Method for preparing an electrode comprising a substrate, aligned carbon nanotubes and a metal oxide deposited by oxidative deposition, the electrode and uses thereof

Abstract

The present invention relates to a method for preparing an electrode comprising a metal substrate, vertically aligned carbon nanotubes and a metal oxide deposited over the entire length of said vertically aligned carbon nanotubes, said method comprising the following consecutive steps: (a) synthesizing, on a metal substrate, a mat of vertically aligned carbon nanotubes; (b) electrochemically depositing the metal oxide on said carbon nanotubes from an electrolytic solution comprising at least one precursor of said metal oxide and at least one nitrate, said electrochemical deposition being carried out by a chronopotentiometry technique. The present invention also relates to the electrode thus prepared and to the uses thereof.

Claims

1. A method for preparing an electrode comprising a metal substrate, vertically aligned carbon nanotubes and a metal oxide deposited over the whole length of said vertically aligned carbon nanotubes, said method comprising: a) synthesizing, on a metal substrate, a carpet of vertically aligned carbon nanotubes; and b) electrochemically depositing said metal oxide on said carbon nanotubes from an electrolytic solution comprising at least one precursor of said metal oxide and at least one nitrate as anion of the electrolyte.

2. The method according to claim 1, wherein said metal substrate comprises titanium, nickel, aluminium, copper, chromium, tantalum, platinum, gold, silver or stainless steel, silicon or carbon.

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

4. The method according to claim 1, wherein said metal oxide is an oxide of at least one transition metal.

5. The method according to claim 1, wherein said metal oxide is at least one selected from the group consisting of a ruthenium oxide, a titanium oxide, a manganese oxide, a copper oxide, a nickel oxide, a cobalt oxide, an iron oxide, a chromium oxide, a vanadium oxide and one of the mixtures thereof.

6. The method according to claim 1, wherein said precursor of said metal oxide is selected from the group consisting of acetates, nitrates, and halides of ruthenium, titanium, manganese, copper, nickel, cobalt, iron, chromium or vanadium.

7. The method according to claim 1, wherein said precursor of said metal oxide is selected from the group consisting of ruthenium biacetate, titanium acetate, manganese(II) acetate, manganese(III) acetate, copper(II) acetate, nickel(II) acetate, cobalt(II) acetate, iron(II) acetate, iron(III) acetate, chromium(II) acetate, chromium(III) acetate, vanadium(III) acetate, titanium nitrate, manganese(II) nitrate, manganese(III) nitrate, copper(II) nitrate, nickel(II) nitrate, cobalt(II) nitrate, iron(II) nitrate, iron(III) nitrate, chromium(II) nitrate, chromium(III) nitrate, vanadium(III) nitrate, titanium chloride, manganese(II) chloride, manganese(III) chloride, copper(II) chloride, nickel(II) chloride, cobalt(II) chloride, iron(II) chloride, iron(III) chloride, chromium(II) chloride, chromium(III) chloride and vanadium(III) chloride.

8. The method according to claim 1, wherein said precursor of said metal oxide is present in said electrolytic solution in a quantity comprised between 1 mM to 1 M.

9. The method according to claim 1, wherein said electrolytic solution comprises at least one polar solvent.

10. The method according to claim 1, wherein following said depositing (b), said method further comprises a rinsing step and optionally a drying step.

11. An electrode, obtained by the method according to claim 1, wherein the metal substrate comprises aluminium.

12. The electrode according to claim 11, wherein a density of said vertically aligned carbon nanotubes is comprised between 10.sup.6 and 10.sup.13 nanotubes.Math.cm.sup.−2 of electrode.

13. A device for storing and restoring electricity, comprising an electrode according to claim 11.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows aluminium pellets after a polarisation at 1.3 V vs a KCl saturated calomel electrode (CSE) for 60 min in electrolytes containing different electrolyte salts.

