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Review Article

Nano Adv., 2016, 1, 50−61.

Nano Advances

http://dx.doi.org/10.22180/na172 Volume 1, Issue 2, 2016

Recent Advances in Heteroatom-Doped Graphene Materials as Efficient Electrocatalysts towards the Oxygen Reduction Reaction

Ruguang Ma, a Yun Ma, b Yucheng Dong a and Jong-Min Lee a*

a School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637459, Singapore. Email: jmlee [at]

ntu.edu.sg b Shandong Liming Polytechnic Vocational College, 389 Jiwei Road, Jinan 250000, Shandong Province, China.

Received April 10, 2016; Revised June 22, 2016

Citation: R. Ma, Y. Ma, Y. Dong and J.-M. Lee, Nano Adv., 2016, 1, 50−61.

The development of cost-effective, efficient and stable electrocatalysts towards oxygen reduction reaction (ORR) has

been an aim to overcome the bottleneck of widespread application of fuel cells and rechargeable metal-air batteries.

Unique physicochemical properties of heteroatom-doped graphene open up a brand-new avenue for design and

application of metal-free electrocatalysts in these electrochemical devices. Given the flourishing research of graphene

and electrochemical energy storage, we critically summarize recent progress in the design, synthesis and application

of various heteroatom (N, S, P, B, etc)-doped graphene materials as the electrocatalysts towards ORR. The distinctive

properties resulting from various dopants and the resultant electrocatalytic activity are highlighted to gain insights

into the mechanism of heteroatom-doped graphene for ORR. We conclude this article with some future trends and

opportunities of developing heteroatom-doped-graphene-based electrocatalysts in the application of the ORR-related

devices.

KEYWORDS: Graphene; Doping; Electrocatalysis; Oxygen reduction reaction; Metal-air battery

1. Introduction

The limited fossil fuels as well as the worsening environmental

pollution stimulate worldwide researchers to explore sustainable

energy conversion and storage technologies.1 Among the

technologies, electrochemical conversion and storage technology

is one of simple, efficient and reliable approaches. For example,

the fuel cell technology can generate electricity from the

electrochemical reaction of hydrogen and oxygen, while

electrochemical water splitting produces oxygen and hydrogen

gases from water. These technologies, which are highly efficient,

cost-effective and environmental friendly, usually involving the

hydrogen evolution reaction (HER), oxygen evolution reaction

(OER) or oxygen reduction reaction (ORR) during the

electrochemical process. In such a process, a high activation

barrier, which is defined by the overpotential or faradic

efficiency, has to be overcome with the assistance of

electrocatalysts.2 As a result, developing efficient and stable

electrocatalysts are central to the efficiency of fuel cells,

metal-air batteries and water electrolyzers.2−3

Actually, precious metal-based materials,4 such as PtPd,5

PtCo,6 Ir@Pt/C,7 and AgAu,8 have long been widely investigated

as the catalysts towards HER, OER or ORR, due to their highly

efficient catalytic activity. However, the scarcity and high cost

are an insurmountable bottleneck for mass production. In

addition, their susceptibility to time-dependent durability and

carbon monoxide (CO) deactivation greatly decreases the

cathode potential and reduces efficiency. On the other hand,

earth-abundant transition-metal compounds, such as metal

oxides/sulfides,9 have been extensively studied as

electrocatalysts towards the abovementioned reactions. But they

frequently suffer from dissolution and agglomeration during the

operation of fuel cells, resulting in a severe degradation.

Moreover, the low electrical conductivity of the electrocatalysts

is not favourable for the electron transport in the electrodes,

constituting a major obstacle for their applications.

With the 2010 Nobel Prize in Physics awarded for

„ground-breaking experiments regarding the two-dimensional

material graphene‟,10 graphene, as a building block for carbon

materials, has emerged as an attractive candidate for energy

applications due to its unique structure and properties.11

Graphene has a carrier mobility up to 10000 cm2 V−1 s−1 and a

thermal conductivity of 3000~5000 W m−1 K−1 at room

temperature, a high surface area of ~2630 m2 g−1, good optical

transparency of ~97.3% and excellent mechanical strength with a

Young's modulus of 1.0 TPa.12 These unique properties,

especially the excellent electrical and thermal conductivities and

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Review Article Nano Advances

Nano Adv., 2016, 1, 50−61.

doi: 10.22180/na172

the huge specific surface area, make graphene either

new-generation catalyst supports or catalyst itself. Furthermore,

chemical doping with heteroatoms is an important strategy to

tailor the electronic property and chemical reactivity of graphene,

because the size and electronegativity of the heteroatoms are

distinctly different from that of carbon atoms. The introduction

of heteroatoms gives rise to new functions for graphene and

greatly broadens its applications. Considerable attempts have

been made to construct a variety of heteroatom-doped graphene

materials or graphene-based nanocomposites, aiming at fully

using their excellent properties in their application of

electrochemical energy devices.

