COMPOSITION FOR AN ORGANIC GEL AND THE PYROLYSATE
THEREOF, PRODUCTION METHOD THEREOF, ELECTRODE FORMED BY
THE PYROLYSATE AND SUPERCAPACITOR CONTAINING SAME
The present invention relates to a 5 noncrosslinked
gelled carbon-based composition and to a
pyrolyzed composition respectively forming an organic
polymer gel and its pyrolysate in porous carbon form,
to the process for preparing same, to a porous carbon
10 electrode consisting of this pyrolyzed composition and
to a supercapacitor incorporating such electrodes. The
invention applies in particular to supercapacitors that
are suitable for equipping electric vehicles.
Supercapacitors are electrical energy storage
15 systems which are particularly advantageous for
applications requiring the conveyance of high-power
electrical energy. Their ability to rapidly charge and
discharge and the increased lifetime compared with a
high-power battery make them promising candidates for a
20 number of applications. Supercapacitors generally
consist of the combination of two conductive porous
electrodes having a high specific surface area, which
are immersed in an ionic electrolyte and separated by
an insulating membrane known as a "separator", which
25 allows ionic conductivity and prevents electrical
contact between the electrodes. Each electrode is in
contact with a metal current collector, making possible
exchange of the electric current with an external
system. Under the influence of a potential difference
30 applied between the two electrodes, the ions present
within an electrolyte are attracted by the electrode
surface exhibiting an opposite charge, thus forming an
electrochemical double layer at the interface of each
electrode. The electrical energy is thus stored
35 electrostatically by charge separation. The expression
of the capacitance C of a supercapacitor is identical
to that of a conventional capacitor, namely:
C = .S/t, where denotes the permittivity of
the medium, S the surface area occupied by the double
- 2 -
layer, and t the thickness of the double layer.
The capacitances achievable within supercapacitors
are nevertheless much higher than those
commonly achieved by conventional capacitors, as a
result of the use of carbon-based electrodes 5 rodes with a
maximized specific surface area and of the extreme
thinness of the electrochemical double layer (typically
a few nanometers thick). These carbon-based electrodes
must necessarily be conductive in order to provide
10 transportation of the electric charges, porous in order
to provide transportation of the ionic charges and the
formation of the electrical double layer over a large
surface area, and chemically inert in order to prevent
any energy-consuming parasitic reactions.
15 The energy E stored within a supercapacitor is
defined according to the conventional expression for
capacitors, i.e.:
E = 1/2. C.V2, where V is the potential of the
supercapacitance.
20 The capacitance and the potential are therefore
two essential parameters which it is necessary to
optimize in order to promote energy performance levels.
The capacitance depends on the porous texture really
accessible by the electrolyte. As it happens, for
25 applications in transportation and in particular for an
electric vehicle, it is necessary to have a high energy
density in order to limit the on-board weight of the
supercapacitor, which imposes having a high mass
capacitance.
30 The potential of a supercapacitor depends
mainly on the nature of the electrolyte used, which may
be organic or aqueous.
There are various possibilities for
incorporating the active material into a supercapacitor
35 electrode. Documents US-B2-6356432, US-A1-2007/0146967
and US-B2-7811337 describe the dispersion of conductive
porous carbons in a non-active organic binder and a
solvent, then the coating of the paste obtained on the
current collector. This method has the drawback of
- 3 -
using a binder which makes the system heavy without
being active for storing energy.
In the context of an application for an
electric vehicle, it is favorable to use, as electrode
active material, a carbon monolith in an a5 queous
electrolyte, in order to maximize the specific energy
of this electrode. In order to achieve operation at
high powers typically greater than 1 kW/kg, it is
necessary for the carbon monolith to be very thin,
10 having a thickness of only a few hundred micrometers
and usually less than or equal to 0.5 mm, while being
sufficiently robust so as not to be brittle and to not
deform at these very small thicknesses.
For the preparation of such a carbon monolith
15 for supercapacitor electrodes, pyrolysis of
resorcinol/formaldehyde (RF) gels is usually carried
out. The RF resins are in fact particularly
advantageous for the preparation of carbon with a high
porosity in monolith form, since they are very
20 inexpensive, can be used in water and make it possible
to obtain various porosities and densities according to
the preparation conditions.