(2) FIG. 2 shows the variation in the potential at the level of the working electrode as a function of time and during deposition by continuous chronopotentiometry carried out on VACNTs/Al using Mnac(III) as precursor.

(3) FIG. 3 shows scanning electron microscopy (SEM) micrographs of bare VACNTs on aluminium (FIG. 3A) and the same VACNTs after an electrochemical deposition using manganese(III) acetate as MnO.sub.2 precursor at two different enlargements (FIGS. 3B and 3C).

(4) FIG. 4 shows SEM micrographs of bare VACNTs on aluminium (FIG. 4A) and the same VACNTs after an electrochemical deposition using manganese(III) acetate as MnO.sub.2 precursor (FIG. 4B).

(5) FIG. 5 shows a transmission electron microscopy (TEM) micrograph of CNTs after an electrochemical deposition using manganese(II) acetate as MnO.sub.2 precursor.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

(6) I. Effect of the Electrolyte on an Aluminium Collector

(7) The carpets of VACNTs on aluminium often disbond and holes may appear in the collector and this because of corrosion of the aluminium, in contact with the electrolyte and under electrochemical stress. Consequently, the inventors have sought an aqueous electrolyte which does not attack aluminium in the potential ranges used during the electrochemical deposition of metal oxide such as manganese oxide.

(8) To this end, several electrolytes listed in Table I hereafter were tested.

(9) TABLE-US-00001 TABLE I Solubility in water Salt Molar mass in g/mol g/L mol/L LiTFSI 287.09 5700 19.9 K2SO4 174.26 120 0.7 LiClO4 106.39 375 3.5 Li2SO4 109.94 385 3.5 LiCl 42.394 769 18.1 Na2SO4 142.04 271 1.9 LiNO3 68.95 1020 14.8

(10) The limited solubility of K.sub.2SO.sub.4 in water implies a limited concentration of salt. Also, all the electrolytes were used at a concentration of 0.5 mol/L.

(11) The aluminium pellets used for this study were pellets of aluminium alloy (DP76) comprising a very high percentage of aluminium (>99%), and all cleaned beforehand. The tests were carried out with cells with 3 electrodes, where the working electrode was the pellet of DP76, the reference electrode a KCl saturated calomel electrode (SCE) and the counter-electrode was an activated carbon electrode.

(12) The first step of the study was the characterisation by measurement C(V) of each system, which provides an insight of the corrosive effect of the electrolyte and makes it possible to identify the corrosion potential. Next, the cell was subjected to a fixed potential of 1.3 V for a duration of 60 min and the evolution of the weight of the pellet was monitored by an ultra-microbalance, in order to determine the variation in the weight of the pellet that would be linked to the dissolution of aluminium following its oxidation in the electrolyte. FIG. 1 gives an overview of the pellets after this experiment.

(13) All the electrolyte salts lead to a dissolution of the pellet except LiNO.sub.3. The salts LiCl, LiClO.sub.4 and Na.sub.2SO.sub.4 seem to be the most corrosive. For these three salts, the loss by weight of the pellet is respectively 85%, 65% and 43%. It is zero in the electrolyte based on LiNO.sub.3.

(14) Furthermore, the variations in weights in the electrolyte based on LiNO.sub.3 are very small, and no apparent damage is present on the aluminium pellets at least up to 1.5V vs SCE which is interesting and sufficient for electrochemical depositions of metal oxide.

(15) Finally, chronopotentiometry tests (imposed current) also show the passive character of the aluminium surface in the presence of LiNO.sub.3 (increase in the potential on the application of the current up to 1.5 V without any lowering thereafter). In the other salts, it is possible to identify easily a phenomenon of trans-passivation and the passage in active zone where the material can oxidize at relatively low potentials.

(16) In conclusion, the salt substrate based on nitrate anions (NO.sub.3.sup.−) such as LiNO.sub.3 is retained for the remainder of the study on aluminium since the presence of Cl.sup.−, SO.sub.4.sup.2−, TFSI.sup.− and ClO.sub.4.sup.− anions leads to corrosion of the aluminium.

