Preparation, Microstructure and Thermal Properties of Aligned Mesophase Pitch-Based Carbon Fiber Interface Materials by an Electrostatic Flocking Method.

The mesophase pitch-based carbon fiber interface material (TIM) with a vertical array was prepared by using mesophase pitch-based short-cut fibers (MPCFs) and 3016 epoxy resin as raw materials and carbon nanotubes (CNTs) as additives through electrostatic flocking and resin pouring molding process. The microstructure and thermal properties of the interface were analyzed by using a scanning electron microscope (SEM), laser thermal conductivity and thermal infrared imaging methods. The results indicate that the plate spacing and fusing voltage have a significant impact on the orientation of the arrays formed by mesophase pitch-based carbon fibers. While the orientation of the carbon fiber array has a minimal impact on the shore hardness of TIM, it does have a direct influence on its thermal conductivity. At a flocking voltage of 20 kV and plate spacing of 12 cm, the interface material exhibited an optimal thermal conductivity of 24.47 W/(m·K), shore hardness of 42 A and carbon fiber filling rate of 6.30 wt%. By incorporating 2% carbon nanotubes (CNTs) into the epoxy matrix, the interface material achieves a thermal conductivity of 28.97 W/(m·K) at a flocking voltage of 30 kV and plate spacing of 10 cm. This represents a 52.1% increase in thermal conductivity compared to the material without TIM. The material achieves temperature uniformity within 10 s at the same heat source temperatures, which indicates a good application prospect in IC packaging and electronic heat dissipation.

The Microstructure and Thermal Conductive Behavior of Three-Dimensional Carbon/Carbon Composites with Ultrahigh Thermal Conductivity.

Carbon-based composite materials, denoted as C/C composites and possessing high thermal conductivity, were synthesized utilizing a three-dimensional (3D) preform methodology. This involved the orthogonal weaving of mesophase pitch-based fibers in an X (Y) direction derived from low-temperature carbonization, and commercial PAN-based carbon fibers in a Z direction. The 3D preforms were saturated with mesophase pitch in their raw state through a hot-pressing process, which was executed under relatively low pressure at a predetermined temperature. Further densification was achieved by successive stages of mesophase pitch impregnation (MPI), followed by impregnation with coal pitch under high pressure (IPI). The microstructure and thermal conductivity of the C/C composites were systematically examined using a suite of analytical techniques, including Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), and PLM, amongst others. The findings suggest that the volumetric fraction of fibers and the directional alignment of the mesophase pitch molecules can be enhanced via hot pressing. The high graphitization degree of the mesophase pitch matrix results in an increased microcrystalline size and thus improved thermal conductivity of the C/C composite. Conversely, the orientation of the medium-temperature coal pitch matrix is relatively low, which compensates for the structural inadequacies of the composite material, albeit contributing minimally to the thermal conductivity of the resultant C/C composites. Following several stages of impregnation with mesophase pitch and subsequent impregnation with medium-temperature coal pitch, the 3D C/C composites yielded a density of 1.83 and 2.02 g/cm3. The thermal conductivity in the X (Y) direction was found to be 358 and 400 W/(m·K), respectively.

Dual interfaces and confinements on Fe2N@Fe3O4/VN heterojunction toward high-efficient lithium storage.

Ferroferric oxide (Fe3O4) as an anode material of lithium-ion battery has been widely investigated due to its high theoretical capacity, environmental friendliness, natural abundance, and low cost. However, it suffers from severe aggregation and volume expansion during energy storage. Herein, we rationally construct an advanced Fe2N@Fe3O4/VN heterostructure via a hydrothermal and followed nitridation process, where the wrapping of conductive Fe2N on the surface of Fe3O4 effectively improves the electron conductivity and alleviates the volume expansion, and VN inhibits the agglomeration of Fe2N@Fe3O4. Benefiting from the dual conductive confinements and promoted interfacial charge transfer, the Fe2N@Fe3O4/VN heterojunction exhibits excellent rate capability and cycling stability. It possesses the highest reversible capacity of 420.8 mAh g-1 at 1 A g-1 after 600 cycles, which is three times that of Fe3O4. Furthermore, a full cell based on a Fe2N@Fe3O4/VN anode and a LiFePO4 cathode delivers considerable electrochemical performance. This work demonstrates that Fe2N@Fe3O4/VN is a potential anode material and provides a model in constructing other high-performance electrode materials.

