| Abstract: | 燃料電池的研究包含了各種物理問題，包括流體力學、質傳學、熱傳學、電化學、二相流與孔隙材料等。本研究討論一個二維陰極側的電池結構，發生在膜電極(MEA)上的電化學反應以及物種輸送機制，此膜電極為五層結構：包括了陽極氣體擴散層、陽極觸媒層、質子交換膜、陰極觸媒層和陰極氣體擴散層。燃料(O2)及生成物(H2O)的進出入均是利用濃度的擴散機制驅動。本文所使用到的統御方程式如下：採用Darcy''s Law來描述孔隙材料內的速度、壓力分佈。用Stefan-Maxwell氣體擴散方程式來描述各種的反應物種在電池內的消長情況，加上Bruggemann模型修正孔隙材料內的有效氣體擴散係數。此外，電化學所產生的電流以Butler -Volmer方程式表示之，並透過電流守恆方程式得到電池內質子、電子電位及活化過電位的分佈情況。而總過電位為活化、歐姆及濃度過電位的總和，燃料電池的操作電壓可由電池的理論電壓扣掉總過電位得到。利用操作電壓與電流密度的關係可以得到電池操作性能曲線，透過比較電池性能曲線分別在模擬與實驗上的誤差，可以瞭解數值方法所造成的誤差可能發生之處，並加以改善研究用的數值模擬模型。
本文在數值求解方面透過COMSOL Multiphysics商用軟體。在數值模擬中，可以從Butler-Volmer方程式中直接控制電池的活化過電位，本研究在模擬結果部分，分別透過不同的活化過電位來觀察各種物理量的變化。在陰極中，越接近大氣開孔端其過電位越低，越遠離大氣開孔端則過電位就越高，結果反應出燃料(O2)的分佈對於電化學反應的影響。當過電位越高也代表著反應越顯得劇烈，反應後生成物(H2O)增加，水的排放也帶動整體電池內部的燃料流動現象，明顯的增加燃料在下游開孔處離開時的速度。此外，本文也探討Butler-Volmer方程式中的參數 Sa、alpha 和 i0 對於電池性能曲線所造成的影響。
The study of fuel cell includes several physical and chemical phenomena, such as fluid dynamics, heat and mass transfer, electrochemical reaction, two phase flow, and porous media. This thesis discussed the phenomena of the electrochemical reaction and species transportation on the MEA for a two-dimensional cathode structure. The MEA is a five layer structure which includes anode gas diffusion layer, anode catalyst layer, a proton exchange membrane, cathode catalyst layer, and cathode gas diffusion layer. The flowing in and out of the fuel of oxygen and product of water are driven by the concentration diffusion mechanism. The related governing equations adopted in the research are illustrated as follows: the distributions of the velocity and pressure inside the porous media are based on the Darcy’s law, and the species transporting in the cathode is described by the Stefan-Maxwell gas diffusion equation and the effective gas diffusion coefficient of the porous material is amended by the Bruggemann model. In addition, the current generated by electrochemical reaction is calculated by the Butler-Volmer equation, and the distributions of the ionic, electronic potential and the activation overpotential are obtained by the current conservation equations. The overall overpotential is the summation of activation, ohmic, and concentration overpotential. The cell voltage can be acquired via the theoretical cell potential minus the overall cell potential. Moreover, the cell performance curve can be derived by the relationship of the operating voltage and current density. The cell performance curve can be utilized to understand the numerical error compared with the experimental results which could help to improve the simulation model.
In this research work, the numerical simulation was accomplished via the COMSOL Multiphysics commercial solver. The cell activation overpotential of the Butler-Volmer equation can be defined directly and the related phenomena variation under different activation overpotential were further observed. In the catalyst layer, the overpotential is decreased when near the opening holes and increased when far away the opening holes. The results also express the effect of oxygen distribution on the electrochemical reaction. If the higher overpotential is higher, the reaction is more active and the product water is also increased. The water discharge also derives the fuel flow inside the cell which enhances the velocity of the downstream where is closed to the opening holes. In addition, this thesis also discusses the effects of Sa, alpha, and i0 on the cell performance curve. |
