|Título/s:||Characterization of an anionic-exchange membranes for direct methanol alkaline fuel cells|
|Autor/es:||Abuin, Graciela C.; Nonjola, Patrick; Franceschini, Esteban A.; Izraelevitch, Federico H.; Mathe, Mkhulu K.; Corti, Horacio R.; Corti, H.|
|Institución:||INTI-Procesos Superficiales. Buenos Aires, AR |
Council for Scientific & Industrial Research. CSIR. Pretoria, ZA
Departamento de Física de la Materia Condensada, Comisión Nacional de Energía Atómica. CNEA. Buenos Aires, AR
|Palabras clave:||Intercambio de aniones; Membranas; Polímeros termoplásticos; Celdas de combustible; Alcohol metílico; Propiedades mecánicas; Conductividad eléctrica|
| Ver+/- |
n A. Franceschini c, Federico H. Izraelevitch c,
a Centro de Procesos Superficiales, Instituto Naciona
Buenos Aires, Argentina
b Council for Scientific & Industrial Research (CSIR)
c Departamento de Fı´sica de la Materia Conde
portable applications, the emphasis has been focused on
lower temperature types (<80 C) including direct alcohol
proton exchange membrane fuel cells (DAPEMFCs) and direct
alcohol alkaline fuel cells (DAAFCs). These types of devices,
for example methanol feed DMFCs, are of primary interest in
extensively studied in fuel cell applications [1,2]. Despite their
advantages of high conductivity and good mechanical and
chemical properties, certain disadvantages exist that restrict
their successful use in fuel cells. These drawbacks include
high cost, high methanol permeability and relative low
* Corresponding author. Fax: þ5411 6772 7121.
Avai lab le a t www.sc iencedi rec t .com
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 e n e r g y 3 5 ( 2 0 1 0 ) 5 8 4 9 – 5 8 5 4
E-mail address: email@example.com (H.R. Corti).
Fuel cell technologies are at the forefront of the effort towards
green and sustainable energy generation. For mobile and
the field of portable devices due to ease and speed of refuelling
and large volumetric energy density of liquid methanol fuel.
Most PEMFCs are currently based on perfluorosulphonic
acid polymers such as Nafion and Flemion, which have been
Alkaline fuel cell
uptakes much less methanol as compared to Nafion. The specific conductivity of the fully
hydrated polysulfone membrane equilibrated with KOH solutions at ambient temperature
increases with the KOH concentration, reaching a maximum of 0.083 S cm1 for 2 M KOH,
slightly less conductive than Nafion 117. The elastic modulus of the polysulfone
membranes inmersed in water is similar to that reported for Nafion membranes under the
same conditions. We concluded that quaternized polysulfone membrane are good candi-
dates as electrolytes in alkaline direct methanol fuel cells.
ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
San Martı´n, Buenos Aires, Argentina
a r t i c l e i n f o
Received 4 December 2009
Accepted 17 December 2009
Available online 13 January 2010
0360-3199/$ – see front matter ª 2010 Professo
l de Tecnologı´a Industrial (INTI), Av. Gral. Paz 5445, B1650KNA, San Martı´n,
, Material Science & Manufacturing, PO Box 395, Brumeria, Pretoria 0001, South Africa
nsada, Comisio´n Nacional de Energı´a Ato´mica (CNEA), Av. Gral. Paz 1499, B1650KNA,
a b s t r a c t
Ammonium quaternized polymers such as poly (arylene ether sulfones) are being devel-
oped and studied as candidates of ionomeric materials for application in alkaline fuel cells,
due to their low cost and promising electrochemical properties. In this work, a quaternary
ammonium polymer was synthesized by chloromethylation of a commercial polysulfone
followed by amination process.
