Proton Conducting Ionic Liquid Doped Nafion ... - ACS Publications

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Proton Conducting Ionic Liquid Doped Nafion Membranes: NanoStructuration, Transport Properties and Water Sorption Rakhi Sood,†,‡ Cristina Iojoiu,*,† Eliane Espuche,‡ Fabrice Gouanvé,‡ Gerard Gebel,§ Hakima Mendil-Jakani,§ Sandrine Lyonnard,§ and Jacques Jestin⊥ †

Laboratoire d’Electrochimie et de Physicochimie des Matériaux et des Interfaces, UMR-5279, 1130 Rue de la Piscine, 38402, St. Martin d’Hères, France ‡ Ingénierie des Matériaux Polymères, UMR-5223, IMP@LYON1, Université de Lyon, Université Lyon 1, 15 Bd. A Latarjet, 69622, Villeurbanne CEDEX France § CEA-Grenoble, INAC/SPrAM, Groupe Polymères Conducteurs Ioniques, UMR-5819, CEA-CNRS-UJF, 17 Rue de Martyrs 38054 Grenoble, CEDEX 9 France ⊥ Laboratoire Leon Brillouin, CEA Saclay, 91191 Gif sur Yvette, France ABSTRACT: The impact of triethylammmonium trifluorosulfonate (TFTEA) concentration on the nanostructuration and electrochemical, thermo mechanical, transport properties of doped Nafion membranes has been studied. The Nafion membranes neutralized with triethylamine (Nafion−TEA) have been doped with various amounts of TFTEA using the swelling method. The effect of the TEA neutralization was first studied. The results suggest a specific arrangement of TEA cations and a quite low water uptake. Concerning TFTEA doped membranes, the evolution trend of nanostructure of TFTEA doped Nafion is very similar to that of acidic Nafion swollen by water, with a slope equal to 1.33. However, at high TFTEA concentration (29 wt %) in this composite membrane, the average hydrophobic−hydrophilic phase separation distance appears to be 59 Å while it is 41 Å in hydrated acidic Nafion at the same volume fraction of polymer, which could be related to a much more heterogeneous distribution of TFTEA in Nafion−TEA than water molecules in acidic Nafion. Increasing TFTEA concentration in Nafion−TEA membranes results in significant increase of anhydrous ionic conductivity but no significant change in gas-permeability coefficients (hydrogen, oxygen). Water sorption experiments at 25 °C show that water uptake increases with increasing water activity and percentage of TFTEA in the membrane. However, the water sorption capacity of the TFTEA molecules within the membrane is limited due to the hindering effect of the polymer matrix.

1. INTRODUCTION

Perfluorosulfonated ionomers consist of an extremely high hydrophobic perfluorinated backbone along with extremely high hydrophilic sulfonic acid functional groups as side chains. The high level of proton conductivity even at low water content has to be attributed to favorable nanoscale morphology as well as very high acidity of fluorosulfonate groups.10 In the nanostructured membrane, the well-connected hydrophilic domain is responsible for the transport of protons and water and the connected hydrophobic fibers of polymer main-chain provide morphological stability and prevent the polymer from dissolution in water. The multiscale structure and its evolution as a function of water content have been largely described in the literature.11−14 Among the most recent structural models, the ribbon-like polymer model13 is one of the best in describing the full set of data obtained with Nafion membranes. This model considers elongated polymer aggregates with a locally

Electrochemical devices capable of converting chemical energy into electrical power, such as proton exchange membrane (PEM) fuel cells, are of great interest to the industry and the scientific community because of their high energy conversion efficiency, low environmental impact, and their possible use in a wide variety of applications from portable electronic devices to light-duty electric vehicles. In this technology, the PEM appears as the central element. It transports protons from the anode to the cathode where oxygen is reduced to produce water. In addition to high proton conductivity, a PEM should exhibit good thermo-mechanical strength, low gas permeability, an outstanding chemical as well as electro-chemical stability and limited water swelling. Despite extensive research on low cost alternative membranes such as sulfonated aromatic polymers1−3 composites membranes (based on blends of polymer and inorganic fillers),4−6 the perfluorosulfonated ionomers7−9 such as Nafion from DuPont, Aciplex from Asashi, Aquivion from Solvay, or 3M membranes remain the reference materials for PEMFC application in terms of performance and stability. © 2012 American Chemical Society

Received: July 4, 2012 Revised: October 8, 2012 Published: October 18, 2012 24413

