Multi-Phase Flow Dynamics in Polymer Electrolyte Fuel Cells

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、汽车和固定应用中具有巨大的前景。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)，而氢聚合物电解质燃料电池(H2 PEFC)是最适合固定和交通应用的候选燃料电池。这些燃料电池的微小长度尺度和复杂的材料提出了许多独特的和具有挑战性的物理化学问题。特别是，两相流在薄膜多孔碳纤维气体扩散层中的流动是实现高功率密度和稳定性的微流体管理的主要瓶颈。GDL是一种关键成分，其功能是将反应物输送到发生电化学反应的电极上，并将产物从电极上送出。如果产品通过GDL或流动通道的传输不充分，反应物的质量传输将受到限制，从而限制性能。例如，DMFC阳极产生的CO2气体会阻碍液态甲醇溶液向反应现场的输送。在DMFC和H2 PEFC的阴极上，来自电化学反应、电渗透阻力和扩散的液态水会淹没阴极，使其缺乏氧气，从而严重限制性能。在DMFC和H2 PEFC中，都存在一个复杂的两相流动挑战，即如何管理反应物和产品通过薄膜孔道和流动通道的传输。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。到目前为止，疏水添加剂的含量是通过低效的反复试验来确定的。现有的文献也沿用了唯象的方法，还没有给出任何关于气泡和液滴在气液界面、气液界面和流道之间或在流道中的基本传输过程的明确的理论基础或基础知识。我们建议开始对气泡和液滴在反应物流道和具有定制润湿特性的薄膜多孔介质中的两相流动进行实验和分析研究。我们将使用高速微流控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、脱离尺寸、传输速率、分布和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，并使用从高仪表化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认识的设计理论，可用于设计燃料电池材料，并为下一代燃料电池系统提供微流体管理。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。我们研究和教育综合方法的目标是：1)为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交汇点创造基础设施；2)培养与当前国家需求高度相关的物理/化学科学领域的毕业生和本科生；3)将尖端研究成果直接整合到宾夕法尼亚州立大学的三门现有课程中，并使受支持的研究生能够在本科课程中展示材料；4)扩大PSU燃料电池和微流体研究项目在本科生和研究生招生过程中的作用；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.