Analytical model of Direct Methanol Fuel Cell

近年來由於能源危機的產生，並且能源持續的消耗也造成自然環境的污染破壞，使得找尋可替代性的綠色能源成為人類的重要課題，其中直接甲醇燃料電池的前景頗被看好。而直接甲醇燃料電池目前還面臨許多問題有待解決，如甲醇燃料會透過膜向陰極滲透(crossover)造成電池電壓下降、甲醇燃料的浪費、甲醇氧化速率緩慢、電池的發電效率、水及熱管理等等…。因此建立一個準確而且計算方便的模型來描述燃料電池的運作情形，對電池的研發至為重要。 在本研究中，我們建構一個甲醇燃料電池的數學模型，並推導得出其解析解。在透過此模型中，我們考慮在恆溫操作下，甲醇燃料電池穩定運作時的物質平衡，並藉此得到電池的各種特性，包括各種物質(如燃料)在電池內部的濃度分佈及流通量(flux)，電池的放電現象(完整的I-V極化曲線)以及電池的放電功率及操作效率，瞭解到對於不同純甲醇進料量時電池有不同的表現以及水分的管理。本模型與文獻中其他模型之最大不同之處，在於我們能夠計算甲醇燃料電池在未使用時(open circuit)的漏電情形。此模型所用到的參數(共43個)，大多由電池的結構與操作條件決定，僅有7個(電子傳遞係數、材料界面電阻、孔隙度、觸媒層厚度、氧氣擴散係散)是透過迴歸實驗數據而得。我們用三種不同的實驗數據(包括不同的電池構造，不同的操作溫度及甲醇濃度)，發現都可以得到很好的描述，證明此模型的可靠性。利用此模型，我們可以了解電化學反應、物質在電池中的質傳現象包括甲醇透過膜的滲透(crossover)、以及反應物在各電極層間的傳遞現象會如何影響電池的整體表現。這些知識將有助於我們尋找進一步改善及提升甲醇燃料電池效能的關鍵。It has become an important issue to search for alternative energy source due to energy crisis and pollution of the natural environment in recent years. Direct methanol fuel cell (DMFC) is one of the attractive energy supplies. There are some technical challenges for DMFC such as low efficiency due to methanol crossover, low methanol oxidation rate, low power density, and the need for excessive water and heat management, etc. For a better description of the cell operation and optimization of performance, it would be important to develop an accurate and quick mathematical model for DMFC. In this research, a 2D analytical mathematical model of a direct methanol fuel cell was developed to describe not only electrochemical reactions on the anode and cathode electrodes, but also transport phenomena within the fuel cell, operating isothermally at steady state. One could use this model to understand the cell performance such as polarization curve, efficiency, power density and concentration profile. Further, we could predict cell performance and understand how to deal water management when methanol input concentration changes. Compared with other models in the literature, our model allows for prediction of the open circuit voltage of the DMFC. This model contains forty three parameters; most of them are decided by cell structure and operation condition. Only seven parameters(transfer coefficient of electron、resistance of material interface、porosity、thickness of catalyst layer and diffusion coefficient of oxygen) are obtained by regressing to experiment data. The theoretical prediction was in good agreement with experiments from three different fuel cells (including different cell structure, operating condition). This indicates that this model is robust and reliable. With this model, one could better understand electrochemical reactions and mass transport phenomena in fuel cell, including methanol crossover and reactant transport in membrane electrode assembly (MEA), and how these phenomena affect cell performance. The knowledge provided from such an analytical model may help one search for the key factors to improve DMFC performance.誌謝 I 中文摘要 II Abstract III 目錄 IV 圖索引 V 表索引 VI 1. 緒論 1 1.1 研究背景簡介 1 1.2 燃料電池的特點 1 1.3 燃料電池的種類 2 1.4 直接甲醇燃料電池 3 2. 直接甲醇燃料電池工作原理與概況 6 2.1. 直接甲醇燃料電池工作原理 6 2.2. 甲燃料電池技術挑戰 7 2.3. Modeling的重要 9 3. 數學模型的建立 11 3.1. 基本假設 11 3.2. 數學模型建立原理與方程式 12 3.2.1 陽極半反應部分 13 3.2.2 陰極半反應部分 17 3.3. 電池整體表現 20 3.3.1 關於開環電路 21 3.3.2 濃度分佈 23 3.4. 模型參數 24 4. 結果與討論 29 4.1. I-V polarization curve 29 4.2. 開環電路電壓、極限電流之預測 33 4.3. 電池的發電功率與放電效率 34 4.4. 燃料於電池內部的分佈 38 4.5. Methanol Crossover與內電流 40 4.6. Methanol crossover and Faraday efficiency 45 4.7. 關於水管理 47 5. 結論 52 參考文獻 54 附錄 5