Aquagel electrode separator for use in batteries and supercapacitors

An electrode separator for electrochemical energy storage devices, such as a high energy density capacitor incorporating a variety of carbon foam electrodes. The separator is derived from an aquagel of resorcinol-formaldehyde and related polymers and containing ionically conducting electrolyte in the pores thereof.

BACKGROUND OF THE INVENTION 
This invention relates to energy storage devices and particularly to 
electrode separators in an energy storage device, such as batteries and 
capacitors, capable of delivering very high specific power and very high 
energy density. More particularly, the present invention relates to an 
aquagel separator utilized between carbon aerogel electrodes, such as used 
in batteries and electrochemical double-layered capacitors. 
Low density organic aerogels have known applications in high energy 
physics, chemical catalysis and ion exchange reactions. U.S. Pat. Nos. 
4,806,290, 4,873,218 and 4,997,804 illustrate the preparation of such 
aerogels. Recently, electrodes formed from machinable, structurally stable 
carbon foams derived from the pyrolysis of organic foams have been 
developed for use in energy storage devices such as batteries, capacitors 
and double-layer capacitors (supercapacitors) as described and claimed in 
parent application Ser. No. 07/822,438, now U.S. Pat. No. 5,260,855. 
Separators used between electrodes in energy storage devices have been 
constructed of rubber, plastic, etc. to prevent the conduction of 
electrons between the electrode, but such prior known separators have had 
a tendency to dry out or breakdown over a period of time, or exhibit poor 
ionic conductivity. 
The need exists for electrode separators with desirable physical and 
chemical properties to enhance the energy storage and dissipation from 
capacitor devices, for example. In particular, a need exists for electrode 
separators in electrochemical energy storage devices which have ionic 
conductivity, low cost, good interfacial contact, improved 
manufacturability, and increased safety. 
SUMMARY OF THE INVENTION 
A general purpose of the present invention is to provide an electrode 
separator for an electrochemical energy storage device such as a battery 
or a capacitor capable of achieving very high energy density. 
An advantage of the present invention is that it functions with electrodes 
in a capacitor having increased specific capacitance as compared with 
conventional separators. 
The invention involves a process for preparing a separator for high density 
carbon foam electrodes from organic gels for use in batteries, capacitors, 
etc. The electrode separators may include aerogels, xerogels and 
aerogel-xerogel hybrids, as well as aerogel hybrid/composites. The 
material used for the separators may be low or high density, electrically 
insulating, dimensionally stable and machinable. 
The present invention provides electrode separators for use such as in 
electrochemical double-layer capacitors utilizing a variety of forms of 
carbon foam electrodes. The separators and the electrodes are formed from 
castable, machinable, structurally stable organic foam. Integration to 
form the capacitor is achieved using lightweight components that help 
minimize internal resistance and maximize energy. 
More specifically, the invention is fabricated from thin films or slices of 
aquagel used in the preparation of the carbon foam electrodes, and then 
immersed in a KOH solution, for example.

DETAILED DESCRIPTION OF THE INVENTION 
Described herein is a high energy electrochemical double-layer capacitor 
that incorporates electrodes based on carbon foams using aquagel 
separators. Also described are carbon aerogels having novel physical and 
chemical properties suitable for use as electrodes in such a capacitor. A 
process for manufacturing high density carbon aerogels is described. Also, 
a process for fabricating the electrode separators of this invention is 
described. 
Component Fabrication and Integration 
A capacitor, cell, or battery generally has two or more conductors or 
electrodes between which a potential difference exists. The conductors or 
electrodes are separated by an electrically insulating material which may 
be composed of an electrolytic solution, porous polymer or inorganic 
materials, or ionic conductive polymers. The function of this separator is 
to allow ions in the solution or in the polymer to flow freely from one 
electrode to the other during the charging/discharging process, but not 
conduct electrons from one electrode to the other since this would 
represent an electrical "short" resulting in loss of energy and heat 
formation. In a typical capacitor, the conductors are electrically 
connected with an external power supply and are known as electrodes. 
As shown schematically in FIG. 1A, capacitor 14 is made up of spaced 
electrodes 22, 26, made of carbon aerogel, separated by electrode 
separator 40. Electrical contacts 16, 18 to electrodes 22, 26 
respectively, provide electrical connection to voltage source 20. 
The electrochemical double-layered capacitor described hereinafter can be 
assembled from a variety of elements. The preferred embodiments 
incorporate, in various combinations, electrodes, electrical contacts to a 
power supply, cell and/or electrode separators, environmental seals, and 
an electrolyte. In general, the elements are desirably lightweight and 
chemically stable with respect to the electrolyte used within the 
double-layered capacitor. The following discussion describes the elements 
and construction of such capacitors that employ carbon aerogels as the 
electrodes using aquagel separators. However, a capacitor of this type may 
also be based on xerogel, xerogel/aerogel hybrids, or hybrid/composites 
described below. 
