Properties of Water in Prestretched Recast Nafion

Abstract

Uniaxially prestretched recast Nafion membranes exhibited an unusual combination of properties (a proton conductivity equal to that of commercial Nafion but with a lower methanol permeability) which make them ideal candidates for use in a direct liquid methanol fuel cell. To better understand the function and underlying morphology of the prestretched membranes, water uptake and mobility data were collected and analyzed for draw ratios ranging from 1 to 7. Macroscopic (gravimetric) water uptake and the water self-diffusion coefficient (measured by NMR) were found to be invariant with membrane elongation and essentially identical to those measured in a commercial Nafion 117 film (a similar behavior was observed for proton conductivity). The ratio of freezable/nonfreezable water in prestretched recast Nafion, the water electro-osmotic drag coefficient, and the spin-lattice relaxation time constant of deuterated water, however, decreased with increasing film elongation, up to a draw ratio of 4. The functional dependence of these properties on draw ratio was similar to that observed for methanol permeability. The combined water results indicated that there were a greater number of smaller ionic/ hydrophilic domains in prestretched recast Nafion as compared to commercial Nafion. Transmission electron microscopy of membrane cross sections confirmed this conclusion.

A Theoretical Study on the Formation, Binding Energy and Monomer Dipole Moment of Small Water Cluster Systems

“…Equation (1) pertains to the average binding energy needed to dissociate or hydrate all water molecules from/to their cluster. Equation (2) represent the local binding energy needed (in equilibrium) to add or remove only one water molecule to or from the cluster.…”

“…Water clusters are involved in various areas of great scientific and technological importance, in atmospheric planetary, environment, biochemistry, and novel energy devices such as in the membrane design of Nafion-based Proton Exchange Membrane for Fuel Cell (PEMFC) [1] applications. Recently, the performance of PEMFC is highly expected to be affected by the clustering behavior of water molecules at the molecular level, which control the humidity conditions (wet/dry) inside the Nafion [2,3] membrane.…”

Diffusion in Solid Proton Conductors: Theoretical Aspects and Nuclear Magnetic Resonance Analysis

“…The authors applied the same technique to BPSH-BPS membranes [58]. Macroscopic water uptake and the water self-diffusion coefficient were instead found to be invariant in pre-stretched recast Nafion and in commercial Nafion 117, although the amount of freezable water in pre-stretched sample decreased and the amount of nonfreezable water increased with increasing film elongation [59].…”

Interpretation of direct methanol fuel cell electrolyte properties using non-traditional length-scale parameters

“…The state of water in PEMs is often probed and related to the properties discussed here: conductivity, methanol permeability and selectivity [27,[56][57][58]. PEMs containing functional groups have decreased water uptakes and exhibit properties consistent with smaller water domains and less free water.…”