METHOD FOR GROWING CARBON NANOTUBES ON THE SURFACE AND IN THE BODY OF A POROUS CARBONACEOUS SUBSTRATE AND USE FOR PREPARING AN ELECTRODE

Abstract

A method for providing a substrate made of a porous carbonaceous material with vertically aligned carbon nanotubes, the method having a first step of depositing a ceramic underlayer on the substrate followed by a second step of synthesizing, by catalytic chemical vapour deposition, the vertically aligned carbon nanotubes on the substrate obtained after the first step, the carbon source necessary for the synthesis during this second step being injected in a direction substantially perpendicular to the plane of the substrate and at a pressure less than 8104 Pa (800 mbar). The use of the substrate for preparing an electrode such as a supercapacitor electrode.

Claims

1) A method for providing a substrate made of a porous carbonaceous material with vertically aligned carbon nanotubes, said method having a first step of depositing a ceramic sublayer on said substrate followed by a second synthesis step, by catalytic chemical vapour deposition, of said vertically aligned carbon nanotubes on the substrate obtained following the first step, wherein a carbon source required for the synthesis during the second step is injected in a substantially perpendicular direction to the plane of the substrate and at a pressure less than 8.10.sup.4 Pa (800 mbar).

2) The method according to claim 1, wherein said substrate made of a porous carbonaceous material is presented in the form of carbon fibres or a carbon foam.

3) The method according to claim 1, wherein said ceramic is an oxide ceramic and particularly silicon oxide (SiO.sub.x where 0<x≤2).

4) The method according to claim 1, wherein said carbon source is co-injected with the catalytic source required for synthesis during said second step.

5) The method according to claim 1, wherein the injection of said carbon source and optionally of said catalytic source is carried out at a pressure between 10.sup.3 Pa (10 mbar) and 7.5.10.sup.4 Pa (750 mbar) and, in particular, between 10.sup.4 Pa (100 mbar) and 7.10.sup.4 Pa (700 mbar).

6) The method according to claim, wherein the injection of said carbon source and optionally of said catalytic source is carried out at a pressure between 3.10.sup.4 Pa (300 mbar) and 6.10.sup.4 Pa (600 mbar).

7) The method according to claim, wherein said first step and said second step are carried out in the same reaction chamber.

8) The method according to claim 1, wherein said first step and said second step are carried out respectively in a first chamber or “pre-treatment chamber” and in a second chamber or “reaction chamber”.

9) A substrate provided with vertically aligned carbon nanotubes obtained following a method as defined in claim 1.

10) A method for preparing an electrode comprising a substrate made of a porous carbonaceous material, vertically aligned carbon nanotubes and an electrically conducting polymer matrix, said method comprising the following successive steps: a) providing a substrate made of a porous carbonaceous material with vertically aligned carbon nanotubes according to the method as defined in claim 1; b) depositing said polymer matrix electrochemically on said carbon nanotubes using an electrolytic solution comprising at least one precursor monomer of said matrix.

11) The method according to claim 10, wherein, following said step (a) and prior to said step (b), the vertically aligned carbon nanotubes are subjected to an oxidising treatment (or pre-treatment).

12) The method according to claim 10, wherein said electrochemical deposition method is carried out using a pulsed or continuous galvanostatic method and/or a pulsed or continuous potentiostatic method.

13) The method according to claim 10, wherein, following said step (b), the method has a rinsing step and optionally a drying step.

14) An electrode capable of being prepared by a method as defined in claim 10.

15) The use of an electrode prepared by a method as defined in claim 10 as a positive/negative electrode of a device for storing and restoring electricity such as a supercapacitor or a battery, as an electrode for a photovoltaic device, in materials for storing CO.sub.2 or as an electrode for electrochemical sensors.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0104] FIGS. 1A and 1B show scanning electron microscopy (SEM) images taken at different magnifications of a VACNT growth on carbon fibre fabric obtained according to the CVD synthesis method in a horizontal configuration (comparative example).

[0105] FIG. 2 presents SEM micrographs showing: the VACNT growth preferentially on the fibres on the surface of the fabric (FIGS. 2A and 2B) and the VACNT growth on the surface fibres but also on those located in the thickness of the fabric (FIG. 2C).

[0106] FIG. 3 shows the cyclic voltammetry curves obtained for the non-annealed carbon fibre fabric during P3MT polymerisation.

[0107] FIG. 4 shows the cyclic voltammetry curves obtained, during P3MT polymerisation, for the carbon fibre fabric provided with VACNTs essentially on the surface.

[0108] FIG. 5 shows the cyclic voltammetry curves obtained, during P3MT polymerisation, for the carbon fibre fabric provided with VACNTs on the surface and in the body.

