Atomic force microscopy studies of conductive nanostructures in solid polymer electrolytes

Abstract

The conductivity of three different sulfonated polymer electrolyte membranes (PEM), two perfluorinated membranes, Nafion® and Aquivion®, and JST, a non-perfluorinated aromatic block copolymer, were 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 were approximately 100–200nm in size, whereas those for the perfluorinated membrane surfaces were 200–300nm 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. The existence of extended water layers in the bulk, even before operation, was confirmed.