Metal-Free and Noble Metal-Free Heteroatom-Doped Nanostructured

Sep 6, 2016 - Road, Piscataway, New Jersey 08854, United States. CONSPECTUS: The large-scale deployment of many types of fuel cells and electrolyzers ...
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Metal-Free and Noble Metal-Free Heteroatom-Doped Nanostructured Carbons as Prospective Sustainable Electrocatalysts Tewodros Asefa* Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, 610 Taylor Road, Piscataway, New Jersey 08854, United States Department of Chemical and Biochemical Engineering, Rutgers, The State University of New Jersey, 98 Brett Road, Piscataway, New Jersey 08854, United States Institute for Advanced Materials, Devices and Nanotechnology (IAMDN), Rutgers, The State University of New Jersey, 607 Taylor Road, Piscataway, New Jersey 08854, United States CONSPECTUS: The large-scale deployment of many types of fuel cells and electrolyzers is currently constrained by the lack of sustainable and efficient catalysts that can replace the less earth-abundant, noble metal-based catalysts, which are commonly used in these renewable energy systems. This burgeoning issue has led to explosive research efforts worldwide to find alternative, metal-free and noble metal-free catalysts that are composed of inexpensive and earth-abundant elements. Hence, the recent discoveries that doping carbon nanomaterials with heteroatoms (such as N, S, B, etc.) can give sustainable materials with good electrocatalytic activity for reactions carried out in fuel cells and electrolyzers have been not only quite exciting but also very promising to address these challenging issues. Interestingly, even though they contain no metals or involve only the inexpensive, more earth-abundant ones, the catalytic activity of some of these materials fares well with those of the commercially used noble metalbased electrocatalysts, such as Pt/C. However, research efforts to improve the catalytic activity, selectivity, and stability of some of these materials for various reactions are still necessary and thus continuing. While some of these efforts have focused on finding synthetic methods that can tune the structures and compositions of already known materials and thereby improve their catalytic properties (activity, selectivity, stability, etc.), others have focused on developing entirely new materials that can exhibit better or superior catalytic properties. In these efforts, additional considerations are also being paid to find facile synthetic routes or renewable and inexpensive precursors that can lead to such types of catalysts in order to make the entire process highly sustainable and widely applicable. In this Account, notable heteroatom-doped carbon catalysts that have been developed for reactions in fuel cells and water electrolyzers, the various synthetic procedures employed to make them, and the challenges involved in their synthesis as well as their characterizations are discussed. The methods used to systematically vary the structures and compositions of the precursors of these materials, as well as the materials themselves, and the different experimental, imaging, and spectroscopic methods used to investigate the properties and structure−property relationships of the materials for various energy related reactions are also included. The discussions focus mainly around the recent notable results reported on these materials by the author’s and other research groups worldwide, albeit not exhaustively. Finally, the author’s perspective about the challenges remaining in the field that need to be addressed, the many existing unanswered questions that beg for more research, and the future prospects for research related to the above topics are also mentioned.

1. INTRODUCTION Sustainable and renewable energy sources that can meet the rising global energy demand, while ameliorating the unabated negative environmental impacts caused by fossil fuels, are currently in dire need. In this regard, fuel cells (which can generate electricity from chemical fuels such as hydrogen)1,2 and water electrolyzers (which can generate the clean fuel hydrogen from water)3 (Figure 1) are among energy systems that stand out to meet these burgeoning challenges; they have, therefore, garnered longstanding and considerable attention. However, the feasibility of large-scale commercialization of fuel cells and water electrolyzers has so far been plagued by the © XXXX American Chemical Society

unavailability of sustainable, efficient catalysts for the reactions involved in these energy systems. Conversely, massive improvements in fuel cells and water electrolyzers require the development of sustainable catalysts based on earth-abundant elements that can promote the reactions running in these systems, at less energy expense or with energy as close as what is thermodynamically required to drive the reactions. If we take the hydrogen fuel cell, which is among the most widely studied, as an example, the pertinent reactions associated Received: June 24, 2016

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Figure 1. Schematic illustration of the working mechanisms of H2/O2 (A) alkaline and (B) acid polymer electrolyte membrane (PEM) fuel cells. The whole assembly, consisting of the two electrodes and a membrane (typically a polymer membrane such as Nafion), is sometimes referred to as membrane electrode assembly (MEA). Reproduced and adopted with permission from refs 1 and 2. Copyright 2013 Royal Society of Chemistry and 2016 Elsevier, respectively. Schematics of the operating principle of (C) alkaline and (D) acid PEM water electrolyzers, respectively, generating hydrogen (H2) and oxygen (O2) from water. Reproduced with permission from ref 3. Copyright 2013 Elsevier Ltd.

