CO2 activation and carbonate intermediates: an operando AP-XPS study of CO2 electrolysis reactions on solid oxide electrochemical cells

Abstract

Through the use of ambient pressure X-ray photoelectron spectroscopy and specially designed ceria-based solid oxide electrochemical cells, carbon dioxide (CO₂) electrolysis reactions (CO₂ + 2e⁻ → CO + O²⁻) and carbon monoxide (CO) electro-oxidation reactions (CO + O²⁻ → CO₂ + 2e⁻) over cerium oxide electrodes have been investigated in the presence of 0.5 Torr CO–CO₂ gas mixtures at ∼600 °C. Carbonate species (CO₃²⁻) are identified on the ceria surface as reaction intermediates. When CO₂ electrolysis is promoted on ceria electrodes at +2.0 V applied bias, we observe a higher concentration of CO₃²⁻ over a 400 μm-wide active region on the ceria surface, accompanied by Ce³⁺/Ce⁴⁺ redox changes. This increase in the CO₃²⁻ steady-state concentration suggests that the process of pre-coordination of CO₂ to the ceria surface to form a CO₃²⁻ intermediate (CO_2(g) + O²⁻_(surface) → CO₃²⁻_(surface)) precedes a rate-limiting electron transfer process involving CO₃²⁻ reduction to give CO and oxide ions (CO₃²⁻_(surface) + 2Ce³⁺ → CO_(g) + 2O²⁻_(surface) + 2Ce⁴⁺). When the applied bias is switched to −1.5 V to promote CO electro-oxidation on ceria, the surface CO₃²⁻ concentration slightly decreases from the equilibrium value, suggesting that the electron transfer process is also a rate-limiting process in the reverse direction.