Nanocomposites of manganese oxides and carbon nanotubes for aqueous supercapacitor stacks

Symmetrical supercapacitors and their serially connected two-cell stacks via a bipolar electrode were constructed with nanocomposites of manganese oxides and carbon nanotubes (MnO_x/CNTs) as the electrode materials. Nanocomposites with different contents of MnO_x were synthesised through the redox reaction between KMnO₄ and CNTs in aqueous solutions. The nanocomposites were characterised by scanning and transmission electron microscopy, BET nitrogen adsorption and X-ray diffraction before being examined in a three-electrode cell with a novel trenched graphite disc electrode by electrochemical means, including cyclic voltammetry, galvanostatic charging-discharging, and electrochemical impedance spectroscopy. The nanocomposites demonstrated capacitive behaviour in the potential range of 0-0.85 V (vs Ag/AgCl) in aqueous KCl electrolytes with less than 9% capacitance decrease after 9000 charging-discharging cycles. Symmetrical supercapacitors of identical positive and negative MnO_x/CNTs electrodes showed capacitive performance in good agreement with the individual electrodes (e.g. 0.90 V, 0.53 F, 1.3 cm²). The bipolarly connected two-cell stacks of the symmetrical cells exhibited characteristics in accordance with expectation, including a doubled stack voltage and reduced internal resistance per cell.     

Iron nanoparticles in aligned arrays of pure and nitrogen-doped carbon nanotubes

Arrays of aligned carbon nanotubes (CNTs) and nitrogen-doped carbon (CN_x) nanotubes have been grown on silicon substrates as the result of thermolysis of ferrocene/toluene and ferrocene/acetonitrile mixture. The microstructure of materials was studied by transmission and scanning electron microscopy, and X-ray diffraction was used to control the carbon and iron forms. The composition and properties of iron nanoparticles developed in the CNT and CN_x nanotube samples were determined from Mössbauer spectroscopy data. The total iron content in CN_x nanotubes was found to be considerably higher than that in CNTs. Three forms of iron nanoparticles α-Fe, γ-Fe, and Fe₃C were detected in CNTs and only two last of them in CN_x nanotubes. In the interior of CNT channels the α-Fe and Fe₃C nanoparticles were observed to be coupled by a strong exchange interaction and to exhibit magnetic behavior at room temperature.     

Coating carbon nanotubes with metal oxides in a supercritical carbon dioxide-ethanol solution

Metal oxide films, including Ce₂O₃ and/or CeO₂, Al₂O₃, La₂O₃, were deposited on the outer surfaces of carbon nanotubes (CNTs) through the decomposition of metal nitrate precursors in supercritical CO₂ modified with ethanol. Transmission electron microscopy showed that CNTs could be coated with metal oxide layers that were nominally complete and uniform. The thickness of the coating could be readily tailored by tuning the ratio of the initial mass of precursors to CNTs. The as-prepared CeO₂-CNT composites showed high sensitivity and selectivity to acetone on the basis of chemiluminescence detection.     

Carbon-based nanostructured catalyst for biodiesel production by catalytic distillation

A promising carbon-based nanostructured catalyst was prepared via the following four steps: (1) thermal decomposition of organometallic compound (C₁₀H₁₄CoO₄) on 304 stainless steel substrate, (2) cracking of benzene to carbon nanotubes (CNTs) on the substrate using Co particle catalyst, (3) sulfurizing CNTs with Na₂S_x, and (4) oxidating the sulfurized CNTs with hydrogen peroxide. The as-prepared carbon-based catalyst was characterized by spectroscopy, scanning electron microscopy, transmission electron microscopy etc. The monolithic catalyst can serve as appropriate filler for a catalytic distillation column. Catalytic activity was examined by catalyzing the transesterification of soybean oil and methanol to biodiesel in the catalytic distillation column.     

