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Journal of
Electroanalytical
Chemistry
Journal of Electroanalytical Chemistry 616 (2008) 117–121
www.elsevier.com/locate/jelechem
Short Communication
Swelling of Nafion in methanol–water–inorganic salt ternary mixtures
Alena Randová a, Štěpán Hovorka a,*, Pavel Izák b, Lidmila Bartovská a
a
Department of Physical Chemistry, Institute of Chemical Technology, Technická 5, 166 28 Prague 6 Dejvice, Czech Republic
b
Institute of Chemical Process Fundamentals, Rozvojová 135, 165 02 Prague 6, Czech Republic
Received 27 August 2007; received in revised form 13 December 2007; accepted 24 December 2007
Available online 11 January 2008
Abstract
The anisotropic swelling of Nafion 112 foil in methanol–water–inorganic salt was examined by the optical method. Our results show
that even small addition of inorganic salt into methanol–water mixture affects kinetics as well as the swelling equilibrium. The effect of
inorganic salts LiCl, NaCl, KCl, CsCl, CaCl2, CdCl2, K2CO3, KNO3, NH4Cl, and AgNO3 was studied. Swelling kinetics of Nafion in
ternary mixtures including salt show maximum suggesting that in the beginning stage of swelling the diffusion of methanol is faster than
ion transport. The experimental data, presented in this work, allow us to assume that swelling of Nafion decreases with increasing ionic
radius of cation.
Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Nafion; Swelling; Methanol; Water; Inorganic salt
1. Introduction
Perfluorosulfonate ionomer membranes are very interesting materials with many attractive commercial applications. These membranes are materials of considerable
commercial significance because of their use as solid polymer electrolytes in fuel cells and various other applications
in electrochemistry and separation technologies. It is usually believed that the useful properties of these materials
are a result of their structure, which is known to be heterogeneous on a microscopic grade. The contrast between the
hydrophobic organic skeleton and hydrophilic ionic side
chains ended by sulfonate groups gives to the perfluorosulfonate ionomer membranes their unusual properties.
This study is specialized in Nafion, a poly(tetrafluoroethylene) (PTFE) polymer with perfluorovinyl pendant side
chains ended by sulfonic acid groups. The PTFE backbone
guarantees a great chemical stability in both reducing and
oxidizing environments. The sulfonic exchange group on
*
Corresponding author. Tel.: +420 220 444 163; fax: +420 220 444 333.
E-mail addresses: [email protected] (A. Randová), Stepan.
[email protected] (Š. Hovorka), [email protected] (P. Izák), Lidmila.
[email protected] (L. Bartovská).
0022-0728/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jelechem.2007.12.018
the side chains have very high acidity. The primary structural formula of Nafion monomer is shown in Fig. 1
[1,2]. In many applications Nafion is immersed in liquid,
which significantly affects the membrane properties, e.g.
conductivity, transport selectivity, density and membrane
mechanical strength [3–5]. Such changes of these properties
have strong influence on processes efficiency. Since Nafion
has wide use in the field of pervaporation separation processes, properties of Nafion in pure organic solvents and
their mixtures have been already studied. In view of its
use in fuel cells (namely DMFCs – direct methanol fuel
cells) the behavior of different cationic forms of Nafion in
alcohol–water mixtures [6–17] or original H+ form of Nafion in ternary liquid alcohol–water–inorganic salt mixtures
[18–22] has been subject of interest. As one of often
encountered problems in praxis is the change of membrane
dimensions, this work presents the results of measurement
of anisotropic swelling of Nafion in liquid methanol–
water–inorganic salt mixtures as a contribution to this kind
of research.
With respect to the intended pervaporation experiments,
demanding low resistance to mass transport, Nafion 112 as
the thinnest one of the available types, was used in our
studies.
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A. Randová et al. / Journal of Electroanalytical Chemistry 616 (2008) 117–121
Fig. 1. The structural formula of Nafion. The index m is usually equal
unity, so that the value of n determines the ratio of polar to nonpolar
material in the membrane and varies from 5 to 11.
2. Experimental
2.1. Materials
NafionÒ 112 membrane (Fig. 1), produced by DuPont,
USA, was used as received, because it was necessary to
have the membrane in equilibrium state at the beginning
of each experiment. When the membrane was pre-treated
by boiling in 10 wt.% H2SO4 (1 h), then in deionized water
(1 h) and subsequently dried (48 h), the problem was how
to prevent the contamination of membrane during the
preparation of experiment. Considering that fuel cells are
not typically kept in gas-tight boxes, using Nafion without
pre-treatment simulates better conditions at its real applications. Moreover, the pre-treatment leads to the distortion of the membrane. This fact is an unpleasant
complication for the swelling measurements by the optical
method employed in these experiments.
