In order to have a better understanding of Nafion behavior in fuel cells operating at temperatures higher than 80 C, the preparation of membranes containing a large amount of layered morphologies prevalently oriented in the direction parallel to the membrane surface (hence of low through-plane conductivity) was attempted. Successful in-plane oriented samples were obtained by forced swelling of membranes between rigid planar constraints. Other than the expected low through-plane conductivity, a first characterization of these modified membranes clearly showed that the dimension change during processes of dehydration and successive hydration takes place essentially in the direction perpendicular to the membrane surface. It was furthermore found that the forced swelling was accompanied by a strong reduction of ionomer density (from an initial value of 2 to about 1.4 g/cm(3)). Finally, evident changes of the n(c)/T plots were also found. A discussion on the formation of these in-plane oriented layered morphologies is reported, giving emphasis to the fact that their formation in working fuel cells is particularly dangerous when they are prevalently oriented in the direction parallel to the membrane surface (large extent of through-plane conductivity decay). Some practical expedients for avoiding the formation of these dangerous in-plane "oriented layered morphologies" under the operative conditions of relative humidity and temperature are also reported. The inter-relations between spectroscopic investigations, recent stochastic simulation processes, and our experimental results are finally discussed.

The cross-link reaction between macromolecular chains of sulfonated poly(ether ether ketone) (SPEEK) by thermal treatment above 150 degrees C in presence of dimethylsulfoxide (DMSO) is investigated by various techniques, including elemental analysis, acid-base titration, infrared spectroscopy, water uptake (WU) measurements, and thermogravimetry. The conditions of thermal treatment are analyzed. The cross-linking reaction occurs in at least three more or less activated and deactivated positions with different activation energies, leading to different time dependencies of the cross-link reaction. The role of residual solvent DMSO is studied particularly: the cross-linking depends significantly on the amount of solvent in the membranes. Implications on WU and thermal stability are of particular importance for the development of high performance proton-conducting membranes.

Thermal cross-linking in presence of residual solvent dimethylsulfoxide without any addition of cross-linker molecules is described. This elegant method increases spectacularly the mechanical and hydrolytic stability of sulfonated aromatic polymers (SAP), making them suitable for use in liquid water also at 145 degrees C. Data on water uptake, mechanical properties and proton conductivity are presented and discussed from a bond energy point of view. The developed method is also inexpensive, being incorporated in the normal membrane casting procedure. This opens new horizons and hitherto conventionally disregarded SAP membranes should be reconsidered as fuel cell membranes. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

The colligative properties of acidic solution inside Nafion (R) 117 membranes have been investigated, in a large temperature range, by two different methods.1) The matrix counter-pressure index of the ionomer, n(c), was first determined and then its relations with the molar fraction and molality of the inner proton solution were found.2) In order to calculate the density of the inner proton solutions, a factor F-e proportional to the solution electro-restriction, was experimentally found. The molarity and molality of the inner proton solution, and hence the inner osmotic pressure, were then calculated.The obtained data confirmed the viscous behaviour of Nafion (R) 117 membranes. Furthermore, the comparison between the molality of inner proton solution obtained with the above different methods showed that to consider a solvation shell for the protons (first method) is equivalent to consider an electro-restriction of the solutions (second method). In other words, hydration number and electro-restriction factor are strictly related and the equation relating them is reported.Finally, T-RH plots at constant n(c) values show that the believing concerning the inability of Nafion (R) membranes to be employed in fuel cells at temperature higher than about 80-90 degrees C is not completely true. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Fuel CellsVolume 10, Issue 2 p. 219-219 Editorial Electrodes for PEM Fuel Cells M. L. Di Vona, M. L. Di VonaSearch for more papers by this authorP. Knauth, P. KnauthSearch for more papers by this authorG. Alberti, G. AlbertiSearch for more papers by this author M. L. Di Vona, M. L. Di VonaSearch for more papers by this authorP. Knauth, P. KnauthSearch for more papers by this authorG. Alberti, G. AlbertiSearch for more papers by this author First published: 30 March 2010 https://doi.org/10.1002/fuce.201090003AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume10, Issue2Special Issue: Materials for Proton Electrolyte Membrane Fuel CellsApril, 2010Pages 219-219 RelatedInformation

