Method for the preparation of an electrode comprising an aluminium substrate, aligned carbon nanotubes and an electroconductive organic polymer, the electrode and uses thereof

Abstract

A method for the preparation of an electrode comprising a substrate made of an aluminium based material, vertically aligned carbon nanotubes and an electrically conductive polymer matrix, the method comprising the following successive steps: (a) synthesising, on a substrate made of an aluminium based material, a carpet of vertically aligned carbon nanotubes according to the technique of CVD (Chemical Vapour Deposition) at a temperature less than or equal to 650° C.; (b) electrochemically depositing the polymer matrix on the carbon nanotubes from an electrolyte solution including at least one precursor monomer of the matrix, at least one ionic liquid and at least one protic or aprotic solvent. Further disclosed is the prepared electrode and a device for storing and returning electricity such as a supercapacitor comprising the electrode.

Claims

1. A method for the preparation of an electrode including a support made of an aluminium based material, vertically aligned carbon nanotubes and an electrically conductive polymer matrix, said method including the following successive steps: a) synthesizing, on a support made of an aluminium based material, a carpet of vertically aligned carbon nanotubes according to the CVD (Chemical Vapour Deposition) technique at a temperature less than or equal to 650° C.; b) electrochemically depositing said electrically conductive polymer matrix on said vertically aligned carbon nanotubes from an electrolytic solution including at least one precursor monomer of said electrically conductive polymer matrix, at least one ionic liquid and at least one protic or aprotic solvent; wherein said at least one ionic liquid comprises an anion, wherein the anion is (CF.sub.3SO.sub.2).sub.2N.sup.−; and wherein during said b) the electrolytic solution contacts the vertically aligned carbon nanotubes and the support.

2. The method according to claim 1, wherein the synthesizing during said a) is carried out at a temperature comprised between 500° C. and 620° C.

3. The method according to claim 1, wherein, following said (a) and prior to said (b), the vertically aligned carbon nanotubes are subjected to an oxidising treatment.

4. The method according to claim 1, wherein said electrically conductive polymer matrix is constituted of at least one polymer or copolymer selected from the group consisting of polypyrroles, polycarbazoles, polyanilines and polythiophenes.

5. The method according to claim 1, wherein said ionic liquid further comprises at least one protic or aprotic cation, substituted or not, selected from the family of pyridinium, pyridazinium, pyrimidinium, pyrazinium, imidazolium, pyrazolium, thiazolium, oxazolium, triazolium, ammonium, pyrrolidinium, pyrrolinium, pyrrolium, piperidinium.

6. The method according to claim 1, wherein said ionic liquid is selected from the group consisting of a dialkylpyrrolidinium bis(trifluoromethylsulphonyl)imide ([DAPyr][TFSI]), 1 ethyl-3-methylimidazolium bis(trifluoromethyl sulphonyl)imide ([EMI] [TFSI]), 1-butyl-3-methylimidazolium bis(trifluoromethylsulphonyl)imide ([BMI][TFSI]), and butyltrimethylammonium bis(trifluoromethylsulphonyl)imide ([BtMA][TFSI]).

7. The method according to claim 1, wherein said aprotic solvent is acetonitrile.

8. The method according to claim 1, wherein a viscosity of said electrolytic solution is comprised between 0.37 mPa.Math.s and 200 mPa.Math.s.

9. The method according to claim 1, wherein the electrochemically depositing of said b) takes place by a cyclic method and/or a galvanostatic method, pulsed or continuous, and/or a potentiostatic method, pulsed or continuous.

10. The method according to claim 1, wherein said electrically conductive polymer matrix comprises up to 99 wt % of a total weight of the vertically aligned carbon nanotubes coated with said electrically conductive polymer matrix.

11. The method according to claim 1, wherein, following said (b), the method has a rinsing step and optionally a drying step.

12. An electrode prepared by a method defined in claim 1.

13. The electrode according to claim 12, wherein a density of said vertically aligned carbon nanotubes is comprised between 10.sup.6 and 10.sup.13 nanotubes per square centimeter of electrode.

14. The use of an electrode according to claim 12 as a positive and/or negative electrode of a device for storing and restoring electricity, as an electrode for a photovoltaic device, in materials for storing CO.sub.2 or as an electrode for electrochemical sensors.