[0109] FIG. 6 shows the cyclic voltammetry curves for a non-annealed crude fibre fabric (1), a carbon fibre fabric provided with VACNTs essentially on the surface (2) and a carbon fibre fabric provided with VACNTs on the surface and in the body (3).

[0110] FIG. 7 shows the cyclic voltammetry curves for a non-annealed crude fibre fabric, coated with P3MT (1′), a carbon fibre fabric provided with VACNTs essentially on the surface, coated with P3MT (2′) and a carbon fibre fabric provided with VACNTs on the surface and in the body, coated with P3MT (3′).

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0111] I. VACNT Synthesis.

[0112] I.1. On the Surface of a Carbon Fibre Fabric (Comparative Example).

[0113] Preliminary Remark

[0114] In order to synthesise VACNTs only on the surface of a carbon fibre fabric, the technique used is the so-called “horizontal configuration” technique described in international application WO 2009/050247 [8] and using the device represented schematically in FIG. 15 of the same application.

[0115] Protocol

[0116] Both in the horizontal configuration and in the vertical configuration (see part 1.2 hereinafter), the synthesis of VACNTs on a carbon fibre fabric includes 2 steps taking place in the same reactor: [0117] depositing a sublayer of SiOx acting as a diffusion barrier of the metallic catalyst required for VACNT growth. The Si-based organometallic precursor used is tetraethyl orthosilicate (TEOS). This deposition takes place at relatively low pressure and at a moderate temperature; [0118] growing the VACNTs using a mixture of precursors comprising a catalytic source and a carbonaceous source which are, in this example, an organometallic precursor which is ferrocene and a liquid hydrocarbon which is toluene. This growth is performed at atmospheric pressure.

[0119] The synthesis conditions are summarised in Table 1 hereinafter.

TABLE-US-00001 TABLE 1 VACNT synthesis parameters on carbon fibre fabrics (injection in horizontal configuration) Injection Injection of of Anhydrous Toluene/ toluene/TEOS Ferrocene Synthesis temperature (° C.) 500 850 Argon flow rate (L/min) 5 3 Pressure (mbar) 100 1000 Mean mass of liquid 0.1 1.06 injected (g/min) Concentration in precursor 1 mol/L of TEOS 2.5 wt % of mixture Ferrocene Injection time (s) 360 450

[0120] Result

[0121] FIG. 1A shows an image of the carbon fibre fabric obtained following the synthesis method described above and shows sheets of carbon nanotubes (CNTs) aligned over the entire fabric surface. FIG. 1B provides an image with a greater magnification of the edge of a CNT sheet deposited on fibre showing the presence of aligned CNTs.

[0122] I.2. On the Surface and in the Body of a Carbon Fibre Fabric.

[0123] Device Used

[0124] In order to synthesise VACNTs on the surface and in the body of a carbon fibre fabric, the technique used in the so-called “vertical configuration” technique.

[0125] This synthesis can use the devices illustrated in FIGS. 1 to 3 of international application WO 2015/071408 [9]. It can also be carried out using a modified device with respect to the device represented schematically in FIG. 15 of international application WO 2009/050247 [8].

[0126] The modified device adapted for vertical configuration synthesis is composed, like the device represented schematically in FIG. 15 of international application WO 2009/050247 [8] (the references hereinafter correspond to the references used in this Figure), of three parts:

[0127] 1) an injection system comprising at least one tank fluidically connected to at least one injector and typically two tanks R1 and R2 fluidically connected to two injectors IN1 and IN2 and for injecting the liquid precursor solutions into the evaporator EV in the form of fine droplets, the latter vaporising the fine droplets previously formed;

[0128] 2) a reactor (or reaction chamber) CR wherein the substrate made of porous carbonaceous material is located typically disposed and held on a substrate-holder such as a stainless steel substrate-holder and wherein the carrier gas stream transports the reactive precursor vapour. This reactor is placed in a tubular or square cross-section furnace FO; and

[0129] 3) a cooling and trapping system SO for processing the gases at the furnace outlet before extraction.

[0130] However, the modified device adapted for vertical configuration synthesis differs from the device represented schematically in FIG. 15 of international application WO 2009/050247 [8] by the following two elements: [0131] the evaporator EV is fluidically connected at the upper part of the reactor CR and no longer at the upstream part (i.e. of the inlet) of the reactor whereby the fine droplets are injected along an essentially perpendicular direction (i.e. 90°±30°), advantageously perpendicular to the plane of the substrate; [0132] the pressure in the reactor at which the second step of the method i.e. the deposition of the VACNTs is carried out is less than 800 mbar and no longer at atmospheric pressure.

