Review Materials Science

Heteroatom-doped graphene for electrochemical energy storage

Yangyang Wen • Congcong Huang •

Lianzhou Wang • Denisa Hulicova-Jurcakova

Received: 13 September 2013 / Accepted: 20 November 2013 / Published online: 26 March 2014

� Science China Press and Springer-Verlag Berlin Heidelberg 2014

Abstract The increasing energy consumption and envi-

ronmental concerns due to burning fossil fuel are key drivers

for the development of effective energy storage systems based

on innovative materials. Among these materials, graphene has

emerged as one of the most promising due to its chemical,

electrical, and mechanical properties. Heteroatom doping has

been proven as an effective way to tailor the properties of

graphene and render its potential use for energy storage

devices. In this view, we review the recent developments in

the synthesis and applications of heteroatom-doped graphene

in supercapacitors and lithium ion batteries.

Keywords Graphene � Graphene oxide �Heteroatom � Doping or doped � Supercapacitors �Lithium ion batteries

1 Introduction

There is no doubt that the development of low-cost, high-

performance, and environmentally friendly energy storage

systems is a great challenge to fully utilise potential of

renewable energy, such as solar, wind, and tide.

Supercapacitors (SCs) and lithium ion batteries (LIBs) have

been considered as the most promising energy storage systems

for these applications [1, 2]. It has been generally agreed that

the performance of an energy storage device is primarily

dependent on the properties of the electrode materials. Highly

porous carbon materials have been traditionally used as

electrode materials of SCs providing high surface area for ion

electrosorption thus leading to large specific capacitance [1].

On the other hand, graphite has been utilized as commercial

anode material of LIBs since their commercialization.

In the last decade, graphene, a single layer of sp2

hybridized carbon atoms arranged in a hexagonal lattice

has emerged as one of the most attractive carbon allotropes

for energy storage applications due to its unique structure

and properties [3–6], including high theoretical surface

area (2,630 m2 g-1) [7], excellent thermal conductivity

(*5,000 W m-1 K-1) [8], high intrinsic carrier mobility

(2 9 105 cm2 V-1 s-1) [8], high optical transmittance

(*97.7 %) [9], and superior mechanical strength [10].

Various morphologies of graphene have been reported to

date, such as zero-dimensional graphene quantum dots [11,

12], one-dimensional graphene nanoribbons [13, 14], two-

dimensional graphene nanosheets, and three-dimensional

(3D) graphene macroscopic structures (like fibers [15, 16],

foams [17], and hydrogels [18, 19] ). Graphene can be

prepared by various approaches, including mechanical

cleavage of graphite with a Scotch tape [3], epitaxial

growth on single-crystal SiC [20], chemical vapor deposi-

tion [21], exfoliation of graphite powder via solution oxi-

dation [22], or high energy ball milling [23].

Like for most materials, it is very rare for a graphene-

based material with desirable bulk properties to also pos-

sess the surface characteristics required for energy storage

applications [2]. Therefore, three main approaches have

been used to improve the energy storage properties for

SPECIAL ISSUE: Advanced Materials for Clean Energy

Y. Wen � C. Huang � L. Wang � D. Hulicova-Jurcakova (&)

ARC Centre of Excellence for Functional Nanomaterials, School

of Chemical Engineering and Australian Institute for

Bioengineering and Nanotechnology, The University of

Queensland, Queensland 4072, Australia

e-mail: d.jurcakova@uq.edu.au

C. Huang

Department of Chemistry, College of Science, Northeastern

University, Shenyang 110819, China

123

Chin. Sci. Bull. (2014) 59(18):2102–2121 csb.scichina.com

DOI 10.1007/s11434-014-0266-x www.springer.com/scp

graphene: (1) development of porosity in graphene.

Improving the specific surface area of graphene materials

without decreasing their electrical properties is an effective

way to enhance the energy storage properties in SCs [24].

This has been achieved by constructing 3D graphene

architectures with open porous structure and high specific

surface area [25], chemical activation [26], and light

treatment [27]; (2) development of heteroatom-doped

graphene (heteroatoms include N, B, P, S). Introduction of

heteroatoms can significantly improve the performance of

the SCs as well as LIBs via altering the electronic prop-

erties of doped graphene [6, 28–30]; (3) development of

graphene-based composites. Combining the high surface

area and electrical conductivity of graphene with the high

electrochemical activity of the second phase (such as

transition metal oxides/hydroxides and conducting poly-

mers) has been reported as an effective way to increase

energy storage capacity of SCs [31, 32].

As mentioned above, chemical doping is an important

approach to tailor the properties of graphene. The hetero-

atom-doped graphene shows different properties compared

with the pristine graphene [24, 33]. For instance, the spin

density and the charge distribution of carbon atoms are

influenced by the neighboring heteroatoms, which increase

the ‘‘activation region’’ on graphene surface thus improving

the energy storage [6, 33, 34]. To date, heteroatom-doped

graphene has been investigated as electrode materials in

numerous applications, including SCs, LIBs, fuel cells, and

field-effect transistors to name a few [28–30, 34–36].

In this review, we focus on the energy storage applica-

tion of heteroatom-doped graphene and review the recent

development in the synthesis approaches and applications

of heteroatom-doped graphene in SCs and LIBs.

2 Synthesis of heteroatom-doped graphene

Heteroatom-doped graphene can be obtained by two dif-

ferent ways: a direct synthesis and a post-treatment [6].

Different methods provide doped graphene with different

levels of doping and distribution of doped atoms.

Direct synthesis includes chemical vapor deposition

(CVD), solvothermal method, segregation growth, and arc-

discharge method [6]. CVD is a common method to produce

heteroatom-doped nanocarbons in which carbon source gas

is mixed with heteroatom-containing precursor at elevated

temperatures [6]. Doped graphene can also be directly grown

on the current collectors used as substrates in the CVD

process as reported by Reddy and colleagues who demon-

strated a controlled growth of nitrogen-doped (N-doped)

graphene layers using acetonitrile as a carbon source. The

N-doped graphene grew directly on copper current collectors

and contained 9 % of nitrogen [37]. By using this method,

active electrode materials can be attached on the current

collectors without any binder which makes the process for

manufacture technology more feasible and efficient.

Solvothermal method was first employed in gram-scale

production of graphene [38]. It is a method with simple

operation, mild synthesis conditions and capability to

deliver relatively large quantities [39]. Deng et al. [39]

prepared N-doped graphene using this method by mixing

lithium nitride and tetrachloromethane with or without

cyanuric chloride, reaching a high nitrogen content of

16.4 %. Another report described the synthesis of N-doped

graphene hydrogel using hydroxylamine hydrochloride and

hydroxylamine through a solvothermal reaction with a

relatively low heteroatom doping (4.32 %) [40].

Compared with direct synthesis, post-treatment approa-

ches seem to attract more attention as an effective way to

introduce heteroatoms into the graphene structure. These

include thermal treatment [28, 30, 41], hydrothermal

method [17, 19, 29], plasma process [42], mechanical ex-

foliation [43], and anodic polarization [44].

For thermal treatment, doped graphene can be prepared by

annealing graphene oxide (GO) in heteroatom-containing gas

at high temperature [41] or carbonizing heteroatom-contain-

ing precursors at high temperature [30]. Wu et al. [41] pre-

pared N-doped or boron-doped (B-doped) graphene through

thermal treatment at 600 or 800 �C using NH3 or BCl3 gas,

showing a small amount of N (3.06 %) and B element

(0.88 %). The phosphorus-doped (P-doped) graphene was

also prepared by a similar approach in which the mixture of

graphene oxide, and triphenylphosphine was heat treated at

elevated temperature, providing phosphorus doping of

1.81 at.% [30]. Thermal treatment as the most frequently used

method shows some advantages like it can control the tem-

perature as well as the flow of the treatment gases easily.

Hydrothermal method is another interesting approach to

prepare doped graphene with desired porous structure and

controlled doping levels. Zhao et al. [17] prepared N-doped

graphene framework with N/C atomic ratio of 4.2 % by

hydrothermal treatment of GO with pyrrole, and the

resulting material showed an ultralow density and high

porosity. Applying the same method, certain reducing

agents were chosen to realize one-step facile process of

doping and reducing the GO. In this way, N-doped

graphene hydrogels were prepared with GO as a raw

material and urea as a reducing-doping agent, obtaining the

controlled nitrogen contents from 3.95 at.% to 6.61 at.%

by controlling GO concentration [19].

3 Heteroatom-doped graphene for supercapacitors

SCs are devices that store and release energy rapidly

and reversibly, providing moderate energy density

Chin. Sci. Bull. (2014) 59(18):2102–2121 2103

123

(1–10 Wh kg-1), high power density (103–106 W kg-1) and

ultralong cycling life ([105 cycles) [24]. SCs have been

applied in electronics, electric vehicles, ac-line filtering, air-

crafts, and so on. Based on the energy storage mechanisms,

SCs can be divided into two types: (1) electrochemical double

layer capacitors (EDLCs) which store energy by charge sep-

aration at the electrolyte/electrode interface and (2) pseud-

ocapacitors that store energy through redox Faradaic

reactions. For EDLCs, porous carbon materials have been

investigated as the electrode materials, while in pseudoca-

pacitors, electrode materials include transition metal oxides/

hydroxides, conducting polymers and carbons containing

oxygen and nitrogen surface functional groups [45].

It has been shown that heteroatom-doped porous car-

bons involve both the EDLCs and pseudocapacitive energy

storage mechanism thus providing high capacitance and

often extended potential window [46]. Understandably, the

heteroatom-doped graphene, as a result of its distinct

electric charge distribution, chemical and electrical prop-

erties, has been increasingly investigated in energy storage

field, particularly in SCs and the next section reviews

recent developments in the synthesis and applications of

heteroatom-doped graphene in SCs.

3.1 N-doped graphene for supercapacitors

Doping of graphene with nitrogen atom that has an extra

valence electron modifies the electronic band structure of

graphene by introducing new energy levels in the lower part of

the conduction band of sp2-bonded carbon atoms [6, 33]. These

new energy levels have been shown to be electrochemically

and catalytically active in energy storage/conversion devices

such as SCs, batteries and fuel cells [6, 33]. Doping with

nitrogen generally results in three common bonding configu-

rations within the carbon lattice, including quaternary N, py-

ridinic N, and pyrrolic N (Fig. 1) [6]. Among these nitrogen

types, pyridinic N and quaternary N are sp2 hybridized, and

pyrrolic N is sp3 hybridized and thus has different charges that

also affect charge of the surrounding carbon atoms.

Thermal treatment, hydrothermal method and plasma

treatment have been the main methods to obtain N-doped

graphene for applications as SCs electrodes and these are

reviewed herein.

3.1.1 N-doped graphene synthesized by thermal treatment

for supercapacitors

Thermal treatment refers to the method which uses high

temperature to produce materials. Using this method,

N-doped graphene was synthesized by treating GO or

graphene with ammonia gas at high temperature.

Following this approach Zhang et al. [47] synthesized

N-doped activated graphene with an ultrahigh specific

surface area of *2,000 m2 g-1 via a microwave exfoliated

process and KOH activation following the annealing in

ammonia gas at 700 �C. The area-normalized capacitance

of the N-doped graphene with similar porous structure

increased from 6 lF cm-2 (160 F g-1) to 22 lF cm-2

(420 F g-1) with 0 at.% and 2.3 at.% N-doping, respec-

tively, at 0.2 A g-1 in 6 mol L-1 KOH electrolyte.