For conventional double-layer capacitors, capacitances are practically 
measured in terms of pico or micro-farads (pF or .mu.F, respectively). In 
an integrated double-layered device capable of delivering very high 
specific capacitance, i.e., energy, and very high power, referred to here 
as a supercapacitor, capacitances on the order of tens of farads/gram of 
aerogel are possible. In particular, by incorporation of lightweight 
materials, thin film contacts and carbon aerogel electrodes, a 
supercapacitor capable of very high energy densities is possible. 
A simple cell 10 illustrated in FIG. 1B consisting of two electrodes and an 
electrode separator as in the FIG. 1A embodiment, and made in accordance 
with the present invention is shown in FIGS. 1B and 2A, discussed in 
detail below. The cells may be repeatedly stacked to obtain higher 
voltages. 
Electrodes 
The electrodes of the electrochemical double-layered capacitor 
(supercapacitor) are desirably carbon aerogels. Compared to materials used 
in conventional electrodes, the aerogels are very lightweight, having 
densities between about 0.3-0.9 g/cc, and have high surface areas, about 
400-1000 m.sup.2 /g. More generally, the densities are about 0.1-1.2 g/cc 
and surface areas are about 100-1200 m.sup.2 /g. These characteristics of 
the aerogel contribute minimally to the weight of the supercapacitor and, 
taken together with the aerogel's electrochemical properties, optimize its 
capacitance. FIGS. 3 and 4 illustrate the charge/discharge characteristics 
of a carbon aerogel having reactant/catalyst ratio, here a 
[Resorcinol]/[Catalyst] or R/C value of 50. In FIG. 3, aerogel samples 
0.15 cm thick with surface areas of 1.58 cm.sup.2 in an inorganic 
electrolyte were charged to 1.0 volts and discharged through a 500 
milliohn resistor. The symmetry between the charging (upper curve) and 
discharging (lower curve) cycles indicates a high degree of reversibility 
of the device and its potentially long life. 
Other physical characteristics of the aerogels permit creation of very thin 
electrodes by conventional methods, with thicknesses ranging from 
fractions of a micron (.mu.m) to several millimeters (mm). The preferred 
thickness of each aerogel electrode is between about 125 .mu.m and 2 mm. 
The electrodes may be formed by slicing the gels before or after solvent 
extraction followed by pyrolysis. Alternatively, these thin electrodes may 
be formed from the aerogel in several ways. In one method, thin layers 
with thicknesses greater than the desired thickness of the electrode can 
be sliced from an organic aerogel monolith before the pyrolysis procedure. 
Since some shrinkage occurs during pyrolysis, very thin carbon aerogel 
electrodes can be formed by subjecting these thin layers of the organic 
aerogel to the pyrolysis procedure. Another method involves slicing the 
thin layers after the carbon aerogel monoliths have been formed. In a 
preferred embodiment, disk-like electrodes are formed. 
To increase mechanical flexibility, the aerogels may be chopped or ground 
into desired size particles, or formed as microspheres, whereafter the 
particles or microspheres are mixed with an appropriate binder to produce 
a composite aerogel electrode. 
The electrodes may also be formed from a lamination of gel layers, for 
example, as in a xerogel-aerogel hybrid consisting of a xerogel layer and 
an aerogel layer joined together. An alternative configuration of a 
xerogelaerogel hybrid is based on alterations of the procedure to form a 
single gel which has characteristics of both a xerogel and an aerogel. 
Specific details about aerogel electrodes are discussed below in connection 
with FIG. 1A. 
In addition, the capacitance of a supercapacitor also depends on the 
composition of the aerogel being used as an electrode. FIG. 5 illustrates 
the capacitance of supercapacitors incorporating electrodes composed of 
one of three carbon aerogel formulations in various potassium hydroxide 
electrolyte solutions. The R/C values of these aerogels were 50, 200, and 
400. The highest capacitance was observed for aerogels having an R/C value 
of 50. For each aerogel formulation, capacitance increased with increasing 
electrolyte concentration. The rate of increase was steepest for the 0-5M 
range of electrolyte concentration. 
Electrode Surface Modifications 
For certain applications, surface modification procedures may be 
incorporated into the above-described aerogel preparation process. For 
example, the energy storage capability of the aerogel when used as an 
electrode can be increased. By binding, at the carbon matrix surface of 
the electrode, electroactive groups that can be reversibly reduced and 
oxidized, the energy storage capability of a capacitor may be increased. 
Such an increase in energy storage capability of capacity is referred to 
here as "pseudo-capacity", distinct from electrolytic double layer 
capacity. The latter terms refer to the capacitance, in a double layer, at 
the interfacial region, associated with charge accumulation at the 
interface of each electrode's carbon matrix with the electrolyte solution, 
and the subsequent charge separation of the ions of which the electrolyte 
is composed. 
