固態氧化物燃料電池陰極形貌對其電化學性質之影響

Effect of Morphology of Cathode on Electrochemical Property of Solid Oxide Fuel Cell

吳鈞聖(Chun-Sheng Wu)

指導教授 ： 洪逸明

元智大學/工程學院/化學工程與材料科學學系/碩士(2012年)

https://doi.org/10.6838/YZU.2012.00032

摘要

本論文研究目的為利用不同形貌之La0.8Sr0.2Co0.2Fe0.8O3-δ (LSCF)陰極材料，降低固態氧化物燃料電池極化現象，以及添加陰極功能層，避免固態氧化物燃料電池Yttria-stabilized Zirconia (YSZ)電解質與混合導體LSCF陰極材料間產生界面反應。利用三種不同粒徑大小之LSCF，探討其對電化學性質之影響。採用與YSZ電解質化學及機械相容性較佳的La0.8Sr0.2MnO3-δ (LSM)陰極材料作為陰極功能層材料，並添加Y0.5Bi1.5O3 (YSB)以增加離子導性，藉由不同的粉末包覆方式，比較其性質差異。
　　首先比較三種不同粒徑大小之LSCF性質，由XRD圖譜得知經不同大小磨球研磨後之LSCF，皆為單相鈣鈦礦結構，其晶粒大小隨研磨球尺寸變小而變小。利用比表面積值計算之粒徑與雷射粒徑分布結果顯示，其粒徑大小隨研磨球尺寸變小而變小。由燒結曲線中得知，LSCF最佳燒結溫度為1000 oC。
　　經1000 oC燒結後之LSCF，其孔隙率及氧空位隨研磨球尺寸變小而增加。導電率皆隨溫度上升而上升，呈現P型導體特性，粒徑較小之LSCF，因在高溫下生成較多氧空位，增加電荷載體在B-site陽離子與氧離子間遷移的障礙，使得活化能提升。在交流阻抗分析中，粒徑較小之LSCF，在800 oC下，極化總阻抗為0.92 ohm‧cm^2，其阻抗為三試樣中最小。但粒徑較小之粉末因燒結收縮率較大，無法完整附著於電解質上，在800 oC下，其電功率僅有20 mW/cm^2，相較粒徑較大之LSCF，性能下降69 %。
　　在陰極功能層方面，由XRD圖譜得知利用Citrate-EDTA合成法製備YSB及LSM皆為單相。將YSB及LSM粉末混合後，經1200 oC熱處理2小時，由XRD圖譜得知，YSB與LSM並未產生化學反應，無其他相生成。
利用LSM/YSB為陰極功能層材料，經1000 oC及1200 oC燒結後，LSM/YSB陰極功能層與YSZ電解質間，附著性不佳，且陰極孔隙率低，因此性能較未添加陰極功能層，分別下降40 %及21 %。以LSM為陰極內層，可增加其附著性，性能僅下降11 %。以YSB-YSZ為陰極內層，不僅能增加附著性，並且使性能大幅提升18 %。
　　利用YSB/LSM為陰極功能層材料，經1000 oC及1200 oC燒結後，性能較未添加陰極功能層，分別下降47%及3%，因LSM燒結溫度為1200 oC，因此經1200 oC燒結後，可明顯改善其附著性。增加LSM為陰極內層，性能提升3 %，以YSB-YSZ為陰極內層，相較使用LSM/YSZ為陰極功能層材料，因相對增加電解質厚度，性能僅提升7 %。

關鍵字

固態氧化物燃料電池 ； 陰極形貌 ； 功能層 ； 阻抗分析 ； 功率密度

並列摘要

The objective of this study was using different morphology of La0.8Sr0.2Co0.2Fe0.8O3-δ (LSCF) cathode materials to reduce solid oxide fuel cell polarization and also adding cathode functional layer to avoid the interfacial reaction between Yttria-stabilized Zirconia (YSZ) electrolyte and LSCF cathode. Three different particle sizes of LSCF were used to explore the effect on the electrochemical property of cathode. LSM with similar coefficients of thermal expansion and chemical stability as YSZ electrolyte was used as cathode functional layer materials. Additional Y0.5Bi1.5O3 (YSB) helped to increase the ionic conductivity of cathode electrode. Powder prepared by different coating methods was also used to investigate the cell performance.
First, the study compared the characterization of LSCF after three different particle size grinding ball milling. The XRD patterns showed LSCF had single-phase perovskite structure which was prepared with three different particle sizes grinding ball milling. XRD and TEM results confirmed that the grain size decreased with the decreasing of grinding ball size. Specific surface area values were used to calculate particle size and laser particle size distribution showed that the particle size decreased with ball size decreasing. The optimal sintering temperature for LSCF was 1000 oC, obtained from sintering curves.
Sintered at 1000 oC, the porosity and oxygen vacancies of LSCF increased with the decreasing of particle size. Conductivity increased with increasing temperature that showed the characteristics of p-type conductor. At high temperature, smaller particle size of LSCF generated more oxygen vacancies which could also increase the energy gap of charge carriers transfer between the B-site cations and oxygen ions, as a result, activation energy increased. AC impedance analysis revealed that smaller particle size of LSCF has smaller polarization resistance. At 800 oC, the total polarization resistance was 0.92 ohm‧cm^2. However, smaller particle size of LSCF had higher sintering shrinkage property. For that, it couldn’t be attached well with the YSZ electrolyte. At 800 oC, the power density based on smaller particle sized LSCF was only 20 mW/cm^2, which was comparatively 69% lower than that of the cell based on larger particle sized LSCF.
The XRD patterns for the cathode function layer materials showed single phase of YSB and LSM prepared by Citrate-EDTA complex method. The YSB and LSM powders were mixed and heated at 1200 oC for 2 hours. The XRD patterns showed no other phase formation between YSB and LSM which confirmed that there was no chemical reaction occurred between them.
LSM/YSB was sintered at 1000 oC and 1200 oC to prepare cathode functional layer, which showed a poor adhesion with YSZ electrolyte, and had lower porosity. Therefore, the power density of cell decreased by 40% and 21%, respectively, compared with the absence of function layer. Using LSM as cathode inner layer could increase the adhesion which showed the power density only dropped 11%. When using YSB-YSZ as cathode inner layer increased not only the adhesion behavior but also power density by 18%.
YSB/LSM was sintered at 1000 oC and 1200 oC to prepare cathode functional layer, the power density decreased by 47% and 3%, respectively, than that of cell without the function layer. Sintering temperature of 1200 oC significantly improved the adhesion property. Using LSM as cathode inner layer increased 3% power density. For YSB-YSZ, as cathode inner layer, with the relatively higher thickness of the electrolyte than LSM/YSZ cathode functional layer materials, the power density only increased by 7%.

並列關鍵字

Power density ； Solid oxide fuel cell ； Morphology of cathode ； Function layer ； Impedance spectroscopy

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