Composition for an organic gel and the pyrolysate thereof, production method thereof, electrode formed by the pyrolysate and supercapacitor containing same

Abstract

A non-crosslinked, gelled carbonaceous composition and a pyrolyzed composition respectively forming an aqueous polymer gel and the pyrolysate thereof in the form of porous carbon is provided. Also provided is a production method thereof, to a porous carbon electrode formed by the pyrolyzed composition, and to a supercapacitor containing the electrodes. The gelled, non-crosslinked composition (G2) is based on a resin created at least partly from polyhydroxybenzene(s) R and formaldehyde(s) F and comprises at least one hydrosoluble cationic polyelectrolyte P. The composition forms a rheofluidifying physical gel. A pyrolyzed carbonaceous composition having a carbon monolith, is the product of coating, crosslinking, drying then pyrolysis of the non-crosslinked gelled composition, the carbon monolith being predominantly microporous and able to form a supercapacitor electrode having a thickness of less than 1 mm.

Claims

1. A non-crosslinked prepolymer precipitate, derived from polyhydroxybenzene(s) R and from formaldehyde(s) F and comprising at least one water-soluble cationic organic polymer polyelectrolyte P and forming a precipitated shear-thinning physical gel which has a viscosity, measured at 25 C. using a Brookfield viscometer, greater than 100 mPa s at a shear rate of 50 revolutions/minute and greater than 200 mPa.Math.s at a shear rate of 20 revolutions/minute, the non-crosslinked prepolymer being wherein the precipitate is a product of a reaction for prepolymerization and precipitation of an aqueous solution of the polyhydroxybenzene(s) R, of the formaldehyde(s) F, of said at least one cationic polyelectrolyte P and of a catalyst C dissolved in an aqueous solvent W, the prepolymerization reaction being carried out in a reactor until precipitation.

2. The non-crosslinked prepolymer precipitate as claimed in claim 1, wherein said product of the reaction comprises said at least one cationic polyelectrolyte P according to a mass fraction of between 0.5% and 5%.

3. The non-crosslinked prepolymer precipitate as claimed in claim 1, wherein said product of the reaction comprises said at least one cationic polyelectrolyte P and said polyhydroxybenzene(s) R according to an R/P mass ratio of less than 50.

4. The non-crosslinked prepolymer precipitate as claimed in claim 1, wherein said product of the reaction comprises said polyhydroxy-benzene(s) R and said aqueous solvent W according to an R/W mass ratio of between 0.2 and 2.

5. The non-crosslinked prepolymer precipitate as claimed in claim 1, wherein said at least one water-soluble cationic polyelectrolyte P is an organic polymer chosen from the group made up of quaternary ammonium salts, poly(vinylpyridinium chloride), poly(ethyleneimine), poly(vinylpyridine), poly(allylamine hydrochloride), poly(trimethylammoniumethyl methacrylate chloride), poly(acrylamide-co-dimethylammonium chloride) and mixtures thereof.

6. The non-crosslinked prepolymer precipitate as claimed in claim 5, wherein said at least one water-soluble cationic polyelectrolyte is poly(diallyldimethylammonium chloride) or poly(diallyldimethylammonium bromide).

7. The non-crosslinked prepolymer precipitate as claimed in claim 5, wherein said at least one water-soluble cationic polyelectrolyte is a salt comprising units resulting from a quaternary ammonium chosen from poly(diallyldimethylammonium halides).

8. The non-crosslinked prepolymer precipitate as claimed in claim 1, wherein the composition is capable of being coated to a coating thickness of less than 2 mm.

9. The non-crosslinked prepolymer precipitate as claimed in claim 1, wherein said product of the reaction comprises said polyhydroxy-benzene(s) R and said aqueous solvent W according to an R/W mass ratio of between 0.3 and 1.3.

