Method for preparing a composite, composite thus obtained and uses thereof

Abstract

The present invention concerns a method for preparing a composite material comprising electrically conductive or semiconductive nano-objects of elongate shape and an electrically conductive polymer matrix, said method comprising a step consisting in electrochemically deposing said matrix on said nano-objects using a pulsed galvanostatic technique. The present invention also concerns the composite material thus obtained and uses thereof.

Claims

1. A method for preparing a composite material comprising electrically conductive or semiconductive nano-objects of elongate shape and an electrically conductive polymer matrix, said method comprising: (i) providing a carpet of electrically conductive or semiconductive nano-objects of elongate shape wherein the nano-objects are aligned in a vertical array; and (ii) electrochemically depositing, via a solution containing monomer(s) precursor(s) of the electrically conductive polymer matrix, said electrically conductive polymer matrix on said carpet of electrically conductive or semiconductive nano-objects in pulsed galvanostatic mode, said pulsed galvanostatic mode comprising an electropolymerization technique with at least two successive applications of a constant current density for a period t.sub.on, separated by a rest period t.sub.off without the application of any current or voltage, wherein the electrochemically depositing in the pulsed galvanostatic mode causes the electropolymerization of the matrix throughout an entire depth of the carpet without any modification of morphology of the carpet; wherein in the pulsed galvanostatic mode the duration of each rest period (t.sub.off) is greater than the duration of each period of application of a constant current density (t.sub.on) by a factor of between 2 and 5.

2. The method according to claim 1, wherein said electrically conductive or semiconductive nano-objects of elongate shape are selected from the group consisting of nanofibers, nanotubes and nanowires.

3. The method according to claim 1, wherein said electrically conductive or semiconductive nano-objects of elongate shape are in a material chosen from the group consisting of carbon, silicon, gold, silver, tantalum, nickel, platinum, copper, molybdenum, palladium, steel, stainless steel, zinc, boron nitride, zinc oxide, manganese oxide, gallium nitride, silicon nitride, tungsten disulfide, molybdenum disulfide, indium phosphide, tungsten selenide, molybdenum selenide, titanium dioxide, silicon dioxide, molybdenum trioxide, and mixtures thereof.

4. The method according to claim 1, wherein said electrically conductive polymer matrix is formed of one (or more) (co)polymers selected from the group consisting of the polyfluorenes, polypyrenes, polyazulenes, polynaphtalenes, polypyrroles, polycarbazoles, polyindoles, polyazepines, polyanilines, polythiophenes, poly(p-phenylene sulfides), polyacetylenes and poly(p-phenylene vinylenes).

5. The method according to claim 1, wherein, in the pulsed galvanostatic mode, a constant current density is applied for periods (t.sub.on) of duration between 10 and 100 s.

6. The method according to claim 1, wherein in the pulsed galvanostatic mode the duration of each rest period (t.sub.off) is greater than the duration of each period of application of a constant current density (t.sub.on) by a factor of 2.

7. The method according to claim 1, wherein step (ii) comprises: a) contacting the electrically conductive or semiconductive nano-objects of elongate shape with the solution (hereinafter designated solution (S)) containing the monomer(s) precursor(s) of said electrically conductive polymer matrix; b) polarising said nano-objects in pulsed galvanostatic mode, after which said electrically conductive polymer matrix is electrochemically deposited on said nano-objects.

8. The method according to claim 7, wherein prior to said step (a), the electrically conductive or semiconductive nano-objects of elongate shape are subjected to an oxidizing treatment (or pre-treatment).

9. The method according to claim 7, wherein said solution (S) is in the form of a pure, protic or aprotic solvent; in the form of an electrolytic solution containing, as solvent, a protic solvent or an aprotic solvent; or in the form of an ionic liquid.

10. The method according to claim 1, wherein, in the pulsed galvanostatic mode, a constant current density is applied for periods (t.sub.on) of duration between 20 and 80 s.

11. The method according to claim 1, wherein, in the pulsed galvanostatic mode, a constant current density is applied for periods (t.sub.on) of duration between 30 and 60 s.

12. The method according to claim 1, wherein in the pulsed galvanostatic mode the duration of each rest period (t.sub.off) is greater than the duration of each period of application of a constant current density (t.sub.on) by a factor of 5.