(17) II. Preparation of VACNTs

(18) II.1. Method of Growth on Stainless Steel

(19) The CVD injection method enables the efficient and reproducible growth of VACNTs on substrates made of 316L stainless steel in the form of pellets having a thickness comprised between 50 and 500 μm and a diameter comprised between 8 and 16 mm.

(20) To do so, the method requires the initial deposition of a sub-layer of ceramic (based on SiOx) obtained from toluene and tetraethyl orthosilicate (TEOS), on the surface of the substrate before the nanotube growth step. Different injection durations were thus tested with the aim of depositing a ceramic sub-layer which is both sufficiently thick to fulfil its role of diffusion barrier for the metal particles and sufficiently thin so as not to increase the electrical resistance of the electrodes. The nature of the reaction atmosphere makes it possible to play on the diameter and the density of the carbon nanotubes of the carpet formed on the metal substrate. The international patent WO 2009/103925 notably describes such a method [10].

(21) Low density VACNTs on stainless steel are obtained at 800 or 850° C. from a toluene/ferrocene solution (2.5% by weight) injected under argon uniquely. The toluene/ferrocene synthesis duration depends on the desired thickness of carpet, typically 3 to 5 min for 100 μm.

(22) High density VACNTs on stainless steel are obtained from the same toluene/ferrocene solution but with a reaction atmosphere based on argon/hydrogen and acetylene, for a temperature varying between 600 and 800° C.

(23) Thus, the deposition of metal oxide (MnO.sub.2) was carried out on the carpet of VACNTs on stainless steel (15 to 200 μM length and 10-40 nm average outer diameter) controlled by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The measured surface densities lie in the range 1 to 5×10.sup.9 VACNT/cm.sup.2 for VACNTs of average diameter of 40 nm (low density) and in the range 1 to 5×10.sup.11 VANTC/cm.sup.2 for VACNTs of average diameter of 10 nm (high density).

(24) II.2. Method of Growth on Aluminium

(25) Concerning the direct CVD growth of VACNTs on aluminium, the lowering of the growth temperature of the carpet of aligned CNTs (580 to 615° C. depending on the reaction atmospheres) is possible thanks notably to the use of a gaseous carbon source (C.sub.2H.sub.2) in the presence of hydrogen and ferrocene as metal precursor.

(26) For high densities on aluminium, one works around 600° C. with a 10% toluene/ferrocene solution and a mixture of argon/hydrogen and acetylene. The duration of the toluene/ferrocene synthesis also depends on the desired thickness of carpet, typically 20 or 30 min to reach 100 μm.

(27) Despite CNT growth speeds lower than at high temperatures (<10 μm.Math.min.sup.−1 at 580° C.), it is possible to produce VACNT/AI electrodes without pre-treatment of the aluminium (crude or alloy) surface. The characteristics (controllable length of 10 to 100 μm, average outer diameter: 6-12 nm) are at the level of the prior art. The measured surface densities are much greater than those obtained on stainless steel without acetylene: in the range 1 to 5×10.sup.11 CNT/cm.sup.2.

(28) The international application WO 2015/071408 notably describes such methods [6].

(29) III. Deposition of Manganese Oxide by Electrochemical Deposition

(30) Different precursors based on manganese acetate were used in order to deposit by oxidation manganese oxides on the surface of the VACNTs.

(31) Depositions were carried out on VACNTs on aluminium, of high densities (10.sup.11 CNT/cm.sup.2) and of 20 μm thickness in an aqueous electrolyte comprising a manganese oxide precursor being in the form of 0.08 mol.Math.L.sup.−1 manganese acetate (Mnac(II)) with LiNO.sub.3 (0.5 mol.Math.L.sup.−1) in H.sub.2O or 0.1 mol.Math.L.sup.−1 Mnac(III) with LiNO.sub.3 (0.5 mol.Math.L.sup.−1) in H.sub.2O.

