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Gas permeation in perflurosulfonated membranes:Influence of temperature and relative humidity

Marco Giacinti Baschetti a,*, Matteo Minelli a,b, Jacopo Catalano c,Giulio C. Sarti a

aDipartimento di Ingegneria Civile, Chimica, Ambientale e dei Materiali (DICAM),

Alma Mater Studiorum e Universita di Bologna, via Terracini 28, I-40131 Bologna, ItalybCentro Interdipartimentale per la Ricerca Industriale e Meccanica Avanzata e Materiali (CIRI-MAM),

Alma Mater Studiorum e Universita di Bologna, ItalycDepartment of Engineering, Aarhus University, Langelandsgade 140, 8000 Aarhus C, Denmark

a r t i c l e i n f o

Article history:

Received 5 March 2013

Received in revised form

13 June 2013

Accepted 24 June 2013

Available online 26 July 2013

Keywords:

Perflurosulfonated membranes

Gas permeability

Nafion

Aquivion

Humid gas permeation

* Corresponding author. Tel.: þ39 (0) 51 2090E-mail address: marco.giacinti@unibo.it (

0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2013.06.1

a b s t r a c t

The permeation of CO2, O2, N2 and He in two perfuorosulfonated ionomer (PFSI) mem-

branes, Nafion� 117 and Aquivion�, has been studied as a function of temperature and

relative humidity. Experiments were carried out in wide ranges of temperatures (25e65 �C)

and relative humidity (0e80%), by means of a manometric apparatus specifically developed

to that aim.

The results showed the marked effect of the presence of water on the gas transport

properties of the hydrophilic materials studied. Indeed, when the relative humidity is

raised to 70%, the permeability of the different gases increases up to two orders of

magnitude with respect to the values obtained in dry conditions.

The two materials showed very similar permeability values, and differences seldom

exceeded 20%. An analogous behavior was also observed for temperature dependence of

gas permeability in the two PSFIs, that indeed followed an Arrhenius behavior, in both dry

and humid conditions. The activation energies were slightly higher for Nafion than for

Aquivion in dry conditions, but very similar values were observed at higher water contents.

At high relative humidity, gas permeabilities in PFSIs are similar to those in pure water,

suggesting that permeation in hydrated PSFI is controlled by sorption and diffusion of the

penetrant in the water-filled channels present inside the matrix.

Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction dioxide, depending on the type of FC considered, and they

Perflurosulfonated ionomers (PFSIs) are of high interest for

energy applications due to their potential use as proton ex-

change membranes (PEMs) in hydrogen or methanol based

fuel cells (FC) [1,2]. In such applications, PEMs are exposed to

different gases like hydrogen, oxygen, nitrogen and carbon

408; fax: þ39 (0) 51 63477M. Giacinti Baschetti).2013, Hydrogen Energy P04

operate at various conditions, both in terms of temperature

and relative humidity (R.H.). For these reasons, the knowledge

of gas permeability of the different penetrants in such mate-

rials as a function of temperature and R.H. is of interest for a

proper physical modeling of the whole system and to obtain a

quantitative estimation of undesired processes, such as

88.

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 1 9 7 3e1 1 9 8 211974

oxygen or hydrogen crossover, which affect FC efficiency.

Different studies available in the technical literature

addressed these issues [3e15], however the complete and

systematic investigation of these features with a thorough

material characterization is still missing for many of the PSFIs

now available.

Apart from Nafion, whose properties have been investi-

gated by many authors [8e14], few reports on mass transport

properties of PSFIs are present in the open literature, espe-

cially concerning the effect of temperature and/or of the R.H.

on the gas permeability behavior. Indeed, such studies are

very limited in number and scope, covering only few exam-

ples of sulfonated materials, such as the already recalled

Nafion [8e14], sulfonated poly(arylene ether sulfone) [15], or

sulfonated poly(ether ether ketone) [11].

