Patent Application
20180025853
The invention relates to components for the storage of energy, in particular capacitors. The capacitors concerned are also known as “supercapacitors”, characterized by a greater energy density than that of dielectric capacitors and a higher power density than that of batteries.
Supercapacitors generally comprise two porous electrodes impregnated with an electrolyte (an ionic salt in generally organic solution, a quaternary ammonium salt, such as tetraethylammonium tetrafluoroborate in acetonitrile or propylene carbonate, for example). These electrodes are generally separated by an insulating and porous membrane which makes possible the circulation of the ions of the electrolyte.
The first supercapacitors, known as “EDLCs” (Electrochemical Double Layer Capacitators), are based on a principle equivalent to that of conventional capacitors with polarizable electrodes and an electrolyte acting as dielectric. Their capacity originates from the arrangement of a double layer of ions and of electrons at the electrolyte/electrode interface. Today, supercapacitors combine, for the storage of energy, a capacitive component resulting from the electrostatic arrangement of the ions close to the electrodes and a pseudocapacitive component due to oxidation/reduction reactions in the capacitor.
The electrostatic component of the storage of energy is produced by a nonhomogeneous distribution of the ions of the electrolyte in the vicinity of the surface of each electrode, under the effect of the difference in potential applied between the two electrodes. The electrostatic component of the storage of energy confers a potentially high specific power and a very good behaviour during the charging and discharging cycles.
Materials having a very high ratio of specific surface to volume, having a porosity suited to ion storage at this scale, have been developed in order to increase the capacity of supercapacitors. The methods for manufacturing these materials have been directed towards the use of fullerenes, carbon nanotubes, activated carbon, carbon nanofibers or CNFs and graphene, which are advantageously light, inexpensive and ecologically appropriate.
Supercapacitors might replace conventional capacitors for applications having a high energy demand, exhibiting in particular extreme temperatures, vibrations, high accelerations or a high salinity. In these environments, batteries may not operate without their lifetime being greatly restricted (these conditions apply to radars, to motor sports, to electrical avionics and to military applications, for example).
Supercapacitors can also be applied to systems which require energy peaks over short times, of the order of the minute, for phases of acceleration of vehicles in ground transportation (motor vehicles, tramways, buses, “stop and start” devices, in which energy is recovered during the deceleration).
Supercapacitors might also be useful for the management of electricity in onboard systems, for rendering electrical installations secure, for rendering the energy supply of sensitive systems secure (radio sets, monitoring systems, military field, data centre), in networks of self-contained sensors for applications in monitoring industrial, complex or sensitive sites (hospitals, avionics, offshore platform, oil prospecting, underwater applications) and finally in renewable energies (wind power, recovery of atmospheric electrical energy).
In order to make an industrial application possible, the energy density and the power of supercapacitors have to be optimized. Furthermore, the internal resistance of a supercapacitor is today too high and poorly controlled. The usual supercapacitors are composed of activated carbons with nonhomogeneous and nonoptimized distributions of the size of the pores and use a polymeric binder to ensure the mechanical strength of their structure. This binder increases the internal electrical resistance of the capacitor and disadvantageously increases its weight. The unsuitable porosity also produces a resistance to ion transfer within the active material.
The publication by Bondavalli, P., Delfaure, C., Legagneux, P. and Pribat, D., 2013, “Supercapacitor electrode based on mixtures of graphite and carbon nanotubes deposited using a dynamic air-brush deposition technique”, Journal of The Electrochemical Society, 160(4), A601-A606, discloses a process for the deposition of graphene nano/microparticles and of carbon nanotubes by hydrodynamic spraying of a suspension over a support. This process makes it possible to manufacture supercapacitors achieving high energy and power densities, without use of a polymeric binder, but requires the use of toxic and polluting solvents, such as N-methyl-2-pyrrolidone (NMP) in order to make possible the suspension of the nano/microparticles.
