A DFT+U Study of the Electrochemical Oxidation of H2 and CO on SrLaFeO4-Δ

Abstract

INTRODUCTION Solid oxide fuel cell (SOFC) anodes are commonly composed of cermet materials, typically Ni-YSZ. Despite success in electrochemical performance, cermet materials suffer from sulfur poisoning, coke formation under low operating temperatures, and metal coarsening. Due to success of material design focused on the cathode such as LSCF, single-phase mixed ionic-electronic conducting (MIEC) materials have displayed promising oxidation activity [1]. The Sr-based Ruddlesden-Popper (RP) family has found recent use as a base stoichiometry in anodes where experimentalists have used both A and B-site doping and nanoparticle exsolvation strategies in order to tune activity [2,3]. In order to understand the fundamental electrochemical activity, we want to elaborate on the mechanism of H2 and CO oxidation on the promising material SrLaFeO4- \U0001d6ff (SLF). In the present study, we used ab initio methods to model H2 and CO oxidation on two FeO2-based models of SLF with and without B-site doping with Co and Ni. COMPUTATIONAL DETAILS All calculations presented in this work were performed using the spin-polarized DFT+U method with periodic boundary conditions as implemented in the VASP 5.4.4 code. We choose the PBE functional to describe exchange and correlation effects. Dudarev’s approach for DFT+U calculations is used with a U-J value of 4.0, 3.32, and 6.2 eV, respectively. We used the projector-augmented wave (PAW) method to represent the inner core potentials. The kinetic energy cutoff was set for all calculations to 700 eV, and integration over the Brillouin zone was performed with 3 × 3 × 1 Monkhorst-Point k-point mesh and the Gaussian smearing method with a sigma value of 0.05 eV. Slab models utilized a 15 Å vacuum layer and the bottom two layers were fixed to bulk position values. The DFT-derived parameters were employed in a microkinetic model to calculate the surface coverage of all adsorbed species for a given reaction condition. RESULTS AND DISCUSSION To begin our study, we tested all non-symmetrical conformers of SLF for the lowest relative energy on a model 2 × 2 × 1 supercell, as displayed in Figure 1a. A total of 3 structures were tested – P4/mmm, P4/nmm, and I4mm. We confirm that the lowest energy structure (I4mm) is equivalent to the experimentally reported structure. Next, we develop two 1.5 × 1.5 × 1 non-identical (001) FeO2-based slab models – so-called FeO2-LaO and FeO2-SrO (Figure 2b). The oxidation mechanism of H2 suggests oxidation begins with a dissociative adsorption step. Subsequent steps include formation of surface H2O, a surface vacancy, and a subsurface vacancy. To complete the catalytic cycle, we computed the bulk vacancy migration of SLF and assumed fast cathode kinetics for the overall cell model. At an operating voltage of 0.7 V, we predict that the highest rate occurs on the FeO2-SrO surface. We predict that H2 dissociative adsorption step is the rate-determining process based on Campbell’s degree of rate control. With the application of a single B-site dopant (Co and Ni), Co and Ni both decrease the barrier of the H2 dissociative adsorption step. Ni causes a slight decrease in energy for the formation energy of H2O. The oxidation mechanism of CO suggests oxidation begins with adsorption of the CO molecule on a reduced Fe site. The CO migrates to a neighboring Fe-O complex in order to form surface CO2. Based on degree of rate control, CO2 formation is likely the rate-determining step for both surface models.In conclusion, this is one of the first mechanism investigations on the anodic oxidation reactions of H2 and CO on RP-based oxides. We find that H2 has promising catalytic performance similar to the experimental evidence. The inclusion of B-site dopants displays that catalytic performance can be tuned with the introduction of different transition metals. Future work will examine the role of exsolved particles for RP-based oxides. ACKNOWLEDGMENTS This work has been funded by the Division of Materials Research, a National Science Foundation organization under Award Number 1832809. Computing resources provided by the San Diego Supercomputer Center (SDSC) and Texas Advanced Computing Center (TACC) under XSEDE grant number TG-CTS090100 and USC High-Performance Computing clusters are gratefully acknowledged. REFERENCES L. Xu, Y. Yin, N. Wang, Z. Ma, Electrochem. Commun., 76, 51–54 (2017)N. Wu, W. Wang, Y. Zhong, G. Yang, J. Qu, Z. Shao, ChemElectroChem, 4, 2378–2384 (2017)H. Chong, H. Chen, Z. Shao, J. Shi, J. Bai, S. Li, J. Mater. Chem. A, 4, 13997–14007 (2016) FIGURE 1a: 2×2×1 supercell model of I4mm SrLaFeO4 FIGURE 1b: Slab models of (001) FeO2-LaO and FeO2-SrO Figure 1