However, since the mixture of resorcinol R and
formaldehyde F precursors in water has a very low
25 viscosity, it cannot be coated with a sufficiently
small thickness, i.e. typically less than 1 mm, and,
instead of such a coating, it is chosen to have the
mixture of R and F precursors in a closed mold so as to
form a gel after polymerization reaction. In order to
30 limit the adhesion of the mixture to the walls of the
mold, it is necessary to provide this mold with a
typically fluorinated, non-stick coating, which
generates a high production cost.
Another drawback of the existing RF gels for
35 supercapacitor electrodes is that they are chemical
gels which are by definition irreversible, since they
are obtained by polycondensation of the liquid
precursors in the mold. Consequently, once formed, the
gel cannot be reused. Furthermore, at high conversion,
- 4 -
this gel becomes hydrophobic and precipitates out,
which induces mechanical stresses in the material and
therefore a greater fragility. Thus, it is necessary to
use a method for drying the water present in the gel
that is sufficiently mild to prevent 5 fracturing or
contraction of the gelled structure, such as
supercritical drying (for the formation of an aerogel),
lyophilization (for the formation of a cryogel), or
very slow drying in a humid chamber (for obtaining a
10 xerogel). The dried gel is then pyrolyzed under
nitrogen at high temperature so as to obtain a
monolithic porous carbon.
As it happens, one limitation of the current
methods is the deformation of the monoliths during
15 pyrolysis, due to the residual stresses when the gel
thickness is less than 2 mm. As it happens, in order to
obtain carbon electrodes having the abovementioned
thickness less than or equal to 0.5 mm, these methods
must also comprise a final polishing/rectifying step
20 which has the drawback of being expensive and difficult
to implement, and of generating considerable losses of
material.
By way of illustration of the prior art
presented above for the preparation of monolithic
25 carbons derived from RF gels for supercapacitor
electrodes, mention may be made of document US-B2-
6 737 445 which teaches the use of a high amount of a
cationic, anionic or nonionic surfactant for forming an
emulsion in water and polymerizing therein the R and F
30 precursors. An irreversible aqueous chemical gel is
obtained which is incapable of being coated with a
small thickness, and after drying of this gel under a
gas stream and pyrolysis, a mesoporous carbon-based
structure, the size of the pores of which corresponds
35 to micelles, is obtained.
Another drawback of this process lies in the
mesoporous structure obtained for the carbon which, in
the case of a supercapacitor, is unfavorable in
comparison with a mainly microporous structure which is
- 5 -
preferred for having a high specific energy and a high
capacitance. Furthermore, the use of a large amount of
surfactant proves to be expensive.
It is also possible to mention, by way of prior
art for the preparation of such electrodes, the 5 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", which discloses a meso10
porous monolithic carbon derived from an RF aqueous
chemical gel comprising, in addition to a sodium
carbonate-based basic catalyst C, a cationic polyelectrolyte
P consisting of poly(diallyldimethylammonium
chloride) which makes it possible to retain
15 the porosity of the gel following air-drying thereof.
The gel is prepared with the molar ratios R:F:C:P =
1:2.5:9 10-3:1.6 10-2 and the corresponding concentrations
[4M]:[10M]:[0.036M]:[0.064M], by polymerizing
R and F from the start in the presence of C and P at
20 70°C for 24 hours.
A major drawback of the irreversible chemical
gels presented in this article lies in their very low
viscosity which makes them totally incapable of being
coated with a thickness of less than 2 mm.
25 An objective of the present invention is to
provide a non-crosslinked, gelled carbon-based
composition and a pyrolyzed composition respectively
forming an aqueous polymer gel and a pyrolysate of said
crosslinked gel in porous carbon form, which remedy the
30 abovementioned drawbacks by making it possible in
particular to use an RF-type gel having a small
thickness directly by coating, and with rapid drying.
