REVIEWS
Carbon-based metal-free catalysts
Xien Liu1 and Liming Dai1,2
Abstract | Metals and metal oxides are widely used as catalysts for materials production,
clean energy generation and storage, and many other important industrial processes.
However, metal-based catalysts suffer from high cost, low selectivity, poor durability, susceptibility
to gas poisoning and have a detrimental environmental impact. In 2009, a new class of catalyst
based on earth-abundant carbon materials was discovered as an efficient, low-cost, metal-free
alternative to platinum for oxygen reduction in fuel cells. Since then, tremendous progress has
been made, and carbon-based metal-free catalysts have been demonstrated to be effective for
an increasing number of catalytic processes. This Review provides a critical overview of this
rapidly developing field, including the molecular design of efficient carbon-based metal-free
catalysts, with special emphasis on heteroatom-doped carbon nanotubes and graphene.
We also discuss recent advances in the development of carbon-based metal-free catalysts
for clean energy conversion and storage, environmental protection and important industrial
production, and outline the key challenges and future opportunities in this exciting field.
BUCT-CWRU International
Joint Laboratory, State Key
Laboratory of OrganicInorganic Composites, Center
for Soft Matter Science and
Engineering, College of
Energy, Beijing University of
Chemical Technology, Beijing
100029, China.
2
Center of Advanced Science
and Engineering for Carbon
(Case4Carbon), Department
of Macromolecular Science
and Engineering, Case
Western Reserve University,
10900 Euclid Avenue,
Cleveland, Ohio 44106, USA.
[email protected];
[email protected]
1
Article number: 16064
doi:10.1038/natrevmats.2016.64
Published online 13 Sep 2016
Three seemingly simple reactions, the oxygen reduction
reaction (ORR), oxygen evolution reaction (OER) and
hydrogen evolution reaction (HER), are critical for clean
and renewable energy technologies, such as fuel cells,
batteries and water-splitting processes. Nevertheless, catalysts are needed to promote the HER for hydrogen fuel
generation via photo-electrochemical water splitting, the
ORR in fuel cells for energy conversion and the OER
in metal–air batteries for energy storage. Metal-based
catalysts, especially noble metals (for example, platinum,
iridium and palladium) or metal oxides, are generally
used in these reactions. However, metal-based catalysts
have several notable disadvantages, including low selectivity, poor durability, susceptibility to gas poisoning
and a negative environmental effect. Furthermore, the
high cost and limited availability of precious metals have
hindered the large-scale commercial application of these
renewable energy technologies.
Along with intensive research efforts to reduce or
replace platinum‑based electrodes with non-precious
metal catalysts in fuel cells, a new class of catalyst based on
heteroatom-doped carbon nanomaterials was discovered
in 2009, which could replace platinum to efficiently catalyse the ORR in fuel cells1,2. Recently, these new metalfree catalysts have been demonstrated to be efficient
for the OER3,4 and HER5,6. They are also effective for
I−/I3− reduction in dye-sensitized solar cells7, CO2 reduction for fuel production8, environmental monitoring
and biosensing 9, and even for the production of commodity chemicals10,11. More recently, co‑doped carbon
nanomaterials were shown to act as efficient metal-free
bifunctional electrocatalysts for the ORR and OER in
rechargeable metal–air batteries4, and for the ORR and
HER in regenerative fuel cells 12. In this Review, we present important developments in carbon-based metal-free
catalysts and discuss the recently gained mechanistic
understanding of metal-free catalysis. The design principles of metal-free catalysts are also elucidated, along
with their structure–property correlations and potential
applications. Finally, challenges and perspectives in this
rapidly developing field are discussed.
Early development and recent advances
Metal-free carbon-based ORR catalysts. The ORR on
the cathode is a key step that limits the energy conversion efficiency of a fuel cell. This reaction requires
a substantial amount of platinum catalyst, and hence
accounts for a large portion of the total cost of the fuel
cell. Platinum nanoparticles have long been regarded
as the best catalyst for the ORR, despite several drawbacks, including time-dependent drift, methanol crossover and CO deactivation13. These, together with the
high cost and scarcity of platinum, have made the use of
platinum the main barrier to implementing fuel cells for
commercial applications, even though alkaline fuel cells
with platinum as an ORR electrocatalyst were developed
for the Apollo lunar mission in the 1960s.
In 2009, nitrogen-doped vertically aligned carbon
nanotubes (VA-CNTs) were discovered to be superior to
platinum for the electrocatalysis of the ORR without CO
deactivation and fuel crossover effects in alkaline media1.
The catalytic mechanism of nitrogen-doped VA-CNTs
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for the ORR was investigated using quantum mechanical calculations based on the B3LYP hybrid density
functional theory (DFT) combined with experimental
data1. It was found that doping-induced charge redistribution facilitated the chemisorption of O2 and electron
transfer for the ORR. Subsequently, nitrogen-doped
graphene was also found to be an efficient metal-free
catalyst for the ORR14. Thereafter, the field of metal-free
catalysis experienced rapid development and various
heteroatom-doped carbon-based catalysts were reported,
including boron-doped CNTs15, sulfur-doped graphene16,
phosphorous-doped graphite17, iodine-doped graphene18
and edge-halogenated (doped with chlorine, bromine or
iodine) graphene nanoplatelets (GnPs)19.
Co‑doping carbon-based metal-free catalysts with
different heteroatoms was found in 2011 to be an
efficient way to further improve the electrocatalytic
activity of the ORR, as exemplified by boron and nitrogen co‑doped VA-CNTs20. Later, boron and nitrogen
co‑doped graphene also showed superior ORR electrocatalytic activity to commercial Pt/C (REF. 21). DFT
calculations revealed that boron and nitrogen doping
can tune the energy bandgap, spin density and charge
density 21, facilitating the ORR through synergistic electron transfer interactions between the dopants and surrounding carbon atoms22. Furthermore, phosphorous
and nitrogen co‑doped VA‑CNTs exhibit significantly
enhanced electrocatalytic activity toward the ORR with
respect to single phosphorous or nitrogen-doped CNTs,
comparable to that of a commercial Pt/C electrode in
alkaline media23. These catalysts also exhibit excellent
long-term stability and good tolerance to methanol
crossover and CO poisoning effects. More interestingly,
sulfur and nitrogen co‑doped CNTs show enhanced
ORR activity in both acidic and alkaline media relative
to nitrogen-doped CNTs, along with a better tolerance
to methanol crossover and long-term stability 24. More
importantly, a rationally designed nitrogen-doped
graphene–CNT–carbon black composite with a welldefined porous structure was recently shown to have
excellent long-term operational stability and high
gravimetric power density in acidic polymer electrolyte
membrane (PEM) fuel cells2 — the mainstream fuel cell
technology with great potential for large-scale applications. Such catalysts may accelerate the delivery of
affordable and durable PEM fuel cells to the marketplace.