In other report, N-doped graphene with 5.2 at.% of

nitrogen by annealing GO at 550 �C under NH3 gas flow

provided a capacitance of *40 F g-1 at 2 mV s-1 in

1 mol L-1 H2SO4. In addition, the effect of graphene

surface functionalization on supercapacitive performance

was also systematically investigated, including GO,

chemically reduced GO (RG), N-doped RG (NG), and

amine-modified RG (NH2-G) [48]. NG loaded with poly-

pyrrole (PPy) provided higher capacitance (as high as

393 F g-1) than the other GO/PPy, RG/PPy, and NH2-G/

PPy composites [49].

Electrospinning combined with thermal treatment in

ammonia gas was used in preparation of carbon nanofibers

with radially grown graphene. This carbon nanofibers/

graphene material was doped with a small amount of

nitrogen and oxygen with different concentrations (N:

1.1 at.%–3.4 at %, O: 2.6 at.%–3.6 at.%) depending on the

annealing temperature. SCs capable of operating at high

voltage of 1.8 V provided energy density of 29.1 Wh kg-1

in a neutral Na2SO4 electrolyte. The SCs also exhibited

excellent cycling performance with the capacity retention

over 93 % after 5,000 charge–discharge cycles [50].

Hierarchically aminated graphene honeycombs with an

N-doping level of 2.79 at.%–3.91 at.% were reported

through vacuum assisted thermal expansion of GO fol-

lowed by amination in NH3 gas. The functional 3D

assemblies of graphene were micrometer-sized curved

flakes with a high transparency chiffon-like texture,

exhibiting high capacitance of 207 F g-1, and long cycle

life in 6 mol L-1 KOH electrolyte. In addition, the surface

chemistry of the aminated graphene treated at different

temperatures (200–600 �C) was studied using X-ray pho-

toelectron spectroscopy (XPS). It was concluded that at

low amination temperature NH3 reacted with the carbox-

ylic acid species to form mainly intermediate amide or

amine like species through nucleophilic substitution. When

the amination temperature increased, the intramolecular

Fig. 1 Bonding configurations of nitrogen atoms in N-doped graph-

ene [6]. Reprinted with permission from Ref. [6], Copyright (2012),

American Chemical Society

2104 Chin. Sci. Bull. (2014) 59(18):2102–2121

123

dehydration or decarbonylation took place to generate

thermally stable heterocyclic aromatic moieties such as

pyridine, pyrrole, and quaternary type N sites [51].

In another study, N-doped graphene with ultrahigh

electrical conductivity (1,000–3,000 S m-1) was synthe-

sized through pre-reduction with hydrazine followed by

thermal nitridation. Even without adding carbon additives

into the electrodes, the doped graphene-based SCs could

deliver high capacitance of 144.9 F g-1 (0.5 A g-1) in

1 mol L-1 Et4NBF4 organic electrolyte and remarkable

energy and power densities (55.3–80.5 Wh kg-1,

0.558–16.68 kW kg-1) at high operating voltage of 4 V

[52]. The resulting curved doped graphene also displayed

high thermal stability and was highly porous when

assembled into a film. These properties resulted from the

restoration of the graphene network by the formation of

C–N bonded groups and N-doping, which are highly

desirable for SCs.

As mentioned above, the advantage of this method is

that the N-doping level of the graphene can be controlled

by varying the flow rate of ammonia gas and further

influence the electrochemical performance, whereas the

disadvantage is the relatively low N content in graphene

materials. Apart from using ammonia atmosphere, N-doped

graphene can be also synthesized by annealing GO in the

presence of various nitrogen-containing reagents at high

temperature, introducing high N contents in graphene

nanosheets.

Highly crumpled N-doped graphene nanosheets with a

high pore volume of 3.42 cm3 g-1 and relative high

nitrogen contents of 5.21 at.%–11.24 at.% were synthe-

sized by thermal annealing cyanamide and GO. The sup-

ercapacitive properties of the crumpled doped graphene

were studied in both organic and aqueous electrolytes,

achieving impressive 248 F g-1 (1 mol L-1 [Bu4N]BF4)

and 302 F g-1 (6 mol L-1 KOH) at 5 mV s-1, respec-

tively. Excellent electrochemical stability was also

observed with 96.1 % capacitance retention after 5,000

cycles [28].

Zheng et al. [53] obtained the KOH-activated N-doped

graphene (aNG) with a specific surface area of up to

555 m2 g-1 through a two-step process, as shown in Fig. 2.

First, GO was activated by KOH followed by the thermal

annealing with melamine that resulted in a very high

nitrogen content of 20.51 %. Electrochemical results

showed that the aNG yielded a capacitance of 132.4 F g-1

at the current density of 0.1 A g-1, which was consider-

ably higher than that without KOH treatment. The work

demonstrated that KOH activation of thermally annealed

N-doped graphene was a promising method for enhancing

the capacitance of the N-doped graphene.

Another approach involved preparation of graphene/N-

doped porous carbon composite by carbonization of the

mixture of GO and ionic liquids (1-ethyl-3-methylimida-

zolium dicyanamide). Electrochemical evaluation showed

that the composite with a high nitrogen content of

13.51 wt% manifested a significantly enhanced capacitive

performance (206 F g-1 at a current density of 0.1 A g-1)

compared with sample with a low nitrogen content

(7.15 wt%, 161 F g-1) and non-doping graphene/porous

carbon composite (69 F g-1) in 1 mol L-1 H2SO4 elec-

trolyte [54]. It is explained that the improved capacitive

performance was attributed to the Faradaic redox reactions

on the nitrogen functional groups as well as enhanced

surface wettability.

3.1.2 N-doped graphene synthesized by hydrothermal

treatment for supercapacitors

Hydrothermal treatment is an important technique for the

synthesis of various kinds of materials [55]. In case of

N-doped graphene synthesized by hydrothermal synthesis,

GO solution and the nitrogen-containing precursors are

typically put into a steel pressure vessel (autoclave) and

treated at a relatively low temperature (generally\200 �C).

During this process, GO can be reduced and doped simul-

taneously by using appropriate reducing nitrogen-containing

agents.

Ammonia and urea are the most common nitrogen-con-

taining agents reported in hydrothermal method. Hassan

prepared N-doped graphene using ammonia and hydrazine,

and systematically studied the hydrothermal temperature

impact on the nitrogen configuration and electrochemical

performance. The result confirmed that N-doped graphene

prepared at 130 �C exhibited the maximum capacitance of

194 F g-1 at 10 mV s-1 and 6.7 Wh kg-1/3.7 kW kg-1 for

the energy density/power density, respectively, in

1 mol L-1 KCl electrolyte [56].

Fig. 2 (Color online) Illustration of the forming process of KOH

activation of N-doped graphene [53]. Reprinted with permission from

Ref. [53], Copyright (2013), Springer

Chin. Sci. Bull. (2014) 59(18):2102–2121 2105

123

In another report, concentrated ammonia-assisted

hydrothermal method was used to obtain N-doped graph-

ene by simultaneous N-doping and reduction of GO. The

effects of different hydrothermal temperatures on the sur-

face chemistry and the structure of N-doped graphene were

also investigated. The results revealed that specimens

prepared at higher hydrothermal temperature had higher N

doping level (up to 7.2 at.%) and possessed high specific

capacitance of 144.6 F g-1 at 0.2 A g-1 in 6 mol L-1

KOH electrolyte [57].

Bai et al. [58] prepared three kinds of graphene through

a hydrothermal reduction under different pH values. The

N-doped graphene was obtained under basic medium using

ammonia solution, giving a specific capacitance of

185 F g-1 at a current density of 1 A g-1 and the energy

density/power density of 17.1 Wh kg-1/20 kW kg-1 in

30 wt% KOH electrolyte.

N-doped graphene nanosheets with a high nitrogen level

of 10.13 at.% were prepared via simple hydrothermal

reaction of GO and urea. It was shown that during the

hydrothermal process, urea released NH3 in a sustained

manner which reacted with the oxygen functional groups of

the GO thus forming nitrogen doping in graphene. Corre-

sponding N-doped graphene had a large surface area of

593 m2 g-1 and exhibited high gravimetric capacitance of

326 F g-1 at low current load in 6 mol L-1 KOH elec-

trolyte, very good cycling stability and coulombic effi-

ciency (99.58 %) after 2,000 cycles. The energy density of

25.02 Wh kg-1 was achieved at power density of

7.98 kW kg-1 by using a two-electrode symmetric capac-

itor [59]. The results also demonstrated that the pyridinic N

and pyrrolic N played a critical role in improving pseudo-

capacitance by the redox reaction, while quaternary N

enhanced the electronic conductivity during the charge/

discharge process.

Using a similar hydrothermal process with urea, doped

graphene with 5.47 at.%–7.56 at.% of nitrogen was

obtained, showing 184.5 F g-1 at 3 A g-1 and 12.4 %

capacitance loss after 1,200 cycles in an aqueous electro-

lyte of 6 mol L-1 KOH [60]. In addition, microwave-

assisted hydrothermal method with GO and urea was used

to synthesize doped graphene, showing that the positive

effect of N-doped groups on capacitance was more pro-

nounced in acidic than in alkaline media [61].

Besides urea, there are numerous other nitrogen-con-

taining agents used in the synthesis of N-doped graphene.

One such example includes hexamethylenetetramine,

which plays double-role, both as the reducing agent for GO

and the nitrogen source. Using hydrothermal conditions

with hexamethylenetetramine and GO, N-doped graphene

with 8.62 at.% of nitrogen doping was reported. The

material provided capacitance of 161 F g-1 at a current

density of 0.5 A g-1 in 6 mol L-1 KOH electrolyte, and

exhibited good cycling stability after 5,000 consecutive

cycles [62].

A pressure-promoted hydrothermal process with

NH4HCO3 as nitrogen-containing agent was used to pre-

pare N-doped graphene, reaching a high N doping level of

9.76 at.%–13.51 at.%. The product exhibited a specific

capacitance of 170 F g-1 at 0.5 A g-1 in 5 mol L-1 KOH,

and a high retention rate of 96.4 % of its initial capacitance

after 10,000 charge/discharge cycles [63].

It has been also demonstrated that by controlling

hydrothermal conditions, N-doped graphene with porous,

or 3D structure can be easily obtained. In this fashion, You

et al. [64] reported a 3D N-doped graphene-carbon nano-

tube (CNT) networks obtained by hydrothermal treatment,

freeze-drying and subsequent carbonization of GO-dis-

persed pristine CNT in the presence of pyrrole. Such

N-doped graphene-CNT used as electrode in alkaline SCs

showed a capacitance of 180 F g-1 at 0.5 A g-1 current

load and approximately 96 % of the initial capacitance

after 3,000 cycles.

Graphene hydrogels were also successfully prepared

using hydrothermal conditions. N-doped graphene hydro-

gels were synthesized via a hydrothermal method using

urea as the reducing/doping agent. The hydrogels contained

*97.6 wt% of water and had a large specific surface area

of [1,300 m2 g-1 in the wet state, showing a high capac-

itance of 326 F g-1 at 1 A g-1 in 6 mol L-1 KOH elec-

trolyte [19]. Using this approach, N contents in hydrogels

can be controlled by several synthesis parameters, such as

the GO concentration, different N-precursors as well as

their concentrations, hydrothermal temperatures, and so on.

