An NMR study on the molecular dynamic and exchange effects in composite Nafion/sulfated titania membranes for PEMFCs

In this work, self-diffusion coefficients data were interpreted in terms of a simple “two-sites” model involving exchange between relatively bound and mobile water sites to discuss the nature of water dynamics and the state(s) of the water (bound and free states) both inside composite and filler-free membranes, as well as its interaction with the hydrophilic polymer sites and particles surface. A quantitative analysis was elaborated to estimate the number of water molecules involved in the hydration shell of the hydrophilic groups of both polymer and filler, as function of the membrane water content. Despite the simplicity of this “bound/free water exchange” model, we obtain consistent values corresponding to about thirteen water molecules per sulfonic group.