This document summarizes research on developing a phosphoric acid doped poly(2,5-benzimidazole) membrane for use in high temperature fuel cells. Key points: - Undoped and phosphoric acid doped poly(2,5-benzimidazole) membranes were prepared using a solvent casting method. Phosphoric acid concentration was varied from 0-60% by volume to improve proton conductivity at high temperatures. - The 60% phosphoric acid doped membrane with a doping level of 1.65 showed the highest proton conductivity of 2.2 x 10-2 S/cm. It exhibited a decrease in crystallinity and mechanical properties compared to the undoped membrane.

1. Phosphoric Acid Doped Poly(2,5-benzimidazole)-Based
Proton Exchange Membrane for High Temperature Fuel
Cell Application
Ratikanta Nayak,1
Tapobrata Dey,1
Prakash C. Ghosh,1
Arup R. Bhattacharyya2
1
Department of Energy Science and Engineering, Indian Institute of Technology Bombay, Powai, Mumbai,
400076, India
2
Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Powai,
Mumbai, 400076, India
Undoped and doped poly(2,5-benzimidazole) (ABPBI)
membrane was prepared by solvent casting method using
methane sulfonic acid as a solvent and phosphoric acid
(H3PO4) as a doping agent. The concentration of H3PO4
was varied from 0 to 60 vol% to enhance the proton con-
ductivity of the ABPBI membrane at higher temperature.
Wide angle X-ray diffraction analysis showed a decrease
in crystallinity in ABPBI membrane with increase in H3PO4
doping concentration. The molecular signature and the
presence of H3PO4 was observed in 1000–1500 cm21
in
the Fourier transform infrared spectra, which was also
supported by a corresponding weight loss at 1808C–2008C
in the thermogravimetric analysis. Undoped ABPBI mem-
brane registered the Young’s modulus (E) and hardness
(H) values of 2.46 and 0.92 GPa, respectively, and the cor-
responding E and H values for 1.65 doping level of 60
vol% H3PO4 doped ABPBI membrane were 0.14 and 0.067
GPa, respectively. The 60 vol% H3PO4 doped ABPBI mem-
brane with doping level of 1.65 showed highest proton
conductivity value of 2.2 3 1022
S/cm. The impedance
spectroscopic analysis and the equivalent circuit model
were discussed to understand the nature of proton con-
duction in H3PO4 doped ABPBI membrane. POLYM. ENG.
SCI., 00:000–000, 2016. VC 2016 Society of Plastics Engineers
INTRODUCTION
Polymer electrolyte doped with sulfonic acid group com-
monly known as perfluorosulfonated solid electrolyte (PFSE)
has been utilized for proton conduction. Nafion is one of such
type of membrane available commercially, which is explored
widely for polymer electrolyte fuel cell (PEFC) application [1].
Sulfonic acid present in the PEFC, provides H1
ion only under
hydrated condition and qualifies for the ion conduction. There-
fore, the operating temperature of the sulfonated membrane
based fuel cell is restricted below 1008C under atmospheric
pressure. Furthermore, the fluorine backbone of this type of
membrane is harmful. Nevertheless, it is a preferred candidate
for low temperature fuel cell applications due to its excellent
mechanical and chemical stability. Due to low temperature oper-
ation, ultra-pure hydrogen gas (CO level <25 ppm) is required
to avoid the poisoning of the platinum catalyst used in PEFC
application. However, the CO tolerance capacity might be
improved up to 3%, when fuel cell was operated at elevated
temperature [2]. High temperature operation further enhances
the reaction kinetics on both the electrodes of the fuel cells.
Moreover, during high temperature operation, the hydrogen
desorption kinetics improves when metal hydride storage is used
for hydrogen storage.
Above the boiling point of water, operation of PEFC
involves only a single phase that is the water vapor, and there-
fore, avoids water flooding. The size of cooling system can be
reduced substantially, which is otherwise very important for the
transport application due to the increased temperature gradient.
The heat can be recovered as steam, which in turn can be used
either for direct heating or steam reforming or for pressurized
operation. In this way, the overall system efficiency can be sig-
nificantly increased. Several efforts have been made to develop
proton-conducting membrane for operation at temperature above
1008C for the fuel cell application [3, 4].
