Mechanical and structural characterization of electrospun PAN-derived carbon nanofibers

The mechanical and structural properties of individual electrospun PAN-derived carbon nanofibers are presented. EELS spectra of the carbonized nanofibers shows the C atoms to be partitioned into ∼80% sp² bonds and ∼20% sp³ bonds which agrees with the observed structural disorder in the fibers. TEM images show a skin-core structure for the fiber cross-section. The skin region contains layered planes oriented predominantly parallel to the surface, but there are some crystallites in the skin region misoriented with respect to the fiber long axis. Microcombustion analysis showed 89.5% carbon, 3.9% nitrogen, 3.08% oxygen and 0.33% hydrogen. Mechanical testing was performed on individual carbonized nanofibers a few microns in length and hundreds of nanometers in diameter. The bending modulus was measured by a mechanical resonance method and the average modulus was 63 GPa. The measured fracture strengths were analyzed using a Weibull statistical distribution. The Weibull fracture stress fit to this statistical distribution was 0.64 GPa with a failure probability of 63%.     

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.     

Multiwall carbon nanotube supported poly(3,4-ethylenedioxythiophene)/manganese oxide nano-composite electrode for super-capacitors

MWCNT-PSS/PEDOT/MnO₂ nano-composite electrodes were fabricated by generating pseudo-capacitive poly(3,4-ethylenedioxythiophene) (PEDOT)/MnO₂ nano-structures on poly(styrene sulfonate) (PSS) dispersed multiwalled carbon nanotubes (MWCNTs). PSS dispersed MWCNTs (MWCNT-PSS) facilitated the growth of PEDOT and MnO₂ into nano-rods with large active surface area and good electrical conductivity. The ternary MWCNT-PSS/PEDOT/MnO₂ nano-composite electrode was studied for the application in super-capacitors, and exhibited excellent capacitive behavior between −0.2 V and 0.8 V (vs. saturated Ag/AgCl electrode) with high reversibility. Specific capacitance of the nano-composite electrode was found as high as 375 F g⁻¹. In contrast, specific capacitance of MWCNT-PSS/MnO₂ and MWCNT-PSS nano-composite electrodes is 175 F g⁻¹ and 15 F g⁻¹, respectively. Based on cyclic voltammetric studies and cycle-life tests, the MWCNT-PSS/PEDOT/MnO₂ nano-composite electrode gave a highly stable and reversible performance up to 2000 cycles. Our studies demonstrate that the synergistic combination of MWCNT-PSS, PEDOT and MnO₂ has advantages over the sum of the individual components.     

A hydroxylamine electrochemical sensor based on electrodeposition of porous ZnO nanofilms onto carbon nanotubes films modified electrode

A novel route (electrodeposition) for the fabrication of porous ZnO nanofilms attached multi-walled carbon nanotubes (MWCNTs) modified glassy carbon electrodes (GCEs) was proposed. The morphological characterization of ZnO/MWCNT films was examined by scanning electron microscopy (SEM) and X-ray powder diffraction (XRD). The performances of the ZnO/MWCNTs/GCE were characterized with cyclic voltammetry (CV), Nyquist plot (EIS) and typical amperometric response (i-t). The potential utility of electrodes constructed was demonstrated by applying them to the analytical determination of hydroxylamine concentration. An optimized limit of detection of 0.12 μM was obtained at a signal-to-noise ratio of 3 and with a fast response time (within 3 s). Additionally, the ZnO/MWCNTs/GCE exhibited a wide linear range from 0.4 to 1.9 × 10⁴ μM and higher sensitivity. The ease of fabrication, high stability, and low cost of the modified electrode are the promising features of the proposed sensor.     

Preparation of highly crystalline carbon nanofibers from pitch/polymer blend

High crystalline carbon nanofibers were prepared by using polymer blend technique. Naphthalene-based mesophase pitch (AR pitch) was dispersed finely in polymethylpentene matrix, spun by using a melt-blown spinning machine, stabilized at 160 °C in an oxygen atmosphere and carbonized at 900 °C in a nitrogen atmosphere. Bundles of the carbon nanofibers with ca. 100 nm in diameter were obtained after removal of polymethylpentene at the carbonization process. No impurity carbon was observed. The carbon nanofibers consisted of fine carbon crystallites with preferred orientation along the fiber axis. After heating to 3000 °C, the carbon crystallites grew drastically to have an interlayer spacing of 0.3367 nm and a crystallite thickness of 56.9 nm, respectively, with remarkable improvement of the preferred orientation of the crystallites. Advantages and disadvantages of the present method were discussed briefly.     