dx.doi.org/10.1021/jp306626y | J. Phys. Chem. C 2012, 116, 24413−24423

The Journal of Physical Chemistry C

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flat interface (ribbon). Parallel polymer ribbons form bundles of 50−100 nm size which are isotropically distributed in the absence of mechanical deformation.15 It has to be noted that a competing model based on parallel cylindrical ionic domains (nanochannels) embedded in the perfluorinated matrix has also been proposed.14 Assuming some shape and size of the hydrophobic and ionic domains as well as polydispersity of the polymer, it is a hard task to differentiate between these two models. However, the high level of proton conductivity at very low water content,16 the effect of confinement on water mobility,17 the evolution of the SAXS and SANS spectra as a function of water content13,18 and the very fast kinetic of swelling19,20 favor the polymer ribbon model. It has to be noted that most of the studies on transport properties were conducted in the presence of a large amount of water21 and the proton conduction mechanism assured by water molecules limits the operating temperature to 100 °C. However, polymer electrolyte membranes capable of working at higher temperatures with dry gases are of great interest to improve the fuel cell performances. Operating temperatures around 120 °C would fasten the electrochemical reactions, increase the tolerance to CO poisoning, simplify the overall system (cooling device minimization and absence of inlet gas hydration), etc.6 In order to increase the working temperature above 120 °C, several approaches have been explored based on doping a polymer network with a proton donating molecule capable of substituting the role of water in the membrane. For instance, phosphoric acid doped polybenzimidazole membranes have been extensively studied for high temperature fuel cell applications.22,23 But this system suffers from certain drawbacks such as leaching phenomenon of the phosphoric acid out of the membrane when the fuel cell is cooled down, condensation of phosphoric acid at high temperature in the absence of water, some corrosive effects on cathode and very large overpotentials at the electrodes.24 Following the pioneering work of Kreuer,25 many works have been devoted to imidazole-doped membranes.26 However, the level of proton conductivity is too low to allow an industrial development (typically from 0.1 to 1 mS/ cm as compared to 100 mS/cm for water swollen membranes). Recently, it has been reported that proton-conducting ionic liquids (PCILs) doped membranes could represent promising systems to address these issues.27−37 The main advantages of PCILs arise from their anhydrous proton conductivity (up to 10−2 S/cm depending of the PCIL structure), negligible vapor pressure and their high thermal and electrochemical stability. Moreover, PCIL can exhibit fast proton transport and facile electrode reactions hydrogen oxidation reaction (HOR) as well as oxygen reduction reaction (ORR) at the electrode interfaces at high temperatures. PCILs consist of a bulky, asymmetric organic cation such as alkyl ammonium, imidazolium, piperidinium, etc. and an inorganic or organic anion35,36 Concerning the fuel cell application where the ORR and HOR take place on the Pt catalyst, the PCILs based on alkyl ammonium seem to be more adapted due to higher proton activities and lesser absorption on the electrode’s surface in comparison to PCILs based on imidazolium.37 Concerning the conductivity mechanism it was recently assumed that the proton conduction takes place by long-range charge transfer process that occurs through proton exchange between cationic PCIL clusters.31 In order to fulfill the requirements of an electrode separator (mechanical strength and gas diffusion barrier), PCILs have to be incorporated in a polymer matrix without losing their conducting properties. The PCIL,

generally, interacts with the polar functions of the polymer leading to its plasticization.27,30−32 In order to maintain reliable mechanical properties, it is crucial to favor the nanoseparation with highly conductive ionic domains and preserved hydrophobic polymer network. In this study, we focused on Nafion membrane as the host matrix because of its well-known outstanding properties and trifluoromethanesulfonate of triethylammonium (TFTEA) as the PCIL due to its conducting properties. Nafion has been utilized in neutralized form to facilitate the compatibility and improve the thermal stability of doped membranes.31 In the first part, Nafion−TEA has been characterized as the reference membrane of the TFTEA-doped membranes in terms of structure, thermo-mechanical, electro-chemical, gas permeation, and water sorption properties. In the second part, the effects of the TFTEA doping level on the membrane structure and properties have been investigated.

2. EXPERIMENTAL SECTION Materials. Triethylamine (TEA) (from Aldrich) was distilled before use. Trifluomethanesulfonic acid (TF) (Aldrich) was used as received. Nafion117 (from Acros) was reactivated and neutralized before use (protocol discussed later). Synthesis. a. PCIL Synthesis. PCIL synthesis was carried out by reaction of TF with distilled TEA (with molar ratio TEA/TF = 1.05) in deionized water at room temperature in an ice bath for 20−30 min. Then, water was evaporated from the synthesized PCIL named as trifluoromethanesulfonate of triethylammonium (TFTEA) using rotavap. After removal of water, activated charcoal and methanol were added to TFTEA and stirred for some time followed by filtration to remove impurities or unreacted products. Methanol was then evaporated followed by drying at 120 °C under vacuum for 48 h, and finally TFTEA was stored in a glovebox under argon atmosphere. The purity of the PCIL was confirmed by NMR techniques. b. Reference Membrane Preparation. Commercially available Nafion117 membrane was treated in refluxing 2 M nitric acid aqueous solution for 1 h (to reactivate all the ionic sites) followed by washing with deionized water up to neutral pH. This membrane is denoted as Nafion−H+. Then, membrane was kept in 1 M TEA in water/ethanol solution (50:50 v/v) under mild stirring at room temperature overnight followed by washing the membrane to neutral pH. The treatment with TEA solution allows the neutralization of the reactivated acidic ionic sites. The neutralized Nafion117 membrane (referred as Nafion−TEA) was dried at 80 °C under vacuum for 48 h and stored in glovebox under argon atmosphere. PCIL-Based-Membrane Elaboration. Nafion−TEA membranes were swollen with different concentrations of TFTEA (by weight) at 80 °C by dipping Nafion−TEA membranes in TFTEA for different periods of time in argon atmosphere. The rate of uptake of TFTEA by Nafion−TEA is fast in the first hour and then reaches a maximum percentage of uptake (24 wt %) after 20 h. The TFTEA weight fraction (with respect to the total weight) within the PCIL-based membranes was determined gravimetrically. The membrane containing more than 24% TFTEA was prepared by swelling at 95 °C. The membranes obtained with different concentrations (shown in Table 1) were then taken for different types of characterization. 24414



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Table 1. Concentration and λ Values for Different Nafion− TEA+x%TFTEA Membranes sample code Nafion−TEA +5%TFTEA Nafion−TEA +8%TFTEA Nafion−TEA +12%TFTEA Nafion−TEA +14%TFTEA Nafion−TEA +17%TFTEA Nafion−TEA +20%TFTEA Nafion−TEA +24%TFTEA Nafion−TEA +29%TFTEA

% TFTEA (by weight)

% TFTEA (by volume)

mol of TFTEA/mol of SO3−of Nafion (λ)