Along with the rapid advances in heteroatom-doped
CNTs and graphene ORR electrocatalysts, graphite-based
catalysts have also been developed in the past few years.
Of particular interest, nitrogen-doped ordered mesoporous graphitic arrays25 and phosphorus‑doped graphite
layers were reported26 in 2010 and 2011, respectively, to
show high catalytic activity, high durability and excellent
tolerance to methanol crossover for the ORR in alkaline
solutions. Carbon nitride (C3N4) — which intrinsically
possesses a very high nitrogen content dominated by
a pyridinic- and graphitic-nitrogen — supported by a
2D graphene sheet 27 or 3D porous graphitic carbon28
shows excellent ORR catalytic activity and good durability. Much like doping-induced intramolecular charge
redistribution to facilitate the ORR process discussed
above, physical adsorption of polyelectrolyte chains onto
undoped all-carbon CNTs and graphene sheets causes
intermolecular charge transfer and results in ORR electrocatalytic activities similar to those of commercial Pt/C
(REFS 29,30).
Pure carbon nanocages without any apparent dopants
or physically adsorbed polyelectrolyte also show good
ORR performance, as supported by DFT calculations
that indicate high ORR activities intrinsically associated with the pentagon and zigzag edge defects31. In this
context, a new class of ORR catalyst based on graphene
quantum dots supported by graphene nanoribbons
was developed through a one-step reduction reaction,
with ORR performance comparable or even better than
that of a Pt/C electrode32. The good electrocatalytic
performance was attributed to the presence of numerous surface and edge defects on the quantum dots and
graphene nanoribbons, respectively 32, coupled with efficient charge transfer between the intimately contacted
quantum dots and graphene nanoribbons. The research
and development of defect-induced ORR catalysis is still
in the early stages, and further mechanistic studies are
desirable.
Carbon-based OER and HER catalysts. In addition
to the electrocatalysis of the ORR, carbon-based catalysts are also promising alternatives to noble metal and
metal oxide catalysts for the OER and HER3,6,33. Similar
to their use in the ORR, noble metals and their oxides,
such as platinum, palladium and IrO2, are regarded
as state‑of‑the-art catalysts for the HER and OER.
Substantial research efforts have focused on the development of OER and HER catalysts based on relatively
inexpensive transition metals and their compounds,
including transition metal oxides, metal-oxide-based
hybrids, substituted cobaltites (MxCo3 − xO4), hydro(oxy)
oxides, phosphates, diselenide, metal-oxide/diselenide
hybrids and chalcogenides34,35. In addition, ordered
Ni5P4 nanoarchitectures with a disc-like morphology
on a nickel foil are effective bifunctional catalysts for
the HER and OER36. However, transition-metal-based
catalysts are prone to gradual oxidation, undesirable
morphological and/or crystalline structure changes,
and uncontrolled agglomeration or dissolution when
exposed to air or aerated electrolytes37.
Recently, nanostructured carbon materials have
emerged as low-cost, metal-free catalysts with good performance for the HER and OER. For example, nitrogen
doping coupled with meso- or macrostructure fabrication enhances both the OER and HER catalytic activities38,39. OER activities exceeding those of traditional
electrocatalysts (for example, IrO2 nanoparticles) in alkaline media have been demonstrated for nitrogen-doped
graphite nanomaterials synthesized from a nitrogen-rich
polymer 3, nitrogen-doped graphene from the pyrolysis of
graphene oxide with polyaniline39 and nitrogen-doped
graphene formed via the hydrothermal method with
ammonia as the nitrogen precursor 40. Doping of CNTs
with heteroatoms other than nitrogen (for example,
boron or oxygen) enhances the catalytic activity for the
OER and HER in water splitting 41,42, and the performance
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can be further improved by co‑doping with other hetero­
atoms. Specifically, nitrogen and sulfur 43, and nitrogen
and phosphorous5,44 co‑doped graphene and other graphitic carbon materials show enhanced catalytic activities
for both the OER and HER.
Recently, nitrogen and phosphorous co‑doped mesoporous nanocarbon foams were synthesized by pyrolysis of polyaniline aerogels in the presence of phytic acid,
resulting in bifunctional catalytic activities towards the
ORR and OER4. These metal-free bifunctional catalysts
show great potential as the air electrode in metal–air
N-doped CNT metal-free ORR catalyst1
2009
2010
N-doped graphene
metal-free ORR catalyst14
Non-N-doped metal-free
ORR catalyst (B-doped CNT)15
Synergetic effect of co-doping20
2011
Metal-free undoped CNT ORR catalyst
by intermolecular charge transfer30
Edge-doped/functionalized
graphene by ball milling45
2012
N-doped graphene foams as
metal-free counter electrodes
for DSSCs7
Metal-free OER catalyst (N/C)3
2013
Metal-free CO2 reduction
catalyst (CNFs)8
Electron spin density found to
have a key role in the ORR16,51
Metal-free HER catalyst
(g-C3N4/N-doped graphene)33
2014
Metal-free catalyst
for acidic PEM cells2
Metal-free ORR and OER bifunctional
catalysts for zinc–air batteries4
Hydrochlorination of acetylene
catalysed by SiC@N–C (REF.10)
Bifunctional metal-free
ORR and HER12
2015
Doping-free, defect-induced
carbon-based ORR catalyst31,32
2016
Pyridine N was determined to
have a key role for the ORR55
O2
Figure 1 | Timeline showing the important developments of carbon-based
Nature Reviews | Materials
metal-free catalysts. CNFs, carbon nanofibre; CNT, carbon nanotube; DSSCs,
dye-sensitized solar cells; g-C3N4, graphitic-C3N4; HER, hydrogen evolution reaction;
OER, oxygen evolution reaction; ORR, oxygen reduction reaction; PEM, polymer
electrolyte membrane. The image of the nitrogen-doped CNT is adapted with permission
from REF. 1, AAAS. The image of the oxygen adsorption onto the carbon atom next to the
pyridinic N is adapted with permission from REF. 55, AAAS.
batteries (discussed later). More recently, 3D porous
carbon networks co‑doped with nitrogen and phosphorus were formed via a simple, template-free approach by
pyrolysis of a supermolecular aggregate of self-assembled
melamine, phytic acid and graphene oxide12. This was
the first metal-free bifunctional catalyst with high activities for both the ORR and HER, making it attractive for
regenerative fuel cells.
