Nickel cobalt sulfide (NCS) was synthesized from nickel and cobalt nitrate (NCS-1) by a hydrothermal method. In another method, nickel cobalt sulfide (NCS-2) was also synthesized from hydrothermally synthesized nickel cobalt oxide (NCO). The syntheses of NCS-1 and NCO were conducted in the presence of glycine as a templating agent.
Trang 1Original Article
electrode material for supercapacitors
M Sathish Kumara,b, N Bhagavatha,b, Sudip K Batabyalc,**, Nikhil K Kothurkara,b,*
a Department of Chemical Engineering and Materials Science, Amrita School of Engineering, Coimbatore, Amrita Vishwa Vidyapeetham, India
b Center of Excellence in Advanced Materials & Green Technologies (CoE-AMGT), Amrita School of Engineering, Coimbatore,
Amrita Vishwa Vidyapeetham, India
c Amrita Center for Industrial Research & Innovation (ACIRI), Amrita School of Engineering, Coimbatore, Amrita Vishwa Vidyapeetham, India
a r t i c l e i n f o
Article history:
Received 4 March 2019
Received in revised form
27 July 2019
Accepted 10 August 2019
Available online 20 August 2019
Keywords:
Nickel cobaltite
Nickel cobalt sulfide
Glycine
Pseudocapacitor
Supercapacitor
a b s t r a c t
Nickel cobalt sulfide (NCS) was synthesized from nickel and cobalt nitrate (NCS-1) by a hydrothermal method In another method, nickel cobalt sulfide (NCS-2) was also synthesized from hydrothermally synthesized nickel cobalt oxide (NCO) The syntheses of NCS-1 and NCO were conducted in the presence
of glycine as a templating agent The NCS and NCO samples were thoroughly characterized by different techniques XRD studies showed that both NCO and NCS consisted of the cubic crystal phases Cyclic voltammetry of NCO and NCS revealed that, with an increasing scan rate within the range of 10e100 mV
s1, the specific capacitance of the samples reduced The specific capacitance of NCS-2, measured by galvanostatic chargeedischarge, was found to be 675 F g1, which is higher than those of NCO (313 F g1) and NCS-1 (500 F g1) The specific capacitance retention of NCS-2 was 88% over 1000 cycles, indicating the good cyclic stability of the material
© 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)
1 Introduction
Supercapacitors are an important emerging energy storage
technology They have many advantages, including high power
density, fast charge and discharge rates, and exceptionally long
cycle stability[1,2], as compared to batteries They are suitable for
many applications, such as in communication, transportation,
consumer electronics and aerospace [2] Supercapacitors can be
divided into two categories, namely, electrical double layer
capac-itors and pseudocapaccapac-itors Pseudocapaccapac-itors exhibit high specific
capacitance, due to the quick reversible faradaic redox reactions
from their electrode materials, which could be metal hydroxides,
oxides or conducting polymers Among the numerous electrode
materials, transition metal oxides, such as NiO [3,4], CuO [5,6],
Ni(OH)2[7], RuO2[8], Co3O4[9,10], Fe2O3[11], and V3O7[12]offer
rich redox reactions and high specific capacitance A few limitations
such as high cost, toxicity and relatively low specific capacitance of RuO2and the low electron mobility of MnO2, NiO and Co3O4exist
[13e15] NiCo2O4is a favorable mixed-metal oxide that has been widely investigated in the field of lithium-ion batteries [16], supercapacitors[17,18], and optoelectronic devices[19]for its po-tential applications It has been reported that, nickel-cobalt binary metal oxides, such as nickel cobaltite (NiCo2O4), possess greater electronic conductivity and electrochemical activity than nickel and cobalt oxides In particular, NiCo2O4 can offer a synergistically greater electrochemical activity as compared to the two single-component oxides[20] The remarkable electrochemical properties
of NiCo2O4have spurred the investigation of spinel NiCo2S4that shows an even greater electrochemical performance [21] Also, NiCo2S4has been reported to be an excellent supercapacitor ma-terial [22] As compared to NiCo2O4, NiCo2S4 has a much lower optical band gap and a much higher conductivity[21] The substi-tution of oxygen with sulfur can lead to a moreflexible structure due to the lower electronegativity of sulfur This may enable the fabrication of electrodes with better electron transport and me-chanicalflexibility[22]
This paper aims to compare two different synthesis methods of NiCo2S4(NCS) and to investigate their effects on the properties and performance of the material Thefirst method is the direct hydro-thermal synthesis of NCS from the respective metal salts in the
* Corresponding author Department of Chemical Engineering and Materials
Science, Amrita School of Engineering, Coimbatore, Amrita Vishwa Vidyapeetham,
India.
