2017 Vol. 32, No. 2

Graphical Contents
2017, 32(2): .
Abstract(171) PDF(1695)
Abstract:
Recent development of polysulfide barriers for Li-S batteries
XU Zhao, YOU Hui-hui, ZHANG Lei, YANG Quan-hong
2017, 32(2): 97-105.
Abstract(712) PDF(1152)
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Li-S batteries have attracted great attention for their high theoretical specific capacity (1 675 mAh·g-1) and energy density (2 600 Wh·Kg-1). However, their low conductivity and poor utilization efficiency of sulfur greatly restrict practical applications. Novel polysulfide barriers have been designed and fabricated to overcome the shuttle effect and to improve cycle stability. In this review, the recent development of polysulfide barrier materials for Li-S batteries is introduced, which include carbon materials, metal oxides and conducting polymers. The development of an integrated electrode consisting of a polysulfide barrier and separator could be a hot research topic for Li-S batteries.
Research progress on electrode materials and electrolytes for supercapacitors
JIAO Chen, ZHANG Wei-ke, SU Fang-yuan, YANG Hong-yan, LIU Rui-xiang, CHEN Cheng-meng
2017, 32(2): 106-115.
Abstract(2162) PDF(3516)
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Supercapacitors have great potential applications for electronic devices, and energy recyling and storage areas owing to their high power density, long cycle life, high safety and excellent performance at low temperatures. The electrode materials and electrolytes are two key factors that influence their performance. The electrode materials used in supercapacitors include carbon materials such as activated carbons, carbon nanotubes, graphene, carbon nanofibers and carbon nano-onions, metal oxides, conductive polymers and their composites. The electrolytes are aqueous electrolytes, organic electrolytes or ionic liquids. Here research progress on the electrode materials and liquid electrolytes for supercapacitors is summarized, their advantages and disadvantages are analyzed, and new electrode materials and electrolytes are suggested.
The hydrothermal synthesis of β-Ni(OH)2 nanoflakes on carbon cloth and their electrochemical properties
LIU Qian-xiu, LU Chun-xiang, LI Qian
2017, 32(2): 116-122.
Abstract(498) PDF(712)
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Carbon cloth (CC) coated with β-Ni(OH)2 nanoflakes was prepared by a hydrothermal method using nickel nitrate, urea and ammonium fluoride as the reaction system, and was characterized by SEM, XPS and XRD. Results indicated that the acid treatment of the CC introduced more active functional groups on its surface, which was beneficial to the coating of the β-Ni(OH)2. The β-Ni(OH)2 nanoflakes were of high purity, and their formation mechanism on the CC was investigated by observing their morphology at different reaction times. It was revealed that the β-Ni(OH)2 were initially present in the form of discrete nanoparticles and nanoflakes and after 6 h formed a continuous and well-crystallized β-Ni(OH)2 nanoflake with diameter and thickness of 1 μm and 10 nm, respectively. Nanoflakes continued to form, grow and overlap and after 12 h the thickness had increased to 200 nm. The sample at 6 h had the highest specific capacitance of 815.67 F·g-1 at a current density of 1 A·g-1, and retained 98.1% of its initial capacitance after 4 000 cycles.
The effects of activation method on the pore structure and electrochemical properties of ordered mesoporous carbons used as the electrode materials of supercapacitors
LIU Mei-jun, FU Dong-mei, WANG Chun-lei, MI Pan-pan, WANG Tong-hua
2017, 32(2): 123-129.
Abstract(362) PDF(721)
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An ordered mesoporous carbon (OMC) synthesized by the soft-template method was activated by (a) CO2 at 900℃ for 4 h, (b) H2O at 800℃ for 20 min, and (c) H2O at 800℃ for 20 min followed by CO2 at 900℃ for 4 h to produce three micro-mesoporous carbons, A1, A2 and A3, respectively. Their pore structures were investigated by XRD, TEM, SEM and N2 adsorption. Their electrochemical properties as the electrode materials of supercapacitors were investigated by cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy. Results indicated that the highly ordered 2D hexagonal original mesostructure of the OMC was well retained after activation. The increasing order of the extent of activation is A1 < A2 < A3. The specific surface area, pore volume and the specific capacitance of A3 compared to OMC respectively increased significantly from 673 to 2404 m2/g, from 0.98 to 2.24 cm3/g and from 70 to 175 F/g at a current density of 500 mA/g. The increasing order of the capacitance retention ratios of the activated samples at high current densities is A1 < A2 < A3.
