Hydrothermal synthesis of cobalt telluride nanorods for a high performance hybrid asymmetric supercapacitor

Article abstract of DOI:10.1039/c9ra08692g

Cobalt telluride nanostructured materials have demonstrated various applications, particularly in energy generation and storage. A high temperature and reducing atmosphere are required for the preparation of cobalt telluride-based materials, which makes this a difficult and expensive process. The development of a facile route for producing the desirable nanostructure of cobalt telluride remains a great challenge. We demonstrated a simple hydrothermal method for preparing cobalt telluride nanorods (CoTe NRs) and telluride nanorods (Te NRs) for supercapacitor applications. The morphology of CoTe NRs and Te NRs was analyzed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The prepared CoTe NR electrode material exhibited a high specific capacity of 170 C g^-1 at a current density of 0.5 A g^-1 with an exceptional cyclic stability. The asymmetric supercapacitor was assembled using CoTe NRs and orange peel-derived activated carbon (OPAA-700) as a positive and negative electrode, respectively. The fabricated device delivered a high energy density of 40.7 W h kg^-1 with a power density of 800 W kg^-1 at 1 A g^-1 current density. When the current density was increased to 30 A g^-1, the fabricated device delivered a high power density of 22.5 kW kg^-1 with an energy density of 16.3 W h kg^-1. The fabricated asymmetric supercapacitor displayed a good cyclic stability performance for 10000 cycles at a high current density of 30 A g^-1 and retained 85% of its initial capacity for after 10000 cycles. The prepared materials indicate their applicability for high performance energy storage devices.

Full text of DOI:10.1039/c9ra08692g

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Hydrothermal synthesis of cobalt telluride
nanorods for a high performance hybrid
asymmetric supercapacitor
Cite this: RSC Adv., 2020, 10, 13632
bc
bc
a
M. Manikandan, †a K. Subramani,
†
M. Sathish
and S. Dhanuskodi
*
*
Cobalt telluride nanostructured materials have demonstrated various applications, particularly in energy
generation and storage. A high temperature and reducing atmosphere are required for the preparation of
cobalt telluride-based materials, which makes this a difficult and expensive process. The development of
a facile route for producing the desirable nanostructure of cobalt telluride remains a great challenge. We
demonstrated a simple hydrothermal method for preparing cobalt telluride nanorods (CoTe NRs) and
telluride nanorods (Te NRs) for supercapacitor applications. The morphology of CoTe NRs and Te NRs
was analyzed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
The prepared CoTe NR electrode material exhibited a high specific capacity of 170 C gꢀ1 at a current
density of 0.5 A gꢀ1 with an exceptional cyclic stability. The asymmetric supercapacitor was assembled
using CoTe NRs and orange peel-derived activated carbon (OPAA-700) as a positive and negative
electrode, respectively. The fabricated device delivered a high energy density of 40.7 W h kgꢀ1 with
a power density of 800 W kgꢀ1 at 1 A gꢀ1 current density. When the current density was increased to
30 A gꢀ1, the fabricated device delivered a high power density of 22.5 kW kgꢀ1 with an energy density of
16.3 W h kgꢀ1. The fabricated asymmetric supercapacitor displayed a good cyclic stability performance
for 10 000 cycles at a high current density of 30 A gꢀ1 and retained 85% of its initial capacity for after
10 000 cycles. The prepared materials indicate their applicability for high performance energy storage
devices.
Received 23rd October 2019
Accepted 14th March 2020
DOI: 10.1039/c9ra08692g
rsc.li/rsc-advances
densities of 4–6 W h kgꢀ1. So, the progress of supercapacitors
continues mostly towards the development of high energy and
low-cost nanostructured electrode materials. The energy density
of the supercapacitor depends on the capacity of the material
and cell voltage. However, the low cell potential of the aqueous-
based symmetric supercapacitor limits its potential application
due to water decomposition. Recently, hybrid asymmetric
supercapacitors in an aqueous electrolytes have been developed
in which the faradaic metal oxide/hydroxide/sulde/telluride
and an EDLC-based carbon electrode materials are used as
the positive and negative electrodes, respectively.
