Method for preparing an electrically conductive composite material, the material obtained thereby and the uses thereof

Abstract

The present invention relates to a method for preparing an electrically conductive composite material, the method comprising the following steps: (a) mixing an alumino-silicate source with electrically conductive carbon-based fillers, whereby a homogeneous solid mixture is obtained; (b) adding the homogeneous solid mixture obtained in step (a) to an activation solution comprising a SiO2 / Na2O molar ratio of between 1 and 1.4, whereby a geopolymer slurry is obtained in which the electrically conductive carbon-based fillers are dispersed; then (c) allowing the geopolymer slurry obtained in step (b) to harden, whereby an electrically conductive composite material is obtained. The present invention also relates to the electrically conductive composite material obtained thereby and to the uses thereof.

Description

[0001] PROCESS FOR PREPARING AN ELECTRICALLY CONDUCTIVE COMPOSITE MATERIAL, SAID MATERIAL AND ITS USES

[0002] TECHNICAL FIELD

[0003] The present invention relates to the field of composite materials and in particular electrically conductive composite materials.

[0004] Thus, the present invention provides a method for preparing an electrically conductive composite material comprising a geopolymer matrix in which electrically conductive carbon fillers are dispersed.

[0005] The present invention also relates to said composite material as well as its various uses, particularly in the field of heat production and energy storage. In particular, the present invention relates to a symmetrical supercapacitor comprising two electrodes made of such a composite material.

[0006] STATE OF THE PRIOR ART

[0007] Numerous works and studies focus on the influence of carbon-based admixtures on the new functional properties of cement matrix or geopolymer matrix materials.

[0008] Thus, Fiala et al, 2019 [1] describe geopolymers in which carbon is dispersed in the form of spherical particles, possibly aggregated, with an average grain size dso equal to 0.52 pm and dgo equal to 17.6 pm. During the preparation of these geopolymers obtained from a granulated blast furnace slag rich in calcium (39.8% CaO), a dispersing agent of the non-ionic surfactant type (Triton™ X-100) is used during the preparation of the suspension containing the carbon particles, which improves the flow properties. In addition, another silicone oil-based surfactant (Lukosan S) is used to eliminate air bubbles generated during high-power mixing of the different ingredients. The activation solution used in [1] includes a SiCh / NazO molar ratio of 2.07.The composite materials described in [1] are used in the field of Joule effect heating for various applications such as collective heating or pavement thawing.

[0009] The use of low-calcium fly ash makes a high-temperature curing step essential to obtain a geopolymer. The work of Cai et al, 2020 [2] therefore aims to provide a self-heating fly ash-based geopolymer to increase curing efficiency and save energy. To achieve this, carbon black and / or steel fibers are added to the fly ash-based geopolymer to increase its electrical properties. From the results obtained, it appears that not only can a conductive geopolymer with self-electrical behaviors be obtained, but also that the fly ash-based geopolymer cured after electrical heating has a denser microstructure and therefore exhibits higher strength and better ductility.When preparing the composite material consisting of a geopolymer with carbon black and / or steel fibers, a water-reducing plasticizing agent of the naphthalene sulfonate type must be used to maintain good fluidity of the geopolymeric grout. In addition, the activation solution was prepared from a combination of NazSiCh comprising 14.7% NazO, 29.4% SiC>2 and 55.9% water and a KOH solution with an alkali concentration of 12 mol / L. Thus, the activation solution implemented by Cai et al, 2020 [2] comprises a SiCh / NazO molar ratio of 2.

[0010] Finally, patent application US 2019 / 0218144 Al [3] describes hydraulic cements and in particular Portland cements made conductive by dispersion of carbon nanoparticles. The use of carboxymethylcellulose as a dispersing agent is necessary to improve the compatibility between carbon nanoparticles considered hydrophobic and Portland cement considered hydrophilic. Resistivity measurements were carried out by the authors with different types of carbon black (CB) and they showed that above a critical concentration, the cement / CB composite became conductive since a percolating path of carbon nanoparticles is created within the material.

[0011] When the composites reach this critical concentration, they can produce heat by Joule effect by passing a low voltage across the sample terminals. Furthermore, given this conductive network, the authors showed that these cement / CB monoliths could be used as EDLC (Electric Double Layer Capacitor) type supercapacitors. The performance of these supercapacitors is, however, far from satisfactory due to high resistance. This is linked to the dispersing agent that partially covers the hydrophobic carbon nanoparticles, thus limiting the access of ions to the surface of these nanoparticles.

[0012] The inventors set themselves the goal of proposing a process for preparing an electrically conductive composite material which does not have the disadvantages of the materials of the prior art.