Among various types of polymer suitable for high-
temperature polymer electrolyte fuel cell (HT-PEFC) application
such as, phosphonated perflurosulfonic acid membrane, sulfo-
nated aromatic hydrocarbon polymer membrane, inorganic–
organic composite membrane [3], and phosphoric acid doped
polybenzimidazole (PBI) are reported as promising candidates
due to its high performance, excellent oxidation and thermal sta-
bility, low fuel permeability, nearly zero water drag coefficient
and high ionic conductivity at temperature up to 2008C.
A majority of the current work is focused towards developing
polybenzimidazole (PBI) based membrane for high temperature
fuel cell application. PBI could absorb up to 75% H3PO4 and
also less expensive as compared with Nafion membrane [5]. It
is also impermeable to fuel gases and methanol and does not
require humidification. Generally, para and meta-PBI are com-
mercially available membranes in the benzimidazole family.
Among the family of benzimidazole series; poly(2,5 benzimida-
zole) (ABPBI) (Fig. 1), is worth investigating for high tempera-
ture fuel cell application [6].
ABPBI exhibits simple structure as shown in the Fig. 1. PBI
synthesis is a tedious process, where two monomers, tetra ami-
nobiphenyl (TAB) and isophthalic acid (IPA) are needed. On
the contrary, the synthesis process of ABPBI is simple and
needs only single inexpensive commercially available monomer,
namely: (3,4 diaminobenzoic acid) even without purification [7].
It is capable of absorbing higher amount acid as compared with
PBI as it has higher concentration of benzimidazole group [8].
Moreover, the preparation method of ABPBI polymer is simple,
safe, and cost-effective. Therefore, ABPBI is considered as a
promising candidate for fuel cell application.
Correspondence to: P.C. Ghosh; e-mail: pcghosh@iitb.ac.in and A. R. Bhat-
tacharyya; e-mail: arupranjan@iitb.ac.in
DOI 10.1002/pen.24370
Published online in Wiley Online Library (wileyonlinelibrary.com).
VC 2016 Society of Plastics Engineers
POLYMER ENGINEERING AND SCIENCE—2016

2. In present work, ABPBI membrane has been prepared
through solvent casting method by considering the appropriate
ABPBI-MSA solution concentration on the basis of relative vis-
cosity, hardness, and membrane formation capacity mentioned
in Table 1. Then the suitable membrane was considered for the
further parametric study [9, 10]. Subsequently, the mechanical
properties of the membrane were studied via Nano-indentation
technique and proton conductivity measurement was carried out
through impedance analysis at high temperature by varying the
doping concentration of H3PO4. The electrochemical response
of the doped membrane has been analyzed by an equivalent cir-
cuit model. In brief, structure–property relationship studies have
been evaluated with commercially available ABPBI to develop
proton conducting membrane for fuel cell application.
EXPERIMENTAL
Materials and Experimental Methods
ABPBI (poly 2,5 benzimidazole) polymer was obtained from
Gharda Chemicals, Mumbai, India (G-5020 with a trade name
GAZOLE, synthesized from 3,4 dibenzoic acid with intrinsic
viscosity of 2.0–2.5 dL/g). ABPBI membrane was prepared by
solvent casting method using methane sulfonic acid (MSA,
MERCK, Germany) as a solvent. The concentration of ABPBI
has been varied from 22 to 78 vol% and the corresponding
membranes were prepared by pouring the solution on a petri-
dish with gradually increasing the temperature up to 2008C for
90 min.
Characterizations
Relative viscosity of ABPBI solution in methane sulfonic
acid was determined with the help of Cannon-Fenske viscometer
by measuring the efflux time of the solvent as well as solution
at different concentration at 308C. The appropriate solution con-
centration was determined by considering the relative viscosity
of the solution, the membrane forming ability(which yields a
membrane of thickness of $60–100 lm) of the solution and the
mechanical properties (modulus and the hardness) of the mem-
brane. Hardness and the Young’s modulus of the corresponding
membrane were determined from nano-indentation technique
described below. Surface morphology of the membrane was per-
formed using an Alicona infinite focus optical three-dimensional
(3D) surface measurement device (Austria). Wide angle X-ray
Diffraction (WAXD) studies were carried out on a Phillips X-
Pert Pro. The incident X-rays (k 5 1.54 A˚ ) were monochromat-
ized using a Ni filter. WAXD patterns were recorded with a
step scan and step size of 0.028 between 58 and 608 (2h). The
elemental analysis of carbon, hydrogen, nitrogen, and sulfur was
calculated from FLASH EA 1112 SERIES (Thermofinnigan
instrument, Italy). Fourier transform infrared spectroscopic
(FTIR) analysis was carried out in the range of 400–4000 cm21
with Vertex 80 FTIR instrument (BRUKER, The United States).