The term pseudo-capacity, on the other hand, refers to the capacitance 
associated with the attachment of surface groups, e.g., hyroquinone, that 
bear oxidizable or reducible components to the carbon matrix surface. 
Application of a potential through the electrolyte changes the oxidation 
state of the surface groups. For a given electrode/electrolyte system, 
this capacitance is in addition to double electrolytic layer capacitance. 
When the electrodes are composed of aerogels having particularly high 
surface areas an electrochemical "formatting" step may be useful to ensure 
the stability of an organic electrolyte. At high voltages, the electrolyte 
can slowly decompose, causing the capacitor to "dry out" as the amount of 
electrolyte decreases. Decomposition of this type can be reduced by a 
careful charging/discharging procedure. One useful procedure involves the 
cycling of the electrode between zero volts and progressively higher 
potentials and then discharging the capacitor, while at the same time 
monitoring the charge efficiency. During this procedure, a uniform, 
nonporous overlayer, itself a decomposition product, is believed to form 
on the carbon matrix surface at the interface with the electrolyte, 
inhibiting further breakdown of the electrolyte. The cycling procedure is 
continued until the charge efficiency approaches unity. As a result of 
this procedure, the interfacial surface is said to be `formatted`. During 
the cycling process, it is believed that a nonaggressive or slow ramp to 
the higher potentials promotes uniform film formation at the interface. 
Using a propylene carbonate/sodium borotetrafluoride electrolyte system as 
an example, the decomposition involves the liberation of propane gas and 
the formation of sodium carbonate. As a result of the cycling procedure, a 
sufficiently thick and uniform overlayer of sodium carbonate forms at the 
carbon matrix-electrolyte interface in about 10 charge-discharge cycles to 
inhibit the subsequent decomposition of the electrolyte. 
Electrical Contacts 
Electrical contacts from the electrodes to an external power supply may be 
provided in several ways. Specific embodiments are discussed below in 
connection with FIGS. 1B and 2A. 
Separators 
The supercapacitor may include either or both of two types of separators. 
An electrode separator 40 electrically insulates adjacent electrodes 
within individual capacitor cells from nonionic conduction of electricity. 
A process for making an embodiment of an electrode separator 40, which 
incorporates a portion of the process for forming the carbon foam 
electrodes, is described in detail hereinafter. A cell separator 24 
provides ionic insulation between individual cells. The separators are 
discussed in greater detail below in connection with FIGS. 1B and 2A. 
Environmental Seal 
To isolate the internal environment of the supercapacitor from ambient 
conditions, the individual cells may be enclosed singly or in groups 
within an environmental seal 52, as illustrated in FIG. 2B. 
Electrolyte 
The primary requirements of electrolytes used in supercapacitors are 1) 
chemical stability; 2) low density (generally less than 2 g/cc); 3) 
relatively low viscosity (less than 4 cp); 4) liquid over a reasonable 
range of temperature near ambient conditions; 5) commercially inexpensive; 
6) environmentally safe; and 7) compatibility with carbon. Some samples of 
inorganic electrolytes which may be used include water, ammonia, sulfur 
dioxide, phosphoryl chloride, thionyl chloride, sulfuryl chloride and 
mixtures of these electrolytes. Examples of organic electrolytes which may 
be used include propylene carbonate, ethylene carbonate, methylformate, 
dimethylsulfoxide, ethylene glycol sulfite, acetonitrile, tetrahydrofuran, 
1,2-dimethoxyethane and mixtures of these electrolytes. Generally, organic 
electrolytes allow for voltage increases about 3 times that for a 
water-based system but lower power by about an order of magnitude. 
Because power and energy of an electrochemical capacitor both increase to 
some extent with increased dissolved ion concentration within the 
electrolyte, it is desirable to have electrolytes which can dissolve large 
amounts of ionic salts. Depending on the electrolyte, a large amount of 
ionic salt can be greater than about 0.1M to 16M. Additionally, it is 
important that the ionic salt is stable, that is, is a nonreducible, 
nonoxidizable salt that is electrochemically stable with respect to the 
electrolyte. This requirement means that the salt has a reduction voltage 
below the solvent reduction potential and oxidation voltage above the 
solvent oxidation potential. A mixed ionic salt or a salt mixed with an 
acid or a base may be introduced into the electrolyte to optimize both 
power and energy density in the supercapacitor. Examples include 
aqueous-based systems where dissolved acids or bases such as sulfuric, 
hydrochloric, nitric or phosphoric acids, or hydroxides of potassium, 
sodium, or lithium, respectively. 
Integration 
The method by which a supercapacitor may be integrated to incorporate the 
above-described elements generally involves constructing one or more 
individual capacitor cells in series, establishing electrical contact 
between the cells and to the power supply, injecting the electrolyte into 
the cell, and sealing the cells. The goals of the integration are to 
establish and maintain electrical and/or ionic conduction processes 
necessary for proper and sustained operation of the supercapacitor, 
incorporate components with minimal contributions to the internal 
resistance of the supercapacitor and thereby help optimize its power, and 
help prolong the life of the supercapacitor. 