In this review, we will focus on the key advancements of

heteroatom-doped graphene materials as efficient

electrocatalysts towards ORR in the field of electrochemistry.

The following sections summarize and discuss recent progress in

the synthesis and elucidation of the electrocatalytic properties of

heteroatom-doped graphene. Finally, the prospects and

opportunities for future research in this field are also discussed.

2. Oxygen reduction reaction (ORR)

2.1 ORR in fuel cells and metal-air batteries

The oxygen reduction reaction (ORR) is an important reaction at

the cathode and a major limiting factor of performance for fuel

cells and metal-air batteries. As schematically shown in Figure

1,13 in a typical fuel cell, fuel molecules (e. g., H2) are oxidized

to protons and electrons at the anode (hydrogen oxidation

reaction, HOR).3c In this process, the electrons flow out of the

anode to provide electrical power, while protons pass through the

electrolyte and react with adsorbed oxygen to produce water at

the cathode (ORR). The reverse process is water electrolysis, in

which water molecules are oxidized to produce oxygen

molecules and release protons and electrons at the anode (OER).

Protons traveling through the electrolyte combine with electrons

to form hydrogen at the cathode (HER).

For metal-air batteries, the forward and backward processes

are very similar to those of fuel cells, except that the metal

replaces hydrogen molecules working as “fuel” and electrolytes

become solutions containing metal ions. As shown in Figure 2,2

in the discharge process, the metal at the anode is

electrochemically oxidized and releases electrons to the external

circuit, while oxygen diffuses into the cathode to accept the

electrons from the anode and is reduced to oxygen-containing

species. Meanwhile, the dissociated metal ions migrate across

the electrolyte and combine with the oxygen-containing species

to form metal oxides. When the cell is charged, the process is

reversed, in which metal plats at the anode and oxygen evolves

at the cathode.

The ORR process involved in fuel cells or metal-air batteries

could proceed through either an efficient four-electron pathway

(Eq. 1 or 3) or a less efficient two-step two-electron pathway (Eq.

2 or 4). The possible reactions for oxygen reduction in alkaline

solution are as follows:14

e

( S )

e

( S ) a

e

( S )

In the four-electron process, the O2 molecules are directly

reduced into OH− at 0.401 V versus Standard Hydrogen

Electrode (vs. SHE), followed by combining with a proton into

water (Eq. 1). In contrast, in the two-step two-electron pathway,

the HO2− as the intermediate specie is firstly formed, followed

by further reduction into OH− (Eq. 2a and 2b).

Similar to the reactions in alkaline medium, there are also two

kinds of pathway in the acidic medium, as shown in Eq. 3−4.

Figure 1. Scheme showing fuel-cell components, unit cell cross-section of

the Nth unit cell in a fuel-cell stack, showing the components of an

expanded membrane electrode assembly. Adapted with permission from

Ref. 13. Copyright 2012, Nature Publishing Group.

Figure 2. Schematic structure of a metal-air battery. Reprinted from Ref. 2

with permission from the Royal Society of Chemistry.

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Nano Adv., 2016, 1, 50−61.

doi: 10.22180/na172

e ( S ) ( )

e ( S ) ( a)

e ( S ) ( )

Oxygen molecules are directly reduced into H2O in the

four-electron process (Eq. 3), while intermediate H2O2 is

produced in a two-electron process (Eq. 4).

2.2 Electrochemical characterization of ORR

The ORR performance usually is evaluated by the onset

potential (Eonset), the electron transfer number (n), half-wave

potential (E1/2), overpotential under a specific current density η ,

the kinetic-limiting current density (JK), Tafel slope, long-term

durability, etc. As mentioned above, the n values per O2

molecule involved in a typical ORR process are of paramount

importance for the efficiency of electrocatalysts. So, to rationally

evaluate the electrocatalyst, there are two methods to calculate

them. One is calculating n value from the slope of the

Koutecky−Levich (K−L) plots by using the following equations

q − :

k

k ⁄

n ⁄ ⁄ (6)

where J, Jk and JL are the measured current density,

diffusion-limiting current density and kinetic-limiting current

density in sequence, is the electrode rotating speed in rpm,

is the reciprocal of the slope, F is the Faraday constant (F =

96485 C mol−1), CO2 is the concentration of O2, DO2 is the

diffusion coefficient of O2, and ν is the kinematic viscosity of the

electrolyte. The coefficient 0.2 is adopted when the rotating

speed is expressed in rpm.

The other is measuring ring and disc currents from a rotating

ring-disk electrode (RRDE) followed by the calculation

according to the following equation (Eq. 7). By this method, the

peroxide (H2O2) yield can also be obtained on the basis of Eq. 8.

n

R ⁄

( ) R ⁄

R ⁄

where IR and ID are the ring and disk currents, respectively, and

N is the current collection efficiency of the Pt ring electrode (N

= 0.37, from the reduction of K3Fe[CN]6).

