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The effect of relative humidity on the gas permeability and swelling in PFSI membranes
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The effect of relative humidity on the gas permeability and swelling in PFSI membranes J. Catalano 1, T. Myezwa, M.G. De Angelis*, M. Giacinti Baschetti, G.C. Sarti Dipartimento di Ingegneria Chimica, Mineraria e delle Tecnologie Ambientali, Universita` di Bologna, Viale Terracini 28, 40131 Bologna, Italy
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abstract
Article history:
The permeation of helium, oxygen and nitrogen in perfluorosulphonic acid ionomeric
Received 8 March 2011
(PFSI) membranes with short and long side chains, namely Aquivion and Nafion 117, was
Received in revised form
studied in the relative humidity range between 0 and 90%, and at temperatures between
12 July 2011
35 C and 65 C. The presence of humidity enhances, up to a factor of 100, the gas
Accepted 13 July 2011
permeability in the membranes, due to the permeation of gas molecules in the hydrophilic
Available online 16 August 2011
domains: the enhancement is rather pronounced for O2 and N2, and less marked for He permeability. The relative permeability increase, Pgas/Pgas,0, shows a complex dependence
Keywords:
on the relative humidity, as the water content in the membrane is itself a non linear
PFSI membranes
function of this parameter. The water volume fraction in the membrane at each activity
Ionomers
was accurately estimated from measurements of vapor-induced swelling, which indicate
Gas permeability
that the partial molar volume of water is smaller than its pure liquid value, in both
Controlled humidity
membranes at 35 C. When plotted against water volume fraction, the gas permeability
Water swelling
increases exponentially in the range between 2% and 20%; the slope of the curve is higher
Fuel cells
for Nafion than for Aquivion, as it is reasonable due to their different microstructures. The ideal selectivity of the two membranes for He over N2, O2 over N2 and He over O2, decreases markedly with increasing water content in the membrane. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The development of effective proton exchange membrane fuel cells (PEMFC) requires polymer membranes with high proton conductivity and low gas permeability, so that a larger fraction of gas can react at the cathode [1]. For this reason the process of gas transport into perfluorosulphonic acid ionomeric (PFSI) membranes plays an important role in the design and control of fuel cell performance. Some authors have determined the transport parameters of the gases of interest for fuel cell operations, namely oxygen, nitrogen and hydrogen, in PFSI membranes of different
equivalent weights [2e5]. However, since the PEMFC normally operates in humid conditions, and in some cases in the presence of membrane solvents such as methanol, the characterization of gas transport has to be carried out preferentially under humid or solvated conditions [6e16]. Indeed the membrane microstructure and all the membrane properties are dramatically affected by the absorption of water or other swelling solvents [17]. Despite the amount of studies, the gas permeation mechanism through solvated PFSI membranes and its relationship with the complex morphology of these materials are still far from being completely understood. A solvated PFSI membrane
* Corresponding author. Tel.: þ39 051 2090410; fax: þ39 051 6347788. E-mail address: [email protected] (M.G. De Angelis). 1 Present address: Center for Inorganic Membrane Studies, Department of Chemical Engineering, Worcester Polytechnic Institute, 100 Institute Rd, Worcester, MA 01609, USA. 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.07.047
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can be depicted as a heterogeneous medium, divided into two phases more or less connected: (I) a hydrophobic phase formed by the main fluorinated backbone and (II) hydrophilic regions formed by polar groups arranged in clusters, plus the water inside the clusters if the membrane is wet. The existence of an intermediate, or (III) interfacial phase has also been hypothesized, formed by the side groups of the fluorinated chains and by free water molecules [17]. The volume fraction, size, shape and connectivity of the different phases varies with the water content in the membrane and strongly affects the transport properties of the polymer. Gebel assumed that, at low water contents, the aqueous phase is formed by isolated spherical domains that at higher water fractions become connected by thin channels and eventually form a continuous aqueous path across the membrane thickness [18]. If one considers that gaseous penetrants normally show higher permeation rates in water than in the dry PFSI matrices, it is obvious to expect that the presence of water enhances markedly the gas permeability, as it is indeed observed empirically [15]. To better represent the complexity of the problem it is worthwhile to mention also that, while the gas permeability in PFSI membranes increases monotonically with water content in all the cases inspected, the diffusion and sorption contributions can show different trends; oxygen, for example, is more soluble in the fluorinated medium than in water, but its diffusivity in liquid water is several times higher than that in a perfluorinated matrix such as poly(tetrafluoroethylene) (PTFE). As a consequence, oxygen diffusivity in hydrated PFSI membranes increases up to a factor of 25 by increasing the water content in Nafion from 0 to 35% in weight [6,8,11,13], while the amount of oxygen absorbed in hydrated PFSI membranes is only slightly higher than that in dry membranes, with a behavior that can be described by an additive model based on the oxygen solubility values in pure PTFE and in pure H2SO4 [11,12]. Up to date the only quantitative approach for mass transport in PFSI membranes was proposed by Pellegrino and Kang [10], that considered the solvated polymer as a heterogeneous medium and adapted equations valid for the corresponding heat conduction problem. Their model however was not suitable to fit the experimental data of CO2 and CH4 permeability in Nafion membranes solvated in different liquids, and the authors explained this result by the fact that their model neglected the enhancement of the permeability of the fluorinated phase induced by solvent swelling [10]. A different approach was presented by Sodaye and coauthors [19,20] and by Mohamed et al. [15] that, using the free volume theory, related the variation of gas diffusivity in Nafion membranes to the fractional free volume of the polymer matrix, which was expressly determined by PALS measurements as a function of water content. Such a free volume was considered to be located between the hydrophobic and hydrophilic phases, in the intermediate region (III), which therefore is assumed to play a predominant role in gas transport. In the present work the gas permeation through Nafion 117 and Aquivion membranes was studied as a function of the water relative humidity, in order to provide experimental support for a more fundamental understanding of the gas
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transport mechanisms in hydrated PFSI membranes. The measurements were performed in a closed-volume permeometer, where a uniform water vapor activity and a gas pressure gradient are maintained during the experiments. The data were obtained for dry gases at temperatures ranging from 35 to 65 C, while data at relative humidities up to 90% were collected at 35 C; three different gases were analyzed: O2, N2 and He. The water vapor swelling was also determined at 35 C, as a function of the relative humidity, in order to obtain the actual volume fraction of water in the membrane.
2.
Materials and methods
2.1.
Materials
The materials studied in this work were Aquivion, formerly known as Hyflon Ion, produced by Solvay Solexis S.p.A. and Nafion 117, produced by Dupont. The chemical formula of the two polymers can be seen in Fig. 1. The Aquivion monomer has a shorter pendant side chain with respect to that of Nafion 117, and a lower equivalent weight: 800 versus 1100 gpol/molSO3H. Aquivion films used in the present work were kindly provided by the producer and had a thickness of about 160 mm. Nafion 117 films were purchased from Aldrich and had a thickness of about 183 mm.
2.2.