This objective is achieved in that the
applicant has just discovered, surprisingly, that prior
35 dissolution in the aqueous phase of the R and F
precursors and of a water-soluble cationic
polyelectrolyte P, followed by precipitation of a prepolymer
obtained from these precursors thus dissolved,
makes it possible to obtain, by means of this
- 6 -
precipitation, a non-crosslinked intermediate physical
gel which is, by definition, reversible and the
specifically shear-thinning nature of which confers on
this gel a viscosity that is sufficiently high to allow
coating therewith according to a very 5 small thickness
suitable for supercapacitor electrodes.
A non-crosslinked gelled carbon-based
composition according to the invention is thus based on
a resin derived at least partly from polyhydroxy10
benzene(s) R and from formaldehyde(s) F and comprises
at least one water-soluble cationic polyelectrolyte P,
and the composition is such that it forms, in the noncrosslinked
gelled state (i.e. before crosslinking of
the gelled composition), a shear-thinning physical gel.
15 According to another characteristic of the
invention, this composition comprises said precipitated
prepolymer forming this shear-thinning gel which is the
product of a reaction for prepolymerization and
precipitation of an aqueous solution containing the
20 polyhydroxybenzene(s) R, the formaldehyde(s) F, said at
least one cationic polyelectrolyte P and a catalyst C
dissolved in an aqueous solvent W.
The term "gel" is intended to mean, in a known
manner, the mixture of a colloidal material and of a
25 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
30 reaction and being, by definition, irreversible, while,
for the second, the aggregation between the macromolecular
chains is reversible.
It should also be recalled that the term
"shear-thinning gel" is intended to mean a gel with
35 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.
- 7 -
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 5 is mixed
with water.
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
10 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
15 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
20 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.
The applicant has in fact discovered that said
25 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
30 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
35 when the gel forms.
As a result, the gelled composition of the
invention can be dried more easily and more rapidly
by simple stoving than the aqueous gels of the prior
art. This oven-drying is in fact much simpler to carry
- 8 -
out and less damaging to the cost of production of the
gel than drying carried out by solvent exchange or by
means of supercritical CO2.
Furthermore, the applicant has demonstrated
that the dried gelled composition (i.e. the 5 xerogel)
does not deform during pyrolysis thereof, even at
thicknesses of less than 1 mm, contrary to the
pyrolyzed gels of the prior art.
It will also be noted that said at least one
10 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
15 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.
Advantageously, said product of the pre20
polymerization 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
25 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
30 and 2 and preferably of between 0.3 and 1.3.
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.
35 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-
9 -
(trimethylammoniumethyl methacrylate chloride), poly-
(acrylamide-co-dimethylammonium chloride), and mixtures
thereof.
Even more preferentially, said at least one
cationic polyelectrolyte P is a salt comprising 5 omprising units
resulting from a quaternary ammonium chosen from poly-
(diallyldimethylammonium halide), and is preferably
poly(diallyldimethylammonium chloride) or poly-
(diallyldimethylammonium bromide).
10 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
15 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
20 advantageously resorcinol (1,3-dihydroxybenzene) or a
mixture of resorcinol with another compound chosen from
catechol, hydroxyquinone and phloroglucinol.
Use may, for example, be made of the polyhydroxybenzene(
s) R and formaldehyde(s) F according to
25 an R/F molar ratio of between 0.3 and 0.7.
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
30 of 50 revolutions/minute, is greater than 100 mPa.s and
is preferably between 150 mPa.s and 10 000 mPa.s, it
being specified that, at 20 revolutions/minute, this
viscosity is greater than 200 mPa.s and preferably
greater than 250 mPa.s.
35 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.
- 10 -
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-5 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.
10 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
15 abovementioned article which are, by definition,
characterized by pore diameters inclusively between
2 nm and 50 nm.
According to another characteristic of the
invention, said composition has, in the pyrolyzed
20 state:
- a density of between 0.1 and 1.2, and/or
- a specific surface area of greater than
400 m2/g, and/or
- a pore volume of between 0.2 and 0.8 cm3/g.
25 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.
