High-Temperature Ionic-Conducting Material - American Chemical

Article pubs.acs.org/JPCC

High-Temperature Ionic-Conducting Material: Advanced Structure and Improved Performance Dominique Langevin,†,‡,§ Quang Trong Nguyen,†,‡,§ Stéphane Marais,†,‡,§ Séma Karademir,†,‡,§ Jean-Yves Sanchez,∥ Cristina Iojoiu,∥ Mathieu Martinez,∥ Régis Mercier,⊥,# Patrick Judeinstein,¶,▽ and Corinne Chappey*,†,‡,§ †

Normandie Univ, France Université de Rouen, PBS, F-76821 Mont Saint Aignan, France § CNRS, UMR 6270 & FR 3038, F-76821 Mont Saint Aignan, France ∥ LEPMI, UMR 5279, CNRS-Grenoble-INP-Université Joseph Fourier-Université de Savoie, BP 75, F-38402 Saint-Martin-d’Hères, France ⊥ LMOPS, CNRS UMR 5041, US, F-73376 Le Bourget du Lac, France ¶ ICMMO, UMR-CNRS 8182, Université Paris-Sud, Bât. 410, F-91405 Orsay, France ‡

S Supporting Information *

ABSTRACT: A new composite proton-conducting material based on the association of an ionic liquid and a porous polymer support was prepared with the aim of applying it as an electrolyte in a proton exchange membrane fuel cell (PEMFC) at elevated temperature (130 °C). The porous support was made from a high glass-transition temperature polymer (Tg) by using the vapor-induced phase separation (VIPS) method in conditions leading to highly interconnected porous films. The ionic liquid tested was obtained by the reaction of a sulfonic acid with a tertiary amine and presents enough high-temperature stability to be used at elevated temperatures. Composite samples were prepared by immersing pieces of porous film in the ionic liquids under test. The porous support was characterized by scanning electron microscopy (SEM), gas permeation, and thermogravimetric analysis (TGA) tests, and the composite samples were characterized by mechanical and proton-conduction measurements. At 130 °C, this new material exhibits proton conductivity (20 mS cm−1) below, but very close to, that of the pure ionic liquid (31 mS cm−1) and presents, up to at least 150 °C, a storage modulus exceeding 200 MPa. This is very promising considering the PEMFC applications. using PVPA as host matrices. Benzimidazole, imidazole11 and triazole12 have been incorporated as heterocyclic compounds in the polymer matrix. These systems show high proton conductivity at high temperatures and in the anhydrous acid state. However, they show poor film forming properties and poor mechanical properties in contact with water vapor.13 Recently, proton-conducting ionic liquids (PCILs) have been proposed to play the role of electrolyte for anhydrous hightemperature fuel cell reactions. Ionic liquids are salts comprising an organic cation with either an inorganic or an organic anion; they are often used as “green” solvents in chemical applications and present very attractive properties, namely low vapor pressure, nonflammability, high thermal stability and high ionic conductivity. Generally liquid at room temperature, protic ionic liquids are synthesized by the neutralization reaction of Brønsted acids and bases;14 this

1. INTRODUCTION Increasing the operation temperature of polymer electrolyte membrane fuel cells (PEMFCs) above 100 °C leads to numerous advantages. Among these are improved cathode kinetics, improved tolerance of the catalyst to pollutants, easier heat exchange and improved water and gas management.1,2 In order to progress toward high temperatures and a low relative humidity domain, several solutions, such as composite, inorganic-polymer membranes3 or polybenzimidazole-based conducting membranes, have been considered.4,5 Several studies on the fabrication of highly proton-conducting materials at low cost and capable of operating above 110 °C showed that the phosphoric/phosphonic polymers are promising membrane materials for PEMFCs, due to their thermal stability and oxidation resistance.6 These membranes are based on an approach in which water, the usual proton carrier, is replaced by alternative protic solvents.7−10 The poly(vinylphosphonic acid) (PVPA) is a proton-conducting polymer having a proton conductivity of 10−3 S cm−1 under 1 bar of H2O vapor.6 Different works have been carried out with polymer blends © 2013 American Chemical Society

