Review pubs.acs.org/CR
Metal-Free Catalysts for Oxygen Reduction Reaction Liming Dai,*,†,‡ Yuhua Xue,†,‡ Liangti Qu,*,§ Hyun-Jung Choi,∥ and Jong-Beom Baek*,∥ †
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, United States § Key Laboratory of Cluster Science, Ministry of Education of China, Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, Department of Chemistry, School of Science, Beijing Institute of Technology, Beijing 100081, People’s Republic of China ∥ School of Energy and Chemical Engineering/Center for Dimension-Controllable Covalent Organic Frameworks, Ulsan National Institute of Science and Technology (UNIST), 100 Banyeon, Ulsan, 689-798, South Korea 4.4. Heteroatom-Doped Carbon Nanomaterials as Metal-Free ORR Catalysts in Acids 4.5. Heteroatom-Doped Carbon Nanomaterials as Bi-/Multi-functional Metal-Free Catalysts 4.5.1. Bifunctional Metal-Free Catalysts for ORR/OER 4.5.2. Bifunctional Metal-Free Catalysts for OER/HER 4.6. Performance Evaluation for HeteroatomDoped Carbon ORR Catalysts in Fuel Cells 5. Concluding Remarks Author Information Corresponding Authors Author Contributions Notes Biographies Acknowledgments References
CONTENTS 1. Introduction 2. Carbon Nanomaterials 2.1. Fullerenes 2.2. Carbon Nanotubes 2.3. Graphene 2.4. Nanostructured Graphite and Nanodiamond 2.5. Heteroatom-Doped Carbon Nanomaterials 2.5.1. Nitrogen-Doped Fullerenes, CNTs, Graphene, and Graphite 2.5.2. Carbon Nanomaterials Doped with Heteroatoms Other than Nitrogen 2.5.3. BCN Nanotubes and Graphene 3. Oxygen Reduction Reaction (ORR) 3.1. Two-Electron and Four-Electron ORR Processes 3.1.1. Metal-Based Catalysts for ORR 3.1.2. Metal-Based Catalysts Supported by Carbon Nanomaterials for ORR 3.1.3. Metal−Nitrogen−Carbon (M−N−C) Nonprecious Metal Catalysts 4. Metal-Free ORR Catalysts 4.1. Intramolecular Charge Transfer 4.1.1. Carbon Nanotubes as Metal-Free Catalysts 4.1.2. Graphene as Metal-Free Catalysts 4.1.3. Graphite as Metal-Free Catalysts 4.1.4. Carbon-Nitride-Based Materials as MetalFree Catalysts 4.1.5. Three-Dimensional (3D) Carbon Nanomaterials as Metal-Free Catalysts 4.2. Intermolecular Charge Transfer 4.3. Spin Redistribution © 2015 American Chemical Society
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1. INTRODUCTION The rising global energy demand and environmental impact of traditional energy resources pose serious challenges to human health, energy security, and environmental protection.1−3 One promising solution is fuel cell technology, which provides clean and sustainable power. Fuel cells directly generate electricity by electrochemically reducing oxygen and oxidizing fuel into water as the only byproduct. This energy conversion technology currently receives intensive research and development focus because of its high energy conversion efficiency, virtually no pollution, and potential large-scale applications.4,5 However, fuel cells require a catalyst for oxygen reduction reaction (ORR). Traditionally, platinum (Pt) has been regarded as the best ORR catalyst. Unfortunately, limited resources and the high cost of Pt have also made the Pt-based catalysts the primary barrier to commercial mass market of fuel cells.6 While facing a prohibitively high cost, the Pt-based electrode also suffers other problems, including its susceptibility to time-dependent drift and CO deactivation.7,8 Thus, efforts are needed to identify alternative catalysts that are readily available, cost effective, and show comparableor even bettercatalytic effects than Pt for cathodic ORR in fuel cells.
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resources, these new classes of metal-free, carbon-based catalysts could enable significant cost reduction while maintaining high efficiency with economic viability for applications in fuel cells and other energy devices (e.g., metal−air batteries). Therefore, the metal-free ORR catalytic technology is having a large impact on the fuel cell field and the energy community, which has led to a huge amount of literature, and the number of publications is still rapidly increasing every year. A timely review on a so rapidly growing field of such significance is highly desirable. The aim of this article is to provide a comprehensive review of the topic by summarizing all the important developments in this emerging field of active research on carbon-based metal-free catalysts for ORR and presenting critical issues, challenges, and perspectives. Through such a critical and comprehensive review, our understanding of metal-free catalytic materials and the associated catalytic processes (e.g., ORR, OER, HER) will significantly increase as will our ability to control and even tailor the characteristics of metal-free catalysts for energy-related electrocatalysis. In what follows, we first provide an overview on the synthesis and heteroatom doping of carbon nanomaterials. Then, we illustrate the importance of ORR to fuel cells and summarize recent progress in the development of metal-based ORR catalysts. This is followed by a comprehensive review of the design and development of metal-free ORR catalysts based on intramolecular and intermolecular charge-spin-redistribution in various carbon nanomaterials, including carbon nanotubes (CNTs), graphene, graphite, carbon nitride, and 3D carbon architectures. Finally, recent progress in the development of carbon-based metal-free bi-/multi-functional catalysts for ORR/ OER/HER and single fuel cell performance evaluation are discussed, along with remarks on challenges and perspectives in this rapidly developing field.
The search for efficient alternatives to replace/reduce Pt for electrochemical reduction of oxygen in fuel cells began in the 1960s and has been growing intensively. Tremendous progress has been made to develop inexpensive nonprecious metal (NPM) ORR catalysts,9−14 and a few recently reported NPM catalysts showed comparable or even better ORR catalytic activities than that of Pt.15,16 Even though the amount of Pt needed for the desired catalytic effect could be reduced by using Pt alloys, commercial mass production would still require large amounts of Pt. Besides, the catalytic performance of many nonprecious metal catalysts still needs to be further improved to meet the requirement for practical applications. Since the focus of this review is not metal-based catalysts, there is no intention for a comprehensive literature survey of the noble and nonprecious metal catalysts, though reference is also made to some of them as appropriate. Therefore, there will be no doubt that the examples to be presented in this paper do not exhaust all significant work on Pt-based and nonprecious metal catalysts. Readers who are interested in the noble and nonprecious metal ORR catalysts are referred to several excellent review articles published recently.10−12,216,217 Along with the intensive research efforts in developing nonprecious metal ORR catalysts, a new class of metal-free ORR catalysts based on carbon nanomaterials has been recently discovered, which, as alternative ORR catalysts, could dramatically reduce the cost and increase the efficiency of fuel cells.8 In particular, it was found that vertically aligned nitrogen-doped carbon nanotube (VA-NCNT) arrays can act as a metal-free electrode to catalyze an ORR process with a 3-times higher electrocatalytic activity and better long-term operational stability than that of commercially available platinum/C electrodes (e.g., C2−20, 20% platinum on Vulcan XC-72R; E-TEK) in alkaline fuel cells.8 They are free from the CO poisoning and methanol cross-over effects. Quantum mechanics calculations and subsequent experimental observations attributed the improved catalytic performance to the N doping, which creates a net positive charge on adjacent carbon atoms in the nanotube carbon plane of VA-NCNTs to change the O2 chemisorption mode and to readily attract electrons from the anode for facilitating the ORR.8 Recent research activities have not only confirmed the above findings but also further proved that the important role of doping-induced charge transfer has a large impact on the design/ development of new metal-free catalytic materials, including various heteroatom-doped CNTs,8,17 graphene,18−20 and graphite,21,22 with a good methanol and CO tolerance and excellent durability for fuel cell and many other applications.23−25 These new findings proved to be sufficiently important to trigger worldwide attention. More recently, it was further found that spin redistribution induced by heteroatom-doping of carbon nanomaterials can also impart high ORR electrocatalytic activities and that carbon nanomaterials can even act as bi/ multifunctional catalysis for ORR, oxygen evolution reaction (OER), and/or hydrogen evolution reaction (HER).26 In addition, three-dimensional (3D) carbon nanostructures were shown to offer additional advantages for metal-free electrolysis.27,28 Metal-free ORR catalysts based on carbon nanomaterials have also been tested in certain practical fuel cells to exhibit even a higher efficiency and longer stability than Pt-based catalysts.29−31 Very recently, carbon-based metal-free ORR catalysts have been demonstrated to show an excellent operational stability and high energy efficiency even in acidic polymer electrolyte membrane fuel cells (PEMFCs)the mainstream fuel cell technology.31a With abundant carbon
2. CARBON NANOMATERIALS Carbon has long been known to exist in three forms, namely, amorphous carbon, graphite, and diamond (Figure 1).32 Depending on how the carbon atoms are arranged, their properties vary. For example, graphite (Figure 1a) is a soft, black, and stable common form of carbon with strong covalent bonding in the carbon plane and the much weaker van der Waals interaction in the transverse direction between the layers. Diamond (Figure 1b) is hard and transparent with each carbon atom bonding to four other carbon atoms in a regular repetitive pattern. It is the Noble-Prize-winning discovery of buckminsterfullerene C60 (Figure 1c)32 that has created an entirely new branch of carbon chemistry.33,34 The subsequent discovery of CNTs (Figure 1d)35 opened up a new era in materials science and nanotechnology.36,37 These elongated nanotubes consist of carbon hexagons arranged in a concentric manner with both ends normally capped by fullerene-like structures containing pentagons (Figure 1d). Graphene, the two-dimensional (2D) single-atom-thick carbon nanosheet (Figure 1e), has recently emerged as a new class of promising carbon nanomaterials.38−43 More recently, hierarchically structured 3D carbon nanomaterials, such as the 3D pillared CNT-graphene architecture (Figure 1f), have attracted considerable attention.44−47 Having a conjugated all-carbon structure with unusual molecular symmetries, fullerenes, CNTs, graphene, and their 3D hybrids show interesting electronic, photonic, magnetic, electrocatalytic, and mechanical properties attractive for a wide range of potential applications, including energy conversion and storage. Of particular interest, carbon nanomaterials have been studied as 4824
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recognized by the 1996 Nobel Prize in Chemistry.54 However, it was not until 1990 when Kratschmer and Huffman55 reported a simple way to produce macroscopic amounts of fullerenes that scientists could finally study the physicochemical properties of the C60 carbon clusters with all of the conventional spectroscopic methods and use fullerenes as useful reagents in synthetic chemistry. Subsequently, other spherical fullerenes (collectively known as buckyballs) were synthesized with a different number of hexagonal faces. This peculiar hollow, sphere-like structure of fullerenes has made them exhibit many new intriguing properties in comparison to those of more conventional members of the carbon family. For instance, interesting photonic, electronic, superconducting, magnetic, and biomedical properties have been observed for fullerene C60 and its derivatives. These special physicochemical properties, in turn, allow for controlled structural modifications of fullerenes, leading to the formation of various advanced fullerene derivatives with appropriate properties for many potential applications.56 2.2. Carbon Nanotubes
Just as the discovery of C60 has created an entirely new branch of carbon chemistry, the subsequent discovery of CNTs by Iijima35 has opened up a new era in material science and nanotechnology.57 CNTs consist of carbon hexagons arranged in a concentric manner with both ends normally capped by fullerenelike structures containing pentagons. They usually have a diameter ranging from a few angstroms to tens of nanometers and a length of up to several centimeters and even a submeter.57−59 There are two distinct types of CNTs. The socalled single-walled carbon nanotube (i.e., SWCNT or graphene tube) is made of one layer of a graphene sheet (Figure 1d), while the multiwalled carbon nanotube (i.e., MWCNT or graphitic tube) consists of more than one layer. Their peculiar hollow geometry, coupled with a conjugated allcarbon structure, has allowed CNTs to exhibit many new intriguing electrical, mechanical, and thermal properties with respect to other members of the carbon family. In particular, CNTs can exhibit semiconducting or metallic behavior depending on their diameter and helicity of the arrangement of graphitic
Figure 1. Carbon materials including (a) graphite, (b) diamond, (c) C60, (d) carbon nanotubes, (e) graphene, and (f) 3D graphene-CNT hybrid materials. The question mark in the middle panel refers to certain types of carbons that are good for ORR but not all (vide infra).
either catalyst supports or catalysts for various applications.48−51 Although not all types of carbon materials mentioned above are good for ORR, publications on CNTs and graphene as ORR catalysts are explosively growing. 2.1. Fullerenes
In 1985, Kroto et al. discovered that fullerene C60 has a soccer ball-like structure consisting of 12 pentagons and 20 hexagons facing symmetrically (Figure 1c).32 This discovery led to an entirely new branch of chemistry,52,53 and its importance was
Figure 2. Structures of fullerene (0D), CNT (1D), graphene (2D), and 3D carbon nanomaterials, indicating that the 2D graphene is a building block for other graphitic carbon materials. 4825
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Table 1. Some Properties of Graphene and CNTs23
rings in the walls. The band gap of the semiconducting SWCNTs is as high as 0.5 eV, while the electrical conductivity of the conducting CNTs is up to 5000 S cm−1.60 The high surface area is another important property of CNTs; the theoretical surface area of SWCNTs can reach up to 1315 m2 g−1.61 Theoretical and experimental results have shown that the tensile modulus of the individual SWCNT is extremely high (up to 1 TPa), and the tensile strength is up to 180 GPa, which is much higher than the strongest steel. Owing to their unique structure and extraordinary electrical, mechanical, and optical properties, CNTs are promising for various applications,62−64 ranging from composite materials to electronic and biomedical devices. The high tensile modulus and tensile strength make CNTs an ideal nanofiller for highperformance nanocomposites.65,66 The outstanding electrical property of CNTs can be applied to electronic devices, such as field effect transistor (FET).67,68 Due to their high specific surface area, mesoporous structure, and good electrical property, CNTs can also be used as efficient catalysts or supports for catalysts69−71 for applications in energy-related areas. For example, CNTs have been used as active electrodes in supercapacitors,72,73 batteries,74 solar cells,75 and fuel cells.76 The applications of CNTs and their derivatives as electrocatalysts for oxygen reduction will be discussed in succeeding sections as appropriate.
properties fracture strength (GPa) density (g cm−3) specific surface area (m2 g−1) thermal conductivity (W m−1 K−1) electrical conductivity (S cm−1) charge mobility (cm2 V−1 s−1)
graphene 124 (modulus ≈ 1100 GPa) >1 2630
CNTs 45
∼5000
1.33 400 (for nanotube “paper”) 3000
106
5000
200 000
100 000
Although the graphene mechanically peeled up from graphite has a very high crystal structure and provides the first platform for the investigation of graphene, the yield of few-layer graphene and single-layer graphene from this method is very low. In general, graphene can be prepared via two strategies: a bottom-up approach and top-down method. The bottom-up approach involves chemical vapor deposition (CVD)93−95 and chemical synthesis.96 CVD is the most widely used bottom-up method to synthesize graphene,94,95 and large pieces of graphene sheets with a size of up to 30 in. have been produced by depositing hydrocarbon on solid substrates.97 Top-down methods have also been used to produce graphene, for example, by exfoliation of graphite into acid-oxidized graphite oxide (GO), followed by chemical reduction of GO.98−100
2.3. Graphene
Graphene, the one-atom-thick planar sheets of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice, is a recent addition to the carbon family.77 Graphene was discovered in 2004 by Geim and Novoselov, who obtained graphene sheets by using adhesive tape to repeatedly split graphite crystals into the increasingly thinner pieces until individual atomic planes were reached.77 This discovery was awarded the Nobel Prize in Physics for 2010 and led to an explosion of interest in graphene.78−84 Consequently, graphene is rapidly becoming a rising star of materials science, physics, and chemistry. As can be seen in Figure 2, graphene can be regarded as the building blocks for all other dimensional graphitic materials because a 2D graphene sheet can be wrapped up into a 0D fullerene, rolled into a 1D CNT, stacked together into the form of 3D graphite, and even other 3D carbon architectures (e.g., 3D pillared CNT-graphene hybrids).85 Graphene has many excellent properties, including an extremely large specific area (2630 m2 g−1),85,86 outstanding thermal conductivity (up to 5000 W m−1 K−1 for a single-layer graphene),87 high Young’s modulus (1.0 TPa),88 good electrical conductivity (106 S cm−1), and charge mobility (200 000 cm2 V−1 s−1).89 Moreover, graphene is also very transparent with the visible light absorption of a single-layer graphene being only ∼2.3% and therefore useful for transparent electrode.90 Table 1 lists some properties of graphene in comparison with those of CNTs. Having many similarities to CNTs in structure and property, graphene is an attractive candidate for potential uses in many areas where the CNTs have been exploited.91 Superior to CNTs in many physical and chemical properties, graphene has attracted huge interest for a host of potential applications, including fuel cells, solar cells, batteries, supercapacitors, FETs, sensors, and actuators.19,76−92 The large surface area and high conductivity of graphene make it particularly attractive for electrochemical applications.
2.4. Nanostructured Graphite and Nanodiamond
Nanostructured graphite with a specific surface area of more than 400 m2/g was produced by mechanical milling in hydrogen atmosphere.101 A large amount of hydrogen could be incorporated into the fragmented nanostructured graphite matrix during mechanical milling.102 The hydrogen content, size, and structure of nanostructured graphite can be controlled by the milling time. With an increasing milling time, the long-range ordering of graphite interlayers disappeared.101 The nanostructured graphite prepared by mechanical milling has specific defective structures with carbon dangling bonds useful for hydrogen storage.103 Using a high-accuracy volumetric measuring system at room temperature, Kajiura et al. demonstrated repeatable hydrogen adsorption and desorption for nanostructured graphite.104 Apart from the mechanical milling, nanostructured graphite has also been produced by hot-filament CVD.105 Nanodiamonds (NDs) are a relatively new class of carbon nanomaterials that have the diamond structure at a nanometer scale. Like CNTs and fullerene C60, NDs also exhibit unique optoelectronic, mechanical, thermal, and biological properties for a variety of important applications.106−113 Single crystals of cubic diamond nanoparticles of an average size of 4−5 nm were first discovered in 1963 by a group of Soviet scientists, who performed detonation of an oxygen-deficient 2,4,6-trinitrotoluene/hexogen composition in inert media without involving any additional carbon source.114 However, this discovery remained unreported and underexploited until recently for military security reasons.114,115 Although the 1963 discovery has opened a new way to prepare nanodiamond particles, aggregation remained as a serious problem to preclude the possibility of their application. A subsequent report on the use of a stirred-media milling technology to disperse detonation ND particles116 has led to novel dispersed ultrananocrystalline diamond (4−5 nm), and the interest in NDs has since grown rapidly. This finding provides 4826
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Figure 3. (a) Doping of graphitic carbon structure with heteroatoms (e.g., N, B, P). (b) Doping of graphitic carbon structure with heteroatoms (e.g., N) by post-treatment. Reprinted with permission from ref 124. Copyright 2013 American Chemical Society. (c) Periodic table and the corresponding electronegativity of elements. Adapted from ref 129.
with the doping-induced defects (Figure 3b), could further change the chemical activity of carbon nanomaterials, thus leading to many novel applications, including electocatalysis for ORR.76,123,124 2.5.1. Nitrogen-Doped Fullerenes, CNTs, Graphene, and Graphite. Nitrogen, one of the neighbor elements next to carbon in the periodic table, has an atomic size similar to that of carbon but different electron configuration. Thus, doping carbon nanomaterials with nitrogen atoms could change their electronic structures while minimizing the lattice mismatch, leading to multifunctional electronic applications.130a Nitrogen-doped fullerenes have been prepared by arc discharge of graphite electrodes,130−132 laser ablation,133 and thermal vaporization of graphite in the atmosphere of nitrogen with and without helium protection.134 Along with the N-doped fullerenes, nitrogen doping of CNTs has been studied for some years with attempts to modulate the nanotube electronic and other properties.135,136 N-doped CNTs have been prepared either by in-situ doping during CNTs synthesis or by postdoping of as-prepared CNTs in nitrogen-containing precursors (e.g., NH3).8,125,136−140 Various N-doped CNTs, in either aligned or nonaligned forms with or without template, have been synthesized by arc discharge, laser ablation, or CVD via in-situ doping67,141−143 in the presence of appropriate nitrogen-containing precursors.144−147 The CVD method is the most commonly used method for synthesis of various carbon nanomaterials with heteroatom doping, including N-doped CNTs. In this context, Tao et al. prepared the nitrogendoped MWCNTs by CVD using methane and ammonia as the carbon and nitrogen sources, respectively.148 On the other hand, Gong et al. synthesized VA-NCNTs by pyrolyzing iron phthalocyanine (FePc) on a quartz glass plate with additional ammonia gas as the nitrogen source.8 By inducing a nitrogen-rich precursor into the anode rods together with graphite and catalyst, Glerup et al. synthesized nitrogen-doped SWCNTs by an arcdischarge process.141
an effective method for producing the well-dispersed ultrananocrystalline diamond.117 Apart from their unusual mechanical properties and environmental stability, nanodiamonds possess a high dielectric constant (3.5 K), high electrical breakdown strength (787 V/μ), high operating temperature (>250 °C), and low dissipation factor of 0.05% (at 25 °C, 1 kHz). These interesting properties make nanodiamonds a promising candidate material for a variety of important applications, such as microelectromechanical systems (MEMS),118 field emission displays,106,107 electrochemical analysis,108 nanomedicine,109−111 and energy storage.112,113 2.5. Heteroatom-Doped Carbon Nanomaterials
The introduction of heteroatoms (e.g., nitrogen, boron, phosphorus) into carbon nanomaterials could cause electron modulation to tune their optoelectronic properties and/or chemical activities useful for many applications.76,119−124 Heteroatom-doped carbon nanomaterials can be prepared either by in-situ doping during the carbon nanomaterials synthesis or through post-treatment of preformed carbon nanomaterials with heteroatom-containing precursors. Postdoping of CNTs often leads to surface functionalization only without altering their bulk properties.125−128 In contrast, in-situ doping can incorporate heteroatoms into the entire structure homogeneously. Heteroatom doping is the replacement of some carbon atoms in the graphitic structure with other atoms. Figure 3a gives a simple illustration of the doping process with heteroatoms, such as B, N, P, S, and/or F, while Figure 3b shows possible graphitic structure changes induced by doping. More than 10 nonmetal elements enclosed by the red line in the periodic table of elements shown in Figure 3c could be doped into carbon nanomaterials. As the size and electronegativity of the heteroatoms are different from those of carbon atom (Figure 3c), the introduction of heteroatoms into carbon nanomaterials could cause electron modulation to change the charge distribution and electronic properties.119−122 This, together 4827
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cells,8,76,138,156 and as the counter electrode for triiodide to iodide reduction in dye-sensitized solar cells (DSSCs).157−159 Although N-doped fullerenes and N-doped CNTs have been studied for some years, the preparation of N-doped graphene (Ngraphene) is only a recent development.19,160−164 For instance, Qu and co-workers are among the first to synthesize the N-doped graphene on a nickel-coated SiO2/Si substrate by CVD of methane in the presence of ammonia gas.19 The resultant highquality nitrogen-doped graphene film can be easily transferred onto another substrate after etching the nickel layer under graphene in an aqueous solution of HCl.95 The N-graphene film thus prepared is flexible and transparent, consisting of only one to a few layers of the graphene sheets with a nitrogen content of ∼4 atom %. In addition, N-doped graphene has also been prepared by non-CVD methods. In this context, Deng et al. synthesized gram-scale N-doped graphene by reacting tetrachloromethane with lithium nitride in the solvothermal process under mild conditions (Figure 5).165,166 The nitrogen content of the resultant N-doped graphene varies from 4.5% to 16.4%. For the first time, these authors observed on a scanning tunneling microscope the detailed electronic structure perturbation induced by the incorporation of nitrogen in the graphene network.165 The resultant nitrogen-doped graphene was demonstrated to show an enhanced performance for ORR in fuel cells with respect to its pure graphene counterpart and commercial Pt on carbon black XC-72 catalyst.165 In addition, N-doped graphene nanosheets have also been synthesized via arc discharge167a and other in-situ doping methods. Recently, a series of nitrogen-doped carbon nanosheets (NDCN) with uniform and tunable mesopores have been prepared by coating polydopamine (PDA) onto an appropriate graphene/silica nanosheet as a template, followed by pyrolysis and removal of the silica template. The resultant NDCNs with well-defined mesopores have highly exposed electroactive and stable catalytic sites (graphitic and pyridinic N atoms) for high ORR performance.167b On the other hand, Baek and coworkers168 also developed a synthetic method for producing high-quality N-graphene films by reacting the pristine graphite with 4-aminobenzoic acid in polyphosphoric acid (PPA) to afford 4-aminobenzoyl-functionalized graphite, followed by thermally annealing the edge-selectively-functionalized graphite (EFG, Figure 6).168,169 The EFG thus produced is readily dispersible in N-methyl-2-pyrrolidone (NMP). Solution casting and subsequent heat treatment can lead to the formation of largearea N-graphene films. The 4-aminobenzoyl moieties (4-H2N− Ph−CO−) at the edges of EFG can act as the in-situ feedstock for carbon and nitrogen sources for simultaneous “C welding” and “N doping” (Figure 6). The resultant N-graphene has been demonstrated to be an effective metal-free catalyst.169
N-doped CNTs (NCNTs) have also been prepared by various postsynthesis methods. For instance, Nagaiah et al. synthesized N-doped CNTs by post-thermal treating oxidized CNTs with ammonia149 and used the resultant NCNTs as efficient catalysts for oxygen reduction in alkaline medium. Chan et al. prepared Ndoped CNTs by exposing CNTs under high input power nitrogen plasma.150 It was found that nitrogen plasma treatment can produce pyridinic and pyrrollic species on the CNT surface, which could act as the anchoring sites for subsequent deposition of nanoparticles.151 The proposed structure models for the bonding configuration of nitrogen doping in CNTs are shown in Figure 4. There are
Figure 4. Bonding configurations for N in CNTs: (a) pyridine-like N, (b) pyrrole-type N, (c) graphite-like or quaternary N, (d) nitrile C N, (e) −NH2, (f) interstitial N, (g) pyridinic N3 vacancy, (h) interstitial divalent N. Reprinted with permission from ref 145. Copyright 2004 American Chemical Society.
three primary candidates: pyridine-like (Figure 4a), pyrrole-like (Figure 4b), and graphitic/quaternary N (Figure 4c). Other nitrogen-vacancy complexes containing pyridinic 2-fold-coordinated nitrogen atoms neighboring a carbon vacancy (V), such as NV and N3V (Figures 4f and 4g)145 as well as nitrile CN (Figure 4d), primary amine (Figure 4e), “interstitial” divalent nitrogen atoms (Figure 4f), and interstitial N (Figure 4h), are also possible.152 The presence of nitrogen in CNTs leads to more chemically active sites, high density of defects,153 high surface areas,154,155 high conductivity, and high electrochemical activity. Owing to the peculiar property changes induced by doping, N-doped CNTs are attractive for a wide range of applications, including their use as metal-free reduction catalysts for ORR in fuel
Figure 5. Possible mechanism for the solvothermal reaction between CCl4 and Li3N to produce N-doped graphene, where the gray balls represent C atoms, blue for N, green for Cl, and purple for Li. Reprinted with permission from ref 165. Copyright 2011 American Chemical Society. 4828
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Figure 6. Functionalization of pristine graphite with 4-aminobenzoic acid to produce EFG with 4-aminobenzoyl groups and subsequent heat treatment to prepare nitrogen-doped graphene (N-graphene). Reprinted with permission from ref 169. Copyright 2011 American Chemical Society.
