Dow Membranes Research Articles

Coarse-grained study of the effect of hydrophobic side chain length on cluster size distributions and water diffusion in (amphiphilic-hydrophobic) multi-block co-polymer membranes

The formation of water clusters and their evolvement towards water containing percolating networks at 16 vol% water contents is studied by dissipative particle dynamics (DPD) for 7 multi-(amphiphilic-hydrophobic)-block co-polymer model membranes. The amphiphilic (A[C]) block is composed of one hydrophobic A backbone bead to which a hydrophilic (acidic) side chain, represented by one C bead, is attached. The architecture of the hydrophobic block is systematically varied and contains x = 4 or 6 A beads and is part of the Ax-p-q[Ap][Aq] family with side chain lengths (p,q)= (0,0), (2,1), (3,0) for x = 4 and (p,q)= (0,0), (3,2), (4,1), (5,0) for x = 6. Water diffusion derived from Monte Carlo trajectory simulations through 700 mapped morphologies reveal the same trends as the water bead diffusivity during the DPD simulations. For similar ion exchange capacity (or x) an increase in hydrophobic side chain length difference, p-q, results in better connected pores, increase of the percolating cluster size, less isolated clusters and enhanced diffusion. These trends are explained by assigning to each polymer A bead a number Nbond which is the number of bonds (DPD springs) between that A bead towards the nearest C bead within the same polymer. The hydrophobic (A-bead_Abead) contact matrices show that high Nbond A beads have a propensity to contact other high Nbond A beads. As a result phase separation and water diffusion become more pronounced with increase of <Nbond>. Based on these findings an alternative to Nafion and Dow membranes can be proposed for which a large fraction of hydrophobic (CF2- CF2-) fragments are distributed along hydrophobic side chains.

Perfluorinated ion-exchange membranes

This review concerns the chemical structure and synthesis of perfluorinated sulfocationite membranes, among which Nafion, MF-4SC, Flemion, Aciplex-S, and Dow membranes are the most well-known representatives. Special attention is given to main mechanisms controlling the formation of microstructure and its relationship with transport processes and selectivity levels of membranes. Key approaches to the modification of ion-exchange materials, above all involving the synthesis of hybrid membranes containing nanoparticles of inorganic compounds, are described. It is noted that increases in the conductivity and selectivity levels of hybrid membranes are primarily related to changes in structure of pores and channels and in the distribution of the concentration of carriers in them. Some examples of the practical use of perfluorinated membranes in modern technologies are highlighted.

Dow membranes recycle cooling tower blow-down water

Dow Water & Process Solutions, a division of US-based Dow Chemical Co, reports that its dual-membrane technology is helping a power plant in China to reuse blow-down water from its cooling towers.

The dependence of the electrochemical properties of perfluorosulfonic acid membrane/water systems on repeat unit structure

We developed a molecular thermodynamic framework to describe the dependence of the electrochemical properties of a perfluorosulfonic acid (PFSA) membrane/water system on the polymer structure. To better understand the behavior of the polymer membrane/water system, we developed a lattice model based on lattice cluster theory. We performed hypothetical calculations for a variety of repeat unit structures. We also investigated the correlation between the interaction energy and extending or reducing the chain length of the polymer repeat unit using COMPASS force fields. Our results indicate that the ionic conductivity of the nonpolar CF(2)CF(2) group in the main chain varies with the length of the chain. In addition, the ionic conductivity of different CF(2)CF(2) group chain lengths fluctuates according to hydration level. When OCF(2)CF(3)CF groups are substituted at 0 and 1, the predictions of the PFSA membrane agreed well with the experimental data from a Dow membrane. To obtain good ionic conductivity at a high vapor equilibrated hydration level, the nonpolar group in the polymer repeat unit and the substituted group chain connecting the nonpolar group length should both be short, and the substituted group chain connected with the sulfonic group should be long.

