A new sequence isomer of AB-polybenzimidazole for high-temperature PEM fuel cells

Article abstract of DOI:10.1002/pola.25034

A new sequence isomer of AB-polybenzimidazole (AB-PBI) was developed as a candidate for high-temperature polymer electrolyte membrane fuel cells. A diacid monomer, 2,2′-bisbenzimidazole-5,5′-dicarboxylic acid, was synthesized and polymerized with 3,3′,4,4′-tetraaminobiphenyl to prepare a polymer that was composed of repeating 2,5-benzimidazole units. In contrast to previously prepared AB-PBI, which contains only head-to-tail benzimidazole sequences, the new polymer also contains head-to-head and tail-to-tail benzimidazole sequences. The polymer was prepared in polyphosphoric acid (PPA) and cast into membranes using the sol-gel PPA process. Membranes formed from the new AB-PBI were found to be mechanically stronger, possessed higher acid doping levels, and showed improved fuel cell performance, when compared to the previously known AB-PBI.

Full text of DOI:10.1002/pola.25034

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A New Sequence Isomer of AB-Polybenzimidazole for High-Temperature
PEM Fuel Cells
Alexander L. Gulledge, Bin Gu, Brian C. Benicewicz
Department of Chemistry and Biochemistry & USC NanoCenter, University of South Carolina, Columbia, South Carolina 29208
Correspondence to: B. C. Benicewicz (E-mail: benice@sc.edu)
Received 18 August 2011; accepted 3 October 2011; published online 28 October 2011
DOI: 10.1002/pola.25034
ABSTRACT: A new sequence isomer of AB-polybenzimidazole
AB-PBI) was developed as a candidate for high-temperature
polymer electrolyte membrane fuel cells. A diacid monomer,
Membranes formed from the new AB-PBI were found to be
mechanically stronger, possessed higher acid doping levels,
and showed improved fuel cell performance, when compared
to the previously known AB-PBI. VC 2011 Wiley Periodicals, Inc.
(
0
0
2
,2 -bisbenzimidazole-5,5 -dicarboxylic acid, was synthesized
0
0
and polymerized with 3,3 ,4,4 -tetraaminobiphenyl to prepare a
polymer that was composed of repeating 2,5-benzimidazole
units. In contrast to previously prepared AB-PBI, which con-
tains only head-to-tail benzimidazole sequences, the new poly-
mer also contains head-to-head and tail-to-tail benzimidazole
sequences. The polymer was prepared in polyphosphoric acid
J Polym Sci Part A: Polym Chem 50: 306–313, 2012
0
0
KEYWORDS: 2,2 -bisbenzimidazole-5,5 -dicarboxylic acid; AB-PBI;
high temperature materials; i-AB-PBI; poly(2,5-benzimidazole);
polybenzimidazole;
polycondensation;
polyelectrolytes;
polymer electrolyte membrane fuel cells; step growth
polymerization
(
PPA) and cast into membranes using the sol–gel PPA process.
ꢀ
INTRODUCTION Polymer electrolyte membrane (PEM) fuel
cells have received much attention due to their potential as
clean energy conversion devices, which can be used for both
S/cm) at elevated temperatures (>110 C) under low rela-
tive humidity conditions and exhibited a high tolerance to
fuel impurities (most notably, CO). Previously, we reported
a new sol–gel method, termed the polyphosphoric acid (PPA)
process, for the preparation of PBI polymer membranes from
PPA solutions that resulted in membranes with high levels of
8
1
mobile and stationary applications. Although PEM fuel cells
are excellent candidates for these devices, one problem asso-
ciated with PEM fuel cells is the availability and development
of appropriate materials that would allow manufacturing
routes to enable the cost of electricity per kWh to compete
9
PA doping. Membranes prepared from the PPA process
showed higher acid doping levels, improved mechanical
properties, higher proton conductivities, and overall
improved fuel cell performance, when compared to conven-
tionally imbibed PBI membranes. Subsequently, many PBI
variants containing a range of structural and functional moi-
eties have been examined to further explore the effects of
2
with existing technologies. Although several types of materi-
als have been developed for PEM fuel cell operation, only
9
few offer the advantages of higher operational temperatures
ꢀ
(
>100 C). Polybenzimidazole (PBI) polymer membranes
doped with phosphoric acid (PA) used in PEM fuel cells are
ꢀ
10–13
capable of operating at temperatures up to 200 C because
polymer structure on membrane properties.
