ARTICLE
Novel Hybrid Nanoparticles of Vanadium Nitride/Porous Carbon
as an Anode Material for Symmetrical Supercapacitor
Yunlong Yang
1
.
Kuiwen Shen
1
.
Ying Liu
1
.
Yongtao Tan
1
.
Xiaoning Zhao
1
.
Jiayu Wu
1
.
Xiaoqin Niu
2
.
Fen Ran
1
Received: 27 June 2016 / Accepted: 5 August 2016 / Published online: 13 September 2016
Ó The Author(s) 2016. This article is published with open access at Springerlink.com
Abstract Hybrid materials of vanadium nitride and porous carbon nanoparticles (VN/PCNPs) were fabricated by a facile
pyrolysis process of vanadium pentoxide (V
2
O
5
) xerogel and melamine at relatively low temperature of 800 °C for
supercapacitor application. The effects of the feed ratio of V
2
O
5
to melamine (r ), and nitrogen flow rate on the
microstructure and electrochemical performance were also investigated. It was found that the size of the as-synthesized
nanoparticles is about 20 nm. Both r value and N
2
flow rate have enormous impacts on morphology and microstructure of
the nanoparticle, which correspondingly dete rmined the electrochemical performance of the material. The VN/C hybrid
nanoparticles exhibited high capacitive properties, and a maximum specific capacitance of 255.0 F g
-1
was achieved at a
current density of 1.0 A g
-1
in 2 M KOH aqueous electrolyte and the potential range from 0 to -1.15 V. In addition,
symmetrical supercapacitor fabricated with the as-synthesized VN/PCNPs presents a high specific capacitance of
43.5 F g
-1
at 0.5 A g
-1
based on the entire cell, and an energy density of 8.0 Wh kg
-1
when the power density was
575 W kg
-1
. Even when the power density increased to 2831 .5 W kg
-1
, the energy density still remained 6.1 Wh kg
-1
.
Electronic supplementary material The online version of this
article (doi:
10.1007/s40820-016-0105-5) contains supplementary
material, which is available to authorized users.
& Fen Ran
ranfen@163.com; ranfen@lut.cn
1
State Key Laboratory of Advanced Processing and Recycling
of Non-ferrous Metals, Lanzhou University of Technology,
Lanzhou 730050, People’s Republic of China
2
Chemistry and Biochemistry, University of California Santa
Cruz, 1156 High Street, Santa Cruz 95064, CA, USA
123
Nano-Micro Lett. (2017) 9:6
DOI 10.1007/s40820-016-0105-5
Graphical Abstract
Keywords Superc apacitors Nanoparticle Vanadi um nitride Porous carbon Hybrid materials
1 Introduction
The rapid development of global economy and growing
human population worldwide followed by environmental
pollution and energy crisis have increased the need for
some clean rene wable energy sources like solar and wind
for powering the electrical grid [
1, 2]. Howeve r, most of
these energy sources cannot become continuously available
on demand because of their intermittence [
3, 4]. As such,
the development of reliable and environmentally friendly
approaches for energy conversion and storage is one of the
key challenges that our society is facing [
5, 6]. Among
various energy storage devices, supercapacitors, also called
electrochemical capacitors (ECs), are generally viewed as a
promising energy storage approach used in hybrid electric
vehicles, stand-by power systems, and other portable elec-
tronics [
3, 7–9]. Despite the fact that supercapacitors
exhibit greater power, longer cycle life, and much faster
response time than batteries, their practical application is
still limit ed by the low energy density, which is signifi-
cantly lower than that of batteries [
10]. According to
E = CV
2
(E is energy density, C is capacitance, and V is
operation voltage window), enhancing C and widening
V can be employed to increase energy density of superca-
pacitors [
11, 12].
As we all know, based on the energy storage mecha-
nisms, ECs has two basic types: electric double-layer
capacitors (EDLCs) and pseudocapacitors. EDLCs store
charge in a thin double layer located at the interface
between the electrolyte and the electrode [
10], while
pseudocapacitance involving surface or near surface redox
reactions through a faradaic process, which offers a means
of achieving high energy density at high charge–discharge
rates [
13]. It has been demonstrated that the performance of
ECs intimately depends on the physical and chemical
properties of their electrode materials [
4, 14], and the
energy density is associated with faradaic reactions which
is at least 10 times greater than that of double-layer pro-
cesses [
10, 15–17]. Therefore, it is necessary to develop
better electrode materials both at storing and delivering
large amounts of energy [
4, 13].