Once the electrodes are formed from the aerogel as described in detail 
below, an electrical contact to the power supply is provided on one side 
of each electrode. In a preferred embodiment, one side of an aerogel 
electrode is compression sealed against a nonporous metal or carbon sheet 
acting as a current collector. The seal between the electrode and the 
metal or carbon sheets is formed as a result of the application of 
pressure, or the use of an electrically conductive adhesive. 
FIG. 1B illustrates an alternative embodiment for establishing electrical 
contact between portions of two adjacent cells 10, 12, with cell 12 being 
partially shown. In the upper cell 10, a solder-wettable layer 32 and an 
adhesive conductive metal layer 30 overlie an electrode 22. An electrode 
separator 40 lies between electrode 22 of the upper cell 10 and electrode 
26 of the upper cell 10. Solder-wettable layer 36 and adhesive conductive 
layer 34 overlie electrode 26. A similar electrode 26 is shown in lower 
cell 12, with adhesive layer 34 and solder-wettable layer 36 formed 
thereon. A cell separator 24 lies between electrode 22 of cell 10 and 
electrode 26 of cell 12. 
Metal adhesive layers 30, 34 may be deposited according to conventional 
methods such as sputtering, vapor deposition, and electroplating methods. 
The thickness of the adhesion conductive layers should be sufficient to 
prevent ionic conduction across the aerogel of the electrolyte through the 
pores. Typically, thicknesses of 500 .ANG. are adequate. The metals should 
be chemically inert to resist corrosion by the electrolyte and provide 
good mechanical adhesion to the aerogel. Suitable metals include titanium, 
nickel, tantalum or alloys such as titanium-tungsten. The adhesive layers 
may also be formed using fired polymeric compounds, sheets or adhesives. 
Noncorrosive solder-wettable metal layers 32, 36 such as gold or copper are 
deposited on top of the adhesion metal layers 30, 34. Layers of 
solder-wettable metal of about 1000 .ANG. or more may be deposited 
according to similar techniques described in connection with the adhesion 
metal layers. If certain types of adhesive conductive layers are used, the 
layers 32, 36 may be omitted. 
Positioned between the cells is a highly electrically conductive cell 
separator 24 that prevents ionic flow but permits electrical conduction 
between the electrodes. The cell separator 24 is preferably lightweight 
and has low density in order to minimize its weight contribution to the 
supercapacitor. In the embodiment, shown in FIG. 2A, the cell separator 24 
is in the form of an o-ring having an outer diameter 24a larger than the 
diameter of the disk-like electrodes 22, 26 and an inner diameter 24b 
smaller than the diameter of the electrodes. The o-ring thickness should 
be as thin as possible. The composition of the cell separator is desirably 
such that it is electrically conductive, but is stable chemically with 
respect to the electrolyte. Suitable cell separators may consist of 
metals, conductive rubbers or plastics, nonporous carbon or metal-plastic 
composites. Also, the cell separators may be made of an adhesive material, 
such as GLYPTAL, made by General Electric Co., which is a low cost, 
carbonizable, organic paste. 
FIG. 2A illustrates an expanded view of portions of the two cells 10, 12 to 
provide further detail. In addition to the elements shown in FIG. 1B, cell 
12 includes an electrode separator 40 which separates electrode 26 from 
the second electrode (not shown) in the cell 12. A portion of solder 42 is 
deposited on solder-wettable layer 36 which is formed on electrode 26. The 
volume of solder 42 should be less than that formed by the inner diameter 
24b of cell separator 24. 
As an additional protection against ionic conduction between the electrodes 
via the electrode separator, the electrode separator may be hermetically 
sealed against the electrodes. Depending on the composition of the 
electrode separator, the hermetic seal may be formed by conventional 
methods, such as compression sealing. Alternatively, for a electrode 
separator consisting of metal-plastic composite, the separator may be 
integrated with an electrode by heating to create the seal. Specifically, 
integration with the electrodes is achieved by heating until the melted 
thermoplastic flows and physically connects both electrodes. The melted 
thermoplastic seals the electrodes to ionically isolate them from one 
another without disturbing electrical conductivity. 
The selection of the solder should take into account the composition of the 
thermoplastic selected as the cell separator. For example, for a cell 
separator composed of polypropylene, melting point approximately 
138.degree. C., a lead/tin alloy solder melting at about 160.degree. C. 
may be suitable so that unnecessary heating is avoided. Melted solder 
flows in the well in cell separator 24 formed by inner diameter 24b and 
connects the second electrode 26 with the cell separator 24 via 
solder-wettable layer 36 and adhesive layer 34 so that electrical contact 
between the electrodes is established. 
