Characterization of Liquid Water Invasion in Gdls Using X-Ray Tomographic Microscopy

The accumulation of liquid water in the gas diffusion layer (GDL) of low temperature polymer electrolyte fuel cells (PEFC) results in increased mass transport limitations [1] and eventually in a significant potential and efficiency penalty. To achieve efficient water removal and maintain high performance, it is necessary to understand the morphological (porosity, pore and throat sizes) and chemical (surface tension) properties of the GDL, which control the water transport [2].

An ex-situ capillary pressure (p_c) – saturation setup was developed in the framework of the IMPALA project [3] for imaging experiments using X-ray tomographic microscopy (XTM) at the Swiss Light Source synchrotron facility as it offers the advantage of well defined capillary pressure boundary conditions. This approach allows a quantitative analysis of the water transport through the GDL by linking the local saturation at the achieved p_c levels with the microstructure of the invaded pores and throats.

Within the setup, a hydrophilic membrane implements a homogeneous boundary condition at the bottom of the GDL. At the top of the GDL either a hydrophobic membrane or a flow field was placed. While the hydrophobic membrane setup allows to study complete liquid wetting of the pore space, the flow field structure allows to implement an anisotropic compression simulating PEFC conditions and to study its consequences on water transport.

GDL samples of different suppliers have been tested. The breakthrough pressure, which corresponds to the capillary pressure when water reaches the opposite side of the injection surface, is a function of the materials. Correlation with the dry GDL microstructure shows that pore (and throat) size is the major controlling parameter. SGL 24BA shows a lower breakthrough pressure than Toray TGP-H060 due to larger throats and exhibits a stronger channel rib deviation of pore and throat sizes as well as of saturation.

References

[1] T. Rosen et al., Journal of the Electrochemical Society, 159 (2012) F536-F544.

[2] R. Flückiger et al., Electrochemica Acta, 56 (2011) 2254-2262.

[3] “IMprove Pemfc with Advanced water management and gas diffusion Layers for Automotive application” within the Fuel Cells and Hydrogen Joint Undertaking (FCHJU) initiative of the European Union. http://www.impala-project.eu

Figure 1: Capillary pressure saturation curves of Toray TGP-H060 and SGL 24 BA measured with the hydrophobic membrane setup (left) and the flow field setup (right). Compared to Toray TGP-H060 SGL 24BA shows a lower breakthrough pressure due to larger throats and exhibits a stronger channel rib deviation in saturation, as well as pore and throat sizes.