Figure 2. Summary of the structural models proposed for some of the most important carbon-based nanostructured materials. Adopted with permission from refs 41 and 42. Copyright 2014 and 2008 Royal Society of Chemistry. When these kinds of materials are doped with heteroatoms using different synthetic methods, various metal-free carbon electrocatalysts for reactions such as ORR and OER can be obtained.

those of noble metals have been well received.6−10 Additionally, these findings have defied conventional paradigms in the stateof-the-art in catalysis where Pt and noble metals had long been deemed the only catalysts that are active enough for use in the reactions carried out in fuel cells and water electrolyzers. While the high catalytic activities exhibited by such metal-free carbon materials have been attributed to the presence of heteroatom dopants within the carbon nanostructures of such materials, which can come in different forms (Figure 2), exactly how the different heteroatomic species help these materials to transform these difficult-to-promote reactions still remains far from being fully understood. Moreover, based on additional experiments carried out by the author’s and a few other research groups, it has been found that adding metals such as Co, which are often suggested to aid the catalytic activity of such metal-free systems, is actually detrimental, or at least not helpful, to the activities of some of these materials.11 All these unexpected results have, thus, called for more research in this area, especially to fully understand how these fascinating materials exactly catalyze reactions and to find facile synthetic routes that can lead to highly efficient and stable metal-free materials with desired catalytic sites and selectivity, particularly for ORR and OER.

with it include not only the two obvious ones, namely, the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR), but also the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), which are needed in order to obtain the hydrogen required to run the fuel cell from a sustainable source, that is, water. Among these four reactions, ORR and OER are particularly the most difficult ones to carry out and also the most responsible for the performance of fuel cells and water electrolyzers, respectively.1,4,5 In other words, these two reactions are the major bottlenecks, critically limiting the efficiency of fuel cells as well as water-splitting electrolyzers. Thus, searching for the most efficient as well as sustainable catalysts for these two reactions is of paramount importance to make these energy systems efficient and widely available. This is also why research on finding the most effective and sustainable catalysts for these two reactions is often regarded as one of the “Holy Grails” in catalysis and renewable energy research today. It is, therefore, not surprising that the recent findings by the author’s and other research groups worldwide on the ability of heteroatom-doped, metal-free nanostructured and nanoporous carbon materials to exhibit unprecedented high electrocatalytic activities for ORR, OER, etc. with comparable efficiencies as B

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2. ISSUES RELATED TO CATALYSIS IN CONVENTIONAL FUEL CELLS AND WATER ELECTROLYZERS The applications of fuel cells and water electrolyzers have so far been limited to relatively small scales. This is mainly because of the challenging issues related to the catalysts and catalysis involved in them, which are summarized below.

The latter is less desirable because it is inefficient for fuel conversion, besides the H2O2 it produces can make many types of metal-based catalysts corrode. Unfortunately, many catalysts yield the partially reduced product, that is, H2O2, in significant amounts during ORR. In the case of OER, the reaction may also face similar undesired, competitive reactions. This is because the oxidation half reaction in water electrolyzers can occur through a twoelectron process, rather than a four-electron process. This means that a H2O molecule undergoes only partial oxidation and produces the intermediate product H2O2, instead of O2. Although the H2O2 can further undergo a two-electron disproportionation reaction to form H2O and O2,24 the process reduces the overall efficiency of the electrolyzers.

2.1. High Cost and Scarcity of Catalysts

The cathode and anode electrodes in many conventional fuel cells as well as water electrolyzers contain electrocatalysts comprised of expensive and less earth-abundant noble metals such as Pt and Ir.12 This makes fuel cells and water electrolyzers costly and unfeasible for large-scale commercial applications. The issue of cost is not exclusive to only noble metals though; some non-noble and metal free catalyst can also be costly. However, the issue of scarcity is a special concern to noble metal catalysts.

3. DEVELOPMENT OF SUSTAINABLE ELECTROCATALYSTS FOR FUEL CELLS AND WATER ELECTROLYZERS Given the issues mentioned above, major technological advances and large scale commercial deployment of fuel cells and water electrolyzers will, therefore, rely on our ability to develop sustainable, metal-free or noble metal-free electrocatalysts that can catalyze ORR and OER with at least the same efficiency, if not better, than the best, benchmark, noble metalbased alloy catalysts known today for these reactions such as Ptalloys, IrRuOx, etc.4,25−28 Currently, three major approaches are being considered worldwide in this regard.