The general properties of polymer electrolyte membranes (PEMs) and the requirements that must be met for their use in fuel cells are reviewed. The influence on water uptake of temperature, relative humidity (RH), and thermal history of the polymeric matrix is critically discussed. On the basis of a simple osmotic model, an index proportional to the counter-elastic force of the polymeric matrix, and hence also to its tensile modulus, is derived. This index can be either experimentally determined from water uptake in liquid water at 20 °C or directly calculated if RH and temperature values at which the membrane has been equilibrated are known. Owing to their good stability, particular attention is paid to perfluorinated ionomers, especially Nafion, Hyflon Ion, and their composite membranes. Morphological conformations and their change with water uptake, as well as recent advances in thermal annealing for increasing their durability in medium temperature PEM fuel cells are discussed. A short review of the synthetic procedures and of the main physicochemical properties of perfluorinated and nonfluorinated aromatic polymers (including polybenzimidazole (PBI), poly(arylene ether)s, poly(imide)s, and poly(phosphazene)s) is also reported.

A major application of polyfunctional acid materials is the fabrication of solid electrolytes exhibiting high proton conductivity in the solid state for operation in fuel cells at medium temperature (<160°C) and low humidity (RH). A most recent trend of research in this field is the search of low vapour pressure doping agents designed to provide liquid-like proton transfer in perflurosulfonic acid membranes. A new polyfunctional acid material made of Nafion doped with difluoromethandiphosphonic (DFMDP) acid is reported in the present work. The DFMDP doping agent, melting at 146°C and stable up to 160°C, has been found to enhance by two orders of magnitude the conductivity of neat Nafion in dry environment at 120°C, and to be stable at this temperature even in the presence of 30% RH. The material chemistry responsible of the enhanced conductivity has been studied by several techniques: i.e., potentiometric titration (PT), scanning electron microscopy, wide (WAXS)- and small (SAXS)-angle X-ray scattering, and X-ray photoelectron spectroscopy (XPS). The results allow a number of conclusions. The PT data prove that a multilevel proton acidity wide gradient is present in the DFMDP doped Nafion samples constituting the driving force for proton mobility through the membrane phase. The WAXS and SAXS data prove that the DFMDP molecules are incorporated into the Nafion SO3H ionic domains. The XPS data provide direct evidence of the interaction between the DFMDP PO3H2 and Nafion SO3H groups which allows the proposition of the establishment of H-bonding network between the proton acceptor P=O and P–OH sites and the proton donor SO3H group. The conductivity data for the DFMDP membranes, compared to other data obtained for membranes made by other sulfonated polymers doped with H3PO4, offer intriguing perspectives for the design of solid electrolytes exhibiting enhanced conductivity in dry or low humidity environment.

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An important problem for medium temperature polymer electrolyte fuel cells (MT PEMFCs) operating in the temperature range 90-1.40 degrees C is the short time-life of proton conducting membranes. To shed some light on the empirical annealing treatments used for increasing the membrane durability, a systematic research on the effects of thermal treatments of Nafion 117 membranes was undertaken with the hope that the information obtained could be useful for a better understanding of the real limits for MT PEMFCs. Kinetic experiments showed that, for each couple of T-RH values, the water taken up from the membrane reaches a constant value only after long times of equilibration (>= 200 h). Taking into account that the enlargements provoked by the water-uptake remain as permanent deformations when the samples are cooled, it was found that the evolution of the deformations provoked by changes in temperature and RH can be conveniently estimated at 20 degrees C by determining the water taken up after equilibration in liquid water. By relating the counter-elastic index of the matrix (n(c(m))) to the extent of these deformations, a set of equations were obtained which allowed us to predict their evolution with changes of temperature and relative humidity. A good agreement with experimental values was found. The importance of this discovery for the development of MT PEMFCs is discussed.

