U.S. patent application number 14/085723 was filed with the patent office on 2015-05-21 for nanoporous electrodes and related devices and methods.
This patent application is currently assigned to Nanotune Technologies Corp.. The applicant listed for this patent is Nanotune Technologies Corp.. Invention is credited to Samir J. ANZ, David MARGOLESE, Vinod M.P. NAIR, Shiho WANG.
Application Number | 20150140476 14/085723 |
Document ID | / |
Family ID | 41400096 |
Filed Date | 2015-05-21 |
United States Patent
Application |
20150140476 |
Kind Code |
A1 |
NAIR; Vinod M.P. ; et
al. |
May 21, 2015 |
NANOPOROUS ELECTRODES AND RELATED DEVICES AND METHODS
Abstract
High surface area electrodes formed using sol-gel derived
monoliths as electrode substrates or electrode templates, and
methods for making high surface area electrodes are described. The
high surface area electrodes may have tunable pore sizes and
well-controlled pore size distributions. The high surface area
electrodes may be used as electrodes in a variety of energy storage
devices and systems such as capacitors, electric double layer
capacitors, batteries, and fuel cells.
Inventors: |
NAIR; Vinod M.P.; (Concord,
CA) ; MARGOLESE; David; (Monrovia, CA) ; ANZ;
Samir J.; (La Crescenta, CA) ; WANG; Shiho;
(Mountain View, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Nanotune Technologies Corp. |
Mountain View |
CA |
US |
|
|
Assignee: |
Nanotune Technologies Corp.
Mountain View
CA
|
Family ID: |
41400096 |
Appl. No.: |
14/085723 |
Filed: |
November 20, 2013 |
Current U.S.
Class: |
429/532 ;
361/502; 427/123; 427/126.3; 427/58; 429/245 |
Current CPC
Class: |
H01M 4/8605 20130101;
Y02E 60/10 20130101; H01G 11/86 20130101; Y02E 60/13 20130101; H01G
11/46 20130101; H01G 11/24 20130101; H01M 4/131 20130101; H01G
11/26 20130101; H01M 4/8846 20130101; Y10T 428/249969 20150401;
H01M 4/66 20130101; H01G 11/32 20130101; Y02E 60/50 20130101; H01G
11/36 20130101 |
Class at
Publication: |
429/532 ;
429/245; 427/58; 427/123; 427/126.3; 361/502 |
International
Class: |
H01G 11/36 20060101
H01G011/36; H01M 4/86 20060101 H01M004/86; H01M 4/88 20060101
H01M004/88; H01M 4/66 20060101 H01M004/66 |
Claims
1-33. (canceled)
34. A method of making an electrode, the method comprising: a)
providing a sol-gel derived silica monolith comprising an open
network of pores; b) coating a surface of the open network of pores
or at least partially filling the open network of pores with a
conductive material; and c) selectively removing the silica
material in the monolith to provide a conductive network.
35. The method of claim 34, wherein the open network of pores are
substantially filled with the conductive material.
36. The method of claim 34, wherein at least partially filling the
open network of pores comprises impregnating the open network of
pores with a colloidal solution of metal and/or metal oxide
particles.
37. The method of claim 34, wherein at least partially filling the
open network of pores comprises impregnating the open network of
pores with one or more precursor to a conducting polymer, and
reacting the one or more precursors to form the conductive
network.
38. The method of claim 37, wherein at least partially filling the
open network of pores comprises impregnating the open network of
pores with one or more carbon precursor materials selected from the
group consisting of furfural, furfuryl alcohol, polyfurfuryl
alcohol, resorcinol formaldehyde, sucrose and glucose, and
converting the one or more carbon precursor materials into carbon
by polymerization and carbonization.
39. The method of claim 34, adapted for making a conductive network
having a conductive surface area of at least about 500
m.sup.2/g.
40. The method of claim 34, wherein the sol-gel derived silica
monolith has an average pore diameter between about 0.3 nm and
about 10 nm.
41. The method of claim 34, wherein the sol-gel derived silica
monolith has a pore size distribution wherein at least about 50% of
pores are within about 20% of an average pore size.
42. An electrode made by the method of claim 34.
43. The electrode of claim 42, configured for use in a capacitor,
an electric double layer capacitor, a battery, or a fuel cell.
44. A method of making an electrode material, the method
comprising: a) providing a silica sol-gel derived monolith
comprising an open network of pores; b) coating a surface of the
open network of pores or at least partially filling the open
network of pores with a conductive material to form a conductive
network; c) selectively removing the silica material in the
monolith to provide a conductive network; and d) making the
resulting material from step c) into a conductive powder.
45. The method of claim 44, wherein at least partially filling the
open network of pores comprises impregnating the open network of
pores with one or more carbon precursor materials selected from the
group consisting of furfural, furfuryl alcohol, polyfurfuryl
alcohol, resorcinol formaldehyde, sucrose and glucose, and
converting the one or more carbon precursor materials into carbon
by polymerization and carbonization.
46. The method of claim 44, wherein the silica sol-gel derived
monolith has an average pore diameter between about 0.3 nm and
about 10 nm, and/or has a pore size distribution wherein at least
about 50% of the pores are within about 20% of the average pore
size.
47. The method of claim 44, adapted for making a conductive network
having a conductive surface area of at least about 500
m.sup.2/g.
48. An electrode material made by the method of claim 44.
49. A method of making an electrode, the method comprising: a)
mixing an electrode material made by the method of claim 44 with a
binder; and b) drying the mixture formed in step a) on a surface to
form a thin film.
50. An electrode made by the method of claim 49.
51. The electrode of claim 50, configured for use in a capacitor,
an electric double layer capacitor, a battery, or a fuel cell.
52. An energy storage device comprising: first and second
electrodes; an electrolyte disposed between the first and second
electrodes; and a separator disposed between the first and second
electrodes; wherein the first electrode and/or the second electrode
comprise a conductive network formed from a sol-gel derived
monolith by coating a surface of an open pore network of the
monolith or at least partially filling an open pore network of the
monolith with a conductive material.