Thermal stability of the various membranes was investigated in
the temperature range between room temperature to 10008C
using a thermogravimetric analyzer (TGA; Mettler Toledo STA
851, The United States); with nitrogen flow rate of 60 mL/min.
Membrane electrode assembly consists of an ion-exchange
membrane sandwiched between a pair of catalyst supported
electrodes under a pressure of 19.6 MPa. Hence, investigations
of the mechanical properties of the doped membranes were
essential to qualify for fuel cell application. Mechanical proper-
ties such as hardness (H) and Young’s modulus (E) of the
H3PO4 doped and undoped ABPBI membrane were determined
using a nano-indenter (TriboIndenter TI-900, Hysitron Inc., Min-
neapolis) with a Berkovich diamond indenter (tip radius of
150 nm and semi apex angle of 658). The Nano-indenter had a
load and depth resolution along z-axis of 1 nN and 0.04 nm,
respectively. Dielectric impedance spectrometer (Concept 80,
Novocontrol Technologies, Germany) was used to measure the
proton conductivity of doped and undoped membrane in the fre-
quency range of 1 Hz–1 MHz from room temperature to 2008C.
The experiment was carried out by placing the ABPBI mem-
brane in between two bronze electrodes by through plane assem-
bly. The experiment was carried out by Novocontrol Alpha-A
Analyzer, Germany in the frequency range of 1 Hz–10 MHz
and Z-view software was used for designing the model equiva-
lent circuit.
RESULTS AND DISCUSSION
The Selection of Appropriate ABPBI Concentration for Membrane
Formation
Acid-doped ABPBI membrane with good proton conductivity
value at higher temperature is considered as the best suitable
membrane for HTPEFC application. The crucial part of the
membrane preparation depends on the specific concentration of
the polymer in a good solvent. It primarily decides the film
forming capability of the solution and also the strength of the
well-prepared membrane. The addition of varying amount of the
polymer to a solvent (viz., methane sulfonic acid), affects the
relative viscosity of the solution. The development of the mem-
brane was carried out with a suitable solution on the basis of
the hardness and film forming capability supported by the rela-
tive viscosity value and the topography associated with the
membrane. Figure 2 shows the variation of hardness and relative
viscosity as a function of molar concentration of ABPBI in
methane sulfonic acid. It is observed that the ABPBI solution of
FIG. 1. Chemical repeat unit of poly(2,5-benzimidazole).
TABLE 1. Hardness and relative viscosity for different concentration of
ABPBI membrane.
Molar concentration
(mol/L)
Relative
viscosity (gr)
Hardness
GPa) Membrane
8.5 705 0.044 Partially formed
6.8 136 0.061 Partially formed
5.6 125 0.074 Formed
4.8 60 0.069 formed
4.2 33 0.051 formed
2 POLYMER ENGINEERING AND SCIENCE—2016 DOI 10.1002/pen

3. molar concentration between 8.5 and 6.8 mol/L could not form
the membrane continuously. However, the ABPBI solution of
molar concentration between 5.6 and 4.2 mol/L yielded continu-
ous membrane. Molar concentration of 5.6 mol/L with relative
viscosity of 125 of the solution having hardness of 0.074 GPa
associated with the membrane was optimized for HTPEFC
application. Table 1 shows the hardness and the relative viscos-
ity values for the various ABPBI membranes and their corre-
sponding solution, respectively.
