CO₂ Reduction on Doped-Ceria Electrodes: Controlling CO selectivity and coking with Applied Bias

Abstract

Mixed ionic-electronic conducting CeOₓ-YSZ electrodes doped with trivalent cations (Pr, Gd) offer an effective strategy to control catalytic activity, syngas stoichiometry, and carbon deposition during CO₂ electrolysis. Operando Raman and thermochemical transient analyses reveal that CO evolution is closely linked to the formation of surface Ce³⁺ sites, which act as the active centers for selective CO₂-to-CO conversion, independent of the thermochemical rWGS pathway. Applying a reducing electrochemical bias enhances Ce³⁺ site concentration, increasing CO yield by ~25% at 2 A cm⁻² and enabling dynamic, bias-dependent tuning of the CO/H₂ ratio in the syngas stream. However, under highly reducing conditions, concurrent formation of M-CO surface species and extensive Ce⁴⁺ → Ce³⁺ reduction trigger a Ce³⁺-CO site-mediated coking mechanism, highlighting the dual role of Ce³⁺ sites in enhancing CO selectivity while influencing carbon stability at the electrode surface.