WAXD/SAXS study and 2D fitting (SAXS) of the microstructural evolution of PAN-based carbon fibers during the pre-oxidation and carbonization process
LI Xiao-yun, TIAN Feng, GAO Xue-ping, BIAN Feng-gang, LI Xiu-hong, WANG Jie
2017, 32(2): 130-137. doi: 10.1016/S1872-5805(17)60110-0
Abstract(940) PDF(906)
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Microstructural evolution in polyacrylonitrile (PAN) fibers at different temperatures during pre-oxidation and carbonization under stretching was studied by synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS). Microvoids were characterized by the classical SAXS method, and were compared with simulation results obtained by fitting 2D SAXS patterns to a model based on a dilute system of cylindrical microvoids randomly distributed and preferentially orientated along the fiber axis and having a log-normal size distribution. The WAXD results showed that the crystal size, d-spacing and preferred orientation decreased during pre-oxidation, and increased during carbonization. A diffraction peak for PAN fibers at 2θ=13.6° disappeared during the final stage of pre-oxidation, meanwhile a new peak at 2θ=23.6° appeared, whose intensity increased during carbonization, indicating the formation of the graphite structure. The average length of the microvoids increased, and new microvoids were formed, which became oriented along the fiber axis as the fiber manufacturing process proceeded. The length of microvoids from simulation results is consistent with that from the classical method, indicating that the model is valid to describe the microvoid structure of fibers.
Microstructures and mechanical properties of pyrocarbons produced from phenolic resin with added Ni(NO3)2
MA Tian-fei, WU Xiao-xian, LI Hong-xia, LIU Guo-qi, YANG Wen-gang
2017, 32(2): 138-143. doi: 10.1016/S1872-5805(17)60111-2
Abstract(422) PDF(620)
Abstract:
A refractory containing graphite is commonly used in the metallurgical industry in locations subject to severe thermal shock because of the high thermal conductivity and good thermal shock resistance of graphite. However, a refractory that uses phenolic resin as the carbon precursor is brittle, and to improve its strength and toughness, Ni(NO3)2 is added to the resin to catalyze the in-situ formation of carbon nanofibers/nanotubes. The microstructure and mechanical properties of the Ni(NO3)2-modified phenolic resin carbons were characterized by XRD, SEM, TEM and mechanical tests. Results indicate that carbon nanofibers/nanotubes (2% by volume) were formed within the pyrocarbons as a result of the nickel catalyst and these are interconnected to form a network structure. The nanocarbon fibers/tubes significantly improve the bend strength, elastic modulus, tensile strength and fracture toughness of the pyrocarbons and their fracture energies are increased accordingly.
Ablation characteristics of a 4D carbon/carbon composite under a high flux of combustion products with a high content of particulate alumina in a solid rocket motor
LIU Yang, PEI Jing-qiu, LI Jiang, HE Guo-qiang
2017, 32(2): 144-151. doi: 10.1016/S1872-5805(17)60112-4
Abstract(572) PDF(669)
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The ablation behavior of a four-directional carbon/carbon (C/C) composite was examined in a lab-scale solid rocket motor under a high flux of combustion products containing a high content of particulate alumina. The composite consisted of three braided carbon fiber bundles at 120° to each other in the XY plane and a hexagonal array of carbon rods in the Z direction, all in a pitch carbon matrix. The rods consisted of a unidirectional array of the same carbon fibers in a pitch carbon matrix The composite was placed in the rocket motor with its XY plane perpendicular to the gas flow and its ablation rate, ablation behavior and microstructure were investigated. The flow field of the combustion products was simulated by solving the Reynolds-averaged Navier-Stokes equations. A deep pit was formed on the surface of the composite, the center of which coincides with the simulated particle accumulation area. The mechanical erosion was significantly increased when the particle impact velocity exceeded 96.82 m/s. The carbon rods were more susceptible to erosion than the surrounding fiber bundles. The maximum ablation rates of the carbon rod and bundles were increased almost by an order of magnitude by increasing the particle impact velocity by a factor of two. Numerous crater-like pores on the ends of the carbon rods were formed by alumina particle impaction, and the tips of the fibers in the carbon rods were almost flat and lower than the surrounding matrix. Heating caused by the particle impact increased the thermal oxidization and hence the overall ablation rate of the composite.
Preparation and properties of graphene/carbon fiber/poly(ether ether ketone) composites
SU Ya-nan, ZHANG Shou-chun, ZHANG Xing-hua, ZHAO Zhen-bo, CHEN Cheng-meng, JING De-qi
2017, 32(2): 152-159.
