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, cat-alysts 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 selec-tivity, 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 cat-alyse the ORR in fuel cells1,2. Recently, these new metal- free 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 reduc-tion for fuel production8, environmental monitoring and biosensing9, and even for the production of com-modity 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 pres-ent important developments in carbon-based metal-free catalysts and discuss the recently gained mechanistic understanding of metal-free catalysis. The design prin-ciples 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 advancesMetal-free carbon-based ORR catalysts. The ORR on the cathode is a key step that limits the energy con-version 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 draw-backs, including time-dependent drift, methanol cross-over 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

1BUCT-CWRU International Joint Laboratory, State Key Laboratory of Organic-Inorganic Composites, Center for Soft Matter Science and Engineering, College of Energy, Beijing University of Chemical Technology, Beijing 100029, China.2Center 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.lxd115@case.edu; liming.dai@case.edu

Article number: 16064 doi:10.1038/natrevmats.2016.64Published online 13 Sep 2016

Carbon-based metal-free catalystsXien 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.

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http://americanhistory.si.edu/fuelcells/alk/alk3.htmmailto:lxd115%40case.edu?subject=mailto:liming.dai%40case.edu?subject=http://dx.doi.org/10.1038/natrevmats.2016.64

for the ORR was investigated using quantum mechan-ical calculations based on the B3LYP hybrid density functional theory (DFT) combined with experimental data1. It was found that doping-induced charge redistri-bution 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 nitro-gen co-doped VA-CNTs20. Later, boron and nitrogen co-doped graphene also showed superior ORR elec-trocatalytic activity to commercial Pt/C (REF. 21). DFT calculations revealed that boron and nitrogen doping can tune the energy bandgap, spin density and charge density21, facilitating the ORR through synergistic elec-tron transfer interactions between the dopants and sur-rounding 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 stability24. More importantly, a rationally designed nitrogen-doped graphene–CNT–carbon black composite with a well- defined 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 appli-cations. 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 meso-porous 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 sheet27 or 3D porous graphitic carbon28 shows excellent ORR catalytic activity and good dura-bility. 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 elec-trocatalytic 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 associ-ated 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 numer-ous surface and edge defects on the quantum dots and graphene nanoribbons, respectively32, coupled with effi-cient 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 cata-lysts 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 devel-opment 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 per-formance for the HER and OER. For example, nitrogen doping coupled with meso- or macrostructure fabrica-tion enhances both the OER and HER catalytic activi-ties38,39. OER activities exceeding those of traditional electrocatalysts (for example, IrO2 nanoparticles) in alka-line media have been demonstrated for nitrogen-doped graphite nanomaterials synthesized from a nitrogen-rich polymer3, 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 precursor40. Doping of CNTs with heteroatoms other than nitrogen (for example, boron or oxygen) enhances the catalytic activity for the OER and HER in water splitting41,42, and the performance

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Metal-free OER catalyst (N/C)3

Metal-free CO2 reduction

catalyst (CNFs)8

Electron spin density found to have a key role in the ORR16,51

Nature Reviews | Materials

2009

2010

2011

2012

2013

2014

2015

2016

N-doped CNT metal-free ORR catalyst1

Synergetic effect of co-doping20

Non-N-doped metal-free ORR catalyst (B-doped CNT)15

Metal-free HER catalyst (g-C

3N

4/N-doped graphene)33

N-doped graphene metal-free ORR catalyst14

Metal-free undoped CNT ORR catalyst by intermolecular charge transfer30

N-doped graphene foams as metal-free counter electrodes for DSSCs7

Edge-doped/functionalized graphene by ball milling45

Hydrochlorination of acetylene catalysed by SiC@N–C (REF.10)

Bifunctional metal-free ORR and HER12

Metal-free catalyst for acidic PEM cells2

Doping-free, defect-induced carbon-based ORR catalyst31,32

Metal-free ORR and OER bifunctional catalysts for zinc–air batteries4

Pyridine N was determined to have a key role for the ORR55

O2

can be further improved by co-doping with other hetero-atoms. Specifically, nitrogen and sulfur43, and nitrogen and phosphorous5,44 co-doped graphene and other gra-phitic carbon materials show enhanced catalytic activities for both the OER and HER.