Gas permeability measurements
The apparatus for the determination of humid gas permeability was adapted from a pure gas, fixed volume permeometer [4] through the addition of a humidifying section on the upstream side of the membrane, of a water vapor reservoir for initial membrane equilibration, and of a purge flow to conduct flow-through experiments (Fig. 2a) [21]. The equipment can be used to detect the permeation rates of any gas in any polymeric material and does not require an analysis of the permeate stream composition but only a measure of the downstream pressure. In the experiments, the humidity was regulated by mixing a dry and a saturated gas stream with a controlled flow rate; a humidity sensor (RH 01: Vaisala RH: 0e100%; T : e 40/þ80 C) was used to monitor the humidity value obtained. The pressure in the upstream side of the system was measured through a high pressure manometer (PI02, Druck f.s. 6 bara), while a low-pressure capacitance manometer was placed in the downstream section of the apparatus (PI03: MKS Baratron f.s. 100 mbar). A differential transducer is also present in the scheme of Fig. 2a (PI04: Druck manometer, f.s. 10 mbar): this sensor is used when temperatures higher than the ones inspected in this work are needed, because it allows to perform accurate measurements in a wide pressure range, according to a procedure already described elsewhere [22]. The volume of the downstream side of the apparatus was carefully measured and resulted to be 35.0 cm3. The whole system was placed in a thermostatic chamber and an additional thermocouple (TI01) was placed in a bubbling vessel (S02, 7 dm3) in order to evaluate the water saturation pressure obtained in the humidified stream. The system was also
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Fig. 1 e Monomer units of the PFSI membranes used in this work.
endowed with two different vessels: S01, that is used for pure gas permeation experiments, and S03 that is the water reservoir (2 dm3) used for the initial stage of water equilibration on both membrane sides, and can be used also for pure water vapor permeation experiments. The experiment is carried out by first putting the membrane in contact, on both sides, with water vapor at fixed
activity, fed from vessel S03, and letting the system reach equilibration. This saturation stage, needed to obtain a uniform water concentration profile in the membrane, normally lasts a few hours in the conditions inspected and is considered to be completed when a constant pressure value is reached on both sides of the membrane. After the initial saturation at the water pressure selected, valves V04, V02 and
Fig. 2 e (a) Schematic of the permeation apparatus for controlled humidity gas permeation experiments (adapted from ref. 21). (b) Scheme of the experiment. (c) Experimental output of a humid gas permeation experiment.
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V03 are closed, in order to isolate the upstream and downstream volumes of the apparatus, and the upstream conditions are varied by feeding a gas stream with a relative humidity, set to maintain the water activity equal to that at the two sides of the membrane. The upstream pressure is set to the desired value by tuning the gas partial pressure, read through PI02, through the pressure reducer in the gas feed (PC600). The stream humidity is adjusted by using the mass flow controllers FC01 and FC02 placed on the pipe connected to the bubbling vessel and on the gas bypass line, respectively; during this operation V01 is open allowing the system to work in a continuous flux mode. Once the relative humidity of the feed stream is stable, with variation less then 0.1% of the presaturation RH value, V01 is closed and, at the same time, valves V02 and V03 are opened. In this way, a driving force for gas permeation is established, while the driving force for water flux is equal to zero as in the pre-equilibration step; the presence of a flow in the retentate side, then, allows to avoid any possible concentration polarization effect. The pressure increase in the downstream volume is due to the permeation of a single gas and allows for a simple quantification of gas permeability under humid conditions with the same equations used for the permeation of pure gases. The scheme of the chemical potential profile during the experiment is represented in Fig. 2b, where the reasonable assumption is made that the presence of gas does not change appreciably the water chemical potential in the membrane. The initial stage of the permeation process was difficult to interpret with the classic time-lag analysis because a nonnegligible water flux was present at the beginning of the experiments, caused by a small difference between the humidity value of the upstream gas and the value of humidity obtained during the previous equilibration stage. This transient stage, however, reached rapidly an equilibrium state so that the gas permeability values obtained in steady state resulted extremely reproducible. It is well known that PFSI membranes may undergo slow relaxation phenomena and reach a true equilibrium state, for several processes, only after long times [23]. However, whether the state reached is a true equilibrium or a pseudoequilibrium one, the values attained at the end of the experiment are meaningful from the practical point of view because the time-scale of PEMFC applications (automotive or portable electronic devices) is of the same order of magnitude of the duration of our experiments. The experimental behavior observed is reported in Fig. 2c: the circles represent the downstream pressure, the squares its time derivative, as a function of time: one can observe the initial peak of the flux, due to simultaneous permeation of gas and water vapor, and the asymptotic value, due to the permeation of the pure gas alone, finally reached when the water concentration is uniform. For Aquivion, dry gas permeation experiments were performed by first treating the membrane under vacuum at 105 C for 4 h in the tightened permeation cell, then testing the membrane at 65, 50 and 35 C. For Nafion, such a procedure caused cracks into the membrane due to the coupling of mechanical and thermal stress and loss of water molecules, thus a milder pre-treatment was applied drying the membrane under vacuum for 12 h at 65 C.
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Humid permeation experiments were performed at 35 C for both Aquivion and Nafion 117 without any thermal pretreatment.
2.3.
Membrane swelling measurements
The swelling induced by water vapor, as a function of activity, in both Aquivion and Nafion 117 membranes, was determined at 35 C via an optical apparatus whose scheme is reported in Fig. 3 [24]. The system is composed of a special sample compartment, formed by a stainless steel cell with two borosilicate glass windows in the opposite sides, to allow optical access for the measurements. The cell is connected to a reservoir containing the penetrant vapor and the pressure transducer, and to a second flask for the storage of liquid penetrant. A vacuum system, with a liquid nitrogen trap and a vacuum pump, is used to evacuate the apparatus before and after each experiment. The sensing element is an optical micrometer (Keyence LS-7030-M) endowed with a high speed linear CCD sensor that ensures accuracy of 1 mm and reproducibility within 0.15 mm in the measurements. The micrometer is fixed on two manual roto-translational optical stages, one for the horizontal alignment of transmitter and receiver with the sample, and one devoted to guarantee that the sample and the light beam are mutually orthogonal. To control temperature, the experimental cell is surrounded by two heating tapes that can reach temperatures up to 200 C; all the instrumentation is inserted in a thermostatic hood to eliminate the fluctuations of room temperature that might affect the response of the electronic devices. The sample elongation is monitored in one planar direction, and the volume dilation is assumed isotropic. Multiple measurements were performed to ascertain an acceptable level of experimental error, in particular reproducibility was checked to be within 95% confidence levels. The data obtained allowed to quantify the volume fraction of water in the polymer and, once the water vapor solubility at each activity is known from independent measurements [25], to compare the swelling behavior to the one predicted by volume additivity.
3.
Results and discussion
3.1.
Dry gas permeability
Initial tests were performed in dry conditions for O2, N2 and He: the permeability of helium represents an upper limit, and the value of hydrogen permeability, according to available data [2] lies between those of oxygen and helium. The experiments were performed on Aquivion and Nafion 117 samples with upstream pressures from 1 to 2 bar and vacuum on the downstream side. In this operative range, the gas permeability showed no appreciable variation with upstream pressure. The data for Aquivion at 35, 50 and 65 C, already presented in a preliminary work [4], are reported in Fig. 4a. Permeability varies from 27.5 to 55 Barrer in the case of helium, from 0.68 to 2.3 Barrer for oxygen and from 0.17 to 0.7 Barrer for nitrogen. In the case of O2 and N2 it was also possible to determine the diffusivity with the time-lag method
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Fig. 3 e Experimental dilation apparatus [24].
obtaining values from 4.2 108 to 1.9 107 cm2/s for oxygen and from 1.8 108 to 9.2 108 cm2/s for nitrogen [4] (Fig. 4b). For He, on the other hand, diffusivity could be only roughly estimated due to the short time-lag values and resulted to lie between 105 and 104 cm2/s. The permeability and diffusivity values vary with temperature according to Arrenhius law and
the resulting activation energies were calculated and listed in Table 1. For the case of Nafion 117, also reported in Fig. 4a, permeability varies between 0.97 and 3.2 Barrer for O2, between 26 and 56 Barrer for He and between 0.18 and 1 Barrer for N2. Diffusivity (Fig. 4b) varies between 4.5 108 and
Fig. 4 e (a) Dry gas permeability in the different PFSI membranes. Filled symbols refer to Nafion (this work), open symbols refer to Aquivion [4] and semi-filled symbols refer to data from J.S. Chiou and D.R. Paul for Nafion 117 [2]. (b) Dry gas diffusivity in the different PFSI. Open symbols refer to Aquivion while filled symbols refer to Nafion 117 films.
