Multi-Phase Flow Dynamics in Polymer Electrolyte Fuel Cells

摘要：燃料电池具有高功率密度和高效率的潜力，在未来的便携式、汽车和固定式应用中有着巨大的前景。在燃料电池类型中，最适合便携式应用的是直接甲醇燃料电池（DMFC），而最适合固定和运输应用的是氢聚合物电解质燃料电池（H2 PEFC）。在这些燃料电池中，微小的长度尺度和复杂的材料提出了许多独特而具有挑战性的物理化学问题。特别是，通过薄膜多孔碳纤维气体扩散层（GDLs）的两相流效应是实现高功率密度和稳定性的主要微流控管理瓶颈。GDL是一个关键部件，其功能是将反应物输送到发生电化学反应的电极，并将产物从电极输送出去。如果通过GDL或流动通道的产物传输不足，则反应物质量传输将受到限制，从而限制性能。例如，DMFC阳极产生的二氧化碳气体会阻碍液态甲醇溶液向反应部位的输送。在DMFC和H2 PEFC的阴极，电化学反应、电渗透阻力和扩散产生的液态水会淹没阴极，使其缺氧，从而严重限制其性能。在DMFC和H2 PEFC中，都存在复杂的两相流挑战，以管理通过薄膜多孔gdl和流动通道的反应物和产物的运输。因此，天然亲水性gdl通常是通过在加工过程中添加疏水材料来定制的。迄今为止，疏水添加剂的分数是通过低效的反复试验来确定的。现有文献也遵循现象学方法，尚未产生气泡和液滴通过GDL，在GDL和流道之间的界面边界或流道中的基本传输过程的任何明确的原理或基本知识。我们提出了一项实验和分析研究的两相流动的气相气泡和液相液滴在反应物流动通道和薄膜多孔介质具有定制润湿特性。我们将使用高速微流体流动可视化技术和模型燃料电池来实验量化参数，如气泡/液滴形状，分离大小，运输速率，分布和多孔材料中的液体饱和度作为操作条件的函数。然后将开发分析模型来描述气泡/液滴动力学，并使用从高度仪表化的燃料电池获得的新数据进行验证。这项研究的最终目标是首次提供基于基本理解的设计理论，可用于设计燃料电池材料和下一代燃料电池系统的动力微流体管理。这将代表着对目前限制燃料电池发展的最关键领域之一的理解的重大飞跃，并将影响便携式，固定式和汽车燃料电池的设计。我们的综合研究和教育方法的目标是：1)创建物理和智力基础设施，以解决下一代氢或甲醇聚合物电解质燃料电池设计中的多学科技术连接；2)培养与当前国家需求高度相关的物理/化学科学领域的研究生和本科生；3)将前沿研究成果直接整合到宾夕法尼亚州立大学的三门既定课程中，并使支持的研究生能够在本科课程中展示材料；4)扩大燃料电池和微流体研究项目在PSU本科生和研究生招聘过程中的作用；5)通过现有和未来的电化学发动机中心行业合作，为本科生/研究生提供与行业互动的重要机会。

ABSTRACT - 0414319With the potential for high power density and efficiency, fuel cells hold tremendouspromise for future portable, automotive and stationary applications. Among fuel cell types, thestrongest candidate for portable applications is the direct methanol fuel cell (DMFC), while thehydrogen polymer electrolyte fuel cell (H2 PEFC) is the strongest candidate for stationary andtransportation applications. The minute length scales and complex materials in these fuel cellspresent many unique and challenging physicochemical issues. In particular, two-phase floweffects through the thin-film porous carbon fiber gas diffusion layers (GDLs) present a majormicrofluidic management bottleneck for achieving high power density and stability. The GDL isa critical component and functions to deliver reactant to, and product away from, the electrodeswhere electrochemical reaction occurs. If product transport through the GDL or flow channels isinsufficient, reactant mass transport will be restricted, limiting performance. For example, CO2gas production at the anode of the DMFC can block transport of liquid methanol solution to thereaction site. At the cathode of the DMFC and H2 PEFC, liquid water from electrochemicalreaction, electro-osmotic drag and diffusion can severely limit performance by flooding thecathode, starving it of oxygen. In both the DMFC and H2 PEFC, there is a complex two-phaseflow challenge to manage the transport of reactants and products through the thin-film porousGDL and in the flow channels. As a result, the naturally hydrophilic GDLs are typically tailoredby addition of hydrophobic material during processing. To date, the fraction of hydrophobicadditive is determined through inefficient trial-and-error testing. The existing literature has alsofollowed a phenomenological approach and has yet to yield any clear rationale or fundamentalknowledge of the basic transport processes of bubbles and liquid droplets through the GDL, at theinterfacial boundaries between the GDL and the flow channel, or in the flow channels.We propose to initiate an experimental and analytical study of the two-phase flow of gasphasebubbles and liquid-phase droplets in reactant flow channels and thin-film porous mediawith tailored wetting properties. We will use high-speed microfluidic flow visualizationtechniques and a model fuel cell to experimentally quantify parameters such as bubble/dropletshape, detachment size, transport rate, distribution, and liquid saturation in the porous material asa function of operating conditions. Analytical models will then be developed to describe thebubble/liquid droplet dynamics and validated using the novel data obtained from a highlyinstrumented fuel cell. The ultimate goal of this research is to provide, for the first time, designtheory based on fundamental understanding that can be used to engineer fuel cell materials andempower microfluidic management for next-generation fuel cell systems. This would represent asignificant leap in understanding of one of the most critical areas currently limiting fuel celldevelopment and would impact portable, stationary, and automotive fuel cell design.The objectives of our integrated approach to research and education are to: 1) create aphysical and intellectual infrastructure for addressing a multidisciplinary technical junction in thedesign of next-generation hydrogen or methanol polymer electrolyte fuel cells; 2) train graduateand undergraduate students in physical/chemical science fields highly relevant to current nationalneeds; 3) integrate cutting-edge research results directly into three established courses at PennState and enable supported graduate students to present material in the undergraduate courses; 4)expand the role for fuel cell and microfluidic research programs at PSU in undergraduate andgraduate recruiting process; and 5) provide significant opportunities for undergraduate/graduatestudents to interact with industry through existing and future Electrochemical Engine Centerindustry collaborations.