ionic liquid composite membranes for fuel cells operating at high temperatures

Electrochimica Acta 130 (2014) 830–840

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Polyimide/ionic liquid composite membranes for fuel cells operating at high temperatures Abdellatif Dahi a,b,c , Kateryna Fatyeyeva a,b,c , Dominique Langevin a,b,c , Corinne Chappey a,b,c , Sergiy P. Rogalsky d , Oksana P. Tarasyuk d , Stéphane Marais a,b,c,∗ a

Normandie Université, France Université de Rouen, Laboratoire Polymères, Biopolymères et Surfaces, Bd. Maurice de Broglie, F-76821 Mont Saint Aignan Cedex, France c CNRS, UMR 6270 & FR 3038, F-76821 Mont Saint Aignan Cedex, France d Institute of Bioorganic Chemistry and Petrochemistry, National Academy of Science of Ukraine, 50, Kharkivske schose, 02160 Kyiv, Ukraine b

a r t i c l e

i n f o

Article history: Received 6 December 2013 Received in revised form 14 March 2014 Accepted 14 March 2014 Available online 27 March 2014 Keywords: High-temperature PEMFC Supported ionic liquid membranes Protic room-temperature ionic liquids Matrimid® membrane Ionic conductivity

a b s t r a c t Supported ionic liquid membranes (SILMs) were prepared by impregnating a porous Matrimid® membrane with protic room-temperature ionic liquids (RTILs): 1-n-methylimidazolium dibutylphosphate ([C1 im][DBP]), 1-n-butylimidazolium dibutylphosphate ([C4 im][DBP]) and 1-n-butylimidazolium bis(2ethylhexyl)phosphate ([C4 im][BEHP]). After immersion of the porous Matrimid® membrane in each RTIL, the prepared SILMs were composed of 53 ± 3% of RTIL phase. The control of the full impregnation of the porous Matrimid® membrane by the RTILs was verified by FTIR spectroscopy. From the bubble point test, the SILMs displayed a good retention of their RTIL phase and, thus, showed a good stability against the increased pressure on one of the faces. The ionic conductivity of SILMs, used as electrolyte membranes for proton exchange membrane fuel cell (PEMFC) was measured by impedance spectroscopy. Unlike the Nafion® , the conductivity of the SILMs still increases at high temperatures (> 80 ◦ C). The [C4 im][DBP]-impregnated SILM showed the best result in terms of conductivity (2.0 10−2 S/cm at 115 ◦ C). Moreover, all SILMs showed satisfactory stability since they retained a large amount of the RTIL phase in the membrane pores. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction Proton exchange membrane fuel cells, also known as polymer electrolyte membrane fuel cell (PEMFC), is a promising technology to reduce pollutant emissions and specifically the greenhouse gas emissions. The polymer electrolyte membrane (PEM) is required to ensure a good chemical, mechanical and electrochemical stability associated with high ionic conductivity. Currently, perfluorinated sulfonic acid membranes satisfy these specifications. The most known of these materials is the Nafion® , considered as the reference electrolyte for PEMFC [1–4]. The proton conductivity of Nafion® membranes is strongly dependent on humidity. Consequently, the Nafion® -based PEMFCs are generally operated at 80 ◦ C under fully hydrated conditions. This is the main limitation for Nafion® membranes since operation in the 100 - 200 ◦ C range is desirable for several reasons, such as the enhancement of the kinetics of fuel oxidation, the increase of the energy efficiency, and the

∗ Corresponding author. E-mail address: [email protected] (S. Marais). http://dx.doi.org/10.1016/j.electacta.2014.03.071 0013-4686/© 2014 Elsevier Ltd. All rights reserved.

reduction of catalyst poisoning phenomena [5,6]. Thus, materials having conductivity less or not dependent on the hydration state of the polymer are desired. During the past years, several approaches have been proposed to remedy the problems of perfluorinated sulfonic acid membranes at high temperatures. These include: (i) the modification of conventional membranes such as Nafion® by incorporating non-aqueous additives [7–9], (ii) the elaboration of new types of membranes using proton-conducting ionomeric polymers different from Nafion® [10] and (iii) the use of inert polymer matrices impregnated with a proton-conducting component as the proton source [9,11–15]. In this latter case, it has been reported that the composite proton conducting membranes containing RTILs as proton carriers are one of the promising alternatives at higher temperatures under anhydrous condition [9,11–15]. The replacement of traditional proton medium (water) by RTILs is challenging and very attractive for proton exchange membrane fuel cells (PEMFCs) operating at high temperature, i.e. at low relative humidity. There is a growing interest to use RTILs as proton carrying medium for the preparation of the composites proton-conducting membranes. RTILs have attractive properties [16–23,28] such as good

