掺杂碳材料在锂空气电池中的应用研究进展

锂空气电池具有理论能量密度高、成本低廉、环境友好等优点,受到学者们的广泛关注。本文首先介绍了锂空气电池的分类,并阐述了非水系锂空气电池的工作原理,回顾了传统碳材料和新型碳材料作为催化剂在锂空气电池中的应用,指出了纯碳材料的优势和不足,综述了杂元素（N、P、S等）的引入对碳材料催化性能的优化。重点讨论了N元素掺杂对氧还原反应的促进作用,强调了金属或金属氧化物的负载有利于氧析出反应,从而构建具有双功能的阴极催化剂。并简要介绍了具有金属-有机骨架等空间结构的新型材料在锂空气电池中的应用,就锂空气电池的发展前景进行了展望。     

锂/空气电池非贵金属催化剂研究进展

锂/空气电池理论能量密度高、体积小、质量轻、价格低、无污染,是极具应用前景的二次电池。本文首先简要介绍了锂/空气电池的基本结构、原理和种类,随后重点讨论了近年来用于锂/空气电池的非贵金属催化剂的研究进展。这些催化剂主要包括过渡金属氧化物、过渡金属氮化物、碳材料以及过渡金属大环化合物等。最后认为,材料化学、纳米技术等学科的发展以及催化机理的阐明对发展高性能的锂/空气电池非贵金属催化剂起至关重要的作用。     

Graphene nanosheets for enhanced lithium storage in lithium ion batteries

Graphene nanosheets were synthesized in large quantities using a chemical approach. Field emission electron microscope observation revealed that loose graphene nanosheets agglomerated and crumpled naturally into shapes resembling flower-petals. High resolution transmission electron microscope analysis, Raman spectroscopy and ultraviolet-visible spectroscopy measurements confirmed the graphitic crystalline structure of the graphene nanosheets. The nanosheets exhibited an enhanced lithium storage capacity as anodes in lithium-ion cells and good cyclic performance.     

Synthesis and characterisation of hydrophilic and organophilic graphene nanosheets

Hydrophilic graphene nanosheets were rapidly synthesized by reacting graphene oxide nanosheets with poly(sodium 4-styrene sulfonate) and simultaneously reducing by hydrazine hydrate under hydrothermal conditions. Organophilic graphene nanosheets were prepared by reacting with octadecylamine and reduction by hydroquinone through a reflux process. Ultraviolet-visible spectroscopy and Fourier transform infrared spectroscopy measurements confirmed the attachment of organic molecules to the graphene nanosheets to achieve hydrophilic and organophilic affinity. X-ray diffraction, Raman spectroscopy, and transmission electron microscopy analysis indicated that the crystal structure of the graphene nanosheets was maintained intact after chemical functionalisation.     

Graphene-supported SnO2 nanoparticles prepared by a solvothermal approach for an enhanced electrochemical performance in lithium-ion batteries

SnO₂ nanoparticles were dispersed on graphene nanosheets through a solvothermal approach using ethylene glycol as the solvent. The uniform distribution of SnO₂ nanoparticles on graphene nanosheets has been confirmed by scanning electron microscopy and transmission electron microscopy. The particle size of SnO₂ was determined to be around 5 nm. The as-synthesized SnO₂/graphene nanocomposite exhibited an enhanced electrochemical performance in lithium-ion batteries, compared with bare graphene nanosheets and bare SnO₂ nanoparticles. The SnO₂/graphene nanocomposite electrode delivered a reversible lithium storage capacity of 830 mAh g⁻¹ and a stable cyclability up to 100 cycles. The excellent electrochemical properties of this graphene-supported nanocomposite could be attributed to the insertion of nanoparticles between graphene nanolayers and the optimized nanoparticles distribution on graphene nanosheets.     

Fischer-Tropsch synthesis by reduced graphene oxide nanosheets supported cobalt catalysts: role of support and metal nanoparticle size on catalyst activity and products selectivity

In this paper,a series of cobalt catalysts supported on reduced graphene oxide(rGO)nanosheets with the loading of 5,15 and 30 wt-%were provided by the impregnation method.The activity of the prepared catalysts is evaluated in the Fischer-Tropsch synthesis(FTS).The prepared catalysts were carefully characterized by nitrogen adsorption-desorption,hydrogen chemisorption,X-ray diffraction,Fourier transform infrared spectroscopy,Raman spectroscopy,temperature programmed reduction,transmission electron microscopy,and field emission scanning electron microscopy techniques to confirm that cobalt particles were greatly dispersed on the rGO nanosheets.The results showed that with increasing the cobalt loading on the rGO support,the carbon defects are increased and as a consequence,the reduction of cobalt is decreased.The FTS activity results showed that the cobalt-time yield and turnover frequency passed from a maximum for catalyst with the Co0 average particle size of 15 nm due to the synergetic effect of cobalt reducibility and particle size.The products selectivity results indicated that the methane selectivity decreases,whereas the C5+selectivity raises with the increasing of the cobalt particle size,which can be explained by chain propagation in the primary chain growth reactions.     

