dc.description.abstract | Solid-oxide fuel cells (SOFCs) are electrochemical power-generation systems characterized by high energy conversion efficiency, low environmental impact, excellent fuel flexibility, and ability to use non-precious-metal catalysts. Typical SOFCs, which operate at a temperature of approximately 1000 ℃, are based on oxygen-ion-conducting electrolytes. Recently, SOFCs based on proton-conducting electrolytes (H+-SOFC) have attracted considerable attention due to their relatively low operation temperature (400-800 ℃) that facilitates the selection of the sealing and interconnection materials, control of the interactions between the electrode/electrolyte, and lowering of the thermal expansion mismatch between the cell components. Moreover, lowering the operation temperature also reduces the capital costs and prolongs cell lifetime. The key issue for H+-SOFC development is finding a suitable proton-conducting oxide electrolyte.
In this study, Ba0.6Sr0.4Ce0.8-xZrxY0.2O3-δ (x=0-0.8) proton-conducting oxides are prepared using a sol-gel complexing process. The effects of the Ce/Zr ratio on various material properties are systematically investigated. The sintered samples show a perovskite crystal structure without impurity phases and have a rather compact interior, making them suitable for use as a fuel cell electrolyte. Increasing the Zr content in the oxides causes lattice constriction and suppresses grain growth during sintering at 1600 ℃. The ionic conductivity of the oxides increases with increasing Ce/Zr ratio. At 800 ℃, Ba0.6Sr0.4Ce0.8Y0.2O3-δ has a conductivity of as high as 0.014 S/cm. However, X-ray diffraction and Raman spectroscopy evaluations show that this oxide cannot withstand a CO2 atmosphere. A suitable substitution of Ce with Zr in the structure significantly improves the chemical stability of the oxide without significantly degrading conductivity.
Ba1-xSrxCe0.6Zr0.2Y0.2O3-δ (0.0 ≤ x ≤ 1.0) proton-conducting oxides have been prepared using a citrate-EDTA complexing sol-gel method. In this study, the relationship between the Sr doping content and microstructure, chemical stability against CO2, and conductivity of the sintered Ba1-xSrxCe0.6Zr0.2Y0.2O3-δ pellets are systematically investigated using XRD, SEM, micro-Raman spectroscopy, and dc two-probe measurements. All sintered Ba1-xSrxCe0.6Zr0.2Y0.2O3-δ oxides exhibit excellent chemical stability after being exposed to the CO2 ambient at 600 ℃ for a long duration; nevertheless, their microstructures and conductivities are very sensitive to the Sr doping amount. The Sr incorporation is found to apparently suppress the formation of CeO2-like second phase, and enhance the grain growth in sintered oxides. Among all sintered samples, the Ba0.8Sr0.2Ce0.6Zr0.2Y0.2O3-δ pellet has the highest conductivity, 0.009 S/cm at 800 ℃. This result can be attributed to the competition between the elimination of CeO2- or (Zr,Ce,Y)O2-like phase inhomogeneity and enhanced grain growth in sintered oxides, both of which adversely influence the ionic conductivity. This work demonstrates that Ba1-xSrxCe0.6Zr0.2Y0.2O3-δ would be a promising electrolyte for H+-SOFC applications if the Sr doping is well controlled.
This study reports the synthesis of proton-conducting Ba1-xKxCe0.6Zr0.2Y0.2O3-δ (x=0.025-0.075) ceramics by using a combination of citrate-EDTA complexing sol-gel process and the composition-exchange method. Compared to the sintered oxides of similar composition prepared from conventional sol-gel powders, Ba1-xKxCe0.6Zr0.2Y0.2O3-δ oxides synthesized by sol-gel combined with the composition-exchange method are found to exhibit improved sinterability, higher conductivity, more homogeneous phase, and excellent chemical stability against CO2. Among all sintered oxides in this study, the Ba0.925K0.075Ce0.6Zr0.2Y0.2O3-δ pellet fabricated by this new method has the highest conductivity, 0.0094 S/cm at 800 ℃, which is higher than those pressed from conventional sol-gel powders in the K doping range of 0-15%. Based on the experimental results, we discuss the mechanism for improvement in these properties in terms of calcined particle characteristics. This work demonstrates that Ba1-xKxCe0.6Zr0.2Y0.2O3-δ oxides synthesized by sol-gel combined with the composition-exchange method would be a promising electrolyte for H+-SOFC applications. More importantly, this new fabrication approach may be applied to other similar material systems, such as Sr-doped Ba(Ce,Zr)O3 ceramics.
This study reports the synthesis of proton-conducting BaZr0.2Ce0.8-xYxO3-δ (x = 0-0.4) oxides by using a combination of citrate-EDTA complexing sol-gel process and composition-exchange method. Compared to those oxides prepared from conventional sol-gel powders, the sintered BaZr0.2Ce0.8-xYxO3-δ pellets synthesized by sol-gel combined with composition-exchange method are found to exhibit improved sinterability, a higher relative density, higher conduction, and excellent thermodynamic stability against CO2. Moreover, the Pt/electrolyte/Pt single cell using such a BaZr0.2Ce0.6Y0.2O3-δ electrolyte shows an obviously higher maximum powder density in the hydrogen-air fuel cell experiments. Based on the experimental results, we discuss the improvement mechanism in terms of calcined particle characteristics. This work demonstrates that the BaZr0.2Ce0.8-xYxO3-δ oxides synthesized by sol-gel combined with composition-exchange method would be a promising electrolyte for the use in H+-SOFC applications. More importantly, this new fabrication approach could be applied to other similar ABO3-perovskite material systems.
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