Carbon-based catalysts for other reactions. Carbonbased catalysts, including the edge-functionalized or
edge-doped graphene produced by ball milling 45, have
also been demonstrated to be efficient for I−/I3− and
Co(bpy)32+/3+ reduction in dye-sensitized solar cells7,46,
the reduction of CO2 for fuel production, environmental monitoring and biosensing 9, and even for the production of commodity chemicals10. Although a more
detailed discussion on particular reactions is given
in subsequent sections, we summarize the important
developments of carbon-based catalysts in FIG. 1. In
TABLE 1, a longer but by no means exhaustive list is given
for carbon-based catalysts with detailed information on
their preparation and performance.
Mechanistic understanding
There are several different nitrogen configurations in a
nitrogen-doped conjugated graphite plane13,47 (FIG. 2a). As
mentioned earlier, the improved ORR catalytic performance for nitrogen-doped carbon catalysts is attributable to the doping-induced charge redistribution (FIG. 2b),
which changes the chemisorption mode of O2 from the
usual end‑on adsorption (Pauling model) at the nitrogen-free CNT surface (FIG. 2c, top part) to a side‑on
adsorption (Yeager model) at the nitrogen-doped CNT
electrode (FIG. 2c, bottom part)1. The nitrogen doping
induces charge transfer, and parallel diatomic O2 adsorption can effectively lower the ORR potential and weaken
the O–O bond, facilitating oxygen reduction at the
nitrogen-doped VA‑CNT electrode.
The configuration of doped nitrogen depends on the
chemical environment and can affect the electronic structure of neighbouring carbon atoms, leading to different
catalytic properties. The doped nitrogen atoms near the
edge provide strong chemical reactivity with enhanced
oxygen adsorption48,49 and hence high catalytic activity
towards the ORR. For the design of efficient catalysts, it is
important to understand the correlation of nitrogen binding configurations with electrocatalytic activity. However,
it is still controversial whether the pyridinic or graphitic
nitrogen is mainly responsible for the active sites for
the ORR. In general, graphitic nitrogen determines the
limiting current density, whereas the pyridinic nitrogen
improves the onset potential for the ORR50. Pyridinic
nitrogen can provide one p electron to the aromatic π
system, with a lone electron pair in the plane of the carbon matrix to enhance the electron-donating capability
of the catalyst. Thus, pyridinic nitrogen can weaken
the O–O bond via the bonding of O with N and/or
the adjacent C atom to facilitate the reduction of O2.
The above mechanistic understanding gained from
experiments is supported by recent theoretical work
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that revealed that N–C active centres in metal-free catalysts can directly reduce oxygen into water through a
four-electron process or a less-effective two-electron
pathway 51,52. However, there are still some concerns about
the possible contribution of metal impurities to the ORR
activity of metal-free catalysts47,53,54. Very recently, based
on studies of a highly oriented pyrolytic graphite model
and nitrogen-doped graphene nanosheet powder catalysts, it was concluded that carbon atoms next to pyridinic nitrogen are the active sites for the ORR55. An oxygen
molecule is first adsorbed at the carbon atom next to the
pyridinic nitrogen (FIG. 2d), followed by proton-coupled
electron transfer to the adsorbed oxygen. This can occur
via either of two pathways. The first is a four-electron
mechanism, in which a subsequent two-proton-coupled,
two-electron transfer breaks the O–OH bond to form a
water molecule. Next, proton-coupled electron transfer
causes breakage of the OH bond to form another water
molecule. The second pathway is a [2 + 2]‑electron mechanism, in which the adsorbed OOH species react with
another proton to form H2O2. The H2O2 is then readsorbed or reduced by two protons and two electrons
(FIG. 2d). Therefore, it is the intrinsic active sites in nitrogen-doped carbons that show efficient electrocatalytic
activities for the ORR.
There is an increasing number of reports of efficient
carbon-based catalysts for the ORR, OER, HER3,5,6,33, as
well as ORR–OER and ORR–HER bifunctional reactions4,12, which cannot be catalysed by trace metal residues. Moreover, the observed CO‑insensitive ORR
activities of carbon-based catalysts do not arise from
metal active centres, which would have otherwise been
poisoned by CO (REF. 1). In addition, the enhanced ORR
activity by the physical absorption of positively charged
polyelectrolytes (for example, poly(diallyldimethylammonium chloride) (PDDA)) onto all-carbon graphene
or nanotubes56 unambiguously demonstrates that ORR
activity in carbon-based catalysts arises from either
doping-induced intramolecular charge transfer or inter­
molecular charge transfer even without doping, rather
than from trace metals.
Compared with ORR studies, OER using carbonbased catalysts has been discussed much less in the literature, although the number of relevant publications has
recently rapidly increased. The mechanism of the OER
on metal-free carbon catalysts is sensitive to the structure of the electrode surface57, and it has been predicted
that the armchair carbon near the nitrogen in graphene
favours the OER 58. For surface-oxidized multiwall
CNTs59, oxygen-containing functional groups (C=O)
on the outer layer change the electronic structure of the
adjacent carbon atoms, facilitating the adsorption of OER
intermediates and hence the OER process.
Although there is still a limited understanding of
the OER process, DFT calculations have been performed to indicate that the HER catalysed by C3N4@
nitrogen-doped graphene is potential-dependent 33.
The Volmer–Heyrovsky mechanism is dominant at
low overpotential, at which electrochemical desorption
is a rate-limiting step. By contrast, the Volmer–Tafel
mechanism becomes dominant at high overpotential33.
Readers that are interested in detailed HER mechanisms
are referred to several recent review articles35,60, and the
mechanisms of CO2 reduction8,61 and the hydrochlor­
ination of acetylene10 by carbon-based catalysts are
discussed next.
