Graphene and carbon nanotube composite electrodes for supercapacitors with ultra-high energy density

We describe a graphene and single-walled carbon nanotube (SWCNT) composite film prepared by a blending process for use as electrodes in high energy density supercapacitors. Specific capacitances of 290.6 F g(-1) and 201.0 F g(-1) have been obtained for a single electrode in aqueous and organic electrolytes, respectively, using a more practical two-electrode testing system. In the organic electrolyte the energy density reached 62.8 Wh kg(-1) and the power density reached 58.5 kW kg(-1). The addition of single-walled carbon nanotubes raised the energy density by 23% and power density by 31% more than the graphene electrodes. The graphene/CNT electrodes exhibited an ultra-high energy density of 155.6 Wh kg(-1) in ionic liquid at room temperature. In addition, the specific capacitance increased by 29% after 1000 cycles in ionic liquid, indicating their excellent cyclicity. The SWCNTs acted as a conductive additive, spacer, and binder in the graphene/CNT supercapacitors. This work suggests that our graphene/CNT supercapacitors can be comparable to NiMH batteries in performance and are promising for applications in hybrid vehicles and electric vehicles.     

Three-dimensional manganese dioxide-functionalized carbon nanotube electrodes for electrochemical supercapacitors

Three-dimensional manganese dioxide (MnO₂)-functionalized multiwalled carbon nanotube (MWCNT) electrodes have been produced by a simple and scalable thermal decomposition process. The electrodes are prepared by treating planar MWCNT sheets with manganese(II) nitrate (Mn(NO₃)₂) solution and annealing at low temperature (200–300 °C) and ambient pressure. The morphology, chemical composition, and structure of the resulting matrices have been investigated with scanning electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and X-ray diffraction. Supercapacitors assembled with three-dimensional electrodes exhibit a 14-fold increase in specific capacitance (C _sp) in comparison to those containing pristine, two-dimensional MWCNT electrodes. C _sp varies linearly with Mn(NO₃)₂ thermal decomposition temperature (from 100 to 61 F/g at 0.2 A/g), a trend that is discussed in the context of nitrate reaction chemistry and MWCNT structure. This efficient and promising approach allows for simultaneous enhancement of electrode–electrolyte contact area and incorporation of redox-based charge storage within electrochemical capacitors.     

Electrochemical sensing of bisphenol A on single-walled carbon nanotube paper electrodes

The electrochemical detection of BPA often requires modification of electrodes to overcome BPA′s slower kinetics and higher oxidation potential. This work reports a modification-free, paper electrode based on vacuum-filtered SWCNT thin film. The prepared electrode does not need to be polished or transferred into the conducting substrates. The linear sweep voltammetric detection showed a linear response from 0.5–10 μM and 25–100 μM with the experimental LOD of 1.0 μM (S/N=3). The interference study and good recovery percentage (93–105 %) in real water samples demonstrated the method‘s selectivity. The sensor can be promising for developing a simple, low-cost, portable, and paper-based BPA monitoring system.     

Dependence of single-walled carbon nanotube adsorption kinetics on temperature and binding energy

We present results for the isothermal adsorption kinetics of methane, hydrogen, and tetrafluoromethane on closed-ended single-walled carbon nanotubes. In these experiments, we monitor the pressure decrease as a function of time as equilibrium is approached, after a dose of gas is added to the cell containing the nanotubes. The measurements were performed at different fractional coverages limited to the first layer. The results indicate that, for a given coverage and temperature, the equilibration time is an increasing function of E/(k(B)T), where E is the binding energy of the adsorbate and k(B)T is the thermal energy. These findings are consistent with recent theoretical predictions and computer simulations results that we use to interpret the experimental measurements.     

Large-scale fabrication of aligned single-walled carbon nanotube array and hierarchical single-walled carbon nanotube assembly

Aligned single-walled carbon nanotubes (SWNTs) and hierarchical SWNT assembly were fabricated by electrospinning. The high fiber elongation and high DC electric field applied during the electrospinning process result in the orientation of the SWNTs along the axial direction of the fiber. The alignment of the electropsun composite fiber transfers this local SWNT orientation to macroscopically aligned SWNTs. After removing the polymer component from the aligned composite fiber, we produced large area aligned SWNTs. The results show that the directional control of SWNT alignment and debundling of SWNTs into individual tubes can be simultaneously realized.     

Detection of dopamine using self-assembled diazoresin/single-walled carbon nanotube modified electrodes

Ultrathin films of diazoresin(DR)/single-walled carbon nanotube(SWNT) were fabricated on thioglycollic acid(TGA) decorated gold(Au) electrodes by the self-assembly method combined with the photocrosslinlcing technique.The electrochemical behavior of dopamine(DA) at the DR/SWNT modified electrodes was studied using the cyclic voltammetry(CV) and differential pulse voltammetry(DPV) methods.Under the optimal conditions,a linear CV response to DA concentration from 1 μmol/L to 40 μmol/L was observed,and the detection limit of DA was 2.1 ×10~(-3) μmol/L via the DPV method in the presence of 10 μmol/L of uric acid(UA) or 2.5 × 10~(-3) μmol/L via the DPV method in the presence of10 μmol/L of ascorbic acid(AA).Moreover,the modified electrodes exhibited good reproducibility and sensitivity,demonstrating its feasibility for analytical purposes.     

