Water In Nafion Research Articles

Water Dynamics and Proton Transfer in Nafion Fuel Cell Membranes

The dynamics of water and its effect on proton transport kinetics in Nafion membranes are compared at several hydration levels. Nafion is the most widely used polyelectrolyte membrane in fuel cells. Ultrafast infrared spectroscopy of the O-D stretch of dilute HOD in H2O provides a probe of the local environment and hydrogen bond network dynamics of water confined in the hydrophilic regions of Nafion. The kinetics of proton transfer in Nafion are tracked by following the excited-state proton transfer and recombination kinetics of a molecular probe, pyranine (HPTS). The hydrophilic domains of Nafion grow with increased hydration, and the interfacial regions reorganize, leading to a changing local environment for water near the interface. Swelling is not uniform throughout the membrane, and heterogeneity is observed in the fluorescence anisotropy decays of the methoxy derivative of pyranine. Measurements of the time-dependent anisotropy of water in Nafion provide a direct probe of the hydrogen bond network dynamics. These dynamics, as well as the rate of proton transport over nanoscopic distances, are observed to slow significantly as the hydration level of the membrane decreases. The results provide insights into the influence of changes in the dynamics of water on the proton-transfer processes.

Diffusion and Sorption of Methanol and Water in Nafion Using Time-Resolved Fourier Transform Infrared−Attenuated Total Reflectance Spectroscopy

Direct methanol fuel cells (DMFCs) are promising portable power sources. However, their performance diminishes significantly because of high methanol crossover (flux) in the polymer electrolyte membrane (e.g., Nafion 117) at the desired stoichiometric methanol feed concentration. In this study, the diffusion and sorption of methanol and water in Nafion 117 were measured using time-resolved Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy. This technique is unique because of its ability to measure multicomponent diffusion and sorption within a polymer on a molecular level in real time as function of concentration. Both the effective mutual diffusion coefficients and concentrations of methanol and water in Nafion 117 were determined with time-resolved FTIR-ATR spectroscopy as a function of methanol solution concentration. The methanol flux, calculated from FTIR-ATR, matched that determined from a conventional technique (permeation cell) and increased by almost 3 orders of magnitude over the methanol solution concentration range studied (0.1-16 M). Furthermore, the data obtained in this study reveal that the main contribution to the increase in methanol flux is due to methanol sorption in the membrane.

Water sorption, desorption and transport in Nafion membranes

Water sorption, desorption, and permeation in and through Nafion 112, 115, 1110 and 1123 membranes were measured as functions of temperature between 30 and 90 °C. Water permeation increased with temperature. Water permeation from liquid water increased with the water activity difference across the membrane. Water permeation from humidified gas into dry nitrogen went through a maximum with the water activity difference across the membrane. These results suggested that the membrane was less swollen in the presence of water vapor and that a thin skin formed on the dry side of the membrane that reduced permeability to water. Permeation was only weakly dependent on membrane thickness; results indicated that interfacial mass transport at the membrane/gas interface was the limiting resistance. The diffusivity of water in Nafion deduced from water sorption into a dry Nafion film was almost two orders of magnitude slower than the diffusivity determined from permeation experiments. The rate of water sorption did not scale with the membrane thickness as predicted by a Fickian diffusion analysis. The results indicated that water sorption was limited by the rate of swelling of the Nafion. Water desorption from a water saturated film was an order of magnitude faster than water sorption. Water desorption appeared to be limited by the rate of interfacial transport across the membrane/gas interface. The analysis of water permeation and sorption data identifies different regimes of water transport and sorption in Nafion membranes corresponding to diffusion through the membrane, interfacial transport across the membrane–gas interface and swelling of the polymer to accommodate water.

