Imidazole-Linked Crystalline Two-Dimensional Polymer with Ultrahigh Proton-Conductivity

Article abstract of DOI:10.1021/jacs.9b06080

Proton-exchange membrane fuel cells are promising energy devices for a sustainable future due to green features, high power density, and mild operating conditions. A facile proton-conducting membrane plays a pivotal role to boost the efficiency of fuel cells, and hence focused research in this area is highly desirable. Major issues associated with the successful example of Nafion resulted in the search for alternate proton conducting materials. Even though proton carrier loaded crystalline porous organic frameworks have been used for proton-conduction, the weak host-guest interactions limited their practical use. Herein, we developed a crystalline 2D-polymer composed of benzimidazole units as the integral part, prepared by the condensation of aryl acid and diamine in polyphosphoric acid medium. The imidazole linked-2D-polymer exhibits ultrahigh proton conductivity (3.2 × 10^-2 S cm^-1) (at 95% relative humidity and 95 °C) in the pristine state, which is highest among the undoped porous organic frameworks so far reported. The present strategy of a crystalline proton-conducting 2D-polymer will lead to the development of new high performing crystalline solid proton conductor.

Full text of DOI:10.1021/jacs.9b06080

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Communication
Imidazole-linked Crystalline Two-Dimensional
Polymer with Ultrahigh Proton-Conductivity
Kayaramkodath Chandran Ranjeesh, Rajith Illathvalappil, Sairam Dnyaneshwar
Veer, Joseph Peter, Vivek Chandrakant Wakchaure, - Goudappagouda,
K. Vipin Raj, Sreekumar Kurungot, and Sukumaran Santhosh Babu
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b06080 • Publication Date (Web): 11 Sep 2019
Downloaded from pubs.acs.org on September 12, 2019
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Imidazole-linked Crystalline Two-Dimensional Polymer with Ultrahigh
Proton-Conductivity
Kayaramkodath Chandran Ranjeesh,†,§ Rajith Illathvalappil,§,‡ Sairam Dnyaneshwar Veer,†,§
Joseph Peter,Ψ Vivek Chandrakant Wakchaure,†,§ Goudappagouda,†,§ K. Vipin Raj,§,‡ Sreekumar
Kurungot,§,‡ Sukumaran Santhosh Babu*,†,§
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†Organic Chemistry Division, National Chemical Laboratory (CSIR-NCL), Dr. Homi Bhabha Road, Pune-4110 08,
India
ΨSacred Heart College, Kochi-682 013, Kerala, India
§Academy of Scientific and Innovative Research (AcSIR), Ghaziabad-201 002, India
‡Physical and Materials Chemistry Division, National Chemical Laboratory (CSIR-NCL), Pune-4110 08, India
Abstract: Proton-exchange membrane fuel cells are promising energy devices for sustainable future due to green features,
high power density, and mild operating conditions. A facile proton-conducting membrane plays a pivotal role to boost
the efficiency of fuel cells and hence focused research in this area is highly desirable. Major issues associated with the
successful example of Nafion resulted in the search for alternate proton conducting materials. Even though proton carrier
loaded crystalline porous organic frameworks have been used for proton-conduction, the weak host-guest interactions
limited its practical use. Herein, we developed a crystalline 2D-polymer comprised of benzimidazole units as the integral
part, prepared by the condensation of aryl acid and diamine in polyphosphoric acid medium. The imidazole linked-2D-
polymer exhibits ultrahigh proton conductivity (3.2 × 10-2 Scm-1) (at 95% relative humidity and 95 °C) in the pristine state,
which is highest among the undoped porous organic frameworks so far reported. The present strategy of crystalline
proton-conducting 2D-polymer will lead to the development of new high performing crystalline solid proton conductor.