Hybrid supercapacitors can efficiently utilize the potential
gap between the two types of electrodes to increase the cell
voltage. The energy density of the supercapacitor materials
depends on the capacity of the electrode material and cell
voltage, while increasing the cell voltage the energy density get
enhanced. Ni-, Co- and Fe-based battery-type electrode mate-
rials are promising materials for high energy storage applica-
tions due to the low cost, environmental sustainability, intrinsic
electronic conductivity, high capacity, good rate performance,
long life span and high redox behavior.12–16 For example, Wang
et al. fabricated a Ni3S4 battery-type electrode supercapacitor
with a specic capacitance of 898.5 C gꢀ1 at 0.5 A gꢀ1 current
density.17 Ni3S2/CdS exhibit a capacity of 545.6 C gꢀ1 at the
1. Introduction
In 21st century, the global demand in the energy sectors and
limited availability of fossil fuels require society to move
towards substantial developments in the energy conversion and
storage devices.1,2 For energy storage, batteries and conven-
tional capacitors are the main devices that can deliver a high
energy and high power, respectively. Currently, batteries
possess a high energy density and low power density, while the
conventional capacitor delivers a high power but limits its
energy storage.3,4 Supercapacitors, also known as ultra-
capacitors, have drawn a great interest in recent years because
of their high power performance, fast charge/discharge ability,
long cycle life and low cost.5–11 Commercial-based symmetric
supercapacitors in non-aqueous electrolytes have delivered
high-power densities in the range of 12–14 kW kgꢀ1 with energy
aSchool of Physics, Bharathidasan University, Tiruchirappalli – 620 024, Tamil Nadu,
India. E-mail: dhanus2k3@yahoo.com
bElectrochemical Power Sources Division, CSIR-CECRI, Karaikudi – 630 003, Tamil
Nadu, India . E-mail: msathish@cecri.res.in
cAcademy of Scientic and Innovative Research (AcSIR), Ghaziabad-201002, India
† These authors contributed equally to the work. All the authors contributed to
the writing of the manuscript.
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current density of 1 A gꢀ1 with the capacitance retention of capacitance of 161 C gꢀ1 with a capacitance retention of 82%
103% aer 5000 cycles.18 Liu et al. reported on Ni–Co@Ni–Co over 1000 cycles at a scan rate of 100 mV sꢀ1 28 In our previous
.
layered double hydroxide for solid state battery-type super- work, we demonstrated a nickel telluride nanorod (NiTe NR) by
capacitor. It gives an energy density of 100 W h kgꢀ1 and cyclic hydrothermal method using ascorbic acid and CTAB as the
stability of 98.6%.19 Ce-doped a-Co(OH)2 shows a specic reducing agent and surfactant, respectively. High specic
capacity of 415 C gꢀ1 with a cyclic stability of 73%, even aer capacitances of 370 C gꢀ1 were achieved at the current density of
2000 cycles.20
1 A gꢀ1 with good electrochemical cyclic stability.29 Here, all
To obtain a new device with a large energy density, power capacitance values are recalculated as capacity values for
density and potential window of the asymmetric supercapacitor consistency.
(ASC) has received a great attention by the combination of two
CoTe has attracted great interest because its thermal, elec-
different types of electrodes to make asymmetric congurations trical and magnetic properties have shown a variety of appli-
resulting in the storage of more energy and a wider cell potential cations such as electrocatalysis,30 water splitting,31 solar cells,32
due to the utilization of faradaic and non-faradaic process in batteries,33 photocatalysis34 and biosensors.35 Cobalt telluride
a single device. In ASC, the faradaic electrode as the positive nanostructure materials have demonstrated various applica-
electrode (energy source) generally uses transition metals such tions particularly in energy generation and storage. High
as Ni(OH)2 and CoO because of their high specic capacity and temperatures and a reducing atmosphere are needed for the
electrochemical performance. The EDLC as a negative electrode preparation of cobalt telluride-based 2-D nanostructure mate-
(power source) uses materials like activated carbon, carbon rials, which makes it difficult and expensive for the large-scale
nanotubes and graphene because of their high surface area, production and fabrication of supercapacitors. Thus, the
high stability and high electrical conductivity.21,22 In this development of a facile route for desirable CoTe nanostructures
scenario, metal chalcogenides are a new type of electrode remains a great challenge. To the best of our knowledge, this is
material for the electrochemical supercapacitor. In the metal the rst report on hydrothermally prepared CoTe and Te NRs as
chalcogenides, the metal ion undergoes fast surface redox electrode materials for supercapacitor applications. The as-
reactions associated with different oxidations of the transition prepared CoTe NR electrode material delivered a high specic
metal ion. Transition metal chalcogenides like NiS, CoS, CuS, capacity of 170 C gꢀ1 at a current density of 0.5 A gꢀ1, which is
MoS2, NiSe, CoSe, and MoSe2 nanostructures are more inter- much higher than the Te NRs. Depending on the growth
esting towards supercapacitor applications due to the high condition of the materials, it can exhibit various surface
surface area, specic capacitance, excellent electrochemical morphologies. The capacity of the materials differs from the
properties, high electrical conductivity and stability.23
different morphologies because of both faradaic and non-
Recently, metal tellurides like CoTe, NiTe, La2Te3, MoTe2 faradaic processes. Here, the CoTe surface area is higher than
and Sm2Te3 have been used as an electrode materials for Te. In the case of morphology, it is the opposite. Asymmetric
supercapacitor applications. Among these metal tellurides, supercapacitors (ASC) were fabricated using CoTe NRs and
CoTe is of immense interest due its high theoretical capacity of orange peel-derived activated carbon (OPAA)36 as positive and
1034 C gꢀ1. CoTe nanowires can extend their potential voltage negative electrodes, respectively. The fabricated device deliv-
to 1.6 V and give energy and power density values of ered a high energy density and power density of 40.7 W h kgꢀ1
32.9 W h kgꢀ1 and 800.27 W kgꢀ1, respectively, even aer 5000 and 22.5 kW kgꢀ1, respectively. In addition, the fabricated
cycles at the current density (5 A gꢀ1) with an initial capacitance device exhibited an excellent rate performance and good cycling
of 90.5%, as reported by Xiao et al.21 Ye et al. synthesized stability, which make it a potential electrode material for energy
a CoTekAC asymmetric supercapacitor electrode grown on storage applications.
