With the potential for high power density and efficiency, fuel cells hold tremendous promise for future portable, automotive and stationary applications. Among fuel cell types, the strongest candidate for portable applications is the direct methanol fuel cell (DMFC), while the hydrogen polymer electrolyte fuel cell (H2 PEFC) is the strongest candidate for stationary and transportation applications. The minute length scales and complex materials in these fuel cells present many unique and challenging physicochemical issues. In particular, two-phase flow effects through the thin-film porous carbon fiber gas diffusion layers (GDLs) present a major microfluidic management bottleneck for achieving high power density and stability. The GDL is a critical component and functions to deliver reactant to, and product away from, the electrodes where electrochemical reaction occurs. If product transport through the GDL or flow channels is insufficient, reactant mass transport will be restricted, limiting performance. For example, CO2 gas production at the anode of the DMFC can block transport of liquid methanol solution to the reaction site. At the cathode of the DMFC and H2 PEFC, liquid water from electrochemical reaction, electro-osmotic drag and diffusion can severely limit performance by flooding the cathode, starving it of oxygen. In both the DMFC and H2 PEFC, there is a complex two-phase flow challenge to manage the transport of reactants and products through the thin-film porous GDL and in the flow channels. As a result, the naturally hydrophilic GDLs are typically tailored by addition of hydrophobic material during processing. To date, the fraction of hydrophobic additive is determined through inefficient trial-and-error testing. The existing literature has also followed a phenomenological approach and has yet to yield any clear rationale or fundamental knowledge of the basic transport processes of bubbles and liquid droplets through the GDL, at the interfacial 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 gasphase bubbles and liquid-phase droplets in reactant flow channels and thin-film porous media with tailored wetting properties. We will use high-speed microfluidic flow visualization techniques and a model fuel cell to experimentally quantify parameters such as bubble/droplet shape, detachment size, transport rate, distribution, and liquid saturation in the porous material as a function of operating conditions. Analytical models will then be developed to describe the bubble/liquid droplet dynamics and validated using the novel data obtained from a highly instrumented fuel cell. The ultimate goal of this research is to provide, for the first time, design theory based on fundamental understanding that can be used to engineer fuel cell materials and empower microfluidic management for next-generation fuel cell systems. This would represent a significant leap in understanding of one of the most critical areas currently limiting fuel cell development and would impact portable, stationary, and automotive fuel cell design.
The objectives of our integrated approach to research and education are to: 1) create a physical and intellectual infrastructure for addressing a multidisciplinary technical junction in the design of next-generation hydrogen or methanol polymer electrolyte fuel cells; 2) train graduate and undergraduate students in physical/chemical science fields highly relevant to current national needs; 3) integrate cutting-edge research results directly into three established courses at Penn State 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 and graduate recruiting process; and 5) provide significant opportunities for undergraduate/graduate students to interact with industry through existing and future Electrochemical Engine Center industry collaborations.