Thermodynamic considerations on the stability of water in Nafion

This work entails modeling the thermodynamic forces contributing to the total free energy of a Nafion membrane to find how liquid water equilibrates and agglomerates inside the membrane. Since the sulfonate acid sites attract water to dissociate, there is a mixture of water and ions within the Nafion membrane. This mixture contributes an entropic term to the free energy of the system which decreases with increasing water content. The hydrophobic Nafion backbone yields a contact angle greater than 90° when in contact with water. This curvature in the surface of the water induces a capillary pressure which can be very high if the size of the agglomeration is small. As the membrane takes on water, polymer chains of the Nafion are stretched and straightened reducing their configurational entropy. Minimizing the free energy under the influence of all these forces at varying pressures and temperatures gives insight into the nature of liquid or vapor filled pores throughout a Nafion membrane. Specifically it indicates a critical pore size in which liquid or vapor is the favorable state.

Nafion–Imidazole– H3PO4 Composite Membranes for Proton Exchange Membrane Fuel Cells

Water in Nafion 115 membranes was replaced by imidazole and then doped with to form imidazole– complexes as proton solvent. Both the Nafion–imidazole and Nafion–imidazole– composite membranes exhibited higher proton conductivity than the plain Nafion membrane under anhydrous conditions at . Although the Nafion–imidazole membranes with Pt catalyst exhibit poor performance in proton exchange membrane fuel cells (PEMFCs) at low temperatures (60 and ) due to the poisoning of the Pt catalyst by imidazole, doping with improves the fuel cell performance significantly due to the suppression of imidazole poisoning through the formation of imidazolium salt with . The imidazole poisoning could also be suppressed further by replacing the Pt catalyst with a newly developed Pd–Co–Mo catalyst. The Nafion–imidazole– membranes with the Pd–Co–Mo catalyst show performance superior to that of Nafion membrane with the Pt or Pd–Co–Mo catalyst in PEMFCs at .

State of Water in Nafion 117 Membranes Studied by Microwave Dielectric Relaxation Spectroscopy

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State of Water in Nafion 117 Proton Exchange Membranes Studied by Dielectric Relaxation Spectroscopy

AbstractDynamics of water in Nafion 117 membranes, in the acid form, at two hydration levels and several temperatures were investigated by means of dielectric relaxation spectroscopy in both, low (10−2to 107Hz, 25 to −80 °C) and microwave (0.045-26 GHz, 35 °C) frequency regions by employing different experimental setups. Three states of water were identified: a) strongly bound to the sulfonic groups (in agreement with other investigations) forming the first hydration layer; b) loosely bound, surrounding the first layer and c) free water, having similar dynamics as in the bulk state.

Thermodynamics and Proton Transport in Nafion: I. Membrane Swelling, Sorption, and Ion-Exchange Equilibrium

A thermodynamic model is proposed to describe the sorption of water in Nafion based on the Flory-Huggins activity model and an appropriate osmotic pressure correction term for the chemical potential of water within the swollen membrane. The key variables for sorption are equivalent weight of ionomer, acid strength of the ionic groups, modulus of polymer elasticity, and interaction between water and polymer. The water uptake per unit mass of dry Nafion increases with the increasing acid strength of the functional groups, decreasing Young’s modulus, and decreasing equivalent weight of Nafion. The model provides insights into the sorption and swelling behavior of ion-exchange membranes, and thus, may be useful in evaluating and designing alternate proton-exchange membranes for fuel cell applications. In a companion paper (Part II), a predictive model is presented for the transport of protons in Nafion. © 2005 The Electrochemical Society. All rights reserved.

Structure and Dielectric Saturation of Water in Hydrated Polymer Electrolyte Membranes: Inclusion of the Internal Field Energy

