中溫型燃料電池鉍基陰極之合成、材料特性及電性研究

Abstract

鑭鍶鈷鐵、鐠鍶鈷鐵及鐵酸鉍基陰極氧化物以EDTA-檸檬酸螯合法合成，其中鑭鍶鈷鐵也以膠粒製程的方法合成，作為對照組。利用掃描式電子顯微鏡的定量成分分析，比較兩種方法產製粉末的均勻性。使用定量X光繞射分析，進行鈷元素摻雜量產生的雜相，以及在高達20mol%的鉍鍶鐵中的固溶極限，以提高其總導電率。此外，為了獲得適當的陰極孔隙率，本研究也進行陰極在不同燒結溫度下的燒結曲線型態，和陰極所產生的裂痕量的關係。對於陰極導電的機制也進行了解，以總導電率、X射線光電子光譜(XPS)，以及熱重分析進行分析。導電率的結果顯示，鈷元素的含量越多,其導電率也越高。最後，X射線光電子光譜結果顯示,本實驗製備的陰極材料表面在室溫下都是以Fe3+離子存在，表面氧空缺以鉍基陰極材料為最多，因此高溫離子導電效能會明顯較佳。

M0.6Sr0.4Co0.2Fe0.8O3 (M=La, Pr, Bi) and BiFeO3-based cathodes were synthesized by a modified EDTA-citrate method. One La0.6Sr0.4Co0.2Fe0.8O3 was synthesized by colloidal mixing/solid state sintering, and used as a comparable case. In order to study the homogeneity, two La0.6Sr0.4Co0.2Fe0.8O3 powders were densified and analyzed by quantitative EDS. Detail study of quantitative XRD was done to analyze the impurity phases and the solid solution limit of Co-doping, so to increase the total conductivity of the BiFeO3-based cathodes. Besides, the porosity of cathode needs control by sintering, of which the behavior has been investigated on sintering curves and correlates to the crack formation in cathode layer. Conduction mechanisms (double exchange, small polaron, and oxygen ions conduction) were studied by measuring total conductivity, X-ray photoelectron spectroscopy (XPS) and TGA analyses on two series cathodes, LnSCF6428 series and BiFeO3-based cathodes. The more the cobalt content is, the higher the conductivity. The results of XPS showed that the dense and porous Bi0.7Sr0.3Co0.04Fe0.96O3 at room temperature was mainly Fe3+ and oxygen vacancies on surface, implying ionic conductivity would dominate at high temperature.