From Carbon-Based Nanotubes to Nanocages for Advanced Energy

Article pubs.acs.org/accounts

From Carbon-Based Nanotubes to Nanocages for Advanced Energy Conversion and Storage Qiang Wu, Lijun Yang, Xizhang Wang, and Zheng Hu* Key Laboratory of Mesoscopic Chemistry of MOE and Jiangsu Provincial Lab for Nanotechnology, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China CONSPECTUS: Carbon-based nanomaterials have been the focus of research interests in the past 30 years due to their abundant microstructures and morphologies, excellent properties, and wide potential applications, as landmarked by 0D fullerene, 1D nanotubes, and 2D graphene. With the availability of high specific surface area (SSA), well-balanced pore distribution, high conductivity, and tunable wettability, carbon-based nanomaterials are highly expected as advanced materials for energy conversion and storage to meet the increasing demands for clean and renewable energies. In this context, attention is usually attracted by the star material of graphene in recent years. In this Account, we overview our studies on carbon-based nanotubes to nanocages for energy conversion and storage, including their synthesis, performances, and related mechanisms. The two carbon nanostructures have the common features of interior cavity, high conductivity, and easy doping but much different SSAs and pore distributions, leading to different performances. We demonstrated a six-membered-ring-based growth mechanism of carbon nanotubes (CNTs) with benzene precursor based on the structural similarity of the benzene ring to the building unit of CNTs. By this mechanism, nitrogen-doped CNTs (NCNTs) with homogeneous N distribution and predominant pyridinic N were obtained with pyridine precursor, providing a new kind of support for convenient surface functionalization via Nparticipation. Accordingly, various transition-metal nanoparticles were directly immobilized onto NCNTs without premodification. The so-constructed catalysts featured high dispersion, narrow size distribution and tunable composition, which presented superior catalytic performances for energy conversions, for example, the oxygen reduction reaction (ORR) and methanol oxidation in fuel cells. With the advent of the new field of carbon-based metal-free electrocatalysts, we first extended ORR catalysts from the electron-rich N-doped to the electron-deficient B-doped sp2 carbon. The combined experimental and theoretical study indicated the ORR activity originated from the activation of carbon π electrons by breaking the integrity of π conjugation, despite the electron-rich or electron-deficient nature of the dopants. With this understanding, metal-free electrocatalysts were further extended to the dopant-free defective carbon nanomaterials. Moreover, we developed novel 3D hierarchical carbon-based nanocages by the in situ MgO template method, which featured coexisting micro−meso−macropores and much larger SSA than the nanotubes. The unique 3D architecture avoids the restacking generally faced by 2D graphene due to the intrinsic π−π interaction. Consequently, the hierarchical nanocages presented superior performances not only as new catalyst supports and metal-free electrocatalysts but also as electrode materials for energy storage. State-of-the-art supercapacitive performances were achieved with high energy density and power density, as well as excellent rate capability and cycling stability. The large interior space of the nanocages enabled the encapsulation of high-loading sulfur to alleviate polysulfide dissolution while greatly enhancing the electron conduction and Li-ion diffusion, leading to top level performance of lithium−sulfur battery. These results not only provide unique carbon-based nanomaterials but also lead to indepth understanding of growth mechanisms, material design, and structure−performance relationships, which is significant to promote their energy applications and also to enrich the exciting field of carbon-based nanomaterials. materials themselves.2,3 For example, the commercial electrocatalyst of fuel cells is platinum supported on carbon black, while the commercial supercapacitors use activated carbon as electrode material. In the past 30 years, great progress has been achieved in carbon-based nanomaterials as landmarked by the discovery of new carbon allotropes of 0D fullerene (1985),4 1D nanotubes (1991),5 and 2D graphene (2004).6 These achievements suggest the great possibility to realize advanced energy

1. INTRODUCTION Advanced energy conversion and storage play the crucial roles in exploiting renewable clean energies, popularizing mobile electric devices, and improving power grid efficiency, which stimulates intensive advancements of the related technologies such as fuel cells, rechargeable batteries, and supercapacitors.1 The key is the development of high-performance materials for energy conversion and storage. The sp2 carbon materials have long taken a dominant position in this field due to their intrinsic high conductivity and stability, abundant microstructures and morphologies, and low cost, which makes them suitable either for catalyst supports or for electrode © 2017 American Chemical Society