Furfuryl alcohol functionalized graphene nanosheets for synthesis of high carbon yield novolak composites

Graphene oxide and furfuryl alcohol modified graphene nanosheets (G‐FA) were used to prepare graphene/novolak composites. Effect of graphene compatibilization on the properties of the composites especially carbon yield value is evaluated. Both types of graphene nanosheets were dispersed uniquely in the novolak matrix as proved by X‐ray diffraction analysis. However, modification of graphene sheets by furfuryl alcohol results in more improved dispersions. Thermogravimetric analysis confirms the elevated thermal stability of the nanocomposites in comparison with the neat novolak. In addition, G‐FA containing composites have higher carbon yield values. A shift in the wave number of characteristic bonds of graphene after oxidation and modification with furfuryl alcohol, O? H, C?O, and C? O bonds, are seen in the Fourier transform infrared spectroscopy spectra. Raman results and scanning electron microscopy images show that graphene nanosheets reduced in size and wrinkled by oxidation and functionalization. Transmission electron microscopy image of the composite with 0.2 wt % of G‐FA reveals the presence of nanosheets with curvature. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40273.     

Graphene nanosheets prepared by low-temperature exfoliation and reduction technique toward fabrication of high-performance poly(1-butene)/graphene films

Thermal exfoliation and reduction of graphene oxide (GO) were performed to prepare graphene nanosheets at 300 °C under the ambient atmosphere without any supplementary conditions. The microstructure and morphology of the resulting graphene nanosheets were characterized with scanning electron microscopy, transmission electric microscopy, atomic force microscopy and X-ray photoelectron spectroscopy. The composite films based on poly(1-butene) (PB) and graphene nanosheets were prepared successfully through solution blending and compression molding. The morphological investigation suggested that the graphene nanosheets with nanoscale thickness achieved a homogeneous dispersion in the PB matrix. The composite films exhibited a sharp transition from insulating state to the conducting one with a low percolation threshold, followed by a high electrical conductivity at graphene content higher than 1.6 vol %. The composite films also achieved high dielectric constant with low dielectric loss due to the effective electrical conductive path established by graphene nanosheets in a local range. Moreover, the mechanical evaluation demonstrated that a considerable reinforcement was achieved for the composite films due to the strong interfaces between the graphene nanosheets and PB matrix. The introduction of graphene nanosheets not only enhanced the nucleation capability and crystallinity of PB domain but also improved the thermal stability of the composite films. In addition, the composite films showed an increase in storage modulus and a decrease in loss factors due to the incorporation of graphene nanosheets.     

Utilization of multiple graphene nanosheets in fuel cells: 2. The effect of oxidation process on the characteristics of graphene nanosheets

Structural properties of graphene nanosheets that will be used as electrode material in fuel cells were investigated at different oxidation times. As the oxidation time was increased, the strong bonding between graphene layers in graphite was reduced and graphene layers started to exfoliate forming clusters with a few number of graphene layers. The variations in interplanar spacings, layer number and percent crystallinity indicated how stepwise chemical procedure influenced the morphology of graphite. It was possible to produce relatively flat graphene clusters with definite number of layers by controlling the oxidation time. Graphene nanosheets were characterized in detail by scanning electron microscopy, atomic force microscopy, X-ray diffraction, Raman spectroscopy, and thermal gravimetric analyzer.     

