(Invited) Mapping Internal Polarization in YSZ Electrolytes Using Grain Size

Abstract

Polarization development near the electrodes is important for the performance of YSZ devices. However, there are very difficult to measure experimentally. Embedded electrodes have been used to obtain a few readings across the electrolyte thickness, but so far there has been no report that provides a continuous mapping of the electrolyte polarization. Here we demonstrate that grain growth provides a means for such mapping. The method is based on the observation that cation mobility in YSZ (as well as other fluorite structured oxides) increases rapidly with decreasing oxygen potential. Placing YSZ under a large current in various atmospheres between electrodes of various configurations, we have created many grain size distributions that vary continuously or discontinuously between the cathode and anode. Aided by the correlation between grain boundary mobility and non-stoichiometry, the grain size map can be converted to a polarization map. Such mapping allows us to quantitatively assess the influence of electrode kinetics and atmosphere on internal polarization under large current conditions. Under the same conditions of enhanced grain growth, we have observed widespread cavitation throughout YSZ in nearly all the samples tested in a reducing atmosphere. This is surprising since the prevailing theory attributes cavitation to oxygen bubbles, and it predicts oxygen bubbles to form inside the YSZ electrolyte only under a large positive oxygen potential. Yet the large grain size in our cavitated samples clearly indicates a large negative oxygen potential. Therefore, we must rule out the oxygen bubble model. Instead, we propose that cavitation in these samples is in the form of voids, which come from condensation of supersaturated oxygen vacancies. These results further suggest strong electronic conduction in the electrolyte. Indeed, the observation of cavitation implies internal reactions between electronic defects and ionic defects, rendering the latter no longer fully ionized. These reactions have the effect of buffering the oxygen potential inside the electrolyte. The observed oxygen potential and grain size transition can only be successfully modeled by taking into account such internal reactions.