Aromatic Ionomers with Highly Acidic Sulfonate Groups: Acidity, Hydration, and Proton Conductivity

A novel sulfonation method that involves iridium-catalyzed aromatic C-H activation/borylation and subsequent Suzuki-Miyaura coupling with sulfonated phenyl bromides was developed for the preparation of aromatic ionomers. Superacidic fluoroalkyl sulfonic acid and less acidic aryl and alkyl sulfonic acids were efficiently incorporated into the aromatic ring of model polystyrene, and the resulting sulfonated ionomers were characterized for their properties as proton-conducting membranes. The membrane properties of ionomers containing sulfonic acid groups with different acidity strengths were compared to study the effect of acidity on the water properties, proton conductivity, and morphology. The superacidic fluoroalkyl sulfonated ionomer (sPS-S-1) exhibited a significantly higher proton conductivity than that of the less acidic aryl and alkyl sulfonated ionomers (sPS-S-2 and sPS-S-3, respectively) at low relative humidity, despite a lower ion exchange capacity and lower water uptake. Hydration behaviors of the ionomers as a function of relative humidity were studied to correlate the acid strength of the sulfonates and water uptake properties. Morphology studies of the sulfonated ionomers show that sPS-S-1 has a larger hydrophilic domain than that of sPS-S-3. Molecular dynamic simulations were performed to understand the origin of the improved proton conductivity of the superacidic ionomer at the molecular level. These simulations suggest that the enhanced proton conductivity of sPS-S-1 is due to the cumulative effect of higher acidity of the sulfonate, which leads to increased dissociation to hydronium ions and a higher degree of ionic character in the sulfonate, and better solvation of the sulfonate with water molecules.