Received: October 29, 2016 Published: February 1, 2017 435

DOI: 10.1021/acs.accounts.6b00541 Acc. Chem. Res. 2017, 50, 435−444

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Figure 1. C6-based growth mechanism and its application for nitrogen doping.11 (a) Structural similarity of benzene ring and the building block of CNTs. (b) Schematic illustration of C6-based growth mechanism: (I) absorption of benzene on catalyst surface; (II) reduction of catalyst; (III) dehydrogenation of benzene and formation of graphene; (IV) continuous growth of CNT; (V) final state of the product. (c) The speculative construction of a graphene sheet from benzene, corresponding to steps II−V in panel b. (d) The NCNT with uniform N distribution stemming from pyridine precursor. (e) Typical NCNT in literature with localized N distribution for comparison. Reproduced with permission from ref 28. Copyright 2006 Wiley-VCH.

mechanism or the new preparation method, a series of unique carbon-based nanotubes and nanocages were designed and obtained, which exhibited excellent performance either as new catalyst supports and metal-free electrocatalysts for energy conversion15−23 or as electrode materials for energy storage.10,13,14,24 In particular, by doping the carbon nanomaterials with electron-rich N, electron-deficient B, or both, the electronic structures were conveniently regulated and the structure−performance relationships were well established by the combined experimental and theoretical study, which is generally interesting for designing advanced carbon-based energy materials.14,15,21−23 We review our progress from carbon-based nanotubes to nanocages in this Account.

conversion and storage by utilizing their advantages such as the available high specific surface area (SSA), well-balanced pore distribution, high conductivity, and tunable electronic structures. Actually, once a new carbon nanostructure was discovered, its potential for energy applications was always intensively explored, just as the case for graphene today.7,8 Meanwhile, we know that nanomaterial synthesis is experiencing a profound evolution from empirical science (“cook-andlook”) to prediction, design, and controllable synthesis, which depends on deep insight into the growth mechanism.9 In addition, more and more research indicates that the performance of a nanomaterial rests not only on what the nanobuilding units are but also largely on how they are assembled into the corresponding mesostructure.1,10 With these in mind, in the past decade, we devoted ourselves to studies on the controllable synthesis of carbon-based nanomaterials toward advanced energy conversion and storage and related mechanisms. Special attention was paid to carbon-based nanotubes and nanocages. Both of them are characterized by an interior cavity, high conductivity, and easy doping, but they have much different SSAs and pore distributions, thus leading to quite different performances and suitability for applications. We demonstrated a six-membered-ring-based (C6-based) growth mechanism of carbon nanotubes (CNTs) with benzene precursor based on the structural similarity of the benzene ring to the honeycomb building unit of CNTs.11,12 In addition, we developed an in situ MgO template method to prepare the novel 3D hierarchical carbon-based nanocages.10,13−15 By application of the growth

2. GROWTH MECHANISM, MATERIALS DESIGN, AND CONTROLLABLE SYNTHESIS In our study, carbon-based nanotubes and nanocages were prepared by chemical vapor deposition (CVD), which usually features mild preparation conditions and potential for mass production. As known, the prediction and design of nanomaterials depends on deep understanding of the growth mechanism and also on the synthesis technique. We paid much attention to this topic and obtained a series of unique carbon-based nanotubes and nanocages, which laid a solid foundation for exploring their performance and related mechanisms as well as potential applications. 436

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Figure 2. Formation diagram and typical characteristics of hCNC.10 (a) Schematic diagram for the formation of hCNC via the in situ MgO template method. (b) Typical scanning electron microscopy and transmission electron microscopy (TEM) images with different magnifications. Arrows indicate the broken fringes. Regions I and II represent the spaces inside the nanocages and interspace between the nanocages, respectively. (c) N2 adsorption/desorption isotherms and the corresponding pore size distributions, featuring with coexisting micro−meso−macropores. Reproduced from ref 10. Copyright 2015 Elsevier.

