Zhaokun Ma

Sn–Co nanoalloys embedded in porous N-doped carbon microboxes as a stable anode material for lithium-ion batteries

For improving the capacity and stability of Sn-based anode materials, a novel Sn–Co nanoalloy embedded in porous N-doped carbon was synthesized using the metal–organic framework ZIF-67 as both the template and carbon source, and SnCl4 as the tin source through carbonization. This composite shows the shape of a microbox with the diameter of about 2 μm in which about 10 nm of Sn–Co nanoalloy particles were uniformly embedded. When used as the anode material for lithium-ion batteries, it exhibits a high capacity of 945 mA h g−1, and 86.6% capacity retention after 100 cycles at 100 mA g−1 as well as an excellent rate capacity of 472 mA h g−1 at a high current density of 2 A g−1. The superior electrochemical performance can be ascribed to the well-dispersed, nano-sized alloy and the buffering effect of porous N-doped carbon coating. Moreover, the uniform particles remain intact upon cycling which gives the material enhanced electrochemical stability.

Lithiation Confined in One Dimensional Nanospace of TiO2 (Anatase) Nanotube to Enhance the Lithium Storage Property of CuO Nanowires

We have fabricated CuO@TiO2 nanocable arrays by a facile method involving in situ thermal oxidation of Cu foil and coating of tetrabutyl titanate solution. The structure of the nanocables has been investigated by various techniques to comfirm that the cores are mainly crystalline monoclinic CuO, and the shells are crystalline tetragonal anatase TiO2. When used as an anode material for lithium-ion batteries, the nanoconfinement effect plays an important role in improving the lithium-ion storage preformance: the lithiation will be confined in one-dimensional space of TiO2 nanotubes to limit the pulverization of CuO, and the phase interface will cause an interfacial adsorption to enrich more lithium ions at some level. Benefiting from the nanoconfinement effect and interfacial adsorption, the reversible capacity does not fade, but rather increases gradually to 725 mAh g(-1) after 400 cycles at a current density of 60 mA g(-1), superior to the theoretical capacity of CuO.

Effects of deformation-induced orientation on cyclization and oxidation of polyacrylonitrile fibers during stabilization process

Orientation of copolymer polyacrylonitrile (PAN) chains during their deformation prior to stabilization and the further effect on the stabilization were investigated in detail. Results reveal that the orientation of PAN chains presents a saturation point of 69.51% when the deformation ratio reaches approximately 1.07, meanwhile the cyclization rather than the oxidation has a stronger dependence on the orientation of PAN chains during stabilization. The cyclization is facilitated that the cyclization degree is increasing while the activation energy is decreasing obviously as a consequence of the developing orientation of PAN fibers before the saturation point; however, it is restrained during the further deformation of PAN fibers after the point. The resulting carbon fibers obtained from the PAN fibers prepared at the saturation point possess the highest mechanical properties of 4.07 GPa in tensile strength and 249.0 GPa in tensile modulus.

The demonstration of a dense and stable circumferential layer in the subsurface of polyacrylonitrile fibers for carbon fibers

The radial structure of polyacrylonitrile (PAN) copolymer fibers was investigated quantitatively by etching layer by layer in an improved permanganic etchant; meanwhile the effect of the etchant on the fiber surface was taken into consideration. The aggregated structure (crystal size, crystallinity, orientation and density) and thermal stability of each circumferential layer of PAN fibers were determined in detail according to a model proposed in the study. A denser layer with a thickness of about 1 µm was observed in the subsurface (2 µm from the PAN fiber surface), possessing a greater crystal size and crystallinity as well as a relatively higher thermal stability than other layers. This layer was considered to be a barrier for the diffusion of oxygen into PAN fibers during the stabilization and accelerated the formation of a core-shell structure in the resulting carbon fibers.

