Preparation And Properties Of LaAlO₃-based Electrolyte Material For SOFC

Perovskite-type LaAl03 system is a cost effective and potential candidate electrolyte material since high stability of crystal structure, strong mechanical property, low cost of aluminum compounds and pure oxygen ionic conductor at low oxygen partial pressure. In the present work, the preparation process of LaA103-based electrolyte material is explored to optimize the microstructure. The effect of microstructure on the electrical property of LaA103-based electrolyte is investigated. Chemical compatibility between LaAlO3-based electrolyte material and anode materials is studied so as to determine excellent chemical compatibility for them. The output characteristics of single cell are tested.Perovskite-type La0.90Sr0.10Al0.97Mg0.03O3-δ (LSAM) powders is synthesized via EDTA-glycine combined process (EGCP). The precursor and calcined powders are characterized by TG-DSC, XRD, ICP-OES, TEM and Acoustic Particle Size Analyzer (APS). The results indicate that perovskite-type LSAM powder can be synthesized at 750 ℃, which is directly converted from amorphous precursor powders without any intermediate phase formation. After calcined at 1000℃ for 3 h, the average particle sizes of 77.2 nm and 136 nm for powders are measured by TEM technique and APS, respectively. The LSAM powders that synthesized by EGCP show accurate stoichiometric ratio and excellent sinterability with the initial sintering temperature at 1000 ℃. The maximum shrinkage value of 20.5% and relative density of 96% are obtained sintered at 1600 ℃ for 5 h for LSAM powdersThe effects of composition, powder prepared process and sintering schedule on the microstructure of LaAlO3-based electrolyte material are investigated by XRD, SEM and Archimedes method. The effect of microstructure on the electrical property of LaAlO3-based electrolyte is investigated. The total conductivities of the specimens are measured using the four-probe d.c. method in air. The results manifest that the sinterability of doped LaA103 material is promoted with Sr content increasing. However, the second phase is obtained when the content exceed solubility of Sr, which deteriorate the electrical property of material. The powders synthesized by EGCP can optimize the microstructure and improve the electrical property of material. The conductivity of LSAM electrolyte prepared by EGCP is about 1.53×10-2 Scm-1 at 800 ℃, which is 50% higher than that synthesized by solid state synthesis route. With increasing sintering temperature and extending holding time, the density and electrical conductivity of material can be promoted in a limited range.LaAlO3-based electrolyte material is prepared by liquid phase sintering with the addition of crystallized-performance Ca-Al-La-Si glass as sintering additive. The sintering additives and their effects on microstructure and electrical property of material are characterized by TG-DSC, XRD, SEM and the four-probe d.c. method. The optimal crystallization temperature of Ca-Al-La-Si glass is 1100 ℃, and the product of crystallization is La9.333+xSi6O26+1.5x with oxygen ionic conductivity. The liquid of glass exhibits excellent wettability with LSAM particles at high temperature. The density and microstructure of material are significantly improved by introducing additives, about 96.2% relative density of material with 1% additive is obtained after sintered at 1500 ℃ for 2 h. The electrical property of material fails to be enhanced contrary to deteriorate through liquid phase sintering, because the insulated phases exist in the material. The total conductivity of sample with 1% additive is about 6.60 × 10-3 Scm-1 at 800 ℃.The chemical compatibility of LSAM with anode materials Ni-GDC, SYT and LSCM is characterized by XRD, SEM and AC impedance spectroscopy. The results indicate that SYT and LSCM have poor chemical compatibility with LSAM since obvious interdiffusion between materials. The interdiffusion of cations is very slight between LSAM and Ni-GDC at 1300 ℃, manifesting excellent chemical compatibility for them. Electrochemical performance for symmetrical cells of anode materials were measured under hydrogen atmospheres, the area-specific polarization resistances (ASRP) of Ni-GDC is 5.12 Ω cm2 at 800 ℃. GDC buffer can improve the performance of combination for anode-electrolyte interfaces and reduce the ASRp. The ASRp of Ni-GDC/GDC/LSAM is 1.67 Ω cm2 at at 800 ℃. Open-circuit voltage of 0.925 V and power densities of 19.5 mWcm"2 are obtained at 800 ℃ for 550 μm-thick LSAM electrolyte-supported single cell (Ni-GDC/GDC/LSAM/GDC/LSF).