13. The method according to claim 8, wherein said solution (S) is in the form of a pure, protic or aprotic solvent; in the form of an electrolytic solution containing, as solvent, a protic solvent or an aprotic solvent; or in the form of an ionic liquid.

14. The method of claim 1, wherein step (ii) comprises impregnating the carpet of electrically conductive or semiconductive nano-objects with the electrically conductive polymer matrix.

15. The method of claim 1, wherein step (ii) is performed in an inert atmosphere.

16. The method of claim 1, wherein the composite material obtained is a self-supported material.

17. A method for preparing a composite material comprising electrically conductive or semiconductive nano-objects of elongate shape and an electrically conductive polymer matrix, said method comprising: (i) providing a carpet of electrically conductive or semiconductive nano-objects of elongate shape wherein the nano-objects are aligned in a vertical array; and (ii) electrochemically depositing, via a solution in the form of an ionic liquid containing monomer(s) precursor(s) of the electrically conductive polymer matrix, said electrically conductive polymer matrix on said carpet of electrically conductive or semiconductive nanoobjects in pulsed galvanostatic mode, said pulsed galvanostatic mode comprising an electropolymerization technique with at least two successive applications of a constant current density for a period t.sub.on, separated by a rest period t.sub.off without the application of any current or voltage, wherein the electrochemically depositing in the pulsed galvanostatic mode causes the electropolymerization of the matrix throughout an entire depth of the carpet without any modification of morphology of the carpet wherein in the pulsed galvanostatic mode the duration of each rest period (t.sub.off) is greater than the duration of each period of application of a constant current density (t.sub.on) by a factor of between 2 and 5.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 schematises the device used to implement the method of the invention.

(2) FIG. 2 illustrates the profiles of the pulses used for electrochemical depositing.

(3) FIG. 3 gives the cyclic voltammetry curves obtained for the carbon nanotubes (CNTs alone, C=3-6 F/g), for the polymer alone (P3MT, C=80-100 F/g) and for the nanocomposite of the invention (nanocomposite, C=130-140 F/g); v=5 mV/s.

(4) FIG. 4 gives transmission electron microscope images of two nanocomposites produced according to two operating conditions: pulsed galvanostatic mode with 50 sequences (FIG. 4A) and 75 sequences (FIG. 4B).

(5) FIG. 5 shows the discharge curves of a P3MT/CNT nanocomposite according to the invention at 10 mA/cm.sup.2 (FIG. 5A) and at 5 mA/cm.sup.2 (FIG. 5B).

(6) FIG. 6 gives photographs of a carpet of carbon nanotubes treated in accordance with the method of the invention, either laid flat (FIG. 6A) or held by a clip (FIG. 6B).

(7) FIG. 7 gives transmission electron microscope images of two nanocomposites according to the invention in which the P3MT was deposited on the CNTs at 2 mA/cm.sup.2 (FIGS. 7A and 7C) or at 4 mA/cm.sup.2 (FIGS. 7B and 7D). FIGS. 7C and 7D are detailed images respectively corresponding to the region materialised by a square in FIGS. 7A and 7B.

(8) FIG. 8 illustrates the evolution of the capacitance of a nanocomposite according to the invention (CNT/P3MT) as a function of the number of pulses.

(9) FIG. 9 illustrates the evolution of the capacitance as a function of the content of conductive polymer (P3MT). The dotted line shows the P3MT content on and after which the sample becomes self-supporting and flexible.

(10) FIG. 10 gives scanning electron microscopy images of NTC/P3MT nanocomposites according to the invention. FIG. 10A corresponds to the image of the surface of a nanocomposite with 80% P3MT and FIG. 10C to the image of nanotubes of this nanocomposite coated with P3MT. FIG. 10B corresponds to the image of the impermeable surface of a nanocomposite with more than 85% P3MT and FIG. 10D to the image of the impermeable layer as seen on the edge of this nanocomposite.

(11) FIG. 11 illustrates pulses applied for electropolymerization with a sufficient rest time of 300 sec (FIG. 11A) or with a shorter, insufficient rest time (FIG. 11B).

(12) FIG. 12 shows the evolution of the capacitance () and the evolution of the polymerization yield (.square-solid.) as a function of pulse time.

(13) FIG. 13 illustrates characterization by cyclic voltammetry of the nanocomposites according to the invention (CNT/P3MT) for short pulse times (FIG. 13A) and for long pulse times (FIG. 13B).