** Corresponding author.
E-mail addresses: s_batabyal@cb.amrita.edu (S.K Batabyal), k_nikhil@cb.amrita.
edu (N.K Kothurkar).
Peer review under responsibility of Vietnam National University, Hanoi.
Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d
https://doi.org/10.1016/j.jsamd.2019.08.005
2468-2179/© 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license
Trang 2presence of glycine The second method is a two-step process In
thefirst step, NiCo2O4(NCO) was hydrothermally synthesized in
the presence of glycine and in the second step, the NCO was
con-verted into NCS by its reaction with sodium sulfide The role of
glycine in both the syntheses is in modifying the morphology of the
material[20] The NCO and NCS samples synthesized by these two
methods were thoroughly characterized and their electrochemical
performance was analyzed
2 Experimental
2.1 Materials and chemicals
Nickel nitrate hexahydrate (Ni(NO3)2.6H2O) (98%) (Rankem),
cobalt nitrate hexahydrate (Co(NO3)2.6H2O) (98%) (Rankem),
so-dium sulfide (Na2S), and glycine (C8H9NO3) (Nice) were used for
synthesizing the samples
2.2 Preparation
All the analytical grade reagents were used as purchased In a
typical experiment, for the hydrothermal synthesis of NiCo2O4,
0.2 M Ni(NO3)2.6H2O, 0.4 M Co(NO3)2.6H2O and 0.2 M of glycine
were added to 30 ml of de-ionized water and stirred until the
solution became homogeneous The mixture solution was then
poured into a 50 mL Teflon-lined stainless steel autoclave kept in a
hot air oven at 200C for 6 h Thefinal product was washed with
DI water several times, filtered and kept in a hot air oven at
100C The obtained product (nickel cobaltite) wasfinely ground
in a mortar and pestle and named as NCO[14] For the synthesis of
NiCo2S4, two methods were followed In thefirst method, 0.2 M of
Ni(NO3)2.6H2O, 0.4 M of Co(NO3)2.6H2O and 0.2 M of glycine were
added to 30 ml of deionized water and stirred until the solution
became homogeneous The above solution was placed in a 50 mL
Teflon lined stainless steel autoclave containing 0.05 M sodium
sulfide solution and maintained at 200C for 6 h[15] Thefinal
product was washed with DI water several times,filtered and kept
in hot air oven at 100 C The obtained product was NiCo2S4
named as NCS-1 In the second method, the NCO synthesized as
described above, was taken along with sodium sulfide 0.05 M and
transferred into a 50 mL Teflon-lined stainless steel autoclave and
maintained at 200C for 6 h The obtained product was washed
with DI water several times and dried at 100C This product was
named as NCS-2
2.3 Characterization
Thermo-Scientific Nicolet IS10 IR spectrometer was used to
measure FT-IR spectra X-ray diffraction (XRD) was characterized by
Bruker D8 Advanced instrument ZIVE SP1 instrument was used to
carry out cyclic voltammetry (CV) and galvanostatic
chargeedischarge measurements FESEM was performed using
30 keV Carl Zeiss Merlin compact and GeminiSEM 300 microscope
3 Results and discussion
FTIR analyses of NCO, NCS-1 and NCS-2 samples were
con-ducted, and the results are shown inFig 1 All the samples showed a
broad band located in the range of 3200e3500 cm1, which is
attributed to the moisture adsorbed on the samples The peak at
1384 cm1is attributed to the presence of physisorbed CO2[23] For
NCS-1 and NCS-2, the bands at 631 cm1(symmetrical stretch) and
1100 cm1(asymmetrical stretch) correspond to the NieS or CoeS
vibrations These groups play an important role in the faradaic
re-actions of these active electrode materials The peak at 641 cm1in
NCO corresponds to the CoeO bond vibration Some residual glycine and its degradation products remained in the samples, which were inferred from the presence of theeC¼Oe stretching band at 1635 cm1and theeC¼Ce stretching bands at 2182 cm1, respectively[15]
Fig 2shows the XRD patterns of NCO, NCS-1 and NCS-2 For NCO, the peaks at 35.2, 36.5, 38.44, 42.69 and 51.57were indexed
to the reflections from the (220), (311), (222), (400) and (422) planes of the cubic phase of NiCo2O4 (JCPDS card no 20-0781)
[16,24] Similarly, for NCS-1 and NCS-2, the peaks at 27.30, 30.77, 33.32, 36.87, 38.89, 42.69, 50.98 and 52.46 correspond to the (220), (311), (222), (400), (235), (422), (511) and (440) planes of cubic NiCo2S4(JCPDS card no 20-0782)[17,25] Due to the appearance of peaks at similar positions, it is difficult to distinguish NCS from NCO
in the XRD patterns So the presence of small quantities of oxide in the sulfide samples (NCS-1, NCS-2) cannot be ruled out As compared to NCS-1, NCS-2 shows some shifting of peaks, which might be due to the presence of an interface between the oxide and sulfide The prominent peak corresponding to (235) in NCS-2 is indicative of the crystal growth leading to the petal-like morphology of the material
FESEM images of NCO (a), NCS-1 (b) and NCS-2 (c) are shown in
Fig 3 NCO showed a flake-like morphology The flakes were roughly 0.6e0.7 mm across and approximately 60e70 nm thick They could be seen to be made of particles ~50e60 nm in size In
Fig 1 FTIR spectra of NCO, NCS-1 and NCS-2.
