Carbon- and Nitrogen-Based Porous Solids: A Recently Emerging ...

Accounts for Nanospace Materials

Carbon- and Nitrogen-Based Porous Solids: A Recently Emerging Class of Materials Ken Sakaushi* and Markus Antonietti Colloid Chemistry Department, Max Planck Institute of Colloids and Interfaces, Am Mühlenberg 1 OT Golm, D-14476 Potsdam, Germany E-mail: [email protected] Received: October 22, 2014; Accepted: November 19, 2014; Web Released: November 27, 2014

In this account, the emergence of a new family of porous solid-state materials based on carbon and nitrogen is discussed. This started with the observation that carbon nitride, a polymeric material with a composition close to C3N4, is not only unexpectedly thermally and chemically stable, but a semiconductor at the same time and able to catalyze a variety of chemical reactions, such as Diels­Alder cyclizations, oxidations, and photochemical water splitting. Carbon nitride is however already sufficiently reviewed but related materials with similar potential have essentially been not sufficiently covered. The remaining structures are manifold and cover the range from on the one hand covalent triazine frameworks (CTFs) as new semiconducting porous solid-state materials to the other porous nitrogen-doped carbons (NdCs) as catalysts which are known to be applicable in a variety of electrocatalytic reactions. CTFs are porous polymeric frameworks constituted of triazine rings and other aromatic rings. Chemical binding motifs and crystal structures can be used to control the band structure of such an (organic) solid-state material. Porous NdCs are usually made by adding nitrogen sources to classical carbonization recipes, and they are attracting a lot of interest because of their catalytic activity for electrochemical processes, such as oxygen reduction reaction (ORR). This account will summarize state-ofthe-art work on those systems being next-generation catalysts for different kinds of reactions, which are affordable and high-performance catalysts with unique principle for emergence of catalytic activities, combining theoretical and experimental studies together with basic and applied science in a range of scientific fields from chemistry to physics. Indeed, the first exploration of the above materials as candidates for components of fuel cells and rechargeable metal­air batteries are discussed.

Introduction Chemistry always seeks to synthesize high value materials from abundant starting products, such as molecules containing nitrogen and carbon.1 As alchemists dreamed of turning ordinary soot into gold a long time ago, modern scientists are following a similar pursuit while synthesizing high-value functional materials, where price nowadays reflects the needs and desires of society.2­17 “Energy” is certainly such an issue, and here, state-of-the-art carbon- and nitrogen-based porous solids can change the way energy is produced and stored. The topic of porous solids is one of the most active frontiers of science, and at the same time, they are important materials for industry because of their high specific surface area and the special physical effects in the pores they contain. Recent investigations suggest that those attractive properties can be even improved by novel accesses and chemistries towards porous solids.11,18­24 The evolution of porous solids goes in our opinion along two main streams: (1) the expansion of openframework crystalline materials from classical porous materi386 | Bull. Chem. Soc. Jpn. 2015, 88, 386–398 | doi:10.1246/bcsj.20140317

als, such as zeolites and aluminum phosphates, to metal­ organic frameworks, and more recently to covalently bonded organic frameworks, and (2) the discovery of ordered mesoporous siliceous materials and production of porous solids from those strategies using hard- and soft-templating strategies (Table 1).9,20 Modern porous solids based on carbon and nitrogen only are developed for further improvement of porous solids aiming affordability, physical and chemical stability together with new, more elaborate functions, especially electronic functions and electrocatalytic activity. For a few decades, photocatalysis and photovoltaics have been of growing interest to produce solar fuels and electric photoenergy.25­27 Properties of these devices depend on electronic properties of materials and especially on band structures.28 The already mentioned ongoing success of graphitic carbon nitride (g-C3N4) has opened a variety of possibilities of metal-free semiconductors, especially in the field of photocatalysis.16,29,30 Since the theoretical prediction of carbon(IV) nitride phase,29 more than 6000 papers on C3N4 have been published.31 Following this paper, a huge amount of funda© 2014 The Chemical Society of Japan

Table 1. Evolution of Functional Porous Solids Before 1990 Zeolite analogues: Crystalline Porous Solids

Templated Porous Carbons

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ion-exchange, molecular sieve, catalysis Discovery of mesoporous silica in 80s.