carbon ber paper by hydrothermal method, which showed
a specic capacitance of 120 C gꢀ1 with an energy density of
2. Experimental
2.1. Preparation of CoTe nanorods
67.0 W h kgꢀ1 22 Cao et al. reported on Te NWs arrays with high
.
conductivity grown on a exible carbon ber to fabricate hybrid
Te/Au/MnO2 core–shell nanostructures as a supercapacitor with Sodium telluride (Na2TeO3, Sigma Aldrich), cobalt acetate
97% capacitance retention for 1000 cycles.24 Zhou et al. (Co(CH3COO)2$4H2O, Sigma Aldrich), ascorbic acid (C6H8O6),
prepared NiTe rods as a positive electrode material for an cetyltrimethylammonium bromide (CTAB, Sigma Aldrich) and
asymmetric supercapacitor (ASC) with a specic capacitance of deionized water from a Milli-Q-ultra pure water system (18.2
804 F gꢀ1 (current density 5 A gꢀ1) and 91% of capacitance MU cmꢀ1) were used as the starting materials. In the hydro-
retention for 1000 cycles.25 Liu et al. recorded that few-layer 1T0 thermal method, 14.2 mmol of C6H8O6 was dissolved in 40 ml of
MoTe2 obtained a high specic capacitance of 835 C gꢀ1 at deionized water. Then, 0.82 mmol of CTAB was added gradually
a current density of 1 A gꢀ1
.
Kumbhar et al. synthesized and stirred for 30 min. In the next step, 1.88 mmol of Na2TeO3
26
Sm2Te3 thin lms through a one-step chemical route with and 1.88 mmol of Co(CH3COO)2$4H2O was dissolved in 30 ml of
a specic capacitance of 129 C gꢀ1 with high energy and power deionized water and slowly added to the above solution and
density values of 25.60 W h kgꢀ1 and 14.18 kW kgꢀ1, respec- a white TeO2 precipitate was obtained immediately. Finally, the
tively.27 Patil et al. successfully synthesized La2Te3 thin lm via resultant solution was transferred into a 100 ml Teon-lined
a successive ionic layer adsorption and reactive method for stainless-steel autoclave that was sealed and maintained at
supercapacitor charge storage properties. It exhibited a specic 180 ꢁC for 24 h in an oven, then allowed to cool to room
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temperature. The obtained black powder was collected and (EIS) study was also carried out by applying an alternate current
washed with deionized water and ethanol several times, and (AC) with a bias voltage of 5 mV amplitude in the frequency
dried at room temperature. For comparison, pure Te was range from 100 kHz to 10 mHz. The specic capacity of
prepared in the absence of cobalt acetate by the same method.37 the electrode materials was calculated from the following
formula:4
2.2. Preparation of orange peel-derived activated carbon
IDt
C ¼
(1)
For the full cell fabrication of ASC, OPAA was used as a negative
electrode material. The detailed synthesis procedure and char-
acterizations are described in our previous publications.36 In
detail, the dry orange peel powder was subjected to carboniza-
tion at 600 ꢁC for 3 h in an argon (Ar) atmosphere and followed
by chemical activation. The carbonized sample was mixed with
KOH in a weight ratio of 1 : 3, and the obtained slurry was again
heat-treated at 700 ꢁC for 1 h in an Ar atmosphere to obtain the
chemically activated carbon. The resulting activated carbon was
washed thoroughly with 1 M HCl solution, followed by DI water
until the pH of the sample became neutral. The resulting acti-
m
where C is the specic capacity of the electrode materials (C
gꢀ1), I is the discharge current (A gꢀ1), Dt is the discharge time
(s), and m is the mass of the active material (g). The
theoretical capacity of the electrode was calculated using the
equation:
n ꢂ F
C ¼
(2)
M
where n is the number of electrons transferred in the redox
reaction, F is the Faraday constant, M is the molar mass and V is
the operating potential window.
ꢁ
vated carbon was dried at 80 C for 5 h.