Mott-Schottky Effect in Core-Shell W@Wx C Heterostructure: Boosting Both Electronic/Ionic Kinetics for Lithium Storage.

The dynamics rate of traditional metal carbides (TMCs) is relatively slow, severely limiting its fast-charging capacity for lithium-ion batteries (LIBs). Herein, the core-shell W@Wx C heterostructure is developed to form Mott-Schottky heterostructure, thereby simultaneously accelerating the electronic and ionic transport kinetics during the charging/discharging process. The W nanoparticles are partially reduced into Wx C to form a particular core-shell structure with abundant heterogeneous interfaces. Benefiting from the Mott-Schottky effect, the electrons at the metal/semiconductor heterointerface can migrate spontaneously to realize an equal work function on both sides. In addition, the independent nanoparticle as well as the unique core-shell structure facilitate the ionic diffusion kinetics. As expected, the W@Wx C electrode exhibits excellent electrochemical stability for LIBs, whose capacity can be maintained at 173.8 mA h g-1 after 1600 cycles at a high current density of 5 A g-1 . When assembled into a full cell, it can achieve an energy density of 360.2 Wh kg-1 . This work presents a new avenue to promote the electronic and ionic kinetics for LIBs anodes by constructing the unique Mott-Schottky heterostructure.

Regulating d-Band Center of Ti2 C MXene Via Nb Alloying for Stable and High-Efficient Supercapacitive Performances.

Ti2 C MXene with the lowest formula weight is expected to gain superior advantages in gravimetric capacitances over other heavier MXenes. Nevertheless, its poor chemical and electrochemical stability is the most fatal drawback and seriously hinders its practical applications. Herein, an alloy engineering strategy at the transition metal-sites of Ti2 C MXene is proposed. Theoretical calculations reveal that the electronic redistribution of the solid-solution TiNbC MXene improves the electronic conductivity, induces the upward d-band center, tailors the surface functional groups, and increases the electron loss impedance, resulting in its excellent capacitive performance and high chemical stability. The as-prepared flexible TiNbC film delivers specific capacitance up to 381 F g-1 at a scan rate of 2 mV s-1 and excellent electrochemical stability without capacitance loss after 10000 charge/discharging cycles. This work provides a universal approach to develop high-performance and chemically stable MXene electrodes.

Zero-strain strategy incorporating TaC with Ta2O5 to enhance its rate capacity for long-term lithium storage.

Ta2O5 holds great potential for lithium storage due to its high theoretical capacity and long-life cycling. However, it still suffers from an unsatisfactory rate capability because of its low conductivity and significant volume expansion during the charging/discharging process. In this study, a zero-strain strategy was developed to composite Ta2O5 with zero-strain TaC as an anode for lithium-ion batteries (LIBs). The zero-strain TaC, featuring negligible lattice expansion, can alleviate the volume variation of Ta2O5 when cycling, thereby enhancing the rate capacity and long-term cycling stability of the whole electrode. Further, the formation of a heterostructure between Ta2O5 and TaC was confirmed, giving rise to an enhancement in the electrical conductivity and structural stability. As expected, this anode displayed a reversible specific capacity of 395.5 mA h g-1 at 0.5 A g-1 after 500 cycles. Even at an ultrahigh current density of 10 A g-1, the Ta2O5/TaC anode delivered a high capacity of 144 mA h g-1 and superior durability with a low-capacity decay rate of 0.08% per cycle after 1000 cycles. This zero-strain strategy provides a promising avenue for the rational design of anodes, sequentially contributing to the development of high-rate capacity and long cycling LIBs.

