Density-Functional-Theory Modeling of Cation Diffusion in Bulk La1-xSrxMnO3 +/-delta ( x=0.0-0.25) for Solid-Oxide Fuel-Cell Cathodes

作者：Lee Yueh Lin^*; Duan Yuhua^*; Morgan Dane; Sorescu Dan C; Abernathy Harry; Hackett Gregory

来源：Physical Review Applied, 2017, 8(4): 044001.

DOI：10.1103/PhysRevApplied.8.044001

In this work, the A- and B-site cation migration pathways involving defect complexes in bulk La1-xSrxMnO3 +/-delta (LSM) at x = 0.0-0.25 are investigated based on density-functional-theory modeling for solid-oxide fuel-cell (SOFC) cathode applications. We propose a dominant A-site cation migration mechanism which involves an A-site cation (e.g., La-A(x)) hop into a V-A ''' of a V-A '''-V-B ''' cluster, where La-A(x), V-A ''', and, V-B ''' are La3+, A-site vacancy, and B-site vacancy in bulk LSM, respectively, and V-A '''-V-B ''' is the first nearest-neighbor V-A ''' and V-B ''' pair. This hop exhibits an approximately 1.6-eV migration barrier as compared to approximately 2.9 eV of the Lax A hop into a V-A '''. This decrease in the cation migration barrier is attributed to the presence of the V-B ''' relieving the electrostatic repulsion and steric constraints to the migrating A-site cations in the transition-state image configurations. The V-A '''-V-B ''' interaction energy is predicted to be weakly repulsive (0.2-0.3 eV) in bulk LSM, which enables the V-A '''-V-B ''' cluster to readily form. The predicted apparent activation energy of D-La* in LaMnO3 +/-delta (LMO) for the A-site migration pathway is about 1.4 eV, in good agreement with the experimental A-site cation impurity diffusivity measurements. By examining the A-site cation migration barriers among different metal cations (Zr4+, Y3+, Gd3+) relevant for SOFC applications, it is demonstrated that migration barriers of the cation impurity in bulk LSM correlate with the ionic charge and ionic radius at a given formal cationic charge. The B-site cation migration barrier takes place by an analogous mechanism that involves a Mn-B(x) (Mn3+ on the B site) hop into a V-B ''' via the Mnx B-V-A '''. V-B ''' path with the same cation transport carrier of V-A '''-V-B '''. This diffusion pathway is found to have a barrier of approximately 1.6 eV, similar to the analogous A-site hop. However, hopping of the Mnx A antisite defect (Mn3+ on the A site) to a nearest-neighbor V-A ''' [Mn-A(x) (V-A ''') mechanism] has a barrier of only 0.5 eV. Such a low Mn-A(x) (V-A ''') migration barrier opens the possibility to activate Mn transport in bulk LSM through the diffusion of the antisite Mn-A(x) (V-A ''') pathway on the A-site lattice, particularly when the concentration of the Mn antisite defect can be altered upon varying the A/B ratio and the activity of MnOy. The increase in Sr-A' doping concentration in bulk La1-xSrxMnO3 +/-delta (x = 0.0-0.25) is found to influence primarily the formation energies of cation transport carriers (cation vacancies), whereas the cation migration barriers exhibit only a weak dependence on the Sr-A' concentration.

• 出版日期2017-10-4
• 单位AECOM