Facile synthesis of a hierarchical CuS/CuSCN nanocomposite with advanced energy storage properties

Article abstract of DOI:10.1039/c8nj02843e

We introduce CuS/CuSCN nanocomposites as active materials in pseudocapacitors, in which the redox reactions of both CuS and CuSCN simultaneously contribute to energy storage. This nanocomposite is prepared using an in situ methodology via facile, low-energy-consuming green nanochemistry. The CuS/CuSCN nanocomposites offer a high capacitance compared to their individual constituents. CuS nanorods (～15 nm) are anchored on the surface of CuSCN nanosheets (～100 nm) and they interconnect the CuSCN nanosheets, producing mesoporous nanoclusters with a large surface area, thus improving the charge transfer efficiency. The CuS/CuSCN nanocomposites exhibit high electrical conductivity and strong redox reactivity, and in particular, the pseudocapacitor with a compositional ratio of 1:1 exhibits the highest charge transfer efficiency. Consequently, the 11 CuS/CuSCN active material exhibits a high energy density (approximately 63 W h kg^-1) and a high power density (1.9 kW kg^-1 at 9.0 W h kg^-1) as a single electrode. The highest specific capacitance is measured to be 1787.3 F g^-1 in the single electrode. Furthermore, an aqueous asymmetric hybrid supercapacitor based on the CuS/CuSCN 1:1//activated carbon (AC) shows an approximately four times increase in the power density (7.9 kW kg^-1), compared to the single electrode.

Full text of DOI:10.1039/c8nj02843e

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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DOI: 10.1039/C8NJ02843E
Facile synthesis of hierarchical CuS/CuSCN nanocomposite
with advanced energy storage properties
1
2
*,1
Yasun Y. Kannangara, Pichaimani Prabunathan, and JangꢀKun Song
1
School of Electronic and Electrical Engineering, Sungkyunkwan University, Suwon,
Gyeonggi-do 16419, South Korea
2
Department of Handloom and Textile Technology, Indian Institute of Handloom
Technology Salem, Salem-636001, India
*
jk.song@skku.edu
Abstract
We introduce CuS/CuSCN nanocomposites as active materials in pseudocapacitors, in
which the redox reactions of both CuS and CuSCN simultaneously contribute to energy storage.
This nanocomposite is prepared using in situ methodology via facile, lowꢀenergyꢀconsuming
green nanochemistry. The CuS/CuSCN nanocomposites offer high capacitance compared to their
individual constituents. CuS nanorods (~ 15 nm) are anchored on the surface of CuSCN
nanosheets (~ 100 nm) and they interconnect the CuSCN nanosheets, producing mesoporous
nanoclusters with a large surface area, thus improving the charge transfer efficiency. The
CuS/CuSCN nanocomposites possess high electrical conductivity and strong redox reactivity,
and in particular, the pseudocapacitor with the compositional ratio 1:1 exhibits the highest charge
transfer efficiency. Consequently, the 1:1 CuS/CuSCN active material exhibits high energy
density (approximately 63 Wh/kg) and high power density (1.9 kW/kg at 9.0 Wh/kg) as a single
electrode. The highest specific capacitance is measured to be 1787.3 F/g in the single electrode.
Furthermore, an aqueous asymmetric hybrid supercapacitor based on the CuS/CuSCN 1:1 //
activated carbon (AC) shows an approximately four times increase in power density (7.9
kW/kg), compared to the single electrode.
Keywords: supercapacitor; CuS/CuSCN; dual semiconductor active material; intercalated
pseudocapacitor

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1
. Introduction
In order to overcome the everꢀworsening energy crisis and global warming issues owing to
the overꢀuse of fossil fuels, various potential solutions, including the development of clean and
highꢀperformance energy technologies, have been intensively studied [1, 2]. Such efforts include
the development of clean energy sources such as solar cells and energy storage devices such as
batteries and supercapacitors [1, 3, 4]. A new version of energy storage devices, supercapacitors,
inherit the properties of both capacitors and batteries and possess significant advantages such as
high specific charge density, long cyclic stability, high specific capacitance, and excellent
charge/discharge behaviour [5, 6]. Two types of energy storage mechanisms are involved:
electrical doubleꢀlayer capacitance and redox electrochemical capacitance (or pseudoꢀ
capacitance); they enable the storage of energy in the form of electrostatic charges and reversible
redox potential, respectively, at the interfaces between electrode and electrolyte.
Recently, nanostructured transitionꢀmetal sulphides such as V O /Ni S , Ni S @CdS, SnS ,
2
5
3
2
3
2
2
CuS/Ni S with reduced graphene oxide (RGO), CuSꢀCNT (CNT: carbon nanotube), Co CuS ,
3
2
2
4
and Ni(OH) have attracted increasing attention as new electrode materials for pseudocapacitors,
2
owing to their high electrical conductivity and stronger redox property compared to transition
metal oxides [1, 5, 7ꢀ10]. CuS is one of the most promising candidates, and it is characterised by
ꢀ3
ꢀ1
its low cost, environmentallyꢀfriendly nature, metalꢀlike electronic conductivity (10 S cm ),
and high theoretical capacity [11]. Popular synthesising methodologies of CuS are the
hydrothermal method and refluxing [5, 12], which usually consume high energy during the
synthesis process.
The attention of researchers has been directed to composite materials rather than a single
metal sulphide compound [12]. CuS can be easily combined with carbon derivatives (CNT,

