Tensile force-induced tearing and collapse of ultrathin carbon shells to surface-wrinkled grape skins for high performance supercapacitor electrodes

Article abstract of DOI:10.1039/c7ta03113k

With glucose as a highly accessible carbon source, mesoporous carbon nano grape skins were successfully fabricated via a facile template-based hydrothermal method and subsequent tensile force-induced tearing and collapse. The resulting carbon nano grape skins possess a novel structure as a nanosheet rolled up from the around, which maintains the ultrathin properties of the nanosheets. However, unlike carbon nanosheets, which are prone to aggregation or restacking, the carbon nano grape skins were separated from each other due to the crimp force induced by the rolled side. The largest specific surface area of 1570.9 m² g^-1 and the total pore volume of 2.429 m³ g^-1 were achieved for the product obtained at 600 °C. Compared to carbon nanosheets and carbon hollow spheres obtained at 700 °C, the products obtained at 700 °C, when used as electrode materials for supercapacitors, showed a much higher specific capacitance of 268 F g^-1 at 0.1 A g^-1, a higher rate specific capacitance of 170 F g^-1 at 10 A g^-1, and better cycling stability with 94.2% capacitance retention after 10 000 cycles at 2 A g^-1 in a two-electrode system.

Full text of DOI:10.1039/c7ta03113k

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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Tensile force induced tearing and collapse of ultrathin carbon
shells to surface-wrinkled grape skins for high performance
supercapacitor electrodes
Guofeng Zhang,a Jing Zhang,a Qing Qin,a Yingxue Cui,a Wenhao Luo,a Yan Sun,a Cen Jina and
Wenjun Zheng*ab
With glucose as highly accessible carbon source, mesoporous carbon nano grape skins were successfully fabricated via a
facile template-based hydrothermal method and subsequently tensile force induced tearing and collapse. The resulting
carbon nano grape skins possess a novel structure as a nanosheet rolled up or folded from the around, which maintains
the ultrathin properties of nanosheets. However, unlike carbon nanosheets, which are prone to aggregate or restack, the
carbon nano grape skins were separated from each other due to the crimp force induced by the rolled side. A largest
specific surface area of 1570.9 m2 g-1 and total pore volume of 2.429 m3 g-1 were achieved for the product obtained at 600
°C. When was used as electrode materials for supercapacitors, the products obtained at 700 °C showed a much higher
specific capacitance of 268 F g-1 at 0.1 A g-1, a higher rate specific capacitance of 170 F g-1 at 10 A g-1, and a more excellent
cycling stability with a 94.2 % capacitance retention after 10000 cycles at 2 A g-1 in a two-electrode system than that of
carbon nanosheets and carbon hollow spheres obtained at 700 °C.
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
performance supercapacitor, such as carbon materials,2
conducting polymers,11 metal oxides,5
and
their
Introduction
compounds.6,12,13 With the advantages of abundant reserve,
innocuity, high special surface area, reasonable conductivity,
and outstanding stability,14-16 carbon materials have been
widely investigated as the activity materials of supercapacitors
electrode. Up to date, over 80 % of commercial supercapacitor
products employ carbon materials as electrode materials.1,17
Carbon materials can be fabricated into various
morphologies, such as carbon nanocages,18,19 carbon
spheres,20,21 carbon nanofibers,22-24 carbon cubes,25 carbon
microtrees,26 two-dimensional carbon nanosheets and carbon
nanohorns.27,28 While these various morphologies of carbon
materials were employed as supercapacitor electrode
materials, they exhibited their respective advantages and
disadvantages. Among these morphologies, two-dimensional
nanosheets have caused great concern due to their particular
characters and potential use in the field of energy storage and
conversion. Two-dimensional carbon materials possess high
surface areas and outstanding electronic transportation
performance. Unlike hierarchical porous carbon which depend
on the pores in a solid support to provide the large special
surface area, the high special surface area of two-dimensional
carbon nanosheets have little relevance to the distribution of
pores in the individual nanosheet, which decreased the ion-
transport resistance and diffusion paths, improving the
electrode kinetics.29 However, carbon nanosheets are prone to
aggregate or restack, significantly comprising the unique
With the quick increasing of energy consumption and
depletion of fossil fuel, the demand for low-cost and
environmentally friendly high-power energy storage devices is
becoming more and more urgent.1-3 As one of the various
power source devices, supercapacitors, also known as the
ultracapacitors, have received extensive attention due to their
excellent performance such as high power, long cyclic life, and
fast charge−discharge rates.4 According to the energy-storage
mechanism, electrochemical capacitors can be classified as
electrical double-layer capacitors (EDLCs), which resulted from
ion adsorption,1-3 and pseudocapacitors, which arise from the
Faradaic redox reactions on the surface of active materials.5,7
In recent years, high-performance supercapacitors have found
some applications, such as the back-up of small current power
supply system, the driving device of hybrid electric vehicle and
the power of some consumer electronics.8-10 Due to the
performance of supercapacitors is mainly determined by the
electrode materials, considerable efforts have been made to
develop advanced materials to meet the need of high-
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properties of individual carbon nanosheet.30-33 Hence, it
seems to be significant to fabricate a novel structure of carbon
materials possess the advantages of nanosheets yet avoid
aggregating or restacking through a facial, economic and
environmentally friendly method.
