PROCESS FOR MANUFACTURING COMPOSITE CONSISTING OF GRAPHENE MONOLITH AND SILICON

Abstract

Disclosed is a process of manufacturing a chemically reduced graphene oxide/silicon nanowire composite. The formation of the three-dimensional monolith and the chemical reduction of graphene oxide by a reducing agent selected from hydrazine hydrate, ethylene diamine and 1,4-diaminebutane are in one step. Also disclosed is a chemically reduced graphene oxide/silicon nanowire composite that can be obtained by the disclosed process. The composite is a three-dimensional monolith in which the two components are covalently linked each other, having a high degree of reduction with a C/O ratio of 1-50, preferably from 10 to 25, more preferably 16.7, having a porous structure and a high specific surface area of 50-5,000 m.sup.2/g, preferably 800-2,500 m.sup.2/g, more preferably 1,433 m.sup.2/g and having a low resistance to charge transfer from 0.1 to 5 , preferably from 0.3 to 1.5 . Also disclosed is a lithium-ion battery or a supercapacitor including the composite (or monolith).

Claims

1. Process of manufacturing a three-dimensional material comprising a monolith of chemically reduced graphene oxide, in which silicon nanowires and gold nanoparticles are dispersed, said process comprising: a step (1) for manufacturing a monolith of chemically reduced graphene oxide in which gold nanoparticles are dispersed, and a step (2) for functionalizing said monolith obtained at step (1), in which silicon is grafted on the surface of said monolith, and for which said monolith is brought to a temperature ranging from 500 to 850 C. in the presence of a silicon gas.

2. Process according to claim 1, wherein the chemically reduced graphene oxide monolith in which gold nanoparticles are dispersed (step 1) is prepared by contacting an aqueous solution of graphene oxide, HAuCl.sub.4 and a reducing agent selected from hydrazine hydrate, ethylene diamine and 1,4-diaminebutane, in a one-step reaction.

3. Process according to claim 1, wherein the functionalization of the chemically reduced graphene oxide monolith in which gold nanoparticles are dispersed (step 2) is carried out by heating said monolith in an LPCVD furnace in the presence of: silicon reactive gas; an additional gas; and a carrier gas; at a pressure in a range of 500 to 1400 Pa.

4. Process for the preparation of a chemically reduced graphene oxide monolith by contacting an aqueous solution of graphene oxide with a reducing agent selected from hydrazine hydrate and ethylene diamine and 1,4-diaminebutane, in a one-step reaction.

5. Chemically reduced graphene oxide monolith, obtainable by the process according to claim 1.

6. Chemically reduced graphene oxide monolith according to claim 5, wherein the chemically reduced graphene oxide monolith: has a high degree of reduction with a C/O ratio ranging from 1 to 50, is porous with a specific surface area ranging from 50 to 5000 m.sup.2/g, and has a low resistance to charge transfer ranging from 0.1 to 5.

7. Monolith according to claim 5, wherein the pores have a diameter ranging from about 1 nanometer to 500 microns.

8. Monolith according to claim 5, wherein the electrical conductivity is from 10 to 2500.

9. Chemically reduced graphene oxide monolith according to claim 5, in which the gravimetric discharge capacity is from 1 to 300 F/g.

10. Lithium-ion battery comprising a monolith according to claim 5.

11. Supercapacitor comprising a monolith according to claim 5.

12. Lithium-ion battery according to claim 10, having a capacity retention ranging from 80% to 99%.

13. Lithium-ion battery according to claim 10, having a volumetric discharge capacity ranging from 100 to 500 F/cm.sup.3.

14. Lithium-ion battery according to claim 10, having a power density ranging from 1 to 100 kW/kg.

15. The process of claim 1, wherein in the step (2), the silicon is in the form of silicon nanowires.

16. The process of claim 15, wherein the monolith is brought to a temperature ranging from 600 to 800 C.

17. The process of claim 15, wherein the monolith is brought to a temperature ranging of about 650 C.

18. The process of claim 3, wherein the silicon reactive gas is SiH.sub.4.

19. The process of claim 18, wherein the additional gas is an acid gas.

20. The process of claim 19, wherein the additional gas is HCl.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0147] FIG. 1 is a photograph showing the scheme of the process for preparing gels with different diamines used as reducing agent.