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RGO, activated carbon), which, however, exhibit shortcomings such as low capacitance and low
conductivity [5]. Goa et al. demonstrated that the agglomeration of graphene limits its
capacitance [1]. Lu et al. reported that CuSꢀCNT composites with organic molecular crosslinks
used as active materials unavoidably reduce the charge transfer capability [5]. Thus, there is a
strong demand for a better composite material with improved conductivity and high capacitance.
In this study, we fabricate a supercapacitor using hybrid nanocomposites of CuS and copper
thiocyanate (CuSCN) as a novel active electrode material. CuSCN is a remarkable inorganic pꢀ
type semiconductor [13ꢀ15], and has good charge mobility and high chemical stability [15ꢀ17].
However, combining two compounds of the same transition metal is not a usual approach to
prepare composite materials, and nanocomposites of CuS/CuSCN for supercapacitor applications
have not been reported to date. Recently, we reported that the conductivity of CuS/CuSCN
composite is better than that of either pure CuS or CuSCN [18]. Here, the CuS/CuSCN
nanocomposites are synthesised via an in situ ratioꢀtuneable synthesis methodology, in which the
compositional ratio of the two compounds is arbitrarily controllable. The CuS/CuSCN
nanostructure provides ultraꢀhigh specific capacitance by increasing the surface area of the active
material exposed to the electrolyte and increasing the conductivity. We clarify the importance of
the boosted charge conductivity for significantly enhancing the specific capacitance. The
morphology, thermal stability, and energy storage behaviour of the developed nanocomposites
are characterised.

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2
. Experimental
.1. Preparation of CuS/CuSCN composite
All the chemicals were of analytical grade and used without further purification. While the
2
conventional methods such as hydrothermal and refluxing methods use toxic hydrazine [5, 12],
we used the nonꢀtoxic Na S O as a reducing agent to prepare the CuS/CuSCN nanocomposite as
2
2
3
presented in Fig. 1. Initially, 100 mL of 0.1 M aqueous CuSO solution was mixed with 100 mL
4
of 0.1 M aqueous Na S O solution, followed by stirring for 30 min; subsequently, the colour of
2
2
3
the solution started to change from blue to light green. Subsequently, varying amounts of 0.1 M
NH SCN were slowly added in a dropꢀwise manner for several hours, and the solution turned
4
slightly whitish owing to the growth of CuSCN crystals. Thereafter, the mixture was stirred for
2
4 h at room temperature, and subsequently, dark green CuS crystals grew on the CuSCN
crystals. In this step, the amount of added 0.1 M NH SCN determined the composition ratio of
4
CuSCN. When NH SCN was not added, pure CuS crystals of green colour were obtained. By
4
adding 25 mL, 50 mL, and 75 mL of NH SCN in this step, the composition ratios of
4
CuS:CuSCN were controlled to be 3:1, 1:1, and 1:3, respectively. When 100 mL of NH SCN
4
was added, we could obtain pure CuSCN crystals. Thus, the compositional ratio could be easily
tuned by adjusting the amount of NH SCN in this step. Subsequently, the dispersion was filtered
4
and washed with deionised water and subsequently anhydrous ethanol several times each to
remove the unreactants. The CuS/CuSCN powder was dried in a vacuum oven at 60 °C for 6 h.

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Figure 1: Schematic diagram of the synthesising process of CuS, CuSCN, and CuS/CuSCN
nanocomposites
2
.2.
Characterizations
The crystalline structures of the materials were determined using powder Xꢀray diffraction
XRD) analysis with an automated Xꢀray diffractometer system (D8 Focus; Bruker, Germany)
(
with CuꢀKα radiation (λ = 1.5418 Å) from 15° to 65°. The morphologies of the nanostructures of
the CuS nanorods, CuSCN nanosheets, and CuS/CuSCN heterostructure were analysed using
fieldꢀemission scanning electron microscopy (FEꢀSEM, JEMꢀ7500F; JEOL Company, Japan)
and the atomic composition was mapped using energyꢀdispersive Xꢀray spectroscopy (EDX).
The compositions of the powder materials were analysed using Xꢀray photoelectron
spectroscopy (XPS) and thermogravimetric analysis (TGA) techniques from 30 °C to 1000 °C by
supplying N (100 mL/min) to ascertain the stoichiometry and chemical state of the species. The
2
XPS data were further processed using CasaꢀXPS software (Casa Software Ltd, UK) to
determine the chemical compositions.

