Perfluoro-sulfonic acid (PFSA) polymers are used as the electrolyte in most solid-state energy-conversion and storage devices including fuel cells, flow batteries, and solar-fuel generators. Of great importance is their role in electrodes or catalyst layers where there is a need for ion conduction and mechanical stability. Recently, we have shown that the properties of Nafion ionomer in catalyst layers is thought to be different than in bulk Nafion, causing high transport resistances. In addition, we have shown how confinement and substrate interactions can alter the morphology and thus transport properties of the ionomer when it exits as a thin-film, which is the case in catalyst layers. In this talk, we will revisit these issues by examining morphology of thin-films of Nafion and other lower equivalent-weight PFSA ionomers, on metallic substrates, mainly through GISAXS as shown in the Figure, where the GISAXS profiles suggest anisotropy and a possible alignment of domains with Nafion on a gold substrate. Also, the transport properties of thin-films will be examined with an aim of relating these observations with fuel-cell performance data and catalyst-layer-ionomer water-uptake data. In addition, we will also examine the impact of hygrothermal ageing on the structure and swelling of the thin-films to initially probe lifetime concerns. Such studies as mentioned above show that there is alignment and ordering of the domains after ageing and as a function of thickness, especially on a platinum substrate. Acknowledgement This work was funded by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Fuel Cell Technologies, of the U. S. Department of Energy under contract number DE-AC02-05CH11231. The work utilizes beamline 7.3.3 at the Advanced Light Source, which is funded by DOE Basic Energy Sciences. We also acknowledge helpful discussions with Steve Hamrock and Mike Yandrasits at 3M, Alex Hexemer at LBNL, and Doug Kushner and Mike Hickner at Penn State University.

Understanding ionomer thin films

249th National Meeting (2015)

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