Improved performance of a CoTe//AC asymmetric supercapacitor using a redox additive aqueous electrolyte

Article abstract of DOI:10.1039/c7ra12919j

Cobalt telluride (CoTe) nanosheets as supercapacitor electrode materials are grown on carbon fiber paper (CFP) by a facile hydrothermal process. The CoTe electrode exhibits significant pseudo-capacitive properties with a highest C_m of 622.8 F g^-1 at 1 A g^-1 and remarkable cycle stability. A new asymmetric supercapacitor (ASC) is assembled based on CoTe (positive electrode) and activated carbon (negative electrode), which can expand the operating voltage to as high as 1.6 V, and has a specific capacitance of 67.3 F g^-1 with an energy density of 23.5 W h kg^-1 at 1 A g^-1. The performance of the ASC can be improved by introducing redox additive K₄Fe(CN)₆ into alkaline electrolyte (KOH). The results indicate that the ASC with K₄Fe(CN)₆ exhibits an ultrahigh specific capacitance of 192.1 F g^-1 and an energy density of 67.0 W h kg^-1, which is nearly a threefold increase over the ASC with pristine electrolyte.

Full text of DOI:10.1039/c7ra12919j

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Improved performance of a CoTe//AC asymmetric
supercapacitor using a redox additive aqueous
electrolyte
Cite this: RSC Adv., 2018, 8, 7997
Beirong Ye, Chao Gong, Miaoliang Huang, * Yongguang Tu, Xuanqing Zheng,
Leqing Fan, Jianming Lin and Jihuai Wu
Cobalt telluride (CoTe) nanosheets as supercapacitor electrode materials are grown on carbon fiber paper
(
CFP) by a facile hydrothermal process. The CoTe electrode exhibits significant pseudo-capacitive
ꢀ1
ꢀ1
properties with a highest C
m
of 622.8 F g at 1 A g and remarkable cycle stability. A new asymmetric
supercapacitor (ASC) is assembled based on CoTe (positive electrode) and activated carbon (negative
electrode), which can expand the operating voltage to as high as 1.6 V, and has a specific capacitance of
Received 30th November 2017
Accepted 10th February 2018
ꢀ1 ꢀ1 ꢀ1
6
7.3 F g with an energy density of 23.5 W h kg at 1 A g . The performance of the ASC can be
improved by introducing redox additive K
that the ASC with K Fe(CN)
4
Fe(CN)
6
into alkaline electrolyte (KOH). The results indicate
DOI: 10.1039/c7ra12919j
ꢀ1
4
6
exhibits an ultrahigh specific capacitance of 192.1 F g and an energy
ꢀ1
rsc.li/rsc-advances
density of 67.0 W h kg , which is nearly a threefold increase over the ASC with pristine electrolyte.
outstanding advantage is to realize the great working potential
window. ASCs are an improvement of the symmetrical super-
1
. Introduction
Recently, as a new type of energy storage device, supercapacitors capacitors, which contain two different type of electrode,
have received extensive attention, owing to their advantages of a capacitor-type electrode (power source) and a battery-type
high specic capacitance, short charging time and excellent faradic electrode (energy source), and can enlarge the oper-
1
–3
cycling ability. The energy storage mechanism of a super- ating voltage in the whole system by take advantage of the
8,9
capacitor is either the accumulation and electrostatic adsorp- different voltage windows of both electrodes. Therefore, the
tion of charge on the interface between the electrode and the key to fabricate an ASC is to select suitable negative electrode
electrolyte (electrical double layer capacitors) or fast, reversible and positive electrode materials which can work in different
redox reaction on or near the surface of the electrode (pseudo- potential windows and can be effectively combined together, for
4
capacitors). Among them, owing to their better cycle stability the same electrolyte.
and higher power density than conventional batteries, electrical
Currently, the negative electrodes of ASC choose all kinds of
double layer capacitors have becoming the most common high surface area of carbon materials, among them, activated
supercapacitors. However, they still present a lower energy carbon is the most common used material. Activated carbon
ꢀ
1
5
density (3–6 W h kg ). The reason is that EDLCs cannot make material with low-cost, stable performance, good conductivity
full use of the body phase of the electrode material for energy and easy preparation, is an ideal negative material. Neverthe-
storage, and they are unable to expand the voltage window of less, the electrochemical capacitance of an ASC relies mainly on
the capacitor by using the difference of the positive and negative the positive electrode materials. Thus, during past few years,
electrode. Hence, for the sake of the needs of practical appli- metal-based electrode materials, such as RuO
2
, MnO
2 3 2
, Ni S ,
10–17
cation, much more efforts have been focused on improving the etc.,
have been exhibited excellent electrochemical proper-
energy density of supercapacitors. According to the calculation ties and especially high specic capacitance.
