Conjugated porous polymers as precursors for electrocatalysts and storage electrode materials

Article abstract of DOI:10.1039/c5cc08148c

Conjugated porous polymers were used as precursors to prepare nitrogen and sulphur doped carbon atoms, which were then used for oxygen reduction and energy storage.

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Conjugated porous polymer as precursor for electrocatalyst and storage
electrode material†
Manik E. Bhosale, Rajith Illathvalappil, Sreekumar Kurungot and Kothandam Krishnamoorthy*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Conjugated porous polymer was used as precursor to prepare
nitrogen and sulphur doped carbon, which was then used for
oxygen reduction and energy storage.
Conjugated porous polymers (CPPs) are synthesized by reacting
10 multifunctional aromatic building blocks, which possess the
advantages of both porous materials and conjugated polymers.1,2
The porosity depends on the length and shape of the aromatic
units.3,4 High fidelity reactions are desirable to achieve highly
porous CPPs. Furthermore, by choosing building blocks with
15 hetero atoms, the properties of the CPPs can be tuned that in turn
impacts the application of the material.5 Due to the control over
Scheme 1. Synthesis of TTB-CPPs
CPPs) was carried out by reacting 2, 4, 6-tris(5-bromothiophen-2-
molecular structure and properties, CPPs have been widely used 50 yl)-1,3,5-triazine with 1,3,5-triethynylbenzene (Scheme 1). The
for applications such as gas storage, chemical encapsulation,
catalysis and energy storage.2,6-13 Usually, the electrical
20 conductivity of the CPPs is not comparable to that of linear
polymers due to the extensive cross linking. However, CPPs have
TTB-CPPs are insoluble, hence solid state 13C NMR was
recorded, which showed six types of carbon atoms (Fig. S5, ESI).
The IR spectra confirm the presence of bis-substituted C-C triple
bond at 2198 cm-1 and the peak at 1370 and 1500cm-1 is due to
been used as electrode material in super capacitors with 55 C=N stretching for triazine unit.
impressive specific capacitance. In these devices the CPPs
electrodes were prepared using acetylene black as conducting
25 additive.11 The CPPs with lower conductivities has been
envisioned to act as precursor to the synthesis of graphene, which
The powder X-ray diffraction pattern of TTB-CPPs doesn’t show
clear peaks indicating the material is amorphous (Fig. S7, ESI).
This is one of the common features of CPPs.11 The SEM image of
TTB-CPPs showed “small petals” like morphology (Fig. 1a).
is an excellent material for variety of applications.14 The 60 Elemental mapping was carried out for carbon, sulphur and
graphene synthesized from CPPs encompasses the advantageous
properties of both graphene and CPPs. For example, porous
30 graphene can be easily synthesized by graphitization of CPPs
prepared using benzene and its derivatives.15 On the other hand,
hetero atom doped graphene can be synthesized by graphitization
of heterocycle containing CPPs.5 It is necessary to note that
synthesis of either nitrogen or sulphur doped graphene is not
35 straight forward by conventional procedures.16,17 Furthermore,
synthesis of graphene, which is doped by both nitrogen and
sulphur is rather difficult. While using heterocycle containing
CPPs, it is possible to impart control over the location of doping
atoms (N and S) in the graphene. This is possible by rationally
40 designing the CPPs and then by a graphitization process.18 Due to
these advantages, we decided to prepare nitrogen and sulphur
doped graphene (NS-G) from CPPs. The CPPs are synthesized by
sonagashira coupling of thiophene connected with triazine and
benzene building blocks. The resultant CPPs was graphetized at
45 three different temperatures and the material was used to study
oxygen reduction reaction. The NS-G was also used as electrode
in supercapacitor.
nitrogen by SEM. These elements are uniformly distributed in the
TTB-CPPs. A representative example of sulphur is shown in Fig.
Fig. 1 SEM image of TTB-CPPs (a), EDAX image showing
Sulphur (b), C 1s XPS spectra of TTB-CPPs (c) and BET curves of
TTB-CPPs (d)
The synthesis of Triazine-Thiophene-Benzene CPPs (TTB-
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1b. XPS was used to study the nature of bonding between the
elements. In the XPS spectra of TTB-CPPs, the C 1s peaks at
284.8, 285.7, 286.8 eV are assigned to sp2 carbon, sp2 C-N and
sp2 C-S, respectively (Fig. 1c).5 The N 1s peak at 398.7 eV is due
to pyridinic nitrogen originating from the triazine. The S 2p peak
appears at 164.3 eV. BET specific surface area calculated by
nitrogen adsorption was found to be 544 m2/g (Fig. 1d). The pore
size was determined to be 1.3 nm. These small pores could not be
visualised by TEM.
5
10
With these information about TTB-CPPs, we proceeded to
synthesize NS-G by carbonization. The TTB-CPPs was
carbonized at three temperatures, 700, 800 and 900 °C in inert
atmosphere to prepare NS-G@700, NS-G@800 and NS-G@900,
respectively. Together, the carbonized materials will be denoted
15 as NS-Gs. The specific surface area determined by BET was
found to be 694 m2/g, 700 m2/g and 624 m2/g for NS-G@700,
NS-G@800 and NS-G@900, respectively (Fig. S8, ESI). From
the surface area measurements, we can conclude that the
carbonization of TTB-CPPs increase the surface area. The
20 increase in area may be due to formation of more pores upon the
loss of nitrogen and sulphur during the carbonization of TTB-
