Recent advances of doped carbon as non

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Recent advances of doped carbon as non
Journal of
Materials Chemistry A
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FEATURE ARTICLE
Cite this: J. Mater. Chem. A, 2014, 2,
15704
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Recent advances of doped carbon as non-precious
catalysts for oxygen reduction reaction
Hao Shi, Yanfei Shen, Fei He, Ying Li, Anran Liu, Songqin Liu and Yuanjian Zhang*
Owing to their remarkable catalytic activities, doped nanocarbon materials have been widely employed as
efficient noble metal-free catalysts for oxygen reduction reaction (ORR) towards the artificial energy
conversion systems, such as fuel cells and a variety of sensors. After several decades of innovative
investigation, the substantial controversies still exist ranging from synthesis strategies to actual active
sites for doped nanocarbon materials, but greatly pave the development of sustainable ORR
Received 3rd June 2014
Accepted 14th July 2014
electrocatalysts with high efficiency. This review mainly focuses on the newly developed synthesis
methods, such as ball milling, co-doping with multi-elements and low temperature preparation with
DOI: 10.1039/c4ta02790f
more predictable structures that were reported in the last five years. Particularly, we have also discussed
www.rsc.org/MaterialsA
the open controversies and mechanism studies of doped carbon for ORR.
Introduction
Due to the rapid depletion of limited fossil fuels and the
subsequently increasingly worsening environmental pollution,
the development of an alternative energy supply has attracted
worldwide attention.1 It is expected to be clean, safe, economic
and renewable. Therefore, several methods have been attempted to address the involved challenges. Among them, the utilizations of renewable energy, such as hydrogen for fuel cells have
Jiangsu Province Hi-Tech Key Laboratory for Bio-Medical Research, School of
Chemistry and Chemical Engineering, Medical School, Southeast University, Nanjing
211189, China. E-mail: [email protected]
Yuanjian Zhang received his BSc
from Nanjing University in 1998
and his PhD in Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences
under Prof. Li Niu in 2007. Aer
that, he joined Prof. Markus
Antonietti's group at MPI of
Colloids and Interfaces (Germany)
as
a
postdoctoral
researcher. In October 2009, he
moved to National Institute for
Materials Science as an ICYS
researcher. He is currently a professor in School of Chemistry and
Chemical Engineering, Southeast University, China. His research
interests include, but is not limited to, chemistry of carbon-rich
materials and their applications in analytical chemistry and
sustainable energy.
15704 | J. Mater. Chem. A, 2014, 2, 15704–15716
recently drawn extensive attentions.2 This innovation offers a
highly efficient way to convert chemical fuels to electricity,
which is the most popular energy resource at present. For this,
polymer electrolyte membrane fuel cell (PEMFC), which utilizes
oxygen reduction reaction (ORR) on the cathode has been
widely used. In theory, oxygen can be reduced in two
different pathways with different standard potentials, as shown
in Table 1.
One is the four-electron (4e) process in which oxygen
molecule directly gets four electrons, generating water or
hydroxyl group depending on pH of the electrolytes. The other is
the two-electron (2e) process in which peroxide is generated as
an intermediate. In fact, ORR is also one of the most common
and crucial metabolic reactions in aerobic organisms.7,8 Though
ORR seems simple, the involved multi-electrons reaction kinetics
is rather complicated and sluggish, which becomes a major
bottleneck in further improving the performance of PEMFC.9
Inspired by the collaboration of multi-enzyme complexes that
catalyzes these reactions in a very high efficient manner,10 great
efforts have been made to explore a comparably efficient catalyst
for ORR, which is the key of articial electrochemical energy
conversion devices, such as fuel cells11 and electrochemical
biosensors as well.12,13 Although the state-of-the-art platinumbased catalyst can catalyze ORR through the four-electron
process,14 the scarcity and the high price of platinum hamper its
large-scale commercialization.15 Moreover, platinum is sensitive
to be poisoned and inactivated by carbon monoxide16/methanol,17,18 which commonly exist in PEMFC. Besides, the stability
of commercial platinum-based catalysts still needs to be
improved.18 In these regards, developing noble metal-free ORR
catalysts with compatible activity is highly anticipated.
In 1964, Jasinski rst reported using cobalt phthalocyanine
as the catalyst for ORR.19 Aer that, carbon-based materials
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Table 1
Journal of Materials Chemistry A
Standard electrode potentials for oxygen reduction reaction3–6
Electrolytes
Four electron
Two electrona
Alkaline
O2 + 4e + 2H2O / 4OH (0.401 V)
Acid
O2 + 4e + 4H+ / 2H2O (1.299 V)
O2 + H2O + 2e 4 HO2 + OH (0.065 V)
HO2 + H2O + 2e 4 3OH (0.867 V)
O2 + 2H+ + 2e 4 H2O2 (0.695 V)
H2O2 + 2H+ + 2e 4 2H2O (1.763 V)
a
Potentials vs. NHE.
have emerged as potential alternatives to substitute platinumbased catalysts20–30 through various current available strategies.
Recently, several studies have focused on the synthesis and
characterization of nitrogen-doped graphene,31,32 inuences of
heat-treating on M–N–C (M : Fe and/or Co)-typed catalysts33,34
and importance of nanostructure engineering in preparing
noble metal-free catalysts for ORR.4,35
In this review article, we mainly update the newly developed
innovations of preparing non-precious doped carbon ORR
catalysts in the last ve years, including those without
commonly used high temperature treatment, and highlight the
role of (multi) dopants in the catalysts. Moreover, the controversy of real active sites in doped carbon with/without metal
containing species, state-of-the-art mechanism studies of doped
carbon for ORR are discussed.
General preparation methods
Briey, the preparation methods for doped carbon catalysts of
ORR can be divided into high temperature treatment (e.g.
pyrolysis, annealing), chemical vapor deposition (CVD), ball
Table 2
milling, arc discharge, plasma, wet chemical synthesis, etc.
