Acid base co-crystal converted into porous carbon material for energy storage devices

Article abstract of DOI:10.1039/c4ra13482f

A simple and facile method is adopted for the synthesis of pure and catalyst free carbon material for supercapacitor applications. In a co-crystal synthesis, the precursors (isophthalic acid and a base, 4,4′-bipyridine) are arranged in regular pattern, followed by carbonization at 600°C under an inert atmosphere to produce pure carbon material, CIN-600. The obtained sample is characterized by many techniques, such as powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and gas adsorption analysis. The gas adsorption and microscopic analysis demonstrated the high porosity of the carbon sample and its irregular geometry. Owing to the excellent porosity and electrical conducting properties, CIN-600 showed enhanced capacitive performance when used as an electrode material in electric double layer capacitors. The specific capacitance of the sample was ca.181.3 F g^-1 at 2 mV s^-1 and maintained 91.3% of its initial capacitance in a long-term cycling test.

Full text of DOI:10.1039/c4ra13482f

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ARTICLE TYPE
Acid base co–crystal converted into porous carbon material for energy
storage devices
a
a
a
b
c
Inayat Ali Khan , Amin Badshah , Ataf Ali Altaf , Nawaz Tahir , Naghma Haider and Muhammad Arif
Nadeem*
a
5
Received (in XXX, XXX) XthXXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
A simple and facile method is adopted for the synthesis of pure and catalyst free carbon material for
supercapacitor applications. In co–crystal synthesis, the precursors (isophathlic acid and a base 4,4´–
bipyridine) are arranged in regular pattern followed by carbonization at 600 °C under inert atmosphere to
pure carbon material CIN–600. The obtained sample is characterized by many techniques like powder
X–ray diffraction (PXRD), transmission electron microscopy (TEM), scanning electron microscopy
1
0
(SEM), energy dispersive X–ray spectroscopy (EDS) and gas adsorption analysis. The gas adsorption and
microscopic analysis have demonstrated the high porosity of the carbon sample with irregular geometry.
Owing to the excellent porosity and electrical conducting properties, the CIN–600 has shown enhanced
capacitive performance when used as electrode material in electric double layer capacitors. The specific
capacitance of the sample is ca.181.3 F g at 2 mV s and maintains 91.3% of its initial capacitance in
long–term cycling test.
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density appliances. Supercapacitors are used in those electronic
equipment’s where tough power supply, high current drain is
required and those that work under extreme environmental
conditions [3].The electrodes of supercapacitor consist of
different carbon materials like carbon nanotubes (CNTs) [4],
graphene [5], activated carbon [6], carbon onion [7], carbon
fibers [8] and mesoporous carbon [9]. These carbon materials
should be predominantly porous in nature to ensure proper
accommodation of electrolyte ions. The surface area and porosity
depends on the synthesis process and the nature of carbon
precursor. Different sources like metal carbides [10–12],
polymers [13], natural gas [14], natural silk [15], sucrose [16],
furfuryl alcohol [17] and metal–organic frameworks (MOFs)
Introduction
Co–crystals are solid materials under ambient conditions. Coꢀ
crystals comprise of two or more molecules known as co–crystal
constructors. According to the Cambridge Structural Database
2
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3
3
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(CSD) survey co–crystal represents less than 0.5% published
crystal structures [1]. Co–crystals can be easily synthesized by
invoking molecular structure, and the molecular interaction like
the hydrogen bonding between the co–crystal formers. So far, co–
crystal synthesis is limited to photopolymerization and
nucleophilic substitution [2]. Their potential impact upon
pharmaceutical formulation and green chemistry can never be
ignored.
In the present study, for the first time, we have found the
application of co–crystal in the synthesis of pure, catalyst free,
cost effective and porous carbon material. The co–crystal (In–3)
was synthesized from two co–formers; isophthalic acid (1,3–
Benzenedicarboxylic acid) and 4,4´–bipyridine. The In–3 was
structurally characterized by X–ray single crystal analysis and it
was found that in In–3, the co–formers are arranged in polymeric
chains via hydrogen bonding.
Supercapacitors or electric double layer capacitors (EDLCs) are
modern electronic devices which work on the principle of double
layer capacitance at the electrode/electrolyte interface where
electric charges are accumulated on the electrode surfaces and
ions of opposite charge are arranged at the electrolyte side.
