研究生: |
陳政緯 Cheng-Wei Chen |
---|---|
論文名稱: |
半電池反應之交流阻抗研究鑭鈣鈷鐵氧化物陰極 Impedance Study of Half-cell Reaction on (La0.75Ca0.25)(CoxFe1-x)O3-δ Cathode |
指導教授: |
蔡大翔
Dah-Shyang Tsai |
口試委員: |
周振嘉
Chen-Chia Chou 周更生 Kan-Sen Chou 陳貞夙 Jen-Sue Chen |
學位類別: |
碩士 Master |
系所名稱: |
工程學院 - 化學工程系 Department of Chemical Engineering |
論文出版年: | 2007 |
畢業學年度: | 95 |
語文別: | 中文 |
論文頁數: | 119 |
中文關鍵詞: | 鈣鈦礦結構 、陰極 、鑭鉬氧化物 、半電池 、氧還原反應 、電化學交流阻抗圖譜 、固態氧化物燃料電池 、極化損失 |
外文關鍵詞: | perovskite, cathode, LAMOX, half-cell, SOFC, electrochemical impedance spectroscopy, oxygen reduction reaction, polarization loss |
相關次數: | 點閱:312 下載:2 |
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本研究主要探討(La0.75Ca0.25)(CoxFe1-x)O3-δ (x=0.1~0.9)接面於(La1.8Dy0.2)(Mo2-yWy)O9 (y=0~1)電解質的半電池反應,尋求最適當的微結構特徵及最佳的電化學表現,進而瞭解影響陰極氧還原反應極化損失的因素,實驗中利用X光繞射(XRD)、掃描式顯微鏡(SEM)分析材料結構,電化學交流阻抗圖譜術(EIS)分析半電池的電化學表現。
(La0.75Ca0.25)(CoxFe1-x)O3-δ經過XRD分析為單一相的斜方晶鈣鈦礦結構,晶格常數a ≈ b ≠ c,由於Co離子半徑小於Fe,所以當Co含量增加,晶格常數與體積縮小,同時晶格常數a與b的差距變大。在微結構上,探討燒結溫度從800-950℃之影響,燒結溫度800℃顯示陰極與電解質的接合較差;950℃則導致孔隙減少及粒子粗化,而最佳之燒結溫度為900℃。陰極粉末煆燒溫度在1200℃與其它較低煆燒溫度相比具有較佳的結晶性,並顯示結晶性對於氧還原反應有相當的影響力。根據TG/DTA測試(La0.75Ca0.25)(Co0.8Fe0.2)O3-與(La1.8Dy0.2) (Mo1.6W0.4)O9混合粉體之結果表示無明顯相變或反應發生。然而,以EDS元素分析半電池,發現有微量的Mo從電解質擴散至陰極。
(La0.75Ca0.25)(CoxFe1-x)O3-δ陰極於Co摻雜量為80mol%(x=0.8)、煆燒溫度1200℃、燒結溫度900℃具有最小的氧還原反應阻抗,(La0.75Ca0.25)(Co0.8Fe0.2)O3-δ / (La1.8Dy0.2)(Mo1.6W0.4)O9半電池於700℃為12 Ωcm2(交換電流密度為3.1 mAcm-2);800℃為0.8 Ωcm2(交換電流密度為30.2 mAcm-2)。以施加偏壓之交流阻抗量測顯示,在600℃與700℃的操作溫度,氧還原反應阻抗明顯降低,上升至800℃則影響不大。(La0.75Ca0.25)(Co0.8Fe0.2)O3-δ之氧還原反應極化損失隨著(La1.8Dy0.2)(Mo2-yWy)O9電解質離子導電率的下降而增加,從全對數圖顯示交換電流密度與電解質離子導電率呈線性的關係,證明電解質離子導電度也是影響氧還原反應極化損失的重要因素之ㄧ。
We investigate the polarization loss of oxygen reduction reaction (ORR) at (La0.75Ca0.25)(CoxFe1-x)O3-δcathode which is interfaced to (La1.8Dy0.2)(Mo2-yWy)O9 electrolyte in the half cell, and search for the optimum electrochemical performance in a wide range of Co content and pore structure. The structure of ((La0.75Ca0.25)(CoxFe1-x)O3-δis studied using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The catalytic activity of (La0.75Ca0.25)(CoxFe1-x)O3-δ is analyzed using electrochemical impedance spectroscopy and verified by Tafel plots of current-voltage curve.
XRD patterns of (La0.75Ca0.25)(CoxFe1-x)O3-δindicate a perovskite phase of orthorhombic cell with cell parameters a ≈ b ≠ c. All cell parameters decrease with the Co content since Co is a smaller ion compared with Fe. The difference between a and b increases with the Co content. The pore structure of cathode varies considerably in the sintering temperature range 800-950℃. Sintering at 800℃ results in a poor interface between cathode and electrolyte. Sintering at 950℃leads to a cathode of low porosity and coarsened grains. Hence the optimum sintering temperature is 900℃. TGA/DTA analysis on the powder mixture of (La0.75Ca0.25)(Co0.8Fe0.2)O3-δ and (La1.8Dy0.2)(Mo1.6W0.4)O9 shows no sign of solid state reaction or phase transformation. However, the elemental analysis using EDS indicates Mo diffusion from electrolyte to cathode after half-cell experiment. (La0.75Ca0.25)(CoxFe1-x)O3-δ powder calcined at 1200℃ is superior to the powder of same composition calcined at lower temperature. It is believed that a high calcination temperature assists in the crystallinity of (La0.75Ca0.25)(CoxFe1-x)O3-δ which is an important factor for this mixed conductor.
The cathode of (La0.75Ca0.25)(CoxFe1-x)O3-δ which was calcined at 1200℃ and sintered at 900℃ shows a minimum ORR polarization loss at x=0.8. (La0.75Ca0.25)(CoxFe1-x)O3-δ/(La1.8Dy0.2)(Mo1.6W0.4)O9 is measured 12 Ωcm2(exchange current density 3.1 mAcm-2) at 700℃, 0.8Ωcm2(exchange current density 30.2 mAcm-2) at 800℃. The imposed dc bias reduces the polarization loss considerably, especially at 600 and 700℃. The influence of dc bias is much less at 800℃. The ion conductivity of electrolyte is also an important factor for ORR polarization loss, as evidenced in the increasing ORR loss of (La0.75Ca0.25)(Co0.8Fe0.2)O3-δ with decreasing ion conductivity of (La1.8Dy0.2)(Mo2-yWy)O9 (y=0.0-1.0). A linear relation was found between exchange current density of the cathode and ion conductivity of the electrolyte in the log-log plot.
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