Surface electrochemistry of CO2 reduction and CO oxidation on Sm-doped CeO2−x: coupling between Ce3+ and carbonate adsorbates

Abstract

The efficient electro-reduction of CO₂ to chemical fuels and the electro-oxidation of hydrocarbons for generating electricity are critical toward a carbon-neutral energy cycle. The simplest reactions involving carbon species in solid-oxide fuel cells and electrolyzer cells are CO oxidation and CO₂ reduction, respectively. In catalyzing these reactions, doped ceria exhibits a mixed valence of Ce³⁺ and Ce⁴⁺, and has been employed as a highly active and coking-resistant electrode. Here we report an operando investigation of the surface reaction mechanism on a ceria-based electrochemical cell using ambient pressure X-ray photoelectron spectroscopy. We show that the reaction proceeds via a stable carbonate intermediate, the coverage of which is coupled to the surface Ce³⁺ concentration. Under CO oxidation polarization, both the carbonate and surface Ce³⁺ concentration decrease with overpotential. Under CO₂ reduction polarization, on the other hand, the carbonate coverage saturates whereas the surface Ce³⁺ concentration increases with overpotential. The evolution of these reaction intermediates was analyzed using a simplified two-electron reaction scheme. We propose that the strong adsorbate–adsorbate interaction explains the coverage-dependent reaction mechanism. These new insights into the surface electrochemistry of ceria shed light on the optimization strategies for better fuel cell electrocatalysts.