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.

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.