A robust hierarchical microcapsule for efficient supercapacitors exhibiting an ultrahigh current density of 300 A g-1

Article abstract of DOI:10.1039/c8ta00255j

A unique three dimensional (3D) hierarchical microcapsule structure (NiS_x@NCV) has been put forward, which is realized by the ensemble of N-doped carbon vesicles encapsulating dual-NiS_x (α-NiS/NiS₂) nanoparticles via an in situ nanospace-confined pyrolysis strategy. The NiS_x@NCV shows a high pseudocapacitance of 1600 F g^-1 at 1 A g^-1 and impressive rate performance (a capacitance retention ratio of 84.5%, from 1 to 25 A g^-1). Benefiting from this intriguing configuration, a kind of high performance asymmetric supercapacitor (ASCs) has been fabricated, using 3D NiS_x@NCV microcapsules and nanoporous carbon (NPC) as positive and negative electrodes, respectively. Remarkably, the fabricated ASCs achieve a capability of 135.06 F g^-1 at 1 A g^-1 and exhibit an outstanding rate capability at 32 A g^-1. More importantly, a high capacitance retention ratio of 87% is still achieved even at current densities from 32 to 300 A g^-1, which exhibits well the ultrahigh rate performance of the NiS_x@NCV//NPC ASCs. In addition, the NiS_x@NCV//NPC ASCs deliver an attractive energy density of 48.02 W h kg^-1 at a power density of 800 W kg^-1, and still maintain 30 W h kg^-1 even at an ultrahigh power density of 240 kW kg^-1. This is attributed to the rich redox reaction and interface effect of dual-NiS_x nanoparticles, abundant active sites and high electrical conductivity from N-doped carbon vesicles.

Full text of DOI:10.1039/c8ta00255j

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Volume
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January 2016 Pages 1–330
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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS nanocages as a lithium ion battery anode material
2
rsc.li/materials-a

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DOI: 10.1039/C8TA00255J
Journal Name
ARTICLE
A Robust Hierarchical Microcapsule for Efficient Supercapacitors
Bearing Ultrahigh Current Density of 300 A/g
Received 00th January 20xx,
Accepted 00th January 20xx
Cheng Yang, Minjie Shi, Xuefeng Song*, Xiaofeng Zhao, Liping Zhao, Jing Liu, Peng Zhang, Lian
Gao*
DOI: 10.1039/x0xx00000x
A unique three dimensional (3D) hierarchical microcapsule structure (NiS
by the ensemble of N-doped carbon vesicles encapsulating dual-NiS (α-NiS/NiS
confined pyrolysis strategy. The NiS @NCV shows a high pseudocapacitance of 1600 F/g at 1 A/g and impressive rate
performance (capacitance retention ratio of 84.5%, from 1 to 25 A/g). Benefiting from the intriguing configuration, a kind
of high performance asymmetric supercapacitors (ASCs) has been fabricated, using 3D NiS @NCV microcapsules and
x
@NCV) has been put forward , which is realized
www.rsc.org/
x
2
) nanoparticles via an in situ nanospace-
x
x
nanoporous carbon (NPC) as positive and negative electrodes, respectively. Remarkably, the fabricated ASCs achieve a
capability of 135.06 F/g at 1 A/g and exhibit outstanding rate capability at 32 A/g. More importantly, a high capacitance
retention ratio of 87% is still achieved even at current density from 32 to 300 A/g, which well displays the ultrahigh rate
performance of NiS
x
@NCV//NPC ASCs. In addition, NiS
x
@NCV//NPC ASCs deliver an attractive energy density of 48.02
Wh/kg at a power density of 800 W/kg，and still maintain 30 Wh/kg even at an ultrahigh power density of 240 kW/kg.
This is contributed to rich redox reaction and interface effect of dual-NiS
electrical conductivity from N-doped carbon vesicles.
x
nanoparticles, abundant active sites and high
Introduction
high charge transfer and ion diffusion rates between electrolyte and
Supercapacitors (SCs) have been considered as a promising electrodes, leading to excellent rate capability21-27. Despite these
power source for applications in portable electronic devices, kinetics advances mentioned above, nickel sulfides inevitably tend
2
electrical vehicles and stand-by power systems, owing to their fast to detach from the carbonaceous scaffold, such as graphene ,
charging/discharging process, long cycle life, security and reliability. carbon fibers and carbon nanotubes
Nickel sulfides, such as NiS , NiS
6, 13, 20
, especially at high
1-7
8-10
11-14
15
16
2
, Ni
3
S
2
, Ni
7
S
6
3 4
and Ni S ,
current densities, due to the unfirm contact between nickel sulfides
have attracted increasing interest as electrodes for SCs because of and carbonaceous species. For example, according to Zhang’s
their high redox activity and theoretical capacitance. Unfortunately, report, NiS nanoparticles immobilized on N-doped carbon fiber
nickel sulfides based electrode material mostly suffers from aerogels (NiS/N-CFA) shows enhanced rate performance compared
6
mediocre rate capability, especially, while the current density is with pure NiS nanoparticles arising from higher conductivity .