Table 1 | Summary of representative carbon-based metal-free catalysts
Materials
Catalyst preparation
Catalytic application
Catalytic efficiency
Refs
N‑doped VA‑CNTs
Pyrolysis of iron phthalocyanine in NH3
ORR
>Pt/C
1
N‑doped graphene
CVD of CH4 and NH3
ORR
>Pt/C
14
B‑doped CNTs
CVD of benzene–TPB–ferrocene mixture
ORR
<Pt/C
15
S‑doped graphene
Ball-milling graphite in S8
ORR
>Pt/C
16
I‑doped graphene
Annealing graphene oxide with I2
ORR
<Pt/C
18
VA‑BCN
Pyrolysis of melamine diborate
ORR
>Pt/C
20
N‑doped and ordered
mesoporous graphitic arrays
N,Nʹ-bis (2,6‑diisopropyphenyl)-3,4,9, 10‑perylenetetracarboxylic diimide with a template
ORR
>Pt/C
25
PDDA–graphene
Reduction of graphene oxide in PDDA using NaBH4
ORR
>graphene
29
Carbon nanocages
MgO template and benzene
ORR
>CNT
31
N-doped graphite
nanomaterials
Melamine formaldehyde
OER
>IrO2
3
C3N4@N‑doped graphene
Dicyandiamide and graphene oxide
HER
~ transition metal
Carbon nanofibers
Pyrolysis of electrospun nanofiber
CO2 reduction
Overpotential (0.17 V)
8
VA‑CNT-carbon fibres
VA‑CNT sheathed carbon fibre via CVD
In vivo monitoring of
ascorbate
–
9
SiC@N–C
Heating a mixture of NH3 and CCl4 on SiC
Hydrochlorination of
acetylene
Conversion of
acetylene (85%)
10
LC‑N
CVD
Oxidation of arylalkanes
Yield (max) >99%
11
33
CVD, chemical vapour deposition; HER, hydrogen evolution reaction; LC, layered carbon; OER, oxygen evolution reaction; ORR, oxygen reduction reaction; PDDA,
poly(diallyldimethylammonium chloride); VA‑BCN, vertically aligned boron and nitrogen co‑doped carbon nanotube; VA‑CNT, vertically aligned carbon nanotube.
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d
a
H+
e–
O2
H2O2
Pyridinic N
H+
2e– process
e–
4e– process
Graphitic N
Pyridinic N
2H+
c
2H+
H+
Pyrrolic N
b
2e–
2e–
2 H2O
e–
e
HCO2–
–0.073
0.005
0.101
0.179
0.017
–0.277
N
–0.181
–0.277
0.171
0.047
0.125
NCNT
Rate determining
–0.071
–0.027
N
0.027
0.231
–0.184
–
H–N
N HCO2
PEI
0.143
2HCO3–
e–
CO2
CO2
e–
–
HCO3
N–CO
0.072
H–N
•–
2
H2O
CO32–
H–N
2CO2 + 2e– + H2O → HCO2– + HCO3–
Figure 2 | Different types of nitrogen-doped carbon and the reaction mechanisms of the Nature
ORR and
CO2 reduction.
Reviews
| Materials
a | Different forms of doped nitrogen in nitrogen-functionalized carbon. b | Calculated charge-density distribution for
nitrogen-doped carbon nanotubes (CNTs). c | Schematic representations of possible adsorption modes of an oxygen
molecule at a non-doped CNT (top) and nitrogen-doped CNT (bottom). d | Schematic representation of the mechanism
of the oxygen reduction reaction (ORR) on metal-free nitrogen-doped carbon catalysts. e | Schematic mechanism for
the selective reduction of CO2 into formate by polyethylenimine-functionalized, nitrogen-doped carbon nanotubes.
NCNT, nitrogen-doped CNT; PEI, polyethylenimine. Panel a is adapted with permission from REF. 47, Wiley-VCH. Panels
b and c are adapted with permission from REF. 1, AAAS. Panel d is adapted with permission from REF. 55, AAAS. Panel e
is adapted with permission from REF. 61, American Chemical Society.
The reduction of CO2 to chemical fuels by carbonbased catalysis has recently emerged as a promising
research focus. The electrocatalytic reduction of CO2 to
CO can be described by the following reaction steps62:
CO2(g) + H+(aq) + e− ↔ COOH*
(1)
COOH* + H+(aq) + e− ↔ CO* + H2O (l)
(2)
CO* ↔ CO(g)
(3)
where the asterisk denotes an adsorbed intermediate;
equations 1 and 2 are two-proton-coupled electron-transfer reaction steps, and equation 3 is non-electrochemical
CO desorption.
CO2 was successfully reduced by a metal-free carbon-­
based catalyst with superior catalytic activity to that of
noble metal catalysts8, whereby pyridinic and quaternary nitrogen were shown not to be catalytically active,
because there was no change in the peak intensities
of the corresponding nitrogen 1s peaks (measured by
X‑ray photoelectron spectroscopy) before and after the
electrochemical reaction. This suggests that positively
charged carbon atoms rather than nitrogen groups are
the active sites directly involved in CO2 reduction. A
mechanism for the reduction of CO2 to CO was proposed that involves the reduction of positively charged
carbon atoms through redox cycling, followed by re­­
oxidizing the reduced carbon atoms to their naturally
oxidized state by the absorbed intermediate complex
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(1‑ethyl‑3‑methylimidazolium–CO2) with the release
of CO (REF. 8). The catalytic cycle can be repeatedly carried out by renewing the redox state of the carbon atoms.
Nitrogen-doped carbon nanomaterials can also selectively reduce CO2 to formate in aqueous media61 using
polyethylenimine as a co‑catalyst to greatly reduce the
catalytic overpotential and to increase the current density
and catalytic efficiency. As shown in FIG. 2e, the polyethylenimine co‑catalyst acts as a stabilizer for the reduced
intermediate CO2•− whilst concentrating CO2 around the
nitrogen-doped CNT metal-free catalyst.
A nanocomposite of nitrogen-doped carbon derived
from silicon carbide was recently demonstrated to directly
catalyse the non-redox hydrochlorination of acetylene10.
Using model catalysts C3N4 (pyridinic and quaternary
nitrogen) and polypyrrole (pyrrolic nitrogen), it was
found that pyrrolic nitrogen has the strongest role in
acetylene hydrochlorination among all nitrogen species,
because the pyrrolic nitrogen associated with an electronic
state of a higher energy level and density is favourable for
the adsorption of acetylene. Both experimental and theoretical studies revealed that the catalytic activity increases
monotonically with the increasing number of accessible
pyrrolic nitrogen sites. Thus, the active sites for non-redox
hydrochlorination of acetylene on the nitrogen-doped
carbon nanocomposite are near to the pyrrolic nitrogen
species. Compared with metal-free electrocatalysis, such
as the ORR, OER and HER, the development of carbonbased metal-free catalysts for non-electrochemical reactions (for example, acetylene hydrochlorination) is still
in its infancy. Further mechanistic understanding in this
important field is therefore necessary.
