質子傳輸型固態氧化物燃料電池關鍵電解質材料之研究;Key Proton-conducting Electrolytes for H+-SOFC Applications

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題名:	質子傳輸型固態氧化物燃料電池關鍵電解質材料之研究;Key Proton-conducting Electrolytes for H+-SOFC Applications
作者:	李侃融;Lee,Kan-rong
貢獻者:	機械工程學系
關鍵詞:	固態氧化物燃料電池;電解質;溶膠-凝膠法;Solid oxide fuel cells;Electrolyte;Sol-gel
日期:	2014-07-16
上傳時間:	2014-10-15 17:18:24 (UTC+8)
出版者:	國立中央大學
摘要:	固態氧化物燃料電池為一種高轉換效率、無汙染、可使用碳氫燃料的發電裝置。且由於其工作溫度高達600-1000 ℃，所以無需貴重金屬當作觸媒。傳統固態氧化物燃料電池為氧離子傳導型固態氧化物燃料電池，因其工作溫度高達1000 ℃，故產生熱膨脹係數匹配、元件需耐超高溫等問題。新型固態氧化物燃料電池為氫質子傳導型固態氧化物燃料電池，其工作溫度可降低至600-800 ℃，對降低成本、延長電池壽命有明顯幫助，故近期受到相當矚目。然而，對於氫質子傳導型固態氧化物燃料電池發展的關鍵問題是找到一個合適的質子導電氧化物電解質。本研究使用溶膠-凝膠法製備鋇鈰鋯釔氧化物並探討其材料與電化學特性。本研究首先探討鈰/鋯比例對Ba0.6Sr0.4Ce0.8-xZrxY0.2O3-δ (0.0≤x≤0.8) 之影響。結果顯示經過高溫1600 ℃燒結後試片呈現單一鈣鈦礦晶體結構、無雜相、緻密度高的試片，適合應用於氫質子傳導型固態氧化物燃料電池之電解質。隨著鋯比例增加會導致晶格收縮，且在1600 ℃燒結時抑制晶粒成長。Ba0.6Sr0.4Ce0.8-xZrxY0.2O3-δ (0.0≤x≤0.8)之導電率隨著鋯比例增加而下降，在800 ℃，導電率最高為為未摻雜鋯之Ba0.6Sr0.4Ce0.8Y0.2O3-δ，其導電率高達0.014 S / cm。但X光繞射和拉曼光譜結果顯示，Ba0.6Sr0.4Ce0.8Y0.2O3-δ對二氧化碳之化學穩定性不佳。可藉由少量摻雜鈰提高其化學穩定性。其次，探討鋇/鍶比例對Ba1-xSrxCe0.6Zr0.2Y0.2O3-δ (0.0≤x≤1.0) 之影響。結果顯示經1600 ℃燒結的Ba1-xSrxCe0.6Zr0.2Y0.2O3-δ試片在二氧化碳環境下表現出優異的化學穩定性。然而，其微觀結構和導電率對鍶摻雜量相當敏感。藉由鍶摻雜可明顯抑制雜相形成，且促進晶粒成長。在800 ℃，Ba0.8Sr0.2Ce0.6Zr0.2Y0.2O3-δ之試片導電率可達0.009 S/cm。推測可藉由鍶摻雜可抑制雜相形成、促進晶粒成長與提高相均勻性。本研究提出，適量的鍶摻雜對Ba1-xSrxCe0.6Zr0.2Y0.2O3-δ之相均勻性等材料特性有明顯幫助，且未來可運用在氫質子傳導型固態氧化物燃料電池之電解質。接著探討以鉀取代鍶之影響，針對鋇/鉀的比例進行研究，並開發新製程：成分交換法。與傳統溶膠-凝膠相比，利用成分交換法所製備出之Ba1-xKxCe0.6Zr0.2Y0.2O3-δ有更好的燒結性、更高的導電率、相均勻性與對二氧化碳有更好的化學穩定性。利用成分交換法所製備出之Ba0.925K0.075Ce0.6Zr0.2Y0.2O3-δ之試片於800 ℃有最高之導電率約0.0094 S/cm。本研究指出利用成分交換法所製備出之Ba1-xKxCe0.6Zr0.2Y0.2O3-δ試片，未來有機會運用在氫質子傳導型固態氧化物燃料電池之電解質。更重要的是，此成分交換法未來亦可運用在其他類似的材料系統。最後探討鈰/釔比例對BaZr0.2Ce0.8-xYxO3-δ (x=0-0.4) 之影響，並利用成分交換法製備出品質更優良之BaZr0.2Ce0.8-xYxO3-δ之試片並分析其差異。與傳統溶膠-凝膠相比，利用成分交換法所製備出之BaZr0.2Ce0.8-xYxO3-δ有更好的燒結性、更高的導電率、相均勻性與對二氧化碳有更好的化學穩定性。此外，並將BaZr0.2Ce0.6Y0.2O3-δ製備為白金/電解質/白金之單電池，進行電解質性能分析。與傳統溶膠-凝膠相比，利用成分交換法所製備出之BaZr0.2Ce0.6Y0.2O3-δ試片，其電解質性能有明顯之提升。本研究指出利用成分交換法所製備出之BaZr0.2Ce0.8-xYxO3-δ試片，未來有機會運用在氫質子傳導型固態氧化物燃料電池之電解質。本研究探討氫質子傳導型固態氧化物燃料電池之電解質材料，針對BaCeO3-δ之基礎材料，藉由元素摻雜提高化學穩定性、導電度與相均勻性等材料特性，並發開新製程：成分交換法。結果顯示：摻雜鋯雖然降低其導電度，但可大幅提升化學穩定性；摻雜鍶可提升其化學穩定性與相均勻性；摻雜鉀可提高導電度，但會造成電解質之孔隙率上升；摻雜釔可提高導電度，但過量摻雜釔會造成晶格嚴重扭曲。利用成分交換法所製備之電解質材料有更好的燒結性、更高的導電率、相均勻性與對二氧化碳有更好的化學穩定性，亦可大幅降低孔隙率與晶格扭曲效應。未來可將成分交換法運用在其他類似的材料系統。 ;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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