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2
.3.
Electrochemical evolution measurements
The working electrode was fabricated by mixing the CuS/CuSCN composite and
polytetrafluoroethylene binder in the mass ratio of 90:10. Subsequently, the mixture was
dispersed in ethanol and loaded into nickel foam (MTI, Korea) as a current collector using the
drop and drying method. The prepared electrode was dried at 160 °C for 2 h. A platinum
electrode and Ag/AgCl electrode were used as the auxiliary and reference electrodes,
respectively, and 3 M KOH solution was used as the electrolyte. The redox capacitive
performances of the electrodes made of CuS/CuSCN composites were investigated using a threeꢀ
electrode system. Electrochemical impedance spectroscopic (EIS) measurements were obtained
using an electrochemical workstation—VSP (Bio Logic SAS, France)—and cyclic voltammetry
(CV) and galvanostatic charge discharge (GCD) measurements were collected using another
electrochemical workstation—WizECM premiumꢀ1600 (WizMAC, Korea). CV and GCD
analyses were performed in the potential window of 0–0.7 V with varying scan rates, and EIS
was performed under the application of 10 mV of AC voltage in the frequency range of 0.01 Hz
to 100 kHz. The specific capacitance, C (F/g), was calculated according to the following
s
equation:
Iꢀt
mꢀV
C =
S
,
(1)
where I (A) represents the discharge current, and m(g), ꢁV(V), and ꢁt(s) indicate the mass of the
active material, potential drop during discharge, and total discharge time, respectively. Other
important parameters of the performance of a supercapacitor are the specific energy density and
power density, which were calculated as follows:
2
0
.5×C ×V
S
ED (W ⋅ h / Kg) =
3
.6
,
(2)

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ED ×3600
PD (W / Kg) =
t
,
(3)
where V (V) is the voltage change during the discharge process after IR drop and t(s) is the
discharge time.
2
.4
Asymmetric supercapacitor
The CuSꢀCuSCN 1:1 // activated carbon (AC) asymmetric supercapacitor was fabricated to
analyse the capacitive behaviour of the asymmetric full cell configuration. The CuSꢀCuSCN 1:1
composite coated Ni foam, 3 M KOH, grade 1 Whatman cellulose filter paper, and AC coated Ni
foam were used as positive electrode, electrolyte, separator, and negative electrode, respectively.
The negative electrode was prepared following the method in the literature [19]. The specific
capacitance of the device was obtained from following formula:[20]
I × ꢀt
mtotal × ꢀV
Cdevice
=
(4)
Here, Cdevice (F/g) is specific capacitance of the asymmetric supercapacitor device; I (A) and ∆t
s) are the current and the time of the discharge process; mtotal (g) is the total mass of both
(
positive and negative electrode active materials; ∆V is the operating potential of the asymmetric
device.
The energy density and power density of the asymmetric supercapacitor device were
calculated using the equations: [21, 22]
1
2
ED = CdeviceꢀV
(5)
(6)
2
3
600× ED
PD =
t

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3
. Results and discussion
.1. Material characterizations
As shown in Fig. 1, the synthesis process is simple and nonꢀtoxic; moreover, the
3
composition ratio of CuS and CuSCN in the nanocomposites is arbitrarily determined by
adjusting the amount of NH SCN. The chemical reactions governing the syntheses of CuS and
4
2
+
CuSCN were precisely described in the literature [23, 24]. For CuS synthesis, Cu ions from
ꢀ
CuSO react with HS ions from Na S O and water. In the same reaction, by adding the same
4
2
2
3
molar fraction of NH SCN as that of CuSO in a dropꢀwise manner, CuSCN is produced instead
4
4
ꢀ
1+
of CuS because SCN has a stronger affinity towards Cu ions. Notably, while the synthesis of
CuS requires approximately 24 h, the synthesis of CuSCN occurs within a short time after adding
NH SCN. The reason for adding NH SCN in a dropꢀwise manner is to prevent the quick growth
4
4
of CuSCN grains and to obtain nanosheets of CuSCN crystals. By controlling the speed of
dropping and stirring, the crystal size can be controlled to some degree. When the molar fraction
of added NH SCN is less than that of CuSO , initially CuSCN nanocrystals are synthesised, and
4
4
subsequently, after fully consuming NH SCN, CuS is synthesised. Notably, CuS nanorods are
4
likely to grow on the surface of CuSCN nanosheets, producing hierarchical nanostructures.
Figure 2 reveals the structures of CuS, CuSCN, and the nanocomposite of CuS/CuSCN.
All the samples were synthesised under the same conditions except for the concentration of
NH SCN. On the one hand, CuS crystals have a petalꢀlike shape (Fig. 2a), and their surface
4
morphology is rough owing to the protruding nanorods but the core is a large lump with no
pores, as indicated in the bottom image of Fig. 2a. On the other hand, fully grown micrometreꢀ
sized CuSCN crystals are prismꢀshaped as shown in Fig. 2b, but the small crystals are round and
flat. In the case of the nanocomposite of CuS/CuSCN (Fig. 2c), larger flat CuSCN particles are

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covered by smaller CuS nanorods. Owing to the lower concentration of NH SCN, the size of the
4
CuSCN crystals is mostly smaller than that of the crystals in the neat CuSCN sample. Notably,
the CuS nanorods do not form a petalꢀshaped lump crystal, but have a distinguished rod shape.
The CuS nanorods cover the CuSCN particles, and many pores exist. Thus, the nanoporous
structure of CuS/CuSCN composite can allow the electrolyte to penetrate into the core particles
of the composite. The EDX spectrum in Supplementary Fig. S1 confirms that the material
compositions are wellꢀmatched to the target chemical structures. The atomic ratios of CuS and
CuSCN in the EDX analysis are also approximately matched to the chemical structures.
Figure 2: SEM images of (a) CuS, (b) CuSCN, and (c) nanocomposite of CuS/CuSCN. The
bottom images are the expanded ones.