Recently, nitrogen and phosphorous co-doped mes-oporous nanocarbon foams were synthesized by pyroly-sis 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

batteries (discussed later). More recently, 3D porous carbon networks co-doped with nitrogen and phospho-rus 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 activ-ities for both the ORR and HER, making it attractive for regenerative fuel cells.

Carbon-based catalysts for other reactions. Carbon-based catalysts, including the edge-functionalized or edge-doped graphene produced by ball milling45, 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, environmen-tal monitoring and biosensing9, and even for the pro-duction 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 understandingThere are several different nitrogen configurations in a nitrogen-doped conjugated graphite plane13,47 (FIG. 2a). As mentioned earlier, the improved ORR catalytic perfor-mance for nitrogen-doped carbon catalysts is attributa-ble 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 nitro-gen-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 adsorp-tion 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 struc-ture 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 bind-ing 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 car-bon 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

Figure 1 | Timeline showing the important developments of carbon-based 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.

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that revealed that N–C active centres in metal-free cat-alysts can directly reduce oxygen into water through a four-electron process or a less-effective two-electron pathway51,52. 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 cata-lysts, it was concluded that carbon atoms next to pyrid-inic 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 mech-anism, in which the adsorbed OOH species react with another proton to form H2O2. The H2O2 is then read-sorbed or reduced by two protons and two electrons (FIG. 2d). Therefore, it is the intrinsic active sites in nitro-gen-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 reac-tions4,12, which cannot be catalysed by trace metal res-idues. 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(diallyldimethylam-monium 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 carbon- based catalysts has been discussed much less in the liter-ature, although the number of relevant publications has recently rapidly increased. The mechanism of the OER on metal-free carbon catalysts is sensitive to the struc-ture of the electrode surface57, and it has been predicted that the armchair carbon near the nitrogen in graphene favours the OER58. 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 per-formed to indicate that the HER catalysed by C3N4@nitrogen-doped graphene is potential-dependent33. 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 16

I-doped graphene Annealing graphene oxide with I2 ORR Pt/C 20

N-doped and ordered mesoporous graphitic arrays

N,Nʹ-bis (2,6-diisopropyphenyl)-3,4,9, 10-perylenetetra-carboxylic 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 33

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

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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a

b c

d

e HCO2–

CO2 CO

2

HCO3

–

2HCO3–

CO32–

2CO2

+ 2e– + H

2O → HCO

2– + HCO

3–

Rat

e de

term

inin

g

H2O

2

O2

2H+

2 H2O

H+

2H+2e–

2e– process

Pyridinic N

4e– p

roce

ss

e–

H+

e–

e–

e–

H+

e–

2e–

Graphitic N

Pyrrolic NPyridinic N

H2O

N

PEINCNT

N H–N

H–N

H–N

N–CO2

•–

N HCO2

–

0.1430.072

0.005

0.101 0.179

0.171

0.2310.0470.027

0.125

0.017–0.073

–0.181–0.277

–0.027

–0.184

–0.071 –0.277

The reduction of CO2 to chemical fuels by carbon- based 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-trans-fer 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 quater-nary 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 pro-posed 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

Figure 2 | Different types of nitrogen-doped carbon and the reaction mechanisms of the ORR and CO2 reduction. 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.

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(1-ethyl-3-methylimidazolium–CO2) with the release of CO (REF. 8). The catalytic cycle can be repeatedly car-ried out by renewing the redox state of the carbon atoms. Nitrogen-doped carbon nanomaterials can also selec-tively 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 polyeth-ylenimine 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 theo-retical 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 carbon- based metal-free catalysts for non-electrochemical reac-tions (for example, acetylene hydrochlorination) is still in its infancy. Further mechanistic understanding in this important field is therefore necessary.