Molecular and structural design
Insight gained from the mechanistic studies described
above can guide the design and development of new
carbon-based catalysts. Despite the diversity of their
molecular architectures, CNTs and graphene possess a
common building block containing a graphitic honeycomb network, with conjugated alternating C–C single
and C=C double bonds to allow for the delocalization
of π electrons. Simply replacing carbon atoms in CNTs
or graphene sheets with heteroatoms (for example,
nitrogen, sulfur, boron or phosphorus) that are different from carbon in electronegativity and size induces
charge redistribution over the graphitic network and
distorts the lattice structure to cause changes in both
physical properties (for example, electronic, magnetic
and photonic properties) and chemical activities, leading to various new applications (for example, metal-free
electrocatalysis)13. Thus, doping carbon nanomaterials
with heteroatoms is an effective strategy for the development of carbon-based metal-free catalysts1–4,13,15,61,63.
In general, there are two pathways towards hetero­
atom-doped carbon nanomaterials: in situ doping during
carbon synthesis and doping during post-treatment of
preformed carbon nanomaterials13. Nitrogen doping of
carbon nanomaterials induces a sufficiently high positive
charge density on surrounding carbon atoms, because
nitrogen has a larger electronegativity (χ = 3.07) than
carbon (χ = 2.55). The charge redistribution induced by
nitrogen doping facilitates the chemisorption of oxygen
and electron-transfer for the ORR (REF. 1).
In the case of boron doping, positively polarized boron
atoms not only adsorb oxygen molecules, but also act as
a bridge to transport electrons from graphitic carbon p
electrons to oxygen molecules15,63. Phosphorus doping can
create a defect-induced active surface for oxygen adsorption because of its larger atomic size and lower electronegativity 17,63. Sulfur doping is considered to be more difficult
than nitrogen doping owing to the larger size of the sulfur
atom. Because a sulfur atom has a similar electro­negativity
(χ = 2.58) to that of carbon (χ = 2.55), intramolecular
charge transfer induced by sulfur doping is insignificant16.
Therefore, the improved ORR activity of edge-sulfurized
GnPs can be attributed to electron spin redistribution,
rather than doping-induced charge transfer 16. Among
the edge-selectively halogenated GnPs (XGnPs, X = Cl,
Br or I), IGnP is the most favourable for charge polarization (because iodine is the largest heteroatom) and has the
best catalytic activity19. In addition to single-atom doping,
co‑doping with different heteroatoms is one of the most
effective methods to improve electrocatalytic activities of
carbon-based catalysts, as a result of the synergistic effects
associated with synergistic electronic interactions between
the different doping heteroatoms and surrounding carbon atoms4,20–24. The key principles based on the hetero­
atom-doping analysis described above for the molecular
and structural design of carbon-based metal-free catalysts
are summarized in FIG. 3 and outlined as follows. First,
the location and configuration of dopants in carbon
nanomaterials are important for controlling the catalytic
performance (FIG. 3a). Although it is still a challenge to
synthesize nitrogen-doped carbon nanomaterials with
a single nitrogen configuration (for example, pyridinic,
pyrrolic or graphitic, as shown in FIG. 2a), carbonization
of covalent organic polymers with well-defined nitrogen
distributions and hole sizes could lead to nitrogen-doped
graphitic carbon materials with tailor-made structure–property relationships for specific applications64.
Therefore, the use of heteroatom-containing molecular
precursors with precisely controlled locations of heteroatom atoms provides a feasible strategy to control the location of heteroatom dopants in the doped carbon structure,
which is impossible to achieve with conventional doping
techniques. Second, the heteroatom content can affect the
catalytic activity, as exemplified by the improved electrocatalytic activity with increasing nitrogen content in nitrogen-doped CNT catalysts65. It is important to optimize
the content of heteroatom dopants by tuning the amount
of precursor and/or the pyrolysis or doping conditions
(for example, synthesis temperature and doping time)
(FIG. 3b). Third, hybridization of carbon-based catalysts
with other electrically conducting materials is a powerful
means of creating highly efficient metal-free electrocatalysts (FIG. 3c), as demonstrated by the excellent ORR and
HER catalytic activities reported for graphitic-C3N4 supported by graphene27,28,33. Fourth, like graphene-supported
hybrids, core–shell composite catalysts can also exhibit
superb catalytic performance with a synergistic effect.
As schematically shown in FIG. 3d, the outer wall surface
(nitrogen-doped CNT) can be decorated by nitrogen
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Heteroatom-doped
carbon nanomaterials
Molecular
compositing
Controlled doping
a Location of dopants
b Content of dopants
c g-C3N4/N-doped
graphene
Controlled 3D
architectures
d Core–shell
structure
e 3D porous
f 3D pillared
Nature
Reviews
| Materials
Figure 3 | Design principles for various carbon-based metal-free catalysts. a,b | Location and
content
control
of
heteroatoms. c,d | Layered molecular composites. e,f | Well-designed 3D architectures. Panel d is adapted with permission
from REF. 66, Wiley-VCH. Panel f is adapted with permission from REF. 67, Elsevier.
dopants as catalytic active sites, while the inner wall can
be used as an electrically conducting channel66. Synergistic
core–shell interactions have been demonstrated to significantly enhance the electrocatalytic activity for the OER
with respect to nitrogen-doped CNTs alone under the
same conditions66. Thus, the core–shell strategy provides
an effective way to enhance the catalytic performance of
nitrogen-doped CNTs. Fifth, 3D ordered porous carbon
catalyst electrodes (FIG. 3e) have many advantages over
1D nanotubes and 2D graphene, including a large surface area with many exposed active sites, good electrical
conductivity and electrolyte diffusibility, low density and
good mechanical strength67. It has recently been demonstrated that graphene–CNT integrated 3D nanomaterials
with a pillared structure (FIG. 3f) can be produced by single- or multistep chemical vapour deposition processes
and even through solution self-assembly 68–70. Besides, 3D
nitrogen-doped carbon nanocages have been prepared
by pyrolysis of pyridine with an in situ generated MgO
template71. With tunable micro-, meso- and macroporous
structures, these 3D pillared graphene nanomaterials and
carbon nanocages have extraordinary surface, mechanical
and electrical properties, and electrolyte transport capability, making them attractive for a large range of applications
from metal-free electrolysis to electrochemical sensing.
