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2021年4期中文目次

2021, (4): 1-1.

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2021年4期英文目次

2021, 36(4): 1-5.

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Phosphorescent carbon dots: Microstructure design, synthesis and applications

KANG Hai-xin, ZHENG Jing-xia, LIU Xu-guang, YANG Yong-zhen

2021, 36(4): 649-664. doi: 10.1016/S1872-5805(21)60083-5

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Phosphorescent carbon dots (CDs) have great potential in energy, information, biomedicine, and other fields because of their long lifetime, long wavelength emission, and low background interference. However, there are still some challenges in their preparation and understanding their luminescence mechanism. For example, their triplet states are easily affected by the external environment, which leads to phosphorescence quenching. The phosphorescence mechanism and the effects of element doping, rigidity of structure, and conjugated structure on their properties are reviewed to address these issues. The synthesis methods include one step and two step methods. The uses of phosphorescent CDs are summarized and include information security, light emitting diodes, ion detection, and biological imaging. The existing problems are discussed and development directions are proposed.

Recent advances in carbon-supported iron group electrocatalysts for the oxygen reduction reaction

LI Ping, WANG Huan-lei

2021, 36(4): 665-682. doi: 10.1016/S1872-5805(21)60072-0

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Metal-air batteries are emerging energy devices that have received worldwide attention. The oxygen reduction reaction (ORR) is the key electrochemical process of metal-air batteries. The sluggish nature of ORR kinetics and the high cost of Pt-based ORR catalysts have severely hindered their large-scale application. As earth-abundant elements, the iron group elements have a variety of hybrid orbitals, and their incorporation into the carbon skeleton achieves good ORR catalytic activity, giving them great potential for substituting for Pt-based catalysts. Here, their uses for ORR and the function of each active site in the ORR process are summarized. The relationship between the microstructure and performance of these catalysts may help us fully understand the role of iron group elements in ORR and provide basic insight into the design of cheap catalysts with outstanding ORR catalytic performance in the future.

Carbon materials for solar-powered seawater desalination

WANG Tian-yi, HUANG Heng-bo, LI Hao-liang, SUN You-kun, XUE Yu-hua, XIAO Shu-ning, YANG Jun-he

2021, 36(4): 683-701. doi: 10.1016/S1872-5805(21)60066-5

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Carbon materials are widely used in solar-powered seawater desalination (SSD) and have attracted a lot of attention in recent years. Recent developments of carbon-based solar absorbers in SSD are reviewed, including composites of carbon materials with other materials such as metal nanoparticles, semiconductors and biomass materials, their photothermal conversion mechanisms, light utilization efficiencies and salt resistance, and the processes of thermal transport and water transfer. The important roles of carbon in SSD are highlighted, including increasing light absorption, improving photothermal conversion efficiency, and balancing water transfer and salt resistance. The key challenges of carbon-based materials in SSD applications are discussed.

Charge storage mechanisms of manganese dioxide-based supercapacitors: A review

TANG Xiao-ning, ZHU Shao-kuan, NING Jian, YANG Xing-fu, HU Min-yi, SHAO Jiao-jing

2021, 36(4): 702-710. doi: 10.1016/S1872-5805(21)60082-3

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Carbon-based materials, such as carbon nanotubes, graphene and mesoporous carbons, are typical electrochemical double-layer capacitive electrodes of supercapacitors (SCs). Although these carbon electrode materials have excellent electrochemical stability, they usually have a low capacitance. Therefore, pseudocapacitive materials are often combined with them to increase capacitance. Among these pseudocapacitive materials, manganese dioxide (MnO₂) has been widely used because of its high theoretical specific capacitance, low-cost, abundance, and environmentally friendly nature. However, the use of MnO₂ often produces rather low actual specific capacitances due to its poor electrical conductivity, phase transformation and large volumetric changes during repeated charge and discharge. To explore high-performance MnO₂/carbon composite electrode materials, it is necessary to understand the charge storage mechanisms of MnO₂. These are analyzed and classified into four types: surface chemisorption of cations, intercalation-deintercalation of cations, a tunnel storage mechanism and a charge compensation mechanism. Although the fourth involves pre-interaction of the cations in MnO₂, the essence of all these mechanisms is the valence transition of manganese atoms between +3 and +4, and many mechanisms are usually involved in MnO₂-based SCs because of the complicated charge storage process. Critical challenges and possible strategies for achieving high-performance MnO₂/carbon-based SCs are discussed and prospective solutions are presented.

