Carbon nano-rod as a structural unit of carbon nanofibers

The structure of carbon nanofiber (CNF) with platelet texture was found to be constructed by a primary structural unit named carbon nano-rod by the authors through comprehensive examination of X-ray diffraction (XRD), high-resolution scanning electron microscopy (HR-SEM), high-resolution transmission electron microscopy (HR-TEM), and scanning tunneling microscopy (STM). Single carbon nano-rod resembles multi-walled carbon nanotube with 4–5 graphene sheets nested (ca. 2.5 nm diameters), but appears to have a hypothetical transverse of hexagonal shape with the inner diameter corresponding to the interlayer spacing. Three-dimensional model of CNF was suggested based on the carbon nano-rods, which are densely stacked perpendicular to the fiber axis to form a typical platelet CNF. Such a structural concept of CNF gives novel views on the correlation between structure and properties of CNFs for potential applications.     

Preparation and characterisation of novel “sea-cucumber”-like structures containing carbon and boron

Spray-pyrolysis of a ferrocene-xylene-triethylborane mixture at 900 °C results in a novel “sea-cucumber”-like structure containing carbon and boron. SEM studies show that these structures are hollow with diameter between 100 and 500 nm and lengths varying from 30 to 40 μm. HRTEM and EELS studies reveal that the hollow structures are partly filled with iron. They are formed by a more graphitic internal core, which hardly contains any B. This core is coated by a more disordered B-containing C material. The amount of B in this coating is around 3 at.%. In addition, at this pyrolysis temperature, the growth of open-ended tubular nanostructures (30-40 nm diameter; 100-200 nm length) on the surface of the sea-cucumber-like structure is observed. These tubular nanostructures are not very graphitic and exhibit a similar composition to the coating of the sea-cucumber-like structure (around 3 at.% of B). The study of material produced at different pyrolysis temperatures show that the growth of these tubular nanostructures and the boron content are temperature dependent. Finally, preliminary results corresponding to oxidation resistance studies are presented.     

Chemical vapor deposition of pyrolytic carbon on carbon nanotubes: Part 1. Synthesis and morphology

The chemical vapor deposition of the pyrocarbon from a CH₄+H₂ mixture is investigated using nanofilamentous substrates. The process consists of growing carbon nanotubes via a catalytic process, which then are thickened by pyrolytic carbon deposition to reach diameters in the nanometer to micrometer range. A key characteristic of the experimental reactor used was the long length of its isothermal zone, preceded (and followed) by a low thermal gradient zone. This allowed us to investigate the role of the variation of the local gas phase composition, which depends on the post-cracking secondary reactions, and on the quantity and quality of the deposited carbon. The ‘time of flight’ of the reactive species was found to be a leading parameter in the pyrolytic carbon deposition process. Various nanometric and micrometric morphologies, several of which are new, were synthesised and found constituted with an association of different sub-morphologies. The various morphologies, that can be sorted following a factor of morphological complexity, were investigated by scanning electron microscopy.     

The ruthenium catalysed synthesis of carbon nanostructures

Reaction of propene over silica-, alumina- or titania-supported Ru catalysts in a tubular quartz flow reactor at moderate temperatures (500-700 °C) and atmospheric pressure produced Ru containing carbon nanofibres/tubes. TEM studies revealed that the fibres/tubes grew away from the support and contained Ru metal particles in their tips. The results indicate that sintering of the Ru during reduction with H₂, to a critical size not less than 30 nm, is required for fibre/tube formation.     

A new air electrode based on carbon nanotubes and Ag–MnO2 for metal air electrochemical cells

A new air electrode using a combination of dual functional silver/manganese dioxide catalysts based on single-walled carbon nanotubes (SWNTs) was developed by chemically reducing silver permanganate with hydrazine. The electrode prepared comprises a high-quality carbon nanotube support with a complex of manganese dioxide and silver catalysts. The electrocatalytical activity of the electrode was examined via a variety of electrochemical testing techniques. Higher electrocatalytical activity for the oxygen reduction reaction and better performance of the zinc air cell, as compared with the catalysts supported on mesocarbon microbeads (MCMB) or commercial graphite, was achieved.     