The publication by Youn, H. C., Bak, S. M., Park, S. H., Yoon, S. B., Roh, K. C. and Kim, K. B., 2014, “One-step preparation of reduced graphene oxide/carbon nanotube hybrid thin film by electrostatic spray deposition for supercapacitor applications”, Metals and Materials International, 20(5), 975-981, discloses the use of graphene oxide and of oxidized carbon nanotubes for an electrostatic spraying of a suspension over a support for the manufacture of supercapacitors. This process uses heating at 300° C. during the deposition, of use in the reduction or in the deoxidation of the carbon-based structures present, but limits the manufacture of thick layers as the solution evaporates before the deposition. In addition, this process uses a water/ethanol mixture as solvent for the suspension of the oxidized particles. This characteristic reduces the evaporation temperature of the solvent, which also promotes evaporation of the solvent before deposition on the substrate and prevents the manufacture of a thick layer. Furthermore, the use of ethanol in the solvent is toxic and is not ecologically appropriate.
A subject-matter of the present invention is a process for the deposition of nano/microparticles, including at least graphene sheets, on a substrate, comprising the steps consisting in:
Advantageously, the said nano/microparticles are suspended in one said solution, the said solvent of which is more than 95% by weight composed of water (H2O) and preferably more than 99% by weight composed of water.
Advantageously, a plurality of said suspensions are simultaneously sprayed over the said substrate.
Advantageously, the nano/microparticles of the deposition process are chosen from carbon nanotubes, carbon nanofibers, carbon nanorods, carbon nanohorns, carbon onions and a mixture of these nano/microparticles, in which the said nano/microparticles are oxidized before spraying them and in which the said deposit, after the said spraying, is annealed at a temperature sufficient to deoxidize the said nano/m icroparticles.
Advantageously, at least one said nano/microparticle is wet oxidized with at least one element chosen from sulphuric acid, phosphoric acid, sodium nitrate, nitric acid, potassium permanganate and hydrogen peroxide.
Advantageously, a heating element brought into contact with a support heats the said substrate and each said part of said suspension sprayed over the said substrate.
Advantageously, the said deposit is annealed at a temperature of between 200° C. and 400° C.
The invention also relates to a process for the manufacture of an electrode comprising, in superimposition, a deposit of nano/microparticles and a substrate, the said substrate comprising a current collector and the said deposit of nano/microparticles being obtained by a deposition process described above.
The present invention also relates to an electrode, the said deposit of nano/microparticles of which is capable of being obtained by a process described above.
Advantageously, the said deposit of the electrode comprises at least graphene and a type of said nano/microparticles chosen from carbon nanotubes, carbon nanofibers, carbon nanorods, carbon nanohorns and carbon onions.
The present invention also relates to a supercapacitor comprising at least one said electrode described above.
The following description exhibits several implementational examples of the device of the invention: these examples are nonlimiting of the scope of the invention. These implementational examples exhibit both the essential characteristics of the invention and also additional characteristics related to the embodiments under consideration. For the sake of clarity, the same elements will bear the same references in the different figures.
“Nanopartcle” is understood to mean particles, at least the smallest of the dimensions of which is nanometric, that is to say of between 0.1 nm and 100 nm. “Microparticle” is understood to mean particles, at least the smallest of the dimensions of which is micrometric, that is to say of between 0.1 μm and 100 μm.
The geometries of nano/microparticles comprise nano/microfibers, nano/microrods, nano/microtubes, nano/microhorns, nano/microonions and nano/microsheets of the monolamellar type comprising a crystalline layer or multilamellar type comprising several stacked lamellae. A nano/microtube is formed of one or more wound nano/microsheets. A nano/microfiber is a solid one-dimensional object of a bulk material. A nano/microrod is a hollow one-dimensional object.
