Showing papers on "Nafion published in 2000"

Methanol transport through Nafion membranes : Electro-osmotic drag effects on potential step measurements

Abstract: This paper describes methanol flux measurements across Nafion, 1100 equivalent weight membranes under conditions of a direct methanol fuel cell but in which methanol is completely electro-oxidized on the opposite side in an inert atmosphere at sufficiently high electrode potential. Both the diffusion coefficient and the methanol concentration in the membrane were determined from the measured transient limiting current density following a potential step. Corrections for electro-osmotic drag effects are developed and found necessary even for low MeOH concentrations. The results agree well with those obtained from nuclear magnetic resonance measurements. The partition coefficient [{rho} = [MeOH]{sub membrane}/[MeOH]{sub solution}] was approximately constant for the membranes in contact with methanol solutions of various concentration and from room temperature to 90 C. The activation energy of methanol diffusion in a fully hydrated Nafion membrane between 30 and 130 C is 4.8 kcal/mol, and that for protonic conduction under the same conditions is 2.3 kcal/mol. For a membrane dried in vacuum at above 100 C, lower values of methanol permeation rate and protonic conductance were found.

Diffusion of water in Nafion 115 membranes

Abstract: In this paper, experimental and simulated data for the diffusion of water across Nafion membranes as a function of the water activity gradient are presented. The gradient in the activity of water across the membrane was varied by changing the flow rate and pressure of nitrogen gas on one side of the membrane. The other side of the membrane was equilibrated with liquid water. It was found that the model predictions are very sensitive to the value of the diffusion coefficient of water in Nafion. Using the Fickian diffusion coefficient extracted from self-diffusion measurements reported in the literature, the model simulations matched experimental data with less than 5% error over a wide range of operating conditions.

Water and Methanol Uptakes in Nafion Membranes and Membrane Effects on Direct Methanol Cell Performance

Abstract: This paper compares direct methanol fuel cells (DMFCs) employing two types of Nafion{reg{underscore}sign} (E.I.DuPont de Nemours and Company) membranes of different equivalent weight (EW). Methanol and water uptakes in 1,100 and 1,200 EW Nafion membranes were determined by weighing P{sub 2}O{sub 5}-dried and methanol solution-equilibrated membranes. Both methanol and water uptakes in the 1,200 EW membrane were about 70--74% of those in the 1,100 EW membrane. The methanol crossover rate corresponding to that in a DMFC at open circuit was measured using a voltammetric method in the DMFC configuration and under the same cell operating conditions. After accounting for the thickness difference between the membrane samples, the methanol crossover rate through a 1,200 EW membrane was 52% of that through an 1,100 EW membrane. To resolve the cathode and anode performances in an operating DMFC, a dynamic hydrogen electrode was used as a reference electrode. Results show that in an operating DMFC the cathode can be easily flooded, as shown in a DMFC using 1,100 EW membrane. An increase in methanol crossover rate decreases the DMFC cathode potential at open circuit. At a high cell current density, the DMFC cathode potential can approach that of a H{sub 2}/air cell.

High‐Temperature Proton Conducting Membranes Based on Perfluorinated Ionomer Membrane‐Ionic Liquid Composites

Abstract: Composite membranes that exhibit fast proton transport at elevated temperatures are needed for proton‐exchange‐membrane fuel cells and other electrochemical devices operating in the 100 to 200°C range. Traditional water‐swollen proton conducting membranes such as the Nafion membrane suffer from the volatility of water in this temperature range leading to a subsequent drop in conductivity. Here we demonstrate that perfluorinated ionomer membranes such as the Nafion membrane can be swollen with ionic liquids giving composite free‐standing membranes with excellent stability and proton conductivity in this temperature range while retaining the low volatility of the ionic liquid. Ionic conductivities in excess of 0.1 S/cm at 180°C have been demonstrated using the ionic liquid 1-butyl, 3-methyl imidazolium trifluoromethane sulfonate. Comparisons between the ionic‐liquid‐swollen membrane and the neat liquid itself indicate substantial proton mobility in these composites. © 2000 The Electrochemical Society. All rights reserved.

Modeling of Conductive Transport in Proton-Exchange Membranes for Fuel Cells

Abstract: The proton-exchange membrane (PEM) fuel cell has lately emerged as a highly promising power source for a wide range of applications. The solid polymer electrolyte utilized in these fuel cells is typically a polyperfluorosulfonic acid (PFSA) membrane ( e.g., Nafion ® , manufactured by DuPont), that provides excellent performance in the presence of water by virtue of its strong acidity, low permeability of hydrogen and oxygen, and good electrochemical stability in the presence of electrocatalysts. This has allowed the development of low-temperature PEM fuel cells with impressive current densities. These membranes have also been widely utilized in the chlor-alkali industry. However, an understanding and modeling of the transport of ionic species through these ion-exchange membranes is not yet adequately developed, especially for proton transport, which is the focus of this paper. There are numerous studies on the nanostructural aspects of the

RRDE study of oxygen reduction on Pt nanoparticles inside Nafion®: H2O2 production in PEMFC cathode conditions

Abstract: Dihydrogen peroxide production on platinum particles supported on carbon inside a proton exchange membrane (Nafion®), that is, under the working conditions of PEMFC cathodes, is rather small at the usual oxygen reduction potentials. As on bulk platinum, a four-electron mechanism appears to be the main pathway, with particle size and carbon substrate effects on the H2O2 production. A large increase in the H2O2 contribution takes place at low potentials, that is, at the working potentials of PEMFC anodes.

