Highly stable hollow bifunctional cobalt sulfides for flexible supercapacitors and hydrogen evolution

Article abstract of DOI:10.1039/c6ta03158g

Hollow structures of NiAs-type cobalt sulfide have been synthesized by a facile hydrothermal method. These hollow structured cobalt sulfides exhibit excellent electrochemical properties for supercapacitor applications (867 F g^-1) and respectable hydrogen evolution activity. The symmetrical supercapacitor device fabricated using cobalt sulfide nanostructures showed an areal capacitance of 260 mF cm^-2 with good flexibility and high temperature stability. The specific capacitance of the supercapacitor is enhanced over 150%, when the temperature is increased from 10 to 70 °C.

Full text of DOI:10.1039/c6ta03158g

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Highly stable hollow bifunctional cobalt sulfides for flexible
supercapacitor and hydrogen evolution
C. Ranaveeraa, Z. Wanga, E. Alqurashia, P. K. Kaholb, P. Dvornica, Bipin Kumar Guptac,
Received 00th January 20xx,
Accepted 00th January 20xx
Karthik Ramasamyd, Aditya D. Mohitee, Gautam Guptae and Ram K. Gupta*a
DOI: 10.1039/x0xx00000x
www.rsc.org/MaterialsA
Hollow structures of NiAs-type cobalt sulfide have been Presently, platinum is the most effective and durable catalyst for
synthesized by a facile hydrothermal method. These hollow hydrogen evolution reaction, but its wide spread use is precluded
structured cobalt sulfides exhibit excellent electrochemical due to its high cost and limited availability.
properties for supercapacitor applications (867 F/g) and
At this juncture it is essential to develop cost effective,
respectable hydrogen evolution activity. The symmetrical sustainable and stable materials for both supercapacitor and HER
supercapacitor device fabricated using cobalt sulfide nanostructure applications. Recently, transition metal chalcogenides have attracted
showed areal capacitance of 260 mF/cm2 with good flexibility and considerable attention for energy applications such as fuel cells,
high temperature stability. Specific capacitance of the supercapacitors and batteries due to their high abundance, low cost
11-15
supercapacitor is enhanced over 150 %, when the temperature is and decent performance2,
Wang et al have synthesized 3D
raised from 10 to 70 °C.
flower-like MoS2 nanostructures and were used as an electrode for
high-performance electrochemical capacitors16. The MoS2 electrode
showed a specific capacitance of 168 F/g and retained 92.6% of
Over the last few decades, the increasing demand for the energy
motivated the scientific community to develop high performance,
sustainable, cost-effective and environmentally friendly materials for
capacitance even after 3,000 cycles. Soon et al have reported a MoS2
17
based supercapacitor having
a
capacitance of
8
mF/cm2
.
energy conversion and storage applications1-4
. Undoubtedly,
Hierarchically porous nickel sulfide has been synthesized for both
HER and oxygen evolution reaction (OER)18. The synthesized nickel
sulfide showed overpotentials of 60 mV for HER and 180 mV for OER
in 1.0 M KOH at a current density of 10 mA/cm2, respectively. Sharma
et al have synthesised flower like ZnS for charge storage
application19. They observed a high specific capacitance (226 F/g)
and specific capacitance was observed to decrease by 60 % on
increasing the scan rate from 20 to 200 mV/s. Other metal sulfide
such as WS2 is also synthesized for HER, supercapacitors and
batteries applications20-22. As an important member of the transition
metal sulfide family, cobalt sulfide is considered to be one of the
promising candidates for such applications. Accordingly, various
phases of cobalt sulfides have been synthesized and studied for
materials for supercapacitors and hydrogen evolution are among the
widely studied ones for these purposes. Supercapacitors are
electrochemical energy storage devices primarily appealed for their
excellent power density and long cycle life4-7. Carbon based
electrodes are suitable for delivering the high power density but have
low energy density8. Nevertheless, in recent years the energy density
of supercapacitors has been boosted by the use of metal oxides,
which in turn sacrifices the cycle life4.
Second, energy generation using a cleaner and cheaper method
is of high importance due to increasing concerns of carbon emission.