Received: December 20, 2012 Revised: July 2, 2013 Published: July 3, 2013 15552

dx.doi.org/10.1021/jp312575m | J. Phys. Chem. C 2013, 117, 15552−15561

The Journal of Physical Chemistry C

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conductivity (>10 mS cm−1 at 130 °C), even at temperatures ranging from 100 to 200 °C, along with good thermal and mechanical properties. In these works, the PCILs were generally used both as a solvent to swell the polymer matrix and as a countercation of the sulfonic sites to replace the protons.25−27 We have recently shown that, for the ionic liquiddoped Nafion membranes, the evolution trend of nanostructure of TriEthylAmmonium TriFluoromethaneSulfonate salt TFSuTEA-doped Nafion is very similar to that of acidic Nafion swollen by water.28 Vito di Noto et al. studied the abovementioned systems and those based on Nafion-TriEthylAmine (Nafion-TEA) + TriEthylAmine TriFlate (TFTEA) by FTIR/ Raman spectroscopies to understand the structure and interactions between different nanophases present in those materials.29 The observed decrease in the crystallinity of the hydrophobic phase in Nafion was explained by the effect of acid group neutralization by amine and ionic liquid addition. The Raman study showed that TF anions aggregate to form micellelike nanoclusters. They also used broadband dielectric spectroscopy (BDS) to study the electrical properties and conduction mechanism for such systems.30 It has been proposed that ionic liquids such as TFTEA form anionic micelle-like nanoparticles (referred to as MTAs) in the hydrophilic domains of Nafion through which proton migration occurs according to a hopping mechanism on ammonium ions, polymer sulfonate sites and MTAs. According to Di Noto et al., the charge transfer rate largely depends on the segmental motion of the polymer chains and the molecular dimensions of the ionic liquid nanoaggregates in such systems. Overall, in all the aforementioned works, there are few examples of functional materials based on the supported liquid principle that associates an inert polymeric porous support and an active ionic liquid phase, though this technique has been used by several authors for the separation of gas, vapors or solute species.31−43 In these references, the ionic liquid phase was impregnating mostly commercial filtration films made of polypropylene, polycarbonate, polyamide, polytetrafluoroethylene, polyethersulfone or polyvinylidene fluoride, with various pore sizes and porosities, and of hydrophilic or hydrophobic character, according to the kind of ionic liquid used. In the present work, the supported liquid system option was selected, targeting proton conductivity at high temperature in anhydrous conditions. An original polymeric porous support has been developed, aiming for high porosity, symmetric structure, efficient pore interconnectivity and favorable thermal and mechanical resistances. High Tg polyimides44 have been tested as porous supports by using the vapor induced phase separation (VIPS) process, which is a common method for producing various types of separation films.45−47 In our selected conditions, as expected from data in the literature, water vapor IPS led to a symmetric macroporous structure throughout the film cross-section.46,47 Attention was paid to finding the adequate conditions of solvent, humidity, temperature and concentration for a percolating porous structure favorable to high proton conductivity. The most satisfactory structure was finally selected for further investigations. The PCILs were synthesized by reacting sulfonic acids with selected amines to obtain high conductivity over a wide range of temperatures.26,27,48 The supported liquid systems were obtained by immersing porous film samples in the PCIL and were then characterized by different techniques to evaluate their performances in view of PEMFC applications. A comparison of conductivity perfor-

