Limitations of Grain Boundary Engineering - Ionic Versus Electronic Conductivity

Abstract

Much attention has recently been focused on the oxygen reduction via Sr-doped lanthanum manganite grain boundaries (GBs) since oxygen transport, as well as surface exchange is approximately 1000x faster at the GBs than for the LSM bulk. With this in mind, there is a strong incentive to tailor properties of thin films by GB engineering. To date, the effect on the el. conductivity has not been studied for such engineered films. In this study, changing microstructure, as well as strain, were found to heavily influence el. charge transport. More specifically, for example, the el. conductivity decreased while ionic conductivity increased with decreasing grain size. For given current collector distance, one could optimize grain size to achieve optimum mixed conductivity. In addition 18O tracer exchange experiments were performed on the same thin films in a novel operando experimental design to study the influence of cathodic bias upon surface exchange and transport properties of both LSM grains and GBs. LSM thin films were deposited by PLD and analyzed by ToF-SIMS, Van der Pauw and impedance spectroscopy. SIMS profiles showed a large increase in 18O concentration in the LSM films with an apparent uphill diffusion. Such experiments were prior performed on microelectrode which require specially designed exchange equipment and a high degree of user expertise. Therefore a novel experimental design was developed which can be performed in conventional set-ups to study the influence of polarization in-operando. This new experimental design facilitates the ability to investigate several voltage changes in every required voltage region under SOFC operation conditions within one and the same thin film sample. The apparent uphill diffusion could be simulated by a 3D finite element model with two parallel and interacting diffusion pathways.