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

1. Description of the Device of the Invention

(14) The device used in the method of the invention schematised in FIG. 1 comprises an electrochemical cell with three electrodes.

(15) This cell comprises (1) a working electrode (WE) formed by the nano-objects of elongate shape, in particular the CNTs and more particularly the carpet of CNTs (2), a counter-electrode (CE) such as a platinum-coated titanium plate and (3) a reference electrode (RE) e.g. a silver wire.

(16) In addition, the electrochemical cell is held in a thermostat-controlled bath, in particular at 30 C. This thermostat-controlled bath may be a water circulation bath with a water inlet (Ee) and water outlet (Se).

(17) The electrolytic solution (S) contacted with these electrodes in the electrochemical cell comprises the monomers which, via polymerization, allow the polymer matrix to be obtained in solution, the monomers and polymer matrix being such as previously defined. In the examples below the solution (S) used is an ionic liquid solution (EMIT-FSI) containing 0.2 M 3-methylthiophene (3MT or MeT). The polymer obtained and deposited on the CNT carpet is poly(3-methylthiophene) (P3MT or PMeT).

(18) When implementing the method of the invention and in particular during polymerization, bubbling of argon (Ba) can be maintained in the solution (S) allowing maintaining of the argon content and gentle mixing of this solution.

2. Embodiments

(19) As previously explained, the essential feature of the method according to the invention is the use of a sequenced galvanostatic mode for depositing, on the working electrode, the polymer obtained from the monomers present in the solution (S).

(20) FIG. 2 shows the pulse profile of the current used. Tests to vary this profile were conducted so that the same quantity of electricity was passed during polymerization so that it was possible systematically to obtain the same m.sub.conductive polymer/m.sub.cnt ratio.

(21) The solid line in FIG. 2 illustrates the conventional profile, the dotted line another profile allowing the same charge to be passed (the areas under curves are identical).

(22) With this method, an electrically conductive polymer/carbon nanotube nanocomposite (ECP/CNT) is obtained in which the CNT weight is of the order of 20 to 25% relative to the total weight of the nanocomposite. This value was verified by TGA analysis and by weighing.

3. Characterisation of the Nanocomposite Obtained

(23) 3.1. Proof of the Nanostructuring Effect on Capacitance Values.

(24) Cyclic voltammetry (CV) studies were carried out (FIG. 3). For example, these studies allowed it to be shown that the area under curve corresponding to the nanocomposite is greater than the sums of the areas of the two other curves i.e. curve of the P3MT polymer and curve of CNTs alone.

(25) This nanostructuring is a first indication of the good distribution of the P3MT along the CNTs.

(26) 3.2. Proof of the Polymerization of the Monomer Along the CNTs.

(27) Measurement of the distribution of the sulphur element by EDX probe on the thickness of the carpet made it possible to verify that the P3MT polymer was present along the entire length of the aligned tubes. On the contrary, in continuous galvanostatic mode, the polymer does not penetrate along the entire length of the carpet which clearly illustrates the benefit of using a pulsed galvanostatic mode.

(28) Similarly, the transmission electron microscopy images (TEM) shown in FIG. 4 clearly show the coating of the CNTs by the conductive polymer (P3MT). The homogeneous distribution of the conductive polymer can distinctly be seen on the tubes and, in addition, it is possible to determine the thickness of the deposited film and this, for two different operating modes.

(29) For example, for a method using a pulsed galvanostatic mode with 50 sequences, the polymer has a thickness of 8 nm (FIG. 4A), whilst, for a pulsed galvanostatic mode with 75 sequences, this thickness is 18 nm (FIG. 4B).

(30) 3.3. Measurement of Capacitances.

(31) The capacitances were measured by conducting charge/discharge cycles with constant current density. FIG. 5 illustrates two examples with J=10 mA/cm.sup.2 (FIG. 5A) and J=5 mA/cm.sup.2 (FIG. 5B), these values frequently being used for electrochemical storage applications.

(32) The capacitances were measured using the slope of the straight line obtained on the discharge curves. These capacitances per weight unit were between 135 F/g and 145 F/g for a P3MT/aligned CNT electrode. Calculated for the weight of P3MT alone, the capacitance is then 180 to 200 F/g.