Degrémont’s largest nanofiltration plant: NOM removal at the Vegi 2000

Nanofiltration (NF) is an alternative for micropollutant and organic matter removal. Some important aspects must be taken into account in the design and the operation (e.g., biofouling). The Vegi 2000 plant with a total production capacity of 110,000 m3/d is the largest NF plant built by Degrémont. Commissioning was scheduled for end of 2008. The first feedback of the plant commissioning of the plant is presented. The choice of the treatment line was linked to pilot tests carried out from 2000 to 2005. The aim was to better understand water quality problems and to compare several alternatives for the treatment; the best dissolved organic carbon (DOC) and especially biodegradable DOC reduction was targeted. Pilots were installed at the Eupen drinking water treatment plant. Ozonation coupled with granular activated carbon (GAC) absorption was tested, and different membrane alternatives were also considered (microfiltration/ultrafiltration) (MF/UF), UF + powderedactivated carbon (PAC), NF). NF was tested with Toray, Hydranautics and Dow membranes and focused on Dow membranes. The first feedback of NF will be presented in the future.

Dynamics of PFSA Polymer Hydration Measured in Situ by SAXS

Proton exchange membranes were cast from three perfluorosulfonic acid (PFSA) ionomers, Nafion®, the 3M PFSA polymer, and the Dow membrane, with side chains -O-(CF2CF(CF3)- O-(CF2)2SO3H, -O-(CF2)4-SO3H, and -O-(CF2)2-SO3H respectively. The materials were heated above the Tg and the reorganization of the polymer morphology was studied in situ on the nanoscale by SAXS using synchrotron radiation. It was shown that both the 3M and the Dow membranes are much more crystalline and featured than the Nafion® membrane. The membranes were then hydrated at 80, 100, and 120ºC, with a humidified gas stream (80ºC dew point), and the kinetics of the morphological changes studied. In general the SAXS features increased in intensity as the ionomer peak grew in and swelled. Various heteropoly acid additives were also added to the membrane which was found to suppress the ionomer peak.

Proton exchange membranes based on the short-side-chain perfluorinated ionomer

Due to the renovated availability of the base monomer for the synthesis of the short-side-chain (SSC) perfluorinated ionomer, fuel cell membrane development is being pursued using this well known ionomer structure, which was originally developed by Dow in the 1980s. The new membranes under development have the trade name Hyflon Ion. After briefly reviewing the literature on the Dow ionomer, new characterization data are reported on extruded Hyflon Ion membranes. The data are compared to those available in the literature on the Dow SSC ionomer and membranes. Comparison is made also with data obtained in this work or available in the literature on the long-side-chain (LSC) perfluorinated ionomer (Nafion). Thermal, visco-elastic, water absorption and mechanical properties of Hyflon Ion are studied. While the general behavior is similar to that shown in the past by the Dow membranes, slight differences are evident in the hydration behavior at equivalent weight (EW) < 900, probably due to different EW distributions. Measurements on dry membranes confirm that Hyflon Ion has a higher glass transition temperature compared to Nafion, which makes it a more promising material for high temperature proton exchange membrane (PEM) fuel cell operation ( T > 100 °C). Beginning of life fuel cell performance has also been confirmed to be higher than that given by a Nafion membrane of equal thickness.

Membrane-Derived Fluorinated Radicals Detected by Electron Spin Resonance in UV-Irradiated Nafion and Dow Ionomers: Effect of Counterions and H2O2

