Liquid-Water Distribution in Compressed Gas-Diffusion Layers Using X-Ray Computed Tomography

Effective liquid-water management is essential for commercializing polymer-electrolyte fuel cells (PEFCs). At lower operating temperatures, there is a need to remove liquid water from the cathode electrode to ensure optimal reactant transport¹. To achieve maximum water permeation, it is necessary to optimize the gas-diffusion layers (GDLs). Currently, there is a lack of understanding of water transport, especially under in-situ conditions wherein the GDLs are selectively compressed under lands to provide good electrical conductivity but remain uncompressed under the gas flow channels. In this study, we use synchrotron X-ray micro computed tomography (CT) to study the water distribution in commercially available GDL materials under various levels of compression. We correlate the GDL’s morphological properties such as local spatially-resolved porosity and void-size distribution to local liquid-water distribution and water-transport capability.

Water imbibition through the GDL driven by a static water column was imaged in a sample holder with a grooved stomp. Controlled levels of GDL compression were achieved by tuning an ultra-fine pitch threaded bolt on top of the stomp. Liquid-water profiles were resolved as local averages in in-plane and through-plane directions. Figure 1 shows local liquid-water distributions for three GDLs under different levels of compression as local averages through the GDL depth. Narrowing of the liquid front was observed with increasing compression (decreasing thickness), where most of the liquid is transported under the channel. It appears that at lower compressions the advance of the liquid front is uniform; this is due to uniform porosity and due to availability of void space at the GDL|injection plate interface that allows liquid-water redistribution. At high compressions, porosity and liquid saturation are directly correlated, something that was not observed for the uncompressed sample. This finding is relevant for the design of GDL architectures with modulated porosity, where liquid-water removal can be directed through controlled porosity levels.

Figure 1 Average YZ liquid saturation as a function of X position for three compressions at capillary pressure of 1.4 kPa Acknowledgement

This work was funded by Assistant Secretary for Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, of the U. S. Department of Energy under contract number DE-AC02-05CH11231. This work made use of facilities at the Advanced Light Source (ALS), supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy.

References

1. A. Steinbach, “High Performance, Durable, Low Cost Membrane Electrode Assemblies for Transportation Applications”, DOE Annual Merit Review (2014).