Preparation of carbon nanosheets
The Se@C composites obtained after 1.5 h hydrothermal
process were placed in a quartz tube furnace, and the tube
was Ar purged for 30 min, then heated to 700
°C at the heating
rate of 10
°
temperature, the final carbon nanosheets were obtained.
C min-1 maintaining for 2 h. After cooled to room
Herein, we present a convenient fabrication of carbon nano
grape skins with ultrathin wall (∼3 nm) from glucose source via
Material characterization
a template-based hydrothermal method and subsequently
tensile force induced tearing and collapse. The novel
morphology seems like a nanosheet was rolled up or folded
from around, which inherits the merit of carbon nanosheets as
supercapacitor, such as providing shorter diffusion paths for
contacting with electrolyte and tiny ion-transport resistance,
and meanwhile generates the three-dimensional porous
structure providing large pore volume of mesopores, which is
favorable to charge accumulation. When used as the electrode
X-ray diffraction (XRD) measurements were performed on a
Bruker D8 Focus Diffraction system with Cu Kα radiation (λ =
0.15418 nm) at V = 40 kV and I = 40 mA. Raman spectroscopy
was performed using Raman spectrometer (Renishaw) with an
argon ion laser (λ = 514 nm). Morphology observations were
performed on a Hitachi S4800 field-emission scanning electron
microscope (FESEM) coupled with energy dispersive X–ray
spectroscopy (EDS). Transmission electron microscope (TEM)
images were recorded on a Tecnai G220STwin transmission
electron microscope at an accelerating voltage of 120 kV. N2-
adsorption/desorption isotherms were collected at liquid-
materials for supercapapcitor, the samples obtained at 700 °C
present the best performance, including large specific
capacitance, high rate capability and excellent long-term cycle
nitrogen temperature on
a Quantachrome Nova 2000e
stability. When compared to carbon hollow spheres, the sorption analyzer. The specific surface area (SBET) of the
samples was calculated by following the multipoint Brunauer–
Emmett–Teller (BET) procedure and the pore size distribution
was calculated by the Barrett-Joyner-Halenda (BJH) model
based on the desorption branch.
carbon nano grape skins exhibit a much more enhancement in
electrochemical performance.
Experimental section
Preparation of C@Se34
Electrochemical measurements
The electrodes were prepared by uniformly mixing active
material, acetylene black, and polyvinylidene fluoride (PVDF) in
N-methylpyrrolidone (NMP) at a mass ratio of 80:10:10 to
obtain a slurry. The slurry was then uniformly pasted onto a
In a typical procedure, 4 g glucose and 36 mg sodium selenite
were added to 70 ml deionized water in a beaker to get a
transparent solution, which was then transferred into a 100 ml
stainless steel autoclave heated for 1.5 h at 180 °C in an oven.
piece of nickel foam (1 cm × 1 cm) and dried at 80 °C overnight.
