Abstract
Visualizing the water distribution in porous gas diffusion layers (GDLs) of operating polymer electrolyte fuel cells (PEFCs) is indispensable to understand the impact of water management on performance. For this purpose, neutron and X-ray transmission imaging have been used for nearly two decades. Certain limitations inherent to attenuation based imaging methods can be overcome by applying neutron dark-field imaging, which has the ability to selectively visualize structures in the micrometer size range. In this study, we compare dark-field images and transmission images of GDLs filled with water through an injection channel. The high contrast of the dark-field value between a heavy water filled and a dry GDL is suitable to reveal water distribution patterns in the GDL. The water present in the 1 mm wide water injection channel of the test device does not alter the dark-field signal, as this technique is selectively sensitive to microstructures. Therefore, neutron dark-field imaging can be applied for the selective analysis of the water distribution in the GDL overlapping with channel water. In addition to the selective visualization of water distributed in a GDL, we show that neutron dark-field imaging can also be used to visualize GDL damages.
Highlights
For almost two decades, neutron and X-ray imaging have been applied for visualizing the liquid water distribution within polymer electrolyte fuel cells (PEFCs)
The mean DF-value is between 0.2 and 0.3 for all samples, and - as expected based on the considerations made in the theoretical background section - it differs slightly for different type of gas diffusion layers (GDLs) and for different coating loads
The DF-value differs for different GDL types and coating loads but it is for all samples in a range where it is possible to perform experiments as dark-field contrast is present, but the signal does not saturate due to too much contrast
Summary
Neutron and X-ray imaging have been applied for visualizing the liquid water distribution within PEFCs. Experiments performed with the beam perpendicular to the cell membrane reveal the water thickness as an integral over all fuel cell components, but does not allow to distinguish water in individual layers.[13] In order to overcome these limitations, the cell membrane can be placed parallel to the beam and the water thickness obtained in this configuration is an integral of the water in the direction along the flow field channels This imaging configuration allows for example to visualize the difference in water distribution in different areas of the GDL (i.e. channel and rib area).[12,23,24,25] to avoid saturation of the signal due to large water thicknesses, the width of the cell is limited to 20–30 mm in this configuration. A fuel cell fed with D2 shows comparable performance to a fuel cell fed with H2 and heavy water is suitable to represent light water in fuel cell experiments
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