15. A device for storing and restoring electricity including: (1) an asymmetric assembly with an electrode according to claim 12 as a positive electrode and a single layer or double layer activated carbon electrode; or (2) an asymmetric type 2 assembly with two electrodes according to claim 12; or (3) an asymmetric type 3 assembly with two electrodes according to claim 12, a p and n dopable conjugated polymer being present on each of the electrodes; or (4) an asymmetric type 4 assembly with two different p and n dopable conjugated polymers on each of two electrodes according to claim 12.

16. A method for manufacturing a device for storing and restoring electricity which includes at least two electrodes and an electrolyte separating the two electrodes, at least one of the electrodes being an electrode according to claim 12, wherein the method includes assembling the two electrodes and the electrolyte to form an assembly, the electrolyte being an interface between the two electrodes, the assembly being contained in an encapsulation package to form said device for storing and restoring electricity.

17. The method according to claim 8, wherein the viscosity of said electrolytic solution is comprised between 1.0 mPa.Math.s and 36 mPa.Math.s at 0° C. and 1 bar.

18. The electrode according to claim 13, wherein the density of said vertically aligned carbon nanotubes is comprised between 10.sup.8 and 10.sup.12 nanotubes per square centimeter of electrode.

19. The electrode according to claim 18, wherein the density of said vertically aligned carbon nanotubes is comprised between 10.sup.11 and 10.sup.12 nanotubes per square centimeter of electrode.

20. The use of an electrode according to claim 14, wherein the device for storing and restoring electricity is a supercapacitor or a battery.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows the typical morphology of a carpet of CNTs aligned perpendicularly to the surface of the Al growth support used within the scope of the present invention.

(2) FIGS. 2A to 2D show scanning electron microscope images of the impact of the corrosion observed electrochemically on an electrode including an aluminium support provided with CNTs aligned perpendicularly to the surface of the support but not having an electroconductive polymer.

(3) FIG. 3 shows the cyclic voltamperometry curves obtained for the aluminium support provided with carbon nanotubes aligned before the deposition of the electroconductive polymer (Al/VACNT) or after this deposition (Al/VACNT/P3MT).

(4) FIGS. 4A to 4C show sectional views of the carpet of carbon nanotubes in three respective zones of this carpet: in the upper part of the carpet opposite the support (FIG. 4A), in the central part (FIG. 4B) and in the lower part (FIG. 4C), whereas FIG. 4D to 4F show the corresponding EDX mapping in the upper part of the carpet opposite the support (FIG. 4D), in the central part (FIG. 4E) and in the lower part (FIG. 4F).

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

(5) I. Preliminary Remarks.

(6) The exemplary embodiments of electrodes according to the invention described hereafter highlight their performance as regards their capacitance, the aim being to propose high capacitance electrodes to be used in devices for storing and restoring electricity of the supercapacitor type.

(7) For all of the exemplary embodiments and comparative examples described hereafter, the synthesis of the CNTs on the aluminium support is carried out by CVD at 615° C. from acetylene as carbon source coupled to ferrocene as catalytic precursor and in the presence of hydrogen and argon. The ferrocene is conveyed into the reactor using a toluene solution (10% by weight of ferrocene dissolved in toluene) in aerosol form in a flow rate range ranging from 0.7 to 7 mL/h. The typical morphology of the carpet of CNTs aligned perpendicularly to the surface of the Al growth support thereby obtained is represented in FIG. 1.

(8) The control of the different lengths of the CNTs aligned on the aluminium support described through the exemplary embodiments described hereafter is established by adjustment of synthesis parameters such as the duration of injection of the precursors or the ratio of the gases constituting the reaction atmosphere (Ar/H.sub.2/C.sub.2H.sub.2). These parameters will not be described for each of the examples and those skilled in the art can find any complementary information necessary to carry out and to control the synthesis of CNTs aligned on aluminium support in the international application WO 2015/071408 [1].

(9) II. Effect of the Electrolyte on a Supercapacitor with Aluminium Collector.

(10) In order to study the effect of the electrolyte in which the electrode is cycled, an aluminium support electrode provided with carbon nanotubes and not comprising any electroconductive polymer was used.

(11) This electrode includes, more specifically, an aluminium support having an active surface of 1.13 cm.sup.2 and vertically aligned carbon nanotubes of 65 μm length and of high nanotube surface density, of the order of 10.sup.11 to 10.sup.12 CNT/cm.sup.2. This electrode was prepared as described in point I above.

