Density Functional Theory Modeling of A-site Cation Diffusion in Bulk LaMnO3 ±δ for Solid Oxide Fuel Cell Cathodes

Abstract

Solid oxide fuel cells (SOFC)s performances and long term degradation are directly and indirectly influenced by cation diffusion in electrodes, electrolytes, and across their interfaces. For SOFC cathodes, due to the sluggish rates of cation diffusion and the complex coupling between the defect chemistry and cation diffusion pathways, the dependences of the cation diffusivities on temperature and O2 partial pressure in the state-of-the-art La1-xSrxMnO3±δ (LSM) remain insufficiently understood based on the available theoretical models. In this work, the interactions of the defect complexes for cation transport and the associated cation diffusion mechanisms in bulk LSM are investigated based on density functional theory simulations. While the active cation diffusion pathways in bulk LSM have been suggested to involve the transport of the cation defect clusters[3,4], this work assessed stability of a number of possible cation vacancy clusters as a function of temperature and P(O2) as well as the associated cation migration barriers, and unveiled new cation diffusion mechanisms with lower migration barriers than those proposed in the previous studies[1-2]. The temperature and P(O2) dependences of the cation self-diffusion coefficients in bulk LSM are further assessed, by incorporating the defect energetics and the transport carrier concentration obtained from the ab initio bulk defect model[5], and the migration barriers of the investigated cation diffusion pathways[6]. The predicted temperature and P(O2) dependences of the cation diffusivities obtained using this model were compared with the experimental tracer diffusion coefficients reported in the literature [2-3], and implications of the new cation diffusion mechanisms will be discussed. The trends in the migration barriers vs. various types of metal cations relevant to SOFC applications, including La3+, Sr2+, Zr4+, Y3+, etc,are also investigated, and the results obtained suggest that both the ionic charge and the ionic radius can correlate with the calculated cation migration barriers [6].