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a
b
1
00
110
1
02 103
CuS
14.2 %
I
7
5
0
06
1
16
1
00
01
108
100
1
105
008
202
6
.2 %
6
.1 %
101
75
2.8 % II
003
100
CuSCN
1
1.8 %
104
107
5.1 % 9.9 %
006
75
III
100
003
1
3.7 %
3.2 %
1
02
75
100
11 %
16 %
CuS/ CuSCN
IV
1
01
1
03
0
06104
107110 108
116
202
1
01
105
19.9 %
1
00
008
75
V
10
20
30
40
50
60
70
150
300
450
600
750
900
o
2
Theta (degree)
Temperature ( C)
c
d
1
000
2
000
500
000
N 1S, -SCN
N 1S, -SCN
8
6
4
2
00
00
00
00
0
1
1
N 1S, -NCS
N 1S, -NCS
5
00
0
4
04
402
400
398
396
394
404
402
400
398
396
394
e3
3
500
000
500
000
500
000
Binding Energy (eV)
f
Binding Energy (eV)
7
000
N 1S, -SCN
6000
N 1S, -SCN
2
5
4
3
2
1
000
000
000
000
000
0
2
1
N 1S, -NCS
1
N 1S, -NCS
5
00
0
404
402
400
398
396
394
404
402
400
398
396
394
Binding Energy (eV)
Binding Energy (eV)
Figure 3: (a) XRD patterns of CuS, CuSCN, and CuS/CuSCN nanocomposite, (b) TGA
curves of (I) CuS nanorods; nanocomposites of ratios (II) 3:1, (III) 1:1, (IV) 1:3; (V) pure βꢀ
CuSCN nanosheets in N environment. (cꢀf) Highꢀresolution XPS spectra for nanocomposites
2
with ratios (c) 3:1, (d) 1:1, (e), 1:3 and (f) βꢀCuSCN nanosheets for the N1s region
The combination of these dual capacitive components in nanoꢀrange is well illustrated by the
crystallite sizes from the XRD patterns shown in Fig. 3a. The sharp peaks of CuS, CuSCN, and
CuS/CuSCN in the XRD pattern clearly demonstrate their good crystallinity. The crystalline
peaks values of CuS and CuSCN at 2θ are consistent with the peaks from the corresponding

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crystal planes shown in JCPDS cards (card no. 06–0464 for CuS and 29ꢀ0581 for CuSCN). The
sharp XRD peaks of the CuS/CuSCN 1:1 nanocomposite shown in Fig. 2 indicate that the
composite is also perfectly crystalline. The diffraction peaks of CuS/CuSCN are almost the same
as the combination of peaks from CuS and CuSCN, indicating no change in the phase structure.
TGA analyses of the composites were performed to study the thermal behaviour and
composition of their nanostructures. All the composites and CuS except CuSCN indicate that the
first weight loss starts in the temperature range of 200–320 ℃ , caused by the dehydration of
water content. CuS nanorods decompose to Cu S and sulphur at approximately 400–450 ℃ (Fig.
2
3
4
bꢀI).[25] As shown in Fig. 3bꢀV, CuSCN undergoes decomposition of approximately 20 % at
33–450 ℃ , producing CuCN and sulphur. The second decomposition with approximately 16 %
of mass loss appears in the range of 580–720 ℃ , which corresponds to the generation of CS2
gas.[26] The TGA curves for the three nanocomposites (CuS:CuSCN = 3:1, 1:1, and 1:3 in Figs.
3
b IIꢀI, respectively) show multiple decomposition drops with varying mass loss depending on
the chemical structures. The mass loss ratios corresponding to the generation of CS gas increase
2
as the composition ratio of CuSCN increases, which is consistent with the expected results. At
approximately 350–500 ℃ , the nanocomposites (Figs.3b II–IV) show two types of phase
decompositions of the two compounds. We believe that the first drop occurs owing to the
decomposition of CuSCN to CuCN and S and the second drop corresponds to the decomposition
of CuS to Cu S and S.[25, 26] As the composition of CuSCN increases, the first drop also
2
increases; this indicates that the compositional ratios of CuS and CuSCN in the nanocomposites
are well controlled as expected.