Research progress on graphene-based materials for high-performance lithium-metal batteries

WANG Xin, HUANG Run-qing, NIU Shu-zhang, XU Lei, ZHANG Qi-cheng, Abbas Amini, CHENG Chun

2021, 36(4): 711-728. doi: 10.1016/S1872-5805(21)60081-1

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Due to their relatively low energy density, commercial lithium-ion batteries (LIBs) have faced difficulty in meeting the increasing requirements of energy storage devices for portable electronics and electric vehicles. Lithium (Li) with a high theoretical specific capacity (3860 mAh g⁻¹) and low density (0.59 g cm⁻³) is regarded as one of the best anodes for next-generation high energy density Li metal batteries, e.g., Li-S and Li-O₂ batteries. However, the safety problems induced by uncontrollable Li dendrite growth and a low Coulombic efficiency caused by an unstable solid electrolyte interphase layer, have limited their practical application. Graphene-based materials (GBMs) with a high specific surface area and controllable structures and chemical properties, have been shown to be important in solving these problems. Various protection strategies for Li metal anodes using GBMs are summarized and the design of GBMs with different roles and functions in Li metal protection is discussed. Challenges and possible solutions for the future development of GBMs used in Li metal anodes are discussed.

Research progress on the effect of graphene oxide on the properties of cement-based composites

WANG Qin, QI Guo-dong, WANG Yue, ZHENG Hai-yu, SHAN Si-han, LU Chun-xiang

2021, 36(4): 729-750. doi: 10.1016/S1872-5805(21)60071-9

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Graphene oxide (GO) has significant strengthening and toughening effects on cement-based composites as a nano-reinforcement filler, and research progress on these materials is presented. The effects of GO on the properties of cementitious composites are summarized, including the dispersion stability of GO in a cement environment, the hydration properties, workability, rheological properties, mechanical properties and durability. Reinforcement and toughening mechanisms are proposed. Prospective research trends are discussed based on the problems already encountered

Non-layered transition metal carbides for energy storage and conversion

GAO Yin-hong, NAN Xu, YANG Yao, SUN Bing, XU Wen-li, Wandji Djouonkep Lesly Dasilva, LI Xuan-ke, LI Yan-jun, ZHANG Qin

2021, 36(4): 751-778. doi: 10.1016/S1872-5805(21)60065-3

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Non-layered transition metal carbides (NL-TMCs) have diverse morphologies and structures, and tunable stoichiometric ratios, giving them many intriguing electrical/catalytic properties such as high gravimetric capacities, high conductivity and excellent stability. The latest progress in the use of NL-TMCs for energy conversion and storage applications is reviewed. Several routes to synthesize NL-TMCs are described, including carbothermal reduction, chemical vapor deposition, a template-assisted method and a hydro/solvothermal method. Their electrochemical performance in lithium-ion batteries, lithium-sulfur batteries and catalytic water splitting are presented. Current challenges for the rational design and fabrication of NL-TMCs for practical applications are discussed and future research suggestions are made.

Research progress on the biomedical uses of graphene and its derivatives

LIU Yang, DING Jing, WANG Qi-qi, WEN Mei-ling, TANG Ting-ting, LIU Yong, YUAN Rong, LI Yong-feng, AN Mei-wen

2021, 36(4): 779-793. doi: 10.1016/S1872-5805(21)60073-2

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Graphene (Gr) is a monolayer of carbon atoms in a two-dimensional honeycomb lattice, and has derivatives of graphene oxide and reduced graphene oxide. Gr is widely used in various fields for its good optics, conductivity, mechanical properties, low toxicity, antibacterial properties, biocompatibility and stability. Graphene oxide and reduced graphene oxide have similar properties to Gr, and all three materials have advantages and disadvantages and are often not used alone but are composited with other materials to improve their properties for a particular application. From the perspectives of the toxicity and antibacterial properties of Gr and its derivatives, this paper reviews their uses in treating skin wounds and tumours, promoting the regeneration of skeletal muscle and bone, and facilitating drug loading and diagnosis. The problems associated with these applications are analyzed and solutions are suggested. Future research and development prospects for Gr-based materials are presented.

