On the origin of the blocking effect of grain-boundaries on proton transport in yttrium-doped barium zirconates

Despite the superior bulk protonic conductivity combined with chemical stability in the temperature region in which intermediate-temperature solid oxide fuel cells (IT-SOFCs) are operated, the use of polycrystalline ceramics of yttrium-doped barium zirconates (BZY) as a solid electrolyte is still largely limited mainly because the grain boundaries in these materials substantially increase the internal resistance. Hence the demand for better understanding of the origin of such high grain-boundary resistance in BZY has grown rapidly. It has long been speculated that the grain-boundary resistance may be attributed to the space charges formed at the grain boundaries. Here we present the results of our investigation on the space-charge effects on the grain-boundary conduction in BZY. We identify the width of the grain boundary as an independent parameter which allows for an unambiguous verification of the space-charge effects in BZY. By quantitatively comparing the values of the widths experimentally determined not only for the 5, 8, and 20 mol% doped BZY samples we prepared but also for various other BZYs previously reported with those calculated based on a space-charge model, we have successfully verified that the grain-boundary conduction in these materials is indeed governed by the space charges.