Surface Morphology of Nafion at Hydrated and Dehydrated Conditions

The relation between proton exchange membrane (PEM) hydration and fuel cell performance has been well documented at the macroscopic scale. Understanding how changes in membrane water content affect the organization of proton conducting domains at the micrometer and submicrometer scales is a sought-after goal in the rational design of higher performing PEMs. Using atomic force microscopy phase and current imaging, we have resolved proton conducting domains at the surface of Nafion membranes at dehydrated, ambient, and hydrated conditions, observing a unique morphology at each membrane water content. At ambient conditions, Nafion's surface morphology resembles that proposed in the parallel-pore and bicontinuous network models, with the exception that hydrophilic domains are larger at the surface of Nafion compared to the bulk. At hydrated conditions, a network of wormlike, insulating domains extends several micrometers over Nafion's surface with more conductive, water-rich regions found between these fibrillar features. Neither the surface morphology observed at ambient conditions nor at hydrated conditions persists in dehydrated membranes, which instead exhibit a low coverage of isolated hydrophilic surface domains that, remarkably, are similar in size to such domains at ambient conditions. These observations affirm properties distinct to Nafion's surface and provide morphological evidence for the low conductivity observed in Nafion at dehydrated conditions and the high conductivity observed at hydrated conditions.