[0148] FIG. 2 is a table showing the elemental composition and the C/O ratio of GH-HD obtained from X-ray photoelectron spectroscopy, the electrical conductivity () of GH-HD and the discharge capacities of GH-HD (C.sub.wt) in F/g at different rates.

[0149] FIG. 3 is a set of 2 scanning electron microscopy images (a and b) of GH-HD 3D structures.

[0150] FIG. 4 is a graph showing a comparison of energy and power densities of GH-HD and GH-ED with references of the literature where graphene-based materials are obtained by various reduction techniques.

[0151] FIG. 5 is a graph showing a cyclic stability study of GH-HD showing discharge capacities after 5000 cycles when performed at a current density of 2 A/g.

[0152] FIG. 6 is a graph showing the specific capacity (expressed in F/cm.sup.3) of GH-HD and CMG in function of current density (expressed in A/g).

[0153] FIG. 7 is a graph showing the cyclic stability of GH-HD and its densified compound GH-HD-AD obtained at a current density of 2 A/g, during 5000 cycles.

EXAMPLES

[0154] The present invention is further illustrated by the following examples which illustrate, without limitation, the methods of the invention.

Example 1

Preparation of Graphene Monolith Decorated with Gold Nanoparticles with Hydrazine Hydrate as Reducing Agent

[0155] An aqueous solution of graphene oxide (GO) is prepared in a range from 2.5 to 10 mg/mL, usually 5 mg/mL. A mass of HAuCl.sub.4 is added, which will depend on the gold nanoparticle loading rate of the final system, for example, 3 or 30 mg. Hydrazine hydrate is added between 3.6 L,/mg of GO and 7.2 L/mg of GO (at lower concentrations the monolith is not formed). The solution is sonicated for about 10 minutes. The flask is closed, placed in an oil bath and heated, without stirring, at about 80 C. for about 24 hours. A block of graphene is obtained, which is then washed by successive immersions in distilled water, freeze-dried at 37 C., then air-dried for about 48 hours, and then heated again in a vacuum oven. at about 60 C., for about one night. The formation of the monolith, the reduction of graphene oxide, and the deposition of gold particles are carried out concomitantly in a one-step process.

[0156] It should be noted that under conditions of GO concentration lower than 2.5 mg/mL and hydrazine hydrate of less than 3.6 L/mg of GO no monolith is formed and the mixture remains in dispersed form after reaction.

[0157] After the reaction, the liquid is removed, and the graphene block is immersed in water for about 30 minutes to dissolve the unreacted hydrazine hydrate. This immersion is repeated at least 3 times. The block is then lyophilized and then dried in a vacuum oven. In its dry form, the block can be broken and cut into pieces.

Example 2

Preparation of Graphene Monolith Decorated with Gold Nanoparticles with Ethylene Diamine as Reducing Agent

[0158] The same operating conditions as those described hereinabove are used. Only the amount of ethylene diamine varies between 5 L/mg of GO and 10 L/mg of GO.

Example 3

Preparation of Graphene Monolith Decorated with Gold Manoparticles with 1,4-Diamine Butane as Reducing Agent

[0159] The same operating conditions as those described hereinabove are used. Only the quantity of 1,4-diamine butane varies between 5 L/mg of GO and 20 L/mg of GO.

Example 4

Preparation of Gold-Free Graphene Monolith with Hydrazine Hydrate, Ethylene Diamine or 1,4-Diamine Butane as Reducing Agents

[0160] The same operating conditions as those described above are used, without HAuCl.sub.4. After the lyophilization phase, hydrogels are obtained: GH-HD with hydrazine hydrate, GH-ED with ethylene diamine and GH-DB with 1,4-diamine butane.

[0161] A dense hydrogel GH-HD-AD is obtained after air drying of GH-HD.

[0162] FIG. 1 shows the scheme of the formation process of gels with different diamines used as reducing agent.

[0163] The technical characteristics of GH-HD and GH-HD-AD are presented below in the results section.

Example 5

Growth of Silicon Nanowires

[0164] A piece of graphene monolith decorated with gold nanoparticles as obtained in Examples 1, 2 or 3 is placed in a quartz crucible placed at the center of an LPCVD furnace. Growth is carried out at a temperature of around 650 C. with a reactive gas SiH.sub.4 (flow rate about 40 cm.sup.3/min), an additional HCl gas (flow rate at 100 cm.sup.3/min about) and an H.sub.2 carrier gas (flow at about 1 L/min). The total working pressure is about 6 Torr. The reaction time is about 20 minutes. This last criterion is only important for the length of the silicon nanowires.