4
, 5, 8, 9, 17
more than 10 A/g
2
. For instance, NiS nanocubes delivered a However, only 60% of the specific capacitance at 1 A/g of NiS/N-
specific capacitance of 695 F/g at 1.25 A/g, whereas the capacitance CFA is retained when current density increases to 20 A/g.
9
only maintained 22.7% at 12.5 A/g . It is ascribed to the insufficient Consequently, it is desirable to develop a kind of nickel sulfides
electron transfer and ion diffusion rates between electrolyte and based electrode material that not only has kinetic virtue, including
electrodes, as well as the lack of structural stability of those high conductivity, high surface area, plentiful active sites and
electrodes.
porous structure, but also possesses sturdy stability of integral
To address these problems, combining nickel sulfides with structure.
carbonaceous materials is regarded as a promising strategy, owing
From the material configurations aspect, it is anticipated that
to their high conductivity, which delivers high electronic/ionic control over the morphology and assembly of the original building
conductivities to reduce capacitance degradation, especially at high blocks to 3D hierarchical hollow microspheres with high surface
1
1, 18-20
current densities
. In general, nitrogen-doped (N-doped) area as well as abundant accessible reaction sites could result in
carbonaceous materials can increase the electrical conductivity of unifying structural stability with long cycle life and high
the electrode, and provide abundant active sites, which allow for electronic/ionic conductivities. More recently, extensive attention
has been paid to the design of 3D hierarchical hollow architectures
composed of subunits (e.g. nanoparticles, nanorods or nanosheets)
State Key Laboratory for Metallic Matrix Composite Materials,
School of Materials Science and Engineering,
Shanghai Jiao Tong University, Shanghai 200240, P. R. China
28-40
in the applications of energy storage . These progresses include
2 5
the synthesis of self-assembled V O hollow microspheres from
*
E-mail: songxfeng@sjtu.edu.cn (X. F. Song); liangao@mail.sic.ac.cn (L. Gao).
28
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
nanorods , the preparation of hydrangea-like multi-scale carbon
30
hollow submicron spheres , and the design of 3D CoS
2
hollow
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33
microspheres constructed by nanosheets . Even though enhanced Experimental
electrochemical performance in some cases, these 3D architectures Synthesis of NiS
still can hardly meet the demand for practical applications NiS hollow spheres (pristine NiS
View Article Online
DOI: 10.1039/C8TA00255J
2
2
2
) were generated through an
especially at high current densities, since the integrating force exchanging reaction between rod-like supermolecular precursor
associating with van der Walls’ interactions or chemical bonding and L-cysteine according to our previous work with some
43
between the contacting subunits in these 3D architectures is not improvement . Firstly,1.5 g NiCl
2
·6H
enough to well maintain the integrity of structure . For instance, were dissolved in 40 ml of ethylene glycol in a warm-water bath at
only 34.6% of the specific capacitance of the 3D CoS hollow 60 °C with constant agitation. Then, 0.8 g of polyethylene glycol and
2
O and 2.4 g sodium acetate
34
2
microspheres constructed by nanosheets is retained as the current 2 g of deionized water were added into the mixture under vigorous
density increases from 1 to 20 A/g. Hence, it is highly desirable but agitation dropwise. The as-prepared green transparent solution was
challenging to develop a kind of electrode material with ultrahigh introduced into a 50 ml Teflon-lined autoclave. The reaction was
rate performance for SCs.
In this work, we develop an in situ nanospace-confined pyrolysis products were collected by centrifugation, washed with ethanol
strategy for 3D hierarchical microcapsule structure (NiS @NCV), in and deionized water for several times and finally dried in a vacuum
which dual-NiS (α-NiS/NiS ) nanoparticle is well confined in the oven at 60 °C overnight. 0.1 g nickel precursor synthesized above
kept at 160 °C for 1 h. After cooling to room temperature, the
x
x
2
compartment of N-doped carbon vesicles. The ultrathin α-NiS layer and 0.4 g L-cysteine were dissolved in 15 ml of deionized water with
was obtained in the control of the space-confinement by the N- constantly stirring at room temperature. The as-formed brown
doped carbon vesicle. Moreover, the interface effect of α-NiS/NiS
2
solution was introduced into a Teflon-lined autoclave (25 ml) and
encapsulated in N-doped carbon vesicles effectively enhances the maintained at 200 °C for 4 h. After cooling to room temperature,
rate performance. Such a design has multiple advantages: (1) the the products were collected by centrifugation and washed with
ultrathin α-NiS layer obtained through a space-confined reaction deionized water several times. Eventually, the obtained products
can be conductive to charge transport and Faradic reactions during were dried in an oven at 60 °C overnight.