锂硫电池石墨烯/纳米硫复合正极材料的制备及电化学性能

利用热解还原将Hummers法制得的氧化石墨烯还原为石墨烯,并采用化学沉淀法将纳米硫成功负载到石墨烯片层上,获得石墨烯/纳米硫（RGO/nano-S）正极复合材料。利用FT-IR、XRD、SEM、TEM和Raman对所制备复合材料的微观结构、形貌等进行表征,采用恒流充放电、循环伏安法和交流阻抗法对复合材料的电化学性能进行研究。研究结果表明,热还原所得石墨烯褶皱的表面形成容纳硫及多硫离子的空间,有助于缓解活性物质溶解和抑制多硫离子迁移;同时,均匀分布的纳米硫能更好地与电解液接触,在石墨烯的导电网络上增大了电化学反应面积,进而改善了该材料作为锂硫电池的实际放比电容量和倍率循环性能。     

Synthesis of enhanced hydrophilic and hydrophobic graphene oxide nanosheets by a solvothermal method

We report that the hydrophilic affinity of graphene oxide nanosheets can be significantly increased by reacting with allylamine. High resolution transmission electron microscopy and electron diffraction analysis confirmed that the graphene oxide nanosheets were amorphous in structure. Hydrophobic graphene oxide nanosheets were also prepared via functionalising with phenylisocynate (C₆H₅NCO) through a solvothermal synthesis process. Hydrophobic graphene oxide nanosheets can be used as additives in polymer-based composites and other functional applications.     

Chemical-free synthesis of graphene–carbon nanotube hybrid materials for reversible lithium storage in lithium-ion batteries

Graphene–carbon nanotube hybrid materials were successfully prepared through the π–π interaction without using any chemical reagent. We found that the ratio between carbon nanotube and graphene had critical influences on the state in aqueous solution and morphology of hybrid materials. Field emission scanning electron microscope and transmission electron microscope analysis confirmed that graphene nanosheets wrap around individual carbon nanotubes and form a homogeneous three-dimensional hybrid nanostructure. When applied as an anode material in lithium ion batteries, graphene–carbon nanotube hybrid materials demonstrated a high reversible lithium storage capacity, a high Coulombic efficiency and an excellent cyclability.     

Highly active nitrogen-doped few-layer graphene/carbon nanotube composite electrocatalyst for oxygen reduction reaction in alkaline media

In this work the electrocatalysis of oxygen reduction on nitrogen-doped few-layer graphene/multi-walled carbon nanotube (FLG/MWCNT) composite catalyst has been investigated. These composite materials were prepared from different nitrogen precursors, acid-treated MWCNTs and graphene oxide (GO), which was synthesised from graphite by the modified Hummers’ method. Urea and dicyandiamide were used as nitrogen precursors and the doping was achieved by pyrolysing the mixture of GO and MWCNTs in the presence of these nitrogen-containing compounds at 800 °C. The N-doped composite catalyst samples were characterised by scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy, the latter method revealed successful nitrogen doping. The oxygen reduction reaction (ORR) was studied in 0.1 M KOH on glassy carbon electrodes modified with N-doped FLG/MWCNT electrocatalysts employing the rotating disk electrode (RDE) method. The RDE results indicated that these metal-free nitrogen-doped nanocarbon catalysts possess remarkable electrocatalytic activity towards the ORR in alkaline media similar to that of commercial Pt/C catalyst. The results obtained in this work are particularly important for the development of non-Pt cathode catalysts for alkaline membrane fuel cells.     

二氧化锰纳米片改性隔膜在锂硫电池中的应用

锂硫电池具有较高的理论能量密度,被认为是最有发展潜力的下一代高能量密度储能器件之一。然而多硫化物穿过隔膜形成的穿梭效应导致电池容量衰减过快、使用寿命降低,严重阻碍了锂硫电池商业化。以层状氧化石墨烯为模板,采用氧化还原法合成了二氧化锰纳米片,通过低压抽滤获得二氧化锰改性隔膜。利用TEM、XRD、FTIR、SEM、AFM等对该二氧化锰纳米片及改性隔膜的微观结构、形貌等进行表征;采用恒电流充放电、循环伏安法、电化学阻抗法对二氧化锰改性隔膜电化学性能进行测试。研究结果表明,二氧化锰纳米片能均匀覆盖聚丙烯隔膜表面的微孔,通过物理阻隔和催化作用,有效抑制了多硫化物的穿梭,提高了锂硫电池的比容量和循环稳定性。     

The effect of graphene nanosheets as an additive for anode materials in lithium ion batteries

A small amount of graphene nanosheets was added to commercial graphite as an anode active material in lithium ion batteries and its effects were examined through a variety of physical and electrochemical characterization techniques: FE-SEM, XRD, Raman, BET, and EIS. Compared to a commercial graphite electrode, a composite electrode containing 1 or 5 wt% graphene nanosheets showed higher reversible capacity and enhanced cyclability. This was attributed to the large surface area and low charge transfer resistance of the graphene nanosheets.     

