Water In Nafion Research Articles

Ex-Situ Investigation of Membrane Swelling in an Electrolyzer Using X-Ray Tomography

In this contribution, we focus on the membrane swelling of a catalyst coated Nafion© 212 in the compound of a polymer electrolyte membrane electrolyzer. So far, the swelling of the catalyst coated membrane (CCM) is mostly investigated isolated from the surrounding components as porous transport layers (PTL) and the flow field plates[1-4]. To explore the swelling, ex-situ tests were carried out, using a special device [5] to control contact pressure and compression and computer tomography. This allowed the swelling of the CCM to be studied at different positions in the structure and at different compression pressures as well as the interaction of the different components. Figure 1 shows a comparison of the dry (a) and (c) and flooded (b) and (d) state of the CCM. The black lines indicate the surface of the CCM. In the flooded state, the thickness of the CCM increases by 20 % from about 50 µm to 60 µm, while the distance of the expanded metal sheets stays constant. This means, that the thickness increase of the CCM results in a decrease of at least one PTL thickness. One main goal of the experiment was to show, whether the swelling of the membrane is dependent or independent of the compression pressure and therefore may cause a different water uptake, which might give an explanation of the decreasing high frequency resistance with increasing pressure [4, 6].[1] Ito, H., Maeda, T., Nakano, A., & Takenaka, H. (2011). Properties of Nafion membranes under PEM water electrolysis conditions. International journal of hydrogen energy, 36(17), 10527-10540[2] Budinski, M. K., & Cook, A. (2010). Osmotic pressure of water in Nafion®. Tsinghua Science & Technology, 15(4), 385-390[3] Kusoglu, A., Kienitz, B. L., & Weber, A. Z. (2011). Understanding the effects of compression and constraints on water uptake of fuel-cell membranes. Journal of the Electrochemical Society, 158(12), B1504[4] Borgardt, E., Giesenberg, L., Reska, M., Müller, M., Wippermann, K., Langemann, M., Lehnert, W., & Stolten, D. (2019). Impact of clamping pressure and stress relaxation on the performance of different polymer electrolyte membrane water electrolysis cell designs. International journal of hydrogen energy, 44(42), 23556-23567[5] Hoppe, E., Janßen, H., Müller, M., & Lehnert, W. (2021). The impact of flow field plate misalignment on the gas diffusion layer intrusion and performance of a high-temperature polymer electrolyte fuel cell. Journal of Power Sources, 501, 230036[6] Stähler, M., Stähler, A., Scheepers, F., Carmo, M., Lehnert, W., & Stolten, D. (2020). Impact of porous transport layer compression on hydrogen permeation in PEM water electrolysis. International journal of hydrogen energy, 45(7), 4008-4014 Figure 1

Self-diffusion micromechanism in Nafion studied by 2H NMR relaxation dispersion.

Field Cycling (FC) 2H nuclear magnetic resonance (NMR) relaxometry was applied to study dynamics in Nafion NR 212 in the temperature range from 300 K to 190 K and water content of λ = 8.2. The sensitive time window of FC was extended up to eight decades using the temperature-frequency superposition principle and master curve. The rotational correlation times obtained from 2H FC NMR coincide with translational correlation times gained from static field 2H NMR diffusometry in the temperature range applied. This fact means that a long-range mass transport in Nafion is coupled to molecular rotations. It is assumed that confined water in Nafion has more ordered oxygen sublattices as compared with bulk water, on a short range is similar to ice. We discuss the possible role of D and L defects, typical for the ordered ice structure and using this concept to describe the processes of self-diffusion of confined water in Nafion, as well as the similarity of temperature and humidity dependence of self-diffusion and proton conductivity.

Bulk-like and Interfacial Water Dynamics in Nafion Fuel Cell Membranes Investigated with Ultrafast Nonlinear IR Spectroscopy.

