Theoretical Investigation of the Electrochemical Oxidation of H₂ and CO Fuels on a Ruddlesden-Popper SrLaFeO_4-δ Anode

The electrochemical oxidation of H₂ and CO fuels have been investigated on the Ruddlesden-Popper layered perovskite SrLaFeO_4-δ (SLF) under anodic solid oxide fuel cell conditions using periodic density functional theory and microkinetic modeling techniques. Two distinct FeO₂-plane-terminated surface models differing in terms of the underlying rock salt layer (SrO or LaO) are used to identify the active site and limiting factors for the electro-oxidation of H₂, CO, and syngas fuels. Microkinetic modeling predicted an order of magnitude higher turnover frequency for the electro-oxidation of H₂ compared to CO for SLF at short-circuit conditions. The surface model with an underlying SrO layer was found to be more active with respect to H₂ oxidation than the LaO-based surface model. At an operating voltage of less than 0.7 V, surface H₂O/CO₂ formation was found to be the key rate-limiting step, and the surface H₂O/CO₂ desorption was the key charge transfer step. In contrast, the bulk oxygen migration process was found to affect the overall rate at high cell voltage conditions above 0.9 V. In the presence of syngas fuel, the overall electrochemical activity is derived mainly from H₂ electro-oxidation and CO₂ is chemically shifted to CO via the reverse water-gas shift reaction. Substitutional doping of a surface Fe atom with Co, Ni, and Mn revealed that the H₂ electro-oxidation activity of FeO₂-plane terminated anodes with an underlying LaO rock salt layer can be improved with dopant introduction, with Co yielding a three orders of magnitude higher activity relative to the undoped LaO surface model. Constrained ab initio thermodynamic analysis furthermore suggested that the SLF anodes are resistant toward sulfur poisoning both in the presence and absence of dopants. Our findings reflect the role of various elements in controlling the fuel oxidation activity of SLF anodes that could aid the development of new Ruddlesden-Popper phase materials for fuel cell applications.