M.S Kumar et al / Journal of Science: Advanced Materials and Devices 4 (2019) 376e380 377
Trang 3some places, the flakes had bunched up to form a somewhat
spherical lump of ~2000e2500mm NCS-1 (Fig 3(b)) showed a
complex morphology consisting of a few irregular structures on the
scale of 1e2mm, in addition to afiner nanostructure consisting of
densely-packed nanoscale protrusions NCS-2 showed (Fig 3(c)) a
complex morphology of interconnected lamellae resembling like
the petals of a marigoldflower The complex morphologies of all
three samples are attributable to the presence of glycine as a
templating agent, and they are likely to enhance ion-transport
leading to good electrochemical performance It should be noted
that the lamellae in NCS-2 are the thinnest at approximately
10e20 nm, which provides the highest surface area, leading to a
stronger interaction with the electrolyte The connectivity of the
nanostructure is also likely to provide good electrical conductivity,
leading to good performance So NCS-2, which was produced by the
two-step method, involving the synthesis of NCO and the
hydro-thermal conversion of the NCO to NCS, is expected to exceed the
other two samples in electrochemical performance Additional SEM
images of NCS-1 and NCS-2 are shown in the Supplementary
Information (Figure SI-1)
Fig 4 shows the FESEM image (a), overlaid EDS (energy dispersive X-ray spectroscopy) elemental maps for Ni, Co, and S (b), the EDS spectrum (c) and the individual elemental maps (def) for NCS-2 The maps indicate that the above three elements were uniformly distributed throughout the sample, which is in agree-ment with the XRD data confirming that the sample is NiCo2S4 The atomic ratios of Ni, Co, and S, as measured by EDS, reveal the compound as a sulfur-deficient material because of the presence of oxygen; probably there were some unconverted NCO along with the NCS-2 Maybe the extra interface between oxide and sulfide helped to enhance the capacitance
Electrochemical measurements were done with the three-electrode cell, which consisted of a working three-electrode, Ag/AgCl as a reference electrode and a platinum counter electrode in 6M KOH electrolyte on the NCO, NCS-1 and NCS-2 samples.Fig 5(a) shows the CV curves of NCO, NCS-1 and NCS-2 at a scan rate of 100 mV s1 NCS-2 possessed a larger included area and consequently a higher specific capacitance as compared to NCO and NCS-1 The shapes of the curves is indicative of a typical faradaic behavior of the materials
[17] Well-defined redox peaks were observed in all CV curves, which
Fig 3 SEM images of (a) NCO, (b) NCS-1, and (c) NCS-2.