53. (canceled)
54. The energy storage device of claim 52, wherein the conductive
network in the first electrode and/or the second electrode is
formed by using the silica sol-gel derived monolith as a template
and subsequently removing at least part of the material of the
monolith so that the conductive network is substantially a
stand-alone conductive network.
55-73. (canceled)
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the priority benefit of U.S.
provisional patent application Ser. Nos. 61/060,451, filed Jun. 10,
2008, and 61/060,449, filed Jun. 10, 2008, which are incorporated
in their entirety by reference.
FIELD OF THE INVENTION
[0002] Described herein are high surface area substrates that can
be used to make high surface area electrodes for use in electrical
energy storage devices, e.g., as electrodes in capacitors such as
electric double-layer capacitors, or in fuel cells, or for battery
electrodes.
BACKGROUND
[0003] A capacitor consists of two spaced apart electrodes with a
potential applied between the electrodes. Capacitance (C) is a
measure of charge (Q) stored on those electrodes at a given applied
potential (V): C=Q/V. A dielectric may be inserted in between the
two electrodes. The energy (E) stored by a capacitor is given by
(1/2)CV.sup.2, which can be approximated by E=.di-elect
cons..sub.0.di-elect cons..sub.1A/d, where .di-elect cons..sub.1 is
a permittivity of a medium between the electrodes, A is an
effective cross-sectional area of the electrodes, and d is an
effective spacing between the electrodes. Thus, to increase energy
storage by a capacitor, a cross-sectional area of one or more
electrodes can be increased, and/or a distance between electrodes
can be decreased. Frequency-dependent equivalent series resistance
(ESR) in a capacitor leads to internal heat losses. Thus, if ESR in
a capacitor is reduced, power losses due to internal heat
generation may be correspondingly reduced, leading to improved
usable power (P) in that capacitor for a given applied voltage (V):
P=V.sup.2/(4ESR).
[0004] Ultracapacitors, also known as supercapacitors or electric
double layer capacitors (EDLC) comprise a cell that, in turn,
comprises two electrodes immersed in an electrolyte with a
separator or membrane that is permeable to ions in the electrode
placed between the electrodes in the electrolyte to divide the cell
into two sections. An insulating separator or membrane that is
permeable to electrolyte ions may be placed in a liquid electrolyte
between the electrodes to prevent the cell from shorting. As a
potential is applied between the electrodes, electrolyte ions can
diffuse to the surface of the electrode. No electron transfer takes
place at either electrode surface; instead electrostatic
interactions between the charged electrode surface and the
electrolyte ions in solution build up an electric double layer at
each electrode. Electrical energy is stored in the electric double
layers from charge separation between the electrolyte ions and the
charged electrodes. Each electrode in an EDLC is a capacitor
distinguishing EDLCs from typical capacitors as described above.
The very small distance between these separated charges can lead to
storage of very high charge densities when high surface area
electrodes are used.
[0005] Batteries rely on electrochemical reactions at electrodes.
Here again, energy storage in a battery, e.g., a lithium ion
battery, may in some cases be increased by increasing a surface
area of an active material at an electrode surface.
[0006] Thus, a need exists for improved high surface area
electrodes for energy storage devices, energy storage devices
utilizing such improved high surface area electrodes, and energy
storage devices exhibiting reduced equivalent series
resistance.
SUMMARY
[0007] The invention provides nanoporous electrode materials, which
are formed using sol-gel derived monoliths as electrode material
substrates or electrode material templates. High surface area
nanoporous electrodes formed using sol-gel derived monoliths as
electrode substrates or electrode templates are also provided here.
Any sol-gel derived monoliths may be used, but in some of the
electrodes and related devices, methods and systems, the sol-gel
derived monoliths may have tunable pore sizes, and well-controlled,
narrow distributions of pore size distributions are possible. Thus,
high surface area, nanoporous electrodes with tunable pore sizes
and well-controlled pore size distributions may be formed by using
the monoliths as substrates or templates for a conducting material.
The high surface area nanoporous electrodes may be used as
electrodes in a variety of energy storage devices such as
capacitors, Electric Double Layer Capacitors (EDLC) (also referred
to as ultracapacitors and supercapacitors), batteries, and fuel
cells.
[0008] Some variations of the nanoporous electrode materials
comprise sol-gel derived monolith which comprises an open network
of pores, and a conductive material disposed on a surface of the
open network of pores or partially filling the open network of
pores to form a conductive network. Some variations of the
nanoporous electrode materials comprise a conductive network which
is obtainable or formed by a) coating a surface of an open network
of pores in a sol-gel derived monolith, and b) selectively removing
the material of the monolith to provide a conductive network. Some
variations of the nanoporous electrode materials comprise a
conductive powder which is obtainable or formed by a) coating a
surface of an open network of pores in a sol-gel derived monolith
with a conductive material, b) selectively removing the material of
the monolith, and c) making the resulting material in step b) into
a powder. Some variations of the nanoporous electrode materials
comprise a conductive network which is obtainable or formed by a)
at least partially filling an open network of pores in a sol-gel
derived monolith with a conductive material, and b) selectively
removing at least part of the material of the monolith. Some
variations of the nanoporous electrode materials comprise a
conductive powder which is obtainable or formed by a) at least
partially filling an open network of pores in a sol-gel derived
monolith with a conductive material, b) removing at least part of
the material of the monolith, and c) making the resulting material
in step b) into a powder.
[0009] Some variations of the nanoporous electrodes comprise a
sol-gel derived monolith that, in turn, comprises an open network
of pores, and a conductive material disposed on a surface of the
open network of pores or partially filling the open network of
pores to form a conductive network. Some variations of the
nanoporous electrodes comprise a conductive network which is
obtainable or formed by a) coating a surface of an open network of
pores in a sol-gel derived monolith with a conductive material and
b) selectively removing the material of the monolith. Some
variations of the nanoporous electrodes comprise a conductive
network which is obtainable or formed by a) at least partially
filling an open network of pores in a sol-gel derived monolith with
a conductive material, and b) selectively removing the material of
the monolith. Some variations of the nanoporous electrodes are made
using a conductive powder described herein.