Aquivion [12] is a short side chain PFSI polymer, formerly

known as HyfloneIon, which has attracted some interest in

recent times due to its similarity to Nafion, in terms of both

structure and physical properties [16e19]. Aquivion has the

samemain chain structure as Nafion, but it is characterized by

shorter sulfonated pendant chains, which confer to the ma-

terial a higher cristallinity. This leads to improved properties

in terms of thermal and mechanical resistance, and, as a

consequence, to higher performances in FC operations

[19e21]. Aquivion has been studied in recent years with the

main focus on thermal and mechanical properties [19,20], on

gas permeation [12,22], as well as on water and methanol

sorption and transport [22e26]. In particular, the effect of

humidity on the permeability of different gases, namely He

(used to mimic H2), N2 and O2, has been recently investigated

at 35 �C, comparing the results with those obtained for Nafion

117 [12]. However, a complete characterization of the effect of

temperature on gas transport properties in Aquivion under

humid conditions is still missing.

In the present work, the systematic analysis of the effect of

temperature on gas permeation in Aquivion and Nafion in dry

and humid conditions has been performed for different pene-

trants. Experimental tests were carried out at different tem-

peratures in the range from 25 to 65 �C, and the data from a

previousworkof permeability ofHe,N2 andO2 at 35 �C [12]were

also included in the analysis of the results. Carbon dioxidewas

considered as further probe molecule, as this gas can also be

present in the fuel cell environment.Moreover, the studyofCO2

permeation is of great importance in view of the need for the

reduction of its emission in the atmosphere, to avoid issues

related to global warming [27,28]. Indeed, PSFIs showed prom-

isingproperties forCO2separation,andhavebeenconsideredas

interesting base materials [5,29e32] to fabricate facilitated

transportmembranes. In view of their potentialities in the field

ofmembranes for CO2 separation, CO2 permeation in Aquivion

and Nafion has also been investigated.

Therefore, permeation data of O2, N2, CO2 and He, in both

Aquivion and Nafion, at temperatures ranging from 25 to 65 �Cand at R.H. up to 80%, are presented and analyzed.

Fig. 1 e Molecular structure of a) Aquivion� and b) Nafion�

117.

2. Materials and methods

The chemical formula of the two materials considered in the

present work, namely Aquivion, produced by Solvay Solexis,

and Nafion 117, produced by Dupont, are shown in Fig. 1. The

two PSFIs posses very similar chemical structures, consisting

in a hydrophobic poly(tetrafluoroethylene) backbone with

pendant perfluorovinyl ether side chains terminated by sul-

fonic acid (�SO3H) groups. The main difference is related to

the length of the side chain, which is longer in Nafion than in

Aquivion, so that the latter is often referred to as a short side

chain PFSI material.

Due to the different molecular structure, Aquivion is

endowed with a lower equivalent weight than Nafion, and it

presentshighercrystallinityatagivenvalueofequivalentweight

[20]. The samples of Aquivion considered in this work were

characterizedbyanequivalentweight of about 860 gpol=molSO3H,

definitely lower than the value of 1100 gpol=molSO3H typical of

Nafion 117. Consequently, the two materials here analyzed are

expected to have comparable crystallinity fractions.

The Aquivion films were obtained by extrusion, and were

kindly provided by the producer, with a thickness of about

160 mm, whereas extruded Nafion 117 films were purchased

from Aldrich and were about 180 mm thick.

The gases used for permeation experiments were chro-

matographic grade, with purity higher than 99.95%, and were

purchased from SIAD S.p.A (Italy).

Two different permeation devices were used in the present

work: a classical close volumemanometric apparatus [33] was

employed to carry out pure gas permeation experiments at

different temperatures, whereas a purposely developed sys-

tem based on the same technique was used for the evaluation

of the humid gas permeability at different temperatures

[12,34].

Experiments were performed by considering a pressure

difference of approximately 2 bar across the membrane, and

the permeability was evaluated once steady state conditions

were attained in the system, through the use of the following

well known equation:

P ¼�dp1

dt

�t/N

$VRT

$lA$

1�p2 � p1

� (1)

where p1 represents the pressure in the calibrated down-

stream volume (V), p2 is the upstream pressure, A and l are

Fig. 2 e He, O2, N2 and CO2 permeability in Nafion and

Aquivion in dry conditions at 25, 35, 50 and 65 �C. Data

points with crossed symbols were retrieved from Catalano

et al. [12].