Another example includes macroscopic 3D N-doped

graphene hydrogels prepared from organic amines and GO

as precursors, as shown in Fig. 3. The results showed that

such hydrogels exhibited excellent supercapacitive perfor-

mance. Even at an ultrafast charge/discharge rate of

185 A g-1, a specific capacitance of 113.8 F g-1, and a

high power density of 205.0 kW kg-1 were obtained in

5 mol L-1 KOH electrolyte. In addition, at a considerably

high current density of 100 A g-1, 95.2 % capacitance was

retained after 4,000 cycles [18]. In this work, organic

amines did not only act as nitrogen precursors but also

controlled assembly of the graphene nanosheets in the 3D

structures. The inner structure of the hydrogels and the

content of nitrogen in the graphene were adjusted by

organic amine.

The highest capacitance for N-doped graphene was

obtained using this method. Zhao et al. [17] prepared 3D

N-doped graphene framework with an ultralow density of

(2.1 ± 0.3) mg cm-3 and a high conductivity of

(1.2 ± 0.2) 9 103 S m-1, as shown in Fig. 4. The

N-doped graphene framework with N/C atomic ratio of

4.2 % was prepared by hydrothermal treatment of aqueous

2106 Chin. Sci. Bull. (2014) 59(18):2102–2121

123

GO suspension with pyrrole to form an N-containing

hydrogel, followed by annealing the freeze-dried hydrogel

at 1,050 �C under argon atmosphere. Such framework

presented high-performance capacitive behaviors due to

the combination of heteroatom doping and 3D porous

structure providing an ultrahigh capacitance of

415–484 F g-1 at the current densities of 1–100 A g-1 in

1 mol L-1 LiClO4 aqueous electrolyte.

Compared with other methods, hydrothermal method is a

facile and one-step method of preparing N-doped graphene.

Using this method, 3D graphene architecture with porous

structure could be easily obtained and the graphene synthe-

sized by this approach is more enriched with nitrogen.

3.1.3 N-doped graphene synthesized by plasma treatment

and other methods for supercapacitors

Choi and co-workers [42] paved way toward improving the

capacitance of graphene-based SCs by using N-doped

graphene. N-doped graphene with nitrogen doping between

1.68 at.% and 2.51 at.% was prepared by nitrogen plasma

process. The results showed that the capacitance of

N-doped graphene (282 F g-1) was about 4 times that of

its pristine graphene-based counterpart (69 F g-1) in

6 mol L-1 KOH electrolyte without sacrificing excellent

cycle life ([2 9 105) and high power capability

(*800 kW kg-1). A possible mechanism of the energy

storage in N-doped graphene was proposed on the basis of

XPS analysis as well as the first principles density func-

tional theory calculation of the ionic binding energy

(Fig. 5). It was concluded that the introduction of hetero-

atom nitrogen manipulated the local electronic structures of

graphene sheets, resulting in increased number of adsorbed

ions to store more energy. The enhanced accommodation

of ions on the electrode surface, especially on the N-doped

sites at the basal planes of graphene sheets through elec-

trostatic interaction, was believed to be the reason for the

high capacitance of N-doped graphene.

A solvothermal method was used in the preparation of

N-doped graphene hydrogel using hydroxylamine

hydrochloride (HACl) and hydroxylamine (HA) as the

chemical dopant and reductant. The N-doped hydrogel

prepared from HA contained more nitrogen (4.32 at.%)

while the N-doped hydrogel prepared from HACl pro-

vided higher capacitance (205 F g-1 at scan rate of

1 mV s-1) and energy density/power density

(3.65 Wh kg-1/20.5 kW kg-1) in 25 wt% KOH electro-

lyte, along with good capacitance retention of 92.6 %

after 3,000 cycles [40].

Fig. 3 Photographs and SEM image of the hydrogel synthesized from GO (G-GH) and N-doped graphene hydrogels (GN-GHs). (a) G-GH and

GN-GHs after hydrothermal process at 180 �C for 12 h using ethylenediamine (1, 2.5 lL; 2, 5.0 lL; 3, 10.0 lL; 4, 20.0 lL); (b) GN-GHs after

hydrothermal process at 180 �C for 12 h using ammonia and different organic amine (5, ammonia; 6, diethylenetriamine; 7, tetraethylene-

pentamine; 8, n-propylamine; 9, n-butylamine); (c) SEM image of the typical GN-GH microstructures [18]. Reprinted with permission from Ref.

[18], Copyright (2013), Elsevier

Fig. 4 a, b Photographs of the N-doped graphene framework and one

with a piece of framework size of 1.8 cm 9 1.1 cm 9 1.2 cm

standing on a dandelion. c, d SEM images of the sample in (a).

Scale bars (c) 10 lm, (d) 100 nm [17]. Reprinted with permission

from Ref. [17], Copyright (2012), WILEY–VCH Verlag GmbH &

Co. KGaA, Weinheim

Chin. Sci. Bull. (2014) 59(18):2102–2121 2107

123

An interesting approach has been reported to prepare

N-modified graphene directly from graphite flakes. With the

aid of melamine, graphite flakes were directly ultrasonicated

into few-layer graphene in acetone solution. The subsequent

annealing process further transformed the melamine absorbed

on the graphene sheets into melon (C6N9H3)x and simulta-

neously doped graphene with nitrogen. When tested in 6 mol

L-1 KOH electrolyte, the sample showed a much higher

specific capacitance (227 F g-1 or 155 F cm-3 at 1 A g-1)

than that of undoped graphene (133 F g-1 at 1 A g-1) [43].

3.2 Other heteroatom-doped graphene

for supercapacitors

Boron has been another atom of choice in an attempt of

altering the electronic structure of graphene and thus

improving the energy storage capacity of SCs made from

corresponding B-doped graphene.

B-doped graphene nanoplatelets with the boron doping

of 0.7 at.%–1.1 at.% were produced via the reduction of

GO by a borane-tetrahydrofuran under refluxing (Fig. 6).

This has been the first report on the production of B-doped

graphene from a solution process and on application of

B-doped graphene in SCs. The B-doped graphene had a

high specific surface area of 466 m2 g-1 and showed good

supercapacitive performance including a high specific

capacitance of 200 F g-1 (43 lF cm-2) at 0.1 A g-1 in

Fig. 5 (Color online) a Gravimetric capacitances of SCs built on nickel and paper substrates measured at a series of current densities. (inset) A photograph

showing that a wearable capacitor wrapped around a human arm could store the electrical energy to light up a LED; b the schematic illustration of the plasma

doping process. By the plasma process with physical momentum, nitrogen atoms replaced the existing carbon atoms. (inset) Possible nitrogen configurations

by the doping treatment; c galvanostatic charging/discharging curves at a current density of 1 A g-1 with the IR drops denoted; d cyclic voltammetric curves

measured at a scan rate of 20 mV g-1. Reprinted with permission from Ref. [42], Copyright (2011), American Chemical Society

Fig. 6 a A borane-tetrahydrofuran (THF) adduct, b a reaction

scheme of the reduction of GO with the borane-THF adduct,

c SEM images of B-doped graphene powder at different magnifica-

tions. Reprinted with permission from Ref. [65], Copyright (2013),

American Chemical Society

2108 Chin. Sci. Bull. (2014) 59(18):2102–2121

123

6 mol L-1 KOH aqueous electrolyte and a good stability

after 4,500 cycles [65].

P-doped graphene was reported from graphene nano-

sheets by the activation with phosphoric acid. When

applies as the electrodes of SCs, a high specific capacitance

of 367 F g-1 at 5 mV s-1 and a high energy/power den-

sities of 59 Wh kg-1/9 kW kg-1 were derived in

1 mol L-1 H2SO4 aqueous electrolyte [66]. However, the

content of the phosphorous in doped graphene was not

reported and the effect of phosphorus doping on the

capacitance was not discussed.

3.3 Co-doped graphene for supercapacitors

3D boron and nitrogen co-doped (B,N-doped) monolithic

graphene aerogels (BN-GAs) were synthesized by com-

bined hydrothermal process and freeze-drying process and

used to fabricate an all-solid-state SCs, as illustrated in

Fig. 7. The nitrogen (3.0 at.%) and boron (0.6 at.%) dop-

ing in carbon networks facilitated charge transfer between

neighboring carbon atoms and thus enhanced the electro-

chemical performance of carbon-based materials. The BN-

GAs based all-solid-state SCs exhibited high specific

capacitance (PVA/H2SO4 gel as the solid electrolyte,

62 F g-1 at 5 mV s-1) and enhanced energy and power

density (8.7 Wh kg-1/1.65 kW kg-1) with respect to

undoped, N-doped, B-doped GAs, or layer-structured

graphene paper. The monolith was also tested in 1 mol L-1

H2SO4 aqueous electrolyte providing capacitance as high

as 239 F g-1 at 1 mV s-1 [29].

In a different approach ammmonia borane was used as

both the reducing and doping agent to synthesize the B,N-

doped graphene by refluxing ammmonia borane and GO.

Concurrently the reduction of GO was studied in aqueous

solution and tetrahydrofuran. The result showed that the

reduction of GO in tetrahydrofuran led to a higher boron-

(1.81 at.%) and nitrogen- (1.57 at.%) content in B,N-doped

graphene and such materials exhibited superior superca-

pacitive performance, with a specific capacitance of

130 F g-1 at 1 A g-1 in organic electrolyte of 1 mol L-1

TEABF4 compared with the sample reduced in aqueous

solution [67].

In another work, anodic polarization was applied in

synthesis of oxygen, nitrogen and chlorine co-doped

reduced graphene oxide films in a neutral nitrogen-deaer-

ated KCl electrolyte. The doped graphene showed a Cl

doping of 0.38 at.% and a N doping of 7.25 at.%, along

with the enhanced capacitance in 1 mol L-1 H2SO4

aqueous electrolyte for the doped film (182 F g-1 at

10 mV s-1 for the doped graphene and 75 F g-1 for the

undoped graphene) [44].

Table 1 summarizes the above described synthesis

methods of heteroatom-doped graphene and their sup-

ercapacitive properties. As one can clearly see, various

methods have been utilised in synthesis of heteroatom-

doped graphene materials for SCs. Clear enhancement in

specific capacitance has been demonstrated in aqueous as

well as organic electrolytes that confirms positive effect of

heteroatom doping on the energy storage capacity of SCs.

However, a caution should be taken when comparing the

energy (E) and power (P) densities of SCs constructed

from different electrode materials since on some occasions

E and P were calculated from the capacitance values

obtained from 3-electrode while other correspond to the

capacitance obtained from 2-electrode tests. In addition,

majority of electrochemical tests have been performed in

very strong alkaline solutions, which are not commercially

and environmentally plausible. More investigation on the

heteroatom doping in graphene on the energy storage

capacity in SCs with commercial organic electrolyte seems

urgent in order to transform research and development of

heteroatom-doped graphene in practical application.

It also can be seen that even though a wide investigation

has been performed in N-doped graphene materials, a

fundamental understanding of energy storage mechanism

for introduced N atoms is still lacking. Besides, most work

has focused on the N-doped graphene materials whereas

few works on other heteroatoms doped graphene have been

reported. The energy storage mechanism for heteroatom-

doped graphene is still needed.