Electron spin resonance (ESR) spectroscopy was used to detect and identify radicals formed by UV irradiation of Nafion and Dow perfluorinated membranes partially or fully neutralized by Cu(II), Fe(II), and Fe(III). This method allowed the monitoring of ESR signals from the paramagnetic counterions together with the appearance of membrane-derived radical species. The most surprising aspect of this study was the formation of membrane-derived radical species only in the neutralized membranes, and even in the absence of H2O2 in the case of Nafion/Cu(II) and Nafion/Fe(III). In Nafion/Cu(II), ESR spectra from radicals exhibiting hyperfine interactions with three equivalent 19F nuclei (the "quartet") and with four equivalent 19F nuclei (the "quintet") were detected. In Nafion/Fe(II) exposed to H2O2 solutions, the formation of Fe(III) was detected. Upon UV irradiation, strong signals from the chain-end radical ROCF2CF2* were detected first, followed by the appearance, upon annealing above 200 K, of the quartet signal observed in Nafion/Cu(II). In subsequent experiments with Nafion and Dow membranes neutralized by Fe(III), the ROCF2CF2* radicals were formed even in the absence of H2O2, indicating that the role of H2O2 is oxidation of Fe(II) to Fe(III); moreover, in these systems small amounts of the chain-end radicals were detected even without UV irradiation. This result validates the method used to form the radicals: the role of UV irradiation is to accelerate the formation of a signal that is produced, albeit slowly, even in the dark, and possibly during fuel cell operation. The major conclusion is that cations are involved in degradation processes; the point of attack appears to be at or near the pendant chain of the ionomer. Therefore when studying membrane stability, it is important to consider not only the formation of oxygen radicals, such as HO*, HOO*, and O2*-, that can attack the membrane but also the specific reactivity of counterions.

燃料電池用高分子膜の最前線 炭化水素系高分子電解質膜の可能性と問題点

This paper presents an overview of the possibility and problems of new proton-conducting polymer electrolyte membranes based on hydrocarbon polymers. Due to their chemical stability, high degree of proton-conductivity, and remarkable mechanical properties, perfluorinated polymer electrolytes such as Nafion®, Aciplex®, Flemion®, and Dow membranes are some of the most promising electrolyte membranes for polymer electrolyte fuel cells. A number of reviews on the synthesis, electrochemical properties, and fuel cell applications of perfluorinated polymer electrolytes have also appeared. While perfluorinated polymer electrolytes have satisfactory properties for a successful fuel cell electrolyte membrane, the major drawbacks to large-scale commercial use involve high cost and low proton-conductivities at high temperatures and low humidities. Presently, one of the most promising ways to obtain high-performance proton-conducting polymer electrolyte membranes is the use of hydrocarbon polymers for the polymer backbone. The present paper attempted to summarize the synthesis, chemical and electrochemical properties, and fuel cell application of new proton conducting polymer electrolytes based on hydrocarbon polymers that have been made during the past decade.

Electro-osmotic Drag of Water in Poly(perfluorosulfonic acid) Membranes

Direct methanol fuel cells (DMFCs) using poly(perfluorosulfonic acid) membranes were used to determine the water electro-osmotic drag coefficients in these ionomeric membranes. In a DMFC fed with 1.0 M aqueous methanol at the anode and dry at the cathode, the ionomeric membrane can be fully equilibrated on both sides with liquid water at a sufficiently high current density. In this special situation, the water flux across the membrane from anode to cathode is driven by protonic drag exclusively. From measurements performed on and membranes, water drag coefficients of all membranes increase with temperature. Compared with a fully hydrated membrane, a dried membrane (105°C in vacuum) known to have a lower water content has a lower water drag coefficient. For a series of Nafion membranes (including C membrane), a membrane with a higher equivalent weight has a lower water drag coefficient. The water drag coefficient of a Dow membrane is unusually low compared to that of a Nafion membrane of similar equivalent weight. These trends of water drag coefficients may correlate with the membrane structure and water content. Implications of these results on water management in polymer electrolyte fuel cells are discussed. © 2000 The Electrochemical Society. All rights reserved.