[0010] The monolith used to form the electrode materials and the
electrodes may be derived from any suitable sol-gel, e.g., a silica
sol-gel. The conductive material may be derived from a
non-conductive material, such as by converting one or more
precursors to a conductive polymer. The conductive material may
comprise any suitable material, e.g., graphite, graphite-like
conductive carbon (carbide), graphene, a graphene-like material,
carbon, activated carbon, conductive carbons derived from the
polymerization and carbonization of carbon precursor materials
like, furfural, furfuryl alcohol (2-furylmethanol), polyfurfuryl
alcohol, resorcinol formaldehyde, sucrose, glucose, a metal (such
as platinum, nickel, gold, palladium, molybdenum), a metal oxide
(such as tin oxide, indium tin oxide, zinc oxide, molybdenum oxide,
ruthenium oxide, tungsten oxide, manganese dioxide, silver oxide,
nickel oxyhydroxide, aluminum doped zinc oxide, titanium oxide,
vanadium pentoxide), sulfides (such as molybdenum sulfide, tungsten
sulfide, iron sulfide), nitrides (such as tungsten nitride,
molybdenum nitride), or combinations thereof, or conductive
polymers (such as poly(3-methylthiophene)). As stated above, the
electrodes may be used in an energy storage device such as a
capacitor, an ultracapacitor, a fuel cell, or a battery.
[0011] The monoliths used in forming the electrode materials and
the electrodes may be selected to have a preselected average pore
size and/or a pore size distribution. For example, for electrodes
that are to be used in an energy storage device utilizing an
electrolyte, an average pore size or pore size distribution in the
monolith may be selected to accommodate an ionic species contained
in the electrolyte. In some applications, the sol-gel monolith may
be selected to have an average pore size in a range from about 0.3
nm to about 300 nm, from about 0.3 nm to about 100 nm, from about
0.3 nm to about 30 nm, or from 0.3 nm to about 10 nm. In some
variations of the electrodes, the monolith may comprise a pore size
distribution where at least about 50% of the pores are within about
30% of an average pore size, within about 20% of an average pore
size, or within about 10% of an average pore size.
[0012] The conductive surface area of the electrodes may be
selected based on the intended application of the electrodes. For
example, some electrodes may have a conductive surface area of at
least about 50 m.sup.2/g, at least about 100 m.sup.2/g, at least
about 500 m.sup.2/g, at least about 700 m.sup.2/g at least about
1000 m.sup.2/g, or even higher, e.g., at least about 2000
m.sup.2/g, at least about 2200 m.sup.2/g, at least about 2500
m.sup.2/g, at least about 3000 m.sup.2/g, at least about 4000
m.sup.2/g, or at least about 5000 m.sup.2/g.
[0013] Energy storage devices are also provided herein. The energy
storage devices may be capacitors, ultracapacitors, batteries, or
fuel cells. In general, the energy storage devices comprise first
and second electrodes, with an electrolyte disposed between the
first and second electrodes. A separator is also disposed in the
electrolyte between the first and second electrodes. If the energy
storage device is a battery, capacitor, or ultracapacitor, the
separator may be permeable to one or more ionic species of the
electrolyte, but the separator may function to prevent electrical
conduction between the electrodes. If the energy storage device is
a fuel cell, the separator may comprise a proton exchange membrane.
In the energy storage devices, any of the electrodes described
herein may be used as the first electrode and/or the second
electrode.
[0014] In some variations of the energy storage devices, the first
electrode comprises a first conductive network formed from a
sol-gel derived monolith by coating an open pore network of the
monolith with a conductive material. In some variations, the second
electrode may also comprise a second conductive network formed from
a sol-gel derived monolith by coating an open pore network of the
monolith with a conductive coating. The monoliths used in these
electrodes may be derived from any suitable sol-gel, e.g., a silica
sol-gel.
[0015] In some variations of the energy storage devices, the first
electrode comprises a first conductive network formed from a
sol-gel derived monolith by partially filling an open pore network
of the monolith with a conductive material. In some variations, the
second electrode may also comprise a second conductive network
formed from a sol-gel derived monolith by partially filling an open
pore network of the monolith with a conductive material. The
monoliths used in these electrodes may be derived from any suitable
sol-gel, e.g., a silica sol-gel.
[0016] In some variations of the devices, the sol-gel derived
monolith used to form the conductive network may remain as part of
the electrode to support the first conductive network. In other
variations, the first conductive network and/or the second
conductive network may be formed by using the sol-gel derived
monolith as a template and subsequently removing completely or at
least part of the material of the monolith so that the first
conductive network is a stand-alone conductive network or a
substantially stand-alone conductive network.
[0017] In some variations of the devices, the first electrode
and/or the second electrode comprise a conductive network formed
with a conductive powder which is obtainable or produced by a)
coating a surface of an open network of pores or at least partially
filling an open network of pores in a sol-gel derived monolith with
a conductive material, b) selectively removing the material of the
monolith, and c) making the resulting material in step b) into a
powder.
[0018] As with the electrodes described above, the first conductive
network (and the second conductive network, if present) in the
energy storage devices may comprise graphite, graphite-like
conductive carbon (carbide), graphene, a graphene-like material,
carbon, activated carbon, conductive carbons derived from the
polymerization and carbonization of carbon precursor materials
like, furfural, furfuryl alcohol (2-furylmethanol), polyfurfuryl
alcohol, resorcinol formaldehyde, sucrose or glucose, a metal or
metal oxide such as platinum, nickel, gold, palladium, molybdenum,
a metal oxide such as tin oxide, indium tin oxide, zinc oxide,
molybdenum oxide, ruthenium oxide, tungsten oxide, manganese
dioxide, silver oxide, nickel oxyhydroxide, aluminum doped zinc
oxide, titanium oxide (e.g., titanium dioxide), vanadium pentoxide,
sulfides such as molybdenum sulfide, tungsten sulfide, iron
sulfide, nitrides such as tungsten nitride, molybdenum nitride, or
combinations thereof, or conductive polymers such as
poly(3-methylthiophene).