After cooled to room temperature, the resultant product was
separated by centrifugation, washed with distilled water and
anhydrous ethanol for several times. The final red product was
In our experiment, about 2.4 mg of active materials was
loaded on the nickel foam; the actual mass is determined by
weighing. The electrochemical test was carried out on a three-
electrode system using platinum plate as the counter
electrode, an Hg/HgO electrode as the reference electrode,
and 6.0 M aqueous KOH solution as the electrolyte. The cyclic
voltammetry (CV) analyses were carried out in a Zahner IM6e
electrochemical workstation and the galvanostatic charge–
discharge (GCD) measurements were performed through a
LAND battery system. The capacitance (Cm) of the electrodes is
calculated from the GCD curves at different current densities
using equation (1):
dried in a vacuum at 60 °C for 12 h. As for the preparation of
carbon hollow spheres, the time of hydrothermal process was
prolonged to 2.5 h.
Preparation of ultrathin mesoporous carbon nano grape skins
The resultant product above was placed in a quartz tube
furnace, and the tube was Ar purged for 30 min, then heated
to 600 °C, 700 °C, 800 °C at the heating rate of 5 °
C min-1
maintaining for 2 h, respectively. After cooled to room
temperature, the final carbon nano grape skins were obtained.
As a contrast experiment, the precursor of carbon hollow
i×∆t
C=
(1)
sphere was calcined only at 700 °C for 2 h.
m×∆V
Where C (F g-1) is the specific capacitance, i (A) is the discharge
current, t (s) is the total discharge time, m (g) is the load
∆
Preparation of carbon hollow spheres
mass of active materials, and
∆V (V) is the discharging
The Se@C composites obtained after 2.5 h hydrothermal
process were placed in a quartz tube furnace, and the tube
potential range.
In the two-electrode system, two CGS-700, CNS-700 or CHS-
700 electrodes made with the same size and weight and
loaded with the same quality of the active material were
separated by a piece of cellulose membrane, which constitutes
the subject part of the capacitor. The CV and GCD
was Ar purged for 30 min, then heated to 700
C min-1 maintaining for 2 h. After cooled to room
°C at the heating
°
temperature, the final carbon hollow spheres were obtained.
rate of 5
measurements were also carried out in
a 6.0 M KOH
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electrolyte. The electrochemical impedance spectrum was spheres with a size of approximately 500 nm. It is remarkable
obtained from the electrochemical workstation, which was set that these spheres have smooth surface and are separated
in a frequency range from 0.01 Hz to 0.1 MHz with the signal from each other. The transmission electron microscopy (TEM)
amplitude of 5 mV. The gravimetric specific capacitance of a image (Fig. S1b insert, ESI†) of a single sphere clearly shows
full cell Ccell (F g-1) was calculated according to the GCD curves the novel core-shell structure with an ultrathin shell of
obtained under different current densities based on the approximately of 10 nm. Through the calcination process,
equation as follow:
elemental Se rushed out from the top of shell of C@Se, which
just tore open a big hole but preserved the flexibility of the
ultrathin carbon wall. The release of the tensile force makes
the shell of C@Se collapsed to grape skins-like structure,
whose wrinkled wall is favorable to accumulate surface charge.
i×∆t
Ccell
=
(2)
mtotal×∆V
Where i (A) is the constant discharging current, ∆t (s) is the
discharge time, mtotal (g) is the total mass of active materials in
the full cell, and ∆V (V) is the discharging voltage range.
Multiplying the ꢀꢁꢂꢃꢃ by four is the specific capacitance of a
single electrode.
