Hai-Wei Liang

Synthesis of Nitrogen-Doped Porous Carbon Nanofibers as an Efficient Electrode Material for Supercapacitors

Supercapacitors (also known as ultracapacitors) are considered to be the most promising approach to meet the pressing requirements of energy storage. Supercapacitive electrode materials, which are closely related to the high-efficiency storage of energy, have provoked more interest. Herein, we present a high-capacity supercapacitor material based on the nitrogen-doped porous carbon nanofibers synthesized by carbonization of macroscopic-scale carbonaceous nanofibers (CNFs) coated with polypyrrole (CNFs@polypyrrole) at an appropriate temperature. The composite nanofibers exhibit a reversible specific capacitance of 202.0 F g(-1) at the current density of 1.0 A g(-1) in 6.0 mol L(-1) aqueous KOH electrolyte, meanwhile maintaining a high-class capacitance retention capability and a maximum power density of 89.57 kW kg(-1). This kind of nitrogen-doped carbon nanofiber represents an alternative promising candidate for an efficient electrode material for supercapacitors.

Mesoporous Metal–Nitrogen-Doped Carbon Electrocatalysts for Highly Efficient Oxygen Reduction Reaction

A family of mesoporous nonprecious metal (NPM) catalysts for oxygen reduction reaction (ORR) in acidic media, including cobalt-nitrogen-doped carbon (C-N-Co) and iron-nitrogen-doped carbon (C-N-Fe), was prepared from vitamin B12 (VB12) and the polyaniline-Fe (PANI-Fe) complex, respectively. Silica nanoparticles, ordered mesoporous silica SBA-15, and montmorillonite were used as templates for achieving mesoporous structures. The most active mesoporous catalyst was fabricated from VB12 and silica nanoparticles and exhibited a remarkable ORR activity in acidic medium (half-wave potential of 0.79 V, only ∼58 mV deviation from Pt/C), high selectivity (electron-transfer number >3.95), and excellent electrochemical stability (only 9 mV negative shift of half-wave potential after 10,000 potential cycles). The unprecedented performance of these NPM catalysts in ORR was attributed to their well-defined porous structures with a narrow mesopore size distribution, high Brunauer-Emmett-Teller surface area (up to 572 m(2)/g), and homogeneous distribution of abundant metal-Nx active sites.

Hierarchically porous carbons with optimized nitrogen doping as highly active electrocatalysts for oxygen reduction

Development of efficient, low-cost and stable electrocatalysts as the alternative to platinum for the oxygen reduction reaction is of significance for many important electrochemical devices, such as fuel cells, metal-air batteries and chlor-alkali electrolysers. Here we report a highly active nitrogen-doped, carbon-based, metal-free oxygen reduction reaction electrocatalyst, prepared by a hard-templating synthesis, for which nitrogen-enriched aromatic polymers and colloidal silica are used as precursor and template, respectively, followed by ammonia activation. Our protocol allows for the simultaneous optimization of both porous structures and surface functionalities of nitrogen-doped carbons. Accordingly, the prepared catalysts show the highest oxygen reduction reaction activity (half-wave potential of 0.85 V versus reversible hydrogen electrode with a low loading of 0.1 mg cm(-2)) in alkaline media among all reported metal-free catalysts. Significantly, when used for constructing the air electrode of zinc-air battery, our metal-free catalyst outperforms the state-of the-art platinum-based catalyst.