3
they do not require water for proton conduction. The ability
Among the various PBI polymers developed for high-temper-
ature fuel cell applications, poly(2,5-benzimidazole) (AB-
PBI), has been examined by several research groups.
AB-PBI has been reported as a promising candidate for high-
temperature PEM fuel cells and is the simplest known PBI
structure prepared from a readily available monomer.
addition to the research devoted to homopolymer AB-PBI,
substituted, crosslinked, and copolymers containing AB-
PBI have also been investigated. AB-PBI membranes that
were imbibed with PA typically showed acid doping levels
from 2 to 10 mol PA/mol polymer repeat unit and corre-
sponding proton conductivities of ꢁ0.05 S/cm at 180 ꢀ
of these systems to operate without the presence of water
eliminates the need for and cost of external humidification
and the related engineering hardware to monitor hydration
levels. Moreover, higher operational temperatures provide
advantages such as faster electrode kinetics, increased resist-
ance to fuel impurities and simplified water and thermal
14–18
19–21
In
4
management.
22
23
2
4
PBI polymers have been investigated for use as high-temper-
ature PEMs because of their known thermal and chemical
5
–7
stabilities under normal fuel cell operation conditions.
These membranes showed high proton conductivity (>0.1
C
V
C
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PPA (115% concentration) was purchased from Aldrich and
used as received. PA (85%) and common solvents(e.g.,
DMSO, MeOH, EtOH, etc.) were purchased and used as
received from Fisher Scientific. Purification of 3,4-diamino-
benzoic acid (DABA; 10 g, Acros, 97%) was conducted via
recrystallization from water/methanol (480 mL/160 mL)
using activated carbon yielding pink/tan crystals after vac-
ꢀ
26
ꢀ
uum drying, 75%, m.p. 216.8 C (Literature, 210–211 C).
Polymerization of Poly(2,5-benzimidazole)
Poly(2,5-benzimidazole) (AB-PBI) was prepared using the
following method: purified 3,4-diamino benzoic acid (3 g)
and PPA (97 g) were added to a 100-mL reactor. The reactor
was equipped with a three-neck reactor head, mechanical
stirrer, nitrogen inlet/outlet, and placed into a silicone oil
bath with a ramp/soak temperature controller. The polymer-
FIGURE 1 Chemical structures of (a) poly(2,5-benzimidazo-
le)(AB-PBI) and (b) new isomeric i-AB-PBI.
ꢀ
ization began with an initial temperature of 60 C and was
ꢀ
raised to 140 C over a period of 1 h. The temperature
2
5
ꢀ
under anhydrous conditions. AB-PBI was prepared using
the PPA process and investigated to determine the effects of
higher PA doping levels on membrane characteristics com-
pared to the conventional imbibing method of preparation.
As previously mentioned, preparation by the PPA process
results in films that contain a much higher concentration of
PA when compared to imbibed films. In addition to a com-
parison of AB-PBI preparation methods, a new sequence iso-
mer of AB-PBI, termed i-AB-PBI, was synthesized and com-
pared to both in-house AB-PBI using the PPA process and
conventional AB-PBI reported in the literature.
remained at 140 C for 2 h to ensure all monomer was dis-
solved. Once the monomer had dissolved, the temperature
was increased stepwise until a final polymerization tempera-
ꢀ
ture of 220 C was obtained. Typically, the polymerization
ꢀ
reaction time at 220 C was between 24 and 36 h, depend-
ing on the viscosity of the polymer. The Weisenberg effect
was observed during the later stages of the polymerization
causing the solution to climb up the stirrer. When this was
observed, PA (85%) was added to the reaction mixture to
adjust the viscosity of the solution for film casting. Analysis:
1
H NMR (400 MHz, DMSO-d
) d 6.7–7.3 (broad multiplet,
6
aromatics, 3H).