(12) The electrolyte in which this electrode was cycled is a mixture known to be very corrosive for aluminium electrode substrates [12] i.e. 1 mol/L lithium bis(trifluoromethylsulphonyl)amide in acetonitrile.

(13) FIG. 2 shows scanning electron microscope images of the impact of the electrochemical corrosion observed on the electrode tested after 2000 cycles. The corrosion on the electrode causes cracking of the CNT carpet (FIG. 2A), disbondments of the carpet (FIG. 2B) and corrosion pits (FIG. 2C) and cracking of the surface of the support (FIG. 2D).

(14) III. Preparation of an Electrode According to the Invention and Characterisation.

(15) III.1. Operating Procedure.

(16) An aluminium support having an active surface of 1.13 cm.sup.2 and including vertically aligned carbon nanotubes of 25 μm length and of high nanotube surface density, of the order of 10.sup.11 to 10.sup.12 CNT/cm.sup.2 was prepared as described in point 1 above.

(17) An electroconductive polymer of the poly(3-methylthiophene) (or P3MT) type is deposited on this aluminium support provided with CNTs by electrochemical process.

(18) The electrochemical device used to carry out this deposition by electrochemical process includes, for example, in a known manner, a stainless steel base on which the sample is deposited surmounted by a cylindrical body made of Teflon.

(19) The electrochemistry technique used is with three electrodes. The working electrode is constituted of the sample with the carbon nanotubes whereas the counter-electrode is a composite paste of carbon black and activated carbon. The assembly is imprisoned in a stainless steel grid. The reference electrode is a silver wire.

(20) The electrolyte contained in the electrochemical device is a 50:50 by volume mixture of acetonitrile and EMITFSI, to which is added the 3-methylthiophene monomer at a concentration of 0.4 M.

(21) The electrochemical technique chosen is cyclic voltammetry. The voltage is imposed between −0.5 V and 1.5 V at a scanning rate of 20 mV/s for 50 cycles.

(22) In these conditions, an electrode is obtained in which the proportion of electroconductive polymer compared to the composite that is to say the carbon nanotubes and the electroconductive polymer is less than 50% by weight.

(23) III.2. Study of the Capacitance of the Electrode Prepared at Point III.1.

(24) In order to characterise the capacitance per surface unit of the electrode according to the invention and to compare it with that of the same electrode (same aluminium support provided with carbon nanotubes and same specific surface) but without electroconductive polymer, the cyclic voltammetry technique was used by imposing a voltage between −0.5 to 1.5 V at a scanning rate of 10 mV/s for 5 cycles.

(25) The characterisation of the capacitance provides a capacitance of 204 mF/cm.sup.2 for the electrode according to the invention (aluminium support provided with carbon nanotubes functionalised by P3MT), whereas the capacitance of the comparative electrode without electroconductive polymer is 29 mF/cm.sup.2 (FIG. 3). The capacitance is increased sevenfold for the electrode according to the invention having an electroconductive polymer associated with the vertically aligned carbon nanotubes on an aluminium support.

(26) The electroconductive polymer makes it possible to provide a much greater capacitance than in the absence of polymer.

(27) IV. Alternative of Preparation of an Electrode According to the Invention and Characterisation.

(28) IV.1. Operating Procedure.

(29) Compared to the operating procedure of point III.1, this illustrative example implements an aluminium support provided with carbon nanotubes having a greater height and another electrolyte to functionalise the carbon nanotubes with P3MT.

(30) To this end, an aluminium support having an active surface of 1.13 cm.sup.2 and including vertically aligned carbon nanotubes of 70 μm length and of high nanotube surface density, of the order of 10.sup.11 to 10.sup.12 CNT/cm.sup.2 was prepared as described in point I above.

(31) The electrochemical device and technique implemented are identical to those used in the operating procedure of point III.1.

(32) Conversely, the electrolyte contained in the electrochemical device is a 50:50 by volume mixture of acetonitrile and EMI-BF4 (1-ethyl-3-methylimidazolium tetrafluoroborate), to which has been added the 3-methylthiophene monomer at a concentration of 0.2 mol/L. In addition, during cyclic voltammetry, the voltage is imposed between −0.2 and 1.4 V at a scanning rate of 5 mV/s for 100 cycles.

(33) In these conditions, an electrode is obtained in which the proportion of electroconductive polymer compared to the composite, that is to say the carbon nanotubes and the electroconductive polymer, is at least 85% by weight.