By repeating these steps, additional cells composed of electrodes with 
similar layers, separators and electrical contacts may be created in order 
to form a multicell stack 50 (FIG. 2B) that may be joined together for a 
high energy density supercapacitor device. The individual cells may have 
electrical contacts prepared according to any of the previous embodiments. 
The intracell seal created by the electrode separator 40 helps prolong the 
life of the device by preventing corrosion of the solder by the 
electrolyte since ionic conduction through the aerogel is prevented. 
In such a stack, individual capacitor electrodes are electrically isolated 
from one another by an electrically nonconductive, but ionically 
conductive electrode separator 40, as illustrated in FIGS. 2A and 1B. 
Electrical nonconductivity is essential in order to maintain intracell 
voltage differences. In addition, the electrode separator 40 must be 
chemically stable with respect to the electrolyte and sufficiently porous 
to facilitate ionic conduction, a major component of the internal 
supercapacitor resistance. Prior known electrode separator materials 
include polypropylene, TEFLON.TM. (commercially available from DuPont 
Company), nylon, and glass fiber filter papers. However, the present 
invention provides an electrode separator composed of an aquagel 
containing KOH, for example, in the pores thereof, as described 
hereinafter. 
Stacking of individual capacitor cells may be performed before or after the 
electrolyte is introduced into the cells. Introduction after stacking is 
preferred in order to avoid degrading the electrolyte while heat is 
applied during the sealing procedure used to join cells in a multilayer 
stack. 
Before introducing the electrolyte, it is important to ensure that neither 
oxidizable nor reducible gases are dissolved in the electrolyte in order 
to minimize the device's self-discharge rates promoted by their presence. 
Removal of these gases may be achieved by bubbling a nonreducible, 
nonoxidizable gas through the electrolyte solution to strip the oxidizable 
and/or reducible dissolved gases from the solution. 
After these dissolved gases are removed, the electrolyte is injected into 
the cells by means known to the art. For example, the supercapacitors may 
be secured in a jig which is in turn immersed in the electrolytic fluid. 
For example, under vacuum, the electrolyte fills the pores of the aerogel, 
separators and other voids. Surfactants in the electrolyte may be used to 
facilitate the introduction of some electrolytes into the aerogel and may 
enhance the capacitance of the supercapacitor. 
Once a cell or stack of cells is prepared, enclosure of the cell or cells 
is necessary to protect the capacitor system from the environment. FIG. 2B 
illustrates a capacitor having a multicell stack 50 enclosed in a 
cylindrical environmental seal 52. The stack 50 is terminated with 
electrical contacts 54, 56, according to any of the materials and methods 
described above. The environmental seal 52 allows electrical contact to be 
established to an external power source, not shown. Formation of the 
environmental seal 52 may be accomplished in several ways. For example, a 
cell or multicell stack may be inserted into a hot sealing cylinder that 
applies heat to create mechanical seals between the melted and protruding 
edges of adjacent thermoplastic o-rings 58, 60, illustrated in FIG. 2A. 
Or, more simply, pressure may be applied to the stack to form a 
compression seal between the o-rings. Alternatively, the cells may be 
enclosed in a c-shaped thermoplastic form which is sealed to form a 
cylindrical environmental seal using a melted bead of thermoplastic. A 
supercapacitor is thereby formed and is ready for charging. 
Preparation Of Carbon Aerogels 
The process in general requires first that the reactants are mixed with a 
catalyst and may include the addition of metals. The reactants include 
resorcinol, phenol, catechol, phloroglucinol, and other polyhydroxybenzene 
compounds that react in the appropriate ratio with formaldehyde or 
furfural. Preferred combinations include resorcinol-furfural, 
resorcinol-formaldehyde, phenol-resorcinol-formaldehyde, 
catechol-formaldehyde, and phloroflucinolformaldehyde. A gel formed by 
polymerization is then dried in a solvent exchange and extraction step. 
The resulting organic aerogel is then pyrolyzed in an inert atmosphere to 
form a carbon aerogel. 
Specifically, the process to prepare the gels of the present invention 
proceeds through a sol-gel polymerization of certain multifunctional 
organic monomers in a solvent, typically water, leading to the formation 
of highly cross-linked, transparent gels. For example, in a preferred 
embodiment, one mole of resorcinol (1,3-dihydroxybenzene) condenses in the 
presence of a basic catalyst with two moles of formaldehyde. Mildly basic 
catalysts such as sodium carbonate are preferred. In this polymerization, 
resorcinol is a trifunctional monomer capable of adding formaldehyde in 
the 2-, 4-, and/or 6-ring positions. The substituted resorcinol rings 
condense with each other to form nanometer-sized clusters in solution. 
Eventually, the clusters crosslink through their surface groups (e.g., 
CH.sub.2 OH) to form an aquagel. A full discussion of the chemistry is not 
provided here since the specific details are described in depth in U.S. 
Pat. Nos. 4,997,804 and 4,873,218, hereby incorporated by reference. 
