Strain Engineering in Thin Film Electrolytes for Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are highly efficient electrochemical energy converters. A crucial component is the electrolyte, a gas tight solid oxide which conducts oxygen ions or protons while it blocks electrons. Tensile strain, i.e. lattice distortions enlarging interatomic distances, has been shown to increase the ion conductivity in typical oxygen ion conducting electrolytes while compressive strain has the opposite effect. However, the magnitude of strain effects reported in literature varies greatly among different studies. In comparison, for proton conducting SOFC electrolytes very few reports on the influence of strain exist. Experimental and theoretical results so far disagreed concerning the influence of compressive strain while tensile strain has not been investigated experimentally yet. The extension of the experimental investigation to probing the effect of tensile strain is highly important as a feedback to the theoretical understanding of the proton conduction mechanism. Here, the fundamental question of how strongly strain affects the ion conduction in a single crystal is addressed. Epitaxial thin films, where well defined strain states are induced through the matching of the crystal lattices of film and substrate, are employed as model systems for the single crystalline bulk. The modification of material properties through lattice distortions in epitaxy is generally referred to as strain engineering for which it is important to understand how the strain is generated and how it evolves during the growth. Therefore, the stress (proportional to the strain) is monitored in situ with a multi beam optical stress sensor during the growth. Simultaneously, the growth mode, which is well known to significantly influence the strain, is monitored with reflection high energy electron diffraction. Ion conductors with the desired strain states for conductivity measurements are engineered by monitoring the strain relaxation and by introducing so-called buffer layers between the single crystalline substrate and the ion conductor to tune the lattice mismatch. Importantly, the unstrained reference samiii ples exhibited the conductivity of the single crystalline bulk reported in literature and transmission electron microscopy revealed a high crystallinity for all films, so that the term "model system" is appropriate to describe the samples. In Sm-doped ceria, a typical oxygen ion conducting SOFC electrolyte, the migration barrier for the thermally activated oxygen ion transport was reduced by 0.05 eV as a result of 0.35% tensile strain. The resulting increase in conductivity is in the lower range of strain effects reported in literature, but in good agreement with several more recent studies and theoretical calculations. Independently of the strain effect, an interface (or surface) effect was found to reduce the conductivity of films thinner than ∼ 50 nm. In a similar investigation carried out for the well-known proton conductor Y-doped BaZrO3, it was shown for the first time experimentally that tensile strain reduces the migration barrier for proton transport which disagrees with previous theoretical calculations. In response to the experimental findings, the group of Prof. Nicola Marzari at EPFL conducted theoretical simulations which showed that the effect of proton trapping and the assumption of isotropic instead of biaxial diffusion are crucial to reproduce the experimentally observed effect. The strain relaxation observed during the epitaxial growth of the materials studied in this thesis is discussed in relation to strongly established theoretical models for semiconductor epitaxy. The strain relaxation in films grown on epitaxial buffer layers showed an initial stage of fast relaxation not found during the growth on single crystals nor fitting to the established models. This is an important observation that has to be taken into account in multilayer heteroepitaxy. In the two studies mentioned above, strain basically introduces an artificial anisotropy in the ionic conduction by distorting the cubic lattice, but the ion conductivity may in general depend on the direction of charge transport in a crystal. As a case study, the brownmillerite structured Ba2In2O5 is investigated, which shows predominant oxygen ion conduction in dry environment and proton conduction in the presence of water vapour. Using epitaxial thin films, the conduction is found to be anisotropic for both types of charge carriers. For oxygen ions this was generally expected, but not necessarily for proton conduction. The anisotropic proton conduction is attributed to a planar order among protons which was recently suggested in literature. For a large part, this thesis addresses fundamental questions about the ion conduction