Trang 4are commonly attributable to the M-O/M-O-OH reversible faradaic
redox processes; where M represents Ni or Co ion[18] No significant
changes in the shape and position of the oxidation and reduction
peaks upon increasing scan rates up to 100 mV s1were observed,
which suggests a fast chargeedischarge response in all the NCO,
NCS-1 and NCS-2 electrodes (Fig 5(b)) The CV curves of NCO and
NCS-1 are included inFigure SI-2 As expected, with increasing scan
rates, (Fig 5 (c)), the specific capacitance decreases due to the
accumulation of ions and the thickening of the diffusion layer at the
electrodeeelectrolyte interface
The redox reactions for this system in an alkaline electrolyte[19]
are given below:
NiCo2S4þ OHþ H2O4NiSOH þ 2CoSOH þ e
CoSOHþ OH4CoSOH þ H2Oþ e
Galvanostatic charge discharge (GCD) studies with the samples
on a glassy carbon electrode were carried out using 6M aqueous
KOH electrolyte solution in a three-electrode system The
synthe-sized samples acted as a working electrode, silver-silver chloride
(Ag/AgCl) was used as a reference electrode and platinum (Pt) was
used as a counter electrode, at 1 A g1current density (Fig 6) The
specific capacitance was calculated using the formula[2]:
Cs¼ i*Dt
DV*m
Fg1
(1)
where,‘i’ indicates current (A), ‘Dt’ indicates discharge time (s), ‘DV’ indicates voltage windows (V) and‘m’ indicates mass (g)
Fig 6(a) shows the GCD curves of NCO, NCS-1 and NCS-2 at a current density of 1 A g1 The triangular shape suggests good reversibility in all three samples NCS-2 showed the highest specific capacitance (675 F g1) among the three due to the favorable ion transport between the electrode and the electrolyte Compara-tively, NCS-1 (500 F g1) showed a lower specific capacitance, and NCO (312.5 F g1) showed an even lower value The results suggest that the anion exchange improves the specific capacitance of the material.Fig 6(b) shows the GCD curve of NCS-2 at different cur-rent densities (1e5 A g1) At every current density, the triangular shape of the GCD curves was retained.Fig 6(c) shows the cyclic stability of NCS-2 over 1000 cycles at 5 A g1 There was only a 12% loss in the specific capacitance over 1000 cycles and 88% of the specific capacitance retention was achieved The GCD and the cyclic stability results of NCS-1 are shown inFigure SI-3 This study shows that NCS-2 is a material with highly reversible pseudo-capacitance and is a promising active electrode material candidate for use in supercapacitors
4 Conclusion
In summary, glycine was used as a templating agent in the hy-drothermal syntheses of nickel cobaltite and nickel cobalt sulfide The materials showed complex morphologies with features on the
Fig 5 (a) Cyclic voltammetry of NCO, NCS-1 and NCS-2 at 100 mV s-1scan rate, (b) cyclic voltammetry of NCS-2 at different scan rates, and (c) a plot of the scan rate vs specific capacitance.
Fig 6 (a) Galvanostatic chargeedischarge analyses of NCO, NCS-1 and NCS-2 at 1 A g 1 current density; (b) Galvanostatic charge discharge tests of NCS-2 at different current densities (1e5 A g 1 ); (c) The cyclic stability of NCS-2 over 1000 cycles at 5 A g1current density.
M.S Kumar et al / Journal of Science: Advanced Materials and Devices 4 (2019) 376e380 379
Trang 5size scale of tens of nanometers to several micrometers The
sam-ples were characterized by different techniques The FTIR studies of
nickel cobalt sulfide showed bands at 631 cm1 (symmetrical
stretch) and 1100 cm1(asymmetrical stretch) corresponding to
the NieS or CoeS vibrations of nickel cobalt sulfide These groups
play an important role in the faradaic redox reaction of the active
electrode material XRD analysis confirmed that the samples were
the cubic phases of nickel cobaltite and nickel cobalt sulfide,
respectively Cyclic voltammetry of both materials showed a typical
faradaic behavior with well-defined redox peaks Galvanostatic
charge and discharge experiments produced triangular plots,
indicating the good reversibility The specific capacitance
calcu-lated from the GCD studies for nickel cobalt sulfide was found to be
higher than that of nickel cobaltite (312.5 F g1) The nickel cobalt
sulfide produced by the two-step method involving an anion
ex-change outperformed (675 F g1) the directly synthesized nickel
cobalt sulfide (500 F g1) in terms of the specific capacitance The
two-step nickel cobalt sulfide maintained a specific capacitance
retention of 88% over 1000 cycles The superior performance of the
two-step nickel cobalt sulfide compared to the directly-synthesized
nickel cobalt sulfide and nickel cobaltite is its more complex
morphology and lower lamellar thickness This morphology yields
a greater interfacial contact between the electrode material and the
electrolyte and improves the ion transport across the interface,
which, in turn, contributes to its higher specific capacitance Thus,
nickel cobalt sulfide, especially when produced by a hydrothermal
anion exchange method, is a promising electrode material for
supercapacitors
Acknowledgements
The authors acknowledge Science and Engineering Research
Board (SERB) of the Department of Science and Technology (DST),
India (Research Grant ECR/2015/000208) and Department of Science
and Technology (DST), India for research grant DST/INT/RFBR/P-241
Appendix A Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jsamd.2019.08.005
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