Post-synthetic Modification of Covalent Organic Frameworks through in situ Polymerization of Aniline for Enhanced Capacitive Energy Storage

Article abstract of DOI:10.1002/asia.202001216

Covalent organic frameworks (COFs) having layered architecture with open nanochannels and high specific surface area are promising candidates for energy storage. However, the low electrical conductivity of two-dimensional COFs often limits their scope in

Full text of DOI:10.1002/asia.202001216

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CHEMISTRY
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Accepted Article
Title: Post-synthetic modification of covalent organic frameworks
through in situ polymerization of aniline for enhanced capacitive
energy storage
Authors: Tapas Kumar Dutta and Abhijit Patra
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Post-synthetic modification of covalent organic frameworks
through in situ polymerization of aniline for enhanced capacitive
energy storage
Tapas Kumar Dutta and Abhijit Patra*[a]
Abstract: Covalent organic frameworks (COFs) having layered
architecture with open nanochannels and high specific surface area
are promising candidates for energy storage. However, the low
electrical conductivity of two-dimensional COFs often limits their
scope in energy storage applications. The conductivity of COFs can
be enhanced through post-synthetic modification with conducting
polymers. Herein, we developed polyaniline (PANI) modified triazine-
based COFs via in situ polymerization of aniline within the porous
frameworks. The composite materials showed high conductivity of
1.4-1.9 x 10-2 S cm-1 at room temperature with a 20-fold enhancement
of the specific capacitance than the pristine frameworks. The
fabricated supercapacitor exhibited a high energy density of 24.4 W h
kg-1 and a power density of 200 W kg-1 at 0.5 A g-1 current density.
Moreover, the device fabricated using the conducting polymer-triazine
COF composite could light up a green light-emitting diode for 1 min
after being charged for 10 s.
lack of control over the pore-size distribution and pore structure
often leads to a sluggish ion-diffusion resulting in an undesirable
capacitance loss during the fast charging and discharging.[9]
Owing to the controlled pore size distribution and special pore
functionality, covalent organic frameworks (COFs), an emerging
class of crystalline porous organic materials obtained through
dynamic covalent chemistry,[10] have been explored for
supercapacitor applications.[11] The layered architecture of 2D
COFs possesses 1D channels that facilitate ion diffusion and
mass transport.[12] In recent years, COFs based organic electrode
materials have been studied extensively due to the precise pore
size for ion-adsorption and inherent ordered columnar structure
for charge transport.[13]
However, the strong π-π stacking of COF layers leads to
inaccessibility of the redox-active sites buried deeply inside the
pore channels, limiting the charge storage capacity.[14,15] On the
other hand, both electron localization on the heteroatom linkages
and defects in the framework arising due to insufficient reversible
condensation interrupt the extended π-electron conjugation.[16]
Consequently, it results in a low electrical conductivity of COFs
retarding practical usefulness in energy storage applications.[16]
The electrical properties of COFs can be improved by doping
conducting polymers into the porous frameworks.[17-19] Dichtel and
co-workers first showed the strategy by introducing
polyethylenedioxythiophene (PEDOT) via electropolymerization
within a redox-active anthraquinone-based COF (DAAQ-TFP
COF) to achieve high volumetric energy storage capacity with fast
charging rates.[17] Awaga and co-workers adopted an in situ solid-
state polymerization of PEDOT inside the nanochannels of
anthraquinone-based COF with the enhancement of electrical
conductivity and high specific capacitance.[18] Later on, Mai and
co-workers grew the COF on the surface of commercial
polyaniline.[19] The coating of COF on conducting polyaniline
Introduction
Porous organic materials due to their low skeleton density, high
specific surface area, tunable pore size, and robust hydrothermal
stability have attracted a great deal of attention for task-specific
applications ranging from gas adsorption, catalysis, and
molecular separation to water purification, sensing, and electrical
energy storage.[1-4] In the context of affordable and sustainable
energy sources, supercapacitors have emerged as promising
energy storage systems in the carbon-neutral economy due to the
high-power density, fast charge/discharge capability, high
coulombic efficiency, and long cycling life.[5,6] The mechanism for
storing charges in a supercapacitor relies on two distinct
processes: nonfaradaic double-layer formation via the
electrostatic adsorption of ions on the electrode/electrolyte
interface (electric double-layer capacitance, EDLC) and faradaic
redox reactions at the surface of the electrode
(pseudocapacitance, PC).[7] Carbon-based amorphous polymeric
porous materials have been studied extensively as
supercapacitive electrode materials due to their large specific
surface area and relatively high packing density.[8] However, the
conceals
the
redox-active
sites
that
diminish
the
pseudocapacitance contribution.