A. Dahi et al. / Electrochimica Acta 130 (2014) 830–840

chemical and thermal stability and high conductivity as they entirely consist of ions (namely, an organic cation and either an organic or an inorganic anion). Besides, RTILs are characterized by broad electrochemical window, negligible vapour pressures, non-flammability, as well as wide temperature liquid range as they are generally liquid under ambient temperature until 200 ◦ C. Moreover, the properties of RTILs such as conductivity can be changed by varying the anion-cation combination. For PEMFC, the use of protic RTILs, also called proton-conducting ionic liquids (PCILs), is motivated by the presence of a protonated cation [11,24]. Indeed, for ionic conductivity, a proton is transferred via hopping among the cations, independently of the presence of water unlike in the case of Nafion® membranes. Lin et al. [9] prepared new protic ionic liquid/poly(styrene-co-acrylonitrile)/silica hybrid PEMs. The membranes showed proton conductivity up to 1.10−2 S/cm at 160 ◦ C and under anhydrous conditions. Deligöz et al. [25] prepared new acid-doped polyimide/IL complex membranes using three ionic liquids [C1 im][BF4 ], [C2 im][BF4 ] and [C4 im][BF4 ]. The long-term conductivity and mechanical properties of these complex membranes were studied depending on the IL’s type and the–COOH/imidazolium molar ratio (n). The acid-doped complex membrane with [C1 im][BF4 ] exhibited the highest conductivity values (8.93 10−4 S/cm at 180 ◦ C). Fernicola et al. [5] obtained conductivity values reaching 1.10−2 S/cm at 140 ◦ C with composite membranes based on PVdF-coHFP (poly(vynilidenfluoride) as polymer matrix and MPyTFSI (1-methylpyrrolidinium bis(trifluromethansulfonyl)imide) as RTIL. Sirisopanaporn et al. [26] prepared ionic liquid-based gel type membranes that can operate without degradation up to a temperature of 110 ◦ C where they reach conductivity values of the order of 10−2 S/cm. All these tested composite membranes containing RTILs have shown excellent conductivity at high temperature (until ∼200 ◦ C) associated to good thermal stability and mechanical properties [5,9,11–14,25,26]. In addition, the conductivity of the composite membranes depended mainly on the nature and content of the RTIL inside the membrane [11,27]. It has also been found that the addition of acids containing common anion with RTILs into certain composite membranes improved the ionic conductivity of the resulting membranes [6,28,29]. This improvement in conductivity is due to the increase in free H+ ions in the RTIL. RTILs have been incorporated in Nafion® membranes [30–33]. Doyle et al. [31] demonstrated that the stability and the conductivity of the Nafion® membrane was enhanced by swollen it with 1-butyl, 3-methyl imidazolium trifluoromethane sulfonate, the ionic conductivity of 0.1 S/cm at 180 ◦ C was reached. Iojoiu et al. [30] focused on the performance of blends involving specifically modified Nafion® and two protic ILs based on triethylamine (TEA). Conductivity values measured at 130 ◦ C were close to those obtained for Nafion® membranes at 80 ◦ C and with 98% relative humidity. In order to improve the performance of IL based composite membranes, conductive polymers were also prepared and especially sulfonated polymers. Sulfonated aromatic polyimides, in comparison to many advanced sulfonated polymers such as poly(ether ketone) [34] and polybenzimidazole (PBI) [35,36], are considered one of the promising PEM candidates to be used in fuel cells [37]. These materials exhibit most of the required properties for this application, including a high level of ionic conductivity, low methanol permeability, and good mechanical properties. Deligoz et al. [38] synthesized and characterized a new type of highly anhydrous conducting complex membrane based on sulfonated polyimide and n-methylimidazolium tetrafluoroborate ([C1 im][BF4 ]). The complex membrane showed a maximum conductivity of 5.6 10−2 S/cm at 180 ◦ C, associated to excellent thermomechanical properties. Chen et al. [39] fabricated composite sulfonated polyimide/protic ionic liquid membranes. Two protic ionic liquids were used (1-vinylimidazolium

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trifluoromethanesulfonate ([ImVH][OTf]) and 1-methylimidazolium trifluoromethanesulfonate ([ImMH][OTf]). The obtained composite membranes exhibited good thermal stability (Td > 350 ◦ C) and high ionic conductivity (3-6 mS/cm at 120 ◦ C) under anhydrous conditions. In literature most works on sulfonated polyimide/IL deal with dense composite membranes [15,25,38,39]. However sulfonated polyimides are more or less sensitive to degradation in cell operations, depending on the environment of imide ring [40]. For instance, the close presence of hydration water to the imide ring enhances the hydrolysis of the latter: phtalic imide type was much more easily degraded than naphtalic imide type. In addition to dense composite membranes, a second approach for elaborating RTIL based membranes is the supported ionic liquid membranes (SILM) made of a porous structure filled with the RTIL. Several papers have shown the stability of SILMs by means of tests in various operating parameters, i.e. the nature of RTILs and the supporting membrane [41–43], the method of preparation [44], the nature of the adjacent phases [42,45–47], the transmembrane pressure [48] and temperature [48,49]. Indeed, the physicochemical properties of RTILs, namely their negligible vapour pressure and high interfacial tension, minimize their loss from the supporting membrane. Fortunato et al. [50] have characterized by impedance spectroscopy different SILMs obtained by immobilizing the RTILs ([Cn C1 im][PF6 ] (n = 4; 8) and [C10 C1 im][BF4 ]) in a poly(vinylidene fluoride) (PVDF) supporting membrane. It has been found that the electrical resistance values of the membranes are comparable to those of a typically charged membrane, such as Nafion® 117. This suggests the possibility of using SILMs in the same type of electrochemical applications that Nafion® membranes, namely for fuel cell applications. Our target was to obtain a membrane with good ionic conductivity under low relative humidity for PEMFC operating at high temperature. For this, a porous support (PI) with good mechanical and thermal properties was swelled in an ionic liquid that imparts its high conductivity after pore filling of PI. Thus, the novelty of the present work consists in the design of new supported ionic liquid membranes (SILMs). Indeed, a new porous structure (high porous volume, double porosity and interconnectivity of pores) of Matrimid film was obtained by using the water vapour phase inversion method. This structure facilitates the impregnation of a large amount of ionic liquid phase and is different from the dense membranes where the IL phase is dispersed in the dense organic matrix. The electrochemical properties and especially conductivity of the SILM can be changed depending on the ionic liquid used. In our case the mechanical support of the composite membrane is provided by Matrimid matrix, while IL in the liquid state (i.e. with high mobility), located in interconnected pores and which is the major phase of the SILM, ensures the conductivity properties. The concept of such membrane structure can be applied to different ionic liquids (protic or aprotic). Moreover, the polyimide macroporous separator was not functionalized by any ionic group so that its hydrophobicity makes it much less sensitive to hydrophilic degradation compared to sulfonated polyimides. Also, in terms of conductivity, the performances of this type of SILM are close to that of Nafion® and sulfonated PI/IL composite dense membranes, but under anhydrous conditions. To our knowledge, such PI/IL composite membranes performed in this work have never been studied previously. Such a concept of composite membranes allows us to optimize by varying separately the polymer host structure (which is a hydrophobic thermostable polymer, no proton conductivity required) and the ionic liquid nature, to a large extent. In this study, we have evaluated by impedance spectroscopy the potential of new PI/protic ionic liquid as electrolyte membranes for anhydrous PEMFC. The supported porous membrane based on Matrimid® 5218 (an aromatic PI; hydrophobic

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A. Dahi et al. / Electrochimica Acta 130 (2014) 830–840

CH3

O

O

N

H3C CH3

O

tissue paper and then stored in a vacuum desiccator over P2 O5 at room temperature (23 ± 2 ◦ C).