2
formula of energy density: E ¼ 1/2 CV , the energy density of
However, compared to transition metal oxides, suldes and
supercapacitors can be improved by increasing capacitance (C) selenides which are frequently investigated, up to now, the
6
,7
or enlarging the operating voltage (V). A promising approach transition metal tellurides as the electrode material have rarely
is to assemble asymmetric supercapacitors (ASCs). Asymmetric been reported. In the periodic table, tellurium is in the transi-
supercapacitors, namely hybrid supercapacitors, their tion of metal to non-metal, and its physical properties and
chemical properties are between metal and non-metal, which
means it having good conductivity. And previous studies
exhibited most of the metal tellurides possess a superior
Engineering Research Center of Environment-Friendly Functional Materials, Ministry
of Education, Institute of Materials Physical Chemistry, College of Materials Science
stability. For instance, a hybrid Te/Au/MnO was fabricated by
2
and Engineering, Huaqiao University, Xiamen, Fujian, 361021, P. R. China. E-mail:
18
Cao et al. and used as supercapacitor electrode with specic
huangml@hqu.edu.cn; Tel: +86-592-6162255
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ꢀ1
capacitance of 930 F g and 3% largest loss of the specic 2.2 Synthesis of CoTe
capacitance compared with the rst cycle during 1000 cycles.
CoTe electrode material was prepared by hydrothermal process.
Typically, the reaction solution of hydrothermal contained
1
9
Zhou et al. synthesized NiTe rods on Ni-foam by a hydro-
thermal method with the highest specic capacitance of 804 F
g
1
0
.3 mmol of Na
2
TeO
3
, 0.3 mmol of Co(NO
3
)
2
$6H
2
O and 55 mL
2
$H O
ꢀ1
and remained 91.3% of initial specic capacitance aer
000 cycles. Some efforts are still needed to enroll conductivity
and energy storage ability of tellurium into intrinsic metal
telluride based supercapacitors. The cobalt being one of the
metal ions with multiple oxidation states (Co , Co and Co )
and high reduction potential, CoTe could serve the purpose of
high potential window, high energy density and power density,
etc. required for supercapacitor. In addition, in analogy to other
of deionized water. Aer magnetic stirring, 15 mL of N
2
H
4
was added with vigorous stirring. Then the aqueous solution
was transferred to a 100 mL Teon-lined autoclave. Aer being
stirred evenly, a piece of washed CFP was installed in the
autoclave and immersed in reaction solution. The autoclave was
2
+
3+
4+
ꢁ
sealed and heated at 140 C for 12 h, and black sample on CFP
was obtained from autoclave aer hydrothermal process.
Finally, product was rinsed several times with water and
20,21
22
transition metal chalcogenides, such as Co
3
O
4
,
CoS and
ꢁ
ethanol, and then dried in vacuum at 60 C for 12 h. The mass of
16
Co0.85Se, which has been demonstrated to possess excellent
pseudo-capacitive performance, CoTe probably is also a kind of
promising electrode material for supercapacitors.
Besides, further to improve the performance of super-
capacitors, an innovative approach is explored that introducing
ꢀ2
CoTe on CFP was about 1.2 mg cm
.
2.3 Fabrication of asymmetric supercapacitor
The as-prepared CoTe on CPF as the positive electrode and
active carbon as the negative electrode were used to construct
the asymmetric supercapacitor (ASC) with 3 M KOH aqueous
solution as the electrolyte. Activated carbon (80%), PEFT
aqueous solution (10%) and acetylene black (10%) was used to
synthesis the activated carbon (AC) electrode, which was then
23
24
redox additive into the electrolytes. Such as VOSO , KI,
4
25
26
27
hydroquinone, indigo carmine and polysulde et al. The
specic capacitance of supercapacitors is notably enhanced as
well as energy density due to the additional pseudo-capacitance
28
deriving from redox additives. Especially, through introducing
ꢁ
pressed on nickel foam and dried at 70 C for 12 h.