Conductivity and Water Uptake of Aromatic‐Based Proton Exchange Membrane Electrolytes

Abstract: Water uptake and proton conductivity as a function of temperature were determined for three aromatic-based, sulfonic acid-bearing polymers, plus the perfluoroalkyl sulfonic acid Nafion{reg_sign} 117. Water uptake of submerged, equilibrated samples ranged from less than five water molecules per acid group for a high equivalent weight, sulfonated polyethersulfone to almost fifty waters per acid for a low equivalent weight, sulfonated polyetheretherketone. The most conductive aromatic-based polymer, sulfonated polyphenylquinoxaline (S-PPQ), had a room temperature conductivity of 9.8 x 10{sup {minus}3} S/cm, about an order of magnitude less than that of a perfluoroalkyl sulfonic acid under identical conditions. The slope of the S-PPQ Arrhenius conductivity plot was sufficiently steep that at 180 C, the proton conductivity, 1.3 x 10{sup {minus}1} S/cm, was only a factor of two lower than that of Nafion under similar conditions. The lower conductivity of the aromatic-based sulfonic acid polymers can be attributed to chain rigidity, lack of ion channels, and lower acidity.

Hydration of Nafion® studied by AFM and X-ray scattering

Abstract: Nafion® is a commercially available perfluorosulphonate cation exchange membrane commonly used as a perm-selective separator in chlor-alkali electrolysers and as the electrolyte in solid polymer fuel cells. This usage arises because of its high mechanical, thermal and chemical stability coupled with its high conductivity and ionic selectivity, which depend strongly on the water content. The membrane was therefore studied in different states of hydration with two complementary techniques: atomic force microscopy (AFM) and small angle X-ray scattering (SAXS) combined with a maximum entropy (MaxEnt) reconstruction. Tapping mode phase imaging was successfully used to identify the hydrophobic and hydrophilic regions of Nafion. The images support the MaxEnt interpretation of a cluster model of ionic aggregation, with spacings between individual clusters ranging from 3 to 5 nm, aggregating to form cluster agglomerates with sizes from 5 to 30 nm. Both techniques indicate that the number density of ionic clusters changes as a function of water content, and this explains why the bulk volumetric swelling in water is observed to be significantly less than the swelling inferred from scattering measurements.

A Comparative Investigation of Proton and Methanol Transport in Fluorinated Ionomeric Membranes

Abstract: Proton conductivities as well as methanol permeabilities were investigated for two commercial partially fluorinated ionomeric membranes: IonClad{reg_sign} R-1010 and R-4010, both manufactured by Pall Company. The investigation was carried out in the temperature range 20 to 60 C. The results were compared with data for Nafion{reg_sign} 117 (DuPont) obtained in the same temperature range. The authors found that IonClad membranes, while exhibiting proton conductivity approximately equal to that of Nafion 117, are considerably less permeable to methanol. The permeability measured for R-4010 membranes was almost four times smaller than for the Nafion membrane. These characteristics together with the low cost make the IonClad membranes interesting potential candidates for application in the direct methanol fuel cell.

Molecular Simulation Study of Nafion Membrane Solvation in Water and Methanol

Abstract: Molecular mechanics and molecular dynamics simulation studies of conformation and solvation of perfluorosulfate oligomers, representing fragments of Nafion membrane, have been performed. Two typical conformations of the oligomer, composed of 10 monomer units, have been found in a vacuum. A stretched geometry of the fluorocarbon skeleton with tortuosity of 2.4 was reached by energy optimization starting from the regular conformation with all CCCC angles in the trans position. A highly folded spiral-like configuration was obtained when the randomly bent chain was taken as the initial configuration. Molecular dynamics simulations of shorter oligomers solvated in water and methanol revealed a noticeable difference between the geometries of the fluorocarbon skeleton in different solvents. The skeleton structure in water was substantially more folded than in methanol. The side chain of the Nafion oligomers was found to be quite stiff; only a few conformational transitions in the side chain were detected. Both w...

High-Resolution Imaging of Ionic Domains and Crystal Morphology in Ionomers Using AFM Techniques

Abstract: AFM methods were applied to resolve the surface and near-surface morphology of the ionic domains in Nafion membranes, Surlyn ionomers, and other ionomers. The ionic clusters were resolved by a new tapping AFM method where low oscillation amplitudes were used, and tip−ionic domain interactions were apparently able to dominate the signal allowing nanometer-level resolution of the domains. In other operating modes, the fluoropolymer crystal or aggregate domains were imaged using tapping AFM by the normal “stiffness” contrast. By sequential images taken under different AFM conditions, the “softer” fluorine-depleted regions were found to contain ionic domains in the same topographical areas, and the changes due to swelling by water were examined in one case. A third AFM operating mode was used to examine the composition in the outermost few angstroms of the surface. Data proving the existence of a very thin fluorine-rich “barrier” layer covering the entire surface of Nafion are obtained, and its relevance to v...