the charge–discharge process, because of the higher conductivity
and more Ni atoms exposed on the surface; (2) the rate Synthesis of NiS
performance is effectively enhanced owing to the interface effect of The preparation of NiS
α-NiS/NiS encapsulated in N-doped carbon vesicles, which provides two steps. Firstly, 0.2 g NiS
x
@NCV microcapsules
@NCV microcapsules was carried out in
hollow spheres was incubated in 100 ml
x
2
2
abundant Faradic reactive sites for pseudocapacitance reaction; (3) of the freshly prepared dopamine (DPA) solution (2 mg/mL) in Tris
the porous N-doped carbon vesicles derived from PDA can be buffer (10 mM, pH 8.5) for 24 h at room temperature.
utilized to improve the electrochemical performance, which not Polymerization of DPA occurred in this incubation and resulted in
only provide plenty of active sites and rich access channels crosslinking of NiS
beneficial for charge storage, but also function as electrical highway carefully cleaning. Secondly, followed by thermal treatment at 500
to ensure all nanoparticles are electrochemically active, allowing for °C for 20 min under nitrogen atmosphere, the NiS @PDA
efficient charge transfer between the electrode and electrolyte; (4) composites were turned into NiS @NCV microcapsules. For
2
@PDA (polydopamine) composites after
2
x
the unique 3D hierarchical hollow structure can provide high comparison, the resultant black powder was collected and stirred in
surface area and effectively enhance the structural integrity of the a 1 M HCl aqueous solution for 8 h to remove NiS, receiving
whole microcapsule, since all nanoparticles are interconnected and NiS
linked up by N-doped carbon.
Benefiting from the intriguing architecture, NiS
2
@NCV. Meanwhile, NiS
annealing the NiS @PDA at 500 °C for 3 h under nitrogen
x
@NCV shows a atmosphere.
x
@NCV-2 was obtained through
2
high pseudocapacitance of 1600 F/g at 1 A/g, impressive rate
performance (capacitance retention ratio of 84.5%, from 1 to Characterization methods
5 A/g) and strong durability (94.4% retention after 5000 cycles). Field emission scanning electron microscope (FESEM, FEI Sirion
Further, the high-performance asymmetric supercapacitors (ASCs) 200) and Transmission electron microscopy (TEM, JEM-2010F) were
were fabricated, using 3D NiS @NCV microcapsules and carried out to characterize the morphology of the samples. X-ray
2
x
nanoporous carbon (NPC) as positive and negative electrodes, diffraction (XRD) patterns were characterized on a powder XRD
respectively. Remarkably, the fabricated ASCs achieve a capability system with Cu Ka radiation. Nitrogen absorption and desorption
of 135.06 F/g at 1 A/g and exhibit outstanding rate capability at 32 measurements were performed with an Autosorb IQ instrument at
A/g. More importantly, an ultrahigh specific capacitance is still 77 K. The specific surface areas were calculated by the Brunauer–
maintained even at a high current density of 300 A/g (capacitance Emmett–Teller (BET) method, and the pore size distributions were
retention ratio of 87%, from 32 to 300 A/g), which well displays the determined from the adsorption branch of the isotherms based on
ultrahigh rate performance of NiS
whole devices deliver a maximum energy density of 48.02 Wh/kg
based on the mass of active materials) at a power density of 800 Ex-situ X-ray diffraction
W/kg and still remain an energy density of 30 Wh/kg even at an Before starting the ex-situ XRD test, cyclic voltammetry (CV)
ultrahigh power density of 240 kW/kg, superior to those of previous scanning with a rate of 50 mV/s has been conducted for 50 cycles to
x
@NCV//NPC ASCs. Moreover, the the density functional theory (DFT).
(
4
1, 42
nickel sulfides based ASCs.
NiS @NCV//NPC ASCs pave a novel and efficient way to fabricate XRD experiment, electrodes were charged and discharged at
high-power energy storage systems.
Thereby, the fabricated stabilize the electrochemical signature of the electrodes. For ex-situ
x
current density of 25 A/g, and the stop positions of the GCD test are
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shown in Fig. 4. Once the electrodes were charged and discharged
to a desired voltage point, the GCD test paused and the electrodes
were lifted out of the KOH solution to be washed clean and then
tested by XRD analysis. After XRD analysis, the electrodes continued
the unfinished GCD test until the next desired voltage point, and
were lifted out of the KOH solution to be washed clean and then
tested by XRD analysis. Above steps were repeated until getting all
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DOI: 10.1039/C8TA00255J
the XRD patterns of pristine NiS
various constant potentials.
