Temperature stable supercapacitors based on ionic liquid and mixed functionalized carbon nanomaterials

The ionic liquid 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide (BDMIM-TFSI) showed a conductivity of 1.65?mS cm^?1 and an electrochemical stability window of 4.4?V at room temperature. Two types of electrodes based on carbon nanomaterials were prepared: (1) with alternating layers of two oppositely charged functionalized double-walled carbon nanotubes (DWCNTs) and (2) with the functionalized DWCNTs and graphene oxide nanoplatelets. The electrodes presented a porous morphology and a connected pathway between the carbon nanotubes and graphene oxide platelets. Electrochemical capacitors based on the carbon nanomaterials and BDMIM-TFSI were produced in a stacking configuration and were characterized at 25?°C, 60?°C, and 100?°C. The supercapacitors with electrodes based on the three alternating layers of two oppositely charged DWCNTs and graphene oxide presented higher values of capacitance, which were attributed to a morphology favorable to providing ionic access to the carbonaceous surface. Box-like voltammetric curves were used to calculate the capacitance in a 4-V potential window at 100?°C.     

Bulk synthesis of linear carbon chains confined inside single-wall carbon nanotubes by vacuum discharge

Carbyne, an infinite carbon chain, has attracted much interest and induced significant controversy for many decades. Recently, the presence of linear carbon chains (LCCs), which were confined stably inside double-wall carbon nanotubes (DWCNTs) and multiwall carbon nanotubes (MWCNTs), has been reported. In this study, we present a novel method to produce LCCs in a film of carbon nanotubes (CNTs). Our transmission electron microscopy and Raman spectroscopy revealed the formation of a bulk amount of LCCs after electric discharge of CNT films, which were used as field emission cathodes. The LCCs were confined inside single-wall CNTs as well as DWCNTs. Furthermore, two or three LCCs in parallel with each other are encapsulated when the inner diameter of CNT is larger than approximately 1.1 nm.     

Synthesis of double-walled carbon nanotubes by catalytic chemical vapor deposition and their field emission properties

Double-walled carbon nanotubes (DWCNTs) were synthesized by catalytic chemical vapor deposition using Fe-Mo/MgO as a catalyst at 1000 degrees C under the mixture of methane and hydrogen gas. The nanotubes were purified by acid but were not damaged. Thermogravimetric analysis revealed the purity of the tubes to be about 90%. The high-resolution transmission electron microscopy image showed that DWCNTs have inner tube diameters of 1.4-2.6 nm and outer tube diameters of 2.3-3.4 nm. We observed radial breathing modes in Raman spectra, which are related to the diameter of inner nanotubes. The purified DWCNTs were mixed with organic vehicles and glass frit, and then they were screen-printed on glass substrate coated with indium tin oxide. The field emission properties of the screen-printed DWCNT films were examined by varying the amount of glass frit ingredient within the DWCNT paste. The results showed that DWCNT emitters had good emission properties such as turn-on field of 1.33-1.78 V/microm and high brightness. When the applied anode voltage was gradually increased, current density and brightness became saturated. We also observed DWCNTs adsorbed on the anode plate; they were DWCNTs peeled off from the cathode plate for field emission measurement.     

Performance of dye-sensitized solar cells with various carbon nanotube counter electrodes

Double-wall carbon nanotubes (DWCNTs), single-wall carbon nanotubes (SWCNTs), and multi-wall carbon nanotubes (MWCNTs) were investigated as an alternative for platinum in counter-electrodes for dye-sensitized solar cells. The counter-electrodes were prepared on fluorine-doped tin oxide glass substrates by the screen printing technique from pastes of carbon nanotubes and organic binder. The solar cells were assembled from carbon nanotubes counter-electrodes and screen printed anodes made from titanium dioxide. The cells produced with DWCNTs, SWCNTs or MWCNTs have overall conversion efficiencies of 8.0%, 7.6% and 7.1%, respectively. Electrochemical impedance spectroscopy measurements revealed that DWCNTs displayed the highest catalytic activity for the reduction of tri-iodide ions. The large surface area and superior chemical stability of the DWCNTs facilitated the electron-transfer kinetics at the interface between counter-electrode and electrolyte and yielded the lowest transfer resistance, thereby improving the photovoltaic activity. A short-term stability test at moderate conditions confirmed the robustness of solar cells based on the use of DWCNTs, SWCNTs or MWCNTs.