The rise in global energy demand and depleting
nonrenewable resources impart serious energy crisis.1 In
this context, significant efforts in identifying alternate
energy sources such as proton-exchange membrane fuel
cells (PEMFCs) have been devoted.2 As an emerging green
and sustainable energy source, PEMFCs have attracted
much attention in the current decade.2 A facile proton
conduction is vital in the performance of FCs and hence
significant amount of research has been directed towards
it.3 Subsequently, a polyelectrolyte membrane, Nafion4
has been emerged as an efficient membrane separator in
FCs due to its robust structure, high proton conductivity
and reusability without significant loss of activity. The
need for more effective PEMs directed the research
towards structurally modified new materials with
enhanced proton conduction.5
pristine state have rarely been used for proton
conduction, instead, mostly served as an insulating 1D-
channel for proton carriers such as phosphoric acid,
imidazole, p-toluene sulfonic acid etc.8 Here the weak
host-guest interaction leads to unavoidable leaching of
proton carriers from the channels. The post
functionalization is also restricted due to the nonuniform
doping of proton carriers and the lack of deeper
understanding of the mechanism. In short, the current
strategies for developing COF-based proton conductors
8,9
rely either on doping with external proton carriers or on
randomly integrated pendant groups in the framework.
10
Even though doping is simple and effective, the overall
control on uniform loading of dopant and achieving a
reliable result is not consistent. In this context, a proton
carrier linked 2D-polymer can be a better candidate for
proton-conduction. Surprisingly, the proton-conducting
feature of an imidazole linked 2D-polymer is hitherto
reported.
Recently, crystalline porous materials such as covalent
and metal-organic frameworks (COFs and MOFs), and
hydrogen-bonded frameworks (HOFs) have gathered
attention due to its framework tunability, chemical
stability, ease of synthesis and feasibility of hybridization
with other materials.6 Moreover, COFs, MOFs, and HOFs
have been attempted for proton-conducting applications.7
As one of the successful strategies, many proton carrier
doped COFs and MOFs exhibited excellent proton-
conducting behavior. However, till this date, COFs in the
We aimed to synthesize a benzimidazole-linked 2D-
polymer by using a monomer 3, which can undergo facile
reaction with 1,2,4,5-tetraaminobenzene 4 in the presence
of polyphosphoric acid (PPA) (Fig. 1a). To confirm the
feasibility of 2D-polymerization, we successfully
synthesized two model derivatives M1 and M2 in good
yield (Fig. 1a, Scheme S1, S2). Even though PPA mediated
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imidazole synthesis is well explored for many small
molecules and linear polymers by the co-condensation
reactions of aryl acid and o-diamine,11 not yet exploited
for a crystalline 2D-polymer synthesis. Moreover, the
formation of two model derivatives motivated to go
further with the same synthetic route.
Page 2 of 6
from carbon atoms of benzimidazole ring and signals at
132, 127, 114 ppm from remaining aromatic rings.
Conclusive evidence is obtained from the peak at 153 ppm
corresponding to carbon atom 12, assigned on the
benzimidazole ring (Fig. 2b, inset). Comparison of the
UV-Vis absorption spectrum of BIP oligomers with model
derivatives that are absorbing in the 280-480 nm range
indicated the presence of benzimidazole moiety (Fig. S4).
A broad peak extending to the near-infrared region in the
solid-state diffuse reflectance spectrum also endorses the
2D-polymerization (Fig. 2c). X-ray photoelectron
spectroscopy (XPS) analysis of BIP, after extensive
washing with organic solvents and water, clearly showed
peaks for C1S, N1S, and O1S and confirmed the absence of
trapped polyphosphoric acid used for synthesis (Fig. 2d).
Thermogravimetric analysis (TGA) revealed the high
thermal stability of BIP, comparable to stable COFs
reported (Fig. 2e).13
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1.6 nm
BIP
150 oC,
24 hr
180 oC,
24 hr
After
Washing
3
b)
c)
d)
e)
C
N
Figure 1. a) Synthetic for BIP (hydrogen atoms have been
removed for clarity), photographs showing the progress of
polymerization of BIP, and chemical structure of monomers
M1 and M2. b) TEM image of BIP showing the presence of
layered sheets. c) TEM-high-angle annular dark-field image
and corresponding element mapping of BIP for d) C, e) N.
As we envisioned, PPA based synthetic route provided a
thermodynamically controlled condensation reaction of
the monomers12 to form stable benzimidazole-linked 2D-
polymer (Fig. 1a, S1). Note that the multilayer stacks of the
2D-polymer structure as revealed by transmission
electron microscope (TEM) images of BIP suspension in
methanol points to the extended 2D-polymerization and
the multilayer stacks could be formed as a result of π-π
stacking of the individual 2D-polymer layers (Fig. 1b, S1).
