Interaction of pore size and hydrophobicity/hydrophilicity for improved oxygen and water transport through microporous layers

Microporous layers consisting of different ratios of acetylene black and carbon fibers with either a hydrophobic polytetrafluoroethylene (PTFE) or a hydrophilic perfluorosulfonic acid (PFSA) ionomer binder are investigated with regards to oxygen and water transport in PEMFCs. For that, the materials are characterized by scanning electron microscopy and mercury porosimetry, revealing an increase of porosity and pore sizes for an increasing carbon fiber content. MPLs, coated onto a commercial hydrophobized non-woven gas diffusion layer substrate, are examined in H₂/air fuel cell tests under differential-flow conditions at various dry and humid operating conditions. For both hydrophobic and hydrophilic MPLs in the presence of significant amounts of liquid water in the diffusion layer substrate, the materials with larger pore sizes, i.e. higher carbon fiber contents, perform superior at 0.6 V and show the lowest oxygen transport resistance. However, at the same carbon composition, hydrophilic MPLs have a lower performance compared to the corresponding hydrophobic MPLs, which is explained by the capillary pressure barriers for different pore properties. At operating conditions relevant for automotive applications, a performance enhancement of 48% could be achieved for a purely carbon fiber based MPL compared to a commercial reference.