Outer Carbon Layer Research Articles

Catalytic activity of tungsten carbide-carbon (WC@C) core-shell structured for ethanol electro-oxidation

In this study, carbon coated WC (WC@C) was synthesized through solvothermal reactions in the presence of reducing agent magnesium (Mg) by employing tungsten oxide (WO3) as a precursor, acetone (C3H6O) as a carbon source. The formation of WC@C nano particles is confirmed by X-ray diffraction and Transmission electron microscopy. The thermal stability of the synthesized powder examined in air shows its stability up to 550 °C. In this method, in-situ produced outer carbon layer increase the surface area of materials which is 52.6 m2 g−1 with pore volume 0.213 cm3 g−1. The Electrocatalytic activity of ethanol oxidation on a synthesized sample with and without Pt nano particles have been investigated using cyclic voltammetry (CV). The CV results show the enhancement in oxidation stability of WC@C in acidic media as well as better CO-tolerance for ethanol oxidation after the deposition of Pt nano particles as compared to without Pt nano particles.

Dual Core-Shell Structured Si@SiOx@C Nanocomposite Synthesized via a One-Step Pyrolysis Method as a Highly Stable Anode Material for Lithium-Ion Batteries.

Silicon (Si) has been regarded as a promising high-capacity anode material for developing advanced lithium-ion batteries (LIBs), but the practical application of Si anodes is still unsuccessful mainly due to the insufficient cyclability. To deal with this issue, we propose a new route to construct a dual core-shell structured Si@SiOx@C nanocomposite by direct pyrolysis of poly(methyl methacrylate) (PMMA) polymer on the surface of Si nanoparticles. Since the PMMA polymers can be chemically bonded on the nano-Si surface through the interaction between ester group and Si surface group, and thermally decomposed in the subsequent pyrolysis process with their alkyl chains converted to carbon and the residue oxygen recombining with Si to form SiOx, the dual core-shell structure can be conveniently formed in a one-step procedure. Benefiting from the strong buffering effect of the SiOx interlayer and the efficient blocking action of dense outer carbon layer in preventing electrolyte permeation, the obtained nanocomposite demonstrates a high capacity of 1972 mA h g-1, a stable cycling performance with a capacity retention of >1030 mA h g-1 over 500 cycles, and particularly a superiorly high Coulombic efficiency of >99.5% upon extended cycling, exhibiting a great promise for practical uses. More importantly, the synthetic method proposed in this work is facile and low cost, making it more suitable for large-scale production of high capacity anode for advanced LIBs.

General synthesis of nano-M embedded Li4Ti5O12/C composites (M = Sn, Sb and Bi) with high capacity and good cycle stability

Nano-metal embedded Li4Ti5O12/C composites (Li4Ti5O12/M/C samples labeled as Li4Ti5O12/Sn/C-L, Li4Ti5O12/Sn/C-H, Li4Ti5O12/Sb/C-L, Li4Ti5O12/Sb/C-H, Li4Ti5O12/Bi/C-L and Li4Ti5O12/Bi/C-H, respectively) as high capacity anode were synthesized by sol-gel and high temperature solid state reaction methods. The physical properties of Li4Ti5O12/M/C were detected by X-ray diffraction (XRD), scanning microscopy (SEM), transmission electron microscopy (TEM) and elemental analysis. All composites were completely coated by a thin carbon layer. Besides the Li4Ti5O12/Bi/C-L, other Li4Ti5O12/M/C samples were composed of spherical-like aggregate particles. Nano-M was embedded between inner Li4Ti5O12 and outer carbon layer. Their electrochemical performances were studied by galvanostatic cycle and cyclic voltammograms (CVs). These analyses show that Li4Ti5O12/Sn/C-L, Li4Ti5O12/Sn/C-H, Li4Ti5O12/Sb/C-H, Li4Ti5O12/Bi/C-H delivered a reversible capacity of 231.9, 308.1, 235.9 and 213.5mAhg−1 at 500th cycle on current density of 200mAg−1, respectively, while Li4Ti5O12/C composite only delivered a capacity of 197.6mAhg−1 at the same conditions. The results demonstrated that metal M can play an important role on high capacity performance of Li4Ti5O12-based anode when it was embedded between inner “zero-strain” Li4Ti5O12 and outer carbon coating layer. Their stable cycle performance owning to the co-effect of Li4Ti5O12 as the structural stabilizer and carbon layer as the volume change buffer for preventing the volume change caused by M during cycling. This work provides an effective way to improve the discharge capacity of Li4Ti5O12-based anodes.

