固態電解質為具有離子導電特性之導電材料，在實際應用上需具備高導電性及穩定性。立方氟化鈣結構的高溫d相的氧化鉍（d-Bi2O3）具有25%氧離子空缺濃度，為氧離子導電率最高之氧化物，然而如此高的氧離子空缺濃度導致其結構在低於723℃以下之溫度並不穩定，要增加氧化鉍系氧離子導體的實用性必須增加立方相的穩定性，因此，本研究針對氧化鉍系氧離子導體，添加不同價數的陽離子(Ca2+、Y3+、Nb5+、W6+)，探討添加物對其結構及導電性質的影響，藉由觀察其在氫氣環境下的穩定性，評估其作為固態氧化物燃料電池電解質材料的可行性，最後將氧化鉍系固態電解質薄膜化並結合陰、陽極，組裝為陽極支撐固態氧化物燃料電池進行電力密度的量測。
以陽離子半徑較小的Y2O3作為添加劑，燒結後雖可形成立方晶系氟化鈣結構(Y0.15Bi0.85)2O3及(Y0.2Bi0.8)2O3，空氣中600°C熱處理10 h，立方晶系氟化鈣結構即相變為菱方結構，而在氫氣環境下350°C熱處理5 h固溶體還原為Bi金屬及Y2O3，顯示其立方晶系氟化鈣結構不穩定。以高於三價之Nb5+及W6+取代Bi3+，將減少氧化鉍系氧離子導體的氧離子空缺濃度，燒結後可形成立方晶系氟化鈣結構(Nb0.2Bi0.8)2O3.4及正方結構(W0.15Bi0.85)2O3.45，其中正方結構(W0.15Bi0.85)2O3.45為立方晶系氟化鈣結構的超晶格，此兩者固溶體在空氣中600°C熱處理1000 h或在氫氣環境下400°C熱處理100 h，皆維持穩定，導電率則略低於Y2O3-Bi2O3系統，700℃約為10-2 S/cm。以Y2O3作為添加劑取代(W0.15Bi0.85)2O3.45中Bi3+的位置，可縮小Bi3+和W6+離子半徑的差異，進而消除Bi3+和W6+有序排列的現象，可獲得穩定的立方晶系氟化鈣結構(Y0.1W0.15Bi0.75)2O3.45，同時可提升導電率，700℃約為2.38 ´ 10-2 S/cm。以部分Ca2+取代(W0.15Bi0.85)2O3.45中Bi3+的位置，將產生氧離子空缺以維持結構之電中性，晶體結構仍為正方結構，然而氫氣環境下400°C熱處理100 h後，出現Bi金屬及CaWO4的生成，顯示氧離子空缺濃度的增加，導致還原反應的發生。
本研究中，單相且在氫氣環境下穩定的(Nb0.2Bi0.8)2O3.4、(W0.15Bi0.85)2O3.45及(Y0.1W0.15Bi0.75)2O3.45有潛力作為固態氧化物燃料電池電解質材料，然而常用之鍶摻雜錳酸鑭(La0.8Sr0.2MnO3-δ)陰極與(Nb0.2Bi0.8)2O3.4及(Y0.1W0.15Bi0.75)2O3.45在900°C持溫20 h後，有反應物LaBiO3.68與Sr0.67Bi1.33O2.67生成，僅(W0.15Bi0.85)2O3.45不與La0.8Sr0.2MnO3-δ反應。將(W0.15Bi0.75)2O3.45固態電解質置於燃料電池測試環境下，於陽極通入不同的氧分壓，其電動勢在400°C皆約為0.6 V，主要原因為(W0.15Bi0.75)2O3.45固態電解質在低氧分壓環境下發生Bi2O3->Bi還原反應，因而在(W0.15Bi0.75)2O3.45固態電解質陽極端形成Bi2O3/Bi平衡氧分壓，導致其電動勢維持一固定值。
本研究最後以Pt-(W0.15Bi0.75)2O3.45做為複合陽極基材，結合60 mm的(W0.15Bi0.75)2O3.45固態電解質及20 mm的(W0.15Bi0.85)2O3.45/La0.8Sr0.2MnO3-δ複合陰極，成為陽極支撐(W0.15Bi0.75)2O3.45系固態氧化物燃料電池，於400及450℃測試時，最大電力密度分別為0.76及1.23 mW/cm2。根據本研究對氧化鉍系氧離子導體結構穩定性及電池測試之結果，高導電之氧化鉍系氧離子導體有潛力運用於固態氧化物燃料電池電解質。

In solid oxide fuel cell (SOFC), solid-state electrolytes are materials possessing defects and high ionic conductivity. For practical applications, solid electrolytes require high ionic conductivity and stability. Up to date, the oxygen ionic conductor with highest ionic conductivity is the high temperature cubic Bi2O3 namely, d-Bi2O3. However, the d phase is not stable below 723℃, and undergoes a phase transformation to a monoclinic phase due to the high oxygen vacancy concentration (25%). The stability of the cubic phase must be enhanced for practical applications. Therefore, the aim of this study was to investigate the effect of aliovalent dopants, Ca2+, Y3+, Nb5+ and W6+ on the crystal structures, conductivities and the stability of Bi2O3-based solid electrolytes after exposed to H2. Finally, the anode-supported Bi2O3-based SOFC was assembled and tested using H2 as the fuel and O2 as the oxidant.