The hybrid asymmetric supercapacitor (ASC) was con-
structed using the faradaic type CoTe NRs as a positive elec-
trode, EDLC-type activated carbon derived from orange peel
biomass-derived activated carbon as a negative electrode with
3.5 M KOH as an electrolyte. The advantages of the ASC are
a higher cell potential with a high specic capacity. To obtain
better ASC performance, it is necessary to maintain the
charge balance between the positive and negative electrodes
using the following relationship: q+ ¼ q , where q+ and q are
2.3. Characterizations
The phase formation and crystalline nature of the synthesized
materials were analyzed by powder X-ray diffraction (XRD)
analysis using PANalytical XRD X'pert pro with Cu Ka radiation
ꢀ
(a ¼ 1.5418 A). The elemental composition and oxidation states
of the metal ions were examined by X-ray photoelectron spec-
troscopy (XPS; MULTILAB 2000, Thermo Scientic) using Mg Ka
(1253.6 eV) as the X-ray source. The surface morphology and
particle size of the synthesized materials have been studied by
scanning electron microscope (SEM), Hitachi S-3000 H and
transmission electron microscope (TEM), Jeol Jem-2100,
respectively. An N2 adsorption–desorption experiment was
carried out using a Micromeritics ASAP 2020 analyzer, and the
surface areas were determined using BET (Brunauer–Emmett–
Teller). The electrochemical performance of the electrode
materials for supercapacitor application was evaluated by cyclic
voltammetry (CV), galvanostatic charge discharge (GCD) and
electrochemical impedance spectroscopy (EIS) analysis using
electrochemical workstation (Biologic- SP300).
ꢀ
ꢀ
the stored charges in the positive and negative electrode
surfaces, respectively. The amount of charge stored on the
electrodes depends on the capacity (C) and mass of the elec-
trode materials (m). Thus, the mass balance equation is
expressed as follows:3,38
q+ ¼ C+ ꢂ m+
¼ C ꢂ m
(3)
(4)
(5)
q
ꢀ
ꢀ
ꢀ
m
C
þ
ꢀ
ꢀ
¼
m
C
þ
where C+ and C are the capacity of the positive and negative
ꢀ
2.4. Preparation of electrode for supercapacitor
electrodes, and m+ and m are the mass of the positive (4.66 mg)
ꢀ
and negative electrodes (2 mg). The capacity of the OPAA
material (specic capacity is 280 C gꢀ1 at 2 A gꢀ1 current
density) was calculated based on our previous work.14 The mass
ratio for optimal electrochemical performance of ASC between
the positive and negative electrodes was calculated to be 2.33.
The energy and power density values were calculated by the
following equations:39
The working electrode was fabricated by mixing the active
materials, acetylene black and polyvinylidene uoride (PVDF)
with a weight ratio of 75 : 20 : 5, respectively. This mixture was
continuously ground in a mortar and 0.5 ml N-methyl-2-
pyrrolidone (NMP) was used as a solvent to make a homoge-
nous slurry. This homogenous slurry was coated on a nickel
2
ꢁ
foam (2 ꢂ 2 cm ) current collector, and then dried at 90 C for
12 h in a vacuum oven. The weight of the active material (ꢃ4
mg) was optimized based on the weight of the Ni foam before
and aer coating. In a three-electrode conguration, active
material-coated Ni foam, Pt foil and Hg/HgO (20% KOH) served
as the working, counter and reference electrodes, respectively.
The CV and GCD measurements were carried out using 3.5 M
KOH as the electrolyte with different scan rates from 5 to
CV
E ¼
(6)
(7)
7:2
E ꢂ 3600
Dt
P ¼
where the energy density (E), power density (P), C is the specic
capacitance (c gꢀ1), DV is the potential window (V), and Dt is the
discharging time (s).
100 mV sꢀ1 and different current densities from 1 to 10 A gꢀ1
,
respectively. The electrochemical impedance spectroscopic
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0.5454 nm, which are coordinated with the standard pattern of
JCPDS no. 34-0420 (ref. 35) shown in Fig. 1b. The diffraction
peaks appearing at an angle of 26.7, 31.6, 32.9, 43.6, 46.8, 58.3,
64.7 and 69.0ꢁ revealed the existence of the (100), (101), (002),
(102), (110), (112), (202) and (004) planes, respectively. No
additional peaks appeared for both samples, which indicate the
high purity of the electrode materials.
The SEM images of CoTe and Te show the formation of
uniform nanorod bundles, consisting of nanorods with an
average length of up to 1 mm (Fig. 2a and b). The average
diameter of the CoTe NRs is less than that of the Te NRs. The
samples were further analyzed by TEM analysis. Fig. 3(a and b)
shows TEM images of the Te NRs at different magnications. A
lattice-resolved higher magnication TEM image in Fig. 3(b)
indicates the interplanar spacing of 0.32 nm, which corre-
sponds to the (101) plane of hexagonal Te and conrms the
formation of the Te NRs. Fig. 3(c–e) shows the TEM images of
the CoTe NRs at different magnications. At higher magni-
cation images (Fig. 3f and g), the formation of nanorods are
clearly indicated with an average diameter in the range of 11–
15 nm, and the surface of each rod was found to be rough. A
lattice-resolved higher magnication TEM image in Fig. 3(h)
indicates the interplanar spacing of 0.28 nm, which corre-
sponds to the (101) plane of hexagonal CoTe and conrms the
formation of CoTe NRs. It was further conrmed that the
average diameter of the CoTe NRs is less than that of the Te
NRs. These results support the previous analysis (e.g., SEM and
XRD analysis).