The energy density (Ecell, Wh kg-1) and power density (Pcell, W
kg-1) of a full cell can be calculated based on the equations as
follow:
Ccell×∆V2×1000
Ecell
=
(3)
2×3600
Fig. 1 Scheme depicting the synthesis of hierarchical porous nano carbon grape skins
from glucose
Ecell×3600
∆t
Pcell
=
(4)
SEM and TEM images (Fig. 2a-f) show that the three samples
obtained at different calcination temperature, which are
assigned as CGS-600, CGS-700, CGS-800 (the numbers
represent the calcination temperature), have grape skins-like
morphology in large domains. These carbon grape skins have
wrinkled surface, which showed considerable flexibility and
scalability. The shell thickness of the CGSs is approximately 3
nm after the calcination process, which can be estimated from
the plicate areas (contrast is deeper) of TEM images. Moreover,
the shell thickness of the three CGSs increased with the
temperature rising. The CGSs possess a novel structure as a
nanosheet rolled up from the around, which maintains the
ultrathin properties of nanosheets. However, unlike carbon
nanosheets, which are prone to aggregate or restack, the CGSs
were separated from each other due to the crimp force
induced by the rolled side. When the CGSs were calcinated at
Where Ccell (F g-1) is the specific capacitance of the full cell,
(V) is the discharging voltage range from the end of internal
∆V
resistance drop to the end of discharge, and
discharge time.
∆t (s) is the
In order to much more directly show the rate capability
through CV curves, we converted the current on the Y-axis to
specific capacitance through the formula as follow:
i
C=
(5)
m×s
Where C (F g-1) is the specific capacitance, i (A) is the current
measured in the CV analyses carried out in a Zahner IM6e
electrochemical workstation, m (g) is the load mass of active
materials, s (V s-1) is the corresponding scan rate.
600
like morphology (inserts of Fig. 2a and c). While, as the
temperature reached 800 C, the structure was destroyed with
°C and 700 °C, the structure maintained the grape skins-
°
Results and discussion
the CGSs separated into several small plates dispersing some
pores. The edge of the small plates was seriously curled into
serration shape (Fig. 2e).
The typical synthetic route for the carbon nano grape skins
(denoted as CGS) was illustrated in Fig. 1. Firstly, an ultrafine
Se@C core-shell composite34 as an intermediate product was
Fig. 3a presents the X-ray diffraction (XRD) of the three
samples obtained at different temperatures. Two obvious
broad peaks at about 24° and 44° can be observed in all the
samples, which are attributed to the (002) and (100)
reflections of graphitic carbon layers, respectively.35 No
obvious peaks of elemental selenium were found,
demonstrating the elemental selenium was nearly eliminated.
obtained through a simple hydrothermal reaction at 180 °C for
1.5 h. Then, the intermediate product was calcined at high
temperature to obtain the final samples. The XRD pattern of
Se@C core-shell composite, as shown in Fig. S1a (ESI†),
exhibits the peaks of amorphous carbon and elemental
selenium without other peaks, demonstrating the merely exist
of C and Se in the composite. A scanning electron microscopy
The energy dispersive X–ray spectroscopy (EDS) in Fig. S2 (ESI†)
(SEM) image (Fig. S1b, ESI†) shows that the Se@C composite
also suggests the rarely presence of elemental selenium in the
synthesized through hydrothermal reaction contains uniform
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CGSs. With increasing the growth temperature, the and 2-32 nm as calculated from the desorption branch by the
crystallinity of CGSs has significant improvement. Fig. 3b shows Barrett–Joyner–Halenda (BJH) method. The major peaks
the Raman spectrum of the fabricated CGSs. Two prominent distributing in 4-8, 4-9 and 4-32 nm, showing progressive shift
peaks can be observed at 1340 and 1596 cm-1 corresponding
as the temperature rise from 600 °C to 700 °C, and 800 °C,
could be attributed to the accumulated pores. The minor
Fig.
3 (a) XRD patterns, (b) Raman spectrums, (c) Nitrogen adsorption-desorption
isotherms and (d) pore size distributions by BHJ method of CGSs.
peaks distributing in 2-4 nm are assigned to the tunnels
through the graphite shells.18As we can see, the quantity of
the tunnels reduces with enhancing the temperature, which is
associated with the fact that the higher temperature could
improve the crystallinity of the carbon layers. The large surface
areas for CGS-600 and CGS-700 are comparable to AC and
graphene, which mainly originated from their ultrathin
graphite shells with tiny tunnels. The high surface/bulk carbon
atoms ratio and numerous tiny tunnels of ultrathin shell
shorten and accelerate the transport of ions or molecules.
