State of Water in Nafion 117 Proton Exchange Membranes Studied by Dielectric Relaxation Spectroscopy

The dynamics and the nature of water environments in a fuel cell proton exchange membrane are studied experimentally. Specifically, the dynamics of water in Nafion 117 membranes, in the acid form, were investigated at two hydration levels and several temperatures by means of dielectric relaxation spectroscopy; two different dielectric spectroscopy experimental setups were employed for low (10-10 Hz, 25 to –80 C) and microwave (0.04526 GHz, 35 C) frequency ranges. Three states of water were clearly identified: (a) water strongly bound to the sulfonic groups (in quantitative agreement with previous investigations) defining the first hydration layer; (b) loosely bound water, surrounding the first layer, and (c) free water, having similar dynamics as in the bulk/liquid water. This is the first time that the dynamics of loosely bound water are experimentally observed.