目前固態氧化物燃料電池(SOFC)在中溫(600oC-800oC)操作時，在陰極普遍觀察到較大極化阻抗，降低整體的轉換效率；將離子導性的電解質材料加入傳統電極材料錳酸鍶鑭（La0.8Sr0.2MnO3-δ，LSM）形成複合電極後，陰極的極化現象可以得到改善，提升SOFC的工作效率。
因此本研究目的為進一步改善陰極的極化現象，分別針對離子傳導與電子傳導之途徑進行分析，首先將不同導電率的電解質加入LSM形成複合電極，比較不同離子導性對界面電阻的影響，為了進一步降低陰極界面電阻，使用高電子導性銀和高離子導性的釔安定氧化鉍（yttria-stabilized bismuth，YSB）形成複合電極，以期獲得更佳的陰極性能；此外，也比較不同導電率之電解質對陰極界面電阻的影響。最後，藉微結構之奈米化，將YSB與奈米銀粒子形成核殼結構電極，大幅增加三相界和反應面積，並探討反應面積對陰極界面電阻之增益效果，利用以上實驗設計，討論高離子導性的YSB、高電子導性的銀、電解質的導電率和三相界長度對陰極氧還原動力之貢獻。
實驗結果發現，單相電子導體LSM陰極出現典型的高界面極化電阻，但在40vol%的電解質相YSZ加入LSM形成複合陰極後，在600oC時，界面電阻從58下降到11 Ωcm2，這是因為三相界從電解質/電極的界面延伸到整體電極當中，使電極反應面積增加，降低界面極化電阻；將高導電率的離子導體YSB加入LSM形成複合電極後，在600oC時，界面電阻下降為1.05 Ωcm2，因為YSB具有高離子導電率和氧氣催化能力，故使陰極的氧還原反應能力大幅提升；為了使陰極性能進一步提升，將Ag取代LSM提升電子導電性，和YSB形成複合陰極後，在600oC時，界面電阻下降到0.55 Ωcm2，顯示Ag的高電子導性，可將電子快速傳遞到電極反應位置，加速陰極反應。
當複合電極LSM-YSZ、LSM-YSB和Ag-YSB的電解質相YSZ或YSB在50vol%和60vol%時，界面電阻皆小於40vol%，推測是因為電解質相較多時，在陰極當中的氧離子傳導較快，故有較好的電化學性質。另一方面，將單相電極和複合電極使用高導電性之離子導體YSB作為電解質基材的電化學阻抗，和使用YSZ電解質基材之結果比較，發現電極LSM的界面電阻下降90%，複合陰極LSM-YSB和Ag-YSB之界面電阻下降約10~15%，這是因為LSM電極反應主要在電極/電解質界面，故使用高離子導性YSB作為電解質基材，可使LSM電極性能大幅提升，而在複合陰極中，由於三相界從電解質/電極的界面延伸到整體電極，故電解質基材之導電率對複合陰極性能影響較小。
從交流阻抗結果得知Ag-YSB複合電極在體積比為4:6和5:5具有最好的電化學表現，這是因為離子導體YSB在60vol%時，提供較多的氧離子傳輸路徑，而離子導體YSB在50vol%時，有最長的三相界提供了更多的氧氣還原反應面積，為了進一步提升陰極的效能，故本研究分別以濕式化學法和離子浸漬法製備以離子導體YSB為基材，在表面披覆奈米銀粒子之核殼結構電極，因為三相界大幅增加的結果，皆具有很好的電化學效能，相對於微米銀的Ag-YSB，其界面電阻在600和650oC下降了30%，然而在700和750oC時，其界面電阻和微米銀的Ag/YSB結果類似，這是因為在700oC時，銀奈米粒子團聚成微米銀粒子，導致三相界延伸效果消失。

The intermediate temperature solid oxide fuel cells (ITSOFC) were developed to reduce the operating temperature of SOFC to 600oC-800oC. However, the performance of IT-SOFC is highly dependent on the interface resistance (or polarization) of SOFC cathode. To reduce interface resistance of SOFC cathode, the concept of composite cathode consisting of ionic conducting and electronic components has been recongnized to be effective to decrease the cathodic polarizaton and improve the performance of SOFC.
Therefore, the objective of this study is to minimize the cathodic polarization by investigating different pathways of electron and ion transport in the composite cathode. The single component Sr-doped LaMnO3 (LSM) cathode and the composite cathodes LSM-YSZ and LSM-YSB were prepared. The effect of different ionic conductors and electronic conductors were characterized by using electrochemical impedance spectroscopy (EIS). Finally, the YSB-silver composite electrode with desired core-shell nanostructure was prepared for further reduction of cathodic polarization by extending triple phase boundary (TPB) where the electrochemical reaction occurs.
The results of this work show that one-component LSM cathode exhibits high polarization. When 40 vol% yttria stabilized zirconia (YSZ) was added to LSM forming the LSM/YSZ composite cathode, the interfacial polarization resistance decreases from 58 to 11Ωcm2 at 600oC due to the extension of TPB length. Moreover, when YSZ is replaced by highly ion-conducting yttria stabilized bismuth oxide (YSB), the interfacial polarization resistance of LSM-YSB composite cathode further decreased to 1.05Ωcm2 at 600oC.
With the understanding of importance of electronic and ionic conduction in the cathode, the performance of composite cathode is further enhanced by the adoption of nanostructure design. YSB used as the matrix was coated with nano Ag forming a core-shell structure by wet chemical process and ion impregnation procedure. As a result, the TPB length of YSB-nano Ag composite cathode was extended to about 10 times longer than that so conventional YSB-Ag composite cathode. Accordingly, the polarization resistance further decreased by as much as 30% with the introduction of YSB-nano Ag cathode with core-shell structure.

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