[0013] STATEMENT OF THE INVENTION

[0014] The present invention makes it possible to achieve the goal set by the inventors since the latter propose a process making it possible to obtain, without using dispersing or compatibilizing materials or plasticizing or superplasticizing materials, an electrically conductive composite material comprising a geopolymer matrix in which electrically conductive carbon fillers are dispersed.

[0015] Indeed, the present inventors have shown that the use of a particular activation solution, namely an activation solution in which the SiCh / NazO molar ratio is moderate, i.e. between 1 and 1.4, makes it possible to greatly improve the wettability of carbon fillers such as carbon nanoparticles, thus limiting the use of organic molecules to promote the mixing of hydrophobic carbon fillers with a hydrophilic binder. By avoiding the addition of dispersing or compatibilizing agents, the preparation process according to the invention is greatly simplified.

[0016] Furthermore, during the process for preparing the electrically conductive composite material according to the invention and in particular thanks to the use of an activation solution whose SiCh / NazO molar ratio is between 1 and 1.4, the geopolymeric grouts obtained have rheological properties allowing easy shaping, without the addition of plasticizer or superplasticizer, and can use additive preparation steps such as 3D printing.

[0017] Furthermore, the method for preparing the electrically conductive composite material according to the invention can use industrial by-products and in particular potassium silicates from the photovoltaic industry, blast furnace slag and carbon blacks. The electrically conductive composite material thus obtained can therefore be 100% derived from by-products.

[0018] In addition, the absence of organic molecules such as dispersing or compatibilizing agents in the final electrically conductive composite material improves the properties and performance of this material compared to those obtained with prior art materials and in particular with the cement-based materials described in [3]. Indeed, as previously explained, the dispersing or compatibilizing agents necessary to have good shaping of the prior art materials reduce the performance of the electrodes prepared from these materials since a portion of these agents covers the carbon nanoparticles, limiting access to ions, thus lowering cell performance.

[0019] In other words, in the present invention, the fact of not using dispersing or compatibilizing agents, nor plasticizing or superplasticizing agents greatly improves the performance of the supercapacitor since no molecule covers the surface of the carbons which leaves the path free for the ions to diffuse in the open porosity of the geopolymer and to migrate to the interface of the conductive elements. A double electrical layer responsible for a high power density is thus created.

[0020] In addition to improved mixing properties and increased storage efficiency, the advantage of using geopolymers also lies in the fact that the porous network is completely open, thereby facilitating ionic transport compared to cements, for example, and controllable from the formulation parameters and in particular by the water / binder ratio, i.e. the water / aluminosilicate source ratio. Indeed, by increasing the water content, the pore volume increases without drastically reducing the mechanical properties of the material. Furthermore, this porosity is filled with a solution rich in dissolved salts (alkaline salts) whose pH is close to 13, which intrinsically gives the material good conductive properties.

[0021] As a consequence of all these advantages, the composite material prepared by the process according to the invention has high cyclability without loss of performance as illustrated in the experimental part below.

[0022] The present invention therefore relates to a process for preparing an electrically conductive composite material comprising the following steps:

[0023] (a) mixing an alumino-silicate source with electrically conductive carbonaceous fillers whereby a solid, homogeneous mixture consisting of an alumino-silicate source and electrically conductive carbonaceous fillers is obtained,

[0024] (b) adding the solid, homogeneous mixture, consisting of an aluminosilicate source and electrically conductive carbon fillers and obtained in step (a) to an activation solution whose pH is greater than 9 and comprising a SiCh / NazO molar ratio of between 1 and 1.4, whereby a geopolymeric slurry in which said electrically conductive carbon fillers are dispersed is obtained, said geopolymeric slurry not comprising any dispersing agent, the Si / Al molar ratio in said geopolymeric slurry being between 1.5 and 2, the water / aluminosilicate source ratio in said geopolymeric slurry being between 0.65 and 1.10, then

[0025] (c) allowing the geopolymeric grout obtained in step (b) to harden at room temperature, whereby an electrically conductive composite material is obtained.

[0026] By "electrically conductive composite material" is meant, in the context of the present invention, a material comprising a geopolymeric matrix (or geopolymeric skeleton) in which electrically conductive carbon fillers are dispersed.

[0027] By "geopolymeric matrix", "geopolymeric skeleton" or

[0028] "geopolymer" means, in the context of the present invention, a solid, inorganic and porous material in the dry state, obtained following the hardening of a mixture containing finely ground materials (i.e. the aluminosilicate source) and a saline solution (i.e. the activation solution), said mixture being capable of setting and hardening over time. This mixture may be referred to as "geopolymeric grout", "geopolymeric mixture", "geopolymeric paste", "geopolymeric composition" or even "geopolymer composition". The hardening of the geopolymer is the result of the dissolution / polycondensation of the finely ground materials of the geopolymeric mixture in the saline solution such as a high pH saline solution (i.e. the activation solution).