[0019] In some variations of the energy storage devices, at least
one of an average pore size and a pore size distribution in the
sol-gel derived monolith may be selected based on a dimension of an
ionic species in the electrolyte. An average pore size in a
monolith may be selected in a range from about 0.3 nm to about 300
nm, about 0.3 nm to about 100 nm, about 0.3 nm to about 30 nm, or
about 0.3 nm to about 10 nm. In certain variations, at least about
50% of pores in the monolith may be within about 30%, or within
about 20% of an average pore size. The first electrode in the
devices may for example have a conductive surface area that is at
least about 50 m.sup.2/g, at least about 100 m.sup.2/g, at least
about 150 m.sup.2/g, at least about 200 m.sup.2/g, at least about
500 m.sup.2/g, at least about 700 m.sup.2/g, at least about 1000
m.sup.2/g, or even higher, e.g., at least about 2000 m.sup.2/g, at
least about 3000 m.sup.2/g, at least about 4000 m.sup.2/g, or at
least about 5000 m.sup.2/g.
[0020] Some of the energy storage devices described here may be
ultracapacitors that have a specific energy of about 0.1 W-h/kg to
about 1000 W-h/kg, e.g., at least about 0.1 W-h/kg, at least about
1 W-h/kg, at least about 10 W-h/kg, at least about 30 W-h/kg, at
least about 50 W-h/kg, at least about 100 W-h/kg, at least about
120 W-h/kg, at least about 150 W-h/kg, at least about 170 W-h/kg,
at least about 190 W-h/kg, at least about 200 W-h/kg, at least
about 300 W-h/kg, at least about 400 W-h/kg, at least about 500
W-h/kg, or as high as about 1000 W-h/kg. Ultracapacitors described
here may have a specific power of about 10 W/kg or higher, e.g.,
about 10 W/kg, about 50 W/kg, about 100 W/kg, about 500 W/kg, about
1 kW/kg, about 5 kW/kg, about 10 kW/kg, about 50 kW/kg, about 100
kW/kg, or even higher.
[0021] Asymmetric ultracapacitors are disclosed herein. In general,
the asymmetric ultracapacitors comprise a first electrode
configured to store charge electrostatically, a second electrode
configured to store charge via a reversible faradaic process, an
electrolyte disposed between the first and second electrodes, and a
separator that is permeable to the electrolyte also disposed
between the first and second electrodes. In an asymmetric
capacitor, the first electrode may be derived from a sol-gel
monolith comprising an open pore network coated or at least
partially filled with a conductive material suitable for
electrostatic charge storage and discharge, and/or the second
electrode may be derived from a sol-gel monolith comprising an open
pore network coated or at least partially filled with a conductive
material suitable for faradaic charging and discharging. Any other
electrodes described herein may also be used for the first
electrode and/or the second electrode.
[0022] The energy storage devices described herein may be used in a
variety of applications, e.g., to provide back up power in
electronic devices such as computers or to function as rechargeable
power sources in handheld devices. Ultracapacitors as disclosed
here may be used in hybrid electric engines such as those used in
hybrid electric vehicles. For example, ultracapacitors may be used
for load-leveling to extend the life of a battery, to supply power
to augment peak power delivery of the battery during startup or
acceleration, or to power auxiliary functions such as power
steering, power windows, or lighting.
[0023] Methods for making nanoporous electrode materials and
electrodes are described here. In general, these methods comprise
providing a sol-gel derived monolith that, in turn, comprises an
open network of pores, and coating a surface of the open network of
pores or at least partially filling the network of pores with a
conductive material to form a conductive network. The methods may
further comprise a step of removing completely or partially the
material of the monolith to create a stand-alone conductive network
or substantially stand-alone conductive network. Methods of making
electrodes using conductive nanoporous powder are also described.
In some variations, the methods comprise a) coating a surface of
the open network of pores in a sol-gel derived monolith or at least
partially filling the network of pores in a sol-gel derived
monolith with a conductive material to form a conductive network;
b) removing completely or partially the material of the monolith to
create a stand-alone conductive network or substantially
stand-alone conductive network; c) making the resulting material in
step b) into a conductive powder; and d) using the powder to make
an electrode. For example, the electrode may be made by mixing the
conductive powder with a binder, and drying the mixture on a
surface to form a film. The methods may be used to make an
electrode for a capacitor, an ultracapacitor, a battery, or a fuel
cell. The monoliths used in the methods may be derived from any
suitable sol-gel, but in some variations, the monoliths are derived
from silica sol-gels.
[0024] Some of these methods for making electrodes may comprise
selecting the sol-gel derived monolith to have a predetermined
average pore size and/or pore size distribution, e.g., an average
pore size and/or pore size distribution selected to accommodate an
ionic species of the electrolyte. For example, the methods may
comprise selecting monoliths having an average pore size in a range
from about 0.3 nm to about 300 nm, about 0.3 nm to about 100 nm,
about 0.3 nm to about 30 nm, or about 0.3 nm to about 10 nm.
Further, monoliths may be selected to have a pore size distribution
such that at least about 50% of pores are within about 30%, or
within about 20% of an average pore size.
[0025] The methods for making an electrode may comprise slicing a
sol-gel wafer from a sol-gel derived monolith, and coating a
surface of the open network of pores or at least partially filling
the open network of pores in the wafer to make a conductive
network. These methods may comprise slicing a sol-gel wafer having
a thickness of about 1 mm or less, e.g., about 800 microns, about
500 microns, about 250 microns, about 200 microns, about 150
microns, or about 100 microns. The wafers may also be made by
methods comprising casting the gel formulation into a mold or
container comprising multiple slots, wherein each slot comprising
opposing parallel sidewalls and hydrophobic inner surfaces, and
shrinking the gel in the slots to form a sol-gel derived
monolith.