H2 and CO2 storage, ionic conductivity 2nd generation porous carbons (Hierarchical structures):

1st generation porous carbons

ORR electrocatalysis of doped carbons.

State-of-the-art C-, N-based porous crystals = CTFs, g-(CxNy)s: semiconductors, photocatalysts, electrocatalysts 3rd generation porous carbon: Doped nanostructured carbons as next-generation ORR, OER electrocatalysts.

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Figure 1. Schematic illustration for a variety of N,C-based materials. (a) g-C12N as the carbon nitride having the highest amount of in-plane nitrogens without defects. (b) One carbon defect surrounded by 6 nitrogen atoms leads to formation of triazine-based graphitic carbon nitride (TGCN). (c) Four carbon defects leads to formation of g-C3N4. (d) Several examples of CTFs. CTFs from monomer c (green part, g-(C7N5H2)). CTFs from monomer d (yellow part, g-(C7N3SH2)). CTFs from monomer f (blue part, g-(C5N6H)). (e) Starting monomer for CTF synthesis. Bull. Chem. Soc. Jpn. 2015, 88, 386–398 | doi:10.1246/bcsj.20140317

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Figure 2. A variety of covalent triazine frameworks. (a) Crystalline CTF-1 and (b) starting monomers for CTF synthesis. Reproduced with permission of Ref. 39. Copyright © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (c) Crystalline Li+ and Cl¹ intercalated poly(triazine imide) frameworks (PTI/Li+Cl¹). Reproduced with permission of Ref. 42. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (d) Crystal structure of TGCN. Reproduced with permission of Ref. 41. Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

mental and applied research has been carried out in the g-C3N4 system not only for photocatalytic reaction but nowadays also towards photovoltaic applications.32 On the other hand, from chemical point of view, there are many possibilities for other graphitic carbon nitrides, so-called g-(CxNy)s. Here, it is shown a principle to predict the undiscovered, therefore, novel g-(CxNy)s by adding N atoms and making periodic C atom defects (of course, a large enough defect becomes a pore) in graphitic framework in order to put nitrogen in the framework as much as possible with consideration of chemical stability (Figure 1). From the experimental point, it seems that the limitation of N-doping within the graphitic framework would be ca. 10 wt %, thus a “g-C12N” can speculatively be the carbon nitride having the highest amount of in-plane nitrogens without defects (Figure 1a). Let us consider to make one C atom defect surrounded by six N atoms (let us call it a “C1N6-defect”). This defect is possible by formation of triazine rings, therefore, this defect leads to form a stable planer periodic g-CxNy with x = 3 and y = 4, which in this case is specially called triazine-based graphitic carbon nitride (TGCN), which has been synthesized recently (Figure 1b). Furthermore 388 | Bull. Chem. Soc. Jpn. 2015, 88, 386–398 | doi:10.1246/bcsj.20140317

another example, if it is formed a four C atom defect surrounded by nine N atoms (C4N9-defect), the whole can be stabilized by formation of s-heptazine units (Figure 1c). This leads to the well covered formation of stable and periodic g-CxNy with x = 6 and y = 8, which is the typical g-C3N4. Another option is the introduction of a six C atom defects surrounded by six N atoms (C6N6-defect), which would lead to the formation of g-C9N4, an undiscovered non-triazine-based g-CxNy so far. It is clear that this construction principle is virtually endless, and that all those regular structures will possess very specific electronic properties and chemical behavior. Allowing surfaces to be partly terminated with hydrogen and changing the chemical strategy, other CN-based porous materials can be achieved. Covalent triazine frameworks (CTFs), which are organic two-dimensional (2D) metal-free semiconductors, can be synthesized from different starting precursors with nitrile functions (Figures 1d and 1e). In the case of CTFs, the modification of chemical functionality is easier than in the case of g-(CxNy) as the choice of monomers is rather flexible. The first CTF was synthesized from 1,4-dicyanobenzene (monomer a in Figure 1e), which is called CTF-1 constituted of © 2014 The Chemical Society of Japan