The size of the clusters is regulated by the concentration of catalyst in 
the resorcinol-formaldehyde (RF) mixture. More specifically, the mole 
ratio of the reactant resorcinol (R) to catalyst (C), R/C, controls the 
surface area and electrochemical properties of the resulting gel. For 
example, as illustrated in FIG. 6, in gels having R/C of 100 and 200, 
electrical conductivity of the gels increases significantly with 
increasing density. 
RF aquagels (and aerogels) are typically dark red in color, and transparent 
because of the extensive network of small pores that are much smaller than 
the wavelength of visible light. 
The next step in aerogel preparation is to dry the aquagel. If the 
polymerization solvent is removed from these gels by simple evaporation, 
large capillary forces are exerted on the pores, forming a collapsed 
structure known as a xerogel. In order to preserve the gel skeleton and 
minimize shrinkage, it is preferable to perform the drying step under 
supercritical conditions. The highly porous material obtained from this 
removal operation is known as an aerogel. By appropriate adjustment of 
drying conditions, a hybrid structure having characteristics of both a 
xerogel and an aerogel may be produced. For example, such a hybrid may be 
produced as a result of a partial evaporation of the gel solvent under 
conditions promoting xerogel formation followed by supercritical 
extraction under conditions promoting aerogel formation. The resulting 
hybrid structure would then be dried under supercritical conditions and 
pyrolyzed. One means for removing water from the water-based aquagel to 
form an organic aerogel is by supercritical extraction of the gel with a 
relatively lower surface tension fluid such as carbon dioxide. Because 
water is immiscible with liquid CO.sub.2, the aquagels are first exchanged 
with an organic solvent such as acetone. The liquid CO.sub.2 is then 
exchanged for the acetone and the gel is supercritically dried inside a 
temperature-controlled pressure vessel. The critical point of carbon 
dioxide (T.sub.c =31.degree. C.; P.sub.C =7.4 MPa) is low enough to 
facilitate its removal without degrading the gel structure. The time 
required for supercritical drying depends on the thickness of the gel. 
In cases where the gels are of sufficiently high density, such as greater 
than about 40 wt. % solids, the pore network may have sufficient inherent 
strength to withstand the drying process without resort to supercritical 
drying conditions. Thus, carbon dioxide may be bled from the vessel under 
nonsupercritical conditions. Nonsupercritical drying is particularly 
attractive because of reduced processing time. Also, low pressure/air 
drying process has been recently discovered, and such is described and 
claimed in copending U.S. application Ser. No. 08/041,503, filed Apr. 1, 
1993, entitled "Method Of Low Pressure And/Or Evaporative Drying Of 
Aerogel". 
Following the solvent exchange/extraction step and the cure cycle, the 
organic aerogel is typically pyrolyzed at elevated temperatures about 
1050.degree. C. in a conventional inert atmosphere of nitrogen, argon, 
neon or helium to form carbon aerogels. Other pyrolysis temperatures 
(500.degree.-3000.degree. C.) can be used to alter the surface area and 
structure of the carbon aerogel. For example, FIG. 7 shows how the BET 
surface area, as measured by nitrogen gas adsorption, varies with 
pyrolysis temperature. In particular, higher surface areas are achieved at 
lower temperatures. The resulting aerogels, independent of the procedure 
by which they are pyrolyzed, are black and no longer transparent due to 
the visible absorption properties of the carbon matrix. 
Although the aerogels have suitably high electrical conductivities to be 
useful as electrodes in a supercapacitor, sometimes it may be desired to 
further increase their electrical conductivities. One means by which this 
may be accomplished is to reduce the resistance of an aerogel electrode by 
incorporating electrically conductive materials into the aerogel during 
its preparation. Any electrically conductive material stable in the 
electrolyte, including metals like tungsten, may be suitable as long as 
the material does not disturb the other chemical or physical 
characteristics necessary to produce an adequate aerogel material. 
The aerogel electrodes may also be formed from aerogels (carbonized or 
uncarbonized) which have been chopped or ground to a desired particle size 
or produced as aerogel microspheres, and then mixed with a binder to 
produce an electrode having greater mechanical flexibility. 
The preparation of several gels according to the above-described process is 
described in the following examples. 
Gel Preparation 
EXAMPLES 1-3 
Resorcinol-formaldehyde-based (RF) gels with varying R/C ratios were 
prepared. The first three reactants were added to a 250 ml beaker in the 
order listed in Table 1 and then mixed until a clear, homogeneous solution 
was formed. The sodium carbonate catalyst was then added. Other catalysts 
such as lithium carbonate (Li.sub.2 CO.sub.3) and potassium carbonate 
(K.sub.2 CO.sub.3) may be used. The solution was then cast in molds, e.g., 
glass vials. The gels were cured according to the following cycle: 24 
hours at room temperature, followed by 24 hours at 50.degree. C., and 72 
hours at 85.degree.-95.degree. C. Densities of the resulting aerogels were 
about 0.3-0.4 g/cc. 
