Visualization of Water in Fuel Cell Gas Diffusion Layers By Neutron Dark Field Imaging
Authors: Siegwart Muriel , Manzi-Orezzoli Victoria , Harti Ralph , Valsecchi Jacopo , Gruenzweig Christian , Schmidt Thomas J. , Boillat Pierre 
Published: February 2020

The distribution of water in operating polymer electrolyte fuel cells (PEFCs) has been and remains an important question, and many researchers in academy and industry have studied the topic over the last decade. Among the methods used to measure this distribution, neutron imaging played a significant role owing to its excellent transparency for fuel cell materials, requiring minimal compromises on the cell design, and its very good sensitivity to liquid water. In classical “through plane imaging” experiments (neutron beam perpendicular to the membrane plane), the contribution due to water in all layers of the cell are superposed, and only the sum of these contributions can be measured. Several approaches have been proposed to tackle this issue. “In plane imaging” (beam parallel to the membrane plane) allows distinguishing the different layers (1-3) and in particular the gas diffusion layers (GDLs), but set constraints on the fuel cell size in the beam direction, which is limited to 20-30 mm. Tomography has been occasionally reported, but it requires either very long total exposure times (4) or has a resolution too low to identify water in GDLs (5), letting aside the important experimental complication introduced by the necessity of rotating the cell. Finally, it is possible to use dedicated purge sequences to identify which portion of the water is contained in the anode and cathode flow fields (6).
Here, we present a new approach to specifically identify the presence of water in gas diffusion layers, with the cell in a classical through plane arrangement setting minimal constraints on the design. We base our analysis on neutron grating interferometry (nGI) and in particular on dark field imaging (DFI). The DFI contrast depends on the size of neutron scattering structures (7), with high contrast values in the size range from micrometers to tens of micrometers, well matching the sizes of fibers and pores in gas diffusion layers. In a first series of experiments, we have investigated the contrast obtained with dry GDLs as well as with GDLs imbibed with light and heavy water. The dry GDLs exhibit a strong contrast for the DFI method, and even material defects such as cracks are clearly visible (see Figure 1a). The measurements with light water resulted in a very slight contrast change compared to the dry material, which is consistent with the minimal change of coherent scattering length density between liquid water and air. Currently, the obtained contrast of a few percent can hardly be used to detect light water in the GDL in a reliable fashion, but future improvements in the experimental setups and/or data analysis may enable the reliable DFI imaging of light water. On the contrary, experiments with heavy water did show an important change of the DFI intensity between the dry and water imbibed structures (see Figure 1b). The important water thickness of the injection channel (1 mm), clearly visible in the transmission image, results in a nearly invisible change in the DFI signal. This indicates that DFI in combination with isotope exchange is perfectly suited for the imaging of water in GDLs with minimal perturbations from water accumulation in the flow channels.

References

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Caption

Figure 1 – Neutron DFI imaging of fuel cell materials. (a) Effect of structural defects. (b) Effect of imbibition with D2O.

Figure 1