Growth of semiconducting single-wall carbon nanotubes with a narrow band-gap distribution.

The growth of high-quality semiconducting single-wall carbon nanotubes with a narrow band-gap distribution is crucial for the fabrication of high-performance electronic devices. However, the single-wall carbon nanotubes grown from traditional metal catalysts usually have diversified structures and properties. Here we design and prepare an acorn-like, partially carbon-coated cobalt nanoparticle catalyst with a uniform size and structure by the thermal reduction of a [Co(CN)6]3− precursor adsorbed on a self-assembled block copolymer nanodomain. The inner cobalt nanoparticle functions as active catalytic phase for carbon nanotube growth, whereas the outer carbon layer prevents the aggregation of cobalt nanoparticles and ensures a perpendicular growth mode. The grown single-wall carbon nanotubes have a very narrow diameter distribution centred at 1.7 nm and a high semiconducting content of >95%. These semiconducting single-wall carbon nanotubes have a very small band-gap difference of ∼0.08 eV and show excellent thin-film transistor performance.

Change in failure mode of carbon nanofibers in nanocomposites as a function of loading rate

Vapor-grown carbon nanofiber (CNF)/epoxy composites are characterized under compression at 5 × 10−3–2800 s−1 strain rates. A difference in the fiber failure mechanism is identified based on the strain rate. CNFs show signs of deformation along their entire length under quasi-static compression. In contrast, the high-strain rate failure results in rupture of outer turbostratic carbon layers, leading to stress transfer to the inner graphite layers. The graphitic layers elongate and rupture, forming a conical tip at the fracture cross section of the CNFs. The strength of nanocomposites at high strain rate is measured to be up to 180 % higher depending on the composite composition and strain rate. CNFs substantially increase the localized plastic deformation of the matrix under quasi-static compression and result in nanoscale deformation features on the failure surface. The observed higher strength and modulus of nanocomposites at high strain rates are attributed to the difference in the matrix and fiber failure mechanisms at different strain rates.

Li3V2(PO4)3 encapsulated flexible free-standing nanofabric cathodes for fast charging and long life-cycle lithium-ion batteries.

Lithiated transition metal phosphates with large theoretical capacities have emerged as promising cathode materials for rechargeable lithium-ion batteries. However, the poor kinetic properties caused by their low intrinsic electronic and ionic conductivity greatly hinder their practical applications. In this work, we demonstrate a novel strategy to prepare monoclinic lithium vanadium phosphate nanoparticles implanted in carbon nanofibers as the cathodes of Li-ion cells with high capacity, flexibility, long cycle stability and significantly improved high-rate performance. The composite nanofibers were obtained by electrospinning using polyacrylonitrile and Li3V2(PO4)3 nanoparticles, followed by annealing and coating with a thin layer of carbon by plasma enhanced chemical vapor deposition. The Li3V2(PO4)3 nanocrystals with the monoclinic phase were uniformly distributed in the composite nanofibers. The electrochemical performances of the as-prepared binder-free fibrous cathodes were characterized by potentiostatic and galvanostatic tests. At the rate of 0.5 C in the range of 3.0-4.3 V, the composite displayed an initial discharge capacity of 128 mA h g(-1) (96.2% of the theoretical capacity). A discharge capacity of 120 mA h g(-1) was observed even at a high rate of 10 C, and a capacity retention of 98.9% was maintained after 500 cycles at 5 C, indicating excellent high-rate capability and capacity retention. Compared to the control samples without a carbon outer-layer, the composite nanofibers with carbon coating demonstrated much better electrochemical performances. It indicates that the carbon coating can further protect the structural integrity of nanofabric electrodes during the charge/discharge processes without hindering the Li-ion mobility and also can prevent undesired side reactions with an electrolyte, thus greatly improving the rate performance and cyclic stability of the cathode.