Fe/N-doped graphene with rod-like CNTs as an air-cathode catalyst in microbial fuel cells

This work proposes a simple and efficient approach for the formation of short carbon nanotubes (CNTs) on graphene sheets. This paper investigates the effect of heat treatment time on the morphology of CNTs. The mechanism of the growth and disappearance of CNTs are also investigated. Graphene is added into ferric trichloride (FeCl3)–melamine solution to obtain a suspension. The suspension is dried with stirring, followed by a carbonization process under N2 atmosphere, resulting in the formation of CNTs on graphene sheets. The thus-prepared carbon material can be used as a kind of durable and efficient non-precious metal oxygen reduction reaction (ORR) electrocatalyst. The ORR activity of the catalyst with favorable performance is characterized and compared with a commercial Pt/C catalyst. The results show that the ORR electron transfer number of Fe–N/G with CNTs is 3.91 ± 0.02. The Fe–N/G-MFC achieves a maximum power density of 1210 ± 23 mW m−2, which is much higher than Pt/C-MFC (1080 ± 20 mW m−2). It demonstrates that Fe–N/G materials with CNTs can be a type of promising highly efficient catalyst and can enhance ORR performance of MFCs. Besides, the reason for the disappearance of CNTs we investigated in this study may provide some ideas for the study of loading metal oxide catalysts on CNTs.

The effect of liquid stabilization on the structures and the conductive properties of polyimide-based graphite fibers

Polyimide (PI) fibers, stabilized under reflux of hot nitric acid, are carbonized and graphitized in an inert atmosphere to obtain polyimide-based graphite fibers (PI-GFs). The results of DSC show that the exothermic interval of stabilized PI fibers becomes broad, accompanied by a reduction in total heat release, from 875.18 mW mg−1 (non-stabilized) to 166.13 mW mg−1 (stabilized for 15 min). And the carbon yields substantially increase, from the initial 15.98% (non-stabilized) to 49.93% (stabilized for 30 min) as proved by TGA. In addition, when the stabilization time is over 15 min, some defects such as cracks will appear on the fiber surface, which have a negative influence on the properties of the resulting PI-GFs. After being graphitized at 2800 °C, the stabilized PI-GFs have a high degree of graphitization and thermal conductivity which could be as high as 415.35 W m−1 K−1. It is indicated that PI fibers may be a good potential candidate for graphite fibers with high conductivity.

The effect of doping graphene oxide on the structure and property of polyimide-based graphite fibre

Herein, graphite fibres were prepared from polyimide (PI) fibres by doping varying contents of graphene oxide (GO) into polyimide (PI) fibres through a carbonization and graphitization process. By in situ polymerization, GO/polyamic acid (PAA) was synthesized and used for preparing GO/PI fibres via dry-jet wet spinning. During the spinning process, the molecular orientation of GO/PI fibres was forced to follow the fibre axis under the strong sheer force at the spinneret. The DSC results show that the exothermic intensity of 1.0 wt% GO/PI composite fibres declined by 69.7% than that of the pure PI fibre; this prevented the breakage of PI molecular chains and maintained the preferred orientation of the GO/PI fibres. During the graphitization process, GO sheets were reduced to grain graphene, acting as nucleus crystals, which could enlarge the size of microcrystals of graphite and increase the degree of graphitization. PI fibres as a carbon precursor showed great potential in the preparation of graphite fibres with high thermal conductivity, and GO doping can improve the thermal conductivity of the composite graphite fibres. When 2.0 wt% GO was added, the thermal conductivity of the GO/PI composite graphite fibre could reach 435 W m−1 K−1, which was twice that of the pure PI-based graphite fibre.

Self-sintering mechanism of stabilising mesophase pitch fibres

Abstract The ribbon mesophase pitch fibres (MPFs) can be sintered into bulk graphite materials with high thermal conductivity by one-step hot pressing method without introducing any binder. The contents of elements and group compositions as well as oxygenic functional groups of MPFs with different oxidation degrees were investigated by means of group composition analysis, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Then, the self-sintering mechanisms of the MPFs with different oxidation degrees were discussed in terms of their sintering ability and the influences on the structural performances of target materials. The results show that the fibres oxidised moderately have appropriate amount of phenol hydroxyl and carboxyl on their surface, which are beneficial to self-sintering. Meanwhile, the axial preferred orientation of aromatic macromolecules obtained during melt spinning can be maintained in fibre under a high temperature. For example, the materials, self-sintered from the ribbon MPFs oxidised at 260°C, have a thermal conductivity of 609 W m−1 K and a bending strength of 126 MPa.