Yuri A. Mastrikov

First-principles modelling of complex perovskite (Ba1-xSrx)(Co1-yFey)O3-δ for solid oxide fuel cell and gas separation membrane applications

The results of the first principles spin-polarized DFT calculations of the atomic and electronic structure of a complex perovskite (Ba1-xSrx)(Co1-yFey)O3-δ (BSCF) used as a cathode material for solid oxide fuel cells (SOFC) and gas separation membranes are presented and discussed. The formation energies of oxygen vacancies are found to be considerably smaller than in other magnetic perovskites, e.g. (La,Sr)MnO3, which explains the experimentally observed strong deviation of this material from stoichiometry. The presence of oxygen vacancies induces a local charge redistribution, associated with the local lattice perturbation, and expansion of the equilibrium volume, in line with the experimental data.

Atomic scale DFT simulations of point defects in uranium nitride

Atomic scale density functional calculations are used to predict the behaviour of defects in uranium mononitride (UN). Two different density functional codes (VASP and CASTEP) were employed with supercells containing from 8 to 250 atoms (providing a significant range of defect concentrations). Schottky and nitrogen Frenkel point defect formation energies, local lattice relaxations and overall lattice parameter change, as well as the defect induced electronic density redistribution, are discussed.

The Structural Disorder and Lattice Stability of (Ba,Sr)(Co,Fe)O3 Complex Perovskites

The structural disorder and lattice stability of complex perovskite (Ba,Sr)(Co,Fe)O3, a promising cathode material for solid oxide fuel cells and oxygen permeation membranes, is explored by means of first principles DFT calculations. It is predicted that Ba and Sr ions easily exchange their lattice positions (A-cation disorder) similarly to Co and Fe ions (B-cation disorder). The cation antisite defects (exchange of A- and B-type cations) have a relatively high formation energy. The BSCF is predicted to exist in an equilibrium mixture of several phases and can decompose exothermically into the Ba- and Co-rich hexagonal (Ba,Sr)CoO3 and Sr- and Fe-rich cubic (Ba,Sr)FeO3 perovskites.

Ab initio thermodynamic study of (Ba,Sr)(Co,Fe)O3 perovskite solid solutions for fuel cell applications

(Ba,Sr)(Co,Fe)O3 (BSCF) perovskite solid solutions are promising materials for solid oxide fuel cell cathodes and oxygen permeation membranes. Cathode performance strongly depends on the morphology of these materials remaining as a single phase or two-phase mixture. Combining ab initio calculations of the atomic and electronic structure of different supercells with thermodynamics of solid solutions, we have constructed and discussed phase diagrams of several important BSCF chemical compositions. It is demonstrated that in BSC cobaltite solid solution the spinodal decomposition may occur already at relatively low temperatures, while ferrite (BSF and SCF) solid solutions decompose at relatively high temperatures forming a two-phase system where the coexisting hexagonal and cubic phases significantly differ in fractions of constituents.

First Principles Modeling of Oxygen Mobility in Perovskite SOFC Cathode and Oxygen Permeation Membrane Materials

Based on first principles DFT calculations, we analyze activation energies of oxygen vacancy migration in several complex ABO3type perovskite candidate materials for SOFC cathodes and permeation membranes (La(Co,Fe)O3- (LCF) and (Ba,Sr)(Co,Fe)O3- (BSCF)). The atomic relaxation, charge redistribution and energies of the transition states of oxygen migration are compared to understand the microscopic origin of the exceptionally low migration barrier (high oxygen mobility) in BSCF. It is shown that the B-O distance is considerably shortened in the transition state for BSCF due to covalency of this chemical bond, which could be a reason for the significant reduction of the oxygen migration energy in this material. Additionally, the Goldschmidt tolerance factor based on Shannon ionic radii is revisited.

Oxygen Incorporation Reaction into Mixed Conducting Perovskites: a Mechanistic Analysis for (La,Sr)MnO3 Based on DFT Calculations

Based on DFT calculations of intermediates and transition states, several hypothetical mechanisms for oxygen incorporation into mixed conducting La1-xSrxMnO3±δ perovskites are discussed. In the most probable mechanism, the rate-determining step comprises the encounter of a highly mobile surface oxygen vacancy and a molecular oxygen adsorbate. Starting from these results, the variation of reaction rates for different materials is explored.

First-Principles Modeling of LaMnO3 SOFC Cathode Material

We present and discuss the results of first principles DFT plane-wave supercell calculations for atomic and molecular oxygen adsorption and diffusion on the LaMnO3 (001) surface which serves as a model material for a solid oxide fuel cell cathode. The dissociative adsorption of O2 molecules from the gas phase is energetically favorable on surface Mn ions even on a defect-free surface. We discuss optimal adsorption sites for oxygen, binding energies, electronic density redistribution as well as charge transfer. We also compare the surface migration energies for adsorbed O atoms and vacancies in the bulk and on the cathode surface

Thermodynamics of ABO3-Type Perovskite Surfaces

The ABO3-type perovskite manganites, cobaltates, and ferrates (A= La, Sr, Ca; B=Mn, Co, Fe) are important functional materials which have numerous high-tech applications due to their outstanding magnetic and electrical properties, such as colossal magnetoresistance, half-metallic behavior, and composition-dependent metal-insulator transition (Coey et al., 1999; Haghiri-Gosnet & Renard, 2003). Owing to high electronic and ionic conductivities. these materials show also excellent electrochemical performance, thermal and chemical stability, as well as compatibility with widely used electrolyte based on yttrium-stabilized zirconia (YSZ). Therefore they are among the most promising materials as cathodes in solid oxide fuel Cells (SOFCs) (Fleig et al., 2003) and gas-permeation membranes (Zhou, 2009). Many of the above-mentioned applications require understanding and control of surface properties. An important example is LaMnO3 (LMO). Pure LMO has a cubic structure above 750 K, whereas below this temperature the crystalline structure is orthorhombic, with four formula units in a primitive cell. Doping of LMO with Sr allows one to increase both the ionic and electronic conductivity as well as to stabilize the cubic structure down to room temperatures necessary conditions for improving catalytic performance of LMO in electrochemical devices, e.g. cathodes for SOFCs. In optimal compositions of

Surface termination effects on the oxygen reduction reaction rate at fuel cell cathodes

The results of first principles calculations of oxygen vacancy and oxygen adsorbate concentrations are analyzed and compared for the polar (La,Sr)O and MnO2 (001) terminations of (La,Sr)MnO3 fuel cell cathode materials. Both quantities strongly depend on the average Mn oxidation state (La/Sr ratio). In thin symmetrical slabs, the cation nonstoichiometry also plays an important role by modifying the average Mn oxidation state. The surface oxygen vacancy concentration for the (La,Sr)O termination is more than 5 orders of magnitude smaller when compared to the MnO2 termination. The vacancy and adsorbed oxygen migration energies as well as the dissociation barriers of adsorbed molecular oxygen species are determined. The encounter of adsorbed atomic oxygen and surface oxygen vacancy is identified as the rate determining step of the oxygen incorporation reaction. Since the increase of atomic and molecular oxygen adsorbate concentration is limited by the typical saturation level in the range of 20% for charged adsorbates, the overall oxygen incorporation rate is predicted to be significantly smaller for the (La,Sr)O termination.