K
3
Fe(CN)
6
/K
4
Fe(CN)
6
couple into KOH aqueous solution for
ꢁ
NiO electrode at ꢀ20 C, the specic capacitance of NiO rose
29
twice and superior to the performance at room temperature.
2.4 Characterization
Later, the electrochemical property of active carbon-based
capacitor has been enhanced through adding two redox addi-
Field emission scanning electron microscopy (FESEM) and
powder X-ray diffractions (XRD) were used to investigate the
morphology and structure characteristics of the samples.
The CPF loaded with CoTe was used as bind-free positive
electrode without further treatment. All the electrochemical
properties (including CV (cyclic voltammetry), GCD (galvano-
static charge–discharge), EIS (electrochemical impedance
spectroscopy)) of CoTe nanosheets electrode were carried out on
a CHI660E electrochemical workstation by using 3 M KOH
aqueous as electrolyte with three-electrode system. The long
cycle life of the CoTe electrode was evaluated using LAND
battery test system (CT2001A). The Hg/HgO and platinum plate
as reference and counter electrode, and CoTe nanosheets on
CFP as working electrode. Furthermore, a two-electrode system
was carried out to measurement the electrochemical properties
of CoTe//AC ASC in a 3 M KOH aqueous solution. In this
process, all the electrochemical measurements of single elec-
trode were performed to value the electrochemical properties of
CoTe//AC ASC except EIS test.
tives of K
into the KNO
4
Fe(CN)
6
and anthraquinone-2,7-disulphonate (AQDS)
30
3
aqueous solution. However, this method so far
has not been applied to the ASC for further improving its
specic capacitance as well as energy density.
Carbon ber paper (CFP) has been oen used as supporter/
or current collector owing to its excellent characteristics of good
conductive, light-weight, high surface area and chemical inert-
20,31,32
ness.
In this paper, cobalt telluride (CoTe) nanosheets
were successfully synthesized on CFP through a simple in situ
growth method, and the CoTe electrode displayed favorable
electrochemical properties. Then, an asymmetric super-
capacitor (ASC) was assembled by employing CoTe as positive
electrode and AC (activated carbon) as negative electrode, which
could extend voltage window to 1.6 V. Finally, potassium hex-
acyanoferrate(II) (K Fe(CN) ) served as redox additive was
4
6
introduced into KOH electrolyte for the ASC to further increase
the value of capacitance and energy density.
m
The specic capacitance (C ) of the single electrode
33
expressed by the following eqn (1):
2
. Experimental section
I ꢂ Dt
C
m
¼
(1)
2
.1 Materials
Co(NO $6H O and Na
Industrial Corporation. Anhydrous ethanol, N
and polytetrauoroethylene (PTFE) aqueous solution (60 wt%) discharge current and the discharge time, respectively.
m ꢂ DV
3
)
2
2
2
TeO
3
were purchased from Aladdin wherein m (g) was the mass of the active substance of electrode.
2
H
4
$H O (80 wt%) DV (V), I (A), and Dt (s) represented the voltage window, the
2
were from Sinopharm Chemical Reagent Co., Ltd. Activated
carbon (AC) (surface area 2167 m g ) was from Fuzhou Yihuan (E, W h kg ) of supercapacitor was calculated using the
Co., Ltd. Carbon ber paper was from Cetech Co., Ltd. None of established eqn (2), and power density (P, W kg ) was obtained
the reagents have been further puried.
Other two key factors to evaluation the energy density
2
ꢀ1
ꢀ1
ꢀ
1
33
according to eqn (3):
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1
2
used to study the composition of samples. As shown in Fig. 1(b)
and (c), the uniform distribution of cobalt and tellurium were
found with their corresponding ratio of 1 : 1 in the sample,
indicating the samples mainly composed of CoTe materials.
The exist of small amount of oxygen which may come from the
2
E ¼
C
m
ðDVÞ
(2)
(3)
E
P ¼
Dt
34
absorption of oxygen and moisture on the samples' surface.
