Carbon-based Metal-free Catalysts - Case Western Reserve University

REVIEWScan be further improved by co‑doping with other hetero atoms. Specifically, nitrogen and sulfur 43, and nitrogenand phosphorous5,44 co‑doped graphene and other graphitic carbon materials show enhanced catalytic activitiesfor 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 theORR and OER4. These metal-free bifunctional catalystsshow great potential as the air electrode in metal–airN-doped CNT metal-free ORR catalyst120092010N-doped graphenemetal-free ORR catalyst14Non-N-doped metal-freeORR catalyst (B-doped CNT)15Synergetic effect of co-doping202011Metal-free undoped CNT ORR catalystby intermolecular charge transfer30Edge-doped/functionalizedgraphene by ball milling452012N-doped graphene foams asmetal-free counter electrodesfor DSSCs7Metal-free OER catalyst (N/C)32013Metal-free CO2 reductioncatalyst (CNFs)8Electron spin density found tohave a key role in the ORR16,51Metal-free HER catalyst(g-C3N4/N-doped graphene)332014Metal-free catalystfor acidic PEM cells2Metal-free ORR and OER bifunctionalcatalysts for zinc–air batteries4Hydrochlorination of acetylenecatalysed by SiC@N–C (REF.10)Bifunctional metal-freeORR and HER122015Doping-free, defect-inducedcarbon-based ORR catalyst31,322016Pyridine N was determined tohave a key role for the ORR55O2Figure 1 Timeline showing the important developments of carbon-basedNature Reviews Materialsmetal-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, polymerelectrolyte membrane. The image of the nitrogen-doped CNT is adapted with permissionfrom REF. 1, AAAS. The image of the oxygen adsorption onto the carbon atom next to thepyridinic N is adapted with permission from REF. 55, AAAS.batteries (discussed later). More recently, 3D porouscarbon networks co‑doped with nitrogen and phosphorus were formed via a simple, template-free approach bypyrolysis of a supermolecular aggregate of self-assembledmelamine, phytic acid and graphene oxide12. This wasthe first metal-free bifunctional catalyst with high activities for both the ORR and HER, making it attractive forregenerative fuel cells.Carbon-based catalysts for other reactions. Carbonbased catalysts, including the edge-functionalized oredge-doped graphene produced by ball milling 45, havealso been demonstrated to be efficient for I /I3 andCo(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 moredetailed discussion on particular reactions is givenin subsequent sections, we summarize the importantdevelopments of carbon-based catalysts in FIG. 1. InTABLE 1, a longer but by no means exhaustive list is givenfor carbon-based catalysts with detailed information ontheir preparation and performance.Mechanistic understandingThere are several different nitrogen configurations in anitrogen-doped conjugated graphite plane13,47 (FIG. 2a). Asmentioned 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 theusual end‑on adsorption (Pauling model) at the nitrogen-free CNT surface (FIG. 2c, top part) to a side‑onadsorption (Yeager model) at the nitrogen-doped CNTelectrode (FIG. 2c, bottom part)1. The nitrogen dopinginduces charge transfer, and parallel diatomic O2 adsorption can effectively lower the ORR potential and weakenthe O–O bond, facilitating oxygen reduction at thenitrogen-doped VA‑CNT electrode.The configuration of doped nitrogen depends on thechemical environment and can affect the electronic structure of neighbouring carbon atoms, leading to differentcatalytic properties. The doped nitrogen atoms near theedge provide strong chemical reactivity with enhancedoxygen adsorption48,49 and hence high catalytic activitytowards the ORR. For the design of efficient catalysts, it isimportant to understand the correlation of nitrogen binding configurations with electrocatalytic activity. However,it is still controversial whether the pyridinic or graphiticnitrogen is mainly responsible for the active sites forthe ORR. In general, graphitic nitrogen determines thelimiting current density, whereas the pyridinic nitrogenimproves the onset potential for the ORR50. Pyridinicnitrogen can provide one p electron to the aromatic πsystem, with a lone electron pair in the plane of the carbon matrix to enhance the electron-donating capabilityof the catalyst. Thus, pyridinic nitrogen can weakenthe O–O bond via the bonding of O with N and/orthe adjacent C atom to facilitate the reduction of O2.The above mechanistic understanding gained fromexperiments is supported by recent theoretical workNATURE REVIEWS MATERIALSADVANCE ONLINE PUBLICATION ehsilbuPnallimcaM6102

