title: 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 + O(2-)) and carbon monoxide (CO) electro-oxidation reactions (CO + O(2-)→ 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 (CO3(2-)) 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 CO3(2-) over a 400 μm-wide active region on the ceria surface, accompanied by Ce(3+)/Ce(4+) redox changes. This increase in the CO3(2-) steady-state concentration suggests that the process of pre-coordination of CO2 to the ceria surface to form a CO3(2-) intermediate (CO2(g) + O(2-)(surface)→ CO3(2-)(surface)) precedes a rate-limiting electron transfer process involving CO3(2-) reduction to give CO and oxide ions (CO3(2-)(surface) + 2Ce(3+)→ CO(g) + 2O(2-)(surface) + 2Ce(4+)). When the applied bias is switched to -1.5 V to promote CO electro-oxidation on ceria, the surface CO3(2-) concentration slightly decreases from the equilibrium value, suggesting that the electron transfer process is also a rate-limiting process in the reverse direction.
authors.affiliation: Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742, USA. eichhorn@umd.edu.
authors.forename: Yi
authors.initials: Y
authors.lastname: Yu
authors.forename: Baohua
authors.initials: B
authors.lastname: Mao
authors.forename: Aaron
authors.initials: A
authors.lastname: Geller
authors.forename: Rui
authors.initials: R
authors.lastname: Chang
authors.forename: Karen
authors.initials: K
authors.lastname: Gaskell
authors.forename: Zhi
authors.initials: Z
authors.lastname: Liu
authors.forename: Bryan W
authors.initials: BW
authors.lastname: Eichhorn