The state of water in Nafion 117 of various cation forms

Abstract

The state of water in Nafion 117 membranes of various alkali and alkaline earth metal cation forms was investigated by means of differential scanning calorimetry (DSC) and infrared spectroscopy (IR). Two kinds of water molecules existed in the membranes: freezing water molecules, which interact with the metal cations and ion exchange sites weakly and could freeze at around -20°C, behaving in a manner close to but not identical to that of bulk water; nonfreezing water molecules which could not transfer to ice even at -120°C, and are considered as both the ones binding strongly with the cations and the ion exchange sites and the ones exposed to fluorocarbon environment. It was found that the fraction of freezing water of the membranes decreased with decreasing the hydrophility of the metal cations in the membranes and that the number of freezing water molecules were almost the same as that of the water molecules pumped by the cations during the cation transport. The water permeability of these membranes were measured using the streaming potential method as well. The results revealed that the water permeability increased with increasing water content of the membrane, and that the membrane equilibrated with alkali metal cations possessed a higher water permeability compared with the membrane equilibrated with alkaline earth metal cations with same water contents. The impedance measurement showed that the membrane conductivity seems to depend on both the hydrophilicity and the valence of the cations. All these results indicated that the hydrophilic domain in Nafion membranes is the determining factor for both cation and water transports in the membranes.