5

5.2

0.25

8

10.3

0.42

12

15.2

0.65

14

17.7

0.78

17

21.3

0.98

20

24.8

1.12

24

29.5

1.51

29

35.1

1.96

force track, and a frequency of 1 Hz. The storage and loss modulus of the composite membranes were determined in the given temperature range. Conductivity in Anhydrous Condition. The conductivity measurements of the membranes were carried out with and Electrochemical Impedance spectroscope using an HP 4192A impedance analyzer in the frequency range of 5−13 MHz. The membranes were placed between two stainless steel electrodes in a Swagelok cell having Teflon joints and spacers and the cells were prepared and closed in the glovebox under argon atmosphere to avoid any contact with humid air. The conductivity measurements were carried out in the temperature range of 20−150 °C with temperature equilibrated for 2 h prior to conductivity measurements. Gas Permeation Analysis. Permeation measurements were performed at 20 °C for H2 and O2. The gas purity was higher than 99%. After a preliminary high-vacuum desorption step, the membrane (effective area 3 cm2) was submitted to an upstream gas pressure fixed at 3 bar. The pressure variations in the downstream compartment were measured as a function of time with a Datametrics pressure sensor. The permeability coefficients expressed in barrer units (1 barrer =10−10 cm3STP cm cm−2 s−1 cmHg−1) were calculated from the slope of the steady-state line. The precision on the permeability value was estimated to be better than 5%. Water Sorption Analysis. Dynamic vapor sorption analyzer, DVS Advantage (Surface Measurement Systems Ltd., London, U.K.) was used to determine the water sorption isotherms of the films. The vapor partial pressure was controlled by mixing dry and saturated nitrogen, using electronic mass flow controllers. The experiments were carried out at 25 °C and the initial weight of the samples was approximately 50 mg. The samples were predried in the DVS Advantage by exposure to dry nitrogen until the dry weight of the samples was obtained. A partial pressure of water was established within the apparatus and the water uptake was followed as a function of time. The equilibrium was considered to be reached when changes in mass with time (dm/dt) were lower than 0.0002 for 5 consecutive minutes. Then, vapor pressure was increased in suitable activity up to 0.9 by step of 0.1. The cycle was ended by decreasing the vapor pressure in steps to obtain, also, desorption isotherms. The sorption rate was estimated at each water activity by the half sorption time t1/2 normalized to the film thickness.

Study of Leaching Phenomenon. Nafion−TEA membranes were swollen with TFTEA under argon atmosphere at 80 °C to the maximum concentration (around 24 wt %). Afterward, a sample was kept in argon atmosphere at 20 °C while another one was kept in ambient atmosphere at the same temperature. The membranes were regularly weighed after being blotted (with paper and a little bit of heating in order to melt the solidified TFTEA on membrane surface). The weight measurements were carried out until stable weights were achieved. TFTEA leaching was observed only for the membranes containing more than 21 wt %. It has been observed that atmosphere has not a significant impact on the leaching phenomenon. For high doping level, the leaching process is pretty slow and 40 days are necessary to obtain a stable weight. Whatever is the initial doping, the stabilization weight was found to be close to 21 wt %. The characterization was carried out principally with membranes presenting a maximum doping level of 20 wt %. However, some experiments have been performed with higher doping levels but only with freshly prepared samples. Small Angle Neutron Scattering (SANS). SANS measurements for the membranes were carried out on the PAXE spectrometer (Laboratoire CEA-CNRS Léon Brillouin, Orphée reactor, Saclay, France). Twelve mm diameter disk-like samples of composite membranes were cut in a glovebox and kept in sealed recipients (to avoid any contact with humid air). The quartz neutron cells were prepared prior to SANS measurements, closed quickly and maintained at room temperature. The neutron scattering intensity was measured as a function of the scattering vector q defined as follows: q = (4π/λ) sin(θ/2), where λ is the wavelength of the incident neutron beam and θ is the total scattering angle. The detector was tilted with respect to the incident beam direction. Two configurations were used to cover the angular range from 8.5 × 10−3 to 0.6 Å−1 (λ = 7.5 Å, D = 5 m and λ = 5 Å, D = 1.3 m, where D is the sample-todetector distance). The SANS spectra were corrected for detector efficiency and background subtraction. Absolute intensities were obtained by measuring a water-sample for calibration (1 mm thick in Helma cell). Dynamic Mechanical Analysis (DMA). The thermomechanical properties of the different membranes were studied using a TA Instruments’ DMA2980. The measurements were carried out in the temperature range of −100 to 150 °C with preloaded force of 0.01N using an amplitude of 20 μm, 200%

3. RESULTS AND DISCUSSION 3.1. Polymer Matrix: Effect of Neutralization. The impact of TEA neutralization on both the nanostructure and the resulting properties of Nafion membranes were first determined and the data were used as reference for the doped samples. The SANS spectrum of Nafion−TEA (Figure 1) is compared to the well-known SANS spectra obtained for dry and low water content-acidic Nafion117. The dry acidic Nafion117 membrane was kept for 48 h under vacuum at 80 °C and the low water content-acidic Nafion117 was obtained by equilibrating the membrane for several days under a relative humidity of 22% (imposed by a saturated salt solution of CH3COO−K+) in a specially designed neutron cell prior to scattering measurements. The corresponding membrane hydration was quantified by the λ parameter which is the number of water molecules per ionic group.38 At 22% RH, λ is equal to 4 at equilibrium. The SANS spectrum of Nafion generally exhibits a correlation peak called ionomer peak in the 24415