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Table 1 e Gas transport parameters in dry PFSI membranes. Membrane
N117 N125 N117 N112 N117 NRE212 N117 Aquivion
T ( C)
35 30 30 35 25 25 35 35 35 35
EW (gpol/molSO3H)
1100 1200 1100 1100 1100 1300 1100 1100 1100 860
Permeability (Barrer) O2
N2
H2
He
1.08 1.4 1.22 1.23 0.9 1 1.2
0.26 0.65
9.3 14 9 8.14
40.9
0.97 0.68
O2
N2
H2
30.5 24.6
27.6
20.1 28.2
Ref.
He [2] [8] [6] [16]
0.22 0.29
[3] 10.7
0.18 0.17
2.1 107 cm2/s for O2, from 9.6 106 to 1.2 105 cm2/s for He and from 2.4 to 8.7 108 cm2/s for N2. Several gas permeability data in dry Nafion 117 can be found in previous works and are listed in Table 1 for the sake of comparison. From this table a large scatter of data can be noticed, which is probably due to the different drying protocols followed by the various authors, that leave different amounts of water in the membrane. This aspect, although not investigated specifically in the present work, is qualitatively proven by tests that we carried on Aquivion samples, which have shown that the gas permeability at room temperature may be up to 45% higher when the membrane is dried at 35 C instead than at 105 C. In any event, our results for the diffusivity of O2 and N2, obtained on a Nafion membrane dried at 65 C for 12 h, are in very good agreement with those obtained by Chiou and Paul [2] (1.73 108 and 4.57 108 cm2/s for N2 and O2, respectively, at 35 C) that were not reported in Fig. 4b for the sake of clarity. Activation energies for permeation in Nafion 117 were evaluated as well, and they are reported in Table 1: their values are similar to those of observed in Aquivion with the exception of N2 activation energy, that is higher for Nafion than for Aquivion.
3.2.
Activation energy EP (kJ/mol)
30.5 26.3 27.5
34.6 34.5
20.4 49.6 41.7
22 20.0
[15] this work [4]
similar but less marked at high humidity and in particular the relative increase of permeability at the same RH (75%) is about 1.7 for He, 20 for O2, 45 for N2, with respect to the permeability obtained on an Aquivion membrane dried at 105 C for 4 h. The observed dependence of permeability on water activity is not linear: the initial increase of permeability with water activity is rather steep and then proceeds more slowly. A similar behavior was already observed by other authors, as discussed in the introduction: some of them have correlated the gas permeability trend to the free volume variation induced by water sorption in the matrix, determined by PALS under humid conditions [15,19,20]. The majority of authors, however, supposed that the behavior of gas permeability in the humidified membranes is due to the fact that a considerable portion of the gas flux occurs via the water phase, that is 10e100 times more permeable than the dry fluorinated matrix, depending on the gas considered. This behavior is also proved
Gas permeability versus relative humidity
The humid gas permeability experiments were performed at 35 C on both Nafion 117 and Aquivion samples and are shown in Fig. 5. For comparison, in the same figure we also reported the oxygen permeability data in Nafion NRE212 determined at 30 C by Mohamed et al. [15], that were measured with a chromatographic device. Such data lie below those measured in this work on Nafion 117, but the difference can be due in part to the lower temperature, and in part to the fact that in ref. 15 they were obtained after heating the sample under vacuum at 80 C for 10 h, while the measurement on our sample was carried out without any previous thermal treatment. As it can be seen from Fig. 5, when the relative humidity is 75%, the permeability increases by a factor of about 3 for He, 30 for O2, 95 for N2 in the case of Nafion, with respect to the permeability measured in the membrane dried at 65 C for 12 h. In the case of Aquivion, on the other hand, the dependence of permeability on relative humidity is qualitatively
Fig. 5 e Permeability for O2, N2 and He at 35 C versus water relative humidity in the different PFSI membranes. Filled symbols refer to Nafion 117 and open ones to Aquivion. The solid line represents the behavior of oxygen permeability data at 30 C in Nafion NRE212 from ref. [15].
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by the fact that gas diffusivity data, plotted at different temperatures, show an inflection point that corresponds to the freezing point of water [8]. Another phenomenon that may cause an increase of gas permeability with water content is the water-induced swelling and plasticization of the fluorinated phase, that contributes to enhance the gas flux in the hydrophobic region, but that is usually neglected, due to its smaller effects with respect to the permeability in the aqueous phase. It is therefore generally accepted that, as far as permeability is concerned, the hydrated PFSI membrane behaves as a twophase structure, with an aqueous phase and a hydrophobic phase. According to these considerations, it is more meaningful to report the permeability data as a function of the water volume fraction inside the membrane rather than the vapor activity, since this parameter better accounts for the relative amount of aqueous domains available in the hydrated matrix. The volumetric fraction of water can be estimated, to a first approximation, by simply assuming volume additivity between the pure polymeric phase and the pure liquid water phase, according to the assumption that water occupies in the matrix the same volume as in its pure liquid state. However, such a procedure does not take into account the fact that the partial molar volume of water sorbed in PFSI may deviate from its pure liquid value.
3.3.
Membrane swelling
In order to evaluate water volume fraction, we have measured the polymer volume increase induced by water vapor sorption at each vapor activity (P/Psat), at 35 C, in both Nafion117 and Aquivion, as it is reported in Fig. 6. The volume dilation is a non linear function of activity, that resembles the shape of a typical water vapor solubility isotherm in PFSI membranes. From the comparison between the two membranes, one notices that in the low activity region the dilation of Nafion
Fig. 6 e Percentage volume dilation induced by vapor sorption, as a function of water vapor activity, measured on Aquivion and Nafion 117 membranes at 35 C.
117 increases more rapidly with activity than that of Aquivion, while is less sensitive to water activity in the intermediate activity range. Interestingly, however, the dilation curves show essentially similar values, at fixed activity, for both Nafion 117 and Aquivion, despite the lower water sorption capacity of Aquivion with respect to Nafion 117 in the low activity range [25]. In any case, the data for both membranes lie below the curve representing volume additivity, which was evaluated considering the partial molar volume of water equal to 18 cm3/ mol, and estimating the water content at each activity from previous works. For Aquivion, the data were measured in a pressure decay equipment at 35 C on a membrane dried at the same temperature, and the actual water amount was estimated by considering the initial presence of one water molecule per sulphonic group in the membrane [25]; for Nafion 117 the data, taken from the work by Pushpa et al., were measured at 25 C on a membrane initially dried at 150 C for 24 h [26]. This result indicates that the partial molar volume of water inside the ionomers is lower than the pure liquid value, and further motivates the experimental analysis of the dilation behavior of PFSI membranes in the presence of vapors. The water volume fraction in the membrane was thus evaluated considering that the polymer volume increase is equal to the volume of water absorbed in the membrane, and was then used to correlate gas permeability data in humid conditions, as explained below.
3.4.
Gas permeability versus water volume fraction
In Fig. 7, the data of humid/dry gas permeability ratio are reported versus the water volume fraction for both membranes at 35 C. The permeability trend can be represented with an exponential curve in the volume fraction range
Fig. 7 e Permeability ratio Pgas/Pgas,0 for various gases in Nafion 117 and Aquivion as a function of water volume fraction, at 35 C. Open symbols refer to Aquivion while filled symbols refer to Nafion 117 films.