30 A process for preparing, according to the
invention, a carbon-based composition as defined above
comprises:
a) dissolution in an aqueous solvent W of said
polyhydroxybenzene(s) R and formaldehyde(s) F, in the
35 presence of said at least one cationic polyelectrolyte
P and of a catalyst, in order to obtain an aqueous
solution,
b) prepolymerization until precipitation of the
solution obtained in a) in order to obtain a
- 11 -
precipitated prepolymer forming said non-crosslinked
gelled composition,
c) coating or molding of the precipitated
prepolymer obtained in b) with a thickness of less than
5 2 mm and preferably less than 1.5 mm,
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
10 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.
Preferably, use is made, in step a), of:
- said at least one cationic polyelectrolyte P
15 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
20 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.
Likewise, preferentially, step a) is carried
25 out:
a1) by dissolving said polyhydroxybenzene(s) R
in said aqueous solvent W, preferably consisting of
water,
a2) by adding, to the solution obtained in a1),
30 said formaldehyde(s) F, said acid or basic catalyst C
and said at least one cationic polyelectrolyte P, then
a3) by stirring the mixture obtained and
adjusting its pH.
Likewise, preferentially, step b) is carried
35 out in a reactor, for example immersed in an oil bath
between 50 and 70°C.
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,
- 12 -
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, 5 sodium hydrogen
carbonate, potassium carbonate, ammonium carbonate,
lithium carbonate, aqueous ammonia, potassium hydroxide
and sodium hydroxide.
It will be noted that this process for
10 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,
15 flat plates of small thickness.
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
20 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.
A supercapacitor according to the invention
25 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
30 collectors respectively covered with these electrodes,
and this supercapacitor is such that at least one of
these electrodes is as defined above.
Other characteristics, advantages and details
of the present invention will emerge on reading the
35 following description of several examples of
implementation of the invention, given by way of nonlimiting
illustration, the description being given with
reference to the attached drawing, in which:
the single figure is a graph showing the change
- 13 -
in viscosity (in mPa.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
5 rate of a Brookfield viscometer.
Example of preparation of carbon-based
compositions:
10 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
15 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".
In order to obtain the gelled compositions G1
to G4 and G0, G0' and G0", the following reagents are
20 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,
25 - catalyst (C) consisting of sodium carbonate,
and
- poly(diallyldimethylammonium chloride) (P),
35% pure (in solution in water W), for gels
G1 to G5.
30
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
35 article "A novel way to maintain resorcinolformaldehyde
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 = 1:2.5:9 10-3:1.6 10-2 and the corres-
14 -
ponding 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.
In order to prepare the compositions G1 to G4
and G0' and G0", the abovementioned reagents 5 were used
according to the following proportions:
- R/F: molar ratio between resorcinol and
formaldehyde,
- R/W: mass ratio between resorcinol and water,
10 - P denotes the mass fraction of polyelectrolyte,
- R/P: mass ratio between resorcinol and polyelectrolyte,
and
- R/C: mass ratio between resorcinol and
15 catalyst.
Firstly, for each composition, the same amount
of resorcinol was dissolved in distilled water. Then,
the following were added to the solution obtained: the
20 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
25 compositions G1 to G4 and G0', and to pH = 6 for the
composition G0", using a 1M solution of Na2CO3. A
nonpolymeric aqueous solution based on the precursors R
and F was thus obtained for each composition G1 to G4
and G0' and G0".
30 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
35 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
- 15 -
approximately 200 mPa.s and 7100 mPa.s at a shear rate
of 50 revolutions/minute for the compositions G1 to G4.
As for the "control" compositions G0' and G0",
they were irreversibly crosslinked with an abrupt jump
in viscosity, without intermediate formation 5 of a
shear-thinning gel contrary to the compositions G1 to
G4.
Table 1 hereinafter gives details of the
conditions followed for preparing the gels G1 to G4 of
10 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 μ of these gels measured at 25°C using a
Brookfield viscometer at a shear rate of
15 50 revolutions/minute.
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
μ
mPa.s 7100 200 400 1600 - 10/-
20 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.