The water confined in the hydrophilic domains of Nafion fuel cell membranes is central to its primary function of ion transport. Water dynamics are intimately linked to proton transfer and are sensitive to the structural features and length scales of confinement. Here, ultrafast polarization-selective pump-probe and two-dimensional infrared vibrational echo (2D IR) experiments were performed on fully hydrated Nafion membranes with sodium counterions to explicate the water dynamics. Like aerosol-OT reverse micelles (AOT RMs), the water dynamics in Nafion are attributed to bulk-like core water in the central region of the hydrophilic domains and much slower interfacial water. Population and orientational dynamics of water in Nafion are slowed by polymer confinement. Comparison of the observed dynamics to those of AOT RMs helps identify local interactions between water and sulfonate anions at the interface and among water molecules in the core. This comparison also demonstrates that the well-known spherical cluster morphology of Nafion is not appropriate. Spectral diffusion of the interfacial water, which arises from structural dynamics, was obtained from the 2D IR experiments taking the core water to have dynamics similar to bulk water. Like the orientational dynamics, spectral diffusion was found to be much slower at the interface compared to bulk water. Together, the dynamics indicate slow reorganization of weakly hydrogen-bonded water molecules at the interface of Nafion. These results provide insights into proton transport mechanisms in fuel cell membranes, and more generally, water dynamics near the interface of confining systems.

Non-invasive macroscopic and molecular quantification of water in Nafion® and SPEEK Proton Exchange Membranes using terahertz spectroscopy

Hydration of Proton Exchange Membranes (PEM) is vital in PEM fuel cells as it is critical in determining the overall proton conductivity. Traditional gravimetric analysis to quantify PEM's hydration is cumbersome, demands contact and importantly cannot differentiate the nature of water - bulk, bound or free water. As terahertz spectroscopy probes bulk and free water relaxations' contributions, it is an attractive non-invasive technique for studying membrane hydration dynamics - water uptake and retention. Here, THz spectroscopy measurements for hydrated Nafion® and sulphonated Polyether Ether Ketone (sPEEK) membranes in ambient atmosphere have been carried out for 100 minutes. Variation in THz absorption by the hydrated membranes with time is used for non-invasive estimation of percentage water content demonstrating good agreement with gravimetric measurements. In addition, proportion of bulk, bound and free water with time has been estimated by fitting the complex permittivity using double Debye model. Results show that % water content in hydrated sPEEK at 0th minute is 2.36 times than hydrated Nafion due to its higher porosity. Further, sPEEK has better water retention capability, especially higher bound water proportion at longer time intervals (85% more at 100th minute) than Nafion due to the presence of widely distributed small pores and comparatively stronger hydrogen bonding between the sulphonic acid and water. Thus, THz spectroscopy can be employed as an effective non-invasive tool for macroscopic and molecular quantification of water in hydrated PEMs.

Probing Acetonitrile Solvation in Perfluorsulfonate Ion Exchange Membrane By in-Situ FTIR