2.2. Deactivation of Catalysts (Due to Sintering, Agglomeration, Leaching, Or Corrosion)

Many of the metals used in conventional fuel cells and water electrolyzers are unstable over time under the catalytic reaction conditions typically used in these energy systems. For example, Pt is susceptible to degradation upon long exposure to the various chemical processes taking place in fuel cells, for example, methanol oxidation in direct methanol fuel cells, which can form CO, a known “poison” to Pt as well as many other metallic catalysts.13,14 The catalytic activity and selectivity of metals such as Pt can also be compromised by the fact that the metal can undergo oxidation, dissolution, and various other degradation processes.15,16 While metal-free catalysts can degrade too, they are relatively less prone to this problem and their specific deactivation processes or mechanisms can be quite different from the ones that can possibly degrade their metallic counterparts.

3.1. Mixing or Alloying Noble Metals with Other Sustainable Elements

In this approach, noble metal catalysts such as Pt and Ir are mixed or alloyed with other inexpensive elements. This way, the overall amount of the noble metals in the catalysts is minimized without reducing or compromising their overall activity,25−27 and in some cases, the specific catalytic activity of the noble metals is even increased through the favorable electronic effects induced by the latter.4,28 Ideally, such materials are prepared with nanoscale sizes with the most active atoms such as Pt being placed on the outer shells to be fully utilized in catalysis while the more sustainable, inexpensive elements filling the remaining volume of space. Nevertheless, since noble metals are still involved in these types of catalysts, many of the abovementioned issues remain. Moreover, the synthesis of such materials is not trivial, since mass diffusion of the atoms between the cores and the shells can occur and compromise the structure and catalytic properties of the materials over time.34

2.3. The Inherent Sluggishness of ORR and OER

As mentioned above, the ORR and OER are the most difficult half reactions to promote in fuel cells and water electrolyzers, respectively.17 For example, while the oxidation half-reaction in fuel cells is facile with many types of catalysts, the second reaction (which almost always involves O2 or ORR) is not, even with noble metal-based catalysts.18 The latter is kinetically slow mainly because of the inherent difficulty in breaking the exceptionally strong OO bond (498 kJ/mol).19 Thus, a great portion of energy in fuel cells goes to activate the bond in O2, or to overcome the ORR half-reaction. Typically, overpotentials as high as 500−600 mV or large amounts of energy have to be supplied to ORR, even in the presence of Pt-based catalysts.20 This severe activation loss associated with ORR and the intrinsic low kinetics of the reaction are responsible for the limited current density and the low cell voltage given off by many conventional fuel cells. Likewise, OER involves very similar, albeit reverse processes, and consequently it goes through similar energy barriers as ORR does. So, unsurprisingly the OER is the half reaction that mainly dictates the energy outputs of water electrolyzers.

3.2. Supported Noble Metal Catalysts

The second approach is centered around finding suitable porous support materials for the noble metal catalysts to increase the metals’ accessible surface area, and thereby catalytic activity per unit mass.29 The synthesis involves either one-pot synthesis or postsynthesis to allow the immobilization of metallic groups into the support materials, in such a way that the metals’ surfaces are as accessible to reactants as possible, while inhibiting their tendency to aggregate or sinter. Furthermore, the support material may need to have good stability and some conductivity; this is why graphitic materials and semiconducting metal oxides are often considered as good support materials for these purposes.

2.4. Selectivity of Catalysts

The selectivity of reactions leading to ORR and OER is another major issue. In the case of ORR, the reaction can proceed through either (i) a more desirable, four-electron process that combines O2 with electrons and protons producing H2O or (ii) a less desirable, two-electron pathway, leading to H2O2.21−23

3.3. Noble Metal-Free Catalysts

These include non-noble metal-based oxides, chalcogenides, etc. Jasinski reported the first interesting finding that Pt-free C

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Figure 3. (A) Typical N 1s XPS spectrum and simulated peaks and (B) schematic illustration of the pyridinic, pyrrolic, graphitic, amino, and pyridinic-N oxide moieties of a N-doped carbon material. Reproduced with permission from ref 47. Copyright 2013 Elsevier.