Multifunctional applications
Carbon-based, metal-free catalysts have attracted great
attention for a wide range of potential applications
owing not only to their large surface area, high mechanical strength, excellent electrical and electrochemical
properties, but also to their low cost and natural abundance. In this section, an overview is provided of these
potential applications in energy conversion and storage,
environmental protection, biosensing and industrially
important chemical production.
Energy applications. Several recent review articles have
covered carbon-based metal-free catalysts for the ORR
in fuel cells13,54,63; hence, in this section we only highlight
cutting-edge research on mono- and bifunctional metalfree catalysis for the OER in metal–air batteries4, the HER
for photo-electrochemical water splitting 5,6, and I−/I3− or
Co(bpy)32+/3+ reduction in dye-sensitized solar cells7,46.
Nitrogen-doped graphite nanomaterials have been
shown to be efficient electrocatalysts for the OER3, with
an overpotential of 0.38 V (versus the reversible hydrogen electrode) at a current density of 10 mA cm−2 in
0.1 M KOH, which is comparable to the performance
of noble metal (such as IrO2 and RuO2) and non-noble
metal (such as cobalt oxides) catalysts3. The active sites
of these materials are the pyridinic and quaternary nitrogen atoms, which are confirmed by X‑ray photoelectron
spectroscopy; inductively coupled plasma-atomic emission spectroscopy revealed that the content of metal species is negligible. Other carbon-based catalysts have also
been reported to be highly efficient OER catalysts, such
as graphitic-C3N4-nanosheet/CNT composites72, graphitic-C3N4 and graphene assemblies derived from 3D
templates of cellulose fibre papers73, nitrogen and sulfur
co‑doped graphite foam74, acid-oxidized carbon cloth75
and surface-oxidized multiwall CNTs76.
Vertically aligned nitrogen-doped coral-like carbon
nanofibre arrays, prepared via chemical vapour deposition, were used as bifunctional catalysts for the ORR
and OER at the cathode in non-aqueous lithium–air batteries77. The nanofibre array cathode operated for 150
reversible charge–discharge cycles with a high specific
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capacity (1,000 mAh g−1) and energy efficiency (90%).
A nitrogen and phosphorous co‑doped carbon-based
bifunctional catalyst for the ORR and OER was later
developed by pyrolyzing polyaniline aerogels in the
presence of phytic acid4. Using the bifunctional catalyst
as the air electrode, both the primary and rechargeable
zinc–air batteries showed performance comparable or
even better than their counterparts based on platinum
for the ORR and RuO2 for the OER, with a durability of
up to 600 charge–discharge cycles (FIG. 4a).
Along with their development as OER catalysts,
carbon-based catalysts have also been used as promising photo- and electrocatalysts for the HER. The use
of carbon-based HER catalysts not only overcomes the
inherent susceptibility of transition-metal-based catalysts to corrosion and oxidation, but also circumvents
the high cost and low abundance of platinum. A highly
active HER catalyst composed of graphitic-C3N4 and
nitrogen-doped graphene33 showed an overpotential of
~240 mV at a current density of 10 mA cm−2 and a Tafel
slope of 51.5 mV dec−1, which compare favourably to
conventional metallic catalysts (FIG. 4b). DFT calculations
combined with experimental data revealed that intrinsic
chemical and electronic coupling synergistically promote
proton adsorption and the reduction kinetics. More
recently, a simple template-free approach was used to
form 3D porous carbon networks co‑doped with nitrogen and phosphorus by self-assembling melamine, phytic
acid and graphene oxide into a supermolecular aggregate12
followed by pyrolysis. This was the first metal-free, bifunctional catalyst with high activities for both the ORR and
HER. The peak power density (310 Wg−1) for the zinc–
air battery using the pyrolyzed aggregate air electrode
is almost two times that of the battery with a Pt/C air
electrode (171 Wg−1). This supermolecular aggregate is a
promising bifunctional catalyst for metal–air batteries and
regenerative fuel cells.
Carbon-based catalysts have also been used to replace
platinum as the counter electrode to catalyse the reduction
of I3− to I− in dye-sensitized solar cells with comparable
performance to that of the platinum-based counterpart7.
Edge-carboxylated GnPs from ball milling exhibited
electrochemical stability comparable to or even better
than that of platinum for the Co(bpy)32+/3+ redox couple46
(FIG. 4c). Carbon-dot/C3N4 composites achieved solarlight-driven water splitting into hydrogen and oxygen
with quantum efficiencies of 16% (λ = 420 ± 20 nm) in
two two-electron steps6 (FIG. 4d). The solar‑to‑hydrogen
conversion efficiency was 2.0%, which is at least one order
of magnitude larger than any stable photocatalyst reported
previously. Furthermore, carbon-dot/C3N4 maintained a
high rate of hydrogen and oxygen production with high
stability during two hundred 24-hour cycles.
Environmental protection. Carbon-based catalysts have
been recently used for the electrochemical reduction of
CO2 into formate or liquid fuels8,61, along with the oxidation of Rhodamine B using H2O2 for the remediation
of wastewaters78, and in vivo monitoring of oxygen in
various physiological processes79 and other biosensing processes9. Fossil fuel combustion is predicted to
produce ~496 gigatonnes of CO2 in the coming 50 years,
with excess emission of CO2 in the atmosphere potentially causing irreversible climate change80. The electro­
catalytic reduction of CO2 to chemical fuels is not only
a promising energy storage route80, but also a potentially
viable solution to decreasing CO2 emission in the atmosphere. Although noble metals (such as silver and gold)
often show good selectivity for the conversion of CO2 to
CO, their high cost and poor durability have limited their
large-scale practical applications.