(Table 2). For high temperature treatment, the advantage of this
method is the capability of facilitating the formation of graphitic
structure, which likely contributes to the enhancement of the
current density. However, the complicated structural variations
would result in difficulties to reveal the active site of catalysts.
CVD is capable of preparing designed structure of high quality
nanocarbon. However, the contradiction is the high cost of
instruments that hampers its large scale application for
preparing ORR catalysts. While the low temperature preparation
(e.g. wet chemical synthesis), which is usually carried out in less
than 200 C, greatly compromises this contradiction. In this way
the original structure of carbon precursors could be largely
retained. Nevertheless, the incomplete carbonization of precursors probably leads to low electrical conductivity, which may
result in low current density. More detailed discussion on these
preparation methods are presented as follows.
(1)
High temperature treatment
Pyrolysis and annealing at high temperature are the two most
popular means to prepare a doped carbon catalyst for ORR with
Selected synthesis methods for doped carbon and their ORR catalytic activity
Synthesis methoda
Precursors
Electrolyte
Onset potentialb
Electron transfer number
HT36
HT37
HT38
HT39
HT17
HT40
HT41
HT25
HT24
HT template42
HT43
HT (template)1
HT (Ball-milling)44
HT (Ball-milling)45
Aniline, ammonium peroxydisulfate, FeCl3, carbon black
g-C3N4, FeCl3$6H2O, GO, NH3, hydrazine hydrate
GO, ammonia
Co(NO3)2$4H2O, imidazole, piperazine
GO, NH3, H3BO3
Oxidized carbon nanotube, Co(OAc)2, NH3
Urea, Fe(OAc)2, exfoliated graphene
GO, cyanamide, FeCl3
Acylonitrile telomere, FeCl3$6H2O, carbon black
GO, benzyl disulde, melamine, colloidal silica
Dicyandiamide, H3PO4, FeCl2$4H2O, CoCl2$6H2O
SBA-15/diamine, aniline
Black Pearls 2000 (BP), phenanthroline, Fe(OAc)2, NH3
ZIF-8 (a microporous host), NH3, 1,10-phenanthro-line,
Fe(OAc)2
Graphite, sulfur trioxide
Graphite, nitrogen (N2)
Iron(II) phthalocyanine, NH3
H2, He, pyridine vapor
Graphite, acetonitrile
GO, dicyandiamide, K4Fe(CN)6$3H2O
0.5 M H2SO4
0.5 M H2SO4
0.1 M KOH
0.1 M HClO4
0.1 M KOH
1 M KOH
0.1 M KOH
0.5 M H2SO4
0.1 M KOH
0.1 M KOH
1 M HClO4
0.1 M KOH
—
—
0.93 V
—
1.08 V
0.83 V
0.91 V
0.93 V
0.99 V
0.68 V
0.92 V
0.92 V
0.80 V
—
—
—
—
3.7 @ 0.1–0.4 V
3.8 @ 0.27 V
3.81 @ 0.56 V
3.97 @ 0.37 V
3.85 @ 0.5 V
3.79–3.99 @ 0.36–0.01 V
3.82 @ 0.41 V
—
3.6 @ 0.38 V
—
3.78 @ 0.5 V
—
—
0.1 M KOH
0.1 M KOH
0.1 M KOH
—
—
0.1 M KOH
0.82 V
0.84 V
—
—
—
0.77 V
3.7
3.9
3.9
—
—
4.1
Ball-milling46
Ball-milling47
CVD48
Arc discharge49
Plasma50
Wet chemical51
a
HT: high temperature treatment; CVD: chemical vapor deposition.
This journal is © The Royal Society of Chemistry 2014
b
@ 0.18 V
@ 0.58 V
@ 0.68 V
@ 0.47 V
Potentials vs. RHE.
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Journal of Materials Chemistry A
high catalytic activity. Though both of them are referred to heattreatment under high temperature, pyrolysis is mainly used in
decomposition of macromolecules, such as metal contained
phthalocyanine and porphyrin or other heteroatom-containing
precursors to expose their inner active sites and thereaer to be
shied to catalyst supports. It is one of the most effective
approaches to optimize catalysts, as previously reported.24,36,43,52
Zelenay et al. prepared the catalyst of PANI–Fe–C with
outstanding activity. As shown in Fig. 1, aniline oligomers were
rst mixed with FeCl3 and immobilized onto the carbon black
support. Aer adding (NH4)2S2O8 (ammonium persulfate, APS),
aniline was oxidized and polymerized into polyaniline (PANI).
Subsequently, the mixture was pyrolyzed in N2 under 900 C,
following the post-treatment of acid leaching and the second
pyrolysis.36 Similarly, Fe–N–C catalyst was prepared by pyrolyzing the complex of FeCl3, graphitic carbon nitride (g-C3N4)
and reduced graphene (rGO) in Ar. The as-obtained catalyst
exhibited high stability and activity in acid electrolytes.37
While annealing in classical is oen utilized in post-treatment to enhance the stability and durability of the as-prepared
metal and alloy in metallurgy. In the eld of chemical preparation, it mostly represents the heat treatment for solid or thin
lm under specic atmosphere. For instance, Sun et al. reported
a catalyst of nitrogen-doped graphene (N–G), which was simply
prepared by annealing GO under high purity ammonia and Ar at
900 C. Compared to Pt/C (E-TEK), the catalyst performed
higher onset potential and larger kinetic limiting current of
ORR, as shown in (Fig. 2).38
However, it should be noted that there are some vital factors,
including temperature and type of precursor, which will greatly
affect the structure and activity of catalysts. One of the most
straightforward evidences toward temperature is the aforementioned case of PANI–Fe–C. Compared to the optimal
Scheme of the synthesis of PANI–M–C catalysts. (A) Mixing of
high surface area carbon with aniline oligomers and transition metal
precursor (M : Fe). (B) Oxidative polymerization of aniline by adding
APS. (C) First heat treatment in N2 atmosphere. (D) Acid leaching. The
second heat treatment after acid leach is not shown.36 Reproduced
from Ref. 36 with permission from The American Association for the
Advancement of Science.
