Dynamics of Fe cations in Nafion membranes studied by nuclear resonant quasielastic scattering of synchrotron radiation

Nuclear resonant quasielastic scattering from ferrous cations in ion-exchan ge Nafion membranes was measured at room temperature, and the intrinsic dyn amics of the cations inside the spherical regions surrounded by the sulfoni c groups of Nafion was studied. For the analysis, the formula of nuclear re sonant absorption for a particle diffusing in a sphere with an impermeable surface was derived. The formula indicates that the effect of the surface u pon the diffusive motion is small if the radius of the sphere is larger tha n a critical value; in the case of Fe-57 ions, the value is about 2 Angstro m. As the radius of the spherical regions in our Nafion samples is estimate d to be about 5 Angstrom, the existence of the surface can be ignored and t he intrinsic dynamics of Fe cations can be extracted. The diffusion coeffic ient estimated from the least squares fit was (1.12 +/- 0.13)x10(-5) cm(2)/ s for 15 wt% water content. This value is in good agreement with that of Fe cations in an HCL solution. As a result, Fe cations in Nafion diffuse as f reely as those in solutions.