According to the manufacturer the membrane Nafion
112 has nominal thickness 51 lm, density 2000 kg m3,
ionic conductivity 8.3 S m1, and acid capacity 0.89
mequiv g1. Experiments were performed with pure water,
pure methanol, binary mixtures methanol–water with
methanol content of 25, 50, and 75 mol.%, and with ternary mixtures methanol–water–inorganic salt (LiCl, NaCl,
KCl, CsCl, CaCl2, CdCl2, K2CO3, KNO3, NH4Cl, and
AgNO3). Pro-analysis grade chemicals and distilled water
were used in all experiments. Swelling experiments were
performed at salt concentration of 0.362 mol.%. This value,
corresponding to the concentration of 0.2 mol dm3 KCl in
pure water, was chosen as convenient one lying in the
region where the swelling degree does not change with
the amount of the salt present in the system.
2.2. Experimental setup
A modification of the optical apparatus, described in
one of our previous papers [23], was used to measure the
change of two dimensions of a flat membrane sample (in
the drawing direction – A, and in the perpendicular direction – B) in liquid mixtures with time. A cold illuminator
was used to light the sample placed in a thermostated vessel, covered with a glass photographic plate to prevent the
evaporation of the liquid. All the experiments were carried
out at 25 °C and atmospheric pressure. A square membrane sample sides were cut off parallely with the edges
of a sheet supplied by manufacturer. The images of a
square 5 mm 5 mm fixed on a spike in the circle Teflon
cell, taken by a digital camera Olympus Camedia 5050,
connected with a computer, were processed using program
QuickPHOTO MICRO. Value of ±1.5 in A and/or B was
determined to be the experimental error.
3. Results and discussion
Typical examples of Nafion swelling kinetics are shown
in the Fig. 2. The curves representing Nafion swelling in
methanol–water mixtures (Fig. 2a) as well as in pure water
(Fig 2b) show monotonous increase of membrane dimensions. The swelling curves obtained in the presence of inorganic salt, however, pass through a maximum before the
dimension reaches its equilibrium value. Comparison of
Figs. 2a and b demonstrates that the maximum is more
pronounced at higher methanol contents. This suggests
Fig. 2. The example of Nafion swelling kinetics in (a) methanol–water (molar fraction xCH3 OH = 0.75), (b) pure water. Lines are added only to connect the
experimental points.
A. Randová et al. / Journal of Electroanalytical Chemistry 616 (2008) 117–121
that short initial stages are more influenced by faster methanol transport whereas the later ones are affected by ion
transport. Practically in all cases the Nafion membrane
exhibits anisotropic swelling; the origin of this effect can
be attributed to the orientation of polymer chains toward
the drawing direction. When the direction of cutting of
the square sample is rotated by 45°, the swelling is
isotropic.
Our results for Nafion swelling can be compared with
the data of Gebel et al. [8] on Nafion expansion in pure
methanol and in pure water. Whereas in B-direction our
data are in good agreement with the values of Gebel
(53% vs. 51% and 12.8% vs. 14%, respectively), the agreement for dimension A is not so good (22.8% vs. 36% and
5.6% vs. 10%, respectively). This discrepancy may be
assigned to the differences in experimental conditions;
whereas Gebel et al. prepared Li+ form of Nafion 117 prior
to swelling experiments in pure methanol or pure water, in
this work the Nafion 112 membrane was used as received
to swell in binary mixtures methanol–LiCl or water–LiCl.
Since Nafion is a cation exchange membrane, it could be
supposed that anions will affect its swelling less than do the
cations. How can be seen from Fig. 3, our experiments confirmed this presumption. Curves, shown in Fig. 3, representing the dependence of equilibrium relative expansion
on molar fraction of methanol in the mixture, xCH3 OH , for
three salts of potassium, are very close.
Fig. 4 represents the dependence of the relative expansion on KCl contents (molar per cent) in the mixtures
25 mol.% of methanol + 75 mol.% of water. It can be seen,
that up to approximately 0.05 mol.% of KCl the extent of
swelling of Nafion membrane rapidly decreases. Above this
concentration the swelling degree does not change with
KCl contents.
Fig. 5 presents the results of relative extension measurements at Nafion swelling in liquid mixtures methanol–
water–alkali metal cation in comparison with the swelling
in mere methanol–water mixtures. It is evident that the
119
Fig. 4. The influence of the salt concentration on Nafion swelling in
methanol–water–potassium chloride mixtures. Lines are added only to
connect the experimental points.
presence of alkali metal ion diminishes the extent of swelling with one exception. Li+ ion has no influence on membrane swelling in both directions (see Fig. 5). The
influence of ion increases with rising ionic radius. Fig. 6
compares the results for cations of approximately the same
ionic radius. It could be seen that the curves of equilibrium
relative expansion are very close, although the atomic
weights are different. The ionic radii (in pm) are: H+ 30,
Li+ 69, Cd2+ 95, Ca2+ 100, Na+ 102, Ag+ 115, K+ 138,
NH4+ 148, Cs+ 170 [24]. Similarity of results for H+ and
Li+ forms of Nafion despite of a large difference between
ionic radiuses can be explained by the fact that H+ forms
larger H3O+ ion in aqueous solutions of methanol.
The swelling of Nafion was measured also in mixtures of
methanol–water–NH4Cl (Fig. 7). The kinetic curves are
similar as with other salts, but the swelling is nearly isotropic (the difference between the equilibrium values A and B
is very small). A possible explanation can be found in the
fact that NH4Cl is a salt of a strong acid and a weak basis
and NHþ
4 hydrolyzes.