Feature Article
Fig. 2 The polarization curves of oxygen reduction on N-graphene
(900 C) and Pt/C (E-TEK) catalysts (0.1 M KOH).38 Reproduced from
Ref. 38 with permission from the Royal Society of Chemistry.
temperature of 900 C, the activity sharply dropped when the
temperature of pyrolysis increased to 1000 C. The phenomena
may be very likely to verify the assumption that optimal
temperature in high temperature treatment is limited in narrow
scope more than wide range.36 Likewise, for another metal-free
catalyst dubbed as PDMC (PANI-derived mesoporous carbon),
the electron transfer number was strongly inuenced by the
pyrolysis temperature (Fig. 3).1
Though the molecular structure of doped carbon prepared
by heat treatment, including pyrolysis and annealing are very
complicated, many efforts have been made to explain the
possible details. Assisted with X-ray photoelectron spectroscopy
(XPS), it revealed that the ratio of nitrogen in nitrogen-doped
graphene signicantly changed between pyridinic and graphitic
nitrogen as a function of annealing temperature.53 Moreover, it
was found that N% was at maximum around 500 C and
decreased with the higher temperature.
In another similar work, it was supposed that a gradual
thermal transformation followed the same trend of thermal
stability of nitrogen bonding congurations from initial amide
to pyrrolic, then to pyridinic, and nally to graphitic nitrogen in
graphene with increasing annealing temperature.54 Thus,
Fig. 1
15706 | J. Mater. Chem. A, 2014, 2, 15704–15716
Fig. 3 The number of electrons transferred as a function of potential
for PDMCs synthesized at different pyrolysis temperatures (0.1 M
KOH).1 Reproduced from Ref. 1 with permission from the American
Chemical Society.
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Feature Article
different temperature used in high temperature treatment will
not only affect the proportion of nitrogen in the composite, but
also transform the different type of nitrogen, which will further
greatly inuence the catalytic activity of the as-prepared doped
carbon.
The type of precursor also has great impact on the activity of
the catalysts as well. For instance, Liu et al. rst reported using
metallic organic framework (i.e. the cobalt imidazolate framework, BET surface area 305 m2 g1, see structure in Fig. 4a and
b) as single carbon/nitrogen/transition metal-containing
precursor to prepare nitrogen-doped carbon via thermal activation (Fig. 4c).39 The electrochemical kinetic studies showed
the as-prepared catalysts had excellent ORR activity in 0.1 M
HClO4 solution, which is comparable to the best C/N/cobaltbased non-precious ORR catalysts. Considering molecular
structure varieties, high surface area of MOF, the work presented a distinct platform with abundant compounds for
studying the correlation between the structure and the catalyst
performance. Dodelet et al. further investigated the precursors
containing not only zinc imidazolate framework (ZnN4C8H12,
ZIF-8, BET surface area as high as 1800 m2 g1 and almost
entirely microporous) but also phen(1,10-phenanthroline) and
iron(II) acetate to prepare iron-based Z8-catalyst.45 A signicant
increased volumetric activity and enhanced mass-transport
properties were observed, which was ascribed to the microporous host offered by zeolitic-imidazolate-framework for phenanthroline and ferrous acetate to form a catalyst precursor that
could be subsequently pyrolyzed.
(2)
Chemical vapor deposition (CVD)
CVD is a facile controllable process that offers high quality
nanocarbon, such as carbon nanotubes55 and graphene.56 It can
enable the realization of designed structure of nanocarbon in
atom scale. Moreover, CVD has been veried as an efficient
method to prepare doped carbon catalyst toward ORR.57 Typically, the process is oen performed on a substrate (e.g., Cu lm
Journal of Materials Chemistry A
on a Si substrate) in a quartz tube of a furnace lled with inert
gas to prevent side oxidation reaction. By pumping carbon and
nitrogen-containing precursors in gas, the precursors will be
decomposed under high temperature and subsequently
deposited on the substrate as seeds to nally grow into the
nitrogen-doped nanocarbon.58 For instance, using iron(II)
phthalocyanine and NH3 as precursors, Dai et al. reported
preparing highly active nitrogen-doped carbon nanotube arrays
(VA-NCNT) for ORR. It shows a signicant current density of 4.1
mA cm3 at 0.22 V (in air-saturated 0.1 M KOH vs. Ag/AgCl), as
shown in Fig. 5.48 Recently, it has been reported that CVD is
even capable of growing single crystal structural nitrogen-doped
graphene arrays with extremely high purity under low temperature of 300 C by the self-assembly method of pyridine molecules on Cu surface, but no test of catalytic ORR activity was
reported in this work.59
(3)
Arc discharge
Arc discharge is the phenomenon of the electrical breakdown of
the gas between two electrodes under high voltage accompanied
with spraying plasma to the counter electrode. This method has
newly been applied to prepare nitrogen-doped nanocarbon. For
instance, two graphite rods were used as anode and cathode in a
sealed chamber, which was lled with pyridine vapor49 or NH3
(ref. 49 and 60) as nitrogen source. Further, H2 functioned as
terminations for dangling carbon bonds and preventing the
newly generated sheets rolling up as closed structures.61 During
the arc discharge, breaking bonds among carbon atoms of
graphite rods occurs with the formation of carbon–nitrogen
bond; thus, nitrogen-doped nanocarbon was nally obtained,49
which was characterized by XPS and electron energy loss spectroscopy (EELS) elemental mapping (Fig. 6). It is noteworthy
that arc discharge can offer large scale production at the level of
grams.60
(4)
Plasma treatment
Plasma refers to an ion and electron-containing charged
mixture, which is commonly in a state of high energy caused
by the ionization of gas through means, such as electrical
(A) SEM image of the as-synthesized VA-NCNTs (B) Rotating
ring disk electrode (RRDE) voltammograms for oxygen reduction in
air-saturated 0.1 M KOH at the Pt–C/GC (curve 1), VA-CCNT/GC
(nitrogen-free nonaligned carbon nanotubes), (curve 2), and VA-NCNT
(curve 3) electrodes.48 Reproduced from Ref. 48 with permission from
The American Association for the Advancement of Science.