Further insights into the dynamics of ORR may be obtained

from the slope of the Tafel plot. The Tafel slope is calculated

according to the Tafel equation as follows:

η log(j j

⁄ )

where η is the overpotential, is the Tafel slope, j is the current

density, and j0 is the exchange current density. For oxygen

electrocatalytic reduction, the Tafel slopes are typically found at

60 mV dec−1 or 120 mV dec−1, where the former corresponds to

a pseudo two-electron reaction as the rate determining step, and

in the latter the rate determining step is presumed to be the

first-electron reduction of oxygen.15

3. Single-doped graphene

Single-doped graphene has made exceptional progress over the

past decade either by in-situ doping during the graphene

synthesis or through post-treatment of graphene with

heteroatom-containing precursors. Comparative study of

single-doped graphene towards ORR has also been extensively

conducted, providing guideline for designing and fabricating

heteroatom-doped graphene.16 And experimentally, the

promising single-doped graphene materials as efficient, stable

electrocatalysts towards ORR have also been demonstrated.

3.1 Nitrogen-doped graphene (NG)

The nitrogen doping modifies materials intrinsically, which

tailors electronic properties, manipulates surface chemistry and

produces local changes to the elemental composition of host

materials. Thus, this doping can consequently improve the

electrocatalytic activity. There are usually several N-containing

species in NG, including pyridinic N (~398.7 eV), pyrrolic N

(~400.3 eV), quaternary N (~401.2 eV), and N-oxides of

pyridinic N (~402.8 eV) as shown in Figure 3.17 Pyridinic N

refers to N atoms at the edges or defects of graphene, which

contri ute one π electron to the aromatic π system Pyrrolic

atoms with higher binding energy bond to two carbon atoms and

contri ute two π electrons to the π system, which are

incorporated into five-membered heterocyclic rings. Quaternary

N atoms are incorporated into the graphene layer by substituting

carbon atoms within the hexagonal ring, which are assigned to

N-Qcenter (graphitic N) and N-Qvalley. N-oxides of pyridinic N,

pyridinic-N+-O−, are N atoms bonded to two carbon atoms and

one oxygen atom.18 Among these nitrogen types, pyridinic N and

quaternary N are sp2 hybridized and pyrrolic N is sp3 hybridized.

By N doping, the spin density and charge distribution of carbon

atoms will be influenced by the neighbour N dopants, which

Figure 3. Schematic representation of different N-functional groups in NG.

Adapted with permission from Ref. 17. Copyright 2013, John Wiley &

Sons, Inc.

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Nano Adv., 2016, 1, 50−61.

doi: 10.22180/na172

provide more active sites on the graphene surface and facilitate

the electron transfer.19 Moreover, nitrogen doping in the

monolayer graphene induces the Fermi level shifting above the

Dirac point and suppresses the density of state near the Fermi

level, thus opening the band gap between the conduction band

and valence band.

Very recently, it has been reported that metal-free N-doped

carbons exhibit excellent ORR catalytic activity, which is

comparable to or better than commercial Pt/C catalysts,28, 34

illustrating an exceptional progress on metal-free catalysts used

in metal-air batteries or fuel cells. For example, Ding et al.

presented a novel strategy for the selective synthesis of

pyridinic- and pyrrolic-nitrogen-doped graphene (NG) by the use

of layered montmorillonite (MMT) as a quasi-closed flat

nanoreactor, as shown in Figure 4.17 The as-prepared NG

exhibits the excellent electronic conductivity, high ORR activity,

and good stability in acidic electrolyte. Table 1 gives a summary

of N-graphene materials recently used as efficient

electrocatalysts toward ORR. Pyridinic and quaternary N species

are generally considered as the active functionalities in NG.

Wang et al. proposed that the activity of N-doped graphite arises

from the electronic states near Fermi level due to the doping of

N atoms.35 This is helpful for the electron transfer from the band

of graphite to the anti-bonding orbitals of O2, which leads to

weakening or breaking of the O–O bond and to the reduction of

O2 Moreover, zkan‟s group found that gross nitrogen content

did not play a role in ORR activity and N was incorporated into

the graphitic matrix, not attached as part of a surface functional

group. Luo et al. further investigated N-doped graphene with

Figure 4. Schematic representation of the synthesis of NG@MMT. SEM (a)

and TEM images (b) of NG@MMT. (c) High resolution TEM image of the

edge of a NG@MMT nanosheet; steady-state plots of ORR polarization for

different catalysts in O2-saturated 0.1 M HClO4. Adapted with permission

from Ref. 17. Copyright 2013, John Wiley & Sons, Inc.

Table 1. Summary of the ORR performance of various N-doped graphene materials.