In the case of carbon, a lamella is denoted by the term “graphene” and exists in the form of a two-dimensional carbon crystal of monoatomic thickness and of nano/micrometric size. The carbon nanotubes are known and formed of a graphene lamella wound into a tube (denoted by the acronym of SWCNT (Single Wall Carbon NanoTube)) or of several stacked graphene lamellae wound into a tube (denoted by the acronym of MWCNT (Multi Wall Carbon NanoTube)).
“Electrode” is understood to mean an assembly comprising a deposit of nano/microparticles on a substrate (comprising a current collector which electrically conducts and optionally a thick material or layer for the mechanical strength of the electrode).
A better understanding of the invention will be obtained and other advantages, details and characteristics of the invention will become apparent during the explanatory description which follows, made by way of example with reference to the appended drawings, in which:
The following description exhibits several implementational examples of the device of the invention: these examples are nonlimiting of the scope of the invention. These implementational examples exhibit both the essential characteristics of the invention and also additional characteristics related to the embodiments under consideration. For the sake of clarity, the same elements will bear the same references in the different figures.
The apparatus 3 comprises a spray nozzle 4, a tank 5 containing a suspension of nano/microparticles and a spray gas source 6. The nano/microparticles comprise oxidized graphene particles and can comprise, in specific implementations of the invention, oxidized carbon nanotubes, oxidized carbon nanofibers, oxidized carbon nanorods, oxidized carbon nanohorns and oxidized carbon onions. Other nanoparticles can be envisaged.
The solvent used for the suspension can advantageously be composed of more than 95% of water (H2O) and more advantageously still composed of more than 99% of water (H2O). In specific implementations of the invention, water can be mixed with other solvents, in proportions which allow them to remain miscible with the water, such as a methanol (CH4O), ethanol (C2H6O), ethylene chloride (DCE), dichlorobenzidine (DCB), N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), hexamethylphosphoramide (HMPA), cyclopentanone (C5H8O), tetramethylene sulphoxide (TMSO), ε-caprolactone, 1,2-dichlorobenzene, 1,2-dimethylbenzene, bromobenzene, iodobenzene and toluene. Other compounds can be envisaged.
The spray gas is, for example, air.
The nozzle 4 is fed with suspension from the tank 5 and with spray gas from the source 6. The nozzle 4 is suitable for spraying the suspension, fed at low pressure, as microdrops using the gas, fed at high pressure. The nozzle 4 is of the airbrush type. The drops are created by hydrodynamic instability between the liquid phase, the gas phase and the nozzle 4, i.e., in a specific implementation of the invention, sprayed by effect of the pressure imposed on the water, on the air and on the geometry of the nozzle.
“Microdrops” is understood to mean drops with a size of a microscopic nature, the diameter of which is between approximately 1 and 100 micrometres.
In a specific implementation of the invention, the apparatus 3 comprises elements 7 for heating the support 8 in the form of resistive heating elements 9 connected to an electrical supply circuit (not represented) so that the resistive heating elements 9 emit heat by the Joule effect when an electric current passes through them. In an alternative form, the apparatus 3 comprises elements 7 for heating the support 8 by induction, for example comprising a plate on which the support 8 is placed with inductors, in order to induce currents in the plate and to generate heat.
The apparatus 3 comprises a temperature sensor 10 positioned so as to measure the temperature of the support 8.
In operation, the nozzle 4 generates a spray jet 11 formed of suspension microdrops projected in the direction of the surface 12 to be covered of the substrate 15. The spray jet 11 reaches the surface 12 to be covered in an impact zone 13, the shape and the dimensions of which depend in particular on the geometry of the nozzle 4, on the adjustment of the nozzle 4 and on the position of the nozzle 4 with respect to the surface 12 to be covered.
The shape and the dimensions of the impact zone 13 depend in particular on the angle a at the top of the cone formed by the spray jet 11 at the outlet of the nozzle 4 and on the distance between the outlet of the nozzle 4 and the surface 12 of the substrate 15. They also depend on the pressure of the spray gas (related to the spray gas flow rate and on the flow rate of each suspension.
