxygen Insertion Reactions

Ion insertion at the interfaces of batteries, fuel cells, and catalysts constitutes an important class of technologically relevant, charge-transfer reactions. However, the molecular nature of charge separation at the adsorbate/solid interface remains elusive. It has been hypothesized that electrostatic dipoles at the adsorbate/solid interface could result from adsorption-induced charge redistribution, preferential segregation of charged,point defects in the solid, and/or intrinsic dipoles of adsorbates. Using operando ambient-pressure X-ray photoelectron spectroscopy, we elucidate the coupling between electrostatics and adsorbate chemistry on the surface of CeO2-x, an excellent electrocatalyst and a model system for studying oxygen-ion insertion reactions. Three adsorbate chemistries were studied-OH-/CeO2-x (polar adsorbate), CO32-/CeO2-x (nonpolar adsorbate), and Ar/CeO2-x (no adsorbate) under several hundred mTorr of gas pressure relevant to electrochemical H-2/CO oxidation and H2O/CO2 reduction. By integrating core-level spectroscopy and contact-potential difference measurements, we simultaneously determine the chemistry and coverage of adsorbates, Ce oxidation state, and the surface potential at the gas/solid interface over a wide range of overpotentials. We directly observe an overpotential-dependent surface potential, which is moreover sensitive to the polarity of the adsorbates. In the case:of CeO2-x Covered with polar OH-, we observe a surface potential that increases linearly with OH- coverage and with overpotential. On the other hand, for CeO2-x covered with nonpolar CO32- and free of adsorbates, the surface potential is independent of overpotential. The adsorbate binding energy does not change systematically with overpotential. From these observations, we conclude that the electrostatic dipole at the adsorbate/CeO2-x interface is dominated by the intrinsic dipoles of the adsorbates, with the solid contributing minimally. These results provide an atomistic picture of the gas/solid double layer and the experimental methodology to directly study and quantify the surface dipole.