Synthesis and characterization of perfluorinated carboxylate/sulfonate ionomer membranes for separation and solid electrolyte applications

A synthesis procedure is given for mixed functionality carboxylate/sulfonate (c/s) perfluorinated ionomer membranes starting from sulfonyl fluoride-form Nafion precursor. The described method produces ionomers with up to 30% carboxylate-substituted equivalents. Attenuated total reflectance and transmission Fourier transform infrared analyses show that the c/s membranes have carboxylate-rich surfaces and a sulfonate-rich center. The surfaces of the c/s membranes are uniform in carboxylate content across the entire membrane areas. The c/s membranes show interesting transport characteristics when compared to Nafion, including proton conductivity that decreases moderately, and water permeability that decreases significantly with increased carboxylate content. As an example, an increase in carboxylate content from 2.4 to 7.5 and 11.4% results in a 26.9 and 32.7% drop in conductivity coupled with a 67.7 and 97% drop in permeability, respectively. Given the dehydration problem in modern H-2/air proton-exchange membrane (PEM) fuel cells and the methanol crossover problem in direct methanol PEM fuel cells, these c/s membranes may offer increased performance in fuel cell applications. The synthesis procedures can be used in general to attach functional molecules to the terminal sulfur atom on the ionomer side chains, creating a number of possibilities in forming dual-functionalized Nafion films.