Coating, in the form of films, of the shear25
thinning reversible gels formed by the compositions G1
to G4 was then carried out using a film spreader at wet
- 16 -
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 5 re not
capable of being coated.
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
10 dried at 85°C and 85% humidity for 6 hours.
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
15 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
20 Micromeritics Tristar 3020 apparatus.
Table 2:
C1 C2 C3 C4 C0' C0"
Density
(g.cm-3)
0.55 0.68 0.85 0.35 0.85 0.40
Minimum dry
thickness
obtained
without
deformation
or breaking
0.5 mm 0.5 mm 0.5 mm 0.4 mm 1.5 mm 2 mm
- 17 -
C1 C2 C3 C4 C0' C0"
Specific
surface
area
(m2.g-1)
including
micro- and
mesoporous
portions
640
including
555
micro
85
meso
640
including
500
micro
140
meso
630
including
450
micro
180
meso
715
including
560
micro
155
meso
650
including
430
micro
220
meso
680
including
450
micro
230
meso
Pore volume
(cm3.g-1)
including
microporous
portion
0.3545
including
0.2138
micro
0.4000
including
0.1981
micro
0.6522
including
0.1786
micro
0.3500
including
0.2200
micro
0.5700
including
0.1700
micro
0.6000
including
0.1800
micro
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 5 olithic 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
10 C1-C4 monoliths were obtained directly at very thin
thicknesses, thereby limiting the losses of material.
The applicant, moreover, compared the shearthinning
gels obtained for the gelled compositions
G1-G4 of the invention with compositions not in
15 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
20 monoliths subsequently obtained by pyrolysis of these
gels.
- 18 -
Average specific capacitances of electrodes
consisting of the pyrolyzed compositions C1 to C4 and
C0' and C0":
The capacitance of the 5 electrodes was
characterized electrochemically, by using the following
device and electrochemical tests.
Two identical electrical electrodes insulated
by a separator were placed in series in a
10 supercapacitor measuring cell containing the aqueous
electrolyte based on sulfuric acid (1M H2SO4) and
controlled by a "Bio-Logic VMP3" potentiostat/-
galvanostat via a three-electrode interface. A first
electrode corresponded to the working electrode and the
15 second electrode constituted both the counter electrode
and the reference electrode.
The device was subjected to charge-discharge
cycles at a constant current I of 0.125 A/g of the
working electrode.
20 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
25 (m1=m2=m), the average specific capacitance Cspe was
defined by: Cspe = 2 C/m.
The performance levels of the various
electrodes are recorded in the following table:
This table 3 shows that the use of a shearthinning
reversible intermediate gel for the synthesis
35 of porous carbons obtained from R and F precursors
makes it possible to confer on these carbons specific
- 19 -
capacitances that are at least similar to if not
greater than those of the prior art carbons, at similar
densities.

CLAIMS
1) A non-crosslinked, gelled carbon-based
composition (G2) forming an aqueous polymer gel, the
composition being based on a resin derived at 5 least
partly from polyhydroxybenzene(s) R and from
formaldehyde(s) F and comprising at least one watersoluble
cationic polyelectrolyte P, characterized in
that the composition forms a shear-thinning physical
10 gel.
2) The non-crosslinked gelled composition (G2)
as claimed in claim 1, characterized in that it
comprises a precipitated prepolymer forming said shear15
thinning gel which is the 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 in an aqueous
20 solvent W.
3) The non-crosslinked gelled composition (G2)
as claimed in claim 2, characterized in that said
product of the reaction comprises said at least one
25 cationic polyelectrolyte P according to a mass fraction
of between 0.5% and 5%.
4) The non-crosslinked gelled composition (G2)
as claimed in claim 2 or 3, characterized in that said
30 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 and preferably of between 10 and 25.
35 5) The non-crosslinked gelled composition (G2)
as claimed in one of claims 2 to 4, characterized in
that said product of the reaction comprises said polyhydroxybenzene(
s) R and said aqueous solvent W
according to an R/W mass ratio of between 0.2 and 2 and
- 21 -
preferably of between 0.3 and 1.3.