Revelation of graphene-Au for direct write deposition and characterization

Graphene nanosheets were prepared using a modified Hummer''s method, and Au-graphene nanocomposites were fabricated by in situ reduction of a gold salt. The as-produced graphene was characterized by X-ray photoelectron spectroscopy, ultraviolet-visible spectroscopy, scanning electron microscopy, and high-resolution transmission electron microscopy (HR-TEM). In particular, the HR-TEM demonstrated the layered crystallites of graphene with fringe spacing of about 0.32 nm in individual sheets and the ultrafine facetted structure of about 20 to 50 nm of Au particles in graphene composite. Scanning helium ion microscopy (HIM) technique was employed to demonstrate direct write deposition on graphene by lettering with gaps down to 7 nm within the chamber of the microscope. Bare graphene and graphene-gold nanocomposites were further characterized in terms of their composition and optical and electrical properties.     

The improved electrocatalytic activity of palladium/graphene nanosheets towards ethanol oxidation by tin oxide

Tin oxide (SnO₂)/graphene nanosheets (GNS) composite was prepared by a simple chemical-solution method as the catalyst support for direct ethanol fuel cells. Then the SnO₂-GNS composites supporting Pd (Pd/SnO₂-GNS) catalysts were synthesized by a microwave-assisted reduction process. The Pd/SnO₂-GNS catalysts were characterized by using X-ray diffraction, transmission electron microscopy and energy-dispersive spectroscopy techniques. The electrocatalytic performances of Pd/SnO₂-GNS catalysts for ethanol oxidation were studied by cyclic voltammetric and chronoamperometric measurements. It was found that compared with Pd/GNS, the Pd/SnO₂-GNS catalyst showed superior electrocatalytic activity for ethanol oxidation when the mass ratio of SnCl₂·2H₂O precursor salt to graphite oxide was about 1:2.     

Status and prospects in sulfur–carbon composites as cathode materials for rechargeable lithium–sulfur batteries

Sulfur stands as a very promising cathode candidate for the next-generation rechargeable batteries due to its high energy density, natural abundance, low cost and environmental friendliness. However, the application of lithium–sulfur batteries suffers from low sulfur utilization and poor cycle life of the sulfur cathode. The problems are mainly ascribed to the electrically insulating nature of sulfur and the discharge products, and to the dissolution of the reaction intermediates of polysulfides. Among various approaches, fabricating sulfur–carbon composite cathodes with sulfur embedded within conductive carbon frameworks has been proven promising. Carbon materials, including nanoporous carbon, carbon nanotubes, graphene nanosheets and some other forms, have excellent conductivity, robust chemistry, good mechanical stability, and great abundance. By constraining sulfur within carbon frameworks, the conductivity of the sulfur electrode can be greatly enhanced, and the dissoluble loss of intermediate sulfur species in the liquid electrolyte can also be restrained due to the sorption properties of carbon, leading to a much improved electrochemical performance. This review summarizes the progresses in the sulfur–carbon composite cathodes for lithium–sulfur batteries in recent years, and introduces the roles and the effectiveness of various carbon structures on the electrochemical properties.     

Identifying the Catalytic Active Sites in Heteroatom‐Doped Graphene for the Oxygen Reduction Reaction

Density functional theory (DFT) calculations can be used to help elucidate the structures of active sites on the surface of fuel cell cathode catalysts, which are exceptionally difficult to identify by experimental techniques. The cathode catalysts were modeled in nitrogen‐, boron‐, sulfur‐, and phosphorus‐doped graphene basal planes. Dually‐doped graphene structures combining nitrogen with phosphorus or sulfur are also studied. Potential energy profiles were obtained, and the energies and activation barriers of molecular oxygen binding to the doped graphene model structures were estimated in order to identify potentially active sites for the oxygen reduction reaction in fuel cells. Among the investigated doped graphene structures, the active sites for molecular oxygen chemisorption are identified in graphene doped with either two nitrogen, or two phosphorus, or one sulfur and one phosphorus atoms. Further, the analysis of atomic spin densities and charges in the model structures enables the correlation of the catalytic activity with electron density distribution.     

RESS制备石墨烯纳米片及防止其再团聚的新方法

本文以天然鳞片石墨为原料,利用超临界状态下二氧化碳的快速膨胀（RESS）来剥离石墨产生石墨烯纳米片。电子显微镜（SEM）表征证实RESS可有效地实现石墨的剥离,并产生了一些石墨烯纳米片层。同时,为了解决再团聚难题,提出利用碳纳米管在产生石墨烯纳米片间穿层的方法和利用小分子包覆法来防止其再团聚,实验证实都起到良好的效果。