dopant atom replacing a C atom in a carbon material can easily donate its electron to the delocalized π-system of the carbon matrix, creating partially positively and negatively charged moieties in the material. The latter, in turn, enable the carbon material to interact strongly with reactants (e.g., O2), activate the bonds in the reactant (e.g., weaken the OO bond in O2), and promote the reactant’s transformations (e.g., O2’s transformation in ORR). In contrast, pure carbon materials possessing no heteroatom dopants do not promote these catalytic processes.

metal-phthalocyanines (also called metal−N4 macrocycles, such as Fe- and Co-macrocycles) could catalyze the ORR as effectively as Pt-based materials do.30 However, such catalysts were also quickly found to be unstable, especially in acidic solutions where most proton-exchange membrane fuel cells are operated. While the stability of these materials could be improved by heat-treatment, the improvement was not large enough for practical applications.31 Ever since Jasinski’s seminal work, various other Ncoordinated transition metals and metal chalcogenides, oxides, oxynitrides, carbonitrides, transition metal-doped conductive polymers, etc. have been explored, and their potential to catalyze ORR, OER, and other reactions have been evaluated, in some cases with sound results.32−36 However, leaching, oxidation, and poisoning of the metals in such materials have made their potential use in practical fuel cells to remain questionable.37,38

4.1. N-Doped Carbons for ORR and OER

It has been well established that the electronic and other properties of carbon materials can be tuned by doping them with various heteroatoms, especially N atoms. Nitrogen is an ideal dopant for carbon materials because its size is similar to that of a C atom and can thus easily replace a C atom in carbon nanomaterials. On the other hand, its higher electron affinity compared with a C atom makes the N dopant atom to easily alter the atomic structures and electron arrangements in carbon materials; this, in turn, results in more charge delocalizations, changes in spin density, and higher density of donor states close to the Fermi level in the carbon nanomaterials.45,46 These further make the carbon materials to have n-type conductivity, increased metallic properties, enhanced electron-transfer rates, better “activation sites” for reactants (such as O2 during ORR and water during water electrolysis), and ultimately improved overall electrocatalytic properties.46 The different synthetic conditions, precursors, catalysts, and procedures employed to make N-doped carbons (see notable examples in section 5 below) result in carbon materials with not only different morphology and structure but also different types and density of N dopant moieties (in terms of oxidation state). The specific types of N species that form, and their oxidation states, can be identified and quantified using X-ray photoelectron spectroscopy (XPS). The high-resolution N 1s XPS signal of N-doped carbon nanomaterials, in particular, can show peaks corresponding to pyridinic N species at ca. 398.4 eV, pyrrolic N species at ca. 399.8 eV, quaternary N species at ca. 401.0 eV, and N-oxidic (pyridine-N-oxide) species at ca. 402.9 eV, as illustrated in Figure 3.47 While the N species incorporated into N-doped carbon materials modulate the structures, properties and potential applications of the carbon materials, not all N dopant-related species constitute highly catalytic active species on the materials.6−13 For example, in many of our and others’ recent works, carbon materials with

3.4. Metal-Free Catalysts

These include entirely new, metal-free (nano)materials that can efficiently catalyze ORR, OER, etc. While the catalytic activity of metal−Nx macrocycles was initially attributed to metal-N2− C species, studies later revealed that N-containing carbon materials obtained by pyrolysis of metal-phthalocyanines can also catalyze ORR, even after completely removing residual metals on them by electrochemical purification.39,40 Numerous studies, especially in recent years, showed that carbon materials with different structures, such as carbon nanotubes, nanotube cups, mesoporous carbons, and graphene oxide (see Figure 2),41,42 can electrocatalyze the ORR and OER on their own when doped with various heteroatoms such as N, S, P, B, I, Li, and Se.43 These kinds of materials will be the subjects of further discussions in the subsequent sections.

4. HETEROATOM-DOPED CARBONS AS ELECTROCATALYSTS The origin of the electrocatalytic activity in heteroatom-doped carbon nanomaterials is now generally believed to be the electronic modulation of the extensive conjugated sp2−sp2 linkages or the delocalization of π-orbital electrons in their graphitic domains due to the presence of heteroatoms.44 Specifically, the presence of heteroatom dopants is suggested to perturb the electronic structure of the carbon lattice, generating partially positively and negatively charged groups on the material without substantially compromising the material’s conductive properties. For example, the electronegative N D

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Figure 4. Synthetic procedure used to make N-, O-, and S-tridoped carbon (NOSC) materials from polypyrrole and NH4S2O8 precursors and colloidal silica nanoparticle templates at different pyrolysis temperatures. The materials were labeled as NOSCx-T, where x represents the amount of colloidal silica (0.5, 2, 4, or 8 g) for 9.2 mmol of pyrrole and T represents the pyrolysis temperature (600, 700, 800, 900, or 1000 °C) used to synthesize the materials. Reproduced with permission from ref 6. Copyright 2014 American Chemical Society.

