Characterization of electrosprayed Nafion films

Nafion has long served as a benchmark material when evaluating the performance of proton conducting membranes used in PEM fuel cells. Traditionally, the membranes are either extruded from dry polymer or cast from a liquid polymer solvent solution. Recently, a different technique for fabricating membranes has been developed. The new approach exploits electrostatic spraying, or electrospraying, to deposit PEM films, with the longer-term goal of fabricating membrane-electrode assemblies using this technique. The focus of this paper is to compare the proton transport and physical properties of electrosprayed membranes with those of extruded Nafion. Audio frequency complex impedance studies of the electrosprayed membranes were conducted at a variety of temperatures and pressures over a wide range of membrane water contents. The results are compared with similar data for Nafion 117 and for membranes cast from the electrosprayed solutions. Water uptake, dimensional changes, and electrical conductivity measurements indicate that extruded, cast, and electrosprayed Nafion films are similar, with the exception that the electrosprayed Nafion absorbs as much as 15 wt.% water more than the other two membranes with only a slight increase in conductivity. The activation volumes for electrosprayed Nafion are also consistent with those for Nafion 117 and concur with proposed mechanisms for proton transport in Nafion membranes. As in Nafion 117, a dielectric loss peak appeared in the electrosprayed Nafion at low temperatures after heating in vacuum at 380 K, further demonstrating the similarity between the two materials.