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Figure 3. Principle of control of electronic structure of CTFs: the ratio of triazine rings and benzene rings influence the electronic structure of CTFs. Reprinted with permission for Ref. 44. Copyright 2013 American Chemical Society.

benzene rings and triazine rings (it could also be described as g-(C9N3H4), Figure 1d). Meanwhile, already several CTFs were proposed and also synthesized, but there are enough openings to produce novel CTFs by using simple monomers such as, 2,5thiophenedicarbonitrile (monomer d in Figure 1e) and 1,2,4triazole-3,5-dicarbonitrile (monomer f in Figure 1e), which will lead to the still hypothetic species g-(C7N3SH2) (yellow part in Figure 1d) and g-(C5N6H) (blue part in Figure 1d), respectively. The strategy described above to tune chemical and electronic functions can be simplified as: addition of N atoms and introduction of regular vacancies (or regular pores) in the graphitic framework, and/or modifications by other aromatic rings lead to formation of the target structures, here g-CxNy and CTFs. Their electronic structures vary because of different crystal structures and different electron densities induced by heteroatoms and periodic porosity. Of course, the Fermi level can be adjusted by the above strategy. Indeed, edge states in graphitic compounds (for example g-C9N4) are known to affect the electronic structure.33,34 Further interesting features in this materials system are expected to also rely on combination with theoretical studies.35 In our opinion, this makes the g-(CxNy)s and CTF system a “treasure box” to find materials with appropriate electronic functions to serve as materials for modern devices, such as photovoltaics, batteries, and even devices based on spintronics or valleytronics.36­38 Disordered versions of those systems, sometimes subsummarized as N-doped carBull. Chem. Soc. Jpn. 2015, 88, 386–398 | doi:10.1246/bcsj.20140317

bons (NdCs) are already attracting broad interest from the scientific community to industrial sections due to their high catalytic activities in various electrochemical reactions. Thus, NdC is one of the top candidates to be an affordable, highperformance catalyst in many energy conversion issues. In the following, we will try to discuss a representative set of those systems and also focus on the already explored application potential of each of the systems. Covalent Triazine Frameworks CTFs can be understood as conjugated, 2D, metal-free organic semiconductors. As compared to g-C3N4, CTFs have a variety of regular structures by covalently bonding triazinebased aromatic rings, such as C3N3 (triazine) or C6N7H3 (heptazine), and other aromatic rings, such as benzene rings (Figures 2a­2c).39,40 C3N3-based g-C3N4 (triazine-based graphitic carbon nitride: TGCN) could be regarded as a bridge between the two families (Figure 2d).41 1,3,5-triazine has the largest electron affinity of +0.46 as compared to other aromatic moieties thus it works as a strong electron acceptor.43 Therefore, a CTF made of C3N3 and benzene rings (so-called CTF-1) has a bipolar electronic character as benzene is an electron donor characterized by an electron affinity of ¹1.15. This means that CTFs are electronically different from other covalent organic frameworks. The first proposal for the possibility of control of band structure was suggested by an integration of theoretical and experimental study using CTF-1 (Figure 3).44 In this work, © 2014 The Chemical Society of Japan | 389

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Figure 4. Synthesis and application of CTFs. (a) Synthetic route of CTFs through ionothermal synthesis and (b) X-ray diffraction pattern of crystalline CTF-1. Reproduced with permission for Ref. 39. Copyright © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (c) X-ray diffraction patterns of crystalline PTI/Li+Cl¹, (d) oxygen evolution property of PTI/Li+Cl¹, and (e) hydrogen evolution property of PTI/Li+Cl¹. Reproduced with permission of Ref. 42. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (f ) X-ray diffraction pattern of crystalline TGCN, (g) UV­vis diffuse-reflectance spectrum with Kubelka­Munk plot (inset), and (f ) DFT calculated band structure for monolayer TGCN. Reproduced with permission of Ref. 41. Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