Figure 7. (A) Pristine graphite, (B) dry ice (solid phase CO2), (C) edge-carboxylated graphite (ECG) prepared by ball milling for 48 h, and (D) schematic representation of physical cracking and edge carboxylation of graphite by ball milling in the presence of dry ice and protonation through subsequent exposure to air moisture. Reprinted with permission from ref 171. Copyright 2012 National Academy of Sciences.
On the other hand, Baek and co-workers170 also developed a low-cost and scalable ball-milling method to produce highquality, edge-functionalized N-graphene films. Figure 7 shows a typical process for ball milling graphite with dry ice in a planetary ball-mill machine to produce edge-selectively carboxylated graphite (ECG), which is highly dispersible in many solvents to self-exfoliate into single- or few-layer graphenes.171 By replacing dry ice with N2 gas, the same authors developed a simple, but versatile, scalable and eco-friendly approach to direct fixation of N2 at the edges of graphene nanoplatelets (GnPs),170 leading to solution-processable edge-nitrogenated graphene nanoplatelets (NGnPs) with superb catalytic performance in both dye-sensitized solar cells and fuel cells to replace conventional Pt-based catalysts for energy conversion. The ball-milling approach to edge-functionalized graphene170 has many advantages over the commonly used solution synthetic method.168 One of the salient features is that the reaction medium does not intercalate into graphite but selectively functionalizes the sp2 C−H defects at the edges of graphite, leading to minimal carbon basal plane damage. In addition to the aforementioned in-situ doping, postsynthesis thermal treatment has also been used to produce nitrogendoped graphite127 and other heteroatom-doped graphene.172,173
In this regard, Geng et al. prepared N-doped graphene with a N content in the range from 2.0% to 2.8% by thermal treatment of graphene in ammonia,174a while Li et al. synthesized N-doped graphene with a nitrogen content as high as 5% by annealing graphene oxide in ammonia gas.174b N-doped graphene has also be prepared by plasma treatment of graphene materials with other N-containing vapors.175 Furthermore, region-selective functionalization/doping can also be achieved, as exemplified by CNTs tip doped with nitrogen by treating the tips of vertically aligned carbon nanotubes (VA-CNTs) with nitrogen plasma.176 2.5.2. Carbon Nanomaterials Doped with Heteroatoms Other than Nitrogen. 2.5.2.1. Boron-Doped Carbon Nanotubes. Apart from the nitrogen doping, carbon nanomaterials doped with other heteroatoms, such as boron-doped CNTs, have also been intensively studied. In fact, boron-doped CNTs were first prepared in 1997 by an electric arc discharge between an anode made of homogeneous BC4N and a cathode made of graphite.177 The content of boron varied from 1% to 5%. Borondoped MWCNTs have also been prepared by the CVD method,178−180 template assistant method,181 and substitution reaction.182 High-quality boron-doped SWCNTs with varying amounts of boron were also produced by laser vaporization183,184 or the CVD method.179 4829
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Figure 8. (A) Phase segregations in the BCN ternary phase diagram. In this diagram, layered architectures could be built in the form of graphene (C), hBN (BN), doped graphene (BC and CN), and BCN composite. C−B and C−N binary chemical compounds are possibly formed with certain stoichiometry, such as BC-3, B-2-C, and CN-3, while BCN, BC-2-N, and BC-4-N are located in the ternary phase diagram (dashed line). All these stoichiometries have specific electronic structure and properties. Reprinted with permission from ref 191. Copyright 2012 Wiley-VCH. (B) BCN graphene models: (a) pure graphene (C100H26), (b−f) B7C87N6H26, (g−k) B12C77N11H26, and (l) B38C28N34H26: C, gray; H, white; B, pink; N, blue. Reprinted with permission from ref 211. Copyright 2012 Wiley-VCH.
Besides, phosphorus can induce local charge density through its lone pair electrons in 3p orbitals and accommodate the lone pairs of O2 to initiate ORR via its vacant 3d orbitals.21 Graphene nanomaterials doped with other heteroatoms, such as halogen, boron, sulfur, and phosphorus, have also been synthesized by ball milling,170,190,191 CVD methods,192−194 and arc discharge.167a 2.5.3. BCN Nanotubes and Graphene. Boron and nitrogen are adjacent to carbon in the periodic table (Figure 3). It is interesting to codope B and N into carbon nanomaterials.195 As shown in Figure 8A, B−C−N has a rich phase diagram with various hexagonal-phase structures because B, C, and N atoms could readily substitute with each other due to their similar atomic characteristics.196 As early as 1994, Stephan et al. synthesized CNTs doped with boron and nitrogen by applying electric arc discharge between a graphite cathode and an amorphous boron-filled graphite anode in the presence of nitrogen.197 On the other hand, Zhang et al. prepared the bamboo-shaped BCN nanotubes by pyrolysis of ethylene diamine, ferrocene, and sodium borohydride over cobalt catalyst in N2/H2 atmosphere.198 These authors demonstrated that the BCN nanotubes produced at 860 °C had a dense bamboo-shaped structure with a high yield. Unlike all-carbon CNTs, the band gap of BCN nanotubes, either in an aligned or in a nonaligned form, are independent of their diameter and chirality,199−201 but tunable by changing their chemical composition.202 This unique structure−property relationship makes BCN nanotubes attractive candidates for potential application in many fields. In particular, the chemically “tunable” band gap of BCN nanotubes, in conjunction with their superb thermal stability, provides tremendous opportunities to tune BCN nanotube electronic properties as multifunctional efficient metal-free electrocatalysts, even at elevated temperatures. Using a CVD method, Dai and co-workers synthesized vertically aligned BCN nanotubes (VA-BCNs) from a melamine diborate as both the nitrogen and the boron sources203,204 and P,N-codoped CNTs with pyridine and triphenylphosphine as the nitrogen and phosphorus source, respectively.205 The VA-BCNs contain both graphitic and pyridinic nitrogen species, showing a
Boron doping has changed both the structures and the properties of CNTs. The effects of boron doping of CNTs include enhanced graphitization177,182 and a reduced roomtemperature resistivity (from 7.4 × 10−7 to 7.7 × 10−6 Ohm m) as compared to pure CNTs (from 5.3 × 10−6 to 1.9 × 10−5 Ohm m).185 Boron doping has also been demonstrated to drastically change the semiconducting behavior of SWCNTs with an unusual abrupt resistance drop at low temperatures, which might imply a possible superconducting transition in boron-doped SWCNTs.186 The Fermi level of boron-doped CNTs was found to shift into the valence band due to an acceptor action of the intrinsic defects and substitutional boron atoms.178 Boron substitution of SWCNTs also led to new features in the optical absorption spectra, attributable to the appearance of an acceptor band at about 0.1 eV above the top of the valence band of the SWCNT. Boron doping has also caused changes in the electrochemical properties of CNTs, leading to efficient catalysts for ORR. Indeed, Yang et al. used boron-doped CNTs as metalfree catalysts for ORR and found that the electrocatalytic performance was improved progressively with increasing boron content.17 Boron-doped CNTs187 can be used not only as catalysts but also as a Pt catalyst support for ORR.188 2.5.2.2. Phosphorus-Doped Graphite. As one of the elements in group V, phosphorus (P) has the same number of valence electrons as nitrogen, and hence similar properties. Liu et al. demonstrated the synthesis of P-doped graphite layers by a pyrolysis approach using toluene and triphenylphosphine (TPP) as carbon precursor and phosphorus source, respectively.21 The P-doped graphite layers thus prepared were found to have more defects in the graphitic structure than the nonphosphorus graphite layers. The resulting P-doped graphite was then used as a metal-free catalyst for ORR. The P-doped graphite layers exhibited a high electrocatalytic activity, long-term stability, and excellent tolerance to cross-over effects of methanol for the ORR in an alkaline medium.21 Using a similar method, Liu et al. also prepared phosphorus-doped graphitic carbon nanospheres without any metal residues.189 Compared to N, phosphorus has a larger atomic size and lower electronegativity, which can introduce defect-induced active surface for O2 adsorption. 4830
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Figure 9. (A) Schematics of a fuel cell. Adapted from ref 220. (B) Oxygen reduction reaction in (a) alkaline and (b) acidic medium. Reprinted with permission from ref 76. Copyright 2010 American Chemical Society.
3. OXYGEN REDUCTION REACTION (ORR)
high specific capacitance (321.0 F/g) with an excellent rate capability and high durability when comparing with nonaligned BCN (167.3 F/g) and undoped SWCNTs (117.3 F/g).203 Compared to undoped VA-CNTs, CNTs doped with only B or N (VA-BCNT, VA-NCNT) and even commercial Pt/C electrocatalysts, VA-BCNs also show significantly improved electrocatalytic activity for ORR.204 Both the enhanced capacitance and the electrocatalytic activity are attributable to a synergetic effect of codoping of CNTs with N and B.203,204 In addition to the BCN nanotubes, Huang et al. predicted the presence of two-dimensional BCN graphitic nanostructures (BCN graphene).206 Subsequently, Panchokarla et al. prepared the boron- and nitrogen-doped graphene either by arc discharge or by transforming from nanodiamond under appropriate atmospheres.207 By reacting high-surface-area activated charcoal with a mixture of boric acid and urea at 900 °C, Raidongia et al. also obtained BCN graphene,208 while Ci et al. and Liu et al. synthesized a new form of 2D hexagonal BCN hybrid structures consisting of a patchwork of BN and C nanodomains by the CVD method using methane and ammonia borane as carbon precursors and BN source, respectively.209,210 More recently, Dai and co-workers developed a facile approach to prepare B,Ncodoped graphene with tunable compositions simply by thermal annealing of graphene oxide in the presence of boric acid and ammonia.211 Three different types of BCN graphene sheets have been prepared of different B- and N-doping levels with their corresponding molecular models shown in Figure 8B. Doping graphene with B and N atoms could open the band gap212 of graphene, leading to new structures and properties.213 First-principles calculations revealed a doping-level-dependent energy band gap, spin density, and charge density.211 BCNgraphene with a modest N- and B-doping level was demonstrated to show the best ORR electrocatalytic activity, fuel selectivity, and long-term durability, along with an excellent thermal stability and porosity. The thermal annealing of graphene oxide (GO) in the presence of boric acid under ammonia can thus provide simple but efficient and versatile approaches to low-cost mass production of BCN graphene as efficient metal-free ORR electrocatalysts for fuel cell and many other applications, including in microelectronic devices,209,210 supercapacitors,214a and batteries.214b
3.1. Two-Electron and Four-Electron ORR Processes
The molecular oxygen reduction reaction is important to many fields, such as energy conversion (e.g., fuel cells, metal−air batteries, solar cells), corrosion, and biology.215−218 For fuel cells, cathodic oxygen reduction plays an essential role in producing electricity and is a major limiting factor on the fuel cell performance.4,219 As schematically shown in Figure 9A, by pumping fuel (e.g., hydrogen gas) onto the anode, hydrogen is split into its constituent electrons and protons. While the electrons flow out of the anode to provide electrical power and end up at the cathode to reduce oxygen, the protons diffuse through the cell toward the cathode to combine with the reduced oxygen species into water. It is well known that the ORR can proceed either by a two-step two-electron pathway with formation of H2O2 (in acidic medium) or HO2− (in alkaline medium) as the intermediate specie or by a more efficient four-electron process to directly reduce O2 into H2O (in acidic medium) or OH− (in alkaline medium) followed by combining with a proton into water (Figure 9B). Therefore, fuel cells directly generate electricity by electrochemically oxidizing fuel and reducing oxygen to produce water as the only byproduct with many advantages, as mentioned earlier (section 1).4,5 For the conventional fuel cell technology, precious Pt is normally required as a catalyst for the ORR at the cathode. In aqueous solutions, the Pt catalyst can act as an active site for efficient adsorption and reduction of molecular O2 through either the two-electron or the four-electron pathway.221 While facing the prohibitively high costs, the Pt-based electrode also suffers other problems, including its susceptibility to timedependent drift, fuel cross-over effect, and CO deactivation.7,222a,b Thus, efforts are needed to identify an alternative material: one that is readily available and cost effective and shows comparable or even better catalytic activities than that of Pt for cathodic ORR in fuel cells. O2 reduction reaction usually involves both the breakage of the O−O bond and the formation of the O−H bond. Adzic and coworkers made an important discovery that platinum supported on appropriate metal substrates (e.g., Au, Fe, ..., other transition metals) could exhibit an excellent electrocatalytic activity even surpassing pure platinum, leading to significantly reduced platinum loadings and the associated cost of fuel cells.223 On this basis, various new ORR catalysts based on non-noble metals 4831
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have been successfully developed. Since the research and development of non-noble metal ORR catalysts have been extensively reviewed,10−12,217−219 only a brief overview on recent progress concerning the non-noble metal ORR catalysts is given below. 3.1.1. Metal-Based Catalysts for ORR. 3.1.1.1. Pt Catalysts. Among all of the pure metal ORR catalysts developed to date, Pt is the most widely used electrocatalyst for ORR.224 The ORR performance of the Pt catalyst depends on its crystallization, morphology, shape, and size. Markovic et al. found that the ORR activity on Pt(100) is much higher than that on Pt(111) in a H2SO4 medium due to the different adsorption rates for the sulfates to be adsorbed on these different facets.225 Therefore, it is critical to control the shape and morphology of Pt nanoparticles for ORR.226 In this context, Wang et al. synthesized monodisperse Pt nanocubes, showing a specific activity over 2 times as high as that of the commercial Pt catalyst.227 The current density for the 7 nm Pt nanocubes is four times that of 3 nm polyhedral or 5 nm cubic Pt nanoparticles, indicating a dominant effect of the Pt shape on ORR.228 3.1.1.2. Pt-Alloyed Catalysts. In the past decades, significant effort has been made to develop Pt-alloyed catalysts to improve their catalytic activity and stability with a reduced usage of Pt. Pt can be alloyed with either noble metals (e.g., Au, Pd, Ru, Ag, Rh) or transition metals (e.g., Cu, Fe, Co, Ni).229−232 For instance, Zhang et al. reported the Pt nanoparticle surface alloyed with Au to enhance the stability for ORR,233 while Lim et al. prepared Pd−Pt bimetallic alloy with high activity for oxygen reduction.234 On the other hand, Stamenkovic et al. demonstrated that the Pt3Ni(111) surface was 10-fold more active for ORR than the corresponding Pt(111) surface and 90-fold more active than the current state-of-the-art Pt/C catalysts for PEMFC.235 This is because the Pt3Ni(111) surface possesses an unusual electronic structure (d-band center position) and arrangement of surface atoms in the near-surface region.235 Stamenkovic et al. also studied the kinetics of oxygen reduction on Pt3Ni and Pt3Co alloy surfaces and found that the order of electrocatalytic activity depended strongly on the nature of anions of supporting electrolytes.236 In H2SO4 electrolyte, the activity decreased in the order Pt3Ni > Pt3Co > Pt. However, the order of activities in HClO4 (at 333 K) was Pt skin > Pt3Co > Pt3Ni > Pt. The catalytic enhancement was greater in 0.1 M HClO4 than in 0.5 M H2SO4 with the maximum enhancement of 3−4 times that for pure Pt observed for the “Pt skin” on Pt3Co in 0.1 M HClO4.236 These authors revealed a fundamental relationship in electrocatalytic trends on Pt3M (M = Ni, Co, Fe, Ti, V) surfaces between the experimentally determined surface electronic structure (the dband center) and the ORR activity.237 Recently, Pt−Cu alloy has also been demonstrated to show a higher catalytic performance for ORR than that of pure Pt catalysts with a good stability.238,239 More recently, Chen et al. modified platinum and nickel alloyed nanoparticles (i.e., crystalline PtNi3 polyhedra) by reacting the nanoparticles with oxygen to dissolve nickel in the particle’s interior to produce Pt3Ni nanoframes.16 The resulting nanoframes of a 3D 12-sided hollow structure retained the edges of the Pt-rich PtNi3 polyhedra (Figure 10). Since both the interior and the exterior catalytic surfaces of the resultant openframework structure are composed of the nanosegregated Pt skin to allow more oxygen to access the Pt atoms, the Pt3Ni nanoframe catalysts were demonstrated to show a factor of 36 enhancement in mass activity and a factor of 22 enhancement in specific activity, respectively, for ORR (with respect to the state-
Figure 10. Transmission electron microscope (TEM) images (scale bars: 50 nm) and corresponding model structures: (A) Initial solid PtNi3 polyhedra. (B) PtNi intermediates. (C) Final hollow Pt3Ni nanoframes. (D) Annealed Pt3Ni nanoframes with Pt(111)-skin−like surfaces dispersed on high-surface-area carbon. Reprinted with permission from ref 16. Copyright 2014 American Association for the Advancement of Science.
of-the-art Pt/C catalyst) with an excellent stability (negligible activity loss after 10 000 potential cycles) at a low cost.16 3.1.2. Metal-Based Catalysts Supported by Carbon Nanomaterials for ORR. 3.1.2.1. Metal/Metal Oxide Catalysts Supported by Carbon Nanomaterials. Owing to their large surface area and good electrical and mechanical properties, carbon nanomaterials, including CNTs, graphene, and graphite, are ideal supporting materials of metal-based catalysts for ORR. The use of carbon supports could significantly increase the stability and activity of metal catalysts (e.g., Pt) for ORR. For instance, Kongkanand et al. reported SWCNT-supported Pt nanoparticles with improved ORR electrocatalytic activity and stability during repeated cycling over a period of 36 h.240 Pt deposited on MWCNTs has also been shown to exhibit high ORR performance.241−243 High activity for oxygen reduction was also observed for Pt-alloy catalyst supported by CNTs,244 along with a higher methanol tolerance for a PtAu alloy supported by CNTs than pure Pt.245 Besides, Zhang et al. reported manganese dioxide supported on CNTs (MgO2/CNTs) as an improved cathodic catalyst for oxygen reduction in a microbial fuel cell.246 Just like CNTs, graphene has recently been used to support metal catalysts for high electrocatalytic performance toward ORR. For instance, Guo et al. assembled Co/CoO and FePt nanoparticles on graphene for electrochemical reduction of oxygen with enhanced performance.247a,b Besides, Choi et al. reported iron catalysts supported by graphene nanosheets for oxygen reduction reaction in PEM fuel cells.247c High stability was observed through an accelerated durability testing (ADT) protocol.247c 3.1.2.2. Metal/Metal Oxide Catalysts Supported by Heteroatom-Doped Carbon Nanomaterials. Since the introduction of heteroatom dopants (e.g., nitrogen) into the carbon nanomaterials could cause electron modulation to provide desirable electronic structures for catalytic and many other processes of practical significance,19,248 heteroatom-doped carbon nanomaterials could be ideal supports for metal catalysts. Indeed, Liang et al. synthesized CoO on the nitrogen-doped CNTs.249 The resulting CoO/nitrogen-doped CNT (NCNT) hybrid showed a 4e− oxygen reduction pathway with a high ORR current density, outperforming Co3O4/graphene hybrid and commercial Pt/C catalyst at medium overpotential.249 Wu et al. reported the use of Fe3O4 nanoparticles supported by a 3D Ndoped graphene aerogel as efficient ORR catalysts with a high 4832
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Figure 11. (a) CV curves of Co3O4/rGO hybrid, Co3O4/N-rGO hybrid, and Pt/C on glassy carbon electrodes in O2-saturated (solid line) or Arsaturated 0.1 M aqueous KOH solution (dash line). (b) Rotating-disk voltammograms of Co3O4/rGO hybrid. (c) Co3O4/N-rGO hybrid in O2saturated 0.1 M aqueous KOH solution with a sweep rate of 5 mV s−1 at the different rotation rates indicated. Insets in b and c show corresponding Koutecky−Levich plots (J−1 vs ω−0.5) at different potentials. (d) Tafel plots of Co3O4/rGO and Co3O4/N-rGO hybrids derived by the mass-transport correction of corresponding RDE data. Reprinted with permission from ref 26. Copyright 2011 Nature Publishing Group.
