Ionomer segregation in composite MEAs and its effect on polymer electrolyte fuel cell performance

Further characterization of the catalyst layer of membrane electrode assemblies (MEAs) prepared by the standard decal/hot-press-transfer method was carried out using atomic force microscopy and scanning electron microscopy coupled with energy-dispersive X-ray analysis. It was discovered that a thin dense skin, consisting primarily of Nation ionomer, formed on the outer surface of the catalyst layer of the completed MEA. The mechanism of the skin formation was attributed to the interaction between the dissolved ionomer in the catalyst ink solution and the hydrophobic Teflon decal surface during the decal fabrication process. The presence of a skin on the catalyst layer was found to diminish the performance of the fuel cell in the mass-transport region of the polarization curve. This suggested a slow rate of diffusion of oxygen through the skin. Removal of water formed in the cathode reaction would be expected to be slowed by the skin as well, leading to potential flooding of the cell at higher current densities. Modifications to the decal preparation procedure to prevent skin formation on the MEA catalyst layer include: use of a decal substrate with a less hydrophobic surface than Teflon, use of a single rather than multiple ink applications, and elimination of hydrophobic lubricants. The reduction/elimination of the skin results in significant improvement in performance in the high current region of the polarization Curve. (C) 2004 The Electrochemical Society.