(354f) A DFT+U Study of the Electrochemical Oxidation of H2 and CO on SrLaFeO4

Solid oxide fuel cell (SOFC) anodes are commonly composed of cermet materials, typically Ni-YSZ. Single-phase electrodes such as the Sr-based Ruddlesden-Popper (RP) family serves as a baseline stoichiometry. Bulk doping strategies and nanoparticle exsolvation are used to tune activity. In this study, we elaborate on the mechanism of H₂ and CO electro-oxidation on SrLaFeO_4-𝛿 (SLF). We used ab initio methods to model H₂ and CO oxidation on two FeO₂-based surface models of SLF with surface Co and Ni.

All calculations were performed using the spin-polarized DFT+U method in VASP 5.4.4. We use PBE and the U-J values of 4.0 (Fe), 3.32 (Co), and 6.0 (Ni) eV. Slab models used a kinetic energy cutoff of 700 eV, a 3 × 3 × 1 Monkhorst-Pack mesh, a 15 Å vacuum layer, and the bottom two layers were fixed to bulk position values.

We tested all non-symmetrical conformers of SLF for the lowest relative energy on a model 2 × 2 × 1 supercell (Figure 1a). We developed two 1.5 × 1.5 × 1 non-identical (001) FeO₂-based slab models – FeO₂-LaO and FeO₂-SrO (Figure 1b). The oxidation mechanism of H₂ begins with a dissociative adsorption step. Subsequent steps include formation of surface H₂O, a surface vacancy, and a subsurface vacancy. We computed the bulk vacancy migration of SLF and assumed fast cathode kinetics to close the catalytic cycle. At V_cell = 0.7 V, we predict that the highest rate occurs on the FeO₂-SrO surface. We find surface H₂O formation to be rate-determining. Co and Ni doping decrease the barrier of the H₂ dissociative adsorption and surface H₂O formation steps. The oxidation mechanism of CO begins with adsorption of the CO molecule where CO migrates to a neighboring Fe-O complex to form surface CO₂. We find surface CO₂ formation to be the rate-determining step.