Fig. 2 SEM and TEM images of (a, b) CGS-600, (c, d) CGS-700 and (e, f) CGS-800. Inserts
are the corresponding magnified images of CGSs.
to the D band and G band, respectively, indicating the
production of amorphous carbons with nanographitic
structures. The D band should be ascribed to the lattice defect
induced by the disordered structure of carbonaceous materials
or graphene edges, while the G band is associated with the
phonon mode with E2g symmetry of graphite. The stronger G
band means the production of graphitation structure in CGSs,
which would enhance the electron transportation by proving
numerous continuous pathways. Moreover, as increasing the
growth temperature of CGSs, the intensity ratio of D band to G
band (ID/IG) decreased from 0.92 to 0.79, indicating the
increase of crystallite size of graphite domains under a higher
When the CGSs were calcinated at 800 °C, the special surface
areas decreased dramatically, resulting from the denser and
thicker shell with fewer tiny tunnels.18
The performance of the CGSs as an electrode material for
supercapacitor was evaluated by cycle voltammetry (CV) and
galvanostatic charge-discharge (GCD) and electrochemical
impedance spectroscopy (EIS) technologies in
a three-
electrode configuration with 6 M KOH as electrolyte. Fig. 4a
shows the CV curves of CGSs obtained under different
calcination temperatures within the potential window of -0.9
to 0 V at the scan rate of 50 mV s-1. Though at the higher scan
rate, it can be seen that all the CV curves of the CGSs display
quasi-rectangular profiles, which exhibit the excellent
capacitive characteristics and the tiny equivalent series
resistance (ESR) with fast charging-discharging properties. As
we know, the quasi-rectangular areas of the CV curves reflect
the specific capacitance of the electrode, which exhibits a
positive correlation at the same scan rate.30,38,39 Therefore,
CGS-700 electrode has a larger special capacitance than those
of the other two electrodes at the same scan rate. The specific
capacitances of these three electrodes calculated from the CV
curves at a scan rate of 50 mV s-1 are 172 (CGS-600), 230 (CGS-
growth
temperature.36,37
N2
adsorption–desorption
measurements were carried out to investigate the
mesoporous structure of the CGSs obtained at different
temperatures. Fig. 3c,
d plotted the corresponding N2
adsorption–desorption isotherms and derived pore size
distributions. All the isotherms are type IV with a large specific
surface area and mesoporous feature. As the temperature rise
from 600 °C to 700 °C，and 800 °C, the Brunauer–Emmett–
Teller (BET) specific surface areas decrease from 1570.9
to1471.1, and 657.1 m2 g-1 ( insert of Fig. 2c), with the
corresponding total pore volume decreased from 2.429
to2.089, and 0.760 m3 g-1 and pore size distributions of 2-8, 2-9
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700) and 50 F g-1 (CGS-800) respectively. In general, the corresponding to the changing character as shown in CV. The
specific capacitance changing character is in agreement with charge/discharge performance of the CGS-700 electrode is
that of the specific surface area. However, CGS-700 has a tested at various current densities from 0.5 to 10 A g-1 as
larger specific capacitance compared with CGS-600, though its shown in Fig. 4e. Even at a high current density of 5 A g-1, no
specific surface area is smaller than that of CGS-600. This obvious ohmic drop is observed, further demonstrating the
result should be ascribed to the larger crystallinity of CGS-700 tiny EIS and excellent capacitive property during a relatively
as revealed by XRD and Raman characterizations, which fast charging-discharging process.40,41 The calculated specific
provides a higher conductivity. Therefore, compared to CGS- capacitances of these three electrodes from the discharge
600, the increased capacitance resulting from the enhanced curves at different current densities are shown in Fig. 4f. For all
conductivity is larger than the decreased capacitance from its the CGSs, with an increase of current density, the specific
reduced specific surface area. So as for the CGS-800, which capacitances decrease progressively, indicating the increasing
possess much higher conductivity
，its relative value of the diffusion limitation. The specific capacitance of CGS-700
specific capacitance is dominated by its specific surface area. It electrode at the current density of 0.5 A g-1 is calculated to be
is the general condition for the carbonaceous materials 315 F g-1, which is relatively higher among the carbonaceous
predominated by electrochemical double layer capacitance. supercapacitor electrodes reported previously, as shown in
Fig. 4b shows the CV curves of CGS-700 electrode at scan rates Table. S1 (ESI†). Moreover, the specific capacitances calculated
from 5 to 100 mV s-1. The specific capacitances of CGS-700 from the discharge curves at the current density of 1 2, 5 and
electrode at different scan rates are calculated as shown in Fig. 10 A g-1 were 282, 267, 258 and 248 F g-1, respectively,
4c. From 5 to 100 mV s-1, the specific capacitance decreased remaining on a relatively high level. The CGS-600 also shows
from 301 to 220 F g-1. The decreasing specific capacitance relatively high capacitance retention at the high charging-
mainly originated from the lag of ion transport behind the discharging rate. In contrast, the CGS-800 possesses a very
potential variation and the retarding of internal resistance to small specific capacitance, which originated from its small
the capacitance performance of electrode at higher scan rate.