Samples Onset Potential ∆E1/2=E1/2(Sample)-E1/2(Pt/C) n Ref.

PANi/RGO

PPy/RGO

BN-RGO

N-RGO-1000

−0.05 V

−0.05 V vs. Ag/AgCl

−0.05 V

−0.1 V

−115 mV

−141 mV

−145 mV

−193 mV

3.65

3.63

3.45

2.85

20

NG1000 −0.1 V vs. Ag/AgCl −73 mV − 21

NCS-800

(N-doped carbon nanosheets)

0.9 V vs. RHE 14 mV − 22

NGSHs

(N-doped graphene/single-walled CNT hybrids)

0.88 V vs. RHE −64 mV 3.3 23

NCM58 (N-graphene) −0.13 V vs. Ag/AgCl −112 mV 3.7 24

NC-900 0.83 V vs. RHE −150 mV 3.3 25

NG-A 0.95 V vs. RHE −62 mV − 26

red-N-GOQD

(reduced N-doped Graphene Oxide Quantum Dots)

0.015 V vs. RHE ~ − 27

GQD-GNR hybrid

(Graphene Quantum Dots Supported by Graphene Nanoribbons)

0 V vs. Ag/AgCl 25 mV 3.91 28

NDCN-22 (N-Doped Carbon Nanosheets) −0.01 V vs. Ag/AgCl −43 mV − 29

N-MG (N-doped mesoporous graphene) 0.92 V vs. RHE −80 mV 2.9 30

dN-C (−100V) (N-doped carbon films deposited at -100V) −0.02 V vs. Ag/AgCl −25 mV − 31

NG-1000 −0.06 V vs. Hg/HgCl2 −81 mV 3.89 32

NG-800 0.97 V vs. RHE −63 mV − 33

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Nano Adv., 2016, 1, 50−61.

doi: 10.22180/na172

XPS (X-ray photoelectron spectroscopy) and UPS (ultraviolet

photoelectron spectroscopy), and concluded that the pyridinic N

efficiently changed the valence band structure of graphene,

including the raising of density of π states near the Fermi level

and the reduction of work function, thus greatly boosting the

ORR activity.36 Qiao group examined three multilayer graphene

samples prepared from graphene oxide using different nitrogen

sources and doping methods for different nitrogen doping

configurations and concentrations.37 They found that the carbon

atom neighbouring pyridinic nitrogen plays an important role in

the ORR process and should be the main active sites. Kim et al.

reported that the outermost graphitic nitrogen sites in particular

enhanced the oxygen adsorption, the first electron transfer, and

also the selectivity toward the four-electron reduction pathway,

but there is an inter-conversion between the graphitic and

pyridinic sites via the ring-opening of a cyclic C−N bond in the

next electron and proton transfer reaction.38 Ruoff‟s group

synthesized a series of NG with different N states as shown in

Figure 5 and studied the catalytic active center for ORR.20 They

pointed out that the electrocatalytic activity of N-containing

metal-free catalysts is highly dependent on the graphitic N

content while pyridinic N species improve the onset potential for

ORR. They also found that the total atomic content of N in the

metal-free, graphene-based catalyst did not play an important

role in the ORR process.

Recently, we also synthesized a series of NG materials by

introducing pyrrole into a Wurtz-type reductive coupling

reaction as shown in Figure 6.26 We found that a remarkable

evolution of N-species from pyrrolic N to pyridinic N upon

annealing, together with improved electrocatalytic activity

happens with a relatively stable content of graphitic N. By

correlating the ORR measurements with the XPS analysis, we

proposed that the pyridinic N plays a more important role than

pyrrolic N and graphitic N in largely improving the catalytic

activity.

3.2 Sulfur-doped graphene

Sulfur-doped graphene also demonstrated improved

electrocatalytic activity toward ORR, compared to its undoped

counterpart. But, different from the mechanism of N-doped

graphene, the enhanced ORR activity of S-doped graphene is

usually attributed to the modifications in spin density instead of

changes in atomic charge density ascribed to N doping, because

sulfur has electronegativity close to that of carbon.16e, 39 Sulfur

atoms have been usually incorporated into the sp2 carbon lattice

by two procedures: (I) chemical vapour deposition (CVD)

technique using liquid organics40 and (II) thermal reaction of GO

and sulfur-containing precursors.21, 41 Pumera‟s group

synthesized a series of sulfur-doped graphene via thermal

exfoliation of graphite oxide in H2S, SO2, or CS2 gas as shown in

Figure 7.42 The shift of the reduction potential by 40 mV from

undoped graphene to S-doped graphene shows that S-doped

graphene provides an electrocatalytic surface of ORR.