EDLCs are superior over modern secondary batteries because of
their potential applications in high power output and high energy
[18–20] etc. have been used for the synthesis of carbon materials.
Among the different methods, carbonization or template
carbonization [21–28] is one of the synthetic rout in which the
carbon material could be obtained by heating the carbon source
precursor at elevated temperatures. But this method suffers due to
complex polymerization process at elevated temperature,
selection of proper template which some time leads to the
contamination of the final product, acid wash of the product to
remove the metal/metal oxide derived from template or catalyst
and low experimental yield. To address these limits there is a
need of suitable carbon source and facile synthetic rout.
In the present study, we are reporting a simple and facile method
for the synthesis of carbon material via carbonization of solid–
state co–crystal at 600 °C. To the best of our knowledge no report
has been done so far where carbon material is derived from co–
crystals. The co–crystal was synthesized under solvothermal
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condition and characterized. The synthesized carbon material was 55 ethanol, the black slurry was pressed between two pieces of
–
2
checked as electrode material in supercapacitor.
nickel foam (1 cm × 1 cm) under 350 kg cm (using hydraulic
presser EQ–HP–88V220). After drying at 100 °C the electrode
was impregnated with electrolyte solution overnight to ensure
complete saturation of the electrode materials with electrolyte
ions. The designed electrode was subjected for electrochemical
Experimental
Synthesis of Co–crystal (In–3)
60
–
1
5
The In–3 was synthesized by mixing solutions of isophthalic acid
studies in a three electrode cell assembly using 6 mol L KOH.
The cell assembly consists of CIN–600 sample electrode as the
working, platinum wire as the auxiliary and Ag/AgCl as the
reference electrode. All the electrochemical measurements were
(IPA) (1.0 mmol) and 4,4´–bipyridine (Bipy) (0.5 mmol) in 20%
ethanol solution of water, stirred and introduced into a Teflon
lined autoclave which was kept at 120 °C for 24 h followed by
slow cooling to get yellow crystals. The product was filtered, 65 carried out by Biologic SP 300 electrochemical analyzer at room
1
1
2
0
washed with water and ethanol and dried under vacuum. Yield:
temperature.
7
6
1
1
9%, Anal. Calcd.: C, 63.93% H, 4.13%, N, 5.74% Found: C,
ꢀ
1
3.84% H, 4.06% N, 5.61.IR data (νmax / cm ): 3447 mbr, 3077 s,
612 s, 1549 m, 1533 s, 1421 s, 1263 m, 1252 s, 1237 s, 1177 s,
141 m, 1051 m, 1041 m, 976s, 968 m, 866.
Results and Discussions
Characterization
5
Pyrolysis of In–3
7
0
The single crystal X–ray structural analysis of In–3 is presented
in Figure 1 and the crystallographic data and structure refinement
parameters are tabulated in Table S1 (supplementary
information). The crystallographic data has been deposited in the
CCDC and can be obtained free of charge from The Cambridge
Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_
request/cif quoting: CCDC 1002755.
The In–3(0.5 g) was grinded to fine powder and transferred into a
ceramic boat which was placed in a quartz tube fixed in a tube
furnace (Nabertherm B 180). Air was flushed away by continuous
flow of Ar for 30 minutes. The sample was heated up to 600 °C
–1
0
(10 °C min ) for 6 h under continuous flow of Ar gas and cooled
slowly to room temperature. The carbon material (0.33 g)
obtained was denoted as CIN–600 with total yield of 66%.