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cules in the ionic phase. A lamellar local geometry has been chosen for the purpose of representing d and d0. Remarkably the SANS spectrum of dry Nafion−TEA is similar to the one obtained with the acidic Nafion117 at low hydration indicating that the highly protonated counterion behaves as water molecules from the scattering point of view. A well-defined and pretty intense scattering peak is thus measured as a consequence of a high contrast between the hydrophobic phase and TEA ions. The ionomer peak position (q* = 0.202 Å−1) for Nafion−TEA is in between the values obtained for the dry and λ = 4 acidic Nafion membranes. The associated separation distance dTEA has been found equal to 31 Å. The sharpness of the peak with respect to the acidic Nafion indicates a better ordering at the nanometric scale. Interestingly, the difference between d0 and dTEA is 4 Å which corresponds to the size of one TEA cation,40 suggesting a string-like organization of TEA ions at the hydrophobic/hydrophilic interface. Figure 2c shows a schematic picture of this optimized packing and organization resulting from this single-layer structure. As a TEA ion contains 16 protons, the corresponding λ of anhydrous Nafion−TEA should be ∼8. However, the volume of a bulky triethylammonium cation is rather comparable to an aqueous cluster of 3−4 water molecules. This is consistent with the observed SANS spectra where the ionomer peak of Nafion−TEA is observed at slightly higher q values than that of a λ = 4 acidic Nafion one. Assuming a lamellar structure with an average thickness of 27 Å for the acidic form, the average distance between two sulfonic groups can be easily calculated. Taking into account the equivalent weight of Nafion117 (EW = 1100 g/equiv) and the matrix polymer density (d = 2−2.1 g/cm3), the average fluorinated volume per sulfonic group is VS = 913.7 Å3. The average surface of interface per sulfonic group in lamellar geometry can thus be deduced. Considering that the 27 Å thick polymer membrane has a sulfonic group on each side, we found an area of 67.6 Å2 per sulfonic group and consequently an average distance between two sulfonate groups at the polymer interface of 8.2 Å. Therefore, it is necessary to pack two TEA counterions with a 4 Å diameter to fill the ionic interface. It follows that the TEA cations have the exact size to produce the structure depicted in Figure 2. We can predict that increasing the size of the ammonium counterion, the dcounterion spacing will increase and the width of the ionomer peak will be affected

Figure 1. SANS spectra of neutralized Nafion−TEA membrane (blue ▲), compared to acidic Nafion in dry state (○) and at low water content ([H2O]/[SO3−] = 4, λ), extracted from ref 17. The scattered intensity is shown on linear scale as a function of the scattering vector q.

q range of 0.1−0.2 Å−1 . This peak is usually considered as a fingerprint of ionomer membrane’s nanostructure.39 This peak arises from the hydrophobic−hydrophilic phase separation at nanometric scale. Its position q* is related to a characteristic distance d = 2π/q* which has been frequently interpreted as the mean separation distance between interconnected ionic domains. As seen from Figure 1, a broad ionomer peak is measured in the hydrated acidic Nafion at position q* = 0.183 Å−1 (d = 34.3 Å) while it is hardly distinguishable in the dried sample (position around 0.23 Å−1, d ∼ 27.3 Å). Under the presented drying conditions, “dry” Nafion can be considered to contain around one water molecule per ionic site (λ = 1), which is not sufficient to produce a significant contrast to generate a well-marked peak. It is thus not possible to experimentally determine the value of d in a completely anhydrous acidic Nafion. However, a limit value at zero λ, called d0, can be extrapolated from the dilution law, i.e., the peak position variation plotted as a function of water content. d0 is the mean correlation distance in the absence of water, and is thus directly related to the mean width of hydrophobic aggregates. A value of d0 = 27 Å has been found in acidic Nafion117. A schematic representation of the local polymer organization is given in Figure 2a,b for ideally dry and low water content Nafion membranes. Sulfonic acid head groups are located at the hydrophobic/hydrophilic interface, protons and water mole-

Figure 2. Schematic representation of the hydrophobic/hydrophilic interface in (a) dried and (b) low water content acidic Nafion, compared to (c) TEA−Nafion. Mean separation distances d, d0, and dTEA obtained from SANS spectra analysis are depicted. 24416



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suggested from the SANS results (Figure 2). This organization along with bulky cation (TEA) should restrict the local mobility of chains yielding a relatively lower population that are capable of activated motions at temperature lower than Tα. Finally, the thermo-mechanical properties of Nafion−TEA, in agreement with the previously published results,32 are very different from those of Nafion neutralized with tetraethylammonium41 where much higher Tα and Tβ were obtained probably due to the cation dissymmetry enhancement, electrostatic interactions and specific organization. The proton conductivities of Nafion−TEA and Nafion−H+ (dried at 80 °C under vacuum) have been measured in anhydrous conditions and presented in Figure 4. The water

because of a less favorable counterion packing, except when doubling the ammonium size. The impact of neutralization process on thermo-mechanical behavior of Nafion117 has been investigated by DMA. The measurements were carried out on Nafion−TEA in comparison to the data obtained with Nafion−H+ in terms of storage modulus and tan δ (Figure 3). The dynamic mechanical

Figure 4. Conductivity vs temperature of Nafion−H+ and Nafion− TEA in anhydrous conditions.

content of the membranes have been determined by quantitative measurement of the NMR signal and it was found to be about 1.5 mols of water/mol of SO3− in the Nafion−H+ and 0.9 mol of water/mol of TEA neutralized SO3− in the Nafion−TEA. While the NMR measurement is rather precise in the case of Nafion−H+ because most of the protons belong to water molecules, it is obviously more difficult and less precise in the case of Nafion−TEA. The much higher conductivity of Nafion−H+, at temperature lower than 120 °C, compared to that of Nafion−TEA can be explained by both: water content and proton (cation) conduction mechanism. Thus, we assume that, in Nafion−H+ with λ = 1.5, jump like motion of the H+ between neighboring SO3− groups is possible in percolated ionic domains.17 However, for Nafion−TEA, a reorganization of triethylammonium is required before a proton (cation) jumps from an anion to another. However the conductivity of Nafion−TEA and Nafion−H+ are close at temperatures higher than 100 °C. This can be attributed to the high chain mobility allowing easier motion and jump of TEA cations leading to an increase in the conductivity in Nafion−TEA while a partial water evaporation can be supposed for Nafion−H+. Because during the PEMFC operation water is formed at the cathode, the ability of the membrane to uptake water is an important asset to be discussed. Thus, the sorption isotherm and the evolution of the square root of half sorption time as a function of the water activity of Nafion−TEA have been studied and compared to Nafion−H+ (Figure 5). The water uptake of Nafion−H+ membrane is about 14 wt % at water activity of 0.9 with a sigmoidal isotherm corresponding to a B.E.T II type isotherm in the Brunauer−Emmett−Teller