Herein, we report the synthesis and characterization of a
0
0
new diacid monomer, 2,2 -bisbenzimidazole-5,5 -dicarboxylic
0
0
0
0
Preparation of 2,2 -Bisbenzimidazole-5,5 -
dicarboxylic acid
To a 1000-mL round bottom flask, purified DABA (10 g,
acid (BBDCA), and its AA-BB polymerization with 3,3 ,4,4 -
tetraaminobiphenyl (TAB) to form a new polymer, which
constitutionally represents a new sequence isomer of AB-PBI,
i-AB-PBI. AB-PBI is a polymer with a head–tail repeating
benzimidazole sequence. The change in orientation of the
benzimidazole groups incorporated into the new sequence
isomer introduces two additional types of chemical bonds. In
the known AB-PBI, benzimidazole groups of the polymer
backbone are linked through benzimidazole–phenyl (2,5)
linkages, whereas the new sequence isomer introduces two
new linkages, phenyl–phenyl (5,5 linkages), and benzimida-
zole–benzimidazole (2,2 linkages) in addition to the benzim-
idazole–phenyl linkage (Fig. 1). Membranes were prepared
from the new polymer using the PPA process and investi-
gated to evaluate their proton conductivity and fuel cell per-
formance properties. The properties of the new membranes
were compared to both the in-house synthesized AB-PBI pre-
pared by the PPA process and literature data on convention-
ally imbibed AB-PBI membranes.
0.066 mol), and methanol (250 mL) were added. The mix-
ture was then placed into an explosion proof refrigerator
ꢀ
and cooled at 10 C for ꢁ20–30 min. The flask was removed
from the refrigerator and methyl-2,2,2-trichloroacedimidate
(
4.1 mL, 0.033 mol) was immediately added dropwise (ꢁ5
min) with stirring. After the addition was completed, the
reaction flask was placed into a preheated oil bath at a tem-
ꢀ
perature of 50 C for 24 h. A dark orange precipitate was
obtained that was filtered and washed with ethanol several
times, during which the product became increasingly lighter
in color. The crude product was dried at 120 C overnight
under vacuum.
ꢀ
The light orange crude product described earlier (5.89 g)
was dissolved in ꢁ200 mL of heated DMSO, followed by the
addition of hot water until the solution became slightly
cloudy. A yellow precipitate was obtained on cooling to room
temperature, which was filtered and washed with cold etha-
EXPERIMENTAL
ꢀ
nol. The product was dried at 220 C under vacuum to
Materials
obtain a yellow powder (4.112 g, 69.8% yield). Analysis:
FTIR: 1671m, 1625m, 1595w, 1497w, 1312s, 1295s, 1206m,
Methyl-2,2,2-trichloroacedimidate (98%) was purchased
from Acros Organics and used as received. TAB monomer,
TAB (polymer grade, ꢁ97.5%) was donated by Celanese Ven-
tures, GmbH (now, BASF Fuel Cell) and used as received.
ꢂ1
1
942m, 769s, 746s, 674m cm
. H NMR (300 MHz, DMSO-
d6) d 13.98 (s, NH), 12.86 (s, COOH), 8.325 (s, 1H), 8.159 (s,
1H), 7.94–7.63 (m, 2H), 7.63–7.60 (d, 1H, J ¼ 8.1). ELEM.
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ANAL: Calcd. for C H N O : C, 59.63; H, 3.13; N, 17.38.
Found: C, 59.42; H, 3.23; N, 17.30.
three separate measurements and were calculated as previ-
ously reported.
1
6 10 4 4
2
7
The PA contents in the membranes were obtained via titra-
tion using a Metrohm 716 DMS Titrino automated titrater
and a standardized 0.1 M sodium hydroxide solution follow-
Polymerization of New Isomeric Poly(2,5-benzimidazole)
In a typical polymerization procedure TAB (1.828 g), PPA
(
70 g), and BBDCA (2.75 g) were added to a 100-mL reactor.
2
8
ing procedures reported previously.