Doping of the ABPBI Membrane
The membrane samples (2 cm 3 2 cm) obtained from sol-
vent casting, were immersed in the varying volume concentra-
tion (20, 40, 60, and 80 vol %) of the phosphoric acid (H3PO4)
solution. Although, it is reported in the literature, that the com-
plete doping could be achieved within 12 h, the membrane was
doped for 15 h to ensure complete doping in the present study
[11, 12]. It has been observed that the membrane was com-
pletely dissolved in 80 vol% H3PO4 solution; however, mem-
brane survived up to 60 vol% H3PO4 solution. Extent of acid
doping was estimated based on the following correlation (Eq. 1)
and it is summarized in Table 2.
Acid dopoing level5
Weight difference
Initial weight
3
Mw of ABPBI repeat unit
Mw of H3PO4
(1)
Membrane Topography
The surface topography of undoped ABPBI membrane
($110 lm thickness) has been investigated via Optical 3D sur-
face measurement. The surface of the ABPBI membrane was
investigated as it was prepared through solvent casting method.
The average roughness and root mean square roughness of the
ABPBI membrane show 832 and 1000 nm, respectively (Fig. 3),
which is in good agreement for the membrane prepared via sol-
vent casting method. The maximum peak to valley height of
roughness and minimum peak to valley height of roughness
found to be 5.9 and 4.5 lm, respectively.
In general, the top surface of a membrane would come in con-
tact with air, whereas the bottom surface stayed in contact with the
plane glass surface of the petri-dish during the solvent casting pro-
cess; however, the SEM images exhibited almost similar features
for both the surfaces [8, 11]. The roughness varies more towards
the end corner of the membrane with 10214
lm height; maximum
peak valley height was found to be 5.9 lm from Fig. 3 with a
mean value to peak height of 4.5 lm. The low peak height value
does not exhibit any major impact on the membrane properties.
Wide Angle X-Ray Diffraction Analysis
ABPBI is semicrystalline in nature, but the doping process
destroys significantly the crystalline nature associated with the
ABPBI membrane. The variation in the crystalline structure has
been studied through WAXD analysis. Figure 4 shows the
WAXD pattern of doped and undoped ABPBI membrane. A sin-
gle peak is observed at 2h 5 278, which corresponds to parallel
benzimidazole ring forming a stacked structure of ABPBI [13,
14]. The sharpness of the peak shows the highly crystalline
nature of the membrane. The peak is broadened with increasing
extent of doping level, referring to an increase in the amorphous
nature, which may lead to reduced mechanical strength. The
decrease in the extent of crystallinity may be due to the discon-
tinuity in the molecular chain arising from the free H3PO4 mole-
cules trapped in the intermolecular free volume space.
Elemental Analysis
The amount of absorbed H3PO4 by the ABPBI membrane
can be calculated via elemental analysis. The membrane is
FIG. 2. Hardness of the ABPBI membrane and the corresponding viscosity
for varying concentration of ABPBI solution. [Color figure can be viewed in
the online issue, which is available at wileyonlinelibrary.com.]
TABLE 2. Doping level for various doped membranes.
Sample Doping (volume of H3PO4, %) Doping level
ABPBI 20% H3PO4 20 0.25
ABPBI 40% H3PO4 40 0.49
ABPBI 60% H3PO4 60 1.65
ABPBI 80% H3PO4 80 4.18
FIG. 3. Surface topography of the undoped ABPBI membrane. [Color fig-
ure can be viewed in the online issue, which is available at wileyonlineli-
brary.com.]
DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—2016 3

4. completely dissolved in 80 vol% H3PO4 solution after 12 h, as
it absorbed 87 wt% of H3PO4 (Table 3) whereas, membrane
swelled up moderately in 60 vol% H3PO4 solution. The residue
is different from C, H, N, and S, which attributes to phosphate
ion (PO32
4 ) content in the membrane. Hydrogen is associated
with both H3PO4 and the polymer, so the phosphate ion concen-
tration only can provide the idea of amount of H3PO4 present in
the doped membrane [15]. During the casting of ABPBI in
MSA, the sulfur present in MSA protonates the benzimidazole
group of the polymer and being displaced by H3PO4 with
increasing doping level. Therefore, only sulfur is present in the
ABPBI membrane, which is mentioned in the Table 3; however,
it is completely disappeared after doping. The residue for
undoped ABPBI is 35.82%, which confirms only about the pres-
ence of oxygen due to MSA but the residue of remaining doped
sample may explain the amount of H3PO4 present in the
membrane.