Figure 3. DMA of Nafion−H+ and Nafion−TEA: (a) storage modulus vs temperature; (b) tan δ vs temperature.

relaxations of the Nafion strongly depend on the strength of interactions between the terminal side-chains. It is well-known that Nafion in acidic form exhibits three kinds of relaxation: γ relaxation (−100 °C) attributed to vibrational and rotational movements of CF2 groups located at short distance from the ionic function; α relaxation (100 °C) related to the physical cross-links associated with the ionic groups; β relaxation (−20 °C) corresponding to the movement of the hydrophobic PTFE main-chain.41 The DMA results (Figure 3) on Nafion−TEA indicate a slight enhancement of E′ between −100 and 50 °C in comparison to Nafion−H+ that can be ascribed to the strong steric hindrance between neutralized side groups of Nafion− TEA. Nevertheless, the value of Tα is slightly lower and much broader in Nafion−TEA than that of Nafion−H+ while the Tβ relaxation is slightly higher and less pronounced. The lower value of Tα can be explained by the replacement of hydrogen bonding networks formed by the strong dipole−dipole interactions of Nafion−H + with weak van der Waals interactions between triethylammonium end-group of sidechains along with more hindered steric interferences. The higher Tβ can be explained through the specific organization of TEA cations at the hydrophobic/hydrophilic interface as 24417



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Figure 5. Water sorption isotherm and kinetic data characteristic of (a) Nafion−H and (b) Nafion−TEA.

and 0.9 barrer, respectively) are in good agreement with those generally reported in the literature.43,46,47 The membrane neutralization by TEA cations induces a significant increase of the membrane permeability with 10.2 and 2.5 barrer as permeability coefficients for hydrogen and oxygen, respectively. This increase points toward an increase in the contribution by ionic phase toward permeability of gases as neutralization step does not affect the hydrophobic phase (as evidenced by SANS). Hence, this result is apparently in contradiction with different published works which suggest that the gas permeation in polymers containing hydrophobic and hydrophilic domains like Nafion can take place mainly in the hydrophobic phase under anhydrous conditions.46,48 Indeed, the ionic character of ionic clusters in Nafion−TEA is much weaker than ionic clusters of Nafion−H+ and the resulting gas permeation through ionic cluster could affect the permeability by a factor up to two. Moreover, it has to be noted that the effect of membrane neutralization by Cs+ and Pt2+ has been already studied and opposite results were obtained, namely a decrease of the gas permeation coefficients.49 The decrease in gas permeability has been attributed to an enhancement in chain stiffness due to stronger ionic interactions when H+ is exchanged by Cs+ or Pt2+. The increase in gas permeability observed on Nafion− TEA can be related to the replacement of the strong ionic interactions by weak van der Waals forces between polymer chains as evidenced from DMA results, in addition to the percolation between the ionic domains by the presence of the bulky TEA amine. The gas permeation is then favored from the contribution of polymer chain mobility as well as from the contribution of free volume in the percolated ionic domains. 3.2. Nafion−TEA + TFTEA. The impact of TFTEA addition as well as its concentration on the nanostructure of Nafion−TEA membrane was studied by SANS. The SANS spectra of Nafion−TEA+xwt%TFTEA composite membranes are presented in Figure 6 on a log−log scale. The q-range of the spectra has been extended with respect to Figure 1. An offset has been applied along the intensity scale for clarity after subtraction of a constant background due to hydrogen incoherent scattering. The structure of Nafion−TEA is clearly maintained upon doping with ionic liquid as revealed by the observation of the typical ionomer peak up to the maximum doping level (29 wt %). Swelling a Nafion−TEA membrane with ionic liquid thus does not profoundly modify the

classification. These results are in agreement with the literature.42−44 The water uptake of Nafion−TEA is much lower than for Nafion−H+ in all the range of water activity with a significantly different shape of the sorption isotherm corresponding to a B.E.T. III type. The concave part observed for Nafion−H+ at low activity corresponding to the Langmuir sites and the formation of the primary hydration sphere of the sulfonic acid groups, is no more observed in Nafion−TEA. A quasi linear part is observed at low activity (aw < 0.7) usually associated with a Henry sorption mode (uniform distribution of the water molecules and a constant diffusion rate). At high water activity (aw > 0.7), the convex curvature is associated with the formation of water clusters. However, the clustering phenomenon is limited due to more hydrophobic nature of Nafion−TEA. The differences between Nafion−TEA and Nafion−H+ can also be clearly observed from the kinetics of sorption phenomenon expressed as the half sorption time, t1/2 in function of water activity (Figure 5). The interpretation of the kinetics of sorption in terms of diffusion coefficient (D) is subject to controversies as it generally leads to very low D values in comparison to the self-diffusion coefficient determined by pulse field gradient NMR (lower by 2 orders of magnitude).45 However, the comparative analysis of the t1/2 experimental values obtained under the same experimental conditions on Nafion−H+ and Nafion−TEA can be performed. In agreement with the previously discussed sorption mechanism, three domains are evidenced for Nafion−H+: a first decrease of the half sorption time value due to the plasticization effect induced by the first water molecules that are sorbed on the ionic sites, then a constant value of t1/2 associated with the Henry type behavior and finally an increase of t1/2 value due to the increasing size of the water clusters formed at high activity. For Nafion−TEA, only the two last domains are observed. Moreover, the water diffusion is much easier in the case of Nafion−TEA in all the range of activity due to the weaker polymer/water interactions involved in the water sorption mechanism at low activity and to the limited clustering phenomenon observed at high activity. In addition to the ionic conduction and the mechanical properties, the gas permeation is a key property for fuel cell application. The hydrogen and oxygen permeability coefficients measured under anhydrous conditions with Nafion−H+ (5.5 24418



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Figure 7. Dilution law log(d) = f(log ϕp) of NafionTEA + TFTEA (light blue ■), d being the characteristic correlation distance obtained as 2π/q* with q* the ionomer peak position, and ϕp the polymer volume fraction. Data measured in acidic Nafion swelled with water19 (○, gray ▲) are also reported for comparison. The data taken from ref 21 were initially plotted as d = f(ϕw).The lines are a guide for the eyes.