Quaternized polysulfone membrane properties such us water and water-methanol
uptake, electrical conductivity and Young’s modulus were evaluated and compared to
Nafion 117, commonly employed in direct methanol fuel cells. The anionic polysulfone
membrane sorbs more water than Nafion all over the whole range of water activities, but it
Mkhulu K. Mathe b, Horaci
. Corti c,*
Graciela C. Abuin a, Patrick Nonjola b, Esteba
Characterization of an anionic
direct methanol alkaline fuel cell
j ourna l homepage : ww
r T. Nejat Veziroglu. Publishe
xchange membranes for
e lsev ier . com/ loca te /he
d by Elsevier Ltd. All rights reserved.
dried at 70 C for 24 h. Amination with 25 wt% trimethylamine
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 e n e r g y 3 5 ( 2 0 1 0 ) 5 8 4 9 – 5 8 5 45850
activity when used in direct alcohol fuel cells (DAFCs) at
temperatures above 80 C [3–5].
The main drawback of current AFCs containing a liquid
electrolyte (aqueous KOH solution) is their sensitivity to
carbon dioxide pollution, which drastically reduces perfor-
mance . A solution to this obstacle could be the replacement
of the KOH solution by an anion conducting polymer electro-
On the other hand, in DAAFCs, OH anions are produced at
the cathode and transported through the membrane to the
anode where they are consumed. Due to the opposite move-
ment of the OH anions through the anion exchange
membrane (AEM), as compared to the transport of proton in
PEM and the direction of methanol flux, an intrinsic reduction
in methanol crossover is expected .
All these facts imply that there is an urgent need for the
development of novel AEMs that can be applied in fuel cells.
Novel methods of synthesis  have also allowed the
production of membrane materials and ionomers that could
facilitate the development of AEMs equivalent to proton
Fig. 1 – Preparation of quaternary ammonium polysulfone derivati
chloromethylated intermediates (2).
exchange membranes (PEMs). These polymeric membranes
are expected to have lower cost and comparable electro-
chemical properties than PEMs. Ion-exchange membranes
have been prepared using polysulfone, a high-performance
engineering thermo-plastic material as base polymer due to
its excellent workability and high mechanical strength .
Many kinds of AEMs have been developed based on qua-
ternized polymers such as polysiloxane containing a quater-
nary ammonium group , poly(oxyethylene) methacrylates
containing ammonium groups , quaternized poly-
ethersulfone cardo anion exchange membranes , quater-
nized poly(phthalazinone ethersulfone ketone) , and
radiation-grafted poly(vinylidene fluoride) (PVDF) and poly
(tetrafluoroethene-hexafluoropropylene) (FEP) [6,15].
AEMs containing a quaternary ammonium group as
a cation exhibit higher thermal and chemical stability than
other quaternary cations such as quaternary phosphonium or
tertiary sulfonium groups .
The aim of this paper is to report the preparation of a new
anion exchange membrane based on a quaternized poly-
sulfone and the characterization of its mechanical properties,
solution at 90 C overnight introduced quaternary ammonium
groups into the membrane, giving quaternary polysulfone
(QPSf) 3 with good yields. Before the membrane is used, it was
water and water-methanol uptake and electrical conductivity
in relation to its possible application in direct methanol
alkaline fuel cells.
All chemicals were purchased from Aldrich and used without
further purification. The quaternary ammonium polymer was
synthesized in two-step sequence [17,18] as depicted in Fig. 1.
Polysulfone (Udel) 1 was dissolved in chloroform and then
treated with chloromethyl ethyl ether in the presence of
Lewis-acid catalysts (SnCl4) at 35 C overnight to obtain the
chloromethylated (CMPSf) intermediate derivative 2. This was
followed by casting a 5 wt% solution of intermediate deriva-
tive 2 in N-methyl-pyrrolidone (NMP) onto a glass plate and
ves (3,4) from commercial Udel polysulfone (1) via
immersed in concentrated KOH solution overnight, to convert
the membrane from Cl form into OH form, and washed with
distilled water until neutral pH. The structure and purity of
the product was confirmed by 1H NMR and FTIR spectra .
The QPSf membranes used for electrical conductivity and
elastic modulus measurements were used as prepared, while
the Nafion 117 membranes were pretreated as described in the
literature . Thin QPSf and Nafion membranes for water and
methanol uptake were prepared by casting from 0.05 g/dm3
aqueous solutions of the respective polymers.