REVIEWSthat revealed that N–C active centres in metal-free catalysts can directly reduce oxygen into water through afour-electron process or a less-effective two-electronpathway 51,52. However, there are still some concerns aboutthe possible contribution of metal impurities to the ORRactivity of metal-free catalysts47,53,54. Very recently, basedon studies of a highly oriented pyrolytic graphite modeland nitrogen-doped graphene nanosheet powder catalysts, it was concluded that carbon atoms next to pyridinic nitrogen are the active sites for the ORR55. An oxygenmolecule is first adsorbed at the carbon atom next to thepyridinic nitrogen (FIG. 2d), followed by proton-coupledelectron transfer to the adsorbed oxygen. This can occurvia either of two pathways. The first is a four-electronmechanism, in which a subsequent two-proton-coupled,two-electron transfer breaks the O–OH bond to form awater molecule. Next, proton-coupled electron transfercauses breakage of the OH bond to form another watermolecule. The second pathway is a [2 2]‑electron mechanism, in which the adsorbed OOH species react withanother 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 electrocatalyticactivities for the ORR.There is an increasing number of reports of efficientcarbon-based catalysts for the ORR, OER, HER3,5,6,33, aswell as ORR–OER and ORR–HER bifunctional reactions4,12, which cannot be catalysed by trace metal residues. Moreover, the observed CO‑insensitive ORRactivities of carbon-based catalysts do not arise frommetal active centres, which would have otherwise beenpoisoned by CO (REF. 1). In addition, the enhanced ORRactivity by the physical absorption of positively chargedpolyelectrolytes (for example, poly(diallyldimethylammonium chloride) (PDDA)) onto all-carbon grapheneor nanotubes56 unambiguously demonstrates that ORRactivity in carbon-based catalysts arises from eitherdoping-induced intramolecular charge transfer or inter molecular charge transfer even without doping, ratherthan 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 hasrecently rapidly increased. The mechanism of the OERon metal-free carbon catalysts is sensitive to the structure of the electrode surface57, and it has been predictedthat the armchair carbon near the nitrogen in graphenefavours the OER 58. For surface-oxidized multiwallCNTs59, oxygen-containing functional groups (C O)on the outer layer change the electronic structure of theadjacent carbon atoms, facilitating the adsorption of OERintermediates and hence the OER process.Although there is still a limited understanding ofthe 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 atlow overpotential, at which electrochemical desorptionis a rate-limiting step. By contrast, the Volmer–Tafelmechanism becomes dominant at high overpotential33.Readers that are interested in detailed HER mechanismsare referred to several recent review articles35,60, and themechanisms of CO2 reduction8,61 and the hydrochlor ination of acetylene10 by carbon-based catalysts arediscussed next.Table 1 Summary of representative carbon-based metal-free catalystsMaterialsCatalyst preparationCatalytic applicationCatalytic efficiencyRefsN‑doped VA‑CNTsPyrolysis of iron phthalocyanine in NH3ORR Pt/C1N‑doped grapheneCVD of CH4 and NH3ORR Pt/C14B‑doped CNTsCVD of benzene–TPB–ferrocene mixtureORR Pt/C15S‑doped grapheneBall-milling graphite in S8ORR Pt/C16I‑doped grapheneAnnealing graphene oxide with I2ORR Pt/C18VA‑BCNPyrolysis of melamine diborateORR Pt/C20N‑doped and orderedmesoporous graphitic arraysN,Nʹ-bis (2,6‑diisopropyphenyl)-3,4,9, 10‑perylenetetracarboxylic diimide with a templateORR Pt/C25PDDA–grapheneReduction of graphene oxide in PDDA using NaBH4ORR graphene29Carbon nanocagesMgO template and benzeneORR CNT31N-doped graphitenanomaterialsMelamine formaldehydeOER IrO23C3N4@N‑doped grapheneDicyandiamide and graphene oxideHER transition metalCarbon nanofibersPyrolysis of electrospun nanofiberCO2 reductionOverpotential (0.17 V)8VA‑CNT-carbon fibresVA‑CNT sheathed carbon fibre via CVDIn vivo monitoring ofascorbate–9SiC@N–CHeating a mixture of NH3 and CCl4 on SiCHydrochlorination ofacetyleneConversion ofacetylene (85%)10LC‑NCVDOxidation of arylalkanesYield (max) 99%1133CVD, 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.4 ADVANCE ONLINE aM6102

REVIEWSdaH e–O2H2O2Pyridinic NH 2e– processe–4e– processGraphitic NPyridinic N2H c2H H Pyrrolic Nb2e–2e–2 7N–0.181–0.2770.1710.0470.125NCNTRate –NN 72H–N –2H2OCO32–H–N2CO2 2e– H2O HCO2– HCO3–Figure 2 Different types of nitrogen-doped carbon and the reaction mechanisms of the NatureORR andCO2 reduction.Reviews Materialsa Different forms of doped nitrogen in nitrogen-functionalized carbon. b Calculated charge-density distribution fornitrogen-doped carbon nanotubes (CNTs). c Schematic representations of possible adsorption modes of an oxygenmolecule at a non-doped CNT (top) and nitrogen-doped CNT (bottom). d Schematic representation of the mechanismof the oxygen reduction reaction (ORR) on metal-free nitrogen-doped carbon catalysts. e Schematic mechanism forthe 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. Panelsb and c are adapted with permission from REF. 1, AAAS. Panel d is adapted with permission from REF. 55, AAAS. Panel eis adapted with permission from REF. 61, American Chemical Society.The reduction of CO2 to chemical fuels by carbonbased catalysis has recently emerged as a promisingresearch focus. The electrocatalytic reduction of CO2 toCO 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-electrochemicalCO desorption.CO2 was successfully reduced by a metal-free carbon- based catalyst with superior catalytic activity to that ofnoble metal catalysts8, whereby pyridinic and quaternary nitrogen were shown not to be catalytically active,because there was no change in the peak intensitiesof the corresponding nitrogen 1s peaks (measured byX‑ray photoelectron spectroscopy) before and after theelectrochemical reaction. This suggests that positivelycharged carbon atoms rather than nitrogen groups arethe active sites directly involved in CO2 reduction. Amechanism for the reduction of CO2 to CO was proposed that involves the reduction of positively chargedcarbon atoms through redox cycling, followed by re oxidizing the reduced carbon atoms to their naturallyoxidized state by the absorbed intermediate complexNATURE REVIEWS MATERIALSADVANCE ONLINE PUBLICATION ehsilbuPnallimcaM6102