Figure 6. SANS spectra of Nafion−TEA+x%TFTEA (4−29 wt %). The reference spectrum of Nafion−TEA without ionic liquid is also shown. The spectra have been arbitrarily shifted for clarity. The constant background due to hydrogen incoherent scattering has been evaluated at high q and subtracted from the data.

polymeric architecture although it improves the transport properties. A second scattering maximum, often referred to as “matrix knee” in the literature, is clearly visible at low scattering vectors. This rather large bump is attributed to correlations between crystalline domains on typical scales of a tenth of nanometres. The effect of increasing the amount of TFTEA within the membrane induces a continuous shift of both the ionomer peak and the matrix knee toward smaller angles similarly to the previously observed effect of water sorption on Nafion structure.15,18,21 The ionomer peak position varies from q = 0.202 Å−1 (free of PCIL) to 0.105 Å−1 (maximum loading 29 wt %), corresponding to characteristic distances increasing from an initial separation of 31 Å in Nafion−TEA to a final value of 59 Å. A q−4 behavior (Porod’s law) is observed at high q values, even if data are scattered, which is the signature of a sharp interface at subnanometre scale, as observed in waterswollen acidic Nafion. The composite membrane is thus a phase-separated system with a well-defined interface between a dense and hydrophobic perfluorinated phase and the ionic domains containing the ionic liquid in addition to the TEA counterions. Overall, the observed behavior is similar to an acidic membrane swollen with a polar solvent such as water. It can be clearly observed in Figure 6 that the ionic liquid insertion induces a significant broadening of the ionomer peak even at the minimum of the doping level (4 wt %). This effect results from the insertion of the bulky ionic liquid molecules which disrupts the particular structure of the Nafion−TEA. Moreover, the bulky TFTEA molecules cannot homogeneously swell the structure of Nafion−TEA at very low doping levels corresponding to one TFTEA molecule for 4 sulfonate groups (Table 1) and hence the distribution in size induces the broadening of the ionomer peak. The swelling behavior of ionic domains can be analyzed through the variation of the characteristic distance d obtained from the ionomer peak position, as a function of TFTEA content of the membrane. The swelling state of a membrane can be characterized either by a local parameter λ defined as the number of doping molecules per sulfonic group or by a macroscopic parameter such as the polymer or solvent volume fractions ϕp and ϕw respectively. Figure 7 shows a plot of log(d) as a function of log(ϕP), a representation that is useful to compare the behavior of different membranes under

different swelling conditions. log(d) presents a linear variation with log(ϕp) as previously observed for Nafion.18 At ϕp = 1, one finds a 4 Å difference between Nafion−H+ in acid form and Nafion−TEA discussed in the previous section, corresponding to the single-layer packing of TEA counterions at the interface. Then, as TFTEA is introduced into Nafion−TEA matrix, the nanometric swelling follows a very similar trend as acidic Nafion, with a slope equal to 1.33. It is worth noting that determination of the peak position for the membranes with high PCIL content is very difficult as the peak has been strongly enlarged due to high PCIL content. Notably, the value obtained at 29 wt % displays important error bars. In this composite, the average separation distance appears to be 59 Å while it is 41 Å in hydrated acidic Nafion at the same volume fraction of polymer. The peak enlargement can result from much more heterogeneous distribution of TFTEA in the ionic domains of Nafion−TEA than that of water molecules in acidic Nafion. As suggested by a recent paper,31 the TFTEA molecules are likely to form micelles of typical size of 16 Å within Nafion. When swelling with TFTEA, the morphology of Nafion−TEA undergoes distortions to accommodate the presence of these micelles. The heterogeneous distributions of TFTEA in Nafion−TEA have also been evidenced by DMA measurements where additional relaxations peaks have been observed close to α and β peaks (Figure 8). The energy dissipated for the α-relaxation (Figure 8b) is higher for Nafion−TEA compared to the doped membranes and the Tα value decreases with the increase of TFTEA concentration. The polar side groups of Nafion−TEA, SO3−N(C2H5)3H+, are solvated and plasticized by TFTEA resulting in the reduction of the strength of the electrostatic interactions within the ionic domains of the membranes. Additionally, for the doped membranes containing more than 18 wt % TFTEA, a second peak is observed in temperature range of 28−35 °C and the temperature of appearance for this relaxation decreases with the increase in TFTEA loading. This small peak could be attributed to the relaxation of TFTEA richphase in the polymer matrix due to concentration gradient of TFTEA (heterogeneous distribution of TFTEA) at high TFTEA content or could be associated with conformational relaxation modes of the fluorocarbon backbone chains of PTFE 24419



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cells were prepared in argon environment and the ionic conductivities were recorded under anhydrous atmosphere. A significant increase in the conductivity of Nafion−TEA with subsequent introduction of TFTEA in the polymer matrix as well as a decrease in the temperature dependence was observed with increasing TFTEA content (Figure 9). The doped membrane with 29 wt % TFTEA content shows anhydrous conductivity of around 6.5 mS/cm at 110 °C.

Figure 9. Conductivity vs temperature of Nafion−TEA+x%TFTEA (4−29 wt %) under anhydrous conditions.