Layer-by-layer assembly of poly(sodium 4-styrenesulfonate) wrapped multiwalled carbon nanotubes with polyaniline nanofibers and its electrochemistry

A simple and versatile method based on noncovalent supramolecular attachment and layer-by-layer (LBL) assembly is proposed to prepare nanostructured hybrid conducting polymer. The negatively charged poly(sodium 4-styrenesulfonate) (PSS) wrapped multiwalled carbon nanotubes (MWCNTs) is doped with cationic polyaniline (PANI) nanofibers via LBL assembly, and a well-defined PANI/MWCNTs composite was obtained. The LBL assembly process is characterized by scanning electron microscopy, energy dispersive spectrometry and electrochemical methods. It was found that PSS wrapped MWCNTs inside the multilayer film can dope nanostructured PANI effectively and shift its electroactivity to a neutral pH environment. Moreover, the conducting composites show amperometric response for hydrogen peroxide with a linear range of 2.0 × 10⁻⁷-1.0 × 10⁻³ mol L⁻¹.     

Ionic liquid assisted fabrication of cellulose-based conductive films for Li-ion battery

An imidazolium-based ionic liquid, 1-ethyl-3-methylimidazolium diethyl phosphate ([Emim]DEP) was used to dispense graphene nanoplates (GN) and multiwalled carbon nanotubes (MWCNTs) as well as dissolve cellulose for fabricating composite conductive films. The effects of GN, MWCNTs, and cellulose mass ratios on the electrical conductivity and morphology of the films were investigated. The interaction between GN, MWCNTs, and cellulose was analyzed by SEM, X-ray diffraction (XRD), TGA, and Raman spectroscopy. The results indicate that [Emim]DEP plays a vital and irreplaceable role in GN and MWCNTs dispersion, cellulose dissolution, and porous formation during the regeneration and drying processes. MWCNTs linked flaky GN and a hybrid structure was constructed elaborately to form a better conductive path and improve the conductivity as well as increase the film stability. For the XRD result, the carbonized GN-MWCNTs-cellulose films exhibited the graphitic peaks, showing that the films still retained the structure of carbon atoms or molecules. Besides, the maximum conductivity of carbonized GN-MWCNTs-cellulose (7:3:2) composite film was up to 9,009 S m⁻¹, due to the small carbon clusters formation and the high degree of graphitization. Further, the carbonized films were applied as anodes in Li-ion battery and showed good electrochemical performance. The best cyclic stability (i.e., discharge/charge capacity) of 438/429 mA h g⁻¹ and coulomb efficiency of 50.2% were obtained. This novel and sustainable design is a promising approach to obtain cellulose-based conductive films and anodes for Li-ion battery applications.     

Factors influencing MnO2/multi-walled carbon nanotubes composite's electrochemical performance as supercapacitor electrode

Poor crystallined α-MnO₂ grown on multi-walled carbon nanotubes (MWCNTs) by reducing KMnO₄ in ethanol are characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and Brunauer-Emmett-Telle (BET) surface area measurement, which indicate that MWCNTs are wrapped up by poor crystalline MnO₂ and BET areas of the composites maintain the same level of 200 m² g⁻¹ as the content of MWCNTs in the range of 0-30%. The electrochemical performances of the MnO₂/MWCNTs composites as electrode materials for supercapacitor are evaluated by cyclic voltammetry (CV) and galvanostatic charge-discharge measurement in 1 M Na₂SO₄ solution. At a scan rate of 5 mV s⁻¹, rectangular shapes could only be observed for the composites with higher MWCNTs contents. The effect of additional conductive agent KS6 on the electrochemical behavior of the composites is also studied. With a fixed carbon content of 25% (MWCNTs included), MnO₂ with 20% MWCNTs and 5% KS6 has the highest specific capacitance, excellent cyclability and best rate capability, which gives the specific capacitance of 179 F g⁻¹ at a scan rate of 5 mV s⁻¹, and remains 114.6 F g⁻¹ at 100 mV s⁻¹.     

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.     

