電解質支撐型微管狀固態氧化物燃料電池之開發與特性研究

Abstract

本研究初期以Zr0.8Sc0.2O2-δ (ScSZ)為電解質材料，以擠出成形法製備電解質支撐微管型之SOFCs。研究顯示塑料在成形時形狀規則且內外表面平整，具有良好的塑化性與成形性。結果顯示於1400oC時具有最佳特性，其外徑約3.8 mm，管壁厚約210 μm，相對密度可達97.1%。該燒結溫度之微管由三點抗彎測試後，其平均抗彎強度可達190 MPa以上，符合目前SOFCs高溫操作之需求，並發現燒結1400oC之微管抗彎強度優於燒結1450oC之微管(171 MPa)，其原因在微管相對密度相近時，平均晶粒越小(晶界比例越多)其抗彎強度值越佳。最佳化之擠出微管使用沉浸法與熱膨脹收縮儀分析出最佳之共燒參數，研究顯示，電解質與陽極共燒雖有較佳的界面燒附性，但易產生裂縫而產生電池破裂，將電解質預燒至1100oC再共燒陽極於1400oC即能獲得燒附性佳且無裂縫之陽極。LSM-GDC/ScSZ/NiO-ScSZ/NiO電池 (Cell A)之最高電功密度於850oC下為0.17 Wcm-2，開路電壓大於1.08 V。多次熱循環後，電池機械結構依然完整。 LSCF-GDC/GDC/ScSZ/NiO-ScSZ/NiO電池 (Cell B)藉由電解質中間層(Ce0.8Gd0.2O0-δ, GDC)隔離LSFC與ScSZ，阻止燒附時SrZrO3及La2Zr2O7之不導電二次相生成。研究顯示，Cell B之歐姆阻抗略高於Cell A，但電池的極化阻抗卻大為下降，使電池整體電阻值下降，於850oC時，其電功率密度為0.26 Wcm-2，其電功率密度比Cell A高出40%。 由上述之研究發現鋯基電解質支撐型微管狀SOFC操作溫度適用於800oC以 上，為降低操作溫度，本研究另使用鈰基電解質取代鋯基電解質，LSCF-GDC/GDC/NiO-GDC/NiO (Cell C)及LSCF-GDC/GDC/ScSZ/NiO-GDC/NiO (Cell D)等電池被製作且探討其微結構與電性能的關係。GDC層及ScSZ層之厚度控制在285 μm及8 μm，加入ScSZ層之微管型SOFC於650、700、750及800oC下，其歐姆阻抗分別上升49.3%、31.4%、19.0%及17.1%；而極化阻抗分別上升220.6%、321.4%、540.0%及566.7%。其歐姆阻抗的增加是因為ScSZ層及界面電阻所致；而極化阻抗的增加則是因為於界面上較低的電荷傳輸及氧擴散速率所致。研究發現Cell D於650到800oC下，其開路電壓僅些微下降(1.06 V至0.98 V)，而Cell C之開路電壓則明顯下降(0.92 V至0.76 V)，這表示ScSZ層確實抑制了因GDC層因電子導電所導致的開路電壓下降。於650、700、750及800oC下，Cell C之最高電功率密度分別為0.20、0.27、0.33及0.36 Wcm-2；而Cell D之最高電功率密度分別為0.16、0.23、0.32及0.42 Wcm-2。Cell D之最高電功率密度於750oC時有明顯的增加，其主因為當操作溫度高於750oC，Cell C開路電壓下降的效應將大於Cell D所增加的電池阻抗。

In this study, dense electrolyte supported micro tubular solid oxide fuel cells (T-SOFCs) were prepared by extrusion and dip-coating. Green Zr0.8Sc0.2O2−δ (ScSZ) electrolyte micro-tubes with a thickness of 300 m were successfully prepared at room temperature. After firing at 1400oC, the bare electrolyte micro-tube with a thickness of 210 μm, a diameter of 3.8 mm, and a length of 40 mm reached a relative density of 96.84% and a flexural strength of 190 MPa. Furthermore, the effects of the GDC-LSCF (Ce0.8Gd0.2O0-δ-La0.6Sr0.4Co0.2Fe0.8O3-δ) cathode layer and the GDC interlayer on the electrochemical performance of the ScSZ (Zr0.8Sc0.2O2−δ) electrolyte supported (270 μm) micro-tubular SOFC cells are investigated. Material formulation and sintering profile for fabricating the micro-tubular SOFC cells are developed to avoid physical defects caused by the large sintering shrinkage mismatch among the layers. Cell B (with the LSCF-GDC composite cathode layer and the GDC interlayer) reports an ohmic resistance slightly higher than that of Cell A (with the GDC-La0.8Sr0.2MnO3-δ, i.e. LSM, composite cathode), while its polarization resistance emerges to be significantly smaller than that of Cell A. In terms of cell performance, Cell B demonstrates an OCV value (> 1.07 V) similar to that of Cell A and a maximum power density (MPD = 0.26 Wcm-2) 44.4% greater than that of Cell A (MPD = 0.17 Wcm-2) at 850oC. It can thus be concluded that using the LSCF-GDC composite-cathode layer and inserting the GDC interlayer help reduce the total cell impedance, thereby improving the power density of the tubular cells. On the other hand, a gadolinia-doped ceria (GDC)-supported micro tubular SOFC was fabricated using extrusion and dip-coating (Cell C). The effects of inserting a scandium-stabilized zirconia (ScSZ) layer as an electron blocking layer between the GDC layer and the GDC-NiO anode layer were also explored (Cell D). The microstructures and electrochemical performances of Cell C and Cell D were investigated and compared. The layer thicknesses of the GDC and ScSZ bi-layer electrolytes were approximately 285 μm and 8 m, respectively. With the inserted ScSZ layer, the ohmic resistance increased by 49.3%, 31.4%, 19.0%, and 17.1%, and the polarization resistance rose by 220.6%, 321.4%, 540.0%, and 566.7% respectively, at the temperatures of 650, 700, 750, and 800oC. The increase in the ohmic resistance of Cell D was predominantly due to the interfacial resistance, while the substantial escalation in the polarization resistance was mainly because of the low bulk oxygen diffusion process in the ScSZ layer and the smaller charge transfer processes occurring at the interfaces. The OCV of Cell D showed a slight decrease from 1.06 to 0.98 V and that of Cell C experienced a dramatic decline from 0.92 to 0.76 V as the temperature rose from 650 to 800oC. The ScSZ layer of Cell D had successfully inhibited the OCV loss caused by the electronic conduction in GDC. The maximum power densities of Cell C and Cell D were measured to be 0.20, 0.27, 0.33, and 0.36 Wcm-2, and 0.16, 0.23, 0.32, and 0.42 Wcm-2 at 650, 700, 750, and 800oC, respectively. The MPD of Cell D was improved at temperatures above 750oC but remained inferior to that of Cell C below 750C. This is due to the fact that, as operating temperature increased above 750oC, the benefit of the higher OCV in Cell D surpassed the deficiency of the higher cell resistance, thereby leading to a higher MPD.