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We also performed a comprehensive XPS study to confirm the compositions of the
nanocomposites. The XPS survey spectra confirm the presence of copper, sulphur, carbon, and
nitrogen, and the results are shown in Supplementary Fig. S2. The accurate quantitative and
qualitative analyses for CuSCN and its composites were performed with their N1s peaks using
XPS, as shown in Figs. 3cꢀ3f. The spectrum of N1s has a major peak at 398.2 eV [13], which
corresponds to N in the nitrile form (N≡C) with a small shoulder. The intensities of these peaks
are proportional to the amount of –SCN groups, providing the information of the amount of
CuSCN. The N1s peak intensities continue to increase and accurately match the concentration of
CuSCN in the materials.
Recently, it was reported that the CuS/CuSCN nanocomposite has significantly improved
conductivity compared to either CuS or CuSCN [18]. The creation of copper vacancies during
the deposition of CuS on CuSCN particles is one of main reasons for the increase in the
conductivity. Smooth interconnection between CuSCN and CuS particles in the nanocomposites
also reduces the resistivity [18].
3
.2.
Electrochemical performance of supercapacitors
The electrochemical performances of supercapacitors with various electrodes were
investigated using threeꢀelectrode systems containing 3 M KOH aqueous electrolyte. For the
purpose of comparison, we also prepared a device with pure nickel metal form electrode (NF)
without any active material, and another device with a manual mixture prepared by grinding both
CuS and CuSCN materials in the composition ratio of 1:1. As shown in Fig. 4a, the CV curves of
all the samples measured at the scan rate of 100 mV/s in 3 M KOH solution confirm that the
mechanism and behaviour of charge storage are completely different from those of electrical
doubleꢀlayer capacitors with an approximately rectangular shape. The curves show that the

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supercapacitors using CuS, CuS/CuSCN, and CuSCN electrodes are all pseudocapacitors having
redox peaks. The corresponding electrochemical reactions for the CuS/CuSCN electrode redox
pair can be explained as follows:
−
−
CuS + OH ↔ CuSOH + e
(7)
(8)
−
−
CuSCN + OH ↔ CuSCNOH + e
b
a
A1
IR
A2
B1
B2
d
c
Figure 4: Comparison of pure CuS, CuSCN, and 1:1 nanocomposite with (a) CV curves under
the scan rate of 100 mV/s with 3 M KOH electrolyte, and (b) GCD curves with the current
density of 1.11 A/g. The comparison of nanocomposites with (c) CV curves under the scan rate
of 100 mV/s with 3 M KOH electrolyte, and (d) GCD curves with the current density of 1.11
A/g.

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As shown in the green CV curve with negligible CV integrated area in Fig. 4a, the NF itself
does not contribute capacitance. Hence, the CV curves of the other electrodes are attributed to
the capacitance contribution of the added active materials. The electrode containing either pure
CuS or pure CuSCN has relatively low specific capacitance, as indicated by the small integrated
areas of the corresponding CV curves in Fig. 4a. Notably, the electrode with the manual mixture
(not composite) exhibits higher capacitance than those of either CuS or CuSCN. Further, the
integrated area in the CV curve of 1:1 nanocomposite cell exhibits dramatic enhancement
compared to those using other materials. Although the manual mixture and 1:1 nanocomposite
have an identical composition ratio of CuS: CuSCN, the nanocomposite electrode exhibits much
higher capacitance than the manual mixture electrode. The nanocomposite has a novel
nanostructure of CuS nanosheets interconnected with CuSCN with a large surface area, which
enhances the charge exchanging ability with the electrolyte. CuS has its cathodic peak (A ) and
1
anodic peak (B ) at approximately 0.4–0.6 V and 0.2–0.3 V, respectively (see Supplementary
1
Fig. S3a for detailed curve structure), and CuSCN has its cathodic (A ) and anodic peak (B ) at
2
2
0
.2–0.4 V and 0.45–0.55 V, respectively (see Supplementary Fig. S3b). The CuS/CuSCN
nanocomposite electrode exhibits two cathodic (A , A ) and two anodic peaks (B , B ), which
1
2
1
2
approximately coincide with the peaks from pure CuS and CuSCN components. Thus, it is
confirmed that both CuSCN and CuS contribute to the redox response of the nanocomposite. The
GCD curves in Fig. 4b exhibit clear deviation of the discharge curve from a straight line,
representing a faradic redox supercapacitor discharge. The GCD curve of the nanocomposite
shows the longest discharging time at the current density of 1.11 A/g. Thus, the specific capacity
of the nanocomposite is remarkably larger than that of pure CuS, CuSCN, and the manual
mixture. This enhancement of the capacitance of the nanocomposite may be due to the

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combination of CuS and CuSCN faradic pseudocapacitance and improved conductivity.
Simultaneously, the novel porous nanostructure increases the contact area between the electrode
material and electrolyte and accordingly exploits the input of the electrochemical active material
to the overall specific capacitance.
Figure 5: (aꢀc) CV curves of 3:1, 1:1, and 3:1 nanocomposites at different scan rates,
respectively, and (dꢀf) the GCD curves of the same materials with different current densities,
respectively.
In order to optimise the compositional ratio in terms of capacitive capability, the
electrochemical behaviours of the three nanocomposites with varying ratios are also evaluated,
as shown in Figs. 4c and 4d. The 1:1 nanocomposite exhibits the largest integrated area
compared to the other two compositional ratios of 3:1 and 1:3 (Fig. 4c). Figure 4d shows the
GCD test of the nanocomposites within the potential window of 0–0.7 V, and the 1:1
nanocomposite exhibits the longest discharging time compared to the 3:1 and 1:3 composites.

