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High-performance asymmetrical supercapacitor composedof rGO-enveloped nickel phosphite hollow spheres andN/S co-doped rGO aerogel
Deyang Zhang1, Yihe Zhang1 (), Yongsong Luo2, Yu Zhang1, Xiaowei Li1, Xuelian Yu1, Hao Ding1,
Paul K. Chu3, and Li Sun1 ()
1 Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials,
School of Materials Science and Technology, China University of Geosciences, Beijing 100083, China 2 School of Physics and Electronic Engineering, Xinyang Normal University, Xinyang 464000, China 3 Department of Physics and Department of Materials Science and Engineering, City University of Hong Kong, Tat Chee Avenue,
Kowloon, Hong Kong, China
Received: 8 May 2017
Revised: 23 July 2017
Accepted: 28 July 2017
© Tsinghua University Press
and Springer-Verlag GmbH
Germany 2017
KEYWORDS
nickel phosphate,
hollow sphere,
graphene,
nitrogen/sulfur
co-doping,
asymmetrical,
supercapacitor
ABSTRACT
An asymmetrical supercapacitor (ASC), comprising reduced graphene oxide
(rGO)-encapsulated nickel phosphite hollow microspheres (NPOH-0.5@rGO) as
positive electrode, and porous nitrogen/sulfur co-doped rGO aerogel (NS-3D rGO)
as negative electrode has been prepared. The NPOH-0.5@rGO electrode combines
the advantages of the NPOH hollow microspheres and the conductive rGO layers
giving rise to a large specific capacitance, high cycling reversibility, and excellent
rate performance. The NS-3D rGO electrode with abundant porosity and active
sites promotes electrolyte infiltration and broadens the working voltage range. The
ASC (NPOH-0.5@rGO//NS-3D rGO) shows a maximum voltage of up to 1.4 V,
outstanding cycling ability (capacitance retention of 95.5% after 10,000 cycles),
and excellent rate capability (capacitance retention of 77% as the current density
is increased ten times). The ASC can light up an light-emitting diodes (LED) for
more than 20 min after charging for 20 s. The fabrication technique and device
architecture can be extended to other active oxide and carbon-based materials
for next-generation high-performance electrochemical storage devices.
1 Introduction
Modern mobile devices require clean and renewable
energy sources and have generated tremendous
research efforts [1–5]. These energy sources including
solar, wind, and hydro are directly accessible, therefore
efficient energy storage/conversion devices are needed
[6]. In this respect, supercapacitors (SCs) have attracted
considerable scientific and technological interest due
to their large energy density, low cost, and being
environmentally friendly [7]. Depending on the charge
storage mechanism and the active materials used as
Nano Research 2018, 11(3): 1651–1663
https://doi.org/10.1007/s12274-017-1780-3
Address correspondence to Yihe Zhang,[email protected]; Li Sun, [email protected]
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1652 Nano Res. 2018, 11(3): 1651–1663
well, supercapacitors can be classified into two types:
electrical double-layer capacitors (EDLCs) which rely
on fast ion adsorption/desorption on the surface of
carbonaceous materials [8–10], and faradaic pseu-
docapacitors based on multiple redox reactions on
the surface or near-surface of the electrodes made of
transition metal oxide, conductive polymers, etc. [2].
Much work has been done on carbon-based materials
such as carbon nanotubes, graphene, and active carbon
for EDLCs, and transition metal oxides like RuO2
[10–12], MnO2 [13–15], NiO [16], ZnCo2O4 [17], and
NiCo2O4 [18, 19] for pseudocapacitors. However, most
materials cannot be used directly as electrodes due to
some limitations in energy storage. Although carbon-
based materials possess high microstructure stability
and long cycle life [20], they tend to have relatively
low capacitance and small energy density. Conversely,
the low conductivity and large volume variation of
transition metal oxides during charging and discharging
result in low rate capability and inferior cycle life, in
spite of having higher capacitance than carbon-based
materials [21]. In order to combine the advantages of
carbon-based materials and pseudocapacitor materials,
efforts have been made to develop carbon-based
electrode materials with higher energy and power
densities, rate capability, and cycle life.
Transition metal phosphides are promising materials
for supercapacitors due to their metalloid properties
and high electrical conductivity [22]. Transition metal
phosphates have the advantages of the open-framework
structure based on the hexagonal crystalline structure,
resulting in the acceleration of the electrolyte penetration.
In fact, the crystalline microporous M11(HPO3)8(OH)6
(M = Zn, Ni, and Co) has attracted much research
interest since the first report in 1993 [23], and the
present work on nickel phosphate Ni11(HPO3)8(OH)6
(NPOH) generally focuses on the design of nano-
materials with a large surface area and well-defined
morphology, for instance microporous NPOH nano-
crystals [24], NPOH nanotubes [25], and NPOH
hexagonal polyhedrons [26]. Similar to metal oxides,
the capacitive performance of the bare NPOH is reduced
by the low electron mobility, which kinetically limits
the rate capability, the large volume change that renders
rapid capacity loss during cycling, and the small
electrochemical potential window that restricts its
application to high-voltage devices. To widen the
practical applications of phosphite, the capacitive per-
formance has to be improved by (i) developing efficient
and simple synthesis of microstructures and (ii)
integrating with carbon-based materials to achieve
large conductivity and high structure stability.
In this work, reduced graphene oxide (rGO) enveloped
NPOH hollow microspheres (NPOH-0.5@rGO) have
been prepared by a surfactant-assisted method. The
hollow NPOH microspheres provide not only abundant
hollow cavities for easy electrolyte infiltration, but also
a large active surface area for efficient ion transfer
[27–29]. The encapsulated rGO thin layers introduced
by solution-based self-assembly further provide high
conductivity, well-dispersed microstructure, and high
structure stability. The NPOH-0.5@rGO delivers excellent
pseudocapacitive performance with high specific
capacitance and rate capability. The asymmetrical
supercapacitor assembled with the NPOH-0.5@rGO
as positive electrode and N/S co-doped 3D porous rGO
as negative electrode shows excellent cycling stability
(capacitance retention of 95.5% in 10,000 cycles) and
rate capability (capacitance retention of 77% when the
current density is increased ten times). The asymmetrical
supercapacitor powers an light-emitting diodes (LED)
for more than 20 min after charging for 20 s.