Molecular and structural designInsight 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 honey-comb 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 differ-ent 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, lead-ing to various new applications (for example, metal-free electrocatalysis)13. Thus, doping carbon nanomaterials with heteroatoms is an effective strategy for the devel-opment 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 adsorp-tion because of its larger atomic size and lower electroneg-ativity17,63. 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 electro negativity (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 transfer16. Among the edge-selectively halogenated GnPs (XGnPs, X = Cl, Br or I), IGnP is the most favourable for charge polariza-tion (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 car-bon 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 struc-ture–property relationships for specific applications64. Therefore, the use of heteroatom-containing molecular precursors with precisely controlled locations of heteroa-tom atoms provides a feasible strategy to control the loca-tion 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 electro-catalytic activity with increasing nitrogen content in nitro-gen-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 electrocata-lysts (FIG. 3c), as demonstrated by the excellent ORR and HER catalytic activities reported for graphitic-C3N4 sup-ported 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-dopedcarbon nanomaterials

Controlled doping Molecular compositing

Controlled 3D architectures

c g-C3N4/N-doped graphene

d Core–shell structure

a Location of dopants b Content of dopants e 3D porous f 3D pillared

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 signif-icantly 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 sur-face area with many exposed active sites, good electrical conductivity and electrolyte diffusibility, low density and good mechanical strength67. It has recently been demon-strated that graphene–CNT integrated 3D nanomaterials with a pillared structure (FIG. 3f) can be produced by sin-gle- or multistep chemical vapour deposition processes and even through solution self-assembly68–70. 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 capabil-ity, making them attractive for a large range of applications from metal-free electrolysis to electrochemical sensing.

Multifunctional applicationsCarbon-based, metal-free catalysts have attracted great attention for a wide range of potential applications owing not only to their large surface area, high mechan-ical strength, excellent electrical and electrochemical properties, but also to their low cost and natural abun-dance. 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 metal- free catalysis for the OER in metal–air batteries4, the HER for photo-electrochemical water splitting5,6, 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 hydro-gen 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 nitro-gen atoms, which are confirmed by X-ray photoelectron spectroscopy; inductively coupled plasma-atomic emis-sion spectroscopy revealed that the content of metal spe-cies is negligible. Other carbon-based catalysts have also been reported to be highly efficient OER catalysts, such as graphitic-C3N4-nanosheet/CNT composites72, gra-phitic-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 depo-sition, were used as bifunctional catalysts for the ORR and OER at the cathode in non-aqueous lithium–air bat-teries77. The nanofibre array cathode operated for 150 reversible charge–discharge cycles with a high specific

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.

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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 prom-ising photo- and electrocatalysts for the HER. The use of carbon-based HER catalysts not only overcomes the inherent susceptibility of transition-metal-based cat-alysts 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 nitro-gen and phosphorus by self-assembling melamine, phytic acid and graphene oxide into a supermolecular aggregate12 followed by pyrolysis. This was the first metal-free, bifunc-tional 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 solar-light-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 oxi-dation of Rhodamine B using H2O2 for the remediation of wastewaters78, and in vivo monitoring of oxygen in various physiological processes79 and other biosens-ing 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 poten-tially 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 atmos-phere. 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 con-version 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 sil-ver8. Positively charged carbons induced by neighbour-ing 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-de-fect structures on the catalytic activity for CO2 reduction was investigated for a 3D nitrogen-doped graphene cat-alyst 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 den-sity. 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 trans-form CO2 to formate61, reaching a higher Faradaic effi-ciency (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. Carbon-based 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, hydro-chlorination of acetylene10, oxidative dehydrogenation of hydrocarbons (for example, the oxidative dehydrogena-tion of ethylbenzene to styrene, which is an important

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Nature Reviews | Materials

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Figure 4 | Applications of metal-free carbon catalysts. a | Charge–discharge cycling curves of a three-electrode zinc–air 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 exam-ple, 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. Fourier-transform 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 cata-lyse the hydrochlorination of acetylene with high activity and selectivity10. Up to 85% of acetylene was successfully converted to acetylene chloride with greater than 98% selectivity10 (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 tradition-ally catalysed by potassium-promoted iron catalysts, which have low energy efficiency84. Although active car-bon can achieve relatively high initial activity in oxida-tive dehydrogenation reactions (for example, the yield of ethylbenzene to styrene is up to 56%), deactivation is una-voidable during the oxidative dehydrogenation process84. By contrast, CNT-based catalysts provide high activity and durability because of their high crystallinity, control-lable 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.