New Journal of Chemistry
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DOI: 10.1039/C8NJ02843E
The specific capacitance of the 1:1 nanocomposite is 1787.3 F/g at 1.11 A/g. Thus, the specific
capacity of the 1:1 nanocomposite is larger than those of the composites with different mixing
ratios. This indicates that the 1:1 nanocomposite has an optimum structure with large specific
surface area and excellent electrical interconnection of CuS nanorods with CuSCN nanosheets.
The IR drop was 0.16 V, as shown in Fig. 4b, and all other GCD curves in Fig. 4d and Figs. 5eꢀf
show a similar level of IR drop.
Furthermore, the GCD curves (Fig. 4b and 4e) exhibited the intercalation pseudocapacitive
behaviour with KOH electrolyte, which has kinetics similar to the typical pseudocapacitive
behaviour [27]. The intercalation pseudocapacitive behaviour has the properties of both
pseudocapacitors and battery, and its electrochemistry is similar to that of battery [28]. The
similar intercalation behaviour with the electrolyte has been reported in the supercapacitors using
CuSbS and MnO [29, 30].
2
2
Figures 5a–5c show the CV curves of the nanocomposite electrodes measured at various
scan rates ranging from 50 mV/s to 500 mV/s in the range of 0–0.7 V with 3 M KOH. These
curves demonstrate the high rate capability and reversibility of the CuS/CuSCN nanocomposite
electrode even at high scan rates. Particularly, the CV curves of the 1:1 nanocomposite (Fig. 5b)
have the largest integrated area for all the scan rates than those of the other two composites (Figs.
5
a and 5c). Their electrochemical behaviour was further evaluated using GCD measurements
over a wide range of current densities. As shown in Figs. 5d–5f, the potential was in the range of
ꢀ1
0
–0.7 V with the current densities ranging from 1.11 to 10.5 Ag . The multiple flat stages in the
discharge part of GCD curves (Fig. 5dꢀ5f) of CuS/CuSCN NCs appear due to the
pseudocapacitive nature, indicating the redox reactions of both CuS and CuSCN [31]. These
discharge curves have two independent flat stages at approximately 0.2–0.3 V and 0.45–0.55 V,

Page 17 of 26
New Journal of Chemistry
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DOI: 10.1039/C8NJ02843E
representing the flat stages of pure CuS (Supplementary Fig. S4a) and CuSCN curves
Supplementary Fig. S4b), respectively. The specific capacitances of the optimum material, i.e.
(
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
the 1:1 composite, are 1787.3 Fg , 1267.5 Fg , 634.07 Fg , 318.2 Fg , 296.82 Fg , and 259.64
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ
Fg at the current densities of 1.11 Ag , 1.48 Ag , 2.00 Ag , 3.32 Ag , 6.18 Ag , and 10.5 Ag
1
,
respectively, as shown in Fig. 5e.
Reversibility and cycle stability are essential factors for the supercapacitor performance. For
further understanding, the 1:1 nanocomposite was evaluated using GCD cycles with different
current densities. As shown in Fig. 6a, the 1:1 nanocomposite at 1.11 A/g in 3 M KOH shows
perfect cycle reversibility. The stability of the 1:1 nanocomposite at the current density of 10.5
A/g was measured using GCD (Fig. 6b). The 1:1 nanocomposite shows the specific capacitance
retention 93.7 % after 2000 cycles, representing the exceptional cycling stability of CuS/CuSCN
nanocomposite. In the first 70 cycles, the capacitance retention was larger than 100 %, which
may be attributed to the gradual activation of the electrode [32].
a
b
c
1
M
1
00
⊗
⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
⊗
⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗
⊗
NCs 3:1
NCs 1:1
NCs 1:3
High Energy
& Power
⊗
⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗
⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗
⊗
⊗
⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗
100k Capasitors
⊗
⊗
⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗
⊗
⊗
⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗⊗
9
8
7
6
0
0
0
0
1
0k
1k
00
93.7 %
1
1
0
Supercapacitors
Batteries
1
Fuel Cells
50
100m
10m 100m
0
50
100
150
200
0
500
1000
1500
2000
1
10 100 1k 10k 100k
Time (s)
Number of Cycles
Energy Density (Wh/ kg)
Figure 6: (a) GCD curve of the 1:1 nanocomposite at the current density of 1.11 A/g, and (b) the
cyclic performance of nanocomposites at the current density of 10.5 A/g within the potential
window of 0ꢀ0.7 V (vs. Ag/AgCl) in 3 M KOH electrolyte. (c) Ragone plot of specific power vs.
specific energy of the CuS/CuSCN nanocomposites within full range of all the positive electrode.