Fig. 5
Fig. 4 (a) Local Co–N4 coordination moiety and (b) structure packing
and (c) proposed structural conversion of cobalt imidazolate framework.39 Reproduced from Ref. 39 with permission from John Wiley and
Sons.
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Journal of Materials Chemistry A
C 1s and N 1s XPS signals of N-doped graphene (NG2) and (b)
EELS elemental mapping of C and N of NG2.49 Reproduced from
Ref. 49 with permission from John Wiley and Sons.
Feature Article
Fig. 6
induction.62 It has been widely used for the modication of
materials on a large scale,63 including functionalized graphene
and its derivatives by replacing carbon with foreign
atoms.32 Recently, it was found that plasma treatment could be
used to prepare nitrogen-functionalized graphene by
simply using graphite and acetonitrile as carbon and nitrogencontaining precursors. First, micro-plasma discharge induced
the exfoliation of the radicalized graphene layer in the acetonitrile solution. Then, it was supposed to form nascent
hydrogen and acetonitrile radicals. At last, N-FG was
formed by the reaction of the radicalized graphene layer
with nascent hydrogen and acetonitrile radicals.50 Moreover,
plasma treatment has been used to assist CVD to grow metalfree nitrogen-doped CNTs and performed good ORR catalytic
behavior.64
(5)
Ball milling
Ball milling is used to grind materials into ne particles (e.g.
down to 1 mm) by a great quantity of balls that heavily fall down
to the bottom of a cabin. During the process, bulk materials
are divided into smaller one; thus, many new interfaces are
created. It was found that ball milling was able to activate
graphite by incorporating active carbon species (e.g., C]O,
–COOH) at the broken edges. Then, the activated graphite
could subsequently react with various gases to obtain corresponding abundant edge-selectively functionalized graphene,
such as halogenated (Cl, Br, I),65 sulfonic acid-46 and
nitrogen-47 functionalized graphene nanoplatelets at an
extremely large scale of grams. The good ORR catalytic activity
and the environmentally friendly nature of synthesis and high
yield highlight its merits.
Besides, Dodelet et al. obtained a highly ORR-active Febased catalyst by ball milling a mixture of carbon support,
phenanthroline and ferrous acetate, aided by successive
pyrolysis in Ar and ammonia. The as-obtained catalyst
exhibited a high performance with catalytic activity of 99 A
cm3 (at 0.8 V iR-free cell),44 which is very close to the 2010
target of 130 A cm3 set by the US. Department of Energy
(DOE), as shown in Fig. 7. The highly catalytic activity of
the catalyst may be assigned to the increased density of the
catalytic site.
15708 | J. Mater. Chem. A, 2014, 2, 15704–15716
Volumetric current density test.44 Reproduced from Ref. 44
with permission from The American Association for the Advancement
of Science.
Fig. 7
Strategies of improving the ORR
catalytic activity
(1)
Optimized temperature for heat treatments
In traditional experiments, a range of high temperatures was
successfully used to screen the best ORR catalysts during
preparation, regardless of obscuring detailed structural properties. Thus, for pyrolysis and annealing, it is widely believed
that these high temperature treatments do play a positive role in
the synthesis of doped carbon catalysts for ORR by introducing
more active sites and enhancing stability. However, it was
reported that the most vital factors, including optimal temperature and treatment time are quite distinctive that hardly can be
clearly formulated.66 Signicantly, the morphological alteration
of metal-containing species (e.g. CoxNy/Co3O4 (ref. 67) and
CFe15.1/Fe3O430) along with the variation of pyrolysis temperature indicating the complicated change induced by high
temperature treatment, revealing the vital effect of temperature
during the procedure. Generally, the temperature must exceed
the bottom line of reaction activation energy to facilitate the
graphitization and doping reaction. But then, it has to be lower
than the limit that would result in the decomposition of
dopants-containing groups. Moreover, it was able to tune the
structure (e.g. porosity) and elemental conguration (e.g. the
transformation among graphitic, pyridinic and pyrrolic
nitrogen) of the nal catalysts.
(2)
Co-doping with multi-elements
Transition metals, such as cobalt and iron have been widely
used as precursors in synthesis of doped-carbon catalysts for
ORR, improving the catalytic activity in many cases due to the
generation of new metal–N active center. Nevertheless, it was
specically noted that the mechanism of ORR that occurred in
nitrogen doped carbon material was considered by the
following steps: rst, adsorption of O2 onto the catalysis surface
was induced by the charged site that originated from the positive charged carbon atoms adjacent to nitrogen atoms. Then,
the O2 would be reduced by the p electrons activated by
conjugating with the long-pair from the nitrogen atom.
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Consequently, two key factors, namely charged sites and activated p electrons were needed in the catalytic reduction
process.