(33) TABLE-US-00001 TABLE 1 Electrochemical data on two nanocomposites according to the present invention, studied at 30 C. (line 1) and 60 C. (line 2) and compared with a prior art nanocomposite. Potential Type Capacitance Electrolyte Forming range P3MT/aligned 140 F/g EMIT-FSI Electro- 0.2 V to CNT at 30 C. at 4 mA/cm.sup.2 polymerization 1.2 V/Ag P3MT/aligned 170 F/g EMIT-FSI Electro- 0.2 V to CNT at 60 C. at 4 and polymerization 1.06 V/Ag 10 mA/cm.sup.2 P3MT at 30 C. 85-100 F/g EMIT-FSI ground P3MT, 0.5 V to at 10 mA/cm.sup.2 AB, PTFE 1 V/Ag

(34) The prior art nanocomposite was prepared by mixing the powder of P3MT (previously ground in a mortar or using specific apparatus) with acetylene black (AB) and a polymer used as a binder (polytetrafluoroethylene or PTFE).

(35) The values obtained with the nanocomposites of the invention are of great interest for ionic liquids.

(36) However an ohmic drop was noted at a current density of 10 mA/cm.sup.2, this drop being inherent in the viscosisty of the ionic liquid.

(37) 3.4. Self-Supported Samples

(38) With respect to P3MT, the amount deposited has a large influence on the possible lift-off of the sample (one of the particular properties of the present invention).

(39) This phenomenon occurs in the region of a P3MT content of 70-75% (content expressed in weight of P3MT to the total weight of the nanocomposite) for CNTs having a length of more than 150 m. Said content is materialized in FIG. 9 below by the vertical dotted line. It is interesting to note that the maximum capacitance is found at around 80%, i.e. the self-supported (detached) material could have this maximum capacitance (cf. FIG. 9).

(40) The samples are self-supported (with no current collector) i.e. it is in no way necessary to metallize the CNTs or to glue aluminium collectors to collect the current along the carpet. In addition, the carpet impregnated with conductive polymer becomes flexible. FIG. 6 shows a photo of a carpet of about 1 cm.sup.2 with the deposited conductive polymer (FIG. 6A). In the photo in FIG. 6B it can be seen that the nanocomposite is very flexible after the depositing of the conductive polymer.

4. Study of the Different Parameters Influencing the Deposit and its Properties

(41) The parameters examined were: The current density: this has an influence on the rate of deposit of the conductive polymer and hence on the quality thereof. the number of pulses: this controls the thickness of a deposit for a given density. the form of the pulse: the rest time may effectively be an influencing factor on the quality of deposit.

(42) 4.1. Current Density Applied During Polymerization.

(43) The current density applied for a pulse is an important factor for the properties of the material. A study was conducted with 3 different current densities: 2; 3; 4 mA.Math.cm.sup.2, and using the same number of pulses and one same quantity of applied charge.

(44) TABLE-US-00002 TABLE 2 Current density J 2 mA/cm.sup.2 3 mA/cm.sup.2 4 mA/cm.sup.2 Rate of 82% 87% 92% electropolymerization Capacitance 85 F/g 105 F/g 110 F/g

(45) The increase in current density leads to an improvement in the rate of polymerization and in the capacitance of the material.

(46) Analysis of Deposits by TEM:

(47) As shown in FIG. 7, the layer deposited at 2 mA/cm.sup.2 (FIG. 7A) is dense and homogeneous, contrary to the layer deposited at 4 mA/cm.sup.2 (FIG. 7B). A larger magnified view of FIGS. 7A and 7B, respectively in FIGS. 7C and 7D, evidences waves on the surface of the sample in FIG. 7D. Since the kinetics of polymerization are more rapid at 4 mA/cm.sup.2, a more porous structure is obtained.

(48) The higher the current density, the more the structure of the deposited polymer will be porous and conversely. Said porosity can account for the improved capacitance. Since the solvent/polymer interface must be maximum, greater porosity improves capacitance.

(49) 4.2. Number of Pulses/Nanotube Length

(50) These two parameters are difficult to be separated from each other. Samples derived not only from different synthesis but also from one same synthesis each have a different CNT length since positioned at a different point in the tubular quartz reactor used for the synthesis of CNTs.