Figure 12. Drawings and micrographs of in-situ nitrogen-doped carbon nanostructures in M−N−C catalysts: CNTs in PANI-derived catalyst (left), onion-like carbon structure in hexamethylenediamine (HDA)-derived catalyst (middle), and graphene formed in PANI-derived catalyst (right). Reprinted with permission from ref 15. Copyright 2013 American Chemical Society.
and N-doped rGO (denoted as Co3O4/N-rGO) could be used as high-performance bifunctional catalysts for ORR and OER.26 As shown in Figure 11a, the Co3O4/N-rGO hybrid material showed a more positive ORR onset potential and higher cathodic current
current density, low ring current, low H2O2 yield, high electron transfer number (∼4), and good durability.250 On the other hand, Liang et al. reported that hybrid material consisting of Co3O4 nanocrystals grown on rGO (Co3O4/rGO) 4833
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Figure 13. (a) Scanning electron microscopy (SEM) image of the as-synthesized VA-NCNTs on a quartz substrate (scale bar, 2 μm). (b) Digital photograph of the VA-NCNT array after having been transferred onto a polystyrene-nonaligned CNT conductive nanocomposite film. (c) Calculated charge density distribution for the NCNTs. (d) Schematic representations of possible adsorption modes of an oxygen molecule at the CCNTs (top) and NCNTs (bottom). Reprinted with permission from ref 8. Copyright 2009 American Association for the Advancement of Science.
performance with excellent durability. Durable performance was observed for nearly 700 h in a H2−air fuel cell operated at a constant voltage of 0.4 V. Wu and co-workers further demonstrated that M−N−C catalysts are rich in carbon nanostructures formed in situ during catalyst synthesis, including carbon tubes, onion-like carbon, and platelets (multilayer graphene) (Figure 12).15 Transition metal was found to be indispensable to catalyze the graphitization of nitrogen−carbon precursor to form the highly graphitized carbon. While CNTs and onion-like carbons appeared when ethylene diamine and Co were used for the high-temperature synthesis, the bamboo-like tubes formed from cyanamide and Fe precursors.15 Although nitrogen species embedded within the in-situ-formed graphitized carbon nanostructures are likely critical to the active-site performance for the M−N−C catalysts, the detailed mechanism for active-site formation and its bonding character/interaction with the carbon nanostructure still remain unknown.
compared with Co3O4/rGO. From the slopes of Koutecky− Levich plots,251 the electron transfer number was determined to be ∼3.9 for Co3O4/rGO (Figure 11b) and ∼4 for Co3O4/N-rGO (Figure 11c). The superior ORR performance of Co3O4/N-rGO to Co3O4/rGO was also evidenced by its much smaller Tafel slope of 42 mV/dec (vs 54 mV/dec for Co3O4/rGO) at low overpotentials in 0.1 M aqueous KOH solution. In addition to the good ORR performance, the Co3O4/N-rGO hybrid was also tested for OER over water oxidation regime in 0.1 M aqueous KOH solution. The Co3O4/N-rGO hybrid was demonstrated to show a current density of 10 mA/cm2 at a small overpotential of ∼0.31 V with a small Tafel slope down to 67 mV/dec, comparable to the performance of the best reported OER catalyst (i.e., Co3O4 nanoparticles) at the same loading. The excellent stability of Co3O4/N-rGO hybrid was reported to be another major advantage for their use as promising candidates for ORR and other important catalytic reactions in alkaline solutions. Although graphene has been used to improve their catalytic performance, most of them are cost inefficient and have poor intermediate tolerance. Therefore, new ORR electrocatalysts that are low cost and more efficient and stable than Pt/C still need to be developed in order to commercialize the fuel cell technology. 3.1.3. Metal−Nitrogen−Carbon (M−N−C) Nonprecious Metal Catalysts. Since Jasinski observed that Co− phthalocyanine catalyzed the ORR in 1964,252a many other nonprecious metal catalysts (NPMCs), including metal oxides252b−f and metal carbides and nitrides,252g−k have been developed. By pyrolyzing carbon-supported N4-macrocyclic complexes between 550 and 1000 °C, the macrocyclic structure of the complex was destroyed to produce a class of promising NPMCs, M−Nx−C catalysts, where M is a 3d transition metal (e.g., Fe, Co, Ni) and x = 1−4.252l Apart from the pyrolysis of carbon-supported nitrogen-rich metal complexes, M−Nx−C catalysts can also be prepared by pyrolysis of a mixture of metal salts, nitrogen- and carbon-containing precursors.252m Recent studies10,15,253−255 revealed that simultaneously heat treating a transition metal, nitrogen, and carbon at 800−1000 °C could also produce promising M−N−C catalysts (M = Fe and/or Co) for ORR (Figure 12). In particular, Wu et al. prepared a new class of nonprecious metal catalysts (NPMCs) via hightemperature synthesis of Fe- and Co-based catalysts in the presence of polyaniline (PANI).253a It was found that PANI− Fe−C could catalyze the ORR in acid media at potentials within ca. 60 mV of that for the Pt/C catalyst and that a binary FeCo catalyst (PANI-FeCo−C) showed much improved ORR
4. METAL-FREE ORR CATALYSTS As can be seen above, carbon nanomaterials are ideal candidates for catalyst supports and even metal-free catalysts due to their wide availability, environmental acceptability, corrosion resistance, and unique surface and bulk properties. While activated carbon and glassy carbon have been long used as catalysts for certain chemical and electrochemical processes,256,257 the recent availability of carbon nanomaterials of various peculiar molecular structures and optoelectronic properties, including fullerenes, CNTs, graphene sheets, and graphite nanoplatelets, offers new opportunities for the development of advanced carbon-based catalysts with much improved catalytic performance.123 The introduction of surface heteroatoms (e.g., nitrogen) into these carbon nanomaterials could further cause electron modulation to provide desirable electronic structures for many catalytic processes of practical significance. Consequently, considerable effort has recently been directed toward the development of metal-free carbon nanomaterials for various catalytic processes involving either oxidation or reduction reactions.123,258,259 Here, we present a comprehensive review on progress in the development of carbon-based metal-free catalysts with emphasis on the use of CNTs and graphene for ORR. Along with the recent intensive research efforts in reducing or replacing Pt-based electrode in fuel cells, Gong et al. for the first time found that vertically aligned nitrogen-containing CNTs (i.e., VA-NCNTs, Figure 13a) could act as extremely effective metal-free ORR electrocatalysts.8 The metal-free VA-NCNTs 4834
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Figure 14. (a) i−t chronoamperometric responses obtained at the VA-NCNT/GC electrode at −0.25 V in 0.1 M KOH under magnetic stirring (1000 rpm) and Ar protection over 0−2400 s, followed by an immediate introduction of air. The arrow indicates the sequential addition of 3.0 M glucose, 3.0 M methanol, and 3.0 M formaldehyde into the air-saturated electrochemical cell. (b) Corresponding i−t chronoamperometric response obtained at the Pt−C/GC electrode with the addition of 3.0 M methanol (as indicated by the arrow) after exposure to air at 1000 s under the same conditions as in a for comparison. Reprinted with permission from ref 8. Copyright 2009 American Association for the Advancement of Science. (c) Chronoamperometric responses in the O2-saturated electrolyte for Pt/C (black) and B-CNT (dark gray) catalysts. Methanol cross-over tests by introducing 1.5 mL of methanol into the electrolyte 1200 s. Reprinted with permission from ref 187. Copyright 2011 Elsevier. (d) Chronoamperometric response for ORR at VA-BCN and Pt/C electrodes on addition of 3 M methanol after about 200 s. Reprinted with permission from ref 204. Copyright 2011 Wiley-VCH. (e) Current density (j)−time (t) chronoamperometric responses obtained at the Pt/C (circle line) and N-graphene (square line) electrodes at −0.4 V in airsaturated 0.1 M KOH. The arrow indicates the addition of 2% (w/w) methanol into the air-saturated electrochemical cell. Reprinted with permission from ref 19. Copyright 2010 American Chemical Society.
Figure 15. (a) CO poison effect on the i−t chronoamperometric response for the Pt−C/GC and VA-NCNT/GC electrodes. The arrow indicates the addition of 55 mL/min CO gas into the 550 mL/min O2 flow; the mixture gas of ∼9% CO (volume/volume) was then introduced into the electrochemical cell. i0, initial current. Reprinted with permission from ref 8. Copyright 2009 American Association for the Advancement of Science. (b) CVs for the ORR at the Pt−C/GC (top) and VA-NCNT/GC (bottom) electrodes before (solid black curves) and after (dotted curves) a continuous potentiodynamic sweep for ∼100 000 cycles in an air-saturated 0.1 M KOH at room temperature. Scan rate, 100 mV s−1. Wavelike bands from −1.0 to −0.5 V seen for the pristine Pt−C/GC electrode are attributable to hydrogen adsorption/desorption. Reprinted with permission from ref 8. Copyright 2009 American Association for the Advancement of Science.
the VA-NCNT/GC electrode remained unchanged after the sequential addition of 0.3 M methanol. However, the corresponding current−time (i−t) chronoamperometric response for a Pt−C/GC electrode given in Figure 14b shows a sharp decrease in current upon the addition of 3.0 M methanol. Compared with Pt/C electrodes, almost all metal-free carbonbased catalysts present greater electrocatalytic selectivity against the electrooxidation of various commonly used fuel molecules, including hydrogen gas, glucose, methanol, and formaldehyde, as exemplified by N-doped CNT (Figure 14a), B-doped CNT (Figure 14c),187 BCN nanotubes (Figure 14d),204 and N-doped graphene (Figure 14e).19
were shown to catalyze a four-electron ORR process (Figure 9B) free from CO “poisoning” with a much higher electrocatalytic activity and better long-term operation stability than that of commercially available Pt-based electrodes in alkaline electrolytes.8 The high surface area, good electrical and mechanical properties, and superb thermal stability of aligned CNTs provide additional benefits for the nanotube electrode to be used in fuel cells under both ambient and harsh conditions (e.g., for hightemperature use). The cross-over and poison effects seriously affect the performance of ORR catalysts. As can be seen in Figure 14a, the strong and stable amperometric response from the ORR on 4835
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Figure 16. (A) CVs for oxygen reduction at the unpurified (upper) and electrochemically purified (bottom) VA-NCNT/GC electrodes in the argonprotected (dotted curves) or air-saturated 0.1 M KOH (solid red curves) at a scan rate of 100 mV s−1. (B) RRDE voltammograms and the corresponding amperometric responses for oxygen reduction in air-saturated 0.1 M KOH at the NA-CCNT/GC (curves 1 and 1′), Pt−C/GC (curves 2 and 2′), and NA-NCNT/GC (curves 3 and 3′) electrodes at a scan rate of 10 mV s−1. The electrode rotation rate was 1400 rpm (rpm), and the Pt ring electrode was poised at 0.5 V. (C) RDE voltammograms for oxygen reduction in air-saturated 0.1 M KOH at the Pt−C/GC (curve 1), VA-CCNT/GC (curve 2), and VA-NCNT (curve 3) electrodes. Because of the technical difficulties associated with sample mounting, amperometic responses with the Pt ring electrode were not measured for the VA-CNTs. Reprinted with permission from ref 8. Copyright 2009 American Association for the Advancement of Science.
the design/development of new catalytic materials for fuel cell applications and even beyond fuel cells, as described below.
The CO poisoning of Pt catalyst is another challenge that blocks the commercialization of the Pt-based fuel cells. The metal-free heteroatom-doped carbon-based catalyst is one of the most effective materials for overcoming the problems that Pt catalysts are facing. Being nonmetallic, the NCNT electrode has been demonstrated to be insensitive to CO poisoning even after adding about 10% CO in oxygen (red line in Figure 15a),8 whereas the Pt−C/GC electrode was rapidly poisoned under the same conditions (black curve in Figure 15a). This result indicates the high CO tolerance of the VA-NCNT/GC electrode for ORR. Other doped carbon-based ORR catalysts also exhibit similar CO tolerance. The doped metal-free carbon-based catalysts also present excellent long-term stability toward ORR. For example, continuous potential cycling between +0.2 and −1.2 V for the VA-NCNT/GC electrode with Pt−C/GC as reference in airsaturated 0.1 M KOH for ∼100 000 cycles has shown that the continuous potential cycling might have caused migration/ aggregation of the Pt nanoparticles and subsequent loss of the specific catalytic activity, while the VA-NCNT/GC electrode possessed almost identical voltammetric responses before and after the continuous potential cycling under the same condition (Figure 15b). This is also the case for most of the doped metalfree carbon-based ORR catalysts. As mentioned above (section 1), the quantum mechanics calculations with B3LYP hybrid density functional theory indicated that the carbon atoms adjacent to nitrogen dopants possessed a substantially high positive charge density to counterbalance the strong electronic affinity of the nitrogen atom (Figure 13c). The high positive charge density induced by the nitrogen atom can change the chemisorption mode of O2. The oxygen adsorption mode at the pure CNT surface is usual end-on adsorption (Pauling model) (top, Figure 13d), which can be changed to a side-on adsorption (Yeager model) onto the NCNT electrode (bottom, Figure 13d). The N-induced charge transfer from adjacent carbon atoms could lower the ORR potential, while the parallel diatomic adsorption could effectively weaken the O−O bonding, facilitating ORR at the VA-NCNT electrode.8,260 Recent research activities in this exciting field have not only confirmed the above findings but also further proved that the important role of nitrogen doping has a large impact on
4.1. Intramolecular Charge Transfer
4.1.1. Carbon Nanotubes as Metal-Free Catalysts. Apart from their use as noble-metal-catalyst supports described in section 3.1.2, aligned CNTs formed by high-temperature treatment of certain metal heterocycle molecules (e.g., ferrocene/NH3) were demonstrated to show some ORR electrocatalytic activities in 2008.261 However, the observed electrocatalytic activity was attributed to the presence of FeN2− C and/or FeN4−C active sites (section 3.1.3) in the nanotube structure containing Fe residues.261 Similarly, N-doped carbon fibers (CNFs), prepared by pyrolysis of FePc, have been demonstrated to show electrocatalytic activities for ORR via a two-step two-electron pathway.262 The N-doped CNFs showed an over 100-fold increase in catalytic activity for H2 O 2 decomposition in both neutral and alkaline conditions,263 albeit still less electroactive than the Pt catalyst. The observed ORR activities were attributed to the presence of the Fe−N4/N2 active sites bound to carbon support and/or the exposed edge plane defects coupled with nitrogen doping to influence adsorption of reactive intermediates.262 Although the N doping has greatly improved the ORR activity, these noble-metal-free catalysts still exhibited poorer, or at the best comparable, ORR performance with respect to Pt catalysts and many even with a two-step, 2e− pathway. It was the discovery of the efficient electrocatalytic activity for metal-free VA-NCNTs with a 4e− ORR process (Figure 13)8 that opened an entire field of metal-free carbon electrocalaysts. Since then, numerous methodologies have been developed for preparing metal-free ORR electrocatalysts based on heteroatom-doped CNTs, graphene sheets, and/or graphite nanoparticles, as we shall see later. 4.1.1.1. Nitrogen-Doped Carbon Nanotubes. In 2009, Gong et al. discovered the high electrocatalytic activity of VA-NCNT arrays for oxygen reduction.8 As the CVD process for producing the VA-NCNTs involves metal catalysts (e.g., Fe),264,265 considerable care has been taken during electrode preparation to completely remove the catalyst residue.8 As can be seen in Figure 16A, the Fe residue has been fully removed. Unlike conventional carbon electrodes, a cathodic process with a rather high reduction potential of about −0.15 V was seen for the ORR 4836
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Figure 17. (a) Schematic illustration for the growth of NCNTs on a pure CNT fiber and the electrocatalytic activity. (b) CVs of the pure CNT and composite fibers in air-saturated 0.1 M KOH aqueous solution both with a scan rate of 100 mV s−1. Reprinted with permission from ref 269. Copyright 2011 Wiley-VCH.
the VA-CCNT/GC electrode (curve 2 in Figure 16C) with respect to its nonaligned counterpart (curve 1 in Figure 16B), albeit with a relatively small effect and at a very high overpotential. Although it is still a challenge to determine the exact locations of nitrogen atoms in the CNT structures and chemical nature of the catalytic sites, recent research activities have clearly indicated that various new metal-free catalytic materials can be developed from heteroatom-doped CNTs for fuel cells and many other applications.76 For instance, Chen et al. synthesized NCNTs by a single-step CVD process using ethylenediamine (EDA-NCNT) or pyridine (Py-NCNT) precursor.137 The resulting EDANCNT exhibited similar ORR performance as that of Pt on carbon support in terms of the onset and half-wave potentials, but superior in terms of the limiting current density, number of electrons transferred, and fuel selectivity. However, the ORR performance of Py-NCNT was inferior to that of EDA-NCNT in terms of onset and half-wave potentials, limiting current density, number of electrons transferred, and fuel selectivity. X-ray photoelectron spectroscopy (XPS) and Raman measurements revealed a higher nitrogen content and more defects for EDANCNT with respect to Py-NCNT, indicating that the higher nitrogen content and more defects could lead to the better ORR performance. These authors further prepared NCNTs with different nitrogen contents by varying the relative amount of pyridine and ethanol in the stock solution during the CVD process.266 It was found that an increase in the pyridine to ethanol ratio of the stock solution produced NCNTs with an increased nitrogen content. On the basis of the RRDE voltammetric measurements, a high nitrogen content in the resultant N-CNTs was found to be critical for ORR. While the onset and half wave potentials for ORR increased slowly with increasing nitrogen content, a higher nitrogen content was shown to have a more significant impact on the limiting current density and fuel selectivity. Apart from the NCNT metal-free ORR electrocatalysts, Tang et al. compared nitrogen-doped CNT cups (NCNCs) to commercial Pt-decorated MWCNTs (Pt-CNTs) and found that they have similar ORR activities,267 though the ORR for NCNCs was via combined two-electron and four-electron pathways, while Pt-CNTs followed a four-electron reduction process. A similar four- and two-electron combined ORR pathway, with the transferred electron number per oxygen molecule calculated to be about 2.6, was also found for metal-free NCNTs synthesized by an unusual detonation-assisted CVD (DACVD) method using melamine as a C/N precursor.268 Recently, Chen et al. designed and fabricated a core−shell composite fiber consisting of a core of aligned undoped CNTs
at both the unpurified and the electrochemically purified VANCNT/GC electrodes. Rotating ring-disk electrode (RRDE) voltammograms (Figure 16B) show the steady-state voltammograms for nitrogen-free nonaligned CNTs supported by a glassy carbon electrode (NA-CCNT/GC, curve 1), commercially available platinum-loaded carbon (Vulcan XC-72R) supported by a glassy carbon electrode (Pt−C/GC, curve 2), and glassy carbon-supported nonaligned nitrogen-containing CNTs (NANCNT/GC, curve 3) in air-saturated 0.1 M KOH electrolyte. The corresponding amperometric responses (curves 1′−3′) for the oxidation of hydrogen peroxide ions (HO2−) measured with a Pt ring electrode at a potential of 0.50 V are also included in Figure 16B. The NA-CCNT/GC electrode showed a two-step process for ORR with an onset potential of about −0.22 and −0.70 V (curve 1 in Figure 16B). The first sharp step over −0.22 V is attributed to the two-electron reduction of O2 to HO2−. Unlike the NA-CCNT/GC electrode, the NA-NCNT/GC electrode exhibited a one-step process for the ORR with a steady-state diffusion current that was almost twice that obtained at the NA-CCNT/GC electrode (curves 1 and 3 in Figure 16B). Like the Pt−C/GC electrode, the observed one-step process suggests a four-electron pathway for the ORR at the NA-NCNT/ GC electrode, as also supported by the corresponding negligible current for HO2− oxidation recorded at the Pt ring electrode (curve 3′ in Figure 16B). The transferred electron number (n) per oxygen molecule involved in the ORR was calculated from eq 1 to be 1.8 and 3.9 for the NA-CCNT/GC electrode (at the potential of −0.40 V) and the NA-NCNT/GC electrode (at the potential of −0.30 V), respectively n = 4ID/(ID + IR /N)
(1)
where N = 0.3 is the collection efficiency, ID is the faradic disk current, and IR is the faradic ring current. The half-wave potentials for ORR at the NA-NCNT/GC (curve 3, Figure 16B) and VA-NCNT/GC electrodes (curve 3 in Figure 16C) are comparable to that at the Pt−C/GC electrode (−0.1 V), but a substantially enhanced steady-state diffusion current (∼0.8 mA) was observed over a large potential range for the VA-NCNT/GC electrodes with respect to the Pt−C/GC electrode (Figure 16C). Thus, the VA-NCNT/GC electrode is much better than the Pt− C/GC electrode for the ORR in an alkaline solution. Compared with the NA-NCNT/GC electrode, the better electrocatalytic performance of the VA-NCNT/GC electrode can be attributed to its well-defined large surface area with all of the nanotube top ends falling on one plane at the interface between the aligned nanotube electrode and the electrolyte solution to further facilitate the electrolyte/reactant diffusion. Similar current enhancement by the alignment structure was also observed for 4837
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Figure 18. (A) Metal-free growth of nitrogen-doped CNTs for the ORR. (B) AFM images of the SiO2/Si surface (a) before and (b) after the waterplasma treatment. SEM images of (c) undoped and (d) N-doped CNTs grown on the water-plasma-treated SiO2/Si substrates. (e) HRTEM image, (f) XPS survey, (g) XPS C 1s, and (h) XPS N 1s spectra of the N-doped CNTs. Reprinted with permission from ref 138. Copyright 2010 American Chemical Society.
sheathed with network-like NCNTs (Figure 17a).269 Owing to the unique combination of the 3D hopping conduction associated with the aligned undoped CNT core and the electrocatalytic activity of the NCNT network shell, the core− shell composite fiber exhibited a good ORR performance. Figure 17b shows a typical CV of an undoped CNT fiber with two reduction peaks at −0.32 and −0.83 V, revealing a two-electron ORR process with the reduction peak at −0.32 V being attributed to the reduction process from O2 to HO2− and the peak at −0.83 V arising from the reduction of HO2− to OH−. Unlike the undoped CNT fibers, the composite fiber with a NCNT shell exhibited only a sharp reduction peak at a higher potential of −0.26 V, indicating a four-electron reduction pathway for the ORR. The composite NCNT fiber showed a much improved catalytic activity for ORR, as reflected by its ca. 60 mV positive shift and more than two-times increase in the current density with respect to the undoped CNT fiber (Figure 17b). More recently, Lepró et al. prepared N-doped CNT sheets by treatment of free-standing sheets of aligned MWCNTs with NH3/He plasma.156 The resultant free-standing N-doped MWCNTs sheets were further spun into the strong twisted Ndoped CNT yarns, which are knottable, wearable, and sewable,270 showing a tunable ORR catalytic activity depending on the plasma treatment time and subsequent thermal annealing conditions.
