Influence of particle size and sintering aid on sinterability and conductivity of BaZr0.1Ce0.7Y0.2O3-δ electrolyte

Abstract

Two BaZr0.1Ce0.7Y0.2O3-δ (BZCY) powders prepared by solid-state reaction (BZCY(sr)) and sol-gel (BZCY(sg)) methods and three NiO powders (two commercial NiO with large (NiO(L)) and small (NiO(S)) particles and one prepared by solution-infiltration method (NiO(I)) are used to study the effect of the particle sizes of BZCY powder and NiO sintering aid on the sinterability and conductivity of sintered BZCY electrolytes for protonic ceramic fuel cells (PCFCs). The BZCY and NiO powders are examined by X-ray diffraction and scanning electron microscopy (SEM). The microstructures of the six BZCY-NiO mixtures sintered at 1500 °C in air for 6 h, named as sr-L, sr-S, sr-I, sg-L, sg-S, and sg-I, are characterized by SEM to compare their pore morphology and grain size, and their conductivities from 550 to 800 °C in a 5 vol% H2–N2 atmosphere are measured using the four-probe and impedance spectroscopy methods, respectively. The results show that sr-L and sg-L are not fully dense with large pores, most of them are at the grain boundaries; sr-S and sg-S have a much denser microstructure; sr-I and sg-I have the highest density with fine pores. The average grain size, the conductivity, and the grain-boundary resistance follow the orders of sr-L (8.16 μm) ≈ sg-L (8.53 μm) < sr-S (10.59 μm) ≈ sg-I (10.65 μm) < sg-S (14.07 μm) < sg-I (26.08 μm), sr-L < sg-L < sr-S < sg-S ≈ sr-I < sg-I, and sr-L > sg-L > sr-S > sg-S ≈ sr-I > sg-I, respectively. The above results indicate that both the density and grain size affect the conductivity critically, and sg-I with the small-particle BZCY and fine and uniformly distributed NiO sintering aid has the highest sintered density and the biggest grains (fewest grain boundaries) and in turn the highest conductivity (0.039 S cm−1 at 800 °C). A cell with the sg-I electrolyte demonstrates electrochemical performance 1.8 times higher than that with the sr-L electrolyte.