10. The non-crosslinked prepolymer precipitate as claimed in claim 1, wherein the precipitated shear-thinning physical gel formed by the non-crosslinked prepolymer has said viscosity, measured at 25 C. using said Brookfield viscometer, which, at a shear rate of 50 revolutions/minute, is between 150 mPa.Math.s and 10,000 mPa.Math.s and which, at a shear rate of 20 revolutions/minute, is greater than 250 mPa.Math.s.

11. The non-crosslinked prepolymer precipitate as claimed in claim 1, wherein the prepolymer is capable of being coated to a coating thickness of less than 1.5 mm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a graph showing the change in viscosity of a non-crosslinked gelled composition in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

(2) The term gel is intended to mean, in a known manner, the mixture of a colloidal material and of a liquid, which forms spontaneously or under the action of a catalyst by flocculation and coagulation of a colloidal solution. It should be recalled that a distinction is made between chemical gels and physical gels, the first owing their structure to a chemical reaction and being, by definition, irreversible, while, for the second, the aggregation between the macro-molecular chains is reversible.

(3) It should also be recalled that the term shear-thinning gel is intended to mean a gel with rheological behavior which is non-Newtonian and independent of time, which is sometimes also described as pseudoplasty and which is characterized in that its viscosity decreases when the shear rate gradient increases.

(4) The term water-soluble polymer is intended to mean a polymer which can be dissolved in water without the addition of additives (of surfactants in particular), unlike a water-dispersible polymer which is capable of forming a dispersion when it is mixed with water.

(5) It will be noted that the composition according to the invention has the advantage, in the non-crosslinked gelled state where it consists of said precipitated prepolymer forming a shear-thinning reversible gel, of being able to be used in the form of a thin layer and of having improved mechanical properties. This intermediate physical gel is thus sufficiently viscous to be coated or molded at thicknesses of less than 2 mm, then crosslinked and dried more easily and more rapidly than a conventional RF gel to give a porous xerogel according to the invention. In comparison, the non-modified RF resins of the prior art directly formed, from their liquid precursors, an irreversible chemical gel which could not be coated in the form of a thin layer and which deformed at small thickness during pyrolysis of the gel.

(6) The applicant has in fact discovered that said cationic polyelectrolyte P has a coagulant effect and makes it possible to neutralize the charge of the phenolates of the polyhydroxybenzene R and therefore to limit the repulsion between prepolymer colloids, promoting the formation and the agglomeration of polymer nanoparticles at weak conversion of the polycondensation reaction. Furthermore, since the precipitation takes place before the crosslinking of the composition according to the invention, the mechanical stresses are weaker at strong conversion when the gel forms.

(7) As a result, the gelled composition of the invention can be dried more easily and more rapidlyby simple stovingthan the aqueous gels of the prior art. This oven-drying is in fact much simpler to carry out and less damaging to the cost of production of the gel than drying carried out by solvent exchange or by means of supercritical CO.sub.2.

(8) Furthermore, the applicant has demonstrated that the dried gelled composition (i.e. the xerogel) does not deform during pyrolysis thereof, even at thicknesses of less than 1 mm, contrary to the pyrolyzed gels of the prior art.

(9) It will also be noted that said at least one polyelectrolyte P makes it possible to retain the high porosity of the gel following this oven-drying and to confer on it a low density combined with a high specific surface area and a high pore volume, it being specified that this gel according to the invention is mainly microporous, which advantageously makes it possible to have a high specific energy and a high capacitance for a supercapacitor electrode consisting of this pyrolyzed gel.

(10) Advantageously, said product of the prepolymerization and precipitation reaction can comprise: said at least one cationic polyelectrolyte P according to a mass fraction of between 0.5% and 5%, and/or said at least one cationic polyelectrolyte P and said polyhydroxybenzene(s) R according to an R/P mass ratio of less than 50 and preferably of between 10 and 25, and/or said polyhydroxybenzene(s) R and said aqueous solvent W according to an R/W mass ratio of between 0.2 and 2 and preferably of between 0.3 and 1.3.

(11) Said at least one polyelectrolyte P which is usable in a composition according to the invention may be any cationic polyelectrolyte which is totally soluble in water and has a low ionic strength.