[0133] Protocol

[0134] As explained above, the VACNT synthesis in the vertical configuration includes the 2 steps taking place in the same reactor as defined for the case of a horizontal configuration synthesis. The conditions of this synthesis are summarised in Table 2 hereinafter.

TABLE-US-00002 TABLE 2 VACNT synthesis parameters on carbon fibre fabrics (injection in vertical configuration) Injection Injection of of Anhydrous Toluene/ toluene/TEOS Ferrocene (5%) Synthesis temperature (° C.) 850 850 Argon flow rate (L/min) 5 3 Pressure (mbar) 400 400 Mass of liquid injected (g/h) 40 70 Concentration in precursor 1 mol/L of TEOS 5 wt % of mixture Ferrocene Injection time (s) 1200 1500

[0135] Result

[0136] FIGS. 2A, 2B and 2C show SEM images of carbon fibre fabrics coated with nanotubes and obtain in the vertical configuration (FIG. 2C), a comparison with the samples obtained in the horizontal configuration (FIGS. 2A and 2B) is also present in these images. Thus, it is obvious that growth in the horizontal configuration gives rise to VACNT formation preferentially on the fibres present on the surface of the fabric, whereas in the vertical configuration and in conjunction with a lowering of the working pressure in the reactor, the aligned nanotubes are formed both on the fibres on the fabric surface but also on the fibres in the fabric thickness. Thus, the method in the vertical configuration thereof and applying a lower working pressure than the atmospheric pressure makes it possible to achieve VACNT growth in the body of the fibrous preform.

[0137] II. Electrodeposition of P3MT and Evaluation of the Capacitance of the Electrodes Produced.

[0138] The purpose of the following four examples is to show the benefit of the presence of nanotube sheets on the surface and in the body of carbon fibre fabrics. In other words, it involves demonstrating the benefit of the nanostructuring on the surface and in the body of carbon fibre fabrics on the deposition of the conducting polymer and the capacitance developed by the electrodes thus produced.

[0139] For this, all the tests were conducted at a constant charge quantity, namely 2 C, in order to be able to compare the different configurations in terms of capacitance values obtained.

[0140] II.1. Electrodeposition of P3MT on Crude Carbon Fibre Fabric and Evaluation of Capacitance (Comparative Example).

[0141] Here, this involves estimating the possibility of depositing conducting polymer on the surface of synthetic crude fibre fabrics, i.e. fibres coated with an organic sizing layer, required for handling carbon fibre fabrics and well known in the aeronautical sector.

[0142] The sample of crude carbon fibres underwent a cyclic voltammetry electrodeposition (−0.5 V to 1.6 V at 20 mV.Math.s.sup.−1, in an equal volume mixture of EMI-TFSI/CH.sub.3CN containing 0.4 M of 3-methylthiophene monomer). In order to attain a charge quantity (Qp) of 2 C, 100 polymerisation cycles were applied, but it proved to be difficult to attain the charge of 2 C envisaged, with the charge quantity reaching an upper limit of 1.75 C (FIG. 3). This is explained by a pronounced resistive effect of the fabric and by a surface area developed by the fabric which remains limited.

[0143] II.2. Electrodeposition of P3MT on Fabric of Crude Carbon Fibres Annealed and Coated with SiOx and Evaluation of Capacitance (Comparative Example).

[0144] For this test, the carbon fibres were annealed in an argon stream at 3 L.Math.min.sup.−1 and at 850° C. then were coated with SiOx according to the first step of the method according to the invention (see Table 2).

[0145] In this case, P3MT deposition proved to be impossible due to overly resistive behaviour explained by the presence of an insulating SiOx film on the surface of the fibres.

[0146] II.3. Electrodeposition of P3MT on VACNTs Distributed Essentially on the Fibres Disposed on the Surface of the Carbon Fibre Fabric and Evaluation of Capacitance (Comparative Example).

[0147] On this sample, the VACNTs are mainly distributed on the surface of the fibres by implementing a horizontal configuration synthesis method. Thus, the fibres in the body of the fabric do not include VACNTs and the surface thereof therefore essentially consists of a layer of SiOx. The nanostructuring of the surface of the carbon fibre fabric therefore enables a slight increase in the active surface area available for P3MT deposition, but the fibres present in the body are inactive in view of the surface area thereof coated with SiOx.

[0148] This sample of VACNTs on a carbon fibre fabric substrate underwent a cyclic voltammetry electrodeposition (−0.5 V to 1.6 V at 20 mV.Math.s.sup.−1, in an equal volume mixture of EMI-TFSI/CH.sub.3CN containing 0.4 M of 3-methylthiophene monomer) of 38 deposition cycles to obtain a charge Qp=2.14 C (FIG. 4). P3MT deposition is then possible, but only on the carbon nanotubes present on the fabric surface fibres, the fibres coated with SiOx in the body of the fabric being too resistive to enable ECP deposition.