2 Experimental
2.1 Synthesis of Ni11(HPO3)8(OH)6 hollow micros-
pheres (NPOH-0.5)
NPOH-0.5 was fabricated by a simple hydrothermal
method. In a typical procedure, 1 mmol of Ni(NO3)2·6H2O,
0.5 mmol of NaH2PO2·H2O, and 0.5 mmol of
C17H35COONa were dissolved in 40 mL of deionized
water and ethyl alcohol with a volume ratio of 1:1.
The solution was transferred to a Teflon-lined stainless
steel autoclave and heated to 180 °C for 12 h. After
cooling to room temperature, the suspension and
green precipitates were separated by centrifugation,
washed with deionized water, benzene, and ethanol
several times, and dried at 80 °C for 5 h to obtain
NPOH-0.5. For comparison, NPOH-0 and NPOH-1
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1653 Nano Res. 2018, 11(3): 1651–1663
were fabricated under the same conditions, except for
the amounts of C17H35COONa which were changed
to 0 and 1 mmol, respectively.
2.2 Synthesis of the graphene-encapsulated NPOH-0.5
composites (NPOH-0.5 @rGO)
The graphene oxide (GO) nanosheets were synthesized
from natural graphite powders by the modified
Hummers method [30]. The NPOH-0.5 was dispersed
sequentially in the following three solutions at different
times at room temperature. Typically, NPOH-0.5 (50 mg)
was first added to 1 g·L−1 polyallylamine hydrochloride
(PAH) solution and dispersed ultrasonically for 1 h.
The solution was poured into 25 mL of 0.2 g·L−1 GO
under sonication and vigorous stirring at 0 °C for 5 h,
10 μL of hydrazine hydrate (N2H4·H2O) was added,
and the solution was heated to 98 °C for 1 h. Finally,
the sample was collected by filtration, washed with
deionized water, and freeze-dried for 48 h to obtain
the NPOH-0.5@rGO composites.
2.3 Synthesis of nitrogen and sulfur co-doped 3D
graphene aerogel (NS-3D rGO)
A GO solution (0.2 mg·mL−1) was prepared by ultrasonic
exfoliation of graphite oxide (20 mg) in deionized water
(100 mL) for 50 min. Then, 5 mg of thiosemicarbazide
(TSC) were dissolved in 40 mL of the GO solution
which was transferred to a Teflon-lined stainless steel
autoclave and heated to 180 °C for 12 h. After cooling
to room temperature, a nitrogen and sulfur co-doped
3D porous graphene hydrogel was obtained. After
washing with deionized water several times, the NS-3D
rGO hydrogel was freeze-dried for 48 h to obtain
the NS-3D rGO aerogel. For comparison, the pure 3D
rGO was prepared under the same conditions but
without TSC.
2.4 Characterization
The morphology and structure of the materials were
characterized by scanning electron microscopy (SEM,
Sirion 200 FEI, Hitachi SU-8020), transmission electron
microscopy (TEM, Hitachi H-8100), and powder X-ray
diffraction (XRD, Rigaku with Cu Kα radiation). The
diffraction patterns were recorded in the 2θ range
from 10° to 80°. The X-ray photoelectron spectroscopy
(XPS) was performed on a Kratos-Axis spectrometer
with monochromatic Al Kα (1,486.71 eV) radiation
(15 kV and 10 mA) and on a hemispherical electron
energy analyzer.
2.5 Electrochemical measurements
The electrochemical measurements were conducted
on a CHI 660E electrochemical workstation. In the
single-electrode tests, a conventional three electrode
system was used. The counter and reference electrodes
were Pt foil and Ag/AgCl, respectively. The working
electrodes were prepared by mixing the samples,
acetylene black, and polyvinylidene fluoride (PVDF)
at a weight ratio of 8:1:1 in N-methyl-2-pyrrolidone
(NMP), forming a slurry which was uniformly pasted
onto the Ni form, and dried in a vacuum oven at
120 °C for 12 h to remove the solvent. The 3 M KOH
solution served as an electrolyte. To fabricate the
asymmetrical supercapacitors, the as-prepared working
electrodes were punched into a disk and assembled
together with a cellulose separator sandwiched between,
followed by encapsulation in a CR2016-type coin
cell. All of the experiments were carried out at room
temperature.
The specific capacitances were calculated using the
equation: C = (I × Δt)/(m × ΔV), where I (mA) represents
the constant discharge current, m (mg), ΔV (V), and
Δt (s) designates the mass of the active materials, the
potential drop during discharge (excluding the IR drop),
and the total discharge time, respectively. The energy
and power densities (E and P) were calculated by using
equations: E = (∫IV(t)dt)/(m) and P = E/Δt, where I is the
discharging current, V(t) is the discharging voltage
excluding the IR drop, dt is the time differential, m is
the total mass of the two active electrode materials,
and Δt is the discharging time.
3 Results and discussion
3.1 Crystal structure of Ni11(HPO3)8(OH)6
The Ni11(HPO3)8(OH)6 has a hexagonal structure with
a = b = 12.6329 Å and c = 4.904 Å (ICSD-72432). The
crystal structure of the Ni11(HPO3)8(OH)6 is schematically
illustrated in Fig. 1, in which the [NiO6] octahedron
and [HPO3] pseudotetrahedron are arranged alternately.
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Each [NiO6] octahedron shares two edges with
two neighboring [NiO6] octahedra. These equivalent
structures are connected with each other in a 2 × 2
manner to create (Ni4O12)n double chains along the
c-axis. The arrangement of the octahedron chains creates
two kinds of pore channels in the crystal structure,
including smaller, triangular ones, surrounded by
three (Ni4O12)n chains that occupy 2/8 of the HPO32−
pseudotetrahedral groups, and larger, hexagonal
ones (channel size: 5.189 Å) located on the remaining
6/8 of the phosphite groups [23]. These channels provide
abundant pathways for diffusion of molecular and
cationic species in the electrolyte, allowing easy
electrolyte infiltration, rapid ion transfer, and favorable
rate performance.
