In this comprehensive review, recent progress and developments on perfluorinated sulfonic-acid (PFSA) membranes have been summarized on many key topics. Although quite well investigated for decades, PFSA ionomers’ complex behavior, along with their key role in many emerging technologies, have presented significant scientific challenges but also helped create a unique cross-disciplinary research field to overcome such challenges. Research and progress on PFSAs, especially when considered with their applications, are at the forefront of bridging electrochemistry and polymer (physics), which have also opened up development of state-of-the-art in situ characterization techniques as well as multiphysics computation models. Topics reviewed stem from correlating the various physical (e.g., mechanical) and transport properties with morphology and structure across time and length scales. In addition, topics of recent interest such as structure/transport correlations and modeling, composite PFSA membranes, degradat...

New Insights into Perfluorinated Sulfonic-Acid Ionomers

New structures for the Pt/C catalyst layer of polymer electrolyte fuel cell electrodes have been developed that substantially increase the utilization efficiency of the catalyst. Fabricating the catalyst layers and gas diffusion backings separately makes it possible to formulate each structure with the properties that are most suitable for its function. In the case of the catalyst layer, the optimal properties are hydrophilicity, thinness, uniformity, and the proper ratio of ionomer and supported catalyst. The catalyst layers are cast from solution as thin films that utilize the ionomer itself as a binder. The thin films are hot pressed directly onto the ionomer membranes, and the hydrophobic gas diffusion backings are inserted when the cells are assembled. The performances of fuel cells based on the thin film catalyst layers are comparable with those of gas diffusion electrode designs that utilize several times as much platinum, thus the specific activities of the Pt catalysts in the new structures are significantly higher.

Thin-film catalyst layers for polymer electrolyte fuel cell electrodes

Fundamental water and salt transport properties of polymeric materials

Polymer electrolyte fuel cells.

Characterization of polymer electrolytes for fuel cell applications

Water sorption as a function of activity (aH2O) by H+-, Li+-, N(CH3)4+- and Cs+-exchanged forms of a perfluorosulphonate ion exchanger (Nafion-117) and by 4 and 8% crosslinked Dowex 50 W resins has been investigated in order to derive sorption isotherms using the isopiestic technique. All exchangers show the sequence of water sorption as H+ > Li+ > N(CH3)4+ > Cs+. However, the H+ and Li+ forms of Nafion-117 absorb significantly larger amounts of water, while the Cs+ form sorbs much less water at all water activities compared to the corresponding ionic forms of Dowex 50W × 8, the N(CH3)+4 isotherms of the two exchangers being quite close to each other. Sorption of water by the 4 and 8% crosslinked Dowex 50W resins is found to be the same at low aH2O( Dowex 50W × 8 > Dowex 50W × 4 (any ionic form). The results are discussed in relation to the relative degrees of hydration of the (respective) ions, including the sulphonate anion, and the differences between the two types of ion exchangers.

Thermodynamics of water sorption by perfluorosulphonate (Nafion-117) and polystyrene–divinylbenzene sulphonate (Dowex 50W) ion-exchange resins at 298 ± 1 K

Hydrogen/deuterium (H/D) fractionation effects in the exchanger-pore water–outer-equilibrium water have been investigated at 298 ± 1 K using H+, Li+ and Cs+ ionic forms of Nafion-117 and Dowex 50W (4%, 8% DVB) resins and waters containing deuterium from 150 to 11 000 ppm. Single-stage separation factors (αH2OHDO) evaluated show preferential enrichment of deuterium in the outer water phase in the sequences: (i) Nafion-117 > Dowex 50W × 8 > Dowex 50W × 4; (ii) H+ > Li+ > Cs+. Whereas αH2OHDO values were nearly unity in the natural D-concentration region, they were appreciably higher in deuterium-rich water systems. A linear relationship was found between log αH2OHDO and inverse deuterium concentration, which has been interpreted in terms of a Born-type equation dealing with the solvation of ions. The selective enrichment of H2O over HDO in resin-pore water conforms with similar observations reported with binary water–organic-solvent systems and with the lesser hydrogen-bonded structure of resin-pore water compared with normal water. Thermodynamic analysis of the present systems has revealed large contributions of swelling pressure terms of αH2OHDO which readily explains the sequences listed above. The role of cationic and sulphonate anion hydration has also been assessed in these fractionation effects. H+– 117, Li+– 117 and H+× 8 exchangers have been suggested as being most suited for deuterium enrichment using the simple technique of equilibrium contacting of these exchangers with waters of high D-concentrations.

Hydrogen isotope effects in the absorption of water by perfluorosulphonate (Nafion-117) and polystyrene divinylbenzene sulphonate (Dowex 50W) ion-exchange resins

K. K. Pushpa

Papers

Thermodynamics of water sorption by perfluorosulphonate (Nafion-117) and polystyrene–divinylbenzene sulphonate (Dowex 50W) ion-exchange resins at 298 ± 1 K

Hydrogen isotope effects in the absorption of water by perfluorosulphonate (Nafion-117) and polystyrene divinylbenzene sulphonate (Dowex 50W) ion-exchange resins