Grain boundaries in solid-oxide electrolytes play an important
role in the oxygen conductivity of the bulk material. Grain
boundaries can be preferential locations for precipitates and can
reduce the oxygen conductivity due to the presence of grainboundary
space-charge layers. It has been shown that for CeO2
the presence of thick siliceous layers can account for the overwhelming
majority of the oxygen ionic resistance,1 but that
even without siliceous impurities the grain boundaries still contribute
a significant fraction of the ionic resistance.2 Controlling
the grain-boundary properties has been proposed as a method
of improving the ionic conductivity of CeO2 electrolytes.3 In
addition, the sintering behavior also strongly depends on the grain-boundary behavior. More generally, interfacial segregation
and its influence on macroscale properties is an important
problem for many ceramic materials4 and in the case of CeO2,
a greater understanding of interfaces should enable improved
performance for SOFCs and catalysts, where the interaction of
oxygen vacancies and surfaces is critical.5
In CeO2 polycrystals with low levels of Si impurities, the
effect of grain boundaries on the oxygen conductivity has
been attributed to the grain-boundary space charge.2,6,7 At
the grain boundary, the segregation of positively charged oxygen
vacancies creates a charged boundary region; this charge
is balanced in the crystal by the formation of neighboring
regions of negative charge. These negatively charged spacecharge
layers are depleted in oxygen vacancies and enriched
in defects such as Ce3+ ions or trivalent dopants (provided
they are mobile). Because oxygen vacancies are the mobile
defects necessary for oxygen conduction, the local depletion
of vacancies will reduce the oxygen conductivity across the
grain boundary.