Interplay of grain size dependent electronic and ionic conductivity in electrochemical polarization studies on Sr-doped LaMnO₃ (LSM) thin film cathodes

Many efforts are being made to tune perovskite thin film cathodes toward improving their oxygen reduction kinetics and thereby improving overall solid oxide fuel cell performance. One approach is to enhance oxygen diffusion via introduction of larger concentrations of grain boundaries during thin film growth. While such grain boundary engineering has been shown to enhance ionic transport and surface reaction kinetics in some cases, little attention has been paid on its corresponding influence on electronic conductivity. To provide insights into the role of grain boundaries and their contribution to the cathode performance, we have investigated separately the electronic and ionic conductivity of La_0.8Sr_0.2MnO₃ (LSM) thin films by Van-der-Pauw and ¹⁸O tracer exchange measurements respectively, as well as their combined contributions by electrochemical impedance spectroscopy. All three types of experiments were performed on the same kind of samples with varying LSM microstructure to illustrate the effects of grain boundaries on both electron and ion conduction. Correlations between active electrode area and microstructure-dependent partial conductivities are presented. The findings can also be used for optimizing current collector spacing in thin film solid oxide fuel cells.