Scheme
nanorods.
1 Schematic representation for the growth of CoTe
3. Results and discussion
Scheme 1 represents the synthesis of CoTe NRs. In the synthesis
process, ascorbic acid covered the surface of the Te nanorods.
Co2+ ions spread over the carbon shells and react with Te to
form CoTe.40 CTAB is a cationic surfactant that controls the
shape, size and aggregation of nanostructures due to the
formation of micelles in the liquid medium. It forms the elon-
gated stable micelles during the addition of CTAB into the
water. This can absorb the CoTe nanoparticles and grows as
nanorods, and also prevents the further aggregation of the CoTe
nanoparticles. The bonding between the CTAB and CoTe
nanoparticles enhances the lengthening of the crystals rather
than the growth in the axial direction.41
3.1. Structural and morphological analysis
Fig. 1a shows the XRD pattern of the Te NRs, and the diffraction
peaks are well matched with the standard pattern of JCPDS no.
36-1452.42 The peaks are well indexed with the hexagonal phase
of tellurium with a lattice parameter of a ¼ 0.4459 nm and c ¼
0.5854 nm. The diffraction peaks appearing at an angle of 23.2,
27.5, 38.3, 40.4, 43.3, 45.9, 49.6, 51.8, 56.5, 62.8, 63.8 and 67.7ꢁ
conrmed the corresponding (100), (101), (102), (110), (111),
(003), (201), (112), (202), (113), (210) and (203) planes, respec-
tively. The diffraction peaks of the CoTe NRs can be indexed to
a hexagonal phase with lattice parameter a ¼ 0.3865 nm and c ¼
Fig. 4a shows Co 2p3/2 at 781.2, 786.4 eV and Co 2p1/2 at
796.9, 801.4 eV. The energy difference is 15.7 eV for the Co 2p3/2
and Co 2p1/2 spin–orbit interaction, which reveals the presence
of Co2+ and Co3+ ions.43 The Te 3d spectrum in Fig. 4b is divided
into four peaks. The satellites at 572 and 583 eV are assigned to
Te 3d5/2 and Te 3d3/2 of the Te2ꢀ ions, and the major peaks at
576 and 586 eV arise due to the formation of the Te4+ ions.44
3.2. BET analysis
Fig. 5(a and b) shows the N2 adsorption–desorption isotherms
of Te and CoTe samples conducted to determine the surface
areas of the materials. The product exhibited a type IV H3
characteristic hysteresis behavior, indicating the formation of
meso- and microporous structures. The specic surface areas of
Te and CoTe were calculated to be 4.8 and 8.4 m2 gꢀ1, respec-
tively. The Barrett–Joyner–Halenda (BJH) pore size distribution
curve indicates the pore radius in the range of 5–14 and 3–
Fig. 1 XRD pattern of (a) Te NRs with standard pattern, (b) CoTe NRs
with standard pattern.
Fig. 2 SEM image of (a) CoTe NRs and (b) Te NR.
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Fig. 3 TEM images of Te NRs at (a) low and (b) high magnifications; (c–h) TEM images of CoTe NRs at different magnifications.
3.3. Electrochemical performance
Initially, the electrochemical properties of the electrode mate-
rials were characterized by CV, GCD and EIS analysis. The CV
prole of the Te NR and CoTe NR electrodes at a scan rate of
50 mV sꢀ1 showed redox-type voltammograms, which demon-
strate the faradaic charge storage mechanism (Fig. 6a). The CV
prole of the CoTe NR electrode displayed a wider and high
current density than the Te electrode. Fig. 6(b and c) shows the
CV prole of the Te and CoTe NR electrode at different scan
Fig. 4 Higher resolution XPS spectra of (a) Co 2p and (b) Te 3d for
rates ranging from 5 to 50 mV sꢀ1. Both electrode materials have
CoTe NR materials.
a strong pair of redox peaks during the CV analysis, which
resulted from the following surface reversible redox reaction
between Co2+ and Te for CoTe NRs and Te NRs in the presence
of OHꢀ ions, respectively. The possible redox reactions in the Te
and CoTe electrodes can be described as follows:22,45
Te + OHꢀ # TeOH + eꢀ
TeOH + OHꢀ # TeO + H2O + eꢀ
CoTe + OHꢀ # CoTeOH + eꢀ
(8)
(9)
(10)
(11)
Fig. 5 BET N2 adsorption–desorption isotherms of the prepared (a) Te
NRs and (b) CoTe NRs with corresponding BJH pore size distribution
curve (inset a and b).