O N O

2.3. Preparation of SILMs

n

Fig. 1. Chemical structure of Matrimid® 5218.

thermostable polymer) was prepared according to the phase inversion technique. Matrimid® 5218 was chosen as polymer material for several reasons. Firstly, this glassy and thermoplastic polymer shows excellent thermal (Tg = 323 ◦ C, Tthermal degradation > 450 ◦ C), chemical and mechanical stability. Secondly, its good solubility and implementation allow to obtain easily durable and resistant membranes. The supported porous membrane was then impregnated with the following protic and hydrophilic RTILs: 1-n-methylimidazolium dibutylphosphate ([C1 im][DBP]), 1-n-butylimidazolium dibutylphosphate ([C4 im][DBP]) and 1n-butylimidazolium bis(2-ethylhexyl)phosphate ([C4 im][BEHP]). These three alkylimidazolium dialkylphosphates are chemically and thermally stable due to the imidazolium cations [16,18,21,51] and do not decompose by hydrolysis unlike RTILs based on [PF6 ] and [BF4 ] anions [16,18,52,53]. Furthermore, in addition to their low toxicity and corrosive-resistant property, their low cost and ease of production make them very attractive for industrial applications [54,55]. SILMs were prepared by impregnation of a porous organic membrane with protic RTILs. FTIR spectroscopy was used to verify the full impregnation of the porous Matrimid® membrane by the RTILs. The stability of prepared SILMs was evaluated by bubble point test. 2. Experimental 2.1. Materials Matrimid® 5218 powder (Fig. 1) (5(6)-amino-1-(4 aminophenyl)-1,3-trimethylindane (BTDA-DAPI)) was kindly procured from Huntsman Advanced Materials (Switzerland). 1-methyl-2-pyrrolidone (NMP, purity 99%) was purchased from Acros Organics and polyvinylpyrrolidone (PVP, purity 90%) from Sigma–Aldrich. All reagents were used without further purification. All water used was milli-Q water (milli-Q Water System, resistivity = 18 M/cm at 25 ◦ C). N2 (ultra-high purity (99.99%)) was used as inert and dry gas for the bubble point test, and was purchased from Air Liquide. [C1 im][DBP], [C4 im][DBP] and [C4 im][BEHP] (Fig. 2) were prepared by mixing equimolar quantities of 1-n-alkylimidazole (1) (alkyl = methyl or butyl) either with dibutyphosphate (2) to obtain [C1 im][DBP] and [C4 im][DBP], or with bis-(2-éthylhexyl)phosphate (3) to obtain [C4 im][BEHP] (Fig. 3). Nafion® 117 membrane of thickness L = 175 ␮m was supplied by Ion Power, Inc. 2.2. Preparation of porous Matrimid® membrane Porous and flat-sheet Matrimid® membranes were prepared by water vapour induced phase inversion from the polymer solution (Matrimid® /NMP/PVP: 14 wt.%/79 wt.%/7 wt.%). The preparation procedure was previously elaborated in our laboratory [56]. The polymer solution was casted on glass plate and was exposed to water vapour-saturated atmosphere (45-55% relative humidity) during 7 h at 20-25 ◦ C, and then was solidified by quenching in a liquid water bath. The membrane (200 ± 20 ␮m) was dried with

SILMs were prepared according to the direct immersion method. In fact, the immobilization of RTILs takes places by impregnating the porous Matrimid® membrane in a given RTIL at a constant temperature (25 ◦ C). After rapid and full immobilization, the membranes were delicately wiped to remove the excess of RTIL from their surfaces. To determine the amount of RTIL in the membrane, all membranes were weighed before and after impregnation. Impregnation kinetics have been realized before the preparation of SILMs to determine the time required for a full impregnation of the porous Matrimid® membrane with RTILs. Mass uptake in RTIL was monitored by periodically weighing of the SILMs until the equilibrium state was reached. The excess of RTIL from the membrane surface was eliminated as mentioned above. 2.4. Characterization methods 2.4.1. Scanning electron microscopy (SEM) Scanning electron microscope (SEM/FIB LEO1530 ZEISS) operating at 5.00 kV (EHT) was used to characterize the porous Matrimid® membrane. Samples for cross-sectional morphology characterization were cryogenically fractured in liquid nitrogen to preserve their structures. Images of membranes coated with a sputtered gold layer were obtained. Moreover, to observe the membranes in their virgin state (i.e. without metallization), an environmental SEM (HITACHI TM3000, Elexience (France)) was also used. 2.4.2. FTIR spectroscopy FTIR spectroscopy was used to control the efficiency of the direct immersion method by verifying the presence of RTILs in the entire cross-section of the porous Matrimid® membrane. Initially, an IR spectrum of the non-impregnated porous membrane was recorded. Then, by keeping the membrane in place, few drops of a given RTIL were poured on the upstream side of the membrane surface. The impregnation through the membrane pores occurred spontaneously and without any external force. After impregnation, an IR spectrum was anew recorded at the downstream side of the membrane allowing to verify the presence of RTIL on both sides of the membrane. IR spectra were measured by an AVATAR 360 FTIR spectrometer (Thermo Fisher) using the ATR mode (equipped with a diamond crystal). 32 scans were collected for each measurement over the spectral range 4000–650 cm−1 with a resolution of 8 cm−1 . The low number of scans allowed several successive measurements in short time duration. 2.4.3. Bubble point test Bubble point method was used to estimate the stability of prepared SILMs by applying a gas pressure on one side of the membrane surface [57,58]. The principle of this method is based on the measurement of the pressure necessary to blow air through a liquid-impregnated porous membrane. In our case, the porous Matrimid® membrane was filled with the RTILs (the wetting fluid). Another less wetting fluid (N2 gas) acts at a given pressure on one side of the membrane and eventually displaces the RTIL phase from the pores. The operating conditions are given in details in a separate paper RTIL-impregnated Matrimid® membranes were mounted on a circular housing (metal grid; 13 mm) in the midst of stainless steel filter holder (Millipore, XX3001200). PTFE O-rings were used in the compression seals. The upstream part of the filter holder was connected to a source of inert gas (N2 ) via a precision pressure

A. Dahi et al. / Electrochimica Acta 130 (2014) 830–840

O

O O

P

O

O

NH

O

833

O

P

O

NH

O

O

P

O

N

N (a)

NH

O N

(b)

(c )

Fig. 2. Chemical structure of the protic RTILs: (a) [C1 im][DBP], (b) [C4 im][DBP], (c) [C4 im][BEHP].

gauge ranging from 0 to 0.1 bar (Air Products). The downstream part of the filter holder was connected to a pipe dived in water by its lower end. At this point, the transmembrane pressure (i.e. difference between the upstream and downstream pressure) is nil. The upstream pressure was gradually and slowly increased in steps of 0.002 bar every 2 minutes. Until the pressure difference over the impregnated membrane reaches the capillary pressure of the pores, RTIL acts as a barrier and no flow through the membrane can occur. After increasing the pressure over this limit, the first gas bubbles appear in water at downstream part. The pressure at this stage corresponds to the bubble point pressure (P) and its value is indicated by the gauge. Every measurement was repeated more than three times to obtain accurate data. The eventual losses in RTIL by blowing were determined by weighing the RTIL-impregnated membranes before and after the testing.