The finding of the C6-based growth model suggested a new approach to design carbon-based nanotubes by using heterocyclic precursors. If the basic idea of the growth model is applicable, the heterocyclic characteristics could be passed down to the final products. Hence, pyridine with the heterocyclic ring of C5N was taken as precursor for the growth of nitrogen-doped CNTs (NCNTs). As expected, the unique NCNTs with a uniform N distribution and an unusual predomination of pyridinic N over graphitic N were obtained with growth temperature below 750 °C (Figure 1d).27 This situation is in contrast to the localized N distribution and predomination of graphitic N over pyridinic N for the common NCNTs in the literature, which were usually synthesized with ammonia or nonheterocyclic N-containing precursors (Figure 1e).28 This means that the pre-existing pyridinic N in the pyridine precursor indeed passed down to the NCNTs product at least to a certain degree. By employing five typical Ncontaining aromatic precursors, the N contents were tuned in the range of 2−9 atom %, and the pre-existing C−N bonds in the precursors could influence the N incorporation in the corresponding NCNTs products. The ring N of the precursor is more likely to incorporate into the matrix of NCNTs as pyridinic nitrogen than the side one.29 This doping strategy was also used for reference in preparing the B and N codoped CNTs with bonded B and N by premixing triphenylborane (B source, a Lewis acid) and benzylamine (N source, a Lewis base) solution to form the stable B−N bonding configuration, which could pass down to the product during CVD growth.22 As contrast, B and N codoped CNTs with separated B and N were obtained by synthesizing B-doped CNTs with triphenylborane as B source in advance,21 followed by NH3 post-treatment.22

2.1. C6-Based Growth Mechanism and Carbon-Based Nanotubes

The CVD growth mechanism of CNTs is a long-standing issue, and little has been learned about the chemistry involved, although some valuable growth models have been speculated based on experimental results.25,26 Considering the structural similarity between the benzene ring and the honeycomb building block of CNTs (Figure 1a), we proposed the C6-based growth mechanism, which involves the selective dissociation of C−H bonds of benzene and the sequential growth of CNTs via the assembly of the dehydrogenated benzene rings (Figure 1b,c).11 A brief thermodynamic estimation also supports this growth model. Actually, the six-membered-ring framework of benzene is unusually stable with each carbon−carbon connection of ∼478 kJ·mol−1, higher than the C−H bond of ∼416 kJ·mol−1. The C6-based mechanism was deduced via in situ study on the chemistry involved in the CVD growth by thermogravimetry−differential scanning calorimetry−mass spectrum coupling technique, which provided all the information about the effluent species, weight variation, and thermal-flow on line.11 The surprising fact that only hydrogen was released while no hydrocarbon species were detected indicated the unbroken six-membered-ring framework during CNTs growth, which strongly reinforced the C6-based growth model (Figure 1b,c). Density functional theory (DFT) calculations further showed that the C−H bond of benzene could be selectively dissociated while the C−C connection still remained with the assistance of catalysts, thus C6 intermediates are generated for forming the graphene sheet through radical incorporation for sequential growth of CNTs.12 437

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Figure 3. Supported and encapsulated Pt-based catalysts. (a,b) Pt3Co alloy nanoparticles (NPs) supported on a NCNT (a) and corresponding schematic diagram (b). The image in panel a is reproduced with permission from ref 17. Copyright 2009 Wiley-VCH. (c) Pt NPs on NCNC. Reproduced from ref 34. Copyright 2016 Elsevier. (d) Pt NPs on N-doped carbon nanofibers. Reproduced from ref 19. Copyright 2009 Royal Chemical Society. (e) The calculated binding energies of different transition metal atoms on NCNTs.35 (f,g) Pt NPs encapsulated inside the nanocages (f) and corresponding construction strategy (g). Reproduced from ref 37. Copyright 2016 American Chemical Society.

2.2. In Situ MgO Template Method and Carbon-Based Nanocages

support interaction could much influence the electronic structure and properties of the catalyst. The carbon-based nanotubes and nanocages demonstrated the excellent performance either as unique functional supports for constructing catalysts or as metal-free electrocatalysts themselves especially for oxygen reduction reaction (ORR) in fuel cells.

Generally, carbon-based nanotubes have SSA less than 300 m2· g−1 with featureless pore distribution, which sometimes limits the applications in some fields, for example, in catalysis and energy storage. We developed an in situ MgO template method to prepare 3D hierarchical carbon nanocages (hCNC) with high SSA and unique pore distribution.10,13 The 3D hierarchical character of basic magnesium carbonate could pass down to MgO template upon decomposition at elevated temperature, and further to the particulate product (Figure 2a). Usually, the micrometer-sized carbon particles are composed of nanosheets with submicrometer-sized interspace. The nanosheets are several micrometers in size and 600 mV vs RHE), while the generation of O2− by outer-sphere electron transfer remains fixed at the potential of ∼310 mV (vs RHE).45 This means that strong O2/catalyst interaction is the prerequisite for the first electron transfer to pristine O2 at low pH. Multidoping heteroatoms into carbon nanomaterials might be a strategy to explore the advanced metal-free ORR catalysts in acidic media.