Fig. 3. The effect of anions on Nafion swelling in methanol–water–0.362 mol.% potassium inorganic salt mixtures for the drawing direction (A) and in the
perpendicular direction (B). Lines are added only to connect the experimental points.
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A. Randová et al. / Journal of Electroanalytical Chemistry 616 (2008) 117–121
Fig. 5. The anisotropic swelling of Nafion in mixtures methanol–water–0.362 mol.% alkali metal chloride in the drawing direction (A) and in the
perpendicular direction (B). Lines are added only to connect the experimental points.
Fig. 6. The anisotropic swelling of Nafion in mixtures methanol–water–0.362 mol.% inorganic salts with cations of approximately same ionic radius in the
drawing direction (A) and in the perpendicular direction (B). Lines are added only to connect the experimental points.
Fig. 7. The swelling of Nafion in mixtures of methanol–water–
0.362 mol.% NH4Cl in the drawing direction (A) and in the perpendicular
direction (B) in comparison with its swelling in mixtures of methanol–
water. Lines are added only to connect the experimental points.
Table 1 presents the series of various ions arranged
according to their effect on Nafion properties. The series,
presented in this work, is in a good agreement with other
authors.
The difference of chemical potential, which induces
osmotic pressure, is driving force of swelling after immersion of Nafion into a liquid [28] and results into a different
composition from composition of bulk of liquid solution. If
the obtained results are supplemented by data about chemical potential of water, methanol and salt in bulk of liquid
solution that is in equilibrium with immersed Nafion membrane, it could be considered to calculate activity of species
and composition inside of the Nafion using a model conception of Nafion. However it is not easy task and it is
not possible with our data only, because we found anisotropic behavior of Nafion and methods for calculations
of chemical potentials in the membrane from the bulk data
(e.g. model of Choi et al. [29,30] based on the Flory–Huggins theory) assume isotropic properties.
A. Randová et al. / Journal of Electroanalytical Chemistry 616 (2008) 117–121
121
Table 1
The effect of cation on Nafion properties
Author
Nafion property
This work
Relative expansion
Nandan et al. [6]
Young et al. [10]
Suresh et al. [13]
Xie and Okada [25]
Solvent uptake
(gravimetric analyses)
Water sorption
(gravimetric analyses)
Cation selectivity
(evalueted from literature data)
Absorbed water
(gravimetric analyses)
Solvent uptake (SANS)
Water uptake
Water content
Okada et al. [26]
Steck et al. [27]
Water content
Water content
Pushpa et al. [7]
Bontha et al. [11]
Cabasso et al. [9]
a
b
In water
+
+
In mixtures alcohol-water
2+
2+
+
Li < H < Cd Ca Na
< NH4+ Ag+ < K+ Cs+
H+ < Li+ < Mg2+ < Sr2+ < Ba2+ < Na+
< K+ < Rb+
H+ < Li+ < N(CH3)4+ < Cs+
Li+ H+ < Cd2+ Ca2+ Na+ NH4+
Ag+ < K+ Cs+a
Li+ < H+ < Na+ < Mg2+ < Ca2+ < Sr2+
< Ba2+ K+ < Rb+a
Ag+ < Li+ < H+ < Na+ < Mg2+ < K+
< Ba2+ < Rb+ < Cs+
H+ < Li+ < Na+ < K+ Cs+
Mg2+ < Zn2+ < Ca2+ < Al3+ < H+ < K+
Cu2+ < Eu3+ < Cs+
Li+ < Mg2+ Ca2+ < Na+ < Sr2+ < Ba2+
< K+ < Rb+ < Cs+
Li+ < Na+ < K+ < Rb+ < Cs+
H+ Li+ < Co2+ < Zn2+ Mg2+ < Na+ < Ag+
Ca2+ < Sr2+ < Ba2+ < K+ < Rb+ Cs+ Tl+
H+ < Zn2+ < Mg2+ < Al3+ < Ca2+ < K+b
Methanol.
Ethanol.
4. Conclusions
The experimental data demonstrate that even a very
small amount of inorganic salt present in methanol–water
mixture is sufficient to cause a restriction of Nafion swelling; in several cases it may even stop the swelling process
at all. Moreover our data confirm that the influence of cation increases with rising ion radius. Major advantage of the
optical method, used in this study, lies in possibility to
study also the anisotropic phenomena and time dependence of swelling.
Acknowledgements
This research was supported partially by Marie Curie
Intra-European and Marie Curie Reintegration Fellowships within the 6th European Community Framework
Programme. P. Izák is grateful also to Purkyne Fellowship
from Academy of Science of the Czech Republic and coauthors from Institute of Chemical Technology acknowledge the financial support from the Grant from Ministry
of Education of the Czech Republic (MSM 6046137307).
The authors would like to thank to the Prof. J.G. Crespo from the Department of Chemistry of the Universidade
Nova de Lisboa, Portugal, for helpful discussions.
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