4.1.1.2. Metal-Free Synthesized Nitrogen-Doped CNTs. As mentioned above (section 2.5.1), the CVD processes for producing the NCNTs used as metal-free catalysts often involve metal residues (e.g., Fe),8 and considerable care has been taken during electrode preparation to completely remove the catalyst residue. However, possible effects of metal contaminates on the observed superb ORR performance could still be a matter of controversy271,272 unless nitrogen-doped carbon materials with excellent ORR electrocatalytic activities can be produced by a metal-free preparation procedure. In this regard, Dai and coworkers developed a simple plasma etching technology to effectively generate metal-free catalysts from efficient metal-free growth of single-walled NCNTs.138 Although the metal-free growth of CNTs reported independently by Liu et al.273 and Huang et al.274 offered an alternative approach to metal-free CNTs attractive for many existing and new applications, including ORR, the possible use of those nitrogen-free CNTs produced by the metal-free growth has been largely precluded by its low growth efficiency.273,274 In order to enhance the metalfree growth yield of CNTs, Yu et al. developed a simple but effective approach to the growth of densely packed undoped and/or nitrogen-doped SWCNTs from metal-free nanoparticles produced by water-plasma etching SiO2/Si wafers.138,275 Figure 18A shows a schematic representation of the metal-free nanotube synthesis procedure. Typically, a SiO2/Si wafer with a 30 nm thick SiO2 coating was water-plasma etched at 30 W, 250 kHz, 4838
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and 0.62 Torr for 20 min to produce SiO2 nanoparticles as the metal-free catalysts on the substrate. The plasma-etched substrate was then placed into a tube furnace for the metal-free growth of CNTs by CVD (Figure 18A). As shown in Figures 18B(a and b), atomic force microscope (AFM) imaging of the water-plasma-etched SiO2/Si substrate clearly shows the formation of homogeneously distributed catalyst particles with an average size of 600 °C), showing the insignificance of this functional group in contributing to ORR.297 Although much progress has been achieved, the exact catalytic role for each of the nitrogen forms in nanocarbon ORR catalysts is still not fully understood.19,137,277,280 In addition, it is a challenge to determine the exact locations of nitrogen atoms in the nanocarbon structures, chemical nature of the catalytic sites, and electrochemical kinetics of the N-doped nanocarbon electrodes. A combined experimental and theoretical approach would be essential. Computer simulation and calculation have proved to be a powerful technique in searching novel electrocatalysts and studying the basic science behind the electrocatalysis. Thus, theoretical calculations should be employed in studying the N-doped nanocarbon catalysts. As mentioned above (section 1), the quantum mechanics calculations with B3LYP hybrid density functional theory (Gaussian 03) have indicated that carbon atoms adjacent to nitrogen dopants possess a substantially high positive charge density to counterbalance the strong electronic affinity of the nitrogen atom (Figure 13).8 A redox cycling process reduces the carbon atoms that naturally exist in an oxidized form, followed by reoxidation of the reduced carbon atoms to their preferred oxidized state upon O2 absorption. Furthermore, the nitrogendoping-induced charge delocalization could also promote a sideon adsorption (Yeager model) of O2 onto the NCNT electrodes (bottom, Figure 13d) to effectively weaken the O−O bonding for efficient ORR. As such, doping carbon nanomaterials with 4842
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Figure 23. Electrocatalytic capabilities of the BCNT catalysts for the ORR in O2-saturated 1 M NaOH electrolyte. (a) CV curves (scan rate 50 mV s−1). (b) RDE voltammetry with a rotation speed of 2500 rpm (scan rate 10 mV s−1). For comparison, corresponding examinations for CNTs and commercial Pt/C catalysts (20 and 40 wt % Pt loading) were also carried out. Reprinted with permission from ref 17. Copyright 2011 Wiley-VCH.
catalyst (Figure 23b), the proportional relationship between ORR performance and boron content suggests the great potential of BCNTs for further improvement. 4.1.1.5. Role of Boron Doping for ORR. To understand the electrocatalytic activity of BCNTs, DFT calculations were performed on boron-doped armchair (5,5) SWCNT (i.e., BCNT(5,5)) before and after O2 adsorption, along with the geometry optimization and the subsequent natural bond orbital (NBO) analysis.17 It was found that B doping induced a transformation of the electron-deficient boron to electrondonating site by taking p electrons of the conjugated carbon network. Consequently, the 2pz orbital of the boron atom holds a fraction of lone pair electrons with an amount of 0.51e, which constitutes the main protruding lobe in the highest occupied molecular orbitals (HOMO-1) of BCNT(5,5) (Figure 24a) to act as the electron-donating site for the ORR. The protruding lobe of the spin-down HOMO-1 of BCNT(5,5) will then have maximal overlap with the lowest unoccupied molecular orbital (LUMO) of a triplet O2 (Figure 24b) to form an end-on adsorption (Figure 24c), facilitating the ORR process. As can be seen above, the doping-induced charge redistribution, regardless of whether the dopants have a higher (as N) or a lower (as B) electronegativity than that of carbon (Figure 24d), could create charged sites (C+ or B+) favorable for O2 adsorption to facilitate the ORR process. This suggests further exploration of the metal-free electrocatalysts based on CNTs doped by atoms (other than N and B) with electronegativities different from that of the carbon atom (Figure 24d).260 4.1.1.6. BCN Nanotubes Codoped with Nitrogen and Boron. Recent research activities performed on heteroatomdoped carbon nanomaterials indicate synergetic effects arising from the codoping of carbon nanomaterials with two heteroatoms, which often show a higher electrocatalytic activity for ORR than their counterparts doped only with either of the two heteroatoms.27,204,205,211,327−330 In this regard, Wang et al. reported the first metal-free ORR catalyst based on carbon nanomaterials (e.g., CNT) codoped with two heteroatoms (i.e., B and N) and elucidated possible synergetic effects of codoping on the ORR activities.204 These authors synthesized VA-CNTs codoped with B and N (i.e., VA-BCN nanotubes) by pyrolysis of melamine diborate, a single compound containing carbon, boron, and nitrogen sources required for the BCN nanotube growth. XPS was used to determine the content of C (85.5%) relative to B (4.2%) and N (10.3%) for VA-BCN. The high C content in the VA-BCN nanotubes ensures a high conductivity,
Figure 22. Minimal energy paths of oxygen molecule dissociation on (a) a pure (8,0) SWCNT, (b) a one nitrogen-atom-substituted (8,0) SWCNT, (c) a two meta-nitrogen-atom-substituted (8,0) SWCNT, (d) a two para-nitrogen-atom-substituted (8,0) SWCNT, (e) a one nitrogen-atom-substituted (8,0) SWCNT with a Stone−Wales defect, and (f) a (8,0) SWCNT with pyridine-like nitrogen atoms. Gray dots, blue dots, and red dots represent carbon, nitrogen, and oxygen atoms, respectively. Reprinted with permission from ref 319. Copyright 2012 Royal Society of Chemistry.
4.1.1.4. Boron-Doped Carbon Nanotubes. Recent research activities have demonstrated that the N-doping-induced chargetransfer mechanism for ORR catalyzed by N-doped carbon nanomaterials can be applied to the design/development of new metal-free catalytic materials for fuel cell and many other applications.76 For instance, Yang et al. recently extended the doping atoms to include boron using boron-doped CNTs (BCNTs) with a tunable boron content of 0−2.24 atom % synthesized by CVD of benzene, triphenylborane (TPB), and ferrocene precursors at different TPB concentrations.17 The resultant BCNTs were subjected to CV testing (Figure 23a). It was found that the maximum peak current, along with the steadystate diffusion current and the onset and half-wave potential, increased with increasing boron content from 2.8 (CNTs, 0 atom % B) through 3.2 (B1CNTs, 0.86 atom % B) and 3.8 (B2CNTs, 1.33 atom % B) to 8.0 mA mg−1 (B3CNTs, 2.24 atom % B). Besides, a progressive positive shift of the peak potentials with increasing boron content from −0.43 (CNTs) through −0.41(B1CNTs) and −0.38 (B2CNTs) to −0.35 V (B3CNTs) in reference to the saturated calomel electrode (SCE) was observed (Figure 23a). Furthermore, RRDE measurements revealed that the transferred electron number per oxygen molecule increased slightly from 2.2 for CNTs to 2.5 for B3CNTs, indicating a dominant two-electron ORR process. Like NCNTs, the BCNT catalysts also show an excellent stability and are free from methanol cross-over and CO poisoning. Although the performance of BCNTs is not yet as good as the commercial Pt/C 4843
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doping, doped with N only (VA-NCNT), and doped with B only (VA-BCNT). As can be seen in Figure 25, all four electrode materials (i.e., VA-CNT, VA-NCNT, VA-BCNT, and VA-BCN nanotubes) showed a substantial reduction process in the presence of oxygen, whereas no obvious response was observed under nitrogen. The onset and peak potentials of ORR on the VA-BCNT, VA-NCNT, and VA-BCN nanotube electrodes are more positive with much higher current densities than those on the VA-CNT electrode, indicating heteroatom doping with N and/or B effectively improved the ORR activity. A simple comparison between the VA-BCNT and the VA-NCNT electrodes shows that N doping is more efficient than B doping for ORR in terms of the onset/peak potential and current density. Among the four electrodes, the VA-BCN nanotube electrode is most active in terms of the onset and peak potentials as well as the current density, suggesting a synergetic effect resulted from codoping of the CNTs with N and B. Finally, the VA-BCN nanotube electrode exhibited a high diffusion current density, high positive half-wave potential, high electron transfer number (ca. 4), and high kinetic current density, outperforming the commercial Pt/C electrocatalysts for ORR in alkaline electrolyte. The VA-BCN electrode was also demonstrated to be highly stable and free from the MeOH cross-over and CO poisoning effects. The observed superior ORR performance with a good methanol and CO tolerance and excellent durability for the VABCN nanotube electrode than the commercial Pt/C electrode opens up avenues for the development of novel efficient metalfree ORR catalysts by codoping CNTs with more than one heteroatoms of electronegativities different from that of carbon atom (Figure 24d). 4.1.1.7. Synergetic Effect by Codoping with Nitrogen and Boron. Apart from the efficient metal-free ORR electrocatalysts based on VA-BCN nanotubes described above, some other Band N-codoped carbon nanomaterials (e.g., BCN graphene, vide infra) did not show good ORR performance even at higher B and N contents.211 In addition to the dopant contents, therefore, there must be an underlying factor that regulates the ORR
Figure 24. Important molecular orbitals involved in the O2 adsorption on BCNT (5, 5). (a) Spin-down HOMO-1 of BCNT (5, 5). (b) LUMO of triplet O2. (c) Spin-down HOMO-2 of O2-BCNT (5, 5). Reprinted with permission from ref 17. Copyright 2011 Wiley-VCH. (d) Electronegativity of elements increases along the Y axis, leading to electron transfer from the carbon atom, C, to the nitrogen atom, N, along the gradient. Adapted from ref 326.
which is a prerequisite for materials to be used for electrochemical applications. The VA-BCN nanotubes were then tested against other electrode materials based on VA-CNT without
Figure 25. CV curves of (a) VA-CNT, (b) VA-BCNT, (c) VA-NCNT, and (d) VA-BCN electrodes in nitrogen- and oxygen-saturated 0.1 M KOH aqueous electrolyte solutions. Scan rate was 50 mV s−1. Reprinted with permission from ref 204. Copyright 2011 Wiley-VCH. 4844
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Figure 26. (A) Schematic structure of VA-BCN. Reprinted with permission from ref 204 (TOC graph). Copyright 2011 Wiley-VCH. (B) Schematic diagrams of two types of B- and N-codoped CNTs and their ORR activities. (a) Separated B- and N-codoped CNTs with high ORR activity. (b) Bonded B−N-codoped CNTs with low ORR activity. Reprinted with permission from ref 331. Copyright 2013 American Chemical Society.
Table 2. Summary of Nitrogen-Doped Graphene as Metal-Free ORR Catalysts materials
synthetic method
nitrogen source
N-doped graphene N-doped graphene N-doped graphene N-doped graphene
CVD CVD thermal annealing thermal annealing
NH3 pyridine and julolidine NH3 NH3
N-doped graphene
thermal treatment
N-doped graphene N-doped graphene
plasma treatment hydrothermal/ solvothermal microwave microwavehydrothermal ball milling KOH activated
melamine, urea, dicyandiamide polyaniline, pyrrole, polypyrrole, ammonium nitrate, cyanamide, 5-aminotetrazole monohydrate NH3, N2 urea, ammonia solution pyrrole ammonia solution
thermal treatment
polydopamine
N-doped graphene N-doped graphene N-doped graphene N-doped holey graphene N-doped porous graphene N-doped graphene quantum dots N-doped graphene ribbon N-doped graphene N-doped mesoporous graphene
solution chemical synthesis thermal treatment thermal treatment thermal treatment
electrolyte
refs
0.1 M KOH 0.1 M KOH 0.1 M KOH TEGDME solution 0.1 M KOH
19 305 333 334
0.1 M KOH
351 310,352−355
0.1 M KOH 0.1 M KOH
356 357
0.1 M KOH
170,358 359,360
melamine, N2
311,335−350
361 0.1 M KOH
362
PANI
0.1 M KOH
363
DCDA glycine
1 M HClO4 0.1 M KOH
364 303
the vacant orbital from B, leading to unfavorable chemisorption of O2 and poor ORR performance on the codoped CNTs. However, the separated B and N codoping can greatly enhance the ORR activity of CNTs.48,331 These results have been confirmed by an independent study332 and demonstrated the importance of the spatial control of dopants to ORR performance of carbon nanomaterials codoped with more than one type of heteroatoms. 4.1.2. Graphene as Metal-Free Catalysts. As a building block for CNTs, graphene is an alternative candidate for potential use as the metal-free ORR catalyst. Indeed, N-doped graphene films produced by CVD in the presence of ammonia have been demonstrated to show a superb ORR performance similar to that of VA-NCNTs with the same nitrogen content in alkaline medium.19 The ease with which graphene materials and their heteroatom-doped derivatives can be produced by various low-cost, large-scale methods, ranging from the CVD to solution exfoliation of graphite (section 2.5), suggests considerable room for cost-effective preparation of metal-free efficient graphenebased catalysts for oxygen reduction. Table 2 lists various metalfree ORR catalysts based on nitrogen-doped graphene materials.
performance of the B- and N-codoped carbon nanomaterials. As schematically shown in Figure 26A, B and N could bond together or isolate from each other in the BCN nanotube. The isolated N and B atoms can both act as active sites for ORR through charge transfer with neighboring C atoms and hence an enhanced ORR performance for the VA-BCN nanotube electrode through synergetic effects. To systematically study the B- and N-codoping effect, Zhao et al. intentionally synthesized two kinds of B- and N-codoped CNTs with one of them being dominated by bonded B and N dopants and the other by separated B and N (Figure 26B).331 The bond B−N-doped CNTs were prepared by simultaneously doping B and N into CNTs in situ during CVD growth by using a solution mixture of triphenylborane (TPB, as B source), benzylamine (BA, as N source), and ferrocene as precursor and catalyst. However, the CNTs codoped with separated B and N dopants were produced by N doping preformed B-doped CNTs with NH3 at 400 °C.331 Both the experimental and the theoretical studies indicated distinct ORR performances for the bonded and separated B- and N-codoped CNTs. In the bonded B−N-codoped CNTs, the extra electron from N is neutralized by 4845
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Figure 27. (A) (a) Digital photoimage of a transparent N-doped graphene film floating on water after removal of the nickel layer by dissolving in an aqueous acid solution. (b, c) AFM images of the N-doped graphene film and the corresponding height analyses along the lines marked in the AFM image (c1−c3 in c). (B) TEM and Raman analyses of the N-doped graphene films. (a) Low-magnification TEM image showing a few layers of the CVD-grown N-doped graphene film on a grid. (Inset) Corresponding electron diffraction pattern. (b−d) High-magnification TEM images showing edges of the Ndoped graphene film regions consisting of (b) 2, (c) 4, and (d) ∼4−8 graphene layers. (e) Corresponding Raman spectra of the N-doped graphene films of different graphene layers on a SiO2/Si substrate. Reprinted with permission from ref 19. Copyright 2010 American Chemical Society.
Figure 28. (a) RRDE voltammograms for the ORR in air-saturated 0.1 M KOH at the C-graphene electrode (red line), Pt/C electrode (green line), and N-doped graphene electrode (blue line). Electrode rotating rate: 1000 rpm. Scan rate: 0.01 V s−1. Mass of catalysts: 7.5 μg. (b) Current density (j)−time (t) chronoamperometric responses obtained at the Pt/C (green line) and N-doped graphene (blue line) electrodes at −0.4 V in air-saturated 0.1 M KOH. The arrow indicates the addition of 2% (w/w) methanol into the air-saturated electrochemical cell. (c) Current (j)−time (t) chronoamperometric response of Pt/C (green line) and N-doped graphene (blue line) electrodes to CO. The arrow indicates the addition of 10% (v/v) CO into air-saturated 0.1 M KOH at −0.4 V; jo defines the initial current. (d) Cyclic voltammograms of N-doped graphene electrode in airsaturated 0.1 M KOH before (red curve) and after (blue curve) a continuous potentiodynamic sweep for 200 000 cycles at room temperature (25 °C). Scan rate: 0.1 V s−1. Reprinted with permission from ref 19. Copyright 2010 American Chemical Society.
30 in. can be prepared by CVD,97 though scalability and cost are often of concern for CVD. However, the ease with which the graphene structure and dopant level can be controlled made the CVD method ideal to produce heteroatom-doped graphene for electrochemical applications. In this regard, Qu et al. synthesized
4.1.2.1. Nitrogen-Doped Graphene by CVD. Among all the methods for the preparation of graphene described in section 2.5, CVD has been most widely used to produce heteroatom-doped graphene in situ during the graphene synthesis. Recent studies have shown that high-quality, large-area thin graphene films up to 4846
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Figure 29. (a) ORR currents at −0.03 V (Hg/HgO). (b) CVs on Pt/C in N2-saturated 0.1 M KOH (50 mV s−1). (c) CVs on N-doped graphene in O2saturated 0.1 M KOH (50 mV s−1). (d) i−t chronoamperometric responses at −0.3 V (Hg/HgO) in N2-saturated 0.1 M KOH on N-doped graphene and Pt/C electrodes (1600 rpm) followed by introducing O2 and CH3OH (0.3 M). ADT (accelerated degradation test): 1000 CVs (−0.3−0.3 V vs Hg/ HgO) in N2-saturated 0.1 M KOH (50 mV s−1). Reprinted with permission from ref 368. Copyright 2010 Royal Society of Chemistry.
the first N-doped graphene by CVD of methane in the presence of ammonia for ORR testing.19 The resulting N-doped graphene film is flexible and transparent (Figure 27A(a)). The AFM image given in Figure 27A(b) shows a smooth surface with ripples due to the instability of a two-dimensional single atomic plane. Figure 27A(c) reveals a layer thickness in the range of 0.9−1.1 nm, consisting of only one layer or a few layers of the graphene sheets as confirmed by Raman and TEM measurements (Figure 27B). Unlike most undoped crystalline graphene films of symmetric hexagonal diffraction patterns,365,366 electron diffraction of the N-doped graphene film (inset of Figure 27B(a)) shows a ringlike diffraction pattern with dispersed bright spots, indicating partial misorientation in the N-doped graphene film due to structure distortions caused by the intercalation of nitrogen atoms into its graphitic plans.19 XPS measurements revealed ca. 4 atom % N, consisting of both pyridine-like (∼398.3 eV) and pyrrolic (∼400.5 eV) nitrogen, which is close to that of the VANCNT.8 Electrochemical performance evaluation indicates that the pure graphene electrode without N doping showed a two-step, two-electron process for oxygen reduction (i.e., graphene in Figure 28a). Unlike the undoped graphene, the N-graphene electrode (i.e., N-doped graphene in Figure 28a) exhibited a onestep, four-electron pathway for the ORR, as is the case with NCNTs.8 The steady-state catalytic current density at the Ndoped graphene electrode was ca. 3 times higher than that of the Pt/C electrode over a large potential range (Figure 28a). The transferred electron number per oxygen molecule at the Ngraphene electrode was calculated to be 3.6−4 at potentials ranging from −0.4 to −0.8 V. These results indicate that the Ndoped graphene electrode is a promising metal-free catalyst for the ORR in an alkaline solution. The corresponding current density (j)-time (t) chronoamperometric responses for the N-
doped graphene and Pt/C electrodes are given in Figure 28b, which shows a 40% decrease in current generated at the Pt/C electrode upon the addition of 2% (w/w) methanol. However, the strong and stable amperometric response from the ORR on the N-doped graphene electrode remained unchanged after the addition of methanol and other fuels (e.g., hydrogen gas, glucose) (Figure 28b). Such high selectivity of the N-doped graphene electrode toward the ORR and remarkably good tolerance to cross-over effect is due to the much lower ORR potential than that required for oxidation of the fuel molecules.8,367 Furthermore, the N-doped graphene electrode was demonstrated to be insensitive to CO (Figure 28c), whereas the Pt/C electrode was rapidly poisoned under the same conditions, and stable over continuous potential cycling even after 200 000 continuous cycles between −1.0 and 0 V in airsaturated 0.1 M KOH (Figure 28d). These results indicate that the N-doped graphene electrode is a promising metal-free catalyst for the ORR in an alkaline solution. 4.1.2.2. Nitrogen-Doped Graphene by Plasma Treatment. Apart from the CVD-generated N-doped graphene as the ORR catalyst described above, N-doped graphene materials synthesized by other approaches, including nitrogen-plasma treatment of graphene,368 thermal treatment of graphene with ammonia,309 and solvothermal treatment of graphene with tetrachloromethane and lithium nitride,165 have also been demonstrated to show good ORR electrocatalytic activities. In particular, Shao and co-workers synthesized N-doped graphene by exposing preformed graphene sheets to nitrogen plasma.368 The XPS measurements indicated a total of 8.5 atom % N for the resultant N-doped graphene, with 26.5 atom % pyridinic N, 48.5 atom % pyrrolic N, 24 atom % quarternary/graphitic N, and 1 atom % pyridinic (N+−O−).368 The observed relatively high pyridinic and graphitic N contents could ensure high ORR electrocatalytic 4847
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Figure 30. Cyclic voltammograms in an O2-saturated and N2-saturated 0.1 M aqueous KOH solution with a scan rate of 0.1 V s−1: (a) EFG and (b) Ndoped graphene (i.e., EFG after the heat treatment). (c, d) RDE voltammograms of sample film/GC electrodes in an O2-saturated 0.1 M aqueous KOH solution with a scan rate of 0.01 V s−1: (c) at a rotation rate of 1600 rpm and (d) at different rotation rates of 100, 400, 900, 1200, and 1600 rpm of the Ndoped graphene film. Reprinted with permission from ref 169. Copyright 2011 American Chemical Society.
on the edge functionalization of graphene provides an effective means for the development of functionalized graphene materials with tailormade chemical structures and electronic/electrochemical properties.168,169 For instance, Jeon et al. prepared nitrogen-doped graphene by edge-selective functionalization of the pristine graphite with 4-aminobenzoic acid via a “direct” Friedel−Crafts acylation reaction in a polyphosphoric acid/ phosphorus pentoxide medium to produce 4-aminobenzoyl EFG (Figure 6).169 Upon solution coating the resultant EFG into a thin film and followed by heat treatment, the EFG film was converted into an N-doped graphene film with outstanding electrocatalytic activity for ORR. Figure 30a reproduces CV curves of the as-cast EFG electrode, which shows featureless voltammetric currents within the potential range from −1.0 to 0.2 V in N2-saturated aqueous KOH solution (0.1 M). Upon saturating the electrolyte solution with O2, however, the reduction of oxygen occurred at a potential of −0.15 V. Similar results were observed for the N-doped graphene film electrode but with a profoundly higher current density than that of EFG in O2-saturated aqueous KOH solution (0.1 M) (Figure 30b). The higher oxygen reduction activity for the N-doped graphene with respect to EFG is also reflected by the RDE voltammograms given in Figure 30c, which clearly shows a more pronounced oxygen reduction activity for the Ndoped graphene, similar to that of N-doped graphene prepared by CVD.19 As expected, the current density increased with increasing rotation rate (Figure 30d). The number of electrons transferred (n) was calculated from the Koutecky−Levich plots to be in the range of 3.2−3.5, which is also similar to the corresponding value for the N-doped graphene prepared by CVD19 and indicates an almost four-electron ORR process, whereas EFG shows a two-electron transfer ORR process. These results indicate that N-doped graphene prepared by thermal treatment of EFG can act as a highly efficient metal-free ORR
activities for the plasma-generated N-graphene. Indeed, the Ndoped graphene thus obtained was demonstrated to exhibit a much higher electrocatalytic activity for ORR than undoped graphene, along with a much higher durability and selectivity for oxygen reduction than the Pt/C electrode. Electrochemical performance of the plasma-induced N-doped graphene electrode was investigated by carrying out an accelerated degradation test (ADT) with potential cycling between −0.3 and 0.3 V (Hg/HgO) in alkaline fuel cells368 using a high-performance hydroxide exchange membrane.369,370 Figure 29a shows the ORR kinetic currents before and after the ADT test, indicating that nitrogen doping greatly increases the electrocatalytic activity of graphene toward ORR and that electrodes based on graphene materials are highly stable in comparison with the Pt/C, although the former exhibit a lower initial electrocatalytic activity than the later. After ADT, ORR activity of the Pt/C degraded by 85% to become even lower than that of the N-doped graphene electrode (Figure 29a), as also reflected by CVs obtained from the Pt/C before and after ADT (Figure 29b), due to the aggregation of Pt nanoparticles. In contrast, the N-doped graphene electrode in O2-saturated 0.1 M KOH showed no change in CVs before and after ADT (Figure 29c) and free from the methanol cross-over effect (Figure 29d), indicating a high stability and selectivity. The high selectivity of N-doped graphene toward ORR makes it very promising as metal-free electrocatalysts in direct methanol fuel cells. 4.1.2.3. Nitrogen-Doped Graphene from Edge Functionalization of Graphite and Subsequent Thermal Treatment. The edge sites of graphene with dangling bonds have been demonstrated to be more reactive than the basal plane of strong covalent bonding with highly delocalized π-electrons over the sp2-hybridized carbon atoms. Thus, the dangling bonds at the graphene edge can be used for covalent attachment of various chemical moieties, including N-containing groups. Recent work 4848
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Figure 31. Illustration of the nitrogen-doping process of melamine into GO layers. (1) Melamine adsorbed on the surfaces of GO at temperature < 300 °C. (2) Melamine condensed and formed carbon nitride at temperature < 600 °C. (3) Carbon nitride decomposed and doped into graphene layers at temperature > 600 °C to produce the N-doped graphene sheet. Typical CVs for ORR obtained at a bare GCE (a), graphene/GCE (b), and NG5/GCE (N% = 7.1%) (c) in O2-saturated 0.1 M KOH aqueous solution. Scan rate: 100 mV s−1. Reprinted with permission from ref 350. Copyright 2011 American Chemical Society.
