質子交換膜中薄膜電極裝置內部傳輸現象之分析

Abstract

本文針對質子交換膜燃料電池中薄膜電極裝置內部傳輸現象進行分析與研究。假設穩態、等溫、且氣體僅有擴散效應，並將氣體擴散層中因電化學反應產生的液態水，使得氣體擴散層內部的孔隙度發生變化時，對整體電池效能的影響考慮進數學模式中。本文引入陰極氣體擴散層層數不同且各層孔隙度為常數，和其孔隙度分佈為不同的連續函數兩種不同概念來進行探討。 在陰極氣體擴散層層數不同，且各層孔隙度為常數的分析中，本文本文分別取四層、五層與六層三種不同層數進行探討。結果顯示，增加陰極氣體擴散層層數對於整體燃料電池效能的影響並不顯著。 在陰極氣體擴散層孔隙度分佈為不同連續函數的分析中，本文取五種不同模式的的連續孔隙度分佈函數來進行探討。結果顯示，五種不同模式的孔隙度分佈對於氧氣質量分率的影響，在較低的表面過電壓下並不明顯；然而，在較高的表面過電壓下，不同孔隙度分佈對於氧氣質量分率的影響就很明顯可以看出。因此，在較大的表面過電壓下不同孔隙度分佈對於氧氣質量分率的影響非常顯著而不可忽視。在表面過電壓等於0.36 V時，最高與最低的電流密度曲線差距竟高達最高電流密度曲線的16﹪，而最高與最低的薄膜相電位曲線差距竟高達最低薄膜相電位的24﹪。由此可知，陰極氣體擴散層中不同孔隙度分佈對於電池整體效能的影響，在高表面過電壓時是不可忽視的。此外，極化曲線的極限電流密度值會隨著氣體擴散層中孔隙度的增大而遞增，而功率密度曲線的最高值亦會隨著氣體擴散層中孔隙度的增大而遞增。因此，在對陰極氣體擴散層孔隙度的限制條件下，陰極氣體擴散層中孔隙度越大，所得到電池的效能就越高。

The primary concern of this study deals with the prediction of fuel cell performance with variable porosity of gas diffuser layer owing to the presence of liquid water in the gas diffuser layer. The half-cell model comprising expressions for the oxygen mass fraction distribution in the gas channel, gas diffuser, and catalyst layer, and current density and membrane phase potential in the catalyst layer and membrane derived from oxygen transport equations and Ohm’s law for proton migration is investigated with the finite-difference scheme numerically. First, the gas diffuser layer was divided into four, five, and six parallel layers defined by different porosity coefficients to investigate that whether the cell performance would become if we increase the number of parallel layers of gas diffuser layers. The results reveal that the increase of parallel layers of gas diffuser layers has insignificant effects on cell performance. The limiting current density increases as the porosity of gas diffuser layer is increased. Second, five different continuous models of porosity distribution are demonstrated. It is found that the differences of oxygen mass fraction among of five models become significant as the surface overpotential □ increases. At □ = 0.36, the difference between the highest and lowest current density can reach 16□ of the highest one and the difference between the highest and lowest membrane phase potential is 24□ of the lowest one. The limiting current density increases as the porosity of gas diffuser layer is increased. The peak power density occurs at larger current density as the porosity of gas diffuser layer is increased. The higher the porosity of gas diffuser layer is, the better cell performance can be obtained.