Hydrogen generation by splitting water is highly desired. Hydrogen
evolution reaction (HER) is one of the key steps in water splitting
process9. Ideally, the thermodynamic potential for hydrogen
evolution reaction should be at 0 V (vs. SHE). However, without an
efficient catalyst, this reaction occurs at high overpotential10.
supercapacitor and HER applications 23,24-29
Xing et al have synthesized octahedron-shaped CoS2 crystals
using hydrothermal method for their application in
.
a
supercapacitors30. These CoS2 crystals showed a specific capacitance
of 237 F/g at 1 A/g with only a loss of 7.4% in the specific capacitance
after 2000 cycles. Sphere like CoS nanostructures were synthesized
and used as an electrode material for charge storage applications31.
These sphere like CoS showed very stable electrochemical
performance with only about 13% reduction in charge storage
capacity of the available capacitance (363 F/g) on increasing the scan
rate by 10 times. Ray et at have used a facile electrochemical
technique to fabricate titania nanotube-CoS composite electrode for
supercapacitor applications32. Cyclic voltammetric measurements
presented a very high specific capacitance (370 F/g) with great cycle
stability in KOH electrolyte. Since various phases of cobalt sulfides
a.Department of Chemistry, Pittsburg State University, 1701 S. Broadway,
Pittsburg, KS 66762, USA, E-mail: ramguptamsu@gmail.com
b.Department of Physics, Pittsburg State University, 1701 S. Broadway, Pittsburg,
KS 66762, USA, E-mail: ramguptamsu@gmail.com
c. National Physical Laboratory (CSIR), Dr K.S. Krishnan Road, New Delhi 110012,
India
d.Center for Integrated Nanotechnologies, Los Alamos National Laboratory,
Albuquerque, NM-87185, USA
e. Materials Physics and Application Division, Los Alamos National Laboratory, Los
Alamos, NM-87545, USA
Electronic Supplementary Information (ESI) available: [synthesis, characterization
and some additional figures]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 2016
J. Mater. Chem. A, 2016, 00, 1-3 | 1
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have been found useful for energy storage and catalytic applications,
it is highly desirable to synthesize cobalt sulfide using a facile and
efficient method to explore its multifunctionality. Herein, we report
a facile one step hydrothermal synthesis of hollow structured NiAs
type cobalt sulfide and investigate the material for supercapacitor
and HER applications. Our investigations show excellent
electrochemical properties for supercapacitor applications with a
specific capacitance of 867 F/g and an electrocatalyst for hydrogen
evolution reaction (Tafel slope of 97 mV/decade). The high
performance of the cobalt sulfide could be due to the unique
morphology.
View Article Online
DOI: 10.1039/C6TA03158G
Hollow structured cobalt sulfide was synthesized by reacting
cobalt nitrate with thioacetamide in ethylene glycol under a
hydrothermal condition at 180 oC for 12 h (details in ESI). Powder
XRD pattern of the black precipitate from the hydrothermal reaction
showed diffraction planes, predominantly matching with NiAs-type
hexagonal Co1-xS phase of cobalt sulfide (JCPDS file: 42-0826). The
diffraction peaks can be indexed to the (010), (011), (012), (120)
planes of Co1-xS (Fig. 1a). Scanning electron microscope image of the
cobalt sulfide obtained by a hydrothermal reaction exhibited
irregular shaped granules (Fig. 1b). The ratio of cobalt to sulfur
determined from SEM-EDX was found to be 0.98, close to the
expected composition for Co1-xS (Fig. 1S, ESI). To gain morphological
insight, TEM imaging was performed. TEM image in Fig. 1c shows
nearly uniform hollow ring like structures that are composed of
pseudo-spherical nanocrystals. It is widely accepted that the hollow
structures are consequence of difference in diffusion rates of
constituent elements33. A similar hollow structure has also been
reported by microwave assisted synthesis of cobalt sulfide33. Lattice
Fig. 2 (a) CV curves at various scan rates, (b) variation of specific
capacitance with applied current (inset figure show potential-
time curves at different applied currents, (c) capacitance
retention as a function of number of charge-discharge cycles
(inset figure shows first few cycles of charge-discharge curves),
and (d) CV curves at various bending angles of cobalt sulfide
electrode.
is much higher than commercial samples of CoS (0.39 m²/g and MoS2
(3.33 m²/g).