[0029] More specifically, a geopolymer or geopolymer skeleton or geopolymer matrix is ​​an amorphous alumino-silicate inorganic polymer. Said polymer is obtained from a reactive material containing essentially silica and aluminum (i.e. the alumino-silicate source), activated by a strongly alkaline solution, the solid / solution mass ratio in the formulation being low. The structure of a geopolymer is composed of a Si-O-AI network formed of silicate (SiC) and aluminate (AIO4) tetrahedra linked at their vertices by sharing oxygen atoms. The substitution of a silicon atom (valence +IV) by an aluminum atom (valence +III) results in a charge deficit compensated by one or more charge compensating cation(s) also called compensation cation(s) which compensate for the negative charge of the AIG complex.The compensation cation(s) is (are) advantageously chosen from the group consisting of alkali metals such as lithium (Li), sodium (Na), potassium (K), rubidium (Rb) and cesium (Cs), alkaline earth metals such as magnesium (Mg), calcium (Ca), strontium (Sr) and barium (Ba) and mixtures thereof. In particular, the compensation cation(s) is (are) advantageously chosen from the group consisting of sodium (Na), potassium (K) and mixtures thereof.

[0030] In the electrically conductive composite material prepared by the method according to the invention, the geopolymer or geopolymer matrix has, due to its preparation method, an intrinsic mesoporous porosity of the order of 10 to 20% by volume relative to the total volume of the geopolymer, the latter being also able to have unconnected macropores. By "macropores" is meant pores or voids having an average diameter greater than 50 nm and in particular greater than 70 nm. By "mesopores" is meant pores or voids having an average diameter of between 2 and 50 nm and in particular between 2 and 20 nm.

[0031] The aluminosilicate source is a reactive material containing essentially silica and aluminum that can be used to prepare the geopolymer, i.e. the geopolymer matrix of the composite material prepared within the scope of the invention. It is a solid source containing amorphous aluminosilicates. These amorphous aluminosilicates are in particular chosen from natural aluminosilicate minerals such as illite, stilbite, kaolinite, pyrophyllite, andalusite, bentonite, kyanite, milanite, grovenite, amesite, cordierite, feldspar, allophane, etc.; calcined natural aluminosilicate minerals such as metakaolin; synthetic glasses based on pure aluminosilicates; aluminous cement; pumice; calcined by-products or residues of industrial exploitation such as fly ash and blast furnace slag respectively obtained from the combustion of coal and during the transformation of iron ore into cast iron in a blast furnace; and mixtures thereof. In a particular embodiment, the aluminosilicate source used is metakaolin.

[0032] By "electrically conductive carbon fillers" is meant electrically conductive carbon fillers whose largest dimension is less than 1 mm. Thus, these fillers may be nanometric sized fillers, micrometric sized fillers or a mixture thereof. The electrically conductive carbon fillers used in the present invention may have any shape such as, for example, a flattened shape, a cylindrical shape, a tubular shape or a spherical shape. They may in particular be in the form of powder, grains, flakes, particles, threads, hollow fibers, filaments, tubes, ribbons, sheets, clusters or a mixture thereof. In a particular embodiment, the electrically conductive carbon fillers used in the present invention are in the form of a powder whose grain size is nanometric.It should be noted that, within the framework of the present invention, we will seek to integrate electrically conductive carbon fillers with the largest possible specific surfaces to improve the storage performance of the composite material obtained.

[0033] As particular examples of electrically conductive carbonaceous fillers which can be used in the present invention, mention may be made in particular of fillers as previously defined in graphite, highly oriented pyrolytic graphite, graphene, graphene oxide, reduced graphene oxide, carbon, activated carbon, vitreous carbon and carbon black.

[0034] As more specific examples of electrically conductive carbon fillers that can be used in the present invention, mention may be made of graphite grains, graphite flakes, graphite particles, highly oriented pyrolytic graphite grains, graphene sheets, graphene oxide sheets, reduced graphene oxide sheets, fullerene, glassy carbon flakes, glassy carbon particles, carbon black powder, single-walled carbon nanotubes (or SWCNTs for "Single-Walled Carbon Nanotube"), functionalized SWCNTs, MWCNTs and functionalized MWCNTs. By “functionalized SWCNT or MWCNT” is meant SWCNT or MWCNT on at least one wall of which one or more reactive unit(s) such as, for example, a vinyl monomer, a carboxyl group, a carbonyl group or a phenol group is / are covalently grafted.

[0035] As even more specific examples of electrically conductive carbon fillers that can be used in the present invention, mention may be made of a carbon black powder such as the carbon black powder marketed by the company Cabot under the name PBX™ 55.

[0036] During step (a) of the process according to the present invention, a solid mixture is obtained, i.e. a mixture comprising only solid elements (aluminosilicate source and electrically conductive carbon fillers) and therefore comprising no liquid element.

