title: In Situ Method for Measuring the Mechanical Properties of Nafion Thin Films during Hydration Cycles.
abstract: Perfluorinated ionomers, in particular Nafion, are an essential component in hydrogen fuel cells, as both the proton exchange membrane and the binder within the catalyst layer. During normal operation of a hydrogen fuel cell, the ionomer will progressively swell and deswell in response to the changes in hydration, resulting in mechanical fatigue and ultimately failure over time. In this study, we have developed and implemented a cantilever bending technique in order to investigate the swelling-induced stresses in biaxially constrained Nafion thin films. When the deflection of a cantilever beam coated with a polymer film is monitored as it is exposed to varying humidity environments, the swelling induced stress-thickness product of the polymer film is measured. By combining the stress-thickness results with a measurement of the swelling strain as a function of humidity, as measured by quartz crystal microbalance (QCM) and X-ray reflectivity (XR), the swelling stress can be determined. An estimate of the Young's modulus of thin Nafion films as a function of relative humidity is obtained. The Young's modulus values indicate orientation of the ionic domains within the polymer films, which were confirmed by grazing incidence small-angle X-ray scattering (GISAXS). This study represents a measurement platform that can be expanded to incorporate novel ionomer systems and fuel cell components to mimic the stress state of a working hydrogen fuel cell.
authors.affiliation: †Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States.
authors.forename: Kirt A
authors.initials: KA
authors.lastname: Page
authors.affiliation: †Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States.
authors.forename: Jae Wook
authors.initials: JW
authors.lastname: Shin
authors.affiliation: ‡United Technologies Research Center, East Hartford, Connecticut 06108, United States.
authors.forename: Scott A
authors.initials: SA
authors.lastname: Eastman
authors.affiliation: †Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States.
authors.forename: Brandon W
authors.initials: BW
authors.lastname: Rowe
authors.affiliation: †Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States.
authors.forename: Sangcheol
authors.initials: S
authors.lastname: Kim
authors.affiliation: §Energy Storage and Distributed Resources Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
authors.forename: Ahmet
authors.initials: A
authors.lastname: Kusoglu
authors.affiliation: ∥Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States.
authors.forename: Kevin G
authors.initials: KG
authors.lastname: Yager
authors.affiliation: †Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States.
authors.forename: Gery R
authors.initials: GR
authors.lastname: Stafford
keywords: Nafion; curvature; humidity; modulus; stress; thin films