Multiwalled carbon nanotubes and nanofibers grafted with polyetherketones in mild and viscous polymeric acid

Direct covalent attachment of amorphous and semicrystalline polyetherketones onto the surface of either an as-received multi-walled carbon nanotube (MWNT) or a vapor-grown carbon nanofiber (VGCNF) in polyphosphoric acid (PPA) with optimized P₂O₅ content resulted in uniform grafting of polyetherketones to these carbon nanoscale materials. Soxhlet extraction experiment, the spectra from FT-IR spectroscopy and the clear images from high-resolution transmission electron microscopy showed that the covalent attachment is effective in uniformly coating the PEK grafts on the surfaces of both MWNT and VGCNF. Additionally, a drastic increase in solution viscosity due to the formation of giant molecules was monitored during polymerization. As such, the resulting nanocomposites were easily fabricated via a simple compression molding technique. The alignment possibility of MWNT and VGCNF grafted with semicrystalline PEK in these thermoplastic nanocomposites via solution fiber spinning was also demonstrated.     

Copper-decorated carbon nanotubes based composite electrodes for non-enzymatic detection of glucose

ABSTRACT: The aim of this study was to prepare three types of multi-wall carbon nanotube (CNT)-based composite electrodes and to modify their surface by copper electrodeposition for non-enzymatic oxidation and determination of glucose from aqueous solution. Copper decorated multi-wall carbon nanotube composite electrode (Cu/CNT-Epoxy) exhibited the highest sensitivity to glucose determination. The reliability of the Cu/CNT-Epoxy electrode was verified by application of this composite electrode for the determination of glucose in real blood serum samples.     

Liquid crystal engineering of carbon nanofibers and nanotubes

Four high-aspect-ratio carbon nanomaterials were fabricated by template-directed liquid crystal assembly and covalent capture. By selecting from two different liquid crystal precursors (thermotropic AR mesophase, and lyotropic indanthrone disulfonate) and two different nanochannel template wall materials (alumina and pyrolytic carbon) both the shape of the nanocarbon and the graphene layer arrangement can be systematically engineered. The combination of AR mesophase and alumina channel walls gives platelet-symmetry nanofibers, whose basic crystal symmetry is maintained and perfected upon heat treatment at 2500 °C. In contrast, AR infiltration into carbon-lined nanochannels produces unique C/C-composite nanofibers whose graphene planes lie parallel to the fiber axis. The transverse section of these composite nanofibers shows a planar polar structure with line defects, whose existence had been previously predicted from liquid crystal theory. Use of solvated AR fractions or indanthrone disulfonate produces platelet-symmetry tubes, which are either cellular or fully hollow depending on solution concentration. The use of barium salt solutions to force precipitation of indanthrone disulfonate within the nanochannels yields continuous nanoribbons rather than tubes. Overall the results demonstrate that liquid crystal synthesis routes provide molecular control over graphene layer alignment in nanocarbons with a power and flexibility that rivals the much better known catalytic routes.     

Chain confinement in electrospun nanofibers of PET with carbon nanotubes

Composite nanofibers of poly(ethylene terephthalate), PET, with multiwalled carbon nanotubes (PET/MWCNT) were prepared by the electrospinning method. Confinement, chain conformation, and crystallization of PET electrospun (ES) fibers were analyzed as a function of the weight fraction of MWCNTs. For the first time, we have characterized the rigid amorphous fraction (RAF) in polymer electrospun fibers, with and without MWCNTs. The addition of MWCNTs causes polymer chains in the ES fibers to become more extended, impeding cold crystallization of the fibers, resulting in more confinement of PET chains and an increase in the RAF. The fraction of rigid amorphous chains greatly increased with a small amount of MWCNT loading: with addition of 2% MWCNTs, RAF increased to 0.64, compared to 0.23 in homopolymer PET ES fibers. Spatial constraints also inhibit the folding of polymer chains, resulting in a decrease in crystallinity of PET. For fully amorphous PET/MWCNT composites, MWCNTs do not affect the chain conformation of PET in the ES fibers. For cold crystallized PET/MWCNT composite nanofibers, more trans conformers were formed with the addition of MWCNTs. The increase of RAF (chain confinement) is associated with an increase of the concentration of the trans conformers in the amorphous region as the MWCNT concentration increases in the semicrystalline nanofibers.     