PA concentrations
The reactor was then equipped with a three-neck reactor
head, a stir rod attached to an overhead stirrer and a nitro-
gen inlet/outlet. A slow nitrogen flow of approximately one
bubble every 2 s was established and monitored with an oil-
filled bubbler. The reactor was placed into an oil bath that
was regulated using a temperature controller with ramp and
soak features. The polymerization used the ramp/soak pro-
file as follows:
were expressed as moles of PA per mole of polymer repeat
unit.
Ionic conductivities were measured using a quadra-probe
alternating current impedance method, which used a Zahner
IM6e spectrometer operating in the frequency range of 1
Hz–100 kHz. A rectangular section of the polymer membrane
2
was cut with the dimensions of 3.5 ꢃ 7.0 cm and was
ꢀ
placed into a glass cell connected to four platinum wire cur-
rent collectors. Current was supplied to the cell through two
outer electrodes, which were set 6.0 cm apart, whereas the
potential drop was measured by two inner electrodes that
were set 2.0 cm apart. The inner and outer electrodes of the
glass cell were positioned on alternating sides of the poly-
mer membrane to obtain through plane bulk measurements
of ionic conductivity. A programmable oven was used to
house the cell, so that a measure of the temperature depend-
ence of the proton conductivity for the cell could be
obtained. Two series of measurements of the conductivity
were conducted subsequently. The first series of measure-
An initial temperature of 60 C was used, and the tempera-
ꢀ
ꢀ
ture was raised to 140 C over a period of 1 h. The tempera-
ture remained at 140 C for 2 h and was then increased to
ꢀ
1
80 C over a period of 30 min. The temperature remained
ꢀ
ꢀ
at 180 C for 10 h, and was then increased to 220 C in a
period of 30 min. The reaction remained at 220 C for the
ꢀ
duration of the polymerization. Twenty hours was the typical
duration of polymerization for the new sequence isomer (i-
AB-PBI). At the end of 20 h the solution was very viscous
and PA (ꢁ15 mL) was back-added to adjust the viscosity.
1
Analysis: H NMR (400 MHz, DMSO-d
6
) d 6.7–7.3 (broad
multiplet, aromatics, 3H), 8.2 (singlet, 1H), 8.5 (multiplet,
H).
ꢀ
ments were conducted from ambient temperature to 180 C,
2
ꢀ
at intervals of 20 C, with a 15 min pause at each tempera-
Polymer Electrolyte Membrane Preparation
ture interval for thermal equilibrium before measurement.
The second series of measurements were subsequently con-
ducted in the same manner to obtain the ionic conductivity
of the membranes under anhydrous conditions. A Nyquist
plot was constructed to fit the experimental curve of the re-
sistance across the frequency range. The conductivities were
calculated at different temperatures from the membrane re-
sistance obtained using the following equation:
Once an appropriate polymer viscosity was obtained, the so-
ꢀ
lution was directly cast at 220 C onto glass plates for mem-
ꢀ
brane preparation. A heated metal casting blade (120 C)
with a casting thickness of 20 mil (508 lm) was used. After
casting, the glass plates were separated and samples were
placed into a humidification chamber maintained at a rela-
ꢀ
tive humidity of 55% at 25 C. The polymer samples were
allowed to hydrolyze overnight. Hydrolysis of the PPA to PA
induced a sol-to-gel transition that resulted in a gel mem-
brane. The samples were then sealed in 3 mil (76.2 lm)
thick polyethylene bags until testing or membrane electrode
assembly (MEA) fabrication.
D
r ¼
W ꢃ T ꢃ R
where r is the ionic conductivity, R is the resistance meas-
ured, W and T are the width and thickness of the membrane,
respectively, and D is the distance between the two inner
electrodes.
Characterization Techniques
FTIR spectra were recorded on a Perkin Elmer Spectrum
1
00 using an attenuated total reflection diamond cell attach-
Membrane Electrode Assembly Fabrication
and Fuel Cell Testing
1
ment. H NMR spectra were recorded using a Varian Mer-
cury 300 spectrometer. Mechanical properties were meas-
ured in tension using an Instron 5543A with an extension
rate of 5.0 mm/min and were preloaded to 0.1 N at 3 mm/
min using a 250 N load cell.