2 x
and NiS @NCV electrodes at
Electrochemical Measurements
All the electrochemical measurements were carried out using a
VMP3 multi-functional electrochemical analysis instrument (Bio-
Logic, France). Electrochemical behaviors were investigated by
cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and
electrochemical impedance (EIS) methods. The CV and GCD tests
were measured at various scan rates and current densities. The EIS
plots were performed in the frequency ranging from 0.05 Hz to 100
kHz with 5 mV AC amplitude. The working electrode was prepared
Fig. 1 (a) Schematic illustrations of the preparation process of
NiS
NiS
NiS
x
@NCV microcapsules. Typical FESEM images of (b) pristine
, (c) NiS @NCV microcapsules and (d) partially broken
@NCV microcapsules, respectively. TEM images (e-f) and
2
x
x
high-resolution TEM (HRTEM) image (g) of NiS
microcapsules.
x
@NCV
by mixing our samples (NiS
NiS @NCV, NiS @NCV-2 or NPC) with acetylene black and
polyvinylidenefluoride (PVDF) as the binder dissolved in N-methyl-
-pyrrolidone (NMP) in a weight ratio of 8:1:1. The resulting slurry
x 2
@NCV microcapsules, pristine NiS ,
2
x
average size of about 1.2 μm. The microcapsules show rough
surface, and are composed of sphere-like subunits. Some of the
microcapsules are partially broken (Fig. 1d and Fig. S1), clearly
demonstrating the hollow nature of the as-prepared microcapsules,
which is further confirmed by the transmission electron microscopy
2
was pressed onto the Ni foam current collector as the working
electrodes. The platinum (Pt) foil and Hg/HgO electrodes were used
as the counter and reference electrodes, respectively. All tests were
(
TEM) images in Fig. 1e and f. It is worth mentioning that each
conducted in
a
6
M
KOH electrolyte. The asymmetric
microcapsule is coated with uniform carbon layer, wherein N-doped
carbon serves as bridges for electrons transportation and ensures
that all nanoparticles are electro-active, interconnected and linked
up, thus preserving enhanced structural integrity compared to
previous 3D hierarchical hollow architectures composed of
supercapacitors were constructed in a coin cell by using a
nanoporous carbon(denoted as NPC) electrode as the negative
electrode, the NiS @NCV microcapsules electrode as the positive
x
electrode and one piece of cellulose paper as the separator in 6 M
KOH.
35,36
, nanorods28,32 or nanosheets
31,33
nanoparticles
. The HRTEM
image (Fig. 1g) displays amorphous carbon and the lattice
interplanar spacings of around 0.27 and 0.296 nm corresponding to
Results and discussion
A schematic illustration for the detailed evolution process of the
8
17
the distances of (200) plane of NiS
respectively, indicating the successfully obtained carbon vesicles
encapsulating α-NiS/NiS in the NiS @NCV microcapsules.
2
and (100) plane of α-NiS ,
3
D hierarchical NiS
carbon vesicles encapsulating dual-NiS
in Fig. 1a. Initially, based on the modified previous method , the
hollow NiS spheres (Fig. 1b) were prepared through
x
@NCV microcapsules assembled by N-doped
x
nanoparticles is presented
4
3
2
x
Meanwhile, the X-ray diffraction (XRD) pattern is consistent with
the result of HRTEM, exhibiting both the typical peaks indexed to
2
a
hydrothermal reaction. Sequentially, the dopamine was used as a
bio-adhesive cross-linker to accomplish the uniform coating of N-
NiS
the NiS
2
(JCPDF11-0099) and α-NiS (JCPDF02-1280) (Fig. 2a). Moreover,
@NCV microcapsules possess higher specific surface area as
4
5
x
doped carbon precursor via an aqueous self-polymerization .
Finally, the NiS /PDA was subjected to a carbonization of PDA layer
and part-decomposition of inner encapsulated NiS , during which
abundant pores/voids were generated in the α-NiS layer (converted
2
3
high as 64.28 m /g, pore volume of 0.208 cm /g and abundant
mesopores ranging from 3 to 12 nm (Fig. 2b), which is favorable for
the electrolyte diffusion in and out of the electro-active materials
2
2
49,
5
0
.
More characterization of NiS
performed to further confirm the fine structure as shown in Fig. S1-
. From characterization for the NiS @NCV, the C coating content,
x
@NCV microcapsules are
45
2
from the incomplete reaction of NiS to α-NiS) . Meanwhile, the
outer PDA-coating layer was converted into amorphous carbon,
making for uniform N-doped carbon-encapsulation of the
5
x
N doping content are 14.82% and 7.72%, respectively. Additionally,
the thickness of the carbon vesicles is about 10 nm. Profiting from
this optimized architecture, it is expected that the robust NiS @NCV
x
microcapsules with high specific surface area and abundant pores
exhibit excellent rate capability and satisfactory cycling stability for
SCs.
46, 47
microcapsules
. It has been reported that crystal structures may
be determined by size constraint through crystallization of
4
8
materials . Clearly, the space-confinement by the N-doped carbon
vesicle is crucial to the formation of α-NiS and prevents its possible
transformation to β-NiS.