4 Heteroatom-doped graphene for LIBs

In comparison with SCs, LIBs have the advantage of high

specific energy density [68]. LIBs typically consist of a

metal oxide-based positive electrode (cathode), graphite-

based negative electrode (anode), a porous membrane

separating the two electrodes, and an electrolyte (typically

LiPF6 or LiClO4 organic electrolytes). The energy storage

mechanism is that lithium ions deintercalate from the sites

between layers of the cathode into electrolytes and inter-

calate into the carbonaceous anode during the charging

process while lithium ions are deintercalated from the

anode and intercalate back to the empty sites between

layers of the cathode during the discharging process [69].

LIBs are irreplaceable in many practical applications

including mobile phones, digital cameras and laptop

computers. However, to extend the application of LIBs into

more wide scale, such as renewable energy, an improve-

ment in cycling life and power density is critically needed.

Extensive research has been performed on the devel-

opment of new and more efficient cathode materials based

on metal oxide materials. At the same time, carbon-based

Chin. Sci. Bull. (2014) 59(18):2102–2121 2109

123

anodes have been investigated with same enthusiasm and

graphene-based anode materials have been attracting

steady interest in past years [69]. Similarly, to the positive

effect of heteroatom doping in graphene on the energy

storage capacity in SCs, heteroatoms can effectively

improve the reversibility and rate performance of anodes of

LIBs [37]. The reason for performance boost is provoked

by doped atoms with different electronegativity (e.g., N:

3.04, B: 2.04, S: 2.58) that can break the electroneutrality

of graphene thus creating the charged sites which improve

the discharge and charge capacity of graphene in LIBs

[30]. Besides, heteroatoms can also induce a large number

of defects onto the graphene surface, which leads to the

formation of a disordered carbon structure that further

enhances the lithium insertion properties [30, 41]. Corre-

spondingly, heteroatom-doped graphene materials

(including N, B, P, and S) have been synthesized as anodes

of LIBs and these are reviewed in the following section.

4.1 N-doped graphene as anode materials for LIBs

In N-doped graphene, the well-bonded nitrogen atom can

drastically alter the electronic properties, provide more

active sites, and enhance the interaction between carbon

Fig. 7 (Color online) a Fabrication illustration of all-solid-state SCs (ASSSs) based on BN-GAs that were involved by a combined hydrothermal

process and freeze-drying process. The as-fabricated SCs with a diameter of 7 mm indicated by the dotted green ring and a simplified schematic

of ASSSs based on aerogels were shown (below left). b Atomic force microscope (AFM) image and c corresponding height profile of the GO

used, with thickness of 1 nm. d Digital images of two pairs of GO solutions with different volumes in the vials and B,N-doped graphene

hydrogels before and after hydrothermal self-assembly. Inset: digital images of the BN-GAs obtained after freeze-drying. e, f) Low- and high-

magnification SEM images of the as-prepared BN-GAs. g, h Low and high-magnification SEM images of the as-prepared BN-GAs as binder/

additive-free electrode upon physical pressing. Reprinted with permission from Ref. [29], Copyright (2012), WILEY–VCH Verlag GmbH & Co.

KGaA, Weinheim

2110 Chin. Sci. Bull. (2014) 59(18):2102–2121

123

Ta

ble

1L

ist

of

the

het

ero

ato

m-d

op

edg

rap

hen

efo

rsu

per

cap

acit

ors

Item

sD

op

ants

/met

ho

ds

Het

ero

XP

S(%

)

BE

T

(m2

g-

1)

Ele

ctro

lyte

Cap

acit

ance

(Fg

-1)

En

erg

yd

ensi

ty

po

wer

den

sity

Ref

.

N-d

op

edg

rap

hen

eN

H3

gas

An

nea

lin

g(7

00

�C)

2.3

1,9

29

6m

ol

L-

1K

OH

42

0F

g-

1o

r2

2lF

cm-

2

(0.2

Ag

-1,

3E

)

–[4

7]

N-d

op

edg

rap

hen

eN

H3

gas

An

nea

lin

g(5

50

�C)

5.2

–1

mo

lL

-1

H2S

O4

40

(0.0

02

Vs-

1,

2E

)–

[48]

N-d

op

edca

rbo

n

nan

ofi

ber

/gra

ph

ene

NH

3g

as

An

nea

lin

g(9

00

–1

,10

0�C

)

1.1

–3

.4–

1m

ol

L-

1

Na 2

SO

4

–2

9.1

Wh

kg

-1

[50]

Am

inat

edg

rap

hen

e

ho

ney

com

bs

NH

3g

as

An

nea

lin

g(2

00

–6

00

�C)

2.7

9–

3.9

12

47

6m

ol

L-

1K

OH

20

7F

g-

1o

r0

.84

Fm

-2

(0.0

03

Vs-

1,

3E

)

3.4

Wh

kg

-1

6.1

93

kW

kg

-1

[51]

N-d

op

edg

rap

hen

eN

H3

gas

Th

erm

aln

itri

dat

ion

(70

0�C

)

26

30

.61

mo

lL

-1

Et 4

NB

F4

14

4.9

(0.5

Ag

-1,

2E

)5

5.3

–8

0.5

Wh

kg

-1,

0.5

6–

16

.68

kW

kg

-1

[52]

N-d

op

edg

rap

hen

eC

yan

amid

e

Car

bo

niz

atio

n(9

00

�C)

5.2

1–

11

.24

46

51

mo

lL

-1

[Bu

4N

]BF

4

6m

ol

L-

1K

OH

24

8(0

.00

5V

s-1,

2E

)

30

2(0

.00

5V

s-1,

2E

)

–[2

8]

N-d

op

edg

rap

hen

eM

elam

ine

Car

bo

niz

atio

n(7

00

�C)

20

.51

55

51

mo

lL

-1

KO

H1

32

.4(0

.1A

g-

1,

3E

)–

[53]

Gra

ph

ene/

N-d

op

edp

oro

us

carb

on

1-E

thy

l-3

-met

hy

lim

idaz

oli

um

dic

yan

amid

e

Car

bo

niz

atio

n(8

00

�C)

13

.51

wt%

40

41

mo

lL

-1

H2S

O4

20

6(0

.1A

g-

1,

3E

)–

[54]

N-d

op

edg

rap

hen

eA

mm

on

iaan

dh

yd

razi

ne

Hy

dro

ther

mal

(80

–1

80

�C)

2.7

7–

3.2

0–

1m

ol

L-

1K

Cl

19

4(0

.01

Vs-

1,

3E

)6

.7W

hk

g-

1

3.7

kW

kg

-1

[56]

N-d

op

edg

rap

hen

eA

mm

on

iaso

luti

on

Hy

dro

ther

mal

(12

0–

18

0�C

)

6.2

–7

.2–

6m

ol

L-

1K

OH

14

4.6

(0.2

Ag

-1,

3E

)–

[57]

N-d

op

edg

rap

hen

eA

mm

on

ia

Hy

dro

ther

mal

(18

0�C

)

–4

80

30

wt%

KO

H1

85

(1A

g-

1,

2E

)1

7.1

Wh

kg

-1

20

kW

kg

-1

[58]

N-d

op

edg

rap

hen

eU

rea

Hy

dro

ther

mal

(18

0�C

)

10

.13

59

36

mo

lL

-1

KO

H3

26

(0.2

Ag

-1,

3E

)2

5.0

2W

hk

g-

1

7.9

8k

Wk

g-

1(2

E)

[59]

N-d

op

edg

rap

hen

eU

rea

Hy

dro

ther

mal

(16

0�C

)

5.4

7–

7.5

6–

6m

ol

L-

1K

OH

18

4.5

(3A

g-

1,

3E

)–

[60]

N-d

op

edg

rap

hen

eU

rea

Mic

row

ave-

assi

sted

hy

dro

ther

mal

(19

0�C

)

3.0

35

51

mo

lL

-1

H2S

O4

1m

ol

L-

1K

OH

23

3(0

.02

5V

s-1,

3E

)

18

6(0

.02

5V

s-1,

3E

)

8.4

Wh

kg

-1

[61]

N-d

op

edg

rap

hen

eH

exam

eth

yle

net

etra

min

e

Hy

dro

ther

mal

(18

0�C

)

8.6

2–

6m

ol

L-

1K

OH

16

1(0

.5A

g-

1,

3E

)–

[62]

N-d

op

edg

rap

hen

eN

H4H

CO

3

Hy

dro

ther

mal

(15

0o

r2

00

�C)

9.7

6–

13

.51

–5

mo

lL

-1

KO

H1

70

(0.5

Ag

-1,

2E

)–

[63]

Chin. Sci. Bull. (2014) 59(18):2102–2121 2111

123

Ta

ble

1co

nti

nu

ed

Item

sD

op

ants

/met

ho

ds

Het

ero

XP

S(%

)

BE

T

(m2

g-

1)

Ele

ctro

lyte

Cap

acit

ance

(Fg

-1)

En

erg

yd

ensi

ty

po

wer

den

sity

Ref

.

N-d

op

edg

rap

hen

e-C

NT

net

wo

rks

Py

rro

le

Hy

dro

ther

mal

and

carb

on

izat

ion

(85

0�C

)

2.6

–6

mo

lL

-1

KO

H1

80

(0.5

Ag

-1,

2E

)–

[64

]

N-d

op

edg

rap

hen

e

hy

dro

gel

s

Ure

a

Hy

dro

ther

mal

(16

0�C

)

3.9

5–

6.6

1[

1,3

00

6m

ol

L-

1K

OH

32

6F

g-

1(1

Ag

-1,

3E

)

30

8F

gv1

(3A

g-

1,

3E

)

–[1

9]

N-d

op

edg

rap

hen

e

hy

dro

gel

Dif

fere

nt

org

anic

amin

e

Hy

dro

ther

mal

(18

0�C

)

3.7

2–

9.6

3–

5m

ol

L-

1K

OH

19

0.1

(10

.0A

g-

1,

2E

)

11

3.8

(18

5A

g-

1,

2E

)

24

5.0

kW

kg

-1

20

5.0

kW

kg

-1

[18

]

N-d

op

edg

rap

hen

e

fram

ewo

rk

Py

rro

le

Hy

dro

ther

mal

(18

0�C

),an

d

carb

on

izat

ion

(1,0

50

�C)

4.2

28

01

mo

lL

-1

LiC

lO4

41

5–

48

4(1

–1

00

Ag

-1,

3E

)

38

4–

45

4(0

.01

–0

.5V

s-1,

2E

)

–[1

7]

N-d

op

edg

rap

hen

eN

itro

gen

gas

Pla

sma

pro

cess

1.6

8–

2.5

1–

6m

ol

L-

1K

OH

1m

ol

L-

1

(C2H

5) 4

NB

F4)

28

2(K

OH

,1

Ag

-1,

2E

)4

8W

hk

g-

1

80

0k

Wk

g-

1

[42

]

N-d

op

edg

rap

hen

e

hy

dro

gel

Hy

dro

xy

lam

ine

hy

dro

chlo

rid

ean

d

hy

dro

xy

lam

ine

So

lvo

ther

mal

reac

tio

n(1

50

�C)