(51) After a certain number of tests, the length of the carpet of carbon nanotubes does not appear to have any effect on capacitance for a small quantity of deposited polymer, almost as if there were no nanostructuring effect in this case. Indeed, irrespective of the length of the sample, the curve of the evolution of the capacitance as a function of the number of pulses will follow the evolution illustrated in FIG. 8, namely a bell-shaped profile.

(52) The drop in capacitance can be accounted for by the fact that at one moment the P3MT will only be deposited on the surface of the carpet: the electrode then loses its 3D structure and capacitance will be strongly affected thereby since the entire inner-side of the carpet no longer takes part in charge storage.

(53) The proportion of P3MT in the nanocomposite of the invention is related to the number of applied pulses. Maximum capacitance is obtained with a sample containing between 80 and 85% P3MT (FIG. 9). This value corresponds to an optimal filling rate of the carpet of aligned nanotubes in which only little space remains between the tubes.

(54) It is therefore assumed that at a certain time, the diffusion of the monomer in the carpet is increasingly slower. When a new pulse is resumed, it is rather more on the upper part of the carpet that polymerization occurs, obscuring the carpet. This phenomenon can account for the sudden drop in capacitance which is observed in FIG. 9.

(55) Therefore on and above 85% of conductive polymer (P3MT) a surface layer is formed forming a shield against the solution and leading to a strong drop in capacitance. The active matter lying at depth in the mat is no longer available and loses its usefulness for storing energy.

(56) The scanning electron microscopy images (SEM) in FIG. 10 of CNT/P3MT nanocomposites with a different number of pulses confirm this fact. In FIGS. 10A and 10C, 225 pulses were used which allowed a nanocomposite to be obtained with 80% P3MT, whereas a nanocomposite with more than 85% P3MT was obtained with 275 pulses (FIGS. 10B and 10D). Observations of the surface and edge under SEM of the nanocomposites thus obtained reveal very high filling of with CNTs coated with P3MT (FIGS. 10A and 10C) and an impermeable surface of the carpet for the nanocomposite with more than 85% P3MT (FIGS. 10B and 10D).

(57) 4.3. Effect of Rest and Pulse Time

(58) Rest Time:

(59) During electropolymerization with a sufficient rest time, the potential that is reached decreases at first and then remains near-constant throughout synthesis. This reached potential is characteristic of the oxidation potential of the polymer generated at the electrode (FIG. 11A).

(60) If, on the contrary, the rest time is not sufficient, diffusion will not have been sufficient to renew the quantity of monomers. The system will increase in potential to oxidize the other species in solution instead of the monomer. This characteristic increase is illustrated in FIG. 11B. It may lead to harmful effects such as over-oxidation of the polymer, adverse reactions, change in morphology . . . .

(61) Pulse Time:

(62) Concerning the effect of pulse time, this is illustrated in FIGS. 12 and 13.

(63) If capacitance is determined as a function of pulse duration, bell-shaped curves are obtained both for capacitance and for polymerization yield as shown in FIG. 12.

(64) Shorter pulse times (5-15 sec) promote the formation of MeT oligomers. Since these are partly soluble in the solvent, they diffuse in solution (since they have time to) and are therefore not counted in the polymerization yield.

(65) The mean optimum capacitance is obtained with pulse times of about 50 sec, which roughly corresponds to the optimum polymerization yield (60 sec pulse time). Over and above this optimum, there is a drop in capacitance. Polymerization time is too long and the phenomenon of the formation of the layer on the surface of the CNT carpet is probably heightened.

(66) FIGS. 13A and 13B also show the variation in the charge passing through the nanocomposite during charge/discharge cycles by scanning cyclic voltammetry, and for short pulse times (i.e. of 45 sec or less) or for longer pulse times (i.e. of 45 sec or longer).

(67) 4.4. Deposit of Other Monomers

(68) Other monomers were used to prepare a nanocomposite according to the invention from long carpets of CNTs by depositing the corresponding polymers thereupon.

(69) These polymers were: polyaniline (PANI) of formula:

(70) ##STR00001## polypyrrole (PPy) of formula:

(71) ##STR00002## poly(3,4-ethylene dioxythiophene) (PEDOT) of formula:

(72) ##STR00003## poly(3-thiophene acetic acid) (PTAA) having an acid function capable of being post-functionalised, of formula:

(73) ##STR00004## poly(carbazole) (PCz) of formula:

(74) ##STR00005##