As shown in the FESEM images (Fig. 1c and d), it is revealed that
x
the NiS @NCV microcapsules are highly uniform spheres with an
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Fig. 2 (a) XRD pattern and (b) nitrogen adsorption/desorption
isotherm of the NiS @NCV microcapsules. Inset in b is the
x
corresponding pore size distribution curve.
x
The electrochemical performance of the NiS @NCV
microcapsules was evaluated by cyclic voltammograms (CV) and
galvanostatic charge-discharge (GCD) measurements in a three-
electrode beaker cell with a 6 M KOH aqueous electrolyte. For
2 2 x
comparison, pristine NiS , NiS @NCV and NiS @NCV-2 were also
carried out at the same test conditions using the identical
assembled configuration. As the annealing time prolonged, the
Fig. 3 (a) CV curves at various scan rates (2-100 mV/s), and (b) GCD
x
curves at different current densities (1-25 A/g) of NiS @NCV
amount of α-NiS increased while the amount of NiS
within obtained product (denoted as NiS @NCV-2). More
information about pristine NiS , NiS @NCV and NiS @NCV-2 are
provided in the Fig. S6-8. The typical CV curves of NiS
@NCV, Inset in d shows the EIS plots of NiS
and pristine NiS electrodes.
2
decreased
electrode. (c) The specific capacitances at various current densities,
d) cycling stabilities at a current density of 1 A/g for 5000 cycles of
NiS @NCV, NiS @NCV, NiS @NCV-2 and pristine NiS electrodes.
@NCV, NiS @NCV, NiS @NCV-2
x
(
2
2
x
x
2
x
2
x
x
2
x
pristine NiS , NiS @NCV and NiS @NCV-2 obtained with a scan rate
2
2
x
2
of 5 mV/s display two peaks in the potential ranging from -0.1 to
.38 V, which demonstrate the pseudocapacitive characteristics of
the samples. The reaction mechanism is similar to those of previous
only 5.64% capacitance loss after 5000 cycles while the pristine NiS
electrode suffers from 30% capacitance loss during the long cycle
life (Fig. 3d). Obviously, NiS @NCV and NiS @NCV-2 exhibit more
2
0
1, 8
literatures , associated with the following reversible reaction:
x
x
-
-
NiS+OH ⇌NiSOH+e
(1)
(2)
durable cycling stability and higher rate capability compared with
pristine NiS and NiS @NCV, attributed to enhanced structural
-
OH+e-
NiS
2
+OH ⇌NiS
2
2
2
Obviously, NiS
x
@NCV microcapsules exhibit larger CV areas than stability and higher electronic/ionic conductivities, which is
, NiS @NCV and NiS @NCV-2, indicating the greatly supported by their FESEM images after 5000 cycles (Fig. S9) and EIS
electrochemical performance
@NCV, the anodic peak shifts
towards positive potential while the cathodic peak shifts towards
pristine NiS
enhanced
microcapsules (Fig. S8). For NiS
2
2
x
of
x
NiS @NCV
plots (inset in Fig. 3d).
x
In order to provide insights into the electrochemical reaction of
negative potential with the increasing scan rate (Fig. 3a), which pristine NiS2 and NiS @NCV in 6 M KOH, ex-situ XRD analysis
x
indicates rapid electronic/ionic transport and plentiful
electrochemical active surface area of the electrode .
coupled with GCD test has been performed. Before starting the ex-
situ XRD test, cyclic voltammetry (CV) scanning with a rate of 50
mV/s has been conducted for 50 cycles to stabilize the
6
x
Fig. 3b shows the GCD curves of the NiS @NCV electrode,
electrochemical signature. The GCD curves for pristine NiS
NiS @NCV at high current density of 25 A/g are shown in Fig. 4a and
b. The XRD diffraction patterns of pristine NiS and NiS @NCV
2
and
wherein the negligible “IR drop” of discharge curve is observed,
strongly evincing the low internal resistance of the electrode.
According to Fig. 3c, the calculated discharge capacitance of
x
2
x
electrodes were recorded at various potentials (A, B, C, D and E)
during the galvanostatic charging-discharging cycle (Fig. 4c and d).