[0037] The quantity of electrically conductive carbon fillers used during step (a) of the process according to the invention will be chosen so as to reach the percolation threshold. In the experimental part below, it has been demonstrated that by using, as an aluminosilicate source, metakaolin and, as electrically conductive carbon fillers, carbon black powder marketed by the company Cabot under the name PBX™ 55, this percolation threshold is reached for a mass ratio (PBX™ 55 carbon black powder) / (metakaolin) greater than or equal to 8%. It is obvious that by using other electrically conductive carbon fillers, the mass ratio necessary to reach the percolation threshold may be different and in particular lower in the case of carbon black powder having a higher specific surface area than that of PBX™ 55 carbon black powder.The person skilled in the art, on the basis of the experimental part below, will be able to determine, without inventive effort, the quantity of electrically conductive carbon charges to be used as a function of the quantity of aluminosilicate source to reach the percolation threshold.

[0038] Prior to step (b) of the process according to the invention, it is appropriate to prepare an activation solution having a SiCh / NazO molar ratio of between 1 and 1.4 and in particular of the order of 1.2 (i.e. 1.2 ± 0.1). The preparation of an activation solution is a classic step in the field of geopolymers.

[0039] The terms "activating solution", "high pH saline solution" and "strongly alkaline solution" are, in the present invention, similar and can be used interchangeably.

[0040] By "strongly alkaline" or "high pH" is meant a solution whose pH is greater than 9, in particular greater than 10, in particular greater than 11 and, more particularly, greater than 12. In other words, the activation solution has an OH- concentration greater than 0.01 M, in particular greater than 0.1 M, in particular greater than 1 M and, more particularly, between 5 and 20 M.

[0041] The activation solution used in the context of the invention is a strongly alkaline aqueous solution which may optionally contain silicate components chosen in particular from the group consisting of silica, colloidal silica and vitreous silica.

[0042] The activation solution used in the context of the invention is an aqueous solution, which means that the solvent it contains is water. By "water", we mean, in the context of the invention, tap water, deionized water (or demineralized water), distilled water, ultrapure water (18.2 MQ) or a mixture thereof.

[0043] In addition, the activation solution implemented in the context of the invention comprises the compensation cation M or the mixture of compensation cations in the form of an ionic solution or a salt. Thus, the activation solution is in particular chosen from an aqueous solution of sodium silicate (NazSiC), potassium silicate (KzSiOz), sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide (CafOH), cesium hydroxide (CsOH) and their derivatives, etc. In particular, the activation solution implemented in the invention is a strongly alkaline aqueous solution of sodium silicate (NazSiC). More particularly, the activation solution implemented in the invention is an aqueous solution of sodium silicate (NazSiC) made alkaline by adding NaOH.More particularly still, the activation solution used in the invention is an aqueous solution prepared from BETOL®39T marketed by the company Woellner and made alkaline by adding NaOH.

[0044] Step (b) of the method according to the invention therefore comprises the preparation of a geopolymeric grout obtained following the addition of the solid and homogeneous mixture comprising an aluminosilicate source and electrically conductive carbon fillers to the activation solution.

[0045] Taking into account any silicate components that the activation solution may contain, the quantity of aluminosilicate source(s) is such that

[0046] - the Si / Al molar ratio in the geopolymeric grout obtained in step (b) of the process of the invention and therefore in the geopolymer matrix of the final electrically conductive composite material is between 1.5 and 2 and, in particular, of the order of 1.8 (i.e. 1.8 ± 0.1) and / or

[0047] - the water / aluminosilicate source(s) ratio in the geopolymeric grout obtained in step (b) of the process of the invention and therefore in the geopolymeric matrix of the final electrically conductive composite material is between 0.65 and 1.10, in particular between 0.88 and 1.00 and, in particular, of the order of 0.94 (i.e. 0.94 ± 0.05).

[0048] The solid and homogeneous mixture comprising an aluminosilicate source and electrically conductive carbon fillers can be poured in one or more times onto the activation solution. In a particular embodiment, this solid and homogeneous mixture can be sprinkled onto the activation solution.

[0049] Advantageously, step (b) of the method according to the invention is carried out in a mixer into which the activation solution has been previously introduced. Any mixer known to those skilled in the art can be used in the context of the present invention. By way of non-limiting examples, mention may be made of a NAUTA® mixer, a HOBART® mixer, a HENSCHEL® mixer and a HEIDOLPH® mixer.

