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Development and characterization of acid-doped polybenzimidazole/sulfonated
polysulfone blend polymer electrolytes for fuel cells
Hasiotis, C.; Li, Qingfeng; Deimede, V.; Kallitsis, J. K.; Kontoyannis, C. G.; Bjerrum, Niels
Published in:
Journal of The Electrochemical Society
Link to article, DOI:
10.1149/1.1366621
Publication date:
2001
Document Version
Publisher's PDF, also known as Version of record
Link back to DTU Orbit
Citation (APA):
Hasiotis, C., Li, Q., Deimede, V., Kallitsis, J. K., Kontoyannis, C. G., & Bjerrum, N. (2001). Development and
characterization of acid-doped polybenzimidazole/sulfonated polysulfone blend polymer electrolytes for fuel
cells. Journal of The Electrochemical Society, 148(5), A513-A519. https://doi.org/10.1149/1.1366621
Development and Characterization of Acid-Doped
PolybenzimidazoleÕSulfonated Polysulfone Blend Polymer
Electrolytes for Fuel Cells
C. Hasiotis,
a
Li Qingfeng,
b,
*
V. Deimede,
a,c
J. K. Kallitsis,
a,c
C. G. Kontoyannis,
a,d,z
and N. J. Bjerrum
b,
*
a
Institute of Chemical Engineering and High Temperature Chemical Processes, GR-265 00 Patras, Greece
b
Department of Chemistry, Technical University of Denmark, DK-2800 Lyngby, Denmark
c
Department of Chemistry and
d
Department of Pharmacy, University of Patras, GR-265 00 Patras, Greece
Polymeric membranes from blends of sulfonated polysulfones 共SPSF兲 and polybenzimidazole 共PBI兲 doped with phosphoric acid
were developed as potential high-temperature polymer electrolytes for fuel cells and other electrochemical applications. The water
uptake and acid doping of these polymeric membranes were investigated. Ionic conductivity of the membranes was measured in
relation to temperature, acid doping level, sulfonation degree of SPSF, relative humidity, and blend composition. The conductivity
of SPSF was of the order of 10
⫺3
Scm
⫺1
. In the case of blends of PBI and SPSF it was found to be higher than 10
⫺2
Scm
⫺1
.
Much improvement in the mechanical strength is observed for the blend polymer membranes, especially at higher temperatures.
Preliminary work has demonstrated the feasibility of these polymeric membranes for fuel-cell applications.
© 2001 The Electrochemical Society. 关DOI: 10.1149/1.1366621兴 All rights reserved.
Manuscript submitted October 19, 2000; revised manuscript received January 30, 2001.
Polymer electrolyte membrane fuel cells 共PEMFCs兲 have at-
tracted much attention during the last decades mainly due to their
possible use in the future as power generators.
1
The potential tech-
nological application of PEMFCs is limited by the poisoning effect
of carbon monoxide. Traces of CO 共5-10 ppm兲 cause a drastic de-
crease in the activity of anode catalysts. Recently, operation of these
cells at higher temperatures, up to 200°C, has been proposed as a
possible way to enhance the tolerance of the catalysts to fuel impu-
rities, e.g.,CO.
2
However, the development of higher temperature
PEMFCs requires an appropriate polymer electrolyte. The ideal
polymer membrane electrolyte in such a fuel cell should exhibit
thermal stability at elevated temperatures, easy preparation of thin
and homogeneous membranes of large area, high ionic conductivity,
low gas permeability, sufficient mechanical strength, and low cost.
Polybenzimidazole 共PBI兲 membranes, being sulfonated, phos-
phonated, or doped with an acid,
3,4
exhibit a proton conductivity at
temperatures up to 200°C. It is reported that this polymer membrane
electrolyte has a high ionic conductivity,
3-8
low methanol crossover
rate,
9
excellent thermal stability,
10
nearly zero water drag
coefficient,
8,11
and enhanced activity for oxygen reduction.
