Journal of Energy Storage 32 (2020) 101840
Contents lists available at ScienceDirect
Journal of Energy Storage
journal homepage: www.elsevier.com/locate/est
A review on Graphitic Carbon Nitride based binary nanocomposites as
supercapacitors
T
M.G. Ashritha, K. Hareesh
⁎
School of Applied Sciences (Physics), REVA University, Bengaluru560064, India
ARTICLE INFO
ABSTRACT
Keywords:
Supercapacitors
Graphitic carbon nitride
Transition metal oxide
Metal sulfides
Conducting polymer
Graphitic carbon nitride (GCN): a structural analogous of graphite with aromatic tri-s-triazine units has attracted
and opened a new arena in energy storage and conversion of supercapacitors owing to its nitrogen rich framework, metal-free characteristic, earth abundance and environmental friendliness in the current decade.
Nevertheless, the supercapacitance performance of GCN can be enhanced by coupling with transition metal
oxide/hydroxides, metal sulfides or conducting polymers, which increases the surface area, number of active
sites and easy diffusion of inserted/de-inserted ions at the electrode-electrolyte interface further facilitating the
faradaic reaction. Therefore, the present review discusses the different GCN based electrode materials with
enhanced supercapacitor performance. Furthermore, the historical development, future prospects and opportunities in supercapacitors are elaborately addressed.
1. Introduction
With ceaseless depletion of fossil fuels, dramatically varying climate
changes, uneven distribution of energy and global concern towards
environmental pollution, there has always been a necessity to develop
cleaner, renewable and reliable energy sources to cope up with ever
growing energy need of human race. To stand up with such indispensable energy demands, it is must to develop more efficient, affordable, alternative energy storage devices to easily harvest energy from
intermittent renewable sources like solar, wind and tidal energies and
to store them effectively [1–4]. Electrochemical energy storage/conversion system is one such technology. Electrochemical capacitors also
termed as supercapacitors or ultracapacitors, gained prominence due to
their unique properties like, they possess higher power densities than
conventional batteries additionally store ten to hundredfold of more
energy delivered in a short interval time than capacitors. They tend to
have thousands of continuous charge discharge cycles which make
them distinctive from other storage devices like batteries and capacitors
[5–9].
In present scenario supercapacitors are widely used where quick
charge/discharging is required like regenerative braking in hybrid
electric vehicles (HEVs). They also used in portable electronics, LEDs,
power back up, power grids and flashlights etc [10,11]. Researchers are
working to improve the efficiency of supercapacitors by using different
electrode materials such as carbon, transition metal oxides, sulfides,
⁎
Mxene or different combination of them depending upon their applications [12–16]. Among these, carbonaceous materials due to their
promising properties towards supercapacitor application like high surface area, remarkable chemical and mechanical stability, ease of
synthesis, availability of precursors due to natural abundance of carbon
based materials and bipolarity leading to continuous charge/discharge
cycles making them a potential material for supercapacitors [7,17,18].
Performance of those carbon-based materials can be enhanced by
doping with heteroatoms which increases the pseudocapacitance of the
system. Heteroatoms like O, N, B, S and P through surface faradaic
reactions, changes acceptor-donor characteristics leading to an increase
in capacitance of system [19,20]. Nitrogen is one such widely incorporated atom into graphene network for supercapacitor application.
Addition of nitrogen to carbonaceous material frame increases the
wettability of electrode, electrical conductivity and hence enhance the
capacitive performance. N doing increase the active surface sites and
being a neighbour of carbon additionally provide an electron, which
improve the stability of material. Moreover, it supplies pseudocapacitance by surface faradaic reactions [21].
Graphitic carbon nitride(GCN) a metal free semiconductor, an
analogous to graphene in its layered structure have high nitrogen
content with C:N ratio of 3:4 has emerged as a most desirable material
in various fields because of its high thermal and chemical stability,
wettability, abundance of raw material, ease of synthesis along with
high nitrogen content makes it more viable in different fields like
Corresponding author.
E-mail address: appi.2907@gmail.com (K. Hareesh).
https://doi.org/10.1016/j.est.2020.101840
Received 20 June 2020; Received in revised form 27 July 2020; Accepted 28 August 2020
2352-152X/ © 2020 Elsevier Ltd. All rights reserved.
Journal of Energy Storage 32 (2020) 101840
M.G. Ashritha and K. Hareesh
photocatalysis, sensing, water splitting and as well as in supercapacitors
[22,23]. In comparison to above investigated areas GCN utilization in
supercapacitor electrodes are limited due to its comparatively lower
surface area and low electronic conductivity [24]. Hence a continuous
effort is going on scientific community to tackle this drawback of GCN.
Although a number of review articles are presented on GCN in various
applications, only a short note is presented on usage of GCN towards
supercapacitors in most of the papers [23,25–29]. Therefore, a detailed
study on usage of GCN is of great concern now. On the other hand, from
the past three to five years only handful of reports are published outlining the drawbacks of GCN and various techniques used to get over it.
Therefore, in this study our aim to insight a note on advances in usage
of GCN and its hybrids which includes at least one GCN material along
with different metal organic framework/carbonaceous material/ conductive polymers or by making appropriate modification in the frame
work as electrode materials for supercapacitors. Considerable enhancement in electrochemical framework of system could be achieved
in some cases. All these different combinations and significant electrochemical property changes are the main focus of this review.
In conclusion and outlook, an advanced overview on different directions and prospects along with some research gaps in the field of
GCN based supercapacitors are briefly discussed.
tri-s-triazine/heptazine aromatic conjugated rings as demonstrated in
Fig. 1. Among all these allotropes, it was proven that graphitic carbon
nitrides is most stable phase in ambient condition [31]. Even though
research in the field of utilization of GCN as a supercapacitive material
is stepping its foot in present decade, it is becoming a new hot spot of
research among scientific and production community.
Graphitic carbon nitride is an n type semiconductor, with 2D
structural analogous of graphene in its stacked layers with a nitrogen
substituted framework which exhibits pi conjugation with sp2 hybridization of carbon and nitrogen atoms in each plane. More interestingly compared to graphite, interplanar distance between layers in
GCN is decreased from 0.335nm to 0.315nm and this can be explained
by altering localization of electrons by substituted nitrogen atom. This
leads to more denser packing of GCN with stronger binding which results in enhanced perpendicular charge transfer between the layers of
GCN which is fruitful for supercapacitors [32].
There are two isomers of GCN, one (Fig. 1a) is condensed s triazine
unit and another (Fig. 1b) is tri-s-triazine unit which are obtained using
different synthesis methods and with whole lots of nitrogen-based
precursors. The latter has larger periodic carbon vacancies in the lattice
compared to earlier. Further DFT calculations demonstrated that tri-striazine isomers are energetically more stable and favourable than other
proposed s-triazine isomer [34]. Therefore, it is recognized as most
productive tectonic unit of GCN frameworks.
GCN is generally a yellow (pale yellow) coloured polymer in powder
form with 0.75 of C:N ratio and that can be varied depending upon the
different precursors and synthesis routes. Graphitic carbon nitride can
be synthesized through various methods. One such most widely used
easy, facile method is pyrolysis (thermal polymerization) of nitrogen
rich precursors. An oxygen free and C-N core structured precursor like
urea [35–40], cyanamide [41,42], thiourea [43,44], melamine [45,46],
dicyandiamide [47,48], guanidinium chloride [49] or different mixture
of them [50] heated at desired temperature to obtain polymeric GCN.
2. History, structure and properties of GCN
Carbon nitride is regarded as one of the earlier developed polymers
in scientific community. History of carbon nitride way back to early
1834 with the synthesis of polymeric tri-s-triazine material called
‘Melan’ by Berzelius [30]. Theoretically through first principle calculation it was found there were seven phases of carbon nitride. It includes alpha, beta, cubic, pseudocubic and graphitic structures (g-htriazine, g-o-triazine, and g-h-heptazine). It has been found that basic
structural building block of all these allotropes are triazine (C3N3) and
Fig. 1. The structure of a) s-triazine, (b) tri-s-triazine, (c) g-C3N4 based on s-triazine units, (d) g-C3N4 based on tri-s-triazine units. Reproduced from ref. [33] with the
permission of Elsevier.
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Journal of Energy Storage 32 (2020) 101840
M.G. Ashritha and K. Hareesh
But these materials yielded a bulk GCN which possess low electronic
conductivity due to stacking of layers which leads to lower surface area
and fast electron hole recombination rates. Various exfoliation techniques were used to obtain GCN with higher surface area with numerous
reaction sites and shorter diffusion pathways leading to increased
conductivity of GCN which is most suitable for supercapacitor applications [16].
As mentioned earlier addition of nitrogen into carbonaceous materials improve their wettability and conductivity. Addition of nitrogen
brings electrons to delocalized pi electron cloud of carbon materials
thus increases the conductivity. Graphitic carbon nitride is chemically
stable that it cannot dissolve in any acidic, alkali or other organic solvents which makes it a powerful material for electrochemical applications. On the other hand due to high contact resistance, low surface
area and large recombination rate it faces serious conductivity issues
affecting storage of supercapacitors [51]. Nevertheless from the view of
non-polar carbons, substitution of nitrogen in graphitic carbon nitride
leads to more heterogenous atoms owing to increased wettability. Injection of electrons increases the density of states near fermi energy
level leading to enhanced quantum capacitance in GCN. Most importantly added heteroatom enriches GCN towards reverse redox faradaic reaction (pseducapacitance) which causes them to possess hundreds of magnitudes of specific capacitance than EDLCs along with
maintaining its cycle life.
As shown in Fig. 2, GCN generally consists of pyrrolic, pyridinic/
pyridone N, quaternary N and oxidized pyridine-N, N rich functional
groups. Pyrrolic N and pyridinic N are present at the edges of GCN
plane whereas quaternary N atoms groups are present inside the plane.
Pyrrolic N and pyridinic N are bonded to two carbon atoms and act as
electron donors and quaternary N are bonded to three carbon atoms
respectively. Quaternary N significantly enhance the electron transfer
property and hence electrical conductivity.
Table 1
Comparison of characteristics of different energy storage devices. Reproduced
from ref. [56] with the permission of Elsevier.
Performance
Battery
Electrostatic
capacitor
Supercapacitor
Charge time
Discharge time
Energy density (Wh/kg)
Power density(W/kg)
Charge- discharge
efficiency
Cycle life
1-5 h
0.3-3 h
1-100
5-200
0.7-0.85
10−3s to 10−6 s
10−3 to 10−6 s
<0.1
>10000
~1
0.3-30 s
0.3-30 s
1-10
~1000
0.85-0.98
500-2000
>50000
>10000
the fields where we have to store a higher amount of energy for long
term use, nevertheless they lack when it comes to higher power delivery
[5,52].