Noncovalent functionalization of multiwalled and double‐walled carbon nanotubes: Positive effect of the filler functionalization on high glass transition temperature epoxy resins

Double‐walled carbon nanotubes (DWCNTs) and multiwalled carbon nanotubes (MWCNTs) were modified using melamine to attach ? NH₂ to the surface of these fillers, without previous oxidation of their graphene layers. FT‐Raman, elemental (chemical) and thermogravimetric analysis, confirmed the modification, which was more extensive for DWCNTs. The potential of this modification was evaluated by adding the melamin‐modified nanotubes to thermosets based on diglycidyl ether of bisphenol A (resin) and polycyclic amine (hardener). Broadening of the glass transition interval and an increase between 7 and 8 °C of the glass transition temperatures show better filler/matrix interaction for the nanocomposites based on melamine‐modified nanotubes. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1860–1868, 2009     

A novel network composite cathode of LiFePO4/multiwalled carbon nanotubes with high rate capability for lithium ion batteries

A novel network composite cathode was prepared by mixing LiFePO₄ particles with multiwalled carbon nanotubes for high rate capability. LiFePO₄ particles were connected by multiwalled carbon nanotubes to form a three-dimensional network wiring. The web structure can improve electron transport and electrochemical activity effectively. The initial discharge capacity was improved to be 155 mA h/g at C/10 rate (0.05 mA/cm²) and 146 mA h/g at 1C rate. The comparative investigation on MWCNTs and acetylene black as a conducting additive in LiFePO₄ proved that MWCNTs addition was an effective way to increase rate capability and cycle efficiency.     

Tolerant cathode catalysts for direct methanol fuel cell

Effect of methanol on the reduction kinetics of oxygen on highly dispersed catalysts 60Pt/C (HiSPEC 9100), 40Pt/carbon nanotubes, and CoFe/carbon nanotubes for the cathode of a direct methanol-oxygen fuel cell was studied. It was shown that the CoFe/carbon nanotubes catalyst surpasses the platinum systems in tolerance to the alcohol. It was found that the tolerance of the cathode catalyst strongly affects the current–voltage characteristics of the fuel cell, which is the principal result of the study and constitutes its scientific novelty. The maximum power density of an alkaline methanol-oxygen fuel cell with nonplatinum cathode (260 mW cm^–2) exceeds the characteristics of similar fuel cells with platinum cathode catalysts, both obtained in the present study and described in the literature, which points to the practical importance of the study.     

Enzymatic electrodes nanostructured with functionalized carbon nanotubes for biofuel cell applications

Nanostructured bioelectrodes were designed and assembled into a biofuel cell with no separating membrane. The glassy carbon electrodes were modified with mediator-functionalized carbon nanotubes. Ferrocene (Fc) and 2,2′-azino-bis (3-ethylbenzothiazoline-6-sulfonate) diammonium salt (ABTS) bound chemically to the carbon nanotubes were found useful as mediators of the enzyme catalyzed electrode processes. Glucose oxidase from Aspergillus niger AM-11 and laccase from Cerrena unicolor C-139 were incorporated in a liquid-crystalline matrix-monoolein cubic phase. The carbon nanotubes–nanostructured electrode surface was covered with the cubic phase film containing the enzyme and acted as the catalytic surface for the oxidation of glucose and reduction of oxygen. Thanks to the mediating role of derivatized nanotubes the catalysis was almost ten times more efficient than on the GCE electrodes: catalytic current of glucose oxidation was 1 mA cm⁻² and oxygen reduction current exceeded 0.6 mA cm⁻². The open circuit voltage of the biofuel cell was 0.43 V. Application of carbon nanotubes increased the maximum power output of the constructed biofuel cell to 100 μW cm⁻² without stirring of the solution which was ca. 100 times more efficient than using the same bioelectrodes without nanotubes on the electrode surface.     