[0026] Some methods for making electrodes may comprise forming a
sol-gel derived monolith having a preselected average pore size
and/or a pore size distribution by reacting a sol-gel precursor
with water in the presence of a catalyst, and controlling a rate of
gelation with the catalyst. The catalyst may comprise hydrofluoric
acid and, in some cases, the catalyst may comprise a second acid in
addition to the hydrofluoric acid. A molar ratio of hydrofluoric
acid to the precursor may be increased to increase an average pore
size. If present, the second acid may be any suitable acid, but in
some variations may be selected from the group consisting of HCl,
HNO.sub.3, H.sub.2SO.sub.4, organic acids, and combinations
thereof. In some cases, the second acid may be a weak acid having a
first pK.sub.a that is about 2 or greater, e.g., about 2 to about
5, or about 2 to about 4.
[0027] In the methods, coating the surface of the open pore network
may comprise synthesizing a layer of graphite or a layer of a
graphite-like material (e.g., conductive carbon) on the surface,
e.g., by polymerizing a polymer material made from the precursors
resorcinol and formaldehyde, furfural, furfuryl alcohol
(2-furylmethanol), polyfurfuryl alcohol, sucrose or glucose on the
surface of the open pore network. In certain variations, coating
the surface may comprise depositing a conductive species, e.g., a
metal and/or a metal oxide, on the surface using chemical vapor
deposition and/or atomic layer deposition. In some circumstances,
the open pore network may be impregnated with a colloidal solution
of a metal, the monolith may be dried to remove liquid from the
colloidal solution, and the metal particles may be coalesced
together, e.g., by melting or using rapid thermal processing, to
form the conductive network in the open pore network.
[0028] The methods may comprise coating the surface of the open
pore network to provide a conductive surface area of at least about
50 m.sup.2/g, at least about 100 m.sup.2/g, at least about 150
m.sup.2/g, at least about 200 m.sup.2/g, at least about 250
m.sup.2/g, at least about 300 m.sup.2/g, at least about 400
m.sup.2/g, at least about 500 m.sup.2/g, at least about 800
m.sup.2/g, at least about 1000 m.sup.2/g, or even higher, e.g., at
least about 1200 m.sup.2/g, at least about 1500 m.sup.2/g, at least
about 1800 m.sup.2/g, at least about 2000 m.sup.2/g, or even
higher, e.g., at least about 3000 m.sup.2/g, at least about 4000
m.sup.2/g, or at least about 5000 m.sup.2/g.
[0029] Additional methods of making electrodes are provided here.
In general, these methods comprise providing a sol-gel derived
monolith comprising an open pore network, coating or at least
partially filling the open pore network with a conductive material,
and selectively removing the sol-gel derived monolith to provide a
conductive electrode. In some variations, at least partially
filling comprises impregnating a material into the open pore
network, and subsequently converting the material into a conductive
material. These methods may be used to make an electrode for a
capacitor, an ultracapacitor, a battery, or a fuel cell.
[0030] The sol-gel derived monoliths used in these methods may be
selected to have an average pore size in a range from about 0.3 nm
to about 300 nm, about 0.3 nm to about 100 nm, about 0.3 nm to
about 30 nm, or about 0.3 nm to about 10 nm. Further, for any
average pore size, the sol-gel derived monoliths may be selected to
have a pore size distribution such that at least about 50% of pores
in the monolith have a pore size that is within about 30% of the
average pore size, within about 20% of the average pore size, or
within about 10% of the average pore size.
[0031] In these methods, at least partially filling the open pore
network may comprise impregnating the open pore network with a
colloidal solution comprising conductive particles, e.g., metal
and/or metal oxide particles. In other variations, at least
partially filling the open pore network may comprise impregnating
the open pore network with one or more precursors to a conductive
polymer, and polymerizing the one or more precursors in situ to
form a conductive electrode comprising the conductive polymer.
[0032] In the methods, impregnating the open pore network may
comprise synthesizing graphite or a graphite-like material (e.g.,
conductive carbon) within the pore structure, e.g., by polymerizing
a polymer material made from the precursors resorcinol and
formaldehyde, furfural, furfuryl alcohol (2-furylmethanol),
polyfurfuryl alcohol, sucrose or glucose within the open pore
network. In some circumstances, the open pore network may be
impregnated partially or fully with a colloidal solution of a
metal, the monolith may be dried to remove liquid from the
colloidal solution, and the metal particles may be coalesced
together, e.g., by melting or using rapid thermal processing, to
form the conductive network in the open pore network.
[0033] Certain variations of these methods may be used to form
electrodes having a conductive surface area of at least about 50
m.sup.2/g, at least about 100 m.sup.2/g, at least about 150
m.sup.2/g, at least about 200 m.sup.2/g, at least about 250
m.sup.2/g, at least about 300 m.sup.2/g, at least about 350
m.sup.2/g, at least about 400 m.sup.2/g, at least about 450
m.sup.2/g, at least about 500 m.sup.2/g, at least about 700
m.sup.2/g, at least about 800 m.sup.2/g, at least about 1000
m.sup.2/g, at least about 1200 m.sup.2/g, at least about 1500
m.sup.2/g, at least about 1800 m.sup.2/g, at least about 2000
m.sup.2/g, or even higher, e.g., at least about 3000 m.sup.2/g, at
least about 4000 m.sup.2/g, or at least about 5000 m.sup.2/g.
