Sandrine Delpeux

Nanotubular materials for supercapacitors

Different types of multi-walled (MWNTs) and single-walled nanotubes (SWNTs) have been considered as active electrode materials for the storage of energy in supercapacitors. Due to their unique mesoporosity, these materials have a high ability for the accumulation of charges in the electrode/electrolyte interface. MWNTs supply twice higher values of capacitance in comparison to SWNTs. The nanotubular materials of high purity point out a box-like shape of voltammetry characteristics that proves an entirely electrostatic attraction. Pseudocapacitance effects are observed if metallic particles are present and after additional functionalization of the nanotubes or deposition of conducting polypyrrole (PPy). The value of capacitance obtained from nanotubes modified by PPy reaches 170 F/g, about twice that given either by the nanotubes (ca. 80 F/g) or by pure PPy (ca. 90 F/g). The open entangled network of the nanocomposite seems to favour a better efficiency for the formation of the electrical layer in PPy.

Enhanced capacitance of carbon nanotubes through chemical activation

Abstract Microporosity of pure multi-walled carbon nanotubes (MWNTs) has been highly developed using chemical KOH activation. Depending on the nanotubular material, the burn-off ranged from 20% to 45% after the activation process. At least twofold increase of surface area has been obtained with maximum values of ca. 1050 m 2 / g for KOH/C ratio of 4:1. The activated material still possesses a nanotubular morphology with many defects on the outer walls that give a significant increase of micropore volume, while keeping a noticeable mesoporosity. Such activated MWNTs have been used as electrode material for supercapacitors in alkaline, acidic and aprotic medium. Enhanced values of capacitance were always observed after activation: in some cases it increased almost seven times from 15 F/g (for non-activated nanotubes) to 90 F/g (after chemical activation).

Nanotubular materials as electrodes for supercapacitors

Different modifications of multiwalled carbon nanotubes (MWNTs) have been performed to increase specific capacitance of this material for supercapacitor application. MWNTs used in this work were obtained by decomposition of acetylene at 600 and 700 °C on cobalt catalyst supported on NaY zeolite, silica or produced from a CoxMg1−xO solid solution. Coating of MWNTs by conducting polymers, e.g. polypyrrole (PPy) has been successfully realized and specific capacitance increased from ca. 50 to 180 F/g demonstrating a synergy between the two components of such nanocomposite. The open mesoporous network of well conducting nanotubular material allows an easy access of ions to the electrode/electrolyte interface and more effective contribution of the pseudofaradaic properties of PPy. Long durability (over 2000 cycles) was obtained for such supercapacitor material. The second way of MWNTs modification was their chemical activation by KOH. In this case, a significant enhancement of microporosity has been obtained and specific capacitance of nanotubular material increased from 15 to 90 F/g.

High yield of pure multiwalled carbon nanotubes from the catalytic decomposition of acetylene on in-situ formed cobalt nanoparticles.

For the first time, multiwalled carbon nanotubes (MWNTs) could be formed selectively in a high yield, free of any disordered carbon by-product, from the catalytic decomposition of acetylene at 600 degrees C on a CoxMg(1-x)O solid solution. Starting from 1 g of catalytic substrate, 4 g of pure MWNTs were obtained after its dissolution in boiling concentrated HCl, without any additional purification in strongly oxidizing medium, as is required for other methods of nanotube production. In situ reduction of CoO by dihydrogen liberated from acetylene decomposition allows highly divided metal particles to be continuously produced as synthesis proceeds. This is undoubtedly the reason for the good performance of the catalyst and for the ability to produce nanotubes in a narrow diameter range, namely from 10 to 15 nm. With the use of acetylene instead of methane, the synthesis proceeds at low temperature, which prevents the growth of carbon shells, in which the metal particles are generally embedded, decreasing their activity. Because of the very low specific surface area of the catalyst support, the amount of disordered carbon by-product formed is negligible.

Functionalization of multiwall carbon nanotubes: Properties of nanotubes-epoxy composites

Multiwall nanotubes were functionalized using plasma treatments, chemical oxidation, ball milling and thermal treatments. In optimized conditions, plasmas modify nanotubes surface chemistry with a great selectivity. Vickers microindentation and tension tests performed on epoxy resin loaded with multiwall nanotubes allow comparison of the influence of nanotubes surface chemistry and microtexture on loaded resin mechanical properties.