75
Characterization
The single crystal X–rays diffraction data was collected on a
Bruker kappa APEXII CCD diffractometer using graphite–
monochromator Mo–Kα radiation (λ = 0.71073 Å) at ambient
25
30
35
40
45
50
temperature. For data collection
ω scan and multi–scan
absorption correction was applied. Final refinement on F2 was
carried out by full–matrix least–squares techniques. Structure
solution and refinements were accomplished with SHELXL–97
[29], WinGx [30], SAINT [31] and PLATON [32]. Wide angle
X–ray diffraction measurements were carried out at a speed of
–1
0
.015 s by PANalytical diffractometer (X’Pert PRO 3040/60)
with a Cu Kα (λ = 1.544206 Å) radiation generated at 40 kV and
30 mA. Surface morphology of CIN–600 was examined by
scanning electron microscope (JEOL–JSM–6610LV) equipped
with energy dispersive X–ray spectroscope (EDS). TEM analysis
of the sample was conducted with FEI Company’s Titan 80–300 80 type hydrogen bonding, the zig zag tape structure of In–3 formed
CT transmission electron microscope with the acceleration
Figure 1. (a) Asymmetric unit of co–crystal In–3, (b) molecular
structure of the In–3 that consists of two acids and one base
molecule, (c) 2D sheet of In–3 formed by the iner–tape CH ꢀꢀꢀ O
by the intermolecular OH ꢀꢀꢀ O and OH ꢀꢀꢀ N type hydrogen
bonding.
The crystal structure reveals that In–3 formed by the
coupling of IPA and Bipy in 2:1 ratio [Figure 1 (b)]. In In–3, two
voltage of 300 kV.N adsorption/desorption measurements were
2
carried out using the Accelerated Surface Area & Porosimetry
System 2020 supplied by Micromeritics Instruments Inc.
Approximately 20 mg of sample was loaded into a glass analysis 85 IPA molecules coupled by OHꢀꢀꢀO type hydrogen bonding and
tube and outgassed for 3 h under vacuum at 200 °C prior to
measurement. The isotherm was measured at 77 K and data was
analyzed using Bruner–Emmett–Teller (BET) model to determine
the surface area. The pore size distribution plot was conducted on
the adsorption branch of the isotherm based on density functional 90 structure has been reported by Shan et al. [33, 34]. The layer by
theory (DFT) model. FTꢀIR analysis was carried out on a
Thermoscientific NICOLET 6700 FTIR.
formed a six member planer ring and this pair form OHꢀꢀꢀN type
hydrogen bond with Bipy which leads to a zigzag tape structure.
CHꢀꢀꢀO type hydrogen bonding assemble these zigzag tapes into
a sheet structure [Figure 1 (c)]. Similar type of co–crystal
layer packing of 2D sheets in the crystal packing motivates us to
carbonize the co–crystal in order to get ordered carbon material.
The PXRD pattern of CIN–600 revealed the formation of pure
carbon material. The heteroatoms like N and O in the In–3 were
Electrochemical studies
For electrode preparation, a black slurry was prepared by mixing 95 converted into NO , O2 and CO and vaporized in Ar flow
x
x
5
mg of CIN–600 with 5 mL ethanol and 10 µL Nafion (5 wt.%)
environment. Broad peaks at 2θ = 25° and 2θ = 45° in the XRD
solution which was sonicated for 30 minutes. After evaporating
pattern of CIN–600 [Figure 2] corresponds to the (002) and (100)
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crystallographic planes of carbon material. To evaluate the degree
show clear and well define pores created at the surface during
of graphitization, the empirical parameter (R) was used, which 60 fabrication of CINꢀ600 carbon sample. These pores and ruptured
can be defined as the ratio of the height of the (002) Bragg peak
to the background [25, 26]. R factor for CIN–600 sample is 0.62
which shows that some graphitic sheets are also present along
with other types of carbon matrix.
surface is developed by the exclusion of gases like CO , NO
x x
during carbonization under Ar atmosphere. The porosity of the
carbon material has a very important role in the capacitance of a
capacitor. Carbon material with proper pore size and distribution
is required to show good capacitive performance. The pores of
carbon material can facilitate the inside diffusion of ions during
charge–discharge operating conditions. Here in the present study,
the CIN–600 sample has exhibited good capacitive performance
due to the porous nature as clear from gas adsorption and
microscopic analysis. EDX analysis was carried out to confirm
the percent composition and purity of CIN–600 sample. The FT–
IR analysis also support the purity of carbon material (Figure S1).
EDX spectrum has [Figure 4 (e)] a single point at ca. 0.25 keV
for carbon which suggests the high purity of the obtained sample.
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70
Figure 2. Powder XRD pattern of CIN–600.