Transition metal nitrides, especially vanadium nitride
(VN), are currently one of the most promising materials
for electrodes of supercapacitors owing to its excellent
VN nanoparticles
This work
Ref.25
VN/CNTs-SCs
Ref.56
Other symmetric-SCs
Ref.57
VOx/VN-ASCs
Ref.57
VN/SSCs
Energy density (Wh kg
−
1
)
Ref.58
TiN-SCs
535 530 525 520 515 510 0.1 1 10 100 1000
gkW(ytisnedrewoP)Ve(ygrenegnidniB
−
1
)
10
1
0.1
O 1s
V 2p
3
V 2p
1
6 Page 2 of 15 Nano-Micro Lett. (2017) 9:6
123
mechanical strength, high electronic conductivity, and
good mechanical strength [
18–21]. Recent years, major
progress in the theoretical and practical research has been
developed to fabricate various VN materials used as
supercapacitor electrode. Choi et al. synthesized nanos-
tructured VN for pseudocapacitor and reported the
highest specific capacitance of 1340 F g
-1
at a scan rate
of 2 mV s
-1
in 1 M KOH electrolyte [22]. The high
capacitance is ascribed to a pseudocapacitance contribu-
tion from the nitride [
23]; however, the rate capability of
these materials still requires further improvement. Lately,
nanocrystalline VN was fabricated by temperature-pro-
grammed ammonia reduction of V
2
O
5
and a capacitance
of 186 F g
-1
in 1 M KOH electrolyte at 1 A g
-1
[18]
was found. Zhou and his co-workers also synthesized VN
powder with a capacitance of 161 F g
-1
at 30 mV s
-1
by
calcining V
2
O
5
xerogel in a furnace under anhydrous
NH
3
atmosphere at 400 °C[24]. In fact, the electronic
conductivity plays a great impact on material’s electro-
chemical performance. For this purpos e, Ghimbeu and
his co-workers synthesized vanadi um nitride/carbon
nanotube (VN/CNTs) composites using a sol–gel
approach in the presence of CNTs [
25]. The VN/CNTs
composites delivered high capacitance retention (58 %) at
high current density (30 A g
-1
) compared with just 7 %
of the pristine VN. More recently, Shu and his co-
workers reported a capacitance of 413 F g
-1
at the
current density of 1 A g
-1
and a retention about 88 % of
its maximal capacitance at a current load of 4 A g
-1
[23, 26, 27]. Unfortunately, despite the fact that these
nanocrystalline VN perform ed an excellent rate capabil-
ity, they still exhibit relatively short cycle life and low
voltage window, which is crucial for the energy density
and applications of supercapacitors. Also, the reactive
process is still unclear, which is crucial for us to find out
the relationship between microstructure and performance.
In this article, vanadium nitride/carbon (VN/C) hybrid
nanoparticles were successfully synthesized by pyrolysis
of V
2
O
5
xerogel and melamine precursor at 800 °C under
N
2
atmosphere. Thermogravimetry–differential scanning
calorimetry was used to simulate the pyrolysis process of
precursor in order to make it clear what is the possible
reaction mechanism and what are the behaviors of reac-
tants during nitration. We also make a detailed discussion
about the relationships between performances and dif-
ferent microstructures obtained by varying the feed ratio
of V
2
O
5
xerogel to C
3
H
6
N
6
and N
2
flow rate during
reaction. The results indicate that the feed ratio of V
2
O
5
xerogel to C
3
H
6
N
6
and N
2
flow rate has enormous
impacts on morphology and microstructure of the
obtained composites, which also greatly influences the
electrochemical performances.
2 Experimental
2.1 Chemicals
Vanadium pentoxide (V
2
O
5
, analytical reagent) and
hydrogen peroxide (H
2
O
2
, analytical reagent) were pur-
chased from Sinoph arm Chemical Reagent Co. Ltd. and
used as received. Vinyl cyanide (AN, analytical reagent)
was purchased from Sinopharm Chemical Reagent Co. Ltd,
and subjected to distillation prior to use. Melamine
(C
3
H
6
N
6
, analyti cal reagent) and all the other chemicals
were purchased from Shanghai Meixing Chemical Reagent
Factory, P. R. China, and used without further treatment.