(12) Preferably, said at least one cationic polyelectrolyte P is an organic polymer chosen from the group made up of quaternary ammonium salts, poly-(vinylpyridinium chloride), poly(ethyleneimine), poly-(vinylpyridine), poly(allylamine hydrochloride), poly-(trimethylammoniumethyl methacrylate chloride), poly-(acrylamide-co-dimethylammonium chloride), and mixtures thereof.

(13) Even more preferentially, said at least one cationic polyelectrolyte P is a salt comprising units resulting from a quaternary ammonium chosen from poly-(diallyldimethylammonium halide), and is preferably poly(diallyldimethylammonium chloride) or poly-(diallyldimethylammonium bromide).

(14) Among the polymers which are precursors of said resin and which are usable in the invention, mention may be made of those resulting from the polycondensation of at least one monomer of the polyhydroxybenzene type and of at least one formaldehyde monomer. This polymerization reaction may involve more than two distinct monomers, the additional monomers optionally being of the polyhydroxybenzene type. The polyhydroxybezenes which are usable are preferentially di- or trihydroxybenzenes, and advantageously resorcinol (1,3-dihydroxybenzene) or a mixture of resorcinol with another compound chosen from catechol, hydroxyquinone and phloroglucinol.

(15) Use may, for example, be made of the polyhydroxybenzene(s) R and formaldehyde(s) F according to an R/F molar ratio of between 0.3 and 0.7.

(16) Likewise advantageously, a composition according to the invention may have, in the non-crosslinked gelled state, a viscosity, measured at 25 C. using a Brookfield viscometer, which, at a shear rate of 50 revolutions/minute, is greater than 100 mPa.Math.s and is preferably between 150 mPa.Math.s and 10 000 mPa.Math.s, it being specified that, at 20 revolutions/minute, this viscosity is greater than 200 mPa.Math.s and preferably greater than 250 mPa.Math.s.

(17) According to another advantageous characteristic of the invention, the composition is capable of being coated in the non-crosslinked gelled state with a coating thickness of less than 2 mm and preferably less than 1.5 mm.

(18) A pyrolyzed carbon-based composition according to the invention, consisting of a carbon monolith which is preferably predominantly microporous, is characterized in that it is the product of coating, crosslinking, drying then pyrolysis of a non-crosslinked gelled composition as defined above, said carbon monolith being capable of forming a supercapacitor electrode having a thickness of less than 1 mm and preferably less than or equal to 0.5 mm.

(19) It will be noted that this essentially microporous structure which can be obtained according to the invention is, by definition, characterized by pore diameters of less than 2 nm, contrary to the mesoporous structures such as those obtained in the abovementioned article which are, by definition, characterized by pore diameters inclusively between 2 nm and 50 nm.

(20) According to another characteristic of the invention, said composition has, in the pyrolyzed state: a density of between 0.1 and 1.2, and/or a specific surface area of greater than 400 m.sup.2/g, and/or a pore volume of between 0.2 and 0.8 cm.sup.3/g.

(21) Advantageously, a composition according to the invention is capable of forming, in the pyrolyzed state, a supercapacitor electrode having a thickness of less than 1 mm and preferably less than or equal to 0.5 mm.

(22) A process for preparing, according to the invention, a carbon-based composition as defined above comprises:

(23) a) dissolution in an aqueous solvent W of said polyhydroxybenzene(s) R and formaldehyde(s) F, in the presence of said at least one cationic polyelectrolyte P and of a catalyst, in order to obtain an aqueous solution,

(24) b) prepolymerization until precipitation of the solution obtained in a) in order to obtain a precipitated prepolymer forming said non-crosslinked gelled composition,

(25) c) coating or molding of the precipitated prepolymer obtained in b) with a thickness of less than 2 mm and preferably less than 1.5 mm,

(26) d) crosslinking and drying, preferably in a humid oven, of the gel coated or molded in c) in order to obtain the dried, crosslinked, gelled composition forming a porous xerogel, and

(27) e) pyrolysis of the dried gel obtained in d), in order to obtain said pyrolyzed composition in the form of porous carbon which is preferably monolithic.