4. ENERGY STORAGE Hierarchical carbon-based nanocages feature large SSA, coexisting micro−meso−macropores, high conductivity and stability, and adjustable wettability, which meet the general requirements for advanced energy storage, that is, abundant electroactive sites, facile mass diffusion of electrolyte, and fast electron transport.49 The inside space further provides opportunity for constructing composites by encapsulation. These characteristics ensure the nanocages as a new kind of electrode materials. 4.1. Electrochemical Double Layer Capacitors (EDLCs)

EDLCs are capable of storing and delivering electricity at ultrafast rates with long cycling stability, but the energy density 441

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300 cycles), much superior to the controls. The fast lithium− sulfur batteries delivered the specific energy of 218 W·h·kg−1, larger than that for current Li-ion batteries (200 W·h·kg−1). Such lithium−sulfur batteries show great promise for mobile devices due to the shortened charging time from hours to minutes.10 Actually, the unique hierarchical structure and convenient doping also make the carbon-based nanocages demonstrate great potential in some other energy storage systems, for example, as anode materials of Li- and Na-ion batteries or as cathode materials of Li-ion batteries by constructing LiFePO4/ hCNC nanocomposites.24,53

is usually limited. Due to the synergism of large SSA, multiscale porous structure and good conductivity (Figure 2), the optimized hCNCs demonstrated excellent supercapacitive performances with a high specific capacitance up to 216 F·g−1 at 1 A·g−1.13 Despite the high-level performance, the areanormalized capacitance is at the ordinary level of carbon materials, ca. 11.8 μF·cm−2 at 1 A·g−1, far below the theoretical value of 21 μF·cm−2 for carbon material.13,50 By N-doping to introduce C−N polar bonds and improve the hydrophilicity (Figure 6a−c), the supercapacitive performance was further boosted to the record-high specific capacitance for carbonbased materials, up to 313 F·g−1 at 1 A·g−1 with the corresponding area-normalized capacitance of 17.4 μF·cm−2 (Figure 6d). The much improved wettability greatly enhanced the electrolyte penetration and increased the ion-accessible surface area, thereby decreasing the charge transfer resistance and equivalent series resistance, leading to state-of-the-art supercapacitive performance (Figure 6e). By a new in situ porous Cu template method, porous 3D few-layer graphenelike carbon with unique interconnected micro−meso−macropores and better conductivity was obtained, which delivers the ultrahigh maximum power densities of 1066.2 and 740.8 kW· kg−1 in aqueous and ionic liquid electrolytes, respectively, with top-level energy density, rate capability, and cycling stability.51 These results demonstrate the great potential of the unique hierarchical carbon-based nanomaterials for applications in EDLCs.

5. SUMMARY AND OUTLOOK This Account reviewed our progress in the discovery of the C6based growth mechanism of CNTs, the invention of the in situ MgO template method for preparing CNCs, and the related designed synthesis from carbon-based nanotubes to nanocages with tunable dopants, SSAs, pore distributions, and wettability. These unique carbon-based nanomaterials demonstrate excellent performances as new catalyst supports or metal-free electrocatalysts for energy conversion such as ORR, MOR, and FTS, and as the electrode materials for energy storage such as EDLC, Li−S, and Li-ion batteries. These achievements not only provide unique energy materials but also deepen our understanding of structure−performance relationships and material design, which much enriches the exciting frontier field of carbon-based energy nanomaterials. It is reasonable to expect that these unique carbon-based nanomaterials could find more space for energy applications. Featuring 3D hierarchical architecture and interior cavities with numerous microchannels, carbon-based nanocages are particularly promising as nanoreactors or nanocontainers to encapsulate different active species with confinement or molecular sieving effects for constructing advanced composite catalysts (e.g., for FTS) and electrode materials. Another attractive topic is developing the carbon-based ORR electrocatalysts in acidic electrolyte and exploring the potential of carbon-based nanocages toward some other key electrochemical energy conversions (e.g., H2 and O2 evolution), where manipulating doping-microstructure and understanding catalytic mechanism are highly desired. This is not easy since the sp2-carbon-based system is usually tangled by dopants, defects, hierarchical porous structures, and even spin density distribution. Thus, new synthesis strategies (e.g., organic synthesis) are expected to exclusively obtain some target configurations such as pure pyridinic nitrogen or graphitic nitrogen to experimentally clarify their contributions, which has confused the field for long time. Meanwhile, the “poisoning experiment” by selectively occupying or excluding a certain kind of active site is another approach to reveal their contributions. Finally, the combination of experimental and theoretical studies is highly recommended to deal with these issues, which is indeed what we are doing now.

