The effects of wetproofing on the capillary properties of proton exchange membrane fuel cell gas diffusion layers

In the analysis of proton exchange membrane (PEM) fuel cell components, the capillary pressure vs. saturation (P-C(S-L)) curve is an increasingly popular tool for understanding the interaction of liquid water with the porous gas diffusion layer(GDL) material. In this study, hysteretic water/air P-C(S-L) measurements were combined with mercury intrusion porosimetry (MIP) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) imaging to quantify the effects of fluoropolymer loading on GDL samples. Commercially wetproofed carbon fiber papers with 0-40 wt.% Teflon loading were investigated. MIP showed a slight reduction in characteristic pore radii and a significant loss of pore volume at the highest Teflon loading. Water/air P-C(S-L) measurements showed a significant reduction in water wetting between samples with 0 and 5 wt.% Tenon loading, but negligible additional wetproofing at loadings from 10 to 40 wt.%. ToF-SIMS imaging, a technique that is sensitive to monolayer surfaces coverages, found that GDL materials with 5 wt.% Teflon loading displayed nearly complete fluoropolymer coverage on the carbon substrate, confirming P-C(S-L) measurements showing that all of the wetproofing occurs in a narrow range of Teflon loadings. Results for P-C(S-L) measurements were fitted using a bundle-of-capillaries model. The apparent water intrusion contact angles fell between 130 degrees and 133 degrees in the rough Teflonated pore space (regardless of loading), whereas the apparent gas intrusion contact angles fell between 66 degrees and 70 degrees for the same materials. (C) 2009 Elsevier B.V. All rights reserved.