[0149] II.4. Electrodeposition of P3MT on VACNTs Distributed Both on the Fibres Disposed on the Surface and in the Body of the Carbon Fibre Fabric and Evaluation of Capacitance.

[0150] On this sample, the VACNTs are distributed on the fibres which are located both on the surface and in the body of the carbon fibre fabric by implementing the method according to the invention. The active surface area developed by the VACNTs is therefore greater than that of the bare or SiOx-coated fibres, and most of the fibres forming the fabric are coated with VACNTs. This sample of VACNTs on a carbon fibre fabric substrate underwent a cyclic voltammetry electrodeposition (−0.5 V to 1.6 V at 20 mV.Math.s.sup.−1, in an equal volume mixture of EMI-TFSI/CH.sub.3CN containing 0.4 M of 3-methylthiophene monomer) of 24 deposition cycles to obtain a charge Qp=2.05 C (FIG. 5). P3MT deposition is then possible, and in view of the electrochemical signal, all of the carbon nanotubes present on the surface and in the volume of the fibrous fabric appears to have been coated with P3MT.

[0151] III. Comparison According to the Different Nanotube Configurations on Carbon Fibre Fabric Substrates and the Electrochemical Conditions.

[0152] All of the results presented for different nanotube configurations on carbon fibre fabric substrates and electrochemical conditions makes it possible to establish points of comparison in terms of performances and particularly in relation to the surface capacitance.

[0153] III.1. Comparison 1: Electrodes with Only the Sheet of Nanotubes Supported on Carbon Fibre Fabric Substrates (without ECP).

[0154] As the capacitance of an electrode free from P3MT is indirectly proportional to the active surface area of the electrode, the comparison of the electrodes under the same electrochemical conditions is a good comparative indicator of the active surface areas of each type of electrode.

[0155] Thus, for the same geometric surface area of the electrode, the electrochemical characterisation was conducted in cyclic voltammetry (−0.5 V to 1.6 V at 20 mV.Math.s.sup.−1, in an equal volume mixture of EMI-TFSI/CH.sub.3CN).

[0156] Table 3 below reports the discharge surface capacitances for each electrode under the same electrochemical conditions, and FIG. 6 reports the voltammograms produced under the analytical conditions.

TABLE-US-00003 TABLE 3 Csd/mF .Math. cm−2 Crude fibre fabric with sizing (non-annealed) 22 Fibre fabric/VACNTs (on surface) 14 Fibre fabric/VACNTs (on surface and in body) 79

[0157] These results show the very pronounced effect of the presence of nanotubes in the body of the fibrous preform which is explained by an increase in the active surface area available. It is also important to note that the capacitance of the fabric containing VACNTs located only on the surface fibres is lower than that of the crude fabric. This is explained in that the fibres which are located in the body are partially coated with a SiOx resistive sublayer which blocks the electrochemical response of the oxide-coated fibre.

[0158] III.2. Comparison 2: Electrodes Composed of VACNTs Supported on Carbon Fibre Fabric Substrates and Coated with P3MT.

[0159] The P3MT deposition is carried out for the same polymerisation charge quantity (Qp ˜2 C) and the storage properties of the electrodes are compared to an equivalent geometric surface area.

[0160] Table 4 below reports the discharge surface capacitances for each electrode under the same electrochemical conditions and is based on the examples of points II.1, II.3 and II.4 as described above. FIG. 7 reports the voltammograms produced under the analytical conditions.

TABLE-US-00004 TABLE 4 P3MT electrodeposition Cyclic voltammetry −0.5 Surface V to 1.6 V capacitance Number Csd/ Qp/C of cycles mF .Math. cm−2 Non-annealed crude fibre fabric 1.75 100 145 Fibre fabric/VACNTs 2.14 38 172 (on surface) Fibre fabric/VACNTs 2.05 24 241 (on surface and in body)

[0161] For a quasi-identical polymerisation charge quantity (Qp ˜2C), the electrode developing the largest active surface area (fibres/VACNTs on surface and in body) shows the highest specific capacitance, and only 24 cycles are required to obtain the charge quantity of 2 C. These results clearly show the important role important of the nanotubes located on the surface and in the body: P3MT nanostructuring via the VACNTs present on the fibres both on the surface and in the body of the fabric makes it possible to improve the performances of the carbon fibre-based electrode significantly.

[0162] Thus, a 40% increase in the capacitance of the fabric including fibres coated with VACNTs on the surface and in the body of the fibrous preform is observed compared to the preform only including surface fibres coated with VACNTs. The increase is even greater (66%) if comparing to the raw fabric with no nanotubes.

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