Proton exchange membrane fuel cells (PEMFCs) are promising power sources for automotive use. Despite the numerous efforts made the recent years, water management still remains a major limiting factor to PEMFC performance and durability. Specifically, the performance of catalyst layers (CLs), where the electrochemical conversion takes place, is heavily affected by mass transport limitations. Understanding liquid-water transport phenomena inside catalyst layers is vital and must be achieved by a basic understanding of the wetting and morphological properties of the pore structure. This requires measuring and quantifying several properties including surface energy and internal contact angle.

In this work we present a method for estimation of the internal contact angle to water (and other different liquids) and the surface free energy of CLs with different ionomer/carbon ratios. The method combines the Washburn technique [1] and the Owens-Wendt model [2]. A similar approach was already used by other researchers to determine the wetting properties of fuel cell gas diffusion layers [3,4] but, to the best of our knowledge, such method was never used for catalyst layers.

The CLs (carbon-ionomer films) used in this study were fabricated by the 3M Company. They consisted of Vulcan™ XC-72 carbon dispersed in a 3M brand ionomer matrix. Being very thin, these films are not stiff enough to immerse into the liquids. Therefore, each sample was prepared by hot-pressing the carbon-ionomer film on both sides of an Ethylene Tetrafluoroethylene (ETFE) film (120 μm thick) with a surface of 10 mm x 5 mm (see Figure 1.a).

The internal contact angle was measured by a dynamic sorption experiment according to the Washburn method [1] using a Krüss tensiometer K100 (Krüss GmbH, Hamburg, Germany) operated at room temperature. A set of six wetting liquids was tested: n-hexane, acetone, methanol, water, Fluorinert™ FC-3283 (3M Company) and toluene. As shown in Figure 1.a, when the sample (10 mm x 5 mm) came into contact with the surface of the desired liquid, the liquid was drawn up as a result of capillary action. The increase in sample mass due to the added liquid weight was determined with respect to time during the measurement. Using the Washburn method [1], internal contact angle can be obtained from the slope of the linear region of the square of the mass against time, Figure 1.b. These contact angles measurements were used to extract both dispersive and polar component of the solid surface energy using the Owens–Wendt model [2-4]. Materials with different ionomer/carbon ratio were used to determine the influence of the ionomer on electrode water uptake and transport. The effect of platinum loading on water transport was also investigated.

Acknowledgement

The authors thank Andrew Haug of 3M Company for providing materials tested in this study.

References

[1] E.W. Washburn, Phys. Rev., 18(1921) 273–283.

[2] Owens and Wendt, J. Appl. Polym. Sci., 13, 1741 (1969)

[3] Wood et al., J. Electrochem. Soc., 157(2) B195-B206 (2010)

[4] Gurau et al., J. Power Sources, 160 (2006) 1156–1162