Showing papers on "Nafion published in 1997"
TL;DR: In this paper, some aspects of the simultaneous optimisation of material properties of proton conductors which are relevant for their use in electrochemical cells such as fuel cells, electrochemical reactors and sensors are discussed.
947 citations
TL;DR: In this paper, gas chromatography was used to study the hydrogen and oxygen permeability of Nafion® 117 membrane and recast nafion film at different values of temperature and gas relative humidity.
Abstract: Oxygen and hydrogen permeability through Nafion® 117 membrane and recast Nafion film has been studied by means of gas chromatography at different values of temperature and gas relative humidity. Water uptake from the vapour phase by Nafion® 117 membrane and recast film has been investigated. It has been shown that oxygen and hydrogen permeability increases both with temperature and relative humidity of the gas, but water vapour uptake by both Nafion® 117 membrane and recast film decreased with increasing temperature. SEM studies have been performed on the membrane-electrode assembly obtained by impregnation/hot-pressing and on carbon/Nafion® composite; these have indicated that the active catalyst layer is porous with channels for the gas transport.
332 citations
TL;DR: In this article, a structural study using both small-angle X-ray and neutron scattering (respectively SAXS and SANS) of water-swollen perfluorosulfonated ionomer (PFSI) membranes is presented.
Abstract: A structural study using both small-angle X-ray and neutron scattering (respectively SAXS and SANS) of water-swollen perfluorosulfonated ionomer (PFSI) membranes is presented. The comparison between the SAXS and SANS spectra combined with the analysis of both the scattering invariant and the Porod behavior of the scattering curves confirms the phase separation between the water pools and the perfluorinated matrix. The experimental data obtained in absolute units were analyzed in terms of the different models proposed for ionomers, and the local order model appears as the best model for describing the structure of swollen PFSI membranes. The size of the water pools and the number of ionic groups per ionic aggregate deduced from the analysis of the asymptotic behavior of the scattering curve at large angles compare well with the values deduced from the local order model. The SAXS spectra of swollen PFSI membranes are presented for the first time over an extended angular range covering 3 decades of angles us...
286 citations
TL;DR: The feasibility of using nonvolatile molten and solid acidic electrolyte impregnated ion exchange membranes in higher temperature proton-exchange membrane fuel cells (PEMFCs) to alleviate their water dependence is investigated in this paper.
Abstract: The feasibility of using nonvolatile molten and solid acidic electrolyte impregnated ion-exchange membranes in higher temperature proton-exchange membrane fuel cells (PEMFCs) to alleviate their water dependence is investigated. Higher temperature PEMFC operation reduces CO poisoning as well as passivation of the Pt electrocatalyst by other condensable species. Further, higher temperature operation could eventually allow direct use of low-temperature reformable fuels such as methanol in the PEMFC. The methodology proposed here involves supporting an appropriate acidic solid, melt, or solution of low volatility within the pores of Nafion{reg_sign} so as to enhance its protonic conductivity at higher temperatures and lower humidity levels. Preliminary experimental results reported here for a PEM fuel cell operating at temperatures of 110 to 120 C based on Nafion supported solutions of heteropolyacid indicate the feasibility of the technique.
233 citations
TL;DR: In this paper, the response to relative humidity of SiO2 composite films was analyzed with their complex impedance spectra at different humidities and the stability of the response against different aging environments was examined.
Abstract: Nafion, sol-gel derived SiO2 and SiO2/Nafion composite films were prepared through casting and dip-coating methods. Impedance response of the films to relative humidity and the stability of the response against different aging environments were examined. The response to relative humidity of each film was analyzed with their complex impedance spectra at different humidities.
180 citations
TL;DR: In this article, a direct methanol fuel cell (DMFC) employing a proton conducting membrane was used to determine the electro-osmotic drag coefficient of water in the ionomeric membrane.
