一氧化碳毒化對質子交換膜燃料電池性能影響之分析

Abstract

近年來由於化石燃料的短缺及有限的蘊藏量，加上傳統應用化石燃料發電方式所產生大量的溫室氣體，造成石油價格攀升及地球暖化等重大議題。因此再生能源及潔淨能源科技的發展成為本世紀最重要的研究課題之一。燃料電池因具有潔淨、高效率及模組化特性，使得燃料電池此一新興能源科技的發展倍受重視。未來質子交換膜燃料電池之重點研究方向有二，一為性能提昇及價格下降，另一為如何提高其可靠度及耐久性 。質子交換膜燃料電池的操作性能與使用期限，與輸入燃氣中所含的不純物質(如陽離子、一氧化碳) 有密切的影響。重組器中所含的一氧化碳由於比氫氣具有與白金更強的鍵結能力，而附著於白金表面造成觸媒參與電化學反應的有效面積降低，即使微量的一氧化碳亦是造成燃料電池性能下降及縮短使用期限的重要因素。因此，本研究主要目的即在建立一套完整的理論模式。探討一氧化碳影響電池性能的主要機制，以及受到一氧化碳毒化後白金觸媒表面受到氫氣與一氧化碳的覆蓋情形與電池內部包含氣體、液體的傳輸現象，研究提升電池抵抗一氧化碳毒化的能力。 首先本文發展出一維暫態的CO毒化理論模式，探討CO在陽極觸媒層中的暫態毒化現象。由理論結果顯示，氫氣在白金表面的覆蓋率隨著時間而降低，這是由於CO佔據白金表面所造成。CO的濃度愈高使得氫氣能參與電化學反應的機會愈小，也縮短達到穩態的時間，亦即縮短燃料電池的使用期限。另外在較低的CO濃度下，增加陽極過電位及增加觸媒的孔隙度明顯增加產生的電流密度。 其次本文推導一維的兩相流理論模式探討CO毒化對PEM燃料電池性能影響之分析。由結果顯示，高濃度的CO造成白金有效反應面積下降。由於電化學反應減緩，陰極觸媒產生的液態水減少使得陽極與陰極的液態水含量降低。在高CO濃度及稀薄的氫氣含量下，由於電滲透效應降低及陰極反應生成的液態水減少，使得薄膜內的液態水分佈梯度降低。CO濃度的改變相較氫氣含量對電池內部液態水的分佈具有較大的影響。CO濃度在10-50 ppm之間電流密度急劇的降低。在不同的CO濃度下，提高氫氣含量均可增加電池輸出之電流密度，提高電池抵抗CO毒化的能力。 接下來本文推導出一維暫態的兩相流理論模式探討CO毒化對PEM燃料電池性能影響之暫態分析。結果顯示，高濃度的CO造成白金有效反應面積下降也同時降低達到穩態所需的時間。在不同的CO濃度下增加氫氣的含量可明顯增加達到穩態的時間。CO濃度在10 ppm，操作電壓在0.6 V以上，可達到較佳的抗CO毒化的能力以維持燃料電池的操作性能。 本文進一步探討高溫型PBI薄膜。由於操作溫度可達200oC，因此水管理及一氧化碳毒化問題可有效解決，並分別由實驗量測及理論分析同時驗證。由實驗結果得知，操作溫度愈高可得愈佳的電池性能，因高溫下可提升化學反應速率。輸入CO濃度高達3000 ppm及40% H2下，電池性能僅下降約26%。在不同的一氧化碳及氫氣濃度下，理論分析結果與實驗非常符合。由本文的研究結果可準確分析燃料成份比例對電池性能的影響，以及電池性能隨時間衰退的變化，提供電池或重組器設計重要參考依據。

Recently, the increase of global energy demand will propel a more rapid depletion of world’s fossil fuel reserves and the burning of the fossil fuels for generating electricity will release greenhouse gases into the atmosphere. The requirements for developing the renewable energy and clean energy technology become the most important issue for the human being in this century. Much attention has been devoted to the developments of the fuel cells because they are clean, high efficient and capable of module. There are two major topics of the R&D programs for the PEM fuel cell systems. First, the improvement of the performance and the decrease of the cost, secondly, enhance the reliability and durability of the fuel cells. Fuel cell performance and life time are strongly influenced by impurities in the fuel gas (cation and CO). Reforming from methanol or gasoline fuels is the most widely used method to generate hydrogen fuel. Even trace amount CO would reduce the hydrogen electro-oxidations effectively by occupying the Pt reacting surface which results in a decrease in the cell performance and life time. To keep a long time and stable operation, how to reduce the CO concentration effectively from the reformer and enhance the tolerance for CO of the fuel cell will become a significant topic. In this work, a comprehensive theoretical model of the poisoning effect of PEM fuel cells by CO is investigated to promote the tolerance for CO and thus elucidate the transport phenomena inside the cell. In the first part of this study, a one-dimensional transient mathematical model is applied to simulate the carbon monoxide poisoning effect on the performance of the PEM fuel cell. The transient behavior of CO poisoning process across the anode catalyst layer is investigated. The results show that the hydrogen coverage, □H, decreases with the time due to the CO adsorption on the catalyst site. A higher CO concentration results in a less available catalyst site for hydrogen electro-oxidation and a more significant decrease in the response time to reach steady state tss. Increasing anode overpotential and gas porosity would result in an increase in the current density, especially at low level of CO concentration. Second, a one-dimensional, two-phase mathematical model was developed to analyze the CO poisoning effect on the performance of a PEM fuel cell. Both vapor and liquid water transport are examined inside the cell. The theoretical results indicate that a higher CO concentration results in large CO coverage across the anode catalyst layer. The slowing of the chemical reactions at both the anode and the cathode reduce the liquid water saturation level in the catalytic layers. At high CO concentration and dilute hydrogen feed, the effect of the electro-osmotic drag is small and less liquid water is generated at the cathode catalyst layer, causing the liquid water distribution to have a small slop across the membrane. The distribution of liquid water depends more strongly on the CO concentration than on dilution of hydrogen in the MEA of the fuel cell. A large dropping rate of the current density is observed in the range between 10-50 ppm CO. Increasing the amount of pure hydrogen drastically increases the current density for a wide range of CO contents, promoting the tolerance for CO of the fuel cell. Third, a one-dimensional, two-phase, transient mathematical model was extended to analyze how carbon monoxide poisoning affects the performance of a PEM fuel cell. The theoretical results indicate that a higher CO concentration results in less hydrogen coverage and a large drop in the time to reach steady state tss. Increasing the amount of pure hydrogen drastically increases tss for a wide range of CO contents. At 10 ppm CO, a long tss can be achieve using pure hydrogen, especially at high cell voltage, promoting the tolerance for CO and providing the desired performance of the fuel cell. Finally, high temperature proton exchange membrane fuel cells have drawn great attentions due to high CO tolerance and overcoming water managements. In this wrok, theoretical and experimental studies were made to analyze the transient CO poisoning process. Experimental results were measured at different temperature and suffered from various CO contents. Higher performance was obtained at elevated temperature due to faster chemical kinetics. Only 26% of performance loss is obtained under 3% CO and 40%H2. The effects of temperature, CO contents and H2 dilutions on the fuel cell performance and the time to reach steady tss are all investigated. The predictions of the degradation of fuel cell performance show good agreements with experimental results under various fuel compositions. Thus, the present results can provide comprehensive information for designing fuel cell system and methanol reformer.