In-situ FTIR is a successful tool to study water absorption and interactions within the Perfluorsulfonate Ion Exchange Membrane (PFSA)(1–4). The versatility of the technique can reveal in-depth information related to proton transport in PFSA, including proton dissociation and solvation. Another desirable feature is that experiments under controlled water content(λ) is also possible. Studies of PFSA membranes equilibrated with different λ was fundamental to understand membrane behavior(5). The volatile nature of acetonitrile and many organic solvents adds to the inaccuracy of many classic membrane characterization methods. To overcome this, techniques tailored for membrane incorporated with organic solvents are necessary. In this study, acetonitrile and lithiated PFSA were chosen to revisit the in-situ IR technique. The goal of this work is to examine acetonitrile’s role in membrane swelling and to create a ‘slow-motion picture’ of how acetonitrile molecules behave upon being imbibed into the membrane, especially at low λ. The vibrational signal of cyanide group (~2250 cm-1)does not overlap with other signals from the membrane and is known to respond to dipole interactions by shifting to higher wavenumber(6). This feature makes an acetonitrile-swollen system an even more favorable candidate for in-situ IR study compared to its aqueous counterpart. For studying proton transport in membrane, initial water buildup in membrane yields a weak and broad signal, making the already obscure -OH stretching region even more complicated to analyze. Without interference from hydrogen bonding, lithium ion solvation with cyanide can be clearly observed. Coupled with monitoring -SO3- area, it is totally feasible to simultaneously see both cation dissociation from membrane (Fig.1A) and cation solvation in solvent (Fig.1B) in a real-time manner to compare with fully swollen state. Acknowledgement We gratefully acknowledge the support of this work by the U.S. Department of Energy, Office of Electricity Delivery and Energy Reliability (Dr. Imre Gyuk). We also thank 3M for providing membranes. References Elabd YA, Baschetti MG, Barbari TA. Time-resolved Fourier transform infrared/attenuated total reflection spectroscopy for the measurement of molecular diffusion in polymers. J Polym Sci Part B Polym Phys. 2003;41(22):2794–807.Hallinan DT, Elabd YA. Diffusion of Water in Nafion Using Time-Resolved Fourier Transform Infrared−Attenuated Total Reflectance Spectroscopy. J Phys Chem B. 2009 ;113(13):4257–66.Hallinan DT, Elabd YA. Diffusion and Sorption of Methanol and Water in Nafion Using Time-Resolved Fourier Transform Infrared−Attenuated Total Reflectance Spectroscopy. J Phys Chem B. 2007 ;111(46):13221–30.Kunimatsu K, Bae B, Miyatake K, Uchida H, Watanabe M. ATR-FTIR Study of Water in Nafion Membrane Combined with Proton Conductivity Measurements during Hydration/Dehydration Cycle. J Phys Chem B. 2011 21;115(15):4315–21.Zawodzinski TA, Derouin C, Radzinski S, Sherman RJ, Smith VT, Springer TE, et al. Water Uptake by and Transport Through Nafion® 117 Membranes. J Electrochem Soc. 1993 ;140(4):1041–7.Barthel J, Buchner R, Wismeth E. FTIR Spectroscopy of Ion Solvation of LiClO4 and LiSCN in Acetonitrile, Benzonitrile, and Propylene Carbonate. J Solut Chem. 2000 ;29(10):937–54. Figure.1A: SO stretching shift due to solvation; Figure.1B: Normalized CN stretching area upon acetonitrile buildup in the membrane. Blue dashline indicates corresponding fully swollen state. Figure 1

Dynamics and spatial distribution of water in Nafion 117 membrane investigated by NMR spin-spin relaxation

The dynamics and spatial distribution of water molecules in Nafion 117 membrane has been investigated by 1H NMR. To this end, 1D and 2D spin-spin (T2) relaxometry experiments were carried out as a function of the relative humidity ranging from 9 to 100%. The inverse Laplace transform was successfully applied to obtain the 1D T2 distributions and 2D relaxation maps. The 1D T2 distributions show two peaks at low RH and mainly one at high water content, which can be associated with the rearrangement of the exchange sites inside the polymeric channels with the amount of water. From the T2 distribution at 70% RH three types of water were identified corresponding to different degree of molecular mobility. The 2D T2-T2 maps show molecular exchange between the two water populations found at low RH, while at 70% the exchange between only the two more mobile water populations takes place.

Insight on Pure vs Air Exposed Hydroxide Ion Conductivity in an Anion Exchange Membrane for Fuel Cell Applications

