More on Nafion conductivity decay at temperatures higher than 80 °c: Preparation and first characterization of in-plane oriented layered morphologies

n order to have a better understanding of Nafion behavior in fuel cells operating at temperatures higher than 80 C, the preparation of membranes containing a large amount of layered morphologies prevalently oriented in the direction parallel to the membrane surface (hence of low through-plane conductivity) was attempted. Successful in-plane oriented samples were obtained by forced swelling of membranes between rigid planar constraints. Other than the expected low through-plane conductivity, a first characterization of these modified membranes clearly showed that the dimension change during processes of dehydration and successive hydration takes place essentially in the direction perpendicular to the membrane surface. It was furthermore found that the forced swelling was accompanied by a strong reduction of ionomer density (from an initial value of 2 to about 1.4 g/cm3). Finally, evident changes of the nc/T plots were also found. A discussion on the formation of these in-plane oriented layered morphologies is reported, giving emphasis to the fact that their formation in working fuel cells is particularly dangerous when they are prevalently oriented in the direction parallel to the membrane surface (large extent of through-plane conductivity decay). Some practical expedients for avoiding the formation of these dangerous in-plane "oriented layered morphologies" under the operative conditions of relative humidity and temperature are also reported. The inter-relations between spectroscopic investigations, recent stochastic simulation processes, and our experimental results are finally discussed.