allows the protons to be mobile without water assistance. One approach for applying PCILs, proposed by Lakshminarayana et al., was to prepare anhydrous proton-conducting inorganic− organic hybrid material by a sol−gel process with tetramethoxysilane/methyl-trimethoxysilane/trimethyl phosphate and 1ethyl-3-methylimida-zolium-bis(trifluoromethanesulfonyl)imide ionic liquid as precursors.15 The thermal stability of the hybrid material was due notably to the presence of an inorganic silica framework, while a conductivity of 5.4 mS cm−1 was obtained at 150 °C in anhydrous conditions. PCILs have also been used in blends with various polymers: Lewandowsky et al. prepared a poly(acrylonitrile)/1-ethyl-3-methylimidazolium tetrafluoroborate polymer composite presenting a rubber-like elasticity and a conductivity of 15 mS cm−1 at 25 °C.16 Sekhon et al. mixed polyvinylidenefluoride-co-hexafluoropropylene with 2,2-dimethyl-1-octylimidazolium trifluoromethanesulfonyl imide. The film obtained showed a conductivity of 2.74 mS cm−1 at 130 °C with good mechanical stability, and was tested in a commercial fuel cell test station at 100 °C in dry conditions.17 Ye et al. developed [acid/ionic liquid/polymer] composite films by combining phosphoric acid, 1-propyl-3-methylimidazolium dihydrogen phosphate and polybenzimidazole. Their films had an ionic conductivity of up to 2 mS cm−1 at 150 °C under anhydrous conditions.18 In order to prevent the possible release of the ionic liquid component, Fernicola et al. incorporated ceramic fillers into their ionic liquid/poly(vinylidenefluoride-co-hexafluoropropylene) formulation.19 This approach was not as effective for retaining the IL, although it was successful in enhancing the conductivity of the films. By combining polymers of different hydrophobic/hydrophilic characters with different PCILs, Tigelaar et al. demonstrated clearly that the conductivity of the polymer/PCIL composite films depends mainly on the PCIL sorption capacity of the polymer, which is governed mostly by the PCIL/polymer matrix affinity.20 The conductivity reached in this study was 50 mS cm−1 at 150 °C with a hydrophobic polymer sample imbibed with 258% of imidazolium trifluoromethanesulfonate. Many studies involve the combination of PCILs with sulfonated polymers to improve PCIL retention by taking advantage of the sulfonic groups. S. Lee et al. prepared composite films made of poly(vinylidenefluoride-co-hexafluoropropylene), sulfonated poly(decafluorobiphenyl-(hexafluoropropylidene) diphenol) and 1ethyl-3-methylimidazolium fluorohydrogenate. These films showed enough thermal stability to operate at temperatures over 100 °C and reached a conductivity of 34.7 mS cm−1 at 130 °C. 21 S. Y. Lee et al. used diethylmethylammonium trifluoromethanesulfonate in a sulfonated polyimide matrix in which the sulfonic acid groups were in diethylmethylammonium form, making the polymer highly compatible with the PCIL. The samples presented good thermal stability and an interesting ionic conductivity (>10 mS cm−1 at 120 °C under anhydrous conditions).22 Sulfonated poly(ether ether)ketones, associated with trifluoroacetic propylamine23 or 1-butyl-3methylimidazolium chloride/hexafluorophosphate and phosphoric acid,24 have also been used successfully as the polymer matrix in preparing composite materials. Conductivities near 20 mS cm−1 at 160 °C were achieved in both cases. As long as there is enough water to allow proton mobility, the perfluorinated sulfonated Nafion membranes have excellent properties, at least up to 80 °C, and are thus widely used as reference material in PEMFC studies. Modifying Nafion membranes with PCILs has led to interesting levels of 15553



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Figure 1. (A) Photograph of viscous solution (Matrimid/PVP/NMP) and scheme for the porous polymer support preparation by the vapor induced phase separation (VIPS) method. (B) Structures of PVP, NMP and Matrimid, photograph of porous support, polymer composite preparation and scheme of the conducting polymer composite.

The shots of the surfaces were analyzed by using ImageJ software to estimate the pore size distribution. In order to test the interconnectivity of the porous network and the availability of conducting paths across the support, two methods were used: in the first (see Figure 2), a weak nitrogen pressure

mances in anhydrous conditions of the two systems would be interesting although the equivalent of the polyelectrolytesmolecular additive systems would be Matrimid blends with ionic liquids.

2. EXPERIMENTAL SECTION 2.1. Materials. N-Methylpyrrolidone (NMP, bp 197 °C), Polyvinylpyrrolidone (PVP, 81440, K 90, molar mass 360 000 g mol−1), as pore-forming agent, were purchased from AcrosOrganics and Fluka, respectively. Polyimide Matrimid, grade 5218 (Tg 323 °C), was kindly furnished free by Huntsman Cie, and the fluorinated polyimide 6FDAmPDA (Tg 304 °C) was synthesized in LMOPS, CNRS UMR 5041, US, F-73376 Le Bourget du Lac, France.49 2.2. Porous Support Preparation. The porous films were prepared by the VIPS process using water vapor as nonsolvent. The polymer (14 wt %) was dissolved in NMP with PVP (7 wt %) then heated (70 °C) and stirred (for about 6 h) in a closed flask until a homogeneous mixture was obtained (Figure 1A). The resulting viscous solution was cast (thickness 300 μm) with a doctor blade on a glass plate then exposed (for 6 h) to a humid nitrogen stream (temperature (25 °C). flow (1 L min−1). controlled relative humidity (50%)) in a glove compartment (volume 30 L) as shown in Figure 1A. After precipitation, the film (final thickness 100 μm) was peeled off in pure water then rinsed thoroughly in a water/ ethanol mixture (50/50 vol.) to eliminate the bulk of PVP and NMP. The porous support was then dried in ambient air and finally stored in a desiccator under vacuum. The Matrimid film was denoted MAT14PVP7 and the Fluorinated polyimide film was denoted PIF14PVP7. 2.3. Film Characterization. The porous film morphology (the two faces and the cross section) was investigated by scanning electron microscopy (SEM JEOL JSM 35CF). For the cross-section observations, the porous support was freezefractured in liquid nitrogen. The samples were coated with a gold conducting thin film before examination. The porosity of the supports was calculated from weight measurements of dense and porous polymer film samples of known volumes.