The SILMs were left overnight in the cell at 25 ◦ C between the two cycles. Before the measurements at each temperature set point, the samples were held at constant temperature (± 2 ◦ C) for at least 10 min. Several measurements were thus carried out at each temperature. The reproducibility of data was verified by repeating each measurement at least two times. Impedance measurements were also performed on the Nafion® 117 membrane used as reference. The Nafion® membrane was first neutralized in 1 M NaOH and, secondly, activated in 1 M HCl. After, the acidified Nafion® membrane was washed with milli-Q water.

2.4.4. Ionic conductivity Ionic conductivity was determined by electrochemical impedance spectroscopy using an Autolab PGSTAT302 N (Metrohm, France) electrochemical interface combined with a Frequency Response Analyser (FRA). The impedance measurements were carried out in a potentiostatic mode in the frequency range of 0.1 Hz–1 MHz. The measurements were performed with a Teflon conductivity cell. The contact between the Pt electrodes and the SILM was ensured by mercury. The cell Pt/SILM/Pt was placed in a programmable thermo-regulated oven to measure the temperature dependence of the conductivity. Ionic conductivity of the samples was calculated from the following equation:

The formation mechanism of the porous Matrimid® membrane depends on the step of phase inversion. During the phase inversion, the continuous exchange between the solvent (NMP) and the non-solvent (water vapour) determines the size and distribution of pores in the resulting membrane. Thus, the polymer solution is composed of a polymer-rich liquid phase and a solvent-rich liquid phase (or polymer-lean). In our case, because the phase inversion is induced by water in vapour state, the separation between these two liquid phases takes place very slowly. The final result is the formation of a membrane with a quasi-symmetric, spongy and interconnected porosity. The SEM photographs of the porous Matrimid® membrane are shown in Fig. 4. All characteristics of the porous structure of the Matrimid® membrane are described in details in a separate paper [59]. The pores on vapour-contacting surface (Fig. 4a) are circular, largely range from 25-35 ␮m and also appear as combinations of several circular pores. The porosity inside the membrane and its interconnection are clearly visible through this surface. On the glass-side surface (Fig. 4b), pores are circular and less large (5-15 ␮m) than the vapour-contacting surface pores. This difference in the pore size between the two surfaces of the membrane is due to very fast and easy evaporation of the solvent (NMP) from the vapour-contacting surface [60]. The porous Matrimid® membrane presents a porosity at two levels: a main macroporous structure (symmetrical and interconnected) and a secondary mesoporous structure within the dense part of the

=

L Rp · S

(1)

where  (S.cm−1 ) is the ionic conductivity, L (cm) and S (cm2 ) are the thickness and the active surface of SILMs, respectively, and Rp (Ohm) is the SILM resistance derived from the impedance value. The conductivity of the SILMs was measured during two consecutive temperature cycles. The first cycle corresponds to a temperature rise from 25 to 115 ◦ C followed by a decrease up to 25 ◦ C, while the second cycle is only a temperature rise from 25 to 115 ◦ C. During the first cycle, the measurements were successively performed at 25, 55, 85 and 115 ◦ C during heating and cooling. During the second cycle, only one measurement was made at 115 ◦ C.

N

R1 R1= CH3 ; C4H9

3.1. Porous Matrimid® membrane: structure and mechanism of formation

+

R2 O

P

H

O

O

N

(1)

3. Results and discussion

OH

OR2

R2= C4H9 (2) ; C8H17 (3)

R2 O

P OR2

O

+

N

N R1

Fig. 3. Reaction scheme of the synthesis of RTIL ([C1 im][DBP], [C4 im][DBP] and [C4 im][BEHP]).

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Fig. 4. SEM photographs (after metallization) of the Matrimid® membrane: a) vapour-contacting surface, b) glass-side surface, c) and d) cross-section.

membrane. The main macroporous structure shows spherical or ellipsoidal macropores largely with similar diameters (∼ 10 ␮m and up to 15-20 ␮m), while the secondary porous structure is composed of closed pores with lower size that is clearly visible on the cross-sectional photographs (Fig. 4c and d). The average diameter of these pores is less than 1 ␮m. This second structure is visible on all the surface of the non-metalized membrane (including the inner walls of pores) as seen in Fig. 5. Thus, the RTILs may also access to this mesoporosity. However, the low-temperature N2 adsorption isotherms have shown that the contribution of mesopores is low and could be negligible in comparison with the main macroporous structure of the Matrimid® membrane [61]. In summary, the main macroporous structure represents the main reception volume for RTILs inside the porous Matrimid® membrane. The porosity ratio (i.e. the pore volume/the total membrane volume) of the porous Matrimid® membrane has been estimated at 66% by means of density measurements [59].

Fig. 5. SEM photographs (without metallization) of the Matrimid® membrane vapour-contacting surface.