Thus, boron that possess mono 2p orbit is also a prospective candidate because the vacant orbital of B could extract and
conjugate with electrons, leading to the charged carbon atom
and activated p electrons.68,69 For instance, compared to those
of carbon nanotubes, Hu et al. showed that the onset and peak
potential of boron-doped carbon nanotubes (BCNT) positively
shi with boron content, boron increasing from 0 to 2.24
atom% (Fig. 8).68 However, it was found that the bonded B and
N in co-doped CNT exhibited antagonism rather than a synergetic effect, indicating that a large scale neutralization of lonepair electrons from N with vacant orbitals of B occurred. This
fact would lead to the decrease of electrons and vacant orbitals,
which result in little chance to conjugate and activate the p
electron.69 Similarly, it was also gured out that for graphene codoped with N and B, the activity was not proportionate to the
content of boron and nitrogen, and it seemed to be that an
optimal ratio existed. This fact was due to the distribution of B
and N. It was assumed that if boron and nitrogen bonded as BN
bonds or BN clusters, it would lead to low conductivity and
would lower the ORR activity.18 Accordingly, aiming to avoid the
generation of inert by-products of hexagonal boron nitride (hBN), Qiao et al. prepared a catalyst of boron and nitrogen codoped graphene without the by-products of h-BN, as shown in
FTIR (Fig. 9a). The catalyst performed with much higher ORR
Journal of Materials Chemistry A
activity compared to the composites containing h-BN (Fig. 9b)
and further proved this assumption.17
Meanwhile, as a member of nitrogen group elements,
phosphorus has many similarities both in physical and chemical properties to nitrogen. Recently, the phosphorus-doped
graphene was reported to be an active catalyst ORR.70 If the
lower electronegativity of P (2.19) is compared to that of C (2.55)
into consideration, P would act as a charged site where the
adsorption of oxygen would occur like C in the N–C bond.
However, the even higher energy of C–C when doped with P
indicates that the mechanism cannot simply be explained by
electronegativity.71 In particular, it was supposed that the
different doping phase was dominated as P–O–C. Namely,
oxygen atom was inserted between P and C. Thus, the oxygen
atoms that possessed higher electronegativity (3.44) would
extract the electron from C. Thus, eventually, C was le as the
charged site.72 Furthermore, P was reported to be co-doped with
N likewise.43,70,71,73–75 Further, P even could be ternary-doped
with B, N, showing 2.3 times higher mass activity at 0.6 V (vs.
RHE) than simply N-doped carbon. It was ascribed to the charge
delocalization of the carbon atoms or split and wrinkled site on
the edge caused by P dopant.72
In addition, carbon materials doped with sulfur or co-doped
with sulfur and nitrogen also have been extensively discussed.24,42,71,76 And these heteroatom (B, N, S, and P)-doped
carbon catalysts also performed good ORR catalytic activity in
acidic media, as concluded in Table 3.
(3)
Fig. 8 RDE voltammetry of BCNT with a rotation speed of 2500 rpm
(B1, B2, B3 stand for boron content of 0.86, 1.33, and 2.24 at%,
respectively).68 Reproduced from Ref. 68 with permission from John
Wiley and Sons.
(a) Fourier transform infrared (FTIR) and (b) linear-sweep voltammetry (LSV) (1500 rpm) in an O2-saturated 0.1 M solution of KOH of
N–graphene, BN–graphene and h-BN–graphene.17 Reproduced from
Ref. 17 with permission from John Wiley and Sons.
Fig. 9
This journal is © The Royal Society of Chemistry 2014
Improving active site density
One bottleneck of the currently available non-precious metal
catalysts is the low turnover frequency (TOF), i.e. the number of
electrons produced per active site in a second under the dened
operating condition, which is much lower than Pt-based materials contemporarily.80 Briey, a higher turnover frequency
could be obtained from more catalytic site density,81 which was
intensely affected by the size of the surface area. Normally, high
surface area could oen increase the opportunity of the
dispersion of catalysts and the density of active sites accessible
to reactants, while pores would promote the transportation of
reactants and intermediate species.82,83 Accordingly, many
works towards high surface area were undertaken by utilizing
support of materials, such as microporous/mesoporous carbon
materials that already have rich pores24,44,84,85 or creating pores
by techniques, such as template1,74,83,86–89 and potassium
hydroxide (KOH) etching-assisted with ball milling.90 For
example, by utilizing a highly microporous carbon black (Black
Pearls 2000) with enormous surface area of 1379 m2 g1
(micropore area of 934 m2 g1) as the host, the as-obtained
catalyst of Fe–N–C exhibited grand performance of volumetric
current density of 99 A cm3. The obtained high activity was
attributed to the microporous carbon black, which was lled
with a large quantity of possible active sites of Fe–N and huge
surface area inherited from the microporous carbon black.44
Similarly, due to the carbon nanober (CNF) possessing a big
amount of exposed edge planes, slits and pores, it was reported
as a very potential catalyst for ORR aer decorated or doped by
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Table 3
Feature Article
Catalytic ORR activity of selected metal-free doped carbon materials in acidic media
Heteroatom(s)
Methoda
Electrolyte
Onset potentialb
Electron transfer number
N64
N77
F78
P, N79
S, N71
N, B, P72
CVD
HT
HT
HT
HT
HT
0.5 M H2SO4
0.5 M H2SO4
0.5 M H2SO4
0.1 M HClO4
1 M HClO4
1 M HClO4
—
0.72 V
0.9 V
0.87 V
0.85 V
0.83 V
3.52 @ 0.41 V
3.49 @ 0.35–0.50 V
3.95 @ 0.8–0.2 V
—
3.83 @ 0.75 V
—
a
CVD: chemical vapor deposition; HT: high temperature treatment. b Potentials vs. RHE.
nitrogen and Fe in cracks.91 In another interesting work, threedimensional (3D) N-doped graphene aerogel (N–GA), which
owned 3D macropores and high specic surface was reported to
support Fe3O4 nanoparticles to form a high efficient catalyst for
ORR.92
Using the ordered mesoporous silica SBA-15 as the hard
template and nitrogen-containing aromatic molecules, i.e. N,N0 bis(2,6-diisopropyphenyl)-3,4,9,10-perylenetetracarboxylic diimide (PDI) and tetrakis(tert-butyl)naphthalocyanine (BNc) as
the C/N precursor, Müllen et al. reported the preparation of
mesoporous N-doped carbon nitrogen-doped ordered mesoporous graphitic arrays with high electrocatalytic activity for
oxygen reduction (see Fig. 10).89 It was also reported that by
impregnating reactants into pores via immersion or ball
milling,44,47,65,93 the reactants could polymerize in the tunnel to
obtain large surface area and gain abundant active sites aer
heat treatment. It was due to the rich pores structure of the
support or template.
