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

Sulfated titania nanoparticles were evaluated as inorganic additives in composite Nafion based membranes, to be considered as advanced electrolyte in proton exchange membrane (PEM) fuel cells (FCs). Three different filler loadings respect to the polymer were comparatively investigated to elucidate the effect of the inorganic particles on membrane peculiar properties and finally establish the most effective electrolyte composition. Water dynamics were investigated by NMR spectroscopy, including pulsed-field-gradient diffusion and 1H spectral analyses conducted as a function of temperature (20-130 °C), and by water content measurements, providing a general description of the water management inside the systems and of the effects of the fillers. Due to its strong acidity and hydrophilicity, sulfated titania was found to improve both the water retention and diffusion in the composite membranes in respect to plain recast Nafion, in the whole range of investigated temperatures, with a significant impact in the region of high temperatures and very low water content. 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.