As for charging section, the XRD patterns recorded in this potential
region (A to C) clearly reveal a shift of the reflections to lower 2θ
values, indicating an increase of the lattice parameters due to the
NiS
F/g at current densities of 1, 5, 10, 15, 20 and 25 A/g, respectively
Fig. 3c). Notably, for the NiS @NCV microcapsules electrode, the
capacitance retention rate as high as 84.25% is much higher than
those of pristine NiS , NiS @NCV, NiS @NCV-2 and previously
reported nickel sulfides based materials (Table S1), such as NiS
nanocubes (capacitance retention ratio of 22.7%, from 1.25 to 12.5
x
@NCV microcapsules is 1600, 1550, 1498, 1466, 1431 and 1348
(
x
2
2
x
expansion of the NiS
2
and α-NiS host lattice caused by the
2
intercalation of OH within the electrodes51-53. Then a progressive
-
9
2
shift of the NiS (200) and α-NiS (100) peaks to higher 2θ values is
observed when discharged from 0.38 V back to -0.1 V, which
evidences a decrease of the lattice spacings, ascribed to the
A/g) , NiS nanoframes (capacitance retention ratio of 33.6%, from 1
4
3 2
to 20 A/g) and Ni S /MWCNT (capacitance retention ratio of 46.8%,
11
from 0.8 to 25 A/g) . Additionally, 80% of the specific capacitance
at 1 A/g of NiS @NCV-2 is retained as the discharge current density
increases to 25 A/g, which is comparable to that observed for
NiS @NCV. However, the specific capacitance of NiS @NCV-2 is
much lower than that of NiS @NCV at the same current densities,
due to the lower theoretical specific capacitance of α-NiS.
Moreover, the NiS @NCV electrode exhibits a durable cycle life with
x
x
x
x
x
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charged to 0.38 V. Then, the (100) and (200) peaks shViiefwt tAortic3le0.O0n7li3ne°
and 31.506° when discharged to -0.1 V, rDeOspI:e1c0t.i1v0e3l9y/,Cw8ThAic0h025a5reJ
nearly the same as their original 2θ positions, demonstrating a
relatively reversible transition within NiS
the charging-discharging cycle at high current density . It is
concluded that the structural stability of NiS @NCV microcapsules
x
@NCV electrode during
51
x
has been effectively enhanced during the electrochemical reactions
especially at high current density, which is contributed to high rate
capability and strong durability.
Nanoporous carbon (NPC) is one of the most promising
candidates for constructing ASCs owing to the high specific surface
area and appropriate pore structure54-62. Herein, metal-organic
frameworks (MOF)-derived NPC obtained via direct carbonization of
ZIF-8, a well-studied MOF material with unique polyhedral
morphology, is prepared as the negative electrode material. It is
Fig. 4 GCD curves at high current density of 25 A/g of (a) pristine
NiS and (b) NiS
diffraction study of (c) pristine NiS
@NCV electrodes. Electrochemical ex-situ X-ray worth mentioning that ZIF-8 derived NPC can not only retain the
and (d) NiS @NCV electrodes at stable porous architecture, but also provide a high specific surface
2
x
2
x
various constant potentials in 6 M KOH electrolyte. The scans
referring to the different potentials marked in panel a and b are
indicated by different colors: A black, B red, C blue, D green and E
magenta.
area, which is beneficial to the electrochemical performances of
electrodes. As shown in the FESEM image of NPC (Fig. 5a), it is
confirmed that the NPC is composed of uniformly sized and
dispersed particles with rhombic dodecahedral shape. The average
particle size is around 1.5 μm. The magnified FESEM image (inset in
Fig. 5a) shows that NPC possesses well-defined polyhedral shape
with sharp edges and smooth surface. Moreover, Fig. 5b gives the
TEM image of NPC, which reveals that NPC has a uniform texture
without large cavity or cracks. The magnified TEM image (inset in
Fig. 5b) on the edge of the NPC clearly presents the existence of
plentiful micro-/mesopores, which indicates the nanoporous
characteristic of NPC. It is worth noting that the BET specific surface
-
deintercalation of OH in the NiS
2
and α-NiS. In the case of pristine
2
NiS electrode, the (200) peak initially observed at 31.580° at -0.1 V
shifts to 31.217° when charged to 0.38 V. However, even when
discharged to -0.1 V, the (200) peak can not reversibly shift back to
the original 2θ position, more specifically about a decrease of
0
.205°. In contrast, as for NiS
NiS initially occurred at 30.160° and shifts to 29.998°, while the
200) peak of NiS exhibits a decrease of 0.171° from 31.580° when
x
@NCV electrode, the (100) peak of α-
(
2
Fig. 5 (a) FESEM image and (b) TEM image of nanoporous carbon particles. Inset in a is the magnified FESEM image of NPC and inset in b is
the magnified TEM image on the edge of the NPC. (c) Nitrogen adsorption/desorption isotherm of nanoporous carbon particles. Inset in c
is the corresponding pore size distribution curve. (d) CV curves at various scan rates (20-500 mV/s) and (e) the specific capacitances at
various current densities (1-25 A/g) of nanoporous carbon particles. Inset in e is the corresponding GCD curves at different current
densities (1-25 A/g). (f) Cycling stability of the NPC electrode at a current density of 1 A/g for 5000 cycles, and the inset shows the
corresponding EIS spectra.