Figure 5. (a) Schematic diagram for the preparation of N-doped graphene

with different N states. N-RG-O 550, 850, and 1000 ˚C are prepared by

annealing of G-O powder at temperatures of 550 ˚C, 850 ˚C, and 1000 ˚C

under a NH3-N precursor. PANi/RG-O and Ppy/RG-O are prepared by

annealing of PANi/G-O and Ppy/G-O composites at 850 ˚C. (b) Linear

sweep voltammograms for N-RG-O 1000 ˚C, bare GC electrode,

PANi/RG-O Ppy/RG-O, BN-RG-O, bare Pt electrode and 20% Pt/C

electrodes in O2 saturated 0.1 M KOH (scan rate: 10 mV s−1, rotation rate

2500 rpm). Reprinted from Ref. 20 with permission from the Royal Society

of Chemistry.

Figure 6. Schematic diagram for the preparation of N-doped graphene by a

Wurtz-type reductive coupling reaction. Reprinted from Ref. 26 with

permission from Springer.

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Review Article Nano Advances

Nano Adv., 2016, 1, 50−61.

doi: 10.22180/na172

More recently, we also fabricated sulfur-doped graphene by a

one-step magnesiothermic reduction strategy to transform

greenhouse gas CO2 (in the form of Na2CO3) in the presence of

small amount of Na2SO4 as an inorganic S source to form

S-doped graphene, as shown in Figure 8.43 At high temperatures,

Mg metal could reduce carbon in the oxidation state of +4 in

CO32− to form black graphene products, while the SO4

2− ions

could supply S atoms to build C−S sp2 bonds integrated into the

graphene lattice. The as-formed S-doped graphene materials

demonstrate good electrocatalytic activity for ORR with a

dominant four-electron reaction pathway as well as excellent

durability. Sulfur-doped graphene derived from cycled lithium–

sulfur batteries was also obtained by Wang and co-workers,

showing a positive shift of about 60 mV and higher current

density compared to the undoped graphene. The doping

mechanism is believed to be similar to other electrochemical

doping methods, but further investigation is still required to fully

understand it.

3.3 Boron-doped graphene

B-doped graphene exhibits tunable band gaps and transport

properties through controllable doping content, especially

tunable p-type electrical properties, which could arise from the

strong electron-withdrawing capability of B atom

(electronegativity: 2.04 on the Pauling scale).44 The increased

carrier concentration and charge polarization in B-doped

graphene highly improve the electrical conductivity and create

more active sites favourable for O2 adsorption. B doping could

boost the reactivity of graphene with oxygen to form covalently

bound bulk borates (BO3−G) in oxygen-rich conditions. These

species are highly important intermediates for breaking O=O

bond in the reduction process of O2 to form H2O.45

Recently, Minett and co-workers reported that boric acid

incorporating into a graphene oxide also favours a 4e− reduction

pathway in ORR, which could be assigned to simultaneous

effects of (i) the surface area augmentation for O2 adsorption and

(ii) the improved electron accepting-donating capability of boron

per oxygen molecule.46 Different from the conventional

synthesis methods, such as CVD47 and thermally annealing GO

and B-containing precursors, Narayanan and co-workers directly

heated rhombohedral B4C at high temperatures ranging from

1200 to 1600 °C to produce few-layer graphene over B4C, with

easily separable phases that enable pure and bulk production of

doped graphene, as shown in Figure 9.47 The as-obtained BG

shows an excellent bifunctional electrocatalytic activity toward

ORR and OER with a four-electron pathway, high methanol

tolerance, high stability, and low yields of side products, which

makes B-doped graphene on par with Pt/C electrode.

Figure 7. Fabrication of sulfur-doped graphene by thermal exfoliation of

graphite oxide prepared by Staudenmaier, Hofmann, and Hummers method

in CS2, H2S, or SO2 atmospheres. Adapted with permission from Ref. 42.

Copyright 2013, the American Chemical Society.

Figure 9. Preparation of bulk graphene from B4C and the phase separation

of unreacted B4C from graphene using water (a two-step process). (a)

Rhombohedral B4C is heat treated at 1400 ˚C in an inert atmosphere (N2

gas at 100 sccm flow rate) and the resultant powder is tried to disperse in

water. The hydrophobic graphene floats on the water surface while the

unreacted B4C is sediment at the bottom of the beaker. (b) XRD image of

bulk B4C or unreacted B4C. (c) XRD image of phase-separated (top in the

beaker) graphene showing a pure hexagonal structure and the inset shows

the bulk graphene powder. Adapted with permission from Ref. 47.

Copyright 2013, John Wiley & Sons, Inc.

Figure 8. Schematic diagram for the preparation of sulfur-doped graphene.

Adapted with permission from Ref. 43. Copyright 2010, Nature Publishing

Group.