PerspectiveOver 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, indus-trial and biomedical applications. However, several key

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 cat-alysts are inferior to Pt/C in acid media, although some of them already show similar or even better ORR activi-ties 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 coun-terparts (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 insight-ful mechanistic understanding1,50,51,55,61. 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 particu-lar, new synthetic or doping strategies must be devel-oped 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 sta-bility 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 computa-tional and experimental approaches for the design and development of new, low-cost, metal-free carbon cata-lysts provides vast opportunities for them to overtake their metal-based counterparts as multifunctional cat-alysts 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 industri-ally important chemical production, and more reliable environmental and health monitoring.

1. 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.

2. 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

densities to the best non-precious metal catalysts in acidic PEM cells.

3. 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.

4. Zhang, J. T., Zhao, Z. H., Xia, Z. H. & Dai, L. A metal-free 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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This work shows the use of metal-free carbon (nitrogen-doped graphene foam) to replace platinum in high-performance dye-sensitized solar cells.

8. Kumar, B. et al. Renewable and metal-free carbon nanofibre catalysts for carbon dioxide reduction. Nat. Commun. 4, 2189 (2013).This paper describes a metal-free catalyst (carbon fibre) that exhibits negligible overpotential (0.17 V) for CO2 reduction.

9. Xiang, L. et al. Vertically aligned carbon nanotube-sheathed carbon fibers as pristine microelectrodes for selective monitoring of ascorbate in vivo. Anal. Chem. 86, 3909–3914 (2014).

10. Li, X. Y. et al. Silicon carbide-derived carbon nanocomposite as a substitute for mercury in the catalytic hydrochlorination of acetylene. Nat. Commun. 5, 3688 (2014).This work shows the use of a metal-free catalyst (nitrogen-doped carbon) to catalyse acetylene hydrochlorination as a potential substitute for mercury.

11. Gao, Y. J. et al. Nitrogen-doped sp2-hybridized carbon as a superior catalyst for selective oxidation. Angew. Chem. Int. Ed. 52, 2109–2113 (2013).

12. Zhang, J., Qu, L., Shi, G., Liu, J., Chen, J. & Dai, L. N,P-codoped carbon networks as efficient metal-free bifunctional catalysts for oxygen reduction and hydrogen evolution reactions. Angew. Chem. Int. Ed. 55, 2230–2234 (2016).

13. Dai, L., Xue, Y., Qu, L., Choi, H. J. & Baek, J. B. Metal-free catalysts for oxygen reduction reaction. Chem. Rev. 115, 4823–4892 (2015).

14. Qu, L. T., Liu, Y., Baek, J. B. & Dai, L. Nitrogen-doped graphene as efficient metal-free electrocatalyst for oxygen reduction in fuel cells. ACS Nano 4, 1321–1326 (2010).

15. Yang, L. J. et al. Boron-doped carbon nanotubes as metal-free electrocatalysts for the oxygen reduction reaction. Angew. Chem. Int. Ed. 50, 7132–7135 (2011).The first carbon-based metal-free catalyst with an electron-deficient dopant (boron-doped CNT).

16. Jeon, I. Y. et al. Edge-selectively sulfurized graphene nanoplatelets as efficient metal-free electrocatalysts for oxygen reduction reaction: the electron spin effect. Adv. Mater. 25, 6138–6145 (2013).

17. Liu, Z. W., Peng, F., Wang, H. J., Yu, H., Zheng, W. X. & Yang, J. A. Phosphorus-doped graphite layers with high electrocatalytic activity for the O2 reduction in an alkaline medium. Angew. Chem. Int. Ed. 50, 3257–3261 (2011).

18. Yao, Z., Nie, H. G., Yang, Z., Zhou, X. M., Liu, Z. & Huang, S. M. Catalyst-free synthesis of iodine-doped graphene via a facile thermal annealing process and its use for electrocatalytic oxygen reduction in an alkaline medium. Chem. Comm. 48, 1027–1029 (2012).

19. Jeon, I. Y. et al. Facile, scalable synthesis of edge-halogenated graphene nanoplatelets as efficient metal-free eletrocatalysts for oxygen reduction reaction. Sci. Rep. 3, 1810 (2013).

20. Wang, S., Iyyamperumal, E., Roy, A., Xue, Y., Yu, D. & Dai, L. Vertically aligned BCN nanotubes as efficient metal-free electrocatalysts for the oxygen reduction reaction: a synergetic effect by co-doping with boron and nitrogen. Angew. Chem. Int. Ed. 50, 11756–11760 (2011).The first paper to show the co-doping effect to enhance the metal-free catalytic activities of carbon-based catalysts.