2.2 Synthesis of Hybrid Vanadium Nitride/Porous
Carbon Nanoparticles (VN/PCNPs)
In a typical synthesis, as shown in Scheme
1,V
2
O
5
xerogel
was prepared by slowly adding 2 g V
2
O
5
powder into
60 mL 30 wt% H
2
O
2
under magnetic stirring until gels
formed, which was dried under vacuum at 40 °C for 24 h.
After that, the ground V
2
O
5
xerogel powder and melamine
were mechanically mixed completely. The mixture used as
precursor was heated up to 800 °C in a tube furnace under
N
2
atmosphere at a temperature rate of 5 °C min
-1
and
kept at 800 °C for 3 h, and then black VN/CNPs were
obtained. The VN/CNPs prepared with different feed ratios
of V
2
O
5
xerogel to melamine of 1:5, 1:10, 1:20, and 1:30
(wt%) were named VN/CNP-5, VN/CNP-10, VN/CNP-20,
and VN/CNP-30, respectively. And VN/CNP-10 samples
prepared at different N
2
flow rates of 20, 40, and
H
2
O
2
V
2
O
5
powder V
2
O
5
xerogel
5 °C/min
N
2
800 °C/min
N
2
4 h
VN/CNPs
N
2
C
3
N
6
H
6
NH
3
N
2
Hydrogenide
2
N
2
Melamine
Scheme 1 Schematic illustration for the preparation of VN/CNPs
Nano-Micro Lett. (2017) 9:6 Page 3 of 15 6
123
70 mL min
-1
were named VN/CNP-10-20, VN/CNP-10-
40, and VN/CNP-10-70, respectively.
2.3 Structure Characterization
The microstructure and morphology were characterized by
transmission electron microscope (TEM, JEOL, JEM-2010,
Japan), field emission scanning electron microscope (SEM,
JEOL, JSM- 6700F, Japan), and energy-dispersive X-ray
(EDX) spectroscopy. X-ray diffraction (XRD) patterns
were recorded with a Rigaku D/MAX 2400 diffractometer
(Japan) with Cu Ka radiation (k = 1.5418 A
˚
) operating at
40 kV and 60 mA. Thermo gravimetric analysis (TGA)
and differential scanning calorimetry (DSC) mea surements
were carried out in air, and in nitrogen at a heating rate of
10 °C min
-1
on a NETZSCH STA 449F3, respectively.
2.4 Electrochemical Tests
All electrochemical measurements were performed using
an electrochemical working station (CHI660E, Shanghai,
China). The electrochemical performances of electrode
materials were tested in a three-electrode system in 2 M
KOH aqueous solution at room temperature with a plat-
inum foil used as counter electrode, and the saturated
calomel used as reference electrode (SCE). The working
electrodes were prepared according to the method reported
in the literature [
28, 29]. Typically, 80 wt% of active
materials was mixed with 7.5 wt% of acetylene black, 7.5
wt% of graphite, and 5 wt% polytetrafluoroethylene, and
then pressed onto a Ni foam current collector at 10 MPa
and dried at 60°C for 8 h. The total quantity of the act ive
substance was 4 mg and had a geometric surface area of
1cm
2
. The electrochemical performances of electrodes
were characterized with cyclic voltammetry (CV), gal-
vanostatic charge–discharge, and impedance spectro scopy
tests in 2 M KOH electrolyte. The corresponding specific
capacitances were calculated from the discharging time and
based on the formula C = (IDt)/(mDV), where C (F g
-1
)is
the specific capacitance, I (A) is the discharge current,
Dt (s) is the discharge time , DV (V) represe nts the potential
drop during discharge process, and m (g) is the mass of the
active material. The cyclic stability measur ement was
carried out on a land cell tester for 1000 cycles.
For the symmetrical supercapacitor, electrochemical
tests were conducted in a traditional two-electrode sym-
metric supercapacitor system with room temperature in
2 M KOH aqueous solution. The measur ements of the
device mainly include CV, galvanostatic charge–discharge,
and impedance spectroscopy. The CV curves were
acquired in the voltage range of 0 to -1.15 V vs Hg/HgO
at the sweep rate range of 5 to 50 mV s
-1
, and the gal-
vanostatic measurement of the cell was characterized at the
current density from 0.5 to 5 A g
-1
. Electrochemical
impedance spectroscopy was measured at a frequency
range of 0.01 to 100 kHz under the current density of
1Ag
-1
.
