Perfluorosulfonic acid (PFSA) polymer membranes have been widely studied in the context of their applications as ion exchange medium/solid electrolytes in proton exchange membrane (PEM) fuel cells and other redox applications since 1970s(1). Water management plays a significant role in the performance of polymer membrane electrolytes, especially in fuel cells, because the ionic conductance dramatically decreases at low relative humidity, while excess water may flood the flow channels and hinder the transport of reagents such as gases (2). The conductivity and water diffusion coefficient of membranes therefore are always of concern because of their function in electrochemical devices.

In this work, Li⁺, Na⁺, NH₄⁺ and TEA⁺ form PFSA membranes were prepared based on H-form 3M PFSA membrane. The water uptake, density and conductivity of these membranes were investigated at various hydration levels. It’s found that the hydrophilicity of cations determines the water uptake of membranes with different cations, showing a series of water uptake in the order of: H⁺ > Li⁺ > Na⁺ > NH₄⁺ > TEA⁺. As expected, H⁺ form membranes show the highest conductivity among all membrane forms due to the smallest charge carrier size and proton hopping transport mechanism. This mechanism may also contribute for NH₄⁺ transport, resulting in a comparably high conductivity of NH₄⁺ form membranes even though the latter take up much less water. Pulsed field gradient (PFG) Nuclear Magnetic Resonance (NMR) was exploited to observe the diffusion coefficient of water in the membranes. It was observed that hydrophilicity of cations and the hydrogen bonding interactions affect the water transport in the membranes. Based on density measurements, a partial molar volume analysis is carried out in this study. This parameter provides insight into the water and polymer interactions in various univalent PFSA membranes.

Acknowledgement.

We gretefully acknowledge the support of this work by the Office of Naval Research. We also appreciate the assistance of 3M company in providing 3M PFSA membrane samples.

References:

1. K. A. Mauritz and R. B. Moore, Chemical Reviews, 104, 4535 (2004).
2. T. A. Zawodzinski, C. Derouin, S. Radzinski, R. J. Sherman, V. T. Smith, T. E. Springer and S. Gottesfeld, J Electrochem Soc, 140, 1041 (1993).