The corresponding chemical reactions to form CoTe were as
3
3
. Results and discussion
.1 Structural and morphological characterization of CoTe
35
follows
2
+
+ 2TeO32ꢀ
+ 3N H $H O /
2
Co
For the sake of investigate the crystal phase and structure
information of products, XRD pattern was measured and shown
in Fig. 1(a). The black samples were scratched from the CFP
substrate to avoid the effect of carbon signal. Obviously, the six
2
4
2
2
Co + 2Te + 3N [ + 6H O (4)
2
2
Co + Te / CoTe
(5)
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
strong diffraction peaks (31.3 , 43.0 , 46.7 , 58.3 , 65.3 , 76.9 )
in this pattern can be readily indexed to CoTe (JCPDS no. 34-
The morphology and microstructure of the CFP and CoTe on
0420) and no impurity diffraction peak was found. The result of
CFP were observed by FESEM. Fig. 2(a) showed the carbon ber
paper (CFP) was made of countless carbon bers to weave into
an alternate network structure, which could provide good
XRD pattern indicated that CoTe had successfully grown on CFP
by a facile hydrothermal method. Moreover, energy dispersive
spectrometry (EDS) analysis and elemental mapping were also
Fig. 1 (a) XRD spectrum of CoTe, (b) EDS pattern of CoTe, (c) elemental mappings with different color for different elements.
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Fig. 2 FESEM images of CoTe on CFP and pure CFP: (a) and (b) pure CFP, (c) and (d) CoTe on CFP (the inset was the FESEM image at high
magnification); (e) and (f) HRTEM image of CoTe.
mechanical exibility and ne transmission performance. That the sheet-like CoTe arrays uniformly covered the surface of CFP
is to say, electron still could successfully pass the network and interconnected with each other, which could conducive to
structure in the case of a minority of carbon ber fracture. the full contact of the electrolyte with the active material surface
Before participating in the hydrothermal reaction, the surface of and promote the rapid, reversible redox reaction of the elec-
CFP was smooth as shown in Fig. 2(b), which was carefully trode, thereby obtaining a larger pseudocapacitance. Trans-
washed by alcohol, acetone and distilled water in turn. Fig. 2(c) mission electron microscopy (TEM) measurements were carried
and (d) showed the FESEM images of as-synthesized CoTe on out to further investigate the structure of the as-synthesized
the CFP substrate at low and high magnication, respectively. CoTe nanosheets. As can be seen from Fig. 2(e) and (f), the
The surface of the CFP substrate became rough aer the CoTe nanosheets exhibit interconnected morphology with
hydrothermal reaction which could be clearly seen in Fig. 2(c). transparent feature, indicating the ultrathin nature.
In order to observe the microstructure of CoTe, we employed the
To further conform that the CoTe was successfully prepared
FESEM image of high magnication (Fig. 2(d)). As can be seen, using this hydrothermal method, X-ray photoelectron
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spectroscopy (XPS) was performed to analyze the surface
Fig. 4(c) presented the GCD curves of CoTe electrode at
elements and their valence state of the CoTe, and the obtained different current densities. All of the curves were to keep good
results were shown in Fig. 3. For Co 2p spectrum (as shown in symmetry during the charge–discharge process, which indi-
Fig. 3(a)), the binding energy at 783.08 eV, 781.05 eV, 801.26 eV, cated CoTe electrode had a good electrochemical capacitive
2
+
3+
2+
3+
16,40
and 797.23 eV were Co 2p3/2, Co 2p3/2, Co 2p1/2 and Co
1/2, respectively, suggesting that the Co atom in the as- specic capacitances (C
synthesized CoTe nanosheets had two valence states (tetrahe-
characteristic and highly reversible nature.
A sequence of
2p
m
) were evaluated based on eqn (1), the
of CoTe electrode was increasing from 553.4 F g to 622.8 F
with the current density ranging from 20 A g to 1 A g
ꢀ
1
C
m
ꢀ1
2
+
3+
ꢀ1
ꢀ1
dral Co and octahedral Co contributing to 2p spectral
prole). For the Te 3d region (Fig. 3(b)), the peaks at 572.8 eV Fortunately, with the current density increase to 20 A g , still
in Te 3d5/2 and 583.3 eV in Te 3d were related to the charac- 88.9% of the Cm remained relative to the value of 1 A g
teristics of Te , and the peaks at 576.2 eV in Te 3d5/2 and indicating that the CoTe nanosheets supported on CFP elec-
g
.
36
ꢀ1
ꢀ1
,
3/2
2
ꢀ
4
+
37
38,41
5
86.6 eV in Te 3d3/2 corresponds to the characteristics of Te .
trode had a high rate performance.
The cyclic stability of
The results of XPS spectrum illustrated the existence of CoTe CoTe electrode was evaluated by a continuous charge–discharge
ꢀ
1
nanosheets.