Double-wall carbon nanotubes: The outer shell may pattern the structure of the inner one

An existing model [L. Bellarosa, E. Bakalis, M. Melle-Franco, F. Zerbetto, Nano Lett. 6 (2006) 1950] predicts that the structures of the seven double-wall carbon nanotubes, DWCNTs, identified by high-resolution transmission electron microscopy [L. Guan, K. Suenaga, S. Iijima, Nano Lett. 8 (2008) 459] are the most stable. Since the samples were obtained from single-wall carbon nanotubes, SWCNTs, filled with ferrocene molecules that were annealed at 1273 K for 24 h, it is concluded that under proper conditions the host SWCNT patterns the formation of the inner one to form the most stable pair.     

Double‐Wall Carbon Nanotube–Porphyrin Supramolecular Hybrid: Synthesis and Photophysical Studies

Double‐wall carbon nanotubes (DWCNTs) with pyridyl units covalently attached to the external wall through isoxazolino linkers and carboxylic groups that have been esterified by pentyl chains are synthesized. The properties of these modified DWCNTs are then compared with an analogous sample based on single‐wall carbon nanotubes (SWCNTs). Raman spectroscopy shows the presence of characteristic radial breathing mode vibrations, confirming that the samples partly retain the integrity of the nanotubes in the case of DWCNTs, including the internal and external nanotubes. Quantification of the pyridyl content for both samples (DWCNT and SWCNT derivatives) is based on X‐ray photoelectron spectroscopy and thermogravimetric profiles, showing very similar substituent load. Both pyridyl‐containing nanotubes (DWCNTs and SWCNTs) form a complex with zinc porphyrin (ZnP), as evidenced by the presence of two isosbestic points in the absorption spectra of the porphyrin upon addition of the pyridyl‐functionalized nanotubes. Supramolecular complexes based on pyridyl‐substituted DWCNTs and SWCNTs quench the emission and the triplet excited state identically, through an energy‐transfer mechanism based on pre‐assembly of the ground state. Thus, the presence of the intact inner wall in DWCNTs does not influence the quenching behavior, with respect to SWCNTs, for energy‐transfer quenching with excited ZnP. These results sharply contrast with previous ones referring to electron‐transfer quenching, in which the double‐wall morphology of the nanotubes has been shown to considerably reduce the lifetime of charge separation, owing to faster electron mobility in DWCNTs compared to SWCNTs.     

The intermediate frequency modes of single- and double-walled carbon nanotubes: a Raman spectroscopic and in situ Raman spectroelectrochemical study

The intermediate frequency modes (IFM) of single-walled carbon nanotubes (SWCNTs) and double-walled carbon nanotubes (DWCNTs) were analyzed by Raman spectroscopy and in situ Raman spectroelectrochemistry. The inner and outer tubes of DWCNTs manifested themselves as distinct bands in the IFM region. This confirmed the diameter dependence of IFM frequencies. Furthermore, the analysis of inner tubes of DWCNTs allowed a more-precise assignment of the bands in the IFM region to features intrinsic for carbon nanotubes. Although the inner tubes in DWCNTs are assumed to be structurally perfect, the role of defects on IFM was discussed. The dependence of IFM on electrochemical charging was also studied. In situ spectroelectrochemical data provide a means to distinguish the bands of the outer and inner tubes.     

Power generation using an activated carbon and metal mesh cathode in a microbial fuel cell

An inexpensive activated carbon (AC) air cathode was developed as an alternative to a platinum-catalyzed electrode for oxygen reduction in a microbial fuel cell (MFC). AC was cold-pressed with a polytetrafluoroethylene (PTFE) binder to form the cathode around a Ni mesh current collector. This cathode construction avoided the need for carbon cloth or a metal catalyst, and produced a cathode with high activity for oxygen reduction at typical MFC current densities. Tests with the AC cathode produced a maximum power density of 1220 mW/m² (normalized to cathode projected surface area; 36 W/m³ based on liquid volume) compared to 1060 mW/m² obtained by Pt catalyzed carbon cloth cathode. The Coulombic efficiency ranged from 15% to 55%. These findings show that AC is a cost-effective material for achieving useful rates of oxygen reduction in air cathode MFCs.     