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between 2% and 20%, for all the gases inspected and for both membranes. The most significant permeability increase is observed in the case of nitrogen, which has the lowest permeability value in the dry state. In the same figure one can also directly compare the behaviors of the two types of membranes inspected. At low water fractions, the gas permeability increase is more marked for Aquivion than for Nafion. That behavior might be due to the fact that the sulphonic sites, that behave as impermeable obstacles for gas diffusion when the matrix is dry, upon hydration suddenly become the nuclei of a highly permeable hydrophilic phase: this effect is more remarkable in Aquivion, which has a larger fraction of sulphonic sites per mass of ionomer (lower EW) with respect to Nafion. At higher volume fractions, we observe the opposite behavior: the gas permeability increase is more marked for Nafion than for Aquivion. One can speculate that, in this range, the water absorbed in Aquivion is more confined, i.e. the water domains are smaller and less interconnected than in Nafion, reasonably due to a different distribution of crystallinity of Aquivion. In this physical representation, the gas permeability in the hydrophilic phase of Aquivion is lower than that in the corresponding phase of Nafion.
3.5.
Gas selectivity
Finally, it is interesting to highlight the effect that the relative humidity has on the selective permeation of the membranes towards gas pairs. In order to do that, we plotted the gas permeability ratio, i.e. ideal selectivity, for the gas pairs He/N2, O2/N2, He/O2, in the various membranes as a function of the relative humidity (Fig. 8). It can be seen in Fig. 8 that the selectivity of He over N2, that is equal to about 100 in dry conditions, decreases exponentially, by more than one order of magnitude, with increasing water activity, due to the fact that permeability of N2 is much more
Table 2 e Gas transport properties in pure liquid water at 35 C. D [27] (cm2/s) O2 N2 He
5
3.35 10 2.95 105 9.45 105
4
3.78 10 1.95 104 1.56 104
P (Barrer) 126.5 57.3 148.1
enhanced by the presence of water inside the membrane than that of helium. The same qualitative behavior is observed for the O2/N2 and He/O2 pairs, although to a smaller extent, in both ionomers. In all cases, the permeability of the less permeable gases is more strongly enhanced, by the presence of water, than the permeability of the more permeable gases. For comparison, we also showed the ideal selectivity that pure liquid water would exhibit towards the same gas mixtures, as evaluated from the diffusivity and solubility data of the various pure gases in liquid water [27,28] (Table 2). After extrapolation to 100% R.H, the humid gas selectivity data seem to approach values comparable to those typical of pure liquid water. The selectivity decreases initially more markedly in Aquivion than in Nafion, while for higher activity values the trend is reversed. This behavior is perfectly consistent with what observed in Fig. 7 for the single gas permeability data and can be interpreted based on the same mechanism. The absorption of moisture therefore causes an enhancement of the permeability with a great loss in selectivity: such a behavior is typical of swollen polymeric membranes and is often observed in gas separation membranes. Indeed in polymeric membranes which show preferential permeation towards of gases of smaller molecular size the swelling phenomena, as for instance those induced by the presence of hydrocarbons, reduce the selectivity by enhancing more significantly the permeation of gases of larger molecular size [29].
4.
Fig. 8 e Selectivity of gas pairs in Nafion 117, Aquivion and in pure liquid water (Table 2) at 35 C, as a function of water activity (relative humidity). Open symbols refer to Aquivion while filled symbols refer to Nafion 117 films.
S [28] (cm3stp/(cm3 cmHg))
Conclusions
The permeability of helium, oxygen and nitrogen at 35 into two PFSI membranes (Aquivion and Nafion 117) of different structure and equivalent weight was determined, as a function of the relative humidity, in the range between 0 and 90%. The dry gas permeability of O2, N2 and He increases with temperature in the range of 35e65 C and the corresponding activation energy was evaluated for the two membranes. When the gaseous streams fed to the membrane are humidified, the gas permeability increases, rather markedly, with relative humidity. That increase is more marked for oxygen and nitrogen and lower for helium. The permeability data were correlated to the water volume fraction in the ionomer, which was estimated from direct dilation experiments with pure water vapor. Such data show that the polymer volume dilation does not follow volume additivity, and that the partial molar volume of water inside the membranes is smaller than its pure liquid value. For all gases, permeability data follow an exponential trend versus water volume fraction, in the range between 2% and 20% vol. The higher
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permeability increase is observed in the case of nitrogen, that has the lowest permeability value in dry conditions. The gas permeability increase, Pgas/Pgas,0 is different for the two types of membranes, at fixed water volume fraction: at low water fractions it is more marked for the less permeable Aquivion than for Nafion, while the opposite behavior is observed at higher humidity. Such a phenomenon has been interpreted on the basis of the different morphology of the hydrophilic clusters inside the two ionomers. In parallel with the permeability enhancement, the presence of water also reduces significantly the ideal selectivity of the membranes for the gas pairs He/N2, He/O2, O2/N2, as it often happens in swollen polymeric membranes.
Acknowledgments This work was carried on with the financial support of the Project “Combined biological production of methane and hydrogen from wastes of the agro-food industry (Bio-Hydro)”, funded by the Italian Ministry of Agriculture, Food and Forestry (MIPAAF). The Erasmus programme is gratefully acknowledged for the financial support provided to Tendekai Myezwa.
references
[1] Peighambardoust SJ, Rowshanzamir S, Amjadi M. Review of the proton exchange membranes for fuel cell applications. Int J Hydrogen Energ 2010;35:9349e84. [2] Chiou JS, Paul DR. Gas Permeation in a dry Nafion membrane. Ind Eng Chem Res 1988;27:2161e4. [3] Lee YT, Iwamoto K, Seno M. Gas permeabilities of perfluorocarboxylate membranes in various ionic forms. J Membr Sci 1990;49:85e93. [4] Catalano J, Giacinti Baschetti M, De Angelis MG, Sarti GS, Sanguineti A, Fossati P. Gas and water vapor permeation in a short-side-chain PFSI membrane. Desalination 2009;240: 341e6. [ 5] He Y, Cussler EL. Ammonia permeabilities of perfluorosulfonic membranes in various ionic forms. J Membr Sci 1992;68:43e52. [6] Sakai T, Takenako H, Wakabayashi N, Kawami Y, Torikai E. Gas permeation properties of solid polymer electrolyte (SPE) membranes. J Electrochem Soc 1985;132:1328e32. [7] Ogumi Z, Takehara Z, Yoshizawa SI. Oxygen permeation through Nafion and NEOSEPTA. J Electrochem Soc 1984;131: 769e73. [8] Sakai T, Takenaka H, Torikai E. Gas diffusion in the dried and hydrated Nafions. J Electrochem Soc 1986;133:88e92. [9] Timashev SF, Vorobiev AV, Kirichenko VI, Popkov YM, Volkov VI, Shifrina RR, et al. Specifics of highly selective ammonia transport through gas-separating membranes based on perfluorinated copolymer in the form of hollow fibers. J Membr Sci 1991;59:117e31. [10] Pellegrino J, Kang KS. CO2/CH4 transport in polyperfluorosulfonate ionomers: effects of polar solvents on permeation and solubility. J Membr Sci 1995;99:163e74. [11] Buchi FN, Wakizoe M, Srinivasan S. Microlectrode investigation of Oxygen permeation in perfluorinated proton exchange membranes with different equivalent weights.