Two-Dimensional Ordered Double-Transition Metal Carbides for the Electrochemical Nitrogen Reduction Reaction.

The electrochemical nitrogen reduction reaction (NRR) provides a green and sustainable strategy as an alternative to the Haber-Bosch process. The development of electrocatalysts with low overpotential, high selectivity, and fast reaction kinetics remains a significant challenge. Here, density functional theory computations are carried out to systematically predict the prospect of 18 two-dimensional (2D) ordered double-transition metal carbides (MXenes) as NRR electrocatalysts. Our results revealed that the basal plane of Mo2Nb2C3 MXene exhibited the most outstanding catalytic activity while effectively suppressed the hydrogen evolution reaction with an overpotential of 0.48 V. The exposed Mo3 moiety moderately regulating the electron transfer between reaction intermediates is answerable for the high activity. Finally, our finding broadens the horizon of 2D materials as NRR electrocatalysts.

Constructing the Bridge from Isotropic to Anisotropic Pitches for Preparing Pitch-Based Carbon Fibers with Tunable Structures and Properties.

Synthetic naphthalene pitches (SNPs) with isotropy and anisotropy were prepared by a simple thermal polycondensation method to fabricate pitch-based carbon fibers. The structural characteristic, thermal stability, phase-separation behavior, and melt-spinnability of the SNPs and the structural properties of the derived carbon fibers were systematically investigated. The results show that spinnable SNPs with controllable mesophase contents ranging from 0 to 100 vol % and softening points (210-290 °C) could be easily obtained by a nitrogen-bubbling treatment to improve their thermal stability and melt-spinnability by avoiding the phase separation of liquid crystal (LC) in the pitch. An experimental phase diagram of spinnability and mesophase content is newly proposed for predicting the spinnability of a mesophase-containing pitch. The LC has a significant influence not only on the constituents, structure, and physical properties of the SNPs but also on the final structure and properties of the corresponding pitch-based carbon fibers. The low ash content (less than 0.15 wt %) in the pitch precursor is found to have no obvious effect on the pitch spinnability and the mechanical properties of derivative large-diameter carbon fibers.

Grain boundary engineering of Co3O4 nanomeshes for efficient electrochemical oxygen evolution.

The development of high-efficiency and stable electrocatalysts is significant for energy conversion and storage. The oxygen evolution reaction (OER), a pivotal half reaction, is seriously limited in its practical applications due to its sluggish kinetics and thus an excellent electrocatalyst for OER is urgently required. In this paper, we design a novel Co3O4 nanomesh (Co3O4 NMs) with high density grain boundaries (GBs), which functions as a highly efficient and steady OER electrocatalyst. The optimal Co3O4 NMs-500 can achieve a low overpotential of 295 mV at a current density of 10 mA cm-2, and a small Tafel slope of 31 mV dec-1, which exceeds the commercial Ir/C, as well as the majority of other catalysts reported in the literature. The Co3O4 NMs-500 also exhibit promising durability, with a negligible decline in activity after 18 h of operation. Detailed studies indicate that the presence of GBs leads to more exposed active sites and the enhanced adsorption of intermediate species on Co3O4 NMs-500, thereby improving the OER's catalytic activity. This work not only relates to the activity-GBs relationship, but also opens up a unique perspective for the design of the next generation of electrocatalysts.

Impact of Microstructure on the Electrochemical Performance of Round-Shaped Pitch-Based Graphite Fibers.