[0034] The invention also provides electrodes made or obtainable by
any of the methods described herein, and electrodes equivalent to
any the electrodes made by any of the methods described herein. In
some variations, the electrodes comprise a continuous skeletal
framework formed of a conductive material. In some variations, the
electrodes comprise a continuous skeletal framework formed of a
conductive material that is formed using a sol-gel derived monolith
as a template. Some variations of the nanoporous electrodes
comprise a monolithic conductive material which comprises an open
network of pores, wherein the monolithic conductive material is
substantially an inverse of a sol-gel derived monolith described
herein.
[0035] Methods for storing energy are provided here. These methods
comprise applying a potential between first and second conductive
electrodes to build up stored charge therebetween, wherein at least
one of first and second conductive electrodes is an electrode
described herein, for example, has been derived from a sol-gel
derived monolith comprising an open network of pores coated or at
least partially filled with a conductive material.
[0036] Energy storage systems are also provided here. These systems
comprise multiple interconnected energy storage cells, wherein each
energy storage cell comprises two electrodes configured to be
oppositely charged and an electrolyte disposed between the two
electrodes. At least one of the two electrodes is an electrode
described herein, such as in at least one of the cells comprises an
electrode derived from a sol-gel monolith comprising an open
network of pores coated or at least partially filled with a
conductive material. In these energy storage systems, the monolith
used may be derived from any suitable sol gel, but in some cases,
it is derived from a silica sol gel. In the systems, at least some
of the multiple cells may be connected in series, or at least some
of the multiple cells may be connected in parallel.
BRIEF DESCRIPTION OF THE FIGURES
[0037] FIG. 1 illustrates an example of a method for making a
sol-gel derived monolith (such as a silica sol-gel derived
monolith) that can be used in the electrodes described herein.
[0038] FIG. 2 illustrates a relationship between a pore size and a
surface area of an open pore network in a sol-gel derived monolith
(such as a silica sol-gel derived monolith).
[0039] FIG. 3 illustrates an example of a silica sol-gel derived
monolith sliced into wafers that can be used in the electrodes
described herein.
[0040] FIGS. 4A-4B illustrate examples of conductive coatings as
applied to open pore networks of sol-gel derived monoliths to form
conductive networks threading through the open pore networks.
[0041] FIGS. 5A-5C illustrate schematically an example of a process
that may be used to synthesize a graphite-like layer in situ to
form a conductive coating on a surface of an open pore network.
FIGS. 5A-5C provides a schematic diagram of a route to a "graphite
like" structure resulting from the polymerization of R and F and
subsequent thermal processing. FIG. 5A shows the pores immersed in
an aqueous solution of monomer and catalyst. FIG. 5B shows a
polymer fragment resulting from the polymerization. FIG. 5C shows a
conception of a "graphite like" material that results from thermal
processing in N.sub.2 and CO.sub.2 as the polymer density
approaches that of graphite.
[0042] FIG. 6A illustrates a variation of an ultracapacitor; FIG.
6B illustrates a variation of an asymmetric ultracapacitor.
[0043] FIG. 7 graphically illustrates combinations of specific
power and specific energy that may be achieved with ultracapacitors
described herein.
[0044] FIG. 8 illustrates an example of an electrode assembly that
may be used in an ultracapacitor.
[0045] FIGS. 9A-9B illustrate examples of energy storage systems
comprising series-connected cells.
[0046] FIG. 10 illustrates an example of an energy storage system
comprising cells connected in parallel.
DETAILED DESCRIPTION
[0047] High surface area electrodes formed using nanoporous sol-gel
derived monoliths as electrode substrates or electrode templates
are provided here. Any sol-gel derived monoliths may be used to
form the electrodes, but in some of the electrodes and related
devices, methods and systems, nanoporous sol-gel derived monoliths
having tunable pore sizes, and well-controlled, narrow
distributions of pore size distributions may be used. Thus,
nanoporous, high surface area electrodes with tunable pore sizes
and well-controlled pore size distributions may be formed by using
such sol-gel derived monoliths as substrates or templates for a
conducting network formed within the continuous open pore network.
The high surface area electrodes may be used as electrodes in a
variety of energy storage devices such as capacitors,
ultracapacitors, batteries, and fuel cells.
[0048] As used herein, the terms "nanoporous materials" and
"nanoporous electrodes" are meant to encompass structures having
pores ("nanopores") having a dimension, e.g., a cross-sectional
diameter, in a range from about 0.1 nm to about 100 nm.
"Nanoparticles" as used herein is meant to encompass materials
having a cross-sectional dimension, e.g., a diameter, in a range
from about 0.1 nm to about 100 nm. Ranges as used herein are meant
to be inclusive of any end points to the ranges indicated, as well
as numerical values in between the end points.
[0049] It should also be noted that as used herein and in the
claims, the singular forms "a," "an," and "the" include plural
referents unless the context clearly indicates otherwise.
[0050] It is understood that aspect and variations of the invention
described herein include "consisting" and/or "consisting
essentially of" aspects and variations.
[0051] In some variations, reference to "about" a value or
parameter refers to variation in the numerical quantity that can
occur, for example, through typical measuring and handling
procedures used; through inadvertent error in these procedures; and
through differences in the manufacture, source, or purity of the
compounds employed to make the compositions or carry out the
methods. Reference to "about" a value or parameter herein includes
(and describes) embodiments that are directed to that value or
parameter per se. For example, description referring to "about X"
includes description of "X".
[0052] As used herein "average pore size" is meant to encompass any
suitable representative measure of a dimension of a population of
pores, e.g., a mean, median, and/or mode cross-sectional dimension
such as a radius or diameter of that population of pores. The mean
pore size, median pore size, and mode pore size of a pore size
distribution in a monolith may in some cases be essentially
equivalent, e.g., by virtue of a very narrow and/or symmetrical
pore size distribution. For convenience only, the following
description includes three headings: electrodes, energy storage
devices, and systems. However, it should be understood that these
organizational headings are not meant to be limiting in any way.
For example, any of the electrodes described herein may be used in
connection with any of the energy storage devices or systems
described herein, and any of the energy storage devices described
herein may be used in connection with any of the systems described
herein.