Integration of carbon nanotubes with controllable inclination angle into microsystems

The possibility of growing carbon nanotubes in the immediate proximity of microstructures on a surface in a controllable way, with a high degree of control over the inclination angle, is demonstrated. Carbon nanotubes synthesised in a plasma-enhanced chemical vapour deposition process are known to grow in the direction of the electrical field. Geometrical features of the conductive substrate holder are used to distort the electrical field, thereby controlling the inclination angle of the carbon nanotubes locally. It is shown that the geometrical features of the microstructures on the silicon wafer do not interfere substantially with the resulting inclination angle. Finite element simulations show good agreement with the experimental observations, thus this is a route towards integrating carbon nanotubes with a special inclination angle on microstructures.     

Cobalt/copper-decorated carbon nanofibers as novel non-precious electrocatalyst for methanol electrooxidation

In this study, Co/Cu-decorated carbon nanofibers are introduced as novel electrocatalyst for methanol oxidation. The introduced nanofibers have been prepared based on graphitization of poly(vinyl alcohol) which has high carbon content compared to many polymer precursors for carbon nanofiber synthesis. Typically, calcination in argon atmosphere of electrospun nanofibers composed of cobalt acetate tetrahydrate, copper acetate monohydrate, and poly(vinyl alcohol) leads to form carbon nanofibers decorated by CoCu nanoparticles. The graphitization of the poly(vinyl alcohol) has been enhanced due to presence of cobalt which acts as effective catalyst. The physicochemical characterization affirmed that the metallic nanoparticles are sheathed by thin crystalline graphite layer. Investigation of the electrocatalytic activity of the introduced nanofibers toward methanol oxidation indicates good performance, as the corresponding onset potential was small compared to many reported materials; 310 mV (vs. Ag/AgCl electrode) and a current density of 12 mA/cm² was obtained. Moreover, due to the graphite shield, good stability was observed. Overall, the introduced study opens new avenue for cheap and stable transition metals-based nanostructures as non-precious catalysts for fuel cell applications.     

Nitrogen-doped carbon nanotubes synthesized with carbon nanotubes as catalyst

Nitrogen-doped carbon (CN_x) nanotubes were synthesized with carbon nanotubes (CNTs) as catalyst by detonation-assisted chemical vapor deposition. CN_x nanotubes exhibited compartmentalized bamboo-like structure. Electron energy loss spectroscopy and elemental mapping studies indicated that the synthesized tubes contained high concentration of nitrogen (ca. 17.3 at.%), inhomogeneously distributed with an enrichment of nitrogen within the compartments. X-ray photoelectron spectroscopy analysis revealed the presence of pyridine-like N and graphitic N incorporated into the graphitic network. The catalytic activity of CNTs for CN_x nanotube growth was ascribed to the nanocurvature and opening edges of CNT tips, which adsorbed C_n/CN species and assembled them into CN_x nanotubes.     

Graphitic carbon nanofibers developed from bundles of aligned electrospun polyacrylonitrile nanofibers containing phosphoric acid

Graphitic carbon nanofibers (GCNFs) with diameters of approximately 300 nm were developed using bundles of aligned electrospun polyacrylonitrile (PAN) nanofibers containing phosphoric acid (PA) as the innovative precursors through thermal treatments of stabilization, carbonization, and graphitization. The morphological, structural, and mechanical properties of GCNFs were systematically characterized and/or evaluated. The GCNFs made from the electrospun PAN precursor nanofibers containing 1.5 wt.% of PA exhibited mechanical strength that was 62.3% higher than that of the GCNFs made from the precursor nanofibers without PA. The molecules of PA in the electrospun PAN precursor nanofibers initiated the cyclization and induced the aromatization during stabilization, as indicated by the FT-IR and TGA results. The stabilized PAN nanofibers possessed regularly oriented ladder structures, which facilitated the further formation of ordered graphitic structures in GCNFs during carbonization and graphitization, as indicated by the TEM, XRD, and Raman results.     