Abstract: A direct methanol fuel cell (DMFC) employing a proton conducting membrane was used to determine the electro-osmotic drag coefficient of water in the ionomeric membrane. Water flux across the membrane in such a cell (operated with 1.0 M aqueous methanol at the anode and dry O{sub 2} at the cathode) is driven by protonic drag exclusively at sufficiently high current densities. This is evidenced experimentally by a linear relationship between cell current and flux of water measured crossing the membrane. Application of the DMFC for such water-drag measurements is significantly simpler experimentally than the approach described by the authors before, particularly so for measurements above room temperature. In measurements the authors performed in the DMFC configuration on Nafion 117 membranes, the water drag coefficient was found to increase with temperature, from 2.0 H{sub 2}O/H{sup +} at 15 C to 5.1 H{sub 2}O/H{sup +} at 130 C. Implications of these new results on water management in DMFCs are briefly discussed.
164 citations
TL;DR: In this article, it was shown that a significant amount of water is transferred across cation exchange membranes from the −ve vanadium half cell electrolyte to the +ve Vanadium half-cell electrolyte by the hydration shells of V 2+ and V 3+ ions which can easily permeate through anion exchange membranes due to their relatively high charge numbers.
145 citations
TL;DR: In this paper, an investigation of the hydrogen oxidation reaction at platinum and at polymer electrolyte interfaces in contact with aqueous solutions composed of mixtures of H 2SO4 and K2SO4 (pH range of 0.2-7.5) was carried out using cyclic voltammetry, rotating disk electrode and potential step chronoamperometry.
123 citations
TL;DR: In this paper, the effect of contamination by Ca{sup 2+} ions in proton conductive membranes for polymer electrolyte fuel cells was investigated systematically by electromotive force analysis.
Abstract: The effect of contamination by Ca{sup 2+} ions in proton conductive membranes for polymer electrolyte fuel cells was investigated systematically. Ion and water transport characteristics of Nafion membranes, which were equilibrated with 0.02 to 0.03 kmol/m{sup 3} of HCl/CaCl{sub 2} mixed solutions of various mixing ratios, were studied by electromotive force analysis. Membrane composition analysis, showed that Ca{sup 2+} has much higher affinity than H{sup +} to the ion exchange sites in Nafion membranes. The water content in the membrane, as expressed by the amount of water per cationic site H{sub 2}O/SO{sub 3}{sup {minus}}, decreased about 19% from 21 for H-form membrane to 17 for Ca-form membrane. The water transference coefficient was obtained from streaming potential measurements of Nafion 115 membranes of various H{sup +}/Ca{sup 2+} cationic compositions. The water transference coefficient increased from 2.5 toward 11 as the Ca{sup 2+} content in the membrane increased, especially when the equivalent fraction of H{sup +} in the cationic exchange sites x{sub HM} became less than 0.5. Ionic transference numbers for H{sup +} in the membrane, determined by a new electromotive force method, showed rapid decrease when the cationic site occupancy by H{sup +} became less than 0.5. Membrane conductivity changed linearlymore » with H{sup +} composition in the membrane. In strong contrast to the interaction mode between H{sup +} and Ca{sup 2+} cations during ionic conduction, which appeared almost independent, a certain extent of interference was observed among water molecules as they were carried along by cations in the membrane. It was predicted that if Ca{sup 2+} ions enter the fuel-cell membrane, they cause serious effects to membrane drying and result in deterioration of fuel-cell performance. The advantage of this methodology in the study of transport characteristics of fuel-cell membranes is stressed due to ease and accuracy of measurements.« less
98 citations
TL;DR: In this article, the voltammetric behavior of dopamine at electropolymerized tetraaminophthalocyanatonickel(II) (p-NiTAPc) film coated electrodes was studied by cyclic voltammetry.
93 citations
TL;DR: In this paper, the overall transport characteristics of different cationic species in Nafion® 117 membrane are presented in order to obtain the membrane chloride concentration, the membrane porosity and membrane water content.
TL;DR: In this article, the anodic oxidn. of various alph. alcs. was carried out on an IrO2 anode under conditions of simultaneous O2 evolution.
TL;DR: In this article, a gas-fuel-feed direct methanol fuel cell is described in detail and the optimisation of the active electrode fabrication process is also reviewed, where the porous carbon electrodes were bonded with Nafion rather than polytetrafluoroethylene (PTFE).
TL;DR: In this paper, the authors measured the methanol uptake from water-methanol mixtures in Nafion®117 membranes initially saturated with water after equilibration for 18 h the membranes were dried at 70°C in vacuum with a cold trap cooled with liquid nitrogen.