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Journal Name
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DOI: 10.1039/C8TA00255J
Fig. 6 (a) Schematic illustration of the asymmetric supercapacitor configuration. (b) CV curves at various scan rates (5-500 mV/s)
and (c) the specific capacitances at various current densities (1-300 A/g) of the NiS @NCV//NPC asymmetric supercapacitor. Inset
in c is the corresponding GCD curves at different current densities. (d) Comparative Ragone plots of the energy density and power
density of our NiS @NCV//NPC ASCs with various previously reported nickel sulfides based supercapacitors. (e) Cycling stability of
x
x
the NiS
x
@NCV//NPC ASCs at a current density of 1 A/g for 10000 cycles, and the inset shows the corresponding EIS spectra.
2
area of NPC is up to 1620 m /g (Fig. 5c). The pore size distribution are fabricated in 6 M KOH solution by using NiS
x
@NCV and NPC as
(
inset of Fig. 5c) reveals the abundant mesopores (about 2.6 nm) of the positive and negative electrodes (with mass loading of 1.5
2
the NPC, which is beneficial to the ionic diffusion and charge mg/cm ), respectively (Fig. 6a). The CV curves of this device exhibit
transport. More characterization of nanoporous carbon particles obvious capacitance from both electric double-layer capacitance
are described in Fig. S10 and 11.
and pseudocapacitance with a wide working voltage window of 1.6
V (Fig. 6b). As shown in the GCD curves (inset in Fig. 6c and Fig.
S13), no obvious “IR” drop is observed, connoting the low internal
resistance of the ASCs (0.9 Ω, inset in Fig. 6e). Calculated from the
GCD curves, the fabricated ASCs achieve a maximum capability of
The CV curves of NPC measured in 6 M KOH solution exhibit the
typical rectangular shapes without obvious distortion even at a scan
rate of 500 mV/s (Fig. 5d), indicating an ideal capacitance behavior
and fast diffusion of electrolyte ions within the electrode. It is also
supported by the near-triangle GCD curves of NPC at various
current densities (1-25 A/g) within a potential window of -1-0 V
1
35.06 F/g at 1 A/g and exhibit high rate capability at 32 A/g. The
specific capacitance was slightly increased with the current density
increasing between 32 and 75 A/g. It might be attributed to the
activation caused by high-rate discharging/charging. With the
progress of charging and discharging, the electrochemical active
channel gradually opened and the electrolyte was fully immersed,
which was conducive to giving full play to the capacitance
characteristics of the electrode material. More interestingly, the
(
inset of Fig. 5e). The negligible “IR drop” of discharge curve
strongly evinces the favorable resistance of the NPC based
electrode (inset in Fig. 5e). By calculating, the NPC based electrode
delivers large specific capacitance as high as 400 F/g at 1 A/g (Fig.
5e), higher than that of previous advanced carbon based negative
electrodes63-65. Furthermore, the NPC electrode shows a superior
cycling stability with high retention of 97.46% of the initial capacity
after 5000 cycles at 1 A/g (Fig. 5f), owing to the unique architecture
of the polyhedron that may avoid the stacking/aggregation problem
x
NiS @NCV//NPC ASCs can achieve ultrahigh capacitance retention
ratio of 87% from 32 to 300 A/g (Fig. 6c), which outperform most
previously reported nickel sulfides based ASCs (Table S2), such as
NiS microflower//AC (capacitance retention ratio of 43.7%, from 1
(
because of multi-face) during the repeated charge/discharge
17
A/g to 10 A/g) , Ni
3
2
S
3
@β-NiS//AC (capacitance retention ratio of
9.8%, from 1 A/g to 30 A/g)14 and Ni
3 2
S /CNF//CNF (capacitance
54
processes (Fig. S12) . Profiting from the excellent electrochemical
performances, it is obvious that NPC has a great potential
application in the field of high-efficient ASCs.
retention ratio of 62.5%, from 1 A/g to 10 A/g)13.
By calculating, the assembled ASCs deliver a maximum energy
density up to 48.02 Wh/kg at a power density of 0.8 kW/kg and still
maintain 30 Wh/kg even at an ultrahigh power density of 240
On the basis of the excellent electrochemical performance of
both NiS @NCV microcapsules and NPC, the NiS @NCV//NPC ASCs
x x
6
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kW/kg (Fig. 6d and Fig. S14), higher than those of previously appealing potential for ultrahigh rate SCs in the vehVicielwesA,rtlicifletsOanlninde
reported nickel sulfides based ASCs (Table S2), such as Ni
S
3 2
//AC the other devices.