The electronic charge density on heteroatoms in the frameworks
having a high specific surface area facilitates the ion-adsorption,
enhancing the double-layer capacitance.[20,21] Besides, the
electroactive building units, like N-rich triazine cores, improve the
relative permittivity of the electrode materials and contribute to the
pseudocapacitance.[22,23] Herein, we synthesized two triazine-
based covalent organic frameworks via a solvothermal Schiff-
base polycondensation- TCOF-1, by combining C2 and C3-
symmetric (triazine) building units, and TCOF-2, connecting two
C3-symmetric triazine units (Figure 1a). We modified TCOFs via
in situ polymerization of aniline using a chemical oxidation method
to improve the electrical conductivity resulting in polyaniline
modified composites, PANI@TCOF-1, and PANI@TCOF-2. The
composite materials showed high electrical conductivity, which
further improved the specific capacitance with cyclic stability. We
fabricated asymmetric supercapacitor devices derived from
[a]
T. K. Dutta, Dr. A. Patra
Department of Chemistry
Indian Institute of Science Education and Research Bhopal
Bhopal Bypass Road, Bhauri, Bhopal 462066, Madhya Pradesh, India
E-mail: abhijit@iiserb.ac.in
Supporting information for this article is given via a link at the end of the
document. See the supporting information for the details of synthesis
and characterizations, powder X-ray diffraction analysis, gas adsorption
studies, electrochemical analysis, and comparative analysis of the
device performance.
1
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Figure 1. a) Synthetic scheme of imine-linked triazine-based covalent organic frameworks obtained from the condensation of p-phenylenediamine and 2,4,6-tris(4-
formylphenoxy)-1,3,5-triazine (TCOF-1) and 2,4,6-tris(4-aminophenyl)-1,3,5-triazine and 2,4,6-tris(4-formylphenoxy)-1,3,5-triazine (TCOF-2). PXRD patterns of b)
TCOF-1 and c) TCOF-2: comparison between the experimental pattern (blue), Pawley refined profile (red), the refinement differences (grey), and the simulated
pattern (green) for eclipsed AA stacking mode of TCOF-1 and slipped AA stacking mode of TCOF-2; space-filling models representing the stacking along the c-axis
with the layer distances are shown. d) Schematic representation depicting in situ polymerization of aniline with TCOFs. e) FTIR spectra of TCOFs and
PANI@TCOFs. f) Pore size distribution plots of TCOFs and PANI@TCOFs obtained through the analysis of respective adsorption isotherms employing the non-
local density functional theory (NLDFT) method.
PANI@TCOF-2 and demonstrated the lighting up of a green light-
emitting diode (LED).
fabricated via Schiff-base polycondensation reactions following
reported procedures (Figure 1a).[25] TCOF-1 was synthesized by
a
solvothermal reaction of 2,4,6-tris(4-formylphenoxy)-1,3,5-
triazine (TFPOT, C3) and p-phenylenediamine (C2), resulting in a
pale yellow solid with 70% yield (Scheme S3). Similarly, TCOF-2
was synthesized using TFPOT (C3) and 2,4,6-tris(4-
aminophenyl)-1,3,5-triazine (TAPT, C′3) to afford a yellow solid
with 75% yield (Scheme S4). The post-synthetic modification of
TCOFs was carried out through in situ polymerization of aniline
via a chemical oxidation method within the porous frameworks
(Figure 1d). The PANI content in the nanocomposite was
calculated considering the weight ratio of resultant polyaniline and
TCOFs, which was found to be 50-54 wt% in PANI@TCOFs.