2.5

–4

.32

–2

5w

t%K

OH

20

5(0

.00

1V

s-1,

2E

)3

.65

Wh

kg

-1

20

.5k

Wk

g-

1

[40

]

N-m

od

ified

gra

ph

ene

Mel

amin

e

Mec

han

ical

exfo

liat

ion

––

6m

ol

L-

1K

OH

22

7F

g-

1o

r1

55

Fcm

-3

(1A

g-

1,

3E

)

–[4

3]

B-d

op

edg

rap

hen

eB

ora

ne-

tetr

ahy

dro

fura

nad

du

ct

Refl

ux

(60

–8

0�C

)

0.7

–1

.14

66

6m

ol

L-

1K

OH

20

0F

g-

1o

r4

3l

Fcm

-2

(0.1

Ag

-1,

2E

)

–[6

5]

P-d

op

edg

rap

hen

eP

ho

sph

ori

cac

id

Sti

ran

dtr

eate

d(2

00

�C)

––

1m

ol

L-

1H

2S

O4

36

7(0

.00

5V

s-1,

2E

)5

9W

hk

g-

1

9k

Wk

g-

1

[66

]

B,N

-do

ped

gra

ph

ene

aero

gel

s

NH

3B

F3

hy

dro

ther

mal

(18

0�C

)B

-0.6

N-3

.0

24

91

mo

lL

-1

H2S

O4

PV

A/H

2S

O4

gel

23

9(0

.00

1V

s-1,

2E

)8

.7W

hk

g-

1

1.6

5k

Wk

g-

1

[29

]

B,N

-do

ped

gra

ph

ene

Am

mo

nia

bo

ran

eR

eflu

xB

-1.8

1

N-1

.57

–1

mo

lL

-1

TE

AB

F4/A

N

13

0(1

Ag

-1,

2E

)–

[67

]

N,C

l-d

op

edg

rap

hen

eA

no

dic

po

lari

zati

on

N-7

.25

Cl-

0.3

8

–1

mo

lL

-1

H2S

O4

18

2(0

.01

Vs-

1,

3E

)[4

4]

2112 Chin. Sci. Bull. (2014) 59(18):2102–2121

123

structure and lithium thus also leading to improved kinetics

of lithium diffusion and transfer. Therefore, N-doped

graphene is believed as an potentially promising anode

material for LIBs [70].

Generally, thermal treatment and hydrothermal

approach are the foremost methods used in the preparation

of N-doped graphene for LIBs.

In early 2010, Reddy et al. [37] demonstrated a controlled

growth of N-doped graphene layers grown directly on copper

current collectors by a liquid precursor (acetonitrile) based

CVD method and studied its reversible Li-ion intercalation

properties as an anode material of LIBs. The N-doped

graphene with 9 at.% of nitrogen provided high discharge

capacity of 0.25 mAh cm-2 in the first cycle and remained at

0.05 mAh cm-2 for the reversible discharge capacity, which

was almost double compared to the undoped pristine

graphene. The performance improvement was concluded

due to induced surface defects and the presence of pyridinic

N atoms in the graphene structure. Besides, such feasible and

efficient process of direct growth of an active electrode

material on the current collector is very adaptable to the

battery manufacturing technology.

In another work, thermal treatment of pristine graphene

at 600 �C under ammonia atmosphere was introduced in

the preparation of N-doped graphene with 3.06 at.% of

nitrogen (Fig. 8). The N-doped graphene used as the anode

material exhibited a high initial capacity of 1,043 mAh g-1

and a reversible capacity of 872 mAh g-1 at low current

density of 50 mA g-1. More interestingly, it could be

quickly charged and discharged in a very short time toge-

ther with high-rate capability and excellent long-term

cyclability [41]. N-doped graphene nanosheets with a rel-

atively low doping of 2 %–2.8 % were also synthesized by

a similar thermal treatment and showed high reversible

capacities [34, 70].

Thermal annealing of graphene nanosheets and mela-

mine resulted in a highly N-doped graphene with 7.04 at.%

of nitrogen. Such material exhibited high reversible

capacity of 1,136 mAh g-1 at the current density of

50 mA g-1 and highly stable capacity of 241 mAh g-1 at

an extremely high current density of 20 A g-1 [71]. The

superior electrochemical performance was attributed to the

high nitrogen-doping level and the existence of pyridinic

nitrogen atoms.

Solvothermally reduced and simultaneously functional-

ized N-doped graphene was prepared by refluxing GO in

dimethyl formamide, exhibiting reversible capacity of

180.1 mAh g-1 at the current density of 50 mA g-1 [72].

The evolution of structural changes with different refluxing

time was also investigated, indicating that the removal of

oxygen-containing functional groups left dangling bonds

during the solvothermal reduction, which was responsible

for the N-doping at longer refluxing time.

4.2 N-doped graphene-based composites as anode

materials for LIBs

Hybridizing heteroatom-doped graphene with high capacity

second-phase materials leads into significantly improved

performance of corresponding composites when studied as

anode materials of LIBs [73–83].

SnO2 is regarded as one of the most promising anode

materials with high theoretical capacity (782 mAh g-1)

and low intercalation potential for lithium ions. However, it

suffers from a severe volume change during lithiation and

delithiation process, which leads to extremely poor cycling

stability [69]. Introducing of N-doped graphene to SnO2

can efficiently solve this problem.

In this fashion, SnO2/N-doped graphene sandwich papers

were fabricated by the pyrolysis of the mixture of graphene

and SnCl2, in which 7,7,8,8-tetracyanoquinodimethane

anion acting as both the nitrogen source and the complexing

agent. The composite with a nitrogen doping of 8 at.%

exhibited a high reversible capacity of 910 mAh g-1 at

50 mA g-1 and good cycling stability [74]. The improved

electrochemical performance was attributed to the structural

features that provided large number of surface defects

induced onto the graphene by N-doping, excellent electronic

conductivity, short transportation length for both lithium

ions, and electrons, and enough elastomeric space to

accommodate volume changes upon Li insertion/extraction,

as depicted in Fig. 9.

A hydrothermal process was used in preparation of

ultrathin SnO2 nanorods/N-doped graphene composite with

4.6 at.% of nitrogen. The composite showed high revers-

ible specific capacity of 803 mAh g-1 even after 100

cycles, superior rate capability and outstanding cycling

stability as anode materials of LIBs [75]. The synergistic

Fig. 8 (Color online) a SEM image of the N-doped graphene. b N1 s

XPS spectrum of the N-doped graphene. Inset schematic structure of

the binding conditions of N in a graphene lattice showing the

pyridinic N (N1) and pyrrolic N (N2), indicated by magenta dotted

rings. Reprinted with permission from Ref. [41], Copyright (2011),

American Chemical Society

Chin. Sci. Bull. (2014) 59(18):2102–2121 2113

123

effect between N-doped graphene nanosheets and SnO2,

which greatly decreased the energy barrier for Li penetra-

tion, the structural defects on the sites of pyridinic nitrogen

atoms and improved electronic properties were concluded

as key reasons for such enhanced performance.

In a different approach, hybrid SnO2 nanocrystals/N-

doped graphene composite with 3 at.% of nitrogen doping

was fabricated from a homogeneous aqueous suspension of

SnO2 nanocrystals and GO by in situ hydrazine monohy-

drate vapor reduction. The very high and stable capacity of

1,021 mAh g-1 for the composite material benefited from

the nano-sized SnO2 particles, the highly conductive

graphene, and the Sn-N bond formed between graphene

and SnO2 nanocrystals [83].

SnO2 nanoparticles/N-doped graphene with high nitro-

gen doping of 7.5 at.% was carried out by a modified

polyol reduction method. The composite was used as an

anode material in LIBs, displaying a superior reversible

capacity (1,220 mAh g-1 after 100th cycle at 90 mA g-1).

In addition, impedance measurements also revealed that N

doping significantly reduced the charge transfer resistance

of graphene-based electrodes [81].

Fig. 9 (Color online) Charge/discharge curves of a SnO2/N-doped graphene papers and b commercial SnO2 nanoparticles (20–50 nm) at various

current densities. c, d Cycling performance of SnO2/N-doped graphene paper (c) and SnO2 nanoparticles (d) at a current density of 50 mA g-1.

e Schematic representation showing paths for lithium ions and electrons in the SnO2/N-doped graphene paper, respectively. Reprinted with

permission from Ref. [74], Copyright (2012), WILEY–VCH Verlag GmbH & Co. KGaA, Weinheim

2114 Chin. Sci. Bull. (2014) 59(18):2102–2121

123

Besides SnO2, other hybrid materials are also performed

as the anodes of LIBs. One example is represented by MnO/

N-doped graphene nanosheets with the nitrogen doping of

1.13 wt% and was prepared by a hydrothermal method fol-

lowed by the thermal treatment in ammonia gas as illustrated

in Fig. 10. MnO nanoparticles were homogenously anchored

on the layers of N-doped graphene to form an efficient

electronic/ionic mixed conducting network, exhibiting a

reversible electrochemical lithium storage capacity as high

as 772 mAh g-1 at 100 mA g-1 after 90 cycles [76].

A hydrothermal condition was applied in the preparation of

Fe2O3/N-doped graphene composite with 2.2 wt% of nitro-

gen. The composite showed improved cycling performance

than those of Fe2O3/pristine graphene composite and pure

Fe2O3, delivering a reversible capacity of 1,012 mAh g-1

after 100 cycles at 100 mA g-1 and a superior rate capability

[73]. The small particle size of Fe2O3, the synergistic effect

between Fe2O3 particles and N-doped graphene, and the good

electronic conductivity by introducing N-doped graphene to

the composite were concluded as main factors for high per-

formance of this anodic LIBs material.

In another work, Fe2O3/N-doped graphene composite

with a higher nitrogen doping of 7 % was produced by an

annealing process and delivered a reversible capacity of

1,100 mAh g-1 after 50 cycles [82].

Co3O4/N-doped graphene was reported by a hydrother-

mal reaction with GO, cobalt acetate and ammonia solu-

tion. Such composite delivered high initial capacity of

1,413 mAh g-1 at 400 mA g-1 and a reversible capacity

of 775.2 mAh g-1 after 50 charge/discharge cycles as the

anode material [78]. In this composite, graphene nano-

sheets buffered the volume change of Co3O4 upon lithium

insertion/extraction thus improving the cycling perfor-

mance of LIBs. Moreover, nitrogen doping not only

improved the conductivity of graphene nanosheets, but also

introduced defects to store lithium and enhanced the con-

nection of the Co3O4 nanocrystals to the graphene nano-

sheets, leading to better distribution of Co3O4 on graphene

nanosheets and enhanced rate performance.

TiO2/N-doped graphene composite with 1.75 wt% N

doping was synthesized by the combination of the ammo-

nia thermal treatment and a gas/liquid interface. The results

revealed that such composite showed a much better elec-

trochemical performance than the TiO2/graphene compos-

ite and the bare TiO2 nanoparticles. Importantly, a high

reversible capacity of 109 mAh g-1 was still obtained even

at a super high current density of 5,000 mA g-1 [77].

Another report describes preparation of anatase TiO2

nanoparticles/N-doped graphene via hydrothermal method.

When applied as anode material of LIBs, such composites

exhibited reversible capacity of 226 mAh g-1 at 0.2 C.

