Showing papers on "Nafion published in 2001"

Synthesis of highly sulfonated poly(arylene ether sulfone) random (statistical) copolymers via direct polymerization

Abstract: Novel biphenol-based wholly aromatic poly (arylene ether sulfones) containing pendant sulfonate groups were prepared by direct aromatic nucleophilic substitution polycondensation of disodium 3,3'-disulfonate-4,4'-dichlorodiphenyl sulfone (SDCDPS), 4,4'-dichlorodiphenylsulfone (DCDPS) and biphenol. Copolymerization proceeded quantitatively to high molecular weight in N-methyl-2-pyrrolidinone at 190°C in the presence of anhydrous potassium carbonate. Tough membranes were successfully cast from the control and the copolymers, which had a SDCDPS/DCDPS mole ratio of either 40:60 or 60:40 using N,N-dimethylactamide; the 100% SDCDPS homopolymer was water soluble. Short-term aging (30 min) indicates that the desired acid form membranes are stable to 220°C in air and conductivity values at 25°C of 0.110 (40%) and 0.170 S/cm (60%) were measured, which are comparable to or higher than the state-of-the art fluorinated copolymer Nafion 1135 control. The new copolymers, which contain ion conductivity sites on deactivated rings, are candidates as new polymeric electrolyte materials for proton exchange membrane (PEM) fuel cells. Further research comparing their membrane behavior to post-sulfonated systems is in progress.

Electrochemical Impedance Study of PEM Fuel Cells. Experimental Diagnostics and Modeling of Air Cathodes

Abstract: The impedance response of a H2/air PEM FC was investigated in conditions relevant for the operation of low-power fuel cells, used for portable applications (cell temperature close to ambient and moderate humidification). The dependence of EIS pattern on cell voltage, humidification temperature, and air flow rate was examined systematically. The spectrum of the air cathode at room temperature was found to contain two arcs, the potential dependence of which was analyzed in terms of the flooded-agglomerate model for gas diffusion electrodes. The high-frequency (HF) loop is responsible for processes occurring in the cathode catalyst layer: interfacial charge transfer and mass transport of air in the pores of the catalyst layer (agglomerate diffusion) and in the Nafion layer surrounding the catalyst particles (thin film diffusion). On the basis of its flow rate dependence, the low-frequency loop (LF) was assigned to the mass transport limitation which appears in the backing due to liquid water accumulation. T...

Evaluation of a Sol-Gel Derived Nafion/Silica Hybrid Membrane for Polymer Electrolyte Membrane Fuel Cell Applications: II. Methanol Uptake and Methanol Permeability

Abstract: Sol-gel derived Nafion/silica hybrid membranes were investigated as a potential polymer electrolyte for direct methanol fuel cell applications. Methanol uptake and methanol permeability were measured in liquid and vapor phase as a function of temperature, methanol vapor activity, and silica content. Decreased methanol uptake from liquid methanol was observed in the hybrid membranes with silica contents of 10 and 21 wt %. The hybrid membrane with silica content of ≈20 wt % showed a significant lower methanol permeation rate when immersed in a liquid methanol-water mixture at 25 and 80°C. Methanol uptake from the vapor phase by the hybrid membranes appears similar to that of unmodified Nafion. Methanol diffusion coefficients, as determined from sorption experiments, were slightly lower in the hybrid membranes than in unmodified Nafion. However, in direct permeation experiments, significantly lower methanol vapor permeability was seen only in the hybrid membrane with silica content of ≈20 wt %. Based on these results, Nafion/silica hybrid membranes with high silica content have potential as electrolytes for direct methanol fuel cells operating either on liquid or vapor-feed fuels. © 2001 The Electrochemical Society. All rights reserved.

Evaluation of a Sol-Gel Derived Nafion/Silica Hybrid Membrane for Proton Electrolyte Membrane Fuel Cell Applications: I. Proton Conductivity and Water Content

Abstract: Sol-gel derived Nafion®/silica hybrid membranes were investigated as a potential polymer electrolyte for fuel cell applications. Membrane proton conductivity and water content were measured as a function of temperature, water vapor activity, and silica content. The hybrid membranes have a higher water content at 25 and 120°C, but not at 150 and 170°C. Despite the higher water content, the proton conductivities in the hybrid membranes are lower than, or equal to, that in unmodified Nafion membranes under all conditions investigated. The proton conductivity of the hybrid membrane decreases with increasing silica content under all conditions. © 2001 The Electrochemical Society. All rights reserved.