12,13
Fuel-cell tests with various types of fuel such as hydrogen,
14
methanol,
15
trimethoxymethane,
16
and formic acid
17
have been re-
ported. Besides, this polymer membrane electrolyte has also been
used for other electrochemical applications.
18
On the other hand, sulfonation of polyaromatic thermoplastics
such as polysulfone has been an active subject in order to develop
ion-exchange membranes for PEMFC
19-21
and other
applications.
22,23
It has been reported that sulfonation of polysulfone
increases the glass transition temperature to higher than 200°C.
24,25
The cast membranes of sulfonated polysulfones 共SPSF兲 exhibit good
mechanical strength, flexibility, and low gas permeability. These
properties are encouraging for further investigation of this polymer.
Blends of PBI with SPSF have been prepared and studied with re-
spect to their miscibility behavior.
24
It was found that blends of PBI
with SPSF show miscibility depending upon the composition of
blends and the sulfonation degree of SPSF. In general, PBI-rich
blends appear to be one compound system, as opposed to the SPSF
rich blends where two phases were observed. The combination of
SPSF with PBI might lead to a promising new polymer electrolyte.
Besides, SPSF is a quite inexpensive polymeric material that results
in lowering the cost of electrolyte. In the present work, an attempt
was made to study the electrochemical behavior of SPSF with vari-
ous sulfonation degrees and its blends with PBI of different compo-
sitions.
Experimental
Polysulfone 共molecular weight ca. 26,000, Aldrich兲 has been sul-
fonated to various degrees using chlorosulfonic acid as sulfonating
agent, according to the procedure proposed by Johnson et al.
26
The
sulfonation degree 共percentage of the introduced sulfonate group to
polysulfone兲 of SPSF was determined by
1
H NMR and Fourier
transform 共FT兲-Raman spectroscopy. Membranes of PBI 共solution in
dimethylacetamide containing 2 wt % LiCl, Celanese兲 with SPSF
were prepared by solution casting from dimethylacetamide 共DMAc,
Aldrich兲. In the case of pure SPSF, dimethylformamide 共DMF, Al-
drich兲 was used as the solvent. The cast membranes were dried at
190°C under vacuum in order to remove residual solvent. The dry-
ness of the membranes is important since plasticization effects due
to remaining solvent may occur.
24
The water uptake and acid doping of PBI-SPSF membranes were
determined by immersing samples in distilled water and acid solu-
tion for a few days. The samples were weighed before and after the
immersion and the weight gain was obtained. The doping level of
phosphoric acid was varied by using H
3
PO
4
of either different con-
centration at room temperature or of 85 wt % acid at various tem-
peratures. The doping level of the membranes is defined as the mo-
lar percent of the acid 共mol%H
3
PO
4
兲 per repeat unit of the PBI
polymer or per average repeat unit of PBI and SPSF, and was de-
termined by comparing the weight changes before and after doping.
In this way the obtained weight gain was due to both water and
phosphoric acid. In order to separate the acid uptake and water
uptake, the doped polymer membranes were dried at 110°C under
vacuum for more than 10 h until an unchanged weight was reached.
In this treatment the water content in the doped polymer membranes
was removed and the acid-doping level was obtained.
Conductivity measurements were performed by means of a four-
probe cell as schematically illustrated in Fig. 1. The cell consisted of
four platinum electrodes placed on one side of the polymeric mem-
brane. Four stainless steel rods pressed the electrodes onto the mem-
brane with the help of springs. In this longitudinal arrangement the
electrodes were in good contact with the membrane while the
sample was exposed to the atmosphere of the cell. The polymeric
membranes were narrow 4 ⫻ 1 cm strips. The thickness of the
doped membranes was of the order of 100 m and measured using
a micrometer before every experiment. Two platinum foils were
used to apply current to the ends of the membrane while the other
*
Electrochemical Society Active Member.
z
E-mail: cgk@iceht.forth.gr
Journal of The Electrochemical Society, 148 共5兲 A513-A519 共2001兲
0013-4651/2001/148共5兲/A513/7/$7.00 © The Electrochemical Society, Inc.
