Atomic force microscopy studies of conductive nanostructures in solid polymer electrolytes

Abstract

tThe conductivity of three different sulfonated polymer electrolyte membranes (PEM), two perfluori-nated membranes, Nafion® and Aquivion®, and JST, a non-perfluorinated aromatic block copolymer, werecompared using advanced material-sensitive and conductive atomic force microscopy (AFM). All of themembranes 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 mem-branes, 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 parallelorientation, was also found for the Aquivion®membrane. AFM high-resolution current images, per-formed under a continuous current flow, were used to distinguish between the conducting network andthe subsurface phase distribution at the membrane surface. The connected subnets of the JST membrane were approximately 100–200 nm in size, whereas those for the perfluorinated membrane surfaceswere 200–300 nm in size. The conductive areas of the Aquivion®and JST membranes exhibited largerhomogeneous conducting areas, corresponding to the smaller correlation lengths of ionic phase separa-tion. Membrane cross sections were analyzed to elucidate the structure of the bulk ionic network of theNafion®membrane, before and after operation. The existence of extended water layers in the bulk, evenbefore operation, was confirmed.