.2 Electrochemical performance of CoTe electrode
process at 1 A g for 2000 cycles. It can be seen from Fig. 4(d),
at the rst 1000 cycles, the capacitance of CoTe electrode
decreased gradually ꢃ10%, and then remained stable value
aer 1300 cycles. And aer 2000 cycles, the capacitance of the
CoTe electrode had still 85% compared with pristine value,
indicating that its excellent cycle stability. Moreover, electro-
chemical impedance spectroscopy (EIS) measurements could be
utilized to measure ion transfer properties and electrical
conductivity of the different electrode materials. Fig. 4(e) pre-
sented the Nyquist impedance plots tested in standard three-
3
The pseudocapacitive properties of as-prepared carbon ber
paper supported CoTe nanosheets electrode were investigated
using CV, GCD and long-term cycling stability tests. In order to
study the contribution of the CFP substrate for the total elec-
trode, the CV curves of CFP and CoTe electrode in 3 M KOH
solution at a voltage window of 0 to 0.6 V with scan rate of
ꢀ
1
100 mV s
by a three-electrode system were exhibited in
ꢀ
2
5
electrode system in the frequency range from 10 to 10 Hz,
and the inset was the corresponding equivalent circuit diagram.
The intercept at the part (Z ) is the internal resistance (R ),
which contains the intrinsic resistance of electrode materials,
the contact resistance at the electrode/electrolyte interface and
the bulk resistance of electrolyte, and the semicircle at high
frequencies region relates to the faradaic charge-transfer resis-
tance (Rct). Besides, the straight line in the low frequency region
represents the Warburg impedance (Z
charge–discharge process of electrode is controlled by ion
Additionally, C and C are the limit capacitance
L dl
and the double-layer capacitance. The tted R , R and Zw of
pure carbon ber paper (CFP) and CoTe coated carbon ber
paper were listed in Table 1.
Fig. 4(a). Obviously, the CV curve of pure CFP electrode was
almost a straight line, indicating that the capacitance behavior
from CFP could be neglected, the electrochemical capacitance
of as-synthesized electrode was dominated by CoTe nano-
0
s
38
sheets. Fig. 4(b) showed CV curves of the CoTe electrode at
different scan rates, where two pairs of redox peaks appeared in
each curve, which correspond to the typical faradic reaction
2
+
3+
4+
among transformation of Co /Co /Co in alkaline solution,
revealing that CoTe electrode had the pseudo-capacitance
performance and was a Faraday-type supercapacitor electrode
material instead of the electric double layer electrode mate-
w
), indicating that the
5,42,43
diffusion.
28
24,39
s
ct
rial.
According to Fig. 4(a), the two pairs of redox peaks at
about 0.21 V and 0.03 V, 0.50 V and 0.41 V, which suggests that
the capacitive characteristics are mainly manifested as pseudo-
capacitance characteristics. The two peaks could be due to the
s
From above results, it can be seen that the value of R and Rct
of CoTe coated CFP was much small than those value of pure
CFP, which means the conductive of CoTe coated CFP was
superior than pure CFP. While the Zw of CoTe coated CFP was
larger than pure CFP, which lead to a better semi-innite
39
20
following redox reactions in analogy with CoS and Co0.85Se.
ꢀ
ꢀ
CoTe + OH 4 CoTeOH + e
(6)
(7)
ꢀ
ꢀ
36
diffusion of the ions in the electrode.
CoTeOH + OH 4 CoTeO + H
2
O + e
Fig. 3 XPS spectra of the (a) Co 2p region, (b) Te 3d region.
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ꢀ1
Fig. 4 Electrochemical performances of the CoTe electrode: (a) CV curves of the CFP substrate and CoTe electrode at a scan rate of 100 mV s
;
(
b) CV curves of the CoTe electrode at different scan rates; (c) GCD curves at different current densities; (d) cycling performances during 2000
ꢀ1
cycles at current density of 1 A g ; (e) corresponding Nyquist plots for composite electrodes prepared by different doping ratios of Se doped
NiTe (the inset were the enlarged curves at a high frequency range and equivalent circuit diagram).
4
4
Table 1 The fitted impedance of pure CFP and CoTe coated CFP
which were conducive to adsorb ions. Besides, the open space
between the nanosheets could promote the electrolyte pene-
2
2
1/2
ꢀ2
Sample
R
s
(U cm )
R
ct (U cm )
o
W, Y (S s cm )
42,44
tration into the inner of the electrode.