New Journal of Chemistry
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DOI: 10.1039/C8NJ02843E
Ragone plots (power density vs. energy density) of the nanocomposites are shown in Fig. 6c.
The plot shows that the pseudocapacitors using the proposed active material are located between
the conventional supercapacitor and battery applications. Researchers in this field aim to achieve
higher power density and high energy density simultaneously, indicated by the arrow in Fig. 6c.
The devices using our nanocomposites exhibit slightly higher energy density than the
conventional supercapacitors and slightly higher power density than the conventional batteries.
Notably, the energy density of the pseudocapacitor using the 1:1 nanocomposite reaches
approximately 62.0 W℃ h/kg at the power density of 198.4 W/kg, which is superior to that of the
other CuS/CuSCN nanocomposites (40.1 W.h/kg at the power density of 182.7 W/kg in the case
of the 3:1 nanocomposite, and 43.3 Wh/kg at the power density of 182.7 W/kg in the case of the
1
:3 nanocomposite). The high energy density in 1:1 nanocomposite is close to the value of usual
battery, as indicated in Fig. 6c, and this may be attributed to the effect of intercalation
pseudocapacitive behaviours. The 1:1 nanocomposite exhibits an outstanding power density of
1
873.5 W/kg at the low energy density of 9.0 Wh/kg, indicating greater power capability.
According to literature, the energy density of CuS nanoꢀhollow spheres is 47. 4 Wh/kg at 0.3
W/kg [33] and that of CuS microspheres is 15.06 Wh/kg at 392.9 W/kg [34]. Thus, the result
obtained using the 1:1 CuS/CuSCN nanocomposite is much greater than that obtained using any
of these materials including CuS. In particular, our material is superior to other typical
supercapacitor materials; for example, the energy density of pomelo peel activated carbon is 12.8
Wh/kg at 3854 W/kg [35], that of Ni(OH) nanospheres is 35.7 Wh/kg at 490 W/kg, that of
2
Ni(OH) /graphene composition is 48 Wh/kg at 0.23 kW/kg, and that of RuO /graphene
2
2
composition is 14 Wh/kg at 21 kW/kg [36].

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The Nyquist plot (Fig. 7) intercepts the Xꢀaxis at R (solution resistance) in the highꢀ
S
frequency area; R includes the sum of electrolyte solution resistance [37], intrinsic resistance of
S
the active material, and contact resistance of the electrode–electrolyte interface [38]. The values
of R of the 3:1, 1:1, and 1:3 nanocomposites are 2 ꢂ, 3 ꢂ, and 3.6 ꢂ, respectively. R gradually
S
S
increases with the increase in CuS content of the nanocomposite from 1:3 to 3:1, which suggests
that CuS acts as the interconnection agent between the composite nanoparticles. The 3:1
nanocomposite has the lowest solution resistance among the nanocomposites. The charge
transfer resistance, R , is represented by the diameter of the semicircle in the highꢀfrequency
ct
region [38, 39]. It is evident that the 1:1 nanocomposite has the lowest R value (0.5 ꢂ) as
ct
compared to the other nanocomposites (2 ꢂ for 3:1 and 7.2 ꢂ for 1:3 composites). This indicates
that the 1:1 composite possesses lower charge transfer resistance. The straight line in the lowꢀ
frequency area appears owing to the ion diffusion behaviour [37]. When the concentration of
CuS is gradually increased, the steepness of the slope of the curves increases, indicating faster
ion diffusion and charge adsorption into the electrode surface at a high concentration of CuS
[
40]. Although the 1:1 nanocomposite has higher R than the 3:1 nanocomposite, R dominantly
S ct
influences the electrochemical performance. Hence, the 1:1 nanocomposite with the lowest Rct
exhibits the highest electrochemical performance and the highest capacitance [41]. These results
reveal that good electrical conductivity and high ion diffusion behaviour of the 1:1
nanocomposite result in its high performance as an active material for a supercapacitor. Even
with a very low internal resistance, the voltage drop can be significant when the current is high,
as shown in Fig. 4b.

New Journal of Chemistry
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DOI: 10.1039/C8NJ02843E
Figure 7: Nyquist plots for the 3:1, 1:1, and 1:3 nanocomposites from left to right
To further validate the practicability of CuSꢀCuSCN nanocomposites for supercapacitor
application, we prepared an asymmetric hybrid supercapacitor using the CuSꢀCuSCN cathode
and an AC anode. The CV curves of the AC (see supplementary Fig. S5) and CuSꢀCuSCN 1:1
NC at 500 mv/s tested in the three electrode system are shown in Fig. 8a, where the AC and 1:1
NC operated at ꢀ0.8 ─ 0.0 V and 0.0 ─ 0.7 V, respectively, revealing the capability of these two
electrodes. The assembled hybrid supercapacitor can be operated in 1.5 V, as confirmed in Fig.
8
b. Degradation of aqueous KOH and gas generation can occur when the applied voltage
exceeds 1.5 V, and so, 1.5 V was selected to investigate the electrochemical performance of the
device. Fig. 8c shows that the supercapacitor can be operated from 0.0 V─ 1.0 V to 0.0 V─ 1.5 V
at 400 mV/s. GCD curves shown in Fig. 8d clearly exhibit the perfect charge/discharge
mechanism. The specific capacitance calculated from the GCD curves at different current
densities showed the highest capacitance 657.86 F/g at the current density of 1.11 A/g. In other
conditions, the capacitances were 538 F/g, 298.66 F/g, 274.5 F/g, 273.9 F/g and 238 F/g for the
current densities of 1.48 A/g, 2.0 A/g, 3.32 A/g, 6.18 A/g and 10.5 A/g, respectively.

