應用薄膜-集塊理論分析質子交換膜燃料電池之暫態行為

Abstract

近年來，燃料電池己經廣泛地應用在可攜式電子產品，如行動電話、筆記型電腦、及數位相機等等。目前研究燃料電池的主要方向大都在於如何降低成本、提高穩定性及續電力。質子交換膜燃料電池具有啟動快速、工作溫度低以及單位功率所需成本低的優點，這是它比其他類型的燃料電池有利的地方。而可攜式電子產品對高動態負載的要求比較高，但是目前的研究大多以穩態為主而非暫態。為了設計出更高性能的質子交換膜燃料電池，所以發展暫態模式的研究乃勢所必然。 首先，本文是以兩相流、半電池的模式進行質子交換膜燃料電池的暫態分析。在觸媒層的電化學反應則以圓柱形薄膜-集塊模式(thin film-agglomerate model)模擬，用以探討質子交換膜燃料電池中氣態水及液態水、氧氣及質子的分佈及變化情形。其中質子的傳輸速度較其他快很多，到達穩態所需時間約在0.1秒左右，而液態水則需要數十秒的時間。質子電位的變化在初期先急劇下降，而在達到極值後，隨時間增加而漸漸上升達到定值。本文也同時探討不同的操作條件下對燃料電池性能的影響，如操作溫度及陰極入口氣體的相對濕度。在60℃~80℃操作溫度下，操作溫度與電池性能呈正比。 此外，對於各項設計參數對電池性能的影響，如孔隙率、觸媒層厚度及集塊半徑(agglomerate radius)本文也有進一步的研究。電流密度隨時間的變化情形如下：在0.01秒內，電流密度的變化非常劇烈。在0.01秒到0.1秒間，則保持定值。在1秒後，它的變化情形則與操作電壓、孔隙率、觸媒層厚度及集塊半徑有關。當氣體擴散層的孔隙率介於0.2和0.5間時，電流密度與孔隙率呈正比。在1秒後，當孔隙率大於0.3以後，電流密度不會受到液態水的影響而下降。而在觸媒層孔隙率方面，當孔隙率在0.06到0.1間時電流密度有最大值。觸媒層厚度則以10um 到13 um間，電流密度最大。而集塊半徑則建議應小於100nm，才能獲得較高的電流密度。 最後本文在觸媒層的電化學反應以更接近實際的球形薄膜-集塊理論進行模擬，探討質子交換膜燃料電池中氣態水及液態水、氧氣及質子的分佈及變化情形。同時透過介面邊界條件結合液態水分量與Nafion相中的液態水，使得的液態水效應能夠更完整地納入研究。根據數值結果顯示，電池的最大質子過電位降隨操作電壓減少而增加，當電池克服其最大活化過電位後，電池只需較小的質子過電位降即可保持原有的電化學反應。當液態水分量及薄膜中的水含量同時偏高時，電池即容易發生水氾濫(flooding)。對於各項參數對電池性能的影響，如氣體擴散層與觸媒層中滲透係數(permeability)，觸媒含量以及氣體擴散層的厚度也一併探討。

Nowadays fuel cells have been enthusiastically developed in order to be used in portable devices, such as mobile phones, notebook computers, power tools and digital cameras etc. Cost, durability and stability are the main concerns of the R&D programs for the fuel cell systems. The advantages of PEM fuel cell include lower cost per kW, and fast start-up, and lower operating temperature. In actual applications, the cell behaviors are highly dynamic. However, most of the recent researches focus on steady state models instead of transient ones. In order to mimic the real performance characteristics of PEM fuel cell, it is crucial to develop the transient model. The first part of this dissertation analyzes the transient behavior for a PEM fuel cell via two-phase and half cell model with a thin film-agglomerate approach to model the catalyst layer. The model includes the transports of gaseous species, liquid water, proton, and electrochemical kinetics. The numerical results reveal that the transport of proton is much faster than the others. The ionic potential reaches the steady state in the order of 10-1sec but that of the liquid water transport takes place is in the order of 10 sec. The ionic potential does not decrease monotonically with time. In the very beginning, the ionic potential rise rapidly, and then reaches the critical value, and then increase till its steady state. We investigate how operating parameters affects the cell performance of a PEM fuel cell, such as the operating temperature and the inlet relative humidity of the cathode streams. For the operating temperature, the higher the operating temperature is, the higher the cell performance will be. Subsequently, we investigate the parameters which can affect the cell performance such as GDL porosity, CL porosity, catalyst layer thickness and agglomerate radius in detail. The transient behaviors show that within 10-2sec, the current density rises rapidly, and there is a plateau between 10-2sec and 10-1sec. After 1sec, the variations of current density depend on cell voltage, gas diffusion layer porosity, catalyst layer porosity, catalyst layer thickness and agglomerate radius. For gas diffusion layer porosity, between =0.2 and =0.5, the higher the GDL porosity is, the higher the cell current density will be. After 1sec, if the GDL porosity is below 0.3, the current density will go down. For catalyst layer porosity, optimum current density appears between =0.06 and =0.1. For the catalyst layer thickness, the optimum values of catalyst layer thickness appears between =10□um and 13□um. For the agglomerate radius, it suggests that the agglomerate radius should smaller than 100nm for a higher utilization of catalyst. Finally, we investigate the transient behavior of a PEM fuel cell by using a one-dimensional, two-phase mathematical model that treats the catalyst layer as a spherical thin file-agglomerate. This method is different from other works that regard it as interface or thin film. Effects of various transport parameters as well as other factors such as catalyst loading, gas diffusion layer thickness and liquid water permeability on the transient evolution of major model properties and cell performance are investigated thoroughly. Numerical results show that the evolution of ionic potential drop and oxygen consumption experience several steps before they reach steady state. The same situation can also be seen for the evolution of water saturation and current density. A close inspection of these phenomena shows a intimate between these transport variables and cell performance. Parametric studies of other design factors’ effects reveal there exists optimum values which lead to a greater current output during its evolution period.