31 Dec 1997
TL;DR: In this paper, a model of a bipolar, Nafion 117 PEM fuel cell stack was developed which predicts equilibrium cell output voltage as a function of current density and partial pressure of CO.
Abstract: The platinum-alloy catalyst used in proton-exchange membrane (PEM) fuel cell anodes is highly susceptible to carbon monoxide (CO) poisoning. CO reduces the catalyst activity by blocking active catalyst sites normally available for hydrogen chemisorption and dissociation. The reaction kinetics at the anode catalyst surface can be used to estimate the decrease in cell voltage due to various levels of CO contamination in the inlet fuel stream. Literature data on the effects of CO-contaminated fuel streams on PEM fuel cell performance have been reviewed and analysed in an attempt to further understand the electrochemical properties of the CO adsorption process. A fuel cell performance model of a bipolar, Nafion 117 PEM fuel cell stack has been developed which predicts equilibrium cell output voltage as a function of current density and partial pressure of CO. The model contains both empirical and mechanistic parameters and evolved from a steady-state electrochemical model for a PEM fuel cell fed with a CO-free anode gas. Reaction kinetics and equilibrium surface coverage have been incorporated into the electrochemical model to predict the decrease in fuel cell performance at equilibrium. The effects of CO were studied at various concentrations of CO in hydrogen as the anode feed gas. Literature data were used to develop the model parameters and the resulting model is used to compare the model-predicted voltages, with and without CO, to data found in the literature.
TL;DR: In this article, a Nafion-coated mercury thin-film working electrode is placed in a specially designed small-volume sonovoltammetric cell, where the preparation and curing procedure for the Nafions film on a glassy carbon electrode substrate as well as ultrasound-enhanced mercury deposition through the nafion film are optimized.
Abstract: The high mass transport caused by ultrasonic radiation in the preconcentration of trace metal species in anodic stripping voltammetry (ASV), leads to an order of magnitude enhancement in sensitivity. A Nafion-coated mercury thin-film working electrode is placed in a specially designed small-volume sonovoltammetric cell. The preparation and curing procedure for the Nafion film on a glassy carbon electrode substrate as well as ultrasound-enhanced mercury deposition through the Nafion film are optimized. The resulting electrode assembly is stable under ultrasonic irradiation. Optical microscopy observations show the formation of closely spaced mercury droplets of micrometer dimensions under the Nafion film. The performance characteristics of ultrasound-enhanced ASV at the Nafion-coated mercury thin-film electrode are studied in detail. The detection limit obtained for lead and cadmium test species is 3 × 10-11 M (30 s preconcentration in the presence of ultrasound). A relative standard deviation of 1.2% is o...
TL;DR: In this article, a carbon-based ink composed of graphite particles and polystyrene was used in association with a manual screen-printer to prepare electrodes on a flexible polyester film.
Abstract: A carbon-based ink composed of graphite particles and polystyrene was used in association with a manual screen-printer to prepare electrodes on a flexible polyester film. The screen-printing step was followed by a drying step which was achieved within 1 h at room temperature. The screen-printed electrode (SPE) was coated by a polyanionic Nafion film in which electroactive cationic species could be accumulated. A detection limit of 10-9 M was thus obtained by cyclic voltammetric (CV) determination of [[(4-hydroxyphenyl)amino]carboxyl]cobaltocenium (P+) after accumulation for 60 min. Since this cationic phenol derivative P+ could be generated from the corresponding anionic ester phosphate (S-) by alkaline phosphatase (AP) hydrolysis, the new S- substrate was synthesized and the sensitive indirect CV determination of AP was performed at a Nafion-coated SPE. The S- substrate did not interfere on the electrochemical response of P+ owing to the permselectivity of Nafion. An AP detection of 4 × 10-16 M was thus ...
TL;DR: An equilibrium ion absorption model that accounts for ion hydration effects, the dielectric saturation of water molecules in a membrane pore, and the neutralization of fixed-charge-site ion pairs, was matched to the experimental data and predicted accurately both the monovalent and divalent cation concentrations.
TL;DR: In this paper, an NO sensor based on a carbon fibre microelectrode modified by a poly(N-methylpyrrole) incorporating [(H2O)FeIIIPW11O39]4− sublayer and coated by a Nafion® external layer is described.