graphene/GCE. This, together with a stronger reduction current for ORR indicates a higher ORR catalytic activity for the NG5/ GCE than the graphene/GCE. Besides, the electron transfer number per O2 molecule for NG5 was found to be about 3.4−3.6 at potentials ranging from −0.3 to −0.8 V, indicating an almost one-step four-electron pathway for ORR. The NG5/GCE was further found to be free from methanol cross-over effect and very stable toward ORR in 0.1 M KOH aqueous solution. Clearly, therefore, NG5 is a promising metal-free catalyst for ORR in alkaline solutions. On the other hand, Lin et al. prepared N-doped graphene by pyrolysis of GO with urea as a solid N precursor at 800 °C in an inert environment.347 The N-doped graphene thus produced was demonstrated to be an efficient metal-free catalyst for ORR via a four-electron transfer process in 0.1 M KOH. Meanwhile, Lai et al. devised two different ways to fabricate N-doped graphene by annealing either GO under ammonia or a N-containing polymer/reduced graphene oxide (RGO) composite (e.g., polyaniline/RGO, polypyrrole/RGO).306 It was found that graphitic and pyridinic N centers preferentially formed by annealing GO with ammonia, whereas annealing of polyaniline/ RGO and polypyrrole/RGO tended to generate pyridinic and pyrrolic N moieties. In this case, the limiting current density for oxygen reduction was found to be determined by the graphitic N content, while the pyridinic N content improved the ORR onset potential with the total N content playing an insignificant role in the ORR process. To gain more insights into the effects of the C−N bonding configuration on ORR, Luo et al. synthesized a single layer of graphene doped with pure pyridinic N by thermal CVD of hydrogen and ethylene on a Cu foil in the presence of ammonia.372 The atomic ratio of N and C can be modulated from 0 to 16% by adjusting the flow rate of ammonia. The ultraviolet photoemission spectroscopic investigation demonstrated that the pyridinic N efficiently changed the valence band structure of graphene by raising the density of p states near the Fermi level and lowering the work function. RDE measurements revealed a 2e− reduction mechanism for ORR on this N-doped graphene,
catalyst. Furthermore, it was demonstrated that the catalytic sites of the N-doped graphene are very stable and free from the methanol cross-over effect. 4.1.2.4. Nitrogen-Doped Graphene by Thermal Treatment of Graphene Oxide. Along with the plasma technique to produce N-doped graphene, N-doped graphene can also be prepared by thermal treatment of graphene materials with ammonia.309 In this context, Sheng et al. prepared N-doped graphene materials with nitrogen contents up to 10.1% by thermal annealing GO with melamine at 700−1000 °C in a tubular furnace.350 As schematically shown in Figure 31, the Ndoped graphene was produced by thermally annealing the mixture of Hummer’s GO99 with melamine under a flow of argon atmosphere. It was found that melamine molecules first adsorbed onto GO surfaces to condense into carbon nitride,371 and that oxygen groups in GO were then removed at high temperature to provide active sites for nitrogen atoms from decomposition of carbon nitride to be doped into the graphene framework. As a result, N-doped graphene with different nitrogen atomic percentages can be prepared by controlling the mass ratio of GO to melamine and annealing conditions (e.g., temperature, heating time). The pristine graphene was also prepared for comparison through a similar procedure shown in Figure 31 but without adding melamine into the GO sample. Figure 31 also shows CVs for oxygen reduction at a bare glassy carbon electrode (GCE), graphene/GCE, and NG5/GCE (NG5 was prepared by annealing at 800 °C for 0.5 h with a mass ratio of melamine:GO = 1:5) in O2-saturated 0.1 M KOH. As can be seen, two-step, two-electron ORR processes with onset potentials at around −0.3, −0.7 V and −0.2, −0.6 V are evident for the bare GCE and pure graphene/GCE, respectively. Along with the more positive onset potential, the much larger reduction current for ORR at the graphene/GCE (curve b) than that of the bare GCE (curve a) clearly indicates much faster electron transfer kinetics for the graphene electrode. It is important to note that the NG5/GCE showed the ORR onset potential at −0.1 V, which is about 0.1 V more positive than that of the 4849
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though doping-induced pyridinic and graphitic N species were both considered to be responsible for their ORR activity (section 4.1.1.3). Compared with the graphitic N,149,308 therefore, the pyridinic N could not be an effective promoter for ORR activity in this case. By heating graphene under high-purity ammonia mixed with Ar at 800−1000 °C, Geng et al. produced N-doped graphene309 and obtained the LSVs, before and after being treated by ammonia at various temperatures, in O2-saturated 0.1 M KOH. As can be seen in Figure 32, the N-doped graphene (900 °C)
Figure 33. (a) CVs of ORR on bare GCE (black), graphene/GCE (red), and BG/GCE (blue) in O2-saturated 0.1 M KOH aqueous solution (scan rate 100 mV s−1). (b) LSV curves of ORR at graphene/GCE (red), BG/GCE (blue), and bulk Pt disk electrode (black) in an O2-saturated 0.1 M KOH aqueous solution (scan rate 10 mV s−1). The rotation rate of RDE was 1200 rpm. Reprinted with permission from ref 194. Copyright 2012 Royal Society of Chemistry.
oxygen adsorption. Superior to the Pt-based catalyst, however, the metal-free B-graphene catalyst showed long-term stability and good CO tolerance. Similar to the reported N-doped graphene,19 therefore, the B-graphene could also be a promising candidate as a metal-free cathode catalyst for ORR in fuel cells. By thermal exfoliation of GO in BF3 at different temperatures, Wang et al. reported a scalable method to prepare B-doped graphene with tunable boron contents within a range from 23 to 590 ppm.373 The current density for ORR increased with increasing boron content. However, it was found that the reduction peak of ORR negatively shifted with increasing boron content in the B-doped graphene, indicating a lower ORR electrocatalytic activity at a higher B-doping level.373 Through a DFT study, Ferrighi et al. revealed how substitutional boron in the B-doped graphene can boost the reactivity for oxygen reduction via the formation of bulk borates covalently bound to graphene (BO3−G) under oxygen-rich conditions.374 These species are interesting intermediates from breaking the OO bond during the reduction of O2 into H2O catalyzed by B-doped graphene catalysts.374 4.1.2.6. BCN Graphene Codoped with Nitrogen and Boron. Following the successful CVD synthesis of VA-CNTs codoped with B and N (i.e., VA-BCN, vide supra) and subsequent demonstration of their superior ORR electrocatalytic activity,204 Wang et al. reported a facile approach to metal-free BCN graphene of tunable B/N codoping levels as efficient ORR electrocatalysts simply by thermally annealing GO in the presence of boric acid and ammonia.211 The resultant BCN graphene was demonstrated to possess superior electrocatalytic activities to the commercial Pt/C electrocatalyst (C2−20, 20% platinum on Vulcan XC-72R; E-TEK) due to the proven synergetic effect associated with N and B codoping (section 4.1.1.7). As expected, a substantial reduction process occurred at about −0.28 V in the presence of oxygen, whereas no obvious response was observed at the same potential range under nitrogen (Figure 34a). The LSV curves given in Figure 34b clearly show that the half-wave potential of the B12C77N11 (Figure 8B) electrode for ORR in 0.1 M KOH solution was at around −0.25 V, which is close to that of the Pt/C but much more positive than those of other BCN graphene electrodes. Furthermore, the current density of ORR on B12C77N11 within almost the whole potential range covered in this study is higher than that of the other BCN and Pt/C electrodes (Figure 34b), and the electron transfer number n for ORR on the B12C77N11 graphene electrocatalyst is close to 4, as determined by the RRDE data.211 The B12C77N11 electrode was further demonstrated to
Figure 32. LSVs of graphene and N-doped graphene prepared under different temperatures. Electrolyte: O2-saturated 0.1 M KOH. Scan rate: 5 mV s−1. Rotation speed: 1600 rpm. Reprinted with permission from ref 309. Copyright 2011 Royal Society of Chemistry.
exhibited a considerably higher ORR activity than its counterparts prepared at different temperatures. A strong temperature dependence on the ORR onset potentials was also observed for graphene (0.046), N-doped graphene (800 °C, 0.184), N-doped graphene (900 °C, 0.308), and N-doped graphene (1000 °C, 0.204 V). As expected, graphene showed an apparent 2e two-step ORR process (Figure 32), while the N-doped graphene (900 °C) exhibited the highest electrocatalytic activity for ORR via a 4e one-step process, presumably due to its well-balanced good conductivity and high quaternary and/or pyridine-like N contents. 4.1.2.5. Boron-Doped Graphene As Metal-Free Catalyst. Just like the evolution from NCNTs to B-doped CNTs, recent studies on heteroatom-doped graphene as metal-free catalysts for ORR have also extended the doping atoms to include boron with a lower electronegativity than that of carbon (Figure 24d). For instance, Sheng et al. prepared boron-doped graphene (Bgraphene, 3.2 atom % B) by thermal annealing GO in the presence of boron oxide,194 which, similar to the Pt catalyst, exhibited excellent electrocatalytic activity toward ORR in alkaline electrolytes. As can be seen in Figure 33a, the GCE and pure graphene/GCE showed two-step, two-electron processes with the ORR peaks at −0.45 and −0.36 V, respectively. The ORR peak (−0.34 V) for B-graphene/GCE (i.e., BG/GCE) is more positive than that of the graphene/GCE, suggesting a higher ORR activity for the former. The corresponding LSV curves in Figure 33b also show the relatively high ORR performance for the B-graphene/GCE with respect to the graphene/GCE, indicating a B-doping-induced enhancement in the ORR performance, though still inferior to the Pt/GCE. This is presumably due to a B-doping-induced acceleration of the 4850
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Figure 34. (a) CV curves of ORR on BCN (B12C77N11) graphene in nitrogen- and oxygen-saturated 0.1 M KOH solutions at a scan rate of 50 mV s−1. (b) LSV curves of ORR on BCN graphene with different compositions (cf., Figure 8B) in oxygen-saturated 0.1 M KOH solution at 10 mV s−1 and compared with the commercial Pt/C electrocatalyst. Reprinted with permission from ref 211. Copyright 2012 Wiley-VCH.
Figure 35. (A) Mechanisms of ORR on nitrogen-doped graphene. Reprinted with permission from ref 286. Copyright 2011 American Chemical Society. (B) Optimized structure of each electron transformation in ORR: (a) Initial position of OOH from nitrogen-doped graphene, (b) OOH adsorbs on the graphene, (c) O−O bond is broken, (d) one water molecule is generated, and (e) C−O bond is broken; the second water molecule is generated. Gray, blue, red, and small white balls represent carbon, nitrogen, oxygen, and hydrogen atoms, respectively. Reprinted with permission from ref 376. Copyright 2012 American Chemical Society.
of the BCN graphene materials can be significantly improved by controlled doping of graphene with B and N. The first-principles calculations were performed to explain the high catalytic capability of the BCN graphene. For the theoretical calculation,211 three BCN graphene models, B7C87N6H26, B12C77N11H26, and B38C28N34H26, along with pure graphene
possess a high selectivity toward ORR and better methanol and CO tolerance than the commercial Pt/C electrocatalyst. These results clearly indicate that the catalytic active sites on the BCN graphene are much more stable than those on the commercial Pt/C electrode. They also indicate that the ORR catalytic activity 4851
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on the graphene at the same carbon atom even when the O2 molecule is put in the range of bonding formation. However, O2 can adsorb on the same site of the negatively charged graphene and further interact with an H+ to form an adsorbed OOH. Thus, the surface charge promotes the adsorption of O2, though the O2 adsorption energy is −0.7 eV, over 10 times smaller than that for OOH+ adsorption (−11.26 eV). Therefore, OOH+ adsorption is a more favorable reaction in the first electron transfer. When an H atom is introduced near the oxygen atom that attaches onto the graphene, a bond is formed between the oxygen and the hydrogen atoms while the O−O bond is broken to form an OH (Figure 35B(c)). During this process, the dissociated OH moves away from the graphene plane, though the other dissociated OH is still bonding to the graphene. After adding two more H atoms to the O atoms in the reaction system, two water molecules are formed and completely departed from the graphene (Figures 35B(d,e)). The third and fourth electrons were then transformed for the ORR. Finally, after the removal of the water molecules, the saddle-shaped graphene recovers to its original shape and is ready for the next reaction cycle. This is a typical four-electron reaction because the O−O bond breaks during the reaction. Furthermore, the identified active sites are closely related to doping cluster size and dopant−defect interactions (Figure 35B). Generally speaking, a large doping cluster size (number of N atoms > 2) reduces the number of catalytic active sites per N atom. Nevertheless, Feng et al. revealed that nitrogen clusters rather than the isolated ones are the most efficient catalytic sites for oxygen reduction.377 Clusters with three or four nitrogen atoms are found to be optimal theoretically, with which catalytic properties similar to or even superior to platinum can be obtained. In combination with N clustering, Stone−Wales defects can strongly promote ORR. For four-electron transfer, the effective reversible potential ranges from 1.04 to 1.15 V (vs SHE), depending on the defects and cluster size. Therefore, the catalytic properties of graphene could be optimized by introducing small N clusters in combination with material defects. During the ORR process, oxygen dissociation on nitrogendoped graphene is of paramount importance. Calculations carried out with DFT implemented in the Vienna ab initio simulation package (VASP) showed that the energy barriers could be reduced efficiently by all types of nitrogen doping in graphene.319 As is shown in Figure 36, the overall energy barrier of the pristine graphene is 2.71 eV (Figure 36a). For the graphene with one graphite-nitrogen-atom substitution, both the intermediate and the final states are less stable than the initial state. The total energy barrier of the one-nitrogen-doped graphene is 1.87 eV (Figure 36b), which is much lower than the pristine graphene case. The graphene with three graphitenitrogen-atom substitutions has also been studied. According to Okamoto’s model,378 two nitrogen atoms occupy a para position and the third nitrogen sits in the meta position. The oxygen molecule is placed over the C−C bond with carbon atoms neighboring nitrogen atoms (Figure 36c); the final overall energy barrier of the three-nitrogen-doped graphene is only 0.19 eV (Figure 36c). When the graphene is substituted with one pyridine-like nitrogen atom, the calculated overall energy barrier is 2.34 eV (Figure 36d), which is only slightly lower than that of the pristine graphene but much higher than that of the graphene with graphite-nitrogen substitution. The graphite-like nitrogen and Stone−Wales defect nitrogen could decrease the energy barrier more efficiently than pyridine-like nitrogen, leading to a dissociation barrier lower than 0.2 eV. For the graphite-like
(i.e., C100H26) with the same size as those BCN graphene samples, were constructed (Figure 8B). As shown in Figure 8B, B38C28N34 is not ideal for the ORR application due to its low conductivity associated with the large BN cluster. In other model structures for BCN, B and N are distributed either randomly without any BN bonds or in small BN clusters or a single relatively large BN cluster, as shown in Figure 8B(b−k). Compared to pure graphene, substitution of C by B and N was found to lead to a smaller energy gap. However, overdoping of B and N, as is the case of B38C28N34H26, results in a significant increase in the band-gap energy. As a result, B38C34N28H26 has the highest band gap, which is nearly two times as high as that of other model structures shown in Figure 8B, leading to a significantly reduced conductivity for B38C34N28H26. This could greatly affect its electron transfer and thus the ORR catalytic activity. Among all those structures shown in Figure 8B, B12C77N11H26 has the lowest band-gap energy, suggesting that B12C77N11H26 should have the highest chemical reactivity or the best catalytic performance, which is consistent with the experimental results.211 Other metal-free BCN graphene materials with high ORR catalytic activities include the B and N self-doped graphene sheets (BNGs) synthesized at high temperature (1000 °C) from borane-tert-butylamine complex precursor,375 B- and N-codoped graphene sheets prepared by two-step boron and nitrogen doping332 and CVD method.27 4.1.2.7. Mechanism for ORR on the Metal-Free Graphene. The observed super ORR performance for heteroatom-doped graphene can be attributable to a charge-transfer effect associated with the doping of graphene, as is the case with the doped CNTs.8 Using DFT, Zhang and Xia286 studied the electrocatalytic mechanism of N-doped graphene in acidic environment. Their energy calculation shows that the ORR could spontaneously occur on the N-doped graphene with a four-electron pathway. The catalytic active sites on single nitrogen-doped graphene were identified to have either a high positive spin density or a high positive atomic charge density, indicating that the doping-induced charge and/or spin redistributions play an important role in enhancement of the ORR electrocatalytic activities for the N-doped graphene (Figure 35A). Subsequently, the same group studied ORR on nitrogendoped graphene in acidic fuel cells using B3LYP hybrid DFT through Gaussian 03.376 In this study, the ORR activities were directly correlated to the material microstructure as well as the number of dopants in cluster and Stone−Wales defects. It was found that the active catalytic sites are more likely to locate at the area with higher positive charge density and/or positive spin density. The optimized structures for OOH, OOH+, or O2 adsorption (ads) to graphene were obtained through structural optimization calculations. As shown in Figure 35B, such structures consist of two combined Stone−Wales defects with two nitrogen atoms in the pyridinic and pyrrolic mixed type of nitrogen atoms, incorporated into the hexagon and pentagon of the graphitic sheet. There are two possible reaction pathways in the first electron transfer: (i) intermediate molecule OOH+ adsorption and (ii) direct O2 adsorption. The simulation shows that both OOH+ and OOH far from the graphene (∼3 Å) can adsorb on the graphene at the carbon atom near to the dopant (Figure 35B(b)), indicating no energy barrier in the reaction. Upon adsorption, the graphene plane distorted into a “saddleshaped” warped surface with the carbon atom attached to the oxygen rising out of the plane to form a tetrahedral structure (Figure 35B(b)). Unlike the OOH adsorption, O2 cannot adsorb 4852
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Figure 36. Minimal energy paths of oxygen molecule dissociation on (a) pristine graphene, (b) one nitrogen-atom-substituted graphene, (c) three nitrogen-atom-substituted graphene, and (d) graphene with pyridine-like nitrogen atoms. Gray dots, blue dots, and red dots represent carbon, nitrogen, and oxygen atoms, respectively. Reprinted with permission from ref 319. Copyright 2012 Royal Society of Chemistry.
Figure 37. Model structures with various geometries, N-doping sites, and edge states. Oxygen molecule is adsorbed at Cad (yellow atom). Nitrogen, oxygen, and hydrogen atoms are colored blue, red, and white, respectively, and the dotted line denotes a periodic boundary. Reprinted with permission from ref 379. Copyright 2011 Royal Society of Chemistry.
nitrogen, the energy barrier reduction is directly proportional to the nitrogen dopant concentration. These observations are closely related to partial occupation of π* orbitals and change of work functions. On the other hand, Kim et al. performed the periodic DFT calculations on ORR at the edge of a graphene nanoribbon (GNR).379 It is found that the edge doping could enhance not only the oxygen adsorption for the first electron transfer but also the selectivity toward the four-electron reduction pathway. The outermost graphitic nitrogen site in particular is demonstrated to give the most desirable characteristics for the improved ORR activity, and hence the active site. As shown in Figure 37, the graphitic nitrogen becomes pyridinic-like in the next electron and proton transfer reaction via the ring opening of a cyclic C−N bond, which may reconcile the controversy of whether the pyridinic, graphitic, or both nitrogens are active sites (section 4.1.1.3). By using DFT calculations and taking the solvent, surface adsorbates, and coverages into consideration, Bao and coworkers also studied the ORR catalyzed by N-doped graphene.380 These authors proposed two mechanisms, namely, dissociative and associative mechanisms, for different N-doping configurations. The associative mechanism was described as follows O2 + → O2(ads) O2(ads) + H 2O + e → OOH(ads) + OH −
OOH(ads) + e → O(ads) + OH −
−
O(ads) + H 2O + e → OH(ads) + OH −
OH(ads) + e → OH + *
−
1/2O2 + * → O(ads)
(7) 380
followed by steps 4 and 5. The full energy profile including all the reaction barriers indicated that the associative mechanism was more energetically favored than the dissociative one and that the removal of O species from the surface was the rate-determining step. The key findings of this particular study include the following. (i) The water effect is essential in constructing a reliable reaction free energy profile as O2 adsorption is significantly enhanced by the polarization of O2 due to hydrogen bonding with H2O. O2 cannot even adsorb on the N-doped graphene surface without the presence of water. (ii) The more energetically favored associative mechanism is dominating for ORR as the dissociation barrier of O2 is too high for the dissociation reaction to be feasible. (iii) The desorption of OOH(ads) to form OOH− is found to be energetically unfavorable compared with the reaction OOH(ads) → O(ads) + OH−, suggesting a “4e− reduction” pathway on N-doped graphene.380 4.1.3. Graphite as Metal-Free Catalysts. As can be seen from the above discussion, heteroatom-doped CNTs and graphene materials can be produced by various physicochemical methods, ranging from the CVD to solution exfoliation of graphite. Heteroatom doping has been demonstrated to induce charge redistribution to facilitate O2 adsorption and oxygen reduction on the carbon-based metal-free catalysts. Since the graphitic carbon structure in graphite can also support charge transfer induced by heteroatom doping, it is also interesting to note that heteroatom-doped graphite materials can also act as metal-free catalysts for oxygen reduction.
(2) (3)
−
(6)
For the dissociative mechanism, the first step is as follows
(1) −
−
OOH(ads) + e− → OOH−
(4) (5)
where the asterisk (*) denotes a free site on the surface. Alternatively, instead of reaction 3, OOH(ads) may desorb from the surface 4853
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Figure 38. (a) Preparation of NOMGAs as metal-free catalysts for the ORR. (b) RDE voltammograms of the series of PDI-NOGMAs and Pt−C supported on GC electrodes at a rotation rate of 1600 rpm in 0.1 M KOH. (c) Electrochemical activity given as the kinetic-limiting current density (JK) at 0.35 V for the PDI-NOGMAs supported on GC electrodes in comparison with that of a commercial Pt−C electrode. Reprinted with permission from ref 280. Copyright 2010 Wiley-VCH.