J Electrochem Soc 1996;143:927e32. [12] Beattie PD, Basura VI, Holdcroft S. Temperature and pressure dependence of O2 reduction at Pt/Nafion 117 and Pt/BAM 407 interfaces. J Electrochem Soc 1999;468:180e92. [13] Lee K, Ishihara A, Mitsushima S, Kamiya N, Ota K. Effect of recast temperature on diffusion and dissolution of oxygen and morphological properties in recast Nafion. J Electrochem Soc 2004;151:A639e45. [14] Ma S, Odgaard M, Skou E. Carbon dioxide permeability of proton exchange membranes for fuel cells. Solid State Ionics 2005;176:2923e7. [15] Mohamed HFM, Ito K, Kobayashi Y, Takimoto N, Takeoka Y, Ohira A. Free volume and permeabilities of O2 and H2 in Nafion membranes for polymer electrolyte fuel cells. Polymer 2008;49:3091e7. [16] James Cw A, RoyMcGrath JE, Marand E. Determination of the effect of temperature and humidity on the O2 sorption in sulfonated poly(arylene ether sulfone) membranes. J Membr Sci 2008;309:141e5. [17] Yeager HL, Steck Cation A, Diffusion Water. In Nafion Ion exchange membranes: influence of polymer structure. J Electrochem Soc 1981;128:1880e4. [18] Gebel G. Structural evolution of water swollen perfluorosulfonated ionomers from dry membrane to solution. Polymer 2000;41:5829e38. [19] Sodaye HS, Pujari PK, Goswami A, Manohar SB. Probing the microstructure of Nafion-117 using Positron Annihilation Spectroscopy. J Polym SCI B Polym Phys 1997;35:771e6. [20] Sodaye HS, Pujari PK, Goswami A, Manohar SB. Measurement of free-volume Hole size distribution in Nafion-117 using Positron Annihilation Spectroscopy. J Polym SCI B Polym Phys 1998;36:983e9. [21] Minelli M, Giacinti Baschetti M, Doghieri F, Ankerfors M, Lindstro¨m T, Siro´ I, et al. Investigation of mass transport properties of microfibrillated cellulose (MFC) films. J Membr Sci 2010;358:67e75. [22] Prabhakar RS, De Angelis MG, Sarti GC, Freeman BD, Coughlin MC. Gas and Vapor Sorption, Permeation, and Diffusion in Poly(tetrafluoroethylene-co-perfluoromethyl vinyl ether). Macromolecules 2005;38:7043e55. [23] Alberti G, Narducci R, Sganappa M. Effects of hydrothermal/ thermal treatments on the water-uptake of Nafion membranes and relations with changes of conformation, counter-elastic force and tensile modulus of the matrix. J Power Sources 2008;178:575e83. [24] Ferrari MC, Piccinini E. Giacinti Baschetti M, Doghieri F, Sarti GC. Solvent-Induced Stresses during sorption in Glassy Polycarbonate: experimental analysis and model Simulation for a Novel Bending Cantilever apparatus. Ind Eng Chem Res 2008;47:1071e80. [25] De Angelis MG, Lodge S, Giacinti Baschetti M, Sarti GC, Doghieri F, Sanguineti A. Water sorption and diffusion in a short-side-chain perfluorosulfonic acid ionomer membrane for PEMFCS: effect of temperature and pretreatment. Desalination 2006;193:398e404. [26] Pushpa KK, Nandan D, Iyer RM. Thermodynamics of water sorption by perfluorosulphonate (Nafion-117) and Polystyrene-Divinyl benzene Sulphonate (Dowex 50W) Ionexchange resins at 298 K. J Chem Soc Faraday Trans 1988; 84:2047e56. [27] Coulson JM, Richardson JF, Backhurst JR, Harker JH. Chemical Engineering Volume 1: Fluid Flow, Heat Transfer and Mass Transfer. Oxford: Butterworh Heinemann; 1999. [28] Perry RH, Green DW. Perry’s Chemical Engineers Handbook. 7th ed. McGraw-Hill; 1999. [29] Bernardo P, Drioli E, Golemme G. Membrane gas separation: a Review/State of the Art. Ind Eng Chem Res 2009;48:4638e63.
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The effect of relative humidity on the gas permeability and swelling in PFSI membranes J. Catalano 1, T. Myezwa, M.G. De Angelis*, M. Giacinti Baschetti, G.C. Sarti Dipartimento di Ingegneria Chimica, Mineraria e delle Tecnologie Ambientali, Universita` di Bologna, Viale Terracini 28, 40131 Bologna, Italy
article info
abstract
Article history:
The permeation of helium, oxygen and nitrogen in perfluorosulphonic acid ionomeric
Received 8 March 2011
(PFSI) membranes with short and long side chains, namely Aquivion and Nafion 117, was
Received in revised form
studied in the relative humidity range between 0 and 90%, and at temperatures between
12 July 2011
35 C and 65 C. The presence of humidity enhances, up to a factor of 100, the gas
Accepted 13 July 2011
permeability in the membranes, due to the permeation of gas molecules in the hydrophilic
Available online 16 August 2011
domains: the enhancement is rather pronounced for O2 and N2, and less marked for He permeability. The relative permeability increase, Pgas/Pgas,0, shows a complex dependence
Keywords:
on the relative humidity, as the water content in the membrane is itself a non linear
PFSI membranes
function of this parameter. The water volume fraction in the membrane at each activity
Ionomers
was accurately estimated from measurements of vapor-induced swelling, which indicate
Gas permeability
that the partial molar volume of water is smaller than its pure liquid value, in both
Controlled humidity
membranes at 35 C. When plotted against water volume fraction, the gas permeability
Water swelling
increases exponentially in the range between 2% and 20%; the slope of the curve is higher
Fuel cells
for Nafion than for Aquivion, as it is reasonable due to their different microstructures. The ideal selectivity of the two membranes for He over N2, O2 over N2 and He over O2, decreases markedly with increasing water content in the membrane. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The development of effective proton exchange membrane fuel cells (PEMFC) requires polymer membranes with high proton conductivity and low gas permeability, so that a larger fraction of gas can react at the cathode [1]. For this reason the process of gas transport into perfluorosulphonic acid ionomeric (PFSI) membranes plays an important role in the design and control of fuel cell performance. Some authors have determined the transport parameters of the gases of interest for fuel cell operations, namely oxygen, nitrogen and hydrogen, in PFSI membranes of different
equivalent weights [2e5]. However, since the PEMFC normally operates in humid conditions, and in some cases in the presence of membrane solvents such as methanol, the characterization of gas transport has to be carried out preferentially under humid or solvated conditions [6e16]. Indeed the membrane microstructure and all the membrane properties are dramatically affected by the absorption of water or other swelling solvents [17]. Despite the amount of studies, the gas permeation mechanism through solvated PFSI membranes and its relationship with the complex morphology of these materials are still far from being completely understood. A solvated PFSI membrane
* Corresponding author. Tel.: þ39 051 2090410; fax: þ39 051 6347788. E-mail address: [email protected] (M.G. De Angelis). 1 Present address: Center for Inorganic Membrane Studies, Department of Chemical Engineering, Worcester Polytechnic Institute, 100 Institute Rd, Worcester, MA 01609, USA. 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.07.047
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can be depicted as a heterogeneous medium, divided into two phases more or less connected: (I) a hydrophobic phase formed by the main fluorinated backbone and (II) hydrophilic regions formed by polar groups arranged in clusters, plus the water inside the clusters if the membrane is wet. The existence of an intermediate, or (III) interfacial phase has also been hypothesized, formed by the side groups of the fluorinated chains and by free water molecules [17]. The volume fraction, size, shape and connectivity of the different phases varies with the water content in the membrane and strongly affects the transport properties of the polymer. Gebel assumed that, at low water contents, the aqueous phase is formed by isolated spherical domains that at higher water fractions become connected by thin channels and eventually form a continuous aqueous path across the membrane thickness [18]. If one considers that gaseous penetrants normally show higher permeation rates in water than in the dry PFSI matrices, it is obvious to expect that the presence of water enhances markedly the gas permeability, as it is indeed observed empirically [15]. To better represent the complexity of the problem it is worthwhile to mention also that, while the gas permeability in PFSI membranes increases monotonically with water content in all the cases inspected, the diffusion and sorption contributions can show different trends; oxygen, for example, is more soluble in the fluorinated medium than in water, but its diffusivity in liquid water is several times higher than that in a perfluorinated matrix such as poly(tetrafluoroethylene) (PTFE). As a consequence, oxygen diffusivity in hydrated PFSI membranes increases up to a factor of 25 by increasing the water content in Nafion from 0 to 35% in weight [6,8,11,13], while the amount of oxygen absorbed in hydrated PFSI membranes is only slightly higher than that in dry membranes, with a behavior that can be described by an additive model based on the oxygen solubility values in pure PTFE and in pure H2SO4 [11,12]. Up to date the only quantitative approach for mass transport in PFSI membranes was proposed by Pellegrino and Kang [10], that considered the solvated polymer as a heterogeneous medium and adapted equations valid for the corresponding heat conduction problem. Their model however was not suitable to fit the experimental data of CO2 and CH4 permeability in Nafion membranes solvated in different liquids, and the authors explained this result by the fact that their model neglected the enhancement of the permeability of the fluorinated phase induced by solvent swelling [10]. A different approach was presented by Sodaye and coauthors [19,20] and by Mohamed et al. [15] that, using the free volume theory, related the variation of gas diffusivity in Nafion membranes to the fractional free volume of the polymer matrix, which was expressly determined by PALS measurements as a function of water content. Such a free volume was considered to be located between the hydrophobic and hydrophilic phases, in the intermediate region (III), which therefore is assumed to play a predominant role in gas transport. In the present work the gas permeation through Nafion 117 and Aquivion membranes was studied as a function of the water relative humidity, in order to provide experimental support for a more fundamental understanding of the gas
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transport mechanisms in hydrated PFSI membranes. The measurements were performed in a closed-volume permeometer, where a uniform water vapor activity and a gas pressure gradient are maintained during the experiments. The data were obtained for dry gases at temperatures ranging from 35 to 65 C, while data at relative humidities up to 90% were collected at 35 C; three different gases were analyzed: O2, N2 and He. The water vapor swelling was also determined at 35 C, as a function of the relative humidity, in order to obtain the actual volume fraction of water in the membrane.