Influence of carbon shell structure on electrochemical performance of multi-walled carbon nanotube electrodes

Core/shell nanostructures have received considerable attention due to the synergistic effect of their combination of materials. In this work, core/shell carbon/multi walled carbon nanotubes (MWNTs) (C-MWNTs) composed of core MWNTs and carbon shells were prepared to obtain a new type of carbon electrode materials. Carbon shells containing nitrogen groups were prepared by coating polyaniline (PANI) onto the MWNTs by in situ polymerization and subsequent carbonization at 850 °C. After carbonization, the C-MWNTs contained 5.84% nitrogen and showed a hollow structure and crystallinity like that of pristine MWNTs. In addition, the C-MWNTs exhibited electrochemical performance superior to that of pristine MWNTs, and the highest specific capacitance (231 F g⁻¹) of the C-MWNTs was obtained at a scan rate of 0.1 A g⁻¹, as compared to 152 F g⁻¹ for pristine MWNTs. This superior performance is attributed to the maintenance of high electrical conductivity by the π–π interaction between the carbon layer and the MWNTs, increased specific surface area of C-MWNTs, and the presence of nitrogen groups formed on the carbon electrode after the carbonization of the shell PANI.     

Exciton energy transfer-assisted photoluminescence brightening from freestanding single-walled carbon nanotube bundles

Photoluminescence (PL) brightening is clearly observed through the direct morphology transition from isolated to thin bundled vertically- and individually freestanding single-walled carbon nanotubes (SWNTs). On the basis of the precise spectra analysis and equation-based estimation of the PL time trace, the origin of the PL brightening is consistently explained in terms of the exciton energy transfer through the tube bundles. The PL brightening is also revealed to obviously depend on SWNT diameters. Only the small-diameter rich sample can realize the PL brightening, which can be explained by the different concentrations of metallic SWNTs causing a PL quenching. Since it can be possible to fabricate brightly illuminating nanotubes on various kinds of substrates, the bundle engineering with freestanding nanotubes is expected to be a potential candidate for realizing the nanotube-based PL device fabrication.     

Glassy carbon electrodes modified with DNA-partly-wrapped single-walled carbon nanotubes

DNA-partly-wrapped single-walled carbon nanotubes (DNA-p-SWCNTs) were separated from the mixtures of calf thymus DNA and SWCNTs in solution by differential centrifugation for the first time. Average mass ratios of DNA to SWCNTs for DNA-p-SWCNTs and DNA-fully-wrapped-SWCNTs (DNA-f-SWCNTs) were determined to be 0.8 and 2.0, respectively. It has been found that DNA-p-SWCNTs could form a uniform and porous film on glassy carbon electrodes due to special structure of them, which could facilitate the electron transfer between positively-charged compounds and electrodes, and showed good enrichment capability at low ionic strength.     

Impact of polypyrrole incorporation on nickel oxide@multi walled carbon nanotube composite for application in supercapacitors

In this article we report the synthesis of polypyrrole incorporated nickel oxide multi walled carbon nanotube (NiO@NMWCNT/PPy) composites by thermal reduction protocol for supercapacitor applications. The structural and morphological properties of the composites were confirmed by the aid of X-ray diffraction (XRD), Field-emission scanning electron microscope (FE-SEM) with energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and Field-emission transmission electron microscopy (FE-TEM) analysis indicating the hexagonal crystal structure of NiO decorated on NMWCNT/Ppy. The electrochemical characteristics of the NiO@MWCNT/PPy composite were analyzed in the presence of 2 M KOH as an electrolyte. The NiO@NMWCNT/PPy nanostructured composite produced a plenty of active sites for ion migration reactions that facilitate the energy storage mechanism. As a proof of concept demonstration, the NiO@NMWCNT/PPy composite was explored as an electrode materials in supercapacitor and exhibited specific capacitance of 395 F g⁻¹ and cyclic stability up to 5000 cycles at 0.5 A g⁻¹. Enhanced performance of composite is attributed to the incorporation of polypyrrole in NiO@NMWCNT. The improved capacitance and cyclic stability demonstrated by the composite indicates the NiO@NMWCNT/PPy to be a promising candidate for supercapacitor applications.     