The presence of water in the membrane induces a significant increase of the conductivity (depending of membrane concentration and relative humidity (RH)).30,32 It is interesting to note that even if the gases would not be humidified, water would still be formed at the cathode. Thus, the conductivity in the real working fuel cell conditions will be much higher. Therefore, we have also investigated the water sorption behavior of the doped membranes at different RH values. The water sorption was limited to the membranes containing less than 21 wt % of TFTEA which do not exhibit a leaching phenomenon with time. In order to check the stability of the membranes upon water vapor exposure, two consecutive cycles of sorption and desorption were performed on each sample. The experiments were perfectly reproducible and the sorption and desorption equilibrium data defined a single isotherm curve for each sample which indicates that the doped TFTEA Nafion−TEA membranes do not present any sorption hysteresis phenomenon. The water sorption isotherms of doped membranes, Nafion− TEA and TFTEA are BET III type (Figure 10). TFTEA exhibits a very large water uptake especially at water activities above 0.7 and consequently the water uptake measured for Nafion−TEA+x%TFTEA membranes increases with the ionic liquid content. At low water activity (below 0.7), the experimental water sorption values and calculated values, obtained with the additive contribution law of each component (Nafion−TEA and TFTEA, respectively), are very close. However, for higher water activity, the experimental data are lower than those calculated and the difference is emphasized as the water activity increases. This behavior underlines the hindering effect of the polymer matrix that limits the water sorption capacity of the TFTEA molecules within the membrane. The water sorption mechanism was analyzed by using Guggenheim Anderson de Boer model (GAB). GAB equation

Figure 8. DMA of Nafion−TEA+x%TFTEA (4−29 wt %) membranes: (a) storage modulus vs temperature; (b) tan δ vs temperature.

domain, specifically a 136 → 157 conformational transition for α and an order−disorder conformational transition for α′, which could be more visible in the presence of high concentration of TFTEA.50,51 The analysis of tan δ profiles in the temperature range of −100 to 0 °C reveals an increase in amplitude and a broadening of β relaxation with increasing TFTEA content signifying an easier relaxation of the hydrophobic main-chains because of decreasing ion-ordering in the ionic domains and interchain interactions. As for β relaxation, a second peak appears around −77 °C from 18 wt % TFTEA content onward. It can be assumed that a specific relaxation associated with TFTEA appears starting from 18 wt %, which could be attributed to the presence of TFTEA aggregates or TFTEA-enriched zones within the membrane. The γ-relaxation for Nafion−TEA observed at −80 °C (Figure 3b) shifts to lower value upon adding TFTEA. On comparing the storage modulus (Figure 8a), a decrease with increasing TFTEA concentration is observed due to the plasticizing effect of the ionic liquid. The monotonic decrease of storage modulus with increasing doping level (from −80 °C to higher than +60 °C) indicates that TFTEA acts like plasticizer even at temperature lower than its melting point which underlines the complete dispersion of TFTEA at the interface between hydrophobic and hydrophilic domains of the doped membrane. The effect of TFTEA addition and its concentration has been investigated through the conductivity measurements as well. In order to evidence the effect of concentration and avoid the major effect of small amounts of water on the conductivity, the 24420



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Figure 10. Water sorption isotherms determined on TFTEA and on Nafion−TEA+x%TFTEA membranes with increasing amounts of ionic liquid.

has already been used to model BET type III isotherms with good accuracy.52 The equation of GAB model is expressed as: M = Mm

Table 2. Water Sorption and Gas Permeation Properties of the Nafion−TEA+x%TFTEA Membranes (0−100 wt %)

CGKas (1 − Kas)(1 + (CG − 1)Kas)

GAB parameters determined from water sorption isotherm modeling

where M is the mass gain and as is the water activity. In this modeling approach, the three parameters, Mm, CG, and K, have a physical meaning. Mm characterizes the availability of membrane ionic sites for the first sorbed water molecules as it represents the saturation of all primary adsorption sites by one water molecule (formerly called the monolayer in BET theory). CG which is the Guggenheim constant is indicative of the binding strength of water to the primary binding sites. K is a correcting factor lower than 1 for the properties of the multilayer molecules with respect to the bulk liquid. The curve fitting efficiency was estimated from the mean relative percentage deviation modulus (MRD), which is defined by MRD =

100 N

N

∑ i=1

gas permeation coefficient

TFTEA weight fraction (%)

Mm

CG

K

MRD (%)

PH2 (barrer)

PO2 (barrer)

0 5 8 12 20 100

0.009 0.0128 0.0140 0.0147 0.0187 0.0491

2.28 2.14 2.40 2.54 2.16 2.81

0.853 0.830 0.848 0.899 0.914 0.981

1.7 1.8 2.5 3.6 2.4 6.6

10.2 10.4 9.6 9.7 − −

2.5 2.3 2.4 2.6 2.7 −

found to be more or less constant whatever the doping level is, despite a significant swelling of the nanostructure as evidenced by SANS. This result can be considered as a confirmation of the already percolated structure of the Nafion−TEA and it suggests that the free volumes are mainly located at the interface between the percolated ionic domains and the hydrophobic phase, thus limiting the influence of the TFTEA content at least for the small sized diffusing molecules utilized for the study.

|mi − mpi| mi

■

where mi is the experimental value, mpi is the predicted value, and N is the number of experimental data. A modulus value below 10% is indicative of an accurate fit of the experimental isotherm curve by the model. We calculated the values of GAB parameters for each film by fitting the isotherm curve according to the software Tablecurve 2D. Determination of Mm, CG, and K values was also performed for neat TFTEA. From the calculated MRD values (Table 2) for the membranes and the ionic liquid, it can be first concluded the GAB model can appropriately describe the water sorption isotherms. All GAB parameters (Table 2) tend to increase with increasing the TFTEA content revealing an increase of the membrane hydrophilicity. In particular, the linear increase of Mm value as a function of TFTEA content in the concentration range from 0 to 100 wt % confirms the validity of the additivity law on water uptake for low activities (Figure 11). The influence of TFTEA concentration on hydrogen and oxygen permeability coefficients are reported in Table 2 for the Nafion−TEA+x%TFTEA membranes containing up to 20 wt % of TFTEA. Surprisingly, the introduction of TFTEA within neutralized Nafion membranes has a negligible effect on the gas permeation properties. The gas permeation coefficients are