Figure 2. Measurement setup for the determination of the gas permeability of the porous supports.

difference was applied across the porous film sample, via a pressure regulator (Air Product R300-0.1), and the resulting gas flow was measured by an electronic flow meter (Honeywell AWM 3300 V). From Darcy’s law,50 the gas permeability coefficient K of a porous sample is given by: K=

Qη Δx S Δp

(1) 3 −1

where K (m ), Q is the flow of gas in (m s ), η is the dynamic viscosity of the gas ((17.8 × 10−6 Pa s) for nitrogen at 25 °C51), S is the effective sample area (9.5 × 10−4 m2), Δp is the pressure difference applied in (Pa) and Δx is the sample thickness (around 10−4 m). The flow-pressure curves allowed the permeabilities of the prepared porous films to be compared. In the second method, a sample of porous film was placed between the platinum plates of a conductivity cell52 immersed in an HCl 0.2 M solution so that the porous volume was filled by the electrolyte (see Figure 3). The rate of ionic conduction, measured by a conductimeter (Genrad RLC Digibridge 1657, 1 kHz), depends both on the conductivity paths crossing the film 2

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Swagelok cell with Teflon joints and spacers. The PCIL filled a special conductivity cell consisting of platinum electrodes. The cell constant of ca. 1 cm−1 was determined using a standard KCl aqueous solution. Measurements were performed from 20 to 150 °C. The temperature was equilibrated for 2 h before each measurement.27 The resistance of the membrane or PCIL is taken at the high frequency intercept with the real axis in the Nyquist plot, which is usually between 106 and 104 Hz. Dynamic mechanical analysis (DMA) measurements were carried out with a TA Instruments DMA Q800 spectrometer working in tensile mode. The strain magnitude was fixed at 0.01%. This value ensured that the tests were done in the linear viscoelastic domain. Measurements were performed in isochronal conditions (1 Hz) and the temperature was varied between (−100 and 150 °C) at 2 °C min−1. The MAT14PVP7 porous support and its composite with TFSu-TEA were also characterized by elongation stress−strain curves. The measurements were performed at room temperature on (3 × 0.5 cm) samples using an Instron 5543 testing machine at a constant crosshead speed of 1 mm min−1. Young’s modulus and Breaking stress and strain were determined from the average of 10 measurements. Oxygen permeability of the polymer composite was investigated with a homemade differential permeation apparatus. The apparatus consists of a measurement cell, oxygen and nitrogen supplies and a Systech 911 oxygen trace analyzer (Gruter et Marchand). U nitrogen, Alpha 2 nitrogen and Alpha 2 oxygen were furnished by Air Liquide. Transport of oxygen through the sample was determined from the increasing oxygen concentration in the carrier gas in the downstream compartment, detected by the oxygen trace analyzer. The experimental data were acquired by recording the oxygen concentration as a function of time. The theoretical oxygen flow rate Q crossing the sample and enriching the carrier gas sweeping the downstream compartment is written:

Figure 3. Measurement setup for the determination of the conductivity of the porous supports filled with HCl 0.2 M electrolyte.

and on the obstruction factor due to the solid phase network. The thermal stability of the prepared porous supports was investigated between 25 and 800 °C by using NETZSCH TG 209 apparatus, with an oxygen flow of (12 mL min−1) and a heating rate of (10 °C min−1). 2.4. PCIL Synthesis. The PCIL synthesis was achieved in the LEPMI Laboratory, UMR 5279 CNRS, Saint-Martin d’Hères.26,27 As the purification of PCILs is not very easy, they were obtained by a thorough acid−base titration, in water or in organic solvents, of an amine by an organic acid, both being previously purified by distillation. Purity was checked by NMR. In the present work, due to its very favorable proton conductivity,26 TFSu-TEA (triethylammonium trifluoromethanesulfonate salt) was selected as the filling phase; some of its characteristics are presented in Table 1. 2.5. Polymer Composite Preparation and Characterization. Composite films were prepared by immersing samples of porous support in the selected PCIL for one night at 60 °C, after which the samples were drained and gently wiped to remove superficial excess of liquid (Figure 1B). The final PCIL content of the film was close to 76% in weight. The TFSu-TEA uptake was checked by NMR spectroscopy. MAT14PVP7 filled with TFSu-TEA (0.1 g) was immersed in 5 mL of deuterated water and stirred during 5 h. To determine the amount of extracted TFSu-TEA by water, 50 μL of trifluoroethanol (TFE) have been added to the solution as internal standard. From 19F and 1H NMR spectra, the molar ratio between TFSu-TEA and then the amount of TFSu-TEA contained in 5 mL were calculated. TFSu-TEA (0.07 g) was found, corresponding to 70% of impregnated ionic liquid component in the membrane, in full consistency with the measured membrane porosity and the TFSu-TEA uptake by the membrane. The conductivity measurements were carried out using a Hewlett-Packard 4192A LF frequency response analyzer in the frequency range (5 Hz to 13 MHz). The films were placed between two stainless steel electrodes, under argon, in a