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 1 9 7 3e1 1 9 8 2 11975

sample area and thickness, respectively, while R is the gas

constant and T is absolute temperature. The leakage in the

system was completely negligible over the entire duration of

the experiments, and thus it is not considered in Eq. (1).

All measurements were replicated at least twice at each

temperature and R.H., for every gas; deviations were usually

below 10%, and were mainly related to the uncertainty in the

R.H. value, which has a significant influence on transport

properties in these highly hydrophilic materials.

The measurements of dry gas permeability were carried

out pre-treating the samples, in order to evacuate residual

moisture from the membrane, following the protocol already

used in a previous work [12]. The specimens were set in the

permeation cell, and then dried for 24 h under vacuum at

different temperatures. Aquivion samples were pre-treated at

about 100 �C, whereas a value of 65 �C was used for Nafion,

because higher temperatures caused a significant crack for-

mation on the membrane, probably due to an excessive

shrinkage of the film inside the permeation cell.

The permeability of the humidified gas has been evaluated

carrying out humid gas permeation experiments in a

purposely-built apparatus, in which the membrane was

maintained at a certain water activity. The polymer mem-

brane was first conditioned with pure water vapor at the

desired R.H., and once equilibrium conditions were reached

and the sample was equilibrated at the desired temperature

and R.H., one side of the membrane was exposed to a stream

of humid gas at the same R.H. of the equilibration step. After a

short transient, only the gas molecules permeated through

the membrane, being water chemical potential equal on both

sides of the specimen. The incoming gas molecules, perme-

ated through the membrane, affected pressure and water

fraction in opposite way thus not altering the partial pressure

of water in the downstream compartment and maintaining

the R.H. value constant in the system throughout the test.

Permeability value can then be retrieved from the pressure

increase in the downstream volume, by means of Eq. (1).

Further details on the apparatus and on the experimental

protocol can be found in previous works [12,34].

3. Results and discussion

3.1. Dry gas permeation

The permeabilities of He, O2, N2 and CO2 in Aquivion and

Nafion, in the absence of water and at different temperatures

from 25 to 65 �C, are presented in the Arrhenius plot of Fig. 2.

The data are fully consistent with those reported in a previous

work [12], also included in the same figure, for the sake of

completeness. In particular, permeability values obtained at

25 �C for He, O2 and N2 closely follow the Arrhenius behavior

described by the data at higher temperatures, reported in

Ref. [12]; the coefficient of determination, R2, is always higher

than 0.985, in the whole temperature range considered

(25e65 �C).CO2 permeability data are not available in the open litera-

ture for Aquivion, while for Nafion 117, the values here ob-

tained at the different temperatures (1.00, 1.63, 3.11 and 5.84

Barrer at 25, 35, 50 and 65 �C, respectively) are in good

agreement with those from previous works [3,9]. Indeed, the

experimental permeability data reported by Chiou et al. for

Nafion 117 at 35 �C (2.43 Barrer) [3], and by Ma et al. at 25 �C in

dry Nafion 111 (2.31 Barrer) [9], are only slightly higher than

those here reported, and such difference is likely related to the

different sample pretreatment. Indeed, the membranes were

dried at lower temperatures, 35 and 40 �C in Refs. [3,9],

respectively, with a procedure not able to guarantee the

complete removal of water from the membrane [35], thus

altering the final permeability values. For the sake of com-

parison, permeation experiments were also carried out drying

the samples at 35 �C, and, as a result, permeability valueswere

about 30% (2.23 Barrer) higher than those showed in Fig. 2 and,

in line with those reported in Refs. [3,9].

The two PSFIs analyzed in this work show rather low gas

permeabilities in dry conditions. Indeed, if no water is present

in the membrane, the material behavior is that of a glassy

polymer with perflourinated chains, and gas transport prop-

erties are further lowered by the rather high content of

crystallinity.

