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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 La0.8Sr0.2MnO3(LSM) thin films by Van-der-Pauw and18O 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. Keywords: grain boundary engineering; LSM; thin film, United States. Department of Energy (Grant DE-SC0002633)

The bulk electrical conductivity of the mixed ionic-electronic conducting perovskite-structured SrSn[subscript 1-x]Fe[subscript x]O[subscript 3-x/2+δ] (SSF) was measured to examine how changes in defect chemistry and electronic band structure associated with the substitution of Ti by Sn impact defect charge carrier density and ultimately electrode performance. These results complement a defect chemical model for SSF investigated and reported in Part I of this study. The electrical properties of SSF were found not to differ significantly from the corresponding composition in SrTi[subscript 1-x]Fe[subscript x]O[subscript 3-x/2+δ] (STF). It is believed that Fe dominates the character of the valence and conduction bands and thus governs the electronic properties in SSF. Though slightly shifted in energy due to the larger size of Sn, the defect equilibria and therefore the electrical conductivity of SSF were found to be largely dominated by Fe and thus differed only in a limited way from that in STF. Key kinetic parameters obtained include the migration enthalpy of oxygen vacancies (0.772 ± 0.204 eV), the activation energy of area-specific-resistance for oxygen exchange (1.65 ± 0.03 eV) and the magnitudes of electron (0.0002 ± 0.00005 cm2/V∙s) and hole (0.0037 ± 0.0015 cm[superscript 2]/V∙s) mobilities. Keywords: Mixed ionic electronic conductor, Cathode, Area specific resistance, Impedance spectroscopy, Perovskite oxide

Functional materials for energy conversion and storage exhibit strong coupling between electrochemistry and mechanics. For example, ceramics developed as electrodes for both solid oxide fuel cells and batteries exhibit cyclic volumetric expansion upon reversible ion transport. Such chemomechanical coupling is typically far from thermodynamic equilibrium, and thus is challenging to quantify experimentally and computationally. In situ measurements and atomistic simulations are under rapid development to explore how this coupling can be used to potentially improve both device performance and durability. Here, we review the commonalities of coupling between electrochemical and mechanical states in fuel cell and battery materials, illustrating with specific cases the progress in materials processing, in situ characterization, and computational modeling and simulation. We also highlight outstanding questions and opportunities in these applications – both to better understand the limiting mechanisms within the materials and to significantly advance the durability and predictability of device performance required for renewable energy conversion and storage., United States. Dept. of Energy (Basic Energy Sciences Division of Materials Sciences and Engineering, grant DE-SC0002633), United States. Dept. of Energy (Office of Science, Graduate Fellowship Program (DOE SCGF)), United States. American Recovery and Reinvestment Act of 2009 (ORISE-ORAU, contract no. DE-AC05-06OR23100)), United States. Dept. of Energy. Division of Materials Sciences and Engineering (MIT/DMSE Salapatas Fellowship), United States. Air Force Office of Scientific Research (Presidential Early Career Award in Science and Engineering (PECASE))