CONCLUSIONS SANS measurements of anhydrous Nafion−TEA suggest a specific arrangement of the TEA counterions. The difference between the characteristic distance, extracted from the ionomer peak of a dry acidic Nafion, d0 and a Nafion−TEA dTEA corresponds to the size of TEA cation (4 Å) suggesting a single layer organization of interdigited TEA cations at the hydrophobic−hydrophilic interface. For the doped membranes, the evolution of nanostructure of Nafion−TEA with TFTEA concentration is very similar to that of acidic Nafion swollen by water. However, the broader peaks at high TFTEA content indicate a more heterogeneous distribution of TFTEA in Nafion−TEA probably due to the micellar organization of TFTEA in the membrane. It has been shown by DMA measurements that the ionic liquid solvates and plasticizes the Nafion−TEA side group reducing the dipolar interactions in the ionic domains inducing more facile mobility of the main fluorocarbon backbone chains. This phenomenon amplifies with the increase of TFTEA concentration. 24421



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Figure 11. Evolution of Mm values determined from GAB modeling as a function of the TFTEA weight fraction. (10) Pathapati, P R.; Xue, X.; Tang, J. Renewable Energy 2005, 30, 1− 22. (11) Gierke, T. D.; Munn, G. E.; Wilson, F. C. J. Polym. Sci. Polym. Phys. 1981, 19 (11), 1687−1704. (12) Litt, M. H. Polym. Prepr. 1997, 38, 80−81. (13) Rubatat, L.; Rollet, A.-L.; Gebel, G.; Diat, O. Macromolecules 2002, 35, 4050−4055. (14) Schmidt-Rohr, K.; Chen, Q. Nat. Mater. 2008, 7, 75−83. (15) Rubatat, L.; Gebel, G.; Diat, O. Macromolecules 2004, 37 (20), 7772−7783. (16) Sone, Y.; Ekdunge, P.; Simonsson, D. J. Electrochem. Soc. 1996, 143, 1254−1259. (17) Perrin, J. C.; Lyonnard, S.; Volino, F. J. Phy. Chem. C 2007, 111 (8), 3393−3404. (18) Gebel, G. Polymer 2000, 41, 5829−5838. (19) Gebel, G.; Lyonnard, S.; Mendil-Jakani, H.; Morin, A. J. Phys. Cond. Matt. 2011, 23, 234107. (20) Kusoglu, A.; Modestino, M. A.; Hexemer, A.; Segalman, R. A.; Weber, A. Z. ACS Macro. Lett 2012, 1 (1), 33−36. (21) Kreuer, K. D.; Paddison, S. J.; Spohr, E.; Schuster, M. Chem. Rev. 2004, 104, 4637−4678. (22) Wainright, J. S.; Wang, J.-T.; Weng, D.; Savinell, R. F.; Litt, M. J. Electrochem. Soc. 1995, 142 (7), L121−L123. (23) Xiao, L. X.; Zhang, H. F.; Scanlon, E.; Ramanathan, L. S.; Choe, E. W.; Rogers, D.; Apple, T.; Benicewicz, B. C. Chem. Mater. 2005, 17, 5328−5333. (24) Oono, Y.; Sounai, A.; Hori, M. J. Power Sources 2009, 189, 943− 949. (25) Kreuer, K. D.; Fuchs, A.; Ise, M.; Spaeth, M.; Maier, J. Electrochim. Acta 1998, 43, 1281−1288. (26) Sekhon, S. S.; Park, J.-S.; Cho, E.-K.; Yoon, Y.-G.; Kim, C.-S.; Lee, W.-Y. Macromolecules 2009, 42 (6), 2054−2062. (27) Di Noto, V.; Piga, M.; Giffin, G. A.; Lavina, S.; Smotkin, E. S.; Sanchez, J.-Y.; Iojoiu, C. J. Phys. Chem. 2012, C 2012, 116, 1361−1369 (28) Martinelli, A.; Iojoiu, C.; Sergent, N. Fuel Cells 2012, 12 (2), 169−178. (29) Di Noto, V.; Piga, M.; Giffin, G. A.; Lavina, S.; Smotkin, E. S.; Sanchez, J.-Y.; Iojoiu, C. J. Phys. Chem. C 2012, 116 (1), 1370−1379Li. (30) Iojoiu, C.; Hanna, M.; Molmeret, Y.; Martinez, M.; Cointeaux, L.; El Kissi, N.; Teles; Leprêtre, J.-C.; Judeinstein, P.; Sanchez, J.-Y. Fuel Cells 2010, 10 (5), 778−789. (31) Di Noto, V.; Negro, E.; Sanchez, J-Y.; Iojoiu, C. J. Am. Chem. Soc. 2010, 132, 2183−2193. (32) Martinez, M.; Iojoiu, C.; Judeinstein, P.; Lepretre, J.-C.; Coiteaux, L.; Sanchez, J.-Y. J. Power Sources 2010, 195 (18), 5829− 5839.

The water sorption of Nafion TEA is not predominated by the presence of specific sites of sorption as demonstrated by BET III isotherms and pretty low water uptakes are obtained. The introduction of ionic liquid within the Nafion membranes significantly boosts the ionic conductivity under anhydrous conditions and the water uptake capability. Surprisingly, the gas permeability of doped membranes is very close to that of polymer matrix, Nafion−TEA whatever is the doping level. The present study demonstrates that an ionic liquid can be introduced in an ammonium neutralized proton exchange membranes and enhance its ionic conductivity in anhydrous conditions by several orders of magnitude. The ionic liquid within the membrane behaves as a polar solvent without disrupting the nanostructure or increasing the gas permeability even at rather higher doping levels. However, the plasticizing effect of TFTEA similar to the polar solvents restricts the application in high temperature fuel cells except using membranes based on high glass transition polymers such as polyaromatic polymers.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the Cluster MACODEV, Rhone-Alpes France for providing financial support to this research work in the framework of the project “Nanostructurated Polymer Electrolytes for HT-PEMFC”.

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Proton Conducting Ionic Liquid Doped Nafion ... - ACS Publications

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