Both these batteries and capacitors uses are limited when it comes
to high energy density along with high power capability application. In
the view of solving this need, supercapacitors gained profound interest
because of its proficiency in high power densities with decent energy
densities along with remarkable cycle life [53,54]. They manage to
supply higher amount of power than battery but they aren't able to store
as much as energy a battery can store in a same volume. They have
advantages of unlimited cycle life without considerable capacitance
loss, with low ESR responsible for high power delivery and quick
charging. As well they work efficiently and safely in low temperatures.
They even possess some limitations like fraction of energy storage with
shorter voltage window in comparison to batteries and cost per Watt is
high compared to capacitors. They also possess high self-discharge rate.
A table of various characteristics of different energy storage devices
are listed in Table 1.
Nevertheless, supercapacitor differs from regular capacitor or battery in its charge storage mechanism also. Supercapacitor can store
charge either electrostatically like capacitor but with higher amount of
capacity (EDLC) or through faradaic charge transfer like batteries but
with no phase change leading to a mechanism called pseudocapacitance
or by combining these two mechanism leading to a hybrid supercapacitor [53].
There are three different classes in hybrid supercapacitor. 1)
Asymmetric supercapacitor-where two different electrodes with differentiating EDLC (carbon based) and Pseudocapacitor (metal oxides or
conducting polymers) are combined to form a cell. 2) Battery type supercapacitor- two different electrode with one battery type and another
3. Supercapacitors
With increase in energy consumption among human trace, as important it is to develop clean, environment friendly energy portfolio it is
also crucial how we efficiently store energy. Supercapacitor is one such
energy storage devices along with commonly used batteries and capacitors. Capacitors are well known for their higher power density and are
generally used in the application where there is need to deliver energy
quickly within seconds which can be termed in other way as, where
there is a need for high power. Whereas the batteries are applicable in
Fig. 2. Schematic of the types of N-doping of GCN.
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Journal of Energy Storage 32 (2020) 101840
M.G. Ashritha and K. Hareesh
Table 2
List of GCN materials for supercapacitors.
Material
Carbon nitride source and
synthesis method
Electrolyte and potential
window in Volt
Specific capacitance in F/g
(at *Cd in A/g)
Cycle number,
capacitance retention
Ref.
g-C3N4 nanofibers
Tubular g-C3N4
Activated carbon nitride (ACN)
Melamine,
Melamine
Melamine, pyrolysis/activation
263.75 (0.1)
233 (0.2)
185(0.5)
2000, 93.6%
1000, No obvious loss
5000 87.2%
69
70
72
Exfoliated g-C3N4
Urea, pyrolysis
113.7 (0.2)
5000 89.3%
73
Mesoporous graphitic carbon nitride
(MGCN)
g-C3N4@G
Melamine, carbonization
Na2SO4, (0.1-0.7)
KOH(-0.2 to 1)
[BMIm]BF4/acetonitrile,
(-1.3 to 1.3)
LiClO4
(0.2 to 0.8)
H2SO4
(-0.2 to 0.8)
LiClO4
(0 to 1)
H2SO4(-0.8 to 0)
KOH(-1 to 0)
KOH(-0.5 to 0.5)
H2SO4(-0.2 to 0.8)
244 (0.5)
5000 100%
74
264 (0.4)
10000, No obvious loss
75
265 (1)
288 (0.5)
379.7 (0.25
403.6 (0.1)
5000, 94%
10000, 85%
5000, 100%
21
76
77
61
Melamine, hydrothermal
GOOCN
rGO/carbon nitride
C-C3N4@rGO
g-C3N4/nitrogen-enriched carbon
spheres
g-C3N4/ mesoporous carbon sphere
Carbon cloth@GCN
(CC@GCN)
g-C3N4 Assembly with graphene oxide
g-C3N4/Ni(OH)2
Melamine, solution based method
Dicyandiamide, hydrothermal
Melamine, solvothermal method
Urea, thermal decomposition
Melamine,
Urea, glucose
annealing
Urea, pyrolysis
Melamine, hydrothermal method
KOH(-0.2 to 0.6)
H2SO4(0 to 1)
352.44 (5 mV/s)
499 (1)
3000, 88.44%
10000,113%
78
79
H2SO4(0 to 0.8)
KOH (-0.1 to 0.5)
936 mF/cm2 (936 mF/cm2)
505.6 (0.5)
1000, No obvious loss
1000, 71.5%
80
85
g-C3N4/NiAl-LDH
Urea, In situ self-assembly
KOH (-0.2 to 0.6)
714 (0.5)
10000, 82%
86
CNRG/Ni(OH)2
KOH (0 to 0.5)
1785 (2)
5000, 71.3%
87
Carbon-doped g-C3N4/MnO2 (CCNM)
Melamine, Hyper acoustic
treatment, oil bath
Melamine, Hydrothermal method
Na2SO4(-0.2 to 0.8)
324 (0.2)
1000, 80.2%
88
K doped g-C3N4/MnO2 (KCNM)
Thiourea, hydrothermal method
Na2SO4(-0.2 to 0.6)
373.5 (0.2)
1000, 95.2%
89
Na doped g-C3N4/MnO2 (Na-CNM)
Thiourea, hydrothermal method
Na2SO4(-0.2 to 0.6)
294.7 (0.2)
1000, 93.7%
89
MnO2/ g-C3N4 NC
Melamine, chemical route
Na2SO4(-0.2 to 0.8)
211 (1)
1000, 100%
90
Nanoneedle-assembled NiCo2O4// gC3N4
Nanosheet assembled NiCo2O4/gC3N4
CN/Ni
g-C3N4/α-Fe2O3
thiourea, Hydrothermal treatment,
calcination
thiourea, oil bath treatment,
calcination
Sonication, heat treatment
Melamine, pyrolysis
KOH(0 to 0.6)
274.8 (1)
1000, 89.4%
91
KOH(0 to 0.6)
118.2 (1)
1000 101%
91
KOH(0 to 0.5)
KOH(0 to 0.5)
1768.7 (7)
580 (1)
4000, 84%
1000, 96%
92
93
Carbon/CuO nanosphere@ g-C3N4
Thiourea, co pyrolysis
decomposition
Melamine, co-precipitation
NaOH(-0.6 to 0.6)
247.2 (1)
6000, 92.1%
94
KOH (0 to 0.5)
780 (1.25)
1000, 80%
95
Melamine, hydrothermal and
lyophilization
Dicynamide, hydrothermal, heat
treatment
Thiourea, hydrothermal
Thiourea, oxidation
reduction reaction
Thiourea, chemical
reduction method
Melamine,
hydrothermal
Melamine, thermal decomposition
Melamine, in situ method
Melamine, hydrothermal
KOH(-1.2 to 0)
704.3 (0.5)
2000, 98%
96
KOH(0 to 0.5)
125.1 (1)
1000, 100%
97
KOH(0 to 1.0)
KOH(0 to 1.0)
79.62 (1)
174 (1)
9000, 92%
6000, 85%
98
99
KOH(0 to 1.0)
64 (1)
6000, 87%
99
KOH(0 to 0.5)
243(1)
1000,81%
100
KOH(0.2 to 0.6)
KOH(0 to 0.6)
KOH(0 to 0.6)
3000(3)
1998(2)
1557 (1)
1000,95.6%
5000 95.2%
10000, 92.6%
101
102
108
Thiourea,
Melamine, solvothermal
Melamine, solvothermal
KOH (-0.1 to 0.5)
KOH (-0.1 to 0.5)
Na2SO4(0 to 0.5)
506C/g (1)
834 (0.5)
210.30 (0.5)
1500 99%
5000 98%
1500 84%
109
16
65
g-C3N4 /MoS2
Thiourea, sonication
Na2SO4-pva(0 to 0.4)
45.5 (0.5)
100 98%
110
PEDOT/ g-C3N4
PEDOT/ g-C3N4
PANI/ g-C3N4
Polypyrrole/ g-C3N4@G
Cellulose /polypyrrole /tubular gC3N4
Melamine, layer-by-layer assembly
Melamine, layer-by-layer assembly
Urea, oxidative polymerization
Melamine, hydrothermal
Melamine, polymerization followed
by self-assembly.
H2SO4(-0.2 to 0.8)
Na2SO4(-0.6 to 0.6)
H2SO4(-0.2 to 1.2)
KOH(-1.0 to 0.2)
KCl(0 to 0.5)
137 (2)
200 (2)
584.3 (1)
260.4 (1)
2.53 F/cm2 (5 mA)
1000,
1000,
1000,
2000,
2000,
114
114
115
116
117
MnS / g-C3N4
Melamine, sol-gel
Na2SO4(-0.2 to 1.0)
403.36 (0.005)
2000, 98.6%
Mesoporous Co3O4/ g-C3N4
RuO2/ g-C3N4@rGO
TiO2/ g-C3N4
TiO2/ g-C3N4
g-C3N4/MnO2
g-C3N4/SnO2
Fe2O3 nanospheres /GCN
GCN/(OZCN)
Ov-NiCo2O4 /ND-g-C3N4
C self-repairing GCN/NiCo2S4 (CPCNNSs/NCS-NPs)
NiCo2S4/p- g-C3N4 nanosheets
Cobalt sulfide/ g-C3N4 (CN/CS)
SnS2– g-C3N4
89%
96.5%
82%
80%
54.9%
67
(continued on next page)
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Journal of Energy Storage 32 (2020) 101840
M.G. Ashritha and K. Hareesh
Table 2 (continued)
Material
Carbon nitride source and
synthesis method
Electrolyte and potential
window in Volt
Specific capacitance in F/g
(at *Cd in A/g)
Cycle number,
capacitance retention
Ref.
Red phosphorus g-C3N4
GCN/ZSO
Melamine, mechanical ball milling
Melamine, ultra-sonication assisted
Na2SO4(0 to 0.4)
Na2SO4(-0.6 to 0.6)
465 (1)
244.7(1)
1000, 90%
No obvious loss
118
119
*Cd- Current density
supercapacitor type are combined. Most important one is 3) Composite
supercapacitor-on a single electrode both chemical (pseudocapacitance) and a static (EDLC) are combined to utilize the advantageous of
both mechanisms to yield both higher energy with power capability
through a single electrode [57,58]. In the following review we mainly
discuss different hybrid composite electrode and their performance.