I. Electrodes
[0053] High surface area electrodes are described herein. In
general, the electrodes comprise a sol-gel derived monolith
comprising an open network of pores, where the open pore network
has been made to be conductive. The high surface area of the open
network of pores can thus be used as a substrate or a template for
the conductive coating or filling that makes up the conductive
backbone of the electrode. In other variations, the high surface
area of the open pore network can be used as a substrate or
template for a conductive coating or filling to allow for faradaic
charge storage. In other variations, the open network of pores in
the monolith may be at least partially filled (including
substantially or completely filled) with a conductive material. The
electrodes may comprise a current-spreading conductive plate in
electrical contact with the conductive network formed in the open
pore network. The sol-gel derived monolith material may either be
left more or less intact in the electrodes, e.g., to support the
conductive network, or the monolith material may be removed, e.g.,
dissolved, to leave behind the conductive framework that has been
formed in the open pore network.
[0054] The monolith used to form the electrodes may be derived from
any suitable sol-gel, e.g., a silica sol-gel derived monolith. The
monolith may be selected to have any desired characteristic or
combination of characteristics, e.g., composition, average pore
size, pore size distribution, surface area, or any combination
thereof. By preselecting an average pore size, pore size
distribution, or surface area, these properties may be at least
partially mapped onto the conductive coating disposed on the open
pore network or conductive framework formed in the open pore
network, thus affecting a resulting conductive surface area of the
electrode. In some variations in which the electrodes are to be
used in combination with an electrolyte, e.g., in a capacitor,
ultracapacitor, battery or fuel cell, an average pore size may be
selected to accommodate an ionic species in the electrolyte.
[0055] Generally, a sol-gel process starts with forming a colloidal
solution (a "sol" phase), and hydrolyzing and polymerizing the sol
phase to form a solid but wet and porous "gel" phase. The gel phase
can be dried in a controlled manner, but generally not under
supercritical conditions, so that fluid is removed to leave behind
a dry monolithic matrix having an open network of pores (a
xerogel). The term "xerogel" as used herein is meant to refer to a
gel monolith that has been dried under nonsupercritical temperature
and pressure conditions. The dry gel monolith can then be calcined
to form a solid glass-phase monolith with connected open pores. The
dry gel monolith can be further densified, e.g., sintered, at
elevated temperatures to convert the monolith into a glass or
ceramic.
[0056] In general, the microstructure of sol-gel derived monoliths
that may be used in the electrodes described herein may be
characterized in terms of a total pore volume, referring to a total
volume of pores per unit mass, a surface area, referring to a
surface area within the open network of pores per unit mass, a
porosity, referring to fraction of the total volume of a monolith
occupied by open pores, an average pore size, referring to an
average (e.g., mean, median or mode) cross-sectional dimension
(e.g., diameter or radius) of pores in a monolith, and a pore size
distribution. The bulk surface area of a monolith may be measured
in m.sup.2/g, and may be measured for example by using B.E.T.
(Brunauer, Emmett and Teller) surface analysis techniques. In
general, multiple point B.E.T. analysis may be performed to
determine the bulk surface area. An average pore size, a pore size
distribution, and a total pore volume may be measured by an
analyzer capable of resolving pore sizes to 0.3 nm or smaller,
e.g., Quantachrome Quadrasorb.TM. SI-Krypton/Micropore Surface Area
and Pore Size Analyzer, available from Quantachrome Instruments,
Quantachrome Corporation (http://www.quantachrome.com, last visited
May 11, 2008). The total pore volume may be measured in cm.sup.3/g,
and is the inverse of the bulk density of a monolith.
[0057] A population of pores can be modeled as a set of spheres
each having a diameter (d) equal to an average pore size for that
population, which may be measured with a pore size analyzer as
described above, an individual pore surface area (A=.pi.d.sup.2),
and an individual pore volume (V=(1/6).pi.d.sup.3). A calculated
bulk surface area (SA) may be determined using the density .rho. of
a material making up the sol-gel matrix (e.g., for silica sol-gel,
the density of silica forming the matrix is 2.1-2.2 g/cm.sup.3) and
the following relationship in Equation 1:
SA=(1/.rho.)[A/V]. (Eq. 1).
[0058] A calculated bulk density (.rho..sub.B) of the monolith may
be determined from the total pore volume (TPV) and the density
.rho. of a material making up the sol-gel matrix using the
following relationship in Equation 2:
.rho..sub.B=1/[(1/.rho.)+TPV] (Eq. 2).
Thus, the fraction of pores (porosity), or % pores (by volume) in a
monolith may be given by TPV/[(1/.rho.)+TPV].
[0059] In general, the monoliths described herein can be formed by
hydrolyzing a precursor. The microstructure of the sol-gel derived
monoliths described here may be affected, and therefore controlled
by, rates of hydrolysis and polymerization. The precursor can be
any suitable precursor, e.g., a metal- or metalloid-containing
compound having ligands or side groups that can be hydrolyzed to
form a sol, and then polymerized (gelled) to form a sol-gel. As is
discussed in more detail herein, the hydrolysis and polymerization
process can be catalyzed using a catalyst in solution.
[0060] FIG. 1 provides a flow diagram of an example of a method for
forming a sol-gel derived monolith (such as a silica sol-gel
derived monolith). There, method 100 comprises preparing a
precursor solution as shown in step 101, and preparing a catalyst
solution as shown in step 102. The precursor solution and the
catalyst solution may be mixed together to form a reaction (step
103). The solution used in the reaction mixture in step 103 may be
aqueous, or may comprise one or water-miscible organic solvents in
combination with water. For example, an alcohol such as methanol,
ethanol, or any alcohol having the general formula
C.sub.nH.sub.2+1OH, where n may be for example 0 to 12.