TABLE 1 
______________________________________ 
Reactants (g) 1 2 3 
______________________________________ 
resorcinol (R) 12.35 12.35 12.35 
formaldehyde (37%) 
17.91 17.91 17.91 
deionized and 
distilled H.sub.2 O 
15.30 13.90 18.10 
sodium carbonate, 
5.58 22.32 2.79 
0.1 --M (C) 
R/C 200 50 400 
______________________________________ 
EXAMPLE 4 
A catechol-formaldehyde-based gel using the reactants listed below was 
prepared and cured according to the procedure used in Examples 1-3. The 
density of the resulting aerogel was about 0.4 g/cc. 
______________________________________ 
Reactants (g) 
catechol (R') 12.35 
formaldehyde (37%) 17.91 
deionized and distilled H.sub.2 O 
15.30 
sodium carbonate, 0.1 --M (C) 
5.58 
R'/C 200 
______________________________________ 
EXAMPLE 5 
Upon completion of the cure cycle, the gels prepared in Examples 1 in an 
organic solvent, e.g., acetone. During a first wash, trifluoroacetic acid 
was usually added to the solvent at a concentration of .ltoreq.0.1 wt. % 
to promote additional crosslinking of the gels. With agitation, the 
solvent diffused into the gels, replacing the water occupying the gel 
pores. After several additional exchanges with fresh acetone, the pores 
were completely filled with the organic solvent and the gels were ready 
for supercritical extraction. 
Supercritical Extraction 
EXAMPLE 6 
Acetone-filled gels prepared according to Examples 1 through 5 were placed 
in open glass containers and covered with additional acetone to ensure the 
gels were submerged. Submersion in the solvent was necessary throughout 
the extraction process to ensure that the gels did not crack. The 
containers were then placed in a jacketed pressure vessel. Trapped air was 
slowly bled from the vessel as the vessel was filled with liquefied carbon 
dioxide at .about.900 psi and a jacket temperature of 14.degree. C. The 
jacket temperature was controlled to .+-.0.5.degree. C. with a circulating 
bath containing a 50/50 solution of ethylene glycol/water. 
The gels were allowed to stand in the liquefied carbon dioxide for a 
minimum of 1 hour. After this initial induction period, the vessel was 
flushed with fresh carbon dioxide for approximately 15 minutes. The carbon 
dioxide was then drained from the vessel to a level just above the 
crosslinked gels. The vessel was then refilled with liquefied carbon 
dioxide. The drain/refill procedure was conducted at least six times 
daily. In this process, diffusion is solely responsible for the 
displacement of acetone from the pores of the gel by carbon dioxide. 
Generally, 2 days of the drain/refill procedure were required to 
completely displace the acetone with carbon dioxide. 
In preparation for supercritical drying [T.sub.C =31.degree. C.; P.sub.C 
=1100 psi], the carbon dioxide level was drained to a level just above the 
gels. All valves to the pressure vessel were then closed and the vessel 
was heated to 50.degree. C. At this temperature, a pressure of 1800 psi 
was usually recorded. The pressure vessel was held at these conditions for 
a minimum of 4 hours, after which time the pressure was slowly reduced by 
bleeding over a period of 8 hours while maintaining the jacket temperature 
at 50.degree. C. The resulting material was an organic aerogel. 
The aerogels produced according to the above-described process and Examples 
have continuous porosity, ultra-fine pore sizes (less than 100 nanometers 
(nm)), high surface areas (400 to 1000 m.sup.2 /g) and a solid matrix 
composed of interconnected colloidal-like particles or polymeric chains 
with characteristic diameters of 10 nm. However, the properties of such 
aerogels may be tailored further according to the desired use by adjusting 
process parameters. For example, the density of the gels may be adjusted 
by altering the polymerization and/or drying conditions to affect solid 
volume fraction and/or pore size. The thus produced aerogels may be also 
chopped or ground to produce particles or powder, and the particles or 
powder can be mixed with an appropriate binder to form the desired 
electrode configuration, for example. As discussed above, electrical 
conductivity may be increased by the introduction of electrically 
conductive materials. In addition, aerogel surface area and functionality 
can also be increased through the proper selection of purge gases employed 
during the pyrolysis cycle. Oxidizing gases such as carbon dioxide and air 
can be slowly purged through a tube furnace at 600.degree.-1200.degree. C. 
to activate the aerogel or alter the reactivity functionality of the 
aerogel surface area. This activation process, for example, may include 
the steps of dipping into fully concentrated 8-16 .mu. nitric acid, 
sealing in a jar, oven heating at &gt;50.degree. C. for at least one day, 
rinsing in water and drying. Various oxidizing agents may be used. While 
an activation process is used commonly to activate powderly carbon blacks, 
it is not typically used to activate bulk materials such as carbon aerogel 
monoliths. In an alternative procedure, the carbon aerogels can also be 
chemically activated through treatment with 35% HNO.sub.3 acid at 
25.degree.-100.degree. C., followed by rinsing with distilled H.sub.2 O, 
and drying. 