4.1.3.1. Nitrogen-Doped Graphite by Thermal Treatment. Using a nitrogen-containing aromatic dye stuff, N,N′-bis(2,6diisopropyphenyl)-3,4,9,10-perylenetetracarboxylic diimide (PDI), as the carbon precursor, Liu et al. reported a facile fabrication route to nitrogen-doped ordered mesoporous graphitic arrays (NOMGAs).280 The resulting NOMGAs with a high surface area (510 cm2/g for PDI-900, vide infra) and a graphitic framework of a moderate nitrogen content (ca. 2.7 wt % for PDI-900) were demonstrated to show ORR electrocatalytic performance better than platinum with excellent long-term stability and high resistance to methanol cross-over effects. Owing to the metal-free preparation procedure, the reported electrocatalytic activity can be attributed exclusively to the incorporation of nitrogen in PDI-NOMGAs (Figure 38). In this study, NOMGAs with different compositions were synthesized by carbonization of PDI/SBA-15 composites at 600, 750, and 900 °C (denoted as PDI-600, PDI-750, and PDI-900), respectively. Both PDI-600/GC and PDI-750/GC showed a two-step process for ORR (Figure 38b). An increase in the pyrolysis temperature led to a clear enhancement of the electrontransfer kinetics of oxygen reduction for PDI-900/GC, as reflected by kinetic-limiting current densities and the electron transfer numbers (Figure 38c). PDI-600 and PDI-750, with a higher nitrogen content (up to 3.5 wt %) than that of PDI-900 (2.5 wt %), actually exhibited lower selectivity and catalytic
activity. This is because the higher pyrolysis temperature could cause the transformation of pyridine-like nitrogen atoms into graphite-like nitrogen atoms381 with a concomitant reduction in the overall nitrogen content. Therefore, the significantly enhanced activity and selectivity of PDI-900 are attributable to the combination of its highly graphitic degree and the increased graphitic N content, indicating, once again, that the nitrogen content does not always directly affect the electrochemical performance. Nitrogen-doped carbon materials with a high proportion of graphite-like nitrogen atoms for enhanced electrochemical performance could be prepared by this metalfree preparation procedure from rationally selecting nitrogenrich aromatic precursors (Figure 38a). In addition to the metal-free precursor route described above, Ma et al. prepared nitrogen-doped hollow carbon nanoparticles (N-HCNPs) simply through metal-free one-pot detonation of 2,4,6-trinitrophenol (TNP) as explosive.382 The resultant NHCNPs exhibited electrocatalytic performance comparable to the commercial Pt/C catalyst for four-electron ORR in alkaline fuel cells. Superior to that of Pt/C, N-HCNPs showed good operation stability and excellent tolerance to methanol crossover and CO poisoning for ORR. 4.1.3.2. Phosphorus-Doped Graphite. The doping of graphite with P could also induce the catalytic activity for ORR. In this context, Liu et al. prepared the P-doped graphite layers by 4854
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pyrolysis of toluene and triphenylphosphine (TPP).21 The ORR onset potential of the P-doped graphite/GC electrode was found to be approximately +0.10 V, which, along with the oxygen reduction current density, is much higher than that of the bare glassy carbon (BGC) electrode at about −0.11 V and the undoped graphite/GC electrode at about −0.10 V (Figure 39).
via the zigzag and armchair cracking, respectively, leading to solution-processable edge-N-doped graphene nanoplatelets (NGnPs) with superb catalytic performance for ORR. As can be seen in Figure 41, the NGnP/GC electrode showed featureless CV in the N2-saturated 0.1 M aq KOH solution (Figure 41a), whereas it exhibited an obvious oxygen reduction peak in the corresponding O2-satureted electrolyte (Figure 41b). The pristine graphite showed a cathodic reduction peak at −0.43 V with a current density of −0.19 mA cm−2 in the O2-saturated 0.1 M aq KOH solution. The corresponding cathodic reduction peak for the NGnPs was positively shifted to −0.28 V with a current density of −0.54 mA cm−2. The Pt/C exhibited a single cathodic reduction peak at −0.10 V with a current density of −0.52 mA cm−2. The current density from NGnP is over 2.8 times that of the pristine graphite and comparable to that of Pt/ C, indicating a good ORR activity for the metal-free NGNP catalyst. In addition to NGnPs prepared by the ball-milling approach, Jeon et al. also produced a series of edge-selectively halogenated (X = Cl, Br, I) graphene nanoplatelets (XGnPs = ClGnP, BrGnP, IGnP) simply by ball-milling graphite in the presence of Cl2, Br2, and I2, respectively.190 The electrocatalytic activities in N2- and O2-saturated 0.1 M aq KOH solutions for the XGnPs thus produced were investigated. CVs given in Figure 42a−d show the obvious oxygen reduction peaks for all four carbon-based electrodes in the O2saturated 0.1 M aq KOH solution but not in the N2-satureted electrolyte under the same condition. Figure 42a shows a single cathodic reduction peak at −0.37 V with a current density of −0.28 mA cm−2 for the pristine graphite electrode in the O2saturated 0.1 M aq KOH solution. The corresponding cathodic reduction peaks for the ClGnP, BrGnP, and IGnP were positively shifted to −0.24, −0.22, and −0.22 V, respectively. The corresponding peak currents for oxygen reduction were determined to be −0.39, −0.60, and −0.78 mA cm−2 for ClGnP, BrGnP and IGnP, respectively. These values are over 1.4, 2.1, and 2.8 times that of the pristine graphite (−0.28 mA cm−2). Thus, the reduction currents of XGnPs gradually increased while their onset potentials positively shifted along the order of ClGnP < BrGnP < IGnP, which can be attributed to the dopant size effect with the bigger edge-functional groups for a more significant graphitic lattice expansion, as schematically shown in Figure 42g, and hence an enhanced electrolyte diffusion and ORR activity. The halogen-doping-enhanced ORR performance for XGnPs was also confirmed by the LSV curves. As shown in Figure 42f, the onset potential for oxygen reduction at the pristine graphite electrode is about −0.33 V, which positively shifted to the range from −0.16 to −0.14 V upon edge functionalization with halogen atoms, though still lower than that of the Pt/C (−0.06 V). The limiting diffusion currents at −0.8 V for the pristine graphite, ClGnP, BrGnP, IGnP, and Pt/C electrodes are −0.09, −0.18, −0.28, −0.40, and −0.30 mA, respectively. These current values for the XGnPs are about 2.0, 3.1, and 4.4 times higher than that of the pristine graphite and 60%, 93%, and 133% that of the Pt/C. These results are consistent with the CV measurements (Figure 42a−e), confirming, once again, the significant ORR electrocatalytic activity of XGnPs. Furthermore, XGnPs showed a high selectivity, good tolerance to methanol cross-over/CO poisoning effects, and excellent long-term cycle stability.190 Clearly, the edge halogenation/doping, particularly Br and I, plays an important role to significantly improve the ORR activity of graphite.
Figure 39. RDE voltammograms of the BGC electrode (1), the graphite/GC electrode (2), the P-doped graphite/GC electrode (3), and the Pt−C/GC electrode (4) in an oxygen-saturated, 0.10 M KOH solution as well as the BGC electrode (1′), the graphite/GC electrode (2′), the P-doped graphite/GC electrode (3′), and the Pt−C/GC electrode (4′) in an oxygen-saturated, 0.10 M KOH solution after addition of 1.0 M methanol at an oxygen flow rate of 20 mL min−1 and a rotation rate of 1600 rpm. Scan rate: 10 mV s−1. Reprinted with permission from ref 21. Copyright 2011 Wiley-VCH.
This finding confirms that doping of phosphorus (electronegativity = 2.19), like boron (electronegativity = 2.04), into the carbon (electronegativity = 2.55) hexagonal network of graphitic sheets caused the charge redistribution in the graphite structure (sections 4.1.1.5 and 4.1.2.5), leading to the observed electrocatalytic activity for ORR with an outstanding durability and almost no methanol cross-over effect (Figure 39). The same authors have also prepared phosphorus-doped carbon nanospheres and phosphorus-doped MWCNTs with good electrocatalytic performance for ORR.189,383 The availability of different P-doped graphite materials provides new opportunities to design and develop various metal-free, efficient ORR catalysts for potential applications in fuel cells. 4.1.3.3. Heteroatom-Doped Graphitic Nanoplatelets (GnPs) by Ball Milling. As mentioned in section 2.5.1, recent studies on ball milling of graphite provide an effective means for the development of edge-functionalized graphene materials, including graphitic nanoplatelets edge doped by nitrogen (NGnPs), with tailormade chemical structures and electrochemical properties.170,171 The edge doping without the basal plane damage, together with a large surface area and high conductivity, makes the heteroatom-doped GnPs ideal for ORR. By replacing dry ice with N2 gas for ball milling of graphite, Jeon et al. demonstrated the direct N doping at the edges of graphene nanoplatelets (GnPs) (Figure 40a)170 involving the breakage of large grain size of graphite (Figure 40b) into small ones (Figure 40c). The calculated energies (ΔE) for the formation of 5- and 6-membered rings at the broken edges were −2.97 (−68 kcal/N2 mol) (Figure 40d) and −3.53 eV/N2 (−81 kcal/N2 mol) (Figure 40e), respectively, indicating an exothermic (spontaneous) reaction favorable for the formation of N-containing aromatic rings at the broken edges (i.e., N doping). This leads to high nitrogen content at the edges, though not all active carbon atoms could react with nitrogen. Both pyrazole and pyridazine rings could form at the edges of NGnPs 4855
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Figure 40. (a) Schematic representation of the physical cracking of graphite flake in a ball-mill crusher (500 mL) containing stainless steel balls (500.0 g, diameter 5 mm) agitated at 500 rpm for 48 h in the presence of nitrogen and subsequent exposure to air moisture to produce NGnPs. SEM images: (b) starting graphite flake; (c) NGnPs after ball milling. Scale bars: 1 μm. The ring formation mechanisms depending upon the cracking patterns of unzipped edges: (d) formation of a 5-membered pyrazole ring after the reaction between the active zigzag-edge carbon atoms and nitrogen; (e) formation of a 6membered pyridazine ring after the reaction between the active armchair-edge carbon atoms and nitrogen. Reprinted with permission from ref 170. Copyright 2013 Nature Publishing Group.
performance (section 2.5). Therefore, it is interesting to test if S doping of carbon materials can induce the ORR activity as the charge polarization stemming from the difference in electronegativity between carbon (χ = 2.55) and sulfur (χ = 2.58) is almost negligible. In this context, Jeon et al. recently prepared edge-sulfurized (i.e., S-doped) graphene nanoplatelets (SGnP) by simply ball milling the pristine graphite in the presence of sulfur (S8) and evaluated their electrocatalytic activity for ORR in alkaline medium.191 Figure 43a schematically shows the preparation process for edge-selectively S-doped graphitic nanoplatelets (SGnP) by ball milling graphite in the presence of sulfur (S8). CVs in Figures 43b and 43c show obvious oxygen reduction peaks for the carbonbased electrodes in the O2-saturated 0.1 M KOH. Specifically, Figure 43b shows a cathodic reduction peak at −0.47 V in an O2saturated solution for the pristine graphite, while the corresponding cathodic reduction peak for the SGnP shifted positively to −0.40 V with an over two-times higher oxygen reduction current as that of the pristine graphite. These results clearly indicate that SGnP has much higher ORR catalytic activity than the pristine graphite. As shown in Figure 43d, the onset potential of oxygen reduction for the pristine graphite was approximately −0.40 V while a significant upshift to −0.22 V was observed for SGnP. As the diffusion current of Pt/C is limited to −0.8 V in the alkaline electrolyte, the limiting currents of the carbon electrodes were measured up to −0.8 V in this particular
Figure 41. CVs of samples on GC electrodes with a scan rate of 0.01 V s−1: (a) in N2-saturated 0.1 M aq KOH solution, (b) in O2-saturated 0.1 M aq KOH solution. Sky blue arrows indicate the contributions of hydrogen adsorption/desorption at around −0.7 V and out of Pt limiting potential (−0.8 V). Reprinted with permission from ref 170. Copyright 2013 Nature Publishing Group.
As can be seen from the above discussions, graphitic carbon materials doped with heteroatoms having either higher or lower electronegativities than that of carbon (2.55), such as N (3.04), Cl (3.16), Br (2.96), I (2.66), B (2.04), and P (2.19), imparted electrocatalytic activities for ORR through the doping-induced charge transfer (section 2.5). Theoretical studies confirmed that it is the difference in electronegativity between the dopant and the carbon atom, regardless of whether the dopant has a higher or lower value of electronegativity than that of C, that breaks the electroneutrality of graphitic materials to create charged sites favorable for O2 adsorption and hence enhanced ORR 4856
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Figure 42. CVs of samples on glassy carbon (GC) electrodes in N2- and O2-saturated 0.1 M aq KOH solution with a scan rate of 10 mV s−1: (a) pristine graphite, (b) ClGnP, (c) BrGnP, (d) IGnP, and (e) Pt/C. Pink arrows indicate the contributions of hydrogen adsorption/desorption at around −0.7 V and out of limiting potential (−0.8 V). (f) LSV at a rotation rate of 1600 rpm and a scan rate of 10 mV s−1, showing a gradual increase in current and a positive shift in the onset potential along the order of the pristine graphite < ClGnP < BrGnP < IGnP < Pt/C (pink arrow). (g) Schematic representation for the edge expansions of XGnPs caused by the edge halogens. Reprinted with permission from ref 190. Copyright 2013 Nature Publishing Group.
case. Figure 43d shows a limiting current of −4.47 mAcm−2 for SGnP, which is 2.7 times higher than that (−1.67 mA cm−2) of the pristine graphite and approaches 93.4% (−4.79 mA cm−2) of the commercial Pt/C. The number of electrons transferred (n) calculated from the Koutecky−Levich equation was 3.3 and 2.0 for the SGnP and pristine graphite, respectively. The electron transfer number (n = 2.0) of the pristine graphite is close to the classical two-electron transfer process, while the corresponding n = 3.3 for the SGnP electrode indicates a combined pathway of two-electron and four-electron transfer processes but closer to the latter. These results confirm that sulfur doping plays the key role for the improved ORR activity of SGnP. Given that the electronegativities of sulfur (χ = 2.58) and carbon (χ = 2.55) are nearly the same, the change of atomic charge distribution for the SGnP is relatively much smaller, compared with nitrogen-doped carbon materials. Therefore, the doping-induced charge transfer may contribute insignificantly to the improved ORR catalytic activity of SGnP, and another new important factor should have contributed to the enhanced ORR activity of SGnP. On the basis of theoretical calculations, the origin of the ORR activity enhancement with the S-doped carbon nanomaterials has been attributed to the doping-induced “electron spin” redistribution, as described in detail in section 4.3.
Although chemical doping of S directly into the framework of preformed graphene would seem to be quite difficult, Yang et al. successfully demonstrated the fabrication of sulfur-doped graphene (S-graphene) by directly annealing GO and benzyl disulfide (BDS) in argon (Figure 44).192 The resultant Sgraphene was demonstrated to show an excellent catalytic activity, long-term stability, and high methanol tolerance in alkaline media for ORR. These authors further found that the graphene doped with selenium (electronegativity of selenium 2.55), another element that has a similar electronegativity as carbon, also showed a high ORR catalytic activity.192 Although they did not give further insight into the ORR mechanism of this Se-graphene, it is believed that the doping-induced electron spin redistribution is responsible for the ORR activity of the Se-doped carbon materials too. S-doped graphene with superb ORR catalytic activity and durability has also been prepared by thermal treatment of exfoliated graphene under CS2 gas flow.384 In this case, the treatment temperature was found to play an important role in controlling the S content in the resultant S-doped graphene with the highest S content of about 2% for the S-graphene treated at 850 °C (SG850). Alternatively, S-doped graphene with good 4857
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Figure 43. (a) Schematic representation of the ball-milling process. CV curves obtained from sample electrodes in N2- and O2-saturated 0.1 M aq KOH solutions at a scan rate of 50 mV s−1: (b) pristine graphite, (c) SGnP, (d) LSVs of the sample electrodes in an O2-saturated 0.1 M aq KOH solution at a scan rate of 10 mV s−1 with a rotation rate of 1600 rpm; (e) Koutecky−Levich plots for the sample electrodes at −0.6 V. Reprinted with permission from ref 191. Copyright 2013 Wiley-VCH.
Figure 44. Schematic illustration of S-graphene preparation for ORR. Reprinted with permission from ref 192. Copyright 2012 American Chemical Society.
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Figure 45. (A) (a, b) TEM images of the as-prepared N−GQDs under different magnifications, (c) AFM image of the N-GQDs on a Si substrate, and (d) height profile along the lines in c. (Insets in b and c) Size and height distribution of N-GQDs. (B) (a, b) CVs of (a) N-GQD/graphene and (b) commercial Pt/C on a GC electrode in N2-saturated 0.1 M KOH, O2-saturated 0.1 M KOH, and O2-saturated 3 M CH3OH solutions. (c) RDE curves for N-GQD/graphene in O2-saturated 0.1 M KOH with different speeds. (Inset) Koutecky−Levich plots derived from the RDE measurements. (d) Electrochemical stability of N-GQD/graphene as determined by continuous CV in O2-saturated 0.1 M KOH. Reprinted with permission from ref 395. Copyright 2012 American Chemical Society.
4.1.3.4. Heteroatom-Doped Graphene Quantum Dots (GQDs). Due to quantum confinement and edge effects, the 0D GQDs have been demonstrated to possess various interesting properties.388 Consequently, various chemical methods, including hydrothermal route,389 solution chemistry,96,390−392 electrochemistry,393 and transforming C60,394 have been developed for controllable synthesis of GQDs. Of particular interest, Li et al. recently reported a facile electrochemical approach to large-scale preparation of functional GQDs.393 By using N-containing tetrabutylammonium perchlorate (TBAP) in acetonitrile as the electrolyte, Li et al. also prepared nitrogen-doped graphene quantum dots (N-GQDs).395 TEM images (Figures 45A(a and b)) show fairly uniform N−GQDs with diameters in the range of ca. 2−5 nm, which are much smaller than those of the N-free counterparts (∼10 nm) synthesized hydrothermally389 but consistent with those of the N-free GQDs prepared electrochemically.393 The corresponding AFM image (Figure 45A(c and d)) reveals a typical topographic height of 1−2.5 nm, suggesting that most of the N-GQDs consist of ca. 1−5 graphene layers. XPS measurements indicated that the as-prepared N-GQDs with an N/C atomic ratio of ca. 4.3% are suitable as a new class of metal-free electrocatalysts for ORR. Indeed, the graphene-supported N-GQDs (N-GQD/graphene) showed a well-defined cathodic peak in the O2-saturated, but not N2-saturated, KOH solution (Figure 45B(a)), exhibiting a stable ORR catalytic activity and remarkable tolerance to the methanol cross-over effect (Figure 45B(b)). The observed electrocatalytic activity for N-GQDs is comparable to those of commercial Pt/C electrodes, N-doped CNTs, or N-doped graphene sheets and can be exclusively attributed to the Ndoping effect. LSV curves of the ORR for N-GQD/graphene in an O2-saturated 0.1 M KOH solution measured on a RDE showed a n value of 3.6−4.4 over the potential range from −0.3 to −0.6 V (Figure 45B(c)), suggesting a four-electron process for the ORR on the N-GQD/graphene electrode. In addition, no obvious decrease in current was observed after 2 days of
ORR performance was prepared by thermal exfoliation of GOs in the presence of S.385 Using GO-mesoporous silica (GO-silica) sheets as the starting material, Yang et al. reported another efficient approach to heteroatom (N or S)-doped graphene via a thermal reaction between GO and guest gases (NH3 or H2S).386 The unique porous silica layer was used at multiple advantages, including (i) enhanced transport of the gas source to the surface of GO to facilitate the thermal reaction and (ii) effective prevention of the irreversible reaggregation of graphene during the heteroatomdoping process at higher temperatures. It was found that the sulfur atoms were mainly doped at the edges and defect sides of the graphene sheet to form the thiophene-like S species at high temperatures (>700 °C) and that the electrocatalytic activity of S-doped graphene depended strongly on the annealing temperature. The electron transfer number (n) per O2 molecule for the S-doped graphene decreased from 3.5 to 3.2 with increasing annealing temperature from 500 to 900 °C, due presumably to the concomitant reduction in the amount of sulfur from 1.7% to 1.2%. By ball milling of graphite and dry ice/sulfur trioxide mixture, Jeon et al. also prepared graphitic nanoplatelets codoped with carboxylic and sulfonic groups (CSGnP), which are highly soluble in various solvents, including neutral water.387 The ball milling of graphite was also carried out in the presence of hydrogen, dry ice, and sulfur trioxide only to produce hydrogen (HGnP), carboxylic acid (CGnP), and sulfonic acid (SGnP) functionalized/doped GnPs, respectively, as references for the electrocatalysis study. It was found that the edge polar nature of these edge-functionalized/doped GnPs played an important role in regulating the ORR efficiency with the electrocatalytic activity in the order of SGnP > CSGnP > CGnP > HGnP > pristine graphite. Furthermore, the sulfur-containing SGnP and CSGnP were found to have a superior ORR performance to the commercially available platinum-based electrocatalyst. 4859
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Figure 46. (a) Structures of N-doped graphene QDs 1 and 2 and an undoped QD 3 for comparison studies, and synthetic routes to the N-doped graphene quantum dots 1−3. Conditions: (i) phenylboronic acid, Pd(PPh3)4, K2CO3, toluene/EtOH/H2O, 80 °C; (ii) a) n-BuLi, THF, −78 °C, b) 2isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, −78 °C to room temperature, c) 1,3-dibromo-5-iodobenzene, Pd(PPh3)4, K2CO3, toluene/EtOH/ H2O, 85 °C; (iii) 9,11-diphenyl-10H-cyclopenta[e]pyren-10-one, Ph2O, 240 °C; (iv) RuCl3, NaClO4, CH2Cl2/CH3CN/H2O, room temperature; (v) benzene-1,2-diamine, EtOH/CHCl3, 65 °C; (vi) Pd(PPh3)4, K2CO3, toluene/EtOH/H2O, 80 °C; (vii) FeCl3, CH2Cl2/CH3NO2, room temperature; (viii) o-xylylenebis(triphenylphosphonium bromide), LiOH (aq, 5 M), CH2Cl2, room temperature.362 (b) CV curves (scanning rate of 50 mV s−1) of 1 on a glassy-carbon RDE in a N2- and O2-saturated 0.1 M KOH solution. (c) LSV curves (10 mV s−1) for 1−3 and Pt/C on a RDE (1600 rpm) in an O2saturated 0.1 M KOH solution. Also shown is the LSV curve for 11, a much smaller N-substituted heterocycle with structure shown in the inset. (d) LSVs (10 mV s−1) for 1 on a RDE in an O2-saturated 0.1 M KOH solution with various rotating speeds. (e) Koutecky−Levich plots for 1 obtained from the LSV curves in d at various voltage values. Global fitting of the plots reveals that the number of electrons transferred per O2 molecule is 3.9. Reprinted with permission from ref 362. Copyright 2012 American Chemical Society.