Synthesis and use of hollow carbon spheres for electric double-layer capacitors

XU Kuang-liang, LIU Jing, YAN Zhao-xiong, JIN Mei, XU Zhi-hua

2021, 36(4): 794-809. doi: 10.1016/S1872-5805(20)60517-0

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Supercapacitors have become an important energy storage device. Based on their energy storage mechanism, supercapacitors are generally categorized into pseudocapacitors and electric double-layer capacitors (EDLCs). Nowadays, carbon materials are used as the electrodes in commercial EDLCs. Hollow carbon spheres (HCSs) have attracted extensive attention for use as the electrode materials of EDLCs because of their large specific surface area, high electrical conductivity, excellent electrochemical stability and high mechanical strength. Progress on the preparation of HCSs is reviewed, including the hard and soft templating methods, template-free methods and the modified Stöber method. Their electrochemical performance as the electrode materials of EDLCs and the effect of their specific surface area, pore size and doped foreign atoms on their electrochemical performance are summarized, which gives insight into their low-cost preparation and high-performance for use in supercapacitors.

Ultra-thin 2D MoO₂ nanosheets coupled with CNTs as efficient separator coating materials to promote the catalytic conversion of lithium polysulfides in advanced lithium-sulfur batteries

KONG Zhen-kai, CHEN Yang, HUA Jing-zhao, ZHANG Yong-zheng, ZHAN Liang, WANG Yan-li

2021, 36(4): 810-820. doi: 10.1016/S1872-5805(21)60080-X

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A severe shuttle effect and the slow kinetics of lithium polysulfide (LiPS) conversion are two major obstacles to the practical use of lithium sulfur batteries. Ultra-thin 2D MoO₂ nanosheets (MoO₂ NSs) have been synthesized by chemical vapor deposition and then mixed with carbon nanotubes (CNTs) for use as coating materials of the Celgard 2400 polypropylene separator to solve these problems. The 2D character of MoO₂ NSs produced high surface/volume ratios and abundant active binding sites for anchoring LiPSs. In addition, the partial reduction of MoO₂ NSs in a H₂/Ar mixture introduced oxygen vacancies in their surface, which acted as catalytic sites for LiPS conversion, while the CNT network ensured rapid electron transfer for LiPS conversion reactions. Symmetric dummy cell tests showed that a 30wt%MoO₂/CNT coated separator reduced the energy barrier for Li₂S nucleation, and first-principles calculations verified its strong binding energy to entrap LiPSs and increase Li₂S precipitation. Because of these features, a cell with a 30wt%MoO₂/CNT coated separator had an improved specific capacity of 738 mAh·g⁻¹ at 1 C with a slow decay rate of 0.053% for 800 cycles.

Magnetic modification of used tea leaves for uranium adsorption

YANG Ai-li, YANG Sheng-ya, ZHU Yu-kuan

2021, 36(4): 821-826. doi: 10.1016/S1872-5805(21)60053-7

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Used tea leaves, or tea waste (TW), were crushed into powder and mixed with graphene oxide (GO) in water, followed by adjusting the pH value of the resulting suspension with ammonia to 11, adding FeCl₂·4H₂O under magnetic stirring, filtration and drying to prepare a rGO/Fe₃O₄/TW (with mass ratios of 1∶2∶1) hybrid material. The structure and crystalline phases of the material were characterized by FTIR and XRD. Isotherms for uranium adsorption were obtained and its kinetics were measured in a conical bottle that was placed in a shaker. The effects of the pH value of the uranium solution, adsorption time and initial concentration on the uranium adsorption were investigated. Results indicate that the hybrid has a much faster adsorption rate than TW with an uranium removal rate up to nearly 100% in 20 min for an initial uranium concentration of 10 mg L⁻¹. The maximum adsorption capacity of the hybrid is 103.84 mg g⁻¹ while that of TW is 97.70 mg g⁻¹. The hybrid with adsorbed uranium can be easily separated from the solution by applying a magnetic field. The isotherms and kinetics of uranium adsorption on the hybrid are best fitted by the Langmuir isotherm model and the pseudo-second-order model, respectively. The hybrid has good reusability with an uranium removal rate of about 85% after 5 cycles.