The permeability of different gases follows a well defined

trend when compared with the molecular size of the pene-

trant, such as its kinetic diameter. The values of kinetic

diameter are reported in Table 1 showing that helium is the

smallest probe followed by CO2, O2 and N2, respectively. The

same trend is observed for permeability, being He the most

permeable penetrant, at all the temperatures inspected, fol-

lowed in a decreasing sequence by carbon dioxide, oxygen and

nitrogen, which has the lowest permeability. Such depen-

dence closely follows the exponential behavior illustrated in

Fig. 3, and it suggests that the permeation process in the dry

polymers is controlled by diffusion, as it is usually observed in

glassy polymers [36].

Among the two materials, Nafion shows the highest

permeability, especially at higher temperatures, although, at

Table 1 e Activation energy of the permeation process for different gases in Nafion 117 and Aquivion. Kinetic diameters ofthe different probe molecules are also reported as taken by Ref. [37].

Permeating gas O2 N2 He CO2

Kinetic diameters (A) 3.46 3.64 2.60 3.30

Membrane EW (gmol=molSO3H) Activation energy EP (kJ/mol) T range (�C) Ref

N117 1100 34.6 49.6 22 # 35e65 [12]

Aquivion 860 34.5 41.7 20 # 35e65 [12]

N117 1100 34.6 49.7 22.1 36.7 25e65 This work

Aquivion 860 36.4 40.1 18.0 35.4 25e65 This work

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 1 9 7 3e1 1 9 8 211976

room temperature, Aquivion is slightly more permeable to N2

and He than Nafion. However, these differences are relatively

small, and deviations can also be due to the different pre-

treatments of the two polymers. Consequently, it can be

concluded that the two PFSIs are nearly identical, as far as the

permeability of dry gas is concerned.

The temperature dependence of dry gas permeability of

both polymers closely follows an Arrhenius behavior, and the

corresponding activation energies, EP, are reported in Table 1.

In the case of helium, oxygen and nitrogen, the values ob-

tained are very close to those already reported in a previous

paper [12], which also included an extensive comparison with

available literature data. For the case of carbon dioxide, the

activation energy was not reported previously, since the effect

of temperature on CO2 permeability was not yet investigated.

The activation energies obtained for CO2 permeability in both

PFSIs are in the same range of values obtained for the other

gases, and are very close to those observed for oxygen, whose

kinetic diameter is similar to that of CO2. Furthermore, as

visible in Table 1, the activation energies also depend on the

penetrant kinetic diameter, and higher values are associated

to larger gas molecules [36], as it is expected when diffusion

process controls the transport mechanism.

The comparison between the two PFSIs indicates that

Nafion has activation energies approximately 20% larger than

those of Aquivion for He and N2, whereas in case of O2 and

Fig. 3 e He, O2, N2 and CO2 permeability in Nafion and

Aquivion in dry conditions at 25, 35, 50 and 65 �C as a

function of the gas kinetic diameter [37].

CO2, differences are smaller (less than 5%), and almost the

same values were obtained for both materials.

3.2. Humid gas permeation

The permeabilities of different gases in Nafion and Aquivion

at water activities in the range 0e80% R.H. and at various

temperatures (25, 35 and 50 �C) are illustrated in Fig. 4aed. The

experimental results obtained in this work are reported in

Fig. 4 together with data retrieved at the intermediate tem-

perature (35 �C) already presented in Ref. [12]. As expected, the

two materials show qualitatively the same trend with respect

to temperature and R.H., and very similar permeability values.

The permeability in Nafion and Aquivion, indeed, increases

significantly for all gases inspected, as temperature and R.H.

increase.

In particular, water activity has a remarkable influence on

the gas transport properties, and permeability values increase

up to two orders of magnitude with respect to those obtained

in dry conditions. The effect of R.H. on the membrane trans-

port properties is clearly non linear: permeability values have

a sharp increase, approximately one order of magnitude,

when water activity is raised from 0 to 5%, and then show an

exponential trend in the range between 10% and 80%. Inter-

estingly, the permeability at the higher water activities seems

to lead toward values comparable with those evaluated in

pure water films, as the product of gas diffusivity and solu-

bility in liquid water [38,39]; the latter values are also included

in Fig. 4 as a reference.

