STATE OF WATER IN PERFLUOROSULFONIC ACID MEMBRANES STUDIED BY MICROWAVE DIELECTRIC RELAXATION SPECTROSCOPY

Open Access
Author:
Lu, Zijie
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 16, 2005
Committee Members:
  • Digby D Macdonald, Committee Chair
  • Evangelos Manias, Committee Chair
  • Harry R Allcock, Committee Member
  • Michael T Lanagan, Committee Member
Keywords:
  • proton exchange membranes
  • Microwave dielectric relaxation spectroscopy
  • Water state
  • fuel cells
Abstract:
The physical state of water in perfluosulfonic acid membranes (e.g., Nafion 117 and Flemion SH150) has been studied by the water sorption isotherms, DSC, 2H NMR, and microwave dielectric relaxation spectroscopy. Different water states have been discriminated in these membranes: 1) the first few water molecules upon absorption from water vapor (  3, where  denotes the number of water molecules per sulfonic acid group) are tightly bound to the sulfonic acid group to form the first hydration water; 2) beyond this water content, water molecules form the second hydration layer of the sulfonic acid group. The amount of this water increases with water content, but reaches a constant value at higher water contents. The microwave DRS measurements showed a relaxation time of about 30 ps for this type of water; 3) the free water, locating at the center of the ion cluster regions where the ion-water interaction is negligible, is present when   6. The free water, with relaxation time   8 ps at 25oC, shows the characteristic dynamics of bulk water; and 4) finally, as water content increases further, water molecules start to be associated with the perfluorinated side chains and even with the fluorocarbon backbone. A much longer relaxation time,   90 ps, is observed for this part of water due to the hydrophobic confinement by the polymer. Microwave DRS measurements showed markedly different dynamics, i.e., negligible activation enthalpy and large negative activation entropy, for the dielectric relaxation of this type of water. The presence of the hydrophobically confined water is found to correspond to the clustering of water and to the appearance of the freezing water in these membranes. The equivalent weight (EW) of the PSA membrane has been found to have considerable influence on the dielectric response of the membranes. Flemion SH150, which has an EW value of 909, shows higher static permittivity than Nafion 117, which has an EW of 1100. This is consistent with the higher water sorption by Flemion SH150 at the same water activity. The “free” and “hydrophobically confined” water are found to start to appear at lower water content in Flemion than in Nafion. These results may suggest that Flemion SH150 membrane is likely to form larger and/or more numerous ionic cluster regions. The free water provides a pathway to the fast proton conduction in the membranes. Optimization of the proton conductivity requires maximization of the free water content. Dimethyl sulfoxide was found to increase the fraction of “free water” in PSA membranes due to its strong ability to form hydrogen bonding with water molecules. This increase leads to increasing conductivity of Nafion/DMSO at lower water content. However, the Nafion/DMSO membrane showed lower conductivity at high water contents. This can be explained by the longer relaxation times for the solvent mixture, compared to pure water.