(32) The electrochemical method used here is the continuous chronopotentiometric method. An intensity of 0.3 mA is applied on the electrode for 60 min in order to deposit manganese oxide on its surface. It is possible to vary the quantity of oxide deposited by increasing or decreasing the duration of the electrodeposition (from 2 to 90 min). After the electrochemical deposition of the oxide layer, the electrodes are rinsed in water to evacuate traces of oxide precursor. To measure the weight of MnO.sub.2 deposited, the rinsing with water is followed by a rinsing with ethanol then drying in an oven at 80° C. under vacuum for one day.

(33) Within the scope of the deposition by continuous chronopotentiometry carried out on VACNTs/aluminium using a Mnac(III) precursor, a rapid increase in the potential takes place up to 0.76 V corresponding to the complete covering of the working electrode by the electrolyte. Another increase, slower this time, corresponds to the deposition of manganese oxide on an electrode more and more covered with this same oxide (FIG. 2).

(34) In these deposition conditions, the carpet of VACNTs is covered in a homogeneous manner by manganese oxide over the whole of its depth (FIGS. 3 to 5). The manganese oxide may be in the form of very porous spheres in the inter-tube space of the carpet of VACNTs and in the whole of its thickness. These spheres have diameters comprised between 150 and 250 nm (FIGS. 3B and 3C).

(35) In order to characterise the capacitance by surface unit of the electrode according to the invention and compare it with that of the same electrode (same substrate provided with carbon nanotubes and same specific surface) but without deposition of metal oxide, the cyclic voltammetry technique was used by imposing a voltage between 0 and −0.8 V vs saturated calomel electrode at a scanning speed of 2 mV/s for 5 cycles, in an aqueous electrolyte (0.5 M LiNO.sub.3).

(36) After the deposition of manganese oxide from Mnac(II), the performances are greatly improved. The capacitances of bare VACNTs of the order of 30 F/g (7 mF/cm.sup.2) pass to 140-170 F/g (120 mF/cm.sup.2). After the deposition of manganese oxide from Mnac(III), the capacitance of the composite was evaluated at 260 F/g, i.e. 8 times more than that of bare VACNTs.

REFERENCES

(37) [1] Fan et al, 2008, «High dispersion of γ-MnO.sub.2 on well-aligned carbon nanotube arrays and its application in supercapacitors», Diamond & Related Materials, vol. 17, pages 1943-1948. [2] Amade et al, 2011, «Optimization of MnO.sub.2/vertically aligned carbon nanotube composite for supercapacitor application», J. of Power Sources, vol. 196, pages 5779-5783. [3] Patent application CN 103346021 in the name of the Aluminium Corporation of China Ltd published on the 9 Oct. 2013. [4] Pinault et al, 2005, «Growth of multiwalled carbon nanotubes during the initial stages of aerosol-assisted CCVD», Carbon, vol. 43, pages 2968-2976. [5] Pinault et al, 2005, «Evidence of sequential lift in growth of aligned multi-walled carbon nanotube multilayers», Nano Lett., vol. 5, pages 2394-2398. [6] International application WO 2015/071408 in the name of the CEA published on the 21 May 2015. [7] Dorfler et al, 2013, «High power supercap electrodes based on vertically aligned carbon nanotubes on aluminum», J. of Power Sources, vol. 227, pages 218-228. [8] Liatard et al, 2015, «Vertically aligned carbon nanotubes on aluminum as a light-weight positive electrode for lithium-polysulfide batteries», Chemical Communications, vol. 51, pages 7749-7752. [9] Arcila-Velez et al, 2014, «Roll-to-roll synthesis of vertically aligned carbon nanotube electrodes for electrical double layer capacitors», Nano Energy, vol. 8, pages 9-16. [10] International application WO 2009/103925 in the name of the CEA published on the 27 Aug. 2009.