Abstract(782) PDF(884)
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Graphene/carbon fiber/poly(ether ether ketone) (GR/CF/PEEK) composites were prepared by directly spraying a GR dispersion onto CF/PEEK prepregs, followed by hot-pressing the stacked prepregs. The structure of the prepregs and cross-section of the composites were characterized by SEM. The mechanical, thermal and electrical properties of the composites were measured to evaluate the influence of GR on their performance. Results showed that the addition of 0.1 wt% GR increases the interlaminar shear strength, the flexural strength and flexural modulus of the composites from 57.3 MPa, 1 226.2 MPa and 64.5 GPa to 77.6 MPa, 1 512.3 MPa and 73.6 GPa, respectively. DSC analysis showed that the crystallinity of the composites increased with the GR content. The thermal conductivity and electrical conductivity of the composites were increased by 15.5 and 73.1%, respectively after 0.5 wt% GR was added. The GR/CF/PEEK composites have much better mechanical, thermal and electrical performance than the CF/PEEK composites.
Graphene oxide: a novel acid catalyst for the synthesis of 2, 5-dimethyl-N-phenyl pyrrole by the Paal-Knorr condensation
CHEN Chun-yan, GUO Xiao-ya, LU Guang-qiang, Christian Marcus Pedersen, QIAO Yan, HOU Xiang-lin, WANG Ying-xiong
2017, 32(2): 160-167. doi: 10.1016/S1872-5805(17)60113-6
Abstract(765) PDF(526)
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Graphene oxide (GO) was used as an efficient and recyclable catalyst for the synthesis of N-substituted pyrrole using a Paal-Knorr condensation reaction between 2,5-hexanedione and aniline. The effects of reaction time, reaction temperature, solvent, catalyst loading and molar ratio of aniline to 2,5-hexanedione on the yield of 2,5-dimethyl-N-phenyl pyrrole were investigated. In situ NMR was used to follow the Paal-Knorr reaction at the molecular level. Results revealed that the oxygen-containing groups of GO, such as sulfonic acid and carboxyl groups, played a key role in this catalytic reaction. Polar protic solvents were favorable for the reaction. The catalytic activity increased with temperature without any side reaction. The GO could be easily recovered and showed remarkable reusability and excellent catalytic performance allowing it to be reused 5 times.
Investigation of defects in a mesophase pitch-based graphite at the atomic scale
RONG Ju, ZHU Yuan-yuan, FAN Zhen, FENG Zhi-hai, HE Lian-long
2017, 32(2): 168-173.
Abstract(376) PDF(721)
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A C/C composite as a thermal conducting material was prepared by the impregnation of unidirectional mesophase pitch-based carbon fibers with mesophase pitch and graphitization at 3 000℃. It was thinned to around 10 nm in the direction parallel to the fiber axis by cutting, grinding and ion beam bombardment. Results indicate that mesophase pitch is transformed into hexagonal graphite (HG). However, there is rhombohedral graphite (RG) tens of nanometer thick mingled with the HG. Defects include the glide and rotation structures and grain boundaries.
Effect of graphite/sodium nitrate ratio and reaction time on the physicochemical properties of graphene oxide
Edwin T. Mombeshora, Patrick G. Ndungu, Vincent O. Nyamori
2017, 32(2): 174-187. doi: 10.1016/S1872-5805(17)60114-8
Abstract(621) PDF(600)
Abstract:
Graphene oxide (GO) was synthesized by the reaction of graphite with sodium nitrate and the graphite/sodium nitrate mass ratio and the reaction time were varied in order to obtain the highest oxygen content. The GO was characterized by TEM, SEM, AFM, XRD, FT-IR, TGA, elemental analysis, and UV-vis and Raman spectroscopy. The effect of oxygen content on the physicochemical properties of GO was investigated. Results indicate that increasing the graphite to sodium nitrate ratio increases the oxygen content, BET surface area, pore volume and pore size but reduces the crystallite size of the GO samples. However, the oxygen content of GO is not directly related to the reaction time. Physicochemical properties such as d-spacing and defect density increase with increasing oxygen content while the thermal stability decreases. The physicochemical properties such as oxygen content, crystallinity, thermal stability and structure can be tailored by varying the graphite/sodium nitrate ratio and reaction time.
Growth of graphene sheets under an oxyacetylene flame without a catalyst
GUO Ling-jun, PENG Jian
2017, 32(2): 188-192. doi: 10.1016/S1872-5805(17)60115-X
Abstract(503) PDF(618)
Abstract:
High quality graphene sheets that had a low ID/IG and a high 2D intensity in Raman spectra were prepared by a catalyst-free acetylene flame method. The sheets were grown vertically on the surface of carbon particles to form a petal-like morphology. A high temperature, high pressure and short residence time of the flame intensified the decomposition and cyclization reactions of acetylene, leading to the formation of graphene sheets in the gas phase. The turbulent flame and the gases released during the formation of the graphene sheets from carbon nuclei could be responsible for their petal-like morphology instead of an onion-like structure.