DOI: 10.1039/C8TA00255J
12
(
34.6 Wh/kg at 0.15 kW/kg) and NiS /ZnS//AC (28 Wh/kg at 0.75
2
10
Conclusions
kW/kg) . Thanks to the high energy density and high power
density, the assembled ASC can light up a 1 W lamp after fully
charged (Fig. S15). As known, the long-term cycling stability is a
critical parameter to evaluate the electrochemical performance of
In conclusion, 3D hierarchical NiS
x
@NCV microcapsules have
been successfully prepared via a hydrothermal method combined
with pyrolysis of PDA. Owing to the high contact area, porous
structure, good electronic conductivity, rich active sites, and the
x
SCs for practical applications. Hence, the NiS @NCV//NPC ASCs are
interface effect of α-NiS/NiS
2
as well as superior structure stability
@NCV
investigated by repeating the galvanostatic charge-discharge
between 0 and 1.6 V at a current density of 1 A/g (Fig. 6e). It can be
clearly observed that the assembled ASCs show a capacitance
retention ratio of 93.9% after 10000 cycles, demonstrating the
excellent cycling stability and superiority to most previously
of 3D hierarchical microcapsules, the as-obtained NiS
x
microcapsules deliver high specific capacitance up to 1600 F/g at 1
A/g and exhibit ultrahigh rate performance (capacitance retention
ratio of 84.5%, from 1 to 25 A/g) as well as durable cycling life,
which contributes to the excellent electrochemical performance of
reported nickel sulfides based ASCs, including NiS/NF//AC (90.6%
x
the ASCs using NiS @NCV and NPC as the positive and negative
1
retention after 1000 cycles) , rod-like NiS
2
//rGO (84.1% retention
electrodes, respectively. As a result, the fabricated ASCs exhibit a
maximum energy density up to 48.02 Wh/kg, maintain 30 Wh/kg at
an ultrahigh power density of 240 kW/kg, and display satisfactory
cycling stability (93.9% after 10000 cycles) as well as remarkable
8
after 4000 cycles) and Ni
3
S
2
//AC (85.7% retention after 1000
@NCV still
retained almost the original topography structure, showing little
12
cycles) . After performing numerous cycles, the NiS
x
collapse. The as-fabricated ASCs inherit the excellent rate performance even at an ultrahigh current density of 300 A/g
electrochemical performance of both NiS @NCV and NPC (capacitance retention ratio of 87%, from 32 to 300 A/g). After
x
electrodes, while display low internal resistance (0.96 Ω) and charging, the assembled ASC can light up a 1 W lamp. The work puts
maintain good conductivity after cycling (Fig. S19) as well as fast forward hierarchical microcapsules and represents a conceptual
leap forward, addressing the moderate rate performance of nickel
sulfides. Therefore, this encouraging novel electrode can open up
charge transfer (inset in Fig. 6e).
The excellent electrochemical behaviors of NiS
x
@NCV//NPC ASCs the possibility of nickel sulfides based materials for numerous
can be attributed to the following aspects: (1) compared with NiS
2
,
applications in SCs with ultrahigh rate performance, high energy
density and power density, and strong durability to meet the
diverse demands of high-power energy storage systems.
the ultrathin α-NiS layer has higher conductivity and more Ni atoms
exposed on the surface (as shown in Fig. S16 and 17), which is
propitious to charge transport and Faradic reactions during the
Acknowledgements
charge–discharge process; (2) the interface effect of α-NiS/NiS
2
encapsulated in N-doped carbon vesicles provides abundant Faradic
reactive sites for pseudocapacitive reaction, which is confirmed by
The authors greatly thank Dr. Zhuang Sun for the valuable
discussion. This work was supported by the Shanghai
Municipal Natural Science Foundation(17ZR1414900), the
Opening Project of State Key Laboratory of High Performance
Ceramics and Superfine Microstructure(SKL201604SIC),the
Shanghai Jiao Tong University Medical (Science) Cross
Fund(YG2017MS02), the Shanghai Aerospace Science and
Technology Innovation Foundation Project, the Advanced
Energy Material and Technology Center of Shanghai Jiao Tong
University, and the National Natural Science Foundation of
China (51302169, 51672174).
comparison of specific capacitance among pristine NiS
2 2
, NiS @NCV
and NiS @NCV (Fig. 3c); (3) the porous N-doped carbon vesicles
x
provide plenty of active sites and rich access channels favorable for
charge storage, and function as electrical highway to make all
nanoparticles electrochemically active, which allows for efficient
charge transfer between the electrode and electrolyte; (4) in a
x
whole NiS @NCV microcapsule, the unique 3D hierarchical hollow
structure can provide high surface area and ensure the ions from
both outer and inner faces to be charged and transferred, while the
structural integrity of the whole microcapsule during the
charge/discharge cycles is effectively enhanced since all
nanoparticles are interconnected and linked up by N-doped carbon, Notes and references
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which conduces to high rate capability and strong durability; (5)
NPC with well-defined polyhedral shape possesses high specific
surface area and plentiful micro-/mesopores, which are beneficial
to the ionic diffusion and charge transport, and may avoid the
stacking/aggregation problem (because of multi-face) during the
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