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

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


Heyden, A., University of South Carolina
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 SrLaFeO4-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 Vcell = 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.