2019 Vol. 34, No. 2

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2019, 34(2): .
Abstract(114) PDF(109)
Abstract:
Carbon materials for use in the electrocatalytic hydrogen evolution reaction
ZHANG Ze-xia, LU Rui-tao, HUANG Zheng-hong, KANG Fei-yu
2019, 34(2): 115-131.
Abstract(921) PDF(489)
Abstract:
As an important clean energy carrier with a high combustion heat value, hydrogen is of great importance for addressing the challenges caused by an increasingly severe energy shortage and environmental pollution. Compared with traditional hydrogen production from fossil fuel (e.g. natural gas, coal) by steam reforming, the electrocatalytic production of hydrogen by water splitting is a renewable and eco-friendly technique. Currently, the widely used catalyst for the hydrogen evolution reaction (HER) is a noble metal (e.g. Pt), which has very limited availability and a high cost. Therefore, it is crucial to develop low-cost and highly effective alternative HER electrocatalysts. In this review, we summarize the use of carbon materials for noble metal-free HER catalysts, in which they play roles including as an active catalyst and a matrix component. By heteroatom doping, the intrinsic activities of carbon materials are optimized and improved, and carbon-based metal-free catalysts are produced. As a matrix, the main roles of the carbon materials are as conductive substrates, highly-dispersed support materials and corrosion resistant layers. The catalytic activity of carbon-based catalysts is improved by a synergistic effect between a co-catalyst and the carbon skeleton. The HER performance of some reported carbon-based catalysts are even close to that of Pt. We summarize the strategies for tailoring the HER performance of carbon-based electrocatalysts and the prospects for future research on developing highly efficient and low-cost catalysts are discussed.
Synthesis of porous carbons from coal tar pitch for high-performance supercapacitors
WEI Feng, ZHANG Han-fang, HE Xiao-jun, MA Hao, DONG Shi-an, XIE Xiao-yu
2019, 34(2): 132-139. doi: 10.1016/S1872-5805(19)60006-5
Abstract(400) PDF(239)
Abstract:
Porous carbons (PCs) for supercapacitors were synthesized by a combined Mg(OH)2 templating and in-situ KOH activation method using coal tar pitch as the carbon precursor, and were characterized by TEM, Raman spectroscopy, XPS and N2 adsorption. Their electrochemical properties were investigated by galvanostatic charge-discharge, electrochemical impedance spectroscopy and cyclic voltammetry. Results show that the specific surface area of the PCs increases with the KOH dosage and exhibits a maximum with an activation temperature at 800℃. The optimum PC has a high surface area up to 3145 m2 g-1 with abundant micropores, and exhibits a high specific capacitance of 272 F g-1 at 0.05 A g-1, a rate capability of 217 F g-1 at 20 A g-1 and a good cycle stability with a 96.69% capacitance retention after 10000 cycles in a 6 M KOH electrolyte. This work provides a simple method for the large-scale production of PCs from pitch-based carbon sources for high-performance supercapacitors.
Recycled silicon powder coated on carbon paper used as the anode of lithium ion batteries
SHEN Chin-Wei, KO Tse-Hao, CHIU Kuo-Feng, LEU Hoang-Jyh, LIAO Ting-Chia, LIU Ching-Han
2019, 34(2): 140-145. doi: 10.1016/S1872-5805(19)60007-7
Abstract(395) PDF(262)
Abstract:
High-capacity electrode materials are critical for portable electronic equipment that requires a high-energy density from lithium ion batteries (LIBs). Many different materials and fabrication methods for such electrodes have been developed for this purpose. Carbon paper coated with recycled silicon powder (CP-RSP) was prepared by coating silicon and pitch powder on the carbon paper, followed by stabilization at 250℃ in air and carbonization at 1 000℃ in N2. The CP-RSP acted as both the current-collector and the active material for the anodes of LIBs. Electrodes with 2.5, 5, and 10 wt.% silicon exhibited capacity increases of 94, 129 and 41%, respectively, compared with the silicon-free electrode. The electrode for the CP-RSP with 5 wt.% silicon exhibited an optimal balance between discharge capacity and stability in long-cycle tests under various charging rates.