Complex impedance studies of proton-conducting membranes

Complex impedance studies have been carried out on Dow 800, Dow 1000 and Nafion 117 membranes at various water contents and a variety of temperatures and hydrostatic pressures. At room temperature and pressure the usual gradual decrease in electrical conductivity with decreasing water content is observed. For very low water content materials the variation of the conductivity with pressure from 0 to 0.2 GPa (2 kbar) is large and gives rise to apparent activation volumes, Δ V, as large as 54 cm 3/mol. In addition, for low water content materials, there is a tendency for smaller equivalent weights (same side chains) or larger side chains to have larger activation volumes. At high water content, Δ V is relatively independent of the host polymer and negative values are observed at the highest water contents. These results provide support for the model where proton transport in high water content sulfonated fluorocarbons is similar to that for liquid water. All results are explained qualitatively via free volume. Ambient-pressure, variable-temperature 2H T 1 and linewidth measurements imply a heterogeneous environment of the water molecules. Proton pulsed field gradient NMR studies in saturated Dow membranes verify the expectation that ionic conductivity is determined primarily by diffusion of water molecules.

Behavior of Raipore Radiation‐Grafted Polymer Membranes in H 2 / O 2 Fuel Cells

The high cost of the membrane materials of the currently favored perfluorinated sulfonic acid membranes such as the Nafions and Dow membranes, is one of the primary obstacles for commercialization of the polymer electrolyte fuel cells. Radiation‐grafted, partially fluorinated polymer membranes are considerably less expensive. Three Raipore membranes R‐1010, R‐4010, and R‐5010M were tested directly in fuel cells and examined by Fourier transform infrared spectroscopy. Under mild conditions, very good polarization results were obtained for cells with R‐1010 and R‐4010 membranes, compared with Nation 117 membranes using Nation‐impregnated Pt/C electrodes and with electrode/membrane assemblies properly hot‐pressed. Membrane degradation was observed from infrared measurements after cell operation, but both R‐4010 and R‐1010 membranes survived long‐term stability tests of ∼1000 h cell operation with reasonably good performance. Further work on both evaluation and modification of the membranes is still required.

Analysis of proton exchange membrane fuel cell performance with alternate membranes

Renewed interest in proton exchange membrane fuel cell technology for space and terrestrial (particularly electric vehicles) was stimulated by the demonstration, in the mid 1980s, of high energy efficiencies and high power densities. One of the most vital components of the PEMFC is the proton conducting membrane. In this paper, an analysis is made of the performances of PEMFCs with Dupont's Nafion ®, Dow's experimental, and Asahi Chemical's Aciplex-S ® membranes. Attempts were also made to draw correlations between the PEMFC performances with the three types of membranes and their physico-chemical characteristics. Practically identical levels of performances (energy efficiency, power density, and lifetime) were achieved in PEMFCs with the Dow and the Aciplex-S ® membranes and these performances were better than in the PEMFCs with the Nafion ®-115 membrane. The electrode kinetic parameters for oxygen reduction are better for the PEMFCs with the Aciplex-S ® and Nafion ® membranes than with the Dow membranes. The PEMFCs with the Aciplex-S ® and Dow membranes have nearly the same internal resistances which are considerably lower than for the PEMFC with the Nafion ® membrane. The desired membrane characteristics to obtain high levels of performance are low equivalent weight and high water content.

Determination of Water Content and Resistivity of Perfluorosulfonic Acid Fuel Cell Membranes

ABSTRACTWater uptake and resistivity have been determined for Dupont's Nafion-115®and Dow membrane 800 EW while in contact with a water-saturated nitrogen atmosphere using Fourier transform near-infrared (NIR), and AC impedance four point techniques in the temperature range of 23°C to 100°C. Results show that at room temperature there is a significant increase in water content and a corresponding decrease in the electrical resistivity as the relative humidity increases from 0% to 100%. Results also indicate that there is a substantial decrease in water uptake from water vapor at 100°C relative to that at 23°C. The water content of Dow membrane is higher than Nafion-115 under all conditions tested. The water contents of Dow PFSA 800 EW and Nafion-115 membranes at about 92% R.H. and 23°C are approximately 25 wt% and 18 wt%, respectively. The corresponding water content values at 100°C are 10 wt% and 8 wt%, respectively. The resistivity of the membranes decreases sharply with the temperature up to 60°C, reaches a minimum near 80°C then increases up to 100°C. The Dow membrane has lower resistivity than Nafion-115 over the entire range.