3.2. Supported ionic liquids membranes The most convenient method to prepare SILMs is the direct immersion of porous membrane into RTILs. However, it is essential to ensure the presence of the RTIL over the whole cross-section of the porous membrane. For this reason, FTIR spectroscopy was used. Fig. 6 shows ATR-IR spectra of the downside of the porous Matrimid® membrane surface before and after deposition of [C4 im][DBP] and [C4 im][BEHP] on its upside. The IR spectrum collected at the downside surface of the membrane after impregnation clearly shows the characteristic absorbance peaks of the RTIL. Consequently, [C4 im][DBP] and [C4 im][BEHP] have successfully penetrated the entire cross-section of the porous Matrimid® membrane. It should be noted that the time duration necessary for each RTIL to cross the porous membrane (i.e. from the upside to the downside) is different. Therefore, in order to estimate the required time for a full impregnation of the porous Matrimid® membrane, it is necessary to study the impregnation kinetics of RTILs. The impregnation kinetics of the porous Matrimid® membrane in [C4 im][DBP] and [C4 im][BEHP] are shown in Fig. 7. In all cases, the first weighing (t1 ) was carried out after ∼ 5 h. For longer time, the mass uptake is practically unchanged. Consequently, 5 hours of impregnation is a sufficient time to prepare the SILMs by the direct immersion method. From Fig. 7 it can be also seen that for the same porous volume fraction of membrane the mass uptake fraction increases in the following order: [C4 im][DBP] (98%) < [C4 im][BEHP] (119%), although [C4 im][BEHP] is more viscous than [C4 im][DBP]. The SILMs based on these RTILs contain a large amount of ionic liquid: 50% v/v of [C4 im][DBP] and 54% v/v of [C4 im][BEHP]. The swelling effect depends on the physicochemical properties of each RTIL, mainly on their density. A study [61] realized with [C4 C1 im][PF6 ], [C6 C1 im][PF6 ] and [C4 C1 im][BF4 ] showed that the highest mass gain was obtained with [C4 C1 im][PF6 ] (0.45 Pa.s (dry) at 25 ◦ C [16]), although it is more viscous than [C4 C1 im][BF4 ] (0.22 Pa.s (dry) at 25 ◦ C [16]). Indeed, as viscosity of each RTIL is different, the impregnation of the porous membrane will be time-dependent. However,

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Fig. 8. Mass uptakes in RTIL of the porous Matrimid® membrane as a function of temperature (measurements made after 5 h of impregnation in the RTILs).

Fig. 6. ATR-IR spectra of the porous Matrimid® membrane before and after impregnation with: (a) [C4 im][DBP] and (b) [C4 im][BEHP].

at equilibrium state, and for or a given porous volume of membrane, it can be expected that the absorbed volume of RTIL should be similar and, thus, the use of RTIL having the highest density should lead to the highest mass uptake. This is exactly what we observed: 56% v/v for [C4 C1 im][BF4 ] (1.12 g/cm3 (dry) at 25 ◦ C [16]) ≤ 57% v/v for [C6 C1 im][PF6 ] (1.30 g/cm3 (dry) at 25 ◦ C [16]) ≤ 61% v/v for [C4 C1 im][PF6 ] (1.37 g/cm3 (dry) at 25 ◦ C [16]) (increasing

Fig. 7. Impregnation kinetics at 25 ◦ C of the porous Matrimid® membrane in [C4 im][DBP] and [C4 im][BEHP].

order of the mass uptake in RTIL). Thus, for a membrane completely impregnated by a RTIL, the mass uptake is linked to the RTIL density. In addition, knowing the densities of the RTILs and taking into account the volume occupied by the RTIL phase in the membranes, we were able to estimate the porosity ratio of the porous Matrimid® membrane at 61 ± 2% [61]. This average value is very close to the value estimated by means of density measurements of the porous Matrimid® membrane (66%) [59]. This indicates a very satisfactory impregnation, i.e. up to 93% on the whole cross-section of the porous Matrimid® membrane. In the present work, and taking into account the volume fraction occupied by [C4 im][DBP] and [C4 im][BEHP], i.e. 50 and 54% v/v respectively, we can note that the porous membrane is not completely filled by the RTILs, with a impregnation on the whole cross-section of the porous Matrimid® membrane of about 84 ± 2%. This value indicates nevertheless a satisfactory impregnation. The slight difference in mass uptakes in RTIL between the two studies appears to be caused by the step of removing the excess of RTIL on the membrane surfaces. We have previously shown that [C4 im][DBP] and [C4 im][BEHP] can penetrate the porous Matrimid membrane from one surface to the other. The high viscosity of these RTILs could prevent them from reaching all free volumes of the porous membrane structure. To answer this question, a study of the influence of the temperature increase (i.e. decrease in the viscosity of RTILs) on the mass uptake of SILM was performed at 70 ◦ C and 103 ◦ C (Fig. 8). In the case of [C4 im][DBP], a very negligible improvement in the mass uptake of SILM was noted after the increase in temperature: 53 and 52% v/v at 70 and 103 ◦ C respectively compared to 50% v/v at 25 ◦ C. In the case [C4 im][BEHP], the mass uptake has not really changed with temperature rising; the mass uptake of SILM remains almost the same: 54% v/v at 25 ◦ C compared to 54% at both 70 and 103 ◦ C. In conclusion, in the temperature range of 25-103 ◦ C, the effect of the decrease in viscosity of RTILs was probably not sufficient to improve more the mass uptake of SILM. Nevertheless, the obtained impregnation rates in the order of 84 ± 2% (at 25 ◦ C) are largely acceptable. Also, the influence of the preparation method on the mass uptake of SILM was studied for [C4 im][DBP], characterized by the lowest mass uptake. The SILMs based on this RTIL were prepared according to the pressure method [41,44]. The obtained result of mass uptake (97%) was exactly the same as determined previously by the direct immersion method (98%). So, one can conclude that the direct immersion method gives very satisfactory results for SILM preparation.

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Table 1 Bubble point pressure P of Supported Ionic Liquid Membranes (porous Matrimid® membranes impregnated by RTILs) and mass losses of RTIL in SILMs after P measurement. RTIL

[C4 im][DBP] [C4 im][BEHP]

Porous Matrimid® membrane

Matrimid® impregnated by RTIL

Thickness (␮m)

Volume fraction of RTIL (%)

P (mbar)