Lastly, the “dead
ꢀ
ꢀ
4
3
surface” of the traditional slurry-derived electrode could be
avoid through directly grown CoTe nanosheets on CFP.
Pure CFP
CoTe coated CFP
2.29
1.04
3.89
1.86
2.14 ꢂ 10
1.25 ꢂ 10
5,44
3
.3 Asymmetric supercapacitor
The outstanding electrochemical performance of CoTe
nanosheet electrode was attributed to the following reasons.
Firstly, the orderly CoTe nanosheets structure gives rise to
a high area, not only improving the utilization of active mate-
rials but also offering a large number of electrically active sites
In order to further research the potential application of CoTe
nanosheets on CFP electrode for energy storage, AC as negative
electrode and CoTe as the positive electrode were used to
fabricate an asymmetric supercapacitor (ASC) with 3 M KOH as
electrolyte. The complementary CV curves of AC and CoTe
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electrode, including AC electrode and CoTe electrode occupy electrochemical performance of the CoTe//AC ASC, the effects of
different voltage windows were ꢀ1 to 0 V and 0 to 0.6 V, different concentration of K Fe(CN) adding into the KOH
4
6
respectively, were shown in Fig. 5(a). Thus, an expanded voltage electrolyte were investigated. Fig. 6(a) exhibited CV and GCD
window of 1.6 V can be achieved by assembled CoTe and AC curves of CoTe//AC ASC with different concentration of
electrode. To maximize the performance of ASC, the mass
4 6
K Fe(CN) adding into the KOH electrolyte at a scan rate of
ꢀ
1
loading of AC was determined by balancing the charges stored 100 mV s with a potential window of 0–0.55 V. The area of the
in each electrode. The mass specic capacitance of CoTe and AC CV curves represents their capacitances. It is noticed that the
ꢀ
1
ꢀ1
electrode was calculated to be 622.8 F g and 228.4 F g
,
area of CoTe//AC ASC with 0.03 M K
4 6
Fe(CN) + 3 M KOH was far
ꢀ1
respectively, at the current density of 1 A g . Therefore, the larger than those of others, which means larger specic
mass loading of AC in CoTe was calculated based on eqn (8).
5,45
capacitance of CoTe//AC ASC with 0.03 M K Fe(CN) + 3 M KOH.
4 6
This fact could also be veried by GCD test. Fig. 6(b) exhibited
(8) GCD curves of different electrode samples at current density of
ꢀ
þ
þ
ꢀ
þ
ꢀ
m
m
C
m
m
ꢂ DV
ꢂ DV
3
2
R ¼
¼
z
C
ꢀ1
1
0
A g . Obviously, the discharge time of CoTe//AC ASC with
.03 M K Fe(CN) + 3 M KOH was longest at the same current
Fe(CN) with 0.03 M was the optimal
concentration. In addition, the electrolyte containing 0.03 M
Fe(CN) exhibited additional redox peak which was due to the
ꢀ
+
mꢀ
where m and m were the mass of activated materials, C and
m
represent mass specic capacitance, DV and DV were the
potential windows of negative and positive electrodes,
respectively.
4
6
+
ꢀ
+
C
density. As a result, the K
4
6
K
4
6
ꢀ
1
3
ꢀ
With a scan rate of 100 mV s , the CV curves of the CoTe//AC possible redox reaction between the redox pair of Fe(CN)
/
6
4
ꢀ
ASC at various voltage windows from 0.8 V to 1.6 V were dis-
played in Fig. 5(b). It is noted that all curves remain similarity
and stable with increasing of the voltage window, indicating the
CoTe//AC ASC can be stably operated at a high voltage of 1.6 V as
Fe(CN) . The possible redox reaction in ASC was given
6
46
below,
4ꢀ
63ꢀ
ꢀ
+ e
Fe(CN)
6
4 Fe(CN)
(9)
4
5
expected. Fig. 5(c) showed the CV curves of CoTe//AC ASC at
different scan rates. Due to the pseudo-capacitive nature of the
CoTe nanosheet electrode, strong redox peaks could be clearly
observed at each curve. At various current densities, the GCD
curves of CoTe//AC ASC in a voltage window from 0 to 1.6 V were
displayed in Fig. 5(d). The GCD curves were nearly symmetrical,
Moreover, by adding K Fe(CN) into pristine electrolyte, the
4
6
CV curve with the enclosed area also increased, which implied
28
that the capacitance was enhanced. Fig. 6(c) was the CV loops
6
of the CoTe//AC ASC at different scan rates in the K Fe(CN) +
4
45
KOH solution, which could easily nd that almost all the CV
curves keeps the same with increasing the scan rate. Galvano-
static charge–discharge test was also executed to measure the
indicating the excellent reversibility of CoTe//AC ASC.