2.
Materials and methods
2.1.
Materials
The materials studied in this work were Aquivion, formerly known as Hyflon Ion, produced by Solvay Solexis S.p.A. and Nafion 117, produced by Dupont. The chemical formula of the two polymers can be seen in Fig. 1. The Aquivion monomer has a shorter pendant side chain with respect to that of Nafion 117, and a lower equivalent weight: 800 versus 1100 gpol/molSO3H. Aquivion films used in the present work were kindly provided by the producer and had a thickness of about 160 mm. Nafion 117 films were purchased from Aldrich and had a thickness of about 183 mm.
2.2.
Gas permeability measurements
The apparatus for the determination of humid gas permeability was adapted from a pure gas, fixed volume permeometer [4] through the addition of a humidifying section on the upstream side of the membrane, of a water vapor reservoir for initial membrane equilibration, and of a purge flow to conduct flow-through experiments (Fig. 2a) [21]. The equipment can be used to detect the permeation rates of any gas in any polymeric material and does not require an analysis of the permeate stream composition but only a measure of the downstream pressure. In the experiments, the humidity was regulated by mixing a dry and a saturated gas stream with a controlled flow rate; a humidity sensor (RH 01: Vaisala RH: 0e100%; T : e 40/þ80 C) was used to monitor the humidity value obtained. The pressure in the upstream side of the system was measured through a high pressure manometer (PI02, Druck f.s. 6 bara), while a low-pressure capacitance manometer was placed in the downstream section of the apparatus (PI03: MKS Baratron f.s. 100 mbar). A differential transducer is also present in the scheme of Fig. 2a (PI04: Druck manometer, f.s. 10 mbar): this sensor is used when temperatures higher than the ones inspected in this work are needed, because it allows to perform accurate measurements in a wide pressure range, according to a procedure already described elsewhere [22]. The volume of the downstream side of the apparatus was carefully measured and resulted to be 35.0 cm3. The whole system was placed in a thermostatic chamber and an additional thermocouple (TI01) was placed in a bubbling vessel (S02, 7 dm3) in order to evaluate the water saturation pressure obtained in the humidified stream. The system was also
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Fig. 1 e Monomer units of the PFSI membranes used in this work.
endowed with two different vessels: S01, that is used for pure gas permeation experiments, and S03 that is the water reservoir (2 dm3) used for the initial stage of water equilibration on both membrane sides, and can be used also for pure water vapor permeation experiments. The experiment is carried out by first putting the membrane in contact, on both sides, with water vapor at fixed
activity, fed from vessel S03, and letting the system reach equilibration. This saturation stage, needed to obtain a uniform water concentration profile in the membrane, normally lasts a few hours in the conditions inspected and is considered to be completed when a constant pressure value is reached on both sides of the membrane. After the initial saturation at the water pressure selected, valves V04, V02 and
Fig. 2 e (a) Schematic of the permeation apparatus for controlled humidity gas permeation experiments (adapted from ref. 21). (b) Scheme of the experiment. (c) Experimental output of a humid gas permeation experiment.
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V03 are closed, in order to isolate the upstream and downstream volumes of the apparatus, and the upstream conditions are varied by feeding a gas stream with a relative humidity, set to maintain the water activity equal to that at the two sides of the membrane. The upstream pressure is set to the desired value by tuning the gas partial pressure, read through PI02, through the pressure reducer in the gas feed (PC600). The stream humidity is adjusted by using the mass flow controllers FC01 and FC02 placed on the pipe connected to the bubbling vessel and on the gas bypass line, respectively; during this operation V01 is open allowing the system to work in a continuous flux mode. Once the relative humidity of the feed stream is stable, with variation less then 0.1% of the presaturation RH value, V01 is closed and, at the same time, valves V02 and V03 are opened. In this way, a driving force for gas permeation is established, while the driving force for water flux is equal to zero as in the pre-equilibration step; the presence of a flow in the retentate side, then, allows to avoid any possible concentration polarization effect. The pressure increase in the downstream volume is due to the permeation of a single gas and allows for a simple quantification of gas permeability under humid conditions with the same equations used for the permeation of pure gases. The scheme of the chemical potential profile during the experiment is represented in Fig. 2b, where the reasonable assumption is made that the presence of gas does not change appreciably the water chemical potential in the membrane. The initial stage of the permeation process was difficult to interpret with the classic time-lag analysis because a nonnegligible water flux was present at the beginning of the experiments, caused by a small difference between the humidity value of the upstream gas and the value of humidity obtained during the previous equilibration stage. This transient stage, however, reached rapidly an equilibrium state so that the gas permeability values obtained in steady state resulted extremely reproducible. It is well known that PFSI membranes may undergo slow relaxation phenomena and reach a true equilibrium state, for several processes, only after long times [23]. However, whether the state reached is a true equilibrium or a pseudoequilibrium one, the values attained at the end of the experiment are meaningful from the practical point of view because the time-scale of PEMFC applications (automotive or portable electronic devices) is of the same order of magnitude of the duration of our experiments. The experimental behavior observed is reported in Fig. 2c: the circles represent the downstream pressure, the squares its time derivative, as a function of time: one can observe the initial peak of the flux, due to simultaneous permeation of gas and water vapor, and the asymptotic value, due to the permeation of the pure gas alone, finally reached when the water concentration is uniform. For Aquivion, dry gas permeation experiments were performed by first treating the membrane under vacuum at 105 C for 4 h in the tightened permeation cell, then testing the membrane at 65, 50 and 35 C. For Nafion, such a procedure caused cracks into the membrane due to the coupling of mechanical and thermal stress and loss of water molecules, thus a milder pre-treatment was applied drying the membrane under vacuum for 12 h at 65 C.
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Humid permeation experiments were performed at 35 C for both Aquivion and Nafion 117 without any thermal pretreatment.
2.3.
