UC Santa Barbara

Nafion represents the most commonly employed and well characterized proton exchange membrane (PEM) used in fuel cells, however structural models which explain its physical properties are oversimplified and incomplete. In this work we use a combination of tapping mode and conductive probe (cp) AFM to investigate the nanoscale morphology and proton conductivity of Nafion. By conducting imaging at a wide range of relative humidity, we were able to characterize Nafion from extensively dehydrated to liquid equilibrated states. Rather than an even swelling of hydrophilic clusters, we see an uneven dehydration of the surface at low hydration and a rearrangement to form a network of cylindrical inverted micelles at high water content. A statistical analysis of these features allowed us to match them with previous x-ray scattering data and develop a model of Nafion which is valid at large length scales and consistent over a wide range of water contents.

Comparison of this model system to new membrane materials has allowed us to better understand the structure-property relationship in these systems. We next investigated Hyflon, a short side chain analog of Nafion and found that in contrast to Nafion it is able to swell reversibly through a dilation and contraction of hydrophilic clusters with minimal rearrangement and retain more hydrophilic character at the surface at dehydrated conditions. Investigation of new multi-acid sidechain membranes was also conducted and showed that while these membranes have excellent water retention and proton conductivity, they swell and form a continuous hydrophilic phase at the surface which may be problematic during fuel cell operation. In addition to these characterization efforts, a number of advances to the cp-AFM technique were demonstrated which allowed for the investigation of new operating conditions and the extraction of new information on electrical and mechanical properties of the membrane.