Electronic properties of single-walled carbon nanotube networks

We present a study on the electronic behavior of films of as-prepared and purified single-walled carbon nanotubes (SWNTs) and demonstrate the important role that chemical functionalization plays in modifying their electronic properties, which in turn throws further light on the mechanism of action of SWNT-based sensors. Films of electric arc SWNTs were prepared by spraying, and optical spectroscopy was used to measure the effective film thickness. The room-temperature conductivities (sigma(RT)) of thin films deposited from as-prepared and purified SWNTs are in the range sigma(RT) = 250-400 S/cm, and the nonmetallic temperature dependence of the conductivity indicates the presence of tunneling barriers, which dominate the film conductivity. Chemical functionalization of SWNTs with octadecylamine (ODA) and poly(m-aminobenzenesulfonic acid) (PABS) significantly decreases the conductivity; sigma(RT) = 3 and 0.3 S/cm for SWNT-ODA and SWNT-PABS, respectively.     

Dispersing as-prepared single-walled carbon nanotube powders with linear conjugated polymers

Suitably modified linear conjugated poly(arylene ethynylene)s are able to assist effective debundling and dispersion of crude as-prepared single-walled carbon nanotube powders in organic solvents, the dispersion of which is effected via a surface coating mechanism and, to some extent, in a size-selective fashion.     

Recent progress in chemical detection with single-walled carbon nanotube networks

Single-walled carbon nanotubes (SWNTs) have had significant impact on the development of gas sensors in the last decade. However, useful applications of SWNTs are limited by the lack of manufacturable routes to device formation. This Highlight article chronicles recent progress in this area and demonstrates the great promise of a new room temperature deposition method for SWNT networks in gas sensing applications. This liquid deposition technique allows the deposition of pre-treated, highly aligned SWNT networks on a wide variety of substrates. A significant advantage of SWNT-network sensors is that fluctuations in the electrical response of individual SWNTs become less important as the size of the network increases. Therefore, device properties can be controlled by the overall density of the network rather than the physical properties of any individual SWNT. At densities where semiconducting pathways dominate, highly sensitive thin-film chemoresistive sensors can be fabricated. Such devices also have higher signal-to-noise ratios and are easier to fabricate than devices based on a single SWNT.     

Direct fabrication of single-walled carbon nanotube macro-films on flexible substrates

We employed a floating chemical vapor deposition technique and applied a liquid (solution)-free precursor system for the fabrication of single-walled carbon nanotube macro-films on various flexible substrates from metallic foils to polymer films.     

Electrochemical and ESR study of the mechanism of oxidation of phenazine-di-N-oxide in the presence of cyclohexanol on glassy carbon and single-walled carbon nanotube electrodes

The mechanism of oxidation of phenazine-di-N-oxide in the presence of cyclohexanol was studied by cyclic voltammetry on glassy carbon (GC) and single-walled carbon nanotube (SWCNT) electrodes in 0.1 M LiClO₄ solutions in acetonitrile. The effect of cyclohexanol on the shape of the cyclic voltammograms of phenazine-di-N-oxide and the intensity of the ESR signal of its radical cation was investigated. It was shown by ESR that the products of the one-electron oxidation and reduction of phenazine-di-N-oxide were radical cations and anions. The catalytic currents were recorded during the oxidation of phenazine-di-N-oxide on the SWCNT and GC electrodes in the presence of cyclohexanol. The results were explained in terms of the E₁C₁E₂C₂ mechanism of the two-stage electrode process characterized by the catalytic current recorded at the second electrode stage. The overall two-electron catalytic oxidation of cyclohexanol in the complex with the phenazine-di-N-oxide radical cation was assumed to occur. It was shown that SWCNT electrodes can be used in the electrocatalytic oxidation of organic compounds in the presence of the electrochemically generated phenazine-di-N-oxide radical cation.     

Human fibrinogen adsorption onto single-walled carbon nanotube films

The adsorption behavior of human fibrinogen (Hfg) on single-walled carbon nanotube (SWNT) films was investigated using scanning electron microscopy (SEM) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. It was shown in the SEM images that fibrinogen was adsorbed strongly on the surface of SWNT when the samples were incubated in the Hfg solutions for 10 min. The dependence of adsorption on the concentration of fibrinogen was also investigated and it was found that adsorption increased with increasing concentration. In order to further explore the adsorption of fibrinogen on SWNT surface, NEXAFS spectra were obtained at the N K-edge and the C K-edge. The results confirmed the conclusion regarding the dependence of adsorption on fibrinogen concentration. It is demonstrated that, due to its high sensitivity to the surface elements, NEXAFS spectroscopy is a powerful tool to investigate the adsorption of fibrinogen on SWNT films.     

Hydrogen-bonded and physisorbed CO in single-walled carbon nanotube bundles

Fourier transform infrared spectroscopy is used to study CO adsorption in single-walled carbon nanotubes. Evidence for adsorption in endohedral and groove/external surface sites is presented through displacement studies involving both CO and CO2. Blue-shifted CO stretching frequencies also indicate that CO hydrogen bonds to hydroxyl functionalities created on the nanotubes by acid purification steps. N2 surface area measurements are used to further understand the porosity of the nanotube samples and to help explain the spectroscopic results.