(34) IV.2. Study of the Capacitance of the Electrode Prepared at Point IV.1.

(35) In order to characterise the capacitance per surface unit of the electrode according to the invention and to compare it with that of the same electrode (same aluminium support provided with carbon nanotubes and same specific surface) but without electroconductive polymer, the cyclic voltammetry technique was used while imposing a voltage between −0.2 to 1.4 V at a scanning rate of 10 mV/s for 5 cycles.

(36) The characterisation of the capacitance provides a capacitance of 1077 mF/cm.sup.2 for the electrode according to the invention (aluminium support provided with carbon nanotubes functionalised by P3MT), whereas the capacitance of the comparative electrode without electroconductive polymer is 27 mF/cm.sup.2. The capacitance is multiplied by 40 for the electrode according to the invention having an electroconductive polymer associated with the vertically aligned carbon nanotubes on an aluminium support.

(37) In this preparation alternative, the electroconductive polymer also makes it possible to provide a much greater capacitance than in the absence of polymer.

(38) IV.3. Characterisation of the Deposition of P3MT According to the Protocol of Point IV.1.

(39) A scanning electron microscope (SEM) analysis associated with analysis by energy dispersive X-rays (EDX) was carried out on the electrode prepared according to the protocol of point IV.1.

(40) The EDX mappings in the upper part of the carpet of carbon nanotubes opposite the support (FIG. 4D), in the central part of the carpet (FIG. 4E) and in the lower part of the carpet of nanotubes (FIG. 4F) all have, as main peak, that of sulphur attesting to the presence of P3MT in all the zones of the carpet of nanotubes tested and thus the homogeneity of the deposition of P3MT over the entire height of the carbon nanotubes. The fluorine peak is, for its part, virtually inexistent making it possible to affirm that there remains, in the composite, virtually no tetrafluoroborate ions initially present in the electrolyte used to deposit the P3MT.

(41) Thus, the method of the invention makes it possible to manufacture electrodes of which the density of vertically aligned carbon nanotubes, the length of these vertically aligned carbon nanotubes and the proportion of electroconductive polymer will be adapted as a function of the capacitance desired for the electrode and, in the end, the capacitance desired for the supercapacitor including at least one electrode according to the invention.

(42) V. Supercapacitor Including an Electrode According to the Invention.

(43) V.1. Preparation of a Supercapacitor S1 According to the Invention.

(44) The supercapacitor, designated supercapacitor or battery S1, is in the form of a button battery of the type CR2032, with asymmetric electrodes, and comprises: an electrode, known as electrode NTC1, of vertically aligned carbon nanotubes on an aluminium support associated with an electroconductive polymer obtained from 3-methylthiophene; an activated carbon electrode; a separator between the two polypropylene electrodes of 25 μm such as the product sold under the name Celgard® 2500; an electrolyte separating the two electrodes based on acetonitrile and EMITFSI.

(45) The electrode NTC1 has a surface of 1.13 cm.sup.2, vertically aligned carbon nanotubes of which the height is around 84 μm, a high nanotube surface density (of the order of 10.sup.11 to 10.sup.12 CNT/cm.sup.2) with a deposition surface of P3MT over the whole of the active surface of 1.13 cm.sup.2.

(46) As regards the preparation of the electrode NTC1, the electrolyte of the electrochemical deposition device of P3MT is a 75:25 by volume mixture of acetonitrile and EMITFSI to which is added the 3-methylthiophene monomer at a concentration of 0.4 M. The electrochemical technique selected is chronoamperometry. The voltage is imposed at 1.55 V per 250 ms pulse and with, between each pulse, an opening of the circuit of 1.5 s until a polymer/composite mass ratio of 20% is obtained.

(47) The electrodes of the battery and the separator were dried in a glass oven, of Bucchi® type, under vacuum between 50 and 80° C.

(48) The electrolyte of the battery S1 is a 40:60 by weight mixture of acetonitrile and EMITFSI with 10% by weight of LiPF.sub.6 (lithium hexafluorophosphate) salt as anticorrosion additive. It should be noted that this electrolyte is comparable to the highly corrosive electrolyte used at point II above: same anion, same solvent and similar transport properties.

(49) The electrolyte was arranged in the package containing the electrodes and the separator under inert argon atmosphere.

(50) V.2. Preparation of a Supercapacitor S2 According to the Invention.