Metal-free carbon nanofibres have recently emerged
as highly efficient electrocatalysts for the selective conversion of CO2 to CO, exhibiting a current density of CO2
reduction at −0.573 V (versus the standard hydrogen
electrode) that is ~13 times higher than that of bulk silver 8. Positively charged carbons induced by neighbouring nitrogen in the unique nanofibrillar structure are
considered as active sites that cause the highly efficient
conversion of CO2 to CO. The effect of the nitrogen-defect structures on the catalytic activity for CO2 reduction
was investigated for a 3D nitrogen-doped graphene catalyst through a combination of experiments and DFT
calculations81. It was found that the reduction of CO2
to CO catalysed by nitrogen-doped CNTs is dependent
upon the nature of nitrogen defects and the defect density. The graphitic and pyridinic nitrogen defects, but
not the pyrrolic nitrogen, were found to significantly
affect the activity. Nitrogen-doped CNTs synthesized at
850 °C using acetonitrile (ACN) as the precursor were
shown to contain graphitic and pyridinic nitrogen,
which effectively reduced the overpotential (–0.18 V)
and increased the selectivity (80% Faradaic efficiency)
for CO formation with respect to the pristine CNTs82
(FIG. 4e). In another study, polyethylenimine-modified
nitrogen-doped CNTs were shown to selectively transform CO2 to formate61, reaching a higher Faradaic efficiency (87%) than those of CNTs and nitrogen-doped
CNTs at current densities of 9.5 mA cm−1.
Recently, carbon-based catalysts have emerged to
be promising for biomedical applications. For example,
pristine microelectrodes of carbon fibres sheathed with
VA‑CNTs were used for in vivo monitoring of ascorbate
in rat brains79. These microelectrodes exhibited a high
selectivity, good reproducibility and stability. Moreover,
Pt/VA‑CNT-carbon-fibre microelectrodes were effective
for the determination of oxygen in the rat brain during
various physiological processes with greater sensitivity
than that of Pt/C fibres79. Furthermore, graphitic-C3N4
can activate H2O2 under visible light, with the resulting
oxy-radicals remediating wastewater by decomposing
organic dyes (for example, Rhodamine B)78 (FIG. 4f).
Industrially important chemical production. Carbonbased metal-free catalysts have also been used for other
reactions normally catalysed by noble or transition
metals, including the hydrogenation of multiple bonds
(often used in fossil fuel and biofuel processing, and the
industrial production of commodity chemicals)83, hydrochlorination of acetylene10, oxidative dehydrogenation of
hydrocarbons (for example, the oxidative dehydrogenation of ethylbenzene to styrene, which is an important
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a
60
96
97
98
99
100
0
–5
1.5
1.0
Current density (mA cm–2)
30
2.0
c
d
0
0
180 360 576
582
588
594
600
Cycle number
16
12
I (mA cm–2geometric)
Potential (V)
2.5
b
Time (h)
–15
Pt
rGO
ECGnP
PEDOT:PSS
8
4
–20
–0.6
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0 days
1 day
5 days
30 days
200 days
–0.5
–0.4
50
5
30
CN600 (dark)
CN500–H2O2 (dark)
CN600 (visible light)
CN500–H2O2 (visible light)
0.6
0.4
0.2
0.0
0
50
100
t (minutes)
150
200
60
40
–0.9
250
–1.0
ACN-750 ACN-850 ACN-950 DMF-850 TEA-850
0
100
100
80
80
60
60
40
40
20
20
0
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Selectivity for C2H3CI (%)
0.8
% conversion of acetylene (a.u.)
g
80
–0.8
Cycle number
1.0
0.0
–0.7
–1.1
200
f
–0.1
–0.6
H2
O2
1
–0.2
e
Onset potential (V)
100
–0.3
E (V versus RHE)
Maximum FE for CO (%)
150
0
C/C0
g-C3N4
N-graphene
C3N4/N-graphene
mixture
C3N4@N-graphene
Pt/C
Voltage (V)
200
Gas evolution (μmol)
–10
0
Partial pressure of HCI (bar)
Figure 4 | Applications of metal-free carbon catalysts. a | Charge–discharge cycling curves ofNature
a three-electrode
zinc–air
Reviews | Materials
battery using a metal-free catalyst as the air electrode at a current density of 2 mA cm−2. b | Hydrogen evolution reaction
polarization curves for four metal-free electrocatalysts and 20% Pt/C. c | Current–voltage characteristics of dye-sensitized
solar cells with different cathode electrodes under one sun illumination (AM 1.5G). d | Typical time course of hydrogen and
oxygen production from water under visible-light irradiation catalysed by C3N4–carbon dots. e | Performance of
nitrogen-doped carbon nanotubes for the electrochemical reduction of CO2. The onset potential and maximum Faradaic
efficiency (FE) for CO formation are shown as a function of nitrogen content in the nanotubes. f | Visible-light photocatalytic
degradation of Rhodamine B with and without H2O2 over graphitic carbon nitride. g | Performance of the metal-free catalyst
based on the nanocomposite of nitrogen-doped carbon derived from silicon carbide for the hydrochlorination of acetylene.
Percentage conversion of acetylene and selectivity to acetylene chloride at varying partial pressures of HCl in the feed stream
are also shown. a.u. arbitrary units; ACN, acetonitrile; C, concentration; C0, initial concentration; CN500 and CN600,
graphitic-C3N4 prepared by thermal polycondensation of dicyandiamide at 500 and 600 °C, respectively; DMF,
dimethylformamide; E, potential; ECGnP, edge-carboxylated graphene nanoplatelet; g-C3N4, graphitic-C3N4; I, current;
N-graphene, nitrogen-doped graphene; PEDOT:PSS, poly(ethylenedioxythiophene):poly(4‑styrenesulphonate); rGO,
reduced graphene oxide; RHE, reference hydrogen electrode; TEA, triethylamine. Panel a is from REF. 4, Nature Publishing
Group. Panel b is from REF. 33, Nature Publishing Group. Panel c is adapted with permission from REF. 46, Royal Society of
Chemistry. Panel d is adapted with permission from REF. 6, AAAS. Panel e is adapted with permission from REF. 82, Wiley-VCH.
Panel f is adapted with permission from REF. 78, Royal Society of Chemistry. Panel g is from REF. 10, Nature Publishing Group.
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monomer for polystyrene)84, aerobic selective oxidation
of benzylic alcohols and activation of the benzylic C–H
bond11,85,86.
In the chemical industry, noble or transition metal
catalysts have been predominant in various applications
for many years, with carbon-based catalysts having only
recently emerged as promising alternatives. For example, nitrogen-doped carbon materials can efficiently
catalyse reductive hydrogen-atom transfer reactions,
including amination of alcohol and benzyl alcohol, and
the reduction of nitro and ketone compounds83. Fouriertransform infrared spectroscopy measurements revealed
that C=O groups were the active sites of the catalysts.
