質子交換膜燃料電池中陰極層汽液界面位置對性能之影響

Abstract

本論文主要是探討三維、多物種、二相混合模式下之質子交換膜燃料電池中，其陰極增濕、陰極氣體擴散層的孔隙度、電池溫度以及增濕溫度的變化對燃料電池的傳輸現象及電池性能之影響。本研究中主要是利用沿著流道方向之汽液界面的位置來說明在這些操作條件下，液態水出現的位置對電池性能的影響。在研究過程中，首先先建立描述質子交換膜燃料電池內部各種傳輸及電化學現象之數學模式，其中以質量、動量、物種、能量及電流守恆方程式做為模式的主要方程式。在電化學反應中，主要的驅動力是利用活化過電位來作為陰極觸媒層中電子相電位與質子相電位的聯繫。而且當電池發生電化學反應時會產生熱能，其熱能的來源包括因電化學反應所造成之不可逆的熱及熵、因質子和電子傳輸所造成的焦耳熱以及因水的蒸發及凝結所產生的潛熱。因為本論文中有考慮二相流動的傳輸，因此在此部分是利用M2模式來說明其各物種之間的相互關係。 本研究探討的議題分為二個部份：第一部份是以等溫系統為主，即是不考慮能量方程式。主要是探討在陰極的流道及擴散層中，陰極的增濕情況以及陰極氣體擴散層的孔隙度改變時對汽液界面位置的影響。數值模擬的結果顯示當陰極的增濕程度逐漸增加以及電池的操作電壓逐漸降低時，汽液界面的位置會逐漸往流道的入口方向移動。這是因為當陰極增濕度增加時，會有較多的液態水產生，而液態水會造成多孔隙材質中的孔穴被液態水佔據，造成燃料氣體傳輸的阻礙，因此電池性能降低。相同的，當電池電壓降低時，即是電流密度較大時，電化學反應較為快速，因此汽液界面的位置也會逐漸往流道的入口方向移動因而造成相同的情況。再者，當陰極的氣體擴散層孔隙度逐漸增加時，無論是水或是燃料氣體，皆較容易通過多孔隙材質，因此，當陰極氣體擴散層的孔隙度增加時，電池性能較佳。另外，沿著流道方向之氧氣分率、水分率以及液態水在陰極流道及擴散層中的分佈及變化也一併提出及探討。 第二部份主要是以非等溫系統為主，因為液態水的蒸發與凝結皆與操作溫度有著密切的關係，因此本部分考慮在不同的操作溫度時其對汽液界面位置的影響。操作溫度主要是分成電池溫度以及增濕溫度來探討。數值模擬的結果顯示當增濕溫度等於或大於電池溫度時，其汽液界面的位置會隨著電池溫度的降低而逐漸往流道的入口處移動，進而造成電池性能的降低。另外也分別探討在薄膜內的溫度分佈、在流道入口處陰極氣體擴散層內溫度以及液態水的分佈。並且清楚的指出燃料氣體在氣體擴散層中是利用擴散方式由流道往勒條的方向移動，反之，液態水在氣體擴散層中則是利用毛細作用力由肋條往流道出口處移動。

This dissertation presents a three-dimensional, multi-component, two-phase model to investigate the transport phenomena and performance of proton exchange membrane fuel cell as the liquid water forms under various cathode humidification conditions, gas diffusion layer porosities, cell temperatures, and humidification temperatures. In this study, the location of the gas-liquid interface along the channel direction is extracted to explain the effects of liquid water appearance that cause the cell performance change. A mathematical model, coupled with the electrochemical process, two-phase flows, species transfer, and heat transfer is developed at first. In the electrochemical reaction of the cathode catalyst layer, the solid phase potential and the electrolyte phase potential is connected by the activation overpotential. Furthermore, thermal energy release and transport is accompanied with the electrochemical reaction and is considered in modeling. The sources of thermal energy accounts for irreversible heat and entropic heat generated due to electrochemical reactions, Joule heating arising from protonic/electronic currents, and latent heat of water condensation and/or evaporation. The multiphase mixture formulation (M2 model) is adopted as it is particularly suitable for two-phase flow modeling in PEM fuel cells. Our quest to the effects of the liquid-water interface location involves the following two parts in the dissertation. First, we considered the isothermal system in the PEM fuel cell, namely the energy conservation equation is not included in it. The objective of this part is to investigate the effects of the location of the gas-liquid interface along the channel direction under various cathode humidification conditions and gas diffusion layer porosities in the conventional flow field. Numerical simulation results indicate that the gas-liquid interface location approaches to the gas flow channel inlet region and cell performance declines gradually as the relative humidity of the cathode is increased. This is because of liquid water may occupy the pores in the porous media, reducing the amount of fuel gas that can reach the cathode catalyst layer to cause the cell performance diminished. Meanwhile, when the cell operating voltage decreases and the current density gets larger, the electrochemical reaction becomes more quickly. Hence, the gas-liquid interface location also moves to the channel inlet region to cause the same consequence. Furthermore, as the gas diffusion layer porosity is increased, the transport of liquid water and fuel gas becomes easier and the cell performance is enhanced. To explain this phenomenon, the oxygen fraction, the water fraction and the liquid water saturation field along the flow channel direction in the flow channel and the gas diffusion layer are presented. The investigation of the location of the gas-liquid interface along the channel direction at various operating temperatures is conducted in the second part of the dissertation, owing to the consideration that the condensation and/or evaporation of the water is related with operating temperature. The effects of two model parameters, namely cell temperature (Tcell) and humidification temperature (Th), on the gas-liquid interface location and cell performance are presented. Simulation results indicate that when the anode and cathode humidification temperatures are equal to or higher than the cell temperature, the gas-liquid interface location moves toward the flow channel inlet region and the cell performance decreases as the temperature is decreased. Additionally, the membrane temperature distribution and the distributions of the liquid water and temperature in the cross-section of the cathode gas diffusion layer in the inlet region are presented. Simulation results indicate that gas-phase fluid diffuses from the channel to the land and that the capillary-driven liquid water is transported in the opposite direction in the cathode gas diffusion layer.