Results and Discussion
Triazine-based covalent frameworks (TCOFs) are the class of
porous organic networks that can be synthesized via ionothermal
trimerization of aromatic polynitriles in the presence of molten
ZnCl2.[24a] Most CTFs obtained under the harsh ionothermal
conditions are amorphous.[24b] We employed a triazine moiety
within the monomeric building unit to get highly ordered crystalline
frameworks with a large specific surface area. The triazine-based
covalent organic frameworks, TCOF-1, and TCOF-2, were
2
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Fourier transform infrared (FTIR) spectroscopy was used to
confirm the formation of TCOFs and PANI@TCOFs. The
disappearance of the C=O (1704 cm-1) and N-H (3460-3210 cm-
1) stretching bands indicated the consumption of aldehyde and
amino groups of the monomers (Figure S1). Whereas, the
appearance of the C=N stretching band at 1622-1626 cm-1
confirmed the imine bond formation in TCOFs via condensation
reaction (Figure S1). Similar stretching bands were observed in a
triazine-based model compound involving TFPOT and p-
phenylenediamine (Scheme S5, Figure S1). The FTIR spectra of
polyaniline-TCOF
composites
(PANI@TCOFs)
showed
characteristic benzenoid–quinonoid nitrogen vibration at 1568 cm-
and aromatic amine peak at 1298 cm-1 (Figure 1e, and S10).
1
The blue shift of characteristic bands of polyaniline (8-12 cm-1) in
composite materials ascertained the inclusion of polyaniline within
the TCOFs (Table S1).[26,27]
Figure 2. Field emission scanning electron microscopy (FESEM) images of a)
TCOFs were characterized by powder X-ray diffraction (PXRD)
analysis. The PXRD pattern of TCOF-1 showed an intense
diffraction peak at 2.68° attributed to the (100) plane (Figure 1b).
The other diffraction peaks appearing at 4.69°, 5.43°, 7.21°, and
9.46° corresponded to the (110), (200), (210), and (220) facets,
respectively. The PXRD profile of TCOF-2 exhibited a prominent
peak at 3.98° and relatively weak peaks at 6.87°, 7.96°, and
10.56°, which were indexed to (100), (110), (200), and (210)
facets, respectively (Figure 1c). The experimental PXRD pattern
accorded well with the simulated pattern based on the AA
stacking for TCOF-1 and slipped AA stacking for TCOF-2 (Figure
1b, 1c, and S2, S3). Pawley refinement of experimental PXRD
data using Materials Studio (version 6.1) resulted in the unit cell
parameters of a = 37.4 Å, b = 37.9 Å, c = 5.7 Å, and α = 90.7°, β
= 90.4°, γ = 117.1° for TCOF-1. The unit cell parameters of TCOF-
2 were found to be a = 26.2 Å, b = 26.3 Å, c = 3.5 Å, and α =90.2°,
β = 89.5°, γ = 120.6°. The interlayer spacings of 5.7 and 3.5 Å
corresponding to (001) plane were observed at 2θ = 16.6° and
25.8° for TCOF-1 and TCOF-2, respectively (Figure 1b, 1c). The
PXRD data and the structural analysis of TCOF-1 and TCOF-2
were consistent with the earlier reports.[25] The suppression of the
high intensity peaks at low 2θ in PXRD suggests a lowering of the
crystallinity in PANI@TCOFs (Figure S4).
TCOF-1, b) TCOF-2, c) PANI@TCOF-1, and d) PANI@TCOF-2; scale = 200
nm.