Specific capacitance (Cs in F/g) of asymmetric supercapacitor is
calculated from GCD data using the formula [59]
Cs =
I× t
ma × V
=
4. Perspectives of GCN
Like aforementioned graphitic carbon nitride being an analogous of
graphite with high nitrogen content(non-graphitic), redox active materials, its application when accessed as a material for electrochemical
electrode is hindered because of its large band gap(2.67eV), low conductivity and low surface area. As though a simple GCN with high nitrogen content can be easily produced by pyrolysis of melamine at
550°C, but it doesn't exhibit a notable energy storage in supercapacitors. Hence, it is important to resolve this problem by altering its
electronic and textural properties of GCN. A various approach has been
put forward including morphological control by varying precursors and
synthesis condition, doping with other atoms, compositing with other
metal organic framework or with conducting polymer materials or even
by combining some of these approaches. Interestingly, GCN have the
potential to integrates the uniqueness of carbon material like low cast,
availability of abundant sources with advantages of nitrogen doping
like surface redox reaction and improved surface wettability along with
benefits from energy storage of supercapacitor. In the following review
we mainly concern on advancements in the field of pristine GCN, GCN/
transition metal oxide/sulfides and GCN/conducting polymer composites for energy storage enhancement in supercapacitors.
where I is applied current (A), Δt is discharge time (s), ma is mass of
active material, ΔV is voltage window (V). The specific energy (Ed in
Wh/kg) and specific power (Pd in W/kg) [58] can be calculated as
follows.
1
Cs ( V )2
2
(2)
Pd =
Ed
t
(3)
The specific capacitance (Cs) of symmetric supercapacitor is calculated using [60,61],
Cs =
4I × t
ma × V
(4)
The corresponding specific energy (Ed) and specific power (Pd) of
symmetric supercapacitor are given by [60,61],
Ed =
Pd =
1
C(
8 s
V )2
4.1. Synthesis of GCN based binary nanocomposites
(5)
3.6
Ed
t
(7)
where td and tc are time taken for charging and discharging taken from
GCD curves.
(1)
Ed =
td
× 100
tc
There are different synthesis methods used for the synthesis of GCN
binary nanocomposites. Some of them are briefly discussed as follows
Hydrothermal/solvothermal methods: It is the most widely used
synthesis process in any nanoparticle or composite preparation. It is
described as a synthesis methods where the chemical reaction takes
place inside a closed Teflon autoclave under high pressure and high
temperature condition [62]. It is admired because there is a control
over different morphology of nanoparticles along with their size, high
purity, and controlled crystallinity. Nevertheless, it lacks control over
particle aggregation. If other solvents instead of water is used in the
pressure vessel(autoclave) it is termed as solvothermal method. Various
(6)
Eddefines how efficiently a device can store energy and thus a device
with higher Ed can store and supply an electronic load for longer than
one with a lower Ed.Pd describes how quickly the device can deliver the
stored energy with respect to time. It is equivalent to the maximum
current you can draw from a device of a given mass/volume A Ragone
plot effectively describe specific energy versus specific power at different current densities. Also, coulombic efficiency (ɳ) is calculated
using [59–61],
Table 3
List of GCN materials and structure for various nanocomposites.
Materials
Structure
Property
Enhancement of Cs
Ref.
Mesoporous GCN
g-C3N4@G
Ni(OH)2/ g-C3N4
g-C3N4/NiAl-(LDH)
MnO2/ g-C3N4
GCN/Ni
Co3O4/ g-C3N4
TiO2/ g-C3N4
g-C3N4/MnO2
g-C3N4/ SnO2
MoS2- g-C3N4
PEDOT/g-C3N4
Nanoflakes
Nanosheet
Nanosheet
Nanosheet
Nanosheet
Nanosheet
Nanosheet
Nanosheet
Nanosheet
Nanosheet
Nanosheet
Nanosheet
Nitrogen content
Specific surface area
Morphology conversion (nanoplates to nanoflowers)
Surface area and pore size
Surface area
Surface area and pore size
Surface are and pore volume
Surface are and pore size
Surface are and pore size
Surface are and pore size
high surface area, increased mass transfer
high surface area, increased mass transfer
3.4-fold higher than bulk GCN
Twice high than pristine graphene
2.2-fold than Ni(OH)2
1.6 fold than pure LDH and 25 fold than GCN
2 fold than MnO2
4.2 fold higher than Ni(OH)2
1.8 fold higher than Co3O4
1.1 fold higher than TiO2
2.6 fold higher than MnO2
1.2 fold higher than SnO2
5 fold higher than MoS2
1.1 fold higher than PEDOT
74
75
85
86
90
92
95
97
99
99
110
114
5
Journal of Energy Storage 32 (2020) 101840
M.G. Ashritha and K. Hareesh
GCN based supercapacitor electrodes are successfully synthesized such
as Cobalt sulfide/graphitic carbon nitride[16], GCN/Ni2CoS4 nanocomposites [63], heterostructure SnO2/GCN [64], 3D flower-like SnS2
on g-C3N4 [65] by hydrothermal method. Co-precipitation method: It is
moderately used synthesis method when it comes for large scale production. Here for the precipitation to takes place, high temperature is
required. It has advantages like high purity, ease and low cost of
synthesis. It disadvantages lies in its uncontrollable morphology because of high rate precipitate formation [66]. Sol-gel method: It is a
method to get uniform particle size with high purity. Here the sol
(particles in a solution) agglomerate and form a gel. Colloidal and
polymeric are the two different methods which differ by precursor used
in solvent. The solvent is usually water and alcohol in respective colloidal and polymeric method. precursor is activated either by using an
acid or base, then with appropriate temperature and reaction time it
forms a network of gel. Materials with different morphologies can be
easily synthesized using this method with high surface area [67]. In-situ
polymerization: Here through sonication, monomers are effectively
dispersed in solution and with addition of oxidising reagents polymerization starts in the solution. These polymerized nanoparticles are
then washed and filtered from the aqueous solution. With modification
in synthesis parameters different morphologies of nanoparticles are
effectively reported with better chemical properties. Chemical vapour
deposition (CVD): This technique is performed in vapour phase where
porosity of synthesized material is of concern. Initial vapour material
subjected to form even morphology nanomaterial under an 800-1000°C
of temperature. For an example synthesis of graphene using CVD
compared to other synthesis route yielded better crystal domain with
less defects and with uniformity [68] Direct coating: It's a commonly
employed fabrication methods when the synthesized nanoparticles are
effectively in the form of slurry. These slurries are directly applied on
the substrate. Generally activated carbon or carbon black is introduced
to enhance electrical conductivity along with polyvinylidene fluoride
(PVDF) as a binder and NMP as a solvent in different electrode preparation process for supercapacitors.
Tahir et al. [69] synthesized nanofibers of GCN(GCNNFs) using a
melamine. SEM images showed that these fabricated GCNNFs have an
average diameter of 10nm and length 20µm as shown in Fig. 5A.
Compared to bulk GCN surface area (4m2/g), GCNNFs possessed a
higher surface area(165m2/g). In aqueous Na2SO4 solution they delivered a specific capacitance (Cs) of 263.73F/g and the cycling life of
GCNNFs was found to be 93.6% when assessed in 1 A/g of current
density by 2000 GCD measurement. The same research group synthesized a tubular GCN using melamine as a precursor which was pretreated with HNO3 and ethylene glycol [70]. With a mean diameter of
0.8 µm of tubular structure, formation of GCN is clearly presented in
Fig. 4. Different classes of supercapacitors.
Fig. 3B. At a discharge current of 0.2 A/g it achieved a specific capacitance of 233 F/g in KOH solution. It exhibited only 10% capacitance
attenuation after 1000 charge/discharge cycles. This difference in Cs of
these structures compared to bulk GCN evidently revealed that structural optimization is one of the effective methods for enhancement in
supercapacitor energy storage. Zhang et al. [71] synthesised a set of
GCN nanosheets using thermal and chemical oxidation methods with
different concentration of H2SO4 and with different temperature. Values found for Cs of GCN is higher than both GCN (127.7 F/g) synthesized at 550 °C using melamine and GCN (133.6 F/g) after chemical
oxidation with 12 M sulfuric acid at a discharge value of 0.5 A/g.
Further, 580 °C CN exhibited energy density of 3.740 Wh/kg at 99.46
W/kg of power output.
Shen et al. [72] synthesized a porous activated carbon nitride (ACN)
via a facile pyrolysis/activation method. A series of ACN is synthesized
by varying the pyrolysis temperature of the precursor melamine and
citric acid, as these precursors can effective form a porous framework.
The yielded ACN at 700°C (ACN-700) exhibited optimal nitrogen content (15.6 at%) with high surface area (716.3 m2/g) [calculated using
BET]. Intense quaternary N group(Q-N) represented in XPS spectra of
ACN-700 predicted the enhanced the specific capacitance of porous
ACN. When the synthesized material used as an electrode it delivered
185 F/g of Cs at the discharge current of 0.5 A/g and offered almost
87.2% of initial capacitance after 5000 GCD cycles. When ACN-700 is
accessed in symmetric cells it delivered 16.9Wh/kg of Pd in an ionic M
[BMIm]BF4/acetonitrile solution and 6.2Wh/kg in KOH solution. The
achieved performance is attributed to pseudo−capacitance of N group
and enhanced surface area after KOH activation.
As well the electrochemical properties of pristine GCN is reported by
Gonçalves et al. [73]. They used a simple pyrolysis of melamine and
exfoliated using ultrasonication using water/isopropanol (2: 1) solution. GCN when accessed exhibited a Cs of 113.7 F/g in LiClO4 acetonitrile solution. It exhibited only 10.7% of capacitance fading after
continuous 5000 GCD measurements indicating its charge/discharge
stability. Furthermore, it delivered an Ed of 76.5 Wh/kg at a power
capability of 11.9 W/kg. Idris et al. [74] is synthesized mesoporous
graphitic carbon nitride (MGCN) using a facile carbonization method.
The methylolated surfactant-polymer composite (reaction between
melamine and glutaraldehyde, in presence of P123, a surfactant) is used
as a precursor to develop MGCN and bulk GCN (BGCN) is developed
without the surfactant in under the same condition. MGCN and BGCN
are tested for electrochemical performance both in acidic (H2SO4) and
alkaline (KOH) electrolyte. Both bulk and mesoporous showed inverted
Fig. 3. Ragone chart: Power density vs energy density for various energy devices. Reproduced from ref. [55] with the permission of Elsevier.