Iron/cobalt-SBA-16 cubic mesoporous composites as catalysts for the production of multi-walled carbon nanotubes

A series of catalysts containing iron and cobalt nanoparticles supported on a highly ordered mesoporous cubic Im3m silica (SBA-16) were prepared by wet impregnation and used for the production of multi-walled carbon nanotubes (MWCNTs) by catalytic chemical vapor deposition (CCVD) of acetylene. The catalysts were characterized by low- and wide-angle X-ray diffraction, N₂ physisorption analysis at 77 K and transmission electron microscopy to study the influence of different metal loading and impregnation time on the CCVD process. Quality and morphology of the MWCNTs was assessed by transmission and scanning electron microscopy, whereas thermal analysis was used to estimate the amount of CNTs produced. It was found that the nanocomposites are catalytically active with particular reference to samples with relatively high metal loading, and are stable under the conditions adopted for the CNT production by the CCVD process.     

碳纳米管改性对Au/CeO2催化剂乙醇部分氧化制氢的影响

采用沉积沉淀法制备了一系列碳纳米管改性的Au/CeO₂催化剂,以乙醇部分氧化制氢为探针反应,研究了碳纳米管对Au/CeO₂催化剂乙醇部分氧化性能的影响,并运用XRD、TPR、BET等方法对催化剂进行了表征。结果表明,碳纳米管的添加提高了Au/CeO₂催化剂的比表面积、孔容和吸氧量,催化剂的氢气选择性先随碳纳米管添加量的增加而大幅增加,碳纳米管的添加量达6%～10%时,氢气选择性达到43%。进一步提高碳纳米管的含量,氢气选择性增加幅度不大。碳纳米管的添加可以有效抑制副产物CO的产生。     

Activity and active sites of nitrogen-doped carbon nanotubes for oxygen reduction reaction

Nitrogen-doped carbon (CNx) nanotubes were synthesized by thermal decomposition of ferrocene/ethylenediamine mixture at 600–900 °C. The effect of the temperature on the growth and structure of CNx nanotubes was studied by transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. With increasing growth temperature, the total nitrogen content of CNx nanotubes was decreased from 8.93 to 6.01 at.%. The N configurations were changed from pyrrolic-N to quaternary-N when increasing the temperature. Examination of the catalytic activities of the nanotubes for oxygen reduction reaction by rotating disk electrode measurements and single-cell tests shows that the onset potential for oxygen reduction in 0.5 M H₂SO₄ of the most effective catalyst (CNx nanotubes synthesized at 900 °C) was 0.83 V versus the normal hydrogen electrode. A current density of 0.07 A cm^?2 at 0.6 V was obtained in an H₂/O₂ proton-exchange membrane fuel cell at a cathode catalyst loading of 2 mg cm^?2.     

Magnesia–Carbon Nanotubes (MgO–CNTs) Nanocomposite: Novel Support of Ru Catalyst for the Generation of COx-Free Hydrogen from Ammonia

Magnesia–carbon nanotubes (abbreviated as MgO–CNTs) nanocomposites were prepared by impregnation of CNTs with Mg(NO₃)₂·6H₂O in ethanol solution, followed by drying at 353 K and calcination at 873 K, respectively. The nanocomposites are thermally more stable than CNTs in a H₂ flow. The use of the nanocomposites as support yielded more efficient Ru catalysts for the generation of CO _x -free hydrogen from NH₃ decomposition.     

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.     

Growth of few-wall carbon nanotubes with narrow diameter distribution over Fe-Mo-MgO catalyst by methane/acetylene catalytic decomposition

Few-wall carbon nanotubes were synthesized by methane/acetylene decomposition over bimetallic Fe-Mo catalyst with MgO (1:8:40) support at the temperature of 900°C. No calcinations and reduction pretreatments were applied to the catalytic powder. The transmission electron microscopy investigation showed that the synthesized carbon nanotubes [CNTs] have high purity and narrow diameter distribution. Raman spectrum showed that the ratio of G to D band line intensities of I_G/I_D is approximately 10, and the peaks in the low frequency range were attributed to the radial breathing mode corresponding to the nanotubes of small diameters. Thermogravimetric analysis data indicated no amorphous carbon phases. Experiments conducted at higher gas pressures showed the increase of CNT yield up to 83%. Mössbauer spectroscopy, magnetization measurements, X-ray diffraction, high-resolution transmission electron microscopy, and electron diffraction were employed to evaluate the nature of catalyst particles.     