Fourier Transform Infrared (FTIR) Analysis
The molecular signature of phosphoric acid in the doped
ABPBI membrane can also be determined by the FTIR spectro-
scopic analysis. FTIR analysis (Fig. 5) shows three major
absorption peaks, which provide the molecular signature for the
undoped and the doped ABPBI membrane. The absorption band
at 3300–3600 cm21
indicates NAH stretching [11]. The broad-
ening of the band may be due to doped sample with the pres-
ence of N1
AH stretching [7]. It implies that proton can jump
from one imidazole group to another non protonated imidazole
group or any phosphoric acid molecule, which is not observed
in the case of undoped membrane [16]. Moreover, FTIR analy-
sis exhibits the OAH stretching vibration (2500–3000 cm21
) as
both are hygroscopic in nature. The most important two peaks
in the range of 1100–1500 cm21
, may indicate the main absorp-
tion peak for H3PO4.
Thermo-Gravimetric Analysis
The physical and chemical changes of the membrane as a
function of temperature are essential to study to evaluate the sta-
bility of the membrane for high temperature fuel cell applica-
tion, which is studied through TGA. Figure 6 explains the mass
loss of ABPBI polymer as a function of temperature of both
doped and undoped state. The initial weight loss starts at 508C–
608C due to the absorbed water as H3PO4 doped ABPBI mem-
brane is very much hygroscopic in nature [17]. During prepara-
tion and handling, it might have absorbed 8% of water. The
second weight loss starts at 1508C up to 2108C due to free phos-
phoric acid elimination from intermolecular free volume spaces.
Phosphoric acid changes to pyrophosphoric acid above 1808C
and then detached from the main chain. The DTG curve (inset
in Fig. 6) also clearly depicts the larger peak above 6008C and
smaller peak at 508C–608C of the ABPBI membrane. The
molecular chain vibrates violently causing weaken of the back-
bone at above 6008C and starts to degrade at 8008C [18].
Young’s Modulus and Hardness of the Doped ABPBI Membrane
Nano-hardness (H) and Young’s modulus (E) of the ABPBI
membrane of varying level of H3PO4 doping along with
undoped ABPBI membrane were determined using nano-
indentation technique. H and E values were evaluated according
to DIN 50359-1 standard from the load (P) versus depth of
FIG. 4. WAXD pattern of undoped and H3PO4 doped ABPBI membranes.
[Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
TABLE 3. Summary of the elemental analysis performed for H3PO4 doped
ABPBI membranes.
Component (wt%)
0%
H3PO4
20%
H3PO4
40%
H3PO4
60%
H3PO4
80%
H3PO4
Nitrogen 12.05 11.15 7.35 5.56 1.52
Carbon 40.15 33.53 23.23 17.26 8.13
Hydrogen 3.35 3.09 3.20 3.52 3.72
Sulfur 8.63 0 0 0 0
Total 64.20 47.78 33.79 26.34 13.37
Residue CH3SO3H 35.82 0 0 0 0
Residue H3PO4 0 52.22 66.20 73.65 86.63
FIG. 5. FTIR spectra of undoped and H3PO4 doped ABPBI membranes.
[Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
4 POLYMER ENGINEERING AND SCIENCE—2016 DOI 10.1002/pen

5. penetration (h) plot using the well-established Oliver and Pharr
method with a fixed constant load of 100 nN. The variations in
depth of penetration with applied load for different undoped and
doped membranes are shown in Fig. 7. The total area under the
P–H plot is much reduced in un-doped membrane as compared
with that of the doped membrane. The hardness and Young’s
moduli data of the doped and un-doped membrane derived from
Fig. 7 are summarized in Table 4. The results clearly demon-
strate that increasing doping concentration results in a decreased
value of H and E. The final depth of penetration in the un-
doped membrane is approximately 25% lower than the 60 vol%
H3PO4 doped ABPBI membrane. For pure ABPBI membrane,
inter molecular H-bonding may act as a dominant force, which
influences the mechanical strength of the ABPBI membrane
[19]. When phosphoric acid is introduced, the molecular cohe-
sion of ABPBI is decreased; however, strong hydrogen bond
persists between AN@ and ANHA group [20].