In this study, three kinds of round-shaped pitch-based graphite fiber with different microstructural features (crystallinity and carbon layer orientation) were fabricated by melt-spinning, preoxidation, carbonization and graphitization. The morphology, crystalline size and carbon layer orientation of carbon fibers from different pitch precursors and spinning rates were characterized through X-ray diffraction, scanning electron microscopy and transmission electron analyses. The correlation of the electrochemical performance and microstructure of graphite fibers as anode materials for lithium-ion batteries was investigated. The results suggest that large-diameter anisotropic graphite fibers (L-AF3000) with a radial texture of the transverse section are more favorable for lithium intercalation storage. The discharge capacity of L-AF3000 is 319.1 mAh∙g-1 at 0.1 C (current density). Nevertheless, the capacity drops to 209.9 mAh∙g-1 at a high current density of 1 C, and the capacity retention is only 82.2% over 100 cycles at 0.1 C. Small-diameter anisotropic graphite fibers (S-AF3000) with a spiral-shaped wrinkle texture of the transverse section possess discharge capacities of 284.1 mAh∙g-1 at 0.1 C and 260.2 mAh∙g-1 at a high current density of 1 C. Meanwhile, the best capacity retention of the fibers is 101.6% over 100 cycles at 0.1 C. The results suggest that the disordered carbon layers in S-AF3000 can retain the structural integrity of fibers as anode material for lithium-ion batteries and thus obtain excellent cycle stability. In addition, larger crystalline sizes of fibers correspond to higher discharge capacity, and a smaller diameter is beneficial to the fast insertion and extraction of lithium-ion in fibers.

Graphene Armored with a Crystal Carbon Shell for Ultrahigh-Performance Potassium Ion Batteries and Aluminum Batteries.

Graphene is of great significance in energy storage devices. However, a graphene-based electrode is difficult to use in direct applications due to the large surface area and flexibility, which leads to the excessive consumption of electrolyte, low Coulombic efficiency, and electrode shedding behaviors. Herein, a special crystal carbon@graphene microsphere (CCGM) composite was successfully synthesized. The scalable carbonaceous microsphere composite displays a small specific surface area and a superior structure stability. As a potassium ion battery electrode in a half-cell, CCGM delivers an initial capacity of 297.89 mAh g-1 with a high Coulombic efficiency of about 99%. It achieves an excellent cyclic stability with no capacity loss after 1250 cycles at the low current density of 100 mA g-1 with a long performing period of more than one year. As the cathode for an aluminum battery, a reversible specific capacity of 99.1 mAh g-1 at 1000 mA g-1 is obtained. CCGM delivers a long cycle performance of about 10 000 cycles at 4000 mA g-1 with a capacity retention of nearly 100%. Our design provides a fresh thought for the improvement of graphene-based materials, and it will greatly facilitate the application of graphene in the field of energy storage.

Ablation Behavior of the SiC-Coated Three-Dimensional Highly Thermal Conductive Mesophase-Pitch-Based Carbon-Fiber-Reinforced Carbon Matrix Composite under Plasma Flame.

This study is focused on a novel high-thermal-conductive C/C composite used in heat-redistribution thermal protection systems. The 3D mesophase pitch-based carbon fiber (CFMP) preform was prepared using CFMP in the X (Y) direction and polyacrylonitrile carbon fiber (CFPAN) in the Z direction. After the preform was densified by chemical vapor infiltration (CVI) and polymer infiltration and pyrolysis (PIP), the 3D high-thermal-conductive C/C (CMP/C) composite was obtained. The prepared CMP/C composite has higher thermal conduction in the X and Y directions. After an ablation test, the CFPAN becomes needle-shaped, while the CFMP shows a wedge shape. The fiber/matrix and matrix/matrix interfaces are preferentially oxidized and damaged during ablation. After being coated by SiC coating, the thermal conductivity plays a significant role in decreasing the hot-side temperature and protecting the SiC coating from erosion by flame. The SiC-coated CMP/C composite has better ablation resistance than the SiC-coated CPAN/C composite. The mass ablation rate of the sample is 0.19 mg·(cm-2·s-1), and the linear ablation rate is 0.52 µm·s-1.

Effect of Liquid Crystalline Texture of Mesophase Pitches on the Structure and Property of Large-Diameter Carbon Fibers.