21. Wang, S. Y. et al. BCN graphene as efficient metal-free electrocatalyst for the oxygen reduction reaction. Angew. Chem. Int. Ed. 51, 4209–4212 (2012).

22. Zheng, Y. et al. Can boron and nitrogen co-doping improve oxygen reduction reaction activity of carbon nanotubes? J. Am. Chem. Soc. 135, 1201–1204 (2013).

23. Yu, D. S. & Xue, Y. H. & Dai, L. Vertically aligned carbon nanotube arrays co-doped with phosphorus and nitrogen as efficient metal-free electrocatalysts for oxygen reduction. J. Phys. Chem. Lett. 3, 2863–2870 (2012).

24. Shi, Q. Q. et al. Sulfur and nitrogen co-doped carbon nanotubes for enhancing electrochemical oxygen reduction activity in acidic and alkaline media. J. Mater. Chem. A 1, 14853–14857 (2013).

25. Liu, R. L., Wu, D. Q., Feng, X. L. & Mullen, K. Nitrogen-doped ordered mesoporous graphitic arrays with high electrocatalytic activity for oxygen reduction. Angew. Chem. Int. Ed. 49, 2565–2569 (2010).

26. Liu, Z.-W. et al. Phosphorus-doped graphite layers with high electrocatalytic activity for the O2 reduction in an alkaline Medium. Angew. Chem. Int. Ed. 50, 3257–3261 (2011).

27. Sun, Y. et al. Chemically converted graphene as substrate for immobilizing and enhancing the activity of a polymer catalyst. Chem. Commun. 46, 4740–4742 (2010).

28. Zheng, Y. et al. Nanoporous graphitic-C3N4@carbon metal-free electrocatalysts for highly efficient oxygen reduction. J. Am. Chem. Soc. 133, 20116–20119 (2011).

29. Wang, S. Y., Yu, D. S., Dai, L. M., Chang, D. W. & Baek, J. B. Polyelectrolyte-functionalized graphene as metal-free electrocatalysts for oxygen reduction. ACS Nano 5, 6202–6209 (2011).

30. Wang, S. Y., Yu, D. S. & Dai, L. M. Polyelectrolyte functionalized carbon nanotubes as efficient metal-free electrocatalysts for oxygen reduction. J. Am. Chem. Soc. 133, 5182–5185 (2011).

31. Jiang, Y. F. et al. Significant contribution of intrinsic carbon defects to oxygen reduction activity. ACS Catal. 5, 6707–6712 (2015).The first experimental and theoretical combined study on the ORR activities induced by intrinsic carbon defects.

32. Jin, H. et al. Graphene quantum dots supported by graphene nanoribbons with ultrahigh electrocatalytic performance for oxygen reduction. J. Am. Chem. Soc. 137, 7588–7591 (2015).

33. Zheng, Y. et al. Hydrogen evolution by a metal-free electrocatalyst. Nat. Commun. 5, 3783 (2014).The first metal-free catalyst (nitrogen-doped carbon) for the HER.

34. Burke, M. S., Enman, L. J., Batchellor, A. S., Zou, S. & Boettcher, S. W. Oxygen evolution reaction electrocatalysis on transition metal oxides and (oxy)hydroxides: activity trends and design principles. Chem. Mater. 27, 7549–7558 (2015).

35. Safizadeh, F., Ghali, E. & Houlachi, G. Electrocatalysis developments for hydrogen evolution reaction in alkaline solutions — A Review. Int. J. Hydrogen Energy 40, 256–274 (2015).

36. Ledendecker, M. et al. The synthesis of nanostructured Ni5P4 films and their use as a non-noble bifunctional electrocatalyst for full water splitting. Angew. Chem. Int. Ed. 54, 12361–12365 (2015).

37. Duan, J., Chen, S., Jaroniec, M. & Qiao, S. Heteroatom-doped graphene-based materials for energy-relevant electrocatalytic processes. ACS Catal. 5, 5207–5234 (2015).

38. Huang, X., Zhao, Y., Ao, Z. & Wang, G. Micelle-template synthesis of nitrogen-doped mesoporous graphene as an efficient metal-free electrocatalyst for hydrogen production. Sci. Rep. 4, 7557 (2014).