densities of states (DOS) of CTFs were determined by using galvanostatic intermittent titration technique (GITT). GITT is often used to investigate a quasi-equilibrium state of electrode materials but at the same time, one can use this technique to have a look on the DOS of a material.45,46 The work suggests that the electronic structure of CTFs can be tuned by the number of stacking layers and the ratio of C3N3 to benzene rings. This work only can point to the rich options in selection of crystal structure and electronic function of CTFs to tune the electronic function of this material system. Indeed, one can organize regular porous structures in CTFs by selecting appropriate monomers, and the tunability of electronic structure and function make CTFs a promising material system towards next-generation metal-free semiconductors. Synthesis and Applications of CTFs A typical synthetic route of CTFs is the ionothermal synthesis using nitrile monomers as starting material and zinc chloride (ZnCl2, Tm = 275 °C) as a molten salt, which was reported in 2008 as the first example of a successful synthesis 390 | Bull. Chem. Soc. Jpn. 2015, 88, 386–398 | doi:10.1246/bcsj.20140317

of CTFs (Figures 4a and 4b).39 This is because nitrile monomers can dissolve in molten ZnCl2 due to a strong Lewis acid­ base interaction, and even more important is that Zn2+ works as a Lewis acid catalyst to drive the reversible trimerization of the above nitrile monomers and the following framework formation at 400 °C. This is usually done for typically 40 h. A binary molten system of LiCl and KCl (45:55 wt %, Tm = 352 °C) with cyanamide or dicyanodiamide (DCDA) as starting precursor can provide highly crystalline Li+ and Cl¹ intercalated poly(triazine imide) frameworks, which were called PTI/ Li+Cl¹. PTI/Li+Cl¹, was proven to work as a photocatalytically active material which generates oxygen and hydrogen from its valence band and conduction band, respectively (Figures 4c­4e).42,47,48 The use of the starting monomer DCDA in LiBr/KBr molten mixture leads to form TGCN at 600 °C for 60 h.41 Theoretical investigation on TGCN indicates that this material is a direct band-gap semiconductor with an optical bandgap of 2.0 eV at AB-stacking arrangement, however, an optical band gap of less than 1.6 eV was identified by UV­vis spectroscopy measurements (Figures 4f­4h). Few reports

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Figure 5. Energy storage principle using CTF-1 electrode and electrochemical properties in Li and Na batteries. (a) Initial discharge process of CTF-1 electrode: the anion will be extracted from CTF-1 association with electron flow; (b) Cation insertion process. After complete extraction of anion electron can still flow into CTF-1 since it can be in a negatively charged state; and (c) Schematic illustration of energy storage principle using CTF-1. Reproduced with permission of Ref. 51. (d) Ragon plot for Li/CTF-1 cell. A full cell is estimated based on the assumption that the cathode material constitutes 35% of the total mass of the active materials in a cell. Reproduced with permission for Ref. 50. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (e) Ragon plot for Na/CTF-1 cell. Reproduced with permission for Ref. 51.

showed that CTFs can be synthesized through an organic synthetic route using an acid catalyst.49 In these reports, semiconducting properties of CTFs are investigated by UV­vis spectroscopy and photoluminescence properties. The authors claim that the obtained CTFs show bright yellowish colors which are in contrast to CTFs synthesized by ionothermal synthesis showing typically dark colors. Some first CTFs were tested as electrodes for rechargeable energy storage devices.50­52 As CTF-1 is constituted of C3N3 as electron acceptors and benzene rings as electron donors, this material shows an interesting electrochemical reaction. Typical Bull. Chem. Soc. Jpn. 2015, 88, 386–398 | doi:10.1246/bcsj.20140317

rechargeable organic batteries use only a cation (Li+, Na+, Mg2+, Zn2+, or Al3+) or an anion (ClO4¹ or PF6¹) as a charge carrier.53,54 However, by using CTF-1, both cations and anions can be used as charge carriers, thus the energy density of a cell can be improved (Figure 5a). Indeed, due to the porous structure of CTF-1, a high specific power is obtained as well. As a result, this electrode shows a remarkably high specific energy of ca. 380 Wh kg¹1 with a high specific power of ca. 4.6 kW kg¹1 by assuming a full cell constituting of the CTF1 cathode and a Li metal anode (Figure 5b).50 Furthermore, the CTF-based electrode works even in a sodium-ion battery (SIB) © 2014 The Chemical Society of Japan | 391