Hydrochlorination of acetylene is generally used to
prepare vinyl chloride as the precursor for polyvinyl­
chloride. A SiC/N–C nanocomposite was shown to catalyse the hydrochlorination of acetylene with high activity
and selectivity 10. Up to 85% of acetylene was successfully
converted to acetylene chloride with greater than 98%
selectivity 10 (FIG. 4g). The conversion and selectivity are
comparable to those of noble metal (such as palladium,
gold and platinum) nanoparticles87. The carbon atoms
next to pyrrolic nitrogen atoms were revealed as the
active sites through a combination of experiments and
theoretical simulations.
Dehydrogenation of hydrocarbons is a well-known
reaction in the petrochemical industry that is traditionally catalysed by potassium-promoted iron catalysts,
which have low energy efficiency 84. Although active carbon can achieve relatively high initial activity in oxidative dehydrogenation reactions (for example, the yield of
ethylbenzene to styrene is up to 56%), deactivation is unavoidable during the oxidative dehydrogenation process84.
By contrast, CNT-based catalysts provide high activity
and durability because of their high crystallinity, controllable homogeneity and chemically uniform active sites.
Aerobic selective oxidation of benzylic alcohols could be
performed by nitrogen-doped graphene nanosheets85,86.
Although carbon-based non-electrochemical catalysts
are still an early development, continued research in
this embryonic field could lead to a flourishing area of
industrially important catalytic technologies.
Perspective
Over the past few years, great progress has been made
in the development of carbon-based metal-free catalysts
for the ORR, OER and HER for energy conversion and
storage, and other areas including environmental, industrial and biomedical applications. However, several key
1.
2.
Gong, K., Du, F., Xia, Z., Durstock, M. & Dai, L.
Nitrogen-doped carbon nanotube arrays with high
electrocatalytic activity for oxygen reduction. Science
323, 760–764 (2009).
The first metal-free catalyst (nitrogen-doped VA-CNT)
that showed superior ORR activity to commercial
Pt/C.
Shui, J., Wang, M., Du, F. & Dai, L. N‑Doped carbon
nanomaterials are durable catalysts for oxygen
reduction reaction in acidic fuel cells. Sci. Adv. 1,
e1400129 (2015).
The first metal-free catalyst (nitrogen-doped
graphene CNT) that showed long-term operational
stabilities and comparable gravimetric power
3.
4.
challenges must be overcome for carbon-based catalysts
to compete with their metal-based counterparts. For
example, most of the carbon-based metal-free ORR catalysts are inferior to Pt/C in acid media, although some
of them already show similar or even better ORR activities to Pt/C in alkaline solutions. For the OER and HER,
the catalytic activities of many carbon-based metal-free
catalysts still do not match those of their metal counterparts (for example, MoS2 for the HER and RuO2 for
the OER). Although an increasing number of metal-free
carbon-based catalysts are emerging for both the OER
and HER, CO2 reduction by carbon-based metal-free
catalysts is much less discussed in the literature. In most
cases, the long-term durability, particularly in practical
devices, has not been tested with a standard evaluation
protocol. Besides, cost-effective large-scale production
of tailor-made catalysts for various specific reactions is
necessary for practical applications4,18,19,29.
To alleviate the aforementioned shortcomings, we
must identify the atomic location and the chemical
nature of the catalytic active centres to gain insightful mechanistic understanding 1,50,51,55,61. Moreover,
molecular and macroscopic structural evaluation and
control are needed to design and fabricate catalytic
materials and electrodes with appropriate multiscale
hierarchical structures and surface characteristics for
optimized catalytic performance5,63,66,69. In particular, new synthetic or doping strategies must be developed to precisely control the location, content and
distribution of dopants in heteroatom-doped carbon
catalysts. A combined experimental and theoretical
approach will be essential to profoundly understand
the structure, mechanism and kinetics of the catalytic
centre, and will guide the design and development of
carbon-based catalysts with a desirable activity and stability for specific reactions crucial in energy conversion
and storage, as well for large-scale chemical synthesis,
biomedical sensing and environmental monitoring.
The availability of powerful, combined computational and experimental approaches for the design and
development of new, low-cost, metal-free carbon catalysts provides vast opportunities for them to overtake
their metal-based counterparts as multifunctional catalysts in clean energy and other technological market­
places. Continued research and development in this
exciting field should result in improved fuel economy,
decreased harmful emissions, reduced cost for industrially important chemical production, and more reliable
environmental and health monitoring.
densities to the best non-precious metal catalysts in
acidic PEM cells.
Zhao, Y., Nakamura, R., Kamiya, K., Nakanishi, S. &
Hashimoto, K. Nitrogen-doped carbon nanomaterials
as non-metal electrocatalysts for water oxidation. Nat.
Commun. 4, 2390 (2013).
The first metal-free catalyst (nitrogen‑doped
carbon) that exhibited comparable activity for the
OER to non-precious metal catalysts.
Zhang, J. T., Zhao, Z. H., Xia, Z. H. & Dai, L. A metalfree bifunctional electrocatalyst for oxygen reduction
and oxygen evolution reactions. Nat. Nanotechnol. 10,
444–452 (2015).
The first metal-free ORR and OER bifunctional
catalyst (nitrogen- and phosphorus-doped carbon
foam) for high-performance rechargeable zinc–air
battery.
5.Zheng, Y. et al. Toward design of synergistically active
carbon-based catalysts for electrocatalytic hydrogen
evolution. ACS Nano 8, 5290–5296
(2014).
6.Liu, J. et al. Metal-free efficient photocatalyst for
stable visible water splitting via a two-electron
pathway. Science 347, 970–974 (2015).
7.Xue, Y. H. et al. Nitrogen-doped graphene foams as
metal-free counter electrodes in high-performance
dye-sensitized solar cells. Angew. Chem. Int. Ed. 51,
12124–12127 (2012).
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Acknowledgements
The authors thank colleagues, collaborators and peers whose
work was cited in this article, and are also grateful for the
financial support from NSF, NSF-NSFC, AFOSR-DoD-MURI,
DAGSI, CWRU, The 111 Project (B14004), The State Key
Laboratory of Organic-Inorganic Composites, Beijing
Advanced Innovation Center for Soft Matter Science and
Engineering, and BUCT.
Competing interests statement
The authors declare no competing interests.
FURTHER INFORMATION
Alkali Fuel Cell History: http://americanhistory.si.edu/
fuelcells/alk/alk3.htm
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