39. Lin, Z., Waller, G. H., Liu, Y., Liu, M. & Wong, C. P. Simple preparation of nanoporous few-layer nitrogen-doped graphene for use as an efficient electrocatalyst for oxygen reduction and oxygen evolution reactions. Carbon 53, 130–136 (2013).

40. Wang, L., Yin, F. & Yao, C. N-Doped graphene as a bifunctional electrocatalyst for oxygen reduction and oxygen evolution reactions in an alkaline electrolyte. Int. J. Hydrogen Energy 39, 15913–15919 (2014).

41. Sathe, B. R., Zou, X. & Asefa, T. Metal-free B-doped graphene with efficient electrocatalytic activity for hydrogen evolution reaction. Catal. Sci. Technol. 4, 2023–2030 (2014).

42. Cheng, N. et al. Acidically oxidized carbon cloth: a novel metal-free oxygen evolution electrode with high catalytic activity. Chem. Commun. 51, 1616–1619 (2015).

43. Ito, Y., Cong, W., Fujita, T., Tang, Z. & Chen, M. High catalytic activity of nitrogen and sulfur co-doped nanoporous graphene in the hydrogen evolution reaction. Angew. Chem. Int. Ed. 54, 2131–2136 (2015).

44. Gong, X., Liu, S., Ouyang, C., Strasser, P. & Yang, R. Nitrogen- and phosphorus-doped biocarbon with enhanced electrocatalytic activity for oxygen reduction. ACS Catal. 5, 920–927 (2015).

45. Jeon, I. Y. et al. Edge-carboxylated graphene nanosheets via ball milling. Proc. Natl Acad. Sci. USA 109, 5588–5593 (2012).

46. Ju, M. J. et al. Edge-carboxylated graphene nanoplatelets as oxygen-rich metal-free cathodes for organic dye-sensitized solar cells. Energy Environ. Sci. 7, 1044–1052 (2014).

47. Masa, J., Xia, W., Muhler, M. & Schuhmann, W. On the role of metals in nitrogen-doped carbon electrocatalysts for oxygen reduction. Angew. Chem. Int. Ed. 54, 10102–10120 (2015).

48. Shen, A. et al. Oxygen reduction reaction in a droplet on graphite: direct evidence that the edge is more active than the basal plane. Angew. Chem. Int. Ed. 53, 10804–10808 (2014).

49. Su, D. S. et al. Metal-free heterogeneous catalysis for sustainable chemistry. ChemSusChem. 3, 169–180 (2010).

50. Lai, L. et al. Exploration of the active center structure of nitrogen-doped graphene-based catalysts for oxygen reduction reaction. Energy Environ. Sci. 5, 7936–7942 (2012).

51. Zhang, L. & Xia, Z. Mechanisms of oxygen reduction reaction on nitrogen-doped graphene for fuel cells. Langmuir 115, 11170–11176 (2011).The first paper to show the doping-induced spin redistribution as the driving force for metal-free catalytic activities of carbon-based catalysts.

52. Yu, L., Pan, X., Cao, X., Hu, P. & Bao, X. Oxygen reduction reaction mechanism on nitrogen-doped graphene: a density functional theory study. J. Catal. 282, 183–190 (2011).

53. Masa, J. et al. Trace metal residues promote the activity of supposedly metal-free nitrogen-modified carbon catalysts for the oxygen reduction reaction. Electrochem. Commun. 34, 113–116 (2013).

54. Zhang, J. T. & Dai, L. Heteroatom-doped graphitic carbon catalysts for efficient electrocatalysis of oxygen reduction reaction. ACS Catal. 5, 7244–7253 (2015).

55. Guo, D. H. et al. Active sites of nitrogen-doped carbon materials for oxygen reduction reaction clarified using model catalysts. Science 351, 361–365 (2016).Detailed experimental elucidation of the ORR mechanism by metal-free nitrogen-doped carbon catalysts.

56. Wang, S., Yu, D. & Dai, L. Polyelectrolyte functionalized carbon nanotubes as efficient metal-free electrocatalysts for oxygen reduction. J. Am. Chem. Soc. 133, 5182–5185 (2011).