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Journal of Energy Storage 32 (2020) 101840
M.G. Ashritha and K. Hareesh
Fig. 5.1. A. (a, b) Low-magnification SEM images of GCNNF and (c, d) high-magnification SEM images of GCNNF. 5.1. B.(a) 1st, 1000th, and 2000th charging−discharging curves for current density 0.5 A/gof GCNNF. (b) 1st, 1000th, and 2000th charging−discharging curves for current density 1.0 A/g of GCNNF.
Reproduced from ref. [69] with the permission of American Chemical Society.
‘V-shape GCD curves. MGCN delivered a higher Cs (244, 56 F/g in acid
and alkaline respectively) in acid medium and compared to BGCN (71
F/g in acid and 27 F/g in alkaline) it is because of the high surface area
and more accessible mesopores possessed by MGCN and being intrinsically basic provides less accessible sites for chemical reaction in
alkaline than in acid medium. Fig. 6: depicts the GCD of BGCN and
MGCN in a) 1 M H2SO4 and (b) 6 M KOH electrolytes. At a power
density of 125 W/kg MGCN exhibited three folds the higher value of
energy density (38.74 Wh/kg) than BGCN (11.39 Wh/kg). Furthermore,
MGCN exhibits no attenuation of Cs after 5000 GCD measurements. The
enhanced electrochemical performance was attributed to uniform mesoporous structure and high accessible surface area of MGCN and different nitrogen groups increased surface redox reactions leading to high
Cs.
Aforementioned few results highlighted electrochemical performance and inherent properties of GCN which are important to develop
composites with other materials to enhance the specific capacitance
along with long lifespan and energy density.
4.2. GCN composite with carbon nanomaterials
Considering the effective stability and high conductivity and ease of
availability carbon material, it is a potential material in supercapacitors. Some of the research groups utilized these properties of
Carbon materials to synthesize composite with GCN to enhance the
conductivity and to yield high specific capacitance of GCN. Graphene, a
class of carbon with two-dimensional hexagonal lattice of carbon
atoms. They stand out in their performance by their large accessible
surface area, thermal as well as electrical properties [75]. As both GCN
and graphene possess similar electronic structure expect GCN having
high nitrogen content, it was found advantageous to modify graphene
with GCN to enhance the performance of composite towards
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Journal of Energy Storage 32 (2020) 101840
M.G. Ashritha and K. Hareesh
Fig. 5.2. A. (a & b) SEM images, (c & d) TEM images of the tubular g-C3N4;B. (a)First five charging discharging curves (b) Last five charging - discharging curve in a
potential window of -0.2–1.0V for current density 0.2 A/g and 0.5A/g of tubular g-C3N4. Reproduced from ref. [70] with the permission of Royal Society of
Chemistry.
supercapacitor electrode application than using bare components. Chen
et al. synthesized a 3D functionalized graphene/GCN binary nanocomposites using a facile hydrothermal process as shown in Fig. 7. Similar to that of graphene, GCN have been successfully formed on graphene layers with loosely formed 3D structure. Even though both gC3N4@G and graphene possess almost same conductivity values (0.38
S/m and 0.35 S/m respectively), g-C3N4@G composite when used as an
electrode material delivered a Cs of 264 F/g which is considerably
greater than pristine graphene (152 F/g). The increase in capacitance
values of almost about 75% is attributed to addition of GCN to graphene
framework. GCN addition was found advantageous because it decreased
the aggregation of graphene sheets and enhanced the capacity along
with cycling stability of composite. At 4 A/g, g-C3N4@G showed no
obvious capacitance decrease even after evaluating over 10000 cycles.Thus, the results demonstrated that synthesized g-C3N4@G composite can be efficiently used as an electrode for electrochemical
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Journal of Energy Storage 32 (2020) 101840
M.G. Ashritha and K. Hareesh
Fig. 6. A.CV of BGCN and MGCN recorded at a scan rate of 5 mV/s 1 in (a) 1 M H2SO4 and (b) 6 M KOH electrolytes; B. Galvanostatic charge-discharge cycles of
BGCN and MGCN recorded at a current density of 0.5 A/g in (a) 1 M H2SO4 and (b) 6 M KOH electrolytes. Reproduced from ref. [74] with the permission of John
Wiley and Sons.
supercapacitors.
Edge nitrogen atoms mainly contribute to pseudo-capacitance of the
system. But a large molecule lacks in number of edge nitrogen. Hence n
number of research groups are working develop a facile GCN with increased number of nitrogen atoms. Hence Lin et al. [21] demonstrated a
facile route by combining graphene with pre-oxidized GCN (GOOCN).
When g-C3N4 treated with HNO3 (oxidizing agent) it cuts the larger
molecule of GCN into smaller pieces, as a results number of edge heteroatoms increases thus, the pseudocapacitance of the system. The
composite in acid and alkaline medium of electrolyte resulted in 265.6
and 243.8 F/g of specific capacitance respectively. With the same
conditions but without pre oxidization of GCN, the composites yielded
40% and 55% decrease in capacitance values in respective acid and
alkaline electrolyte medium. The composite exhibited an excellent cycling life of 94% over 5000 GCD cycles. As shown in Fig. 8c, the surface
capacitive effects are found to take over the major part of Cs of the
composite with surface capacitive effects being 86.6% and diffusioncontrolled processes being 13.4% of total capacitance of electrode. It is
proved that the oxidation of g-C3N4 leads to higher electron conductivity and enhance ion diffusion of the composite by developing a
uniform dispersion of oxidized GCN on graphene. When GCN is treated
with strong oxidizing agents, it breaks the polymeric structures and give
rise to higher amount of edge N along with O doping of GOOCN which
leads to higher pseudocapacitance hence the conductivity.
Wen et al. [76] synthesized graphene/GCN composites with various
ratios of GCN in the composite using a one-step hydrothermal method.
In 6M KOH electrolyte the as synthesized RGO/GCN is used as a negative electrode and Ni-MOF/CNT as positive electrode for calculating
the Cs of the system. For RGO/GCN -X (X¼ 2, 3, 4, 5) is an abbreviation
used all over the paper where X stands for concentration of GCN in
composite. Pristine rGO was also prepared without the addition of GCN
nanosheets for comparison. at the same scan rate, rGO/GCN delivered a
higher current compared to the pristine rGO, indicating the higher
specific capacitance. This is because the added nitrogen in the GCN,
enhanced electron transfer of the composite and also increased the
wettability of electrode. The incorporated nitrogen atoms induced the
pseudocapacitance, which is in well agreement with the CV curve and
its shape. Higher values of g-C3N4 (from 4 mg to 5 mg of doping) induced smaller electrical double-layer contribution thus a decreased
power density compared to bare rGO. All the galvanostatic charge/
discharge curves exhibited a symmetrical triangular shape with a slight
deflection at 0.1 A/g of current density as shown in the following Fig. 9.
Symmetrical curve represented a good reversibility and linearity interpreted the good capacitive properties of electrode material. It is
observed that specific capacitance of rGO/GCN composite increased
from 212F/g to 288 F/g with the decrease in discharge current from 10
A/g to 0.1 A/g. It offered over 95% retention rate of Cs after 5000 GCD
cycles.
A porous structure of nitrogen-rich carbon material is synthesized
and reported by Ding et al. from carbon self-repairing GCN (C–C3N4)
assembled with 2D graphene oxide [77]. They developed a facile route
for the fabrication of porous nitrogen- rich carbon through a colloidal
solution of self-assembly of (C–C3N4) and GO. at 0.25A/g of current
density synthesized material exhibited a high Cs of 379.7 F/g. Energy
densities are calculated and represented in Ragone plots using GCD
tests and it was found to be 52.7 W h/kg at 0.25 A/g. At 10 A/g of
constant discharge current, stability tests showed that the material retained almost 85% of its initial specific capacitance even after
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Journal of Energy Storage 32 (2020) 101840
M.G. Ashritha and K. Hareesh
Fig. 7. A. (a) Schematic of synthesis of g-C3N4@G; (b) photograph of g-C3N4@G; (c) SEM image of g-C3N4@G; B. (b) CV curves with different scan rates and (c)
galvanostatic charge-discharge curves for g-C3N4@G at different discharge current densities. Reproduced from ref. [75] with the permission of Royal Society of
Chemistry.
continuous 10,000 charge–discharge cycles. Pristine rGO was also used
as electrode material and it exhibited only 195.1 F/g of specific capacitance. These specific capacitances values when compared illustrated
the role of GCN in enhancement of performance. The enhanced capacitance is due to the improved electrical conductivity, which attributed
from expanding of delocalized π-electron cloud by insertion of GCN
through the formation of substitutional C configuration and also by the
larger specific surface area, enhanced contact between rGO and C–
C3N4 of hybrid material. Nitrogen doped carbon spheres are successively introduced into interlayers of GCN to form g-CN/NCS composite
using co-calcination of precursors [61] as shown in Fig. 10. Synthesized
hierarchical porous structure in turn possess high nitrogen content
(along with added NCS) which is making them ideal for supercapacitor
material. Introduction of NCS expanded interlayers of GCN and thus,
more area was available for conductivity which was proved by BET
results of composite (448m2/g). When used as an electrode material the
as prepared composite exhibited 403.6 F/g of specific capacitance in
1M H2SO4 electrolyte solution. It also exhibited excellent cycling stability of almost zero % capacitance loss even after 5000 cycles. The
notable improved performance of composite is ascribed to its high nitrogen doping, improved specific surface area (electro active sites) and
also because of enhanced conductivity which imparts efficient pathways for electron and ion transportation in the conductive network of
GCN by introduction of NCS. As a result, g-CN/NCS material can be
efficiently utilized as a material for supercapacitor electrodes owing to
the synergistic effects.