A quartz crystal microbalance (QCM) was used to measure
the water and water-methanol uptake of the QPSf
membranes. A thin QPSf film, in the range of 13–40 nm in
thickness, was prepared by direct casting on the quartz
crystal. As the membrane became rigid it was mounted on the
QCM holder contained within a sealed chamber with a dry
nitrogen stream, to obtain the mass of the dry film. Once the
equilibrium resonance frequency has been reached, the dry
nitrogen was replaced by a stream of nitrogen previously
bubbled across saturated aqueous salt solutions at 20 C, and
the change in the resonance frequency was measured. The
other properties of the membrane, under different operation
m ¼ ww/wo, the mass (in grams) of water uptaked per gram of
dry membrane, are shown in Fig. 3. It is observed that the water
uptake by QPSf membranes is higher than in Nafion over the
entire range of water activities considered. The effect of the
membrane thickness on the water sorption in extruded and
cast Nafion membranes is not clear. Some authors concluded
that the sorption of water in Nafion thin membranes is similar
to extruded membranes , while others [26,27] observed that
water or organic vapors uptakes on thin perfluorosulfonic acid
membranes prepared by casting are lower than those reported
for thicker commercial (extruded) membranes.
We observed that thin cast Nafion membranes (28 nm
thick), sorbed less water than commercial Nafion 117
membranes (178 mm thick) . However, the comparison of
water and methanol uptake by thin Nafion and QPSf
membranes is still valid and the measurement is much less
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 e n e r g y 3 5 ( 2 0 1 0 ) 5 8 4 9 – 5 8 5 4 5851
procedure was repeated using different saturated salt solu-
tions, in order to measure the water uptake at controlled
water activity, in the range from 0.33 to 1. Water-methanol
uptake was determinated by exposing the films to nitrogen
saturated with methanol aqueous solution in the range of
methanol concentration 0–100 wt%.
The water and methanol/water mixture uptake of Nafion
films were also measured for comparison.
The mass change when dry membranes are equilibrated
with water or methanol–water mixtures were calculated from
the measurement of the resonant frequency shift, Df, by
means of the Sauerbrey’s equation :
Df ¼ Dm 2nf
where n is the order of the harmonic, f0 the base frequency of
the crystal, A the electrode area and r and m the density and
shear modulus of the crystal, respectively.
The local elastic properties of QPSf and Nafion
membranes immersed in water were measured by means of
force spectroscopy using an atomic force microscope (AFM)
with a cell for liquids. The model developed by Stark et al.
, was used to obtain Young’s modulus from the linear part
of the force curves, which relates the loading force and
indentation depth when the AFM tip indents the membrane
A commercial AFM (Veeco-DI Multimode Nanoscope IIIa)
equipped with 150 mm lateral scan range and a 5 mm z-scanner
was employed for the measurements. The elasticity
measurements were done with a Si3N4 tip with a spring
constant of 0.46 N/m (Nano Devices, Veeco Metrology, Santa
Barbara, California, pyramidal tip shape, cone half angle
a ¼ 18, tip curvature radius r < 10 nm, resonant frequency
nominal: 57 kHz, measured: 47.50 kHz).
The indentation of an AFM tip fixed to a cantilever (spring
constant k) into a soft sample (Young’s modulus E, Poisson’s
ratio n) can be modeled using the Hertzian contact mechanics
. This theory provides a very simple but direct approach to
the material elasticity for a sample with a semi-infinite
thickness. The procedure to obtain the force curves has been
described in detail elsewhere .