Q=J×S=q×C

(2)

So that J=

q×C S

(3)

where Q is expressed in (cm3 STP s−1), S (cm2) is the sample surface area, J is the flux crossing the interface (cm3 STP cm−2 s−1), q (cm3 s−1) and C (cm3 STP cm−3) are respectively the flow rate of the carrier gas and its oxygen concentration.53 From the ideal gas law, the oxygen concentration in the carrier gas is given by:

C=

X × ptd R×T

(4)

T is the absolute temperature and R the ideal gas constant, X is the oxygen mole fraction in the carrier gas, measured by the analyzer, and ptd is the total pressure of the carrier gas in the

Table 1. Proton Conducting Ionic Liquid

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Figure 4. (A, B) SEM images of the cross sections showing the spongy structure of the porous supports (A) MAT15 × 577 and (B) PIF15 × 702 prepared without PVP. (C, D, E, F) SEM images of the cross section showing the interconnected structure of the porous supports. (C) MAT14PVP7 × 1000; (D) MAT14PVP7 × 4000; (E) PIF14PVP7 × 1000; (F) PIF14PVP7 × 5000. (G, H, I, J) SEM images of the “gas” and “glass” faces of the porous supports. (G) MAT14PVP7 × 1000 (gas); (H) MAT14PVP7 × 1000 (glass); (I) PIF14PVP7 × 1000 (gas); (J) PIF14PVP7 × 10 000 (glass).

Mitutoyo 0−25 mm) evenly distributed at the surface of the sample film.

detector. From eq 3, and considering the definition of the coefficient of permeability P = J((L)/(Δp)), where Δp/L is the pressure gradient, the oxygen permeability coefficient PO2 can be calculated by: PO2 =

3. RESULTS AND DISCUSSION 3.1. Porous Support Characterization. 3.1.1. Porous Morphology. Figure 4C and E shows SEM images of the crosssection of the porous supports prepared by using the phase inversion technique described above. The temperature, humidity, polymer and porogen agent concentrations were optimized after many trials according to the objectives indicated in the Introduction. The Matrimid (MAT14PVP7) and 6FDAmPDA (PIF14PVP7) films present similar morphologies, that is, an open and percolating structure, practically symmetric, with large and tortuous pores. It can be noticed that the pores developed in Matrimid are larger (around 8 μm in size) than in 6FDAmPDA (around 4 μm). Symmetric morphologies obtained by water VIPS have previously been mentioned for polysulfone47,57 and polycarbonate58 films but they were only spongy structures with surface skin layers. In the walls of the main structures of these two polymers, it is easy to discern, at stronger magnification, the presence of another type of porosity with smaller-diameter pores (see Figure 4D and F); this secondary structure is spongy and shows small juxtaposed cells of about 0.5 μm in size. Figure 4A and B shows the importance of the role of the porogen agent PVP in the final morphology of the porous films.59,60 Figure 4A and B presents SEM images of the cross sections of MAT15 and PIF15 films (15% polymer in NMP) obtained by the VIPS method without addition of PVP. The porous structures obtained are spongy and consist of juxtaposed closed cells with sizes between 3 and 10 μm. Such morphology, consisting mostly of isolated cells, is not favorable to conducting paths. Figure 4G−J shows SEM images of the two faces (gas-side and glass-side) of the prepared supports. Although the porous structure is practically symmetric across the film cross sections, a difference of surface porosity appears between the two faces. According to Park et al., this difference is attributable to differences in the kinetic transport parameters

q × L × X × ptd (p1 − p2) × S × R × T

(5)