A513
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two platinum probe wires spaced 1 cm apart were employed to
measure the potential drop along the film near the center of the
sample. This cell was introduced in a stainless steel vessel that was
immersed in an oil bath so as to control the temperature. The poly-
meric membrane was placed on a Teflon plate in order to be isolated
from the stainless steel vessel. Also, ceramic tubes of alumina were
used for the isolation of the extension of the electrodes. A thermo-
couple was arranged near the sample for monitoring its temperature.
The relative humidity was controlled by passing a mixture of hu-
midified and dry nitrogen through the cell and was monitored using
a hygrometer. Two flow meters were used to control the ratio of dry
and humidified nitrogen that were mixed in a chamber placed just
before the cell. The measurements were carried out by a current
interruption method using a potentiostat/galvanostat 共EG&G model
273兲 and an oscillator 共Hitachi model V-650F兲.
Mechanical properties of the membranes were measured by
means of tensile strength, i.e., the ultimate tensile stress when the
sample breaks. The sample had an original cross section of about
1 ⫻ 0.008 cm and was tested in a glass furnace, where the tempera-
ture was controlled. The tensile strength was measured as a function
of doping level at different temperatures.
Platinum catalysts 共20% Pt兲 supported by carbon black 共Vulcan
XC-72R, Cabot兲 were prepared by chemical reduction of platinum
chloroacid. The catalysts were applied onto a wet-proofed carbon
paper 共Toray TGP-H-120兲 by a tape-casting technique. Thus ob-
tained gas diffusion electrodes were further impregnated with a 5%
PBI solution in DMAc. The loading of the polymer in the catalyst
layer was controlled in a range from 0.6 to 0.8 mg cm
⫺2
. Assem-
blies from the acid-doped polymer membranes and the impregnated
electrodes were prepared by means of a hot-press at 150°C for 10
min. A single test cell 共5cm
2
兲 was made of graphite plates with gas
channels. Two aluminum end plates with attached heaters were used
to clamp the graphite plates. Fuel and oxidant gases were supplied
by means of mass-flow controllers. Performance curves were ob-
tained by a current step potentiometry. Potential values at various
current densities were then taken from chronopotentiometric curves
when a steady state was reached.
Results and Discussion
Water uptake and acid doping of PBI and blend mem-
branes.—The water uptake of polymer membranes is of special im-
portance, since water is involved in the electrode reactions and pro-
ton conduction through the membrane of PEMFCs. It is well known
that PBI has a high affinity for moisture and is hydrophilic. This is
possibly due to an intermolecular hydrogen bonding between N and
N-H groups in PBI and water.
27
By immersing PBI membranes in
distilled water for several days at room temperature, it is found in
the present work that up to 20-21 w/w % 共with a dry polymer as
basis兲 water can be absorbed by the pristine PBI membranes, corre-
sponding to 3.4-3.6 water molecules per repeat unit of PBI.
Water absorption is also observed for SPSF membranes. The
water uptake of the SPSF membranes might be attributed to the
interactions between H
2
O and the sulfonate groups in SPSF. The
sulfonation degree of the studied SPSF membranes was 36%, cor-
responding to an ion exchange capacity 共IEC兲 of 0.81 mequiv/g. It is
found that, at room temperature, 8-10 w/w % water is absorbed by
the dry SPSF membranes, which have not experienced any pretreat-
ment. This corresponds to about seven water molecules per sul-
fonate group. Brousse et al.
25
claimed that the moisture gain by
SPSF of IEC 0.635 and 0.9 mequiv/g 共corresponding to sulfonation
degrees around 30 and 40%兲 at room temperature is 7 and 7.4 mol
H
2
O per ionic site, respectively. Using SPSF of sulfonation degree
40.3% at room temperature, Lufrano et al.
20
reported a water uptake
of about 9 mol H
2
O/SO
3
H for the rehydrated membranes after dry-
ing at 30°C for 18 h. This value became about 12 mol H
2
O/SO
3
H
for the rehydrated membrane after drying at 70°C for 18 h. As they
suggested, these water uptake results confirmed the history depen-
dence of the ionomer membranes, well known to perfluorosulfonic
acid polymer membranes, e.g., Nafion. As suggested by Zawodzin-
ski et al.