The formation of NaxC72(1 ≤ x ≤ 7) on a single layer graphene surface:a first-principles study
SUN Wen, YANG Shao-bin, SHEN Ding, DONG Wei
2019, 34(2): 146-152.
Abstract(530) PDF(395)
Abstract:
The formation of NaxC72 (1 ≤ x ≤ 7) on a single layer graphene surface was investigated using a first-principles study based on density functional theory. Adsorption sites, the lowest energy structures, the adsorption energy, Mulliken population, density difference, electron localization function (ELF) and the partial density of states (PDOS) of NaxC72(1 ≤ x ≤ 7) were calculated. It was found, based on the calculation of absorption energy, that the optimum formation mode of NaxC72(1 ≤ x ≤ 7) is that Na atoms are adsorbed on graphene bilaterally when x<5 and a Na cluster is formed when x ≥ 5. The average voltages of NaxC72 decrease with x when x<5 and increase with x when x ≥ 5. The maximum sodium storage capacity is 124 mAh/g, corresponding to a structure of Na4C72. The weak ionic bonds between Na atoms and graphene are formed by charge transfer from Na 3s to graphene π*, and metallic bonds are found between Na atoms in Na cluster as revealed by density difference, ELF and Mulliken population. Moreover, PDOS results show that the Fermi level of NaxC72(1 ≤ x ≤ 7) shifts to the graphene π* with increasing x, resulting in an increase of electronic conductivity.
Improved corrosion resistance of copper coated by graphene
ZHANG Hai-xia, MA Qiong, WANG Yong-zhen, XU Bing-she, GUO Jun-jie
2019, 34(2): 153-160. doi: 10.1016/S1872-5805(19)60008-9
Abstract(413) PDF(121)
Abstract:
High quality graphene on a Cu foil was synthesized by atmospheric pressure chemical vapor deposition. The morphology and microstructure of specimens were characterized by optical microscopy, scanning electron microscopy, high-resolution transmission electron microscopy, Raman spectroscopy, ultraviolet-visible spectroscopy and X-ray photoelectron spectroscopy. The corrosion resistance of specimens was tested by potentiodynamic polarization and electrochemical impedance spectroscopy. Results show that monolayer and high quality tri-layer graphene are synthesized at 1000℃ for 5 and 15 min depositions, respectively. The monolayer graphene cannot protect Cu efficiently from oxidation at 300℃ in air and from electrochemical corrosion in a 0.1 M NaCl solution due to its abundant defects and grain boundaries. The tri-layer graphene effectively increases the oxidation resistance and the electrochemical corrosion resistance. The protective performance of tri-layer graphene for a Cu foil is significantly better than that of monolayer graphene.
The effects of graphene content on the corrosion resistance, and electrical, thermal and mechanical properties of graphene/copper composites
WANG Jian, GUO Li-na, LIN Wan-ming, CHEN Jin, ZHANG Shuai, CHEN Shao-da, ZHEN Tian-tian, ZHANG Yu-yang
2019, 34(2): 161-169. doi: 10.1016/S1872-5805(19)60009-0
Abstract(841) PDF(222)
Abstract:
Graphene-reinforced copper matrix (G/Cu) composites were prepared by temperature-programmed sintering of their mixtures in molds under pressure. The effects of graphene content on the microstructure, and electrical, thermal, mechanical and corrosion properties of the G/Cu composites were investigated. Results show that the hardness, tensile strength, yield strength, thermal and electrical conductivities, and corrosion resistance of the composites all reached maxima at a graphene content of 0.5 wt.%. The addition of graphene increased the thermal and electrical conductivities, tensile and yield strengths, and hardness of the composites, but led to defect formation in the graphene due to the thermal expansion mismatch between graphene and copper. Therefore, an optimal graphene content was needed to obtain the best improvement of these properties. Tafel and electrochemical impedance tests using the composite as the working electrode, Pt as the counter electrode and a saturated calomel electrode as a reference electrode showed that the composite with a graphene content of 0.5 wt% had the lowest corrosion current of 3.45×10-6μA/cm2 and the highest charge transfer resistance of 1 705 Ω·cm2.
