Theoretical Investigation of the Electrochemical Oxidation of H and CO Fuels on a Ruddlesden-Popper SrLaFeO Anode.

The electrochemical oxidation of H and CO fuels have been investigated on the Ruddlesden-Popper layered perovskite SrLaFeO (SLF) under anodic solid oxide fuel cell conditions using periodic density functional theory and microkinetic modeling techniques. Two distinct FeO-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 H, CO, and syngas fuels. Microkinetic modeling predicted an order of magnitude higher turnover frequency for the electro-oxidation of H 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 H oxidation than the LaO-based surface model. At an operating voltage of less than 0.7 V, surface HO/CO formation was found to be the key rate-limiting step, and the surface HO/CO 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 H electro-oxidation and CO 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 H electro-oxidation activity of FeO-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.