2
5
To evaluate the surface area and the pore size
distribution N adsorption/desorption analysis were performed
2
and the isotherm is shown in Figure 3. Type–IV isotherm is found
for CIN–600 sample suggesting the existence of different pore
sizes ranging from micro to mesopores. At high relative pressure
hysteresis between adsorption and desorption branches
demonstrates the presence of mesopores in the surface texture.
3
0
ꢀ
1
The BET surface area of CIN–600 sample is 1230m² g with
pore diameter of 2.13 nm.
3
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5
0
5
7
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Figure 4. TEM micrographs (a, b), SEM images (c, d) and EDX
spectrum of CIN–600 (e).
Electrochemical Evaluation
The working principle of supercapacitor is the accumulation of
electrolyte ions in the electric double layer developed by the
electrostatic force. The capacitance (energy stored by electrode)
largely depends on the surface interaction of electrode and
electrolyte. There is direct relationship between specific
capacitance and surface area: C = εS/d; (C = specific
8
8
9
0
5
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sp
sp
capacitance, ε = permittivity of electrolyte, S = surface area of the
electrode–electrolyte interface, distance between the
d
=
polarized carbon surface and maximum charge density of
solvated ions) [27, 28]. However, the surface which can be
electrochemically accessed by the solvated ions only makes
contribution to the capacitance. The pore size of the electrode
materials should be properly matched with the size of the
solvated ions for excellent capacitive performance. In addition,
the capacitance of a capacitor also depends on surface wettability,
applied pressure for packing and electrical conductivity of the
electrode materials.
5
0
Figure 3. N2 adsorption/desorption isotherm and pore size
distribution (inset) of CIN–600 carbon sample.
To characterize the surface morphology of the sample,
TEM and SEM analysis was conducted and the obtained images
are presented in Figure 4. TEM images [Figure 4 (a, b)] shows
micro–porous nature of amorphous carbon. It is obvious that the
surface texture of samples is porous with monolithic irregular
shape. The SEM images of CIN–600 sample [Figure 4 (c, d)]
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The voltammograms of the CIN–600 sample are
Galvanostatic charge/discharge tests were also carried
presented in Figure 5 (a) with different voltage scan rates from 2
mV s to 150 mV s in the potential range of –0.1 V to +0.5 V. 45 current density from 0.250 to 1.5 A g to evaluate the capacitive
out in the potential window of –0.1 V to +0.5 V at different
–1
–1
–1
The sample has excellent capacitive behavior as obvious from the
fairly rectangular shape of voltammograms in the given range of
potential scan rates. The box like rectangular shape of
voltammograms with increasing voltage sweep rate suggests that
performance of the carbon material and the results are shown in
Figure 5(b). Specific capacitance was calculated by the equation;
5
0
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0
5
0
5
C = (I×ꢁt)/(ꢁV×m), where ‘I’ is the constant discharge current,
sp
‘ꢁt’ is the discharge time, ‘ꢁV’is the voltage difference, and ‘m’
the electrode material is quite suitable for quick charge–discharge 50 is the mass of the carbon material. The specific capacitance from
operation condition. Generally, better capacitive behavior of a
sample at high voltage sweep rate is attributed to the better
accessibility of ions to the electrochemically active surface area.
It is believed that the capacitive performance of the carbon
charge/discharge measurements at different current densities are
given in Table S2.It is clear that the charging curve is almost
symmetrical with their corresponding discharge counterpart and
the linear voltage–time relationship with no obvious voltage drop
1
1
2
2
3
3
materials depends on the pore size which favors the penetration 55 indicates good capacitive behavior of the CIN–600 carbon
of solvated ions. The excellent capacitive performance of CIN–
600 sample can also be attributed to its large porosity as
presented in gas adsorption and microscopic studies. The specific
sample. The galvanostatic charge/discharge results are well
consistent with voltammetric measurements illustrating an ideal
capacitor behavior. Cycling charge/discharge measurements were
ꢀ
1
–1
capacitance (F g ) from CV curves were calculated using the
conducted at 0.500 A g to test the long–term cycling stability of
following equation; C = (ꢁQ)/(ꢁV×m) where ‘ꢁQ’ is the charge 60 the carbon electrode material [Figure 5 (d)]. The CIN–600 sample
sp
(C) integrated from the whole voltage range, ‘ꢁV’ is the whole
maintains almost 91.3% of their initial capacitance after 3000
cycles presenting its good long–term cyclic stability. Figure 6
shows that voltammetric currents are almost proportional to the
scan rate of CV. The slop of i–υ lines is dependent upon the
voltage (V) difference, and ‘m’is the mass (g) of carbon on the
electrode. The calculated values of specific capacitance are given
in Table S2 (supplementary information) for CIN–600 sample.