Oh et al. [78] successfully dispersed mesoporous carbon spheres
(MPCS) on GCN to synthesis g-C3N4 /MPCS composites via a three- step
carbonization method. In this report they highlighted the effect of
dispersed MPCS on the electrochemical performance of electrode material. It was shown in the paper that only a proper ratio or amount (1:2
by weight) of MPCS can enhance the capacitive behaviour by increasing
surface area but a higher amount of MPCS in composite hinders the
electrolyte towards accessing GCN thus, decreasing the performance. A
Cs of 352.4 F/g was exhibited at a potential sweep of 5 mV/s by the
GCN/MPCS composites. Almost 89% of Cs retention was found after
3000 cycles. Zhu et al. [79] synthesised a carbon cloth (CC) supported
GCN nanosheet electrode for supercapacitor. Zhu et al. coated GCN
synthesized using urea and glucose on CC through annealing treatment
at a temperature of 900°C. taking advantage from binder free character,
favourable, ease of coupling and high N content of GCN, CC@GCN-900
delivered 499 F/g of Cs in discharge value of 1 A/g. moreover a symmetric supercapacitor exhibited moderate energy storage of about 10.1
Wh/kg at a high power output of 10000 W/kg. zero capacitance loss
was found even after 10000 GCD cycles. Thus, it showed potential to
become a binder free electrode for supercapacitors. Graphitic carbon
nitride in its bulk structure possess low conductivity may be because of
its high nitrogen level (greater than 50%) or by failure of thickness
control which is leading to agglomeration (because of some conventional synthesis methods). There are n numbers of works are going in
this field to increase the conductivity of GCN. Lu et al. [80] synthesized
an assembly of graphene-templated, conductive graphitic carbon nitride using epitaxial strategy. The reported assembly exhibited a large
specific surface area of about 724.9 m2/g with moderate nitrogen
doping of 18% along with 12.2 S/cm of high conductivity compared to
bulk GCN (1 S/m). The as prepared assembly delivered a specific capacitance of 936 mF/cm2. They exhibited excellent charge discharge
cycling stability of over 10000 cycles. In the synthesized assembly
graphene acted as a current collector and the structure provided more
active sites along with many channels for storing and transport. This
report promotes the use of GCN as an efficient material in energy storage in supercapacitors.
4.3. Carbon nitride composite with transition metal oxides and hydroxides
Transition-metal oxides such as manganese dioxide (MnO2),Fe3O4,
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Journal of Energy Storage 32 (2020) 101840
M.G. Ashritha and K. Hareesh
Fig. 8. (a) Synthetic route of GOOCN composites; (b) schematic illustration of predicted g-C3N4 before and after oxidation (c) kinetic analysis of the GOOCN24
electrode from CV curves. Reproduced from ref. [21] Published by The Royal Society of Chemistry. This article is licensed under a Creative Commons Attribution 3.0
Unported Licence.
nickel oxides (NiO), ruthenium oxide (RuO2) and cobalt oxide (Co3O4)
have been studied as a pseudocapacitive material for energy storage
applications in supercapacitors because of its multiple oxidation sates
[81–83]. Inherently they possess pseudocapacitive nature. They can be
used to improve specific capacitance of g-C3N4 by developing a binary
nanocomposite within appropriate potential window. Hence they endows a great potential to yield high capacitance supercapacitors and are
discussed as follows [84]. Shi et al. [85] reported g-C3N4/Ni(OH)2
composite by a facile single step hydrothermal method. Two sets of gC3N4/ Ni (OH)2 were prepared by adding different quantity of nickel
acetate (0.5 and 1 g) to GCN to form composites. Added GCN
successfully convert nanoplates like morphology of Ni(OH)2 into flower
like g-C3N4/Ni(OH)2 composite proved by TEM images. Synthesized
composited when used as an electrode material exhibited notable enhancement in specific capacitance (505.6F/g) compared to pristine Ni
(OH)2 (228F/g) in 6M KOH electrode solution. Here flower like Ni
(OH)2 enhanced the electron diffusion and ion transport in the composite hence exhibited an improved performance. It also showed about
71% of capacitance retention over 1000 cycles. It 17.56Wh/kg of energy density along with 125.43W/kg of power density. Thus, the synthesized composite can be used as an electrode material. A g-C3N4/
NiAl-layered double hydroxide (LDH) composites are synthesized
Fig. 9. Electrochemical performances. (a and b) CV and GCD of rGO/GCN composites with different contents of g-C3N4, respectively. (c) Specific capacitance as a
function of current density. Reproduced from ref. [76] with the permission of Royal Society of Chemistry.
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Journal of Energy Storage 32 (2020) 101840
M.G. Ashritha and K. Hareesh
Fig. 10. A. Schematic illustration of the preparation process of the g-CN/NCS; B. (a) CV (b) GCD of g-CN/NCS-2. Reproduced from ref. [61] with the permission of
Royal Society of Chemistry.
through a in situ method, where of NiAl-LDH were successfully grown
on GCN. Mixture of Al(NO3)3 9H2O and Ni(NO3)2 6H2O along with gC3N4 nanosheets and urea was refluxed to form the composite. Bare
LDH possess platelets stack like structure and the prepared nanocomposite exhibits a porous structure with curved nanosheets as shown
in TEM Fig. 11. Different amounts of GCN are added LDH to form
composites. g-C3N4 in composite improve the dispersibility of the LDH
and the formation of mesoporous structures which enhance the supercapacitor performance. When the composite is used as an electrode it
showed 714 F /g of specific capacitance which is considerably greater
than LDH 446 F /g at a discharge current of 0.5 A/g. After 10000 GCD
evaluations the composite exhibited only 18% of capacitance fading
indicating its higher stability. Excellent Cs demonstrated that prepared
nanocomposite can be used as a potential electrochemical electrode
material [86].
Li et al. [87] synthesized CNRG/Ni(OH)2 via a facile hydrothermal
method. Porous GCN/rGO was synthesized with the aid of glucose
molecules and Ni(OH)2 nanosheets are successively grown on GCN/rGO
assembly as clearly illustrated in the Fig. 12. It is clearly indicated that
g-C3N4 layers are uniformly coated on both sides of the rGO, to form a
sandwich structure upon which Ni(OH)2 are successively deposited.
Here GCN avoid agglomeration Ni(OH)2 and also leads to high dispersity of the composite. The as prepared hybrid when accessed as an
electrode exhibited excellent Cs of 1785F/g in KOH solution. At 3 A/g
they delivered only 71% of cycling capacitance lifespan after assessing
over 5000 GCD measurement. It retained almost 900 F/g of specific
capacitance when the current density increased from 2A/g to 20A/g
owing to its excellent rate stability. Such enhanced electrochemical
performance signifies that this hybrid could be a prominent electrode
for supercapacitors.
Carbon-doped g-C3N4/MnO2 (CCNM) was reported using facile hydrothermal synthesis route to investigate the advantages of using 2D
architectures. Carbon is doped into GCN network by calcination of
porous melamine in presence of thiourea. Then it is mixed with KMnO4
solution and through hydrothermal reaction, carbon-doped g-C3N4/
MnO2 composite was prepared. Series of composites are prepared by
varying temperatures during hydrothermal process (CCNM 1- 120°C,
CCNM 2- 140°C, CCNM 2-180°C). The specific capacitance of carbon
doped g-C3N4/MnO2 (CCNM 2) composite delivered a Cs of 324 F/ g in
a Na2SO4 electrolyte which is found to be much higher in comparison to
the specific capacitance exhibited by undoped g-C3N4/MnO2 (89 F/g).
At a discharge current of 0.2 A /g, a retention rate of almost 80% was
found over 1000 cycles. Results indicated doping is an alternative way
of increasing the surface area and charge transfer in GCN and the GCN
improves the wettability thus enhances the specific capacitance of the
composite [88]. K or Na doped g-C3N4/MnO2 composite was synthesized by Shen et al. [89] using thiourea and KBr/NaBr by thermal
treatment. As prepared K/Na doped g-C3N4 hydrothermally treated in
presence KMnO4 solution to from composite material containing K/Na
doped g-C3N4/MnO2 as shown in Fig. 13. All electrochemical characterization was done for the samples with and without doping. GCD
results showed better specific capacitance for the materials with metal
doping. Specific capacitance evaluated from GCD at various current
densities represented improved performance of doped composite than
the bare g-C3N4/MnO2 composite. The structure of self-assembled Na
and/or K doped g- g-C3N4/MnO2 improved electron and ion transportation as well as utilization of MnO2 towards specific capacitance of
electrode. Both K doped g-C3N4/MnO2 and Na doped g-C3N4/MnO2
exhibits Cs of 373.5 F/g and 294.7 F/g respectively in Na2SO4 electrolyte solution at 0.2A/g of current density. Higher value of specific
capacitance in K doped g-C3N4/MnO2 is because of K doping which is
intercalated into the layers of GCN, provided numerous delivery
channels thus improve charge transfer. Results exhibited (Fig. 13)
doping is an efficient way to improve the energy storage of electrode
materials. Chang et al. [90] fabricated graphitic carbon nitride/MnO2
nanocomposite, by sandwiching MnO2 nanorods (NRs) on upper and
lower surfaces of GCN layers via chemical route. GCN layered sheets
provided numerous sites towards chemical reaction (nucleation) for
growth of the MnO2 nanorods on both sides to form a sandwich like
structure of the composite g-C3N4/MnO2 NCs. To study the effect of
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Journal of Energy Storage 32 (2020) 101840
M.G. Ashritha and K. Hareesh
Fig. 11. (A) SEM image of LDH, (B) SEM image CL10 (g-C3N4/NiAl-LDH10), (C) TEM images of LDH and (D) CL10, (E) Cyclic voltammograms of LDH, g-C3N4 and
C3N4/LDH composites (CL5, CL10, CL20) at scan rate of 10 mV/s, (F) GCD of LDH, g-C3N4 and C3N4/LDH composites (CL5, CL10, CL20) at 1 A/g. Reproduced from
ref. [86] with the permission of Elsevier.
sandwiching on the composite performance, a bare MnO2 and one more
composite of MnO2 but with graphene oxide (MnO2/GO NC) was prepared under same condition. When the synthesized material is used an
electrode material the MnO2/g-C3N4 NC (211 F/g) composite exhibited
a specific capacitance much higher than other two compounds. Enhanced specific capacitance of MnO2/ g-C3N4 NC compared to MnO2
/GO NC is attributed to the unique 2D sandwich like structure on gC3N4 sheets and the presence of heteroatom of g-C3N4. This work
clearly demonstrated that improvement in the performance of MnO2 as
well as other transition metal oxides can be found using GCN than the
commonly used GO sheets. Future more to study the dependence of
structure of nanocomposite on performance of electrode material Guan
et al. [91] fabricated nanoneedle and nanosheets of NiCO2O4 on GCN.
g-C3N4 prepared via thermal decomposition of thiourea. Nanosheet and
nanoneedle NiCO2O4/ g-C3N4 (NNC) are synthesized using oil bath
(NNC-I) and hydrothermal method (NNC-II) respectively which are
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Journal of Energy Storage 32 (2020) 101840
M.G. Ashritha and K. Hareesh
Fig. 12. (A) Schematic illustration of the preparation of CNRG/Ni(OH)2 composite;(B) (a) Cyclic voltammograms of CNRG/Ni(OH)2 electrodes measured at scan
rates from 2–50 mV/s, (b) charge-discharge curves of CNRG/Ni(OH)2 measured at various discharge current. Reproduced from ref. [87] of Nature. This work is
licensed under a Creative Commons CC BY license.
followed by a calcination process. NNC-I exhibited nanosheets like
structure which staggered on the GCN, whereas NNC-II exhibited uniform nanoneedles structure on GCN substrate as shown in Fig. 14. The
surface area and pore distribution values of nanosheet and nanoneedle
like morphologies of NNC are calculated to be 77.5 m2/g, 5nm and
105.7 m2/g, 8nm respectively. At a discharge current of 1 A/g, NNC-II
composite exhibited a higher Cs of (274.8 F/g) than NNC-I (118.2 F/g)
in 1 M KOH solution. Results are accredited to the higher surface area
and availability of more redox sites provided by nanoneedle structure.