28,29
and Hinatsu et al.,
30
the water uptake by the polymer
membranes is essential for proton conductivity. As a comparison,
Nafion membranes have a water uptake of about 22 mol H
2
O/SO
3
H
from the liquid phase and about 16 mol H
2
O/SO
3
H from the vapor
phase at room temperature,
28-30
being much higher than the water
uptake values for SPSF obtained in this work. It has been observed
that the water uptake of the SPSF increases with the sulfonation
degree or the ion-exchange capacity,
20,21
the latter being found to
increase linearly with the sulfonation degree.
20
In the present work
the water uptake is found to increase with increasing PBI content in
the PBI-SPSF blends. At room temperature without any pretreat-
ment, blends rich in PBI 共75 w/w %兲 absorb 18-19 w/w % water. In
contrast, 10-12 w/w % water can be absorbed by the dry blends rich
in SPSF 共75 w/w %兲.
Since PBI possesses both donor and acceptor hydrogen-bonding
sites, it is capable to participate in specific interactions. In the pres-
ence of acids or bases, a PBI polycation can be formed, resulting in
acid or base neutralization and formation of a salt with the imidazole
ring structure. An electrolyte active species dispersed within the
polymer structure is a necessity for proton conduction. The proton
conducting active species is phosphoric acid in the present study.
By immersing PBI membranes in a phosphoric acid solution, the
weight gain of the polymer is due to both acid and water uptake.
In the previous work,
8
it was assumed that the water uptake by PBI
membranes remained unchanged in the presence and absence of the
acid. In the present work, efforts were made to determine the water
contribution in the acid-doped PBI membranes. This was done by
drying the doped polymer membranes at 110°C under vacuum while
the weight of the doped polymer membranes was measured periodi-
cally. It appeared that an unchanged weight of the doped polymer
membranes was reached after about 6 h under this condition. The
weight loss is assumed to be due to the water uptake and the remain-
ing weight is assumed to be the polymer purely doped with the acid.
Figure 1. Setup used for conductivity measurements.
Journal of The Electrochemical Society, 148 共5兲 A513-A519 共2001兲A514
Downloaded 28 Jun 2010 to 192.38.67.112. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp
The water-uptake by PBI membranes from a phosphoric acid
solution is found to be around 3.5 mol H
2
O per repeat unit of PBI as
the concentration of the acid increases from zero 共pure water兲 to 6.0
M. Further increase in the acid concentration results in an increase
in the water uptake. For 7.0 and 9.7 M H
3
PO
4
, for example, the
water uptake by PBI membranes is found to be 4.0 and 5.6
mol H
2
O, respectively.
The acid-doping level of the PBI membranes was obtained ac-
cordingly, as shown in Fig. 2 as a function of the acid concentration
at room temperatures. For pure PBI membranes 共䊐兲, as can be seen
from the figure, a doping level of around 500 mol % H
3
PO
4
can be
achieved by using 9-11 M H
3
PO
4
solution at room temperature,
which is in good agreement with other reports.
3,6,9,12
For SPSF membranes of a sulfonation degree lower than 50%, as
studied in the present work, the acid doping level was found to be
lower than 50 mol % H
3
PO
4
. Direct doping of SPSF with phos-
phoric acid was also attempted by addition of the desired amount of
H
3
PO
4
in a solution of SPSF in DMF before the membranes were
cast. It is noteworthy that direct doping allows us to prepare self-
standing doped SPSF membranes with high sulfonation degree
共higher than 50%兲. These polymers have a brittle nature and there-
fore they do not form freestanding films either from solution or from
melt pressing. By increasing the doping level to more than 300
mol % H
3
PO
4
, nonhomogeneous membranes exhibiting patches of
different physical characteristics 共e.g., thickness, coloration, etc.兲,
were obtained using the direct-doping procedure.