The nature of the water in hydrated polymer electrolyte membranes (PEMs) is distinct from that of bulk water and affects the rate of diffusion of protons in the material. Traditional methods used in the study of the phenomenon of dielectric saturation involve the assumption of the presence of a homogeneous or slowly varying external field, conditions that are hardly appropriate for PEMs. If the condition of field homogeneity is relaxed, a nonphysical divergence in the permittivity results. In this paper we show that through proper inclusion of the internal field energy the difficulty is rectified. Our recently derived equilibrium statistical mechanical model for the calculation of the spatial variation in the permittivity of water within PEMs has been extended to include the field energy. Using simple model calculations, we demonstrate that the interaction of the water molecules with the anionic sites constitute a more important contribution to the energy than the intermolecular forces. We have recalculated pore-radial profiles of the dielectric constant of the water in Nafion and 65% sulfonated PEEKK (poly arylene ether ketone) polymer electrolyte membranes over a range of hydration levels obtaining qualitative agreement with experiments. These profiles quantitatively show the increased ordering of the water in the neighborhood of the anionic sites, and although only the affect due to these sites is specifically computed, confinement effects on the water is implicitly included. In separate investigations, it was determined that both the uniformity in the distribution of the sulfonate groups, and the intrusion of these groups into the pore volume, will dramatically affect the permittivity of the water. Together our results help to elucidate the impact of structure and ordering of the water due to the effect of the fixed anionic groups on the underlying mechanisms of proton conduction in PEMs.

Diffusion of Water in Nafion 115 Membranes

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. © 2000 The Electrochemical Society. All rights reserved.

Oxygen permeability and the state of water in Nafion ® membranes with alkali metal and amino sugar counterions

The mobility of hydration water and the dissolved oxygen permeability through different cation forms of the Nafion ® membranes were determined. Two alkali metals (Na and K) and two amino sugars (an equivalent molar mixture of d-glucopyranosyl-α(1′ → 6)-2-amino-2-deoxy- d-mannitol and its sorbitol (GPA)and d-glucosamine (GLU)) were used as counterions. Based on the two-state model, the content and mobility of hydration water were determined using DSC and 17O NMR. The dissolved oxygen permeability through the Nafion ® membrane containing GPA was the lowest value in this study because, for the membrane, the fraction of hydration water was the greatest and the mobility of hydration water was the lowest. The amount and location of the hydrophilic group contained in substances as well as the kind of hydrophilic group affected the fraction and mobility of hydration water and dissolved oxygen permeability through the membranes.

The State of Water in Nafion 117 of Various Cation Forms

The State of Water in Nafion 117 of Various Cation Forms

Orientation effects in the deuterium NMR spectroscopy of perfluorinated ionomer membranes

The properties of water in Nafion ® and Dow perflourinated ionomer membranes in their protonated forms were investigated by proton and deuterium NMR spectroscopy. The 2H quadrupolar splittings were shown to be dependent on the orientation of the membranes in the magnetic field as well as on their water content. The larger quadrupolar splittings found for Nafion are attributed to a greater degree of orientational order compared to the Dow samples. Hydrogen bonding, as evidenced by 1H and 2H chemical shifts, was very similar in the two types of membrane for a given relative humidity. The 2H spin-lattice relaxation times ( T 1) were 93 and 109 ms for Nafion and Dow respectively at 300 K and 100% relative humidity, compared to 612 ms for D 2O in 1:9 D 2O:H 2O. The temperature dependence of the T 1 values was similar for the two membranes in the range 280–320 K and activation energies for molecular reorientation were estimated at 22±2 kJ mol -1, comparable to that for pure water in the same temperature range.

Water‐sorption and transport properties of Nafion 117 H

AbstractThe density and dimensional changes of Nafion 117 H have been measured as a function of the water content. Swelling of Nafion commences at N (≡ mol ratio water to hydrogen ion) ∼ 1.9. Some anisotropy of dimensional changes was observed. Water‐sorption isotherms obey Henry's Law with a nonzero intercept indicating some water retention. The diffusion coefficient of water in Nafion and the electrical conductivity of Nafion are strong functions of the water content. The latter is exploited for the development of a humidity sensor. © 1993 John Wiley & Sons, Inc.