MEA’s were fabricated by hot pressing a membrane (ꢁ20%
compression) sample between two platinum doped carbon
2
2
electrodes, and were 50 cm in area (45.15 cm active area).
Gas diffusion electrodes, with a platinum loading of 1.0 mg/
2
Inherent viscosity (IV) measurements were conducted using
a Canon Ubbelohde (size 200) viscometer at a concentration
of 0.2 dL/g in methane sulfonic acid (MSA) at 30.0 C. A
small amount of polymer solution was precipitated in water
and neutralized with 0.1 N ammonium hydroxide. After
thoroughly washing with water, the sample was dried under
cm , were obtained from BASF Fuel Cell. A Kapton frame-
work was also incorporated to add stability to the MEA, and
to allow handling without damaging the MEA. The MEA was
placed into a single-cell fuel cell apparatus for testing. The
gas flow plates used were graphite with dual gas channels.
Steel endplates were used to clamp the gas flow plates
together. Heating pads were attached to the steel plates to
regulate and monitor temperature. A commercially available
ꢀ
ꢀ
vacuum overnight at 120 C and then dissolved in MSA using
a mechanical shaker. Recorded IV values are an average of
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fuel cell testing station (Fuel Cell Technologies) equipped
with mass flow regulators was used to conduct and record
measurements. Stoichiometric gases were supplied to the an-
ode and cathode at a stoichiometric of 1.2 and 2.0, respec-
tively, without applied backpressure. Fuel/oxidant studies
were performed with hydrogen gas, oxygen gas, and refor-
mate gas, all without humidification. The composition of
reformate gas used consisted of ꢁ70% hydrogen, 28% CO2,
and 2% CO.
membranes was adjusted by soaking the membranes in PA
baths with lower PA concentrations than that of the original
film. Membrane samples with doping levels of 29.1, 22.7,
and 14.5mol PA/mol polymer repeat unit were obtained.
However, the films with lower PA content still did not main-
tain a gel state at higher temperatures, and therefore, high-
temperature anhydrous conductivity and fuel cell perform-
ance could not be obtained for AB-PBI membranes with
these moderate levels of PA prepared using the PPA process.
Synthesis and Purification of
2
RESULTS AND DISCUSSION
0 0
,2 -Bisbenzimidazole-5,5 -dicarboxylic Acid
Typical synthesis of bisbenzimidazoles involves a reaction
Polymerization of Poly(2,5-benzimidazole)
between o-phenylenediamines with a diacid or diamide in
Poly(2,5-benzimidazole) (AB-PBI) was prepared by polymer-
izing DABA in PPA. Polymerization conditions for AB-PBI
prepared in PPA were experimentally determined. Initial
monomer concentrations in PPA were observed to have a
direct effect on the molecular weight of the resulting poly-
mer. A study to examine the effect of monomer concentration
on polymer IV was conducted through a series of polymer-
izations to determine the optimal concentration for the for-
mation of high-molecular-weight polymer and is shown in
Figure 2. The optimal monomer concentration was ꢁ3 wt %
yielding polymer samples with inherent viscosities up to
2
9
HCl or PPA (PPA). The synthesis of BBDCA has not been
previously reported. However, synthesis of 2-substituted
benzimidazoles from the reaction of o-phenylendiamine with
3
0
an imidate is well known.
Synthesis of BBDCA was
achieved by reacting purified DABA and methyl 2,2,2-tri-
chloroacetimidate in a 2:1 stoichiometric ratio, respectively.
Synthetic procedures involving 2-trichlorobenzimidazoles are
often conducted at room temperature after the exothermic
3
1
reaction has subsided to avoid unwanted side reactions.