specific surface areas and bad wettability.18 The long-term
Fig. 4d shows the GCD curves of these three electrodes at a cyclic stability of the capacitance performance of CGS-700 was
current density of 2 A g-1. These GCD profiles show triangular investigated by testing of 10000 cycles at a current density of 2
symmetrical distribution between -0.9 and 0 V corresponding A g-1, as shown in Fig. 5. The specific capacitance of CGS-700
to the potential window in the CV measurement, exhibit fine remains 94.4 % after 10000 cycle tests, showing excellent
capacitive properties for EDLCs. The highly linear and electrochemical stability and prominent reversibility. The
symmetrical show the excellent electrochemical reversibility insert in Fig. 5 shows the charge-discharge curves of the last 10
and charge–discharge properties. The calculated special cycles, which remain the highly linear and symmetrical and
capacity from the discharge curves of these electrodes are 220 have no obvious ohmic drop.
(CGS-600), 267 (CGS-700), and 75 F g-1 (CGS-800), which is
Fig. 4 Electrochemical performance characteristic of CGSs measured in a three-electrode system on the 6 M KOH electrolyte: (a) CV curves at 50 mV s-1 of CGSs; (b) CV
curves of CGS-700 at different scan rates; (c) specific capacitance of CGSs at different scan rates; (d) galvanostatic charge/discharge curves of CGSs at 2 A g-1; (e)
galvanostatic charge/discharge curves of CGS-700 at different current densities; (f) special capacitance of CGSs at different current densities.
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of the CHS-700, CNS-700 and CGS-700 electrodes at scan rates
from 5 to 200 mV s-1 under the two-electrode system,
respectively. The CV curves of these two electrodes all present
quasi-rectangular shape. As for CGS-700, no obvious distortion
was observed even at a high sweep rate of 200 mV s-1 further
demonstrate its tiny ESR and excellent capacitive property
during a high rate charging-discharging process. While, the
CNS-700 and CHS-700 exhibited distorted CV shapes when the
scan rate enhanced to 200 mV s-1 due to their delayed ion
transmission rate induced by the destruction of their structure.