So templating method was also used to create porosity for
N-doped carbon.94 For instance, starting with Fe(NO3)3,
Mg(NO3)2 and gelatin, we obtained self-expanded foam, which
could be further carbonized into N-doped carbon with accessible trimodal porosity aer etching out Fe-containing species
(see Fig. 11).94
Moreover, there are some works that demonstrated that
doped carbon ORR catalysts with high surface area could be
obtained without the support or template as well. For example,
owing to high surface area and porous structure, self-supported
Fig. 11 (a) Schematic diagram showing the one-pot route to C/Fe3C/
MgO foams and etching to generate carbon with trimodal porosity. (b)
Gelatin sponge. (c) SEM and (d) TEM images of the calcined foam.94
Reproduced from Ref. 94 with permission from the Royal Society of
Chemistry.
of porphyrin-based catalysts prepared from porous polyporphyrin exhibited excellent catalytic activity.81 Using the
approach of template-free precursor-controlled pyrolysis of
polymer frameworks, the obtained porous cobalt porphyrinbased electrocatalyst exhibited high activity in acidic media
with very positive onset potential 0.74 V (vs. RHE) and high
electron transfer number (3.94), which was comparable with
Pt/C.95 Additionally, KOH etching and ball milling,90 were also
used to efficiently enlarge the surface area due to the decrease of
intersheet interactions and formation of three-dimensional
porous. For instance, the better ORR activity of nitrogen doped
holey graphene (NHG) compared to the untreated nitrogen
doped graphene (NG) revealed the advantage of porous
structure.90
(4)
Fig. 10 Preparation of ordered mesoporous carbon using the silica
SBA-15 as the hard template.89 Reproduced from Ref. 89 with
permission from John Wiley and Sons.
15710 | J. Mater. Chem. A, 2014, 2, 15704–15716
Biomimetic approaches
Inspired by biochemical oxygen activation process, Chen et al.
prepared iron phthalocyanine-based ORR catalyst (Fe-SPc)
(Fig. 12a) without pyrolysis. It was subsequently conrmed that
the iron ion was active center with assistances of exact structure
information obtained by Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR) and high
resolution electron spray ionization mass spectroscopy (ESIMS). Owing to the carefully designed preparation scheme, thioether functional groups served as electron-providing sites to
facilitate electron transfer of the reduction process, preventing
accumulation of the by-products and intermediates.
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Journal of Materials Chemistry A
Fig. 12 (a) Structure of Fe-SPc and (b) polarization curves for the ORR
of Fe-SPc/KJ300 on RRDE voltammograms.20 Reproduced from
Ref. 20 with permission from the American Chemical Society.
(a) Representative synthetic pathway for ORR catalytic active
N-doped carbon and (b) RDE voltammograms at the rGO, rGO–N,
rGO–N–Fe, and HiSPEC™ Pt/C (20 wt%) in 0.1 M KOH.51 Reproduced
from Ref. 51 with permission from the Royal Society of Chemistry.
Fig. 14
Fig. 13 (a) Structure of FePc–Py–CNTs and (b) durability test. (0.1 M
KOH).21 Reproduced from Ref. 21 with permission from the Nature
Publishing Group.
Additionally, bulky diphenyl thiophenol groups provided steric
hindrance, avoiding an overlap of the active sites in Fe-SPc and
suppressing its weakness of rapid degradation inherited from
phthalocyanine, displaying good durability aer 100 potential
sweeps of RRDE voltammograms, as shown in Fig. 12b.20
Likewise, in order to enhance the conductivity and stability
of the active site, Thapa et al. incorporated CNTs as providers of
electron transfer channel and steric hindrance to prepare a FePc
and pyridine-functionalized single-walled CNTs (FePc–Py–
CNTs) electrocatalyst. The as-prepared catalyst exhibited
impressive catalytic activity with extraordinary durability in 0.1
M KOH (Fig. 13).21
(5) Creating dopants with more predictable molecular
structure
Differing from pyrolysis and annealing, which use high
temperature treatment to increase the graphitization of carbon
of reagents to enhance conductivity,37 low temperature
synthesis usually uses precursors [e.g., graphene, GO, rGO,
carbon nanotube (CNT), oxidized carbon nanotube (OCNT)]
that contain abundant sp2 carbon. Arising from excluding
violent pyrolysis procedure around 1000 C, the best advantage
of low temperature synthesis is it being much more facile and
controllable.
Our group has used a wet chemical way to prepare nitrogendoped carbon catalyst (rGO–N) without any ercely high
temperature treatments, such as pyrolysis and annealing. GOcontaining native graphitic carbon was utilized as the carbon
source; thus, high temperature graphitization could be avoided.
Moreover, oxygen-containing defects in GO would be the place
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where nitrogen-dopants could be introduced. It was found that
using dicyandiamide as the nitrogen source, N-doped carbon
with similar fair catalytic activity could be prepared at a lowtemperature (180 C) instead of commonly used high temperature treatment. It provided a new way to prepare doped carbon
materials with high catalytic ORR activity with more predicable
structure (Fig. 14a), a favorable onset potential and large kinetic
current (Fig. 14b).51
Other types of chemical reactions were also used to tailor
carbon-based materials. Acid-catalyzed dehydration was reported to prepare the aromatic pyrazine rings-containing nitrogendoped graphene from double-condensation reaction between
ortho-diamine and diketone in GO.96 Likewise, tetrachloromethane and lithium nitride were also able to prepare N-doped
graphene in one-pot direct solvothermal synthesis below
350 C.97
Moreover, it was conrmed by isotope labeling DFT theory
that hydrazine treatment to GO would not only result in the
insertion of an aromatic N2 moiety into a ve-membered ring as
pyrazole formation at the platelet edges, but also retained the
original basal structure of GO.98 Therefore, regardless of toxin
and ammability, hydrazine treatment is a convenient method
to reduce and nitrogenate GO in just one step. Furthermore, the
low operating temperature, commonly below 100 C, makes the
reaction controllable, which offers the possibility to detect the
mechanism in detail.