It is also observed that the doped membrane shows less elas-
tic recovery than the corresponding undoped membrane. It may
be also due to the fact that the excess free acid present between
the intermolecular chains, enhances the chain flexibility and acts
like a plasticizer in ABPBI membrane. For the same reason, the
open space volume of polymer increases [21] and the membrane
swells more and more with increasing doping concentration,
which causes the reduction of E and H value [22]. Undoped
ABPBI membrane shows (Table 4) the E and H value of 2.46
[19] and 0.92 GPa, respectively, and the corresponding E and H
values for 1.65 doping level of 60 vol% H3PO4 doped mem-
brane exhibits the value of 0.14 and 0.067 GPa, respectively.
Proton Conductivity and Activation Energy
Proton conductivity is increased with increasing temperature
for all the membranes expect for the undoped ABPBI mem-
brane. Figure 8a shows that proton conductivity remains con-
stant up to 1008C for the undoped and doped ABPBI samples
until 40% H3PO4 concentration or there is a marginal increase
in proton conductivity for all the membranes, which indicate
Grotthus hopping mechanism of proton transfer, where the hop-
ping between two molecules (acid–acid, acid–water, acid–benz-
imidazole ring) may take place in these membranes. This
mechanism exhibits less significant role below 1008C. Even the
undoped membrane exhibits very less proton conductivity,
which matches well with the reported value of 10212
S/cm [12].
However, the proton conductivity exponentially increases with
increase in temperature for all the doped membranes. 20%
H3PO4 doped ABPBI membrane with 0.25 doping level shows a
proton conductivity of 1028
S/cm and increases to 1027
S/cm
with an increasing doping level of 0.49. The 60% H3PO4 doped
membrane with doping level of 1.65 shows highest proton con-
ductivity value of 2.2 3 1022
S/cm (Fig. 8b). This suggests that
the excess H3PO4, which leads to higher doping level in ABPBI
membrane, may help in proton conduction. At higher tempera-
ture, proton transfer occurs primarily through one NAH site to
H3PO4 anion and H3PO4 to H3PO4 for contributing conductivity.
The increase in proton conductivity in ABPBI membrane with
increasing doping level may be due to the presence of excess
acid around the molecular chain. It is to be noted that 80%
H3PO4 doped ABPBI membrane swelled up significantly and
dissolved when it was impossible to measure proton
conductivity.
The activation energy (Ea) can be calculated from the Arrhe-
nius plot of various ABPBI membranes, which is shown in Fig.
9. Undoped ABPBI membrane registers activation energy of
147.58 KJ/mol; whereas it is decreased gradually to 20.85 KJ/
mol for 60% H3PO4 doped ABPBI membrane. Activation
energy of ABPBI 40% H3PO4 doped ABPBI membrane shows a
lower value (31.6 KJ/mol) as compared with ABPBI 20%
FIG. 6. TGA and DTG plot for undoped and H3PO4 doped ABPBI mem-
branes. [Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
FIG. 7. Load (P) versus depth (H) plots for undoped and H3PO4 doped
ABPBI membranes. [Color figure can be viewed in the online issue, which
is available at wileyonlinelibrary.com.]
TABLE 4. Young’s modulus and hardness of undoped and doped ABPBI
membrane.
Sample Young’s modulus (GPa) Hardness (GPa)
ABPBI 0% 2.46 6 0.34 0.147 6 0.03
ABPBI 20% 1.85 6 0.35 0.089 6 0.028
ABPBI 40% 1.31 6 0.16 0.096 6 0.011
ABPBI 60% 0.92 6 0.13 0.067 6 0.006
DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—2016 5

6. H3PO4 doped ABPBI membrane (33.7 KJ/mol). In general,
undoped membrane shows higher activation energy than doped
ABPBI membrane. It implies that energy needed to transport
proton requires less energy for doped ABPBI membrane as com-
pared with undoped membrane. The lowest activation energy of
20.85 KJ/mol corresponding to 60% H3PO4 doped ABPBI mem-
brane suggests that the proton conduction would be faster than
other corresponding membrane [14].