Preparation of an activated carbon artifact: factors influencing strength when using a thermoplastic polymer as binder

An activated carbon artifact was prepared through mixing, moulding, curing and carbonizing, using polyvinylbutyral resin (PVB) as the binder, dibutyl phthalate (DBP) as a plasticizing agent and isocyanuric acid ester as a cross-linking agent to clarify influential factors on its strength. Preparation conditions such as moulding pressure, temperature and time of curing, carbonization and the amount of cross-linking agent were varied to find their influences on the strength of the resultant form. The form was observed under SEM of wide scope to find correlations between its morphology and strength. The closed packing of the activated carbon filler and the plastic binder was always favorable to develop the strength of the form. The curing extent of PVB, which was influenced by curing atmosphere, temperature and time, and cross-linking agent, was found to govern the strength of the forms. Air or oxygen is very essential for the curing. The optimum temperature was found to be 200°C and longer curing time is beneficial to improve the strength. The cross-linking agent improved the strength of the form up to 7000 kPa through accelerating the cross-linkage of PVB resin. Sufficient curing allows the rapid heating up to 10°C/min for the development of the strength by maintaining the shape of the form. The thermoplastic powders are highly dispersed onto the surface of activated carbons and are cured sufficiently there to adhere the activated carbon grains. Sufficient curing stabilizes the thermoplastic polymer to be thermosetting, anchoring the grains through the carbon bond for higher strength of the carbonized form. PVB resin is cured into heat-resisting cross-linked chains through oxidative condensation onto the activated carbon surface where the oxygen functional groups appear to play important roles in the curing.     

TEM evidence for factors affecting the genesis of carbon species on bare and K-promoted Ni/MgO catalysts during the dry reforming of methane

The structure and morphology of carbon species generated under dry reforming of methane (DRM) at 650 and 800°C on ‘bare’ and ‘K-doped’ Ni/MgO catalysts have been comparatively investigated by Transmission Electron Microscopy (TEM) analyses of ‘used’ samples. K-addition (K_at/Ni_at, 0.125) strongly improves the resistance of the Ni/MgO catalyst to coking and sintering phenomena at any temperature. At 650°C, an extensive formation of filamentous (whisker carbon) carbon species on bare Ni/MgO catalyst causes the detachment of a large number of Ni particles from the support with a consequent destruction of the structure and remarkable sintering phenomena of the active phase. Considerably lower amounts of carbon deposits with a shell-like (encapsulating carbon) morphology, forming at 800°C on both catalysts, point to the Bouduard reaction as the main route of carbon deposition on Ni-based catalysts during DRM. The electronic effect induced by potassium on the active phase of the Ni/MgO system, timely monitored by a rise in E_app of DRM from 50 to 70 kJ/mol, markedly hinders the rate of coking also affecting the morphology of carbon whiskers, by inhibiting the processes of C diffusion and nucleation across Ni particles under steady-state conditions.     

Synthesis of carbon nanotubes on metallic substrates by a sequential combination of PECVD and thermal CVD

Carbon nanotubes were synthesized from acetylene and hydrogen gas mixture directly on stainless steel plates by sequential combination of rf PECVD and thermal CVD. PECVD was used for nucleation and initial growth of carbon nanotubes while thermal CVD was utilized for further growth of them. In this way decoupling of nucleation and growth of carbon nanotubes was realized, and growth of carbon nanotubes was enhanced compared to growth by PECVD. Synthesized carbon nanotubes were curly in shape, and proper pretreatment of the substrate surface was required for the satisfactory growth of carbon nanotubes. Carbon nanotubes could be fabricated into electrodes for electric double layer capacitors without any further treatment. With an electrolyte composed of lithium hexafluorophosphate, ethylene carbonate and diethyl carbonate, charge/discharge test showed specific capacitance in the range of 33-82 F/g.     