3.2 Characterization of the morphologies and
structures of NPOH-0.5@rGO
The NPOH hollow microspheres (NPOH-0.5) are
synthesized hydrothermally with a suitable amount
of surfactants (0.5 mmol). NPOH-0.5 has a diameter
of 1–2 μm (Fig. 2(a)) and a shell thickness of about
500 nm (Fig. 2(b)). The shells of the NPOH-0.5 are
constructed by numerous interconnected grains, each
with a diameter of about 300 nm (Fig. 2(c)). This special
microstructure gives the hollow spheres rough interior
and outer surfaces, thereby it can provide plenty of
holes for easy electrolyte infiltration and large active
surface areas for efficient ion transfer. Figure 2(d)
shows the energy dispersive X-ray spectroscopy (EDS)
elemental maps of NPOH-0.5 and Ni, P, and O are
uniformly distributed. The TEM image in Fig. 2(e)
reveals the hollow structure and the uniform shell
thickness. Clear grain lattices can be seen in the high-
resolution TEM image shown in Fig. 2(f), and it shows
that the typical lattice distance of 0.27 nm agrees
with the d-spacing of the (400) facet of the hexagonal
Ni11(HPO3)8(OH)6 (JCPDS card No. 81-1065). The
corresponding selected area diffraction (SAED) results
of the NPOH-0.5 (Fig. 2(g)) corresponding to the
hexagonal Ni11(HPO3)8(OH)6, indicate the crystallization
of NPOH-0.5 grains.
The morphology of the synthesized NPOH depends
on the amount of surfactants in the hydrothermal
treatment. When no surfactant is used (NPOH-0), the
NPOH grains tend to form irregular sheets which are
unstable and easily agglomerate or break down during
cycling (Figs. S1(a)–S1(c) in the Electronic Supplemen-
tary Material (ESM)). When an excessive amount of
surfactant is added (1 mmol), the porosity among
NPOH grains is eliminated and the rather smooth
surface (Figs. S1(d) and S1(e) in the ESM) limits the
electrolyte infiltration and lowers the NPOH utilization.
Compared with NPOH-0 and NPOH-1, NPOH-0.5 is
more favorable as electrode material in supercapacitors.
The interior space and stable microstructure of
NPOH-0.5 allow efficient electrolyte accommodation
and high cycling reversibility. The rough surface
on NPOH-0.5 provides a larger specific surface area
providing more active sites for the fast electrochemical
Figure 1 Schematic crystal structure of the Ni11(HPO3)8(OH)6 based on data from ICSD-72432 with the unit cell (left) and super cell (4 × 4 × 1) viewed along the (001) plane (right).
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1655 Nano Res. 2018, 11(3): 1651–1663
reaction. As shown in Fig. S2 in the ESM, the abundant
holes on the shells of NPOH-0.5 provide ion penetration
paths in an alkaline electrolyte.
A possible formation mechanism of NPOH-0.5 is
described in Fig. 3. Sodium stearate (Ste-Na) is chosen
both as surfactant and as soft template which inherently
forms spherical vesicles in the aqueous solution with
proper concentration [29].
The Ni2+ deposited on the surface sites of these
vesicles increases the Ni2+ concentration near the
hydrophilic parts of the Ste-Na. Meanwhile, H2PO2−
decomposes into HPO32−, and P3− and HPO3
2− coordinates
with Ni2+ around the vesicles forming a nickel phosphate
precursor, which further condenses onto the hollow
microspheres due to Ostwald ripening. The associated
reactions are described as follows [31]
3H2PO2− + OH− = 2HPO3
2− + PH3↑+ H2O (1)
11Ni2+ + 8 HPO32− + 6OH− = Ni11(HPO3)8(OH)6 (2)
To encapsulate NPOH-0.5, clean and ultrathin GO
sheets were synthesized by the modified Hummer’s
method. The TEM image of the as-prepared GO
(Fig. S3(a) in the ESM) reveals the ultrathin, transparent,
Figure 2 (a)–(c) Low-magnification SEM images of the NPOH-0.5 hollow microspheres, (d) EDS elemental maps of Ni, P, and O, (e) low- and (f) high-magnification TEM images of the NPOH-0.5 hollow microspheres, (g) SAED patterns of the NPOH-0.5.
Figure 3 Possible formation mechanism of the NPOH-0.5 hollow microspheres.
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1656 Nano Res. 2018, 11(3): 1651–1663
and gauze-like features. The atomic force microscopy
(AFM) image and the corresponding height profile
(Fig. S3(b) in the ESM) shows smooth nanosheets
with an average height of 1.731 nm. The graphene-
enveloped NPOH-0.5 (NPOH-0.5@rGO) was prepared
by self-assembly of NPOH-0.5 and GO, and in situ
reduction of GO in the aqueous solution. As shown
in Figs. 4(a) and 4(c), both the outer surface of the
NPOH-0.5 spheres and the interior surface of some
hemispheres are uniformly encapsulated by a layer
of ultrathin rGO nanosheets as bridges between
neighboring NPOH-0.5 hollow spheres (Fig. 4(b)).
The enveloped rGO protects the inner crystal grains
from breaking, and facilitates rapid electron transfer
between the microspheres to improve the rate pro-
perties. The Brunauer–Emmett–Teller (BET) analysis
also reveals that the NPOH-0.5@rGO has a high specific
surface area of ~ 155 m2·g–1 (Fig. S4 in the ESM).
Figures 4(d) and 4(e) display the SEM image and the
corresponding EDS of NPOH-0.5@rGO showing the
rGO layers and uniform distribution of Ni, P, O, and
C. The matching lattice structure is theoretically
confirmed by the microcosmic interface model of
NPOH and graphene in the top view (Fig. 4(f1)) and
side view (Fig. 4(f2)). A lattice mismatch of only 2.6%
can be seen from the 5 × 5 graphene supercell for the
matching of one nickel phosphite unit cell.
The crystal structure of the NPOH samples and
NPOH-0.5@rGO composite was investigated by XRD.
As shown in Fig. 5(a), diffraction peaks of the hexagonal
Ni11(HPO3)8(OH)6 (JCPDS card No. 81-1065) can be seen.
After encapsulation with graphene, a broad peak at
2θ ≈ 23° corresponding to the (002) plane of graphene
can be observed. The XPS survey spectra of NPOH-0.5
and NPOH-0.5@rGO in Fig. 5(b) reveal the presence
of Ni, P, O, and C [32].
Following the introduction of rGO, the intensity of
the C 1s peak increases, and the intensities of the Ni 2p,
O 1s, and P 2p peaks decrease further, corroborating
the existence of rGO layers around the NPOH-0.5
hollow microspheres. The high-resolution Ni 2p
spectrum (Fig. 5(c)) can be deconvoluted into four
sub-peaks at 857.2 eV (Ni 2p3/2), 863.4 eV (satellite peak
attributed to multi-electron excitation) [33], 875.2 eV
(Ni 2p1/2), and 881.5 eV (shake-up peak of Ni 2p1/2).
