K. Méténier

Supercapacitor electrodes from multiwalled carbon nanotubes

Electrochemical characteristics of supercapacitors built from multiwalled carbon nanotubes electrodes have been investigated and correlated with microtexture and elemental composition of the materials. Capacitance has been estimated by cyclovoltammetry at different scan rates from 1 to 10 mV/s, galvanostatic discharge, and impedance spectroscopy in the frequency range from 100 kHz to 1 mHz. The presence of mesopores due to the central canal and/or entanglement is at the origin of an easy accessibility of the ions to the electrode/electrolyte interface for charging the electrical double layer. Pure electrostatic attraction of ions as well as quick pseudofaradaic reactions have been detected upon varying surface functionality. The values of specific capacitance varied from 4 to 135 F/g, depending on the type of nanotubes or/and their posttreatments. Even with moderate specific surface area (below 470 m2/g), due to their accessible mesopores, multiwalled carbon nanotubes represent attractive materials for su...

Electrochemical insertion of lithium in catalytic multi-walled carbon nanotubes

Electrochemical lithium insertion was studied into purified and heat-treated catalytic multi-walled carbon nanotubes. It appears that the irreversible capacity for the MWNTs is relatively large, but decreasing with annealing temperature. This clearly shows that the intrinsic entanglement and the microtexture of the nanotubes must be responsible for this drawback of any potential application as an anode. The crucial role of the charge cut-off on the «traditional» intercalation was underlined and the reversible capacity was assigned to particular Li sites by high resolution NMR spectroscopy.

Lithium Insertion in Carbon Nanotubes

Abstract Different types of multi-walled carbon nanotubes obtained by catalytic decomposition of acetylene or pyrolysis of propylene on an alumina template have been investigated for lithium storage in aprotic medium. The samples presented a high reversible storage capacity of lithium, ranging from 350 mAh/g to 780 mAh/g, depending on their physico-chemical characteristics. It has been found that the mesoporous character of the nanotubes is responsible for an important overvoltage between insertion and extraction of lithium. Especially swelling mesopores, created by the entanglement of flexible nanotubes, facilitate charging of an electrical double layer. This contributes to the important reversible capacity observed in the range from 1 V to 3 V vs Li (up to 580 mAh/g). On the other hand, solvated lithium cations easily penetrate the mesopores and the solvent molecules are decomposed at the carbon surface forming the solid electrolyte interphase (SEI). That is at the origin of the important irreversible capacity ranging from 570 mAh/g to 1080 mAh/g. Due to their pseudo-capacitance properties. these nanotubes are very promising as electrode materials of electrochemical capacitors.

Capacitance properties of carbon nanotubes

Electrochemical cumulation of charges has been investigated in capacitor electrodes made from carbon multiwall nanotubes (MWNT) produced either by catalytic decomposition of acetylene on supported cobalt or by template carbonization of propylene. A good correlation has been found between the values of capacitance and the microtextural characteristics of the nanotubes. The highest specific capacitance, of the order of 70 F/g in 6M KOH electrolyte, could be reached for catalytic MWNT presenting mesopores due to the canal and to entanglement.

Doping of Carbon Nanotubes by Heavy Alkali Metals

Abstract Multiwall (MWNT) and single wall (SWNT) carbon nanotubes were intercalated with heavy alkali metals. From the point of view of their composition, alkali 2D superlattice, EPR and 13C NMR characteristics, the intercalation compounds of MWNT (1st and 2nd stage) are close to their parent GIC. An expansion of the 2D triangular lattice of SWNT bundles was clearly detected, showing that the alkali atoms are intercalated in the free space between the tubes.

Designing nanostructured carbons for the negative electrode of lithium batteries

Various parameters such as mesopore volume, microtexture, surface area, dangling bonds which can affect the value of irreversible capacity C irr of a carbon anode have been considered. A new hypothesis is suggested and confirmed that active surface sites are at the origin of C irr but not the specific surface area of carbon. An ideal proportional dependence has been found between C irr and mesopore volume in the case of nanotubular materials. Surface modification of carbon fibres by a pyrolytic carbon coating has been successfully realized with an improvement of lithium insertion characteristics.

Intercalation reactions in catalytic multiwall carbon nanotubes

Heat-treated catalytic multiwall carbon nanotubes (MWNTs) were intercalated by K and FeCl3 in vapor phase, using the two-bulb technique. A first stage KC9 intercalation compound was formed with potassium. After elimination of potassium, the tubular morphology is still preserved showing that intercalation is a reversible phenomenon. In the case of FeCl3, the saturated compound is less rich than with graphite. However, well defined in plane hk bands prove the intercalation. Due to the position of the 002 line at 0.345 nm, it is likely that intercalation is incomplete and that the material is a mixture of intercalated and non intercalated zones. A model of catalytic nanotubes is presented which accounts for the reversibility of the intercalation reactions.

Intercalation of heavy alkali metals (K, Rb and Cs) in the bundles of single wall nanotubes

The electric-arc discharge carbon deposits (collaret) containing Single Wall Carbon Nanotubes (SWNTs) were heat treated at 1600 °C during 2 days under N2 flow in order to eliminate the Ni catalyst by sublimation, without modifications of the SWNTs ropes. Sorting this deposit by gravity enabled to obtain in the coarsest particles higher amount of SWNTs ropes than in other particle sizes. The coarser particles of the carbon deposits were reacted with the alkali metals vapor giving intercalated samples with a MC8 composition. The intercalation led to an expansion of the 2D lattice of the SWNTs so that the alkali metals were intercalated in between the tubes within the bundles. Disordered lattices were observed after intercalation of Rb and Cs. The simulations of the X-ray diffractograms of SWNTs reacted with K, gave the best fit for three K ions occupying the inter-tubes triangular cavities. The investigations by EPR, and 13C NMR, showed that doped carbon deposits are metallic.

Alkali-metal intercalation in carbon nanotubes

We report on successful intercalation of multiwall (MWNT) and single wall (SWNT) carbon nanotubes with alkali metals by electrochemical and vapor phase reactions. A LiC10 compound was produced by full electrochemical reduction of MWNT. KC8 and CsC8-MWNT first stage derivatives were synthesized in conditions of alkali vapor saturation. Their identity periods and the 2×2 R 0° alkali superlattice are comparable to their parent graphite compounds. The dysonian shape of KC8 EPR line and the temperature-independent Pauli susceptibility are both characteristic of a metallic behavior, which was confirmed by 13C NMR anisotropic shifts. Exposure of SWNT bundles to alkali vapor led to an increase of the pristine triangular lattice from 1.67 nm to 1.85 nm and 1.87 nm for potassium and rubidium, respectively.