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Investigations on (Ba,Sr)FeO3 cathode materials stability and the fuel-cell properties of co-firing with CeO2-based electrolyte (LSBC)
|Contributors: ||NTOU:Department of Marine Engineering|
solid oxide fuel cell;(Ba0.5Sr0.5)FeO3-δ;interface impedance;core-shell
|Issue Date: ||2013-10-07T03:01:17Z
|Abstract: ||固態氧化物燃料電池(Solid Oxide Fuel Cell : SOFC)為一個陰極、陽極與電解質所組成的全陶瓷組件。多孔陰、陽極結構有利氣體催化性能；兩極之間的緻密電解質為高離子導體並做連結兩極氣體氧化還原的橋樑，陰極、陽極與電解質間的界面性質決定了離子催化與導電特性的好壞。(Ba,Sr)FeO3-δ是一具高效率催化與高電子傳導性，不含昂貴Co成份的中低溫陰極材料。然而其高燒結收縮率(25~1150℃，14%)，不易完整接合電解質，易造成應力龜裂與電解質脫離。(Ba,Sr)FeO3-δ為B晶格位置多價數系統，易受氧化還原氣氛與水氣影響其結構穩定性而使材料崩解。本論文針對(Ba,Sr)FeO3-δ系陰極與CeO2電解質(LSBC)間的接合匹配性；克服其結構不穩定；提升其離子/電子導電性能進行研究。 本研究採用添加30mol%電解質LSBC粉末與(Ba0.5Sr0.5)FeO3-δ形成混合陰極；製作純(Ba0.5Sr0.5)FeO3-δ極薄緩衝層在陰極與電解質界面，皆可增進界面接合特性與有效降低界面阻抗。本研究藉由對ABO3系統之A晶格位與B晶格位摻雜，解決(Ba,Sr)FeO3-δ材料的結構不穩定性。La摻雜的(La0.5Sr0.5)FeO3-與Nb摻雜的(Ba0.5Sr0.5)(Nb0.1Fe0.9)O3-有效穩定陰極材料結構的環境穩定性並維持良好的離子/電子導電特性。 純氧離子導體La0.2Ce0.8O2-δ(LC)與良電子導體(La0.5Sr0.5)FeO3-(LSF)混合或形成LSF的殼層，由於高達25 mol% LC含量，不論LC粒徑大小，皆無法有效達成同時提升離子催化與提升導電特性的目標。以LC或La2O3披覆在(Ba0.5Sr0.5)(Nb0.1Fe0.9)O3- (BSNF)顆粒表面，發現La與Ce對BSNF不同的擴散特性形成漸次梯度的殼層，調整其殼層相組成，能有效提升離子催化與提升導電特性的目標。3.75 mol% LC與10 mol% La2O3殼層，大大降低BSNF-LSBC半電池極化阻抗，在650C操作溫度下，分別提升88.6%與60.7%的功率密度。|
Solid oxide fuel cell (SOFC) is a full ceramic device consisted of cathode, anode and electrolyte components. Both porous electrodes possess beneficial gas electrocatalytic characteristics. The densified electrolyte with high ionic conductivity between electrodes acts as reduction-oxidation (redox) connected medium. The interface compactness between electrode and electrolyte will determine the performance of electrocatalysis and conductivity of electrodes. (Ba,Sr)FeO3-δ is a cathode material without Co composition and with high ion catalytic and high electron conducting properties in intermediate temperature SOFC. However, it has disadvantages of high sintering shrinkage, difficult co-firing with electrolyte and stress-cracking from electrolyte material. (Ba,Sr)FeO3-δ is also a muti-valent material in B-site. Its crystal structure is unstable under high redox atmosphere and humidity environment. This thesis is emphasized on the cathode material modifications and co-firing matching between (Ba,Sr)FeO3-δ cathode and CeO2-based electrolyte (LSBC), to overcome the structural unstability and to improve the ionic/electronic conductivity as well as to promote the power density of cathode-electrolyte half cell. The interface co-firing improvement and interface impedance reduction effectively were completed by addition 30 mol% LSBC powders to (Ba0.5Sr0.5)FeO3-δ (BSF) powders to form mixed cathode and by preparing a thin buffer layer of pure densified BSF between cathode and electrolyte. The A-site or B-site doping to (Ba,Sr)FeO3-δ material provided a solution to the crystal structural unstability. The compositions of (La0.5Sr0.5)FeO3- (LSF) and (Ba0.5Sr0.5)(Nb0.1Fe0.9)O3- (BSNF) exhibited stable cathode structures under high redox atmosphere and humidity environment, and maintained good ion/electron conducting properties. Pure ionic conductor La0.2Ce0.8O2-δ (LC) was mixed with good electronic conductor (La0.5Sr0.5)FeO3- (LSF) or was formed as a shell layer on LSF. No matter what kind of particle size of LC particles did not promote ion catalytic and electron conducting properties simultaneously under the high 25 mol% LC content. When coating LC or La2O3 on BSNF particle surface, different diffusion characteristics of La and Ce on BSNF formed gradient shell layer. The composition of shell could be adjusted to improve the electrocatalytic and conducting characteristics of (Ba,Sr)FeO3-δ cathode. Use the core-shell cathodes of BSNF-3.75 mol% LC and BSNF-10 mole% La2O3 can largely reduce the interface polarization impedance of original BSNF-LSBC half cell. Therefore, the power density of half cell is promoted with 88.6% and 60.7% for BSNF-3.75 mol% LC and BSNF-10 mole% La2O3, respectively, under operation temperature of 650C.
|Appears in Collections:||[輪機工程學系] 博碩士論文|
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