Similarly, the O 1s spectrum (Fig. 5(d)) can be resolved
into four components, with the low binding energy
component at 530 eV attributed to O2− forming oxide
with nickel, and the other three at 531.2, 533, and
534.7 eV assigned to OH−, C–O and O–C=O, and H2O,
respectively [34]. The high-resolution C 1s spectrum
can be fitted by one dominant component for C–C
(284.6 eV), and there are smaller components for C–O
(286.7 eV), C=O (288.0 eV), and O–C=O (289.2 eV),
Figure 4 (a)–(d) Low-magnification SEM images of NPOH-0.5@rGO, (e) EDS elemental maps of Ni, P, C, and O. Schematic illustration of graphene/NPOH-0.5 interface in a top view (f1) and side view (f2).
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Figure 5 (a) XRD patterns of the NPOH-0, NPOH-0.5, NPOH-1, and NPOH-0.5@rGO composites, (b) XPS survey spectra of NPOH- 0.5 and NPOH-0.5@rGO. High-resolution (c) Ni 2p, (d) O 1s, (e) C 1s, and (f) P 2p XPS spectra of NPOH-0.5@rGO.
revealing the existence of rGO in the composite
(Fig. 5(e)) [35]. The P 2p spectrum in Fig. 5(f) presents
one peak at 133.5 eV, indicating that P has a +3
oxidization state [36].
3.3 Electrochemical performance of NPOH-0.5@ rGO
The electrochemical performance of the NPOH and
NPOH-0.5@rGO electrodes is assessed to use the
three-electrode configuration. Figure 6(a) shows a pair
of redox peaks in the potential range between 0
and 0.5 V (vs. Ag/AgCl) related to the faradaic redox
reactions in the alkaline electrolyte [24]
HP-Ni(II) + OH− ↔ HP-(OH−)Ni(III) + e− (3)
where HP represents the phosphite hydroxide group
[-(HPO3)8(OH)6]. Owing to the hollow microstructure,
the bare NPOH-0.5 sample exhibits a larger capacitance–
voltage (CV) area (Fig. 6(a)) than NPOH-0 and NPOH-1,
and the CV curve area of NPOH-0.5@rGO increases
drastically.
The advantage of the NPOH-0.5@rGO electrode is
more apparent at large scanning rates (Fig. S5 in the
ESM). Compared with the bare NPOH electrode with
the obvious polarization near 0.5 V at large scanning
Figure 6 Comparison of (a) CV, (b) galvanostatic charging–discharging curves, and (c) specific capacitance of the NPOH-0, NPOH-0.5,NPOH-1, and NPOH-0.5@rGO electrodes. (d) Possible ion penetration and electron transmission mechanism of the NPOH-0.5@rGO electrodes in an alkaline electrolyte.
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1658 Nano Res. 2018, 11(3): 1651–1663
rates (Figs. S5(a)–S5(c) in the ESM), the shape of the
CV curves of NPOH-0.5@rGO (Fig. S5(d) in the ESM)
remains unchanged, which indicates fast charge transfer
and reversible redox reactions due to the conductive
rGO [37, 38]. The galvanostatic charging/discharging
(GCD) curves in Fig. 6(b) show an obvious voltage
plateau at 0.18–0.25 V for all electrodes, further con-
firming the significant contribution to pseudocapaci-
tance. Similar to the CV curves, the longest discharge
time can be discovered from NPOH-0.5@rGO for
different current densities (Fig. S6 in the ESM), indicating
the synergistic effects between NPOH-0.5 and rGO.
As shown in Fig. 6(c), the NPOH-0.5@rGO electrode
exhibits a specific capacitance of 1,120 F·g−1 at 0.8 A·g−1,
which is nearly twice of that of NPOH-0.5 (669 F·g−1),
three times of that of NPOH-0 (392 F·g−1), and four
times of that of NPOH-1 (288 F·g−1). As the current
density is increased to 8 A·g−1, a large specific capaci-
tance of 672 F·g−1 is observed from NPOH-0.5@rGO
and it is much higher than 184 F·g−1 of NPOH-0.5,
163 F·g−1 of NPOH-0, and 99 F·g−1 of NPOH-1. The
superior performance of the NPOH-0.5@rGO stems
from the special microstructure illustrated in Fig. 6(d).
The graphene nanosheets surrounding the NPOH-0.5
serve as active materials contributing to the capacitance
and conductive agents facilitating electron transfer
between the microspheres.
3.4 Morphology, structure, and electrochemical
performance of NS-3D rGO
The nitrogen/sulfur co-doped rGO aerogel (NS-3D
rGO) was prepared as the negative electrode by
one-step hydrothermal reduction of the GO aqueous
dispersion, with TSC as the N/S source followed by
freeze-drying. The photograph of the NS-rGO hydrogel
(Fig. 7(a1)) indicates a small density and integrated
3D structure after the freeze-drying (Fig. 7(a2)). The
SEM images of the NS-3D rGO (Fig. 7(b)) present a
highly porous 3D network composed of interconnected
graphene nanosheets with pore sizes ranging from
sub-micrometers to several micrometers.
Figure S7 in the ESM shows the XRD patterns of
the pure 3D rGO and NS-3D rGO, and that the structure
of graphene is intact after N/S doping. Compared
with the 3D rGO aerogel without doping (Figs. S8(a)
and S8(b) in the ESM), several tiny holes can be
observed on NS-3D rGO (Figs. S8(c) and S8(d) in the
ESM). These were introduced during nitrogen and
sulfur doping and are expected to facilitate electrolyte
penetration in the NS-3D rGO aerogel and to improve
both the specific capacitance and the rate capability.
BET analysis reveals that the NS-3D rGO has a high
specific surface area of ~ 228 m2·g–1 (Fig. S9 in the ESM).
The EDS spectrum (Fig. S10 in the ESM) confirms the
Figure 7 Photographs of the as-prepared NS-3D rGO (a1) hydrogel and (a2) aerogel obtained by freeze-drying. (b) Low-magnificationSEM images of the NS-3D rGO aerogel; (c) EDS elemental maps of C, N, and S. (d) XPS survey spectrum of the 3D rGO and NS-3D rGO and high-resolution. (e) N 1s and (f) S 2p XPS spectra of NS-3D rGO.