Example 6

Supercapacities Manufacturing

[0165] The GH-HD graphene monoliths as obtained in Example 4 are cut into several slices about 1 mm thick. Two slices of the monolith, of the same mass, are pressed on nickel foams under a pressure of 10 MPa. A piece of filter paper (Whatman filter paper) was used as a separator between the electrodes. Electrodes and filter paper, quenched overnight in 6M KOH electrolyte, were assembled in a layered structure in a Swagelok-type two-electrode cell configuration with stainless steel as the current collector.

[0166] The technical characteristics of the supercapacitor obtained are presented below in the results section.

[0167] Materials and Methods

[0168] Electrochemical Measurements

[0169] The supercapacitor performance of Example 6 was evaluated using cyclic voltammetry, galvanostatic charge-discharge cycles, and electrochemical impedance spectroscopy.

[0170] A VMP3 multi-channel potentiostat/galvanostat equipped with EC-Lab software (Biologic) was used for all electrochemical techniques.

[0171] The cyclic voltammetry and charge-discharge measurements were made between 0 and 1 V with scanning rates from 100 mV/s to 1000 mV/s and from 0.5 A/g to 100 A/g respectively.

[0172] The electrochemical impedance spectroscopy test was performed in a frequency range between 400 kHz and 40 mHz and an AC disturbance of 10 mV.

[0173] The gravimetric capacitances (C.sub.wt) of graphene monoliths derived from galvanostatic discharge curves were calculated using the equation: C.sub.wt=2 I/m (V/t)), where I is the constant discharge current, m is the mass of an electrode and V and t represent the voltage change (except for V.sub.drop) on the discharge and the full discharge time, respectively.

[0174] The corresponding volumetric capacities (C.sub.vol) were calculated as follows: C.sub.vol=C.sub.wt, where is the conditioning density of graphene.

[0175] Gravimetric energy (E.sub.wt) and power densities (P) were calculated as E.sub.wt=C.sub.wtV2/8 and P=E.sub.wt/t.

[0176] Conditioning densities were obtained by calculating the dried gel mass with an accuracy of 0.01 mg and measuring the dimensions using scanning electron microscopy.

[0177] Technical Characterizations

[0178] A Metler-Toledo XPE205 weighing scale was used to obtain the weight of the samples with an accuracy of 0.01 mg.

[0179] The modifications of the chemical bond were analyzed by Fourier Transform Infrared Spectroscopy (FT-IR, Thermofischer ES 50) in the frequency range from 4000 to 400 cm.sup.1.

[0180] The materials were tested with KBr pellet.

[0181] The crystallographic structures of the materials were determined by a Wide Angle X-Ray Diffraction (XRD) system on a Panalytical Xpert PRO X-ray diffractometer using a Co K radiation source (=1.79 ).

[0182] Thermogravimetric analysis (TGA) of all samples was performed with Setaram TGA 92 equipment with a heating ramp of 5 C./minute over a temperature range of 30 C. to 800 C. under a nitrogen atmosphere.

[0183] Electrical conductivity values were obtained using a four-point measurement technique. Pellets prepared by pressing a piece of gel under 10 MPa were used for these measurements.

[0184] Conditioning densities were also calculated by measuring the mass and volume of these films.

[0185] X-ray photoelectron spectroscopy (XPS) analyzes were performed using a Versa Probe II PHI spectrometer with a monochromatized (1 486.6 eV) Al K X-ray source focused on a 100 m spot and with an electronic take-off angle of =45.

[0186] Global spectra of photoelectrons were recorded with a pass energy of 117 eV and high resolution spectra with a pass energy of 23.5 eV. Deconvolution of the C1s and N1s level spectra was performed by fitting the individual components to values obtained from previous reports using the Casa XPS software. The spectra were adjusted in Gauss-Lorentz curves with maximum values of total width less than 1.5 in all cases.

[0187] The morphology of graphene monoliths was characterized using a Zeiss Ultra 55 electron microscope at an acceleration voltage of 7 kV.

[0188] Results

[0189] GH-HD

[0190] The physical and chemical properties of the GH-HD graphene monoliths of Example 4 were characterized by FT-IR, TGA, XRD, XPS and conductivity measurements. Scanning electron microscopy was performed to analyze the three-dimensional structures.