Ar

700-900 oC

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Nano Adv., 2016, 1, 50−61.

doi: 10.22180/na172

3.4 Phosphorus-doped graphene

Phosphorus, an element of the nitrogen group, often shows the

similar chemical properties due to the same number of valence

electrons as N. But it has lower electronegativity (2.19) and

larger covalent radius (107±3 pm) than N. Therefore, the polarity

of the C−P bond is opposite to that of the C−N bond and favours

the sp3-orbital configuration in molecules with larger structural

distortion. The larger P−C bond length (1.79 Å) than 1.42 Å for

C−C sp2 bonds combined with the difference in bond angles,

forces P to protrude from the graphene plane.48 These different

features also facilitate the adsorption of O2 and result in

improved reaction rate of the overall oxygen reduction process.

Poyato and co-workers showed that the four-electron transfer

mechanism is the thermodynamically predominant pathway in

the ORR on the studied P-doped graphene surfaces because the

two-electron transfer mechanism is energetically less favourable.

The comparative study of the ORR on N-doped, B-doped, and

P-doped graphene surfaces demonstrates that the highest

efficiency is obtained for the P-doped surface.16a

Liu et al. demonstrated the synthesis of P-doped graphite

layers by pyrolysis of toluene and triphenylphosphine with

improved electrocatalytic activity toward ORR.49 ou‟s group

also showed that P-doped graphene exhibits remarkable catalytic

activity, outstanding tolerance to methanol crossover effect and

excellent long-term stability.50 Li et al. reported metal-free

P-doped graphene nanosheets by annealing a homogeneous

mixture of graphene oxide and an ionic liquid (BmimPF6), as

shown in Figure 10.51 P atoms in the as-prepared PG are partially

oxidized and existed as tetrahedral forms such as C3PO, C2PO2,

and CPO3. The O atom with the higher electronegativity will

first make the P atom polarized, and then withdraw electrons

from the carbon atoms with the polarized P atom as a bridge,

creating a net positive charge on the carbon atoms adjacent to

the P atom. As a result, the positively charged carbon atoms

become the active sites favourable adsorption of O2, weakening

the O−O bonding, and attracting electrons from the anode to

facilitate the reduction of O2 to H2O through an efficient

four-electron pathway. A latest article reports that ORR on

P-doped graphene could proceed firstly by a 2e− process to form

an OOH intermediate, followed by a 4e− process to break the

O−O bond of OOH.52 Along this reaction path, the reduction of

the second OH to H2O is the rate-limiting step with the largest

barrier of 0.88 eV.

3.5 Halogen-doped graphene

Halogen-doped graphene can be obtained by several strategies,

including CVD,53 liquid-phase exfoliation,54 mechanical

exfoliation,55 etc. Recently, Jeon et al. synthesized

edge-halogenated graphene nanoplatelets (ClGnP, BrGnP, and

IGnP) and demonstrated remarkable electrocatalytic activities

toward ORR with a high selectivity, good tolerance to methanol

crossover/CO poisoning effects, and excellent long-term cycle

stability, as shown in Figure 11.56 The order of ORR activities is

lGnP ˂˂ rGnP ˂ GnP, which seems to e contradicted to the

doping-induced charge-transfer mechanism (electronegativity:

Cl = 3.16, Br = 2.96, and I = 2.66).

But it can be explained that the atomic sizes of Br and I are

larger than that of Cl, and hence the valence electrons of Br and I

are much loosely bound than those of Cl for facilitating charge

polarization in the BrGnP and IGnP electrodes. Unlike Cl, Br

and I can form partially ionized bonds of –Br+– and –I+– to

Figure 10. (a) XRD patterns of as-prepared P-TRG, TRG, and GO as well as the original graphite powder. (b) Typical SEM and (c) TEM images of

the P-TRG sample. The inset in (c) shows the SAED pattern of the rectangle area. (d) HRTEM image of the circled area in panel (c). (e) STEM

image and (f-h) the corresponding elemental mappings of the rectangle area in (e). (i) LSV curves of the ORR at various electrodes in O2-saturated

0.1 M KOH solution at a scan rate of 5 mV s−1 and rotation rate of 1600 rpm. (j) Schematic illustration of the possible ORR process catalyzed by the

P-doped graphene. Reprinted from Ref. 51 with permission from the Royal Society of Chemistry.

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Nano Adv., 2016, 1, 50−61.

doi: 10.22180/na172

further enhance the charge-transfer because of their relatively

large atomic sizes. Gentle and co-workers also reported that both

Br-RGO and I-RGO facilitated a more efficient ORR than

Cl-RGO and the undoped RGO at a higher reaction onset

potential.57 Yao et al. showed that I-doped graphene by utilizing

GO and iodine at 500−1100 ˚ in Ar has a long-term stability

and an excellent resistance to crossover effects for ORR,

suggesting that the formation of the I3 structure plays a crucial

role for enhancement of the ORR activity of graphene.58

4. Dual-doped graphene

Dual doping of foreign atoms into pristine graphene could

induce a unique electronic structure in graphene and create a

synergistic coupling effect between heteroatoms. From this point

of view, dual doping is a promising approach to tailor the

physical and chemical properties of graphene for obtaining

excellent metal-free electrocatalysts. Several kinds of dual-doped

graphene have been demonstrated to possess better

electrocatalytic activity than those single-doped counterparts,

which will be discussed below.