Controversies
(1)
The role of transition metals, such as Fe for catalytic ORR
Though iron dopant has greatly been regarded as a signicant
contributor to ORR activity, the confusion of real active center of
Fe–N–C remains ercely debate. Many studies have demonstrated that Fe and N have synergistic effects.99,100 For example, a
metal-free N-doped graphene (NG) catalyst with a high content
of N (7.86 at%, XPS) was synthesized by the pyrolysis of urea and
GO. It was found that the oxygen reduction onset potential was
0.14 V (vs. SCE in 0.1 M KOH).101 Interestingly, Fe/N-doped
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Journal of Materials Chemistry A
graphene (N 5.81%, Fe 0.89 at%, XPS) was synthesized in
similar route by pyrolyzing urea and exfoliated graphene with
Fe(OAc)2. It showed an enhancement of the ORR activity with an
onset potential at 0.023 V (vs. SCE in 0.1 M KOH),41 indicating
the important role of Fe. However, there existed a view that only
coordinated Fe made contribution to ORR, and the rest which
may have existed as iron-containing nanoparticles were not
helpful for the catalytic activity.25 Dodelet and Jaouen reported
that the activity of the Fe-containing carbon catalyst increased
with Fe content up to 0.2%. Beyond that, the activity did not
signicantly increase.102 Coincidentally, another Fe-containing
(0.05 wt%) nitrogen-doped carbon catalyst exhibited excellent
ORR activity. On the contrary, the activity sharply dropped when
the content of Fe doubled to 0.1 wt%.99
Others have shown that Fe may only catalyze the formation
of graphite carbon and the activity centers (C–N) during pyrolysis thereaer being inactive during ORR.103,104 For instance,
some researchers assumed Fe as the activity center by considering the fact that CO was evidently able to block the Fe site and
poison the catalysis.3 However, it was assumed that CO was not
a convincing proof to verify the assumption.105 Considering that
CN can effectively poison heme proteins, which is historically
well-dened in biochemistry, Chen et al. divided the Fe–C–N
activity center into two categories and hypothesized that if Fe
suited as in-plane structure within, macrocyclic would render it
resistant to CN replacement because of the steric hindrance of
this complex.106 Star et al. evaluated the ORR catalytic activities
based on a variety of transition metals, including Fe, Co and Ni
used in the synthesis procedure of carbon nanotubes with/
without N-dopants. It was revealed that the quantity of active
sites dictated the half-wave potential, while the type of metal
affected the electron transfer number of ORR.55 Nevertheless,
Dai et al. reported that nitrogen-doped carbon nanotube
without metals was an excellent metal-free catalyst for ORR.
They found that aer completely leaking-out iron-containing
impurities, no signicant decrease of catalytic ORR activity was
observed, indicating the iron did not facilitate the catalytic
ORR.48 Moreover, it was also reported that nitrogen-doped
carbon nanotube arrays showed favorable activity in ORR, and
the activity was independent of trace iron, which may originate
from FeCl3 (a precursor of CVD) in this case.107 In recent years,
although many efforts have been made aiming to reveal the
activity sites of doped carbon in catalytic ORR, this issue is still
an ongoing debate. One conceivable challenge is that a high
temperature treatment of a diversity of precursors makes the
molecular structure of doped carbon rather complicated and
difficult to predict and characterize.
(2)
N-dopants in catalytic ORR
Graphene without any impurity exhibits excellent conductivity,
which endows it with great electric potential, but it has poor
performance of activity compared to the commercial Pt/C in
ORR. Therefore, abundant attempts, including doping of
foreign atoms have been investigated. Briey, compared to
graphene, nitrogen-doped graphene was reported to facilitate
activity, not only did it doubled the limiting-current, but it also
15712 | J. Mater. Chem. A, 2014, 2, 15704–15716
Feature Article
improved the onset potential to a great degree.108 Moreover,
nitrogen-doped nanocomposites with high current, no less than
or even higher that of Pt/C, further conrmed the feasibility of
N-doped carbon material for efficient ORR catalysts.109,110
However, there are some arguments regarding the nitrogen
dopants for catalytic ORR in carbon materials. Three types of
nitrogen including pyridinic N, pyrrolic N and quaternary N
primarily have been considered. Some researches demonstrated
that pyridinic-type nitrogen dominated the catalytic activity.111
However, another investigation showed that pyridinic-N in a
series of nitrogen-doped graphene with N content ranging from
0% to 16% did not work as an effective active site.112 Meanwhile,
in some studies, both pyrrolic and quaternary N in doped
carbon were reported to show exclusive catalytic activities48,113
and inertness in ORR.114 While other studies focused on the
increase of content of nitrogen,115 and another study demonstrated that the optimal N content was around 25%.28 Accordingly, many studies have been devoted to exploring the activity
center of N-doped carbon material.116 For instance, by annealing the three combinations of composites (GO/NH3, polyaniline/rG-O and polypyrrole/rG-O), the product of GO/NH3 mainly
possessing graphitic and pyridinic N showed higher limiting
current density, while the products of polyaniline/rGO and
polypyrrole/rG-O mainly possessed pyridinic, and pyrrolic N
moieties had more positive onset potential. Thus, it was
supposed that the graphitic N determined the electrocatalytic
activity, namely, the limiting current density, while the pyridinic N respond to the onset potential and tend to drive ORR
through 4e process.117
Mechanism studies
In a four-electron reduction pathway, O2 is rst adsorbed to the
surface of the activity site, then combined with an H probably
offered by H2O via hydrogen bond to form OOH as intermediate. Aer that, the O (ads) stretched an H and converted to OH
(ads), which would nally desorb from the activity site with
assistance of OH in an ambient solution. The signicant
symbol of four-electron pathway is the bond break of O–O, while
the less efficient two-electron pathway ends with the formation
of H2O2.118 Meanwhile, it was reported that being doped with
nitrogen, the intrinsic sluggishness of carbon materials toward
ORR could be improved into in a more preferred manner of the
four-electron pathway. To explore the nature of the mechanism
around the doping process, density functional theory (DFT) has
been extensively used to mimic the electronic structure of
composites, which have been commonly stimulated as specic
models by reasonable simplication and further optimization
for practical reaction.