Improvement of silicon-based electrode for Li-ion batteries by formation of Si-TiB2-C nanocomposites

In an attempt to improve silicon-based electrode for lithium ion batteries, a series of core-shell-type nanocomposites, consisting of rigid, nano-sized grains of TiB2, which are covered by electrochemically active silicon layer and enclosed in outer carbon layer, were prepared by a relatively simple, two-step ball milling method. The composites, having different TiB2:C ratios, were characterized in terms of their lattice structural, microstructural and electrochemical properties, by means of X-ray diffraction, Raman spectroscopy, scanning electron microscopy, particle size distribution, transmission electron microscopy, as well as by electrochemical impedance spectroscopy and galvanostatic cycling tests. It is shown that with the assist of rigid TiB2 particles, silicon and graphite can be easily refined to nanoscale, and both become amorphous during the milling process, forming desired nanocomposites with TiB2-core/Si-shell-type grains covered and embedded in carbon. The amorphous Si, which is uniformly covering TiB2 cores, is electrochemically active, while both, C and TiB2 contribute to the improved electronic conductivity of the nanocomposites. Si-TiB2-C composite with 30mol.% of TiB2 content (STC-433) delivers the best electrochemical performance, with a high reversible capacity on the order of 660 mAh g-1 after 60cycles. A model depicting microstructural changes of the considered nanocomposites with increasing TiB2 content was elaborated, with the low ionic conductivity of titanium diboride being the limiting factor, which causes a decrease of the reversible capacity of the materials with excessive TiB2 amount.

Preparation of the Coated Nuclear Fuel Particle Using the Fluidized Bed-chemical Vapor Deposition (FB-CVD) Method

The tristructural-isotropic (TRISO) type coated fuel particle, which has been commonly used in the current high temperature gas-cooled reactor consists of a microspheric UO2 fuel kernel surrounded by four coated layers: a porous buffer pyrolysis carbon layer (buffer PyC), an inner dense pyrolysis carbon layer (IPyC), a silicon carbide layer (SiC) and an outer dense pyrolysis carbon layer (OPyC). The fluidized bed-chemical vapor deposition (FB-CVD) method was selected for preparing the coated nuclear fuel particle. In this paper, the special design for FB-CVD process is given. The gas distributors with multi-ring inclined holes and the coating furnace with vertical inhomogeneous heating design are developed. Based on experimental study, the TRISO coated nuclear fuel particle has been successfully prepared, and the appropriate parameters in the coating process are obtained.

Enhanced Properties of Sulfur-Infiltrated Carbon Sphere with Conductive Polymer Coating for Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) battery is promising energy system with a high theoretical energy density (2500 Wh kg-1), which is ~7 times larger than those of the current LIBs (~387 Wh kg-1) utilizing the unparalleled theoretical capacity of sulfur (1675 mAh g-1). Sulfur as a cathode material also has other advantages such as non-toxicity, low cost, and abundance of raw materials.For all its virtues, several challenges such as an insulating nature of sulfur (conductivity ~10-30), a large volume expansion by up to 80 % upon full lithiation and a severe capacity fading during cycling still remained unsolved. Sulfur, unlike the insertion based electrode, suffers a serious of structural and morphological changes during cycling with the formation of high-order lithium polysulfides (Li2Sn, 2<n≤8) and low-order lithium polysulfides (Li2S2 and Li2S), which are insulators. Besides, the high-order lithium polysulfides are soluble in current liquid electrolyte at the early stage of discharging. The dissolution of high-order polysulfides results in the so-called shuttling effects.Those problems should be addressed for utilization of lithium-sulfur battery in consumer electronics.In this present investigation, we developed sulfur infiltrated hollow carbon sphere with conductive polymer layer. The numerous small pores in carbon sphere were formed by evaporation of inner materials without damages of outer carbon layer. The conductive polymer layer (PEDOT) coated on the surface of carbon sphere could improve in the trapping of polysulfides and play a role in buffer layers against the volume changes of sulfur. The electrode design could improve the electrical performance with resolving the aforementioned issues.