As can be seen in the data that the sample CIN–600 have the 65 measured potential. The linear dependency of current on scan
–
1
–1
highest capacitance of 181.3 F g at sweep rate of 2 mV s and
rates of CV illustrates the high–power characteristics of the CIN–
600 sample.
–
1
lowest capacitance value of 122.3 F g at voltage sweep rate of
50 mV s . At low sweep rate the ions have sufficient time to
–1
1
diffuse into the pores of sample while at high voltage scan rate
the ions can only penetrate into some external large pores with
fast movement.
The relationship between the ratio of retained
capacitance and voltage scan rate is plotted and presented in
Figure 5 (c). There is a decrease in the ratio of retained
capacitance with increase in voltage scan rate, showing the
limited use of electrochemically active surface area at high scan
rate in comparison to low scan rate. However, the sample retains
about 66% of its initial capacitance even at high scan rate of 150
–
1
mV s .
Figure 6. Plot of current density versus potential scans rates(i–υ)
from CV results where the current was obtained at: 0.0 V, 0.2 V,
and –0.4 V.
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0
5
0
The electrochemical performance of CIN–600 is
comparable with most of the recently reported porous carbon
materials synthesized by complicated and tedious process. A few
reports are described here for reference. Lu et al., have obtained
ordered mesoporous carbon (OMCs) from sucrose with
controllable pore sizes in the range of 4–10 nm by a template
procedure using 2D hexagonal MSU–H and 3D cubic KIT–6
silica as hard templates and boric acid as the pore expending
agent. The prepared OMCs have exhibited the specific
–1
–1
capacitance values from 143 to 205 F g at 5 mV s [38]. Li et
al., synthesized nitrogen enriched microporous carbon spheres
(MCS) by the polymerization–induced colloid aggregation
–
Figure 5. Cyclic voltammetric curves from2 to 150 mV s
1
40
potential sweep rates(a), galvanostatic charge/discharge curves at
different current densities (b), plot of retain capacitance versus
scan rates (c), and long–term cycling (d) of CIN–600 sample.
2
–1
method with BET area of ca. 1330 m g . As electrode material
85 for electric double layer capacitors the MCS have shown specific
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–
1
capacitance value of up to 211 F g at a discharge current density
electrode materials which may be due to the proper combination
–
1
of 1 A g [39]. Song and coworkers have prepared phenolic 60 of micro and meso–pores in which the former will favor a better
based carbon nanofiberswebs (PCNFWs) by electrospining
resole–type phenolic resin/PVA blend solution, followed by
curing and carbonization [40]. The specific surface area of the
efficiency of ionic access to the electrochemically active surfaces
at low frequencies while the later will facilitate ionic diffusion at
high frequencies.
5
2
–1
carbon was 416 m g and the specific capacitance value of up to
–
1
–1
1
71 F g at 5 mV s . Jin and coworkers [41] have reported novel
carbon–based microporous nanoplates containing numerous
heteroatoms (H–CMNs) from regenerated silk fibroin. The BET
2
–1
1
1
2
2
3
3
4
4
5
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surface area of the carbon product was 2557.3 m g similar to
pristine graphene sheet. The H–CMNs have exhibited specific
–
1
–1
capacitance up to 264 F g at 0.1 A g current density in 1 mol
–
1
L
H SO . Guo et al., [42] have prepared microporous carbon
2 4
materials by carbonization of sulfuric acid pretreated sucrose. The
pore size and BET surface area were in the ranges of 0.7–1.2 nm
Figure 7. The electrochemical impedance spectrum (a),Bode
angle plot from impedance measurements(b)of CIN–600.