It is noteworthy that the specific capacitance of CGS-700
remains as high as 178 F g-1 compared to that of CNS-700 (120
F g-1) and CHS-700 (129 F g-1) at a high scan rate of 100 mV s-1,
showing fast charging-discharging characteristic. The enhanced
capacitance performance could be ascribed to the novel
structure, which provided ultrathin carbon wall as that of
nanosheets and restrain the aggregation of CGSs in electrolyte,
decreasing the ion-transport resistance and diffusion paths. In
addition, the proper pore distribution originated from the
novel structure also enhanced the electrode kinetics. The
capacitive properties of CGS-700, CNS-700 and CHS-700 were
Fig. 5 Cycle stability of CGS-700 at the current density of 2 A g-1
To further investigate the electrochemical behavior of CGS-
700 electrode as real applicable capacitor in 6 M KOH aqueous
electrolyte, we constructed
a symmetrical two-electrode
capacitor, which reproduce the physical construction, charge
transfer resistance, and internal voltage. Meanwhile, to exhibit
the advantage of the structure of carbon grape skins as
supercapacitor electrodes, we synthesized carbon nanosheets
and carbon hollow spheres and studied their electrochemical
behavior in the two-electrode system. The carbon nanosheets
further
investigated
by
chronopotentiometry
(CP)
measurement. As shown in Fig. 6c and d, the GCD curves show
triangular symmetric without an obvious ohmic drop at lower
current density, suggesting high Coulombic efficiency and good
reversibility. The specific capacitance values calculated from
and carbon hollow spheres obtained at 700 °C were denoted
Fig. 6 Electrochemical performance characteristic of CGS-700, CNS-700 and CHS-700 measured in two-electrode system on 6 M KOH electrolyte: CV curves of (a) CHS-700, (b)
CNS-700 and (c) CGS-700 at different scan rates; galvanostatic charge/discharge curves of (d) CHS-700, (e) CNS-700 and (f) CGS-700 at different current densities
as CNS-700 and CHS-700, respectively, their SEM and TEM the discharge curves at the current density from 0.1 to 10 A g-1
images were shown in Fig. S5 and Fig.S6 (ESI†). As we can see, are shown in Fig. 7a. As for CGS-700, a high specific
the carbon nanosheets have lost the character of grape skins capacitance of 268 F g-1 was obtained at the low current
with less folded section and carbon hollow spheres showed a density of 0.1 A g-1, which is larger than most cases of
thick and frangibility shell. Fig. 6a, b, and c show the CV curves mesoporous carbon (Table. S1, ESI†). In contrast, the CHS-700
6 | J. Name., 2012, 00, 1-3
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merely exhibited a specific capacitance of 150 F g-1, which
The electrochemical impedance spectroscopy was measured
originated from its thicker wall and the overall fracture of the to further understand the ion and electron transport behavior
shell of the CHS-700 (Fig. S5, ESI ). As for carbon nanosheets, of CGS-700, CNS-700 and CHS-700 electrodes in the two-
†
the specific capacitance was 190 F g-1, which is higher than electrode system. Fig. 7c gives the Nyquist plots of the CGS-
that of CHS-700 due to its ion-transport resistance and 700, CNS-700 and CHS-700 electrodes based two-electrode
diffusion paths induced by their ultrathin carbon wall. The supercapacitor in a frequency range of 100 kHz-0.01 Hz, which
long-term cyclic stability test was also carried out by a can be divided into three parts, including a semicircle in the
constant current charging-discharging process at the current high frequency region, a line with a slope about 45° in the
density of 2 A g-1 for 10000 cycles, as shown in Fig. 7b. After middle frequency region and a nearly vertical line in the low
10000 cycles, the CGS-700 exhibited a specific capacitance of frequency region. The semicircle, which comprehensively
170 F g-1, with 94.2 % of the specific capacitance preserved, reflects the adsorption kinetics of the electrolyte ions at the
directly indicating the fine cycle durability of the CGS-700 as mesoporous carbon electrode, series resistance of porous
electrode material for supercapacitor. While, only 69.8 % and
carbon, and charge transfer resistance inside the carbon
structure at higher frequency, was much smaller in CGS-700
and CNS-700 compared to CHS-700, suggesting the faster ion
diffusion toward the surface of the CGS-700 and CNS-700
electrodes resulted from their higher electric conductivity.42
Besides, the equivalent series resistance (ESR) of the three
electrodes based capacitor are 0.15 Ω, which can be obtained
from the intercept of the EIS curves at the Z’ axis in the high
frequency region, including the ohmic resistance induced by
electrolyte , the contact between the active material and
current collector, and the intrinsic nature of the active material.
Moreover, it was can be seen that the 45° slop lines of CGS-
700 and CNS-700 showed a much shorter diffusion pathway
compared to CHS-700 due to the ultrathin carbon wall as that
of nanosheet. The much higher slope in low frequency region
part of the Nyquist plots indicated the CGS-700 electrode have
much more ideal EDLC behavior than the CNS-700 and CHS-
700 electrode.