Figure 5. (a) CVs of NOSC8-900 in O2- and N2-saturated KOH (0.1 M) solutions, (b) polarization curves of ORR over NOSC8-900 placed on a rotating disc electrode (RDE), rotating at different speeds, (c) polarization curves of ORR over NOSCx-900 materials at 1600 rpm, (d) electron transfer number and H2O2 yield in ORR on NOSC8-900 material, (e) kinetic current density (Jk) of ORR at different potentials on NOSC8-T materials, and (f) Tafel plots of NOSC8-900 and NOSCB-900(A) (a carbon black control material) at 1600 rpm. Reproduced with permission from ref 6. Copyright 2014 American Chemical Society.

relatively high density of pyridinic N moieties have been found to exhibit higher catalytic activity toward ORR.

However, given their difference in size and electronegativity, the different types of dopant elements in carbon nanomaterials can give different results too. For instance, S is larger in size and has greater polarizability than N, and so, a S dopant atom renders carbon materials a higher density of spin, edge strain, and charge delocalization than a N dopant atom does.51 Due to their relatively lower electronegativity, higher atomic size and lone pair electrons in their 3p orbitals, P and S dopants in carbon nanomaterials can lead to materials with defect-induced active sites that are more suitable to accommodate the lone pair electrons of O2 or help with the adsorption of O2 during ORR.52,53 Similarly, due to its relatively more electropositivity compared with a C atom, a B dopant atom in a carbon nanomaterial can polarize the negatively polarized O atoms of O2 and enhance the chemisorption of O2 on the surface of the

4.2. Other Heteroatom- and Multielement-Doped Carbon Materials

While N-doped carbon nanomaterials have traditionally been the most extensively investigated, carbon materials doped with S, P, B, Se, and I have also been explored for their potential applications for electrocatalysis.48,49 Because the electronegativity of these elements (for example, S = 2.58 and P = 2.19) is different from that of C (2.55), when used as dopants, these elements can also perturb the electroneutrality and thereby the structures and charge densities and electrocatalytic activities of the materials, for example, by assisting with the adsorption of •OOH or O2 and cleavage of the OO bond during ORR.50 E

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Accounts of Chemical Research carbon material during ORR.54 The B atom can also act as electron donor for ORR, as its free pz orbital can hold some of the electron density of the graphitic p-electron system in the Bdoped carbon material. Recently, it has also been shown that the co-incorporation of two or more types of heteroatom dopants (e.g., N and S, N and P, etc.) into carbon materials would lead to materials with large density of electrocatalytic active sites for four-electron ORR and OER processes and better operational stability than noble metal catalysts.55 In one specific example, nanoporous N-, O-, and S-tridoped carbons (NOSCs), which are prepared from persulfate-containing polypyrrole in the presence of colloidal silica nanoparticle templates (Figure 4), are found to electrocatalyze ORR with a low onset potential and Tafel slope in basic solutions.6 The materials also give different electron-transfer numbers (or different ratios of H2O/H2O2 products) depending on their structure or the relative amount of colloidal silica templates used to make them (Figure 5). Although heteroatom-doped carbon materials have been found to catalyze ORR largely in basic solutions, there are now some that are reported to do so under acidic conditions as well. For example, the Dai group recently reported metal-free, vertically aligned N-doped carbon nanotube (VA-NCNT) arrays, and N-doped graphene/CNT composite (N-G-CNT) that can serve as cathode catalysts in acidic PEM fuel cells with significantly better long-term operational stabilities and comparable gravimetric power densities with respect to the best nonprecious metal catalysts (NPMCs) (Figure 6).56 Furthermore, because carbon is much more resistant to acids than most transition metals, the VA-NCNT array and N-GCNT catalysts further showed a significantly durable performance, even with pure H2/O2 gases, in acidic PEM fuel cells, outperforming their NPMC counterparts.

Figure 6. Synthesis of N- and O-doped polyaniline (PANI)-derived mesoporous carbons (named as PDMCs) by polymerizing PANI within the channel pores of mesoporous silica, then carbonizing the resulting PANI/SBA-15 composite material, and finally etching the mesoporous silica off of the carbonized product. The materials are found to electrocatalyze ORR well. Reproduced with permission from ref 57. Copyright 2013 American Chemical Society.