Xia et al. reported nitrogen dopants with lone-pair electrons
altering the electron distribution of the original one of C46H20.
Compared to C46H20 in acid, the model of pyridine containing
C45NH20 had an improved ORR activity attributed to the
reduction of HOMO–LUMO gap, indicating greater opportunities of electrons excited from valence band to conduction band.
While in the case of C45NH18 containing pyrrole, rather than the
similarity above, nitrogen gave rise to asymmetric spatial
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Distributions in two 6 6 supercells of (a) S1 and (b) S2. The
gray and blue spheres represent C and N atoms, respectively. And the
free energy diagram for O2 reduction on (c) S1 and (b) S2 under the
condition of 0.04 V and pH ¼ 14. The black line indicates the intermediates and reaction barriers of associative mechanism. The blue line
indicates the dissociative barrier of O2 and the intermediates. The red
line indicates the formation of OOH.118 Reproduced from Ref. 118 with
permission from Elsevier.
Fig. 15
distributions of a electron and b electron. It was concluded that
the catalytic sites were probably the carbon atoms that possess
highest spin density or the carbon atoms with small negative
value of spin density but owns large atomic positive charge
density. That is, the promotion, more than irreplaceable of
nitrogen, is ascribed to a high asymmetric spin density and
atomic charge density on graphene induced by active chemical
species.119
However, there are numerous factors, including solvent
effect that complicate the stimulation of oxygen reduction. By
considering the solvent, surface adsorbents, and coverage, Bao
et al. built two congurations with same N concentration but
different N distribution in 6 6 supercells in alkaline. It was
found that compared to the conguration of S1 (see Fig. 15a),
the decentralization of N distribution in S2 (see Fig. 15b) was
more energy favorable of catalyzing oxygen reduction. This
active gap was mainly interpreted by the lower formation
energies of O (ads) on S1, illustrating stronger intermediatesurface bonding on S1. However, the barrier of O (ads)
desorption occupied maximum in all steps, indicating ratedetermining step itself in ORR. Moreover, according to calculating energy barriers of oxygen reduction, though with slightly
inferiority (0.08 eV higher than the corresponding in S1) in
hydrogenation of O (ads), S2 possessed superiorities in most
steps, especially the higher formation energy of O (ads)
compared to S1, arguing weak bond energy between individuals
that facilitated removing O (ads) and outperformed S1 overall
(Fig. 15c and d).118
Apart from single dopant of nitrogen, metal elements,
including Fe and Co are oen co-doped with nitrogen into
carbon materials, forming the composites classied as Fe (Co)–
N–C. For instance, by stimulating the M–N4 (M ¼ Fe or Co)
cluster bridged two pores that were located in the edge of graphene plane, it was reported that the central metal atom was
important for adsorbing O2 and breaking O–O bond. And for the
This journal is © The Royal Society of Chemistry 2014
Journal of Materials Chemistry A
Fig. 16 Correlation plot of O2 binding energy with d-orbital center of
the central transition metal in the Fe–N4, Co–N4, and Ni–N4 clusters
embedded between the graphitic pores. The d-orbital center was
calculated by integrating the electronic density of states (DOS) of the
transition metal d-orbitals (excluding dx2–y2 orbitals) in the energy
range of 5 to 5 eV relative to Fermi energy.120 Reproduced from
Ref. 120 with permission from the Royal Society of Chemistry.
vital step of adsorbing O2 to the surface of the catalyst, it was
found that the binding energy of M–O2 had a correlation with
the d-orbital center of the central transition metal in this case,
indicating the possibility to optimize other metals containing
catalysts by tuning their electronic structures or supports
(Fig. 16).120
Meanwhile, other than verifying assumptions and
accounting for facts in the past, it is also essential to apply DFT
to contemporary relevance as forecast, namely, digging up the
unexploited candidates for ORR. Recently, based on the principles of energy favorable of crucial steps, including low
formation energy of impurity atom(s) at vacancy(ies) on graphene and affordable but excessively strong adsorption of O2 on
the defect, Kaukonen et al. stimulated several conditions of
metal clusters that formed transition metal-vacancy complexes
without the assistance of nonmetals. It was suggested that the
feasibilities of P2 (phosphorus) and Ag2 (silver) doped in a
divacancy of graphene, which have been seldom reported.
Additionally, though the superiority of Zn met most requirements, the high formation energy hindered its potential applications in ORR. And Ga performed favorably except weak
affinity to O2, which may lead to the bond break and be replaced
by H2O in a lower adsorption energy.121
Conclusion
It is important to enhance the kinetics of oxygen reduction
reaction (ORR) for several articial energy conversion devices
and sensors. Therefore, many efforts have been expanded on
exploring highly active ORR catalysts. Not only being supported
by a vast range of experiments but theoretically veried, doping
carbon with foreign elements, such as N, B, P, or some additional transition metals has emerged as a very competitive
candidate for outstanding catalytic performance that mostly
attributed to optimized electronic structures. Despite remarkable catalytic activity, the actual active site remains extensively
controversial among a variety of bonding patterns, which is
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presumably ascribed to the complicated molecular structure of
most doped carbon by high temperature heat treatment. Thus,
developing a new strategy to prepare doped carbon with more
predictable molecular structures and high ORR catalytic activity
is highly anticipated in the future.
Acknowledgements
This work was supported in part by the National Natural Science
Foundation of China (21203023, 21305065, 21005016), NSF of
Jiangsu province (BK2012317, BK20130788, BK2011591), and
Fundamental Research Funds for the Central Universities,
China, for nancial support.
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