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Understanding Ionomer-Related Transport and Morphology in Polymer-Electrolyte Fuel Cells

Speaker: Adam Weber, Lawrence Berkeley National Laboratory
Series: CBE Departmental Seminars
Location: Elgin Room (E-Quad A224)
Date/Time: Wednesday, October 3, 2012, 4:00 p.m. - 5:00 p.m.

Perfluorosulfonic-acid (PFSA) membranes are still considered to be the benchmark material for proton-exchange-membrane fuel cells. Their performance is controlled by their transport properties and sorption behavior, which are strongly correlated through physiochemical interactions. Furthermore, mechanical properties and membrane stability, which are important for fuel-cell durability, are also governed by the nanomorphology and chemical interactions. Thus, when the membrane is in equilibrium, there exists a balance between the chemical and mechanical energies. This balance can be affected by various internalities and externalities including compressive loads, annealing, humidity, temperature, thickness, etc.  In particular, it is of great interest to understand how these interactions occur within fuel-cell catalyst layers where the ionomer exists as nanometer-thickness films over the catalytic sites. 

In this talk, the ionomer’s structure/function relationships within the fuel-cell cathode catalyst layer will be discussed.  Of importance is how the water sorption decreases with film thickness, resulting in experimentally observable mass-transport limitations on fuel-cell performance.  The transport changes will be examined through morphology studies using grazing-incidence small-angle X-ray scattering (GISAXS) and transmission electron microscopy (TEM) of different ionomer film thicknesses on model hydrophilic and hydrophobic substrates.  It will be shown how the ionomer film becomes more resistive as film thickness decreases owing to a less-defined nanophase-separated morphology.  This structure and the film interactions with both the solid substrate and external environment (e.g., water vapor versus liquid water) results in much lower water sorption and hence higher transport resistance.  In addition, linkages between the ionomer thin films and the bulk membrane’s interfacial properties will be introduced.