Unveiling Direct Electrochemical Oxidation of Methane at the Ceria/Gas Interface

Solid oxide fuel cells (SOFCs) stand out in sustainable energy systems for their unique ability to efficiently utilize hydrocarbon fuels, particularly those from carbon-neutral sources. CeO_2-δ (ceria) based oxides embedded in SOFCs are recognized for their critical role in managing hydrocarbon activation and carbon coking. However, even for the simplest hydrocarbon molecule, CH₄, the mechanism of electrochemical oxidation at the ceria/gas interface is not well understood and the capability of ceria to electrochemically oxidize methane remains a topic of debate. This lack of clarity stems from the intricate design of standard metal/oxide composite electrodes and the complex nature of electrode reactions involving multiple chemical and electrochemical steps. This study presents a Sm-doped ceria thin-film model cell that selectively monitors CH₄ direct-electro-oxidation on the ceria surface. Using impedance spectroscopy, operando X-ray photoelectron spectroscopy, and density functional theory, it is unveiled that ceria surfaces facilitate C─H bond cleavage and that H₂O formation is key in determining the overall reaction rate at the electrode. These insights effectively address the longstanding debate regarding the direct utilization of CH₄ in SOFCs. Moreover, these findings pave the way for an optimized electrode design strategy, essential for developing high-performance, environmentally sustainable fuel cells.