[0050] Step (b) of the method according to the invention comprises mixing or kneading the activation solution with the solid and homogeneous mixture comprising an aluminosilicate source and electrically conductive carbon fillers. This mixing or kneading is initially carried out at a relatively slow speed. By "relatively slow speed" is meant, in the context of the present invention, a rotation speed of the mixer rotor less than or equal to 500 rpm / min, in particular greater than or equal to 100 rpm / min and, in particular, between 150 and 300 rpm / min. By way of non-limiting example, in the case of a standardized mixer, the stirring speed is 200 rpm / min. Once the mixing has been carried out and to obtain optimal homogenization, the stirring speed can be increased to a sustained speed, i.e. a speed greater than 700 rpm / min, in particular greater than 800 rpm / min and, in particular, of the order of 1000 rpm / min (i.e. 1000 rpm / min ± 100 rpm / min).This stirring at sustained speed typically lasts between 1 min and 15 min and in particular around 3 min (i.e. 3 min ± 1 min).

[0051] Step (b) of the process according to the invention is carried out at a temperature between 10°C and 40°C, advantageously between 15°C and 30°C and, more particularly, at room temperature (i.e. 23°C ± 5°C). Step (b) of the process according to the invention is carried out for a sufficient time to obtain a homogeneous geopolymeric paste.

[0052] The conditions allowing the geopolymeric grout to harden during step (c) of the method according to the invention include the conditions typically implemented during a curing step of a geopolymer. This hardening can be carried out in the open air, under water, in various hermetic molds, by humidifying the atmosphere surrounding the grout and / or by applying an impermeable coating or impermeable material such as a parafilm to this grout. Typically, in the context of the method according to the invention, this hardening can be carried out at a temperature between 10°C and 50°C, in particular between 15°C and 40°C and, in particular, at room temperature. It can last between 12 h and 2 days and in particular of the order of 24 h (i.e. 24 h ± 2 h).

[0053] As previously mentioned, no dispersing or compatibilizing agent is used in any of the steps of the process according to the invention. By "dispersing or compatibilizing agent" is meant any agent promoting the dispersion of a material, in this case electrically conductive carbon fillers, in a medium, in particular a liquid medium. Examples of dispersing or compatibilizing agents include surfactants, in particular non-ionic surfactants such as, for example, Triton™ X-100 or Lukosan S. Similarly, no plasticizing or superplasticizing agent is used in any of the steps of the process according to the invention. Examples of plasticizing agents include naphthalene sulfonate.

[0054] The present invention also relates to an electrically conductive composite material prepared by a method as previously defined. More particularly, the electrically conductive composite material according to the invention is in the form of a geopolymeric matrix in which electrically conductive carbon fillers are dispersed. The composite material according to the invention does not comprise any dispersing or compatibilizing agent, nor any plasticizing or superplasticizing agent.

[0055] The electrically conductive composite material according to the invention and in particular the geopolymeric matrix which it comprises has all of the following characteristics:

[0056] - a SiOz / AhOs molar ratio between 3.2 and 3.8 and, in particular, of the order of 3.6 (i.e. 3.6 ± 0.1);

[0057] - a FbO / NazO molar ratio greater than or equal to 12 and, in particular, of the order of 14 (i.e. 14 ± 1); and

[0058] - an AIzOî / NazO molar ratio between 0.8 and 1.2 and in particular of the order of l (i.e. l ​​± 0.l).

[0059] Due to the properties of the electrically conductive composite material, the present invention relates, firstly, to the use of such a material to produce heat by the Joule effect by applying an electrical potential difference to the terminals of this material and in particular a low electrical potential difference to the terminals of this material.

[0060] By "low electrical potential difference" is meant an electrical potential difference of less than 20 V, in particular less than or equal to 15 V and, in particular, less than or equal to 10 V. A person skilled in the art will be able to determine, without inventive effort and by means of a routine test, the electrical potential difference to be applied to the terminals of the electrically conductive composite material as a function of the desired temperature difference. As a particular example, if a temperature delta of the order of 10°C is desired, the applied electrical potential difference will be of the order of 5 V.

[0061] In other words, the present invention relates to a structure such as a monolithic structure, heatable by Joule effect, comprising at least one electrically conductive composite material and at least two terminals configured to receive the application of an electrical potential difference between these at least two terminals. This structure such as a monolithic structure may comprise at least a portion of a wall, a ground, a basement, a floor, a sidewalk, a road or an airport runway.

[0062] The present invention also relates to an electrode comprising or consisting of an electrically conductive composite material as previously defined.

[0063] Due to the properties of the electrically conductive composite material that it comprises or of which it is made, the electrode according to the invention makes it possible to store energy by capacitive effect. Also, the present invention relates to the use of such an electrode as a positive / negative electrode of a device for storing and restoring electricity such as a supercapacitor and in particular a symmetrical supercapacitor as defined in Example 3 below or a battery, as an electrode for a photovoltaic device, in materials for storing CO2 or as an electrode for electrochemical sensors.Thus, the present invention relates to a device comprising an electrode according to the present invention, said device being chosen from the group consisting of a device for storing and restoring electricity such as a supercapacitor and in particular a symmetrical supercapacitor as defined in example 3 below or a battery; a photovoltaic device; a material for storing CO2 and an electrochemical sensor.