improved catalytic activity for larger N-GQDs, though the electrocatalytic activities of these N-GQDs are not as good as that of Pt/C. The undoped GQD 3 shows appreciable activity only at a much more negative potential (Figure 46c), illustrating the importance of the nitrogen doping, even at a doping level (N/C atomic ratio) as low as ∼1%, to the ORR activity. The above results seem not to support the positive correlation between the catalytic activity and the nitrogen content that has been suggested previously,19,137,277,280,297,301,395 and the largest NGQD 1, with the lowest doping level, shows the highest activity. This is most likely because the larger N-GQD possesses a higher conductivity. From the slope of the Koutecky−Levich plots (Figure 46e) obtained from the data in Figure 46d, the number of electrons transferred per oxygen molecule in the ORR was calculated to be about 3.9, consistent with a four-electron process that has been reported for other N-doped carbon nanomaterials (sections 4.1.1 and 4.1.2). Therefore, similar catalytic mechanisms may also apply to the N-GQD metal-free ORR catalysts. 4.1.3.5. Mechanism for ORR on the Metal-Free Graphite. Just like N-doped CNTs and graphene, nitrogen doping has been demonstrated to play an important role in the mechanism of O2 reduction on the heteroatom-doped graphite electrodes.396,397 In particular, Kondo et al. characterized the nitrogen-doped graphite at atomic scale by STM, STS, and XPS and investigated the effects of nitrogen dopant on the local electronic structure of the surrounding carbon atoms.398 Their experimental characterization was complemented by first-principles calculations. It was found that pyridinic N defects could induce an atomic rearrangement to form a pentagon, while graphitic N affected the structure only slightly. In both cases, however, the electronic
continuous cycling in O2-saturated 0.1 M KOH (Figure 45B(d)), indicating no loss of catalytic activity for the N-GQD/graphene electrode. N-GQDs have also been synthesized via a solution chemistry approach in the form of N-doped colloidal GQDs with welldefined structures.362 In this case, N doping was demonstrated to significantly affect the properties of the GQDs, including the emergence of size-dependent electrocatalytic activity for oxygen reduction. Figure 46a shows structures of N-GQDs 1 and 2 and an undoped GQD 3 for comparison studies, along with the synthetic routes to the N-GQDs 1−3. In addition to two nitrogen atoms, the conjugated cores (marked blue in Figure 46a) of N-GQDs 1 and 2 contain 176 and 128 carbon atoms, respectively. GQD 3, an undoped analogue of 2 that contains 130 conjugated carbon atoms in the graphene core (marked blue in Figure 46a), was also synthesized as reference for understanding the doping effects in the GQDs. With the solution chemistry approach, the numbers of nitrogen atoms and their bonding configurations in the GQDs can be precisely controlled, which otherwise is difficult, if not impossible, with other doping methods. Figure 46b shows a well-defined reduction peak at around −0.3 V (vs SCE) in CVs for 1 deposited on glassy carbon in an O2-saturated 0.1 M KOH aqueous solution but not in the N2saturated one. Figure 46c shows LSV curves for ORR in O2saturated 0.1 M KOH aqueous solution on the GQDs/GC 1−3 and commercial Pt/C (20% wt Pt on Vulcan XC-72R). The Ncontaining 1, 2, and 11 (inset of Figure 46c) show ORR onset potentials at −0.04, −0.10, and −0.14 V, respectively, relative to that for the commercial Pt/C catalyst. The increased cathodic current density with increasing size of the QDs indicates the 4860
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Figure 47. (A) Optimized structure for the N-substituted graphite model, C41NH16. Larger gray circles are carbon atoms, and smaller white circles are hydrogen atoms. (B) Spin and charge distributions for C41NH16 in its optimized structure. Both values are shown along the symmetry axis. The first number in each pair refers to the spin density, and the underlined numbers refer to the charge density. The cluster is symmetric, so spin densities are given on the top half and charge densities on the bottom half. For the edge H atoms, the spin and charge densities are, on average, 0.1. The highest bond strengths within the cluster for H and OOH are 2.238 and 0.891 eV, respectively, on the carbon atom that is directly bonded to N.22 As shown in Figure 47C(a and b), the changes in the hybridization of the adsorption site atoms caused by adsorption of a radical on the radical sheet could distort the planar geometry to push the adsorption site carbon out of the plane toward a tetrahedral structure. When the adsorption site is an edge carbon atom, it can relax to a nearly tetrahedral structure. This indicates that the edge and basal-plane
structure of graphite close to the Fermi energy was modified by the defects. The localized π states appeared in occupied and unoccupied regions near the Fermi level around pyridinic and graphitic N species, respectively, facilitating the ORR process. On the other hand, quantum calculations on cluster models of graphite sheets with and without N doping showed that carbon radical sites formed adjacent to substitutional N in graphite are active for O2 electroreduction to H2O2 via adsorbed OOH intermediate.22 Figure 47A shows the N-doped graphite model used in this work, which contains 14 hexagonal rings with delocalized π electrons and terminated with C−H bonds and a nitrogen atom placed substitutionally within the basal plane sheet of graphite. Adsorption of reaction intermediates on sites a−d and others of the graphite and N-doped graphite models was examined. Figure 47A shows the fully optimized geometry for the 14-ring C41NH16 sheet radical in the absence of an adsorbate and in the doublet state, which has C2v symmetry along the Ca−N axis and is flat with no more than ±0.001 Å variations from planarity. Because of the symmetric distribution of spin and charge densities along the Ca−N axis, Figure 47B shows a simplified model structure with the top half displaying only spin densities and the bottom half for only the charge densities, while both spin and charges are given along the Ca−N axis. In this model, the N 4861
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doping could also cause different ORR activities for the N-doped graphite, as is the case with N-doped graphene (section 4.1.2). The weak catalytic effect of undoped carbon was attributed to weaker bonding of OOH to the H-atom-terminated graphite edges. Adsorption of the neutral molecules O2 and H2O2 on the radical sheet are very weak (Figure 47D), which does not cause any significant effect on the planar geometry of the sheet radical. Subsequently, Kurak and Anderson further used the linear Gibbs energy relationship to predict the reversible potentials for forming intermediates during O2 reduction in acid over graphene doped with two N atoms substituting for adjacent edge CH groups.302 This procedure, generally accurate within ∼0.2 V, is useful for estimating overpotentials for electrode surfacecatalyzed reactions. Using bond strengths from VASP slabband DFT calculations, it is predicted that one of the edge nitrogens has H bonded to it at potentials of ∼1.70 V and below. In the first reduction step, the OOH that forms then dissociates on the edge into O that bonds strongly to N with OH weakly associated with it. The calculated reversible potential is ∼0.89 V. The OH is proposed to abstract H from an edge NH, forming H2O. The H is then replaced in a reduction reaction. 4.1.4. Carbon-Nitride-Based Materials as Metal-Free Catalysts. Graphitic carbon nitride, g-C3N4, is a carbonaceous material that has a planar phase analogous to graphite.283 Unlike graphite, however, g-C3N4 has both 3-fold-coordinated (graphite-like) and 2-fold-coordinated (pyridine-like) nitrogen atoms, and every carbon atom is bonded to three nitrogen atoms (Figure 48), including both pyridinic and graphitic nitrogen moieties. Two structural isomers of g-C3N4 are commonly known, including the one comprising condensed melamine units with a periodic array of single carbon vacancies (Figure 48a) and another consisting of condensed melem (2,5,8,-triamino-tri-striazine) subunits with larger periodic vacancies in the lattice (Figure 48b). The melem isomer is predicted to be slightly more stable.399 Although the planar phase (g-C3N4) of both isomers is relatively stable,400 its vacancy-free analogue of graphite is predicted to be energetically unstable.401 g-C3N4 can be readily obtained through the pyrolysis of cyanamide,402 melamine,403 ethylenediamine/carbon tetrachloride,404 or s-triazine derivatives405,406 via condensation. Depending on reaction conditions, g-C3N4 materials with different degrees of condensation, properties, and reactivities can be produced in large quantities. Various nanostructured g-C3N4 materials,407−411 such as nanoparticles and mesoporous powders, have also been fabricated, allowing for additional tuning of properties, intercalation of foreign nanomaterials, and surface active sites for heterogeneous reactions. Because of the high nitrogen content and special semiconducting properties, gC3N4 has been demonstrated to serve as a metal-free catalyst for a variety of reactions,403 such as the activation of benzene, trimerization reaction, activation of carbon dioxide, and ORR. Below, we provide a critical review of recent studies on g-C3N4 as metal-free ORR electrocatalysts. 4.1.4.1. Graphitic-C3N4. Using rotating electrode voltammetry to determine the catalytic activity in oxygen-saturated sulfuric acid, Lyth et al. reported the first study on carbon nitride for catalyzing ORR.283 A much higher onset potential of 0.69 V (vs NHE) was found for oxygen reduction on the carbon nitride electrode compared to 0.45 V for a carbon black reference electrode (XC-72R, Cabot). The relatively low current density for the carbon nitride electrode (0.72 vs 0.91 mA/cm2 for the carbon black electrode) could be attributed to its low surface area (5 m2/g) and poor electric conductivity. A significant improve-
Figure 48. Two predicted structures of g-C3N4 made up of (a) condensed melamine subunits and (b) condensed tris-triazine subunits. Reprinted with permission from ref 283. Copyright 2009 American Chemical Society.
ment in both current density (2.21 mA/cm2) and onset potential (up to 0.76 V) was achieved by blending the carbon nitride with a high surface area carbon black support (50 wt %). In order to overcome the shortcomings of g-C3N4 as an electrocatalyst for ORR, Zheng et al. incorporated g-C3N4 into mesoporous carbon with a large surface area to enhance the electron transfer efficiency of g-C3N4.412 The resulting g-C3N4@ carbon composite (C3N4@CMK-3) was demonstrated to exhibit a significantly improved catalytic activity with a kinetic-limiting current density of 11.3 mA/cm2 at −0.6 V for a nearly 100% fourelectron ORR process and superior methanol tolerance compared to a commercial Pt/C catalyst. As shown in Figure 49a, the pristine mesoporous g-C3N4(m) exhibited two obvious ORR peaks at cathodic voltages of about −0.4 and −0.6 V, respectively, corresponding to a two-step 2e− ORR process. In contrast, the CV curve recorded for g-C3N4@CMK-3 is similar to those obtained on other carbon-based metal-free electrocatalysts with only one single ORR peak at a more positive potential (−0.25 V) and a higher cathodic current, indicating a better electrocatalytic performance for ORR on g-C3N4@CMK-3 over g-C3N4(m). LSV curves given in Figure 49b further revealed the more positive onset potential and higher ORR current density on g-C3N4@CMK-3 than g-C3N4(m) and mixed g-C3N4 + CMK-3 electrodes. This catalyst exhibited high stability with an almost 93% current retention after continuous potentiodynamic sweeps for 45 h (Figure 49c). The CVs in the inset of Figure 49c also revealed the good stability of g-C3N4@CMK-3 with less than 4862
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Figure 49. (a) CVs of ORR on various electrocatalysts in O2-saturated 0.1 M KOH solution. (b) LSV of various electrocatalysts on RDE at 1500 rpm in O2-saturated 0.1 M KOH solution. (c) Current−time (i−t) chronoamperometric response of g-C3N4@CMK-3 at −0.3 V; (inset) cyclic voltammograms under continuous potentiodynamic sweeps. (d) Chronoamperometric responses of Pt/C and g-C3N4@CMK-3 at −0.3 V in O2saturated 0.1 M KOH solution without methanol (0−3 h) and with adding methanol (3−6 h). (e, f) LSV of various electrocatalysts on RDE at different rotating rates (500−2000 rpm) and corresponding Koutecky−Levich plots at −0.6 V. Reprinted with permission from ref 412. Copyright 2011 American Chemical Society.
10% cathodic current loss during ∼10 000 continuous potential cycling. The g-C3N4@CMK-3 is also free from the methanol cross-over effect (Figure 49d). The electron transfer number (n) per O2 for ORR derived from Figure 49e and 49f is 2.6, 1.7, and 4.0 for g-C3N4(m), mixed g-C3N4+CMK-3, and g-C3N4@CMK3, respectively, indicating a one-step 4e− ORR process for ORR on g-C3N4@CMK-3. Subsequently, Liang et al. designed and prepared the macroporous g-C3N4/C with 3D-ordered interconnected structures using silica microspheres as hard templates,281 which possesses prominent ORR catalytic activity comparable to commercial Pt/C in both reaction current density and onset potential. Furthermore, the macroporous g-C3N4/C showed much better fuel cross-over resistance and long-term durability than the commercial Pt/C in alkaline medium. Since inexpensive cyanamide was used as a precursor and easily fabricated silica microspheres as a template, this simple synthetic approach holds great promise for low-cost large-scale production. The pore size of g-C3N4/C can be easily tailored by using silica spheres of
different sizes as templates. For more information on 3D carbonbased metal-free catalysts; please see section 4.1.5. 4.1.4.2. Graphene/Carbon Nitride Nanosheets. As can be seen from the above discussion, g-C3N4 can be prepared by polymerization of cyanamide or melamine at low cost283,402,413 as promising metal-free catalysts for ORR. However, the electron transportation during the ORR process is strongly restricted by its low conductivity and hence a reduced electrocatalytic activity. To prepare carbon nitride materials with enhanced electrical conductivity and surface accessibility, Sun et al. immobilized gC3N4 onto chemically converted graphene (CCG) sheets to form a composite (G-g-C3N4) by polymerizing melamine molecules adsorbed on CCG at a high temperature of 823 K.414 The resultant G-g-C3N4 composite was found to contain 9.5−11.5 wt % nitrogen and exhibited an ORR electrocatalytic activity comparable to that of a CCG composite with ca. 23 wt % Pt nanoparticles (G-Pt). Figure 50a shows broad oxygen reduction peaks at about −0.45 and −0.30 V for the g-C3N4/GC and G-g-C3N4/GC 4863
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graphene-based carbon nitride (G-CN) nanosheets with graphene interdispersed between the g-C3N4/C nanosheets via a nanocasting route.315 The resulting G-CN nanosheets were demonstrated to possess a high nitrogen content, high surface areas, large aspect ratio, and enhanced electrical conductivity attractive for electrocatalysis. As shown in Figure 51a, G-CN800
Figure 51. (a) CVs of G-CN800 at a scan rate of 100 mV s−1 in O2(solid line; lower lying curve) and Ar-saturated (solid line; higher lying curve) 0.1 M KOH solution as well as O2-saturated 0.1 M KOH solution with 3 M methanol (dashed line; the two lower lying curves for the two O2-saturated solutions are fully overlapped). (b) Current−time (I−t) chronoamperometric responses at 0.25 V in O2-saturated 0.1 M KOH on G-CN800 and Pt−C electrode (1600 rpm) followed by introduction of O2 and methanol (0.3 M). Reprinted with permission from ref 315. Copyright 2011 Wiley-VCH.
Figure 50. (a) CVs of g-C3N4/GC, G-g-C3N4/GC, and G-Pt electrodes at a scan rate of 100 mV s−1. (b) RDE voltammograms of g-C3N4/GC, CCG, G-g-C3N4/GC, and G-Pt electrodes at a scan rate of 10 mV s−1 and a rotation rate of 1500 rpm in O2-saturated 0.1 mol L−1 KOH solution. The weight of catalyst was controlled to be the same for each electrode. (c) RDE voltammograms of G-g-C3N4/GC at different rotating rates in 0.1 mol L−1 KOH solution saturated with O2. Scan rate: 10 mV s−1. (d) Koutecky−Levich plot of j−1 vs ω−1/2 obtained from the RDE data of (a) at −0.6, −0.75, and −0.9 V. Dashed lines are calculated for the diffusion−convection-controlled reduction of O2 by 2 (n = 2) or 4 (n = 4) electrons. Reprinted with permission from ref 414. Copyright 2010 Royal Society of Chemistry.
(annealed at 800 °C) showed a featureless CV in an argonsaturated solution and a well-defined cathodic peak in a O2saturated electrolyte solution, arising from oxygen reduction. Furthermore, Figure 51b shows a strong methanol cross-over effect for the Pt−C catalyst but not G-CN800. Superior to the commercial Pt−C catalyst, therefore, G-CN800 nanosheets exhibited a higher selectivity for ORR with a remarkably good tolerance to cross-over effects. Like the G-g-C3N4 discussed above,414 the introduction of graphene into G-CN nanosheets significantly enhanced the electrocatalytic performance of gC3N4 for ORR. Given the low-cost and easy processes for producing the G-g-C3N4 and G-CN composites with high catalytic performance, these graphene and carbon nitride composites are promising metal-free ORR catalysts of practical significance. 4.1.4.3. Mechanism of ORR on Carbon-Nitride-Based Materials. Zheng et al. carried out first-principles calculations to understand the fundamental steps of ORR on g-C3N4.412 As mentioned earlier (Figure 9B), the standard ORR process in alkaline solutions can be described as either a direct 4e− pathway
electrodes, respectively. Compared to the g-C3N4 electrode, the observed 0.15 V positive shift in potential and several times increase in the corresponding peak current for the G-g-C3N4 composite electrode implied that the electrocatalytic activity of gC3N4 was significantly enhanced by CCG (Figure 50a). Furthermore, the ORR electrocatalytic activity of G-g-C3N4 is comparable to that of G-Pt composite with Pt weight content as high as 23%, as can be seen from the similar ORR peak currents for the G-g-C3N4 and G-Pt electrodes. LSV curves given in Figure 50b show a more positive onset potential for the G-g-C3N4 electrode (ca. −0.14 V) than that of the g-C3N4 (−0.35 V), CCG (−0.19 V), and G-Pt (−0.16 V) electrodes with a stronger reduction current at a given applied potential, though still slightly weaker than that of the G-Pt electrode. Figure 50c shows the limiting currents of ORR at G-gC3N4-modified RDE recorded at various rotating rates, from which the Koutecky−Levich plots (Figure 50d) were constructed and the electron transfer number (n) per O2 was calculated to be 2.5−2.7 over potentials from −0.6 to −0.9 V, indicating a combined two-electron and four-electron ORR pathway. The G-g-C3N4 electrode was also shown to be insensitive to CO with a good durability. These results clearly indicate that the CCG support strongly enhanced the ORR catalytic activity of g-C3N4, due presumably to the strongly improved conductivity of g-C 3 N 4 ( C-COP-T > C-COP-P (Figure 60), as predicated by the DFT calculations.449 Compared to C-COP-P, the more positive ORR peak potential (Figure
60B(a)) and slightly higher diffusion current density (Figure 60B(b)) for C-COP-T could be attributed to its slightly higher graphitization degree. Overall, the ORR performance of the CCOP graphitic carbon materials correlated well to the calculated electronic properties for the COP precursors with well-defined N locations and hole sizes. It can be seen in Figure 60B(b) that the LSV curve of the C-COP-4 shows a one-step 4e− process, as is the case of the Pt/C catalyst. C-COP-4 exhibits a similar onset potential as the Pt/C catalyst, and the half-wave potential of the C-COP-4 electrode reaches 0.78 V, which is also very close to 0.80 V for Pt/C. The transferred electron number (n) per O2 4872
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Figure 61. (a) Illustration of the charge-transfer process and ORR on PDDA-CNT. Reprinted with permission from ref 458. Copyright 2011 American Chemical Society. (b) Polyelectrolyte-functionalized graphene as metal-free electrocatalyst for oxygen reduction. Reprinted with permission from ref 459. Copyright 2011 American Chemical Society.
carbon CNTs or graphene, in either an aligned or a nonaligned form, could also act as metal-free electrocatalysts for ORR through the intermolecular charge transfer from the all-carbon CNTs or graphene to the adsorbed PDDA (Figure 61).458,459 In particular, these authors found that quaternary ammonium functional groups along the PDDA backbone have a strong electron-accepting ability to withdraw electrons from carbon atoms in the conjugated nanotube carbon plane to induce the net positive charge, facilitating the ORR catalytic activity of the nitrogen-free CNTs adsorbed with the PDDA chains (Figure 61a).458 It is notable that the PDDA-adsorbed VA-CNT electrode possesses remarkable electrocatalytic properties for ORRsimilar to that of commercially available Pt/C electrodebut a better fuel selectivity and long-term durability. Furthermore, this work clearly indicates that the important role of intermolecular charge transfer to ORR of nitrogen-free CNTs can be applied to other carbon materials for the development of various other metal-free efficient ORR catalysts. Wang et al. further found that physical adsorption of PDDA onto dopant-free graphene could also create net positive charge on carbon atoms in the all-carbon graphene plane via the intermolecular charge transfer (Figure 61b).459 In a typical experiment, the adsorption of PDDA onto the surface of graphene was conveniently performed during the process of reducing GO into graphene by sodium borohydride (NaBH4) in the presence of PDDA. Similar to the case of PDDA-CNT,458 the PDDA-graphene also shows remarkable ORR electrocatalytic activities with a better fuel selectivity, more tolerance to CO poisoning, and higher long-term stability than that of commercially available Pt/C electrode. It is believed that the active site for ORR is still carbon atoms in the graphene sheets with the adsorbed PDDA to create somewhat delocalized positive charges on the conjugated carbon surface of graphene to alter the electronic properties of graphene and its adsorption behavior toward oxygen for facilitating the ORR process. The ease with which all-carbon CNTs and graphene can be converted into efficient metal-free ORR electrocatalysts simply by the adsorption-induced intermolecular charge-transfer suggests considerable room for cost-effective preparation of various metal-free catalysts for oxygen reduction and even new catalytic materials for applications beyond fuel cells.
molecule for C-COP-4 calculated by the Koutecky−Levich (K− L) equation is 3.90 at 0.55−0.70 V (Figure 60B(c)), which is similar to 3.88 calculated by the RRDE curves.449 The first-principles calculations indicate that O2 molecules prefer to be adsorbed on the top of the phenyl group adjacent to triazine groups via the Yeager model (section 4.1.1.3) on the CCOP-4 graphitic carbon (Figure 60B(d)). Since the N atom in the triazine group possesses a strong electron affinity with a substantially high negative charge density to counterbalance C atoms, the O atoms with a negative charge density in the adsorbed O2 molecules prefer to stay around the C atoms in phenyl groups with a relatively lower positive charge density rather than those C atoms adjacent to N atoms. Moreover, the bond length of O2 molecules adsorbed on the C-COP-4 graphitic carbon is elongated from 1.216 Å in pure O2 molecules to 1.232 Å at the γ site, suggesting that the parallel diatomic adsorption could effectively weaken the O−O bonding to facilitate ORR at the C-COP-4 electrode. Therefore, the N-doped C-COP-4 electrode can efficiently create the metal-free active sites for electrochemical reduction of O2 through the charge redistribution (section 4.1.1). The C-COP-4 electrode was further demonstrated to be free from the methanol cross-over effect (Figure 60B(e)). Although the C-COP-4 electrode showed a weak CO-poisoning effect, possibly due to the hole (edge) adsorption, its electrocatalytic activity can be self-recovered to 90% within a short period of time (∼5 min) (Figure 60B(f)). Furthermore, the C-COP-4 electrode exhibited a remarkably better long-term stability than the commercial Pt/C catalyst. These results clearly indicate that the C-COP-4 graphitic carbon is a promising metal-free ORR catalyst for fuel cells. Apart from the demonstrated high electrocatalytic activities toward ORR, the observed electrocatalytic activities of these N-doped holey graphitic carbon materials can be correlated to the N locations in their COP molecular precursors, once again showing the technological power and importance of the N-location control for tailoring the structure and property of N-doped carbon nanomaterials. 4.2. Intermolecular Charge Transfer
In addition to the intramolecular charge transfer that imparts ORR electrocatalytic activities to heteroatom-doped CNTs, graphene, and graphite described above, Wang et al. demonstrated that certain polyelectrolyte (e.g., poly(diallyldimethylammonium chloride), PDDA) adsorbed pure 4873
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4.3. Spin Redistribution
that the incorporation of nitrogen functional groups within the carbon structure improved the oxidative stability of the NCNTs under ORR conditions even in the acidic medium. On the other hand, Wang et al. demonstrated that nitrogen-doped OMCS via NH3 activation can also act as alternative metal-free catalysts with a high ORR activity and better stability than Pt-based electrocatalysts in 0.05 M H2SO4.20 Recently, Li et al. showed that few-walled CNTs with the outer wall exfoliated via oxidation and high-temperature reaction with ammonia could act as an ORR electrocatalyst in both acidic and alkaline solutions.465 Under controlled oxidation conditions, these authors created nanoscale sheets of graphene, containing extremely small amounts of iron residues and nitrogen impurities, attached to the inner tubes of the few-walled CNTs by partially unzipped their outer walls. In comparison to other nonprecious metal catalysts, the resulting CNTs−graphene complexes exhibited a high ORR activity, excellent tolerance to methanol, and superior stability in acidic media. The graphene sheets formed from the unzipped part of the outer wall of the nanotubes are responsible for the catalytic activity; the inner walls remain intact and retain their electrical conductivity to facilitate charge transport during electrocatalysis. More recently, Shi et al. prepared sulfur- and nitrogencodoped CNTs (SN-CNTs) by annealing N-CNTs with sulfur and found that the as-prepared SN-CNTs exhibited enhanced ORR activity in both acidic and alkaline media compared with NCNTs.327 Figure 63 shows CV of the SN-CNTs in comparison with those of N-CNTs in acidic and alkaline media. As shown in Figure 63a and 63b, there are notable oxygen reduction peaks at about +0.29 (vs Ag/AgCl) and −0.3 V (vs SCE) for the SNCNTs in O2-saturated acid and alkaline solutions, respectively, with the strongest peak current among CNTs, N-CNTs, and SCNTs, indicating that S and N codoping significantly increased ORR activity. Moreover, the reduction potentials and corresponding peak currents of SN-CNTs in the aqueous solution of 1 M HClO4 are slightly lower than those in 0.1 M KOH aqueous solution. LSV curves for SN-CNTs in 1 M HClO4 and 0.1 M KOH are given in Figure 63c and 63e, respectively, with the corresponding Koutecky−Levich plots (J−1 vs w−1/2) in Figure 63d and 63f. The electron transfer numbers (n) of all samples at different potentials were calculated according to the Koutecky− Levich equation. The calculated average value of n for the SNCNTs in acidic medium is 3.42 (Figure 63d), indicating a mixture of two- and four-electron processes. In alkaline solution, the average n of SN-CNTs is 4 (Figure 63f), which is the same value as Pt−C (4.00) and higher than that of N-CNTs (3.83), indicating a full four-electron pathway for ORR on SN-CNTs. Furthermore, the SN-CNT catalysts exhibited an excellent durability with a high current retention of 89.6% even after 10 000 cycles, whereas Pt−C catalyst showed a gradual decrease with a low current retention of 49.3% under the same condition. The enhanced ORR activity of SN-CNTs can be attributed to the codoping effect with introduction of S to change the state of N species in the N-doped CNTs and create asymmetrical spin and charge density, and hence improved ORR performance. Therefore, the reported SN-CNTs are promising ORR catalysts for PEMFCs, and the codoping with synergistic effects provides a new approach to explore low-cost electrocatalysts for practical fuel cell applications even in acidic media. More examples of metal-free electrocatalysts for ORR in acidic electrolytes include the metal-free synthesized N-doped SWNTs,138 vapor-phase-polymerized PEDOT,216 space-confinement-synthesized pyridinic and pyrrolic N-doped gra-
As can be seen from the above discussion, the doping-induced charge polarization due to the difference in electronegativity between carbon (χ = 2.55) and heteroatoms is accountable for the ORR activities of N-doped (χ = 3.04) CNTs, graphene and graphite. Since the charge polarization stemming from the difference in electronegativity between carbon and sulfur (χ = 2.58) is almost negligible, a different mechanism must have contributed to the enhanced ORR activity of SGnP discussed above (section 4.1.3.3). In this context, theoretical calculations were conducted to investigate the origin of high ORR activity of SGnP.191 The calculated results show that the electronic spin density, in addition to generally considered charge density, plays a key role in the high ORR activity of SGnP. Thus, the S atoms doped at the edges of the graphene nanoplatelets strongly promote the ORR activity.191 The covalently bonded sulfur or sulfur oxide at the zigzag and armchair edges of graphene has induced both charge and spin density on the graphene (Figure 62). Therefore, carbon atoms with either high spin densities and/
Figure 62. (a) HOMO and (b) LUMO distributions of SGnP with doping-induced charge and spin redistributions. Reprinted with permission from ref 191. Copyright 2013 Wiley-VCH.