Impedance Spectroscopic Analysis
The impedance spectra (Fig. 10) exhibit a suppressed semi-
circle at high frequency domain, which may be due to the con-
tribution of the membrane resistance coupled with constant
phase element (CPE), which may be in parallel combination.
Further, a straight line in low frequency region corresponds to
the linear diffusion process of charged particle and described by
Warburg element [23]. The model circuit (Fig. 10a) is consid-
ered based on the experimental data obtained from the sample
at 119.958C. The trend of slowly increasing conductivity from
undoped ABPBI to 40% H3PO4 doped ABPBI membrane and
then sudden increase to 60% H3PO4 doped ABPBI membrane
matches well with the fitting value, which is seen from Fig. 10.
An electron cannot cross from the electrode to the electrolyte
membrane, so the electrode/electrolyte interface is likely to be
presented as double layer capacitance [24]. However, an ideal
capacitance is generally expressed by a vertical line in Z0
versus
Z00
plot. Here, it deviates for solid electrolyte membrane due to
irregularities on the surface of the electrode. Therefore, it is rep-
resented as CPE mentioned in (Eq. 2), the CPE behavior is con-
tributed by surface inhomogeneity, reactivity, porosity, current,
and potential distribution associated with electrolyte geometry,
which is known as fractal geometry [25].
ZCPE5
1
QðjxÞ1
(2)
Where, ZCPE is the impedance of CPE, x is the rotational fre-
quency, and U is the CPE component When U 5 1, ZCPE
behaves like a complete capacitor and when U 5 0, it is com-
pletely independent of frequency and the value of U should
always be less than 1. The resistance value at 119.958C is con-
sidered for modeling analysis. The output of modeling result is
compared with the experimental value. The %error in resistance
(R) of undoped ABPBI, 20% H3PO4 doped ABPBI membrane,
40% and 60% H3PO4 doped ABPBI membrane show 3.60%,
3.25%, 7.21%, 5.87% of error for constant phase element (CPE)
shows 1.55%, 4.26%, 3.22%, 2.92%, respectively. It implies
modeling results are appreciably fitting with the experimental
results.
The suitability of H3PO4 doped ABPBI membrane as a pro-
ton conducting membrane for fuel cell application is elucidated
via various characterization techniques employed in the present
work and is compared in Table 5 considering some of the repre-
sentative reported literature. It is observed from Table 5 that
extensive characterization techniques have been utilized to ana-
lyze the H3PO4 doped ABPBI membrane in the present investi-
gation, which shows high proton conductivity. Moreover, nano-
indentation technique has been utilized for evaluating mechani-
cal properties of the H3PO4 doped ABPBI membrane. Further,
an extensive impedance analysis has been used to understand
the various parameters, which affect the proton conduction in
the H3PO4 doped ABPBI membrane.
FIG. 8. Proton conductivity of (a) undoped, 20 vol%, 40 vol% and (b) 60 vol% H3PO4 doped ABPBI membrane.
[Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
FIG. 9. Arrhenius plot of logr versus 1000/T for undoped and H3PO4
doped ABPBI membranes. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
6 POLYMER ENGINEERING AND SCIENCE—2016 DOI 10.1002/pen

7. CONCLUSIONS
The highest doping level of 1.65 of phosphoric acid was
achieved for the investigated ABPBI membrane. Presence of
H3PO4 molecule is detected by FTIR peak at 1300 and
1400 cm21
also it is supported by the elimination of same
molecule at 1808C and 2008C through TGA.The nano indenta-
tion analysis clearly demonstrated that an increasing doping
concentration might result in a decreased value of H and E.