The electrical conductivity measurements on the QPSf and
Nafion 117 membranes were carried out with a two point-
probe conductivity cell designed and constructed in our
laboratory, shown in Fig. 2. The two point probe conductivity
cell frame was made of two Teflon blocks, with an open
‘window-like structure’ employed to allow determinations of
conductivity of fully and partially hydrated membranes. Two
wire electrodes (1 cm apart) were used to apply current and
measure the voltage drop along the film. The membrane
sample (3 3 cm) was sandwiched between the two blocks
that were pressed together by four screws fastened with
approximately the same torque to ensure good electrode-
Impedance measurements were carried out in the frequency
range between 1 Hz and 1 MHz at open circuit potential with ac
voltage of 5 mV by means of Frequency Response Analyzer
(Autolab PGSTAT 302). The specific conductivity, s, was calcu-
lated according to the following relationship:
condition in a fuel cell. Moreover, the selective sorption of
methanol and water give us some insights into the perme-
ability of methanol through the membrane, which is
a fundamental parameter for application in direct methanol
AFC. For comparison, the water and methanol sorption of thin
films of Nafion were also determined.
The water sorption isotherms for QPSf and Nafion
membranes expose to the vapour phase at 20 C, expressed as
where l is the thickness of the membrane, A the membrane
surface area exposed to the electric field, and R is the bulk
resistance of the membrane sample obtained from the real
axis intercept of the impedance Nyquist plot. The membrane
samples were immersed in water for at least 24 h before
measurements were performed.
3. Results and discussion
3.1. Water and methanol uptake
The water and methanol uptake of the QPSF membranes is of
great importance, since the water content as a function of the
relative humidity determines the electrical conductivity and
Fig. 2 – Schematic view of conductivity cell for two-point
probe electrochemical impedance spectroscopy technique.
time consuming that the sorption experiments on thicker
A typical force curve in the approach direction for a 100 mm
0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Fig. 3 – Water sorption isotherms at 20 8C: (B) QPSf (40 nm);
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 e n e r g y 3 5 ( 2 0 1 0 ) 5 8 4 9 – 5 8 5 45852
The methanol/water uptake of QPSf membranes from
nitrogen equilibrated with methanol aqueous solutions at
20 C, expressed as m ¼ ws/wo, the mass (in grams) of solvent
mixture uptaked per gram of dry membrane, are shown in
Fig. 4 at methanol concentrations in the range 0–100 wt%.
It can be seen that the uptake of pure methanol by QPSf is
almost three times lower than by Nafion, while the sorption of
water by QPSf doubled that by Nafion. Therefore, the perme-
ability of methanol in QPSf would be lower than in Nafion,
provided that the diffusion coefficient of methanol in both
membranes is similar. Experiments are in progress in order to
confirm this conclusion.
The sorption behaviour of QPSf as a function of the water/
methanol composition is opposite to Nafion. The total sorp-
tion of water-methanol in QPSf, expressed as mass of sorbate
by mass of dry membrane, increases monotically with the
water content, and this trend is more pronounced when the
uptake is measured in moles of mixture per gram of
(,) Nafion (28 nm).
membrane. Thus, it is clear that water is preferently sorbed
over methanol by QPSf. In Nafion the total sorption increases
with methanol content in the equilibrating water-methanol
0 20 40 60 80 100
Fig. 4 – Methanol/water sorption isotherms at 20 8C: (B)
QPSf (40 nm); (,) Nafion (13 nm).
mixture, as already found in the literature [29,30]. However,
when expressed as moles of mixture per gram of dry
membrane, the uptake of pure methanol and pure water are
Also, the different water/methanol sorption characteristics
of Nafion and QPSf could impact on the low temperature
behavior of the membranes. Corti et al.  have demon-
strated that the freezing of water in a Nafion membrane in the
presence of methanol could increase up to 65%, probably as
a result of the exclusion of water out of the ionic cluster
regions. It is expected that this would not be the case in QPSf
membranes due to its sorption selectivity toward water.
3.2. Mechanical properties
-90 -80 -70 -60 -50 -40 -30 -20 -10 0
Piezo extend / nm
Fig. 5 – Indentation curves (solid line) on the QPSf
membrane surface. The dashed curve was obtained by
fitting the experimental data to the Stark model.
QPSf membrane is showed in Fig. 5, where it can be seen
an excellent agreement with the model proposed by Stark
et al. .
