Characterization of the Nafion Fuel Cell Membrane Via Observation of Surface Wetting Properties

Sharonmoyee Goswami, Chemical Engineering, Princeton University, Princeton, NJ 08544, Shannon Klaus, Chemical Engineering, University of Missouri, Columbia, MO 65201, and Jay B. Benziger, Department of Chemical Engineering, Princeton University, Princeton, NJ 08544.

A primary difficulty in using fuel cells as a viable energy alternative is that the Nafion membrane has dynamic physical and mechanical properties that depend upon hydration. Nafion undergoes microphase separation due to energy differences between the hydrophobic backbone and the hydrophilic sulfonic acid side chains. We examine here the effect of microphase separation on the surface wetting of Nafion and Nafion composite films. A combination of Wilhelmy plate and sessile drop measurements were carried out with Nafion membranes of varying thickness (0.002 to 0.010), amount of composite material (0,3,and 10% TiO2), and pre-strain levels. These sessile drops on Nafion membranes were monitored through time-lapse photography to observe physical changes at the Nafion-water interface.

Wilhelmy plats measurements showed the Nafion was hydrophobic (contact angle of ~110) in the descending mode but was hydrophilic (contact angle of ~0) in the ascending mode. Sessile drops were hydrophobic on Nafion, but remained pinned in position when the drop evaporated, indicating the Nafion surface under the drop must be hydrophilic. Water droplets on the membrane immediately induced local swelling; the maximum state of protrusion increased with membrane thickness, and drop size. After complete liquid evaporation, the membrane reverted to its pre-wetted state.

Wetting of Nafion was also examined with octane and ethanol. Octane wet the Nafion surface with a contact angle ~0, but the octane did not cause any measurable deformation of the Nafion. Ethanol induced the most dramatic response; it resulted in enormous film deformation.

Nafion deformation due to sessile drops of water did not show any significant effect due to pre-straining, or adding 3% and 10% TiO2 nanoparticles to the membrane.

The results suggest that Nafion surfaces can be switched from hydrophobic to hydrophilic due to exposure to hydrogen bonding liquids: water and ethanol. Swelling properties originate from the solvent penetration into the membrane causing a differential expansion of the membrane in contact with the liquid. Even though octane wets the Nafion surface it does not appear to penetrate into the membrane suggesting that the solvent interactions are with the hydrophilic sulfonic acid domains.