Understanding the role of water in anion exchange membrane (AEM) used in fuel cells is essential for commercial application.1 Many literatures published have emphasized the important of water to optimize membrane properties such as ionic conductivity, ion transport, and mechanical and chemical stability.1-3 Recently, the idea of using thin layers of catalyst and solubilized ionomer is very common and demands the greater understanding of hydration behavior of AEMs.4 Several techniques have been used to understand the diffusion of water in membranes. Elabd et al5 have summarized the diffusion coefficient values from different methods and mentioned the in-consistency among different methods. A time resolved Fourier transform infrared- attenuated total reflectance (FTIR-ATR) spectroscopy is a less common method to measure the water diffusion through AEMs although this method has been recently used in other materials.6, 7 FTIR-ATR spectra were collected using Nexus 4700 (Thermo-fisher) with an ATR accessory (Specac, Inc). A liquid nitrogen cooled mercury cadmium telluride (MCT) detector was used to collect a spectrum with 128 scans every 25 seconds with a resolution of 4 cm-1. A custom built design was used to control RH of the membrane sample during experiments. In order to verify the experimental setup, the diffusion coefficient of water through Nafion 117® membrane was observed at the same order of magnitude (1.63±0.27 x 10-7 cm2/s vs 3.71x 10-7 cm2/s observed by Elabd et al7) when the relative humidity of the sample was changed from 0 to 100 %RH. Water diffusion coefficient was then measured for AEMs as a function of temperature and RH. The Fickian water diffusion coefficient for AEMs was observed to be lower than proton exchange membranes by one order of magnitude. Figure 1 shows the comparison of water diffusion kinetics between one of the anion exchange membranes and Nafion 117® at 30 oC.In this work, water behavior of different AEMs as a function of temperature and RH will be presented, the energy barrier for the water diffusion through these membrane will be discussed and the science behind a slow water diffusion through AEMs will be explored.Figure 1. Time-resolved normalized absorbance for O-H stretching vibration collected using FTIR-ATR at 30oC when RH was increased from 0 to 100%. Acknowledgement The authors would like to thank the US Army Research Office for the funding under the MURI #W911NF-10-1-0520and the purchase of FTIR microscope from ARO DURIP #W911NFNF-0462 References Merle, G.; Wessling, M.; Nijmeijer, K., Anion exchange membranes for alkaline fuel cells: A review. Journal of Membrane Science 2011, 377, 1-35.Duan, Q. J.; Ge, S. H.; Wang, C. Y., Water uptake, ionic conductivity and swelling properties of anion-exchange membrane. Journal of Power Sources 2013, 243, 773-778.Myles, T. D.; Kiss, A. M.; Grew, K. N.; Peracchio, A. A.; Nelson, G. J.; Chiu, W. K. S., Calculation of Water Diffusion Coefficients in an Anion Exchange Membrane Using a Water Permeation Technique. Journal of the Electrochemical Society 2011, 158, B790-B796.Mehta, V.; Cooper, J. S., Review and analysis of PEM fuel cell design and manufacturing. Journal of Power Sources 2003, 114, 32-53.Hallinan, D. T.; Elabd, Y. A., Diffusion of Water in Nafion Using Time-Resolved Fourier Transform Infrared−Attenuated Total Reflectance Spectroscopy. The Journal of Physical Chemistry B 2009, 113, 4257-4266.Sundfors, F.; Lindfors, T.; Hofler, L.; Bereczki, R.; Gyurcsanyi, R. E., FTIR-ATR Study of Water Uptake and Diffusion through Ion-Selective Membranes Based on Poly(acrylates) and Silicone Rubber. Analytical Chemistry 2009, 81, 5925-5934.Hallinan, D. T.; Elabd, Y. A., Diffusion and Sorption of Methanol and Water in Nafion Using Time-Resolved Fourier Transform Infrared−Attenuated Total Reflectance Spectroscopy. The Journal of Physical Chemistry B 2007, 111, 13221-13230.

NMR Relaxometry Study of the Interaction of Water with a Nafion Membrane under Acid, Sodium, and Potassium Forms. Evidence of Two Types of Bound Water

Through (1)H NMR relaxometry techniques (determination of the spin-lattice relaxation time as a function of the NMR measurement frequency), we have investigated, on a molecular scale, the water behavior in Nafion NRE 212 under acid, sodium, and potassium forms, the latter arising from different chemical treatments (with and without EDTA). Quantitatively, it turns out that (i) EDTA removes unwanted cations that may affect water mobility and (ii) the natural countercations (sodium and potassium) also affect water mobility according to their size. In order to go further, we have developed a new methodology that rests on the comparison between samples prepared with H2O and D2O. For the latter, residual protons allow us to exclusively access intermolecular contribution to proton relaxation and, thus, enable us to deduce the intramolecular contribution of proton relaxation in H2O. The analysis of this contribution reveals, for the first time, two types of bound water in Nafion.