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1659 Nano Res. 2018, 11(3): 1651–1663
existence of C, N, and S in the NS-3D rGO without
impurities, and the EDS maps in Fig. 7(c) reveal
homogeneous distributions in the aerogel. The elemental
composition and chemical bonding states of the NS-3D
rGO were determined by XPS (Fig. 7(d)). Apart from
the C 1s and O 1s peaks originating from rGO [39],
the characteristic peaks of S 2p and N 1s are emerge
from NS-3D rGO, which is in good agreement with
other N/S co-doped carbon materials reported in the
literature [40]. The N 1s spectrum (Fig. 7(e)) is com-
posed of three peaks: pyridinic N, pyrolic N, and
graphitic N at 398.2, 399.3 and 401.2 eV, respectively
[41]. Similarly, the high-resolution S 2p peak can be
fitted with two peaks (Fig. 7(f)) at 163.9 (S 2p3/2) and
165.2 eV (S 2p1/2), indicating C–S and conjugated
–C=S– bonds, respectively [42].
The electrochemical performance of the rGO and
NS-3D rGO electrodes was evaluated. Similar to the
literature, typical rectangular CV curves (Fig. S11(a)
in the ESM) and symmetrical triangular GCD curves
(Fig. S11(b) in the ESM) can be seen from 3D-rGO,
indicating the electrical double-layer capacitive
behavior. In contrast, the CV and GCD curves of NS-3D
rGO have an irregular shape, probably because of
some irreversible faradic processes [43]. Compared
with 3D-rGO without doping, the NS-3D rGO shows
larger CV curve areas and longer discharge time
suggesting an increased capacitance. The shape of
the CV (Fig. S11(c) in the ESM) and GCD (Fig. S11(d)
in the ESM) curves is essentially unchanged at larger
scanning rates or current densities, indicating fast ion
adsorption/desorption and reversible charge transfer
processes. Figure S12 in the ESM shows the specific
capacitance of NS-3D rGO at different current densities,
and the specific capacitance of NS-3D rGO at 1 A·g−1
is 249.4 F·g−1. The superior performance of NS-3D
rGO stems from the highly porous feature ensuring
effective electrolyte access, large surface area providing
abundant adsorption sites for substantial EDLC
capacitance, and introduced functional groups/
heteroatoms (including N and S) contributing to the
additional faradic pseudocapacitance [44].
3.5 ASC device NPOH-0.5@rGO//NS-3D rGO
To further evaluate the electrodes in real applications,
an ASC device (denoted as NPOH-0.5@rGO//NS-3D
rGO), consisting of a NPOH-0.5@rGO (mNPOH-0.5@rGO ≈
6.32 mg) as positive electrode, an NS-3D rGO as
negative electrode, a 3 M KOH as electrolyte, and one
piece of cellulose paper as the separator was produced
(Fig. 8(a)). As for the ASC, to balance the charge storage
(Q+ = Q−) between the two electrodes, the masses of
the electrode materials need to follow the equation:
(m−/m+) = (C+ × ΔV+)/(C+ × ΔV+) [45], where m is the
mass of electrode, C is the specific capacitance, ΔV is
the potential range for the charge–discharge process,
and subscripts "+" and "−" are the positive and negative
charge carriers. Referring to the specific capacitance
calculated from the above CV, ~ 7.39 mg of NS-3D
rGO is required. Figure S13(a) in the ESM shows the
CV curves of the NPOH-0.5@rGO and NS-3D rGO
electrodes. Stable voltage windows of ~ 0–0.5 V and
~ −1–0 V are identified from NPOH-0.5@rGO and NS-3D
rGO, respectively. The CV curves were obtained between
0 and 1.4 V (Fig. 8(b)) and they remained rectangular
as the scanning rate was increased, suggesting an
excellent rate capability. Besides, as the current density
increased from 1 to 10 A·g−1, the discharging curves
remained almost symmetrical to the charging curves
(Fig. 8(c)), reflecting excellent capacitive behavior. The
specific capacitances of the ASC device at 1, 2, 3, 5, and
10 A·g−1 were 42, 41, 40, 37, and 32 F·g−1, respectively
(Fig. S13(b) in the ESM).
Excellent rate capability and cycle performance are
also achieved by the ASC device. Figure 8(d) shows
the rate capability as the current density was increased
progressively. During the first 500 cycles at 1 A·g−1,
the ASC exhibited an initial capacitance of 42 F·g−1
and as the current density increased, the capacitance
remained stable at 41.4, 39.7, and 37 F·g−1 at 2, 3 and
5 A·g−1, respectively. When the current density was
10 A·g−1, a high capacitance of 32.5 F·g−1 corresponding
to a retention of 77.38% was still achieved. Moreover,
when the current density lowered back to 1 A·g−1, a
capacitance of 41.7 F·g−1 was recovered, demonstrating
high reversibility. Apart from the excellent rate
capability, the ASC device delivered outstanding
cycling performance (inset in Fig. 8(d)), when cycling at
5 A·g−1, capacitances of ~ 37.9–36.2 F·g−1 were achieved
in 10,000 cycles, corresponding to a capacitance
retention of 95.5%. It is noted that the capacitance
increased during the initial cycles in both rate and
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1660 Nano Res. 2018, 11(3): 1651–1663
cycling tests due to an “activation process”. The
Ragone plot describing the relationship between
the energy density and power density is shown in
Fig. S13(c) in the ESM. The ASC device shows an
energy density of 13.125 Wh·kg–1 at a power density
of 750 W·kg–1 and 10 Wh·kg–1at a high power density
of 7,500 W·kg–1.
For further demonstration, the ASC device is used to
power LED. The ASC devices were connected in series
in order to output the required voltage. According to
the GCD curves in Fig. 8(e), the operating voltages
can be extended to 1.4, 2.8, and 4.2 V for a single
ASC, two ASCs in series, and three ASCs in series,
respectively, and it is consistent with the voltage
monitored by a digital multimeter (Fig. 8(f) and
Fig. S13(d) in the ESM). As shown in Fig. 8(f), the two
ASC devices in series can power a 3-mm-diameter blue
LED (2.2–2.4 V) and the three ASC devices in series
with a higher output voltage can power one blue LED
and one red LED simultaneously (4–4.5 V). In fact,
after charging for 20 s, the LEDs were lit for as long
as 20 min, providing experimental evidence of the
excellent performance of the devices.