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Figure 6. Use of CTFs as solid catalysts. (a) Synthesis of Pt-CTF catalyst and (b) catalytic activity of Pt-CTF in the direct oxidation of methane to methanol over several recycling steps n. Reproduced with permission for Ref. 56. Copyright © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (c) Pt-CTF as oxygen reduction reaction electrocatalyst. The authors and the original source of the Ref. 57 are fully acknowledged for the distribution of this work under a Creative Commons CC-BY license.

system, which is more affordable. The electrochemical properties of CTF-based electrodes in SIBs are even more promising: They show both a high energy density of 500 Wh kg¹1 and a 10 kW kg¹1 based on the mass of the active material (Figure 5c).51 Only a few organic compounds show high specific capacities as electrodes in SIB, and most of them are quinone-based compounds.55 From this point of view, CTF-based electrodes are quite interesting for application also in other metal battery systems even they will require large amounts of electrolytes compared to the rocking-chair system. CTFs can also work as solid catalysts for a wide spectrum of chemical reactions.56,57 For example, platinum coordinated at CTF (Pt-CTF) shows a promising performance as a solid catalyst for direct low-temperature oxidation of methane to methanol with high selectivity, activity, and stability. This catalyst shows a turnover frequency of ca. 300 h¹1 even after several recycles (Figures 6a and 6b). Indeed, very recently, Pt-CTF/ carbon nanoparticle composites proved to show a selective activity of electrochemical oxygen reduction reaction (ORR) in an acidic solution even in the presence of a high concentration of methanol (Figure 6c).57 The reason for this interesting selective catalytic activity could originate from a Schottky metal­semiconductor heterojunction at the Pt/CTF interface, which can facilitate charged surface by a charge transfer and can trigger a different catalytic activity (Figure 7).58­61 392 | Bull. Chem. Soc. Jpn. 2015, 88, 386–398 | doi:10.1246/bcsj.20140317

Nanoporous and High Surface Area Nitrogen-Doped Carbons Carbon is a well-known material because of its number of polymorphs and intermediates, but also affordability and high controllability of structure and morphology. Materials scientists will agree that heteroatom doping is a very successful, and at the same time general way to change materials’ functions dramatically.62,63 The properties of carbon can be tuned just by adding another atom into a graphene structure, and this leads heteroatom-doped carbons to have different kinds of catalytic activities compared to nondoped carbons, induced by different electronic structures (Figure 8).64,65 Especially, nitrogen-doped carbons (NdCs) are the most successfully heteroatom-doped carbons having promising catalytic activities for a wide variety of catalytic reactions.66 The strategy towards control of NdCs’ catalytic activity is based on the nitrogen content, chemical structure of the doping nitrogen, and morphology. There are two major approaches to dope nitrogen into carbon: posttreatments or in situ doping. For the post-treatment approaches, N-doped carbon materials can be obtained by being subjected to nitrogen-containing gas, in most case ammonia (NH3), together with heat treatment.67,68 In situ doping is a method to obtain N-doped carbons from N-containing precursor, such as melamine,69 acetonitrile,70 and phthalocyanines.71 These pre-

© 2014 The Chemical Society of Japan

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Figure 7. Schematic illustration for changed surface induced by metal­semiconductor heterojunction. (a) A metal and n-type semiconductor contact, (b) A metal and ptype semiconductor contact, and (c) metal­semiconductor ohmic contact. Reproduced with permission for Ref. 61. Copyright 2013 Royal Society of Chemistry. Detailed theory is described in Ref. 59.