The composition of BIP is confirmed by TEM elemental
mapping (Fig. 1c-e). Scanning electron microscope (SEM)
image showed the bulk structure of BIP composed of
layered-sheets (Fig. S3). A comparison of the Fourier-
transform infrared (FT-IR) spectrum of BIP (Fig. 2a) with
all the starting materials and model derivatives reveals N-
H stretching as a broad peak ranging from 3412 cm-1 (free
N-H) to 3200 cm-1 (hydrogen-bonded N-H), while intense
new bands appeared at 1634 cm-1 (C=N), which can be
assigned to vibrations of the benzimidazole ring skeleton.
The intensity of the C=O band at 1691 cm-1 in 3 is
substantially disappeared in BIP, which suggested the full
Figure 2. a) FT-IR spectra of 3, 4, M1, M2, and BIP. b) 13C
CP-MAS NMR spectrum of BIP, asterisks denote spinning
sidebands. c) Solid-state diffuse reflectance UV-Vis-NIR
spectra of M1, M2, and BIP. d) XPS and e) TGA of BIP.
The permanent porosity of BIP evaluated by measuring
N2 adsorption isotherm at 77 K showed reversible type I
adsorption isotherm for the microporous structure of BIP
(Fig. 3a).14 The Brunauer-Emmett-Teller (BET) surface
area of the activated BIP sample was found to be 510 m2/g
with a narrow pore size distribution profile centered at 1.6
nm (Fig. 3b), which is matching with the calculated
distance as shown in Fig. 1a. The powder X-ray diffraction
(PXRD) pattern of BIP exhibited an intense peak at 4.2°,
corresponding to the reflection from the (100) plane (Fig.
3c). The peak at 2θ = 25° is due to the reflection from the
(001) plane.13 The corresponding π-π stacking distances
between 2D-polymer layers were calculated to be 3.8 Å
from the d-spacing between 001 planes (Fig. 3c).
Structural simulations with an eclipsed AA-stacking
structure generated an XRD curve that is consistent with
the experimental profile (Fig. 3c-3e, S5, 6). It is clear from
the PXRD analysis that PPA assisted synthetic route helps
to undergo thermodynamically controlled condensation
13
consumption of monomers during 2D-polymerization. C
cross-polarization magic angle spinning nuclear magnetic
resonance (13C CP-MAS NMR) spectrum of BIP (Fig. 2b)
contains signals located at 153, 142 and 99 ppm arising
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reaction of the monomers,12 which resulted in a crystalline
with all well-known proton-conducting porous materials,
reported to date (Table S4). We have investigated the
time-dependent proton conductivity of BIP and as shown
in Fig. 4e, a negligible difference observed over 10 hours
of measurement implies that efficient proton conductivity
can be retained for longer hours. In addition, the proton
conductivity of BIP is supported by the low electrical
conductivity (~5.2 x 10-8 S/cm), which has a negligible
effect on proton conductivity of BIP (Fig. S10).16
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framework.
a)
b)
1.6 nm
Adsorption
Desorption
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Pore Width (nm)
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0.0
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1.0
The role of water molecules in high proton conductivity
Relative Pressure (P/P0)
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was confirmed by comparison of the conductance of BIP
c)
d)
e)
BIP
o
by exposing it to H2O and D2O vapor at 25 C and 98%
RH (Fig. 4f). The conductivity values were varied as 2.9 ×
10-3 (H2O) and 2.0 × 10-3 (D2O) Scm-1. The observed drop in
conductivity for D2O compared to that of the H2O
exposed sample under identical condition can be
attributed to the heavier isotopic effect of D2O. Deuteron
conductivity experiment points to Grotthus-type
mechanism, supported by a decrease in charge carrier
velocity upon replacing proton with deuteron. The lack of
anhydrous conductivity and available deuteron
conductivity at a slower rate, in turn, ascertain the vital
role of water molecules in the conduction process. The
porous nature of the framework allows diffusion of water
molecules through the nanochannels, making a network
of hydrogen bonds available for the facile proton
conduction. Density functional theory calculations on a
fragment consists of two layers also support this kind of a
mechanism assisted by hydrogen-bonded water molecules
in the nanochannels (Fig. S11-13).
Simulated
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2 (degree)
0.38 nm
Figure 3. a) BET N2 sorption isotherm curves of BIP
measured at 77 K. b) Pore-size distribution profile of BIP. c)
PXRD profiles of BIP; simulated pattern for an AA stacking
mode of BIP is provided for comparison. d) Top and e) side
views of the layered 2D-polymer resulted from eclipsed AA
stacking.