L is the thickness of the sample, p1 and p2 are partial oxygen pressures on both sides of the film (p1 ≫ p2). PO2 is expressed in Barrer (1 Barrer = 10−10 cm3 STP cm cm−2 s−1 cmHg−1). The quantity R = L/P is the resistance of the sample to oxygen permeation. In the case of a multilayer sample, the total resistance is the sum of the resistances (Ri = Li/Pi) of each layer.54 In order to prevent expulsion of the ionic liquid from the porous support due to a fortuitous pressure difference during measurement, the composite system sample is inserted between two films of silicon (s). The silicon film used in this study is a commercial silicon referenced as pure silicon (thickness, Ls is (0.100 cm ± 0.005 cm), Solutions Élastomères Cie, St Etienne, France). This silicon is very permeable, Ps(O2) close to 800 Barrer,55 and generally interacts weakly with ionic liquids.56 The resistance Rs= Ls/Ps of a silicon bilayer sample (Ls/s = 0.2 cm) is first determined, then the resistance RPC = LPC/PPC of the polymer composite (PC) confined between two layers of silicon. The resistance LPC/PPC of the polymer composite can be deduced from the global resistance by: L(s/PC/s) P(s/PC/s)

PPC =

=

Ls L L + PC + s Ps PPC Ps

L PC L(s/PC/s) P(s/PC/s)

−

2Ls Ps

(6)

(7)

and thus the permeability coefficient PPC, knowing the thickness LPC. The circular films (3.5 cm in diameter) were dried in a vacuum for 12 h at room temperature. The thickness, L (cm), was an average of 5 measurements (micrometer 15556



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Figure 5. Surface pore size distribution (estimated by using Image J software). (A) MAT14PVP7. (B) Surface porosity analysis. (C) PIF14PVP7; (D) PIF14PVP7 glass face, thin pores only. (E) Porosity rate.

of the components (polymer, solvent and nonsolvent, porogen agent) that govern the final structure induced by VIPS.36 Figure 5 presents some characteristics of the surface porosities obtained using ImageJ software analysis. The outlet pore size is noticeably higher on the gas-side than on the glass-side (see Figure 4G/I or H/J and also Figure 5A− C). This phenomenon is even more significant with the 6FDAmPDA support although the preparation protocol was the same for the two polymers. Considering the porous area fraction (Figure 5B), the porosities of the two faces of the MAT14PVP7 film are practically equivalent (33.7 and 27.7% for gas and glass faces, respectively), whereas the difference is notable between the two faces of the PIF14PVP7 film (area fractions equal to 24.8 and 4.6% for gas and glass faces respectively). Figure 4J shows the glass-side of the PIF14PVP7 film at high magnification (×10000) and reveals a multitude of small pores of 100 to 250 nm in diameter (see also Figure 5D). This porosity also opens conduction paths (area fraction around 3%), but to a lesser extent compared to the Matrimid film. This will induce some differences of performance between the two porous supports. Figure 5E shows the porous volume fractions calculated from weight and volume measurements achieved on dense and porous samples of Matrimid and 6FDAmPDA (PIF). It appears that MAT14PVP7 and PIF14PVP7 support membranes exhibit equivalent total porosity, around 72%. We supposed that for the ionic liquids that are not too viscous, the rate of pore filling by the liquid phase is similar. However, when the mean pore size at the membrane surface changes, the filling rate can also change, especially for ionic liquids of high viscosities. The viscous flow of the ionic liquid through the surface toward the support membrane core will be reduced because of the surface transfer resistance. 3.1.2. Interconnectivity. Figure 6A presents the gas flow observed across the porous supports under a nitrogen pressure difference (Δp up to 0.2 bar). The MAT 15 film, of spongy

Figure 6. (A) Gas flow across the porous supports as a function of the nitrogen pressure applied. (B) Conductivity of the porous supports filled with 0.2 M HCl electrolyte.

structure prepared without addition of a porogen agent, seems to be a full barrier to gas since the flow remains null, whatever the pressure (gas permeability coefficient K ≈ 0). The more permeable support is the MAT14PVP7 film (K ≈ 7.6 × 10−9 m2), which is about twice as permeable as the 6FDAmPDA film (K ≈ 3.5 × 10−9 m2). In spite of its percolating structure, PIF14PVP7 seems to be hindered by the low porosity of its skin, so when filled with a PCIL, MAT14PVP7 will have the better conductivity. Figure 6B presents the conductivity values 15557



