The Relationship Between Segmental Motions, Hydration and Overall Mass Transport in Pfsas

A great many membranes have been studied for use in fuel cells and other electrochemical devices. Prominent amongst these are the perfluorosulfonic acids, including Nafion as an archetype and materials prepared by Dow (later others) and 3M presented over time.  Generally, similar mass transport (conductivity and water transport) phenomena have been observed, with high transport rates closely tied to high levels of hydration. However, maintaining high hydration of the membrane by maintaining an environment of high relative humidity requires system level measures that represent parasitic losses.

Achieving high conductivity with less hydration requires more substantial understanding and then control of the transport mechanisms involved at low water content.  There are clear differences in the behavior of materials at low and high hydration, with a line of demarcation at hydration levels of ca. 5 or 6 waters per sulfonate.  Above this level, the water in the membrane is more continuous across multiple pores and protons move in an environment that increasing resembles that in aqueous solutions as hydration levels increase.  Below this level, such factors as the acidity of the sulfonic acid, the mobility of water within the first or second hydration spheres of the sulfonate and proton and possibly sidechain movements in the polymer all have been suggested to play roles in the transport processes.

In this contribution, we will explore various processes in the low water content regime.  For a series of PFSAs, we will gather experimental information to reveal the approximate rate of side chain motion, use vibrational spectroscopy and related methods to provide an in-depth view into the hydration of ions and water interactions in the membrane and present information on the energetics of hydration.  These will be synthesized into an experimentally supported picture allowing us to assess the relative importance of the motions of anions, cations and water molecules in determining transport in PFSAs at RH < 50% under different conditions.