Preparation and microwave absorption properties of magnetic carbon nano-onion matrix composites
DENG Chuan, ZHANG Wei-ke, YANG Yan-qing, GAO Ze-yu, Wang Jia-wei, WEI Xian-xian
2019, 34(2): 170-180.
Abstract(621) PDF(195)
Abstract:
Magnetic carbon nano-onions (MCNOs) and multi-wall carbon nanotubes (MWCNTs) were prepared by chemical vapor deposition using a stainless steel mesh as the substrate and a sol-gel derived LaFeO3 as the catalyst. They were dispersed in water with a mass ratio of 1:1, and spray-dried with the assistance of ultrasonication to form hybrid granules. The morphology and magnetic permeability of the granules were investigated by XRD, SEM, TEM, Raman spectroscopy and a vibrating sample magnetometer. The granules were coated on a glass plate to evaluate their microwave absorption properties with a microwave vector network analyzer. Results showed that the absorption ability of the hybrid granule coatings was significantly improved in the frequency range 2-18 GHz compared with MCNOs or MWCNTs alone under the same conditions. With an increase of the coating thickness, the maximum absorption peak shifts to a lower frequency. The coating shows the best absorption properties when the thickness is 3.5 mm, where the maximum peak value reaches -25.6 dB and the effective bandwidth of less than -10 dB reaches 2.2 GHz.
Fabrication, microstructures and properties of SiCf/SiC composites prepared with two kinds of SiC fibers as reinforcements
WANG Hong-lei, ZHOU Xin-gui, PENG Shu-ming, ZHANG Hai-bin, ZHOU Xiao-song
2019, 34(2): 181-187. doi: 10.1016/S1872-5805(19)60010-7
Abstract(517) PDF(201)
Abstract:
Three-dimensional (3D) SiC/SiCf composites were fabricated by a polymer impregnation and pyrolysis (PIP) method using 3D four directional braided preforms from two kinds of SiC fibers as the reinforcements and liquid polycarbosilane as the SiC precursor. Both kinds of SiC fibers were prepared from the same precursor and process but with different heat treatment temperatures. The one prepared at a higher temperature (No.1) had a higher crystallinity than the another one (No.2). A thin pyrolytic carbon (PyC) interlayer was coated on the preforms by a methane CVD method before impregnation. Their microstructures and mechanical properties were investigated by TEM, SEM, Raman spectroscopy and mechanical tests. Results showed that both had a polycrystalline structure with grain sizes of 8-15 nm. Fiber pullout was seen from the fracture cross-section of the composites after a bending test, indicating pseudo-ductile fracture behavior. The composite using No.1 SiC fibers had an average flexural strength, elastic modulus and fracture toughness of 955.0±42.8 MPa, 110.3±1.7 GPa and 28.5±2.8 MPa·m1/2, respectively, which are superior to those of the material formed using No.2 SiC fibers. This was ascribed to the high modulus and excellent thermal resistance of the stoichiometric No.1 SiC fibers. The PyC layer adhering to No.1 SiC fibers was ordered and smooth while that deposited on No.2 SiC fibers had a loose and granular microstructure, which was attributed to the different surface chemistries of the two types of SiC fibers.