TCOF-1 and fused flake-like morphology for TCOF-2 (Figure 2a,
2b).
A distinctly different morphology was noticeable for
PANI@TCOFs compared to the pristine TCOFs (Figure 2c, 2d).
PANI@TCOF-1 and PANI@TCOF-2 showed the well-distributed
intricate network structure attributing to polyaniline loading with
the porous framework. PANI@TCOF-2 exhibited whisker-like
morphology of polyaniline that grew over the framework of TCOF-
2, resulting in faster ion diffusion through the nanochannels.[26]
The transmission electron microscopy (TEM) images of TCOFs
and PANI@TCOFs further revealed the porous nature of the
frameworks (Figure S8).
The heteroatom rich porous framework structure of TCOFs and
PANI@TCOFs motivated us to explore the electrochemical
performance of TCOFs and PANI@TCOFs as active electrode
materials
for
supercapacitors.
The
electrochemical
measurements were carried out using a typical three-electrode
electrolytic cell with 1 M H2SO4 as an aqueous electrolyte. TCOFs
and PANI@TCOFs were coated on a platinum-foil and used as
the working electrode. A platinum wire and saturated calomel
electrode (SCE) were employed as the counter and reference
electrodes, respectively. The dispersion of composite
PANI@TCOF, PVDF binder, and acetylene black (mass ratio of
3:1:1) was drop-casted, having the PANI content on the working
electrode as 30-32 wt%. The electrochemical performances of the
electrodes were investigated by cyclic voltammetry (CV),
galvanostatic charge-discharge (GCD) measurements, and
electrochemical impedance spectroscopy (EIS).
Thermogravimetric analysis (TGA) profiles of TCOFs showed
high thermal stability up to 400 ℃ (Figure S5). The surface area
and porosity of TCOFs and PANI@TCOFs were estimated by the
nitrogen adsorption−desorption isotherms at 77 K (Figure S6 and
S7). The sorption curves of both TCOFs showed type-IV
isotherms (Figure S6a, S6c). The TCOF-1 sorption curves having
the H1 hysteresis loop indicated the cylindrical-like pore
channels.[28] The H4 hysteresis loop of TCOF-2 suggested the
The cyclic voltammetry was carried out at different scan rates,
and specific capacitance from the cyclic voltammograms was
calculated using the following equation.
presence
of
narrow
slit-shaped
pores.[28a]
The
Brunauer−Emmett−Teller (BET) specific surface area of TCOF-1
and TCOF-2 were found to be 1756 and 1110 m2 g−1, respectively.
The pore volumes were 1.64 cm3 g−1 for TCOF-1 and 0.85 cm3
g−1 for TCOF-2, estimated from the nitrogen adsorption amount at
P/P0 = 0.95 (Figure S6b, S6d). The uniform pore size distribution
(2.6 nm for TCOF-1; 1.4 and 3.9 nm for TCOF-2) was obtained
based on the non-local density functional theory (NLDFT) method.
The lowering of BET specific surface area and the changes in the
pore size distribution of PANI@TCOFs infer the successful
integration of PANI within the porous framework of TCOFs (Figure
1f, and S7).
퐼 . 푑퐸
∫
퐶푠 =
[1]
2휈푚퐸
Cs, I, ν, m, and E denote the specific capacitance, current, scan
rate, mass of active material deposited on the electrodes, and the
potential applied, respectively. Figure 3a and 3c depict the cyclic
voltammograms of PANI@TCOF-1 and PANI@TCOF-2,
respectively, recorded at different scan rates from 10 to 100 mV
s−1 in the potential window from 0 to 0.8 V. The CV curves of the
composite materials showed a rectangular shape featuring redox
peaks. As revealed from cyclic voltammograms, the capacitive
response was due to the combined effect of both electric double-
layer capacitance and pseudocapacitance (Figure S10c). Here,
The field emission scanning electron microscopy (FESEM)
images depict the aggregated-particles like morphology for
3
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