Recent advances in perfluorinated ionomer membranes : structure, properties and applications

▪ Abstract The need to operate polymer electrolyte membrane (PEM) fuel cells at temperatures above 100°C, where the amount of water in the membrane is restricted, has provided much of the motivation for understanding the mechanisms of proton conduction at low degrees of hydration. Although experiments have not provided any direct information, numerous theoretical investigations have begun to provide the basis for understanding the mechanisms of proton conduction in these nano-phase-separated materials. Both the hydrated morphology and the nature of the confined water in the hydrophilic domains influence proton dissociation from the acidic sites (i.e., −SO3H), transfer to the water environment, and transport through the membrane. The following molecular processes are discussed in connection to their role in the conduction of protons in sulfonic acid–based polymer electrolyte membranes (PEMs): (a) local chemistry of the hydrophilic side chains; its effect on the dissociation of the proton and eventual stabi...

Proton Conduction Mechanisms at Low Degrees of Hydration in Sulfonic Acid–Based Polymer Electrolyte Membranes

This review presents a description of what is known about ionomer morphology. All papers on ionomer morphology are not included in this review; but all relevant questions about ionomer morphology are posed and answered as completely as possible. The review is critical; that is, reasons are given for morphologies that are deemed to be more or less likely. The review is organized along three length scales: sub-nanometer, nanometer to 10 nm, and greater than 10 nm. Within each length scale, all three phases are considered: crystalline, amorphous, and ionic aggregate. The purity of the three phases, the arrangement of atoms in the three phases, the size and shape of the three phases, and their arrangements in space relative to each other are explored in this review. A model for the shape of aggregates in ionomers that have well-ordered internal environments is presented. This model proposes that aggregates are fundamentally planar, but will “roll up” into tubes or other related structures in order to reduce the number of atoms at aggregate edges. This model was developed primarily based on the varied morphologies ionomers have shown in electron micrographs, but is consistent with other indirect measurements of ionomer morphology. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/pen.21024

Review and critical analysis of the morphology of random ionomers across many length scales

This paper deals with the structure and chemistry of Nafion-H, the acidic form of Nafion, which is the trade name for a perfluorinated polymer manufactured by E. I. du Pont de Nemours & Co., Inc. Nafion-H and its salts are available from Dupont in both membrane and powder forms. Nafion composites with Teflon mesh (polytetrafluoroethylene) added to give increased mechanical strength are also available. In addition, sulfonated fluorocarbons modified with other chemically active polymers have been made by Dupont and by competing companies such as Ashai Chemical Industry Co., Ashai Glass Co., Tokuyama Soda Co., and Toya Soda Co.

Structure and chemistry of nafion-h: a fluorinated sulfonic acid polymer

Pillared smectites in which the pillars consist of iron oxide are expected to have interesting and unusual magnetic properties. Several possible routes by which such materials might be made have been investigated, namely intercalation of hydroxy-Fe(III) polycations, mixed hydroxy-Fe(III)/Al polycations, phenanthroline-Fe(II) cations, and trinuclear Fe(III) acetato cations into Na-montmorillonite. Only the last of these yielded a pillared clay (PILC) on calcination. The products have been characterized using X-ray powder diffraction and 57Fe Mossbauer spectroscopy. The precursor Fe-PILC has a d-spacing of 21 A and expands to 23 A on solvation with glycol. The calcined Fe-PILC has a d-spacing of 19 A (gallery height 9·4 A) and does not expand with glycol, confirming cross-linking of the layers. From Mossbauer spectra at 4·2 K it is estimated that there are of the order of some hundred Fe atoms per pillar.

Preparation and characterization of iron oxide pillared montmorillonite

Mossbauer spectra of “Nafion” perfluoronated acid membranes exchanged with iron or europium have been studied as a function of ion concentration, water content, temperature, and applied magnetic field in order to characterize the ionic phase in these materials In every sample, the recoil-free fraction falls to zero at a temperature which decreases from 280 to 220 K with increasing water content The sudden fall in recoil-free fraction corresponds to a glass transition in the ionic phase rather than its melting point In Fe2+ Nafion, the spectrum is independent of water content above 6 wt%, and is typical of fully hydrated, isolated Fe2+(H2O)6 species The isomer shift, quadrupole splitting, and linewidth all change continuously below 6 wt % water, indicating a range of environments for the iron For Fe3+ Nafion, ions in several distinct environments can be identified on the basis of their magnetic properties Isolated Fe3+ species (Fe–Fe distances greater than 12 A) showing paramagnetic hyperfine structure in their 42-K spectra predominate in slightly neutralized samples, and in those with high water content The proportion of dimers, whose quadrupole splitting is about 17 mm/s, increases with increasing iron content or decreasing water content A third component of the spectra has a quadrupole splitting of 04 mm/s It is associated with paramagnetic iron having other iron ions in the neighborhood, and also with iron ions located in the interior of small groups or chains formed from dimers There is no iron belonging to magnetically ordered clusters involving hundreds of ferric ions of the type found in some other ionomers All the exchanged ions belong to the ionic phase, and none are associated with the polymer backbone

M. Pineri

Papers

Möussbauer spectroscopy of nafion polymer membranes exchanged with Fe2+, Fe3+, and Eu3+

Microstructure of copper(II) α,ω-dicarboxylato polybutadiene

Microstructure of iron(III) α,ω-dicarboxylatopolybutadiene

Mössbauer Spectroscopy of Perfluorosulfonated Polymer Membranes: Structure of the Ionic Phase