4.1 N, B-doped graphene

ai‟s group demonstrated that N, B-doped graphene (NBG),

which was synthesized by thermal annealing graphene oxide in

the presence of boric acid under ammonia atmosphere, showed

ORR electrocatalytic activities even better than the commercial

Pt/C electrocatalyst and also investigated the electrocatalytic

activity theoretically by density functional theory (DFT)

calculations.59 Compared to pure graphene, the substitution of C

by B and N leads to a smaller energy gap, but over-doping of B

and N resulting in a significant increase in the energy gap is not

favourable for the electrocatalytic activity. Therefore, a modest

N- and B-doping level is necessary to improve the catalytic

activity of graphene.

Qiao and co-workers also incorporated N and B sequentially

into selected sites of graphene domain to facilitate the

electrocatalytic ORR.60 The resultant NBG exhibited much

improved electrochemical performance as compared to that of

single-doped graphene and the hybrid electrodes, suggesting a

synergistic coupling effect between N and B atoms as shown in

Figure 12. Dual doping of N and B atoms could cause offset of

asymmetric charge density because N and B atoms have higher

and lower electronegativity than C atoms, respectively. However,

the dual doping could bring out high asymmetric spin density of

C atoms, which plays a much more important role than the

atomic charge density in determining the catalytic active sites.16e,

61 Agnoli and co-workers attributed the better performance of N,

B co-doped graphene than that of N or B doped counterparts to

two factors (i) the twice higher concentration of active sites for

ORR (B atoms and C atoms neighbouring to N) and (ii) an

enhanced positive charge on B caused by the electron

withdrawing properties of N.27

Figure 11. (a) A schematic representation for mechanochemically driven edge-halogenation reaction between the in-situ generated active carbon

species (gold balls) and reactant halogens (twin green balls). Active carbon species were generated by homolytic bond cleavages of graphitic

C-C bonds and reacted with halogen molecules top roduce edge-halogenated graphene nanoplatelets (XGnPs) in a sealed ball-mill capsule and

the remnant active carbon species are terminated upon subsequent exposure to air moisture. Red and gray balls stand for oxygen and hydrogen,

respectively; The optimized O2 adsorption geometries onto XGnPs, in which halogen covalently linked to two sp2 carbons. (b) ClGnP; (c) BrGnP;

(d) IGnP. Adapted with permission from Ref. 56. Copyright 2010, Nature Publishing Group.

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Nano Adv., 2016, 1, 50−61.

doi: 10.22180/na172

4.2 N, S-doped graphene

N, S co-doped graphene (NSG) is usually synthesized by CVD

method62 or thermal reaction of graphene oxide and

solid/gaseous N/S-containing precursors21, 63 (e. g., thiourea or

NH3/H2S gas). The synergistic effect between heteroatoms was

also reported for N and S dual-doped graphene, which is

eneficial for improving the RR or example, Qiao‟s group

displayed a very high ORR onset potential of NSG (−0.06 V vs.

Ag/AgCl), very close to that of commercial Pt/C (−0.03 V vs.

Ag/AgCl) and much more positive than that of NG, SG or G at

1600 rpm.64 DFT calculations indicate that simultaneously

incorporating of S and N into graphene induces a substantially

uplift of spin density, indicating higher ORR catalytic

performance. Recently, Jeon and co-workers prepared a series of

heteroatoms (N and/or S) doped graphene in different ratios

from various doping precursors (pyridine, thiophene and

bithiophene combined separately with dipyrrolemethane as

single N and/or S precursor) by thermal reaction.65 They found

that the N and S dual-doped graphene has better catalytic activity

Figure 12. (a) Energies of HO2 adsorption (Ead) on N- and/or B-doped graphene. N(p) and N(g) indicate pyridinic and graphitic N bonding,

respectively. (b) Optimized configuration of HO2 adsorbed on B, N-graphene. (c) Ead on various B, N-graphene models with pyridinic or graphitic N

groups. (d) Ead on various B, N-graphene models with B active sites as a function of the distance to a pyridinic N atom (sites I–VI as marked in

inset). B-graphene with a single B atom was considered as a reference with an infinitely large distance. Adapted with permission from Ref. 60.

Copyright 2013, John Wiley & Sons, Inc.

Table 2. Summary of electrochemical performance of NSG.

Samples Precursors Onset Potential ∆E1/2=E1/2(Sample)-E1/2(Pt/C) n Ref.