(51) The supercapacitor, designated supercapacitor or battery S2, is distinguished from the supercapacitor S1 at the level of the characteristics relative to the carbon nanotubes, the weight ratio of P3MT and the electrolyte of the battery which does not include any anticorrosion additive.

(52) The battery S2 comprises an electrode, designated battery NTC2, which has a surface of 1.13 cm.sup.2, vertically aligned carbon nanotubes of which the height is around 80 μm, a high nanotube surface density (of the order of 10.sup.11 to 10.sup.12 CNT/cm.sup.2) with a deposition surface of P3MT over the whole of the active surface of 1.13 cm.sup.2 and this is so in a polymer/composite weight ratio of 40%.

(53) The two electrodes NTC1 and NTC2 are extremely similar with regard to the length of the carbon nanotubes and the density (2.25 and 3.48 mg/cm.sup.2). On the other hand, the amount of electroconductive polymer is two times greater for the electrode NTC2.

(54) The electrolyte of the battery S2 is the same as that of the battery S1 i.e. a 40:60 by weight mixture of acetonitrile and EMITFSI but, on the other hand, without anticorrosion additive.

(55) V.3. Preparation of a Comparative Supercapacitor Scomp.

(56) The comparative supercapacitor, designated supercapacitor or battery Scomp, corresponds to a button battery such as the batteries S1 and S2, apart from the aluminium support electrodes provided with carbon nanotubes, i.e. the positive and negative electrodes NTCcomp, do not comprise any electroconductive polymer.

(57) The electrodes NTCcomp have a surface of 1.13 cm.sup.2, vertically aligned carbon nanotubes of which the height is around 65 μm and a high nanotube surface density (of the order of 10.sup.11 to 10.sup.12 CNT/cm.sup.2).

(58) The two electrodes NTCcomp are similar with regard to the length of the carbon nanotubes and the weight per unit area of the electrodes NTC1 and NTC2.

(59) The electrolyte of the battery Scomp is the same as that of the battery S2 i.e. a 40:60 by weight mixture of acetonitrile and EMITFSI without anticorrosive additive.

(60) V.4. Charge/Discharge Capacitance and Coulombic Efficiency of the Supercapacitors S1, S2 and Scomp.

(61) Tables I, II and III hereafter resume the charge and discharge capacitances and the coulombic efficiency at the 1.sup.st cycle and after 1500 cycles, as well as the capacitance retention rate of the battery S1, the battery S2 and the battery Scomp respectively. The capacitance retention rate is the normalised percentage of capacitance after cycling. “Cycle” is taken to mean the fact of charging and discharging totally the system.

(62) TABLE-US-00001 TABLE I (Battery S1) After 1500 Cycle 1st cycles Retention % C(system-charge)/mF/cm.sup.2 133.9 111.9 83.5 C(system-discharge) mF/cm.sup.2 132.9 110.0 82.8 Coulombic efficiency/% 99.27 98.34 —

(63) TABLE-US-00002 TABLE II (Battery S2) After 1.sup.st 1500 Cycle cycle cycles Retention % C(system-charge)/mF/cm.sup.2 170.3 128.1 75.2 C(system-discharge) mF/cm.sup.2 168.0 126.0 75.0 Coulombic efficiency/% 98.68 98.59 —

(64) TABLE-US-00003 TABLE III (Battery Scomp) After 1.sup.st 1500 Cycle cycle cycles Retention % C(system-charge)/mF/cm.sup.2 29.8 24.4 81.9 C(system-discharge) mF/cm.sup.2 26.5 24.0 90.5 Coulombic efficiency/% 89.11 98.47 —

(65) The batteries S1 and S2 thus provide a cyclability over more than 1500 cycles without dysfunction.

(66) The performance of the battery S2 with regard to its charge and discharge cycles is comparable to that of the battery S1. Moreover, the presence of an anticorrosion additive is not strictly necessary despite the use of a potentially corrosive electrolyte for the aluminium collector.

(67) The performance of the battery Scomp with regard to its charge and discharge cycles and in the absence of an anticorrosion additive in the electrolyte implemented is well below that of the batteries S1 and S2.

(68) Consequently, the electrode of the invention based on an aluminium support makes it possible to provide a lightened supercapacitor, while providing good performances with regard to the capacitance without risk of corrosion of the electrode thanks to the presence of vertically aligned carbon nanotubes associated with the electrically conductive polymer.

REFERENCES

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