Thickness of membrane ~ 50µm
There are some disadvantages of the solution casting method that the cold cast films are generally mechanically weak, susceptible to crack formation and soluble in many polar solvents, especially in water at room temperature. Generally, the annealed films undergo morphological changes when heated above the glass transition temperature (Tg=109oC for protonated Nafion) leading to improvement of mechanical stability of the membrane. This phenomenon was clearly explained by Zook et al.  by observing that cold cast Nafion films have a micellar configuration with the sulfonic acid groups on the outside and tetraflouroethylene backbone inside of the micelle; by the heat treatment, the micelle structure was changed into an inverted micelle with the sulfonates on the interior. the curing procedure may seal pinholes or cracks present in the Nafion membrane, in addition to increasing crystallinity of Nafion http://www.eng-tips.com/faqs.cfm?fid=957
Crossover of all the cast membranes was higher than that of Nafion 112 since they were home-made and possibly contained defects. the one that cured at 150oC exhibited the best performance.
6h is the best curing time. Even though curing membrane for longer than 6 h would produce highly crystalline and mechanically stable membrane, ionic cluster could shrink, resulting in lower ionic conductivity of cast membrane and lower cell performance. Curing a membrane for longer time produces migration of sulfonic acid groupfrom bulk to surface of the membrane; this in turn may reduce the charge transfer resistance between membrane and catalyst layer.With increasing curing time, crossover current density decreased because longer curing produces a mechanically stable and crystalline membrane.
FT-IR study of the microstructure of Nafion® membrane Fabricated at low temperature,...