A novel method for carbon modification with minute polyaniline deposition to enhance the capacitance of porous carbon electrodes

A novel method was developed for minute deposition of polyaniline onto microporous activated carbon fabric to enhance the capacitance of the carbon serving as electrodes for electrochemical capacitors. The deposition consisted of pre-adsorption of monomer into carbon micropores followed by electrochemical polymerization of the adsorbed monomer in a monomer-free H₂SO₄ solution at 0.85 V vs. Ag/AgCl. In comparison with the conventional polymerization in a monomer solution, the developed deposition resulted in a polymer framework distributed over the vast surface in carbon micropores, thus leading to a lower resistance for ion binding with the polymer in H₂SO₄ during charge-discharge. The lower resistance gave rise to a higher specific capacitance for the deposited polymer. In the assembled two-electrode capacitors, the usage of polyaniline redox reactions to store charges was more prominent for polymer-carbon composite electrodes from the developed method because of the higher electrode open circuit potentials. The present work has demonstrated that a capacitance enhancement of >50% in comparison with bare carbon can be achieved with minute polyaniline deposition (<5 wt.%) using the developed method, while only 22% was reached using the conventional method.     

Carbon/carbon-boron nitride composites with improved wear resistance compared to carbon/carbon

This paper describes the fabrication of a carbon fiber reinforced/carbon-boron nitride (C/C-BN) hybrid matrix composite for possible use in aircraft brakes. These composites were fabricated via liquid infiltration of a liquid crystalline borazine oligomer into a low-density carbon fiber/carbon matrix (C/C) composite. The friction and wear properties of the C/C-BN were explored over the entire energy spectrum for aircraft braking using an inertial brake dynamometer. The C/C-BN composites with densities of 1.55 g/cc displayed wear rates 50% lower than values observed with C/C samples with densities of approximately 1.75-1.8 g/cc. This includes the near elimination of wear from 300 to 600 kJ/kg, which represents the normal landing regime for aircraft brakes. This encouraging behavior is attributed in part to the improved oxidation resistance of the BN at high energy levels and the ability of the BN to facilitate formation of a stable wear film at lower energy levels. The coefficient of friction, while being slightly lower than the values for C/C, appeared much less sensitive to changes in energy level.     

Mechanism of lithium storage in low temperature carbon

Through measurement of the intensity of the EPR signal of carbon anodes at different discharge and charge potentials, a micropore mechanism is suggested for the storage of lithium in low temperature carbons (LTCs), and it is further confirmed by results from the addition of pore-genic agent and introduction of crosslinker DVB into addition polymers PAN and P(4-VP). The size of micropores acting effectively as ‘reservoirs’ for lithium storage is suggested to be below 100 nm. The phenomena, which are characteristic in LTCs such as voltage hysteresis and capacity fading, are explained through the suggested mechanism.     

Preparation of an activated carbon artifact: oxidative modification of coconut shell-based carbon to improve the strength

Coconut shell-based activated carbon was oxidized in aq. H₂SO₄, HNO₃ and H₂O₂ to induce surface oxygen functional groups on its surface and to increase the mechanical strength of the resultant activated carbon artifact with PVB as a binder. Although all oxidation was confirmed to significantly increase the strength, aq. H₂O₂ was found to be most effective, giving strength as high as 6000 kPa, which is believed to be sufficient for the electrode of an electric double layer capacitor (EDLC). The increase of CO₂ evolving groups induced on the surface of activated carbon appears to be responsible for the strength increase. There was an optimum extent of oxidation for the strength as well as the performance of the electrode. Too much oxidation reduces the electrical conductivity of the activated carbon. Facile oxidation by aq. H₂O₂ can be recommended as a practical modification of the surface since it takes place safely below 100°C without releasing any harmful gas.     

