Solution Casting and Thermal Annealing of Nafion Ionomer for Fuel cell Applications

Fuel cells are a promising technology to produce and develop clean and sustainable energy. Nafion has been regarded as the current benchmark for polymer electrolyte membranes (PEM). The structure of Nafion plays an important role in its unique properties such as excellent thermal stability and high proton conductivity. However, researchers are seeking alternative ways to further develop PEM membranes that can have higher conductivities at higher temperatures and low relative humidity. Pre-treatments are often employed to improve the properties of PEMs by altering their microstructure. In this study, Nafion membranes were created by solution casting in Dimethylformamide (DMF) at 90 ^OC followed by annealing at different temperatures (140^OC and 170^OC) and rapid cooling. The microstructure of the membranes was investigated using transmission SAXS and was found that the annealing had no effect on the through-plane orientation of the Nafion membranes. However, the water uptake studies showed that when compared to unannealed membranes, membranes annealed at 170^OC had a reduced water uptake of 65.3%. This can be attributed to large structural modifications with enhanced crosslinking formed due to annealing. Heat treatment at higher temperature induces free volume in the membrane and make the molecular segments more organized. Hence it becomes harder for the water molecules to diffuse into the hydrophobic zone and water uptake and swelling decreases with increase in the annealing temperature. It is postulated that optimizing the annealing and cooling rates would induce more compact chain packing structure which would significantly impact the diffusion and proton conductivity.