Investigation of the Transversal Water Profile in Nafion Membranes in Polymer Electrolyte Fuel Cells

Abstract: The in situ resistance of Nafion membranes with different thickness was measured in one-dimensional fuel cells as a function of current density. Except for the thin Nation I 12 membrane, an increase of the ionic resistance with current density (in the range 0 to I A/cm 2 ) was found. The thicker the membrane, the stronger the increase in the same current density interval. The resistance distribution across the thickness of membranes was determined by using membranes composed from several thin sheets with interlying thin gold wires as potential probes. It was found that the increase of the resistance is always confined to the membrane sheet contacting the anode electrode. These measurements, combined with the results from experiments with membranes of different water content, lead to the conclusion that the resistance increase at the anode side is due to the insufficient compensation of the electro-osmotic drag by the hack transport of water to the anode. Based on a solution diffusion mechanism of the water motion in the membrane, the experimental results may he explained by a mechanism whereby the electro-osmotic drag coefficient is independent of the local membrane hydration and the water diffusion coefficient D H2O , is a strong function of the local membrane water content. The experimental data would, qualitatively, also he in line with a model proposing hack transport of water to the anode by convection of water in the submicropores of the membrane.

Molecular Dynamics Simulation of Microstructure and Molecular Mobilities in Swollen Nafion Membranes

Abstract: The microphase segregation in the Nafion (DuPont trademark) perfluorinated membrane at different water contents was studied using molecular dynamics simulations. As the degree of solvation increased, we observed the formation of water clusters containing up to ca. 100 water molecules. In contrast to the conventional network models, the water clusters do not form a continuous hydrophilic subphase. The cluster size distribution is rather wide and evolves in time due to formation and break-up of temporary bridges between the clusters. This dynamic behavior of the cluster system allows for the macroscopic transfer of water and counterion. The calculated diffusion coefficients of water were found to be on the same order as the experimental ones.

A Phenomenological Model of Water Transport in a Proton Exchange Membrane Fuel Cell

Abstract: A steady-state, two-dimensional model is presented and discussed that describes the water transport in a proton exchange membrane fuel cell. Concentrated solution theory is used to describe the transport of water in the membrane, and of water vapor and liquid water in the electrodes. The inclusion of the liquid water transport into the model turned out to be essential for explaining recent experimental results on the effective drag coefficient and its dependence on operating conditions as well as on variations of the components that constitute the membrane electrode assembly.

Polystyrene-Bound Tetrafluorophenylbis(triflyl)methane as an Organic-Solvent-Swellable and Strong Brønsted Acid Catalyst

Abstract: Several advantages over inorganic solid acids such as zeolites and perfluororesinsulfonic acids such as Nafion are offered by the new reusable polystyrene-bound catalyst 1: a broader range of applications, improved yields, improved selectivity, and milder reaction conditions. Tf = F3 CSO2 .

Electrocatalytic oxidation and sensitive detection of cysteine on a lead ruthenate pyrochlore modified electrode

Abstract: Electrocatalytic oxidation of cysteine (CySH) at Nafion/lead ruthenate pyrochlore (Py) chemically modified electrodes was thoroughly studied. Electrochemical ac impedance spectroscopy analysis indicated the formation of Py microparticles in the interfacial galleries of Nafion. Experiments with benchmark systems of Fe(CN)63-/4- and Ru(bpy)2+/3+ reveal the suppression of Nafion's anionic character after the in situ precipitation of Py. Michaleis−Menten-type kinetics with the rate determination step of CyS−Py−RuVI → Py−RuIV + CyS−SCy was proposed for this catalytic oxidation. The electrocatalytic behavior is further developed as a sensitive detection scheme for CySH by square-wave voltammetry (SWV) and flow injection analysis (FIA). Under the optimized conditions, the calibration curve is linear up to 560 μM with a detection limit (signal/noise 3) of 1.91 μM in SWV. The detection limit can be improved to 1.70 nM (i.e., 24.22 ng in a 20-μL sample loop) in FIA. This is the lowest value ever reported for direct...

Modified Nafion 117 as an Improved Polymer Electrolyte Membrane for Direct Methanol Fuel Cells

Abstract: Modification of the surface structure of the commercial ion exchange membrane Nafion®1 17, by low dose electron beam (EB) exposure, to produce an improved polymer electrolyte membrane for direct methanol fuel cells (DMFC) is described. Nation 117 film was exposed to low dose EB irradiations at an accelerating voltage of 35 kV. Subsequently the properties of the film itself, in terms of conductance, methanol permeability, percentage water uptake and shrinkage, together with the performance of its membrane electrode assembly in the DMFC were analyzed and contrasted with the untreated material. Low-dose EB treatment is shown to be effective in the reduction of methanol crossover, 600 μC/cm 2 exposure reducing crossover to 7% of that of the parent material. In terms of overall DMFC performance (maximum power output). improvements of up to 51% are reported in comparison to the use of untreated Nafion 117. A simple analytical protocol, allowing film properties to he directly related to subsequent DMFC performance, is also reported. IR reflectance (attenuated total reflectance) spectroscopy was used to study film surface composition and determine the effect of low-dose EB exposure on Nafion 117 structure. These observations are contrasted with previous findings using traditional EB systems.

Kinetics of Water Sorption in NafionThin Films − Quartz Crystal Microbalance Study

Abstract: Quartz crystal microbalance measurements are used to investigate the water sorption in the cast Nafion films exposed to water vapor. Equilibrium and kinetic sorption data are reported for H- and Na...