14N NMR spectroscopy of NH4+ as a probe of ion-exchange membranes

14N NMR Spectroscopy of NH4+ is a convenient and sensitive probe of structure and mobility in Nafion, Dow and Neosepta cation-exchange membranes. NH4+ ions bound to these commercially produced membranes give rise to small 14N quadrupolar splittings ranging from 10 to 160 Hz. Moderate stretching of Nafion and Dow membranes induces much larger splittings of up to 2 kHz. The splitting has also been found to be dependent on the orientation of the membranes in the magnetic field B0. In Nafion it reaches a minimum at an angle of 45–55° between the direction of stretch and B0. The quadrupolar splittings also increase with increase in temperature. 14N spin–lattice relaxation times in all three membranes at 300 K are about 20 times shorter than in aqueous solution, consistent with strongly bound NH4+ ions. The apparent activation energies for 14N relaxation, derived from the temperature dependence of the T1, data, show that reorientation of the bound NH4+ ions requires ca. 20 kJ mol−1 in Nafion and Dow membranes and ca. 30 kJ mol−1 in Neosepta, compared with 6.5 kJ mol−1 for free NH4+ in aqueous solution.

Oxygen Reduction on Ruthenium‐Oxide Pyrochlore Produced in a Proton‐Exchange Membrane

Ruthenium‐oxide pyrochlore particles were synthesized inside a Dow developmental fuel‐cell membrane (the Dow membrane is a perfluorosulfonated ion‐exchange polymer) by treatment of Ru3+, Pb2+‐exchanged membrane in alkaline aqueous solution with purging of at 75°C for at least 2 days. Compared to our previous work of ruthenium‐oxide pyrochlore electrode attached to a Dow membrane, this work took advantage of incorporating Dow gel, eliminated the reaction‐irrelevant Teflon binder, and simplified the electrode fabrication procedure. Interesting oxygen reduction activity at room temperature was obtained, and the response time for the oxygen reduction reaction was very fast.

AC Impedance and Conductivity Study of Alkali Salt Form Perfluorosulfonate Ionomer Membranes

AC impedance studies were performed on Na+ and K+ alkali salt forms of the short sidechain perfluorosulfonate ionomer (PFSI) membrane films manufactured by Dow Chemical Company. With impressive performances of 4 A/cm2current density and power densities near 2.5 W/cm2, the acid forms of these short sidechain PFSI are very promising candidates for use in fuel cells for future electric vehicles.1 Since, at present, little is known about the exact transport mechanisms for the ionic species within PFSIs, an ac impedance study of the Na+ and K+ forms has been performed. It is hoped that this will provide some insight and understanding of the transport mechanisms in the PFSI and thus will aid in the development and optimization of fuel cells. Results suggest that there are marked differences with respect to host environments within the Dow membrane as compared to Nafion® long sidechain PFSI membrane films. Impedance spectra of the Dow salt form membranes displaying two distinct relaxation peaks while the spectra for all forms of Nafion reveal only a single peak. This second low temperature peak in the Dow membrane has been attributed to a much larger ratio, possibly greater than one, existing within the Dow membrane as compared to Nafion, which contains an ratio between 0.23 and 0.29. The temperature behavior of the dc conductivity was extracted from the temperature Cole‐Cole plots and was found to be non‐Arrhenius. Room temperature conductivities of the order of 10−2S/cm were achieved for the Na+ and K+ salt forms of the XUS membrane. It was observed that for all , . It has been proposed that this behavior arises from the hydration phenomena of the alkali ions. These results suggest that substantial differences exist between host environments of the Dow and Nafion PFSI.