Alcohol dehydrogenase amperometric biosensor based on a colloidal gold-carbon nanotubes composite electrode

A novel ethanol biosensor based on the bulk incorporation of alcohol dehydrogenase (ADH) into a colloidal gold (Au_coll)-multiwalled carbon nanotubes (MWCNTs) composite electrode using Teflon as binding material is reported. The composite Au_coll-MWCNTs-Teflon electrode exhibited significantly improved electrooxidation of NADH when compared with other carbon composite electrodes, including those based on carbon nanotubes. Amperometric measurements for NADH at +0.3 V showed significant differences in sensitivity between Au_coll-MWCNTs-Teflon and MWCNTs-Teflon composite electrodes. Incorporation of ADH into the bulk electrode material allowed the construction of a mediatorless ethanol biosensor. Both the enzyme loading and the NAD⁺ concentration in solution were optimized. The ADH-Au_coll-MWCNTs-Teflon biosensor allowed a limit of detection for ethanol of 4.7 μmol l⁻¹, which is remarkably better than those reported for other CNTs-based ADH biosensors. The apparent Michaelis-Menten constant was 4.95 mmol l⁻¹, which is much lower than that reported by immobilization of ADH onto a gold electrode. Both repeatability of the ethanol amperometric measurements, reproducibility with different biosensors, lifetime and storage ability can be, in general, advantageously compared with other ADH-CNTs biosensors. The biosensor was applied for the rapid determination of ethanol in commercial and certified beer samples.     

Carbon nanotube array anodes for high-rate Li-ion batteries

The electrochemical performance of carbon nanotube array (CNTA) and entangled carbon nanotube (ECNT) electrodes are studied as anodes for Li-ion batteries. CNTA anodes display higher capacity (373 mAh g⁻¹) and much better rate and cycle performances than ECNT anodes. The performance of CNTA electrode shows length dependencies, i.e., shorter CNTA electrodes present higher specific capacity and better rate performance. The energy storage characteristics of CNTA electrodes are discussed on the basis of experimental results of SEM, TEM, and Raman spectra. The inner graphene layers of CNTs in CNTA electrode, which can form electron conductive paths and ensure a high conductivity, are retained during Li-ion insertion/extraction. These mechanically robust inner graphene layers can avoid the loss of outer active materials during Li-ion insertion/extraction, which, in turn, results in a good cycle performance.     

Electrostatic heterocoagulation of carbon nanotubes and LiCoO2 particles for a high-performance Li-ion cell

Nanotubes were coated on the surface of active LiCoO₂ particles using electrostatic heterocoagulation to enhance the electrochemical properties of a Li-ion battery. Only 0.5 wt% of multiwalled carbon nanotubes (MWCNTs) was added as a conducting agent into the LiCoO₂ cathode, which had a density of 4.0 g cm⁻³. We found that our electrode that was prepared using heterocoagulation with 0.5 wt% of thin MWCNTs maintained a volumetric capacitance of 403 mAh cm⁻¹ after 40 cycles from the initial 624 mAh cm⁻¹, compared with previous result of 310 mAh cm⁻¹ obtained from simple mixing with 3 wt% MWCNTs. The high volumetric capacity with smaller swelling using less amount of MWCNTs was attributed to the self-assembled nanotube network formed between active particles during coagulation, which was maintained with volume expansion during cycle testing.     

Dependencies of the open-circuit voltage of Au|YSZ|Pt single-chamber fuel cell on the composition of the uniform CH4 + O2 gas mixture with solid carbon deposition

Single-chamber solid oxide fuel cell is a device where two electrodes of different materials contacting a solid oxide ionic conductor, may generate a considerable potential difference and electrical power, when supplied by a common fuel + oxidant gas mixture. The Au|YSZ|Pt system in the CH₄ + O₂ gas mixture is one of the simplest examples of such a cell. In this article the open-circuit voltage (OCV) of this cell, supplied with the gas mixture xO₂ + aCH₄ + (1 − x − a)Ar (where a = 0.01, 0.1 or 0.5), is investigated. On the basis of the obtained results, as well as those for the xCH₄ + (1 − x)(0.2O₂ + 0.8Ar) (0 ≤ x ≤ 1) gas mixture, reported in our previous work [Electrochim. Acta, 50 (2005) 2771], we postulate that the OCV of the above system arises as a result of electrode modification resulting from solid carbon deposition in the cell. After oxidation of the carbon deposit, the system, once treated by the gas mixture enabling the formation of the carbon phase, shows more and more tendency to generate the OCV. The open-circuit potential of the Au electrode depends only on the O₂ concentration in the initial gas mixture, whereas in the case of the Pt electrode it becomes dependent on chemical equilibria determining the O₂ content in the converted gas mixture. Our results reveal that the OCV achieves a reproducible limiting value of ∼650 mV, which is lower by ∼400 mV than the calculated equilibrium value.     