It also improved wettability (electrode- electrolyte contact) and space
between synthesized nanoneedles improved the diffusion kinetics of
electrode material. Nanosheet-assembly of NiCO2O4/ g-C3N4 exhibited
no capacitance loss even after continuous 1000 GCD cycles. Thus, this
report successfully demonstrated the dependence of specific capacitance on structure of electrode material.
A g-C3N4/Ni(OH)2 [CN/NI] material was synthesized by dong et al.
[92] as they exhibit high theoretical specific capacitance. Via a GCN
template the composite was synthesized using nickel nitrate as a main
precursor in oil bath treatment. The yielded nanocomposite possessed
honeycomb structure comprising of Ni(OH)2 nanosheets on GCN template. TEM image showed the successful formation of Ni(OH)2
nanosheets connected to GCN sheets with 20 nm thickness. CN/NI
when used as an electrode materials showed excellent specific capacitance of 1768.7 F/g which is much higher than bare Ni(OH)2 (968.9 F/
g) and GCN (416.5 F/g) respectively in aqueous electrolyte at a current
density of 7 A/g. CN/NI also showed a very high 2667 F/g of specific
capacitance at a scan rate of 3 mV/s in three electrode system. In 6M
KOH, two electrode supercapacitor was fabricated using graphene as
negative and CN/NI as positive electrode. Typical CV curves with a
variation in scan rate from 0 to 1.3V of CN/Ni//G asymmetric supercapacitor. They exhibited a moderate Cs of 51 F/g along with almost
72% of its initial capacitance retention after 8000 charge/discharges
kinetics. Furthermore, they achieved a high Ed of 43.1Wh/kg and
power capability of 9126W/kg in a two-electrode configuration. The
notable performance of the CN/NI hybrid could be used as an excellent
electrode for supercapacitor. Liu et al. [93] synthesized graphitic
carbon nitride/iron oxide(GCN/ α-Fe2O3) hybrid by simple one step
pyrolysis using Prussian blue (PB) and melamine as precursors at 550C.
Here α-Fe2O3 which has successfully anchored on GCN prevents it from
agglomeration and expanding. In this report the synthesized composite
was accessed for supercapacitors and also for sensing application of
non-enzymatic glucose. When used as supercapacitor electrode material
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M.G. Ashritha and K. Hareesh
Fig. 13. (A)(a) Schematic representation of the preparation of K-CNM and Na-CNM (b) SEM images of K-CNM and Na-CNM; (B). (a) CV curves at the scan rate of 100
mV/s; (b) GCD curves at 0.5 A/g of K-CNM and Na-CNM. Reproduced from ref. [89] with the permission of Elsevier.
they exhibited 580 F/g of specific capacitance in 1M KOH at 1 A/g.
With increase in current density (1 A/g to 4A/g) only a fractional
amount of capacitance loss was found owing to its rate capability
property of composite. At a discharge current of 2 A/g they exhibited a
nominal cycle life up to 1000 cycles. Vattikuti et al. [94] synthesized
carbon (C) and copper oxide (CuO) nanospheres on GCN nanosheets to
form a hybrid C/CuO@GCN assembly through co-pyrolysis method. A
precursor mixture of CuSO4, D-glucose and thiourea was heated in a
muffle furnace at 550o (10°C/min), to yield a yellow coloured product
of C/CuO@GCN. C/CuO@ g-C3N4 was found to have higher specific
surface area than bare GCN owing to more reaction sites of hybrid for
electrochemical reaction. Higher specific capacitance of hybrid is
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Journal of Energy Storage 32 (2020) 101840
M.G. Ashritha and K. Hareesh
Fig. 14. (A). (a) Low and high (the inset) magnification SEM images of the g-C3N4; (b) low and high (the inset) magnification SEM images of NNC-I; (c) Low and (d)
high magnification SEM images of NNC-II.(B). (a, b) CV curves, (c, d) charge-discharge curves of NNC-I and NNC-II. Reproduced from ref. [91] with the permission of
Elsevier.
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Journal of Energy Storage 32 (2020) 101840
M.G. Ashritha and K. Hareesh
Fig. 15. (A). SEM images of (a) pure Co3O4 (b) g-C3N4 and (c) Co3O4/g-C3N4; (B). (a) CV curves of the Co3O4/g-C3N4 in 2M KOH. (b) Charge-discharge curves of gC3N4, Co3O4 and Co3O4/g-C3N4 composites with current density of 1.25 A/g (c) GCD curves of Co3O4/g-C3N4 composites at different current densities. Reproduced
from ref. [95] with the permission of Elsevier.
attributed to anchoring of C/CuO nanospheres on GCN surface. When
this novel nanocomposite is used as an electrode material in 0.5 M
aqueous NaOH electrolyte for supercapacitor exhibited 247.2 F/g of Cs
at 1 A/g of current density. They delivered 92% of the initial Cs after
6000 cycles determining the long-term stability of the nanocomposite
electrode. Co3O4 on graphitic carbon nitride (GCN) was successfully
anchored by Zhu et al. [95] using a co-precipitation method. Precursors
of Co3O4 and GCN are heated at 400°C in muffle furnace to form Co3O4/
g-C3N4 powder. Fig. 15 presents the SEM images of GCN, Co3O4 and
Co3O4/g-C3N4. GCN showed an aggregated layered structure and Co3O4
exhibited polyhedral aggregation with 10-200nm. Co3O4/g-C3N4
clearly indicated the physical interaction between Co3O4 and g-C3N4
structures. Specific surface area of Co3O4/g-C3N4 and bare Co3O4 nanoparticles are calculated using Brunauer-Emmett-Teller theory. It demonstrated that Co3O4/g-C3N4 composite possessed a higher surface
area of 12.6m2/g with a pore volume of 0.05cm3/g compared to surface
area of Co3O4(11.8 m2/g and pore volume is 0.03 cm3/g), which attributed to the enhanced electrochemical behaviour of composite
electrode. Using GCD curve of synthesized nanocomposite, calculated
specific capacitance was found to be 780 F/g in 2 M KOH electrolyte at
1.25 A/g of current density. There is 80% of initial capacitance retention after 1000 continuous cycles of charging and discharging. The Cs of
the Co3O4 (419 F/g at 1.25 A /g) is much lower than Co3O4/g-C3N4
nanocomposite proofing the utilization of GCN in the nanocomposite.
Zhang et al. fabricated a three-dimensional RuO2/g-C3N4@reduced
graphene oxide aerogel composite (RCGA) through an electrostatic
assembly of positively charged colloidal nanoparticles of RuO2, negatively charged graphene oxide and GCN [96]. These precursors are
successfully transformed into a 3D RuO2/g-C3N4@reduced graphene
oxide (rGO) aerogel (RCGA) composite through a facile hydrothermal
and simple lyophilization processes. The fabricated RCGA composite
showed 704.3F/g of specific capacitance which is a high value for the
use of such electrode in supercapacitors at 0.5A/g of current density.
Galvanostatic charge discharge curves of these nanocomposites also
exhibited usual triangular charge/discharge kinetics demonstrating the
reversibility of electrochemical process. Excellent cycling charge
discharge stability of almost 98% of its initial specific capacitance was
retained after 2000 cycles. This excellent electrochemical behaviour
showed potential approach towards supercapacitor application. To
study the practical application of composite an asymmetric SC was
developed by using activated carbon as the second electrode in two
electrode system. AS exhibits a the specific capacitances of 23.2 F/g at
0.5 A/g. They delivered a high energy and power characteristics of
80.64 Kw/h and 450 W/kg respectively.
A new cost-effective nanocomposite was prepared using TiO2 nanoparticles. TiO2/g-C3N4 hybrid was synthesized via a simple, facile
hydrothermal method. GCN layers are prepared by heating dicyanamide followed by a hydrothermal method to yield amorphous TiO2/gC3N4 composite upon heating leading to crystal form of the composite.