Tetrahydrofuran-functionalized multi-walled carbon nanotubes as effective support for Pt and PtSn electrocatalysts of fuel cells

A novel and simple method to functionalize multi-walled carbon nanotubes (MWCNTs) is developed using tetrahydrofuran (THF) solvent as the functionalization and anchoring agent. The effectiveness of the method is demonstrated by the synthesis of uniformly distributed Pt and PtSn nanoparticles on THF-functionalized MWCNTs. Transmission electron microscopy and X-ray diffraction results indicate that Pt and PtSn nanoparticles with a narrow particle size distribution and an average particle size of ∼4 nm are synthesized on THF-functionalized MWCNTs. The lattice parameter of PtSn/MWCNTs increases with the Sn content, indicating the successful formation of PtSn binary nanoparticles. The results demonstrate the applicability and effectiveness of the THF-functionalized MWCNTs as effective catalyst supports in the development of highly dispersed and active Pt and Pt-based electrocatalysts for low temperature fuel cells. The successful functionalization of MWCNTs by THF also indicates that there could be a strong σ-π interaction between the MWCNTs and the THF.     

Electrochemical stability of carbon nanofibers in proton exchange membrane fuel cells

This fundamental study deals with the electrochemical stability of several non-conventional carbon based catalyst supports, intended for low temperature proton exchange membrane fuel cell (PEMFC) cathodes. Electrochemical surface oxidation of raw and functionalized carbon nanofibers, and carbon black for comparison, was studied following a potential step treatment at 25.0 °C in acid electrolyte, which mimics the operating conditions of low temperature PEMFCs. Surface oxidation was characterized using cyclic voltammetry, X-ray photoelectron spectroscopy (XPS), and contact angle measurements. Cyclic voltammograms clearly showed the presence of the hydroquinone/quinone couple. Furthermore, identification of carbonyl, ether, hydroxyl and carboxyl surface functional groups were made by deconvolution of the XPS spectra. The relative increase in surface oxides on carbon nanofibers during the electrochemical oxidation treatment is significantly smaller than that on carbon black. This suggests that carbon nanofibers are more resistant to the electrochemical corrosion than carbon black under the experimental conditions used in this work. This behaviour could be attributed to the differences found in the microstructure of both kinds of carbons. According to these results, carbon nanofibers possess a high potential as catalyst support to increase the durability of catalysts used in low temperature PEMFC applications.     

Pt-Co supported on single-walled carbon nanotubes as an anode catalyst for direct methanol fuel cells

Catalyst of Pt-Co supported on single-walled carbon nanotubes (SWCNTs) is prepared using mixed reducing agents. The SWCNTs were pretreated in a microwave oven to enable surface modification. Pt-Co nanoparticles with narrow particle size distribution around 5.4 nm were uniformly deposited onto the SWCNTs. Under same Pt loading mass and experimental conditions, the SWCNTs-Pt-Co catalyst shows higher electrocatalytic activity and improved resistance to CO poisoning than the SWCNTs-Pt catalyst.     

In-situ growth of bamboo-shaped carbon nanotubes and helical carbon nanofibers on Ti3C2Tx MXene at ultra-low temperature for enhanced electromagnetic wave absorption properties

The potential of two-dimensional layered MXenes in electromagnetic wave (EMW) absorption needs further development. Herein, we carried out the in situ growth of carbon nanotubes (CNTs) on the surface of Ti₃C₂T_x MXene at ultra-low temperature via chemical vapor deposition. The obtained CNTs exhibited a bamboo-like structure and were accompanied by helical carbon nanofibers. The ultra-low temperature solved the problem that the high temperature required in the traditional CNT growth process would destroy the structural integrity of MXene. The lush CNT forest cross-linked the MXene layers, transforming the two-dimensional layered structure into a three-dimensional conductive network, providing abundant conductive channels for carriers, optimizing the impedance matching of the CNT/MXene hybrid, and resulting in a significant dielectric loss. The as-prepared CNT/MXene hybrid exhibited a minimal reflection loss of ?52.56 dB (99.9994% EMW absorption) in the X-band. This work proposes a new idea to enhance the EMW absorption properties of Ti₃C₂T_x MXene and fabricate high-performance MXene-based EMW absorbers.     