Molding versus dispersion: effect of the preparation procedure on the capacitive and cycle life of carbon nanotubes aerogel composites

Binderless carbon nanotubes aerogel (CNAG) composites represent a new class of high-performing electrodes for energy storage applications such as electrochemical double layer capacitors. The composites developed here differ significantly from these previously prepared with dispersion processes. The CNAG material was prepared by a molding procedure that is the synthesis by a chemical vapor deposition method to grow carbon nanotubes directly onto a microfibrous carbon paper substrate. Then the carbon aerogel is synthesized on the carbon nanotubes. The key feature of the method is eliminating the need of controlling the carbon nanotube concentration, which permits optimized dispersion processes to reinforce the aerogel's networks. The CNAG electrode delivered very high specific capacitances of 524 F g⁻¹ in KOH electrolyte and 280 F g⁻¹ in H₂SO₄ electrolyte. Furthermore, this better integration of carbon nanotubes in the matrix of carbon aerogel improved its resistance to the attack by the electrolyte and conferred an excellent cycle life over 5,000 cycles of charge–discharge in both electrolytes.     

Effect of conducting additives on the properties of composite cathodes for lithium-ion batteries

In an attempt to achieve lithium-ion batteries with high rate capability, the effect of conducting additives with various shapes and contents on the physical and electrochemical performances was evaluated. Although the density of the cathode decreased upon the addition of the additives, the electrical conductivity and electrochemical performance were greatly improved. The composite cathodes with well-dispersed multi-walled carbon nanotubes (MWCNTs) exhibited excellent high rate capabilities and cyclabilities. In the case of cathode with 8 wt.% of MWCNTs (low density—LD), the highest discharge capacity of 136 mAh/g was obtained at 5 C-rate and capacity retention of 97% for 50 cycles was observed at 1 C-rate of discharge. The cathode with a mixture of 2 wt.% of Super P and 4 wt.% of MWCNTs (LD) also exhibits improved cycle performances. The volume changes in the charge and discharge processes were successfully controlled by the bundles distributed between the host particles.     

Multi-walled carbon nanotubes as solid-phase extraction adsorbents for the speciation of cobalamins in seafoods by liquid chromatography

Multi-walled carbon nanotubes (MWCNTs) were evaluated as potential adsorbents for miniaturized solid-phase extraction coupled to liquid chromatography. The adsorption capacity of this sorbent was applied to assess the speciation of four cobalamins representing the various forms of vitamin B₁₂. The preconcentration on the MWCNTs was based on the retention of analytes by introducing the sample online into the mini-column system. Dimethyl sulfoxide was used to elute the retained vitamins for liquid chromatographic analysis. The experimental conditions of the continuous flow device, which affect the enrichment procedure, such as the type and amount of nanotubes, the volume, pH and flow rate of the sample solution, and the eluent and its volume, were optimized. For detection purposes, a diode array device was used and good resolution was obtained with a mobile-phase acetonitrile–phosphate buffer and gradient elution. Specificity was demonstrated by the retention characteristics and UV spectra and by comparing the peak purity index with commercial standards. Linearity, precision, recovery, and sensitivity were satisfactory. Detection limits ranged from 0.35 to 30 ng mL⁻¹. The method was successfully applied to the determination of cobalamins in seafoods, which were extracted from the sample with a buffer solution using an ultrasonic probe. The reliability of the procedure was checked by analyzing a certified reference material.     

Carbon nanotube enhanced mediator-type biosensor for real-time monitoring of glucose concentrations in fish

We have developed a mediator-type biosensor to rapidly monitor blood glucose concentrations in fish, which are an indicator of stress. Glucose oxidase was used to detect glucose concentrations and ferrocene was used to limit the effect of oxygen. We also improved the sensitivity and durability of the sensor for better performance. Single-walled carbon nanotubes were used to enhance sensor sensitivity. Affixing the carbon nanotubes (30 mg ml^-1) to the working electrode increased the sensor sensitivity to 61.9 mM nA^-1 mm^-2, twice the value for the sensor without single-walled carbon nanotubes. A fabricated mediator-type biosensor sensor was used to perform real-time in vivo measurements. The sensor was implanted into the interstitial fluid of a fish eyeball, and detection was transmitted to a personal computer by a wireless potentiostat. Continuous measurement of the glucose concentration was possible for 78 hours. Stress was artificially applied to the fish during the measurement, and the change of blood glucose concentrations were observed. Our proposed sensor is applicable for effectively monitoring stress in free-swimming fish.     