57. Mom, R. V., Cheng, J., Koper, M. T. M. & Sprik, M. Modeling the oxygen evolution reaction on metal oxides: the infuence of unrestricted DFT calculations. J. Phys. Chem. C 118, 4095–4102 (2014).

58. Li, M., Zhang, L., Xu, Q., Niu, J. & Xia, Z. N-Doped graphene as catalysts for oxygen reduction and oxygen evolution reactions: theoretical considerations. J. Catal. 314, 66–72 (2014).

59. Lu, X., Yim, W. L., Suryanto, B. H. & Zhao, C. Electrocatalytic oxygen evolution at surface-oxidized multiwall carbon nanotubes. J. Am. Chem. Soc. 137, 2901–2907 (2015).

60. Zheng, Y., Jiao, Y., Jaroniec, M. & Qiao, S. Z. Advancing the electrochemistry of the hydrogen-evolution reaction through combining experiment and theory. Angew. Chem. Int. Ed. 54, 52–65 (2015).

61. Zhang, S. et al. Polyethylenimine-enhanced electrocatalytic reduction of CO2 to formate at nitrogen-doped carbon nanomaterials. J. Am. Chem. Soc.136, 7845–7848 (2014).

62. Wu, J. J. et al. Achieving highly efficient, selective, and stable CO2 reduction on nitrogen-doped carbon nanotubes. ACS Nano 9, 5364–5371 (2015).

63. Paraknowitsch, J. P. & Thomas, A. Doping carbons beyond nitrogen: an overview of advanced heteroatom doped carbons with boron, sulphur and phosphorus for energy applications. Energy Environ. Sci. 6, 2839–2855 (2013).

64. Xiang, Z. H., Cao, D. P., Huang, L., Shui, J. L., Wang, M. & Dai, L. Nitrogen-doped holey graphitic carbon from 2D covalent organic polymers for oxygen reduction. Adv. Mater. 26, 3315–3320 (2014).

65. Mo, Z. Y., Liao, S. J., Zheng, Y. Y. & Fu, Z. Y. Preparation of nitrogen-doped carbon nanotube arrays and their catalysis towards cathodic oxygen reduction in acidic and alkaline media. Carbon 50, 2620–2627 (2012).

66. Tian, G. L. et al. Toward full exposure of “active sites”: nanocarbon electrocatalyst with surface enriched nitrogen for superior oxygen reduction and evolution reactivity. Adv. Funct. Mater. 24, 5956–5961 (2014).

67. Sihn, S., Varshney, V., Roy, A. K. & Farmer, B. L. Prediction of 3D elastic moduli and Poisson’s ratios of pillared graphene nanostructres. Carbon 50, 603–611 (2012).

68. Yu, D. S. et al. Scalable synthesis of hierarchically structured carbon nanotube-graphene fibres for capacitive energy storage. Nat. Nanotechnol. 9, 555–562 (2014).

69. Du, F., Yu, D., Dai, L., Ganguli, S., Varshney, V. & Roy, A. K. Preparation of tunable 3D pillared carbon nanotube–graphene networks for high-performance capacitance. Chem. Mater. 23, 4810–4816 (2011).

R E V I E W S

NATURE REVIEWS | MATERIALS ADVANCE ONLINE PUBLICATION | 11

© 2016

Macmillan

Publishers

Limited,

part

of

Springer

Nature.

All

rights

reserved. ©

2016

Macmillan

Publishers

Limited,

part

of

Springer

Nature.

All

rights

reserved.

70. Xue, Y. et al. Rationally designed graphene-nanotube 3D architectures with a seamless nodal junction for efficient energy conversion and storage. Sci. Adv. 1, e1400198 (2015).

71. Chen, S et al. Nitrogen-doped carbon nanocages as efficient metal-free electrocatalysts for oxygen reduction reaction. Adv. Mater. 24, 5593–5597 (2012).

72. Ma, T. Y., Dai, S., Jaroniec, M. & Qiao, S. Z. Graphitic carbon nitride nanosheet–carbon nanotube three-dimensional porous composites as high-performance oxygen evolution electrocatalysts. Angew. Chem. Int. Ed. 53, 7281–7285 (2014).

73. Chen, S., Duan, J. J., Ran, J. R. & Qiao, S. Z. Paper-based N-doped carbon films for enhanced oxygen evolution electrocatalysis. Adv. Sci. 2, 1–2 (2015).