Electrochemical properties of electrospun PAN/MWCNT carbon nanofibers electrodes coated with polypyrrole

The multi-walled carbon nanotube (CNT)-embedded activated carbon nanofibers (ACNF/CNT) and activated carbon nanofibers (ACNF) were prepared by stabilizing and activating the non-woven web of polyacrilonitrile (PAN) or PAN/CNT prepared by electrospinning. Both ACNF and ACNF/CNT were partially aligned along the winding direction of the drum winder. The average diameter of ACNF was 330 nm, while that of ACNF/CNT was lowered to 230 nm with rough surface. This was attributed to the CNT-added polymer solution in the electrospinning process providing finer fibers by increasing the electrical conductivity compared with the CNT-free one. The specific surface area and electrical conductivity of ACNF were 984 m²/g and 0.42 S/cm, respectively, while those of ACNF/CNT were 1170 m²/g and 0.98 S/cm, respectively. PPy was coated on the electrospun ACNF/CNT (PPy/ACNF/CNT) by in situ chemical polymerization in order to improve the electrochemical performance. The capacitances of the ACNF and PPy/ACNF electrodes were 141 and 261 F/g at 1 mA/cm², respectively, whereas that of PPy/ACNF/CNT was 333 F/g. This improvement in capacitance was attributed to the following: (i) the preparation of aligned nano-sized ACNF/CNT by electrospinning and the addition of CNT and (ii) the formation of a good charge-transfer complex by the PPy coating on the surface of the aligned nano-sized ACNF/CNT. The former leads to a good morphology and superior properties, such as a higher surface area, the formation of mesopores and an increase in electrical conductivity. The latter offers a refined three-dimensional network due to the highly porous structure between ACNF/CNT and PPy.     

Electrospun carbon nanofibers decorated with MnO nanoparticles as a sulfur-absorbent for lithium-sulfur batteries

Shuttle effect of the dissolved polysulfide is a main disadvantage for Li-S batteries, which has been explored by several polar materials to absorb lithium polysulfide with physical and chemical effect. Herein, for the first time, a composite of carbon nanofibers decorated with MnO nanoparticles (CNF-MnO) has been prepared by the facile electrospinning method followed by thermal treatment. SEM and TEM characterization delivered that the MnO NPs on CNF did not change the morphology but decrease the electronic conductivity of CNF-MnO composite. The CNF-MnO composite exhibited excellent electrochemical cyclic stability because of its strong chemical absorption for polysulfide. Interestingly, CNF-MnO composite served as both cathode as well interlayer for Li-S batteries. The CNF-MnO-S as cathode material showed an initial discharge capacity of 683.2 mAh g^-1 at 1.0?C and remained 592.0 mAh g^-1 even after 250 cycles with the capacity decay of 0.053% per cycle. As well, CNF-MnO as interlayer delivered superior cycling stability even at high current density of 3.0?C, where the capacity still maintained 542.2 mAh g^-1 over 200 cycles.     

Growth of carbon nanofibers on activated carbon fiber fabrics

Activated carbon fiber fabrics, an excellent adsorbent, were used as catalyst supports to grow carbon nanofibers. Because of the microporous structure of the activated carbon fibers, the catalysts could be distributed uniformly on the carbon surface. Based on this concept, the carbon nanofibers can be grown directly on the activated carbon fiber fabrics. We demonstrate that carbon nanofibers with a diameter between 20 and 50 nm for most of the fibers can be synthesized uniformly and densely on activated carbon fiber fabrics, impregnated by nickel nitrate catalyst precursor, using catalytic chemical vapor deposition. Although the carbon nanofibers are not straight with a crooked morphology, they form a three-dimensional network structure. Structure characterizations by TEM and XRD indicate that the carbon nanofibers have a turbostratic graphite structure and the graphite layers are stacked with a herringbone structure.     

Electrocatalytic properties of Pt/carbon composite nanofibers

Pt/carbon composite nanofibers were prepared by electrodepositing Pt nanoparticles directly onto electrospun carbon nanofibers. The morphology and size of Pt nanoparticles were controlled by the electrodeposition time. The resulting Pt/carbon composite nanofibers were characterized by running cyclic voltammograms in 0.20 M H₂SO₄ and 5.0 mM K₄[Fe(CN)₆] + 0.10 M KCl solutions. The electrocatalytic activities of Pt/carbon composite nanofibers were measured by the oxidation of methanol. Results show that Pt/carbon composite nanofibers possess the properties of high active surface area and fast electron transfer rate, which lead to a good performance towards the electrocatalytic oxidation of methanol. It is also found that the Pt/carbon nanofiber electrode with a Pt loading of 0.170 mg cm⁻² has the highest activity.     