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of 0.2 M HCl-filled porous supports, measured at 30 and 60 °C. As expected, the conductivity increases with the temperature and is the highest for the MAT14PVP7 support compared with the PIF14PVP7 and the MAT 15 films. This result is in agreement with the values of gas permeability observed previously. 3.1.3. Thermal Stability. Thermogravimetric analysis of MAT14PVP7 and PIF14PVP7 supports are compared with the source polymer (MAT and PIF respectively), the porogen agent (PVP) and the porous support prepared without PVP (MAT15 and PIF15 respectively) (Figure 7A and B). In

Figure 8. Temperature dependence of the proton conductivity of TFSu-TEA and MAT14PVP7/TFSu-TEA. Comparison of NafionTEA/TFSu-TEA with MAT14PVP7/TFSu-TEA.

conductivity of the film remains lower than that of the pure ionic liquid. At 130 °C, for example, the MAT14PVP7/TFSuTEA film conductivity reaches about 20 mS cm−1, that is, 64% of the TFSu-TEA conductivity 31 mS cm−1. Figure 8 allows us to compare, in terms of conductivity, the performances of the MAT14PVP7/TFSu-TEA film and of a composite film resulting from the neutralization by TEA and swelling by TFSu-TEA of an acidic Nafion membrane.19 It is clear that the MAT14PVP7/TFSu-TEA curve is well above that of the Nafion-TEA/TFSuTEA. At 130 °C, the Nafion composite membrane shows conductivity near to 1.8 mS cm−1 against 20 mS cm−1 for MAT14PVP7/TFSu-TEA, which is a difference of 1 order of magnitude. This result is in agreement with the values of gas permeability observed below. 3.2.2. DMA Measurements and Elongation Stress−Strain Measurements. Figure 9A presents the temperature dependence of the storage modulus of samples of MAT14PVP7/ TFSu-TEA and Nafion-TEA/TFSuTEA. In the range 30−150 °C, while the modulus of MAT14PVP7/TFSu-TEA decreases slightly from 296 to 202 MPa, that of the Nafion composite membrane falls dramatically from 49 to 1 MPa. This result shows that in contrast to the perfluorinated ionic matrix, the mechanical resistance of the rigid Matrimid is affected very little by temperature, and remains high even at the temperature of 130 °C targeted in this work. It is clear that pore impregnation by the ionic liquid tends to make the film brittle, as confirmed by the statistical results calculated from 10 samples and collected in Figure 9B.61,62 The breaking stress and the breaking strain are decreased by 17 and 52%, respectively, while Young’s modulus increases by 5%. When the pores are filled with the IL, the polyimide matrix is in equilibrium with the ionic liquid phase so that it absorbs a quantity of salt able to antiplastisize it. 3.2.3. Oxygen Permeability. Three successive series of differential permeation experiments were performed: (i) three experiments with two silicon films stacked in the cell permeation, (ii) three experiments with the empty porous support inserted between both films of silicon, and (iii) three experiments with the composite system inserted between both films of silicon. The experiments were performed in the

Figure 7. Thermogravimetric analysis of (A) MAT14PVP7 and (B) PIF14PVP7.

comparison with the pure polymers, the thermograms of the porous materials prepared without PVP are slightly affected by the residual solvent (NMP), while the degradation temperatures of MAT14PVP7 and PIF14PVP7 are significantly decreased, due to residual trapped NMP and PVP. The main mass loss is situated around 400 °C for MAT14PVP7 and around 500 °C for PIF14PVP7; keeping in mind that the temperature of 130 °C was targeted for using the supports prepared, their thermal stability is considered to be satisfactory. 3.2. Conducting Polymer Composite Characterization. 3.2.1. Polymer Composite Conductivity. Figure 8 presents the temperature dependence of the proton conductivity of the supported PCIL system prepared by impregnating the MAT14PVP7 support with (triethylammonium trifluoromethanesulfonate salt, TFSu-TEA (72 wt %)). The conductivity of the PCIL is also represented; the conductivity increases as the temperature increases, and because of the hindrance due to the polymer structure the 15558



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Figure 9. (A) DMA measurements, storage modulus. Comparison of NafionTEA/TFSu-TEA with MAT14PVP7/TFSu-TEA. (B) Comparison of Nafion, dense MAT14, MAT14PVP7 and MAT14PVP7 + TFSu-TEA stress−strain measurements (1Averaged from 10 measurements at room temperature). Figure 10. *Mean Oxygen permeability P(O2) for silicon films, silicon/porous support/silicon and silicon/composite membrane/ silicon systems at 25 °C (averaged for 3 measurements). (Lsilicon = 0.1 cm; Lporous support and Lcomposite membrane = 0.0127 cm). **1 Barrer = 1 × 10−10 cm3 STP cm cm−2 s−1 cmHg−1.