CoTeOH + OHꢀ # CoTeO + H2O + eꢀ
While the scan rate increased, the oxidation and reduction
peaks shied to negative and positive potentials, respectively,
because the ion diffusion rate had to be limited to satisfy elec-
tronic neutralization during the redox process. At a slow scan
rate, the OHꢀ ions have enough time to intercalate with the
inner and outer surfaces of the electrode. But, in the case of
a high scan rate, the OHꢀ ions have less time to intercalate with
the surfaces of the electrode.
12 nm for Te NRs and CoTe NRs (Fig. 5(a and b) (inset)),
respectively. The unique meso- and microporous nature with
the high specic surface area of the CoTe NRs could improve
the mass transport and electron transfer as an electrode in the
electrochemical process; this may be an advantage for the
excellent electrochemical performance. Therefore, it is esti-
mated that the CoTe NRs can offer higher electrochemical
activity than the Te NRs.
The electrode materials were further investigated by GCD
analysis at various current densities from 1 to 10 A gꢀ1 at the
potential range of 0.0 to 0.5 V vs. Hg/HgO (20% KOH).
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Fig. 6 CV profile of the (a) Te and CoTe NRs at a scan rate of 50 mV sꢀ1, (b) Te NRs, (c) CoTe NRs at different scan rates; GCD profile of (d) Te and
CoTe NRs at 1 A gꢀ1 current density, (e) Te NRs and (f) CoTe NRs at different current densities.
Compared to the CV prole, the active potential window was attain the minimum utilization of the active surface areas of the
slightly decreased for GCD analysis because of gas evolution at electrode materials. Here, we have compared the specic
low current density. Fig. 6d shows the GCD prole of the Te and capacities of some metal chalcogenide-based electrode mate-
CoTe electrodes. CoTe had a higher discharge time compared to rials with our synthesized materials. Although the specic
the Te electrode at the current density of 1 A gꢀ1. The GCD capacity of CoTe was slightly lower than that of the reported
prole exhibits non-linear charge/discharge proles that materials, its capacity retention was much higher even aer
conrm the faradaic nature of the electrode materials, which is 5000 cycles. The interfacial resistance of the electrode materials
in good agreement with the CV analysis. The calculated specic at the electrode and electrolyte interfaces was evaluated using
capacities of the Te NRs and CoTe NRs were 87 and 148 C gꢀ1
,
the EIS technique. Fig. 7b reveals the Nyquist plot of the CoTe
respectively, at 1 A gꢀ1 current density. The GCD prole of the NR and Te NR electrode materials tted with an appropriate
Te NR and CoTe NR electrode materials at different current equivalent circuit model. The equivalent circuit model con-
densities are shown in Fig. 6(e and f). The specic capacity of all tained solution resistance (Rs), charge-transfer resistance (Rct),
electrode materials as a function of different current densities is double layer capacitance (Cdl), Warburg impedance (W) or
displayed in Fig. 7a. The calculated specic capacities of the Te diffusion resistance and faradaic capacitance (CF). From the
NRs from the GCD prole are 87, 86, 80, 65, 60, 53, 48, 42, 37, tting analysis, the CoTe NR and Te NR electrodes showed Rs
and 34 C gꢀ1 at current densities of 1, 2, 3, 4, 5, 6, 7, 8, 9 and values of 0.18 and 0.21 U, respectively. The Rct values were 2.3
10 A gꢀ1, respectively. The calculated specic capacities of the and 2.7 U for the CoTe NR and Te NR electrodes, respectively. In
CoTe NRs from the GCD prole are 170, 148, 120, 98, 86, 83, 75, addition, the CoTe electrode materials showed a vertical line
71, 66, 62, and 61 C gꢀ1 at current densities of 0.5, 1, 2, 3, 4, 5, 6, along the y-axis in the low frequency region of more than 45ꢁ,
7, 8, 9 and 10 A gꢀ1, respectively. Even at the high current which implied the ideal capacitive behavior of the electrode
density of 10 A gꢀ1, the CoTe NRs and Te NRs delivered a good materials with less interfacial charge transfer resistance at the
rate performance of 36 and 39%, respectively. The specic electrode/electrolyte interface.
