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 (PC(SL)) 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 PC(SL) 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 PC(SL) measurements showed a significant reduction in water wetting between samples with 0 and 5 wt.% Teflon 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 PC(SL) measurements showing that all of the wetproofing occurs in a narrow range of Teflon loadings. Results for PC(SL) measurements were fitted using a bundle-of-capillaries model. The apparent water intrusion contact angles fell between 130° and 133° in the rough Teflonated pore space (regardless of loading), whereas the apparent gas intrusion contact angles fell between 66° and 70° for the same materials.