The final depth of penetration in the undoped membrane was
approximately 25% lower than the 60% H3PO4 doped ABPBI
membrane. For pure ABPBI membrane, inter molecular H-
bonding might act as a dominant force, which influenced the
mechanical strength of the ABPBI membrane. Undoped
ABPBI registered E and H value of 2.46 and 0.92 GPa,
which are much higher than 60% H3PO4 doped ABPBI mem-
brane. 60% H3PO4 doped ABPBI membrane achieved only
0.14 and 0.067 GPa value for E and H because of its highest
doping level of 1.65. The undoped ABPBI membrane exhib-
ited very low proton conductivity; however, the proton con-
ductivity exponentially increased with increase in temperature
for all the H3PO4 doped ABPBI membranes. The 20% H3PO4
doped ABPBI membrane with 0.25 doping level showed a
proton conductivity of 1028
S/cm and increased to 1027
S/
cm, with an increasing doping level of 0.49. About 60%
H3PO4 doped ABPBI membrane with doping level of 1.65
exhibited highest proton conductivity value of 2.2 3 1022
S/
cm. Undoped ABPBI membrane registered an activation
energy of 147.58 KJ/mol; whereas it was decreased gradually
to 20.85 KJ/mol for 60% H3PO4 doped ABPBI membrane.
The lowest activation energy of 20.85 KJ/mol corresponding
to 60% H3PO4 doped ABPBI membrane might suggest that
the proton conduction would be faster than other correspond-
ing membranes. Equivalent circuit model was employed in
order to understand the important circuit elements (viz., elec-
trolyte resistance, constant phase element, and Warburg ele-
ment) influencing the proton conductivity of H3PO4 doped
ABPBI membrane. It was proposed that the circuit involved
resistance, which was parallel to the constant phase element,
and was in series with Warburg element that matched per-
fectly with the experimental results.
FIG. 10. (a) Equivalent model circuit for proton conduction in undoped and doped ABPBI membranes, where R1,
CPE1, and Ws1 represent the membrane resistance, constant phase element and diffusion element Warburg; Complex
impedance of ABPBI membrane: (b) undoped ABPBI, (c) 20 vol% H3PO4 doped ABPBI, (d) 40 vol% H3PO4 doped
ABPBI, (e) 60 vol% H3PO4 doped ABPBI. The dotted line shows the experimental result, whereas the continuous
line represents the fitted result. [Color figure can be viewed in the online issue, which is available at wileyonlineli-
brary.com.]
DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—2016 7

8. In brief, H3PO4 doped ABPBI membrane was developed as
proton conducting membrane for fuel cell application, which
showed the proton conductivity of 2.2 3 1022
S/cm at 1.65
doping level and showed E and H values of 0.14 and 0.067
GPa, respectively.
ACKNOWLEDGMENTS
The authors would like to acknowledge Nano-indenter and
Broad-band Dielectric Spectroscopy Central Facilities at
MEMS, IIT Bombay. The authors would like to thank Mr. Jay-
ant Kolte (MEMS, IIT Bombay) for his valuable suggestions on
impedance analysis. The authors would also acknowledge
Gharda Chemicals, Mumbai for supplying ABPBI and
methane-sulfonic acid for carrying out this research work.
NOMENCLATURE
ABPBI Poly(2,5-benzimidazole)
DAB 3,4 Diaminobenzoic acid
H3PO4 Phosphoric acid
HT-PEFC High-temperature polymer electrolyte fuel cell
IPA Isophthalic acid
PEFC Polymer electrolyte fuel cell
PFSE Perfluorosulfonated solid electrolyte
TAB Tetra aminobiphenyl
WAXD Wide angle X-ray diffraction
CPE Constant phase element
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ParametersPresentwork
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J.A.Asensioetal.[7]
J.A.Asensioetal.[14]
Directacidcasting
J.A.Asensioetal.[18]
SulfonatedABPBI
HaitaoZhengetal.[19]
PorousABPBI
Viscosity0.52Pas2.4dL/g2.3–2.4dL/g2.4dL/g–
DopingABPBI.4.18H3PO4ABPBI.3.0H3PO4ABPBI.3.0H3PO4SABPBIÁ4.6H3PO4–
TopographyAverageroughness5832.2nm––––
MechanicalpropertiesHardness:0.06760.006GPa
Modulus:0.9260.13GPa
–––Stressatbreak0.6MPa
XRDpeak278268258––
ModelEquivalentcircuitParallelcombinationofresistance
andconstantphaseelement,then
serieswithWarburgelement
––––
Protonconductivity(S/cm)2.231022
at2008C6.231022
at1508C
and30%RH
1.531022
at1808C3.531022
S/cmat1858C2.2331022
1808C
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