6) The non-crosslinked gelled composition (G2)
as claimed in one of claims 1 to 5, characterized in
that said at least one water-soluble cationic 5 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-
10 (trimethylammoniumethyl methacrylate chloride), poly-
(acrylamide-co-dimethylammonium chloride) and mixtures
thereof.
7) The non-crosslinked gelled composition (G2)
15 as claimed in claim 6, characterized in that said at
least one water-soluble cationic polyelectrolyte is a
salt comprising units resulting from a quaternary
ammonium chosen from poly(diallyldimethylammonium
halide), and is preferably poly(diallyldimethylammonium
20 chloride) or poly(diallyldimethylammonium bromide).
8) The non-crosslinked gelled composition (G2)
as claimed in one of the preceding claims, characterized
in that it has a viscosity, measured at 25°C
25 using a Brookfield viscometer, which, at a shear rate
of 50 revolutions/minute, is greater than 100 mPa.s and
is preferably between 150 mPa.s and 10 000 mPa.s.
9) The non-crosslinked gelled composition (G2)
30 as claimed in one of the preceding claims, characterized
in that the composition is capable of being
coated with a coating thickness of less than 2 mm and
preferably less than 1.5 mm.
35 10) A pyrolyzed carbon-based composition
consisting of a carbon monolith, characterized in that
the pyrolyzed composition is the product of coating,
crosslinking, drying then pyrolysis of a non-crosslinked
gelled composition as claimed in one of the
- 22 -
preceding claims, said carbon monolith being
predominantly microporous and capable of forming a
supercapacitor electrode having a thickness of less
than 1 mm and preferably less than or equal to 0.5 mm.
5
11) The pyrolyzed composition as claimed in
claim 10, characterized in that it has:
- a density of between 0.1 and 1.2, and/or
- a specific surface area of greater than
10 400 m2/g, and/or
- a pore volume of between 0.2 and 0.8 cm3/g.
12) A process for preparing a non-crosslinked
gelled composition (G2) as claimed in one of claims 1
15 to 9, characterized in that it comprises:
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 C, in order to obtain an aqueous
20 solution,
b) prepolymerization until precipitation of the
solution obtained in a) in order to obtain a
precipitated prepolymer forming said non-crosslinked
gelled composition (G2), then
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.
13) The process for preparing a non-crosslinked
30 gelled composition (G2) as claimed in claim 12,
characterized in that 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
35 - 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
- 23 -
solvent W according to an R/W mass ratio of between 0.2
and 2 and preferably between 0.3 and 1.3,
in that step a) is carried out:
a1) by dissolving said polyhydroxybenzene(s) R
in said aqueous solvent W, preferably consisting 5 nsisting of
water,
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
10 a3) by stirring the mixture obtained and
adjusting its pH,
and in that step b) is carried out in a
reactor, for example immersed in an oil bath between 50
and 70°C.
15
14) A process for preparing a pyrolyzed carbonbased
composition as claimed in claim 10 or 11,
characterized in that it comprises:
a) dissolution in an aqueous solvent W of said
20 polyhydroxybenzene(s) R and formaldehyde(s) F, in the
presence of said at least one cationic polyelectrolyte
P and of a catalyst C, in order to obtain an aqueous
solution,
b) prepolymerization until precipitation of the
25 solution obtained in a) in order to obtain a
precipitated prepolymer forming said non-crosslinked
gelled composition (G2),
c) coating or molding of the precipitated
prepolymer obtained in b) with a thickness of less than
30 2 mm and preferably less than 1.5 mm,
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
35 e) pyrolysis of the dried gel obtained in d),
in order to obtain said pyrolyzed composition in the
form of monolithic porous carbon.
15) A porous carbon electrode which is usable for equipping a supercapacitor cell while being
immersed in an aqueous ionic electrolyte, the electrode
covering a metal current collector, characterized in
that the electrode consists of a pyrolyzed carbon-based
5 composition as claimed in claim 10 or 11 and has a
thickness of less than 1 mm and preferably less than
0.5 mm.
16) A supercapacitor comprising 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, characterized in that at
least one of these electrodes is as defined in claim
15.