Selective and Scalable Chemical Removal of Thin Single‐Walled Carbon Nanotubes from their Mixtures with Double‐Walled Carbon Nanotubes

Double‐walled carbon nanotubes (DWCNTs) are materials in high demand due to their superior properties. However, it is very challenging to prepare DWCNTs samples of high purity. In particular, the removal of single‐walled carbon nanotubes (SWCNTs) contaminants is a major problem. Here, a procedure for a selective removal of thin‐diameter SWCNTs from their mixtures with DWCNTs by lithium vapor treatment is investigated. The results are evaluated by Raman spectroscopy and in situ Raman spectroelectrochemistry. It is shown that the amount of SWCNTs was reduced by about 35 % after lithium vapor treatment of the studied SWCNTs–DWCNTs mixture.     

Electrochemical reduction of oxygen on double-walled carbon nanotube modified glassy carbon electrodes in acid and alkaline solutions

The oxygen reduction reaction has been investigated on double-walled carbon nanotube (DWCNT) modified glassy carbon (GC) electrodes in acid and alkaline media using the rotating disk electrode (RDE) method. The surface morphology and composition of DWCNT samples was examined by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Aqueous suspensions of DWCNTs were prepared using Nafion and non-ionic surfactant Triton X-100 as dispersing agents. The RDE results indicated that the DWCNT modified GC electrodes are active catalysts for oxygen reduction in alkaline solution. In acid media DWCNT/GC electrodes possess poor electrocatalytic properties for O₂ reduction which indicates lack of metal catalyst impurities in the DWCNT material studied. The oxygen reduction behaviour of DWCNTs was similar to that of multi-walled carbon nanotubes (MWCNTs) observed in our previous studies.     

Electrophoretic Deposition of Multi-walled Carbon Nanotube on a Stainless Steel Electrode for use in Sediment Microbial Fuel Cells

Sediment microbial fuel cells (SMFCs) could be used as power sources and one type of new technology for the removal of organic matters in sediments. In order to improve electrode materials and enhance their effect on the performance, we deposited multi-walled carbon nanotube (MWNT) on stainless steel net (SSN). Electrophoretic deposition technique as a method with low cost, process simplicity, and thickness control was used for this electrode modification and produced this novel SSN-MWNT electrode. The performances of SMFCs with SSN-MWNT as electrode were investigated. The results showed that the maximum power density of SMFC with SSN-MWNT cathode was 31.6 mW m^?2, which was 3.2 times that of SMFC with an uncoated stainless steel cathode. However, no significant increase in the maximum power density of SMFC with SSN-MWNT anode was detected. Further electrochemical analysis showed that when SSN-MWNT was used as the cathode, the cathodic electrochemical activity and oxygen reduction rate were significantly improved. This study demonstrates that the electrophoretic deposition of carbon nanotubes on conductive substrate can be applied for improving the performance of SMFC.     

Functionalizing Single-Wall Carbon Nanotubes in Hollow Cathode Glow Discharges

A hollow cathode glow discharge was used to functionalize single-wall carbon nanotubes. This low temperature, solvent free, facile and fast process may be used to efficiently attach various functional groups (COOH, OH, CH, NH₂, NO₂ and NO) to the open ends and sidewalls of carbon nanotubes. The presented technique yields a broader set of functional groups being attached to the tubes as compared to other discharge routes. A rich functionalized surface provides an attractive scaffold for the further coupling of complex molecules, e.g., enzymes, antibodies. In situ optical emission spectroscopy investigations provided detailed information of the dynamic processes within the plasma itself. The findings show a gas temperature of 480 K and suggest the functionalization occurs through radical addition channels that are assisted by N₂ ⁺ radical ion collisions viz. N₂ ⁺ ion radical bombardment breaks C–C bonds on SWNTs surface opening a path for subsequent addition and quenching for other radical species.