Proton and deutron NMR study of PTFE ionomer membranes

Proton and deutron NMR have been conducted to investigate the ionic motion in perfluorinated ionomer membranes from Dow Chemical (XUS) and DuPont (Nafion R). Two proton relaxation peaks were found in the XUS specimen absorbed with H 2O. The major (narrow) peak presented a spin-lattice relaxation time ( T 1) of 107 ms while the minor (broader) one gave much longer T 1. While the former was attributed to the water molecules involved in restricted motion, the latter was expected to be associated with the protons located in the vicinity of the sulfonate groups. Similar to the previous results from the others, only a single peak was detected in Nafion R in 1H spectra, indicating that the protons in the different environments were engaging rapid exchange within NMR time scale. In contrast to the inverse proportion dependence of the linewidth on the water sorption in Nafion R, the major line of the XUS membrane exhibited insensitive linewidth dependence on the variation of H 2O concentration. The difference was attributed to the existence of narrow breaths of the pores in XUS sample, such that free water contribution to the enhancement of proton mobility was limited. The 2H spectra of Nafion R were found to possess a doublet, due to nuclear quadrupolar interaction. Dow (XUS) membrane treated in at 100% relative humidity (RH) D 2O presented a single peak with the linewidth insensitive to the amount of heavy water absorbed. An additional rise emerged on the “shoulder” of this single peak when treated at 33% RH. It is concluded that XUS membrane does not provide strong hydrogen bonding to eliminate the rapid motion average over the nuclear quadrupole interaction.

Comparative laboratory evaluation of six chlorine monitoring devices

The performances of six chlorine monitoring methods/devices have been compared to expedite the development of several novel personal monitoring devices. The devices evaluated included a diffusional colorimetric dosimeter badge from Moleculon Research Corporation, three diffusional electrochemical sensors (the Dow membrane sensor, the Dow glass frit sensor, and the Compur MonitoxTrademark of Compur Electronic Gmbh, Munich, West Germany sensor), the sulfamic acid bubbler/ion specific electrode method, and the InterScan voltammetric instrument. To be useful for personal monitoring, the electrochemical sensors must be coupled with a digital readout or microprocessor-based unit to display, store, or average chlorine concentrations detected by the sensor. A Dow built microprocessor unit and the Compur Mini-Dosimeter were evaluated. The major evaluation parameters included: applicable concentration range, linear range, accuracy, precision, effects of humidity, temperature and air velocity, and interferences from Br2, N02, SO2, H2S, and HC1. While all of the methods are useful for chlorine monitoring, each device has certain advantages and limitations. The successful utilization of these devices for determing time-weighted-average and excursion concentrations requires calibration of sensors with a reliable dynamic calibration unit, determination of field conditions encountered during sampling, and an understanding of the advantages and limitations of these devices with respect to the field conditions.

Comparative laboratory evaluation of six chlorine monitoring devices

The performances of six chlorine monitoring methods/devices have been compared to expedite the development of several novel personal monitoring devices. The devices evaluated included a diffusional colorimetric dosimeter badge from Moleculon Research Corporation, three diffusional electrochemical sensors (the Dow membrane sensor, the Dow glass frit sensor, and the Compur MonitoxTrademark of Compur Electronic Gmbh, Munich, West Germany sensor), the sulfamic acid bubbler/ion specific electrode method, and the InterScan voltammetric instrument. To be useful for personal monitoring, the electrochemical sensors must be coupled with a digital readout or microprocessor-based unit to display, store, or average chlorine concentrations detected by the sensor. A Dow built microprocessor unit and the Compur Mini-Dosimeter were evaluated. The major evaluation parameters included: applicable concentration range, linear range, accuracy, precision, effects of humidity, temperature and air velocity, and interferences from Br2, N02, SO2, H2S, and HC1. While all of the methods are useful for chlorine monitoring, each device has certain advantages and limitations. The successful utilization of these devices for determing time-weighted-average and excursion concentrations requires calibration of sensors with a reliable dynamic calibration unit, determination of field conditions encountered during sampling, and an understanding of the advantages and limitations of these devices with respect to the field conditions.