It can be noted that during the indentation does not appear
neither, signals of a structural breakdowns of the sample, nor
inelastic deformations, as revealed by the common slopes in
the extension and retraction curves. These facts indicate that
the samples have a purely elastic response to the interaction
with the tip.
The results for the Young modulus of QPSf and Nafion
membranes immersed in water are summarized in Table 1,
Table 1 – Young’s modulus of hydrated QPSf and Nafion
Membrane E (GPa)
QPSf 0.25 0.11
Nafion 117 0.29 0.08
Nafion 117a 0.09–0.25
a Ref. .
3.3. Conductivity measurements
membranes have promising barrier properties against meth-
membrane, comparable to that of Nafion. Therefore, it can be
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 e n e r g y 3 5 ( 2 0 1 0 ) 5 8 4 9 – 5 8 5 4 5853
Electrical conductivity measurements are used to corroborate
if the quaternized polysulfone are suitable for fuel cell appli-
cations. Using the impedance measurements by the way of
a Nyquist plot, the bulk resistance of the membrane samples,
R, were obtained and specific ionic conductivities were
calculated using equation (2). The results of conductivity
measurements at 20 C are summarized in Table 2. Fully
hydrated Nafion 117 membrane displays higher conductivity
(0.112 S cm1) than QPSf membranes equilibrated with water,
but increasing the KOH concentration increases the specific
conductivity of the membrane, with the maximum of
0.0833 S cm1 at a KOH concentration around 2.0 mol dm3.
Clearly, this is a consequence of the higher ionic conductivity
of the HO ions relative to chloride ion, and the maximum
along with data reported in literature for Nafion . Young’s
modulus obtained for Nafion immersed in water (0.29 GPa) is
significantly smaller than the obtained for the dry membrane
which is 1.59 GPa . This behavior can be explained by the
increases of the membrane elasticity upon swelling by sorp-
tion of water, as observed by Tang et al.  for Nafion on the
basis of tensile tests.
The elastic modulus of QPSf, as compared to Nafion, would
indicate that these membranes are mechanically feasible to
construct MEAs for AFC.
Recently, Huang and Xiao  have reported mechanical
properties of QPSf and benzoil guar gum blends. For pure
QPSf, they reported a tensile strength of 40.7 MPa and elon-
gation at break of 6.1%, although the Young modulus was not
Table 2 – Specific conductivities of QPSf and Nafion
Membrane cKOH (mol dm3) s (S cm1)
QPSf 0 (H2O) 0.0016
Nafion 117 H2O 0.112
conductivity would correspond to the complete replacement
of chloride ions by HO- in the quarternary sites. The decrease
of the conductivity at higher KOH concentrations is probably
due to the displacement of the ammonium group by HO- via
the nucleophilic displacement reaction, as postulated by Fang
and Shen  for quaternized poly(phthalazinon ether sulfone
The ratio of the specific conductivity of QPSf to Nafion
membranes is 0.74, at the optimum KOH concentration,
which is higher than the ratio of ionic conductivities of Hþ to
HO in water, which is 0.57 . This result would suggest
that the higher water content in the QPSf membranes play
a role to reduce the conductivity difference between the
alkaline polysulfone and the Nafion proton conducting
concluded that QPSf membrane prepared as in this work are
good candidate for MEAs preparation for DMFC.
Ionic conductivity as a function of temperature as well as
alcohol permeation measurements of QPSf membrane will be
addressed in a forthcoming study.
The authors acknowledge MINCyT/NRF Cooperation Program
RSA-Argentina (grant number 67370), ANPCyT (PICT 35403),
and Consejo Nacional de Investigaciones Cientı´ficas y Te´c –
nicas (PID 5977) for financial support. MM and PN thanks CSIR
(MSM) for support. HRC is a member of Consejo Nacional de
Investigaciones Cientı´ficas y Te´cnicas (CONICET). EF thanks
a fellowship by CONICET. FI thanks a fellowship by University
of Buenos Aires (UBA).
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