In order to further improve the performance of CoTe//AC
ASC, K Fe(CN) as a single redox additive was introduced into
alkaline electrolyte (3 M KOH). To obtain the optimized
4
6
specic capacitance value of ASC in K Fe(CN)6 + KOH
4
ꢀ1
Fig. 5 (a) CV curves of the CoTe and AC electrodes collected at 100 mV s in a three-electrode system; (b) CV curves of CoTe//AC ASC at
ꢀ1
different potential windows at 100 mV s ; (c) CV curves of CoTe//AC ASC at different scan rates; (d) GCD curves of CoTe//AC ASC at different
current densities.
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ꢀ
1
ꢀ1
Fig. 6 (a) CV curves of ASCs at 100 mV s in KOH and K
curves of ASC at different scan rates in K Fe(CN) + KOH; (d) GCD curves of ASC at different current densities in K
specific capacitance of ASCs at different current densities in KOH and K Fe(CN) + KOH; (f) Nyquist plots of ASCs in KOH and K
4
Fe(CN)
6
+ KOH; (b) GCD curves of ASCs at 1 A g in KOH and K
4
Fe(CN)
+ KOH; (e) the mass
Fe(CN) + KOH.
6
+ KOH; (c) CV
4
6
4
Fe(CN)
6
4
6
4
6
electrolyte, as presented in Fig. 6(d). The corresponding calcu- KOH were clearly higher than those of ASC without redox
lated value of specic capacitances in K Fe(CN) + KOH elec- additive. And the maximum energy density of ASC in K Fe(CN)
6
4
6
4
ꢀ
1
ꢀ1
trolyte, as shown in Fig. 6(e), was increased from 177.8 F g to + KOH electrolyte could reach 67.0 W h kg , which was nearly
92.1 F g at current densities changed from 5 A g to 1 A g
ꢀ
1
ꢀ1
ꢀ1
ꢀ1
1
,
threefold increase than pristine electrolyte (23.5 W h kg ) at
ꢀ
1
respectively. The calculated results displayed that the specic almost the same power density (ꢃ793.5 W kg ). The highest
capacitance of CoTe//AC ASC with adding extra redox additive energy density was superior to those earlier reported for Te-
K
4
Fe(CN)
6
into KOH was huge increased and almost triple based or Co-based components and (as is shown in Table 2),
ꢀ
1
19
0
higher than the specic capacitance of ASC in pristine electro- such as NiTe//AC ASC in KOH electrolyte (33.6 W h kg ), 1T -
lyte (57.3 F g to 67.3 F g with current density from 5 A g to MoTe //AC in KOH electrolyte (56.4 W h kg ), Co0.85Se//AC in
1
introducing 0.03 M K
possible redox reaction (eqn (9)). EIS was performed to further
explore the improved capacitive behavior of the CoTe//AC ASC applicability of CoTe//AC ASCs, which was shown in Fig. 7(b). It
in K Fe(CN) + KOH solution. The resulting Nyquist plots were can generally nd that the ASC without redox additive exhibited
ꢀ
1
ꢀ1
ꢀ1
ꢀ1 37
2
ꢀ
1
ꢀ1 16
A g ). Therefore, the performance of ASC was enhanced by KOH (45.0
W
h
kg
)
and Co0.85Se//AC in KOH
ꢀ
1
47
4
Fe(CN)
6
into electrolyte through its (39.7 W h kg ).