Bioelectrochemistry and enzymatic activity of glucose oxidase immobilized onto the bamboo-shaped CNx nanotubes

The novel bamboo-shaped CN_x nanotubes, synthesized by nitrogen atoms doping into carbon nanotubes, were used for the immobilization of a relatively large enzyme glucose oxidase (GOx) and its bioelectrochemical studies. The morphologies and adsorptions of GOx immobilization onto CN_x nanotubes were clearly observed by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). Electrochemical impedance spectroscopy (EIS) was also used to feature the GOx adsorbed onto the surface of CN_x nanotubes. The immobilized GOx incorporated into CN_x nanotubes films exhibited a well-defined nearly reversible cyclic voltammetric peaks for the electroactive centers of GOx and a fast heterogeneous electron transfer rate with the rate constant (K_s) of 1.96 s⁻¹. The immobilized GOx onto the CN_x nanotubes exhibited its bioelectrocatalytic activity for the oxidation of glucose. The obtained results suggest that with a large amount of defective/active sites on the tube surfaces, a special bamboo structure and a suitable C-N microenvironment introduced by nitrogen doping, CN_x nanotubes could not only facilitate the direct electron transfer between the enzyme and electrode, but also retain the high enzyme loading and the enzymatic bioactivity.     

Purity-controllable growth of bamboo-like multi-walled carbon nanotubes over copper-based catalysts

The growth of bamboo-like multi-walled carbon nanotubes (CNTs) without the formation of amorphous carbons was performed using copper-based catalysts by catalytic chemical vapour deposition (CVD) with diluted ethylene at 700–900 °C. The as-grown CNT soot was characterised by transmission electron microscopy, thermogravimetric analysis and Raman spectroscopy. The weak metal–support interaction of a sulphate-assisted copper catalyst (CuSO₄/SiO₂) can provide high-purity growth with remarkable yields of CNTs (2.24–6.10 CNT/g Cu·h) at 850–900 °C. Additionally, hydrogen-assisted CVD can activate inert copper catalysts, e.g., Cu(NO₃)₂/SiO₂ or Cu(CH₃COO)_2/SiO₂, for the growth of CNTs.     

Alkaline carbon nanotubes as effective catalysts for H2S oxidation

Carbon nanotubes (CNTs) were made alkaline by impregnation with Na₂CO₃ and used for the direct oxidation of H₂S into sulfur at 30 °C. The alkaline CNTs before and after H₂S oxidation were characterized by N₂ adsorption, scanning and transmission electron microscopy, X-ray diffraction, and thermogravimetry analysis. The effects of Na₂CO₃ loading and the structure of the CNTs on the catalytic activity of alkaline CNTs were investigated. Results indicated that the saturation sulfur capacity of alkaline CNTs was up to 1.86 g H₂S/g catalyst, which was about 3.9 times higher than that of a common commercial H₂S oxidation catalyst. The introduction of Na₂CO₃, which provided alkalinity needed for H₂S dissociation, significantly enhanced the catalytic performance. The optimum content of Na₂CO₃ loading was determined to be 20 wt.% in the CNTs. The catalytic performance was also dependent on the structure of the CNTs, and the single-walled CNTs with the smallest tube diameter exhibited the highest sulfur capacity. Tangled CNTs provided their external voids to store the sulfur produced, which was a key feature of this high-performance CNT catalyst.     