4 Conclusions
An efficient strategy to synthesize rGO-enveloped
NPOH hollow spheres and N/S co-doped rGO aerogels
as electrodes in supercapacitors is described. The
ASC device composed of NPOH-0.5@rGO//NS-3D
rGO has been assembled to demonstrate the highly
reversible capacitance, excellent cycling ability, and
Figure 8 (a) Schematic illustration of the ASC configuration. (b) CV curves of the ASC device at different scanning rates in the voltagewindow between 0 and 1.4 V. (c) Galvanostatic charging/discharging curves at different current densities. (d) Rate and cycling performanceof the device (insert in (d)). (e) Galvanostatic charging–discharging curves of the single ASC, two ASCs in series, and three ASCs inseries at 1 A·g−1. (f) Voltage test of single ASC and optical images of two (three) ASCs in series lighting up a blue (a blue and a red)LED indicator simultaneously.
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1661 Nano Res. 2018, 11(3): 1651–1663
good rate capability. The excellent electrochemical
performance can be attributed to several reasons.
First of all, the unique crystal structure of the NPOH
provides pore channels enhancing diffusion of cationic
electrolyte species and resulting in a more effective
ion transfer. Secondly, the NPOH-0.5 hollow spheres
have a large surface area and a highly porous structure
ensuring large specific capacitance. Thirdly, the
conductive rGO nanosheets enveloping the NPOH-0.5
contribute to the capacitance, facilitate electron transfer
between the microspheres, consequently improving
the rate and cycle performance. Fourthly, the nitrogen
and sulfur increase the electron mobility in the rGO
aerogels and contribute to additional faradic pseudo-
capacitance, which further facilitate in gel electrolyte
penetration in the NS-3D rGO to improve the
capacitance and rate capability. The strategy and device
architecture can be extended to other active oxide and
carbonaceous materials to create new opportunities
in the design of future high-performance electrochemical
storage devices.
Acknowledgements
This work was jointly supported by the National
Natural Science Foundation of China (No. 51572246),
Fundamental Research Funds for the Central Universi-
ties (Nos. 53200859565, 53200859500 and 2652015425),
as well as City University of Hong Kong Applied
Research Grant (ARG) (No. 9667122) and Strategic
Research Grant (SRG) (No. 7004644).
Electronic Supplementary Material: Supplementary
material (the SEM and TEM of the NPOH-0 and
NPOH-1; the XRD, SEM and electrochemical perfor-
mance of the rGO and NS-3D RGO aerogel; the BET
of the NPOH-0.5@rGO and NS-3D RGO) is available
in the online version of this article at https://doi.org/
10.1007/s12274-017-1780-3.
References
[1] Miller, J. R.; Simon, P. Electrochemical capacitors for energy
management. Science 2008, 321, 651–652.
[2] Simon, P.; Gogotsi, Y. Materials for electrochemical capacitors.
Nat. Mater. 2008, 7, 845–854.
[3] Sun, L.; Li, M.; Jiang, Y.; Kong, W. B.; Jiang, K. L.; Wang,
J. P.; Fan, S. S. Sulfur nanocrystals confined in carbon
nanotube network as a binder-free electrode for high-
performance lithium sulfur batteries. Nano Lett. 2014, 14,
4044–4049.
[4] Sun, L.; Wang, D. T.; Luo, Y. F.; Wang, K.; Kong, W. B.;
Wu, Y.; Zhang, L. N.; Jiang, K. L.; Li, Q. Q.; Zhang, Y. H.
et al. Sulfur embedded in a mesoporous carbon nanotube
network as a binder-free electrode for high-performance
lithium-sulfur batteries. ACS Nano 2016, 10, 1300–1308.
[5] Hou, X. Y.; Peng, T.; Cheng, J. B.; Yu, Q. H.; Luo, R. J.;
Lu, Y.; Liu, X. M.; Kim, J. K.; He, J.; Luo, Y. S. Ultrathin
ZnS nanosheet/carbon nanotube hybrid electrode for high-
performance flexible all-solid-state supercapacitor. Nano Res.
2017, 10, 2570–2583.
[6] Luo, Y. S.; Luo, J. S.; Jiang, J.; Zhou, W. W.; Yang, H. P.;
Qi, X. Y.; Zhang, H.; Fan, H. J.; Yu, D. Y. W.; Li, C. M. et al.
Seed-assisted synthesis of highly ordered TiO2@α-Fe2O3
core/shell arrays on carbon textiles for lithium-ion battery
applications. Energy Environ. Sci. 2012, 5, 6559–6566.
[7] Xu, Y. X.; Huang, X. Q.; Lin, Z. Y.; Zhong, X.; Huang, Y.;
Duan, X. F. One-step strategy to graphene/Ni(OH)2 composite
hydrogels as advanced three-dimensional supercapacitor
electrode materials. Nano Res. 2013, 6, 65–76.
[8] Chen, J.; Li, C.; Shi, G. Q. Graphene materials for electro-
chemical capacitors. J. Phys. Chem. Lett. 2013, 4, 1244–1253.
[9] Bose, S.; Kuila, T.; Mishra, A. K.; Rajasekar, R.; Kim, N. H.;
Lee, J. H. Carbon-based nanostructured materials and their
composites as supercapacitor electrodes. J. Mater. Chem.
2012, 22, 767–784.
[10] Wang, H. L.; Liang, Y. Y.; Mirfakhrai, T.; Chen, Z.;
Casalongue, H. S.; Dai, H. J. Advanced asymmetrical
supercapacitors based on graphene hybrid materials. Nano
Res. 2011, 4, 729–736.
[11] Zhang, J. T.; Jiang, J. W.; Li, H. L.; Zhao, X. S. A high-
performance asymmetric supercapacitor fabricated with
graphene-based electrodes. Energy Environ. Sci. 2011, 4,
4009–4015.
[12] Wu, Z. S.; Wang, D. W.; Ren, W. C.; Zhao, J. P.; Zhou, G. M.;
Li, F.; Cheng, H. M. Anchoring hydrous RuO2 on graphene
sheets for high-performance electrochemical capacitors. Adv.
Funct. Mater. 2010, 20, 3595–3602.
[13] Zhang, D. Y.; Zhang, Y. H.; Luo, Y. S.; Chu, P. K. Highly
porous honeycomb manganese oxide@carbon fibers core–shell
nanocables for flexible supercapacitors. Nano Energy 2015,
13, 47–57.