Membrane swelling measurements
The swelling induced by water vapor, as a function of activity, in both Aquivion and Nafion 117 membranes, was determined at 35 C via an optical apparatus whose scheme is reported in Fig. 3 [24]. The system is composed of a special sample compartment, formed by a stainless steel cell with two borosilicate glass windows in the opposite sides, to allow optical access for the measurements. The cell is connected to a reservoir containing the penetrant vapor and the pressure transducer, and to a second flask for the storage of liquid penetrant. A vacuum system, with a liquid nitrogen trap and a vacuum pump, is used to evacuate the apparatus before and after each experiment. The sensing element is an optical micrometer (Keyence LS-7030-M) endowed with a high speed linear CCD sensor that ensures accuracy of 1 mm and reproducibility within 0.15 mm in the measurements. The micrometer is fixed on two manual roto-translational optical stages, one for the horizontal alignment of transmitter and receiver with the sample, and one devoted to guarantee that the sample and the light beam are mutually orthogonal. To control temperature, the experimental cell is surrounded by two heating tapes that can reach temperatures up to 200 C; all the instrumentation is inserted in a thermostatic hood to eliminate the fluctuations of room temperature that might affect the response of the electronic devices. The sample elongation is monitored in one planar direction, and the volume dilation is assumed isotropic. Multiple measurements were performed to ascertain an acceptable level of experimental error, in particular reproducibility was checked to be within 95% confidence levels. The data obtained allowed to quantify the volume fraction of water in the polymer and, once the water vapor solubility at each activity is known from independent measurements [25], to compare the swelling behavior to the one predicted by volume additivity.
3.
Results and discussion
3.1.
Dry gas permeability
Initial tests were performed in dry conditions for O2, N2 and He: the permeability of helium represents an upper limit, and the value of hydrogen permeability, according to available data [2] lies between those of oxygen and helium. The experiments were performed on Aquivion and Nafion 117 samples with upstream pressures from 1 to 2 bar and vacuum on the downstream side. In this operative range, the gas permeability showed no appreciable variation with upstream pressure. The data for Aquivion at 35, 50 and 65 C, already presented in a preliminary work [4], are reported in Fig. 4a. Permeability varies from 27.5 to 55 Barrer in the case of helium, from 0.68 to 2.3 Barrer for oxygen and from 0.17 to 0.7 Barrer for nitrogen. In the case of O2 and N2 it was also possible to determine the diffusivity with the time-lag method
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Fig. 3 e Experimental dilation apparatus [24].
obtaining values from 4.2 108 to 1.9 107 cm2/s for oxygen and from 1.8 108 to 9.2 108 cm2/s for nitrogen [4] (Fig. 4b). For He, on the other hand, diffusivity could be only roughly estimated due to the short time-lag values and resulted to lie between 105 and 104 cm2/s. The permeability and diffusivity values vary with temperature according to Arrenhius law and
the resulting activation energies were calculated and listed in Table 1. For the case of Nafion 117, also reported in Fig. 4a, permeability varies between 0.97 and 3.2 Barrer for O2, between 26 and 56 Barrer for He and between 0.18 and 1 Barrer for N2. Diffusivity (Fig. 4b) varies between 4.5 108 and
Fig. 4 e (a) Dry gas permeability in the different PFSI membranes. Filled symbols refer to Nafion (this work), open symbols refer to Aquivion [4] and semi-filled symbols refer to data from J.S. Chiou and D.R. Paul for Nafion 117 [2]. (b) Dry gas diffusivity in the different PFSI. Open symbols refer to Aquivion while filled symbols refer to Nafion 117 films.
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Table 1 e Gas transport parameters in dry PFSI membranes. Membrane
N117 N125 N117 N112 N117 NRE212 N117 Aquivion
T ( C)
35 30 30 35 25 25 35 35 35 35
EW (gpol/molSO3H)
1100 1200 1100 1100 1100 1300 1100 1100 1100 860
Permeability (Barrer) O2
N2
H2
He
1.08 1.4 1.22 1.23 0.9 1 1.2
0.26 0.65
9.3 14 9 8.14
40.9
0.97 0.68
O2
N2
H2
30.5 24.6
27.6
20.1 28.2
Ref.
He [2] [8] [6] [16]
0.22 0.29
[3] 10.7
0.18 0.17
2.1 107 cm2/s for O2, from 9.6 106 to 1.2 105 cm2/s for He and from 2.4 to 8.7 108 cm2/s for N2. Several gas permeability data in dry Nafion 117 can be found in previous works and are listed in Table 1 for the sake of comparison. From this table a large scatter of data can be noticed, which is probably due to the different drying protocols followed by the various authors, that leave different amounts of water in the membrane. This aspect, although not investigated specifically in the present work, is qualitatively proven by tests that we carried on Aquivion samples, which have shown that the gas permeability at room temperature may be up to 45% higher when the membrane is dried at 35 C instead than at 105 C. In any event, our results for the diffusivity of O2 and N2, obtained on a Nafion membrane dried at 65 C for 12 h, are in very good agreement with those obtained by Chiou and Paul [2] (1.73 108 and 4.57 108 cm2/s for N2 and O2, respectively, at 35 C) that were not reported in Fig. 4b for the sake of clarity. Activation energies for permeation in Nafion 117 were evaluated as well, and they are reported in Table 1: their values are similar to those of observed in Aquivion with the exception of N2 activation energy, that is higher for Nafion than for Aquivion.
3.2.
Activation energy EP (kJ/mol)
30.5 26.3 27.5
34.6 34.5
20.4 49.6 41.7
22 20.0
[15] this work [4]
similar but less marked at high humidity and in particular the relative increase of permeability at the same RH (75%) is about 1.7 for He, 20 for O2, 45 for N2, with respect to the permeability obtained on an Aquivion membrane dried at 105 C for 4 h. The observed dependence of permeability on water activity is not linear: the initial increase of permeability with water activity is rather steep and then proceeds more slowly. A similar behavior was already observed by other authors, as discussed in the introduction: some of them have correlated the gas permeability trend to the free volume variation induced by water sorption in the matrix, determined by PALS under humid conditions [15,19,20]. The majority of authors, however, supposed that the behavior of gas permeability in the humidified membranes is due to the fact that a considerable portion of the gas flux occurs via the water phase, that is 10e100 times more permeable than the dry fluorinated matrix, depending on the gas considered. This behavior is also proved
Gas permeability versus relative humidity
The humid gas permeability experiments were performed at 35 C on both Nafion 117 and Aquivion samples and are shown in Fig. 5. For comparison, in the same figure we also reported the oxygen permeability data in Nafion NRE212 determined at 30 C by Mohamed et al. [15], that were measured with a chromatographic device. Such data lie below those measured in this work on Nafion 117, but the difference can be due in part to the lower temperature, and in part to the fact that in ref. 15 they were obtained after heating the sample under vacuum at 80 C for 10 h, while the measurement on our sample was carried out without any previous thermal treatment. As it can be seen from Fig. 5, when the relative humidity is 75%, the permeability increases by a factor of about 3 for He, 30 for O2, 95 for N2 in the case of Nafion, with respect to the permeability measured in the membrane dried at 65 C for 12 h. In the case of Aquivion, on the other hand, the dependence of permeability on relative humidity is qualitatively
Fig. 5 e Permeability for O2, N2 and He at 35 C versus water relative humidity in the different PFSI membranes. Filled symbols refer to Nafion 117 and open ones to Aquivion. The solid line represents the behavior of oxygen permeability data at 30 C in Nafion NRE212 from ref. [15].
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by the fact that gas diffusivity data, plotted at different temperatures, show an inflection point that corresponds to the freezing point of water [8]. Another phenomenon that may cause an increase of gas permeability with water content is the water-induced swelling and plasticization of the fluorinated phase, that contributes to enhance the gas flux in the hydrophobic region, but that is usually neglected, due to its smaller effects with respect to the permeability in the aqueous phase. It is therefore generally accepted that, as far as permeability is concerned, the hydrated PFSI membrane behaves as a twophase structure, with an aqueous phase and a hydrophobic phase. According to these considerations, it is more meaningful to report the permeability data as a function of the water volume fraction inside the membrane rather than the vapor activity, since this parameter better accounts for the relative amount of aqueous domains available in the hydrated matrix. The volumetric fraction of water can be estimated, to a first approximation, by simply assuming volume additivity between the pure polymeric phase and the pure liquid water phase, according to the assumption that water occupies in the matrix the same volume as in its pure liquid state. However, such a procedure does not take into account the fact that the partial molar volume of water sorbed in PFSI may deviate from its pure liquid value.