On the Diffusion Coefficient of Water in Polymer Electrolyte Membranes

The Fickian diffusivity of water in Nafion has been shown to exhibit a local maximum. In the present research effort it is shown that this spike vanishes if the equilibrium water content and chemical diffusivity are modeled carefully. Further, it is shown that permeation experiments falsely can generate a local maximum, if a wrong equilibrium water content relation is used. Finally, in order to study the obtained diffusivity model a parameter variation is carried out using a one-dimensional steady-state model. The effect of diffusivity model, surface roughness and water content driving force is studied.

NMR and Electrochemical Investigation of the Transport Properties of Methanol and Water in Nafion and Clay-Nanocomposites Membranes for DMFCs.

Water and methanol transport behavior, solvents adsorption and electrochemical properties of filler-free Nafion and nanocomposites based on two smectite clays, were investigated using impedance spectroscopy, DMFC tests and NMR methods, including spin-lattice relaxation and pulsed-gradient spin-echo (PGSE) diffusion under variable temperature conditions. Synthetic (Laponite) and natural (Swy-2) smectite clays, with different structural and physical parameters, were incorporated into the Nafion for the creation of exfoliated nanocomposites. Transport mechanism of water and methanol appears to be influenced from the dimensions of the dispersed platelike silicate layers as well as from their cation exchange capacity (CEC). The details of the NMR results and the effect of the methanol solution concentration are discussed. Clays particles, and in particular Swy-2, demonstrate to be a potential physical barrier for methanol cross-over, reducing the methanol diffusion with an evident blocking effect yet nevertheless ensuring a high water mobility up to 130 °C and for several hours, proving the exceptional water retention property of these materials and their possible use in the DMFCs applications. Electrochemical behavior is investigated by cell resistance and polarization measurements. From these analyses it is derived that the addition of clay materials to recast Nafion decreases the ohmic losses at high temperatures extending in this way the operating range of a direct methanol fuel cell.

Steady-State Diffusion Coefficients for Water in Nafion in the Absence of Inert Gas

Steady-state diffusion coefficients of water in Nafion were determined in the absence of inert gases at 25°C. A large range of water activity was investigated to determine average and differential diffusion coefficients. Flux decreased toward zero as the water activity gradient between water vapor and liquid water approached zero, indicating no Schroeder's paradox. The average diffusion coefficients varied between 1 × 10−6 and 5 × 10−6 cm2/s, with a minimum at 14 moles H2O/mol SO3−. Differential diffusion coefficients ranged between 4 × 10−7 and 2 × 10−5 cm2/s, with a minimum at 14 moles H2O/mol SO3−.

2H double- and zero-quantum filtered NMR spectroscopy for probing the environments of water in Nafion

Multiple quantum 2H NMR spectroscopy is used to study the structure and dynamics of D2O in Nafion membranes as a function of membrane hydration. By employing both double- and zero-quantum filtered experiments, residual quadrupolar coupling constants and T2 relaxation values are obtained. The residual couplings vary from 240 to 20 Hz and the T2 values range from 20 to 180 ms, with the high hydration values having smaller couplings and longer T2 values. Analysis of the data using a water-exchange model suggests that the changes in parameters arise from a change in the fraction of time water spends in the anisotropic environments and not from changes in the order parameters that characterize the anisotropic sites. It has been found that a two-site model is needed to accurately fit the spectra above a hydration level of 10 D2O per sulfonate, with the second site having a negligible residual quadrupolar coupling. The data supports a model with two different hydration layers at high hydration and can be understood in terms of the recently proposed parallel-channel model for Nafion hydration.

Effect of temperature on structure and water transport of hydrated sulfonated poly(ether ether ketone): A molecular dynamics simulation approach