(28) Preferably, use is made, in step a), of: said at least one cationic polyelectrolyte P according to a mass fraction of between 0.5% and 5%; and/or said at least one cationic polyelectrolyte P and said polyhydroxybenzene(s) R according to an R/P mass ratio of less than 50 and preferably between 10 and 25, and/or said polyhydroxybenzene(s) R and said aqueous solvent W according to an R/W mass ratio of between 0.2 and 2 and preferably between 0.3 and 1.3.

(29) Likewise, preferentially, step a) is carried out:

(30) a1) by dissolving said polyhydroxybenzene(s) R in said aqueous solvent W, preferably consisting of water,

(31) a2) by adding, to the solution obtained in a1), said formaldehyde(s) F, said acid or basic catalyst C and said at least one cationic polyelectrolyte P, then

(32) a3) by stirring the mixture obtained and adjusting its pH.

(33) Likewise, preferentially, step b) is carried out in a reactor, for example immersed in an oil bath between 50 and 70 C.

(34) By way of catalyst which is usable in step a), mention may, for example, be made of acid catalysts, such as aqueous solutions of hydrochloric acid, sulfuric acid, nitric acid, acetic acid, phosphoric acid, trifluoroacetic acid, trifluoromethanesulfonic acid, perchloric acid, oxalic acid, toluenesulfonic acid, dichloroacetic acid or formic acid, or else basic catalysts, such as sodium carbonate, sodium hydrogen carbonate, potassium carbonate, ammonium carbonate, lithium carbonate, aqueous ammonia, potassium hydroxide and sodium hydroxide.

(35) It will be noted that this process for preparing the pyrolyzed gelled composition according to the invention has the advantage of being simple and inexpensive to carry out, in order to obtain a carbon which is advantageously monolithic and essentially microporous making it possible to obtain, by coating, flat plates of small thickness.

(36) A porous carbon electrode according to the invention is usable for equipping a supercapacitor cell while being immersed in an aqueous ionic electrolyte and covers a metal current collector, and this electrode is such that it consists of a carbon-based composition in the pyrolyzed state as defined above and that it has a thickness of less than 1 mm and preferably less than or equal to 0.5 mm.

(37) A supercapacitor according to the invention comprises cells each comprising at least two porous electrodes, an electrically insulating membrane separating these electrodes from one another and an ionic electrolyte in which these electrodes are immersed, each cell comprising at least two current collectors respectively covered with these electrodes, and this supercapacitor is such that at least one of these electrodes is as defined above.

(38) Other characteristics, advantages and details of the present invention will emerge on reading the following description of several examples of implementation of the invention, given by way of non-limiting illustration, the description being given with reference to the attached drawing, in which:

(39) the single FIGURE is a graph showing the change in viscosity (in mPa.Math.s) of a non-crosslinked gelled composition G2 according to the invention and of a control non-crosslinked gelled composition G0, measured at 25 C., as a function of the rotational shear rate of a Brookfield viscometer.

Example of Preparation of Carbon-Based Compositions

(40) The examples which follow illustrate the preparation of four gelled compositions G1 to G4 according to the invention and of four pyrolyzed compositions C1 to C4 according to the invention respectively obtained by pyrolysis of the compositions G1 to G4, in comparison with three control gelled compositions G0, G0 and G0 and respective control pyrolysates C0 and C0 of G0 and G0.

(41) In order to obtain the gelled compositions G1 to G4 and G0, G0 and G0, the following reagents are used for the polycondensation of the resorcinol R with the formaldehyde F: resorcinol (R) from Acros Organics, 98% pure, formaldehyde (F) from Acros Organics, 37% pure, catalyst (C) consisting of sodium carbonate, and poly(diallyldimethylammonium chloride) (P), 35% pure (in solution in water W), for gels G1 to G5.