Encapsulation of manganese oxides nanocrystals in electrospun carbon nanofibers as free-standing electrode for supercapacitors

Flexible composites with manganese oxides (MnO_x) nanocrystals encapsulated in electropun carbon nanofibers were successfully fabricated via a simple and practical combination of electrospinning and carbonization process. The as-formed MnO_x/carbon nanofibers composites have a rough surface with MnO_x nanoparticles well embedded in the carbon nanofibers backbones. When used as electrodes for supercapacitor, the resulting MnO_x/carbon nanofiber composites exhibit good electrochemical performance with a specific capacitance of 174.8 F g⁻¹ at 2 mV s⁻¹ in 0.5 M Na₂SO₄ electrolyte, a good rate capability at high current density and long-term cycling stability. It is expected that such freestanding composites could be promising electrodes for high-performance supercapacitors.     

Formation and electrochemical performance of copper/carbon composite nanofibers

Copper-loaded carbon nanofibers are fabricated by thermally treating electrospun Cu(CH₃COO)₂/polyacrylonitrile nanofibers and utilized as an energy-storage material for rechargeable lithium-ion batteries. These composite nanofibers deliver more than 400 mA g⁻¹ reversible capacities at 50 and 100 mA g⁻¹ current densities and also maintain clear fibrous morphology and good structural integrity after 50 charge/discharge cycles. The relatively high capacity and good cycling performance of these composite nanofibers, stemmed from the integrated combination of metallic copper and disordered carbon as well as their unique textures and surface properties, make them a promising electrode candidate for next-generation lithium-ion batteries.     

Carbon nanofibers supported Pt-Ru electrocatalysts for direct methanol fuel cells

Oxidized and reduced carbon nanofibers (OCNF and RCNF) were used as supports to prepare highly dispersed PtRu catalysts for the direct methanol fuel cells (DMFC). The structural and surface features and electrocatalytic properties of bimetallic PtRu/OCNF and PtRu/RCNF were extensively investigated. FT-IR spectra show that carboxyl groups exist on the surface of the OCNF, which greatly influence the morphology and crystallinity of the electrocatalysts. Transmission electron microscopy and X-ray diffraction consistently show that PtRu/RCNF has a smaller particle size and more uniform distribution than PtRu/OCNF. However, both catalysts have very similar methanol oxidation peak current densities that are significantly lower than commercial catalyst based on current-voltage (CV) results. These two catalysts also give very similar single cell performance except for some difference in the resistance polarization region. The OCNF supported catalysts give better performance than commercial catalysts when current density is higher than 50 mA cm⁻² in spite of low methanol oxidation peak current density. These results can be ascribed to the specific surface and structural properties of carbon nanofibers.     

Synthesis of Pt nanocatalyst with micelle-encapsulated multi-walled carbon nanotubes as support for proton exchange membrane fuel cells

Micelle-encapsulated multi-walled carbon nanotubes (MWCNTs) with sodium dodecyl sulfate (SDS) were used as catalyst support to deposit platinum nanoparticles. High resolution transmission electron microscopy (HRTEM) images reveal the crystalline nature of Pt nanoparticles with a diameter of ∼4 nm on the surface of MWCNTs. A single proton exchange membrane fuel cell (PEMFC) with total catalyst loading of 0.2 mg Pt cm⁻² (anode 0.1 and cathode 0.1 mg Pt cm⁻², respectively) has been evaluated at 80 °C with H₂ and O₂ gases using Nafion-212 electrolyte. Pt/MWCNTs synthesized by using modified SDS-MWCNTs with high temperature treatment (250 °C) showed a peak power density of 950 mW cm⁻². Accelerated durability evaluation was carried out by conducting 1500 potential cycles between 0.1 and 1.2 V with 50 mV s⁻¹ scan rate, H₂/N₂ at 80 °C. The membrane electrode assembly (MEA) with Pt/MWCNTs showed superior performance stability with a power density degradation of only ∼30% compared to commercial Pt/C (70%) after potential cycles.