Electrochemical characterization of TiO2/WOx nanotubes for photocatalytic application

TiO₂/WO_x nanotubes have unique photo-energy retention properties that have gathered scientific interest. Herein, we report the synthesis, morphological characterization, and the electrochemical characterization of TiO₂/WO_x nanotubes compared with pure TiO₂ nanotubes, prepared by anodization technique. Significant structural differences were not observed in TiO₂/WO_x nanotubes as observed by using scanning electron microscopy and transmission electron microscopy. The charge transfer resistance of TiO₂/WO_x before and after photo irradiation determined by using electrochemical impedance spectroscopy proves the inherent energy retention property which was not observed in pure TiO₂ nanotubes.     

High impact strength epoxy nanocomposites with natural nanotubes

Epoxy-based nanocomposites were prepared with natural nanotubes from halloysite, a clay mineral with the empirical formula Al₂Si₂O₅(OH)₄. The morphology of the nanotubes was examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and was found geometrically similar to multi-walled carbon nanotubes. The thermal and mechanical properties of the nanocomposites were characterized by thermogravimetric analysis, dynamic mechanical analysis, Charpy impact and three-point bending tests. The results demonstrated that blending epoxy with 2.3 wt% halloysite nanotubes increased the impact strength by 4 times without scarifying flexural modulus, strength and thermal stability. Unique toughening mechanisms for this improvement were investigated and discussed. It was proposed that impact energy was dissipated via the formation of damage zones with a large number of micro-cracks in front of the main crack. The micro-cracks were stabilized by nanotube bridging. Nanotube bridging, pull-out and breaking were also observed and proposed as the major energy dissipating events. The findings of this work suggest that halloysite nanotube may be an effective impact modifier for epoxy and other brittle polymers.     

Thermal and electrical properties of carbon nanotubes based polysulfone nanocomposites

Carbon nanotubes (CNTs)-reinforced polysulfone (PSU) nanocomposites were prepared through solution mixing of PSU and different weight percent of multi-walled carbon nanotubes (MWCNTs). Thermal properties of nanocomposites were characterized using thermo-gravimetric analysis (TGA) and differential scanning calorimetry (DSC). TGA studies revealed an increase in thermal stability of the PSU/MWCNTs nanocomposites, which is due to the hindrance of the nanodispered carbon nanotubes to the thermal transfer in nanocomposites and also due to higher thermal stability of CNTs. Morphological properties of nanocomposites were characterized by high resolution transmission electron microscopy (HRTEM) and field emission scanning electron microscope (FESEM). The influence of CNTs loading on electrical properties of PSU/MWCNTs nanocomposites was studied by the measurement of AC and DC resistivity. Dielectric study of nanocomposites was carried out at different frequencies (10 Hz–1 MHz) by using LCR meter. An increase in dielectric constant and dielectric loss was observed with increase in CNTs content, which is due to the interfacial polarization between conducting CNTs and PSU.     

Visualization and functions of surface defects on carbon nanotubes created by catalytic etching

Surface defects were created on carbon nanotubes (CNTs) by catalytic steam gasification or catalytic etching with iron as catalysts. The structure and morphology of the etched CNTs were studied by transmission electron microscopy (TEM) and scanning tunneling microscopy (STM). The electronic structure of the etched CNTs was investigated by ultraviolet photoelectron spectroscopy (UPS). The etched CNTs were treated by nitric acid to obtain oxygen-containing functional groups. The amount and the thermal stability of these groups were studied by temperature-resolved X-ray photoelectron spectroscopy (XPS). Temperature-programmed desorption with ammonia as a probe molecule (NH₃-TPD) was employed to investigate the interaction of the surface defects with foreign molecules in gas phase. TEM and STM studies disclosed the presence of surface defects especially edge planes on the etched CNTs. Etching of CNTs led to a less pronounced p-π band than the as-is CNTs, as evidenced by UPS studies. The XPS and NH₃-TPD studies demonstrated that the defects on the CNTs enhanced the reactivity of the exposed surfaces allowing obtaining a higher degree of oxygen functionalization and more active adsorption sites.     