4.2. Lithium−Sulfur Batteries

Lithium−sulfur batteries are very attractive owing to the high theoretical capacity and low cost, while their performance is hindered mainly by the insulation of sulfur, the dissolution of intermediate polysulfides, and large volumetric expansion during lithiation.52 The large interior cavities and high conductivity of hCNCs exhibits great advantages to tackle these challenges.10 Through the microchannels across the shells, sulfur can be filled into the cavities by melt-diffusion with a maximum loading of 79.8 wt % (Figure 7). The sulfur

Figure 7. Construction and characterizations of the 79.8%S@hCNC composite. (a) A schematic diagram of constructing S@hCNC composite via melt-diffusion and subsequent lithiation. (b−e) TEM image (b) and corresponding elemental mapping for C (c), S (d), and integrated C and S (e). Scale bar: 100 nm. Reproduced from ref 10. Copyright 2015 Elsevier.

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encapsulation ensured small sulfur grains, enhanced conductivity, and significantly suppressed polysulfide dissolution. Meanwhile, the hierarchical pore structure facilitates the high accessibility of electrolyte throughout the hCNC host. These features endowed the 79.8%S@hCNC composite with excellent performance, including large discharge capacity, high-rate capability, and long cyclability (558 mA·h·g−1 at 1 A·g−1 over

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zheng Hu: 0000-0002-4847-899X 442

DOI: 10.1021/acs.accounts.6b00541 Acc. Chem. Res. 2017, 50, 435−444

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The authors declare no competing financial interest. Biographies Qiang Wu received both his B.S. (1999) and his Ph.D. (2004) in chemistry from Nanjing University. He became an associate professor of Nanjing University in 2006 and full professor in 2015. As a HuaYing Scholar, he visited Stanford University for one year. His scientific interests focus on semiconductor nanomaterials, energy storage, and electrocatalysis. Lijun Yang received his Ph.D. in solid mechanics from Harbin Institute of Technology in 2006, and gradually converged to chemistry after two postdoctoral periods in IMEC Belgium and Nanjing University. Now he is an associate professor in Nanjing University and mainly focuses on the theoretical understanding of the mechanisms in energy conversion and storage systems, such as fuel cells, supercapacitors, and lithium batteries. Xizhang Wang received his Ph.D. in chemistry from Nanjing University in 2001. He was appointed an associate professor of Nanjing University in 2003 and full professor in 2011. He was a JSPS Fellow in Tokyo University (2003−2005). His scientific interests mainly focus on nanomaterial chemistry, sustainable energy, and heterogeneous catalysis. Zheng Hu received both his B.S. (1985) and Ph.D. (1991) in physics from Nanjing University. After two-year’s postdoctoral research in Department of Chemistry, he became an associate professor (1993), full professor (1999), and Cheung Kong Chair professor (2007) of Nanjing University. He is the recipient of the NSFC fund for outstanding young scientists of China (2005). He spent two years in Research Center of Karlsruhe, University of Cambridge, and MIT as a postdoctoral fellow and Hua-Ying Scholar, respectively. Hu is engaged in the research field of physical chemistry and materials chemistry addressing the growth mechanism, materials design, and energy applications of a range of nano- and mesostructured materials, especially the carbon-based materials, group III nitrides, and transition metal oxides.

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ACKNOWLEDGMENTS This work was jointly supported by NSFC (Grants 51232003, 21373108, 51571110, 21473089, 21573107) and MOST of China (Grant 2013CB932902).

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DOI: 10.1021/acs.accounts.6b00541 Acc. Chem. Res. 2017, 50, 435−444