Investigation of the Reaction Mechanisms in La1-XSrxCo1-YFeyO3 Electrode by DFT Calculations

Abstract

Solid Oxide Cells (SOCs) are electrochemical devices working at high temperatures for a clean hydrogen and electricity production in electrolysis and fuel cell modes, respectively. This technology has attracted a growing attention thanks to its high efficiency without the use of expensive catalysts. Although SOCs could play a key role in the energy transition, its commercialization at large scale is still limited due to performance and degradation issues. The state of the art oxygen electrode, La1-xSrxCo1-yFeyO3 (LSCF) exhibits a high electrochemical activity combined with a good mixed ionic and electronic conductivity. However, LSCF is prone to decompose under operation with a Strontium segregation at the surface reducing the cell performance over time [1].To date, the driving force of Sr segregation in LSCF and its impact on the cell performance is not precisely understood. However, to be able to discern these phenomena, it is still needed to better understand at atomic scale the mechanisms occurring at the electrode in the absence of Sr segregation. Therefore, we carried out a first study on the bulk properties of the parent material LSF [3] and LSCF using Density Functional Theory (DFT) calculations. Now we turn our attention to the surface properties of LSCF. In particular, the oxygen reduction reaction is decomposed in fuel cell mode into a sequence of elementary reactions. Thereby, a special focus lies on the identification of the rate determining steps limiting the overall reaction.To shade light into this, we modeled the A-site terminated (100) surface of LSCF using a symmetric slab. Our calculations revealed the existence of peroxide species at the surface arising from the reductive adsorption of a dioxygen molecule into a surface oxygen vacancy. Incorporation and dissociation of the peroxide requires a surface oxygen vacancy or sub-surface oxygen vacancy. To make this reaction happen, the peroxide and the vacancy have to encounter each other. Thereby it is still unclear weather the oxygen vacancy diffuses towards the peroxide, or vice-versa. Due to the particular orientation of the peroxide, which does not stand perpendicularly out of the (100) plane, but leans towards the direction between two A-site surface ions (cf. Fig. 1), four possible orientations of the peroxide exist. Thus, on-site rotations of the peroxide and its hopping to nearest neighbor sites are considered, as well as the diffusion of oxygen vacancies on surface and sub-surface. The Nudged Elastic Band (NEB) method is used to calculate the activation barriers. Diffusion coefficients are then determined using Transition State Theory (TST) combined with Kinetic Monte Carlo (KMC) calculations. These diffusion coefficients will be used as inputs to improve the existing elementary kinetic models [3] in order to investigate more precisely the performance and durability of SOCs under working conditions.[1] Chen, K., Jiang, S.P. Surface Segregation in Solid Oxide Cell Oxygen Electrodes: Phenomena, Mitigation Strategies and Electrochemical Properties. Electrochem. Energ. Rev. 3, 730–765 (2020). https://doi.org/10.1007/s41918-020-00078-z[2] Hartmann, C., Geneste, G., Laurencin, J., Hole polarons in LaFeO3 and La1 − x Sr x FeO3 − δ : Stability, trapping, mobility, effect of Sr concentration, and oxygen vacancies. Accepted in Phys. Rev. B. [3] E. Effori, J. Laurencin, E. D. R. Silva, M. Hubert, T. David, M. Petitjean, G. Geneste, L. Dessemond, and E. Siebert, J.Electrochem. Soc. 168, 044520 (2021). Figure 1