Although these types of reactions are typically done at room
temperature, an increase in reaction yield of ꢁ10–15% was
4.63 dL/g. Attempts below 2 wt % monomer concentration
observed in this work, when the reaction was performed at
under these polymerization conditions yielded lower molecu-
lar weight polymers and unstable films, when the PPA solu-
tions were cast and hydrolyzed. Polymerizations conducted
above 3.5 wt % resulted in films with higher rigidity and
less favorable mechanical properties. AB-PBI membranes
prepared using the PPA process were found to have much
higher acid doping levels (22–35 mol PA/mol polymer repeat
ꢀ
50
C. It is surmised that the exothermic reaction coupled
with the increased reaction temperature provided the system
with enough energy to induce reaction and ring closure of
the intermediate 2-trichloro-5-carboxyl benzimidazole with a
second equivalent of DABA. The crude product of this mono-
mer was obtained in yields of 50–60% after 24 h reaction
time. The diacid monomer was found to be insoluble in most
common organic solvents, but soluble in DMSO and dimethy-
lacetamide. Purification of the diacid monomer was achieved
using a mixed solvent recrystallization from DMSO and
distilled water. The purified diacid monomer was a yellow
colored powder after drying and was typically obtained in
yields of ꢁ60–70%. The compound was also determined to
be slightly hygroscopic, and therefore, thorough drying was
required before polymerization to achieve high-molecular-
weight polymer. In addition to standard characterization
techniques, FTIR analysis was used to confirm the complete
removal of the intermediate after purification by the absence
unit) when compared to conventional imbibing methods (2–
5
1
0 mol PA/mol polymer repeat unit). However, AB-PBI
membranes prepared with elevated PA doping levels (>14
mol PA/mol polymer repeat unit) were found to be unstable
ꢀ
at elevated temperatures (>130 C). The PA level in the
ꢂ1
of a peak at ꢁ829cm , which correlates to the C–Cl stretch
of the intermediate.
Polymerization of i-AB-PBI
The new sequence isomer of AB-PBI, i-AB-PBI, was synthe-
sized using the synthetic scheme displayed in Figure 3(b).
The synthesis of i-AB-PBI is an AA-BB step polymerization
and, as defined by the Carother’s equation, an exact stoichio-
metric ratio of BBDCA with TAB was required to achieve
high-molecular-weight i-AB-PBI. Polymerization conditions
for the new sequence isomer were experimentally deter-
mined and were conducted over a range of monomer con-
centrations as shown in Figure 4. A dramatic increase in
polymer IV occurred, when monomer concentration was
increased from 5 to 6 wt %. The reason for this abrupt
FIGURE 2 Effect of monomer concentration on IV for the poly-
ꢀ
merization of DABA in PPA at 220 C.
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FIGURE 3 Synthetic scheme for (a) two step synthesis of BBDCA and (b) synthesis of new isomeric i-AB-PBI.
change in IV is not completely understood, and this type of
behavior has not been observed in our previous and exten-
sive investigations of PBI polymers. Although such abrupt
changes in IV could be indicative of a transition to an
ordered or liquid crystalline phase, all other observations
were not supportive of a liquid crystalline phase. Further
increases in monomer concentration led to polymers with
slightly lower inherent viscosities, which we attributed to
increased solution viscosity and difficulties in stirring during
the later stages of the polymerization. Subsequent polymer-
izations were conducted at 6 wt % monomer concentration
to obtain high-molecular-weight polymer.
lm) casting blade. The solutions then underwent a sol-to-gel
transition as the PPA of the membrane hydrolyzed into PA
and the system temperature decreased to room temperature.
The membrane composition data show that the typical com-
positions of i-AB-PBI membranes prepared using the PPA
process are greater than 50% PA and have a polymer con-
tent between 7 and 10.5 wt %. Mechanical property evalua-
tions of the i-AB-PBI doped membranes typically yielded
Young’s moduli ranging from 0.9 to 1.6 MPa and tensile
strengths between 0.43 and 1.11 MPa. Membrane composi-
tion data along with other significant properties for i-AB-PBI
and the in-house AB-PBI prepared using the sol–gel method
are shown in Tables 1 and 2, respectively.