Phase change materials coated with modified graphene-oxide as fillers for silicone rubber used in thermal interface applications
FENG Jing, LIU Zhan-jun, ZHANG Dong-qing, HE Zhao, TAO Ze-chao, GUO Quan-gui
2019, 34(2): 188-195. doi: 10.1016/S1872-5805(19)60011-9
Abstract(397) PDF(182)
Abstract:
Graphene oxide prepared by the Hummers method was chemically modified by 3-aminopropyltriethoxysilane and spherical paraffin@modified graphene-oxide particles (P@m-GO) with a core-shell structure were obtained by an emulsion method. P@m-GO filler/silicone rubber (SIR) matrix composites (P@m-GO/SIR) were prepared by dispersing different amounts of P@m-GO in the SIR precursors, followed by curing and were used as thermal interface materials (TIMs). Results indicate that the best TIM had a P@m-GO loading of 60 wt.% and had both a high thermal conductivity (1.248 W·m-1·K-1) and a high latent heat (88.7 J·g-1). Its compression elastic modulus (1.01 MPa) was only one-eighth of that of the pristine SIR (8.16 MPa) due to the plasticity of the paraffin. The paraffin leakage under pressure was low (below 3.98 wt.%) before and after thermal cycling 50 times. These favorable thermal and mechanical properties together with the good cycling stability make it a promising TIM for electronic devices.
High-softening-point pitches prepared by the Br2 modification of low-temperature coal tar pitch and their mesophase transformation behavior
YANG Hai-xiao, HAN He-xiang, WANG Ji-tong, QIAO Wen-ming, LING Li-cheng
2019, 34(2): 196-204.
Abstract(707) PDF(195)
Abstract:
High-softening-point pitches were prepared from refined low-temperature coal tar pitch (R-CTP) by thermal bromination, followed by dehydrobromination/polymerization. R-CTP was first brominated in the presence of 5-20 wt.% bromine, and then heat-treated at 250-350℃ for dehydrobromination/polymerization. The structures of the dehydrobrominated pitches were characterized by 1H NMR, LDI-TOF/MS and FT-IR. The introduction of bromine significantly facilitated polycondensation of the component molecules and Br2 modification apparently increased the softening point, coking values and molecular weight of the obtained pitches compared with that formed by direct thermal condensation. The dehydrobrominated pitches showed relatively low melt viscosities and shearing-thinning rheological behavior with obvious plateau regions. An optimized dehydrobrominated pitch produced from R-CTP modified with 15 wt.% bromine (BRC-15%) by heat-treatment at 350℃ for 6 h had a softening point of 232℃ and a carbon yield of 55.2 wt.%. Polarized optical microscopy showed that the semicoke produced by the carbonization of BRC-15% at 410℃ appeared to be the most anisotropic with a large domain texture, and XRD patterns indicated that this sample also had the largest crystallite size (Lc=117 Å) of all samples examined.
Beneficiation of ultra-large flake graphite and the preparation of flexible graphite sheets from it
LI Ji-hui, HOU Shi-yu, SU Jia-rui, LI Kuan, WEI Lu-bin, MA Li-qiang, SHEN Wan-ci, KANG Fei-yu, HUANG Zheng-hong
2019, 34(2): 205-210. doi: 10.1016/S1872-5805(19)60012-0
Abstract(395) PDF(249)
Abstract:
A graphite ore with ultra-large flakes (ULFG) found in China (Wulate County, Inner Mongolia Autonomous Region, China), was first jaw-crushed and pneumatically separated to remove rocks, before being subjected to two froth-flotations and finally leached with NaOH and HCl to a purity of 99.9%. The purified ULFG was used to prepare exfoliated graphite by a combined chemical intercalation and rapid heating method and the latter was rolled into flexible graphite sheets of different densities. Results show that the beneficiation and purification methods preserve the graphite crystallites. The purified ULFG has a high degree of graphitization (99.9%). The volume of the ULFG-based exfoliated graphite is more than 400 mL/g, and some randomly selected individual exfoliated graphite particles are larger than 40 mm. A ULFG-based flexible graphite sheet with a density of 1.8 g/cm3 shows an excellent electrical conductivity of 2.78×105 S/m.

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