Characterizing Water Transport in the Microporous Layers of Polymer Electrolyte Fuel Cells
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Date
2023
Publication Type
Doctoral Thesis
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Abstract
Microporous layers (MPLs) are an essential part of the gas diffusion layers (GDLs), which improve polymer electrolyte fuel cell (PEFC) performance, particularly at high current density and humid operating conditions. One of the ways to reach future PEFC performance and cost target is through better design and integration of this layer. To achieve this, understanding of the water transport in MPLs is important.
This thesis aims at obtaining insight to the morphology of and water transport in the MPLs by means of X-ray tomographic microscopy (XTM). The opening three chapters provide the general background of this thesis study. Chapter 1 lays out the motivation for this work. Chapter 2 gives a basic overview of PEFC thermodynamics, efficiency and major loss mechanisms. Chapter 3 summarizes the methods used in this thesis work, including XTM principles and fundamentals, illustration of the laboratory computed tomography (CT) and beamline XTM setups and the image processing steps.
First, MPL properties and water saturation within are characterized in an ex situ setup (Chapter 4 and Chapter 5). In Chapter 4, the use of polychromatic XTM is extended for the quantitative determination of water saturation in MPLs through establishing a calibration relation between grayscale values and linear attenuation coefficients. From there on, MPL properties and porosity are characterized based on laboratory CT imaging. The results show that the porosity distribution of MPLs is not homogeneous and therefore, for accurate determination of the spatial water saturation distribution, porosity distribution must be obtained first. In Chapter 5, fifteen commercially available MPL materials are characterized for their total porosity, the microporosity, the crack volume, thickness and porosity heterogeneity. Some of these properties (especially the porosity and its heterogeneity) were previously unknown. This provides a new insight to the properties of MPLs and reveals the diversity of commercial MPLs with respect to thickness, porosity and morphology, which cannot be summarized in a single morphological category as often done previously.
Operando water transport in MPLs and GDLs is characterized in Chapter 6 and Chapter 7. In Chapter 6, high porosity MPLs with different pore size distribution (PSD) are investigated, and the work is focused on the first minute fuel cell operation from dry state as this allows for easier characterization of the water transport modes. Data show that the addition of both kind of high porosity MPLs (with different PSD) resulted in enhanced vapor transport at the beginning of fuel cell operation. Therefore, it was hypothesized that the (low) thermal conductivity of MPLs is one of the important properties determining water transport mechanisms. In Chapter 7, water transport in MPLs of the same material but with different morphology (sheet and intruding) is investigated. Here, results show that an intruding MPL morphology leads to lower liquid water saturation in the GDL, especially the MPL/GDL mixed region. This is proposed to be the reason for the better performance of the PEFC with at highly humid and high current density conditions.
Chapter 8 summarizes the results and conclusions from this work and motivates further XTM studies on water transport in PEFCs, operated at even more realistic operating conditions with further developed MPLs and GDLs.
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ETH Zurich
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Polymer electrolyte fuel cell (PEFC); Polymer electrolyte membrane fuel cell; X-ray tomography; X-ray Tomographic Microscopy (XTM); Microporous layer; Water transport properties; Microporous layer porosity; Electrochemistry; X-ray imaging
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03910 - Schmidt, Thomas J. / Schmidt, Thomas J.