RTIL mass losses %

252.2 ± 20.7 210.2 ± 22.3

50 54

19 ± 1 21 ± 3

5.5 5.2

3.3. Stability of SILMs The stability of RTILs into porous membrane is namely due to their negligible vapour pressure and high interfacial tension which allow the porous structure to be more or less filled, but also to the chemical affinity/interactions between RTILs and the polymer. It is clear that the presence of ionic groups in the polymer, that is the case of polyelectrolyte (as sulfonated PI) favour the compatibility with RTIL due to the enhanced interactions between ionic species of RTIL and those of PI. In our case, despite the absence of sulfonated groups in PI, IL contains imidazolium groups and, thus, may interact with imid groups of Matrimid. Also, the strong interactions between the carbonyl group of Matrimid and the NH group of IL would be the cause for the enhanced chain stability of hydrogenbonding interactions. In addition to these interactions, the internal interactions in IL (linked to surface tension) lead to the improvement of IL stability inside PI. Indeed, the greater the surface tension of the RTIL is, the more stable is the SILM. Table 1 shows the thickness of the non-impregnated porous Matrimid® membranes. After impregnation by the direct immersion, the SILMs were weighed before and after the measurement of the bubble point pressure (P). This enabled us to estimate the quantity of [C4 im][DBP] and [C4 im][BEHP] released from the SILMs at the bubble point. One can see that under the experimental conditions the SILMs displayed similar bubble point pressure (∼ 20 mbar), while their thickness are slightly different and the RTIL used is not exactly the same. For this low thickness range, the porous structure of both membranes is quite similar, i.e. quasi-symmetric, spongy and interconnected, with pores of similar sizes. Therefore, the P value should be more dependent on the capillary forces between the membrane pores and the RTILs. The bubble point is a static equilibrium test, so the capillary force is the property of interest. Since the surface tension is a measure of the surface cohesive energy, it is thus related to the strength of the interactions that are established between the anions and cations in a RTIL [62]. The literature data shows that both the anion and cation have an influence on the surface tensions [62,63]. For example, Freire et al. [62] showed that the increase of the cation alkyl chain length within the imidazolium ring reduces the surface tension values. The literature data presents surface tension values of imidazolium based RTILs well above those of conventional organic solvents (COVs), as well as those of n-alkanes, but still lower than those of water. The high surface tension of RTILs explains partly their use as alternatives to COVs in the preparation of the stable supported liquid membranes. The IL used in this study ([C4 im][DBP] and [C4 im][BEHP]) have the same cation and the only difference between them is the alkyl chain of the anion (Fig. 2). This difference has no effect on the capillary forces, because the obtained results showed the same P values for both RTILs (Table 1). After the determination of P, the amount of RTIL loss was estimated at ∼ 5 wt.%. For both studied ionic liquids (Table 1) such low value indicates that RTILs are well trapped in the porous structure of the membrane. We have noticed that after the measurement, the RTIL presence was observed on the metal grid, more precisely on the membrane side after opening the filter holder. Therefore, it can be supposed that at the bubble point the pressure accumulated on the upside of SILM creates diffusion pathways in the RTIL phase for its evacuation without causing a loss of RTIL. In other words,

the low loss of RTIL can be explained by contact of the SILM with the parts of the measuring cell. So, SILMs would retain all RTIL and showed very satisfactory stability towards the increased pressure on one membrane faces. 3.4. Ionic conductivity of the SILMs The ionic conductivity of the SILMs and Nafion® 117 was measured by impedance spectroscopy according to two consecutive temperature cycles. The first cycle corresponds to a temperature rise from 25 to 115 ◦ C followed by a decrease to 25 ◦ C, while the second cycle is only a temperature rise from 25 to 115 ◦ C. The results obtained are discussed here taking into account the performance of the Nafion® 117 membrane used as a reference. Fig. 9 shows the Arrhenius plot of ionic conductivity as a function of temperature for the SILMs and for the Nafion® 117 membrane determined from the first cycle. The ionic conductivity of Nafion® 117 measured in our conditions is lower when compared to literature data [64–66]. This difference can be due to either measuring devices or more probably to the activation method of the Nafion® 117 membrane. It is well-known that the proton conductivity highly depends on the hydration level of the membrane. The highest conductivity of Nafion® is obtained when the membrane is immersed (the highest hydration condition) during the measurement [66] and also when the membrane is under fully humidified condition (100% relative humidity with deionized water at bottom of the test container) [65]. However the conductivity is found to be lower when the pre-hydrated membrane (membrane soaked in deionized water before measurement) is measured without humidified generator [64] leading to a medium hydration state during the measurement. In our case, the conductivity measurements for Nafion membranes were performed exactly under the same conditions as for SILMs, so not in water. These medium conditions of very low relative humidity explains the lowest conductivity values obtained with our Nafion® 117 membrane compared with the literature data (Fig. 9). Nevertheless, the ionic conductivity values of SILMs were of the same order of magnitude as those of Nafion® measured in the same conditions and even exceeded them in the case of [C1 im][DBP] and [C4 im][DBP] (Fig. 9).

Fig. 9. Arrhenius plots of conductivity for SILMs during the first temperature cycle.

