Formation and mobility of protonic charge carriers in methyl sulfonic acid–water mixtures: A model for sulfonic acid based ionomers at low degree of hydration

The formation of ionic charge carriers and their transport in methyl sulfonic acid (CH3SO3H)–water mixtures (MSA–H2O) have been investigated by 1H NMR and impedance spectroscopy. The dissociation constant is close to K = 1.8 at high water contents, and the maximum molar concentration of ions (H3O+ and CH3SO3−) is found at a water hydration number of λ = [H2O]/[MSA] = 1.5. For this concentration also minima in the molecular diffusion coefficients and ionic mobilities are observed. Protonic conductivity is closely related to molecular diffusion (vehicle mechanism) in the entire composition range (λ = 1–8). The data have some relevance for the understanding and optimization of the properties of sulfonic acid based ionomer (such as Nafion). They provide interesting insight into the humidity dependence of the viscoelastic properties, the proton conduction mechanism and the conductivity limit, especially at low degrees of hydration.