The introduction of nitrogen in graphene was proven to

increase the conductivity of graphene as well as providing

more structural defects on N-doped graphene where more

lithium ions could be accommodated thus leading to the

good electrochemical performance [79].

VO2 nanoparticles/N-doped graphene synthesized by a

hydrothermal method also showed an improved LIBs per-

formance, with an enhanced reversible capacity of

251 mAh g-1 (50 mA g-1) compared to the pure VO2 [80].

Apart from metal oxides, other anode materials with

N-doped graphene have also been studied. N-doped

graphene frame supported silicon@graphitic carbon gran-

ules with a nitrogen doping of 3.35 % was synthesized by

one-step solid-state reaction using iron phthalocyanine

(FePc) and silicon nanoparticles (Si NPs) as the precursors.

The self-assembled granules realized a high reversible

capacity up to 1,065 mAh g-1 after 200 cycles at

0.28 A g-1 when applied as anode material in LIBs [84].

VN/N-doped graphene hybrid composites were synthe-

sized by a sol–gel method followed by thermal treatment at

ammonia atmosphere (800 �C). In the composites, VN

nanoparticles were adhered to the surface of N-doped

graphene nanosheets providing efficient electronic and

ionic conducting network and exhibited a capacity of 983

mAh g-1 at 42 mA g-1 [85].

Phase-controlled Ni3S4/N-doped graphene composite

with nitrogen of 6.05 at % was prepared during the hydro-

thermal process. The composite with a high content of the

electrochemically favorable pyridinic nitrogen (up to

81.93 %) showed high discharge capacity of 558.2 mAh g-1

at high discharge rate of 4 C and excellent capacity retention

of 95.19 % after 100 cycles [86].

A fast microwave-assisted route was employed using

ethylenediamne as the nitrogen source to produce the

Zn2GeO4/N-doped graphene nanocomposites. GO sheets

transformed in situ into N-doped graphene through the

Fig. 10 (Color online) Schematic illustration for the preparation of

N-doped MnO (N-MnO)/N-doped graphene hybrid material [76].

Reprinted with permission from reference 76, Copyright (2012),

American Chemical Society

Chin. Sci. Bull. (2014) 59(18):2102–2121 2115

123

reduction and doping of N atoms by the resulting amine

under microwave irradiation. The as-formed nanocom-

posite with nitrogen doping of 1.98 wt% exhibited high

capacity of 1,463 mAh g-1 for the initial discharge

capacity and 1,044 mAh g-1 after 100 cycles at a current

density of 100 mA g-1 and stable rate performance [87].

4.3 Other heteroatom-doped graphene as anode

materials for LIBs

B-doped graphene with a small amount of boron ele-

ment (0.88 at.%) was synthesized by thermal treatment

of GO with BCl3 gas (Fig. 11). When tested as anode

in LIB, this material exhibited an ultrahigh initial

capacity of 1,549 mAh g-1 and high reversible capacity

of 1,227 mAh g-1 at low current density of 50 mA g-1.

More importantly, as-produced B-doped graphene could

be quickly charged and discharged in a very short time

together with high-rate capability and excellent long-

term cycleability. A high capacity of 235 mAh g-1 was

obtained even at high current density of 25 A g-1 with

just 30 s to the full charge [41]. The superior electro-

chemical performance of this B-doped graphene was

attributed to the increased conductivity and electro-

chemical activity, increased disordered surface mor-

phology, produced topological defects by doping and

higher hydrophobicity as well as better wettability

toward organic electrolytes than the pristine graphene,

all of which were favorable for Li storage and conse-

quently improved the reversible capacity of this anodic

material.

In another report, P-doped graphene was synthesized

and for the first time applied as the anode materials in

LIBs. The disordered carbon structure upon doping with

phosphorus was concluded as beneficial to the Li storage.

P-doped graphene with 1.81 at.% of phosphorous was

synthesized after thermal annealing of GO and triphenyl-

phosphine. Enhanced reversible capacity and rate perfor-

mance reaching a capacity of 460 mAh g-1 at 0.1 A g-1

with almost no loss over 80 cycles were observed com-

pared with that of the un-doped graphene (280 mAh g-1)

[30].

Hierarchically porous nitrogen (4.2 %) and sulfur

(0.94 %) co-doped graphene was fabricated through a

facile in situ constructing strategy via nickel foam tem-

plating and using GO, sulfonated polystyrene spheres, and

poly(vinyl pyrrolidone) as precursors. Benefitting from the

synergistic effect of hierarchically porous structure and

nitrogen and sulfur co-doping, the novel electrodes deliv-

ered reversible capacities of 1,137 and 957 mAh g-1 at

first and tenth cycles, respectively, and high-power density

and energy density of 116 kW kg-1 of 322 Wh kg-1 at

80 A g-1, respectively [88].

Table 2 lists all the above mentioned heteroatom-doped

graphene materials applied as anode materials of LIBs.

Similar to SCs, heteroatom doping clearly affects the

performance of graphene in LIBs and improved electro-

chemical properties including capacity, rate performance

and cycling stability are demonstrated. Particularly.

promising is attempt of combining heteroatom-doped

graphene with high capacity inorganic anodic materials

such as SnO2 and Si where doped graphene not only

prevents mechanical deterioration of inorganic phase

during charge/discharge but also improved electronic and

conductivity properties of such composite electrode

materials.

Fig. 11 (Color online) a STEM image of B-doped graphene and b C- and c B-elemental mapping of the square region in (a). d B1 s XPS

spectrum of B-doped graphene. Inset: schematic structure of the binding conditions of B in a graphene lattice showing BC3 (B1) and BC2O (B2),

indicated by magenta dotted rings. Reprinted with permission from Ref. [41], Copyright (2011), American Chemical Society

2116 Chin. Sci. Bull. (2014) 59(18):2102–2121

123

Ta

ble

2L

ist

of

the

het

ero

ato

m-d

op

edg

rap

hen

ein

LIB

s

Item

sD

op

ants

/met

ho

ds

Het

ero

XP

S

(at.

%)

BE

T

(m2

g-

1)

Ele

ctro

lyte

Rev

ersi

ble

cap

acit

yE

ner

gy

den

sity

po

wer

den

sity

Ref

.

N-d

op

edg

rap

hen

eA

ceto

nit

rile

CV

D

9–

1m

ol

L-

1L

iPF

60

.05

mA

hcm

-2

–[3

7]

N-d

op

edg

rap

hen

eN

H3

Th

erm

altr

eatm

ent

(60

0�C

)

3.0

62

90

1m

ol

L-

1L

iPF

68

72

mA

hg

-1

(50

mA

g-

1)

22

6W

hk

g-

1

29

.1k

Wk

g-

1

(25

Ag

-1)

[41

]

N-d

op

edg

rap

hen

eN

H3

Th

erm

altr

eatm

ent

(80

0�C

)

2–

1m

ol

L-

1L

iPF

69

00

mA

hg

-1

(42

mA

g-

1)

–[7

0]

N-d

op

edg

rap

hen

eN

H3

Th

erm

altr

eatm

ent

2.8

59

91

mo

lL

-1

LiP

F6

68

4m

Ah

g-

1(1

00

mA

g-

1)

–[3

4]

N-d

op

edg

rap

hen

eM

elam

ine

Th

erm

altr

eatm

ent

(70

0�C

)

7.0

4–

1m

ol

L-

1L

iPF

61

,13

6m

Ah

g-

1(5

0m

Ag

-1)

–[7

1]

N-d

op

edg

rap

hen

eD

imet

hy

lfo

rmam

ide

Refl

ux

(14

0�C

)

––

1m

ol

L-

1L

iPF

61

80

.1m

Ah

g-

1(5

0m

Ag

-1)

–[7

2]

Sn

O2/N

-do

ped

gra

ph

ene

7,7

,8,8

-tet

racy

ano

qu

ino

dim

eth

ane

anio

n

Th

erm

altr

eatm

ent

(80

0�C

)

8–

1m

ol

L-

1L

iClO

49

10

mA

hg

-1

(50

mA

g-

1)

–[7

4]

Sn

O2/N

-do

ped

gra

ph

ene

Ure

a

Hy

dro

ther

mal

(12

0�C

)

4.6

–1

mo

lL

-1

LiP

F6

80

3m

Ah

g-

1(1

00

mA

g-

1)

–[7

5]

Sn

O2/N

-do

ped

gra

ph

ene

Hy

dra

zin

em

on

oh

yd

rate

Vap

or

(12

0�C

)

3–

1m

ol

L-

1L

iPF

61

,02

1m

Ah

g-

1(5

00

mA

g-

1)

–[8

3]

Sn

O2/N

-do

ped

gra

ph

ene

Po

lyp

yrr

ole

ther

mal

trea

tmen

t(8

00

�C)

7.5

–1

mo

lL

-1

LiP

F6

1,2

20

mA

hg

-1

(90

mA

g-

1)

–[8

1]

N-M

nO

/N-d

op

edg

rap

hen

eN

H3

Th

erm

altr

eatm

ent,

80

0�C

1.1

3w

t%–

1m

ol

L-

1L

iPF

67

72

mA

hg

-1

(10

0m

Ag

-1)

–[7

6]

Fe 2

O3/N

-do

ped

gra

ph

ene

Fe(

NH

4) 2

(SO

4) 2

Hy

dro

ther

mal

(18

0�C

)

2.2

wt%

–1

mo

lL

-1

LiP

F6

1,0

12

mA

hg

-1

(10

0m

Ag

-1)

–[7

3]

Fe 2

O3/N

-do

ped

gra

ph

ene

7,7

,8,8

-etr

acy

ano

qu

ino

di-

met

han

ean

ion

Th

erm

altr

eatm

ent

(50

0�C

)

7–

1m

ol

L-

1L

iClO

41

,10

0m

Ah

g-

1(0

.2C

)–

[82

]

Co

3O

4/N

-do

ped

gra

ph

ene

Am

mo

nia

solu

tio

n

Hy

dro

ther

mal

(15

0�C

)

––

1m

ol

L-

1L

iPF

67

75

.2m

Ah

g-

1(4

00

mA

g-

1)

–[7

8]

TiO

2/N

-do

ped

gra

ph

ene

NH

3

Th

erm

altr

eatm

ent

(80

0�C

)

1.7

5w

t%–

1m

ol

L-

1L

iPF

61

09

mA

hg

-1

(5A

g-

1)

–[7

7]

TiO

2/N

-do

ped

gra

ph

ene

Hy

dra

zin

ean

hy

dro

us

Hy

dro

ther

mal

(20

0�C

)

––

1m

ol

L-

1L

iPF

62

26

mA

hg

-1

(0.2

C)

–[7

9]

Chin. Sci. Bull. (2014) 59(18):2102–2121 2117

123

Ta

ble

2co

nti

nu

ed

Item

sD

op

ants

/met

ho

ds

Het

ero

XP

S

(at.

%)

BE

T

(m2

g-

1)

Ele

ctro

lyte

Rev

ersi

ble

cap

acit

yE

ner

gy

den

sity

po

wer

den

sity

Ref

.