[0064] Other characteristics and advantages of the present invention will become apparent to those skilled in the art upon reading the examples below given for illustrative and non-limiting purposes, with reference to the appended figures.

[0065] BRIEF DESCRIPTION OF THE DRAWINGS

[0066] Figure 1 is a photograph of the geopolymeric grout used to prepare a composite material of geopolymer with carbon black of molar composition of 3.6 SiOz / l AI2O3 / I Na2O / 14 H2O + 9% by mass of carbon black relative to the mass of metakaolin.

[0067] Figure 2 shows the influence of the composition of the composite material according to the invention on mass loss.

[0068] Figure 3 shows the pore size distribution as a function of the composition of the composite material according to the invention.

[0069] Figure 4 shows the nitrogen adsorption isotherms with, in insert, the evolution of the specific surface area and the pore volume of the composite material according to the invention as a function of the carbon content.

[0070] Figure 5 shows the influence of the carbon content of the composite material according to the invention on its resistivity.

[0071] Figure 6 shows the temperature variation for a composite material according to the invention, dried and comprising 8% by mass of carbon black relative to the mass of metakaolin.

[0072] Figure 7 is a schematic diagram of a supercapacitor.

[0073] Figure 8A shows the cyclic voltammetry of a supercapacitor comprising electrodes made of a composite material according to the invention as a function of the voltage scan rate. Figure 8B shows the capacitance of a supercapacitor comprising electrodes made of a composite material according to the invention as a function of the voltage scan rate.

[0074] Figure 9A shows a galvanostatic cycle of a supercapacitor comprising electrodes made of a composite material according to the invention.

[0075] Figure 9B shows the calculation of the capacity of a supercapacitor comprising electrodes made of a composite material according to the invention.

[0076] Figure 10 shows the performance evaluation of the supercapacitor comprising electrodes made of a composite material according to the invention by galvanostatic cycling.

[0077] DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0078] Example 1: Synthesis of a composite material according to the invention.

[0079] I. General principle.

[0080] First, a premix of dry matter is prepared. A quantity of carbon black and aluminosilicate powder (metakaolin in this case) is weighed. The two products are mixed by hand to obtain a premix that is visually homogeneous.

[0081] The activation solution is prepared separately. It consists of a mixture of alkali hydroxide, alkali silicate, and water. In practice, a quantity of sodium hydroxide in the form of pellets is weighed, to which a precise quantity of sodium silicate and water is added. The solution is left stirring until the sodium hydroxide pellets are completely dissolved and to wait for the solution to return to room temperature.

[0082] When the solution and the premix are ready, the solid premix is ​​added to the activation solution under stirring (200 rpm) until the solid phase is completely integrated into the liquid phase. Then the stirring speed is increased to 1000 rpm for 2 to 3 minutes. All these operations are carried out at room temperature. The mixture is poured into containers of the desired size and covered with paraffin to prevent evaporation. Curing is carried out at room temperature and, after 24 hours, the composite material according to the invention is hardened and ready.

[0083] II. Particular example.

[0084] To synthesize a geopolymer composite material with carbon black of molar composition of 3.6 SiCh / l AI2O3 / I Na2O / 14 H2O + 9% by mass of carbon black relative to the mass of metakaolin, the raw materials used are:

[0085] - Betol 39T from Woellner,

[0086] - soda tablets (NaOH),

[0087] - distilled water,

[0088] - metakaolin M1000 from Imerys and

[0089] - PBX55 carbon black from Cabot.

[0090] Preparation of the solid premix

[0091] 65.43 g of metakaolin M1000 is weighed and mixed with 5.89 g of PBX black 55. A visually homogeneous mixture is obtained after manual stirring.

[0092] Preparation of the activation solution

[0093] 12.86 g of NaOH pellets are mixed with 63.89 g of betol 39T and 21.25 g of distilled water. The mixture is stirred using a magnetic bar until the sodium hydroxide pellets are completely dissolved and the temperature returns to room temperature.

[0094] Synthesis of composite material

[0095] The solid premix is ​​poured in very slowly with moderate stirring until a homogeneous mixture of the geopolymeric grout type is obtained (Figure 1).

[0096] The cure is carried out at room temperature for at least 24 hours.

[0097] Example 2: Characterization and properties of the composite material according to the invention.

[0098] I. Water content and porosity of the composite material according to the invention.

[0099] For some synthesized samples, the water content was determined by thermogravimetric analysis (TGA). We found that it is only very slightly influenced by the incorporated carbon content. The water content is thus around 40% by mass (Figure 2), which corresponds to a pore volume of approximately 45-50%.

