标题: | 质子交换膜燃料电池中传输现象效应对性能影响之研究 Investigation of the Effects of Transport Phenomena on the Performance of Proton Exchange Membrane Fuel Cells |
作者: | 江木胜 Mu-Sheng Chiang 陈俊勋 曲新生 Chiun-Hsun Chen Hsin-Seng Chu 机械工程学系 |
关键字: | 质子交换膜;燃料电池;性能;传输现象;过电位;极化;proton exchange membrane;fuel cell;performance;transport phenomenon;overpotential;polarization |
公开日期: | 2006 |
摘要: | 本论文探讨质子交换膜燃料电池于各种传输元件设计与水热操作模式下的传输现象与性能影响。研究中首先建置描述质子交换膜燃料电池内部各种传输及电化学现象之数学模式,其中以质量、物种、动量与电流守恒方程式做为模式主要架构。在电化学反应部份以Bulter-Volmer 方程式加以描述,方程式使用活化过电位联系触媒层中电子相与质子相电位,为反应的主要驱动力。为考虑电化学反应所产生热能,模式中同时纳入系统能量守恒方程式,其中焓值源项考虑电流焦耳热、过电位以及水之相变化潜热。本研究中含盖三个质子交换膜燃料电池传输现象与电化学反应之主要议题, 分别为温湿度梯度、传输元件效应以及新型阴极流道设计对性能影响之研究。 在探讨各式加湿与温度梯度的影响时,数值计算结果以极化曲线与局部性质分布说明各种加湿与温度梯度效应。结果显示阴阳极加湿温度变化对电池性能产生不同效果。而在温度梯度效应方面,依据梯度大小与梯度方向亦有不同的现象表现。在任一较高的边界温度下,由于触媒反应速率及薄膜导电度的提升,电池性能随另一边界温度之增加而提升。从局部气体浓度及液态水分布情况绘图中,显示出其与温度及加湿梯度具有密切关系,进而对电池性能产生影响。 于传输元件设计效应之研究中,藉由改变各种流道高宽比与气体扩散层厚度的分析结果显示,细高型的流道设计适合于中电流密度的电池操作情况,而宽扁型的流道则于高电流密度下有较佳的性能输出。经由特定位置横向氧气浓度与相电位图形显示在燃料电池中的氧气传输与电子传导对局部电化学反应呈现竞争效应,其相对强度与操作条件与位置有关系。于电池极化曲线中显示最佳气体扩散层厚度随操作电压减少而增大;然而在最小的考虑电压0.14V时,由于液态水的累积与传输路径增长而逆转此一趋势。 本论文第三部份针对一具新型阴极流道燃料电池之电化学反应与性能进行探讨。藉由流道出口宽度改变,而考量渐扩、直管与渐缩等三种外型流道的效应。研究结果显示渐缩型流道于中电池电压情况下,由于其具有增进电子传输之效果而有较佳性能表现。反之,当电池操作电压下降反应速率提升时,渐扩型流道因为可以提供较高浓度氧气至触媒层反应位址,致使其产生较大电流输出。藉由检视触媒层中局部电流密度与氧气及各项电位分布情形,可以获得燃料电池中内部电化学反应的变化趋势。此一结果可以使我们从一般所熟知的整体性能曲线变化机制中,进一步了解电池内部由于各种物种传输现象特性差异,所产生反应主导机制不同的原因,对于研发者及工程师具有重要帮助。 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. |
URI: | http://140.113.39.130/cdrfb3/record/nctu/#GT009014814 http://hdl.handle.net/11536/81324 |
显示于类别: | Thesis |
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