A. Dahi et al. / Electrochimica Acta 130 (2014) 830–840

At 25 ◦ C (before heating), the ionic conductivity of SILMs increases according to the following order: [C4 im][BEHP] < [C1 im][DBP] < [C4 im][DBP]. The reduction of the alkyl side-chain in the [Cn im] cation (i.e. from [C4 im][DBP] to [C1 im][DBP]) induces a slight decrease of conductivity. But the lengthening and the branching of the alkyl side-chains on the anion (i.e. from [C4 im][DBP] to [C4 im][BEHP]) leads to a significant decrease in conductivity. Thus, when the alkyl chain of the anion increases, the conductivity decreases while the opposite effect is observed with the increase of the cation alkyl chain. This latter effect seems surprising when compared to some literature. For example, Deligöz et al. [25] show for acid-doped polyimide/IL membranes containing imidazolium-based ILs (Rim-BF4 ) with different alkyl chain, the conductivity decreases with the alkyl chain increase. However, for acid free PI/IL membranes, the same authors show some reverse results, in particular for temperatures higher than 120 ◦ C. The conductivity of PI/ButIm-BF4 membrane is found to be higher than that for membranes containing IL of shorter alkyl chain length, i.e. EtIm-BF4 and MeIm-BF4 . This latter situation is more consistent with the results obtained in the present work since the elaborated SILMs are not doped and, so, are acid-free. In other words, the changes of the proton conductivity of the PI/IL membranes depend not only on the length of the IL alkyl chain, but are more complex because of other concomitant effects such as interactions between ionic groups of IL, mobility of ionic species, possible interactions between IL and PI knowing that these interactions are higher when PI is sulfonated because of the presence of ionic groups on the PI backbone. The ionic conductivity is dependent on the RTIL viscosity [50,67,68] that influences the mobility of ionic species [24,28,30,69]. Martinelli et al. [11] indicated that motion of the charge species is controlled by the viscous properties of the ionic liquid. The lower the viscosity is, the faster the mobility of conductive species is and consequently, the protic conductivity found should be higher. This partly explains why [C4 im][BEHP] is less conductive than [C1 im][DBP] and [C4 im][DBP] à 25 ◦ C, due to its high viscosity. This result is in accord with the study of Sanchez et al. [27] who have studied the conductivity of several proton conducting ionic liquids (PCILs) based on triethyl ammonium cation and different perfluorinated sulfonyl anions. They noticed that the conductivities decrease significantly with lengths of anionic chains and are partly related to their viscosity. [C4 im][BEHP] is more viscous than [C1 im][DBP] and [C4 im][DBP] due to the branched and long side-chains in the [BEHP] anion. Huddleston et al. [16] indicated that geometry and molar mass of the anions have a strong influence on the viscosity. They also added that the imidazolium RTILs with longer alkyl substituents produce more viscous RTILs. However, other parameters also affect the ionic conductivity, and not only the viscosity. Benhôte et al. [19] related the conductivity of RTILs to a combination of four parameters: their viscosities, formula weight, densities and to the radii of their ions. All these parameters are of course dependent on the nature of anion and cation. Thus, the obtained result at 25 ◦ C highlights the effect of the chemical structure of a RTIL (Fig. 2) on its ionic conductivity. When the temperature increases up to 115 ◦ C, the ionic conductivity of SILMs linearly increases. This is due to an increase in mobility, as viscosity and mobility are inversely related to each other, which also enhances ionic conductivity [24,28,30,69]. The conductivity of the SILMs impregnated with [C1 im][DBP] and [C4 im][DBP] increases at the beginning of the heating. At 85 ◦ C, these SILMs display conductivities of 6.1 10−4 S/cm and 4.8 10−3 S/cm, respectively. Beyond this temperature, it is known that the Nafion® membrane gradually loses its performance due to dehydration. This is what we also observed (Fig. 9) while the conductivity of the SILMs impregnated with [C1 im][DBP] and [C4 im][DBP] continues to increase and reaches a high

837

Fig. 10. Arrhenius plots of conductivity for SILMs for the two temperature cycles.

conductivity value (2.0 10−2 S/cm at 115 ◦ C) in the case of [C4 im][DBP]. This result is very promising since it is comparable to the performance of the Nafion® membrane in wet conditions. The ionic conductivity of SILM filled with [C4 im][BEHP] increases only slightly with temperature increase. It is clear that [C4 im][BEHP] is less conductive than [C1 im][DBP] and [C4 im][DBP] in the temperature range of 25-115 ◦ C. By cooling SILMs from 115 to 25 ◦ C, the viscosity of the RTILs increases (the mobility decreases) that explains the observed decrease of the conductivity (Fig. 9). This decrease is not superimposed to the increase of the first heating cycle. In the case of [C4 im][DBP], a significant loss in conductivity is observed at the beginning of cooling, while in the case of [C1 im][DBP] and [C4 im][BEHP] the conductivity remains practically constant until 55 ◦ C and then decreases gradually and linearly. In all cases, the conductivity measured at the end of the cooling (at 25 ◦ C) is below than that recorded at the beginning of the experiment (at 25 ◦ C; before heating). The greatest difference corresponds to the case of the [C4 im][DBP]-impregnated SILM, which showed the best performance in terms of conductivity with temperature rising. Therefore, one can suppose that a portion of the RTIL was lost during the first cycle (heating and cooling), since the measured conductivity is directly linked to the total quantity of RTIL in the SILM [50]. This loss is probably due to the mechanical stress created by the electrochemical cell and facilitated by the decrease in viscosity of the RTILs at high temperature. After the first cycle, the SILMs were left in the electrochemical cell at room temperature until the next day. They were again heated to 115 ◦ C and then their ionic conductivity was measured at this temperature (Fig. 10). The second cycle of temperature allowed us to confirm the mass loss of RTIL observed during the first cycle. The ionic conductivity increases by heating the SILMs from 25 to 115 ◦ C. The difference of ionic conductivity observed between two heating cycles is practically the same on the whole range of temperature (25 ◦ C - 115 ◦ C), the highest gap in conductivity being obtained in the case of [C4 im][DBP] (Fig. 10). Since the gap is proportional to the mass loss in RTIL [50], this result confirms that the [C4 im][DBP]impregnated SILM is the membrane characterized by the most RTIL lost. Although the conductivity values measured during the second cycle are lower than those measured during the first cycle, the obtained conductivity is sufficient to confirm the presence of large amount of RTIL in the porous membrane (a necessary condition to the establishment of contact between the two electrodes). Consequently, the SILMs show both satisfactory stability (by retaining their RTIL phase) and high ionic conductivity values. In the case of Nafion® , the proton conduction is known to occur via two mechanisms, i.e. i) the hopping mechanism and ii) the vehicle mechanism [70]. According to the hopping

838

A. Dahi et al. / Electrochimica Acta 130 (2014) 830–840

Table 2 Activation energy of SILMs and Nafion® 117.

Ea (kJ/mol)

[C4 im][DBP] based SILM

[C1 im][DBP] based SILM

[C4 im][BEHP] based SILM

Nafion® 117

51.7

34.5

14.2

10.9

mechanism (also called Grotthuss mechanism), the conductivity is due to jumping of protons on ionic sites and sorbed water molecules through hydrogen bonds, while in the case of the vehicle mechanism, the protons, associated with water into a complex, diffuse in hydrophilic domains usually considered as hydrated channels. The first mechanism usually requires higher activation energy (Ea ∼1540 kJ/mol) than that necessary for vehicle mechanism [64,71–73]. The activation energy of SILMs was obtained from the following Arrhenius equation:  = 0 exp(−

Ea ) RT

(2)

