Fundamental study of two-phase flow in the porous media of polymer electrolyte membrane fuel cells (PEMFC)
Cho, Kyu Taek
M.S. (Master of Science)
Department of Mechanical Engineering
Polymer electrolyte membrane fuel cells (PEMFC) is an electrochemical device that converts the chemical energy of fuels into electrical energy through electrochemical reactions. Due to the zero emissions and high efficiency, PEMFC has been regarded as one of the most promising future energy sources. However, water management in the PEM fuel cell is a critical issue for the effective cell operation and full-scale application. Sufficient water must be maintained in the membrane to facilitate proton transfer, but if it exists in excess, the water will flood the catalyst layer. The amount of water generation is directly and entirely contingent upon operating conditions. When liquid droplets form, the pore paths for transporting reactant through a porous diffusion media as well as a catalyst layer may be blocked resulting in the degradation of the cell's performance. Therefore, it is important to understand the two-phase (i.e. mixture of liquid and gas) flow in the porous diffusion media to resolve issues due to the excess water in the cell. In this study, a physics-based one-dimensional isothermal mathematical model was developed to understand the effects of operating conditions (humidity conditions and current density) on the water distribution of the fuel cell. It was found that gas humidity conditions and operating current density has a significant impact on water distribution and should be considered during cell design. The transport of liquid water in equilibrium with water vapor and reactant gases in the porous diffusion media of the PEMFC was studied by implementing physics of two-phase flow in porous soil media into the model. The effects of surface properties (i.e. hydrophobicity and hydrophilicity), wetting characteristics, and pore structures (micro-pore and macro-pore) of porous diffusion media on liquid water transport were explored. It is evident that two-phase flow should be considered after reaching threshold operating conditions and hydrophobic diffusion media with larger contact angles enhances residual water removal from the cell. Composite pore structures (multi-diffusion layer) enhance water transport and reduce liquid saturation at catalyst layer. Since the liquid water formation depends on saturation pressure, which is a strong function of temperature, a thermal analysis model was studied to analyze temperature variation during operation. The effects of operating current densities and thermal conductivities of porous diffusion media on the temperature variation were investigated. The additional liquid water transport originated from temperature variation in the cell, known as phase change induced (PCI) flow, were introduced in the two-phase flow model.
Marma, Kyamra, "Fundamental study of two-phase flow in the porous media of polymer electrolyte membrane fuel cells (PEMFC)" (2017). Graduate Research Theses & Dissertations. 3224.
ix, 64 pages
Northern Illinois University
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Advisors: Kyu Taek Cho.||Committee members: Sun Ung Kim; Iman Salehinia; John Shelton.||Includes bibliographical references.||Includes illustrations.