The electrochemical properties of electrode prepared using Co1-
xS hollow structures were studied using cyclic voltammetry (CV),
galvanostatic charge-discharge (GCD), and electrochemical
distance measured from HRTEM image (Fig. 1d) of Co1-x
S
impedance spectroscopy (EIS). The CV measurements of Co1-x
S
nanocrystals was close to 0.293 nm corresponding to (010) plane. In
addition, we have carried out BET nitrogen adsorption/desorption
isotherm measurements to determine the surface area of the sample
(Fig. 2S, ESI). The hysteresis loop in the isotherm of the synthesized
Co1-xS is the indication of capillary condensation, which arises due to
different mechanisms of adsorption/desorption in micropores. The
synthesized Co1-xS showed a specific surface area of 23.9 m²/g which
electrode in alkaline electrolyte (3M NaOH) displayed non-
rectangular shape curves with quasi-reversible redox waves in
potential range of 0 to 0.6 V (Fig. 2a). The unambiguous appearance
of redox waves, confirms the pseudocapacitive behaviour of Co1-xS.
As evident, the charge-storage mechanism in CoS is significantly
Faradic in nature than capacitive type. The Fadadic reaction
occurring on the surface of CoS electrode in alkaline media is given
below31:
퐶표푆 + 푂퐻− ↔ 퐶표푆푂퐻 + 푒−
퐶표푆푂퐻 + 푂퐻− ↔ 퐶표푆푂 + 퐻2푂 + 푒−
….. (1)
….. (2)
The specific capacitance of Co1-xS electrode estimated from CV
curves was found to be 867 F/g at 5 mV/s (Fig. 3S, ESI). Further, GCD
measurements exhibit symmetrical and non-linear charge-discharge
characteristics, confirming the pesudocapacitance behaviour of the
electrode (inset of Fig. 2b). The specific capacitance of cobalt sulfide
electrode determined from GCD measurements is about 420 F/g at 1
A/g, which is significantly higher than the value obtained for CoS
hollow nanosheets and hollow nanoprisms31, 33. Interestingly, the
specific capacitance value was nearly retained (402 F/g at 6 A/g) even
at higher current density (Fig. 2b), suggesting excellent rate stability
of Co1-xS hollow structures. In addition, we have evaluated the cyclic
stability of the cobalt sulfide electrode using GCD measurements.
From Fig. 2c, it can be witnessed that the specific capacitance was
almost preserved even after 5,000 charge-discharge cycles,
confirming their high electrochemical stability. It is noteworthy that
electrode using CoS2 ellipsoids lost nearly 50
% of specific
capacitance within 1,000 charge-discharge cycles34. Furthermore, we
have tested the flexibility of the cobalt sulfide electrodes by carrying
out CV measurements at different bending angles. The shape and
size of the CV curves were found to be identical at different bending
angles (Fig. 2d), suggesting that Co1-xS hollow structures are
exceptional candidates for flexible energy storage devices. The
Fig. 1: (a) XRD pattern (Inset: crystals structure of Co1-xS, (b)
SEM image, (c) TEM image (Inset: Schematic of hollow
structure composed of nanocrystals), and (d) HRTEM image of
synthesized cobalt sulfide (inset: Low magnification TEM
image of hollow structure.
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variation of the specific energy density versus power density
(Ragone plot) for cobalt sulfide is shown in Fig. 4S, (ESI). The highest
specific energy density of 19.4 Wh/kg and power density of 2644
W/kg was observed.