Modeling and analysis of UN TRISO fuel for LWR application using the PARFUME code

The Idaho National Laboratory (INL) PARFUME (PARticle FUel ModEl) code was used to assess the overall fuel performance of uranium nitride (UN) tristructural isotropic (TRISO) ceramic fuel under irradiation conditions typical of a Light Water Reactor (LWR). The dimensional changes of the fuel particle layers and kernel were calculated, including the formation of an internal gap. The survivability of the UN TRISO particle was estimated depending on the strain behavior of the constituent materials at high fast fluence and burn-up. For nominal cases, internal gas pressure and representative thermal profiles across the kernel and layers were determined along with stress levels in the inner and outer pyrolytic carbon (IPyC/OPyC) and silicon carbide (SiC) layers. These parameters were then used to evaluate fuel particle failure probabilities. Results of the study show that the survivability of UN TRISO fuel under LWR irradiation conditions might only be guaranteed if the kernel and PyC swelling rates are limited at high fast fluence and burn-up. These material properties have large uncertainties at the irradiation levels expected to be reached by UN TRISO fuel in LWRs. Therefore, a large experimental effort would be needed to establish material properties, including kernel and PyC swelling rates, under these conditions before definitive conclusions can be drawn on the behavior of UN TRISO fuel in LWRs.

Mesoporous CNT@TiO2-C Nanocable with Extremely Durable High Rate Capability for Lithium-Ion Battery Anodes

A well-designed nanostructure CNT@TiO2-C with fine anatase TiO2 particle (< 8 nm), good electronic conducting network (inner CNT core and outer carbon layer), and mesoporous structure was prepared by a simple and green one-pot hydrothermal reaction. The utilization of glucose in the hydrothermal process not only solves the interfacial incompatibility between CNTs and titanate sol and controls the nucleation and growth of TiO2 particles, but also introduces a uniform, glucose-derived, carbon-layer on the TiO2 particles. The nanosized TiO2 particle, high conducting network, and interconnected nanopores of the CNT@TiO2-C nanocable greatly improve its electrochemical performances, especially rate capability. The CNT@TiO2-C nanocables show remarkable rate capability with reversible charge capacity of 297, 240, 210,178 and 127 mAh g−1 at 1C, 5C, 10C, 20C and 50C, respectively, as well as excellent high rate cycling stability with capacity retention of 87% after 2000 cycles at 50C.

Promoting long-term cycling performance of high-voltage Li2CoPO4F by the stabilization of electrode/electrolyte interface

High-voltage Li2CoPO4F (∼5 V vs. Li/Li+) with double-layer surface coating has been successfully prepared for the first time. The Li3PO4-coated Li2CoPO4F shows a high reversible capacity of 154 mA h g−1 (energy density up to 700 W h kg−1) at 1 C current rate, and excellent rate capability (141 mA h g−1 at 20 C). XRD and MAS NMR results show that Li2CoPO4F can be indexed as an orthorhombic structure with space group Pnma and coexists with Li3PO4. The XPS depth profiles and TEM analysis reveal that the as-prepared material has a double-layer surface coating, with a carbon outer layer and a Li3PO4 inner layer, which greatly enhances the transfer kinetics of the lithium ions and electrons in the material and stabilizes the electrode/electrolyte interface. Using LiBOB as an electrolyte additive is another way to further stabilize the electrode/electrolyte interface, and the LiBOB has a synergistic effect with the Li3PO4 coating layer. In this way, the Li2CoPO4F cathode material exhibits excellent long-term cycling stability, with 83.8% capacity retention after 150 cycles. The excellent cycling performance is attributed to the LiBOB electrolyte additive and the Li3PO4 coating layer, both of which play an important role in stabilizing the charge transfer resistance of Li2CoPO4F upon cycling.

Fission product release and survivability of UN-kernel LWR TRISO fuel

A thermomechanical assessment of the LWR application of TRISO fuel with UN kernels was performed. Fission product release under operational and transient temperature conditions was determined by extrapolation from fission product recoil calculations and limited data from irradiated UN pellets. Both fission recoil and diffusive release were considered and internal particle pressures computed for both 650 and 800μm diameter kernels as a function of buffer layer thickness. These pressures were used in conjunction with a finite element program to compute the radial and tangential stresses generated within a TRISO particle undergoing burnup. Creep and swelling of the inner and outer pyrolytic carbon layers were included in the analyses. A measure of reliability of the TRISO particle was obtained by computing the probability of survival of the SiC barrier layer and the maximum tensile stress generated in the pyrolytic carbon layers from internal pressure and thermomechanics of the layers. These reliability estimates were obtained as functions of the kernel diameter, buffer layer thickness, and pyrolytic carbon layer thickness. The value of the probability of survival at the end of irradiation was inversely proportional to the maximum pressure.