PBI-SPSF blend membranes can be readily doped with H
3
PO
4
after immersion in the acid solution. It was found that the doping
level increases upon increasing the acid concentration, temperature,
and the PBI/SPSF ratio. In the studied blend composition 共from 0 to
50% SPSF兲, the doping levels of blend membranes from a certain
concentration of the acid are lower than that for pure PBI, as seen
from Fig. 2.
Ionic conductivity of SPSF.—Ionic conductivity of the mem-
branes under investigation was measured as a function of relative
humidity, acid-doping level, sulfonation degree of SPSF, and tem-
perature. When the relative humidity increases in the range between
30 and 80% the conductivity was found to increase slightly 共⬃2-
4%兲. Poinsignon et al.
21
also reported a slight increase in the proton
conductivity with relative humidity from 70 to 110% at 50°C for
SPSF of IEC 1.8 mequiv/g 共corresponding to a sulfonation degree
around 80%兲, though a dramatic increase in the proton conductivity
is observed for SPSF filled with phosphatoantimonic acid at a rela-
tive humidity near 100%.
The ionic conductivity of acid-doped SPSF membranes increases
upon increasing temperature 共Fig. 3兲. In a range from room tempera-
ture to 160°C, conductivity of the order 10
⫺3
Scm
⫺1
was obtained.
The relatively low ionic conductivity should be attributed to the low
sulfonation degree and the low phosphoric acid doping level. In the
studied range of the sulfonation degree 共from 10 to 44%兲,asmall
dependence of conductivity on sulfonation degree of SPSF was ob-
served, though a high conductivity was reported at much higher
sulfonation degrees.
20,21
Actually, the conductivity of SPSF was
found to increase exponentially with the increase of sulfonation
degree.
20
In the previous works,
20,21
the conductivity was studied in
relation to the water uptake of SPSF membranes. In the present
work, the SPSF membrane was doped with phosphoric acid and the
doping level seems more dominant for affecting the conductivity. A
small decrease in the doping level reduces the conductivity even
when the sulfonation degree is higher 共Fig. 3兲. However, higher
acid-doping levels cannot be achieved. As stated above a doping
level of SPSF membranes of 300 mol % H
3
PO
4
was achieved when
they were directly cast from a solution in which phosphoric acid had
been added. These membranes, however, exhibited no conductivity.
As shown in Fig. 3, there are two regions where the conductivity
increases linearly with temperature. The slopes of the Arrhenius
plots of conductivity between 298-363 and 363-433 K 共not shown兲
give the activation energy of 12.8-14.8 and 6.1-6.7 kJ/mol, respec-
Figure 2. Doping levels of the blend membranes in molar percent of H
3
PO
4
per repeat unit of the polymer as a function of concentration of the doping
acid at room temperature. The composition of the blend polymers is indi-
cated in the figure. The sulfonation degree of the SPSF is 36%.
Figure 3. Temperature dependence of ionic conductivity of acid-doped
SPSF: 共䊐兲 sulfonation degree 44% and doping level 30 mol % H
3
PO
4
and
共䊊兲 sulfonation degree 10% and doping level 45 mol % H
3
PO
4
. The relative
humidity is 80%.
Journal of The Electrochemical Society, 148 共5兲 A513-A519 共2001兲 A515
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tively. The change of the activation energy might be connected with
the presence of water in the membranes. As reported,
20
water is
involved in the conduction mechanism. The presence of water in
SPSF membranes results in a phase separation between the SPSF
backbone and the sulfonate-water clusters that are formed. These
ionic clusters permit the ion migration through the membranes. In
the present work the SPSF membranes were doped with H
3
PO
4
共85%兲. It means that not only phosphoric acid but also water is
present in the membranes. Consequently, both phosphoric acid and
sulfonate-water clusters are the conducting active species. As stated
previously, the conductivity increases slightly when the relative hu-
midity increases in the range between 30 and 80%. This behavior
implies that in this range, the atmospheric humidity does not seem to
influence significantly the retention of water in the membrane. The
change of the activation energy, which occurs at ⬃90°C, might be
attributed to the phase transition of water at elevated temperatures.