SNGL-20 GO, pyrimidine and

thiophene

−0.11 V vs. Ag/AgCl 10 mV 3.7 62b

NSG-700 GO and thiourea −0.12 V vs. SCE −150 mV 3.91 63

NSG GO, melamine and benzyl

disulfide

−0.06 V vs. Ag/AgCl −156 mV 3.6 64

NSG GO and 2-aminothiophenol −0.09 V vs. Ag/AgCl - 2.9–3.2 67

NSG GO, cysteine and

polydopamine

−0.002 V vs. Hg/HgO −65.8 mV 2.98–3.36 68

NSG-1000 GO, and poly

[3-amino-5-mercapto-1, 2,

4-triazole]

−0.1 V vs. SCE −70 mV 3.59–3.63 69

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Nano Adv., 2016, 1, 50−61.

doi: 10.22180/na172

on ORR than only N-doped graphene, probably due to the

synergistic effect of dual-doping and superiority of S than N

with two lone-paired electrons. The improved electrocatalytic

activity caused by N and S atoms paves the way of practical

application of NSG in the Li-air batteries, as demonstrated by

Kim et al.66 The electrochemical performance of NSG reported

in recent years is summarized in Table 2.

4.3 N, P-doped graphene

Dual doping of N and P atoms into graphene is also a promising

approach to improve the electrocatalytic activity of graphene

towards ORR. Heteroatomic P, N defect is more stable than the

P defect in graphene, which creates localized electronic states

that modify the electron transport properties by acting as

scattering centers.48 Similar to N, B and N, S doping, the

incorporation of N and P atoms also induces asymmetric spin

density of C atoms, making C atoms more active for adsorption

of O2 molecule. It was reported that the P, N co-doped graphene

synthesized by a CVD route exhibits a remarkably stable n-type

behaviour at ambient conditions and reasonable charge carrier

mobility.70 Woo‟s group reported , P co-doped graphene

exhibits the higher activity towards ORR in acidic medium

compared with single N-doped graphene.

We also fabricated N, X (X = B, S or P) dual-doped graphene

by a simple solvothermal method, which is facile and scalable,

as shown in Figure 13.71 We found that the electrocatalytic

activity is in the order of NSG < NBG < NPG, which is

consistent with the reported theoretical and experimental results.

The improved electrochemical performance of all co-doped

graphene samples indicates that dual doping actually exerts a

positive influence on the electrochemical activity. Such

influence could be regarded as the so-called synergistic effect,

namely, N atom polarizes the adjacent C atom, while X (S, P, or

B) atom facilitates the adsorption of and bonding with HO2.60

Furthermore, additional doping of B or P into N-doped graphene

could also result in the improvement of asymmetry of charge

distribution, a facile electron transfer on the basal plane of

graphene, and a decrement of energy gap between highest

occupied molecular orbital and lowest unoccupied molecular

orbital.

5. Ternary-doped graphene

Earlier study shows that ternary doping of B and P with N into

carbon induces remarkable performance enhancement, due to the

charge delocalization of the carbon atoms and increase of edge

sites of the carbon.61 Recently, Yu and co-workers incrementally

doped P atoms into N, S co-doped graphene to achieve a ternary

(N, S and P)-doped graphene, demonstrating an excellent ORR

activity, which is 2 times better than that of binary (N and

S)-doped RGO, and 5 times better than that of single (P)-doped

graphene and even exceeds the commercial Pt in alkaline

medium.72

6. Summary and perspectives

In this review, we have summarized recent advances in the

development of heteroatom-doped graphene towards ORR. The

discussion of electrocatalysis has focused on the influence of

heteroatoms on the electrochemical properties and performance

of graphene. Doping-induced charge/spin redistribution make

the C atoms in the graphene more active for O2 adsorption to

facilitate the ORR process, which make doped graphene

promising in the electrocatalysis, either as a catalyst support or

catalyst itself. In this exciting field of heteroatom-doped

graphene materials, although significant progress has been made,

challenges are still remained. Firstly, large-scale and low-cost

fabrication method is a prerequisite to utilize graphene in the

electrocatalytic reactions, because most of synthetic methods are

involved in high temperature or toxic precursors, which increase

the cost and difficulty for mass production. Secondly, reaction

mechanism of doped graphene is still further elucidated by

exactly controlling the doping configuration experimentally and

carefully model construction and simulation by quantum

calculation theoretically. At last but not least, the exploration of

doped graphene used in practical fuel cells and metal-air

batteries still needs to be extended, because most reports put

emphasis on the estimation of sole ORR performance. We

believe that the ever-increasing understanding and exploration of

heteroatom-doped graphene will open up avenues for the

development of future energy systems.

Figure 13. (a-c) A proposed mechanism on the synthesis of N, X (B, P or S)

dual-doped graphene. (d) LSV curves of various graphene products at 1600

rpm in O2-saturated 0.1 M KOH solution.71

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doi: 10.22180/na172

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