The influence of temperature on a nutty-cake structural material: LiMn1−xFexPO4 composite with LiFePO4 core and carbon outer layer for lithium-ion battery

The extremely low electronic conductivity, slow ion diffusion kinetics, and the Jahn–Teller effect of LiMnPO4 limit its electrochemical performance. In this work, a nutty-cake structural C–LiMn1−xFexPO4–LiFePO4 cathode material is synthesized by hydrothermal method and further calcined at different temperatures. The influence of calcination temperature on the electrochemical behavior is investigated by X-ray diffractometer, scanning electron microscope, field-emission high-resolution transmission electron microscope, energy-dispersive X-ray spectroscopy, electrochemical impedance spectroscopy and charge–discharge tests. And the performance of C–LiMn1−xFexPO4–LiFePO4 materials has a relationship with its crystal structure. The well-crystallized Sample-600 calcined at 600 °C shows the smallest charge transfer resistance, the largest lithium ion diffusion coefficient (DLi) and the best cycling stability. The discharge capacity of Sample-600 holds around 112 mAh g−1 after the 3rd cycle at 0.1 C rate. The performances improvement of C–LiMn1−xFexPO4–LiFePO4 material can be mainly attributed to the iron diffusion from the LiFePO4 core to the outer LiMnPO4 layer under appropriate calcination temperature.

Fracture behaviour of SiC monofilament reinforced copper – an acoustic emission study

SiC monofilament-reinforced copper is a potential heat sink material for the divertor of future fusion reactors, where heat loads up to 15 MW/m2 must be removed. The composite combines high mechanical strength (300 MPa at 300℃) with sufficient thermal conductivity (at least 200 W/mK) at temperatures up to 550℃. The bonding between fibres and matrix is essential for the mechanical behaviour. We investigated two different fibre types (SCS0 and SCS6, Specialty Materials Inc.) with different interlayers in a galvanic copper matrix. Tensile tests in combination with acoustic emission analysis showed the dependence of fracture behaviour of the composite material on the fibre type. The average ultimate tensile strength of composite samples with 20% fibre reinforcement was in the range of 600 MPa, with a Young's modulus of 160 GPa. The tensile tested composite with SCS6 fibres generated roughly twenty times the number of acoustic emission events in comparison with the SCS0 fibres composite. Acoustic emission events in tensile test specimens with SCS6 fibres are correlated with the failure of the carbon-enriched surface layer of the fibre. In the case of SCS0 fibre-reinforced specimens, acoustic emission events were only caused by fibre failure. Fracture area analysis after tensile tests showed the failure of the SCS6 fibres at the interface between the two outer carbon layers and no interface failure for the composite with SCS0 fibres.

Encapsulation of TRISO particle fuel in durable soda-lime-silicate glasses

Tri-Structural Isotropic (TRISO) coated particle-fuel is a key component in designs for future high temperature nuclear reactors. This study investigated the suitability of three soda lime silicate glass compositions, for the encapsulation of simulant TRISO particle fuel. A cold press and sinter (CPS) methodology was employed to produce TRISO particle–glass composites. Composites produced were determined to have an aqueous durability, fracture toughness and Vickers’ hardness comparable to glasses currently employed for the disposal of high level nuclear wastes. Sintering at 700°C for 30min was found to remove all interconnected porosity from the composite bodies and oxidation of the outer pyrolytic carbon layer during sintering was prevented by processing under a 5% H2/N2 atmosphere. However, the outer pyrolytic carbon layer was not effectively wetted by the encapsulating glass matrix. The aqueous durability of the TRISO particle–glass composites was investigated using PCT and MCC-1 tests combined with geochemical modelling. It was found that durability was dependent on silicate and calcium solution saturation. This study provides significant advancements in the preparation of TRISO particle encapsulant waste forms. The potential for the use of non-borosilicate sintered glass composites for TRISO particle encapsulation has been confirmed, although further refinements are required.