The effects of temperature on hydrated sulfonated poly(ether ether ketone) are studied using molecular dynamics. Three different temperature conditions (298 K.15 K, 323.15 K, and 353.15 K) with two different water contents (10 wt. % and 20 wt. %) are simulated. Analyzing the pair correlation functions, it is found that there is limited temperature effect on the distribution and solvation of the sulfonate groups. The structure factor analysis shows that the temperature dependence of the nanophase-segregated morphology is not significant in the simulated temperature range. On the contrary, the structure factors S(q) at ∼30 Å (q = ∼0.2 Å−1) and ∼13 Å (q = ∼0.5 Å−1) clearly increase with water content, indicating that the development of water channels is mostly affected by the water content. Within such water phase in the nanophase-segregated structure, the internal structure of water phase becomes more developed with decreasing temperature and increasing water content. By analyzing the mean square displacement of the water molecules, it is also found that self-diffusion of water is enhanced with the increasing temperature. From the observation that the activation energies calculated from such temperature dependency are very similar (Ea = 25.7 kJ/mol and Ea = 24.9 kJ/mol for 10 wt. % and 20 wt. %, respectively), it is inferred that the extent of the structural change in the water phase as a function of temperature is very similar between the 10 wt. % water content and the 20 wt. % water content. Compared to the bulk water (13.2 kJ/mol) and the water in Nafion (16.7-18.9 kJ/mol), it is confirmed that more nanophase-segregation enhances water transport through the membrane.

Diffusion and Interfacial Transport of Water in Nafion

Water absorption, membrane swelling, and self-diffusivity of water in 1100 equivalent weight Nafion were measured as functions of temperature and water activity. Free volume per water at 80 °C, determined from water uptake and volume expansion data, decreases with water content in the membrane from 12 cm(3)/mol at λ = 0.5 H(2)O/SO(3) to 1.5 cm(3)/mol at λ = 4. The change in free volume with water content displays a transition at λ = 4. Limiting water self-diffusivity in Nafion was determined by pulsed gradient spin echo NMR at long delay times. The limiting self-diffusivity increases exponentially with water activity; the rate of increase of diffusivity with water content shows a transition at λ = 4. The tortuosity of the hydrophilic domains in Nafion decreased from 20 at low membrane water activity to 3 at λ = 4. It suggested a change in the connectivity of the hydrophilic domains absorbed water occurs at λ ∼ 4. The diffusivity results were employed to separate the contributions of diffusional and interfacial resistance for water transport across Nafion membranes, which enabled the determination of the interfacial mass transport coefficients. A diffusion model was developed which incorporated activity-dependent diffusivity, volume expansion, and the interfacial resistance, and was used to resolve the water activity profiles in the membrane.

Impact of chemical treatments on the behavior of water in Nafion® NRE-212 by 1H NMR: Self-diffusion measurements and proton quantization

In this article, we have studied by 1H NMR the behavior of water in Nafion under acid, sodium and potassium forms which arises from different chemical treatments with or without EDTA. Self-diffusion coefficients were measured by PGSTE (Pulsed Gradient STimulated Echo) whereas integration of the proton spectrum provides a quantitative information about water content. Examination of the self-diffusion coefficients of water in Nafion shows clearly that: (i) in the acid form, the use of a chelating agent does not affect translational diffusion of water, (ii) regardless of sample conditioning, water within Nafion in acid form diffuses more rapidly than in sodium and potassium forms, in particular at low water content (at λ=3.4, D=5.4×10−7cm2s−1 vs. 7.2×10−8cm2s−1), and (iii) for a given water amount, water diffusion is identical in sodium and potassium forms. Point (ii) is due to the presence of oxonium ions within Nafion in acid form, favoring both vehicle and Grotthuss mechanisms. It is also due to a higher number of protons available for diffusion (we find an average value of 1.19 in acid form vs. 0.91 in sodium form). Our results suggest in the case of Nafion in acid form that, at low water content, diffusion occurs by jumps between different sites. Moreover, plotting self-diffusion coefficients vs. relative humidity leads to a clear differentiation between sodium and potassium cations, related to their respective solvation number.

Osmotic Pressure of Water in Nafion®

Mechanical failure modes leading to cracks or breeches in proton exchange membrane fuel cells are driven by mechanical forces associated with swelling from water uptake and shrinkage from dehumidification. To determine the magnitude of compressive mechanical stress imposed by water swelling in a proton exchange fuel-cell membrane, the osmotic pressure of water in a perfluorosulfonic acid ionomer (Nafion® N 117) membrane was measured using a hydrostatic piston-cylinder device with an in-situ hydrophilic frit. Experiments indicate that hydrostatic stresses greater than 103.5 MPa are created in a membrane when swollen with water at 23°C suggesting that pressure from water swelling can distort Nafion N 117-based structures as the osmotic pressure is of the same order of magnitude as the flow stress of Nafion N 117.