5. NOTABLE SYNTHETIC STRATEGIES FOR HETEROATOM-DOPED CARBON ELECTROCATALYSTS The synthetic methods used to make heteroatom-doped carbon nanomaterials can be broadly classified into two groups. The first one involves pyrolysis of carbon and heteroatom containing precursors that can give high yields of carbon along with heteroatom dopants. The second method generally involves postsynthetic modification of premade carbon nanomaterials with heteroatom-doping agents (e.g., melamine or ammonia) chemically or thermally. To create high surface area and thereby higher catalytic outputs per unit mass, the materials are often synthesized with high surface area using some sacrificial templating agents such as silica nanoparticles and mesoporous materials along the way. Notable specific synthetic methods are described below.

pyrrolic, pyridinic, pyrodinic, etc.), they contain. With a similar synthetic procedure but using polypyrrole as the precursor, Ndoped mesoporous carbons that show good electrocatalytic activity for hydrazine oxidation reaction are also obtained.58 5.2. Synthesis Using Colloidal Silica as Templates

In this case, nanoparticles (typically colloidal silica) are used as templates with carbonizable molecular or polymeric precursors. After removal of the colloidal silica templates from the colloidal silica-containing carbonized material using NaOH or HF solution, heteroatom-doped nanoporous carbon materials that are highly active electrocatalysts for ORR, OER, etc. are obtained (as in Figure 4).6 The yields, structures, and properties of the resulting materials depend on the relative amount of the templates, besides the synthetic protocols and precursors employed to make the materials.

5.1. Synthesis Using Nanoporous Materials As Templates

5.3. Synthesis Using Core−Shell Nanostructures

In one of the most commonly applied synthetic methods within for the first group, mesoporous materials are used as sacrificial templates. This is depicted in Figure 6 for polyaniline (PANI) precursor.57 After carbonization of the precursor-immobilized mesoporous silica and then etching of the silica framework (typically using NaOH or HF solution), PANI-derived mesoporous carbon materials (denoted as PDMCs), which show electrocatalytic activity for ORR, are obtained. Their electrocatalytic activities are found to depend on the final pyrolysis temperature with which the materials are synthesized, and the type of heteroatom dopant-related species (e.g.,

Core−shell nanostructures, in which carbonizable precursors are coated with thermally robust silica shells, can also be used to make electrocatalytically active carbon nanomaterials. In one example, edge-plane-rich N-doped carbon nanoneedles (CNNs) are synthesized by using cellulose nanowhiskers (CeNWs) both as carbon sources and as templates (Figure 7).9 High aspect ratio CeNW precursors with negatively charged surfaces and high crystallinity are obtained by kinetically controlled hydrolysis of natural cellulose in acidic solution. Their surfaces are coated with metal−amine F

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Figure 7. A core−shell synthetic route to produce an electrocatalytically active, heteroatom-doped, edge plane-rich carbon nanoneedle (CNN) from a cellulose nanowhisker (CeNW) precursor. Reproduced with permission from ref 9. Copyright 2012 Wiley-VCH. Figure 9. STEM and SEM images and elemental mapping of C, N, P, and O atoms in HCSC. The scale bars in all the images represent 1 μm. Reproduced with permission from ref 8. Copyright 2015 American Chemical Society.

complexes by stirring the CeNWs in solutions containing metal salts and ammonia and then with silica shells via the sol−gel process. The resulting CeNWs/metal−amine/silica nanoneedles are then treated at high temperature under inert atmosphere. It is worth noting that the silica shells assist the graphitization process by trapping any volatile carbon species and increasing the local pressure inside the cores of the core− shell−shell nanostructures, besides keeping the original nanoneedle shapes of the precursors intact. If necessary, the amount of the metal−amine complexes sandwiched in CeNWs/metalcomplex/silica core−shell−shell nanoneedles can be increased or intentionally not etched and left behind, to further tune the properties of the carbon materials.59 By application of the core−shell synthetic strategy to biomaterial precursors, which contain heteroatoms or whose surfaces can be modified with heteroatomic species, catalytically active heteroatom-doped carbon materials can also be synthesized. For example, by carbonization of silica-coated metal−amine complex-modified yeast cells or yeast cell walls, and then etching of the silica shells, catalytically active heteroatom (or N and P)-doped hollow core−shell carbon (HCSC) and yeast cell wall derived carbon (YCWC) materials, respectively, are successfully synthesized (Figures 8 and 9).8 HCSC possesses more nonmetallic dopants and shows higher electrocatalytic activity than YCWC, due indirectly to the biomacromolecules present in the inner parts of the yeast cells.