Furthermore, the long cycle stability was test to evaluated the
4
6
presented in Fig. 6(f), and the inset was the tted equivalent excellent cycling performance with 92.3% retention of capaci-
circuit. As is shown in Fig. 6(f), the CoTe//AC ASC in K Fe(CN) + tance aer 3000 cycles, which owing to the prominent electro-
4
6
KOH solution had smaller intercept of the real axis and semi- chemical stability of both AC and CoTe electrodes. The specic
circle compared with CoTe//AC ASC in KOH solution, which was capacitance of ASC in K Fe(CN) + KOH electrolyte had
4
6
veried by the tted value of R
s
and Rct, meaning the small bulk a signicantly reduced in the rst 1250 cycles, but still remains
resistance and lower electronic resistance of this improved 80.7% of initial capacitance aer 3000 cycles. Meanwhile, the
ꢀ
1
electrolyte. According to the tted results, the ASC in K
+
0
4
Fe(CN)
KOH electrolyte not only had the lower internal resistance (R
.77 U vs. 1.11 U), but also possess of a smaller interfacial indicating that the ASC with K
6
capacitance value of ASC with redox additive reaches 149.8 F g
and signicantly higher than that of the ASC without K Fe(CN)
Fe(CN) had a great prospect of
s
,
4
6
,
4
6
charge transfer resistance (R , 0.49 U vs. 0.56 U), which means the application. The capacitance retention rate of ASC con-
ct
adding K Fe(CN) into KOH electrolyte could lead to an increase taining redox additive was distinctly lower than those without
4
6
of the conductivity which has been conrmed by EIS test. This K Fe(CN) , which could be attributed to the reduction of the
4
6
25,26
was the positive effect which was the reason why the electro- activity of the redox additive in the redox process.
This could
chemical performance of ASC had signicantly promoted by be veried by rate capability of ASCs with different electrolyte at
introducing
electrolyte.
K
4
Fe(CN)
6
as redox additive into pristine different current densities. Fig. 7(c) revealed the current density
dependence of the cycling performance. During the process, ve
The energy and power densities of the CoTe//AC asymmetric steps of charge–discharge rates were changed successively from
ꢀ
1
supercapacitors were calculated according to the GCD curves 1 to 5 A g . At the rst 500 cycles with charge–discharge current
ꢀ
1
and their Ragone plots were shown in Fig. 7(a). The energy densities of 1 A g , both ASCs showed unsteady specic
densities of CoTe//AC ASC with redox additive of K Fe(CN) into capacitances with a relatively low coulombic efficiency. In the
4
6
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4 6 4 6
Fig. 7 (a) Ragone plots of ASCs in KOH and K Fe(CN) + KOH electrolyte; (b) cycling performance of ASCs in KOH and K Fe(CN) + KOH
ꢀ1
ꢀ1
ꢀ1
electrolyte at current density of 1 A g ; (c) cycling performance at various rates from 1 to 5 A g after 50 cycling at 1 A g
.
Table 2 The comparison energy density of Te-based components
K Fe(CN) + KOH electrolytes with a high voltage of 1.6 V. The
4 6
redox additive could provided additional pseudo-capacitance by
Energy density
(W h kg )
redox reaction. ASC with K
4
Fe(CN)
6
+ KOH delivered the specic
ꢀ1
Components
Electrolyte
3 M KOH
ꢀ1
ꢀ1
capacitance of 192.1 F g with energy density of 67.0 W h kg
ꢀ
1
CoTe
CoTe
23.5
67.0
33.6
56.4
45.0
39.7
at 1 A g was almost threefold increase than ASC with pristine
ꢀ1
ꢀ1
0.03 M K
3 KOH
4
Fe(CN)
6
+ 3 M KOH
electrolyte (67.3 F g , 23.5 W h kg ). Therefore, it was a highly
promising pathway by introducing the redox additive to
improve the performance of ASC for practical application.
NiTe
0
1
T -MoTe
2
2 M KOH
3 KOH
3 KOH
Co0.85Se
Co0.85Se
Conflicts of interest
following 1100 cycles at large current densities, the ASC with There are no conicts to declare.
KOH as electrolyte demonstrates a stable performance at each
situation while the ASC with K Fe(CN) + KOH as electrolyte
4
6
ꢀ1
shows a little decay. When the current turns back to 1 A g
,
Acknowledgements
a fully recovered ASC with KOH as electrolyte with a coulombic
efficiency of z91.6% was observed in the following 400 cycles
while the retention capacitance of ASC with K Fe(CN) + KOH as
4 6
electrolyte is calculated to be z88.2% with a coulombic effi-
This work was jointly supported by the National Natural Science
Foundation of China (No. U1205112) and Provincial Natural
Science Foundation of Fujian, China (No. D0010011).
ciency of 94.9%.
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