Membrane Formation and Properties
ꢀ
As previously mentioned, the new i-AB-PBI contains two
additional types of chemical bonds when compared to
AB-PBI, benzimidazole–benzimidazole and phenyl–phenyl
Polymer solutions were directly cast at 220 C onto heated
ꢀ
glass plates (120 C oven temperature) using a 20 mil (508
1
linkages. H NMR analysis was conducted for both AB-PBI
and the new i-AB-PBI. On the basis of chemical structures
presented the i-AB-PBI spectrum would contain additional
peaks, which arise from the CAH bonds in the biphenyl
moiety of the polymer. Spectral analysis did in fact show
additional peaks at 8.2 and 8.5 ppm, which are consistent
0
with the assignments of the biphenyl moeity in 2,2 -di-
0
32
phenyl-5,5 -bibenzimidazole, a model compound. Prelimi-
nary molecular modeling using MOPAC 2009 Semi Empiri-
cal PM6 indicated slightly increased chain stiffness for i-
AB-PBI as compared to AB-PBI. Experimentally, we
observed a lower solubility for i-AB-PBI in PA at elevated
ꢀ
temperatures (130–180 C), when compared to the known
AB-PBI. The lower solubility of the i-AB-PBI resulted in
films with higher gel stability at higher temperatures,
even with higher acid doping levels. Thus, films made
with i-AB-PBI and doped with high levels of PA were suf-
ficiently stable for conductivity measurements and fuel
cell performance evaluations.
FIGURE 4 Effect of monomer concentration on IV for the new
ꢀ
isomeric i-AB-PBI at a polymerization temperature of 220 C.
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TABLE 1 Characterization Results for i-AB-PBI Polymers and Membranes
Anhydrous
Membrane Composition
(as cast) (wt%)
Mol PA/Mol
Polymer
Conductivity
ꢀ
Sample
Number
Monomer
Wt%
IV
at 180 C
(dL/g)
Repeat Unit
(S/cm)
PA
H
2
O
Polymer
a
1
2.92
4.11
4.77
5.18
6.21
6.92
8.21
1.0
N/A
N/A
N/A
N/A
N/A
.202
.216
.195
N/A
N/A
N/A
N/A
N/A
7.67
7.26
10.49
8.02
a
2
1.1
N/A
N/A
N/A
a
3
1.03
1.05
3.05
2.84
2.60
N/A
N/A
N/A
4
5
6
7
a
34.76
36.52
24.5
32.12
55.96
54.98
53.92
54.08
36.37
37.76
35.59
37.90
Low molecular weight samples were obtained and resulting films were unstable for testing.
Membrane Characterization and Fuel Cell Testing
reformate gas used in these polarization studies consisted of
The ionic conductivity data for i-AB-PBI are shown in Figure
ꢁ70% hydrogen, 28% CO , and 2% CO. In the reformate/air
2
ꢀ
5
along with the literature data for AB-PBI. At 180 C under
polarization curve, it is shown that i-AB-PBI membranes
ꢀ
anhydrous conditions, i-AB-PBI showed
a
nearly 10ꢃ
maintained high performance at 180 C, even in the presence
increase in conductivity, from 0.02 to 0.2 S/cm, when com-
pared to the reported literature on the conventionally
of carbon monoxide, a well-known fuel contaminate. At
lower temperatures and in the presence of CO (2%), a signif-
icant decrease in performance was observed, as would be
expected from decreased electrode kinetics and irreversible
Pt–CO binding. A lifetime performance study was conducted
on i-AB-PBI membranes and is shown in Figure 9. An
observed break-in period of around 500 h was observed
before the cell reached and maintained an average output of
2
5
imbibed AB-PBI. This is largely due to the stability of i-AB-
PBI with higher acid doping levels at increased
temperatures.
The high conductivity (ꢁ0.2 S/cm) for i-AB-PBI indicated
these membranes would be an excellent candidate for high-
temperature PEM fuel cells. Fuel cell performance studies
were conducted on i-AB-PBI membranes that included polar-
ization curves with hydrogen/air and reformate/air fuel/oxi-
dant combinations over a range of temperatures (120–180
0
.65 V. The test was conducted over a period of 3500 h with
2
a relatively constant output voltage of 0.65 V at 0.2 A/cm .