(42) The control gelled composition G0 consisting of a gel of resorcinol R and of formaldehyde F was prepared by rigorously following the experimental protocol described in the abovementioned prior art article A novel way to maintain resorcinol-formaldehyde porosity during drying: Stabilization of the sol-gel nanostructure using a cationic polyelectrolyte, Mariano M. Bruno et al., 2010, i.e. the molar ratios R:F:C:P=1:2.5:910.sup.3:1.610.sup.2 and the corresponding concentrations [4M]:[10M]:[0.036M]:[0.064M], by immediately polymerizing R and F in the presence of C and P at 70 C. for 24 hours.

(43) In order to prepare the compositions G1 to G4 and G0 and G0, the abovementioned reagents were used according to the following proportions: R/F: molar ratio between resorcinol and formaldehyde, R/W: mass ratio between resorcinol and water, P denotes the mass fraction of polyelectrolyte, R/P: mass ratio between resorcinol and polyelectrolyte, and R/C: mass ratio between resorcinol and catalyst.

(44) Firstly, for each composition, the same amount of resorcinol was dissolved in distilled water. Then, the following were added to the solution obtained: the formaldehyde, the solution of calcium carbonate and the polyelectrolyte consisting of a solution of poly-(diallyldimethylammonium chloride) at 35% for only the compositions G1 to G4. After magnetic stirring for 10 minutes, the pH was adjusted to pH=6.5 for the compositions G1 to G4 and G0, and to pH=6 for the composition G0, using a 1M solution of Na.sub.2CO.sub.3. A nonpolymeric aqueous solution based on the precursors R and F was thus obtained for each composition G1 to G4 and G0 and G0.

(45) Secondly, a prepolymerization of each aqueous solution thus obtained was carried out in a reactor immersed in an oil bath between 50 C. and 70 C. until precipitation of the prepolymer obtained after a reaction time ranging, as appropriate, approximately from 5 minutes to 1 hour, so as to form an intermediate white gel of shear-thinning, homogeneous and reversible nature. The viscosity of each shear-thinning gel obtained was measured at 25 C. using a Brookfield viscometer, and this viscosity was between approximately 200 mPa.Math.s and 7100 mPa.Math.s at a shear rate of 50 revolutions/minute for the compositions G1 to G4.

(46) As for the control compositions G0 and G0, they were irreversibly crosslinked with an abrupt jump in viscosity, without intermediate formation of a shear-thinning gel contrary to the compositions G1 to G4.

(47) Table 1 hereinafter gives details of the conditions followed for preparing the gels G1 to G4 of the invention and the three control gels G0 (according to the abovementioned article by Mariano M. Bruno et al.), G0 and G0, and also the respective viscosities of these gels measured at 25 C. using a Brookfield viscometer at a shear rate of 50 revolutions/minute.

(48) TABLE-US-00001 TABLE 1 G1 G2 G3 G4 G0 G0/G0 R 152.8 g 152.8 g 152.8 g 152.8 g 152.8 g W 95.5 g 47.8 g 0 251.5 g 251.5 g F 225.3 g 225.3 g 225.3 g 225.3 g 225.3 g P 23.9 g 23.9 g 23.9 g 23.9 g 0 R/F 0.5 0.5 0.5 0.5 0.5 R/W 0.67 0.84 1.14 0.4 0.4 R/P 18.4 18.4 18.4 18.4 43 R/C 172 157 170 157 122 600/ pH 6.5 6.5 6.5 6.5 6.5-6 7100 200 400 1600 10/ mPa .Math. s

(49) These gels G1 to G4 exhibited polymer particle sizes of about 100 nm, measured by dynamic light scattering by means of a Malvern zetasizer nano ZS device.

(50) Coating, in the form of films, of the shear-thinning reversible gels formed by the compositions G1 to G4 was then carried out using a film spreader at wet thicknesses of 1 mm to 2 mm, and the irreversible gels formed by the compositions G0 and G0 were placed in Teflon-coated steel molds according to a wet thickness of 2 mm. It will be noted that these G0 and G0 gels can be processed only in a mold because they are not capable of being coated.