capacitance value decreased while increasing the current
The electrochemical stability is one of the vital properties for
density; this may be attributed to the diffusion of ions at a supercapacitor. To assess the cyclic stability of the electrodes,
different current rates. At a lower current density, the electrolyte a GCD test was carried out up to 5000 cycles at a high current
ions have a sufficient time to diffuse to the inner pores of the density of 10 A gꢀ1 as shown in Fig. 7c. Aer 5000 cycles, 98.5%
electrode surfaces to attain the maximum utilization of the (Te) and 99% (CoTe) of the initial specic capacitance was
active surface areas of the electrode materials.46,47
retained. For all 5000 cycles, the Coulombic efficiency of the
At high current densities, the electrolyte ions can access only electrode materials remained constant (ꢃ98%), which conrms
the surface of the electrode materials. They do not have enough the high reversibility of the charge storage process. This good
time to intercalate the inner pores of the electrode surfaces to stability reveals that the highly reversible surface redox reaction
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Fig. 7 (a) Specific capacity of CoTe NRs and Te NRs as a function of current density; (b) Nyquist plot of CoTe NRs and Te NRs with corresponding
equivalent circuit (inset) and (c) specific capacity retention and coulombic efficiency (inset) of CoTe NRs and Te NRs as a function of the cycle
number.
took place between the electrolyte and electrode surface. To this neutralization during the redox process. At a slow scan rate, the
end, the CoTe NR electrode shows higher electrochemical OHꢀ ions have enough time to intercalate with the inner and
activity than the Te NR electrode. For the full cell analysis, CoTe outer surfaces of the electrode. However, in the case of a high
NRs was used as a positive electrode and OPAA was used as the scan rate, the OHꢀ ions have less time to intercalate with the
negative electrode for further studies.
surfaces of the electrode.
Fig. 8d reveals the GCD curves of the hybrid ASC device
performed at a current density of 1 A gꢀ1 in different cell
potentials (1.2 to 1.6 V). From the discharge prole, the calcu-
lated specic capacity of the hybrid ASC cell was 144, 160, 169,
181 and 183 C gꢀ1 at cell potentials of 1.2, 1.3, 1.4, 1.5 and 1.6 V,
respectively. The specic capacity as a function of the cell
potential is shown in Fig. 8e, where the capacity value increased
with increasing cell potential. Even at 1.6 V cell potential, the
CD prole shows a peak-like charge–discharge prole without
any gas evolution. Fig. 8f demonstrates the GCD prole of the
hybrid ASC cell with a cell potential of 1.6 V at different current
densities. The calculated specic capacity of the hybrid ASC cell
was 183, 160.4, 154, 146, 140.5, 120, 92 and 78 C gꢀ1 at the
3.4. Hybrid asymmetric supercapacitor
Fig. 8a represents the CV prole of OPAA and CoTe electrodes
occupying different potential windows of ꢀ1.0 to 0 V and 0 to
0.6 V at the scan rate of 50 mV sꢀ1, respectively. The CV proles
of the hybrid ASC (CoTekOPAA) at different cell potentials from
1 to 1.6 V at the scan rate 50 mV sꢀ1 are shown in Fig. 8b. The
device remained stable with increasing cell potential up to 1.6 V.
The set of redox peaks with a wider CV prole indicates the
existence of faradaic and EDLC nature properties. At the cell
voltage of 1.6 V, the CV curves of the CoTekOPAA hybrid ASC at
different scan rates from 5 to 100 mV sꢀ1 are given in Fig. 8c.
While the scan rate increased, the oxidation and reduction
peaks shied to negative and positive potentials because the ion
diffusion rate had to be limited to satisfy electronic
current density values of 1, 2, 3, 4, 5, 10, 20 and 30 A gꢀ1
,
respectively. When the current density was increased to
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Fig. 8 (a) CV profile of OPAA and CoTe electrodes from the half-cell (three electrode configuration) analysis at a scan rate of 50 mV s; CV profile
of CoTekOPAA ACS (b) at the scan rate of 50 mV sꢀ1 with different cell potentials and (c) at different scan rates with a cell potential of 1.6 V. GCD
profile of the fabricated ASC at (d) different cell potentials at 1 A gꢀ1 current density and (e) specific capacity obtained as a function of different cell
potentials, (f) at different current densities with the cell potential of 1.6 V as a cell voltage.
conrmed that the fabricated hybrid ASC cell had excellent
electrical contact with minimum ionic resistance.
For viable applications, a supercapacitor should have high
electrochemical stability. In order to nd the electrochemical
stability of the fabricated hybrid ASC cell, cycling studies were
carried out for 10 000 cycles at the high current density of
30 A gꢀ1. Fig. 9c shows the capacity retention and coulombic
efficiency of the hybrid ASC cell as a function of the cycle
number. The fabricated hybrid ASC cell delivered an excellent
electrochemical stability of 85% of the initial specic capacity
aer 10 000 charge–discharge cycles with a coulombic efficiency
of 99.9%. The fabricated hybrid ASC cell with CoTe NRs and
OPAA electrode materials as the positive and negative electrodes
shows good discharge reversibility and outstanding cycle
stability, respectively. These electrochemical stability values are
higher than the recently reported materials, such as CoTekAC
(90% for 5000 cycles),21 CoTekAC (80% for 3000 cycles),22 NiTe
(81% for 3000 cycles),25 1T-MoTe2 (87% for 5000 cycles).26
Fig. 9 (a) Specific capacity curve of fabricated ACS as a function of
The Ragone plot is a typical tool to evaluate the super-
capacitor device with other types of energy storage devices in
different current densities, (b) Nyquist plot of CoTe NRskOPAA ASC
with the magnified view (inset), and (c) the capacity retention and
terms of their energy and power density values. Fig. 10 displays
the Ragone plot of the fabricated CoTekOPAA hybrid ASC
device. The fabricated hybrid ASC device exhibited high energy
density values of 24, 28.9, 32.9, 37.7 and 40.7 W h kgꢀ1 with cell
potential values of 1.2, 1.3, 1.4, 1.5 and 1.6 V, respectively.