Ultralight, Flexible, and Fire-Resistant Carbon Nanofiber Aerogels from Bacterial Cellulose†

Carbon-based aerogels, composed of interconnected threedimensional (3D) networks, have attracted intensive attention because of their unique physical properties, such as low density, high electrical conductivity, porosity, and specific surface area. As a result, carbon-based aerogels are promising materials used as catalyst supports, artificial muscles, electrodes for supercapacitors, absorbents, and gas sensors. Especially, ultralight or flexible carbon-based aerogels have many potential applications. For example, ultralight nitrogen-doped graphene framework, used as an absorbent for organic liquids or the active electrode material, exhibits a high absorption capacity and specific capacitance; stretchable conductors, fabricated by infiltrating flexible graphene foam with elastic polymers, show high stability of electronic conductivity even under high stretching and bending strain. Traditionally, to fabricate carbon aerogels, resorcinol– formaldehyde organic aerogels were pyrolyzed in an inert atmosphere to form a highly cross-linked carbon structure. 12] The carbon aerogels always have a high density (100–800 mgcm ) 13] and tend to break under compression. Carbon nanotube (CNT) sponges, graphene foam, and CNT forests have been prepared through chemical vapor deposition (CVD). Meanwhile, CNTs and graphene can be employed as building blocks and assembled into macroscopic 3D architectures. However, the harmful and expensive precursors or complex equipments involved in these syntheses dramatically hamper the large-scale production of these carbon-based aerogels for industry application. Recently, we have developed a template-directed hydrothermal carbonization process for synthesis of carbonaceous nanofiber hydrogels/aerogels on macroscopic scale by using glucose as precursors. However, the use of expensive nanowire templates in this synthesis pushes us to explore a facile, economic, and environmentally friendly method to produce carbon-based nanostructured aerogels. Nowadays, there is a trend to produce carbon-based materials from biomass materials, as they are very cheap, easy to obtain, and nontoxic to humans, etc. Bacterial cellulose (BC), a typical biomass material, is composed of interconnected networks of cellulose nanofibers, 22] and can be produced in large amounts in a microbial fermentation process. Recently, we reported a highly conductive and stretchable conductor, fabricated from BC, shows great electromechanical stability under stretching and bending strain. Herein, we report a facile route to produce ultralight, flexible, and fire-resistant carbon nanofiber (CNF) aerogels in large scale from BC pellicles. When used as absorbents, the CNF aerogels can absorb a wide range of organic solvents and oils with excellent recyclability and selectivity. The absorption capacity can reach up to 310 times the weight of the pristine CNF aerogels. Besides, the electrical conductivity of the CNF aerogel is highly sensitive to the compressive strain, thereby making it a potential pressure-sensing material. For fabricating the CNF aerogels, a piece of purified BC pellicle with the size of 320 240 12 mm was first cut into rectangular or cubic shape and then freeze-dried to form BC aerogels (see the Supporting Information). The dried BC aerogels were pyrolyzed at 700–1300 8C under argon atmosphere to generate black and ultralight CNF aerogels. After pyrolysis, the volume of obtained CNF aerogel is only 15% of that of the original BC aerogel. Meanwhile, the density decreases from 9–10 mg cm 3 for BC aerogels to 4–6 mgcm 3 for CNF aerogels, owing to evaporation of volatile species. The macroscopic sizes of the as-synthesized CNF aerogels are dependent on the sizes of the BC pellicles cut in the fabrication procedure. It is well-known that temperature has a great influence on pyrolysis products. To create ideal CNF aerogels, BC aerogels were pyrolyzed separately at different temperatures. Scanning electron microscopy (SEM) images show that BC aerogels exhibit a porous, interconnected, well-organized 3D network structure, which was formed through self-assembly in the bacteria culture process (Figure 1a). A high-magnification SEM image indicates that the nanofibers with a diameter of 20–80 nm are highly interconnected with large numbers of junctions (see the Supporting Information, Figure S1). After the pyrolysis treatment, the porous 3D structure of BC aerogels was maintained, and the diameter of the nanofibers decreased to 10–20 nm (Figure 1b, also see the Supporting [*] Z. Y. Wu, C. Li, Dr. H. W. Liang, Prof. Dr. J. F. Chen, Prof. Dr. S. H. Yu Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemistry, CAS Key Laboratory of Mechanical Behavior and Design of Materials, the National Synchrotron Radiation Laboratory, University of Science and Technology of China Hefei, Anhui 230026 (P.R. China) E-mail: shyu@ustc.edu.cn Homepage: http://staff.ustc.edu.cn/~ yulab/

Bacterial‐Cellulose‐Derived Carbon Nanofiber@MnO2 and Nitrogen‐Doped Carbon Nanofiber Electrode Materials: An Asymmetric Supercapacitor with High Energy and Power Density

A new kind of high-performance asymmetric supercapacitor is designed with pyrolyzed bacterial cellulose (p-BC)-coated MnO₂ as a positive electrode material and nitrogen-doped p-BC as a negative electrode material via an easy, efficient, large-scale, and green fabrication approach. The optimal asymmetric device possesses an excellent supercapacitive behavior with quite high energy and power density.