Effect of the pitch-based carbon anode on the capacity loss of lithium-ion secondary battery

The total capacity loss of lithium-ion secondary cell is reduced by engineering the pitch-based carbon anode through one or more of the following methods: attaining a high degree of graphitization, minimizing the surface oxygen concentration, attaining a large crystal size L_c, degassing, and use of PVDF in place of teflon as the binder.     

New origin of spirals and new growth process of carbon whiskers

A new kind of carbon whisker, different from others, has been found by means of high resolution electron microscopy and field emission scanning electron microscopy. Carbon layers are almost perpendicular to the whisker axis and a spiral structure is formed around it. Structure analysis indicates the growth process consists of two stages; firstly, carbon layers stacked in turbostratic manner, and then gradually graphitized. Accordingly, the final structure of whiskers is perfect graphitized texture. The spirals are not similar to the so-called ‘growth spirals’ formed by the screw dislocation mechanism, and a more reasonable growth mechanism of whiskers is suggested in accordance with the analysis.     

Synthesis and properties of carbon nanospheres grown by CVD using Kaolin supported transition metal catalysts

Carbon spheres with diameters between 400 and 2000 nm were synthesized in large quantities by catalytic chemical vapor deposition (CCVD) method using Kaolin supported transition metal salts (M=Fe, Co, Ni, etc.) as catalysts. The reaction conditions for the synthesis of carbon spheres in different sizes are described. The reactivity of the carbon spheres in various organic solvents is discussed. The as-synthesized carbon spheres are composed of unclosed graphene layers with the interlayer distances 0.33-0.35 nm. These carbon spheres have been characterized by SEM, TEM, HRTEM, XRD, Raman, ESR and SQUID magnetization techniques. From the ESR and SQUID the metallic nature of the carbon spheres is described.     

Influence of activated carbon pore structure on oxygen reduction at catalyst layers supported on rotating disk electrodes

Carbon materials are often used as catalyst supports, and for catalysts in electrodes of a polymer electrolyte fuel cell, carbon black has been used. Recently, it was found, however, that activated carbon could replace carbon black and besides, significantly improve the activity of the electrode catalyst layer for oxygen reduction. In the present study, to optimize the pore structure of activated carbon for further activity improvement, the influence of the pore structure on the activity was investigated using activated carbon of various specific surface areas and mean pore diameters. A catalyst layer was formed from activated carbon loaded with platinum and a polymer electrolyte. The activity of the layer was measured in an oxygen-saturated perchloric acid solution, supporting the layer on a rotating glassy carbon disk electrode. We found that increases in the specific surface area and mean pore diameter increased the activity and that the latter was more effective than the former mainly due to the enhanced mass-transfer in the pores; the catalyst layer formed from activated carbon with the largest mean pore diameter was the most active. Unless pores excessively develop and lose connections between particles, a large pore diameter is therefore desired for the fuel cell electrodes.     

Formation of filamentous carbon during methane decomposition over Co-MgO catalysts

The carbon formation during methane decomposition was investigated at 900 °C over the 48 wt% Co-MgO catalysts as a function of the calcination temperature T_c used in their preparation. Examination of the carbonaceous deposits by transmission electron microscopy revealed three kinds of structures: shapeless tangles, shell-like materials, and carbon filaments. In another set of experiments, the structural characteristics of the calcined catalysts were investigated using temperature-programmed reduction (TPR) and X-ray diffraction (XRD). Co₃O₄, Co₂MgO₄, and (Co, Mg)O (solid solution of CoO and MgO) were identified for T_c≤700 °C, Co₃O₄ and (Co, Mg)O for T_c=800 °C and only (Co, Mg)O for T_c=900 °C. It was found that the metal particles originated from the reduction of the solid solution favored the formation of filamentous carbon. A possible explanation is proposed.