Gelation Behavior of Nafion Dispersion

Perfluorinated Nafion possesses unique properties that are valuable for electrochemical devices.¹ Nafion is industrial standard material for the proton exchange membrane and electrode binder of proton exchange membrane fuel cells (PEMFCs). The solution processing techniques for Nafion have been developed since the mid-1980s and currently several versions of solution-cast Nafion membranes and liquid dispersions are commercially available. For solution processing techniques, understanding Nafion particle morphology in liquid media^2,3 and the gelation process are of particular interest. Recently small angle neutron scattering (SANS) and ¹⁹F NMR studies found that the Nafion in dilute solvent systems can have three distinctive morphology, depending on the dispersing solvent⁴: i) a well-defined cylindrical dispersion in polyhydric alcohols ii) a random-coil conformation in aprotic solvents and iii) a less-defined highly solvated large particle in water-monohydric alcohol mixtures. In this presentation, the gelation behavior of Nafion from dilute dispersions and their impact on fuel cell membrane and electrode properties are investigated. 

Three types of gelation for Nafion dispersion were observed, depending on the dispersion solvent. For Nafion dispersion in polyhydric alcohols such as ethylene glycol or glycerol, inhomogeneous thermal irreversible gelation was observed. In this process, thermal irreversible gel with remaining concentrated liquid is formed during the solvent evaporation. The formed gel was not transformed back to the sol phase upon temperature increase or dilution. For Nafion dispersion in aprotic solvents, thermal reversible homogeneous gelation was observed. In this gelation process, an instant gelation occurred at a certain polymer concentration. At this concentration, the sol-gel transition occurred reversibly with temperature change. For Nafion dispersion in water/monohydric alcohols, Nafion was precipitated during the solvent evaporation process. The white fine string/flake precipitates were aggregated in the bottom of the vial and solidified with further solvent evaporation. 

The gelation process of Nafion strongly impacts the properties of solution-cast membrane and electrodes. The tensile toughness of Nafion changed from 0.001 to 20 MPa (over 4 orders of magnitude difference), depending on gelation type. Nafion membranes cast from homogeneous gelation exhibited the highest mechanical properties while Nafion membranes cast from precipitation process exhibited an extreme brittle nature. The electrode durability during potential cycling also strongly depends on gelation type. The electrode durability prepared from inhomogeneous thermal irreversible gelation exhibited the best durability. Microelectrode experiments indicated that the Nafion ionomers cast from irreversible gelation have more resistance to the polymer deformation in the presence of water, that allowed maintaining the electrode structure during the potential cycling experiments. This presentation explains further aspects of the importance of gelation behavior. 

Acknowledgment

U.S. Department of Energy Fuel Cell Technologies Program (Technology Development Manager: Dr. Nancy Garland) funded this research.

References

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