Local electrical and dielectric properties of nanocrystalline solid oxide fuel cell electrolytes

Reducing the operating temperature of solid oxide fuel cells (SOFCs), to improve durability and lower cost, requires an increase in the low temperature oxygen-ion conductivity of the electrolyte. This work investigates whether the electrolyte conductivity could be increased by decreasing the grain size into the nanoscale.Bulk electrolytes - cubic yttria-stabilized zirconia (YSZ, with 8mol% Y2O3), tetragonal zirconia polycrystal (TZP, with 3mol% Y2O3), and Sr- and Mg- co-doped LaGaO3 (LSGM) - were fabricated with grain sizes ranging from 10nm to 3mum, using commercial or sol-gel-derived nanopowders and various sintering techniques. Local grain boundary and grain core conductivities and dielectric constants were analyzed over a range of temperatures and atmospheres using AC-impedance spectroscopy and our novel nano-Grain Composite Model, and interpreted in terms of grain-size dependent defect chemistry (e.g. space charge models, local thermodynamics, and impurity/ acceptor segregation).All three oxides exhibited qualitatively similar electrical/ dielectric behavior. Their single crystal/ grain core dielectric constants exhibited an upturn with temperature, which was attributed to the onset of dipolar relaxation. Grain boundary dielectric constants were consistently higher than grain core dielectric constants at the nanoscale. n-GCM-derived electrical grain boundary half-widths agreed well with measured acceptor dopant segregation widths at grain boundaries. The local grain boundary conductivity was consistently increased in nanocrystalline vs. microcrystalline samples, although the mechanisms responsible for this behavior differed in each material. Grain core conductivity did not change with grain size in each case.Despite the increase in local grain boundary conductivity at the nanoscale, the total conductivity decreased monotonically with decreasing grain size in all three electrolytes the grain boundaries remain barriers to transport (relative to grain cores), and there are many more of them at the nanoscale. Based on this research, it appears that these acceptor-doped nanocrystalline oxygen ion conductors in bulk form may not show improved ionic conductivity for use in reduced-temperature SOFCs.Nonetheless, potentially beneficial effects of nanocrystallinity were observed: nanocrystalline LSGM exhibited mixed ionic and electronic conductivity in oxidizing environments with electronic transference numbers that are orders of magnitude higher than in microcrystalline LSGM, and YSZ and TZP enabled significant protonic conductivity at the nanoscale.