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Figure 8. Schematic illustration for change of electronic structure of doped graphene by change of chemical structure of doped nitrogen. Reprinted with permission for Ref. 64. Copyright 2012 American Chemical Society.

cursors were used for instance to synthesize N-doped carbon nanotubes. Nanostructured N-doped carbons were achieved by direct polymerization of N-containing polymers, such as, poly(vinylpyrrolidone),72 polypyrrole,73 and polyacrylonitrile,74 in a various of porous templates. Synthesis of N-Doped Carbons from Ionic Liquids and Poly(ionic liquids) Ionic liquids (ILs) and poly(ionic liquid)s (PILs) are promising precursors for the synthesis of these special carbons.17,66,75 Bull. Chem. Soc. Jpn. 2015, 88, 386–398 | doi:10.1246/bcsj.20140317

The first report using imidazolium- or pyridinium-based ILs for a functional carbon synthesis was independently reported by two groups.76,77 The use of IL- and PIL-based precursors has several benefits for nanostructured NdCs synthesis: (1) they easily wet surfaces of various materials thus, they can form homogenous films or coatings or enable the creation of nanohybrids; they are not volatile and are essentially position-stable until they are condensed towards higher entities, (3) they enable us to obtain not only N-doped carbons having different chemical structures but codoped carbons like N,S-doped or N,B-doped carbons by selection of ILs and PILs (Figure 9).66 The effective IL-/PIL-based precursors for functional caboneous materials are imidazolium-, or pyridinium-based cations with nitrilebased anions such as, dicyanamide (dca), tricyanomethinide, or tetracyanoborate cations.75 These cyano-based cations can undergo trimerization to form triazine rings at relative lowtemperature region, therefore, they can condense before full decomposition of the precursors. In fact, this idea is inspired by the formation process of CTFs, described above.39 The intermediary formation of triazine rings leads to electron poor carbon sites in the resultant graphitic carbon materials, which was confirmed by 13C magic angle spinning solid state NMR, and this is one reason for a high oxidation resistance of ILs/ PILs-delivered carbons.76,78 A detailed reaction mechanism of condensation and carbonization is proposed in the Ref. 76. The carbons synthesized from those ILs showed ca. 10 wt % of nitrogen content even after carbonization at 1000 °C (Figure 10). PILs were applied to synthesize mesoporous, graphitic NdCs in 2010.79 This report indicates that PILs can form a functional nanostructured carbon with high yield even without nitrile-based cations since the additional backbone provides the necessary immobilization. The chemical structures of ILs/PILs-based NdCs were studied by X-ray photoemission spectroscopy (XPS) measurements: This revealed that nitrogen atoms are well incorporated in the graphitic network, mainly as a mixture of the edge terminating pyridinic nitrogen and the in-plane included quarternary nitrogen form.76,78 Indeed, it is suggested that the ratio between pyridinic nitrogen and quarternary nitrogen can be modified by carbonization temperature, while the general considerations given above are still valid: It is practically impossible to include more than 7% of quarternary nitrogen into a structure.80 This tunable chemical structure of doped nitrogen of Il/PILs-delivered NdCs leads also to different chemical properties, especially different catalytic activity. Nanoarchitectonics of Functional Carbons Using ILs/PILs and Their Applications The good affinity and adhesion ILs/PILs for oxide surfaces enables construction of nanostructured and/or porous NdCs by hard-templating (Figure 11).78 PILs precursors have additional advantages compared to IL precursors due to their polymeric properties. Electrospinning was applied to PILs precursors, such as vinylimidazolium- or vinylpyridinium-type cations having secondary allyl functions as well as dca anion, and NdC-based fibers and membranes were obtained.81 A variety of sizes of NdC-based hollow spheres were synthesized by PILsprecursors through a facile method.82 Silica templates were coated by PILs by one-step, radical-free polymerization of IL monomers in ethanol. PILs are insoluble in ethanol and have © 2014 The Chemical Society of Japan | 393