[14] Park, S.; Shim, H. W.; Lee, C. W.; Song, H. J.; Park, I. J.;
Kim, J. C.; Hong, K. S.; Kim, D. W. Tailoring uniform
γ-MnO2 nanosheets on highly conductive three-dimensional
| www.editorialmanager.com/nare/default.asp
1662 Nano Res. 2018, 11(3): 1651–1663
current collectors for high-performance supercapacitor
electrodes. Nano Res. 2015, 8, 990–1004.
[15] Peng, Y. T.; Chen, Z.; Wen, J.; Xiao, Q. F.; Weng, D.; He,
S. Y.; Geng, H. B.; Lu, Y. F. Hierarchical manganese oxide/
carbon nanocomposites for supercapacitor electrodes. Nano
Res. 2011, 4, 216–225.
[16] Yan, H. L.; Zhang, D. Y.; Xu, J. Y.; Lu, Y.; Liu, Y. X.; Qiu,
K. W.; Zhang, Y. H.; Luo, Y. S. Solution growth of NiO
nanosheets supported on Ni foam as high-performance
electrodes for supercapacitors. Nanoscale Res. Lett. 2014,
9, 424.
[17] Zhang, D. Y.; Zhang, Y. H.; Li, X. W.; Luo, Y. S.; Huang, H.
W.; Wang, J. P.; Chu, P. K. Self-assembly of mesoporous
ZnCo2O4 nanomaterials: Density functional theory calculation
and flexible all-solid-state energy storage. J. Mater. Chem.
A 2016, 4, 568–577.
[18] Zhang, D. Y.; Yan, H. L.; Lu, Y.; Qiu, K. W.; Wang, C. L.;
Tang, C. C.; Zhang, Y. H.; Cheng, C. W.; Luo, Y. S.
Hierarchical mesoporous nickel cobaltite nanoneedle/carbon
cloth arrays as superior flexible electrodes for supercapacitors.
Nanoscale Res. Lett. 2014, 9, 139–147.
[19] Zhang, D. Y.; Yan, H. L.; Lu, Y.; Qiu, K. W.; Wang, C. L.;
Zhang, Y. H.; Liu, X. M.; Luo, J. S.; Luo, Y. S. NiCo2O4
nanostructure materials: Morphology control and electro-
chemical energy storage. Dalton Trans. 2014, 43, 15887–
15897.
[20] Huang, Y.; Liang, J. J.; Chen, Y. S. An overview of the
applications of graphene-based materials in supercapacitors.
Small 2012, 8, 1805–1834.
[21] Wu, Z. S.; Zhou, G.; Yin, L. C.; Ren, W.; Li, F.; Cheng, H.
M. Graphene/metal oxide composite electrode materials for
energy storage. Nano Energy 2012, 1, 107–131.
[22] An, C. H.; Wang, Y. J.; Wang, Y. P.; Liu, G.; Li, L.; Qiu,
F. Y.; Xu, Y. N.; Jiao, L. F.; Yuan, H. T. Facile synthesis
and superior supercapacitor performances of Ni2P/rGO
nanoparticles. Rsc Adv. 2013, 3, 4628–4633.
[23] Marcos, M. D.; Amoros, P.; Beltran-Porter, A.; Martinez-
Manez, R.; Attfield, J. P. Novel crystalline microporous
transition-metal phosphites M11(HPO3)8(OH)6 (M = Zn, Co,
Ni). X-ray powder diffraction structure determination of the
cobalt and nickel derivatives. Chem. Mater. 1993, 5, 121–128.
[24] Gao, Y. P.; Zhao, J. H.; Run, Z.; Zhang, G. Q.; Pang, H.
Microporous M11(HPO3)8(OH)6 nanocrystals for high-
performance flexible asymmetric all solid-state supercapacitors.
Dalton Trans. 2014, 43, 17000–17005.
[25] Pang, H.; Wei, C. Z.; Ma, Y. H.; Zhao, S. S.; Li, G. C.;
Zhang, J. S.; Chen, J.; Li, S. J. Nickel phosphite superstructures
assembled by nanotubes: original application for effective
electrode materials of supercapacitors. ChemPlusChem 2013,
78, 546–553.
[26] Pang, H.; Yan, Z. Z.; Wei, Y. Y.; Li, X. X.; Li, J.; Zhang, L.;
Chen, J.; Zhang, J. S.; Zheng, H. H. The morphology
evolution of nickel phosphite hexagonal polyhedrons and
their primary electrochemical capacitor applications. Part.
Part. Syst. Char. 2013, 30, 287–295.
[27] Lai, X. Y.; Halpert, J. E.; Wang, D. Recent advances in
micro-/nano-structured hollow spheres for energy applications:
From simple to complex systems. Energy Environ. Sci. 2012,
5, 5604–5618.
[28] Xu, S. M.; Hessel, C. M.; Ren, H.; Yu, R. B.; Jin, Q.; Yang,
M.; Zhao, H. J.; Wang, D. α-Fe2O3 multi-shelled hollow
microspheres for lithium ion battery anodes with superior
capacity and charge retention. Energy Environ. Sci. 2014, 7,
632–637.
[29] Wang, X. J.; Feng, J.; Bai, Y. C.; Zhang, Q.; Yin, Y. D.
Synthesis, properties, and applications of hollow micro-/
nanostructures. Chem. Rev. 2016, 116, 10983–11060.
[30] Hummers Jr, W. S.; Offeman, R. E. Preparation of graphitic
oxide. J. Am. Chem. Soc. 1958, 80, 1339–1339.
[31] Liao, K. M.; Ni, Y. H. Synthesis of hierarchical
Ni11(HPO3)8(OH)6 superstructures based on nanorods
through a soft hydrothermal route. Mater. Res. Bull. 2010,
45, 205–209.
[32] Tong, Y. Y.; Gu, C. D.; Zhang, J. L.; Huang, M. L.; Tang, H.;
Wang, X. L.; Tu, J. P. Three-dimensional astrocyte-network
Ni-P-O compound with superior electrocatalytic activity
and stability for methanol oxidation in alkaline environments.
J. Mater. Chem. A 2015, 3, 4669–4678.
[33] Gu, Z. J.; Zhai, T. Y.; Gao, B. F.; Zhang, G. J.; Ke, D. M.;
Ma, Y.; Yao, J. N. Controlled hydrothermal synthesis of nickel
phosphite nanocrystals with hierarchical superstructures.
Crystal Growth Design 2007, 7, 825–830.