The spray jet 11 is, for example, a cone of revolution, so it forms an impact zone 13 of circular general shape. In an alternative form, the spray jet 11 might define an oblong impact zone 13, which is more elongated in a first direction than in a second direction perpendicular to the first.
In a specific implementation of the invention, the nano/microparticles used to form a deposit 1 can be graphene sheets and single wall carbon nanotubes (SWCNT).
In a first stage, the carbon-based nano/microparticles are oxidized. The carbon-based nano/microparticles are, for example, SWCNTs. SWCNTs are dispersed in a mixture of equal volumes of sulphuric acid and nitric acid for 30 minutes. The mixture is subsequently refluxed for 3 hours. The SWCNTs are then oxidized. They can be recovered by filtering the mixture under vacuum and by washing them with several hundred millilitres of water until a neutral pH of the filtrate is obtained. The product is dried under vacuum at 70° C. for several days.
The graphene oxide particles can be obtained commercially.
In a second stage, suspensions of each of the different particles in deionized water can be prepared by sonication for one hour, at a concentration of between 5 μg·ml−1 and 50 mg·ml−1 and preferably between 50 μg·ml−1 and 5 mg·ml−1. It is possible subsequently to combine together the different suspensions into just one suspension and to place the suspension under ultrasound for one hour.
In a third stage, the nano/microparticles are deposited on the current collector of the substrate 15. Deposition is carried out by spraying the suspension by hydrodynamic instability over a substrate 15 heated to a temperature preferably of greater than 100° C. and preferably of less than or equal to 200° C., indeed even 150° C.: the temperature has to be sufficient to make possible rapid evaporation of the drops deposited by spraying and to thus prevent the “coffee stain” effect, that is to say a nonhomogeneous surface distribution of adsorbed nano/microparticles. On the other hand, an excessively high temperature, such as that presented in the process presented by Youn et al., would bring about a complete evaporation of the drops during their journey between the nozzle 4 and the support 8, thus preventing a controlled and efficient adsorption or attachment. As a minimum, the process of Youn et al. requires the use of a high suspension volume in order to compensate for the total evaporation, induced by a high temperature, of a high proportion of the sprayed suspension.
In a fourth stage, the deposit 1 is annealed at a temperature of greater than 200° C. in order to deploy the surfaces accessible by the electrolyte 2 in the deposit of nano/microparticles 1, to reduce or deoxidize the graphene oxide and the oxidized nanotubes and to increase the conductivity of the deposit of de nano/microparticles 1. This step is necessary as the deposition temperature is too low to reduce or to deoxidize the nano/microparticles of the deposit 1. This step exhibits two distinct advantages with respect to the process presented by Youn et al.; on the one hand, the annealing makes it possible to deoxidize the nano/microparticles at an effective temperature while retaining a lower temperature during the spraying (and the advantages which are related thereto and presented in the preceding paragraph). On the other hand, the annealing can be carried out in a controlled manner, by applying, for example, an equal annealing time for all of the particles to be deposited. Disadvantageously, in the process presented by Youn et al., the particles deposited at the start of the spraying will be subjected to a different annealing time from the particles deposited at the end of the spraying.
During this implementation of the process in accordance with the invention, the two types of carbon-based structures are organized into a hierarchy during the deposition by spraying over the substrate 15 heated by the support 8, which makes it possible to instantaneously evaporate the water. This organization into a hierarchy is illustrated by
The rectangular shape of the different cyclic voltammograms of
For all of the cycling rates,
The crossing of the curves (f) and (g) shows the advantage of an interaction between oxidized graphene and oxidized SWCNT nano/microparticles in order to retain a high specific capacity even at a high cycling rate. Furthermore, the curve (f) illustrates that the interaction between oxidized graphene and oxidized SWCNT nano/microparticles makes it possible to retain relatively stationary specific capacity values.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1500231 | Feb 2015 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2016/052541 | 2/5/2016 | WO | 00 |