The sharp increase in permeability at the very low water

activity (up to 5%), corresponding to water content in the

polymer of approximately 3% on a mass basis, is likely

related to significant changes in the PFSI matrix, as also

observed by other authors from different points of view

[40e44]. Indeed, quite interesting analogies are observed with

the experimental behavior of proton conductivity with R.H.,

as reported by Yang et al. [43], who showed a sharp increase

in the conductivity value in the range 0e5% R.H. and an

exponential rise between approximately 20% and 80%, in a

fashion very similar to the permeability trends illustrated in

Fig. 4. On the other hand, Majsztrik et al. [40] and Zhao and

Benziger [41] showed that the mechanical and viscoelastic

behaviors of Nafion obtained by stress strain measurements

are also strongly influenced by R.H. and temperature.

Furthermore, Rivin et al. [42] carried out sorption and

permeation experiment of water in Nafion, revealing the

large effect of the water content in the membrane on the

transport parameters (i.e. penetrant diffusivity and

Fig. 4 e Permeability of He (a), N2 (b), O2 (c) and CO2 (d) in humid Nafion and Aquivion membranes at 25, 35 and 50 �C. Data

points with crossed symbols (at 35 �C for He, N2 and O2) were retrieved from Catalano et al. [12].

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 1 9 7 3e1 1 9 8 2 11977

permeability), with a significant increase in diffusivity in the

activity range between 0 and 5%.

Several authors [4e15] have already analyzed and re-

ported the permeability behavior of different gases in hu-

midified Nafion membranes. In particular, the data obtained

in this study for oxygen and carbon dioxide permeability in

Nafion 117 can be compared with the permeation results

reported in the open literature [4e15] for the same material,

or in similar PFSI membranes, such as Nafion 111 or NRE 212,

having the same equivalent weight of the materials here

inspected.

In the case of oxygen, a number of experimental works can

be considered for comparison, although the data reported

show a very large variability, as often observed for transport

properties in PSFIs. Sakai et al. [7,8], for instance, measured

oxygen permeability in Nafion 117 at different temperatures

and humidities, reporting at 50%R.H. values of about 25 and 45

Barrer at 30 and 50 �C, respectively. On the other hand, Gode

et al. [13] reported O2 permeabilities in Nafion 117 at 25 and

60 �C that arewell below that range, not exceeding the value of

20 Barrer obtained at 60 �C for a water activity of 75%. In the

present work, the oxygen permeability in Nafion membranes

at 50% R.H. increases from 8 to 19 Barrer, when the tempera-

ture is raised from 25 to 50 �C. Therefore, the experimental

results obtained in this work are definitely lower than those

reported by Sakai et al. [8], but are in good agreement with the

work by Gode et al. [13], as well as with those byMa et al. [9] for

Nafion 111, andMohamed et al. [10] for NRE212 at 25 and 30 �C,respectively; the latter are not explicitly considered here for

the sake of brevity.

Carbon dioxide permeability in Nafion 111 at 25 �C is also

reported in Ref. [9] showing an increase from 66 to 260

Barrer, when R.H. is increased from 52% to 100%. Once again,

these results are somewhat lower, but still in reasonable

agreement with the experimental data obtained in this work,

in which permeability values increase from 90 up to 160

Barrer, as the humidity increases from 50 to 75% at 25 �C.Concerning Aquivion, no other studies on humid gas

permeability have been reported, and a direct comparison

with previous data is not possible. However, it is reasonable to

expect approximately the same behavior observed for Nafion,

due to the very similar structure of the twomaterials, and also

in view of the results obtained in dry conditions.

