Study of Transient Behavior of Water Molecules in Membrane Electrode Assembly of PEM Fuel Cells

本研究以數值模擬，針對質子交換膜燃料電池內部的水傳輸現象進行理論分析，此物理模型包含陰陽極氣體擴散層、陰陽極觸媒層和質子交換膜..等五層。在質子交換膜中，只考慮電滲透效應和逆向擴散效應來控制內部水含量之暫態分佈情形，而在陰陽極觸媒層和陰陽極氣體擴散層中，僅考慮擴散效應影響該層內的氣態水濃度之暫態分佈。並把質子交換膜會因為水合的作用而造成體積發生膨脹的效應考慮進數學模型中，使得整個數學模型更加接近實際情況，及利用不同的操作條件，來探討各層內水濃度在暫態和穩態時的分佈情況。 研究結果顯示，當陰陽極入口氣態水相對加濕量不足時，不但會造成各層水濃度下降，還會使得各層達穩態的時間拉長。當使用孔隙度較大的氣體擴散層，可以防止薄膜靠陽極側乾化，薄膜靠陰極側淹水，並可把各層達穩態的時間縮短。而若把薄膜厚度減小，會讓薄膜內部水含量更加均勻，也可讓各層達穩態的時間減短。另外，若要降低薄膜內電阻值和薄膜相電位，可以朝著增加陰陽極入口氣態水相對加濕量、提高氣體擴散層的孔隙度和減少薄膜厚度的方向進行。就薄膜膨脹部份而言，可以發現到，當薄膜的整體厚度膨脹比率相同時，在不同的操作電流密度下，其薄膜內水含量會有不同的分佈，此易造成薄膜膨脹不均勻現象。若薄膜的整體厚度膨脹比率過大，則薄膜靠陰極側之水含量會超過16.8，此表示薄膜靠陰極側有大量水產生，形成淹水現象。若考慮薄膜有無膨脹對燃料電池達穩態時間影響，可以發現到，薄膜有膨脹時達穩態的時間較無膨脹時還要多。The behaviors of the water transport in the proton exchange membrane fuel cells are considered by numerical simulation in this study. The mathematical model includes the gas diffusion layers of the anode and cathode, the catalyst layers of the anode and cathode, and the membrane. The water content distribution in the membrane based on the diffusion of water and electro-osmotic water drag is taken into account. The water concentration distributions in the gas diffusion layers and catalyst layers are only considered the diffusion of water. The volume of membrane is assumed to be changing with the variation of hydration, and this hypothesis can make this model much approach the reality. By using different operational conditions, the water transport transient behaviors are investigated. Results from the model show that when the water concentrations of relative humidification are insufficient, the water concentration in each layer reduces and the time needed to reach steady state is longer. By using the larger porosity of the gas diffusion layer, it can effectively prevent the anode side of the membrane from being dry, the cathode side of the membrane from flooding. When the thickness of the membrane is thinner, the water content in the membrane is more uniform and higher, and the time needed to reach stable is shorter. Furthermore, increasing the water concentrations of relative humidification, raising the porosity of the gas diffusion layer, and lessening the thickness of the membrane can make membrane resistance and membrane overvoltage lower. From aspect of swell of the membrane, the water content of the membrane has difference distribution at difference operational current densities in the same membrane expansion coefficient, and this phenomenon will not bring about expansion identically in the membrane. If membrane expansion coefficient is so large, that value of water content at the cathode side of the membrane is higher than 16.8, and this means that the situation of the flooding at the cathode side of the membrane will be occurred. The time needed to reach stable when the membrane is expansive is longer than it when the membrane is not expansive.目 錄 中文摘要 iii 英文摘要 v 表目錄 vii 圖目錄 viii 符號說明 xv 一、緒論 1 1.1燃料電池發展歷史與簡介 1 1.2 燃料電池的基本原理與構造 2 1.3 燃料電池的種類 2 1.3.1 液態電解質燃料電池 2 1.3.2 固態電解質燃料電池 4 1.4 燃料電池的極化性能曲線 6 1.5 文獻回顧 8 1.6 本文探討主題 13 二、理論分析 17 2.1 基本假設 17 2.2 統御方程式 17 2.2.1 薄膜內之水傳輸方程式 18 2.2.2 氣體擴散層內的水傳輸方程式 20 2.2.3 觸媒層內的水傳輸方程式 21 2.2.4薄膜內電阻及薄膜相電位 22 2.3 邊界條件 23 2.4 初始條件 25 三、數值方法 27 3.1 有限差分法 27 3.2 統御方程式之差分法 29 3.3 穩態條件 29 3.4 格點測試 30 四、穩態情況下之結果與討論 35 4.1 各層水濃度之穩態分析 35 4.2 薄膜內電阻之分析 52 4.3 薄膜相電位之分析 53 五、暫態情況下之結果與討論 60 5.1 各層水濃度之暫態分析 60 5.2 各層水濃度達穩態之時間分析 75 六、結論與建議 83 6.1 結論 83 6.2 建議 85 參考文獻 8