When used as electrode it represented a high Cs of 125.1 F/g compared
to bare TiO2 nanoparticles (106.7 F/g). As prepared TiO2/g-C3N4
showed no capacitance loss after 1000 cycles illustrating its stability as
an electrode material in 2M KOH solution. It was found that improved
electrochemical behaviour of electrode is related to its pore size (9.3
nm) and specific surface area (128 m2/g) of TiO2/GCN which was reported by Zhao et al. [97]. Kavil et al. also synthesized a TiO2 composite
with GCN but with 1D nanotubes of TiO2 supported by two dimensional
(2D) GCN nanosheets [98]. GCN layers are prepared by simple pyrolysis
of thiourea followed by a hydrothermal treatment to yield TiO2/g-C3N4
assembly. The electrochemical electrode based on 1: 4 weight ratios of
TiO2 and GCNexhibited an improved specific capacitance compared to
bare TiO2 and GCN (79.62, 15.02 and 50.22 F/g respectively). BET
surface area of TNT (TiO2 anatase phase) and TiO2/GCN respectively
are 25 and 74 m2/g. The improved specific capacitance of composite is
because of improved specific surface area of composite which can be
attributed to the spacing effect of 1D nanotubes towards layered network of 2D GCN. Pseudocapacitive MnO2 and SnO2 anchored GCN
nanosheets, are reported by Kavil et al. They are synthesized through a
one-pot simple chemical reduction. The enhanced performance of gC3N4 /MnO2 is attributed to successful incorporation of MnO2 on bare
GCN. The Cs delivered by g-C3N4, g-C3N4 /SnO2 and g-C3N4 /MnO2 are
found to be 50.22, 64, and 174 F/g respectively which are obtained
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Journal of Energy Storage 32 (2020) 101840
M.G. Ashritha and K. Hareesh
from GCD curves of two electrode configurations. They respectively
preserved 94%, 85% and 87% of Cs after 6000 cycles. Improved performance of g-C3N4 /MnO2 composite is accredited to enhanced surface
area as a result of spacing effect exhibited by MnO2 between the layers
of GCN consequently a greater charge transport at interfaces of electrode and electrolyte [99]. Through a facile hydrothermal synthesis
method SnO2/g-C3N4 nanocomposite was synthesized and reported by
Xu et al. [64]. This SnO2/g-C3N4 composite exhibited a flower like
structure with an average of 15µm in diameter. When the prepared
composite accessed as an electrode material, delivered a 488 F/g of Cs
within 0 to 1 V potential range of 1M H2SO4 electrolyte solution at a
discharge current of 1A/g. After a continuous 5000 GCD cycles, SnO2/
g-C3N4 composite represented 97.2% initial capacitance at 10 A/g and
around 73.4% at 20A/g illustrating a good capacitive retention. Furthermore, in symmetric cell it showed 153F/g of Cs nevertheless, exhibited energy density of 2.8 Wh/kg at operating power density of 5684
W/kg. In the composite GCN reduces the agglomeration of SnO2 hence
an enhanced performance is found compared to both bare GCN and
SnO2. Zhang et al. [100] synthesised Fe2O3 /GCN by anchoring Fe2O3
nanospheres on GCN which are surface oxidised using HNO3 through
one step hydrothermal method. The as prepared 10%OCN/ Fe2O3
(10%:1 weight ratio of oxidised GCN and Fe2O3) delivered a 243 F/g of
Cs at 1A/g of current. A capacitive retention loss of almost 19% was
exhibited by 10%OCN/ Fe2O3 composite after 1000 GCD cycles. OCN
which was well dispersed on GCN effectively prevent the aggregation of
nanospheres which lead to an enhanced specific capacitance. The
maximum Ed of 25.7 7 Wh/kg is exhibited at power characteristic of
833.8 W/kg. One more TMO based GCN nanocomposite using ZnO has
been prepared by shen et al. [101]. An oxygen vacancy rich zinc oxide
with GCN is synthesized using a novel thermal decomposition method
using melamine as a precursor for GCN and zeolitic imidazolate framework precursor for ZnO. Here this novel assembly exhibited an excellent Cs value of 3,000 F/g at 3 A/g of discharge current. The assembled asymmetric supercapacitor delivered 680 F/g of capacitance
and retained almost 86.2% of its initial Cs at a discharge current of 3 A/
g after 1000 GCD cycles. They represent higher energy as well as power
characteristics of 100.9Wh/kg and 3,466W/kg. This excellent value is
accredited to enhanced ion exchange rate and carrier concentration
between the assemblies of OZCN leading to an improved charge-discharge mechanism. Hou et al. [102] developed an oxygen-vacancy-rich
NiCo2O4/ nitrogen-deficient GCN hybrids (Ov- NiCo2O4 /ND-g-C3N4)
using a facile in-situ method using nickel and cobalt nitride. The as
prepared hybrid delivered a high Cs of 1998F/g at 2A/g of current
density. The fabricated asymmetric supercapacitor using Activated
carbon as second electrode represented Ed and Pd of 70.22 Wh/kg and
800W/kg respectively. They also delivered 95.2 % of initial capacitance
after 5000 GCD cycles.
electrolyte solution. CPCN-NSs/ NCS-NPs deliver almost 93% retained
Cs after 10000 cycles. CV curves represented pair of cathodic and
anodic peaks corresponding to oxidation states of Ni and Co. the results
clearly demonstrated incorporation of CPCN-NSs on the surface NCSNPs is an effective and viable way of improving the capacitive behavior
of CPCN-NSs/ NCS-NPs composite [108].
A new electrode material prepared by compositing nickel cobalt
sulfide (NiCo2S4) on GCN to form (NiCo2S4 NSs/P-g-C3N4) nanocomposite by LI et al. [109]. Here NiCo2S4 nanosheets are successfully
grown on porous GCN sheets using hydrothermal method. Along with
TEM, EDS mapping successfully proven the formation of hybrid material showing Ni, Co, and S elements of NiCo2S4 and C, N of porous GCN.
For understanding the effect of GCN on the composite and for electrochemical behavior comparison, nickel sulfide, cobalt sulfide along with
different cobalt, nickel and Sulphur stoichiometric (NixCoxSx) ratios of
composite with GCN nanosheets were examined. Among all, NiCo2S4@
g-C3N4 delivered highest electrochemical performance. They composite
delivered a Cs of2210 F/g at 1 A/g. The composite retained 100% capacitance even after 1500 GCD cycles exhibiting notable cycling stability. The excellent exhibited performance is attributed to shorting of
electron and ion diffusion pathways owing to the presence of ‘sheet on
sheet’ morphology. Two electrode configuration supercapacitor fabricated using activated carbon and the aforementioned composite delivered an Ed of 16.7 Wh /kg and Pd of 200 W/kg. It affords 99% capacitive retention even evaluating after 5000 cycles. Jiang et al. [16]
synthesized a 2Dcobalt sulfide/graphitic carbon nitride (CN/CS) nanocomposite through a facile single step solvothermal method by successfully anchoring of cobalt sulfide nanosheet on GCN nanosheets.
Fig. 17 illustrates the synthesis of CN/CS composite. Using GCD curves
it is shown that with increase in GCN content Cs increased till 50% then
after that a decrease in Cs is found because of high GCN content agglomerated CS and thus reduced the effective electron transfer and
active sites. Calculated surface area of CN/CS (52.84 m2/g) using BET
was found to more than bare GCN (41.12 m2/g) and CS (14.80 m2/g)
attributed nanosheet architecture of CS on GCN sheets. Hybrid nanosheet architecture possessed larger surface area for electrochemical
reaction hence exhibited a 668F/g of Cs at a discharge current of 2A/g.
furthermore CN/CS showed no attenuation in Cs after 5000 cycles.
These excellent electrochemical performances attributed to symbiotic
nature of both cobalt sulfide and GCN nanosheets. CN/CS delivered an
Ed of 18.5 Wh/kg and 99.8 W/kg of power capability according to a
discharge current of 0.5 A/g. No absolute capacitance loss was found
after 5000 GCD cycles.
Kavil et al. [110] synthesized a TMS based g-C3N4/MoS2 binary
nanocomposite using advantages of both electrostatic and electrochemical behavior of GCN and MoS2 respectively. When this 2D nanohybrid is employed as an electrode material, it delivered a Cs of 45.5
F/g which is greater than both pristine GCN (10F/g) and MoS2 (14 F/g).
They also represented only 2% capacitance attenuation after only 100
cycles of GCD in Na2SO4 gel electrolyte solution. These performances
are ascribed to synergist effect and also due to spacer act of MoS2 between GCN layers.
Ansari et al. [65] reported a SnS2 TMS on GCN sheets to form a 3D
SnS2/GCN binary composite using solvothermal route. SEM images
represented flower like morphology of SnS2 and successful formation of
these SnS2 on GCN sheets to form a 3D structure. SnS2/GCN delivered a
Cs of 210.30 F/g at a discharge current of 1 A/g which is found to be
higher than pristine GCN or the physical mixture of powders of GCN
and SnS2 in Na2SO4 solution as shown in Fig. 18. The synthesized
material exhibited almost around 16% capacitance loss after continuous
1500 GCD cycles represented its cycling stability.
4.4. GCN composites with transition metal sulfides
Transition metal sulfides (TMSs) are other class materials which are
used in place of TMO because of their high conductivity along with
excellent mechanical stability in energy storage systems [103–105]. A
nanocomposite of TMSs such as sulfides of Co, Mn, Ni or Cu (CoS,
MO2S, NiS or CuS) [106,107] with GCN could be an ideal option to
enhance the electrochemical behavior of pristine GCN. Some of the
binary metal sulfide/ GCN hybrids are discussed as follows
NiCo2S4 nanoparticle (NCS NPs) composite with porous GCN (PCN)
nanosheets were synthesized using a hydrothermal route. Carbon selfrepairing PCN (CPCN) are prepared using solvothermal method followed by heat treatment to form the CPCN-NSs/ NCS-NPs hybrid as
shown in Fig. 16. The successful anchoring of NCS nanoparticles on
PCN and acted as a conductive linker are showed in TEM images in
Fig. 16. By TEM images CPCN-NSs which are having a surface area of
about 220.7 m2/g are showed a particle size of average 50 nm. The as
synthesized nanocomposites delivered a Cs of 1557 F/g in NaOH
4.5. GCN composites with conductive polymer
Conducting polymers (CP) are the materials which provide pseudocapacitance to the system (bulk of the material undergo redox
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Journal of Energy Storage 32 (2020) 101840
M.G. Ashritha and K. Hareesh
Fig. 16. (A) (a) Schematic representation of the carbon self-repairing porous g-C3N4 nanosheet / NiCo2S4 nanoparticle (CPCN-NS/NCS-NP) hybrid composite, (b)
TEM image of CPCN-NSs/NCS-NPs; (B) (a) Comparison of cyclic voltammogram (CV) curves of CPCN-NSs, pristine NCS-NPs, PCN-NSs/NCS-NPs and CPCN-NSs/NCSNPs at same scan rate of 10 mV/s; (b) Comparison of galvanostatic charge-discharge (CD) curves at a current density of 10 A g-1; (c) CV curves of CPCN-NSs/NCS-NPs
at various scan rates; (d) GCD curves of CPCN-NSs/NCS-NPs at different current densities. Reproduced from ref. [108] with the permission of Elsevier.
reaction). Till date, CP are standing as an attractive material in supercapacitor application because of their prominent properties such as
superior conductivity, facile redox capability and also flexibility
[111,112]. PANI, PPy and PTh are important CPs. Nonetheless, they
face low power performance and also a short cycle life (cycling stability) to successfully use as a single component for electrode of supercapacitors because of its ion and charge transportation in conductive
network of polymer is hindered during charge/discharge cycles. Furthermore, compositing with other carbon material will address this
disadvantage towards improved performance of the electrode. In this
section, we will review advancement in GCN based composite with
different conducting polymers such as PANi, PPy or PTh and derivatives
of them [113]. One such group Chen et al. [114] synthesized poly(3,4ethylenedioxythiophene)
(PEDOT):poly(styrenesulfonate)
and
GCNnovel assembly. Here PEDOT was successfully deposited on GCN
layers via an electrodepositing method on a glassy carbon nitride. When
used as an electrode, the as prepared PEDOT/g-C3N4 composite in
electrochemical cell exhibited excellent rate capability, cycling stability
and electrochemical performance (energy and power density) in both
Na2SO4 and H2SO4 electrolyte solution than bare PEDOT. At a discharge current of 2A/g electrode delivered a Cs of 137 F/g and 200 F/g
in H2SO4 and Na2SO4 solution respectively. As described in the Ragone
plot with increase in current density, power density increased at different current densities. It is also observed that at higher current
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Journal of Energy Storage 32 (2020) 101840
M.G. Ashritha and K. Hareesh
Fig. 17. (a) Preparation process of CN/CS nanocomposites (b) cycling performance and coulombic efficiency of the 50%-CN/CS electrode at 10 A/g. (b) CV curves of
50% CN/CS electrode at various scan rates (b) Galvanostatic discharge of 50%-CN/CS at various current densities. Reproduced from ref. [16] with the permission of
Elsevier.
density, energy density is decreased because of less accessibility towards pores and active sites. The equivalent series resistance PEDOT/gC3N4 is comparatively lower than bare PEDOT and is attributed to excellent interaction between PEDOT and GCN.Cycling stability of the
composite electrode in H2SO4 and Na2SO4 solution are found to kept at
89% and 96.5% of its initial capacitance respectively. in H2SO4 electrolyte solution energy and power density are found to be 9.65 Wh/kg
and 4000.8 W/kg respectively. At an Ed of 17.5 Wh/kg they represented
an excellent power characteristic of 5000W/kg because of larger hole
and accessibility towards ions in Na2SO4.