Fig. 7 (a) Calculated specific capacitances of the CGS-700 and CHS-700 electrodes from
the discharge curves at different current densities; (b) Cycle stability of CGS-700 and
CHS-700 at the current density of 2 A g-1 for 10000 cycles. (c) Nyquist plot in the
frequency range from 0.01 Hz to 100 kHz of CGS-700 and CHS-700 electrodes; (d)
Ragone plots of the CGS-700 and CHS-700 measured in the two-electrode cell; insert is
the digital photograph of a LED lamp lighted by two symmetry supercapacitors
Fig. 7d shows the Ragone plot of the CGS-700, CNS-700 and
CHS-700 electrodes based symmetry supercapacitor, which
present the relationship of energy density and power density.
The energy density of CGS-700 can reach to 9.0 Wh kg-1 at the
power density of 51.9 W kg-1, which is higher than that of CNS-
700 (6.6 Wh kg-1, 50.6 W kg-1) and CHS-700 (5.2 Wh kg-1, 45.7
W kg-1). As shown in the insert, a low voltage LED lamp can be
lighted, although the lights were dim.
78.2 % of the specific capacitance of CNS-700 and CHS-700 was
retained after 10000 cycles, exhibiting much lower cyclic
stability. This is because the CGS-700 was formed by the rush
out of elemental Se from the top of shell of C@Se, which just
tore open a big hole but preserved the flexibility of the
ultrathin carbon wall. The release of the tensile force makes
the shell of C@Se collapsed to grape skins-like structure,
whose wrinkled wall is favorable to accumulate surface charge.
As for the formation of CHS-700, the elemental Se can’t rush
out from one side due to the thicker carbon wall. So the
elemental Se rushed out from all around of the shell of C@Se,
which destroyed the flexibility of the carbon wall, leading the
frangibility of the overall carbon shell. However, CNS-700 was
obtained by the enhancement of heating rate, which induced
large scale of tearing. Fast rushing out of the elemental Se
produced carbon nanosheets, however, the surface of
ultrathin carbon shell lost curvature, which reduced the
structure stability of carbon shell. So after long-term cyclic
stability test, its specific capacitance declined severely.
Therefore, the CGS-700 exhibited a much higher cyclic stability
than that of CNS-700 and CHS-700.
Conclusions
In summary, a grape skins-like carbon material with ultrathin
wall (<3 nm) was fabricated via a facile template-based
hydrothermal method using glucose as carbon source. The
resulting CGSs possess a novel structure as a nanosheet rolled
up from the around, which maintains the ultrathin properties
of nanosheets. However, unlike carbon nanosheets, which are
prone to aggregate or restack owning to the intersheet van der
waals’ force, the CGSs were separated from each other due to
the crimp force induced by the rolled side. Benefiting from the
novel structure, the CGSs have plenty of mesopores and a
large specific surface area (up to 1570.9 m2 g-1 for CGS-600).
When was used as electrode materials for supercapacitors, the
CGS-700 showed a much higher specific capacitance of 268 F g-
1 at 0.1 A g-1 in a two-electrode system (or 315 F g-1 at 0.5 A g-1
in a three-electrode system), a higher rate specific capacitance
of 170 F g-1 at 10 A g-1 (or 248 F g-1 at 10 A g-1 in a three-
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22 W. Li, F. Zhang, Y. Q. Dou, Z. X. Wu, H. J. Liu, X. F. Qian,
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94.2 % capacitance retention after 10000 cycles at 2 A g-1 (or
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Therefore, the glucose-derived CGSs exhibited application
potentials in economic and eco-friendly supercapacitor
electrode materials.
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Tensile force induced tearing and collapse of ultrathin carbon shells
to surface-wrinkled grape skins for high performance supercapacitor
electrodes
Guofeng Zhanga, Jing Zhanga, Qing Qina, Yingxue Cuia, Wenhao Luoa, Yan Suna, Cen
Jina, Wenjun Zheng*ab
Mesoporous carbon nano grape skins can be successfully fabricated via a facile
template-based hydrothermal method followed by a calcining process. The resulting
carbon nano grape skins possess a novel structure as a nanosheet rolled up or folded
from the around, which maintains the ultrathin properties of nanosheets. As electrode
materials for supercapacitors, they show large specific capacitance, high rate
capability and excellent long-term cycle stability.