Blocking in Ionic Conduction By Defect Segregation Near Grain-Boundaries in Nanocrystalline Yttria-Doped Ceria Thin Films

Abstract

Nanocrystalline materials have unusual and exceptional electrical and chemical properties, some of which are due to their nanoscale grain sizes and highly dense grain boundaries. Compared to bulk materials, nanocrystalline films used in solid oxide fuel cells have an extremely high density of grain boundaries, and therefore, understanding the role of grain boundaries in ion transport is crucial. There is evidence that grain boundaries affect the electrochemical properties and behavior of thin film solid oxide fuel cell (SOFC) components in two main ways. First, in terms of cathode interfacial kinetics, grain boundaries at the external surface of the solid oxide electrolyte, e.g.,yttria-stabilized zirconia (YSZ), exhibit a higher surface exchange coefficient for oxygen than the bulk. Due to dopant segregation, grain boundaries contain a higher population of oxygen vacancies than the bulk, and thus facilitate low activation energy for oxygen incorporation. Second, previous work studying the effects of dopant distribution, space charge, as well as grain size on ionic transport across grain boundaries revealed that grain boundary resistivity is usually several orders higher than that of the bulk. Thus, understanding the blocking effect of grain boundaries on ionic conductivity in nanocrystalline materials requires a study of how grain size and thermal history affect ion transport in nanocrystalline films. Yttrium-doped ceria (YDC) is a promising electrolyte material for low temperature solid oxide fuel cells (LT-SOFCs) because of its superior ionic conductivity and lower activation energy for ionic transport than YSZ, which is the electrolyte material most commonly used in SOFCs. Previous studies on nanocrystalline YDC examined the effects of grain size on ionic conductivity by varying the grain size using different thermal conditions to generate different grain boundary densities. However, even though the effects of heat treatment on grain boundary density were considered in these studies, the effects of heat treatment on dopant segregation were not. In this paper, nanocrystalline YDC thin films with grain-sizes ranging from 38 to 93-nm were prepared using pulsed laser deposition followed by thermal annealing. The blocking effect of the grain boundary on ionic conduction in nanocrystalline YDC films was investigated with different grain sizes and thermal histories. Through high resolution TEM analysis accompanied with EDS capability, we found out that dopant segregation at the grain boundary core region became significant after annealing while it was not observed in as-deposited YDC film. Concentration of oxygen vacancies due to dopant segregation may hinder ionic transport by a space charge layer effect, decreasing the ionic conductivity of annealed YDC films. In electrochemical impedance spectroscopy analysis, as-deposited YDC film showed comparable ionic conductivity and activation energy to epitaxial YDC film, but ionic conductivity decreased as the annealing temperature increased. Spectroscopic evidence presented in this study suggests that dopant segregation has a more pronounced impact on ionic conductivity than grain size, i.e., grain boundary density [1]. Reference [1] J. An, et al., "Grain Boundary Blocking of Ionic Conductivity in Nanocrystalline Yttria-doped Ceria Thin Films", Scripta Materialia, Vol. 104, pp. 45-48 (2015)