Speaker
Description
One of the key issues related to long-term polymer electrolyte membrane (PEM) fuel cell durability is water management within the porous gas diffusion layer (GDL). When too much water accumulates within the GDL, the flow of the gases to the catalyst layer becomes restricted, which can lead to the degradation of key cell components or even result in total cell failure. Therefore, understanding the water transport phenomena within the porous GDL is a key component to optimal fuel cell design. In this work, the characterization and visualization of water transport within the GDL of a PEM fuel cell is investigated from macro (e.g. fuel cell performance) and micro (e.g. pore scale) perspectives. Results from these experimental investigations are integrated into a computational fluid dynamics (CFD) model of water vapor transport across the membrane electrode assembly (MEA). At the macroscale, a novel technique is used to determine the effective diffusivity by experimentally measuring water transfer rates across an electrochemically active PEMFC. This technique expands upon a previously published anode water removal (AWR) diagnostic protocol, which functions to remove excess cathode GDL water. This protocol allows for the introduction of an evaporative gradient which drives water transport across the MEA into an initially dry anode stream. The strength of this gradient is controlled by the flow rate of the anode stream. In this work, the resulting relative humidity of the anode stream is determined through analysis of the anode pressure drop, allowing for the quantification of the water flux. This approach allows for a combined analysis of fuel cell performance and net water vapor transport. Through analysis of the water vapor transport rates and water concentrations of the anode and cathode streams, overall effective diffusivities are calculated during in-situ operation. However, at the macroscale the fuel cell components are opaque, making it difficult to resolve the water flooding issues locally. The lack of microscale data in the porous GDL hinders optimized operating conditions and material designs. To address this issue, synchrotron radiography is utilized at the Canadian Light Source to investigate and visualize GDL de-saturation at the micro scale. The high x-ray flux provided by synchrotron radiography allows for rapid x-ray computed tomography scans, which are used to visualize the transient de-saturation process. This ex-situ procedure mimics the desaturation that occurs in the macroscale tests. Two different GDL samples are investigated to determine the contributions of convection and evaporation, specific water removal pathways, and overall de-saturation rates. Results show that the major de-saturation mechanism is evaporation, but that convection also plays a minor role. Results agree with previous studies showing that water is removed preferentially under the channels, and then from underneath the ribs. An initial CFD model is developed based on the experimental fuel cell architecture to verify water transfer rates and effective diffusivities obtained in the macro study.
References
1) Battrell, L., A. Trunkle, E. Eggleton, L. Zhang, and R. Anderson, Investigation of Water Transport Within a Proton Exchange Membrane Fuel Cell by Diffusion Layer Saturation Analysis. ASME. International Conference on Fuel Cell Science, Engineering and Technology, ASME 2016 14th International Conference on Fuel Cell Science, Engineering and Technology ():V001T05A002. doi:10.1115/FUELCELL2016-59408.
2) Battrell, L., A. Trunkle, E. Eggleton, L. Zhang, and R. Anderson, Quantifying Cathode Water Transport via Anode Relative Humidity Measurements in a Polymer Electrolyte Membrane Fuel Cell. Energies, 2017. 10(8): p. 1222. doi:10.3390/en10081222
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