The differences between permeability values in Nafion and

Aquivion are more pronounced in the presence of water than

in dry conditions. Indeed, relative deviations among the

permeability valuesmeasured in the two PSFIs are often above

20%, with also peaks up to 70%. In particular, at the lower

temperatures (25 and 35 �C) and at low activity, Aquivion

presents higher permeability than Nafion for all the gases but

helium, for which the two materials show very similar values

and behaviors. At high temperature (50 �C) and high R.H., on

the other hand, Nafion shows higher permeabilities with

respect to Aquivion, for all the gases inspected.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 1 9 7 3e1 1 9 8 211978

Thesedifferences between the two PFSIs in the behaviors of

permeability with water activity are likely related to the

different water contents in the twomembranes. Experimental

measurements clearly indicate that, on a weight basis, water

solubility in Aquivion is higher than in Nafion, due to its lower

equivalent weight [12,35]. Hence, at a given activity, the larger

water content inAquivion, enhances the gas permeability and,

consequently, higher P values are detected. Gas transport in

humid PSFI is known to occur preferentially in the hydrophilic

domains that are present in the material [44e48], and the

higher is thewater uptake, the higher is the volume fraction of

the hydrophilic phase, and consequently the gas permeability.

The difference in equivalentweight, and the corresponding

water solubility, plays a significant role at lowactivity, because

the hydrophilic domains are still rather small and not inter-

connected. At medium and high R.H., the water absorbed in

the polymer produces larger and more interconnected water-

like regions, and differences between the two materials are

less important, since continuous hydrated pathways in the

membrane guarantee high permeation rate in both cases. In

these conditions, therefore, other factors are responsible of the

larger gas permeabilities observed in Nafion with respect to

Aquivion, such as differences in crystallinity, which is gener-

ally lower in Nafion, or in the mobility of perflurosulfonated

side chains, which are longer and consequently more flexible

in Nafion than in Aquivion.

Fig. 5 e N2 permeability in Aquivion and Nafion

membranes hydrated at 5, 15, 30 and 70% R.H.: effect of

temperature.

3.3. Activation energy

The above qualitative analysis offers a rationale for the

description of experimental behaviors at low temperatures. At

higher temperatures, however, gas permeability in Nafion is

higher than in Aquivion at all R.H. inspected, indicating the

need to complete the analysis of the temperature effect on

permeation; such temperature effect seems more significant

for Nafion than for Aquivion.

A quantitative determination of the temperature effect is

often provided by the activation energy of permeation, EP. In

dry permeability, a significant difference between the two

polymer was found for helium and nitrogen, for which EP was

20% larger in Nafion than in Aquivion, whereas oxygen and

carbondioxidepresentedapproximately thesametemperature

behavior in the twomaterials (EP differencesbetween3and5%).

A similar analysis can be carried out in humid conditions,

through an effective activation energy of humid gas perme-

ability of species i, EeffP;i , defined as:

EeffP;i

R¼ �

�dlnPi

dð1=TÞ�

RH

(2)

The activation energy thus defined is rigorously an effective

quantity, since it is calculated at constant R.H. in the external

phase rather than at constant composition in the polymer,

although minor differences are expected between the two

cases. Indeed, EeffP;i provides a rather accurate estimation of the

real activation energies in view of the following

considerations:

i) the gas concentration in the membrane is actually

negligibly small due to the low pressures considered

during experiments, never exceeding 2.5 bar on the up-

stream side of the membrane, and almost zero in the

downstream compartment;

ii) in the activity range inspected, water solubility in PSFI

shows a slight dependence on temperature; conse-

quently, at each temperature value, the same water

content is found in the membrane at any given activity.

It has been clearly shown that water solubility in Aqui-

vion is basically constant with temperature, whenever

the samples have been fully dried by a proper thermal

pretreatment [35]. Similarly, the analysis of several

different experimental data in Nafion [42,49e55] showed

that a clear trend with temperature can hardly be found,

and most likely the temperature dependence of water

solubility at a given R.H. is very weak. In fact, within the

fewworks specifically devoted to the investigation of the

temperature effect on water solubility in Nafion, con-

tradictory results were obtained. The comparison

among solubility isotherms from different experimental

works is not feasible for Nafion, because a significant

scatter of data is observed, likely associated to the

different thermal pretreatments of the membranes and

to the different experimental conditions. The pretreat-

ment of the polymer membranes seems to be the pri-

mary source of the observed differences, as it generally

covers any possible temperature dependence of water

solubility in Nafion.

