Investigation of the direct methane oxidation over Sm-doped ceria for application in SOFC system

One of the key advantages of solid oxide fuel cells is that they can use hydrocarbon fuels, and CeO_2-δ (ceria)-based oxides play an important role in hydrocarbon activation and carbon coking inhibition. However, even for the simplest hydrocarbon molecule, CH₄, the mechanism of electrochemical oxidation on the ceria surface remains largely unknown. This is due to the complex architecture of typical metal/oxide composite electrodes and the heterogeneity of electrode reactions involving multiple chemical/electrochemical steps. Here, we present a Sm-doped ceria thin-film model electrochemical cell capable of selectively monitoring CH₄ direct-oxidation on the ceria surface. Combined impedance spectroscopy, in operando X-ray photoelectron spectroscopy and DFT calculations reveal that the ceria surface catalyzes the C-H cleavage and that the overall electrode reaction rate is dominantly determined by the H₂O formation step. These observations end the longstanding academic debate over the direct use of CH₄ and provide an ideal electrode design for high-performance fuel cells.