The bulk polymerisation of N-vinylcarbazole in the presence of both multi- and single-walled carbon nanotubes: A comparative study

The bulk polymerisation of N-vinylcarbazole (NVC) at an elevated temperature in the presence of both multi- and single-walled carbon nanotubes (CNTs) leads to the formation of two different types of composite materials, the morphology and properties of which were characterised by a field emission scanning electron microscopy (FE-SEM), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis, and electrical property measurements. The efficiency of CNTs to initiate the NVC polymerisation was investigated using both multi-walled CNTs (MWCNTs) and single-walled CNTs (SWCNTs). The focus was on three major aspects: the degree of polymerisation, the morphology and the properties of the resulting nanocomposite materials. Results showed that SWCNTs were more efficient in initiating NVC polymerisation than MWCNTs, and the morphology of resultant nanocomposites revealed wrapping and grafting of some poly(N-vinylcarbazole) (PNVC) chains on the SWCNT surfaces. The morphology of the PNVC/MWCNT nanocomposites showed only homogeneous wrapping of the outer surfaces of MWCNTs by PNVC chains. The direct current (dc) electrical conductivity of pure PNVC improved dramatically in the presence of both MWCNTs and SWCNTs, however, the extent of improvement is higher in the case of PNVC/MWCNT nanocomposites.     

Experimental assessment on potential for processiblity of carbon nanotubes/epoxy system used as nanocomposites

In this study, carboxylic acid functionalized carbon nanotubes (CNTs) were used to modify epoxy with intent to develop a nanocomposite matrix for hybrid multiscale composites combining benefits of nanoscale reinforcement with well‐established fibrous composites. CNTs were dispersed in epoxy by using high energy sonication, followed by the fabrication of epoxy/CNTs composites. The processibility of CNTs/epoxy systems was explored with respect to their dispersion state and viscosity. The dependences of viscosity, mechanical and thermomechanical properties of nanocomposite system on CNTs content were investigated. The dispersion quality and reagglomeration behavior of CNTs in epoxy and the capillary infiltration of continuous fiber with the epoxy/CNTs dispersion were characterized using optical microscope and capillary experiment. As compared with neat epoxy sample, the CNTs nanocomposites exhibit flexural strength of 126.5 MPa for 1 wt% CNTs content and impact strength of 28.9 kJ m^?2 for 0.1 wt% CNTs content, respectively. A CNTs loading of 0.1 wt% significantly improved the glass transition temperatures, T_g, of the nanocomposites. Scanning electron microscopy (SEM) was used to examine the fracture surface of the failed specimens. It is demonstrated that the properties of CNTs/epoxy system are dispersion‐dominated and interface sensitive. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

CeOx/TiO2-yFy nanocomposite: An efficient electron and oxygen tuning mechanism for photocatalytic inactivation of water-bloom algae

Eutrophication raises a widespread problem for rivers and lakes all over the world, which serves as a seedbed for uncontrollable growth of harmful algae. Here, we prepare CeO_x/TiO_2-yF_y nanocomposites by a facile sol-gel method to offer an efficient solution for inactivation of harmful algae such as Microcystis aeruginosa with visible light irradiation. The results show that the nanocomposites hold ideal CeO_x coupling and F doping forms when the calcination temperature is 550?°C (CeO_x/TiO_2-yF_y-550). The CeO_x/TiO_2-yF_y-550 nanocomposite exhibits a higher light absorption capacity and lower recombination efficiency for photogenerated carriers in comparison to the CeO_x/TiO_2-yF_y nanocomposite. Moreover, it exhibits an excellent photocatalytic inactivation efficiency of 100% following 4?h irradiation. We also find that the photosynthetic efficiency of algal cells reduces in the inactivation process, and the electron transport process in the photosynthetic system is inhibited, which are ascribed to the formation of hydroxyl radicals (·OH) and superoxide radical (·O₂^-). Such an efficient photocatalytic inactivation performance for the CeO_x/TiO_2-yF_y-550 can be attributed to the outstanding contribution of ·O₂^- resulting from the electron and oxygen tuning by CeO_x coupling and F doping.