The present experimental results are consistent with that

interpretation. In fact, by reporting gas permeability at constant

R.H. versus reciprocal absolute temperature for both PFSIs, a

clear Arrhenius type behavior is observed, confirming the val-

idity of the proposed assumption. In Fig. 5, Arrhenius plot for N2

permeability inAquivion andNafion at different R.H. is reported

as an example; similar behaviors are obtained also for the other

gases here investigated, not explicitly reported for the sake of

brevity (R2 values always higher than 0.95).

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 1 9 7 3e1 1 9 8 2 11979

The effective activation energies EeffP are calculated as

illustrated above, for both materials and for all gases inspec-

ted as a function of R.H.; their values are reported in Fig. 6a for

He and CO2, and in Fig. 6b for N2 and O2. The results show that,

in all conditions, higher values are obtained for Nafion than

for Aquivion, and the larger differences are observed for lower

water contents. Indeed, at low values of R.H., Aquivion data

show a sharp decrease of the effective activation energy with

respect to the corresponding value in dry conditions, whereas,

the trend in Nafion membranes presents much smoother

behaviors.

For both PFSIs, the activation energy ranges between the

value obtained in the dry state and the value obtained for the

gas permeability in pure water, which is also included in

Fig. 6a and b for the sake of completeness. The pure water

values, calculated from solubility and diffusivity data avail-

able [38,39], are lower than the dry polymer activation energy

for all gases but helium. Therefore, with the only exception of

helium, the increase of R.H. lowers the gas activation energy,

EeffP , toward the values typical of the permeation in pure liquid

water, with rather sharp variations in Aquivion and smoother

changes in Nafion. At low R.H., indeed, sharp changes in EeffP in

Nafion are observed only in the case of carbon dioxide, while

for other gases a smoother decrease is found.

TheEeffP values in the twopolymers are approximately equal

for all gases at about 50% R.H.. Fig. 6a and b, indeed, clearly

illustrate that the differences between the two polymers tend

to disappear at high activity, and the EP values approach the

value of activation energy in purewater. This seems to confirm

that permeation inhumid PFSI ismainly drivenby gas sorption

and diffusion in the hydrophilic regions of the materials, and

ultimately in the water channels formed at high R.H. in the

hydratedmatrix. These water-like domains are very similar in

the two materials, and, consequently, gas probes can diffuse

basically in the same environment. At high water activity, the

slight differences observed in permeability are likely related to

the hydrated polymer microstructure, as for instance the

channel tortuosity, which could be different in the two mate-

rials. However, the structure of the water-like domains in the

hydrated polymers are not expected to change significantly in

the range 25e50 �C, consequently not altering the values ofEeffP .

Fig. 6 e Effective activation energy of permeation of different gas

humidity: a) helium and carbon dioxide; b) nitrogen and oxyge

3.4. Selectivity

The presence of water in the membranes and in the gaseous

stream affects the transport properties of each penetrant in a

different way. Gas permeability increases for all gases with

increasing R.H., but the behaviors of the different penetrants

are quantitatively different. Among the penetrants consid-

ered, carbon dioxide presents the largest increases in

permeability followed by nitrogen, oxygen and helium, whose

values are only slightly affected by the presence of water. In

particular, while the permeability of CO2 increases of about 2

orders of magnitude in the range 0e80% R.H., the permeability

of He increases only five times.

In dry PFSIs, the permeability is directly related to the ki-

netic diameter of the gaseous species, with the smaller

molecule, He, which is the fastest diffusing in the polymeric

matrix. On the contrary, in humid conditions, gas solubility

may become the controlling factor, and themost condensable

penetrant and most water soluble, CO2, becomes the fastest

permeating species, already at 30% R.H.. Indeed, the perme-

ability behaviors of the other penetrants, which have a much

lower condensability, are less affected by the formation of the

hydrophilic phase in the PSFIs.

This different behavior is clearly illustrated by evaluating

the CO2/gas selectivity of the two membranes at different

water activities. The gas selectivity toward gas i with respect

to gas j, ai,j, is a key parameter for gas separation applications,

and it is defined as follows:

ai;j ¼ Pi

Pj(3)

According to its definition, the gas selectivity should be

calculated from the permeabilities of single penetrants of the

mixed gas streams, although in most of the cases it is esti-

mated from pure gas permeation measurements only, for

simplicity sake. In this work, the selectivity is evaluated from

the measurement of single gases (with or without water

vapor), neglecting thus the effect of the gas/gas interactions.