View Article Online
DOI: 10.1039/C6TA03158G
We fabricated the supercapacitor device by sandwiching an ion
transporting layer between two cobalt sulfide electrodes (inset of
Fig. 3b). Fig. 3a shows the CV curves of the supercapacitor device at
different scan rates. The shape of the voltammogram at various scan
rates are similar in nature, suggesting high charge transfer stability
of the device even at high scan rates. The near rectangular shape of
the CV curves indicates a near ideal capacitive behaviour. The
variation of specific capacitance as a function of scan rate for the
device is shown in Fig. 3b. The specific capacitance of the device was
observed to decrease with increase in the scan rate. The highest
areal capacitance of 261 mF/cm2 was observed at a scan rate of 5
mV/s. It was observed that specific capacitance of the devices
decreases with increasing scan rate, which is due to insufficient time
for the electrolyte to form an electrochemical double layer capacitor
at cobalt sulfide electrode. The galvanostatic charge-discharge
measurements were also performed to study the charge storage
capacity of the device. The charge storage capacity of the device
decreases with increasing current density (Fig. 5S, ESI). A specific
capacitance of 60 mF/cm2 was observed at 2 mA/cm2 of current
density. Further, the effect of temperature on the performance of
the supercapacitor device was studied. The CV curves of the device
at various temperatures are shown in Fig. 3c. The area under the CV
curves increases with increasing temperature, indicating
improvement in charge storage capacity of the device. It was
observed that the charge storage capacity of the device increases
Fig. 4 (a) Polarization curves for synthesized cobalt sulfide (CoS),
commercially samples of molybdenum sulfide (MoS2-CS) and
cobalt sulfide (CoS-CS) (inset figure show the Tafel plots) and (b)
Zreal vs. Zimag plots at different potentials for cobalt sulfide.
impedance of the supercapacitor device at various temperatures is
shown in Fig. 3d. The real and imaginary impedance of the device
decreases with increasing temperature. The equivalent series
resistance (ESR) of the supercapacitor device decreases with
increasing temperature (Fig. 7S, ESI). The decrease in ESR with
temperature could affect the charge storage properties of the
device. The increase in the temperature enhances the mobility of the
electrolyte ions and thus increases the conductivity of the electrolyte
ions35. The effect of temperature on the total impedance of the
device is shown in Fig. 8S, ESI. It was observed that the impedance of
the cobalt sulfide based supercapacitor device decreases with
increasing temperature. The total impedance of the supercapacitor
device was higher at lower frequency compared to that of at higher
frequency. Our observations based on electrochemical
characterizations suggest that the cobalt sulfide based
supercapacitor device performs even better at elevated temperature
than at room temperature.
The HER activity of the synthesized cobalt sulfide was also
studied using glassy carbon working electrode with a standard
rotating disk electrode instrument. All measurements were
performed in 1N H2SO4 (aq) electrolyte under continuously purging
with nitrogen gas (details in ESI). We have compared the HER activity
of commercially available molybdenum sulfide (MoS2-CS, Sigma,
USA) and cobalt sulfide (CoS-CS, Strem Chemicals, USA) with our
cobalt sulfide hollow structures. The polarization curves for the
samples are shown in Fig. 4a. Cobalt sulfide showed a lower onset
potential (0.21 V vs RHE) compare to MoS2-CS (0.31 V vs RHE) and
CoS-CS (0.35 V vs RHE). Our synthesized cobalt sulfide displayed
about 60 and 30 times higher current density at 0.30 V (vs RHE)
compared with MoS2-CS and CoS-CS, respectively. The HER activity
was further analysed using corresponding Tafel plots (inset of Fig.
