Pathways for Oxygen Incorporation in Mixed Conducting Perovskites: A DFT-Based Mechanistic Analysis for (La, Sr)MnO_3−δ

Abstract

An extensive set of DFT calculations on LaMnO 3 slabs has been generated and used as a basis to identify the most probable reaction mechanism for oxygen incorporation into (La, Sr)MnO 3-δ cathode materials. MnO 2 [001] is found to be the most stable surface termination under fuel cell operation conditions (high temperature, high pO 2 , cubic unit cell). Chemisorption leading to the formation of O 2 -, O 2 2-, and O -atop Mn is exothermic, but due to the negative adsorption entropy and electrostatic repulsion the levels of coverage of molecular oxygen adsorbates are low (in the few percent range). Under typical solid oxide fuel cell conditions, a mechanism in which the encounter of O -with a surface oxygen vacancy at the surface is rate-determining exhibits the fastest rate. The variation of the reaction rate and preferred mechanism(s) with adsorbate and point defect concentrations is discussed.

Oxygen partial pressure dependence of surface space charge formation in donor-doped SrTiO3

“…Setvin et al showed the existence of oxygen molecules forming superoxides at TiO 2 surfaces below 300 K. 23,24 Similarly, charged oxygen adsorbate species are proposed on perovskite surfaces at comparably high temperatures, their concentration diminishing with increasing temperature though. 25 Far above room temperature, the presence of any of those oxygen species on n-SrTiO 3 surfaces is still unconfirmed and any effect on the space charge formation has not been quantified so far. Thus, up to now, it is not known, whether intrinsic electronic surface states, 11 or unintentional defect layers due to carbonate adsorbates, 4 or possibly chemisorbed charged oxygen molecules such as observed for TiO 2 , 23,24,26 or intrinsic ionic surface defects, namely, V Sr , 15 can cause the observed surface space charge layer effect in n-SrTiO 3 13 and its dependence on the oxygen partial pressure (pO 2 ).…”

Approaching chemical accuracy with density functional calculations: Diatomic energy corrections

“…Unfortunately, these XC approximations often fail badly on molecular species, giving errors of up to 10 or 20% with respect to experimental bond lengths, bond angles, and formation energies [1][2][3][4] . However, we require simultaneously accurate energetics for solids and molecules when calculating the thermodynamics of many technologically relevant reactions, in application areas such as catalysis, hydrogen storage, fuel cells, and batteries [5][6][7][8] . Chemical accuracy (commonly taken as 1 kcal/mol ≈ 40 meV/formula unit), the accuracy of many thermochemical experiments, represents the ultimate goal for first-principles calculations, although it remains elusive in many systems.…”

Stoichiometry of theLaFeO3(010) surface determined from first-principles and thermodynamic calculations

“…1 Despite the merit of electronic structure calculations to directly compare material stabilities, 2,3 relatively few studies on SOFC cathode materials have been devoted to surface stability as a function of oxygen stoichiometry. For instance, Mastrikov et al showed that the MnO 2 termination with adsorbed oxygen is the most stable form of (010) surface for cubic LaMnO 3 at p(O 2 ) ≈ 0.21 atm and T=1200 K, but only MnO 2 -and LaO-terminated surfaces with and without adsorbed oxygen atoms were considered 4 . In addition, the stabilization mechanisms due to electron redistribution within LFO (010) surfaces with different surface oxygen stoichiometries were explored in a previous paper 5 .…”