Figure 9. Different ILs and PILs for carbon synthesis. Reproduced with permission for Ref. 66. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

strong interaction with silica surfaces, thus very homogeneous PILs film on the silica templates are formed, in strict absence of secondary nucleation of pure polymer particles and uncoated templates. This thin layer was found to keep its morphology after carbonization and removal of the templates, and this results in NdC-based hollow spheres having 5-nm thin layer thicknesses. All these systems are unique nanostructured carbons, such as one-dimensional, membrane, and hollow sphere carbons, and are attractive materials for electrode materials and separators in rechargeable electrochemical energy storage devices.83­88 A salt-templating method was introduced to obtain micro- as well as mesoporosity in NdCs by a simple one-step process.89 In this method, the pore structures and specific surface areas are tuned by just changing the type of salts and salt concentrations. As a result, a variety of NdC and codoped carbons with very high specific surface areas up to 3600 m2 g¹1 were obtained. A detailed explanation of salttemplating method is available in the Ref. 89. 394 | Bull. Chem. Soc. Jpn. 2015, 88, 386–398 | doi:10.1246/bcsj.20140317

To illustrate the potential of ILs/PILs-based nanostructured/ porous NdCs in energy applications, a spotlight on those NdCs is put towards electrocatalytic reactions, such as ORR and oxygen evolution reaction (OER). Up to now, most effective ORR and OER catalysts are solely found among expensive and nonsustainable noble metals like platinum and gold, respectively.90,91 Therefore, improvement of NdCs as an abundant, affordable catalyst system is one of the most important basic and, at the same time, applied research topics. Actually, NdCs are reported in past papers as ORR catalyst with a fourelectron-mechanism, however, their efficiency was not high enough to compete with Pt.92 Our group has introduced ILsbased porous NdCs with addition of nucleobases as a preorganized nitrogen-source while applying hard-templating for the generation of the required porosity (note that electrocatalysis is a heterogeneous reaction which always relies on high specific surface areas of the catalytic system).93 This metal-free electrocatalyst exhibited a high nitrogen content of

© 2014 The Chemical Society of Japan

Figure 10. Course of nitrogen (above) and hydrogen (below) composition of the material at different reaction temperature. Reproduced with permission for Ref. 76. Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

12 wt % after carbonization at 1000 °C with a high specific surface area of 1500 cm2 g¹1 on the basis of 1-ethyl-3-methylimidazolium-dca (EMIM-dca) as an IL precursor. The ORR performance of this porous NdC catalyst showed a high selectivity for the four-electron mechanism with a high catalytic activity and long-term stability (Figures 12a and 12b). At the same time, a selective two-electron mechanism could be addressed by a NdC catalyst synthesized from 3-methylN-butylpyridinum-dca (3MBP-dca) after carbonization at 800 °C.80 These two reports are suggesting that the choice of IL-precursors, addition of preorganized nitrogen-sources, such as nucleobases, and carbonization temperature tunes the chemical structure and patterns of nitrogen atoms in the graphitic network and modifies the electrocatalytic properties. The origin of high catalytic activity of NdCs is still unknown, but the first groundbreaking papers have started to shed light onto some of the relevant descriptors.94,95 In these papers, the effect of heteroatom doping to graphene towards the ORR activity in the 0.1 M KOH solution was assessed by a combination of experimental and computational studies as a model case (Figures 12c­12f ). Recently, NdCs were found to show a highly catalytic activity not only for the ORR but also the OER reaction.96 This bifunctional catalytic activity is important for metal­oxygen batteries, which undergoes ORR and OER at the discharging and recharging stroke, respectively. Especially, lithium­oxygen batteries (LOBs) have a very high theoretical energy density comparable to internal combustion engines.97,98 The development of LOBs is crucial issue for practical installation of electronic vehicles (EVs) in our society. The perfor-

Figure 11. Nanostructured porous N-doped carbons. (a­c) Nanostructured porous N-doped carbon obtained by templating with porous alumina membrane. (d) Ludox-templated IL-derived porous N-doped carbon. (e) SBA-15 templated porous N-doped carbon. (f ) Nakanishi-type Silica monolith and (g) templated porous N-doped carbon. Reproduced with permission for Ref. 78. Copyright 2010 Royal Society of Chemistry. Bull. Chem. Soc. Jpn. 2015, 88, 386–398 | doi:10.1246/bcsj.20140317

