In situ rehydration of perfluorosulphonate ion-exchange membrane studied by AFM

Nafion(R) is a commercially available perfluorosulphonate cation exchange m embrane commonly used as a perm-selective separator in chlor-alkali electro lysers and as the electrolyte in solid polymer fuel cells. This usage arise s because of its high mechanical, thermal and chemical stability coupled wi th its high conductivity and ionic selectivity, which depend strongly on th e water content. The membrane was therefore studied in different states of hydration obtained by placing the membrane and atomic force microscope (AFM ) in a specially constructed environmental chamber to control the humidity. Tapping mode phase imaging was successfully used to identify the hydrophob ic and hydrophilic regions of Nafion, The images support a cluster model fo r the hydrophilic regions of Nafion at humidities of (9-34) +/- 2%. The clu sters have a range of sizes from 5 to 30 nm, significantly larger than the similar to 4 nm structures proposed from X-ray studies, which is probably d ue to the formation of cluster agglomerates. Phase images were interpreted semi-quantitatively in terms of energy loss, typically 3 kJ m(-2), and the number and size of energy dissipative features. The number of clusters decr eased while the average cluster size increased with increasing humidity. (C ) 2000 Elsevier Science Ltd. All rights reserved.