Synergy of components in supercapacitors based on nanotube/polypyrrole composites

Multiwalled nanotubes (MWNTs) composites with electrodeposited polypyrrole (PPy) have been proposed as electrode materials for supercapacitors. The total capacitance of the nanocomposite combines a pure electrostatic attraction of ions and the electrochemical redox reactions of the -conjugated system of PPy. A synergy effect has been found between the two components of the nanocomposite. The mesoporous network of the nanotubular material supplies very good transport conditions for ions, and PPy enhances its conducting properties. A high ability of charge accumulation in such volumetric capacitor was obtained with capacitance values of 165 F/g of material.

High yield of multiwalled carbon nanotubes from the decomposition of acetylene on Co/MgO catalyst

A new catalytic method of carbon nanotubes synthesis, by the decomposition of acetylene at 600 °C over particles of CoxMg(1−x)O solid solution, gives high yield of carbon nanotubes. CoMgO catalyst have been prepared from Co nitrate hexahydrate, Mg nitrate hexahydrate and citric acid dissolved in water solution then gently evaporated to solid solution and finally calcinated under N2 during 5 hours at 700 °C. The influence of different parameters on the quality and quantity of the produced nanotubes was investigated such as the nature of support, reaction time and velocity of hydrocarbon along the catalyst. Separation of the nanotubes from the catalyst particles was carried out by 12 mol.l−1 hydrochloric acid treatments at 110 °C during 12 hours. The quality of the nanotubes production was characterized by scanning electron microscopy (SEM) and the nanotexture was investigated by transmission electron microscopy (TEM). The standard external diameters obtained, range between 10–20 nm. Presented method is an ...

Electrochemical Application of Carbon Nanotubes

Different types of nanotubes have been used for storage of energy, e.g. lithium insertion and electrochemical capacitors. In both cases the electrochemical properties of nanotubes are dominated by their mesoporous character mainly due to the entanglement and a presence of a central canal. The most promising electrochemical application of carbon nanotubes seems to be their usage for supercapacitors. Capacitance values of pristine nanotubes varied widely from 10 to 80 F/g depending mainly on their microtexture, presence of central canal, amorphous carbon and catalyst impurities. Special modification has been performed to increase the capacitance values. For instance, a coating of nanotubes by conducting polymers, e.g. polypyrrole (PPy) supply pseudocapacitive effects connected with quick faradaic reactions. The open entangled network of the nanotubular composite seems to form a volumetric electrochemical capacitor where the charge has a threedimensional distribution. On the other hand, a significant increase of capacitance was reached by development of nanotubes specific surface area, i.e. micropore volume, through chemical KOH activation. Modified nanotubes and their nanocomposites with PPy supplied capacitance values of 80 to 170 F/g, hence, they are attractive electrode materials for supercapacitors.

Functionalisation of carbon nanotubes for composites

Catalytic multiwall carbon nanotubes (MWNT) were functionalized by low-pressure ammonia plasma and chemical oxidation, and their surface groups were identified by X-ray Photoelectron Spectroscopy (XPS) and acid-base titration. The reactivity of MWNT with ammonia plasma largely depends on their microtexture and on residual oxygen pressure in the reactor. Using catalytic MWNT presenting numerous dangling bonds on their outer part, the N/C atomic ratio could reach 0.18. Oxidation by sodium chlorate was very efficient (O/C atomic ratio=0.2) for the creation of surface carboxylic groups.

Characterization and Gas Adsorption on Multi‐Walled Carbon Nanotubes Before and After Controlled Chemical Opening

We report on the surface characterization of multi‐walled carbon nanotubes (MWNTs) before and after chemical opening using an HNO3 oxidation followed by a CO2 treatment. Transmission electron microscopy (TEM) and image analysis were performed to characterize the structural evolution. Adsorption isotherms were measured for Kr at 77.3 K to study the effect of this treatment on the surface crystallinity and to give evidence of the accessibility of the inner channels of the tubes. The specific surface area was determined for each sample as was the presence or absence of a hysteresis loop in the adsorption‐desorption cycle. The results lead to concluding that this treatment is very efficient for purification and selective opening but also results in significant decrease of the surface crystallinity.