Where ␴ is the conductivity, ␴o is a preexponentoal factor, R is the ideal gas constant, Ea is the activation energy of ionic conduction and T is the temperature. Ea values were determined from conductivity plots for the first temperature cycle. As one can see from Table 2, the Ea values of SILMs are higher than that of Nafion® . The low value of Ea calculated for Nafion® (10.9 kJ/mol) is in good agreement with data usually observed in literature (910.5 kJ/mol) [64]. The higher Ea values calculated for studied SILMs (> 14 kJ/mol) confirm that the ionic conduction is governed by the hopping mechanism. This result is consistent with the fact that the ionic conductivity is not really affected by the hydration state of SILMs which is not the case for Nafion® 117 characterized by the conductivity decrease at 115 ◦ C after few hours (Fig. 9). Indeed, at this temperature and due to the dehydration of the Nafion® membrane, the percolating hydrophilic network through which the protons are conducted is reduced. Proton is transferred via hopping among the protonated imidazolium cations [11,12,24,28]. Despite the fact that anion accept the proton, it is clear that the cation, as well as anion, play a role in the conductivity. Indeed, as observed from Figs. 9 and 10, the conductivity between [C1 im][DBP] and [C4 im][DBP] is not the same, the cations being different. This is also in accordance with the study of Iojoiu et al. [74] on NMR measurements of ion transport in several ILs based on a variety of amines and three perfluorosulfonate anions. They indicated that the protonated ammonium cations NH(R’R”R”’)+ are those that mainly ensure proton transport. Thus, because of the very low water content in SILMs, the vehicle mechanism as considered in Nafion® is not really present. However, for the proton conductivity, it must be considered that the mobility of ionic groups in RTIL (mobile phase) and in polyelectrolyte (sulfonated polymer, Nafion® . . .) is quite different. Therefore, in the case of SILMs, in addition to the hopping mechanism, the high mobility of ionic groups in RTIL should also contribute to the proton conductivity. This is why the conductivity of SILMs is in part due to the hopping mechanism but also due to the mobility of the ionic groups which is dependent on the viscosity of the RTIL and the interaction strengths between ionic species that enable them to be more or less separated. 4. Conclusions Three supported ionic liquid membranes were elaborated and characterized as electrolyte membranes for PEMFC. The Matrimid® membrane used as mechanical support, with spongy, interconnected and symmetrical porous structure was impregnated with [C1 im][DBP], [C4 im][DBP] and [C4 im][BEHP]. The SILMs prepared by direct immersion method are composed of at least 50% v/v of RTIL phase and show very good retention of the RTIL phase.

The ionic conductivity of the SILMs was compared to that of Nafion® . Preliminary results are very encouraging, especially with the [C4 im][DBP]-impregnated SILM, which shows very good conductivity at high temperature, i.e. 2.0 10−2 S/cm at 115 ◦ C. Unlike the Nafion® , the conductivity of the SILMs increases at high temperature (> 80 ◦ C). Indeed, the conduction mechanism in the RTILs is different from that in the Nafion® and depends on the physicochemical properties of RTILs such as viscosity. The viscosity of RTILs decreases at high temperature, thereby favouring the mobility of conductive species in the RTILs and thus the increase in the conductivity of SILMs was observed. However, the decrease in viscosity at high temperature can also lead to release a part of the RTIL that induces a reduction of conductivity. Despite this lack of RTIL, the conductivity values of SILMS remain sufficiently high compared to Nafion® measured in the same experimental conditions. Finally, the prepared SILMs have both acceptable stability and good conductivity values, showing that this type of composite membranes is promising candidate as electrolyte membranes for PEMFC. The success of these membranes arises from the RTIL properties and also from the polymer Matrimid® support which is thermally and mechanically stable and offers a particular porous structure (interconnected and quasi-symmetric) allowing RTIL to be trapped in a high free volume. The use of other protic ionic liquids might improves the performance of such conductive membranes. Acknowledgment The authors gratefully acknowledged the financial support provided by the Réseau Polymères Innovants (France). We also thank Mr Jean-Jacques Malandain (Groupe de Physique des Matériaux, UMR 6634 CNRS, France) and Mr Michel Gomes (Elexience, France) for the realization of the SEM images. References [1] L. Li, Y.M. Zhang, Chemical modification of Nafion membrane with 3,4ethylenedioxythiophene for direct methanol fuel cell application, J. Power Sources 175 (2008) 256. [2] L. Maldonado, J.C. Perrin, J. Dillet, O. Lottin, Characterization of polymer electrolyte Nafion membranes: Influence of temperature, heat treatment and drying protocol on sorption and transport properties, J. Membr. Sci. 389 (2012) 43. [3] D.K. Paul, A. Fraser, K. Karan, Towards the understanding of proton conduction mechanism in PEMFC catalyst layer: Conductivity of adsorbed Nafion films, Electrochem. Commun. 13 (2011) 774. [4] M. Lavorgna, M. Gilbert, L. Mascia, G. Mensitieri, G. Scherillo, G. Ercolano, Hybridization of Nafion membranes with an acid functionalised polysiloxane: Effect of morphology on water sorption and proton conductivity, J. Membr. Sci. 330 (2009) 214. [5] A. Fernicola, S. Panero, B. Scrosati, Proton-conducting membranes based on protic ionic liquids, J. Power Sources 178 (2008) 591. [6] S.S. Sekhon, P. Krishnan, B. Singh, K. Yamada, C.S. Kim, Proton conducting membrane containing room temperature ionic liquid, Electrochim. Acta 52 (2006) 1639. [7] K. Fatyeyeva, J. Bigarré, B. Blondel, H. Galiano, D. Gaud, M. Lecardeur, F. Poncin-Epaillard, Grafting of p-styrene sulfonate and 1,3-propane sultone onto Laponite for proton exchange membrane fuel cell application, J. Membr. Sci. 366 (2011) 33. [8] K.T. Adjemian, S.J. Lee, S. Srinivasan, J. Benziger, A.B. Bocarsly, Silicon Oxide Nafion Composite Membranes for Proton-Exchange Membrane Fuel Cell Operation at 80-140 (C, J. Electrochem. Soc. 149 (2002) A256. [9] B.C. Lin, S. Cheng, L.H. Qiu, F. Yan, S.M. Shang, J.M. Lu, Protic Ionic LiquidBased Hybrid Proton-Conducting Membranes for Anhydrous Proton Exchange Membrane Application, Chem. Mater. 22 (2010) 1807. [10] M.L.D. Vona, Z. Ahmed, S. Bellitto, A. Lenci, E. Traversa, S. Licoccia, SPEEK-TiO2 nanocomposite hybrid proton conductive membranes via in situ mixed sol–gel process, J. Membr. Sci. 296 (2007) 156.

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