74. Yu, X. W., Zhang, M., Chen, J., Li, Y. R. & Shi, G. Q. Nitrogen and sulfur codoped graphite foam as a self-supported metal-free electrocatalytic electrode for water oxidation. Adv. Energy Mater. 6, 1501492 (2016).

75. Cheng, N. Y. et al. Acidically oxidized carbon cloth: a novel metal-free oxygen evolution electrode with high catalytic activity. Chem. Commun. 51, 1616–1619 (2015).

76. Lu, X. Y., Yim, W. L., Suryanto, B. H. R. & Zhao, C. Electrocatalytic oxygen evolution at surface-oxidized multiwall carbon nanotubes. J. Am. Chem. Soc. 137, 2901–2907 (2015).

77. Shui, J. L., Du, F., Xue, C. M., Li, Q. & Dai, L. Vertically aligned N-doped coral-like carbon fiber arrays as efficient air electrodes for high-performance

nonaqueous Li–O2 batteries. ACS Nano 8, 3015–3022 (2014).

78. Cui, Y. J. et al. Metal-free activation of H2O2 by g-C3N4 under visible light irradiation for the degradation of organic pollutants. Phys. Chem. Chem. Phys. 14, 1455–1462 (2012).

79. Xiang, L. et al. Platinized aligned carbon nanotube-sheathed carbon fiber microelectrodes for in vivo amperometric monitoring of oxygen. Anal. Chem. 86, 5017–5023 (2014).

80. Davis, S. J., Caldeira, K. & Matthews, H. D. Future CO2 emissions and climate change from existing energy infrastructure. Science 329, 1330–1333 (2010).

81. Wu, J. J. et al. Incorporation of nitrogen defects for efficient reduction of CO2 via two-electron pathway on three-dimensional graphene foam. Nano Lett. 16, 466–470 (2016).

82. Sharma, P. P. et al. Nitrogen-doped carbon nanotube arrays for high-efficiency electrochemical reduction of CO2: on the understanding of defects, defect density, and selectivity. Angew. Chem. Int. Ed. 54, 13701–13705 (2015).

83. Yang, H. M., Cui, X. J., Dai, X. C., Deng, Y. Q. & Shi, F. Carbon-catalysed reductive hydrogen atom transfer reactions. Nat. Commun. 6, 6478 (2015).

84. Qi, W. & Su, D. S. Metal-free carbon catalysts for oxidative dehydrogenation reactions. ACS Catal. 4, 3212–3218 (2014).

85. Long, J. L. et al. Nitrogen-doped graphene nanosheets as metal-free catalysts for aerobic selective oxidation of benzylic alcohols. ACS Catal. 2, 622–631 (2012).

86. Patel, M. A. et al. P-Doped porous carbon as metal free catalysts for selective aerobic oxidation with an unexpected mechanism. ACS Nano 10, 2305–2315 (2016).

87. Hu, J. Y. et al. Confining noble metal (Pd, Au, Pt) nanoparticles in surfactant ionic liquids: active non-mercury catalysts for hydrochlorination of acetylene. ACS Catal. 5, 6724–6731(2015).

AcknowledgementsThe 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 statementThe authors declare no competing interests.

FURTHER INFORMATIONAlkali Fuel Cell History: http://americanhistory.si.edu/fuelcells/alk/alk3.htm

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http://americanhistory.si.edu/fuelcells/alk/alk3.htmhttp://americanhistory.si.edu/fuelcells/alk/alk3.htm

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 durEarly development and recent advancesFigure 1 | Timeline showing the important developments of carbon-based 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 rMechanistic understandingTable 1 | Summary of representative carbon-based metal-free catalystsFigure 2 | Different types of nitrogen-doped carbon and the reaction mechanisms of the ORR and CO2 reduction. a | Different forms of doped nitrogen in nitrogen-functionalized carbon. b | Calculated charge-density distribution for nitrogen-doped carbon nanMolecular and structural designMultifunctional applicationsFigure 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, WileyFigure 4 | Applications of metal-free carbon catalysts. a | Charge–discharge cycling curves of a three-electrode zinc–air battery using a metal-free catalyst as the air electrode at a current density of 2 mA cm−2. b | Hydrogen evolution reaction polarizatPerspective