3 Results and Discussion
3.1 Effects of [V
2
O
5
]/[C
3
H
6
N
6
] Amount and N
2
Flow on Microstructure of VN/PCNPs
In order to understand the growth mechanism of VN/CNPs,
the effects of [V
2
O
5
]/[C
3
H
6
N
6
] and N
2
flow on
microstructure of VN/CNPs were investigated in detail.
Figure
1 depicts the typical high-and-low magnification
SEM images of VN/CNP-5, VN/CNP-20, and VN/CNP-30
at different [V
2
O
5
]/[C
3
H
6
N
6
] values of 1:5, 1:20, and 1:30,
revealing a porous network microstructure feature and that
agglomerates are mainly composed of numerous uniformly
sized spherical nanoparticles with a average diameter of
20 nm. However, ther e were differences among these VN/
CNPs prepared at different [V
2
O
5
]/[C
3
H
6
N
6
] values. When
the amount of C
3
H
6
N
6
was small ([V
2
O
5
]/
[C
3
H
6
N
6
] = 1:5), limited gas and carbon were produced
during the pyrolysis process, so the obtained VN/CNP-5
was similar to a blocky shaped aggregation with few pores
accordingly (Fig.
1a, b). With the increase of C
3
H
6
N
6
amount, the independent nanoparticles with porous struc-
ture were obtained (Fig.
1c, d). All of these changes were
usually ascribed to the released gas and remained carbon
during pyrolysis, which simultaneously inhibited the for-
mation of hard agglomerates and recrystallization of the
VN particle [
26, 27]. Besides, the boundary between
nanoparticles also became more obvious with the increase
of C
3
H
6
N
6
amount, indicating a high active surface area.
When the [V
2
O
5
]/[C
3
H
6
N
6
] value decreased to 1:30, the
obtained VN/CNPs had a porous network morphology and
packed with count less particles with a grain size of
regarding 20 nm (Fig .
1e, f), which may be an advantage in
electrochemical application.
The effect of N
2
flow on microstructure of VN/CNPs
was further studied. Figure
2 shows the SEM images of
VN/CNP-10-20, VN/CNP-10-40, and VN/CNP-10-70
prepared at different N
2
flow rates of 20, 40, and
70 mL min
-1
. Slowly N
2
(20 mL min
-1
) flow was virtu-
ally impossible to extract plenty of released gas from the
reacting phase within a short time; therefore, much pores
formed from the escaping gas was rarely observed (Fig.
2a,
b). The morphology of VN/CNP-10-20 was full of rugosity
and presents a rippled or flaky structure [
30]. As the N
2
flow increased to 40 mL min
-1
, the morphology was the
combination of both aggregates of nanoparticles and the
remaining flaky structure (Fig. 2c, d). Afterward, as the N
2
6 Page 4 of 15 Nano-Micro Lett. (2017) 9:6
123
flow further increased (40 mL min
-1
), the rippled and
flaky structure was replaced by numerous completely,
which was also accompanied with the growing pores and
grain boundary (Fig.
2e, f).
Figure S1 shows the N
2
adsorption–desorption iso-
therms of the as-prepared VN/CNP-10-20, VN/CNP-10-40,
and VN/CNP-10-70. All of N
2
adsorption–desorption
isotherms (Fig. S1a, c, e) display combined characteristics
of type I/IV, which indicates a hierarchical porous structure
combined of micro-, meso-, and macropores [
31, 32]. The
BET surf ace area of VN/CNP-10-20, VN/CNP-10-40, and
VN/CNP-10-70 are 206.7, 184.0, and 217.0 m
2
g
-1
,
respectively. In fact, according to the analysis of N
2
adsorption–desorption isotherms for these samples, the
Fig. 1 SEM images of a, b VN/CNP-5, c, d VN/CNP-20, and e, f VN/CNP-30
Fig. 2 SEM images of VN/CNP-10-20, VN/CNP-10-40, and VN/CNP-10-70
Nano-Micro Lett. (2017) 9:6 Page 5 of 15 6
123