Macroscopic‐Scale Template Synthesis of Robust Carbonaceous Nanofiber Hydrogels and Aerogels and Their Applications

Hydrogels and aerogels are two typical families of gels, classified according to the medium they encompass, that is, water and air, respectively. Hydrogels have not only pervaded our everyday life in a variety of forms (e.g., fruit jellies, toothpaste, contact lenses, and hair gel), but have also been extensively explored as functional soft materials for use in various scientific fields. Replacing the liquid solvent in hydrogels or other wet gels by air without collapsing the network structure can lead to a new type of porous materials, namely, aerogels. Particularly, 3D nanoscale networks with open pores in the gels allow access and fast diffusion of ions and molecules, and thus hydrogels/aerogels have exhibited excellent performance as super adsorbents, electrode materials for batteries and supercapacitors, catalyst supports, and chemical and biological sensors. Despite their outstanding potential, several challenges in aerogel synthesis still must be addressed prior to their extensive practical application. The major problem associated with conventional aerogels is poor mechanical stability. The mechanical strength of aerogels could be enhanced by nanocasting conformal polymer coatings on preformed 3D networks, but this was accompanied by dramatic decreases in their porosity. Furthermore, to prevent the network from collapsing in a gel, supercritical drying is the most widely used technique for solvent removal. It is difficult to prepare low-cost aerogels on a large scale due to the limitations of industrial supercritical drying. Although several nanomaterials including carbon nanotubes, cellulose nanofibers, and the newly discovered graphene have been recently used as building blocks and assembled into monolithic gels, there is a lack of precise control of their physicochemical properties, particularly the size of building blocks, the porosity, and their surface chemistry, which are crucial in the further design and functionalization of aerogels for various applications. Here we report a new class of monolithic hydrogels/aerogels consisting of highly uniform carbonaceous nanofibers (CNFs), based on the recent, well-developed templatedirected hydrothermal carbonization (HTC) process. Compared with the conventional process for aerogel preparation, our synthetic method has some significant advantages: 1) Direct scaleup from 30 mL to 12 L just by using a large autoclave and without changing reactant concentrations and reaction time; 2) Easy and precise control of the structural parameters and mechanical strength of the CNF hydrogels/ aerogels over a wide range; and 3) Extraordinary flexibility and high chemical reactivity of the CNF gels give them great application potential. The synthesis of CNF gels is illustrated in Figure 1a. Ultrathin Te nanowire (TeNWs) templates are first dispersed in glucose solution to form a homogenous mixture (step 1 in Figure 1a). Hydrothermal treatment of the mixture at 180 8C for 12–48 h results in a mechanically robust monolithic gel-like product, which occupies the whole Teflon container and can be taken out directly without any damage (step 2 in Figure 1a; see also Supporting Information Figure S1a). The as-prepared wet gel can be easily cut into the desired shape (Supporting Information Figure S1b). After washing and chemical etching to remove TeNWs (Supporting Information Figure S2), the CNF hydrogel is formed (step 3 in Figure 1a). To obtain the CNF aerogel, water in the hydrogel is removed by freeze-drying (step 4 in Figure 1a and Supporting Information Figure S1c). A low-magnification SEM image of the aerogel reveals a highly porous network structure consisting of disordered nanofibers with uniform size (Figure 1c, left). There is no apparent difference in CNF size and distribution over the whole monolithic gel (Supporting Information Figure S3), that is, the network structure is homogeneous. Further SEM observations indicate that these highly uniform nanofibers interconnect with each other to a high degree through numerous junctions (Figure 1c, right). We hypothesize that these junctions are responsible for the outstanding mechanical properties of the gels. Formation of junctions between CNFs is not difficult to understand. In the original mixture before hydrothermal treatment, it was unavoidable that the TeNWs physically contacted or approached each other if their concentration reached a critical [*] Dr. H. W. Liang, Q. F. Guan, L. F. Chen, Z. Zhu, W. J. Zhang, Prof. S. H. Yu Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemistry, National Synchrotron Radiation Laboratory, University of Science and Technology of China Hefei, Anhui 230026 (China) E-mail: shyu@ustc.edu.cn Homepage: http://staff.ustc.edu.cn/~ yulab/