The narrow pressure range inspected (up to 2.5 bar) corre-

sponds to limited gas solubilities in the polymers, and the

effect of the presence of large portions of water-like domains

es in Nafion and Aquivion membranes at different relative

n.

Fig. 7 e Ideal gas selectivity as function of water humidity for the gas pair CO2/He, CO2/O2, CO2/N2 at different temperatures:

a) 25 �C; b) 35 �C; c) 50 �C.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 1 9 7 3e1 1 9 8 211980

on the permeability of the different penetrants is likely

predominant.

The selectivity aCO2,j of CO2 over the other gases is then

reported in Fig. 7aec, at the three temperatures inspected, as

function of water activity. Although some of the data calcu-

lated for aCO2,j appear quite scattered, it is possible to observe

rather clear trends of gas selectivity versus R.H.. Indeed, in all

cases, CO2 selectivity increases rather continuously with

water content, and inmost of the cases, it reaches amaximum

at the highest activity inspected, leading to values that, once

again, are close to those obtained for pure water. Only

exception seems to be the CO2/N2 and CO2/O2 selectivity in

Nafion at low temperature, in which a decrease in aCO2,j is

observed when the R.H. is increased above 50% (above 30% in

the case of nitrogen at 25 �C). As a consequence, at the higher

activities, Aquivion presents larger selectivity values with

respect to Nafion.

When water is absorbed in the polymer, therefore, the two

PSFIs seem to act as supported liquid membranes, and due to

its high solubility in water, CO2 permeation in the hydrophilic

channels is extremely fast with respect to other gases.

It is noteworthy that perfluorosuphonated materials

have been extensively investigated also in the field of

membranes for gas separation, in view of their ability to be

selectively permeable to carbon dioxide [5,31,32]. In the case

of the two materials here studied, Aquivion showed a lower

CO2 permeability at high R.H., but the best separation

performance.

4. Conclusions

The permeability of He, O2, N2 and CO2 in two PSFIs, Aquivion

and Nafion 117, has been investigated as a function of tem-

perature and R.H.. In particular, temperatures ranging from 25

to 65 �C and R.H. up to 80% were explored. The two materials

showed quite similar behaviors for permeability trends and

values.

Humidity has a significant effect on gas permeability, with

increments up to two orders of magnitude with respect to

values observed in the dry gas conditions, and a substantial

change of the gas transport behavior through themembranes.

Indeed, indry conditions, thePSFIsbehaveasunswollenglassy

polymers, the permeation mechanism is mainly controlled by

diffusion, and permeability values scale with the kinetic

diameter of the diffusingmolecules, with Hemore permeable,

followed in sequence by CO2, O2 and N2. Conversely, the pres-

ence of water vapor, able to swell the hydrophilic matrix, pro-

duces water-like domains characterized by rather high gas

permeability. Therefore, gas permeability largely increases up

to values close to those encountered for the permeation in

liquid water, and shows a behavior completely different from

that of the dry materials. At R.H. larger than approximately

30%, the highest permeabilities are observed for CO2, followed

by He, O2, and N2. Hence, the permeation process is closely

related to the solubility of the different gases in water, rather

than to their molecular sizes.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 1 9 7 3e1 1 9 8 2 11981

The comparison between the two materials, shows higher

gas permeabilities in Aquivion than in Nafion 117 at low

water activity and at low temperatures, while at high activity

and especially at the higher temperatures, the opposite

behavior is observed, and gas permeabilities are larger in

Nafion than in Aquivion. The differences between the two

materials, however, rarely exceed 20%, consistently with the

very similar chemical structures of the two PSFIs.

The permeability dependence on temperature is well

described by an Arrhenius relationship, which holds in both

materials in dry as well as in humid conditions. Activation

energies are often higher for Nafion than for Aquivion, with

differences that decrease and seem to vanish when the water

content increases. The activation energy is itself a function of

R.H. and changes with increasing water content, toward the

values typically encountered in pure water.

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