Two types of carbon fibers with a large diameter of ∼22 µm, derived from unstirred and vigorously stirred mesophase pitch melts with different liquid crystalline mesophase textures, were prepared by melt-spinning, stabilization, carbonization, and graphitization treatments. The morphology, microstructure, and physical properties of the carbon fibers derived from the two kinds of mesophase precursors after various processes were characterized in detail. The results show that the optical texture (i.e., size and orientation) of the liquid crystalline mesophase in the molten pitch is obviously modified by thermomechanical stirring treatment, which has a significant effect on the texture of as-spun pitch fibers, and finally dominates the microstructure and physical properties of the resulting carbon and graphite fibers. These large-diameter fibers expectedly maintain their morphological and structural integrity and effectively avoid shrinkage cracking during subsequent high-temperature heat treatment processes, in contrast to those derived from the unstirred pitch. This is due to the smaller crystallite sizes and lower orientation of graphene layers in the former. The tensile strength and axial electrical resistivity of the 3000 °C-graphitized large fibers derived from the unstirred pitch are about 1.8 GPa and 1.18 µΩ m, respectively. In contrast, upon melt stirring treatment of the pitch before spinning, the resulting large-diameter graphite fibers possess the corresponding values of 1.3 GPa and 1.86 µΩ m. Despite the acceptable decrease of mechanical properties and axial electrical and thermal conduction performance, the latter possesses relatively high mechanical stability (i.e., low strength deviation) and ideal morphological and structural integrity, which is beneficial for the wide applications in composites.

Cs2CdV2O6Cl2 and Cs3CdV4O12Br: two new non-centrosymmetric oxyhalides containing d0 and d10 cations and exhibiting second harmonic generation activity.

Two new oxyhalides including d0 and d10 cations, Cs4Cd2V4O12Cl4 (1) and Cs3CdV4O12Br (2), were successfully synthesized via a solid phase reaction. Their crystal structures have been determined by X-ray single crystal diffraction. Compound 1 crystallizes in space group Cm (no. 8), whereas compound 2 is found in space group Cmm2 (no. 35). The corner-shared V3O8 units and CdO2Cl4 units in the compound 1 bridge a three-dimensional network, whereas the corner-shared V4O11 polyhedron and CdO4Br2 octahedron in the compound 2 form a three-dimensional structure. All the polar groups align in one direction, which results in a favorable net polarization. Powder second harmonic generation, using 1064 nm incident radiation, indicates that they are phase-matchable with observable and strong SHG response (5 and 7 times KH2PO4, respectively). The UV-vis-NIR diffuse reflectance spectra indicate that the band gaps of the compound 1 and 2 are 3.00 eV (413 nm) and 3.13 eV (396 nm), respectively. Based on the IR and UV-vis-NIR data, the transparent range of both compounds is 0.4-10.4 µm. Furthermore, the electronic structure was also investigated by the first-principles calculations.

A Comparison of Ethylene-Tar-Derived Isotropic Pitches Prepared by Air Blowing and Nitrogen Distillation Methods and Their Carbon Fibers.

Two isotropic pitches were prepared by air blowing and nitrogen distillation methods using ethylene tar (ET) as a raw material. The corresponding carbon fibers were obtained through conventional melt spinning, stabilization, and carbonization. The structures and properties of the resultant pitches and fibers were characterized, and their differences were examined. The results showed that the introduction of oxygen by the air blowing method could quickly increase the yield and the softening point of the pitch. Moreover, the air-blown pitch (ABP) was composed of aromatic molecules with linear methylene chains, while the nitrogen-distilled pitch (NDP) mainly contained polycondensed aromatic rings. This is because the oxygen-containing functional groups in the ABP could impede ordered stack of pitch molecules and led to a methylene bridge structure instead of an aromatic condensed structure as in the NDP. Meanwhile, the spinnability of the ABP did not decrease even though it contained 2.31 wt % oxygen. In contrast, the ABP had narrower molecular weight distribution, which contributed to better stabilization properties and higher tensile strength of the carbon fiber. The tensile strength of carbon fibers from the ABP reached 860 MPa with fiber diameter of about 10 µm, which was higher than the tensile strength of 640 MPa for the NDP-derived carbon fibers.