質子交換膜燃料電池中傳輸現象效應對性能影響之研究

Abstract

本論文探討質子交換膜燃料電池於各種傳輸元件設計與水熱操作模式下的傳輸現象與性能影響。研究中首先建置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、物種、動量與電流守恆方程式做為模式主要架構。在電化學反應部份以Bulter-Volmer 方程式加以描述，方程式使用活化過電位聯繫觸媒層中電子相與質子相電位，為反應的主要驅動力。為考慮電化學反應所產生熱能，模式中同時納入系統能量守恆方程式，其中焓值源項考慮電流焦耳熱、過電位以及水之相變化潛熱。本研究中含蓋三個質子交換膜燃料電池傳輸現象與電化學反應之主要議題, 分別為溫溼度梯度、傳輸元件效應以及新型陰極流道設計對性能影響之研究。 在探討各式加濕與溫度梯度的影響時，數值計算結果以極化曲線與局部性質分佈說明各種加濕與溫度梯度效應。結果顯示陰陽極加溼溫度變化對電池性能產生不同效果。而在溫度梯度效應方面，依據梯度大小與梯度方向亦有不同的現象表現。在任一較高的邊界溫度下，由於觸媒反應速率及薄膜導電度的提升，電池性能隨另一邊界溫度之增加而提升。從局部氣體濃度及液態水分佈情況繪圖中，顯示出其與溫度及加濕梯度具有密切關係，進而對電池性能產生影響。 於傳輸元件設計效應之研究中，藉由改變各種流道高寬比與氣體擴散層厚度的分析結果顯示，細高型的流道設計適合於中電流密度的電池操作情況，而寬扁型的流道則於高電流密度下有較佳的性能輸出。經由特定位置橫向氧氣濃度與相電位圖形顯示在燃料電池中的氧氣傳輸與電子傳導對局部電化學反應呈現競爭效應，其相對強度與操作條件與位置有關係。於電池極化曲線中顯示最佳氣體擴散層厚度隨操作電壓減少而增大；然而在最小的考慮電壓0.14V時，由於液態水的累積與傳輸路徑增長而逆轉此一趨勢。 本論文第三部份針對一具新型陰極流道燃料電池之電化學反應與性能進行探討。藉由流道出口寬度改變，而考量漸擴、直管與漸縮等三種外型流道的效應。研究結果顯示漸縮型流道於中電池電壓情況下，由於其具有增進電子傳輸之效果而有較佳性能表現。反之，當電池操作電壓下降反應速率提升時，漸擴型流道因為可以提供較高濃度氧氣至觸媒層反應位址，致使其產生較大電流輸出。藉由檢視觸媒層中局部電流密度與氧氣及各項電位分佈情形，可以獲得燃料電池中內部電化學反應的變化趨勢。此一結果可以使我們從一般所熟知的整體性能曲線變化機制中，進一步了解電池內部由於各種物種傳輸現象特性差異，所產生反應主導機制不同的原因，對於研發者及工程師具有重要幫助。

This dissertation presents a numerical investigation of the transport phenomena and performance of proton exchange membrane fuel cell (PEMFCs) with various transport component design parameters as well as water and thermal management schemes. A three-dimensional fuel cell model, incorporating conservations of species, momentum, as well as current transport, is developed at first. The Bulter-Volmer equation that describes the electrochemical reaction in the catalyst layer is adopted with the activation overpotential as the connection between the solid phase potential and that of the electrolyte phase. To ensure the conservation of enthalpy, the energy equation is employed to the model domain with three sources of enthalpy generation: ohmic heating, activation overpotential as well as phase change of water harnessed in modeling. Three major topics concerning the transport phenomena as well as electrochemical reaction in PEMFCs are presented in this dissertation. This includes the water and thermal management, transport component design effect as well as a novel cathode channel shape effect. In the investigation of cell temperature and humidification effects, numerical prediction results are presented using polarization curves and contour plots. Findings show that humidification level perturbation on the anode or cathode side creates an individual effect. Mechanisms influencing performance variation tendencies are interpreted. Also, modeling results with existing temperature gradient exhibit different trends on the overpotentials according to the slop and magnitude. At a higher boundary temperature on either side, cell performance increases according to the temperature increase on the other side because the reaction kinetic and ionic phase conductivity is promoted. Contour plots of local concentration and saturation level show the close relation between the existence of both temperature and humidification gradients and the local properties variations. Through cell performance simulation with various channel aspect ratios and gas diffusion layer (GDL) thicknesses, a slender channel is found suitable for cells operating at moderate reaction rate, and a flat channel produces more current at low cell voltage. Plots of transverse oxygen concentration and phase potential variation indicate that these oppositely affect the local current density pattern. The relative strengths of these two factors depend on the transport component position and geometry, as well as on the cell operating conditions. Consequently, the curves of cell output current density demonstrate that the optimal GDL thickness increases as the cell voltage decreases. However, at the lowest considered cell voltage of 0.14V, optimal thickness decreases as that of a thick GDL the oxygen deficiency caused by long traveling length and clogging effect of liquid water reverses this relationship. The electrochemical reaction and performance of PEMFCs with a novel cathode channel shape is proposed in the third part of this dissertation. This channel geometry has a characteristic of continuous variation of shoulder/channel ratio along the main stream direction. Three types of channel configurations: convergent, straight and divergent channels are investigated. The calculation results demonstrate the effects of the channel configuration on the transport phenomena and cell reaction. As the cell operates at a medium cell voltage, the convergent channel behaves better because the electrons transport is enhanced and the corresponding ohmic overpotential is small. On the contrary, the divergent shape channel performs better at higher reaction rate as it is able to deliver sufficient oxygen concentration to the reaction sites. Through the distribution inspections of the local current density, oxygen concentration and potential fields in the catalyst layer, variation trend of the electrochemical reaction in the fuel cell is available. With the knowledge of the well comprehended determining reasons of global performance variation, these results can further offer the explanations, which are of great important to the researchers and engineers, of the different dominant mechanisms resulting from the characteristic differentials of transport phenomena for the various species inside the fuel cell.