[0100] Nitrogen adsorption / desorption isotherms were also measured on the same samples. It is found that the capillary condensation phenomenon appears at a lower relative pressure for samples containing carbon, indicating a decrease in the average mesopore size, which is verified by the BJH (for "Barret-Joyner-Halenda") method (Figure 3).

[0101] The insert in Figure 4 shows the evolution of the specific surface area calculated by the BET (for “Brunauer-Emmet-Teller”) method and the pore volume as a function of the carbon content. The pore volume decreases by about 15% when carbon black is added but the specific surface area increases by about 20%. This is related to the fact that carbon intrinsically has a specific surface area of ​​around 55 m 2 / g thus leading to an increase in this surface area for the bulk sample. The pore size distribution is refined and shifted towards small sizes by the addition of carbon black.

[0102] I Carbon content and percolation threshold.

[0103] One of the challenges, whether for the production of heat by Joule effect or for the storage of energy by capacitive effect, is to determine the minimum carbon content to reach the percolation threshold.

[0104] One way to determine this threshold is to measure the resistance of an electrode on samples saturated with their pore solution (i.e. directly after synthesis) and on dried samples.

[0105] In the experimental setup used for this measurement, the electrode is sandwiched between two flexible graphite sheets to ensure good contact with the electrode faces. Two thermocouples are inserted into the sample to measure the temperature rise when the current passes through. The electrode faces are previously polished to have a controlled surface condition. A voltage (U) is then passed across the electrode terminals and the current (I) is measured. It is well known that there is a linear relationship between U and I and that the coefficient of proportionality is the resistance of the material: U=RL When the samples are filled with a poral solution, the current passes through the sample via this solution or via the carbon network if it is percolating.

[0106] As shown in Figure 5, for carbon contents lower than 8% by mass relative to the mass of meta kaolin, there is no percolating carbon network because the measured resistivity (p=RA / l with R the resistance, A the surface and I the electrode thickness) on dry samples is three orders of magnitude higher than on wet samples. It is clear that, for these contents, the current does not pass through the carbon network.

[0107] On the other hand, from 8% by mass of carbon relative to the mass of metakaolin, the resistances between dry and wet samples are similar and fall with the increase in carbon content. The more it increases, the greater the number of contacts between the carbons, facilitating the passage of electricity.

[0108] III. Temperature increase by Joule effect.

[0109] When electricity flows through a conductive material, a temperature rise can occur due to the Joule effect. To quantify this, simply apply a constant current between the electrode terminals and measure the temperature rise within the sample over time. The tests were carried out between 1 and 10 volts for 15 minutes on wet and dry samples.

[0110] Figure 6 shows the temperature variation (AT = Tfinal - Tinitial) for the composite dried at 8% by mass of carbon black relative to the mass of metakaolin and for 5 applied tensions. The composite was dried at 250°C for 24 h in agreement with the mass loss measurements (Figure 2). The evolution of this temperature difference follows a U-shaped law 2 . In fact, the thermal power P = Ul = U 2 / R.

[0111] The results are comparable to what can be found in the literature even if a strict comparison is difficult given the differences in terms of formulation, geometry and voltages applied to the electrode terminals.

[0112] Example 3: Supercapacitor comprising electrodes made of a composite material according to the invention. I. Schematic diagram.

[0113] One of the challenges of the present invention lies in the synthesis of electrodes for the development of low-carbon supercapacitors based on activated materials to which geopolymers belong.

[0114] The schematic diagram is shown in Figure 7. A separator is sandwiched between two composite electrodes and the whole is sealed and pressurized between two brass blocks. Since brass is conductive, applying a voltage from a generator allows a voltage or current to pass through the EDLC system. The separator is fiberglass but could be any other porous insulator that allows the ions to diffuse without causing a short circuit. The electrolyte is potassium chloride at a concentration of 1 M. Any other current-conducting electrolyte could be used if the pH is above 6. Several tests have been carried out with this configuration.

[0115] II. Cyclic voltammetry tests.

[0116] Initially, cyclic voltammetry tests were carried out for various potential scan rates. The raw data are given in Figure 8A. The results are close to a perfect supercapacitor (rectangular voltammogram) but are more similar to the operation of a real supercapacitor with curvatures related to resistive effects. It is interesting to note that, for these electrodes, it is possible to charge and discharge in 1 s. Indeed, given that we scan from 0 to 1 V, a potential speed of 1000 mV / s results in a charge and discharge time of 1 s. This shows the very "reactive" nature of the synthesized electrodes.

[0117] The cell efficiency can be estimated from these data by calculating the area under the curves which is proportional to the stored charge. As described in Figure 8B, the capacity of the active material decreases by about a factor of 2 as the potential scan rate increases, but this is still quite reasonable given the scan rates.

