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.

Starting from the analysis of the hierarchy of equations for many-point reactant densities involved in three kinds of basic bimolecular reactions, A + A + B, A + B + C, A + B + B, in condensed media, a review is given of a new class of self-organisation phenomena. Unlike the usual synergetic effects, these phenomena are characterised by the appearance of microscopic dynamical clustering of similar reactants, which, however, does not violate the macroscopic homogeneity of the system. The many-particle effects are described in terms of the correlation length and critical exponents in much the same way as is done in the theory of critical phenomena (phase transitions) developed in statistical physics. It is shown that microscopic self-organisation results in asymptotic decay laws for reactant densities which are unusual for standard physical and chemical kinetics. The corresponding reduction of reaction rate with time is due to the emergence, in the course of biomolecular reaction, of a non-Poisson fluctuation spectrum of reactant densities governing the time development of average quantities. The universal character of the newly discovered self-organisation phenomena has been demonstrated to occur not only in numerous kinds of diffusion-controlled reactions, but also for static reactions at low temperatures, including reactant accumulation, when there is a source creating them (e.g. irradiation) and a long-range (tunnelling) recombination of immobile donors and acceptors in crystals. The mathematical formalism developed is applied to the two-stage bimolecular processes using the Lotka and Lotka-Volterra models as examples. Their analysis has revealed that in this case the generally accepted viewpoint on the self-organisation phenomena fails. This review was received in its present form in June 1988. 'Everything should be done as simply as possible but not simpler.' A Einstein 'Entia non sunt multiplicanda sine necesitate.

We present and discuss the results of calculations of SrTiO 3 ͑100͒ surface relaxation and rumpling with two different terminations ͑SrO and TiO 2 ͒. These are based on ab initio Hartree-Fock method with electron correlation corrections and density functional theory calculations with different exchange-correlation functionals, including hybrid exchange techniques. Both approaches use the localized Gaussian-type basis set. All methods agree well on surface energies and on atomic displacements, as well as on considerable increase of covalency effects nearby the surface. More detailed experiments on surface rumpling and relaxation are necessary for further testing theoretical predictions.

We present theoretical support for a mass storage anomaly proposed for nanocomposites in the context of lithium batteries which forms the transition between an electrostatic capacitive mechanism and an electrode mechanism. Ab initio atomic and electronic structure calculations, performed on the Ti(0001)/Li2O(111) model interface, indicate the validity of the phenomenological model of interfacial Li storage and provide a deeper insight into the local situation. Beyond the specific applicability to storage devices, the possibility of a two-phase effect on mass storage generally highlights the availability of novel degrees of freedom in materials research when dealing with nanocomposites.

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