A double-shelled yolk-like structure as an ideal magnetic support of tiny gold nanoparticles for nitrophenol reduction

A facile method is proposed to fabricate yolk–shell microspheres consisting of a movable silica core, a multifunctional double-layered shell, and plenty of tiny gold nanoparticles (Au NPs, ∼2 nm) confined within the interior cavity and the mesoporous shell. The presented strategy involves the one-step coating of a Fe3O4/carbon double-layered shell, the partial etching of the silica cores and the in situ immobilization of Au NPs. The inner Fe3O4 layer of the double-layered shell endows the composites with superparamagnetism and thereby simplifies the introduction procedure of a magnetic component. The outer carbon layer not only protects the Fe3O4 layer from outside harsh conditions but also provides additional adsorption sites for Au NPs besides the interior space. The large number of catalytic active sites together with the advantages of the yolk–shell architecture make the nanocomposite a perfect catalyst for the reduction of 4-nitrophenol to 4-aminophenol in the presence of NaBH4 (TOF value is 17.4 min−1). Moreover, the synthesized catalyst can be easily recovered and reused for at least nine cycles due to its magnetically separable feature and good stability. These confirm that the as-prepared yolk-like nanocomposites are promising candidates for catalytic application.

Nano-structured carbon-coated CuO hollow spheres as stable and high rate anodes for lithium-ion batteries

Carbon-coated CuO hollow spheres with a thin layer of CuO nanoparticles anchored to the outer carbon layer were synthesized using a one-step, fast, scalable, and low-cost aerosol spray pyrolysis method. The hollow spherical particles provide a capacity of 670 mA g−1 at 1 C and maintain the capacity for 300 cycles and 400 mA h g−1 at 50 C. This is the best performance for CuO anodes reported to date. The exceptional electrochemical performance is attributed to the unique architecture of the carbon-coated hollow structure.

Controlled synthesis of α-FeOOH nanorods and their transformation to mesoporous α-Fe2O3, Fe3O4@C nanorods as anodes for lithium ion batteries

α-FeOOH nanorods with varying lengths of 200–500 nm and axis ratios of 2–8 have been successfully synthesized by a simple hydrothermal method with different amounts of hydrazine. Hierarchical nanostructured mesoporous α-Fe2O3 and Fe3O4@C, which combined both mesoporosity and carbon coating, were fabricated by transformation of these α-FeOOH nanorods. The mechanism of formation has been described in detail. The α-FeOOH, mesoporous α-Fe2O3 and Fe3O4@C samples with the longest nanorods exhibited excellent cycle and rate performance, as the long nanorods could ensure many fast and convenient electron transport pathways, thus enhancing the electronic conductivity. The as-synthesized mesoporous α-Fe2O3 and Fe3O4@C nanorods showed large specific surface area and porosity due to the inner mesoporous structure, effectively increasing the contact area between the electrode materials and electrolyte, shortening the diffusion length of Li+ and alleviating the stress from volume changes during the charge–discharge process. Mesoporous Fe3O4@C nanorods exhibit high reversible capacities of 1072 mAh g−1 after 50 cycles, demonstrating the superior electron transport and fast Li+ diffusion ability combination of outer carbon layer and mesoporous microstructure.

Morphologies and dispersion stabilities of carbon-encapsulated metal (Ag and Cu) nanoparticles synthesized by pulsed-wire evaporation (PWE)

Carbon-encapsulated Ag and Cu nanoparticles were synthesized using pulsed-wire evaporation (PWE). All as-made materials were composed of nanocapsules with a uniform particle size at and below 50 nm. The nanocapsules consisted of an outer carbon layer shaped multi-shell. The dispersion stability of the solvent depends on the measured zeta potential for the particles in water, ethanol, ethylene glycol (EG), and polyethylene glycol (PEG). In the case of Ag@C, a stable dispersion was observed in the PEG suspension only. The stable dispersions of Cu@C were observed in EG and PEG without showing any clarification or sedimentation. Flocculation by a coalescing reaction between the nanoparticles was insignificant due to carbon layers on the surfaces of the metal (Ag and Cu) particles.