TL;DR: In this paper, the effects of the content and hydrophilicity of the hydrophilic group in PVA, ionization of TA, and the cation in Nafion have significant effects on sensing properties of the films.
Abstract: Films of differently modified PVA (polyvinyl alcohol), TA (phthalocyaninosilicon) and Nafion in different ionic forms (H+, Li+, and Na+) have been prepared by using casting methods. Impedance changes of the films with relative humidity are examined. The results demonstrate that the content and hydrophilicity of the hydrophilic group in PVA, ionization of TA, and the cation in Nafion have significant effects on sensing properties of the films.
TL;DR: In this article, an advanced kinetic model of the coupled diffusion of two counterions in a fixed-site ion exchange membrane is developed considering the effect of the varying ionic composition on the membrane water content.
Abstract: An advanced kinetic model of the coupled diffusion of two counterions in a fixed-site ion-exchange membrane is developed considering the effect of the varying ionic composition on the membrane water content. The transport problem is solved numerically for a set of ratios of the diffusion coefficients of the two counterions and 1:1 ion-exchange stoichiometry. The model is used to evaluate the diffusion coefficients of alkali metal cations in the as-received and expanded H- and M-form Nafion® (M = Li, Na, K, Rb, Cs) from ion-exchange measurements. Owing to a compensating effect of the electro-osmotic pore fluid flow, the initial rates of ion exchange correspond to a fixed water content which, however, is different in H- and M-form membranes. A strong correlation is revealed between the ratio of the membrane to aqueous ion diffusion coefficients and the polymer-phase volume fraction. It is concluded that the polymer phase presents mainly a steric effect without significantly changing the mechanism of transport of alkali metal cations or protons, which resembles that in bulk water. The different behaviors of the as-received and expanded Nafion® forms are probably associated with the prevailing cluster- or pore-network morphology, respectively
TL;DR: In this paper, cyclic voltammetry was used to investigate the electrochemical behavior of a nafion/ruthenium modified glassy carbon towards the electrooxidation of hydrazine compounds in perchloric acid solutions.
TL;DR: In this paper, photophysical probes were used to interrogate structural diversity within Nafion/[silicon oxide] and [2] Nafions/[ORMOSIL] hybrids.
Abstract: Pyrene (Py) photophysical probes were used to interrogate structural diversity within [1] Nafion/[silicon oxide] and [2] Nafion/[ORMOSIL] hybrids. The interior of the silicon oxide phase in [1] has the most polar environment wherein Py is trapped in silicon oxide cages in the vicinity of −SO3- groups. Polar/nonpolar interphase regions in [1] are next in order of decreasing polarity. The interior of the ORMOSIL phase in [2] displays lowest polarity, reflecting CH3 groups in Py-encapsulating ORMOSIL cages, and the interphase ranks somewhat higher in polarity. Water uptake is ordered: Nafion/ORMOSIL < unfilled Nafion-H+ < Nafion/[silicon oxide]. The hydrophilicity of unfilled Nafion-H+ is adjustable by tailoring the organic content of the incorporated sol−gel-derived nanophase as polarity determined from fluorescence emission and water uptake correlate well. Fluorescence emission of Py monitored nanostructural polarity shifts that result from secondary in situ gel growth induced by annealing. Additional con...
TL;DR: In this article, the authors demonstrate the suitability of electron transfer between immobilized horseradish peroxidase (HRP) and a glassy carbon electrode via N-methyl phenazine methosulfate (NMP) in a Nafion film.
TL;DR: In this paper, the electrochromic properties of poly(aniline N-butylsulfonate)s (PANBUS) coatings on indium-tin oxide (ITO) glasses exposed to an ion conducting polymer films were investigated.