Several facility events occurred during the lifetime study of
i-AB-PBI, which led to uncontrolled shutdowns. However, a
full recovery of the cell was observed after normal restart
operations. The long (500 h) break-in period for i-AB-PBI
membranes was attributed to the higher polymer content
ꢀ
C). The performance of i-AB-PBI in comparison to reported
literature of AB-PBI, using hydrogen/oxygen with 1.2:2.0
stoichiometric flows is shown in Figure 6. Significant
increases in fuel cell performances for i-AB-PBI were
(
ꢁ7–10.5%), which introduces a resistance to the transfer of
ꢀ
observed over a range of temperatures (120–180 C). At a
2
internal PA of the film to the catalytic interface of the elec-
trodes during initial fuel cell operation.
current density of 0.2 A/cm , a voltage of ꢁ0.65 V was
observed with hydrogen and air supplied at 1.2 and 2.0 stoi-
chiometric flows. The polarization curves for the new
sequence isomer with hydrogen/air and reformate/air are
shown in Figures 7 and 8, respectively. As would be
expected, a slight decrease in performance was observed,
when the oxidant was changed from pure oxygen to air. The
CONCLUSIONS
AB-PBI membranes were prepared using the PPA process
and compared to reported literature on the known conven-
tionally imbibed AB-PBI. A new diacid monomer was
TABLE 2 Characterization Results for AB-PBI Polymers and Membranes
Membrane Composition
as cast) (wt%)
Mol PA/Mol
Polymer
(
Sample Number
Monomer wt%
IV (dL/g)
Repeat Unit
PA
2
H O
Polymer
8
9
1.46
2.76
2.7
1.56
4.63
3.59
2.57
1.8
35.28
24.71
29.07
31.82
22.56
60.22
62.17
62.59
57.27
73.82
37.67
34.36
34.83
40.49
22.25
2.11
3.47
2.58
2.24
3.93
1
1
1
0
1
2
2.86
4.84
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FIGURE 5 Effects of temperature on anhydrous ionic conduc-
tivity. (Squares: new isomeric i-AB-PBI (Sample 5 from Table
FIGURE 7 Polarization curves for i-AB-PBI membranes with
2
5
1), circles: reported literature values for AB-PBI.
hydrogen/air at 1.2:2.0 stoichiometric flows, respectively.
ꢀ
ꢀ
ꢀ
(
Squares: 180 C, triangles: 160 C, circles: 140 C, inverted tri-
ꢀ
angles: 120 C.)
synthesized from an imidate reaction with DABA. Polymer-
candidates for high-temperature polymer membrane fuel
cells.
0
0
ization of this diacid monomer with 3,3 -4,4 -tetraminobi-
phenyl yielded a new sequence isomer of AB-PBI. Polymer-
ization studies were conducted, which showed an unusual
dependence on monomer concentration, but revealed condi-
tions where high IV polymers could be prepared. Mem-
branes prepared from the PPA process exhibited higher lev-
els of PA than typically reported for conventionally imbibed
AB-PBI. The membrane mechanical property characteriza-
tion and proton conductivity measurements indicated
that the membranes formed from i-AB-PBI were suitable
The new sequence isomer, i-AB-PBI, was found to be less
soluble in PA and mechanically stable at elevated tempera-
ꢀ
tures (120–180 C) with high PA doping levels (24–35 mol
PA/mol polymer repeat unit). The changes in chain sequence
are believed to affect chain stiffness, which may contribute
to decreased solubility and increased gel stability in PA at
high temperatures. Membranes with higher polymer content
(ꢁ7–10.5 wt %) than those prepared from the traditional
FIGURE 6 Polarization curves for i-AB-PBI membranes with
FIGURE 8 Polarization curves for i-AB-PBI membranes with
hydrogen/oxygen at 1.2:2.0 stoichiometric flows, respectively.
reformate/air at 1.2:2.0 stoichiometric flows, respectively.
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
(
Squares: 180 C, circles: 160 C, triangles: 140 C, inverted tri-
(Squares: 180 C, circles: 160 C, triangles: 140 C, inverted tri-
ꢀ
ꢀ
angles: 120 C, diamonds: reference data for AB-PBI at 130
ꢀ
angles: 120 C; Reformate composition: 70% hydrogen, 28%
25
C. ).
2
CO , and 2% CO.).
312
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POLYMER SCIENCE
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ARTICLE
8
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The increased solids content of the i-AB-PBI films resulted in
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process. Fuel cell performance evaluations were conducted
using i-AB-PBI membranes and an output voltage of 0.65 V
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