An Aerogel Hybrid/Composite 
Thin film aerogel hybrid/composite electrodes can be used. Thin film 
fabrication of aerogel hybrid composites can be accomplished by 
infiltration of highly porous carbon papers, membranes, felts, or fabrics 
with a resorcinol-formaldehyde (RF) sol. The RF sol is prepared by mixing 
12.35 g resorcinol and 17.91 g formaldehyde (37% solution) at room 
temperature until a clear solution is formed. Next, 5.58 g 0.1 M sodium 
carbonate is added to the mixture, which is then heated at 50.degree. C. 
for 10 mins. to form the RF sol. We have demonstrated that the RF sol (60% 
w/v) can be infiltrated into a 125 .mu.m thick carbon paper (Textron C.05; 
.about.0.15 g/cc), dried, and pyrolyzed at 1050.degree. C. into a carbon 
aerogel hybrid/composite. This material has high surface (&gt;500 m.sup.2 
/g), a density of .about.0.5 g/cc, and a capacitance of .about.20 F/cc 
when used as an electrode in an aqueous-based (4M KOH) supercapacitor. 
The Aquagel Electrode Separator 
The electrode separator, such as illustrated at 40 FIG. 1A, for example, is 
an ionic conductive polymer. As pointed out above, the function of an 
electrode separator is to allow ions to flow freely from one electrode to 
the other, while preventing the flow of electrons from one electrode to 
the other. By way of example, a resorcinol-formaldehyde (RF) polymeric 
aquagel which was immersed in a 5 molar potassium hydroxide (KOH) solution 
to fill the pores thereof has been experimentally tested and shown that it 
has better ionic conductivity than commercial materials previously used as 
separators, and it can be cast directly onto the electrode as a thin film, 
thereby increasing power capability. The aquagels may be made from the 
various mixtures of reactants and catalysis described above. The aquagel 
separators may also incorporate an ionic conductive electrolyte, for 
increased manufacturability. 
Preparation Of The Aquagel Electrode Separator 
The aquagel electrode separator 40 may be produced by the abovedescribed 
process of Examples 1-4, whereafter slices (thickness of 1 to 50 mil) of 
the thus produced aquagel were immersed in a potassium hydroxide (KOH) 
solution in order to replace the water with KOH and fill the pores 
thereof. This was carried out by exchanging water in the aquagel with KOH 
of a 5 mole (M) concentration. The water to KOH exchange was carried out 
at a temperature of 25.degree. C. and a time period of 1 hour. 
While the experimental verification utilized potassium hydroxide to fill 
the aquagel pores, other suitable electrolyte-type materials may be used. 
Examples include aqueous-based systems of dissolved acids or bases such as 
sulfuric, hydrochloric, nitric or phosphoric acids, or other hydroxides, 
such as sodium or lithium. 
In tests to verify this invention, the resulting separator was placed 
between two carbon aerogel electrodes and a voltage was placed across the 
cell, such as illustrated in FIG. 1A. Upon removal of the applied 
potential, the voltage output of the cell was stable, indicating little 
electron conduction through the separator. Upon shorting the cell a 
current of 1.35 amps was measured. Calculations based on resistance 
measurements indicate a specific conductivity of 72/mohm.cm.sup.-1. The 
following chart is a comparison of specific resistance of separation 
materials, each in 5M KOH. 
______________________________________ 
RESISTANCE 
MATERIAL (m.OMEGA..mil/cm.sup.2) 
______________________________________ 
Aquagel 100.ANG. pore size (R/C 200), 26 mil 
72 
Glass Microporous Filter, 10 mil 
154 
Cellulose Acetate, &lt;1 .mu.m pore, 8 mil 
243 
Polypropylene 10 .mu.m pore 
328 
Ashless filter paper, 7 mil, med. course 
772 
Nylon, 4000.ANG. pore, 4.5 mil 
860 
______________________________________ 
In summary, the capacitor achieves very high energy density by 
incorporation of high density carbon aerogel electrodes and utilization of 
lightweight carbon aerogel electrodes and utilization of lightweight 
packaging techniques. Specific capacitance is increased substantially 
compared to conventional capacitance devices. One or more cells may be 
connected in series to achieve high voltage stacks. 
Industrial Applicability 
While the invention has been described in connection with specific 
embodiments and uses thereof, it will be understood that the process is 
capable of further modification, and this application is intended to cover 
any variations, uses, or adaptations of the invention following, in 
general, the principles of the invention and including such departures 
from the present disclosure as come within known or customary practice in 
the art to which the invention pertains and as may be applied to the 
essential features herein before set forth, and as fall within the scope 
of the invention and the limits of the appended claims.