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Figure 12. Application of N-doped carbons towards ORR electrocatalysts. (a) Linear sweep voltammogram (LSV) for IL-derived N-doped carbon for ORR in 0.1 M KOH solution with a rotation speed of 1600 rpm. The result showed a comparable electronic activity of N-doped carbons to Pt/C composite catalyst and (b) Current­time response of porous N-doped carbon. Reprinted with permission from Ref. 80. Copyright 2011 American Chemical Society. (c) Volcano plot between j0theory and ¦GOOH* with chargetransfer coefficient ¡ = 0.5 (red dashed line). Blue hollow squares are j0expt obtained from Tafel plots and DFT-derived ¦GOOH* for each doped graphene catalyst, (d) LSV at the ORR initial region for different catalysts on RDE at 1600 rpm in an O2-saturated 0.1 M solution of KOH. Inset illustrates the first electron transfer step that is O2 to adsorbed OOH*, (e) potential corrected free energy diagram for gN-G at experimentally observed on-set potential UNon-set (red) and theoretically predicted UN1 and UN2 which meet ¦G(UN1) = 0.43 eV and ¦G(UN2) = 0.22 eV, respectively (blue). Inset shows the atomic configuration of gN-G cluster, and (f ) experimentally derived on-set potentials of doped graphenes (red squares), and the predicted values (blue bars). Reprinted with permission from Ref. 94. Copyright 2014 American Chemical Society.

mance of LOBs using IL-based porous NdCs cathode showed a promising bifunctional electrocatalytic performance, while being conductive electrode support at the same time. Especially, the OER occurs with very low overpotential comparable to porous Au electrodes.82,99 The main issue for further development of the porous NdCs cathode in LOB system is a strategy to achieve a long cycling stability.

structured of NdCs together with controlling the chemical structures and amount of doped nitrogen in graphitic networks, and this was already shown to lead to a very broad range of different catalytic activities. Current targets on the NdC are further improvement of electrocatalytic activity and stability of NdCs and synthesis of affordable ILs from sustainable precursors in order to reduce the price of the overall materials.

Conclusion

K.S. acknowledges the Max Planck Society for financial support and for providing an opportunity for precious scientific activities. We are thankful to the colleagues in the institute for their cooperation.

In this account, potentialities of a recent development, Nand C-based porous solids, were highlighted. As a subclass, CTFs are promising candidates for next-generation metal-free semiconductors with tunable electronic functions. Fascinatingly, CTFs were also already proven to be electrocatalysts with selective catalytic activity. Further research on new syntheses and formation routes of CTFs is however necessary since structure control is just at its infancy, and it is important to obtain highly crystalline CTFs for detailed investigations on their crystal structure, chemical structure, electronic function, and thereby correlation between physical structures and electronic properties. N-Doped carbons were successfully synthesized with a variety of architectures, porosity, and high specific surface area. The ILs/PILs approach enables us to tune morphology and 396 | Bull. Chem. Soc. Jpn. 2015, 88, 386–398 | doi:10.1246/bcsj.20140317

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Markus Antonietti has been Director of the Max Planck Institute of Colloids and Interfaces since 1993. He has published about 650 papers in the fields of polymer science, materials chemistry, and sustainable chemistry. His interests are devoted to a sustainable future of mankind using novel materials and processes based upon them.

Ken Sakaushi studied solid-state physics (B.Sc. in 2008), and inorganic chemistry and electrochemistry (M.Sc. in 2010) at Keio University, Japan. In 2013, He received his Ph.D. in chemistry from the Dresden University of Technology, Germany, as a German Academic Exchange Service (DAAD) Fellow. Currently, he is a member of the Colloid Chemistry Department in the Max Planck Institute of Colloids and Interfaces directed by Prof. Dr. Markus Antonietti. Since 2014, he has been a team leader there. His research focuses on synthesis of a variety of nanostructured inorganic materials and novel functional materials, such as C- and N-based porous catalysts and electroactive covalent polymeric frameworks, in order to study chemical transformations in energy. https://sites.google.com/site/sakaushiken/.

398 | Bull. Chem. Soc. Jpn. 2015, 88, 386–398 | doi:10.1246/bcsj.20140317

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