Ionic conductivity of PBI-SPSF blends.—Dependence of ionic
conductivity on doping level and relative humidity.—The conductiv-
ity of blends was found to be higher than 10
⫺2
Scm
⫺1
and increase
with temperature. These polymeric materials are conductive after
doping with an acid. The acid-doping level is therefore expected to
influence the ionic conductivity of blends. For blend membranes
rich in PBI 共Fig. 4兲 doped with 500 mol % H
3
PO
4
, the conductivity
was found to be in the range 2 ⫻ 10
⫺2
and 10
⫺1
Scm
⫺1
. The
conductivity increases up to 2 ⫻ 10
⫺1
Scm
⫺1
at 160°C for a dop-
ing level of 1000 mol % H
3
PO
4
. Further increase of the doping level
results in significant increases of conductivity. Using hot phosphoric
acid 共130°C兲, a doping level as high as 2300 mol % can be achieved,
which seems to be the limit since at higher doping level degradation
of the membranes occurs. Under these extreme conditions the con-
ductivity is measured up to 7 ⫻ 10
⫺1
Scm
⫺1
at 160°C. However,
high doping levels 共⬎1000 mol % H
3
PO
4
) are not desirable for fuel-
cell applications, since membranes of very low mechanical strength
are produced.
Relative humidity does not seem to influence significantly the
conductivity of blends since a ⬃3-5% increase in ionic conductivity
was recorded when the relative humidity was gradually increased
from 30 to 80%. This is in accordance with previous results since
PBI can be used with low humidification
6
while conductivity of
SPSF, as stated above, is slightly dependent on relative humidity. As
a result, the blends of PBI with SPSF could also be expected to be
operational with low humidification.
Dependence of ionic conductivity on sulfonation degree and PBI/
SPSF ratio.—In the case of pure SPSF, the dependence of conduc-
tivity on sulfonation degree was not studied in the present work. The
blends rich in PBI are readily impregnated with H
3
PO
4
and the
conductivity is found to increase upon increasing sulfonation degree
of SPSF. This behavior is illustrated in Fig. 5 and implies that not
only the acid-doped PBI but also sulfonate sites of SPSF are in-
volved in the conduction mechanism. The sulfonation degree of
SPSF should not be lower than 20%, since SPSF with very low
sulfonation degree is not miscible with PBI.
24
When the sulfonation
degree is equal to 20%, the conductivity is found to be 7 ⫻ 10
⫺2
Scm
⫺1
at elevated temperatures for blends rich in PBI 共Fig. 5兲. For
blends with the same composition and doping level but higher sul-
fonation degree 共70%兲, the conductivity is found to be 10
⫺1
Scm
⫺1
at 160°C. Since for blends rich in PBI, even with low sulfonation
degree of SPSF, the conductivity is found to be in the range of 10
⫺2
to 10
⫺1
Scm
⫺1
, these blends are preferable because of their im-
proved mechanical behavior compared to those with high sulfona-
tion degree.
The ionic conductivity was also measured as a function of com-
position of the blends. As depicted in Fig. 6 for PBI-SPSF mem-
branes with sulfonation degree of 36%, even for blends with low
Figure 4. Dependence of ionic conductivity of PBI-SPSF blends on acid-
doping level. The content of PBI in the blends is 75 wt %. The sulfonation
degree of the SPSF is 36%. Doping level 共mol % H
3
PO
4
兲: 共䊐兲 500, 共䊊兲
1100, and 共䉭兲 2300. Relative humidity 80%.
Figure 5. Dependence of ionic conductivity of PBI-SPSF blends on sulfona-
tion degree of SPSF. The doping level is 500 mol % H
3
PO
4
. The content of
PBI in the blends is 75 wt %. The sulfonation degree of the SPSF is: 共䊐兲 20,
共䊊兲 36, and 共䉭兲 70%. Relative humidity 80%.
Journal of The Electrochemical Society, 148 共5兲 A513-A519 共2001兲A516
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