以週期性的空氣吹離法改善燃料電池一氧化碳毒化容忍度與微型燃料電池設計與性能分析

Abstract

本論文分為兩部分。第一部分為運用不同週期的空氣吹離法改善被一氧化碳毒化之質子交換膜燃料電池性能。陽極燃料中如含有一氧化碳，一氧化碳會吸附白金觸媒造成燃料電池毒化現象產生，因此本論文研究當電池電位固定於0.6伏特時，以4個不同週期的空氣吹離法分別輸入含有不同濃度之一氧化碳的(20 ppm, 52.7 ppm和100 ppm)陽極燃料中觀測電池性能回復情形。由實驗結果可知，使用空氣吹離法可以明顯地增進燃料電池一氧化碳容忍度；除了一氧化碳濃度100 ppm外，當循環週期定為輸入空氣10秒接著關閉10秒時，改善電池性能的效果接近於持續地輸入空氣於被毒化的燃料電池中。 第二部分為研究設計微型質子交換膜燃料電池(反應面積為2.5 cm2)，微型燃料電池的流場板是使用微機電製程加工於矽晶圓材料上以蝕刻出流道，流場板表面沒有鍍金增加導電性，因此流場板並非如傳統燃料電池亦當雙極板使用，本論文改變習慣上組裝的順序，將電流收集板移至流場板與氣體擴散層之間。本論文亦以模擬燃料電池電化學反應來觀測微型燃料電池內部質子交換膜電化學反應分布情形，電池內部的傳輸現象會遵守質量守恆、動量守恆、物種守恆、電荷守恆和能量守恆方程式，模擬以商業套裝軟體CFD-ACE+完成，模擬的極化曲線與實驗的結果相互驗證，三維模型模擬結果分析兩個不同化學劑量比流量(3和5)的微型燃料電池，其觸媒層上電流密度、質子交換膜上水和溫度的分布情形。模擬結果發現微型燃料電池內部溫度分布受質子交換膜上氣態水分布所影響，模擬結果說明，當操作電壓定在0.4 V時，觸媒層上較低的化學劑量比流量會有較佳的電化學反應，因此會有較均勻的電流密度分布。另外，較低的化學劑量比流量會讓質子交換膜上水較多並均勻分布，造成較低且較均勻的溫度分布，模擬的結果可闡明微型燃料電池內部物理現象。

The dissertation consists of two subjects. Subject one investigates the improvement of carbon monoxide (CO) poisoning in a proton exchange membrane fuel cell (PEMFC) by using a periodic air dosing. The CO in the fuel gas leads to a significant loss in current density due to CO poisoning in the anode. The method of this experiment involves bleeding air into the anode fuel stream (H2-CO), which contains CO in various concentrations (20 ppm, 52.7 ppm and 100 ppm). In the transient CO poisoning test, air bleeding is performed for four different periodic air dosing and cell voltage is fixed at 0.6 V. The results show that air bleeding technique can improve CO tolerance. The CO tolerance of a dosing air for 10 s in intervals of 10 s is similar to that of continuous air bleeding except 100 ppm CO. The CO tolerance of the fuel cell and cell performance recovery from poisoning can be improved by air bleeding. Subject two designs a novel single micro proton exchange membrane fuel cell (active area is 2.5 cm2) which is manufactured by applying Micro-electro-mechanical Systems (MEMS) technology on silicon substrates to etch flow channels without a gold-coating. A theoretical analysis is performed also in this study for such micro PEMFC. The three dimensional mathematical model transport phenomena in the micro PEMFC consists of the conservation equations of mass, momentum, energy, species and charge in a fully integrated finite-volume solver using the CFD-ACE+ commercial code. The computed polarization curves are agreed well with experimental data. In addition, the predictions and analyses of micro PEMFC current density, water and temperature distributions in two different stoichiometric H2/O2 flows are given (3 and 5). The numerical results show that temperature distribution is affected by water distribution in the membrane. They also indicate that much more active electrochemical reaction in the catalyst layer at low flow rate leads to a superior current density distribution under 0.4 V operating voltage. In addition, a more and uniform water distribution induces a low and uniform temperature distribution in the membrane at low flow rate. Those illustrate that the model predictions are well correlated with the known phenomena of experimental mechanism.