4a). Tafel slope of 97 mV/decade, 174 mV/decade and 113
mV/decade was observed for synthesized cobalt sulfide, MoS2-CS
and CoS-CS, respectively. Lukowski et al have reported a Tafel slope
of 112 mV/decade for as such synthesized nanostructured MoS236. A
Tafel slope of 75 mV/decade and 90 mV/decade was reported for
CoS synthesized using microwave assisted and solvothermal
methods, respectively33. Sun et al have used electrodeposited cobalt-
sulfide for HER application and reported a Tafel slope of 93 mV/s in
phosphate buffer solution26. CoS2 synthesized using hydrothermal
method showed a Tafel slope of 72 mV/s in acidic media37. Peng et al
have synthesized CoS2/RGO/CNT nanocomposites for HER38. A Tafel
slope of about 51, 82 and 148 mV/s was observed for CoS2/RGO/CNT,
CoS2/RGO and CoS2, respectively. The rate of hydrogen evolution is
limited either by proton adsorption onto an active site or evolution
of the formed hydrogen from the surface. A high Tafel slope (120
mV/ decade) is indicative of proton adsorption (Volmer step) as the
rate-limiting step, while a lower Tafel slope (30 or 40 mV/decade)
o
o
over 150 % by increasing temperature from 10 C to 70 C (Fig. 6S,
ESI). The improved performance of the cobalt sulfide based
supercapacitor device could be due to activation of the cobalt sulfide
sites at higher temperature. In addition, the improvement could be
due to and improved mobility of the electrolyte ions and lower series
resistance at higher temperature.
The effect of temperature on the electrochemical behaviour of
the supercapacitor was further investigated using electrochemical
impedance spectroscopy (EIS). The variation of real and imaginary
Fig. 3 (a) CV curves of supercapacitor device at various scan rate
at room temperature, (b) variation of specific capacitance as a
function of scan rate (inset figure show schematic diagram of the
supercapacitor device), (c) CV curves of supercapacitor device at
different temperature, and (d) Zreal vs. Zimag plots for the device
at different temperature.
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indicates that the evolution of molecular hydrogen from the catalyst 4.
is rate limiting (Heyrovsky or Tafel step, respectively). Tafel slope of
97 mV/decade for Co1-xS suggest typical Volmer–Tafel route with the 5.
Volmer step as the rate determining step39. Electrochemical
impedance spectra of the cobalt sulfide were recorded at various 6.
potentials (Fig. 4b). As seen in the EIS spectra, the arc at zero
potential converges to a semi-circle at negative potentials. A 7.
decrease in semi-circle diameter is observed with increase in
negative potential. The charge-transfer resistance determined from
the semicircle registered at low frequencies was found to be 8.
overpotential-dependent. Similar observation was reported for
molybdenum phosphide9.
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behaviour up to 5,000 cycles of charge-discharge study with highly
flexible nature. The symmetrical supercapacitor device showed an 12.
areal capacitance of about 260 mF/cm2 which improved over 150 %
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available molybdenum sulfide. Our study suggests that cobalt sulfide
could be used for high temperature operable flexible energy storage
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14.
Acknowledgements
15.
16.
Dr. Ram K. Gupta expresses his sincere acknowledgment to the
Polymer Chemistry Initiative, Pittsburg State University for providing
financial and research support. This material is partly based upon
work supported by the National Science Foundation under Award
No. EPS‐0903806 and matching support from the State of Kansas
through the Kansas Board of Regents. Dr. Ram K. Gupta
acknowledges support from Department of Energy for Visiting
Faculty Program. Authors thank Dr. Lifeng Dong for recording
SEM/EDX. Drs. G. Gupta and A. Mohite thank los Alamos LDRD.
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Author contributions
RKG designed the experiments, run some structural and
electrochemical characterization of the devices, interpret and
analyze the data, and prepared the manuscript. CR, CZ, and EA
helped in synthesis and performed some electrochemical
characterizations. KR performed TEM measurement. GG and AM
guided the hydrogen evolution studies. PK, PD, KR, BKG and GG
reviewed the manuscript. All the authors reviewed and commented
on the manuscript.
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Additional information
The authors declare no competing financial interests. Requests for
materials should be addressed to RKG.
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Journal of Materials Chemistry A
Page 6 of 6
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DOI: 10.1039/C6TA03158G
Graphical abstract
Highly stable hollow bifunctional cobalt sulfides for flexible supercapacitor
and hydrogen evolution
C. Ranaveeraa, Z. Wanga, E. Alqurashia, Bipin Kumar Guptab, Karthik Ramasamyc, Aditya D. Mohited,
Gautam Guptad and Ram K. Gupta*a
Hollow structured cobalt sulfide exhibits excellent electrochemical properties for highly stable flexible
supercapacitor and hydrogen evolution reaction (HER).