Nitrogen‐Doped Carbon Nanosheets with Size‐Defined Mesopores as Highly Efficient Metal‐Free Catalyst for the Oxygen Reduction Reaction

Nitrogen-doped carbon nanosheets (NDCN) with size-defined mesopores are reported as highly efficient metal-free catalyst for the oxygen reduction reaction (ORR). A uniform and tunable mesoporous structure of NDCN is prepared using a templating approach. Such controlled mesoporous structure in the NDCN exerts an essential influence on the electrocatalytic performance in both alkaline and acidic media for the ORR. The NDCN catalyst with a pore diameter of 22 nm exhibits a more positive ORR onset potential than that of Pt/C (-0.01 V vs. -0.02 V) and a high diffusion-limited current approaching that of Pt/C (5.45 vs. 5.78 mA cm(-2) ) in alkaline medium. Moreover, the catalyst shows pronounced electrocatalytic activity and long-term stability towards the ORR under acidic conditions. The unique planar mesoporous shells of the NDCN provide exposed highly electroactive and stable catalytic sites, which boost the electrocatalytic activity of metal-free NDCN catalyst.

Flexible all-solid-state high-power supercapacitor fabricated with nitrogen-doped carbon nanofiber electrode material derived from bacterial cellulose

To meet the pressing demands for portable and flexible equipment in contemporary society, it is strongly required to develop next-generation inexpensive, flexible, lightweight, and sustainable supercapacitor systems with large power densities, long cycle life, and good operational safety. Here, we fabricate a flexible all-solid-state supercapacitor device with nitrogen-doped pyrolyzed bacterial cellulose (p-BC–N) as the electrode material via a low-cost, eco-friendly, low-temperature, and scalable fabrication hydrothermal synthesis. The pliable device can reversibly deliver a maximum power density of 390.53 kW kg−1 and exhibits a good cycling durability with ∼95.9% specific capacitance retained after 5000 cycles. Therefore, this nitrogen-doped carbon nanofiber electrode material holds significant promise as a flexible, efficient electrode material.

High‐Performance Electrocatalysts for Oxygen Reduction Derived from Cobalt Porphyrin‐Based Conjugated Mesoporous Polymers

A cobalt-nitrogen-doped porous carbon that exhibits a ribbon-shape morphology, high surface area, mesoporous structure, and high nitrogen and cobalt content is fabricated for high-performance self-supported oxygen reduction electrocatalytsts through template-free pyrolysis of cobalt porphyrin-based conjugated mesoporous polymer frameworks.

A Free‐Standing Pt‐Nanowire Membrane as a Highly Stable Electrocatalyst for the Oxygen Reduction Reaction

Proton exchange membrane fuel cells (PEMFCs) have attracted a great deal of attention because of their high energy conversion effi ciency, low operation temperature, and low pollution emission. [ 1 ] However, there are still some challenges, such as sluggish kinetics and poor electrocatalyst durability of the oxygen reduction reaction (ORR) at the cathode, that limit the effi ciency and commercial viability of PEMFCs. [ 2,3 ] Therefore, it is necessary to develop a highly active and durable catalyst to improve the ORR performance of PEMFCs. [ 4,5 ]