Molecular Dynamics Simulation Study On Sulfonated Polystyrene Membrane with Different Sulfonate Groups: Effect of Acidity

The structure and transport properties of sulfonated polystyrene membranes with different sulfonate groups were investigated using classical molecular dynamics simulation method.  Particularly, the effect of acidity of sulfonate groups was characterized by comparing superacidic fluoroalkyl sulfonic acid and less acidic alkyl sulfonic acid both of which are attached on the syndiotatic polystyrene backbone. The superacidic fluoroalkyl sulfonated ionomer exhibits a significantly higher proton conductivity than that of the less acidic alkyl sulfonated ionomer at low relative humidity, despite a lower ion exchange capacity and lower water uptake. The internal structure of water phase and water distribution do not show significant difference between these two ionomer membranes. These simulation results are in a good agreement with the experimental observations. Our experimental and simulation study suggests that the enhanced proton conductivity of fluoroalkyl sulfonated ionomer is due to the higher acidity of the sulfonate, which leads to increased dissociation to hydronium ions, a higher degree of ionic character in the sulfonate, and better solvation of the sulfonate with water molecules.