Sulfonated poly(ether ether ketone) (SPEEK) membranes were thermally treated at temperatures between 120 and 160 degrees C. Water uptake measured at different relative humidity values or by full immersion in water between 25 and 145 degrees C was found to depend very strongly on previous thermal treatment and casting solvent. Water-uptake coefficient values as low as 10-15 even upon immersion in water at 100 degrees C were obtained with membranes treated at 160 degrees C. This effect is related to cross-linking by SO2 bridges between macromolecular chains. An important role is also played by the casting solvent: among the investigated solvents, dimethylsulfoxide (DMSO) gave the best results. A chemical kinetics model is outlined that permits the estimation of the relevant kinetic parameters, especially the activation energy of the cross-linking reaction, which was found to be about 60 kJ/mol. These results are of significant importance for the improvement of proton-exchange membrane fuel cells.

This paper will present a survey of new zirconium phosphates and phosphonates, recently appeared in the literature, with structure different from that of the traditional α- and γ-type zirconium phosphates. The structure and properties of compounds with different dimensionality, such as one-dimensional chains, new layered compounds, and even open-framework 3-D solids, will be discussed. In many cases the presence of groups different from phosphate or phosphonate tetrahedra, coordinated to zirconium, such as fluoride, chloride, or hydroxyl groups, and neutral ligands such as dimethylsulfoxide, was found. Some of these new compounds showed unusual characteristics and reactivity; they can offer new possibilities to the materials chemists for the preparation of tailor made compounds, with structure and reactivity that can be tuned for specific purposes.

A kinetic investigation of the water-uptake of Nation 117 membranes after different hydrothermal and thermal treatments was performed. Long equilibration times (150-225 h) and a decrease of kinetic rate with the increasing of temperature were found. This behaviour suggests that the kinetics of the water-uptake is the result of two distinct processes: one very fast that can be attributed to the time of osmotic equilibration necessary for the water diffusion within the thin membrane and the other, very slow, that has been associated with a slow modification of the Nation conformation with the temperature.The irreversibility of the hydration process with the temperature was related to the irreversibility of the conformational changes. The memory of the thermal treatment is due to the fact that ionomers are essentially constituted as an amorphous matrix in which some microcrystalline phases are embedded. All the amorphous ionomers can give metastable phases that can appear kinetically stable although, in fact, they are thermodynamically unstable.Previous osmotic models for ionomers were re-visited in order to relate the water-uptake isotherms of the various treated samples to the counter-elastic force of their matrix. It was found that an index proportional to the counter-elastic force of the matrix can be simply derived by determining the water-uptake of the samples in liquid water at 20 degrees C. Furthermore, some useful relations between this index and tensile modulus and the shape of water-vapour sorption isotherm of the samples are reported and discussed.We believe that the researches on the modifications of PFSA membranes by thermal and hydrothermal treatments are very important: (1) in order to have a better understanding of the fundamental properties of this important class of membranes; (2) for obtaining reliable comparisons between the properties of different PFSA membranes and (3) for improving the stabilization of the Nafion membranes at temperatures higher than 90 degrees C. (C) 2007 Elsevier B.V. All rights reserved.

The irreversible conductivity decay exhibited by Nafion 117 membranes above certain values of temperature and relative humidity (RH) has been investigated by two-probe impedance measurements carried out at 120°C with the electric field normal to the membrane surface, under controlled applied pressure on the electrodes. The analysis of the evolution of both frequency response and normal conductivity during the decay has suggested that the decay arises from changes in the bulk transport properties of the Nafion membrane. This has been confirmed by determining, under stability conditions, the conductivity of membranes pre-treated under decay conditions. The results of these measurements have shown that the decay occurs only if the membrane undergoes an anisotropic deformation along the direction parallel to the electrode surface. Four-probe impedance measurements with the electric field parallel to the membrane surface have also been carried out to determine the membrane tangential conductivity before and after the decay. Comparison of normal and tangential conductivity has indicated that the decay is associated, to a certain extent, with an increase in the conduction anisotropy.