Solid oxide fuel cells (SOFC) are under intensive investigation since the 1980's as these devices open the way for ecologically clean direct conversion of the chemical energy into electricity, avoiding the efficiency limitation by Carnot's cycle for thermochemical conversion. However, the practical development of SOFC faces a number of unresolved fundamental problems, in particular concerning the kinetics of the electrode reactions, especially oxygen reduction reaction. We review recent experimental and theoretical achievements in the current understanding of the cathode performance by exploring and comparing mostly three materials: (La,Sr)MnO₃ (LSM), (La,Sr)(Co,Fe)O₃ (LSCF) and (Ba,Sr)(Co,Fe)O₃ (BSCF). Special attention is paid to a critical evaluation of advantages and disadvantages of BSCF, which shows the best cathode kinetics known so far for oxides. We demonstrate that it is the combined experimental and theoretical analysis of all major elementary steps of the oxygen reduction reaction which allows us to predict the rate determining steps for a given material under specific operational conditions and thus control and improve SOFC performance.

The atomic and electronic structure, formation energy, and the energy barriers for migration have been calculated for the neutral O vacancy point defect ($F$ center) in cubic $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ employing various implementations of density functional theory (DFT). Both bulk and $\mathrm{Ti}{\mathrm{O}}_2$-terminated (001) surface $F$ centers have been considered. Supercells of different shapes containing up to 320 atoms have been employed. The limit of an isolated single oxygen vacancy in the bulk corresponds to a 270-atom supercell, in contrast to commonly used supercells containing $\sim 40–80$ atoms. Calculations carried out with the hybrid B3PW functional show that the $F$ center level approaches the conduction band bottom to within $\sim 0.5\mathrm{eV}$, as the supercell size increases up to 320 atoms. The analysis of the electronic density maps indicates, however, that this remains a small-radius center with the two electrons left by the missing O ion being redistributed mainly between the vacancy and the $3d\left(z^2\right)$ atomic orbitals of the two nearest Ti ions. As for the dynamical properties, the calculated migration energy barrier in the low oxygen depletion regime is predicted to be $0.4\mathrm{eV}$. In contrast, the surface $F$ center exhibits a more delocalized character, which leads to significantly reduced ionization and migration energies. Results obtained are compared with available experimental data.

The formation and migration of oxygen vacancies in the series of (La,Sr)(Co,Fe)O_3−δ perovskites, which can be used as mixed conducting SOFC cathode materials and oxygen permeation membranes, are explored in detail by means of first principles density functional calculations. Structure distortions, charge redistributions and transition state energies during the oxygen ion migration are obtained and analyzed. Both the overall chemical composition and vacancy formation energy are found to have only a small impact on the migration barrier; it is rather the local cation configuration which affects the barrier. The electron charge transfer from the migrating O ion towards the transition metal ion in the transition state is much smaller in (La,Sr)(Co,Fe)O_3−δ compared to (Ba,Sr)(Co,Fe)O_3−δ perovskites where such a charge transfer makes a significant contribution to the low migration barriers observed (in particular for high Ba and Co content).

We present and discuss the results of ab initio DFT plane-wave supercell calculations of the atomic and molecular oxygen adsorption and diffusion on the LaMnO₃ (001) surface which serves as a model material for a cathode of solid oxide fuel cells. The dissociative adsorption of O₂ molecules from the gas phase is energetically favorable on surface Mn ions even on a defect-free surface. The surface migration energy for adsorbed O ions is found to be quite high, 2.0 eV. We predict that the adsorbed O atoms could penetrate the electrode first plane when much more mobile surface oxygen vacancies (migration energy of 0.69 eV) approach the O ions strongly bound to the surface Mn ions. The formation of the O vacancy near the O atom adsorbed atop surface Mn ion leads to an increase of the O–Mn binding energy by 0.74 eV whereas the drop of this adsorbed O atom into a vacancy possesses no energy barrier. Ab initio thermodynamics predicts that at typical SOFC operation temperatures (∼1200 K) the MnO₂ (001) surface with adsorbed O atoms is the most stable in a very wide range of oxygen gas pressures (above 10⁻² atm).