Interpretation of the Small-Angle X-ray Scattering from Swollen and Oriented Perfluorinated Ionomer Membranes

Abstract: Small-angle X-ray scattering (SAXS) and bulk volumetric measurements were performed on the perfluorinated ionomer membrane “Nafion”. The swelling experiments were conducted using controlled environments of different relative humidities. The membranes were oriented using uniaxial draw, both before and after conversion of the precursor material, as well as both sequential and simultaneous biaxial draw. Using a novel model-independent maximum entropy method, it is shown that the most statistically probable scattering model for Nafion is an ion clustered morphology with a hierarchical scale of structure. The smallest scale is comprised of arrays of roughly spherical ionic clusters. The clusters are agglomerated into higher-order structures, whose shape is determined by the spatial coherence of the clusters, as inferred from the SAXS measurements. Anomalies in the magnitudes of the microscopic and macroscopic swellings are shown to be caused by the rearrangement of ionic material, producing changes in the numb...

Electrogenerated chemiluminescence of tris(2,2'-bipyridyl)ruthenium(II) ion-exchanged in nafion-silica composite films

Abstract: The voltammetry and electrogenerated chemiluminescence (ECL) of tris(2,2‘-bipyridyl)ruthenium(II) (Ru(bpy)32+) ion-exchanged in Nafion and Nafion−silica composite materials have been investigated. The major goal of this work was to investigate and develop new materials and immobilization approaches for the fabrication of ECL-based sensors with improved reactivity and long-term stability. Nafion−silica composite materials with varying contents of Nafion (53−100 wt % relative to silica) were prepared via the two-step acid/base hydrolysis and condensation of tetramethoxysilane. The Nafion doped sols were spin cast on glassy carbon electrodes, dried, and then ion-exchanged with Ru(bpy)32+. The shapes of the cyclic voltammetric curves and the amount of Ru(bpy)32+ exchanged into the films strongly depends on the amount of Nafion incorporated into the hybrid sol. Nafion−silica films with a low content of Nafion ion-exchanged less Ru(bpy)32+ and exhibited tail-shaped voltammetry at 100 mV/s. The ECL of immobilize...

Electro‐oxidation of Dimethyl Ether in a Polymer‐Electrolyte‐Membrane Fuel Cell

Abstract: The possibility of using dimethyl ether (DME) as a fuel in direct oxidation polymer‐electrolyte‐membrane (PEM) fuel cells is investigated. A mechanism for DME electro‐oxidation is proposed based on the results of half‐cell experiments using cyclic voltammetry combined with gas‐chromatographic (GC) analyses of a single direct DME fuel cell. It is shown that, as a consequence of this mechanism, there is an additional overpotential at the anode of a direct DME fuel cell which is related to the initial adsorption step on the catalyst surface. DME is typically not oxidized at the cathode of a PEM fuel cell. This minimizes unwanted effects of fuel crossover, leading to improved fuel‐cell efficiencies compared to direct methanol fuel cells, especially at low‐to‐medium current densities. © 2000 The Electrochemical Society. All rights reserved.

Equilibrium and diffusion of methanol and water in a nafion 117 membrane

Abstract: The uptake concentrations of methanol and water in a Nafion membrane are measured as a function of methanol mol fraction from a liquid methanol-water mixture. The results are fitted by a thermodynamically consistent equation, derived from an expression of the Gibbs energy. The uptake of pure methanol and pure water from the vapor phase is measured as a function of vapor pressure. A discontinuity, known in the literature as Schroeder's paradox, between membrane uptakes of pure methanol and pure water from the liquid and vapor phases is noted. Regions of unconditional instability are enveloped by spinodal loci for the ternary mixture in the membrane when equilibrated with liquid mixtures. The effective diffusion coefficient is measured at 60°C for diffusion from a vapor into a saturated Nafion membrane. The effective diffusion coefficients are found to be on the order of 10 -11 m 2 /s, which is comparable to other measurements made for diffusion of a vapor through the membrane.

In vitro and in vivo degradation of glucose oxidase enzyme used for an implantable glucose biosensor.

Abstract: Background: The degradation of the glucose oxidase (GOD) enzyme, commonly used in the construction of glucose sensors has been of concern for scientists for decades. Many researchers have found that GOD deactivates over time, mostly due to H2O2 oxidation. This decay can lead to the eventual failure of the sensor. However, these findings are controversial, because other researchers did not find this degradation. Methods: The goal of this study was twofold. The first goal was to evaluate the in vitro and in vivo stability of two commercially available GOD enzymes and the second goal was to evaluate Nafion as a protective coating of GOD. Crosslinked GOD samples were sandwiched between two 10- μ m pore polycarbonate membranes (Nafion coated or uncoated) and placed in custom designed Lexan chambers. Chambers were then exposed to a total of five different environments: Dulbecco's Modified Eagle Medium (DMEM) or phosphate buffered saline (PBS) with and without a 5.6-mM glucose concentration, as well as the subcu...