Abstract: Electrochromic properties of poly(aniline N-butylsulfonate)s (PANBUS) coatings on indium-tin oxide (ITO) glasses exposed to an ion conducting polymer films were investigated. The ion conducting polymer films were prepared via photocross-linking reactions of methoxy polyethylene glycol-mono methacrylate with tri(propylene glycol) diacrylate in the presence of a photoinitiator and LiClO{sub 4}. Mechanical properties of the electrolyte film were enhanced by introducing styrene or butylmethacrylate into the polymer network. Color contrast and optical response of the PANBUS-based window were improved by adding Nafion into the electrolyte. The electrochromic (EC) window assembled with PANBUS, Nafion containing polymer electrolyte film, and tungsten trioxide coated ITO glass (type 2) required less energy for operation ({+-}1.5 V) compared to the EC window without tungsten trioxide film (type 1, {+-}2.0 V). By applying {+-}1.5 V, optical density of the type 2 window changed from zero to maximum of 1.2, corresponding transmission change of higher than 95%. Lifetime tests show that the type 2 window could support more than 2.3 {times} 10{sup 3} cycles, of 60 s duration.
TL;DR: In this paper, a Pt-black catalyst film was used as a working electrode in a Nafion 117 solid polymer electrolyte cell to investigate the kinetics of H{sub 2} oxidation by gaseous O(sub 2) mixture.
Abstract: The kinetics of H{sub 2} oxidation by gaseous O{sub 2} were investigated on a Pt-black catalyst film also serving as a working electrode in a Nafion 117 solid polymer electrolyte cell. The Pt-covered side of the Nafion 117 membrane was exposed to the flowing H{sub 2}-O{sub 2} mixture and the other side was in contact with a 0.1 M KOH aqueous solution with an immersed Pt counterelectrode. It was found that the applied current and the catalyst-electrode potential affect the catalytic rate of H{sub 2} oxidation in a very pronounced and reversible manner. Positive currents enhance the rate of H{sub 2} oxidation by up to 2,000%. The increase in the rate of H{sub 2} oxidation is up to 300 times larger than the electrochemical rate, I/2F, of H{sub 2} oxidation. This shows that the catalytic properties of the Pt film are significantly modified by varying catalyst potential, as also manifested by the observed potential-induced change in the functional dependence of the catalytic rate on the partial pressures of H{sub 2} and O{sub 2}. This novel application of solid polymer electrolytes to act as electrochemically activated catalyst supports, and thus promote and control heterogeneous catalytic reactions, is of considerable theoretical andmore » possibly practical interest.« less
TL;DR: Permeation of different inorganic anions through NafionTM 117 ionomer membrane has been studied as a function of pH and salt concentrations in feed solution at low pH, fluoride and nitrite ions showed higher permeation than other anions This has been attributed to a higher association of HF and HNO2 in the membrane phase Easy diffusion of the undissociated acids and their subsequent permeation has been explained on the basis of the morphology of the ionomer.
TL;DR: In this paper, the performance of three novel fuels for direct-oxidation-type fuel cells is described. But these new fuels are best used without further processing in direct liquid-feed polymer electrolytemore fuel cells.
Abstract: Significant advantages in weight, volume, and overall system performance of fuel cells would result if high-energy organic substances such as methane and its derivatives can be made to undergo direct electro-oxidation at the electrodes of the fuel cell without having to be converted by some catalytic process to hydrogen. This study demonstrates three novel fuels for direct-oxidation-type fuel cells. These new fuels are dimethoxymethane (DMM, dimethyl orthoformate), trimethoxymethane (TMM, trimethyl orthoformate), and trioxane (1,3,5-trioxane). The electro-oxidation and electrosorption characteristics of these new fuels at Pt, Pt-Sn, and Pt-Ru electrodes and the performance of these fuels in direct-oxidation fuel cells is described. Sustained direct electrochemical oxidation of aqueous solutions of DMM, TMM, and trioxane at high current densities has been demonstrated for the first time in half-cells and liquid-feed polymer electrolyte fuel cells. The oxidation of these fuels leads to the formation of methanol and ultimately carbon dioxide. Cyclic voltammetry and steady-state studies suggest that the electro-oxidation processes occur by chemisorption steps followed by surface reactions of adsorbed intermediates. The electro-oxidation of trioxane is preceded by an acid-catalyzed hydrolysis step on Nafion and in sulfuric acid solutions. These new fuels are best used without further processing in direct liquid-feed polymer electrolytemore » fuel cells.« less
TL;DR: In this paper, a quartz crystal microbalance was employed to monitor the changes in mass of thin protonated Nafion® coatings during the replacement of the protons by other cations.