Experimental Determination of the Transport Number of Water in Nafion 117 Membrane

The transport number of water in Nafion® 117 membrane over a wide range of water contents is determined experimentally using a concentration cell. The transport number of water, the ratio , is about 1.4 for a membrane equilibrated with saturated water vapor at 25°C, decreases slowly as the membrane is dehydrated, and falls sharply toward zero as the concentration of water approaches zero. The relationship between the transference number, the transport number, and the electro‐osmotic drag coefficient is presented, and their relevance to water management in solid‐polymer‐electrolyte fuel cells is discussed. Results are compared with other data available in the literature and with the theoretical maximum.

Nafion® Membranes: Molecular Diffusion, Proton Conductivity and Proton Conduction Mechanism

AbstractH-PFG-NMR diffusion and proton conductivity data for NAFION® 117 is presented as a function of temperature and water content. From this, H2O diffusion on a molecular scale is concluded to be similar to the one in bulk water. But “geometrical” restrictions of diffusion lead to a decreasing pre-exponential factor, i.e. decreasing macroscopic diffusion coefficient with decreasing water content. It is shown that proton conductivity is correlated with water diffusion for low degrees of hydration (vehicle mechanism). With increasing hydration there is an increasing contribution from “structure diffusion” culminating in an amplification factor of A = 2.5 for fully hydrated protonic NAFION® 117 at room temperature. The bonding of water in NAFION® as well as the local environment (hydration) of the acidic proton are concluded to be similar as in acidic aqueous solutions.

Conductivity of ion-exchange membranes—II. Mobilities of ions and water

The mobilities of Na +, Cl − and OH − ions and water in Nafion 120 membrane have been discussed. According to the frictional formalism they have been expressed in terms of friction coefficients describing single interactions between components of the system. The significant influence of the movement of water on mobilities of Na + and Cl − ions has been quantitatively shown. Due to a different transport mechanism of OH − ions their mobility is influenced to a smaller degree than other ions.

An infrared study of water in perfluorosulfonate (Nafion) membranes

We have studied the infrared spectra of H2O, D2O, and HDO in hydrated Nafion membranes in the sodium form. The spectrum of isotopically isolated HDO exhibits two distinct absorption bands in the OH stretching region and similarly in the OD stretching region. These bands have been taken to correspond to two distinct environments of the OH groups in Nafion, identified as [Formula: see text] and[Formula: see text]. It appears that a substantial proportion of water molecules have an OH group (or occasionally both OH groups) exposed to the fluorocarbon environment. This implies that the hydrated ion clusters are either much smaller than previously estimated or, more likely, are non-spherical in shape with frequent local intrusions of the fluorocarbon phase. Water in Nafion is much less strongly hydrogen-bonded than water in aqueous salt solutions at the same temperature and may be expected to have distinct physical properties.

Physical properties and supermolecular structure of perfluorinated ion-containing (nafion) polymers

The supermolecular structure and viscoelastic and diffusion properties of a perfluorinated polymer containing sulfonic acid (Nafion) were investigated. The breakdown of time–temperature super-position for the dry salt and and acid in the presence of 0.5 H2O/SO3H as well as the results of small-angle x-ray scattering suggest that the ions in this material are clustered. Above 180°C, the reestablishment of the time–temperature superposition in the salt suggests that ions in the clusters become mobile. Dynamic mechanical studies were performed over a temperature range from −190°C to above the glass transition temperatures Tg of the materials. The Tg of the salts is found at ca. 220°C, while in the acid it occurs at 110°C. A β peak in the acid is found at ca. 20°C, while in the salts it occurs between 140°C and 160°C. The β peak shifts to a lower temperature with the addition of water in both the acid and the salts, while the α and γ peaks are unaffected. The latter is located at ca. −110°C at 1 Hz. Dielectric behavior has also been studied as a function of water content for the acid sample and the potassium salt at frequencies of 100 Hz to 10 kHz. Two relaxations with different activation energies were observed. The position of both peaks shifts to a lower temperature as the water content increases. Finally, the diffusion of water in Nafion in the acid form has been determined. The diffusion coefficient can be represented by the equation