Y2O3 was selected as dopants due to the smaller cation radius than Bi3+. The samples were synthesized by solid state reaction. The as-sintered (Y0.15Bi0.85)2O3 and (Y0.2Bi0.8)2O3 exhibited a cubic lattice. However, the cubic (Y0.15Bi0.85)2O3 and (Y0.2Bi0.8)2O3 transformed to rhombohedral phase when annealed at 600 ℃ for 10 h. Additionally, after annealing in H2 at 350°C for 5 h, (Y0.15Bi0.85)2O3 and (Y0.2Bi0.8)2O3 were reduced to metallic Bi and Y2O3. The addition of Nb2O5 or WO3 into Bi2O3 decreased the oxygen vacancy concentration. The as-sintered (Nb0.2Bi0.8)2O3.4 exhibited a cubic lattice while as-sintered (W0.15Bi0.85)2O3.45 exhibited a tetragonal structure derived from the fluorite subcell. Both cubic (Nb0.2Bi0.8)2O3.4 and tetragonal (W0.15Bi0.85)2O3.45 were stable after annealing at 600℃ for 1000 h in air or at 400°C for 100 h in H2. The addition of Y2O3 is capable of minimizing the mismatch in ionic radius between Bi and W ions. Therefore, the co-addition of Y2O3 and WO3 is able to stabilize cubic (Y0.1W0.15Bi0.75)2O3.45. Moreover, the conductivity is 2.38 ´ 10-2 S cm-1 at 700°C and slightly higher than that of (WO3)0.15(BiO1.5)0.85(1.5 ´ 10-2 S cm-1). Cubic (Y0.1W0.15Bi0.75)2O3.45 was stable after annealing at 600 ℃ for 1000 h in air or at 400°C for 100 h in H2. (Ca0.1W0.15Bi0.75)2O3.35 exhibited a tetragonal structure derived from the fluorite subcell. However, (Ca0.1W0.15Bi0.75)2O3.35 was reduced to Bi and CaWO4 after annealing at 400°C for 100 h in H2. The addition of CaO into (W0.15Bi0.85)2O3.45 increased oxygen vacancy concentration. It is suggested that the increase of oxygen vacancy concentration resulted in reduction.
In this Study, (Nb0.2Bi0.8)2O3.4, (W0.15Bi0.85)2O3.45 and (Y0.1W0.15Bi0.75)2O3.45 were potential electrolyte materials. Moreover, no reaction between La0.8Sr0.2MnO3-δ cathode and (W0.15Bi0.85)2O3.45 was observed at 900℃. The e. m. f. of (W0.15Bi0.85)2O3.45 electrolyte was about 0.6 V at 400 ℃ when anode was in different oxygen partial pressures. The reason was that the reduction of Bi2O3®Bi occurred for (W0.15Bi0.85)2O3.45 electrolyte in low oxygen partial pressure. Therefore, the anode side of (W0.15Bi0.85)2O3.45 was under the equilibrium partial pressure of Bi2O3/Bi.
In this study, the 60 mm (W0.15Bi0.85)2O3.45 electrolyte was deposited onto the (W0.15Bi0.85)2O3.45-Pt anode using tape casting method. The 20 mm (W0.15Bi0.85)2O3.45-La0.8Sr0.2MnO3-δ cathode was applied on (W0.15Bi0.85)2O3.45 using screen printing technique. Finally, the anode-supported Bi2O3-based SOFC was tested using H2 as the fuel and O2 as the oxidant. The power densities were 0.76 and 1.23 mW/cm2 at 400 and 450℃, respectively. According to these results, Bi2O3-based oxygen ion conductor is a potential electrolyte for SOFC.

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