Effective catalyst layer liquid permeability

The gas diffusion layers, one next to the anode and the other next to the cathode, are usually made of a porous carbon paper or carbon cloth, typically 100 pm to 300 pm thick. Fig. 14 shows a porous GDL made of carbon paper, which is partially covered by catalyst layer. The porous nature of the backing layer ensures effective diffusion of feed and product components to and from the electrode on the MEA. The correct balance of hydrophobicity in the backing material, obtained by PTFE treatment, allows the appropriate amount of water vapor to reach the MEA, keeping the membrane humidified while allowing the liquid water produced at the cathode to leave the cell. The permeability of oxygen in the GDL affects the limiting current density of ORR, and thus the performance of PEMFC.[ l... 

The work of Adachi et al. (2009) represented a first attempt to correlate and validate ex situ and in situ water permeation phenomena in PEMs. Water permeabilities of Nafion PEMs and water transport in operating PEFCs were investigated under comparable ex situ and in situ values of temperature and RH. The examined parameters included the type of driving forces (RH, pressure), the phases of water at PEM interfaces, PEM thickness, and the effect of catalyst layers at the membrane interfaces. Several experimental setups and schemes were designed and explored. Water permeability at 70°C was determined for Nafion membranes exposed to either liquid or vapor phases of water. Chemical potential gradients of water across the membrane are controlled through the use of differences in RH (38-100%), in the case of contact with water vapor, and hydraulic pressure (0-1.2 atm), in the case of contact with liquid water. Three types of water permeation experiments were performed, labeled as vapor-vapor permeation (VVP), liquid-vapor permeation (LVP), and liquid-liquid permeation (LLP). Ex situ measurements revealed that the flux of water is largest... 

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