Comparative Studies of Conductive Nanostructures and Activation Properties in Solid Polymer Electrolytes

Kurzfassung

Proton-conducting membranes are a key component of PEM fuel cells: the properties of these membranes at the nanoscale influence fuel cell performance and durability. Nafion® (DuPont) remains the most common membrane type and is therefore used as a reference system to which novel membrane materials are compared, and novel membranes are expected to exhibit at least an equivalent performance. The conductivity on a nanostructure scale of different sulfonated polymer electrolyte membranes (PEM) is discussed in comparison. In particular, two perfluorinated membranes, Nafion® and Aquivion®, and JST, a non-perfluorinated aromatic block copolymer, are compared using advanced material-sensitive and conductive atomic force microscopy (AFM). All of the membranes required activation by a current flow to reach significant conductivity for the AFM analysis, indicating the existence of a highly resistive surface skin layer. The two perfluorinated sulfonic acid membranes, a membrane with long side-chains (Nafion®) and a membrane with short side-chains (Aquivion®), exhibited similar properties. A lamellar surface structure, with polymer bundles or micelles in a parallel orientation, was also found for the Aquivion® membrane. AFM high-resolution current images, performed under a continuous current flow, were used to distinguish between the conducting network and the subsurface phase distribution at the membrane surface. The connected subnets of the JST membrane are approximately 100 - 200 nm in size, whereas those for the perfluorinated membrane surfaces are 200 - 300 nm in size. The conductive areas of the Aquivion® and JST membranes exhibited larger homogeneous conducting areas, corresponding to the smaller correlation lengths of ionic phase separation. Membrane cross sections were analyzed to elucidate the structure of the bulk ionic network of the Nafion® membrane, before and after operation. At a protonated and water-soaked but not activated Nafion® sample, the conductivity measured by conductive AFM is barely detectable. The common activation procedure is to force a flow of current by, e.g., electrolysis. Without activation only a current of a few pA is detected for all membranes. The often-observed lamellar structure in the topography which is also visible in the conductivity images supports a parallel orientation of polymer structures with respect to the surface. The investigated membranes show a distinct activation behavior which is interpreted as as indicative of different skin layer thickneses.