6
5
2
–1
and 178–603 m
g
respectively. It was concluded that the
sample carbonized at 800 °C displayed the highest surface area
–
1
–1
Conclusions
and highest specific capacitance up to 232 F g at 0.1 A g
current density. Xu and coworkers [43] used metal–organic
framework (MOF) as a template and furfuryl alcohol as a
precursor for the synthesis of nanoporous carbon (NPCs). The
BET surface area of the NPCs fall in the range of 1140 to 3040
m g and the pore size distribution centered at about 3.9 nm.
They have concluded that the NPCs samples obtained at a
temperature higher than 600 °C have almost constant specific
Porous carbon material was obtained by the simple carbonization
of rigid co–crystal using the self–degradation template method.
The carbonization was carried out at 600 °C in an argon
atmosphere for 6 h in order to obtain pure carbon product. During
gas adsorption and SEM analysis it was found that the sample
CIN–600 is very porous. Due to the porous nature the sample was
subjected as electrode material in EDLCs. In CV studies the
sample has exhibited good voltammetric behaviour with specific
7
7
8
0
5
0
2
–1
–
1
–1
capacitance up to 100 F g at 5 mV s while the NPC sample
obtained at 530 °C gives rise to specific surface area of 3040 m
2
–1
–1
–1
capacitance of 181.3 F g at 2 mV s and 122.3 F g at 150 mV
–
1
–1
–1
–1
g
but lowest specific capacitance of 12 F g at 5 mV s .
The electrochemical behavior of carbon sample CIN–
600 was further evaluated by impedance spectroscopy. Generally
the Nyquist spectrum is divided into high frequency semicircle
region attributed to the charge transfer resistance occurring at the
electrode/electrolyte interface and the low frequency curve with
s . The good frequency response during electrochemical
impedance measurement further confirms the excellent capacitive
candidature of the sample. Given the broad range of acids and
basis, a great variety of co–crystals with different dimensionality
and porosity can be synthesized and converted into broad range
of carbon materials which can also be structurally tuned via
judicious choice of co–formers. These preliminary results open
up a new research gateway towards synthesis of multifunctional
carbon materials.
4
5° slope representing the Warburg diffusion resistance [35]. The
–1
impedance study of CIN–600 carbon electrode in 6 mol L KOH
solution in the frequency range of 1 to 100,000 Hz (at 10 mV as
potential amplitude) is shown in Figure 7. The Nyquist plot
[
Figure 7 (a)] consists of two parts, the horizontal line (with real
85
Acknowledgments
axis) at high frequency region and the 45° slope at low frequency
region. The absence of the semicircle in the spectrum suggests
very good ionic conductivity at the electrode/electrolyte interface
The work was financially supported by Higher
Education
Commission
(HEC)
of
Pakistan
(PD–
IPFP/HRD/HEC/2013/1933 and 20–1638/R&D/09/2900).
[
36, 37]. Between the horizontal line and the 45° slope, there is a
Notes and references
transition zone (at resistance of 2.3 Ω) which is believed to be
due to the ionic mobility in the pores of carbon, which affect the
capacitive performance of the electrode at high current drain [25].
The 45° slope at low frequency region indicates an ideal
capacitive behavior of the carbon material. This further
demonstrates that the carbon material can store significant value
of electrical energy at low frequency in the electric double layer.
The frequency response (FR) of the electrode material has been
depicted from the impedance measurement and the plot is
a
9
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0
5
Catalysis and Nanomaterials Lab 27 Department of Chemistry Quaidꢀiꢀ
Azam University, Islamabad 45320, Pakistan, Tel: +92ꢀ51ꢀ90642062; Eꢀ
mail: arifchemist@hotmail.com
b
Department of Physics University of Sargodha, Sargodha 40100,
Pakistan
c Geoscience Advanced Research Laboratories, Geological Survey of
Pakistan, Chakshahzad Town, Islamabad, Pakistan.
[
1] M. L. Cheney, G. J. McManus, J. A. Perman, Z. Wang andM. J.
Zaworotko,Cryst Growth Des, 2007, 7, 616–617.
presented in Figure 7 (b). The FR reflects the number of solvated 100[2] M. C. Etter, G. M. Frankenbach and J. Bernstein, Tetrahedron, 1989,
3
0, 3617–3620
ions reached at the porous surface at a specific frequency of
alternating current. It is clear from Figure 7 (b) that the imaginary
part of impedance (capacitive part) decreases with increase in
[
[
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