or positive charge could serve as the active sites, which promotes catalytic activity for ORR.191 The theoretical prediction is consistent with the experimental results on the S-doped graphene.191 4.4. Heteroatom-Doped Carbon Nanomaterials as Metal-Free ORR Catalysts in Acids
Alkaline fuel cells with Pt-loaded carbon as an electrocatalyst for the four-electron ORR were first developed for the Apollo lunar mission in the 1960s.460 Although their large-scale commercial application has been precluded by the high cost of the Pt-based electrode, the alkaline fuel cell system not only provides the onboard power system for the Apollo lunar missions but also stimulates enormous, long-lasting interest in fuel cell technology. As can be seen from the above discussions, most recent studies on the metal-free electrocatalysts based on heteroatom-doped carbon nanostructures focused on the ORR reaction in alkaline electrolytes. However, fuel cells that operate with acidic electrolytes, particularly the polymer electrolyte fuel cell (PEMFC),461−464 could have a more significant economic impact. As discussed in preceding sections,31a,138,283,448 some of these metal-free heteroatom-doped nanocarbons have also been demonstrated to exhibit ORR activities even in acidic electrolytes. In addition, Kundu et al. found that NCNTs prepared by pyrolysis of acetonitrile could show a considerably higher and more stable ORR activity in 0.5 M H2SO4 compared with their undoped counterparts.277 These authors pointed out 4874
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Figure 63. Typical CVs for the ORR of (a) N-CNTs and the annealed N-CNTs catalysts in the aqueous solution of 1 M HClO4, (b) CNTs, S-CNTs, NCNTs, and SN-CNTs catalysts in 0.1 M KOH aqueous solution. LSV curves of SN-CNTs catalysts in oxygen-saturated 1 M HClO4 solution (c) and 0.1 M KOH solution (e) at different rotation rates at a scan rate of 10 mV s−1. Koutecky−Levich plot of J−1 versus w−1/2 at different electrode potentials of SN-CNTs obtained in oxygen-saturated 1 M HClO4 solution (d) and 0.1 M KOH solution (f). Reprinted with permission from ref 327. Copyright 2013 Royal Society of Chemistry.
phene,307 and electrostatically induced graphene/CNT layer-bylayer self-assembly.364 However, the catalytic performance of these reported N-doped carbon nanomaterials in acidic medium still needs to be further improved to meet the requirement for practical applications.
materials which are earth-abundant, cost-effective, stable, and very active in catalyzing the ORR/OER/HER reactions for the renewable energy technologies. Recent experiments indicate that hybrid materials consisting of metal oxide (e.g., Co3O4) nanoparticles on N-doped graphene have unusual bifunctional catalytic activity for ORR and OER, arising from synergetic chemical coupling effects between the particles and the N-doped graphitic carbon plane.26 Since the optimal active-site structures for different energy-related reactions (e.g., ORR, OER, HER) are often not identical, bifunctional or multifunctional catalysts are needed, especially for rechargeable Li−O2 batteries in nonaqueous media and electrochemical water splitting. This requires designing catalytic structures that contain more than one type of active sites. In spite of the long history for the development of individual ORR and OER electrocatalysts based on metal/metal oxides, however, the preparation of ORR and OER bif unctional electrocatalysts based on metal/oxide hybrids is only a recent development.26 Below, recent progress in the emerging ORR/OER and OER/HER
4.5. Heteroatom-Doped Carbon Nanomaterials as Bi-/Multi-functional Metal-Free Catalysts
Just like the fact that ORR is paramount to fuel cells, OER and HER are at the heart of metal−air batteries and electrochemical water-splitting systems. Catalysts are required to promote these electrochemical reactions that generate power or fuels. Platinum group metal catalysts are used in fuel cells and lithium−air batteries to accelerate the ORR electrochemical process,7,222,234,466 while many metal oxides are used as catalysts (e.g., vanadium oxides in batteries) for OER and HER.467,468 As seen above, however, these metal-based catalysts often suffer from multiple disadvantages, including their high cost, low selectivity, poor durability, and sometimes detrimental environmental effects.123 It is highly desirable to develop alternative 4875
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Figure 64. (a) Rate performance of the VA-NCCF electrode under current densities of 100, 600, and 1000 mA g−1. (b) Discharge/charge voltage profile of the VA-NCCF as a function of specific capacity. Cutoff voltages were 2.2 V for discharging and 4.4 V for charging. Current density was 500 mA g−1. (c) Cycling performance of the VA-NCCF electrode on the Toray carbon paper substrate. (d) Representative discharge−charge curves from the 1st to the 200th cycle. Current density was 250 mA g−1, and a controlled capacity was 500 mAh g−1. Reprinted with permission from ref 477. Copyright 2014 American Chemical Society.
aqueous Li−O2 batteries.477 An extremely narrow voltage gap (0.3 V, Figures 64a and 64b) between the charge and the discharge plateaus and an unusually high energy efficiency of 90% were obtained (Figure 64c). More than 150 highly reversible cycles under a specific capacity of 1000 mAh g−1 were also demonstrated (Figure 64c). Due to the low overpotential of the VA-NCCF electrode, the electrolyte decomposition could be minimized, leading to a long device cycle life. To investigate the stability of the VA-NCCF electrode, continuous charge/discharge cycling of one cell was performed using the SS cloth substrate for more than 2 months and 150 cycles at a specific capacity of 1000 mAh g−1 per discharge/charge cycle. As can be seen in Figure 64c and 64d, the cell based on the SS-supported VA-NCCF electrode showed an almost constant capacity over 150 cycles and the voltage profile was fairly stable with little change in overpotential after the 10th cycle (Figure 64d). It is the interplay of the N-doping-induced high catalytic activity, the coral-like microstructure, and the highly conductive microporous SS cloth support that provides the VA-NCCF oxygen electrode with high energy efficiency, low overpotential, and long cycle life. This work clearly demonstrated that the performance of Li−O2 batteries could be dramatically improved by using rationally designed ORR/OER bifunctional electrodes with well-defined hierarchical structures and heteroatom-doping-induced catalytic activities, which represents a significant advance in the research and development of Li−O2 batteries and other energy devices. Having exhibited good ORR catalytic activities, certain carbon nanomaterials, including CNTs and graphene, with high surface areas have also been demonstrated to be alternative electrocatalyst for OER.25,396,474,478−487 Metal-free bifunctional ORR/ OER catalysts were also prepared by an injection CVD of
bifunctional metal-free catalysts based on heteroatom-doped carbon nanomaterials is reviewed. 4.5.1. Bifunctional Metal-Free Catalysts for ORR/OER. The key charge/discharge reactions that take place at the oxygen electrode in Li−air batteries469 involve oxygen reduction by electrons from the current collector to combine with Li+ ions dissolved in the electrolyte to produce solid Li2O2 on the cathode during discharging.470 Subsequent charging causes a reverse reaction (i.e., OER). Carbon materials in oxygen electrodes have been proven to decompose accompanying the electrolyte decomposition, particularly at high overpotentials.471 Generally speaking, an ideal oxygen electrode requires a highly conductive porous structure to facilitate both electron and oxygen transportations. A large specific surface area is also desirable for the electrode to show a high Li2O2 update. Spherical-shaped Li2O2 particles often grow on the fiber surface of most currently used fibrous electrodes with a very limited contact to the conductive substrate,472−474 while a homogeneous coating layer of Li2O2 over the electrode material is more favorable than separated large particles for a uniform electrochemical reaction on the electrode. Very recently, noncrystalline Li2O2 has been demonstrated to form on MWCNTs decorated by noble metal catalysts with the catalytically functionalized fiber surface to facilitate the ORR all over the fiber surface.475 The continuous coating layer of Li2O2 over the electrode material was found to be also favorable for the subsequent OER because the improved contact between the electrode and Li2O2 led to a low overpotential.475,476 In this context, Shui et al. recently prepared vertically aligned nitrogen-doped coral-like carbon nanofiber (VA-NCCF) arrays by CVD and then transferred the as-prepared VA-NCCF onto a piece of microporous stainless steel (SS) cloth as a binder-free ORR/OER bifunctional electrode for non4876
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Figure 65. (a, b) HER polarization curves and Tafel plots for four metal-free electrocatalysts and 20% Pt/C (electrolyte 0.5 M H2SO4, scan rate 5 mV s−1). Curve referring to C3N4@NG was recorded for the sample with 33 wt % of g-C3N4 in the hybrid. (c) Polarization curves recorded for C3N4@NG hybrid before and after 1000 potential sweeps (from +0.2 to −0.6 V versus reversible hydrogen electrode) under acidic and basic conditions. (d) Electrochemical impedance spectroscopy data for C3N4@NG hybrid and C3N4/NG mixture in H2SO4; data were collected for the electrodes under HER overpotential = 200 mV. Reprinted with permission from ref 492. Copyright 2014 Nature Publishing Group.
applied,491 and hence, catalysts for high photovoltaic efficiency (e.g., in DSSCs) are also needed for photoelectrochemical water hydrolysis. In this context, Zheng et al. demonstrated that graphitic-carbon nitride supported by nitrogen-doped graphene (g-C3N4@NG) could act as a metal-free HER catalyst with an unexpected hydrogen evolution reaction activity comparable to some well-developed metallic catalysts.492 Their experimental observations were complemented by density functional theory calculations, which reveal that the observed unusual electrocatalytic properties are originated from an intrinsic chemical and electronic coupling to synergistically promote the proton adsorption to enhance HER kinetics. As shown in Figures 65a and 65b, the polarization curve (i−V) recorded on C3N4@NG shows an overpotential of ∼240 mV, 10 mA cm−2 HER current density, a Tafel slope of 51.5 mV dec−1, and an HER exchange current density (i0) of 3.5 × 10−7 A cm−2 (calculated from the Tafel plot by extrapolation), which are comparable to those of the well-developed nanostructured MoS2-based metallic catalysts.493−496 Because of the interfacial covalent bonds between g-C3N4 and N-doped graphene layers, the g-C3N4@NG catalyst showed a robust stability in both acidic and alkaline solutions (Figure 65c). In contrast, the nonconductive pure g-C3N4 or the nonactive N-doped graphene showed negligible HER activity (Figure 65a). Besides, the gC3N4@NG hybrid exhibited a higher proton reduction current and electrical conductivity (Figure 65d) than the g-C3N4/NG mixture, indicating a synergistic interaction between g-C3N4 and N-doped graphene in the former. To elucidate the mechanism of HER on the metal-free catalysts, Zheng et al. studied two generally accepted HER mechanisms,492 namely, Volmer−Heyrovsky and Volmer−Tafel
ethylene-diamine-based NCNT on thermally reduced graphene oxide (TRGO), and its electrocatalytic activities for oxygen reduction and evolution reactions were investigated for metal− air battery applications.488 The resulting TRGO/NCNT composite was demonstrated to exhibit not only ORR performance similar to that of commercial Pt/C catalyst but also a superior OER activity with an excellent electrochemical durability. 4.5.2. Bifunctional Metal-Free Catalysts for OER/HER. H2 can be used as a fuel in fuel cells to directly and efficiently produce electricity, which is the ultimate source of clean carbonfree chemical energy. If H2 gas can be produced from water and sunlight, it is inherently sustainable. Solar hydrogen production, however, has not been practical in a sustained manner,489,490 though conceptually it is promising. This is because the overall reaction is thermodynamically highly unfavorable. Consequently, the use of appropriate OER and HER bifunctional catalysts to reduce both overpotentials is essential. Ideally, the catalyst materials to be developed can be used in a single photoelectrochemical cell with integrated HER and OER catalysts contained in a shared aqueous solution. However, it is rare to find OER and HER catalysts, which can be coupled in the same pH range to work together efficiently for water splitting as almost all the best OER catalysts, particularly based on earthabundant elements, work well only in neutral or basic media,491 whereas most of the HER catalysts are only good in acidic media.491 Thus, finding OER/HER catalysts that can work efficiently in a wide range of pHs has been among the “holey grails” of chemistry for decades. To achieve a practical rate of H2 generation, voltages above the thermodynamic potentials for both half reactions need to be 4877
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Figure 66. Reaction pathways of HER on C3N4@NG according to the Volmer−Heyrovsky route (a) and Volmer−Tafel route (b). Dashed lines are activation barriers for each reaction step as reported in ref 498. Reprinted with permission from ref 492. Copyright 2014 Nature Publishing Group.
Figure 67. (a) Steady-state voltammograms showing the oxygen reduction reaction taking place on VA-NCNT at various oxygen partial pressures. Potential scan rate = 5 mV cm−1 with the third scans shown. (b) ORR electrocatalytic profile of Pt black on glassy carbon electrode (red) and a VANCNT on Pt mesh (blue) electrode immersed in an air-saturated solution containing H2SO4 (pH = 3). Potential scan rate = 5 mV s−1 with the third scans shown. Reprinted with permission from ref 506. Copyright 2010 American Chemical Society.
reactions.497 On the basis of the DFT calculation-derived HER pathway for the Volmer−Tafel mechanism, Figure 66b shows that there is a 0.33 eV difference in free energy for the secondstep Tafel reaction under equilibrium potential, which is higher than that of the Heyrovsky reaction (0.19 eV, Figure 66a). Such energy difference can be eliminated at higher overpotential, for example, 0.33 V, under which the free energy of the second and third reaction steps is the same (the red line in Figure 66b). Therefore, there is a potential-dependent pathway selectivity on g-C3N4@NG: at low overpotential the Volmer−Heyrovsky mechanism with a rate-limiting step of electrochemical desorption is the most probable, whereas it becomes the Volmer−Tafel mechanism at high overpotential. Also included in Figure 66 are the corresponding potential-dependent barrier values derived from a previously reported Pt surface (dashed lines) for comparison. By assuming that g-C3N4@NG possesses the same energy barriers for each reaction step as those on Pt surface, it was found that the Volmer−Tafel mechanism is much faster than the Volmer−Heyrovsky one at low overpotentials, and they become equally fast around −1.0 V versus reversible hydrogen electrode.492
instant power output with high energy conversion efficiency and high power density without any detrimental effect on environment (e.g., flame, combustion, noise, or vibration). Instead of burning fuel to create heat, fuel cells convert chemical energy directly into electricity by electrochemically oxidizing fuel (e.g., H2) and reducing oxygen into water. This energy conversion technology currently receives intensive research and development focus because of its high energy conversion efficiency (typically, 40−60% or up to 85% efficiency if waste heat is captured for use), virtually no pollution, and potential large-scale applications.4 However, catalysts are required for hydrogen oxidation at the anode and ORR at the cathode.503−505 Traditionally, Pt has been regarded as the best catalyst for fuel cells. While the very facile H2 oxidation kinetics greatly reduces the amount of catalyst (e.g., platinum) at the anode, the slow ORR on the cathode is a key step to limit the energy conversion efficiency of a fuel cell and requires a substantial amount of platinum catalyst. The large-scale practical application of fuel cells will be difficult to realize if the expensive Pt-based electrocatalysts for ORR cannot be replaced by other efficient, low-cost, and stable electrodes. Thus, the heteroatom-doped carbon ORR electrocatalysts have recently received intensive research and development focus because of their low cost, high electrocatalytic activity, and scalable production. Although excellent ORR performance, particularly in alkaline
4.6. Performance Evaluation for Heteroatom-Doped Carbon ORR Catalysts in Fuel Cells
A fuel cell, in particular PEMFC,13,499−502 is one of the most probable options to alternative energy sources that can provide 4878
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Figure 68. (a) Anion-exchange membrane fuel cell performance of the MEA with N-CNT (blue curves) and Pt/C (red curves) as cathodes at 323 K (open and closed symbols corresponding to the cell voltage and power density, respectively), and (b) corresponding cell voltage−time curves at 20 mA/ cm2 for 30 h. Reprinted with permission from ref 29. Copyright 2012 American Chemical Society.
Figure 69. (A) Schematic drawings for the fabrication of membrane electrode assembly (MEA) from VA-NCNT arrays (0.16 mg cm−2) and the electrochemical oxidation to remove residue Fe. C.E., counter electrode; R.E., reference electrode; W.E., working electrode. (B) Typical SEM image of the VA-NCNT array. (C) Digital photoimage of the used MEA after a durability test with the cross-section SEM images shown in the insets. (D) Polarization curves as a function of the areal current density after accelerated degradation by repeatedly scanning the cell from the open circuit voltage (OCV) to 0.1 V at a rate of 10 mA s−1. (E) Polarization and power density as a function of the gravimetric current density. Cathode catalyst loading = 0.16 mg cm−2; Nafion/VA-NCNT = 1/1. H2/O2, 80 °C, 100% relative humidity, back pressures 2 bar. Reprinted with permission from ref 31a. Copyright 2015 American Association for the Advancement of Science.
5.38 × 10−3 cm s−1, which compared favorably with those obtained on Pt-based catalysts. A critical comparison of the catalytic performance for VA-NCNT was made in Figure 67b against the commonly used Pt black catalyst containing 2.2 nm Pt particles dispersed onto a carbon black support. About 90 μg of Pt black generated a ∼20 μA peak transient ORR current. In contrast, a sum of 50 μg (electrochemically accessible) of VANCNT yielded a ∼120 μA steady-state ORR current. In addition to the significantly greater current output, the VA-NCNT also resulted in a ∼47 mV anodic shift in oxygen reduction potential. More importantly, the volumetric current density for the metalfree VA-NCNT catalyst is 2108.3 A cm−3, which represents a
media,8,19,368 has been demonstrated for many of the carbonbased ORR catalysts, the performance evaluation of these nanocarbon catalysts in practical fuel cells should be performed. Xiong et al. investigated nitrogen-doped CNT arrays as an ORR catalyst in a PEMFC analogous acidic medium and demonstrated a strong ORR signal at the favorably positive potential.506 In an aqueous solution mimicking the chemical environment of a PEMFC, a discrete reduction current was seen to be proportional to the concentration of O2 (Figure 67a). The voltammograms exhibited virtually steady-state features, including the reduction current plateau and literally zero reoxidation current. In this case, the ORR rate constant was estimated to be 4879
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Figure 70. Cross-section SEM images of (A, B) the densely packed catalyst layer of N-G-CNT/Nafion (0.5/0.5 mg cm−2) and (C, D) porous catalyst layer of N-G-CNT/KB/Nafion (0.5/2/2.5 mg cm−2). (D) Purple arrows indicate the parallelly separated N-G-CNT sheets with interdispersed porous KB agglomerates. (E) BET surface areas and (F) pore volume distributions of a piece of 5 cm2 gas diffusion layer (GDL), GDL with KB (2 mg cm−2), GDL with N-G-CNT (0.5 mg cm−2), and GDL with N-G-CNT/KB (0.5/2 mg cm−2) as indicated in the figures. (G, H) Schematic drawings of the MEA catalyst layer cross-section, showing O2 efficiently diffused through the carbon black-separated N-G-CNT sheets (G) but not the densely packed N-GCNT sheets (H). Reprinted with permission from ref 31a. Copyright 2015 American Association for the Advancement of Science.
Figure 71. (A) Polarization curves of N-G-CNT with loadings of 2, 0.5, or 0.15 mg cm−2 plus KB 2 mg cm−2 for each cathode. Weight ratio of (N-GCNT/KB)/Nafion = 1/1. (B) Cell polarization and power density as a function of gravimetric current for the N-G-CNT/KB (0.5/2) mg cm−2 with the weight ratio of (N-G-CNT/KB)/Nafion = 1/1. (C) Durability of the metal-free N-G-CNT in a PEM fuel cell measured at 0.5 V compared with a Fe/N/ C catalyst. Catalyst loading of N-G-CNT/KB = 0.5 mg cm−2, and Fe/N/C = 0.5 and 2 mg cm−2. Test condition: H2/O2, 80 °C, 100% relative humidity, back pressures 2 bar. Reprinted with permission from ref 31a. Copyright 2015 American Association for the Advancement of Science.
and substantially higher ethanol tolerance than Pt/C. On the basis of these results, the VA-NCNT metal-free electrocatalysts were evaluated in anion-exchange membrane fuel cells (AEMFCs).370,508,509 Figure 68a reproduces the steady-state polarization curves for the VA-NCNT and commercial Pt/C (ETEK) cathodes, respectively, in AEMFCs under identical testing conditions, which shows an open-circuit voltage of ∼0.87 and 1.0 V, respectively. The membrane−electrode assembly (MEA) with NCNT exhibited Pt-like behavior in the current density region of 0−50 mA/cm2 but with a low cell voltage. The
62% higher performance than the entrance benchmark commonly accepted for the ORR catalysts.507 This work indicates that N-doped CNTs are promising metal-free ORR catalysts for fuel cell applications. On the other hand, Rao and Ishikawa29 prepared metal-free, VA-NCNTs with a N content of 8.0 atom % by pyrolyzing a suitable polymer precursor within an alumina template. Using RDE voltammetry, these authors investigated electrocatalytic ORR activities of the resultant VA-NCNTs in alkali and found an ORR activity comparable to Pt/C with a four-electron pathway 4880
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Figure 72. (a) Schematic pathway of oxygen reduction in the NCNTs-MFCs. (b) Power densities and cell voltages in N-CNTs-MFCs and Pt/CMFCs. Error bars represent standard deviations of duplicate tests. Performance of NCNTs-MFCs (c) and Pt/C-MFCs (d) at 1000 Ω during the first four filling cycles. Arrows indicate when reactor was fed with fresh medium. Reprinted with permission from ref 510. Copyright 2011 Royal Society of Chemistry.
that a 5 cm2 porous cathode has a surface area of 155 m2 g−1 (or 1161 m2 g−1 for the N-G-CNT/KB after taking off the weight of GDL and Nafion) and a significant number of pores from microto macrosizes (Figures 70E and 70F). The presence of pores, as shown in Figures 70C and 70D, was demonstrated to significantly facilitate the mass transfer of O2 gas in the porous N-G-CNT/KB catalyst layer (Figures 70G) with respect to a densely packed N-G-CNT@GDL MEA without interspersed carbon black particles (Figure 70H). Consequently, the addition of carbon black into the N-G-CNT catalyst layer in the MEA was found to cause ∼85% improvement on the delivered current density at low voltage range (