[0191] Analysis of the GH-HD samples synthesized with the optimized reaction conditions of Example 4 revealed that the resulting hydrogels are exceptionally conductive with excellent power capabilities. In addition, instead of using large amounts of hydrazine monohydrate as described in the literature, one simply uses a molar equivalent of reducing agent corresponding to the amount of graphene oxide used.

[0192] Hydrazine, which is a strong reducing agent, promotes the formation of the monolith via the assembly of reduced graphene oxide by non-covalent interactions such as - interactions. The synthesized GH-HD therefore has a high degree of reduction with a significant C/O ratio of 16.7 (see FIG. 2), an interlayer spacing of 3.76 and a thermal behavior identical to graphite.

[0193] GH-HD has a porous morphology (see FIG. 3) and has a high specific surface area of 1433 m.sup.2/g.

[0194] The macroporous network also allows a high conductivity of GH-HD of 1141 S/m (see FIG. 2).

[0195] Finally, the synthesized GH-HD has a low resistance to charge transfer of 0.62 offering real high-speed performance.

[0196] The synthesis process according to the invention is very simple, in one step, and makes it possible to obtain these very reduced graphene monoliths which are likely to be used in lithium ion batteries and low cost supercapacitors having high powers.

[0197] Supercapacitor

[0198] The synthesized GH-HD was tested in a supercapacitor configuration according to Example 6.

[0199] Charge transfer and ion transfer within the highly conductive and porous GH-HD have been found to be responsible for the good electrochemical performance of supercapacitor.

[0200] The synthesized GH-HD, in supercapacitor configuration, demonstrates high capacities of 190 F/g at a current density of 0.5 A/g and 123 F/g at a current density of 100 A/g (see FIG. 2).

[0201] These performances are attributed, on the one hand, to three-dimensional networks with large pore volumes that facilitate the rapid transfer of ions on the electrode interface, on the other hand, to the excellent electrical conductivity of GH-HD which allows fast charge transfer to the electrode/electrolyte interface.

[0202] These results corroborate the positive impact of the electrical conductivity and the porosity of GH-HD on the charge transfer and the ion diffusion of the electrodes.

[0203] When recycled at a high current density of 100 A/g, with a secondary discharge time of 0.4 s, GH-HD in supercapacitor configuration offers a high power density of 38 kW/kg while providing an energy density of 4.3 Wh/kg (see FIG. 4). GH-HD thus exceeds the graphene monoliths obtained by reduction of graphene oxide by hydrothermal treatment, heat treatment and L-glutathione.

[0204] Finally, GH-HD in supercapacitor configuration exhibits excellent cyclic stability with a 93% capacity retention observed after 2000 cycles at a high current density of 10 A/g. Full capacity retention was observed after 5000 cycles at a current density of 2 A/g (see FIG. 5).

[0205] Graphene Oxide Reduced with Gaseous Hydrazine (CMG)

[0206] CMG was synthesized under reaction conditions similar to those of GH-HD of Example 4 with continuous stirring (ESI procedures).

[0207] CMG has an electrical conductivity of 1832 S/m, which is higher than that of GH-HD (1141 S/m); this can be understood by improving the reduction in homogenized reaction media. FIG. 6 shows the specific capacity of GH-HD and CMG in function of current density.

[0208] GH-HD Densified Gel (GH-HD-AD)

[0209] Finally, given the importance of good volumetric capacitances for compact supercapacitors, we synthesized a dense GH-HD gel by following a simple drying procedure.

[0210] After hydrazine gel synthesis, the GH-HD was densified from a density of 0.58 g/cm.sup.3 to 1.56 g/cm.sup.3 by drying under ambient conditions. The gradual removal of the water molecules from the inter-layer spaces reduces the gel to nearly one-tenth of its original size.

[0211] FIG. 7 shows the cyclic stability of GH-HD and its densified counterpart GH-HD-AD at a current density of 2 A/g. GH-HD-AD provides gravimetric and volumetric discharge capacities of 130 F/g and 200 F/cm.sup.3 respectively at a high current density of 2 A/g.

[0212] Although gravimetric capabilities are typically used as a criterion for evaluating supercapacitors, volumetric capacity is considered a crucial measure for compact portable applications. By a simple drying technique, it is possible to double the volumetric capacity of freeze-dried GH-HD from 110 F/cm.sup.3 to 257 F/cm.sup.3 at a current density of 0.5 A/g.