3.3.
Membrane swelling
In order to evaluate water volume fraction, we have measured the polymer volume increase induced by water vapor sorption at each vapor activity (P/Psat), at 35 C, in both Nafion117 and Aquivion, as it is reported in Fig. 6. The volume dilation is a non linear function of activity, that resembles the shape of a typical water vapor solubility isotherm in PFSI membranes. From the comparison between the two membranes, one notices that in the low activity region the dilation of Nafion
Fig. 6 e Percentage volume dilation induced by vapor sorption, as a function of water vapor activity, measured on Aquivion and Nafion 117 membranes at 35 C.
117 increases more rapidly with activity than that of Aquivion, while is less sensitive to water activity in the intermediate activity range. Interestingly, however, the dilation curves show essentially similar values, at fixed activity, for both Nafion 117 and Aquivion, despite the lower water sorption capacity of Aquivion with respect to Nafion 117 in the low activity range [25]. In any case, the data for both membranes lie below the curve representing volume additivity, which was evaluated considering the partial molar volume of water equal to 18 cm3/ mol, and estimating the water content at each activity from previous works. For Aquivion, the data were measured in a pressure decay equipment at 35 C on a membrane dried at the same temperature, and the actual water amount was estimated by considering the initial presence of one water molecule per sulphonic group in the membrane [25]; for Nafion 117 the data, taken from the work by Pushpa et al., were measured at 25 C on a membrane initially dried at 150 C for 24 h [26]. This result indicates that the partial molar volume of water inside the ionomers is lower than the pure liquid value, and further motivates the experimental analysis of the dilation behavior of PFSI membranes in the presence of vapors. The water volume fraction in the membrane was thus evaluated considering that the polymer volume increase is equal to the volume of water absorbed in the membrane, and was then used to correlate gas permeability data in humid conditions, as explained below.
3.4.
Gas permeability versus water volume fraction
In Fig. 7, the data of humid/dry gas permeability ratio are reported versus the water volume fraction for both membranes at 35 C. The permeability trend can be represented with an exponential curve in the volume fraction range
Fig. 7 e Permeability ratio Pgas/Pgas,0 for various gases in Nafion 117 and Aquivion as a function of water volume fraction, at 35 C. Open symbols refer to Aquivion while filled symbols refer to Nafion 117 films.
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between 2% and 20%, for all the gases inspected and for both membranes. The most significant permeability increase is observed in the case of nitrogen, which has the lowest permeability value in the dry state. In the same figure one can also directly compare the behaviors of the two types of membranes inspected. At low water fractions, the gas permeability increase is more marked for Aquivion than for Nafion. That behavior might be due to the fact that the sulphonic sites, that behave as impermeable obstacles for gas diffusion when the matrix is dry, upon hydration suddenly become the nuclei of a highly permeable hydrophilic phase: this effect is more remarkable in Aquivion, which has a larger fraction of sulphonic sites per mass of ionomer (lower EW) with respect to Nafion. At higher volume fractions, we observe the opposite behavior: the gas permeability increase is more marked for Nafion than for Aquivion. One can speculate that, in this range, the water absorbed in Aquivion is more confined, i.e. the water domains are smaller and less interconnected than in Nafion, reasonably due to a different distribution of crystallinity of Aquivion. In this physical representation, the gas permeability in the hydrophilic phase of Aquivion is lower than that in the corresponding phase of Nafion.
3.5.
Gas selectivity
Finally, it is interesting to highlight the effect that the relative humidity has on the selective permeation of the membranes towards gas pairs. In order to do that, we plotted the gas permeability ratio, i.e. ideal selectivity, for the gas pairs He/N2, O2/N2, He/O2, in the various membranes as a function of the relative humidity (Fig. 8). It can be seen in Fig. 8 that the selectivity of He over N2, that is equal to about 100 in dry conditions, decreases exponentially, by more than one order of magnitude, with increasing water activity, due to the fact that permeability of N2 is much more
Table 2 e Gas transport properties in pure liquid water at 35 C. D [27] (cm2/s) O2 N2 He
5
3.35 10 2.95 105 9.45 105
4
3.78 10 1.95 104 1.56 104
P (Barrer) 126.5 57.3 148.1
enhanced by the presence of water inside the membrane than that of helium. The same qualitative behavior is observed for the O2/N2 and He/O2 pairs, although to a smaller extent, in both ionomers. In all cases, the permeability of the less permeable gases is more strongly enhanced, by the presence of water, than the permeability of the more permeable gases. For comparison, we also showed the ideal selectivity that pure liquid water would exhibit towards the same gas mixtures, as evaluated from the diffusivity and solubility data of the various pure gases in liquid water [27,28] (Table 2). After extrapolation to 100% R.H, the humid gas selectivity data seem to approach values comparable to those typical of pure liquid water. The selectivity decreases initially more markedly in Aquivion than in Nafion, while for higher activity values the trend is reversed. This behavior is perfectly consistent with what observed in Fig. 7 for the single gas permeability data and can be interpreted based on the same mechanism. The absorption of moisture therefore causes an enhancement of the permeability with a great loss in selectivity: such a behavior is typical of swollen polymeric membranes and is often observed in gas separation membranes. Indeed in polymeric membranes which show preferential permeation towards of gases of smaller molecular size the swelling phenomena, as for instance those induced by the presence of hydrocarbons, reduce the selectivity by enhancing more significantly the permeation of gases of larger molecular size [29].
4.
Fig. 8 e Selectivity of gas pairs in Nafion 117, Aquivion and in pure liquid water (Table 2) at 35 C, as a function of water activity (relative humidity). Open symbols refer to Aquivion while filled symbols refer to Nafion 117 films.
S [28] (cm3stp/(cm3 cmHg))
Conclusions
The permeability of helium, oxygen and nitrogen at 35 into two PFSI membranes (Aquivion and Nafion 117) of different structure and equivalent weight was determined, as a function of the relative humidity, in the range between 0 and 90%. The dry gas permeability of O2, N2 and He increases with temperature in the range of 35e65 C and the corresponding activation energy was evaluated for the two membranes. When the gaseous streams fed to the membrane are humidified, the gas permeability increases, rather markedly, with relative humidity. That increase is more marked for oxygen and nitrogen and lower for helium. The permeability data were correlated to the water volume fraction in the ionomer, which was estimated from direct dilation experiments with pure water vapor. Such data show that the polymer volume dilation does not follow volume additivity, and that the partial molar volume of water inside the membranes is smaller than its pure liquid value. For all gases, permeability data follow an exponential trend versus water volume fraction, in the range between 2% and 20% vol. The higher
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permeability increase is observed in the case of nitrogen, that has the lowest permeability value in dry conditions. The gas permeability increase, Pgas/Pgas,0 is different for the two types of membranes, at fixed water volume fraction: at low water fractions it is more marked for the less permeable Aquivion than for Nafion, while the opposite behavior is observed at higher humidity. Such a phenomenon has been interpreted on the basis of the different morphology of the hydrophilic clusters inside the two ionomers. In parallel with the permeability enhancement, the presence of water also reduces significantly the ideal selectivity of the membranes for the gas pairs He/N2, He/O2, O2/N2, as it often happens in swollen polymeric membranes.
Acknowledgments This work was carried on with the financial support of the Project “Combined biological production of methane and hydrogen from wastes of the agro-food industry (Bio-Hydro)”, funded by the Italian Ministry of Agriculture, Food and Forestry (MIPAAF). The Erasmus programme is gratefully acknowledged for the financial support provided to Tendekai Myezwa.
references
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