Creep and stress-rupture of Nafion® membranes under controlled environment

Nafion® as a polymer shows time-dependent behavior including creep and relaxation. Application of this material in automotive fuel cells and targeted lifetimes require consideration of time-dependent failure of Nafion® in fuel cell design. In this work, creep tests for Nafion® were conducted in a controlled environment at different hydrations and temperatures. Stress-rupture curves for Nafion® were obtained for different hydrations (RH = 5%, 50% and 90%) at room temperature and also for two different temperatures (25 °C & 80 °C) at 50% relative humidity. Time-dependent failure was observed for all these conditions regardless of temperature and hydration level (state of water in Nafion®). Ionic forces created by cross linkings (by introducing water to membrane) do not seem to prevent disentanglements of the chains since there is no change in rate of time-dependent failure results. A time-hydration/temperature equivalence method (Sherby–Dorn) is suggested as a method to use short-term test data to estimate rupture times for other conditions.

Non-Fickian Diffusion of Water in Nafion

Understanding the dynamics of water in solid-state polymer electrolytes (e.g., Nafion) is important for a variety of applications ranging from membrane-based water purification to hydrogen fuel cells. In this study, the dynamics of water in Nafion was investigated at both low and high humidities with time-resolved Fourier transform infrared−attenuated total reflectance (FTIR−ATR) spectroscopy; a technique that provides a molecular fingerprint of both the diffusant and the polymer simultaneously in real time. At low humidities (0−22% RH), an extended initial time lag resulted in non-Fickian behavior, where dynamic infrared data provided evidence for a reaction between water and sulfonic acid. A diffusion−reaction model was developed and predicted this anomalous behavior, where the time lag was a function of water content. At high humidities (0−100% RH), a slow approach to steady state resulted in non-Fickian behavior, where dynamic infrared data provided evidence of water-induced relaxation in the polymer backbone. A diffusion−relaxation model was developed and regressed well to both the polymer relaxation and water diffusion data, where only one fitting parameter was used for each data set to determine both a relaxation time constant and diffusion coefficient. This approach differs significantly from previous work on non-Fickian behavior in glassy polymers, which, consisted of regressing gravimetric data to models with a minimum of six fitting parameters. Not only do the diffusion coefficients from these two models compare well with Fickian diffusion coefficients from experiments with small water concentration gradients, but also the results in this study provide physical insight into the transport mechanisms of water and relaxation phenomena in solid-state polymer electrolytes.

Coherent neutron scattering study of confined water in nafion

Coherent neutron scattering study of confined water in nafion

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

Hydrogen fuel cells are attractive alternative power sources for applications such as transportation; however, fuel cell performance is a strong function of water equilibrium content and water sorption and desorption kinetics in polymer electrolyte membranes (e.g., Nafion). Although similar water sorption isotherms for Nafion have been reproduced in many laboratories, reported diffusion coefficients of water in Nafion vary by 4 orders of magnitude. In this study, sorption and desorption dynamics of water vapor in Nafion were measured as a function of water vapor activity and flow rate using time-resolved Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy. Both integral and differential experiments were performed, where integral experiments consisted of increasing the vapor activity from 0% RH to one of five values (22, 43, 56, 80, or 100% RH), while in differential experiments the activity was sequentially increased in smaller steps from 0 to 22 to 43 to 56 to 80 to 100% RH. For integral experiments, non-Fickian behavior was observed at both low and high vapor activities, while Fickian behavior was observed at moderate vapor activities. For differential experiments, Fickian behavior was observed at all vapor activities except at low vapor activities (0-22% RH). Sorption kinetics was found to be a function of flow rate, where mass transfer resistance at the vapor/polymer interface was significant at low flow rates but was insignificant at high flow rates. Accurate sorption and desorption diffusion coefficients were calculated in this study (measured at high flow rates with no mass transfer resistance) and were similar, on the order of 10(-7) cm(2)/s, and weak functions of water vapor activity.