5.4. Synthesis by Using Metal−Organic Frameworks (MOFs) as Precursors

MOFs, which are formed by the spontaneous assembly of metallic ions or metal clusters (composed of Ni, Zn, Fe, Co, etc.) and organic ligands, have ordered nanometer pores and the three major ingredients required to make heteroatomdoped carbon electrocatalysts (i.e., heteroatoms, metals, and hydrocarbons). So, not surprisingly, recently, a large number of them have been successfully used as precursors and selftemplates to make heteroatom-doped nanoporous carbon materials via carbonization, for electrocatalysis of ORR, HER, OER, etc.60−62 Because of their nanoscale structure, they have an added advantage to serve both as templates and as carbon and heteroatom sources, and lead to hierarchically porous, high surface area, heteroatom-rich carbon electrocatalysts (Figure 10).60 Furthermore, the inherent presence of metallic species in them make them ideal to generate carbon-based materials with a high degree of graphitization or electrical conductivity as well as electrocatalytically active species, such as Fe(Co)−Nx, for ORR, OER, etc. Since a variety of MOFs with different compositions and structures are known, the synthesis of a variety of MOF-derived carbon electroctocatalysts is possible by using them as precursors or templates. For example, the Zhang group

Figure 8. Schematic illustration of the synthesis of heteroatom-doped, yeast cell-derived hollow core/shell carbon (HCSC) microparticles and yeast cell wall-derived carbon (YCWC), respectively. (a) Adsorption of [Fe(NH3)6]3+ ions around yeast cells and yeast cell wall particles, (b) deposition of silica shells, (c) high temperature treatment of the yeast/metal ion/silica, and (d) removal of the silica shells. Reproduced with permission from ref 8. Copyright 2015 American Chemical Society. G

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Figure 10. (A) Schematic illustration of the synthesis of highly graphitized N-doped porous carbon nanopolyhedra from MOFs. (B,C) TEM images of typical zeolitic imidazolate framework (ZIF-8) MOF and N-doped graphitic porous carbon (NGPC) polyhedral nanoparticle obtained from it. (D) TEM image of a single NGPC polyhedron. Adopted with permission from ref 60. Copyright 2014 Royal Society of Chemistry.

Heteroatom-doped carbon materials can also be synthesized from premade carbon materials either by thermally treating them after mixing them with heteroatom-doping agents or by thermally treating them in the presence or in a flow of heteroatom-doping agents (e.g., ammonia gas). In one specific example, premade carbon particles are coated with melamine formaldehyde (MF) polymer and nickel nitrate, and the resulting metal-salt/MF-polymer-coated carbon particles are pyrolyzed.64 After etching the resulting N-doped carbon/NiOx (N/C−NiOx) composite material with acid, N-doped carbon (N/C) particles that are proven to be very good electrocatalysts for OER, with activity better than that of IrO2/C, are obtained (Figure 12).

reported N-doped nitrogen-doped porous carbon sheets by using two-dimensional (2-D) sandwich-like zeolitic imidazolate framework (ZIF) grown on graphene oxide (GO) as a precursor.61 The resulting material showed high catalytic activity for ORR due to the synergistic catalytic effects produced between the N-doped carbon and the graphene in the materials. In another work, Liu et al.62 showed that a similar material can serve as a bifunctional electrocatalyst for both ORR and OER. 5.5. Synthesis by Postmodification of Premade or Pre-existing Carbon Nanomaterials

Another method that has substantially been used for making heteroatom-doped carbon nanocatalysts involves the treatment of premade carbon materials with heteroatom-containing reagents under different synthetic conditions. For instance, the chemical treatment of defective graphene, which can be prepared from graphite with borane tetrahydrofuran (BH3THF), leads to electrocatalytically active, B-substituted graphene (B-SuG) (Figure 11).63 The resulting material serves