Zhou et al. [115] synthesized a composite of polyaniline and GCN
(PANI/g-C3N4) using oxidative polymerization of aniline along with
GCN. SEM images of PANI showed a nanorod like morphology with
50nm diameter and 400nm length as shown in Fig. 19. GCN exhibited
regular stacked layer structure, where as the synthesized nanocomposite showed a flower-like morphology. CV curves of composite exhibits
a deviation from normal rectangular curves indicating its pseudocapacitive nature and area under CV increased with increase in scan rate
with alike shapes illustration stability of electrochemical reactions. In
1M H2SO4 electrolyte solution at a current density of 1A/g PANI/GCN
exhibited a Cs of 585.3 F/g. they exhibited a high power as well as
energy density of 81.15 Wh/kg and 500.04 W/kg respectively. Furthermore, at a high-power density of 4998.28 W/kg, the Ed of composite remained at 51.94 Wh/kg, which illustrate an excellent range of
power can be maintained even at moderate energy densities. At a
current of 1A/g the synthesized composite exhibited almost 82% of
capacitance retention after 1000 continuous GCD cycles which is notably higher than bare PANI when used as electrode (75.1%). There are
also some researches reporting ternary composite by compositing GCN/
conducting polymer with other carbon-based materials towards enhancing the super capacitive performances. Two such composites like
Conducting poly-pyrrole/g-C3N4@ graphene(PPy/g-C3N4@G) and cellulose/polypyrrole/tubular GCN are presented by Alshahrie et al. [116]
and Li et al. [117] respectively. Conducting poly-pyrrole/g-C3N4@
graphene (PPy/g-C3N4@GN) ternary composite delivered a Cs of 260.4
F/g at 1 A/g and at 5mA/cm2 of current density, nano fibrillated cellulose/polypyrrole/GCN exhibited 2.53 F/cm2 of specific capacitance
[116,117].
The below table summarizes the materials properties and structure
for various GCN based nanocomposites.
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M.G. Ashritha and K. Hareesh
Fig. 18. A. (a and b) SEM of SnS2/GCN nanocomposite; (c) schematic illustration of the formation of 3D flower-like SnS2 on the surface of g-C3N4 sheets; B. CD curves
for (a) g-C3N4-AP, SnS2-g-C3N4-St, and SnS2-g-C3N4-Pm heterostructure electrodes at a current load of 1 A/g, (b) g-C3N4-AP, (c) SnS2-g-C3N4-St heterostructure, (d)
SnS2-g-C3N4-Pm heterostructure. Reproduced from ref. [65] with the permission of Royal Society of Chemistry.
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M.G. Ashritha and K. Hareesh
Fig. 19. A.SEM images of (a) g-C3N4 (b) PANI, (c) PANI/ g-C3N4 (e) PANI, (f) PANI/ g-C3N4.B. (a) CV and (b) GCD curves of g-C3N4, PANI and PANI/ g-C3N4 at a
current density of 1 A/g in 1 M H2SO4 solution. Reproduced from ref. [115] with the permission of Royal Society of Chemistry.
4.6. Some other GCN based composites
5. Outlook and conclusion
Different materials other than transition metal oxide, conducting
polymer and carbon based materials are also synthesized and used as
electrode material for supercapacitors and they exhibited potential results. A short note on different GCN composites are briefly described.
The research group Ragupathi et al. [67] fabricated g-C3N4 doped MnS
nanoparticles through a sol-gel synthesis method. There is formation of
spherical particles around 40nm revealed by SEM images. When synthesized material is evaluated it presented a Cs of 463.32 F/g at a very
low discharge current of 0.005 A/g. an excellent stability (98.6%) over
2000 cycles is found and attributed to the existence of spherical nanoparticles and doping of GCN which enhance the electron transport. A
red phosphorus GCN (RPh-g-C3N4) composited was synthesized by
Ansari et al. [118] through simple ball milling. They exhibited a fair
amount of Cs of 465 F/g at a 10 A/g with a decent 90% of cycling life
was found after 1000 GCD measurement. Balasubramaniam et al. [119]
reported a composite of GCN with zinc antimonate-ZnSb2O6 (ZSO)
nanorods. Here ZSO nanorods are successfully integrated on GCN using
ultrasonication-assisted method. A well-developed assembly of GCN/
ZSO exhibited a Cs of 557.5 F/g at 2.5 A/g of discharge current which is
higher than GCN nanosheets synthesized using melamine precursor
(244.7 F/g at 1 A/g of discharge current). They tend to possess no capacitance loss at 10 A/g of current density. Relatively higher Cs is attributed to synergistic effect of GCN and ZSO and also the presence of
active sites on nanorod and nano porous structure of GCN and ZSO. The
copper-manganese alloy nanoparticles decorated GCN was developed
by Siwal et al. [120] which showed specific capacitance of 817.85 F/g
with capacitance retention of 91% up to 1000 cycles attributed to the
quicker diffusion extent of ions and higher surface area leading to enhanced electrochemical response. Talukdar et al. [121] have reported
the enhanced the supercapacitance performance of GCN/FeNi3 heterostructure depicted a maximum areal capacitance of 19.21 mF/cm2 at
0.05 mA/cm2. They have also argued that the enhanced areal capacitance of developed heterostructure was due to the generation of the
delocalized energy states, which consequently enriches the overlapping
states in band structure near Fermi level, in turn providing the several
electron channels for ion transfer within the system.
Among the various challenges faced across the globe, energy crisis is
undoubtedly the major problem which is affecting the modern society
to a greater extent. Hence, there is a need for benign, low cost, efficient
energy storage and conversion.Electrochemical energy storage is one
such storage mechanism which have the potential to turn down energy
outbreak. In the list of electrochemical devices, supercapacitor stands
first because of its unique advantageous properties like desirable power
density, fast charge/discharge rates, and long-life cycles which could
effectively intermediate conventional battery and dielectric capacitor.
Performance of ultracapacitors (supercapacitor) mainly depends on
nature of electrode materials, electrolyte and voltage window of material used. n number of research studies in the field of supercapacitors
are going on both in academic and industrial community to enhance
supercapacitive performance. Development of new electrode materials
is becoming a hotspot of research for enhancing the supercapacitive
performance. Graphitic carbon nitride (g-C3N4) is one such material, a
unique analogous of graphene with nitrogen as a heteroatom has gained
attention as a novel material due to its prominent properties like ease of
synthesis, unique electronic feature with 2D structure. Furthermore,
with its high nitrogen content it increases wettability of electrode and
also increases the number of active sites available for pseudocapacitive
reaction, thereby, enhancing the efficiency of electrode. Even though
bulk GCN is not efficiently used in supercapacitor applications on account of its shortcomings like low surface area along with low conductivity nevertheless, it exhibited excellent mechanical stability
(withstand elastic deformation without damaging structure) and flexibility. Hence studies are carrying out to alleviate the above-mentioned
disadvantages either by modifying the structural network of GCN or by
coupling with other nanocomposites. Strategies like synthesizing a nanoscale hybrid material possessing high electrical conductivity has
proved as an efficient way of upsizing the electrochemical performance
of materials. A single layer GCN faced limited application in high power
performance supercapacitors as a result of their inherent stacking due
to van der Waals force. Hence development of a facile composite of
GCN with pseudocapacitive material is of more importance. The combination of two different materials possessing two different storage
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M.G. Ashritha and K. Hareesh
mechanism leads to an enhancement in the performance through the
formation of a new material due to synergistic effect. Therefore, herein
we discussed about GCN composites with different carbon materials,
some of transition metal oxides and conducting polymers. The different
GCN binary nanocomposites with their diverse configuration and synergistic effects between the individual components leading to enhancement in surface area, redox reaction at the electrode-electrolyte
interface, non-dissolution of electrode material into electrolyte during
chare-discharge processes, in turn enhancing electrochemical behaviour which can be effectively used in supercapacitor as a robust material of high performance. Adhering to their superior features like
stability, hydrophobicity along with high amount of nitrogen, these
GCN and their composites exhibits excellent supercapacitive performance. However, there is a broad spectrum which has been left for
research studies towards use of SCs in practical applications. There
aren't many reports on composite of GCN with metal phosphides which
left a space for further exploration. It is also expected that these different combinations of GCN with TMO/hydroxides, CPs, carbon materials and some other functional groups to enhance of electrochemical
performance of SCs. Additionally, a ternary composite by the combination of GCN with two different materials also started grounding its
roots towards application of supercapacitors. Nevertheless, more information about optimized synthesis temperature, different exfoliation
methods, different template approaches and electrical conductivity towards alleviating the shortcomings of GCN can be well explored.
Future scope can be assigned to development a theoretical computation towards understanding different mechanisms of GCN and GCN
composites required to understand relationship between different
electrochemical variables such as composition, actives sites. There exist
one more direction in extending the development of GCN with different
C/N ratios of 3:5, 3:6 and also 3:7 which may exhibit improved electrochemical performance. From the future prospective GCN opens an
array of opportunities either in academic and industrial fields. We hope
this present review gives an essence about GCN and its nanocomposites
which could be a beacon for next generation of researchers in the field
of supercapacitors.
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
Declaration of Competing Interest
[23]
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence the work reported in this paper.
[24]
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