Notably, the energy density of the device increased with
increasing cell potential. Similarly, the power density also
coulombic efficiency of fabricated ASC as a function of cycle numbers.
30 A gꢀ1, the hybrid ASC cell retained 42.6% of the initial
specic capacity (Fig. 9a). Fig. 9b Illustrates the Nyquist plot of
the fabricated hybrid ASC cell, which exhibited Rs and Rct values
of ꢃ0.4 and ꢃ0.6 U, respectively. From the EIS studies, it was
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shows a high specic capacity of 170 C gꢀ1 at a current density
of 0.5 A gꢀ1 and excellent life cycle (99%). The fabricated
CoTekOPAA ASC achieved a specic capacity of 183 C gꢀ1 with
a high-energy density of 40.7 W h kgꢀ1 and power density of
800 W kgꢀ1 at the current density of 1 A gꢀ1. The device deliv-
ered a high power density of 22.5 kW kgꢀ1 with an energy
density of 16.3 W h kgꢀ1 at a high current density of 30 A gꢀ1
.
The fabricated hybrid ASC cell delivered an excellent electro-
chemical stability of 85% of the initial specic capacity even
aer 10 000 charge–discharge cycles with a coulombic efficiency
of 99.9%. The prepared materials have shown a promising
results as an electrode for supercapacitor applications. The
proposed simple hydrothermal method could be scaled up for
a large-scale preparation of CoTe and other Te-based materials
for their viable applications in energy storage. Furthermore,
these materials have a great potential in other applications such
as lithium-ion batteries, catalysts and data storage devices.
Fig. 10 Ragone plot for the fabricated ACS as a function of different
cell potentials and different current densities using CoTe NRs and
OPAA as the positive and negative electrodes, respectively.
increased to 600, 650, 701, 750 and 800 W kgꢀ1 with increasing
cell potential at 1.2, 1.3, 1.4, 1.5 and 1.6 V, respectively (Fig. 10).
The high energy and power density values of the fabricated
hybrid ASC at a current density of 1 A gꢀ1 at the cell potential of
1.6 V were 40.7 W h kgꢀ1 and 800 W kgꢀ1, respectively. When
the current density increased to 30 A gꢀ1, the fabricated hybrid
ASC device delivered a high-power density of 22.5 kW kgꢀ1 and
retained an energy density of 16.3 W h kgꢀ1. The above observed
high energy and power densities were signicantly higher than
the reported values for the metal oxide-based electrode mate-
rials in ASC using an aqueous electrolyte. For instance, Ye et al.
synthesized CoTekAC ASC that delivered an energy density of
67 W h kgꢀ1 for the pristine electrolyte.22 Quan et al. fabricated
a CuS-CNTs@NFkAC ASC showing the specic capacitance of
Conflicts of interest
There are no conicts to declare.
Acknowledgements
Dr K. S (IF131153) thanks DST, India for the INSPIRE Fellow-
ship. Dr K. S and Dr M. S thank the Science & Engineering
Research Board, Department of Science and Technology (DST-
SERB, File No EMR/2016/006807, GAP 25/17), India for nan-
cial support. The authors thank the Central Instrumentation
Facility, CSIR-CECRI, Karaikudi.
164 F gꢀ1 at a current density of 0.5 A gꢀ1 with an energy density
Notes and references
of 22.14 W h kgꢀ1
.
The assembled Ni3NkPRPC-1K ASC pos-
48
sessing a high energy and power density of 30.9 W h kgꢀ1 and
412 W kgꢀ1, respectively, have been reported by Peng et al.49
Wang et al. constructed a NiCo LDHkAC ASC displaying
a specic capacitance of 58 F gꢀ1 at the current density of
1 A gꢀ1 with an energy and power density of 18 W h kgꢀ1 and
750 W kgꢀ1, respectively.50 The remarkable electrochemical
performance of the fabricated hybrid ASC can be attributed to
the good combination of the faradaic-based CoTe NRs and
EDLC-based high surface area activated carbon (OPAA) CoS
nanoparticles and activated carbon electrode. In addition, this
methodology could be readily extended to the preparation of
other metal tellurides.
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