(51) The coated gelled compositions G1 to G4 were then crosslinked in a humid oven at 90 C. for 24 hours. The resulting crosslinked gelled compositions were then dried at 85 C. and 85% humidity for 6 hours.

(52) These crosslinked gelled compositions G1 to G4 and G0 and G0 were then pyrolyzed at 800 C. under nitrogen in order to obtain respective monolithic carbons C1 to C4 and C0 and C0. The flat monoliths which are considered to be usable for forming electrodes were machined at fixed thickness, and were characterized by measuring the density of the carbons via the mass/volume ratio of the monolith, the specific surface areas and the pore volumes by means of the Micromeritics Tristar 3020 apparatus.

(53) TABLE-US-00002 TABLE 2 C1 C2 C3 C4 C0 C Density 0.55 0.68 0.85 0.35 0.85 0.40 (g .Math. cm.sup.3) Minimum dry 0.5 mm 0.5 mm 0.5 mm 0.4 mm 1.5 mm 2 mm thickness obtained without deformation or breaking Specific surface 640 640 630 715 650 680 area (m.sup.2 .Math. g.sup.1) including including including including including including including 555 500 450 560 430 450 micro- and micro micro micro micro micro micro mesoporous 85 140 180 155 220 230 portions meso meso meso meso meso meso Pore volume 0.3545 0.4000 0.6522 0.3500 0.5700 0.6000 (cm.sup.3 .Math. 9.sup.1) including including including including including including including 0.2138 0.1981 0.1786 0.2200 0.1700 0.1800 microporous micro micro micro micro micro micro portion

(54) As shown in table 2, in particular by the comparison between the pyrolyzed compositions C1 to C4 and C0 and C0 (see, for example, C2 and C0), essentially microporous C1-C4 monolithic carbons having densities and specific surface areas similar to those of the monolithic carbons prepared from RF gels of the prior art were obtained by simple coating of a thin film of shear-thinning gel G1-G4. Furthermore, these C1-C4 monoliths were obtained directly at very thin thicknesses, thereby limiting the losses of material.

(55) The applicant, moreover, compared the shear-thinning gels obtained for the gelled compositions G1-G4 of the invention with compositions not in accordance with the invention, differing therefrom by the addition of various shear-thinning polymers to the gels obtained with the compositions of G0 and G0. Whatever the shear-thinning agent thus incorporated into these gels, this each time led to breaking of the monoliths subsequently obtained by pyrolysis of these gels.

(56) Average Specific Capacitances of Electrodes Consisting of the Pyrolyzed Compositions C1 to C4 and C0 and C0:

(57) The capacitance of the electrodes was characterized electrochemically, by using the following device and electrochemical tests.

(58) Two identical electrical electrodes insulated by a separator were placed in series in a supercapacitor measuring cell containing the aqueous electrolyte based on sulfuric acid (1M H.sub.2SO.sub.4) and controlled by a Bio-Logic VMP3 potentiostat/-galvanostat via a three-electrode interface. A first electrode corresponded to the working electrode and the second electrode constituted both the counter electrode and the reference electrode.

(59) The device was subjected to charge-discharge cycles at a constant current I of 0.125 A/g of the working electrode.

(60) Since the potential changes linearly with the charge conveyed, the capacitance C of the supercapacitive system was deduced from the slopes p during charging and discharging (knowing that C=I/p). Since the system is symmetrical in terms of masses (m.sub.1=m.sub.2=m), the average specific capacitance C.sub.spe was defined by: C.sub.spe=2C/m.

(61) The performance levels of the various electrodes are recorded in the following table:

(62) TABLE-US-00003 TABLE 3 C0 C0 C1 C2 C3 C4 Specific 125 145 145 128 119 244 capacitance (F/g)

(63) This table 3 shows that the use of a shear-thinning reversible intermediate gel for the synthesis of porous carbons obtained from R and F precursors makes it possible to confer on these carbons specific capacitances that are at least similar to if not greater than those of the prior art carbons, at similar densities.