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,zyh@cugb.edu.cn; Li Sun, sunli@cugb.edu.cn

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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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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,zyh@cugb.edu.cn; Li Sun, sunli@cugb.edu.cn

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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.