Regulating the radial structure of polyacrylonitrile fibers during pre-oxidation and its effect on the mechanical properties of the resulting carbon fibers

WANG Yun-feng, WANG Yi-wei, XU Liang-hua, WANG Yu

2021, 36(4): 827-834. doi: 10.1016/S1872-5805(20)60516-9

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The radial structure of polyacrylonitrile fibers oxidized before carbonization and its distribution directly affect the performance of the resulting carbon fibers. Optimizing the radial distribution of the oxidized structure and establishing a relationship between this structure and the mechanical properties of the final carbon fibers will help optimize the oxidation conditions for the preparation of high-performance carbon fibers. Solid-state nuclear magnetic resonance spectroscopy, optical microscopy, thermogravimetric analysis, and mechanical tests were used to investigate the effect of the oxidation reaction rate on the radial distribution of the structure of the oxidized fibers and the mechanical properties of the resulting carbon fibers. The oxidation reaction rates were controlled by regulating the oxidation temperature gradient. Results show that the degree of oxidation increases with both the average and initial oxidation rates. By increasing the average oxidation reaction rate, the oxidized structure penetrates deeper into the core region of the fibers, the content of oxygen-containing functional groups increases, the thermal stability of the fibers decreases, and the degree of graphitization of the final carbon fibers increases, but the density of the fibers is decreased and their mechanical properties are degraded. Compared with sample obtained with the lower initial oxidation rate, the number of oxygen-containing functional groups, thermal stability, degree of graphitization and density of the final carbon fibers of the sample with the higher initial oxidation rate are higher, and its tensile strength and modulus are respectively 4.2% and 2.2% higher. A new type of carbon fiber with high strength, medium modulus and a relatively large diameter is obtained under the optimized oxidation conditions.

Conversion of carbon nanotubes into curved graphene with improved capacitance

LI Xiao-yan, WANG Qiang, WANG Huan-wen

2021, 36(4): 835-842. doi: 10.1016/S1872-5805(21)60086-0

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Multi-wall carbon nanotubes (MWCNTs) have achieved mass production, but their lengths are in the millimeter range, which is unfavorable for the diffusion of electrolyte ions into their innermost tube. We report an oxidation method to simultaneously cut and unzip MWCNTs along transverse and longitudinal directions, which leads to the formation of curved graphene sheets (CGSs). SEM shows that the curved morphology was retained but the diameters were large after unzipping. This could be caused by the interaction of oxygen-containing functional groups between layers on the edges of the CGSs. Because of the larger number of active sites the specific capacitance is improved. To further increase the capacitive performance, a sample was put into a 0.1 mol L⁻¹ KMnO₄ to incorporate MnO₂. The microstructure of the resulting CGS-MnO₂ hybrid was revealed by electron microscopy, Raman spectroscopy and powder X-ray diffraction. The results indicate that amorphous MnO₂ successfully grew on the surface of the CGSs. The capacitive behavior was measured by cyclic voltammetry in a 1 mol L⁻¹ Na₂SO₄ solution. The CGS-MnO₂ had a specific capacitance of 236 F g⁻¹ at 2 mV s⁻¹ (even 127 F g⁻¹ at 100 mV s⁻¹), which is superior to that of MWCNTs (15 F g⁻¹), CGS (88 F g⁻¹) and MWCNT-MnO₂ (111 F g⁻¹). In addition, excellent cycling performance was achieved for the CGS-MnO₂ hybrid electrode with a 97% capacitance retention over 1000 cycles.

树脂基球状活性炭用于焦油加氢后油品脱色研究

武俊成, 王建龙, 管涛涛, 张果丽, 李开喜

2021, 36(4): 843-850. doi: 10.1016/S1872-5805(21)60056-2

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采用悬浮聚合、水蒸气活化方法制备了树脂基球状活性炭（ACS），并进行硝酸氧化改性（NACS），用于焦油加氢后油品的脱色研究。通过SEM、N₂吸附-脱附、FTIR、XPS、TG等技术对所制样品ACS和NACS进行结构性质表征。结果显示，经硝酸氧化改性后，NACS样品的表面形貌和孔结构并未受到显著影响，但球状活性炭表面的含氧官能团明显增加。选取两种典型的显色化合物对苯醌（PBQ）和N,N-二仲丁基对苯二胺（DBD）配置一定浓度的模型油进行吸附脱色，考察了吸附时间、吸附温度和吸附剂用量对吸附剂性能的影响。研究表明，NACS样品展现出良好的吸附性能，在一定的吸附条件下，对DBD和PBQ的脱色率分别达到94.5%和96.6%，除了球状活性炭表面微孔提供的活性位点之外，NACS表面官能团与有色物质形成的氢键可能对吸附性能的提升起着关键作用。重复使用6次后，吸附剂对两者的脱色率仍能达到90%以上，展现出良好的可再生性能。在对真实加氢油品脱色后，脱色效果显著，验证了所制备吸附剂在实际应用中的可行性。

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