following order: silicon/silicon, silicon/porous support/silicon, silicon/composite system/silicon; the results are presented in Figure 10. The insertion of the porous support between both silicon films did not affect the oxygen transport, which indicates that the resistance of the porous support to oxygen permeation is negligible compared to silicon (PO2 ≈ 636 Barrer). On the other hand, the presence of TFSu-TEA modified the transport of the oxygen through the multilayer. The oxygen permeability of the composite system was calculated using the multilayer resistance model and was found to be equal to 14 Barrer. The low value of oxygen permeability is very promising considering the PEFMC applications.

on temperature. The oxygen permeation measurements show that the low value of oxygen permeability is very promising. Tailoring a proton-conducting ionic liquid, in such a way to provide high enough conductivities while exhibiting a hydrophobic character, is like squaring the circle. Due to the risk of elution of the water-soluble protonconducting component out of the membranes in fuel cell operations, it is necessary to operate the MEAs based on ionic liquid membranes only at high temperatures (e.g., at temperatures higher than 120 °C) in order to avoid its progressive elution by its dissolution in the liquid water produced by the electrochemical reaction. Such a fuel cell operation requires a hybridization of the PEMFC either with a rechargeable battery or with a small internal combustion engine. Thus, the PEMFC could be started and stopped above 100 °C, the hybridization allowing the PEMFC to be started once its temperature reaches 100 °C and then the battery to be recharged during the PEMFC operation. We must emphasize that most of the electric vehicle prototypes powered by PEMFC use lithium-ion batteries. Keeping in mind that further investigations, in particular tests in a fuel cell test station, are still necessary to supplement these first experiments, the results are very promising for application in high temperature PEMFCs. The interest of this new hightemperature conducting material is also due to this new concept of film based on supported ionic liquid that can be applied in other industrial processes such as separation of gas, VOC, water purification (heavy metals) but also in biomedical applications as selective film for controlled releasing, decontamination, etc. The success of this advanced system is in the

4-. CONCLUSIONS In this article, the possibility of producing a high-temperature (130 °C) proton-conducting material based on the supported ionic liquid system concept has been investigated. A macroporous polymer support (MAT14PVP7) was designed by the VIPS method from a high Tg heatproof industrial polymer, the Matrimid 5218 polyimide. This porous structure is highly interconnected, with a bimodal porosity, and able to develop high conductivity if filled with a conducting phase. A composite material was prepared by impregnating MAT14PVP7 with an ionic liquid that is highly proton conductive (TFSu-TEA) synthesized from triethylamine and trifluoromethanesulfonate acid. This material presents very interesting performances in terms of mechanical resistance and proton conductivity compared to the composite Nafion-TEA/TFSu-TEA membrane previously obtained by impregnation of the well-known perfluorinated Nafion 117 membrane. Impregnation of the MAT14PVP7 material with TFSu induced a slight embrittlement of the macroporous material; however, MAT14PVP7/ TFSu-TEA exhibits a conductivity of 20 mS cm−1 at 130 °C, that is, 64% of the conductivity of the pure TFSu-TEA, and a storage modulus near 200 MPa at 150 °C, weakly dependent 15559



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particular porous structure (dual porosity, high interconnectivity, symmetric structure) obtained with a polyimide matrix that allows maintenance of the performance (conductive and selective properties) of the ionic liquid used at high temperatures (>100 °C).

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ASSOCIATED CONTENT

S Supporting Information *

Complete author list of the references with more than 10 authors. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Corinne Chappey, Laboratoire PBS UMR 6270 - Université de Rouen-CNRS-INSA Bâtiment Pierre-Louis DULONG, n°23 Bd. Maurice de Broglie, 76821 Mont-Saint-Aignan Cedex, France. Tel.: (33)2.35.14.66.97. Fax: (33)2.35.14.67.04. E-mail: [email protected]. Present Addresses #

R.M.: IMP, UMR 5223 CNRS/UCB Bât. Raulin, 69622 Villeurbanne, France. ▽ P.J.: LLB, UMR CNRS-CEA 12, CEA Saclay, F-91191 Gif sur Yvette, France. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Many thanks to ANR who supported the PAN’H CLIPPAC project. The authors thank I. Zimmerlin and J.J. Malandain for the Scanning Electron Miscroscopy technical support.

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REFERENCES

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High-Temperature Ionic-Conducting Material - American Chemical

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