加壓鈕扣型陽極支撐SOFC實驗量測與活化和濃度過電位分析計算

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题名:	加壓鈕扣型陽極支撐SOFC實驗量測與活化和濃度過電位分析計算
作者:	吳佩真;Wu,Pei-Chen
贡献者:	機械工程學系
关键词:	高壓固態氧化物燃料電池;固態氧化物燃料電池;電化學阻抗頻譜;鈕扣型固態氧化物燃料電池;陽極支撐;加壓;High-Pressure SOFC;SOFC;EIS;impedance spectroscopy;Anode-supported;elevated pressure;pressurization;pressure effect;Electrochemistry impedance spectroscopy;high pressure double chamber testing platform;button SOFC;anode-supported button cell
日期:	2013-08-07
上传时间:	2013-10-08 15:28:17 (UTC+8)
出版者:	國立中央大學
摘要:	本論文首次設計鈕扣型SOFC全/半電池之實驗載具，將之置於本實驗室已建立之雙腔體高壓設備內，針對加壓型陽極支撐SOFC (Ni-YSZ/YSZ/LSM)全電池，量測其電池性能與電化學阻抗頻譜，以詳細探討壓力和溫度效應對陽極支撐全電池之影響。實驗條件為固定氣體流率(陽極200ml min-1 H2/陰極200ml min-1 Air)，含四個不同操作溫度(700oC, 750oC, 800 oC, 850 oC)，並且每個操作溫度均包含5組不同壓力(1~5atm)之量測，故共有20組不同壓力和溫度之實驗數據。結果顯示，當壓力由1atm增加至5atm時，在任一溫度與任一定電壓/定電流條件下，功率密度皆會隨壓力增加而增加；而在任一固定壓力條件下，功率密度會隨操作溫度增加而增加，且溫度增益效應大於壓力效應。有關電化學阻抗頻譜結果，加壓效應可使總極化阻抗減小，其中又以其對低頻弧之影響較高頻弧來得顯著。因高頻弧特徵頻率在常壓時，約為100 ~ 1000Hz，其對應之機制應為陰極活化極化(Nielsen et al. 2011)，而低頻弧在常壓時則約為10 Hz，其所對應之機制應為陽極孔隙擴散(Leonide et al. 2008)，所以我們所得之結果顯示，加壓效應可同時改善減小陽極支撐全電池之陰極活化與陽極擴散濃度極化阻抗。除了前述實驗量測電池性能曲線與電化學阻抗頻譜外，本論文並利用Butler-Volmer方程式，來分析實驗所得之數據，以計算活化過電位，進而以一維擴散方程式來計算陽極濃度過電位。結果顯示，不論活化還是濃度過電位，皆會隨著壓力的升高而下降。其中影響活化和濃度過電位有兩個重要參數，即交換電流密度(Exchange current density)和有效擴散係數(Effective diffusion coefficient)，兩者均由分析計算中代入實驗數據加以求得，故我們可以分析加壓效應對此兩重要參數之影響。由分析計算結果顯示，加壓效應可使交換電流密度增加，這表示電化學反應速率會隨壓力增加而有所提升，而加壓效應雖然會使陽極有效擴散係數下降，但壓力的提升同時也會使體積莫爾濃度增加，導致整體反應氣體擴散率的增加。我們並將前述過電位等之分析計算結果與先前數值模擬結果(Henke et al. 2011)作比較，兩者有不錯的吻合性，但其與先前對稱型陽極半電池之實驗所得之結果(Kikuchi et al. 2004)相異，後者提出濃度過電位會隨壓力增加而增加之結果。最後，本論文也討論加壓效應對電解質支撐全電池之影響。綜合而論，加壓效應可增進SOFC電化學反應率與氣體孔隙擴散率，故可提升SOFC之電池性能；此研究結果，應有助於了解加壓效應對於SOFC極化機制之影響。 A high-pressure full/half button solid oxide fuel cell (SOFC) experimental setup is designed using a recently-established dual-chamber high-pressure testing platform to measure current-voltage curves and AC impedance spectra (EIS) of anode-supported (Ni-YSZ/YSZ/LSM) full button cells. In this study, we applying constant gas flow rates, anode: 200ml min-1 H2 and cathode: 200ml min-1 Air, including four different operating temperatures (700oC, 750oC, 800 oC, 850 oC). Each operating temperature has five measurements at different pressures (1~5atm). Thus, there are totally 20 sets of different pressure and temperature experimental data to be presented and discussed. Results show that the power densities increase with increasing pressure at any fixed operating temperatures and at any fixed voltage/current densities. As to the effect of operating temperature, power densities increase noticeably with increasing temperature at any fixed pressures. It should be noted that the enhancement of power densities due to the temperature effect is more profound than that owing to the pressure effect. Concerning EIS data, both high and low frequency arcs of impedance spectra decrease with increasing pressure, so that the total polarization resistances are reduced by increasing pressure. However, the high frequency arcs decrease rather weakly with increasing pressure when comparing with that for the low frequency arcs. Because for high frequency arcs at normal pressure, the characteristic frequencies occur around 100~1000Hz that is attributed to the cathode activation polarization (Nielsen et al. 2011) and for low frequency arcs the characteristic frequencies occur at about 10Hz that corresponds to the diffusion process in the anode porous electrode (Leonide et al. 2008) Hence, pressurization can simultaneously decrease the cathode activation polarization impedance and the anodic diffusion concentration polarization impedance of the anode-supported full button cell. In addition to the aforesaid measurements of I-V curve and EIS, further analyses on the activation and concentration overvoltages are made by using the Butler-Volmer equation and the related concentration overvoltage equation based on one-dimensional diffusion model. It is found that both activation and concentration overvoltages decrease with increasing pressure. There are two important parameters, the exchange current density and the anodic effective diffusion coefficient, which are also calculated using the present experimental data. Results show that pressurization can enhance the exchange current density, leading to an increase of the electrochemical reaction rate. Though pressurization tends to reduce the anodic effective diffusion coefficient, it can also increase the molar concentration and as such the overall gas-phase diffusion rate in porous electrode can be increased. Our analytical results of overvoltages are compared with numerical results obtained by (Henke et al. 2011) and with previous experimental data measured by (Kikuchi et al., 2004). The former shows a good agreement with our present data, but the latter shows an opposite trend in which the concentration overvoltage was found to be increased (not decreased) with increasing pressure. Finally, this study also discusses the influence of pressurization on the electrolyte-supported SOFC. In short, pressurization can improve both the electrochemical reaction rate and the gas-phase diffusion rate, and thus it can increase the performance of SOFC. These results should be useful for our understanding of the effect of pressurization on the polarization mechanisms in SOFCs.
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