Page 21 of 26
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DOI: 10.1039/C8NJ02843E
a
b
c
0
.03
0.03
0.02
0.01
0.00
-0.01
-0.02
-0.03
0
0
0
0
0
.8
.6
.4
.2
.0
1.0 V
AC
1
1
.2 V
.4 V
0.02
CuS/CuSCN 1:1 NC
0
.01
.00
1.5 V
0
4
00 mv/s
300 mv/s
00 mv/s
-
0.2
0.4
0.6
-0.01
0.02
0.03
2
-
-
100 mv/s
50 mv/s
-
-
-
0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
Potential (V)
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
Potential (V)
Potential (V) vs Ag/AgCl
d
e
f
1
1
1
1
0
0
0
0
0
.6
.4
.2
.0
.8
.6
.4
.2
.0
3
1
1
.11 A/g
.48 A/g
8.0x10
CuS-CuSCN 1:1 NC // AC
CuS-CuSCN 1:1 NC
2.00 A/g
3
3
6
.32 A/g
.18 A/g
6.0x10
10.5 A/g
3
4
.0x10
3
2
.0x10
0.0
0
400
800
1200
1600
0
2
4
6
8
10
Time (s)
Current density (A/g)
Figure 8: (a) CV curves for AC and CuSꢀCuSCN 1:1 NC electrodes at the scan rate of 500
mV/s. (b) CV curves for the CuSꢀCuSCN 1:1 NC//AC hybrid supercapacitor at various scan
rates. (c) CV curves in different potential windows for the hybrid supercapacitor at 400 mV/s. (d)
GCD curves for the hybrid supercapacitor device at various current densities. (e) Power densities
as a function of current densities for the hybrid supercapacitor and a CuSꢀCuSCN single
electrode. (f) A photograph of two CuSꢀCuSCN 1:1 NC//AC hybrid supercapacitors connected to
2
47 LEDs.
The power density as a function of current density is presented in Fig. 8e. The maximum
power density of 7.87 kW/ Kg was recorded for the hybrid device at the current density of 10.5
A/g. It clearly shows that the power density increases roughly 4 times by assembling the CuSꢀ
CuSCN 1:1 NC cathode and the AC anode, compared to the single electrode measurement. It is
known that carbon based material is suitable for an anode for supercapacitor with high power
density [42, 43]. In addition, the use of hybrid device increases the operating potential window,

New Journal of Chemistry
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DOI: 10.1039/C8NJ02843E
which is essential to expand its applicability. Using two hybrid supercapacitors connected in
series, a panel of 247 LEDs composed of 134 red LED (L813SRDꢀC, Kingbright Electronics Co.
Ltd., Taiwan) and 113 green LED (L813GD) was easily lighted up, as shown in Fig. 8f. The
operational voltages of green and red LEDs are around 2.2 V and 1.8 V, respectively. The 247
LEDs were arranged in a parallel connection, and so, the minimum operational voltage to light
the LED panel is 2.2 V. The CuS/CuSCN 1:1 NC//AC hybrid supercapacitor device has the
maximum operational voltage of 1.5 V. Therefore, we connected two hybrid supercapacitor
devices in series to double the maximum operational voltage, which can easily light the LED
panel.
4
. Conclusion
We synthesised a dual pseudocapacitive CuS/CuSCN nanocomposite using an inꢀsitu
methodology in which the compositional ratio between the CuS and CuSCN in the
nanocomposites could be controlled in a facile manner, and mesoporous CuSꢀcoveredꢀCuSCN
nanocrystals with large surface area could be obtained. The novel heteroꢀnanostructure could be
characterised as CuSCN nanosheets wrapped by CuS nanorods. We used the CuS/CuSCN
nanocomposites as an active material for pseudocapacitors. Both CuS and CuSCN demonstrated
pseudocapacitive behaviours independently and their nanocomposites achieved high
electrochemical performance. Consequently, the fabricated CuS/CuSCN 1:1 nanocomposite
electrode exhibited a high specific capacitance of 1787.3 F/g at 1.11 A/g and high energy density
(
approximately 63 Wh/kg) at the operating voltage of 0.7 V in 3 M KOH for the halfꢀcell test. In
the asymmetric hybrid cell combined by AC electrode, the specific capacitance decreased to
57.86 F/g at 1.11 A/g, but the power density increased approximately four times. The high
6

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energy density may be partially attributed to the effect of intercalation pseudocapacitive
behaviours. The supercapacitor reported in the present study has unique characteristics that
exceed those of some of the best reported supercapacitors operated in aqueous electrolytes. The
superior long GCD cycles demonstrated that it can be applied in both supercapacitors and
batteries. Such superior capacitive behaviour is attributed to the hierarchical novel nanostructure
assembled from CuS nanorods with CuSCN nanosheets. Thus, this study paves the way for the
use of CuS/CuSCN electrodes in electrochemical energy storage materials with high capacity
and fast rate capability.
Acknowledgement
This work was supported by the Korea Research Fellowship Program through the National
Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (NRFꢀ
2
016H1D3A1938043).
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Table of Contents
Facile fabrication of CuS/CuSCN nanocomposite electrode for a supercapacitor with high specific
capacitance (1787.3 F/g)