Enhanced ionic conductivity and grain boundary conduction mechanisms in YSZ electrolyte via fast hot-press sintering

Y 0.15 Zr 0.85 O 1.93 (YSZ) is a key electrolyte material for solid oxide fuel cells (SOFCs), and its performance is strongly governed by the sintering process. Conventional sintering (CS) and ultrafast sintering (US) typically yield fine-grained YSZ microstructures in which the high density of grain boundaries impedes O 2− migration. In this study, a fast hot-press sintering (FHPS) process is proposed, which combines rapid resistive heating of a graphite die with the simultaneous application of uniaxial pressure. At temperatures 200°C and 100°C lower than those of the CS and US processes, respectively, this integrated process produced high-quality YSZ ceramics with a relative density of 99.2% and an average grain size of 25.68 μm. Electrochemical impedance spectroscopy revealed that at 500°C, the ionic conductivity of the FHPS sample was approximately 10 and 3 times higher than those of the CS and US samples, respectively; at 800°C, the ionic conductivity of the FHPS sample reached 0.04 S cm −1 . This significant enhancement in conductivity is primarily attributed to the substantial reduction in grain boundary density resulting from pronounced grain growth: the high-density, large-grain microstructure obtained through the FHPS process effectively reduces the number of grain boundaries per unit transport path, lowers the grain boundary resistance to O 2− migration, and decreases the migration activation energy. Mott–Schottky analysis further reveals that FHPS-processed samples exhibit a reduced space charge potential at grain boundaries, which additionally contributes to the observed improvement in grain boundary conductivity. These results demonstrate that the FHPS process significantly enhances the ionic conductivity of YSZ electrolytes while substantially reducing the sintering temperature and shortening the sintering time, offering a promising technical pathway toward the scalable fabrication of high-performance SOFC electrolyte ceramics.