[0118] III. Galvanostatic cycling test. The other commonly used test to characterize a supercapacitor is galvanostatic cycling, in which, this time, a current is applied to the terminals of the device. A cycle corresponds to Figure 9A where a rise to IV (in agreement with Figure 8A) followed by a discharge takes place in 4 s. The potential drop (AV) observed during current reversal is solely due to the equivalent series resistance (ESR).

[0119] As with cyclic voltammetry tests, cell capacities can be calculated and the capacities obtained and presented in Figure 9B are in perfect agreement with the data in Figure 8B.

[0120] IV. Long-term performance.

[0121] One advantage of supercapacitors over batteries is their long-term performance. It is therefore important to evaluate performance monitoring over a large number of cycles. Figure 10 shows that the capacity decreases by a few percent (a decrease certainly linked to a drop in resistance) at the very beginning of cycling, stabilizing at 96.8% after 100,000 cycles.

[0122] As a reminder, a variation in performance (decrease in capacity and increase in resistance) of 10% over 10,000 cycles validates the cyclability tests for commercial supercapacitors.

[0123] References

[0124] [1] Fiala et al, 2019, « Self-heating ability of geopolymers enhanced bu carbon black admixtures at different voltage loads », Energies, vol. 12, 4121.

[0125] [2] Cai et al, 2020, « Fly Ash-based geopolymer with self-heating capacity for accelerated curing », Journal of Cleaner Production, vol. 261, 121119.

[0126] [3] Demande de brevet US 2019 / 0218144 publiée le 18 juillet 2019.

Claims

CLAIMS 1. A method of preparing an electrically conductive composite material comprising the following steps: (a) mixing an alumino-silicate source with electrically conductive carbonaceous fillers whereby a solid, homogeneous mixture consisting of the alumino-silicate source and the electrically conductive carbonaceous fillers is obtained, (b) adding the solid, homogeneous mixture, consisting of the aluminosilicate source and the electrically conductive carbon fillers and obtained in step (a) to an activation solution whose pH is greater than 9 and comprising a SiCh / NazO molar ratio of between 1 and 1.4, whereby a geopolymeric slurry in which said electrically conductive carbon fillers are dispersed is obtained, said geopolymeric slurry not comprising any dispersing agent, the Si / Al molar ratio in said geopolymeric slurry being between 1.5 and 2, the water / aluminosilicate source ratio in said geopolymeric slurry being between 0.65 and 1.10, then (c) allowing the geopolymeric grout obtained in step (b) to harden at room temperature, whereby an electrically conductive composite material is obtained.

2. Preparation process according to claim 1, characterized in that said aluminosilicate source is metakaolin.

3. Preparation process according to claim 1 or 2, characterized in that said electrically conductive carbonaceous fillers are graphite, highly oriented pyrolytic graphite, graphene, graphene oxide, reduced graphene oxide, carbon, activated carbon, vitreous carbon and carbon black.

4. Preparation process according to any one of claims 1 to 3, characterized in that said electrically conductive carbonaceous fillers are a carbon black powder.

5. Preparation process according to any one of claims 1 to 4, characterized in that said activation solution is an aqueous solution of sodium silicate (NazSiCh) made alkaline by adding NaOH.

6. Preparation process according to any one of claims 1 to 5, characterized in that the quantity of aluminosilicate source is such that - the Si / AI molar ratio in the geopolymeric grout obtained in said step (b) is of the order of 1.8 (i.e. 1.8 ± 0.1) and / or - the water / aluminosilicate source ratio in the geopolymeric grout obtained in said step (b) is between 0.88 and 1.00 and in particular of the order of 0.94 (i.e. 0.94 ± 0.05).

7. Electrically conductive composite material prepared by a process as defined in any one of claims 1 to 6, said composite material being in the form of a geopolymeric matrix in which electrically conductive carbon fillers are dispersed, said geopolymeric matrix having all of the following characteristics: - a SiOz / AhOs molar ratio between 3.2 and 3.8 and, in particular, of the order of 3.6 (i.e. 3.6 ± 0.1); - a HzO / NazO molar ratio greater than or equal to 12 and, in particular, of the order of 14 (i.e. 14 ± 1); and - an AIzOî / NazO molar ratio of between 0.8 and 1.2 and in particular of the order of 1 (i.e. 1 ± 0.1) and said geopolymeric matrix not comprising any dispersing agent.

8. Use of an electrically conductive composite material according to claim 7, for producing heat by Joule effect by applying an electrical potential difference across the terminals of said material.

9. Electrode comprising or consisting of an electrically conductive composite material according to claim 7.

10. Device comprising an electrode according to claim 9, said device being chosen from the group consisting of a device for storing and restoring electricity such as a supercapacitor and in particular a symmetrical supercapacitor or a battery; a photovoltaic device; a material for storing CO2 and an electrochemical sensor.

11. Symmetrical supercapacitor comprising two electrodes made of a composite material according to claim 7.

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