[34] Luo, Y. S.; Luo, J. S.; Zhou, W. W.; Qi, X. Y.; Zhang, H.;
Yu, D. Y. W.; Li, C. M.; Fan, H. J.; Yu, T. Controlled synthesis
of hierarchical graphene-wrapped TiO2@Co3O4 coaxial
nanobelt arrays for high-performance lithium storage. J.
Mater. Chem. A 2013, 1, 273–281.
[35] Ai, W.; Luo, Z. M.; Jiang, J.; Zhu, J. H.; Du, Z. Z.; Fan,
Z. X.; Xie, L. H.; Zhang, H.; Huang, W.; Yu, T. Nitrogen
and sulfur codoped graphene: Multifunctional electrode
materials for high-performance Li-ion batteries and oxygen
reduction reaction. Adv. Mater. 2014, 26, 6186–6192.
[36] Pelavin, M.; Hendrickson, D. N.; Hollander, J. M.; Jolly,
W. L. Phosphorus 2p electron binding energies. Correlation
with extended Hueckel charges. J. Phys. Chem. 1970, 74,
1116–1121.
[37] Zhang, G. Q.; Wu, H. B.; Hoster, H. E.; Chan-Park, M. B.;
Lou, X. W. D. Single-crystalline NiCo2O4 nanoneedle arrays
www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research
1663 Nano Res. 2018, 11(3): 1651–1663
grown on conductive substrates as binder-free electrodes for
high-performance supercapacitors. Energy Environ. Sci. 2012,
5, 9453–9456.
[38] Xu, Y. X.; Lin, Z. Y.; Huang, X. Q.; Wang, Y.; Huang, Y.;
Duan, X. F. Functionalized graphene hydrogel-based high-
performance supercapacitors. Adv. Mater. 2013, 25, 5779–
5784.
[39] Sun, Y. M.; Hu, X. L.; Luo, W.; Huang, Y. H. Self-assembled
hierarchical MoO2/graphene nanoarchitectures and their
application as a high-performance anode material for
lithium-ion batteries. Acs Nano 2011, 5, 7100–7107.
[40] Liang, J.; Jiao, Y.; Jaroniec, M.; Qiao, S. Z. Sulfur and
nitrogen dual-doped mesoporous graphene electrocatalyst for
oxygen reduction with synergistically enhanced performance.
Angew. Chem., Int. Ed. 2012, 51, 11496–11500.
[41] Wang, Y.; Shao, Y. Y.; Matson, D. W.; Li, J. H.; Lin, Y. H.
Nitrogen-doped graphene and its application in electrochemical
biosensing. ACS Nano 2010, 4, 1790–1798.
[42] Bearinger, J. P.; Terrettaz, S.; Michel, R.; Tirelli, N.; Vogel,
H.; Textor, M.; Hubbell, J. A. Chemisorbed poly(propylene
sulphide)-based copolymers resist biomolecular interactions.
Nat. Mater. 2003, 2, 259–264.
[43] Zhang, L.; Shi, G. Q. Preparation of highly conductive
graphene hydrogels for fabricating supercapacitors with high
rate capability. J. Phys. Chem. C 2011, 115, 17206–17212.
[44] Yan, J.; Wang, Q.; Wei, T.; Fan, Z. J. Recent advances in
design and fabrication of electrochemical supercapacitors with
high energy densities. Adv. Energy Mater. 2014, 4, 1300816.
[45] Zhu, J. H.; Jiang, J.; Sun, Z. P.; Luo, J. S.; Fan, Z. X.;
Huang, X. T.; Zhang, H.; Yu, T. 3D carbon/cobalt-nickel
mixed-oxide hybrid nanostructured arrays for asymmetric
supercapacitors. Small 2014, 10, 2937–2945.
Nano Res.
Electronic Supplementary Material
High-performance asymmetrical supercapacitor composedof rGO-enveloped nickel phosphite hollow spheres andN/S co-doped rGO aerogel
Deyang Zhang1, Yihe Zhang1 (), Yongsong Luo2, Yu Zhang1, Xiaowei Li1, Xuelian Yu1, Hao Ding1,
Paul K. Chu3, and Li Sun1 ()
1 Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials,
School of Materials Science and Technology, China University of Geosciences, Beijing 100083, China 2 School of Physics and Electronic Engineering, Xinyang Normal University, Xinyang 464000, China 3 Department of Physics and Department of Materials Science and Engineering, City University of Hong Kong, Tat Chee Avenue,
Kowloon, Hong Kong, China
Supporting information to https://doi.org/10.1007/s12274-017-1780-3
Figure S1 Morphology of (a-c) NPOH-0 and (d-f) NPOH-1.
Address correspondence to Yihe Zhang,[email protected]; Li Sun, [email protected]
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Nano Res.
Figure S2 Possible ion penetration mechanism of NPOH-0.5 electrodes in an alkaline electrolyte.
Figure S3 (a) TEM image, (b) AFM image, and (c) Height profile of the synthesized graphene oxide.
Figure S4 Nitrogen adsorption and desorption isotherms and BJH pore distribution of the NPOH-0.5@rGO.
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Nano Res.
Figure S5 CV curves of (a) NPOH-0, (b) NPOH-1, (c) NPOH-0.5, and (d) NPOH-0.5@rGO at different scanning rates.
Figure S6 Galvanostatic charging-discharging curves of (a) NPOH-0, (b) NPOH-1, (c) NPOH-0.5, and (d) NPOH-0.5@rGO at different current densities.
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Nano Res.
Figure S7 X-ray diffraction patterns of pure 3D rGO and NS-3D rGO.
Figure S8 Low-magnification and high-magnification SEM images of (a, b) Pure 3D rGO and (c, d) NS-3D rGO.
Figure S9 Nitrogen adsorption and desorption isotherms and BJH pore distribution of the NS-3D rGO.
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Nano Res.
Figure S10 EDS spectrum of NS-3D rGO.
Figure S11 (a) CV and (b) Galvanostatic charge-discharge curves of pure 3D rGO and NS-3D rGO. (c) CV curves of the NS-3D rGO at different scanning rates in the voltage window between 0 and 1 V. (d) Galvanostatic charging/discharging curves of the NS-3D rGO at different current densities.
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Nano Res.
Figure S12 Specific capacitance of NS-3D rGO electrode at different current density.
Figure S13 (a) Comparison of CV curves of NS-3D rGO porous rGO and NPOH-0.5@rGO electrodes at a scanning rate of 10 mV s−1; (b) Specific capacitance of the ASC device at different current density; (c) Ragone plots of the ASC device. (d) Voltage test of two ASCs in series.