A first comparison is made of the heterogeneous porosity distributions of various polymer electrolyte membrane fuel cell ͑PEMFC͒ gas diffusion layer ͑GDL͒ materials. Microscale computed tomography imaging is performed for three main categories of commercially available GDL materials: carbon fiber paper, felt, and cloth. The methodology to analyze the through-plane and in-plane porosities of various materials is presented, and the relationship between heterogeneous porosity distributions and manufacturing techniques is discussed. This work also provides insight into the manufacturing process employed for GDLs.

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This is the first investigation of the liquid water saturation profile dependence on empirically determined heterogeneous polymer electrolyte membrane fuel cell ͑PEMFC͒ gas diffusion layer ͑GDL͒ porosity distributions. An unstructured, two-dimensional pore network model using an invasion percolation algorithm is presented. Random fiber placements are based on the heterogeneous porosity distributions of six commercially available GDL materials recently obtained through X-ray-computed tomography visualizations. The pore space is characterized with a Voronoi diagram, and simulations are performed with a single inlet liquid water cluster. Saturation profiles are also computed for GDLs with uniform, sinusoidal, and square-wave porosity distributions. Liquid water tends to accumulate in regions of high porosity due to the associated lower capillary pressures. The results of this work suggest that GDLs tailored to have smooth porosity distributions have fewer pockets of high saturation levels within the bulk of the material. Finally, a study on theoretical surface modifications demonstrates that low porosity surface treatments at the catalyst layer͉GDL interface result in greatly reduced overall saturation levels of the material.To operate polymer electrolyte membrane fuel cells ͑PEMFCs͒ over a wide range of operating conditions, liquid water within the fuel cell must be actively or passively managed, maximizing reactant diffusion pathways and preventing premature mass transport limitations. 1 Water is introduced into the cathode of the PEMFC at the catalyst layer ͑CL͒ both as a product of the electrochemical reaction and via electro-osmotic drag through the polymer electrolyte membrane. Water is also introduced at the flow field ͑FF͒ inlet from humidified oxidant streams. In the studies of Basu et al. 2 and Yamada et al., 3 saturated relative humidity levels have been predicted at the gas diffusion layer ͑GDL͉͒CL boundary, indicating that liquid water streams within the cathode may originate near the GDL͉CL boundary under specific operating conditions. Pore network models have been employed to describe the liquid water saturation patterns generated from the invasion of hydrophobic, porous materials, 4-14 where heterogeneity is often provided by randomizing pore and throat radii. Two-phase flow for GDL materials has recently been modeled within three-dimensional pore spaces found from either microcomputed tomography 15 or stochastic geometry generation. [15][16][17] The modeling techniques employed were the Lattice-Boltzmann method 16,17 and pore morphology modeling. 15,17 While pore space heterogeneity is obtained intrinsically with these methods, the detailed, three-dimensional images are required for all simulations and transport calculations, which can result in high computational costs. More recently, a topologically equivalent pore network model by Luo et al. 18 has been developed for three-dimensional stochastic models of GDL materials that applies the maximal ball method 19 to reduce the pore space into a pore n...

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Synchrotron X-ray radiography, due to its high temporal and spatial resolutions, provides a valuable means for understanding the in operando water transport behaviour in polymer electrolyte membrane fuel cells. The purpose of this study is to address the specific artefact of imaging sample movement, which poses a significant challenge to synchrotron-based imaging for fuel cell diagnostics. Specifically, the impact of the micrometer-scale movement of the sample was determined, and a correction methodology was developed. At a photon energy level of 20 keV, a maximum movement of 7.5 µm resulted in a false water thickness of 0.93 cm (9% higher than the maximum amount of water that the experimental apparatus could physically contain). This artefact was corrected by image translations based on the relationship between the false water thickness value and the distance moved by the sample. The implementation of this correction method led to a significant reduction in false water thickness (to ∼0.04 cm). Furthermore, to account for inaccuracies in pixel intensities due to the scattering effect and higher harmonics, a calibration technique was introduced for the liquid water X-ray attenuation coefficient, which was found to be 0.657 ± 0.023 cm(-1) at 20 keV. The work presented in this paper provides valuable tools for artefact compensation and accuracy improvements for dynamic synchrotron X-ray imaging of fuel cells.

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Synchrotron X-ray radiography on beamline 05B1-1 at the Canadian Light Source Inc. was employed to study dynamic liquid water transport in the porous electrode materials of polymer electrolyte membrane fuel cells. Dynamic liquid water distributions were quantified for each radiograph in a sequence, and non-physical liquid water measurements were obtained. It was determined that the position of the beam oscillated vertically with an amplitude of ~25 µm at the sample and a frequency of ~50 mHz. In addition, the mean beam position moved linearly in the vertical direction at a rate of 0.74 µm s(-1). No evidence of horizontal oscillations was detected. In this work a technique is presented to account for the temporal and spatial dependence of synchrotron beam intensity, which resulted in a significant reduction in false water thickness. This work provides valuable insight into the treatment of radiographic time-series for capturing dynamic processes from synchrotron radiation.