6. CONCLUSIONS, FUTURE OUTLOOK, AND PERSPECTIVES Owing to their sustainability and unprecedented catalytic activity for various electrocatalytic reactions in fuel cells and electrolyzers, heteroatom-doped metal-free and noble metalfree materials have stimulated a great deal of interest in recent years. The electrocatalytic properties of these materials are known to depend heavily on the structures and compositions of materials. This, in turn, depends on the carbon precursors and the synthetic conditions used to make the materials, and whether or not metals are present during their synthesis. However, more research is still needed to determine the exact catalytic active sites responsible for their catalytic properties and to probe the effects of the different parts of the materials, including possible trace metals, on their catalytic activities. More research is also needed to improve the electrocatalytic performances (activity, stability and selectivity) of the materials. Additionally, making heteroatom-doped carbon materials with the highest possible density of the most catalytically active groups is currently among the major interests in this field of research, but it is also a relatively challenging one to attain. Another area of major interest is the possible use of various biologically derived materials as precursors for making heteroatom-doped carbon materials. This is interesting because there are millions of microorganisms in nature that can serve as renewable precursors for a broad range of bioderived carbons with electrocatalytic activity. In particular, by coupling of carbonization synthetic methods with genetic engineering of different mutated microorganisms (e.g., by inserting unnatural amino acids in them), carbon materials with the most desired dopants and the best electrocatalytic performances can potentially be synthesized.

Figure 11. Schematic representation of the various steps employed in the synthesis of B-substituted graphene (B-SuG): (a) oxidative exfoliation of graphite to graphene oxide (GO), (b) chemical reduction followed by thermal treatment of GO to form defective graphene (DeG), and (c) finally, borylation of DeG using BH3-THF to dope the defective sites of DeG with B atoms and to produce the desired material (B-SuG). Reproduced with permission from ref 63. Copyright 2014 Royal Society of Chemistry.

as an efficient metal-free electrocatalyst for HER. Notably also, compared with other commercially available borylating agents such as NaBH4, B(OH)3, carborane, B2O3, and NH3-BH3, BH3THF is found to be most effective in producing such materials from graphene oxide. H

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Figure 12. (I) Synthetic procedure for N-doped carbon particles by postsynthetic modification: (1) synthesis of melamine formaldehyde (MF) polymer with nickel nitrate and carbon particles; (2) pyrolysis of metal-salt/MF-polymer precursor; (3) acid leaching of the pyrolyzed samples. Materials are (a) carbon particles (black dot), (b) carbon particles covered with MF polymer (orange sphere) and nickel nitrate (green dot), (c) N/ C-NiOx catalyst (gray dot, NiOx), and (d) N/C catalyst. (II) Catalytic activities of the materials and their precursors for OER in KOH electrolyte (pH 13) (N/C, N/C−NiOx, IrO2/C (20 wt %), and Pt/C (20 wt %); catalyst loading = 0.2 mg·cm−2; rotation speed = 1500 rpm; scan rate = 5 mV s−1). Adopted with permission from ref 64. Copyright 2014 Nature Publishing Group.

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This author believes that by combining experimentalspectroscopy-microscopy-theory investigations and with continued research efforts, the interplay among the precursor, structure, composition, active sites, and catalytic properties of these fascinating materials will be more understood. This, in turn, may allow the development of the most sustainable, efficient, and inexpensive metal-free catalytic systems based on earth-abundant elements that can catalyze ORR, HER, OER, and other related reactions relevant for renewable energy systems.

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AUTHOR INFORMATION

Corresponding Author

*Tewodros Asefa: [email protected]. Notes

The author declares no competing financial interest. Biography Tewodros (Teddy) Asefa was born in Ethiopia. He received his B.Sc. degree from Addis Ababa University (Ethiopia) with distinction, M.Sc. from SUNY-Buffalo (USA), and Ph.D. from University of Toronto (Canada). He is currently a Full Professor in the Department of Chemistry and Chemical Biology and the Department of Chemical and Biochemical Engineering at Rutgers University in New Brunswick, New Jersey, USA. He served as a Visiting Professor at Kyoto University (Japan, 2014), Maringá State University (Brazil, 2014− 2017), and ETH Zurich (Switzerland, 2016). His group’s research involves the development of synthetic methods of a wide array of advanced functional nanomaterials and the investigation of their potential applications in catalysis, electrocatalysis, targeted drug delivery, solar cells, and environmental remediation.

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REFERENCES

ACKNOWLEDGMENTS

The author acknowledges the financial support of the US National Science Foundation (NSF) through the CAREER Grant CHE-1004218, NSF DMR-0968937, NSF NanoEHS1134289, NSF American Competitiveness and Innovation Fellowship (NSF-ACIF), NSF Special Creativity grant, and NSF DMR-1508611, which have all enabled parts of the research work made possible in the author’s group over the past several years that have been used for the Account here. I

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