A hierarchy of material structures and molecular interactions influence bulk transport in ionomer membranes used in applications such as reverse-osmosis, fuel cells, mechanical actuators, and batteries. Mobile species within these membranes (e.g., ions, water, alcohols) move according to local chemical interactions as well as morphological features (e.g., ion channels). The fundamental processes of ion transport occur on the scale of several molecules (< 1 nm), which is a lengthscale that is experimentally difficult to access. Here, we show that the Arrhenius activation energy of diffusion (Ea) provides insight into the fundamental nature of ion transport on the < 1 nm scale. Ea is obtained through temperature-dependent pulsed-field-gradient (PFG) NMR, allowing for chemically specific measurements on intact, hydrated membranes. We have explored the activation energy of diffusion in the perfluorosulfonate ionomer (PFSI) Nafion� as a function of hydration level (see figure), allowing us to draw conclusions about the local interactions that govern ion transport. We also find quantitative parallels between experimental activation energies for the ionomer and hydrated triflic acid (CF3SO3H), thus emphasizing the importance of the polymer ion moiety on transport. We will further discuss the physical significance of Ea and the pre-exponential factor D0 in liquid and soft solid systems.

Unraveling the local energetics of molecular transport in a polymeric ion conductor

245th National Meeting (2013)

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