Cyclic durable high-capacity Sn/Cu6Sn5 composite thin film anodes for lithium ion batteries prepared by electron-beam evaporation deposition

Sn/Cu₆Sn₅ alloy composite thin films were directly prepared by electron-beam deposition for anodes of lithium ion batteries. The thin film was comprised of micro/sub-microcrystalline Sn and Cu₆Sn₅, where the polyhedral micro-sized Sn grains were uniformly dispersed in the loose Cu₆Sn₅ matrix. Lithiation reaction kinetics were confirmed to be controlled by a diffusion step and the diffusion coefficient of Li⁺ in the thin film anode was determined to be 1.91 × 10⁻⁷ cm²/s. The galvanostatic cycling behavior of Sn/Cu₆Sn₅ composite thin film anodes was studied under different conditions. Stable capacities of more than 370 mAh/g were obtained by discharging from 1.25 to 0.1 V. Structure changes and fading mechanism of the thin film electrodes was discussed based on SEM, XRD and EDX investigations. The present results demonstrated that the multi-phase composite structure can improve electrochemical performance of the Cu-Sn alloy thin film electrodes.     

Horizontally aligned single-walled carbon nanotube field emitters fabricated on vertically aligned multi-walled carbon nanotube electrode arrays

Vertically aligned multi-walled carbon nanotube (MWCNT) arrays were fabricated on an anodic aluminum oxide membrane bonded to a Si wafer. After obtaining a protruding tip for the MWCNTs by etching away some oxide, they were used as electrodes in the fabrication of carbon nanotube field emitters. Long single-walled carbon nanotubes (SWCNTs) were spin coated on the MWCNT arrays of uniform height. Clean SWCNTs were suspended by attaching them to the tips of the vertically aligned MWCNT arrays. The spin coated SWCNTs function as emitters, while the MWCNT arrays function as electrodes. The field emission was greatly improved by coating gold on the MWCNT arrays and annealing at 400 °C. Our field emitter exhibits good field emission properties such as a low turn-on field (1.4 V/μm), high current density (122 mA/cm²), and good stability (55 h for 10% degradation of current density from 400 μA/cm²).     

Oxygen functionalization of multiwall carbon nanotubes by Ar/H2O plasma treatment

To increase the applicability of multiwall carbon nanotubes (MWCNTs), oxygen-containing functional groups were introduced on the surfaces of MWCNTs by using microwave-excited Ar/H₂O surface-wave plasma. X-ray photoelectron spectroscopy and Raman spectroscopy were used to determine dependencies of Ar/H₂O gas partial pressure, treatment time and microwave power. The oxygen functionalization of MWCNTs by plasma can be achieved very rapidly, about 10 min. The C-O and O-C═O fractions firstly increase and then decrease with increasing Ar partial pressure. The C-O and O-C═O fractions increase with increasing microwave power from 400 W to 700 W. A slight increase of the R (I_D/I_G ratio) value for the treated MWCNTs indicated disordering in the surface microstructure of MWCNTs coincident with the introduction of surface oxygen. The oxygen-containing groups introduced on the surfaces of MWCNTs by plasma treatment are hydrophilic. The dispersion of plasma treated MWCNTs is therefore improved.     