VO

2/N

-do

ped

gra

ph

ene

Am

mo

niu

mv

anad

ates

Hy

dro

ther

mal

(18

0�C

)

––

1m

ol

L-

1L

iClO

42

51

mA

hg

-1

(50

mA

g-

1)

–[8

0]

N-d

op

edg

rap

hen

efr

ame

sup

po

rted

sili

con

@g

rap

hit

icca

rbo

ng

ran

ule

s

Th

erm

altr

eatm

ent

(95

0�C

)3

.35

–1

mo

lL

-1

LiP

F6

1,0

65

mA

hg

-1

(0.2

8A

g-

1)

33

0m

Ah

g-

1(N

G)

–[8

4]

VN

/N-d

op

edg

rap

hen

eN

H3

Th

erm

altr

eatm

ent

(80

0�C

)

––

1m

ol

L-

1L

iPF

69

83

mA

hg

-1

(42

mA

g-

1)

–[8

5]

Ni 3

S4/N

-do

ped

gra

ph

ene

Am

mo

nia

solu

tio

n

Hy

dro

ther

mal

(20

0�C

)

6.0

5–

1m

ol

L-

1L

iPF

65

58

.2m

Ah

g-

1(4

C)

–[8

6]

Zn

2G

eO4/N

-do

ped

gra

ph

ene

Eth

yle

ned

iam

ne

Mic

row

ave

irra

dia

tio

n

1.9

8w

t%1

mo

lL

-1

LiP

F6

1,0

44

mA

hg

-1

(10

0m

Ag

-1)

–[8

7]

B-d

op

edg

rap

hen

eB

Cl 3

Th

erm

altr

eatm

ent

(80

0�C

)

0.8

82

56

1m

ol

L-

1L

iPF

61

,22

7m

Ah

g-

1(5

0m

Ag

-1)

–[4

1]

P-d

op

edg

rap

hen

eT

rip

hen

ylp

ho

sph

ine

Th

erm

altr

eatm

ent

1.8

1–

1m

ol

L-

1L

iPF

64

60

mA

hg

-1

(10

0m

Ag

-1)

–[3

0]

N,S

-do

ped

gra

ph

ene

Su

lfo

nat

edp

oly

sty

ren

esp

her

esan

d

po

ly(v

iny

lp

yrr

oli

do

ne)

Th

erm

altr

eatm

ent

(80

0�C

)

N-4

.2

S-0

.94

70

1m

ol

L-

1L

iPF

69

57

mA

hg

-1

(10

0m

Ag

-1)

32

2W

hk

g-

1

11

6k

Wk

g-

1

(80

Ag

-1)

[88

]

2118 Chin. Sci. Bull. (2014) 59(18):2102–2121

123

5 Conclusion and future prospects

In this paper, we aimed to summarize recent developments

in the synthesis of heteroatom-doped graphene materials

and their application as electrode materials for the energy

storage systems including SCs and LIBs.

Since graphene discovery a large number of methods

have been explored and successfully applied in preparation

of the heteroatom-doped graphene including direct syn-

thesis and post-synthesis treatments. Graphene-based

materials with different levels of doping have been

obtained but the process for a large-scale production of

doped graphene is still lacking thus providing an oppor-

tunity for further exploration and development. Another

important aspect is the ability to control the level of doping

as well as the type of introduced functionalities to the

graphene materials. It is the feeling of authors that the

strategy in most reported researches is to randomly select

precursors and the synthesis methods to produce graphene

materials with some sort of doping. The systematic studies

on the mechanism of heteroatom doping in graphene from

model precursors (such as urea or melamine in case of

N-doped graphene production for instance) and at the

conditions of standard synthesis routes (such as hydro-

thermal or thermal methods for instance) could be very

enriching in terms of good fundamental understanding of

the formation of the heteroatom-doped graphene thus

providing more standardized routes for doped graphene

with desired level and type of doping.

This is particularly important for the application of

heteroatom-doped graphene and its derivatives as electrode

materials of SCs and LIBs. As the studies clearly show,

heteroatom doping greatly enhances the energy storage

capacity and rate performance of graphene materials. Par-

ticularly, the N-doped graphene has been extensively

investigated as the electrode material of SCs and improved

performance has been reported each time. The variations in

graphene electronegativity upon doping, different charge

on doped heteroatoms and surrounding carbon atoms as

well as changes in physical and chemical properties of

graphene have been found as key reasons for improved

energy storage capacity of doped graphene in SCs. How-

ever, a profound fundamental understanding of the effect of

nitrogen doping on the energy storage mechanics is still

somehow lacking, especially the effect of various types of

nitrogen functionalities. It is our opinion that computa-

tional chemistry combined with the state of the art ana-

lytical methods will play a critical role in clarifying the

fundamentals lying behind the energy storage capacity in

N-doped graphene and will lay foundation fordesigning

synthesis methods for specifically enriched N-doped

graphene.

With regards to application of heteroatom-doped

graphene in LIBs, a great amount of interest has been

devoted to N-doped graphene and its composites as anode

materials. The greatly enhanced performance is often

attributed to the alteration of the electronic properties of

graphene upon doping, formation of defects and thus

improved storage capacity for lithium ions and enhanced

bonding capabilities between N-doped graphene and

composite counterparts. Similar to the SCs, the LIBs

research and development would also benefit from better

understanding of the mechanism of the Li ion energy

storage in N-doped graphene and its composites that would

consequently allow designing anode materials with supe-

rior Li ion storage capacity.

The majority of the work has been focused on N-doped

graphene and its application in SCs and LIBs, however,

some reports also show that other heteroatoms including

boron and phosphorus as well as co-doping of nitrogen and

boron have positive effect on the energy storage capacity of

corresponding doped graphene in both the supercapacitors

and Li-ion batteries. While the mechanism of the energy

storage capacity in such graphene materials is yet to be

clarified, results clearly indicate that doping of graphene

with nitrogen, boron, phosphorus, or their mixtures is an

effective way for increasing the specific energy storage

capacities and rate performance, both of which are criti-

cally important in designing effective energy storage sys-

tems for large-scale applications including renewable

energy. Therefore, once the fundamentals behind energy

storage mechanism in heteroatom-doped graphene are

better understood, the new large-scale synthesis routes can

be developed, and the heteroatom-doped-based graphene

materials will play major role as commercial electrode

materials of SCs and LIBs in the future.

References

1. Arico AS, Bruce P, Scrosati B et al (2005) Nanostructured

materials for advanced energy conversion and storage devices.

Nat Mater 4:366–377

2. Dai L (2012) Functionalization of graphene for efficient energy

conversion and storage. Accounts Chem Res 46:31–42

3. Novoselov KS, Geim AK, Morozov SV et al (2004) Electric field

effect in atomically thin carbon films. Science 306:666–669

4. Huang X, Yin Z, Wu S et al (2011) Graphene-based materials:

synthesis, characterization, properties, and applications. Small

7:1876–1902

5. Huang X, Qi X, Boey F et al (2012) Graphene-based composites.

Chem Soc Rev 41:666–686

6. Wang H, Maiyalagan T, Wang X (2012) Review on recent pro-

gress in nitrogen-doped graphene: synthesis, characterization, and

its potential applications. ACS Cataly 2:781–794

7. Stoller MD, Park S, Zhu Y et al (2008) Graphene-based ultra-

capacitors. Nano Lett 8:3498–3502

Chin. Sci. Bull. (2014) 59(18):2102–2121 2119

123

8. Balandin AA, Ghosh S, Bao W et al (2008) Superior thermal

conductivity of single-layer graphene. Nano Lett 8:902–907

9. Nair R, Blake P, Grigorenko A et al (2008) Fine structure con-

stant defines visual transparency of graphene. Science 320:1308

10. Lee C, Wei X, Kysar JW et al (2008) Measurement of the elastic

properties and intrinsic strength of monolayer graphene. Science

321:385–388

11. Ponomarenko LA, Schedin F, Katsnelson MI et al (2008) Chaotic

Dirac billiard in graphene quantum dots. Science 320:356–358

12. Ritter KA, Lyding JW (2009) The influence of edge structure on

the electronic properties of graphene quantum dots and nano-

ribbons. Nat Mater 8:235–242

13. Barone V, Hod O, Scuseria GE (2006) Electronic structure and

stability of semiconducting graphene nanoribbons. Nano Lett

6:2748–2754

14. Kosynkin DV, Higginbotham AL, Sinitskii A et al (2009) Lon-

gitudinal unzipping of carbon nanotubes to form graphene

nanoribbons. Nature 458:872–876

15. Shin MK, Lee B, Kim SH et al (2012) Synergistic toughening of

composite fibres by self-alignment of reduced graphene oxide and

carbon nanotubes. Nat Commun 3:650

16. Zhao Y, Jiang C, Hu C et al (2013) Large-scale spinning

assembly of neat, morphology-defined, graphene-based hollow

fibers. ACS Nano 7:2406–2412

17. Zhao Y, Hu C, Hu Y et al (2012) A versatile, ultralight, nitrogen-doped

graphene framework. Angewe Chem Int Ed 124:11533–11537

18. Chen P, Yang JJ, Li SS et al (2013) Hydrothermal synthesis of

macroscopic nitrogen-doped graphene hydrogels for ultrafast

supercapacitor. Nano Energy 2:249–256

19. Guo HL, Su P, Kang X et al (2013) Synthesis and characteriza-

tion of nitrogen-doped graphene hydrogels by hydrothermal route

with urea as reducing-doping agents. J Mater Chem A 1:2248

20. Berger C, Song Z, Li T et al (2004) Ultrathin epitaxial graphite:

2D electron gas properties and a route toward graphene-based

nanoelectronics. J Phys Chem B 108:19912–19916

21. Kim KS, Zhao Y, Jang H et al (2009) Large-scale pattern growth

of graphene films for stretchable transparent electrodes. Nature

457:706–710

22. Park S, Ruoff RS (2009) Chemical methods for the production of

graphenes. Nat Nanotechnol 4:217–224

23. Jeon IY, Shin YR, Sohn GJ et al (2012) Edge-carboxylated

graphene nanosheets via ball milling. Proc Natl Acad Sci USA

109:5588–5593

24. Chen J, Li C, Shi G (2013) Graphene materials for electro-

chemical capacitors. J Phys Chem Lett 4:1244–1253

25. Cao X, Shi Y, Shi W et al (2011) Preparation of novel 3D graphene

networks for supercapacitor applications. Small 7:3163–3168

26. Zhu Y, Murali S, Stoller MD et al (2011) Carbon-based sup-

ercapacitors produced by activation of graphene. Science

332:1537–1541

27. El-Kady MF, Strong V, Dubin S et al (2012) Laser scribing of

high-performance and flexible graphene-based electrochemical

capacitors. Science 335:1326–1330

28. Wen Z, Wang X, Mao S et al (2012) Crumpled nitrogen-doped

graphene nanosheets with ultrahigh pore volume for high-per-

formance supercapacitor. Adv Mater 24:5610–5616

29. Wu ZS, Winter A, Chen L et al (2012) Three-dimensional

nitrogen and boron co-doped graphene for high-performance all-

solid-state supercapacitors. Adv Mater 24:5130–5135

30. Zhang C, Mahmood N, Yin H et al (2013) Synthesis of phos-

phorus-doped graphene and its multifunctional applications for

oxygen reduction reaction and lithium ion batteries. Adv Mater

25:4932–4937

31. Wu ZS, Ren W, Wang DW et al (2010) High-energy MnO2

nanowire/graphene and graphene asymmetric electrochemical

capacitors. ACS Nano 4:5835–5842

32. Xu J, Wang K, Zu SZ et al (2010) Hierarchical nanocomposites

of polyaniline nanowire arrays on graphene oxide sheets with

synergistic effect for energy storage. ACS Nano 4:5019–5026

33. Liu H, Liu Y, Zhu D (2011) Chemical doping of graphene.

J Mater Chem 21:3335

34. Li X, Geng D, Zhang Y et al (2011) Superior cycle stability of

nitrogen-doped graphene nanosheets as anodes for lithium ion

batteries. Electrochem Commun 13:822–825

35. Some S, Kim J, Lee K et al (2012) Highly air-stable phosphorus-doped

n-type graphene field-effect transistors. Adv Mater 24:5481–5486

36. Yang S, Zhi L, Tang K et al (2012) Efficient synthesis of het-

eroatom (N or S)-doped graphene based on ultrathin graphene

oxide-porous silica sheets for oxygen reduction reactions. Adv

Funct Mater 22:3634–3640

37. Reddy ALM, Srivastava A, Gowda SR et al (2010) Synthesis of

nitrogen-doped graphene films for lithium battery application.

