CO2 reduction on doped-ceria electrodes: controlling CO selectivity and coking with applied bias

Abstract

Mixed ionic and electronic conducting ceria electrodes are promising for high-temperature CO₂ electrolysis. Doping these electrodes with Pr/Gd enhances both electrochemical and thermochemical CO-producing reactions. The improved electrodes demonstrated excellent performance even under extreme currents of ∼2 A cm⁻² without much overpotential penalty. The decoupling of electrochemical and thermochemical CO production is demonstrated via mass transient studies. CO electrolysis was directly linked to the number of Ce³⁺ sites. Reduction bias creates Ce³⁺, which in turn produces CO without affecting the underlying thermochemical reverse water gas shift reaction. High bias essentially enhanced electrochemical CO production without affecting the thermochemical counterpart. This allows us to control the syngas output composition by adjusting the bias. Operando Raman, optical microscopy, and online mass spectrometry were employed to track structure–property relationships. CO production correlated with the reduction of Ce⁴⁺ centers to Ce³⁺. Coking was observed under certain conditions. The presence of surface M–CO species was found to be a key prerequisite for possible coke formation. Only on samples that show M–CO species, the onset of coke formation was possible when nearly all of the Ce⁴⁺ sites disappeared. This could be controlled with bias. A “Ce³⁺–CO” site-mediated coking mechanism has been reported.