CPPs at high temperatures. Please note that this effect is
pronounced while preparing graphene from oxygen containing
Fig. 2 Pore size distribution of TTB-CPPs and NS-G@800 (a),
Raman spectra of NS-Gs (b), cyclic voltammogram showing the
capacitive behaviour of NS-G@800 (c) and charge discharge curves
of NS-G@800 (d)
TTB-CPPS showed extremely poor charge-discharge behaviour.
On the other hand, NS-G@700 showed improved charge
heterocycles.19 The pore size of TTB-CPPs and NS-Gs remained 50 discharge characteristics. The capacitor showed linear increase
25 around 1.3 nm (Fig. 2a). This indicates that the graphetization
process didn't collapse the pores of the polymer. The SEM
images of NS-Gs showed morphology similar to that of TTB-
CPPs with negligible amount of “small petals” (Fig. S9, ESI).
and decrease in voltage while charging and discharging,
respectively (Fig. 2d). The charge-discharge curve was
symmetric irrespective of the current densities used for charging
and discharging the capacitor. The specific capacitance (C)
The SEM elemental mapping of NS-Gs showed uniform 55 decreased from 211 to 25 F/g, while charge-discharge rate was
30 distribution of C, N and S. This experiment indicates that the
carbonization didn’t completely remove the hetero atoms in the
carbon material. In the Raman spectra, the intensity of D band
was stronger than the G band. This indicates the presence defects
increased from 0.5 to 5 A/g (Table 1). The C of NS-G@800 was
found to be 75 F/g while charging-discharging at a very high
current density of 5 A/g, which is three times higher than that of
NS-G@700. Interestingly, in case of NS-G@900, the C was 102
over graphitic carbon (Fig. 2b). The intensity ratio of G to D was 60 F/g while discharging at 5 A/g. This is four times higher than the
35 found to be 0.85, 0.88 and 0.91 for NS-G@700, NS-G@800 and
NS-G@900, respectively. The C 1s XPS spectra of NS-G@700
showed peaks corresponding to C-N (285.7 eV) and C-S (286.8
eV) indicating the presence of nitrogen and sulphur in the
carbonized material. The decrease in the intensity of these two
40 peaks indicates the inevitable loss of these elements during the
carbonization process. This compares well with the increase in
surface area due to loss of nitrogen and sulphur. Same peaks were
observed for NS-G@800, but with the increased intensity of
graphitic nitrogen.
C observed for NS-G@700. This indicates that the NS-Gs are
useful electrode materials in supercapacitor in general and the
performance is better with increased graphitic carbon (NS-
G@900).
While this work was in progress, nitrogen, sulphur and oxygen
tridoped graphene synthesized by carbonization of polypyrrole
has been found to be a useful electrocatalyst for oxygen
reduction.20 Interestingly, the presence of nitrogen and sulphur
found to enhance the electrocatalytic activity.21 Thus, we
70 envisioned our material to be suitable for oxygen reduction
because nitrogen and sulfur modulates the spin density and
charge distribution of carbon lattice. This results in the formation
of active sites for electrocatalysis.22 For the NS-Gs, the oxygen
65
45
The cyclic voltammograms of NS-Gs showed typical
capacitive voltammograms without any Faradaic peaks (Fig. 2c).
Supercapacitors were fabricated with TTB-CPPs and NS-Gs. The
Table 1. Charge storage and materials property of NSGs
Sample
BET (m2/g)
Pore size
(nm)
IG/ID ratio
Specific Capacitance (F/g)
0.5 A/g
211
1 A/g
138
2 A/g
90
5 A/g
25
NS-G@700
NS-G@800
NS-G@900
694
700
624
1.3
1.3
1.3
0.854
0.888
0.910
198
151
118
136
75
190
149
103
2
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be in the range 3.3 to 3.5, which is comparable to that of
theoretical number of electrons transferred (4 e-). In case of NS-
G@800, maximum number of electrons were transferred. The
40 approach to synthesize nitrogen and suphur doped materials from
CPPs, opens up the possibility of synthesizing graphene doped
with
d block elements by synthesizing d block element
comprising conjugated porous polymers.
We thank the Department of Science and Technology for
45 financial support. MEB thank Council of Scientific and Industrial
Research for scholarship.
Notes and references
a CSIR-Networks institutes of solar energy, Polymer science engineering
division, CSIR-National Chemical Laboratory, Dr Homi bhabha road,
50 Pune, 411008, Maharashtra, India Tel:+91-20-25903075;
Academy of scientific and innovative research, New Delhi, India
E-mail: k.krishnamoorthy@ncl.res.in
Fig. 3 Cyclic voltammogram of NS-G@800 in N2 and O2 atmosphere
(a), LSV of NS-G@800 in presence of oxygen (b), Koutecky-Levich
plot of NSGs (c) and number of electrons transferred as a function of
applied potential (d).
† Electronic Supplementary Information (ESI) available: [Experimental
procedures, synthetic schemes, NMR, XRD, XPS, IR, capacitor curves
55 and oxygen reduction voltammograms are available]. See
DOI: 10.1039/b000000x/
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Conclusions
Conjugated porous polymer comprising triazine and thiophene
30 were synthesized as a precursor to prepare nitrogen and sulphur
doped graphene. The heteroatom doped graphene exhibited a
capacitance of 211 F/g indicating the materials ability to store
charges. It should be noted that the NS-G@900 exhibited higher
storage due to the increased contribution of graphene.
35 Furthermore, these materials also acted as electrocatalyst to
reduce oxygen. The number of electrons transferred was found to
100
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