REVIEWS(1‑ethyl‑3‑methylimidazolium–CO2) with the releaseof 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 usingpolyethylenimine as a co‑catalyst to greatly reduce thecatalytic overpotential and to increase the current densityand catalytic efficiency. As shown in FIG. 2e, the polyethylenimine co‑catalyst acts as a stabilizer for the reducedintermediate CO2 whilst concentrating CO2 around thenitrogen-doped CNT metal-free catalyst.A nanocomposite of nitrogen-doped carbon derivedfrom silicon carbide was recently demonstrated to directlycatalyse the non-redox hydrochlorination of acetylene10.Using model catalysts C3N4 (pyridinic and quaternarynitrogen) and polypyrrole (pyrrolic nitrogen), it wasfound that pyrrolic nitrogen has the strongest role inacetylene hydrochlorination among all nitrogen species,because the pyrrolic nitrogen associated with an electronicstate of a higher energy level and density is favourable forthe adsorption of acetylene. Both experimental and theoretical studies revealed that the catalytic activity increasesmonotonically with the increasing number of accessiblepyrrolic nitrogen sites. Thus, the active sites for non-redoxhydrochlorination of acetylene on the nitrogen-dopedcarbon nanocomposite are near to the pyrrolic nitrogenspecies. Compared with metal-free electrocatalysis, suchas the ORR, OER and HER, the development of carbonbased metal-free catalysts for non-electrochemical reactions (for example, acetylene hydrochlorination) is stillin its infancy. Further mechanistic understanding in thisimportant field is therefore necessary.Molecular and structural designInsight gained from the mechanistic studies describedabove can guide the design and development of newcarbon-based catalysts. Despite the diversity of theirmolecular architectures, CNTs and graphene possess acommon building block containing a graphitic honeycomb network, with conjugated alternating C–C singleand C C double bonds to allow for the delocalizationof π electrons. Simply replacing carbon atoms in CNTsor graphene sheets with heteroatoms (for example,nitrogen, sulfur, boron or phosphorus) that are different from carbon in electronegativity and size inducescharge redistribution over the graphitic network anddistorts the lattice structure to cause changes in bothphysical properties (for example, electronic, magneticand photonic properties) and chemical activities, leading to various new applications (for example, metal-freeelectrocatalysis)13. Thus, doping carbon nanomaterialswith 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 duringcarbon synthesis and doping during post-treatment ofpreformed carbon nanomaterials13. Nitrogen doping ofcarbon nanomaterials induces a sufficiently high positivecharge density on surrounding carbon atoms, becausenitrogen has a larger electronegativity (χ 3.07) thancarbon (χ 2.55). The charge redistribution induced bynitrogen doping facilitates the chemisorption of oxygenand electron-transfer for the ORR (REF. 1).In the case of boron doping, positively polarized boronatoms not only adsorb oxygen molecules, but also act asa bridge to transport electrons from graphitic carbon pelectrons to oxygen molecules15,63. Phosphorus doping cancreate 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 difficultthan nitrogen doping owing to the larger size of the sulfuratom. Because a sulfur atom has a similar electro negativity(χ 2.58) to that of carbon (χ 2.55), intramolecularcharge transfer induced by sulfur doping is insignificant16.Therefore, the improved ORR activity of edge-sulfurizedGnPs can be attributed to electron spin redistribution,rather than doping-induced charge transfer 16. Amongthe 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 thebest catalytic activity19. In addition to single-atom doping,co‑doping with different heteroatoms is one of the mosteffective methods to improve electrocatalytic activities ofcarbon-based catalysts, as a result of the synergistic effectsassociated with synergistic electronic interactions betweenthe different doping heteroatoms and surrounding carbon atoms4,20–24. The key principles based on the hetero atom-doping analysis described above for the molecularand structural design of carbon-based metal-free catalystsare summarized in FIG. 3 and outlined as follows. First,the location and configuration of dopants in carbonnanomaterials are important for controlling the catalyticperformance (FIG. 3a). Although it is still a challenge tosynthesize nitrogen-doped carbon nanomaterials witha single nitrogen configuration (for example, pyridinic,pyrrolic or graphitic, as shown in FIG. 2a), carbonizationof covalent organic polymers with well-defined nitrogendistributions and hole sizes could lead to nitrogen-dopedgraphitic carbon materials with tailor-made structure–property relationships for specific applications64.Therefore, the use of heter

Carbon-based metal-free catalysts Xien Liu 1 and Liming Dai 1,2 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