Preparation of a carbon nanotube/carbon fiber multi-scale reinforcement by grafting multi-walled carbon nanotubes onto the fibers

To prepare a carbon nanotube (CNT)/carbon fiber multi-scale reinforcement (MSR), multi-walled carbon nanotubes (MWCNTs) functionalized at the end caps with hexamethylene diamine (HMD) are grafted onto the surfaces of carbon fibers treated with acyl chloride. The surface element concentrations, surface functional groups and morphology of the MSR were examined by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). XPS spectra indicate that sp² and sp³ carbon atoms are major components in the MSR surface, and the carbon fiber surface structure is not destroyed. There is 17.41% of C-NH_x in the surface of the MSR, which suggests that MWCNTs are covalently grafted onto carbon fiber surfaces. SEM shows that the grafted MWCNTs stick to the carbon fiber surface at different angles, and are uniformly distributed along the outer edges of the grooves in the fiber surface. The grafted MWCNTs are 50-200 nm in length and around 14 nm in diameter. It was found that the grafting increases the weight of carbon fiber by 1.2%, which implied that a considerable amount of MWCNTs were grafted onto carbon fiber surfaces.     

Desirable electrical and mechanical properties of continuous hybrid nano-scale carbon fibers containing highly aligned multi-walled carbon nanotubes

The continuous highly aligned hybrid carbon nanofibers (CNFs) with different content of acid-oxidized multi-walled carbon nanotubes (MWCNTs) were fabricated through electrospinning of polyacrylonitrile (PAN) followed by a series of heat treatments under tensile force. The effects of MWCNTs on the micro-morphology, the degree of orientation and ordered crystalline structure of the resulting nanofibers were analyzed quantitatively by diversified structural characterization techniques. The orientation of PAN molecule chains and the graphitization degree in carbonized nanofibers were distinctly improved through the addition of MWCNTs. The electrical conductivity of the hybrid CNFs with 3 wt% MWCNTs reached 26 S/cm along the fiber direction due to the ordered alignment of MWCNTs and nanofibers. The reinforcing effect of hybrid CNFs in epoxy composites was also revealed. An enhancement of 46.3% in Young’s modulus of epoxy composites was manifested by adding 5 wt% hybrid CNFs mentioned above. At the same time, the storage modulus of hybrid CNF/epoxy composites was significantly higher than that of pristine epoxy and CNF/epoxy composites not containing MWCNTs, and the performance gap became greater under the high temperature regions. It is believed that such a continuous hybrid CNF can be used as effective multifunctional reinforcement in polymer matrix composites.     

Modification of glassy carbon electrode with multi-walled carbon nanotubes and iron(III)-porphyrin film: Application to chlorate, bromate and iodate detection

In this study, multi-wall carbon nanotubes (MWCTs) is evaluated as a transducer, stabilizer and immobilization matrix for the construction of amperometric sensor based on iron-porphyrin. 5,10,15,20-Tetraphenyl-21H,23H-porphine iron(III) chloride (Fe(III)P) adsorbed on MWCNTs immobilized on the surface of glassy carbon electrode. Cyclic voltammograms of the Fe(III)P-incorporated-MWCNTs indicate a pair of well-defined and nearly reversible redox couple with surface confined characteristics at wide pH range (2-12). The surface coverage (Γ) and charge transfer rate constant (k_s) of Fe(III)P immobilized on MWCNTs were 7.68 × 10⁻⁹ mol cm⁻² and 1.8 s⁻¹, respectively, indicating high loading ability of MWCNTs for Fe(III)P and great facilitation of the electron transfer between Fe(III)P and carbon nanotubes immobilized on the electrode surface. Modified electrodes exhibit excellent electrocatalytic activity toward reduction of ClO₃⁻, IO₃⁻ and BrO₃⁻ in acidic solutions. The catalytic rate constants for catalytic reduction of bromate, chlorate and iodate were 6.8 × 10³, 7.4 × 10³ and 4.8 × 10² M⁻¹ s⁻¹, respectively. The hydrodynamic amperometry of rotating-modified electrode at constant potential versus reference electrode was used for detection of bromate, chlorate and iodate. The detection limit, linear calibration range and sensitivity for chlorate, bromate and iodate detections were 0.5 μM, 2 μM to 1 mM, 8.4 nA/μM, 0.6 μM, 2 μM to 0.15 mM, 11 nA/μM, and 2.5 μM, 10 μM to 4 mM and 1.5 nA/μM, respectively. Excellent electrochemical reversibility of the redox couple, good reproducibility, high stability, low detection limit, long life time, fast amperometric response time, wide linear concentration range, technical simplicity and possibility of rapid preparation are great advantages of this sensor. The obtained results show promising practical application of the Fe(III)P-MWCNTs-modified electrode as an amperometric sensor for chlorate, iodate and bromate detections.     

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.