A proton conduction study was carried out by the AC
impedance method using compacted pellets of BIP
having a thickness of 1.44 mm after saturating for 12 h
(Fig. 4). The proton conductivity was measured at
different temperatures (from 30 to 95 ᵒC) under varying
relative humidity (RH) (from 50 to 95%). Even though
BIP exhibited lower proton conductivity below 60 oC (Fig.
S7), the measurements below 60% RH resulted in only
noisy signals (Fig. S8). Hence, BIP is not an active proton
transporter under anhydrous condition. The conductivity
value increased with an increase in temperature and RH,
and concluded that BIP exhibited highest proton-
conductivity of 3.2  10-2 Scm-1 at 95 °C under 95% RH
(Fig. 4, Table S1,2). The Nyquist plots were obtained at
different RHs (from 60 to 95%) by keeping a constant
temperature of 95 oC, and at different temperatures (from
60 to 95 ᵒC) under constant RH, 95% (Fig. 4b,c). Nyquist
plots displayed a noticeable decrease of resistance with
increasing RH, indicates that the conductivity of BIP
increases with a rise in humidity and the high water
affinity is confirmed by the contact angle and water vapor
sorption measurements (Movie S1 and Fig. S9). It
indicates that water plays a significant role in the proton-
conduction pathway. The activation energy determined
using temperature-dependent conductivity profiles of BIP
was found to be 0.31 eV (Fig. 4d), which suggests the
possibility of a Grotthuss-type mechanism.15 BIP pellet
retains excellent proton conductivity and Table S3
summarises the reliable performance of BIP from three
different batches under the best performing condition.
Interestingly, for the first time, a pristine 2D-framework
shown high proton conductivity, which is comparable
a)100
100
90
80
70
60
b)
90
2k
80
1k
60% RH
70% RH
80% RH
95% RH
70
60
0
0
1000
Z' ()
2000
0.00
0.01
0.02
0.03
Conductivity (S/cm)
40
20
-3
c)
d)
60 o
70 o
80 o
95 o
C
C
C
C
-4
-5
Ea = 0.31 eV
0
-6
0
20
Z' ()
40
2.7
2.8
2.9
3.0
1000/T (K-1)
40
200
e)
f)
After 1h
After 2h
After 4h
After 8h
After 10h
H2O 2.9 x 10-3 S/cm
2.0 x 10-3 S/cm
D2O
20
0
100
0
0
20
Z' ()
40
0
100
Z' 
200
Figure 4. a) Variation of temperature and humidity
dependent proton conductivity of BIP. Nyquist plots of BIP
measured b) under 95 ᵒC at different RH and c) 95 % RH at
different temperatures. d) Arrhenius plots for BIP at
different temperatures. e) Time-dependent Nyquist plots of
BIP measured at 95 oC and under 95% RH. f) Comparison of
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o
the Nyquist plots of BIP measured at 25 C and under 98%
RH using H2O and D2O.
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To establish the advantages of new synthetic protocol,
we compared the proton conductivity of BIP and the
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value 1.2  10-4 Scm-1 for the latter is far inferior to BIP
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In conclusion, we introduced a new protocol to develop
a porous, crystalline benzimidazole-linked 2D-polymer
through PPA assisted synthetic route. A unique design of
long-range ordered 2D-framework having interspersed,
nonconducting (pyrene) and functionally proton hoping
(benzimidazole) regions to facilitate proton transport. In
principle, our demonstration might shift the current focus
of (unevenly) proton carrier doped or post functionalized
porous organic materials to a new class of porous
crystalline 2D-polymers having
a precise molecular
structure, which impart high pristine proton conductivity.
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AUTHOR INFORMATION
Corresponding Author
*sb.sukumaran@ncl.res.in
Funding Sources
This work is supported by Science and Engineering Research
Board (SERB), Govt. of India, EMR/2014/000987).
ACKNOWLEDGMENT
KCR, VCW, SDV and Goudappagouda acknowledge
University Grants Commission (UGC), India for fellowship.
Prof. R. Vaidhyanathan and Debanjan Chakraborty, IISER,
Pune is acknowledged for water vapor sorption studies.
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