ACS Nano 4:6337–6342

38. Choucair M, Thordarson P, Stride JA (2009) Gram-scale pro-

duction of graphene based on solvothermal synthesis and soni-

cation. Nat Nanotechnol 4:30–33

39. Deng D, Pan X, Yu L et al (2011) Toward N-doped graphene via

solvothermal synthesis. Chem Mater 23:1188–1193

40. Chang Y, Han G, Yuan J et al (2013) Using hydroxylamine as a

reducer to prepare N-doped graphene hydrogels used in high-

performance energy storage. J Power Sources 238:492–500

41. Wu ZS, Ren W, Xu L et al (2011) Doped graphene sheets as

anode materials with superhigh rate and large capacity for lithium

ion batteries. ACS Nano 5:5463–5471

42. Jeong HM, Lee JW, Shin WH et al (2011) Nitrogen-doped graph-

ene for high-performance ultracapacitors and the importance of

nitrogen-doped sites at basal planes. Nano Lett 11:2472–2477

43. Xiao N, Lau D, Shi W et al (2013) A simple process to prepare

nitrogen-modified few-layer graphene for a supercapacitor elec-

trode. Carbon 57:184–190

44. Wang DW, Wu KH, Gentle IR et al (2012) Anodic chlorine/

nitrogen co-doping of reduced graphene oxide films at room

temperature. Carbon 50:3333–3341

45. Huang Y, Liang J, Chen Y (2012) An overview of the applica-

tions of graphene-based materials in supercapacitors. Small

8:1805–1834

46. Hulicova-Jurcakova D, Puziy AM, Poddubnaya OI et al (2009)

Highly stable performance of supercapacitors from phosphorus-

enriched carbons. J Am Chem Soc 131:5026–5027

47. Zhang LL, Zhao X, Ji H et al (2012) Nitrogen doping of graphene

and its effect on quantum capacitance, and a new insight on the

enhanced capacitance of N-doped carbon. Energ Environ Sci

5:9618

48. Lai L, Yang H, Wang L et al (2012) Preparation of supercapacitor

electrodes through selection of graphene surface functionalities.

ACS Nano 6:5941–5951

49. Lai L, Wang L, Yang H et al (2012) Tuning graphene surface

chemistry to prepare graphene/polypyrrole supercapacitors with

improved performance. Nano Energy 1:723–731

50. Zhao L, Qiu Y, Yu J et al (2013) Carbon nanofibers with radially

grown graphene sheets derived from electrospinning for aqueous

supercapacitors with high working voltage and energy density.

Nanoscale 5:4902–4909

51. Chen CM, Zhang Q, Zhao XC et al (2012) Hierarchically ami-

nated graphene honeycombs for electrochemical capacitive

energy storage. J Mater Chem 22:14076

52. Qiu Y, Zhang X, Yang S (2011) High performance supercapac-

itors based on highly conductive nitrogen-doped graphene sheets.

Phys Chem Chem Phys 13:12554–12558

53. Zheng B, Chen TW, Xiao FN et al (2013) KOH-activated

nitrogen-doped graphene by means of thermal annealing for

supercapacitor. J Solid State Electrochem 17:1809–1814

2120 Chin. Sci. Bull. (2014) 59(18):2102–2121

123

54. Zhou M, Li X, Cui J et al (2012) Synthesis and capacitive per-

formances of graphene/N-doping porous carbon composite with

high nitrogen content and two-dimensional nanoarchitecture. Int J

Electrochem Sci 7:9984–9996

55. Hu B, Wang K, Wu L et al (2010) Engineering carbon materials

from the hydrothermal carbonization process of biomass. Adv

Mater 22:813–828

56. Hassan FM, Chabot V, Li J et al (2013) Pyrrolic-structure enri-

ched nitrogen doped graphene for highly efficient next generation

supercapacitors. J Mater Chem A 1:2904

57. Jiang B, Tian C, Wang L et al (2012) Highly concentrated, stable

nitrogen-doped graphene for supercapacitors: simultaneous dop-

ing and reduction. Appl Surf Sci 258:3438–3443

58. Bai Y, Rakhi RB, Chen W et al (2013) Effect of pH-induced

chemical modification of hydrothermally reduced graphene oxide

on supercapacitor performance. J Power Sources 233:313–319

59. Sun L, Wang L, Tian C et al (2012) Nitrogen-doped graphene

with high nitrogen level via a one-step hydrothermal reaction of

graphene oxide with urea for superior capacitive energy storage.

RSC Adv 2:4498

60. Su P, Guo H, Peng S et al (2012) Preparation of nitrogen-doped

graphene and its supercapacitive properties. Acta Phys-Chim Sin

28:2745–2753

61. Lee YH, Chang KH, Hu CC (2013) Differentiate the pseudoca-

pacitance and double-layer capacitance contributions for nitro-

gen-doped reduced graphene oxide in acidic and alkaline

electrolytes. J Power Sources 227:300–308

62. Lee JW, Ko JM, Kim JD (2012) Hydrothermal preparation of

nitrogen-doped graphene sheets via hexamethylenetetramine for

application as supercapacitor electrodes. Electrochim Acta 85:

459–466

63. Cao H, Zhou X, Qin Z et al (2013) Low-temperature preparation of

nitrogen-doped graphene for supercapacitors. Carbon 56:218–223

64. You B, Wang L, Yao L et al (2013) Three dimensional N-doped

graphene-CNT networks for supercapacitor. Chem Commun

49:5016–5018

65. Han J, Zhang LL, Lee S et al (2012) Generation of B-doped

graphene nanoplatelets using a solution process and their sup-

ercapacitor applications. ACS Nano 7:19–26

66. Karthika P, Rajalakshmi N, Dhathathreyan K (2013) Phosphorus-

doped exfoliated graphene for supercapacitor electrodes. J Nano-

sci Nanotechnol 13:1746–1751

67. Pham VH, Hur SH, Kim EJ et al (2013) Highly efficient reduction

of graphene oxide using ammonia borane. Chem Commun 49:

6665–6667

68. Scrosati B, Garche J (2010) Lithium batteries: status, prospects

and future. J Power Sources 195:2419–2430

69. Xu C, Xu B, Gu Y et al (2013) Graphene-based electrodes for

electrochemical energy storage. Energy Environ Sci 6:1388

70. Wang H, Zhang C, Liu Z et al (2011) Nitrogen-doped graphene

nanosheets with excellent lithium storage properties. J Mater

Chem 21:5430

71. Cai D, Wang S, Lian P et al (2013) Superhigh capacity and rate

capability of high-level nitrogen-doped graphene sheets as anode

materials for lithium ion batteries. Electrochim Acta 90:492–497

72. Shen Y, Zhang HB, Zhang H et al (2013) Structural evolution of

functionalized graphene sheets during solvothermal reduction.

Carbon 56:132–138

73. Du M, Xu C, Sun J et al (2012) One step synthesis of Fe2O3/

nitrogen-doped graphene composite as anode materials for lith-

ium ion batteries. Electrochim Acta 80:302–307

74. Wang X, Cao X, Bourgeois L et al (2012) N-doped graphene-

SnO2 sandwich paper for high-performance lithium ion batteries.

Adv Funct Mater 22:2682–2690

75. Xu C, Sun J, Gao L (2012) Controllable synthesis of monodis-

perse ultrathin SnO2 nanorods on nitrogen-doped graphene and

its ultrahigh lithium storage properties. Nanoscale 4:5425–5430

76. Zhang K, Han P, Gu L et al (2012) Synthesis of nitrogen-doped

MnO/graphene nanosheets hybrid material for lithium ion bat-

teries. ACS Appl Mater Interfaces 4:658–664

77. Cai D, Li D, Wang S et al (2013) High rate capability of TiO2/

nitrogen-doped graphene nanocomposite as an anode material for

lithium ion batteries. J Alloy Compd 561:54–58

78. Li D, Shi D, Chen Z et al (2013) Enhanced rate performance of

cobalt oxide/nitrogen doped graphene composite for lithium ion

batteries. RSC Adv 3:5003

79. Li D, Shi D, Liu Z et al (2013) TiO2 nanoparticles on nitrogen-

doped graphene as anode material for lithium ion batteries.

J Nanopart Res 15:1–10

80. Nethravathi C, Rajamathi CR, Rajamathi M et al (2013) N-doped

graphene-VO2(B) nanosheet-built 3D flower hybrid for lithium

ion battery. ACS Appl Mater Interfaces 5:2708–2714

81. Vinayan BP, Ramaprabhu S (2013) Facile synthesis of SnO2

nanoparticles dispersed nitrogen doped graphene anode material

for ultrahigh capacity lithium ion battery applications. J Mater

Chem A 1:3865

82. Wang X, Tian W, Liu D et al (2013) Unusual formation of a-

Fe2O3 hexagonal nanoplatelets in N-doped sandwiched graphene

chamber for high-performance lithium ions batteries. Nano

Energy 2:257–267

83. Zhou X, Wan LJ, Guo YG (2013) Binding SnO2 nanocrystals in

nitrogen-doped graphene sheets as anode materials for lithium ion

batteries. Adv Mater 25:2152–2157

84. Zhang L, Hao W, Wang H et al (2013) Porous graphene frame

supported silicon@graphitic carbon via in situ solid-state syn-

thesis for high-performance lithium ion anodes. J Mater Chem A

1:7601

85. Zhang K, Wang H, He X et al (2011) A hybrid material of

vanadium nitride and nitrogen-doped graphene for lithium stor-

age. J Mater Chem 21:11916

86. Mahmood N, Zhang C, Hou Y (2013) Nickel sulfide/nitrogen-

doped graphene composites: phase-controlled synthesis and high

performance anode materials for lithium ion batteries. Small

9:1321–1328

87. Zou F, Hu X, Sun Y et al (2013) Microwave-Induced in situ

synthesis of Zn2GeO4/N-doped graphene nanocomposites and

their lithium storage properties. Chem Eur J 19:6027–6033

88. Wang ZL, Xu D, Wang HG et al (2013) In situ fabrication of

porous graphene electrodes for high-performance energy storage.

ACS Nano 7:2422–2430

Chin. Sci. Bull. (2014) 59(18):2102–2121 2121

123