Alternatively or in addition, formamide may be used in reaction
mixture. The hydrolysis and polymerization reaction process may be
allowed to proceed (step 104). After the wet gel is formed, the gel
may be dried to form a monolith (step 105). It should be noted that
the steps illustrated in method 100 need not be performed in any
particular order, and steps may be combined together. For example,
steps 101 or 102 may be reversed, or steps 101 and 102 may be
combined into a single step, or steps 101, 102, and 103 may be
completed simultaneously. Each of the steps in the methods is
described in more detail below.
[0061] In some variations, the methods of making a silica sol-gel
derived monolith comprising hydrolyzing a SiO.sub.2 precursor with
water in the presence of a catalyst to form a sol; gelling the sol;
and drying the gelled sol. In some variations, the catalyst is
preselected to obtain a porous SiO.sub.2-containing monolith having
a pore volume of between about 0.3 cm.sup.3/g to about 2.0
cm.sup.3/g, and a predetermined average pore diameter in a range
from about 0.3 nm to about 30 nm with at least about 60% of pores
having a pore size within about 20% of the average pore size.
[0062] Non-limiting examples of hydrolyzable side group that can be
used in precursors include hydroxyl, alkoxy, halo, and amino side
groups. In many cases, silica (SiO.sub.2) sol-gels may be formed,
e.g., using alkylorthosilicate, fluoralkoxysilane, or
chloroalkoxysilane precursors. However, in other cases, sol-gels
based on germanium oxide, zirconia, titania, niobium oxide,
tantalum oxide, tungsten oxide, tin oxide, hafnium oxide, alumina,
or combinations thereof may be formed using appropriate precursors.
For example, germanium alkoxides, e.g., tetratheylorthogermanium
(TEOG), zirconium alkoxides, titanium alkoxides, vanadium
alkoxides, or aluminum alkoxides may be used as precursors to form
sol-gels incorporating the respective metal or metalloid
elements.
[0063] As stated above, silica sol-gels may be formed using
alkylorthosilicates as precursors, e.g., tetraethylorthosilicate
(TEOS) or tetramethylorthosilicate (TMOS). In general, the
stoichiometric hydrolysis reaction to form the sol can be described
as:
(RO).sub.4Si+4H.sub.2O.fwdarw.4ROH+Si(OH).sub.4,
where R may for example be an ethyl group or a methyl group.
Following this hydrolysis step, gelation can occur, in which the
Si(OH).sub.4 condenses and polymerizes to form a network of
SiO.sub.2 and H.sub.2O. The SiO.sub.2 network so formed comprising
open-necked pores, and H.sub.2O may be present in the open pores.
The reaction as described above is aqueous, and may comprise one or
water-miscible organic solvent in combination with water. For
example, an alcohol such as methanol, ethanol, or any alcohol
having the general formula C.sub.nH.sub.2n+1OH, where n may be for
example 0 to 12. Alternatively or in addition, formamide may be
used in the hydrolysis of the precursors. Two competing mechanisms
may be operative that affect the microstructure of the monolith:
formation of isolated silica particles, and formation of silica
chains that form a fibril-like network.
[0064] The SiO.sub.2 precursor may be hydrolyzed under either
nonstoichiometric or stoichiometric hydrolysis conditions. In some
variations, the molar ratio of water to precursor is about 3:1 or
less, about 2.5:1 or less, about 2.25:1 or less, or about 2:1. In
some variations, hydrolysis is performed directly with water and no
solvent (such as an alcohol, including methanol and ethanol) is
added into the reaction.
[0065] The microstructure of a sol-gel derived monolith that can be
used to form an electrode for an energy storage device may be
controlled by varying any one or any combination of several
reaction parameters. For example, U.S. Pat. No. 4,851,150, U.S.
Pat. No. 4,849,378, U.S. Pat. No. 5,264,197, U.S. Pat. No.
6,884,822, U.S. Pat. No. 7,001,568, U.S. Pat. No. 7,125,912, PCT WO
2006/068797, U.S. provisional application Ser. No. 61/060,449
(filed Jun. 10, 2008), and U.S. patent application Ser. No. ______
entitled "Nanoporous Materials and Related Methods" (Attorney
Docket No. 64334-20001.00, filed on Jun. 10, 2009), each of which
is hereby incorporated by reference herein in its entirety,
describe a variety of methods for making sol-gel derived monoliths
wherein in one or more reaction parameters is varied to control an
average pore size and/or a pore size distribution.
[0066] Catalysts can be used to adjust, e.g., increase, rates of
hydrolysis and polymerization, and correspondingly adjust the rate
of gel formation, which can affect the microstructure in the
resulting monolith. Further, a reaction temperature or temperature
profile may be used to adjust a rate of gel formation. A catalyst
may be an acid or a base. In some variations, the catalyst
comprises an organic acid (such as formic acid, acetic acid, citric
acid, or mixtures thereof). In some variations, a catalyst may
comprise a first acid and a second acid, where the second acid
catalyzes the hydrolysis reaction, and the first acid is capable of
etching, dissolving, and/or redepositing in the sol matrix (e.g., a
SiO.sub.2 matrix), which may have the effect of increasing size of
redeposited nanoparticles in the sol-gel matrix formation, leading
to correspondingly increased nanopores size. Thus, the second acid
of the catalyst may be added first to the sol-gel precursors to
activate hydrolysis, and the first acid may be added subsequently
to tune the pore size in the sol-gel. In other variations, the
first and second acids of the catalyst may be added simultaneously.
Of course, the first acid and/or the second acid of the catalyst
may comprise a mixture of acids. In certain instances, the first
acid, e.g., a matrix (e.g., SiO.sub.2) dissolving component, of the
catalyst may comprise hydrofluoric acid (HF), or a source of HF. HF
sources that may be used include suitable fluorine-containing
compounds that can produce HF during hydrolysis, or during
polymerization (gelation).
[0067] Fine tuning of an average pore size and/or a pore size
distribution in the resulting monolith may be accomplished by
varying any one or any combination of the following reaction
conditions: an amount of HF relative to a precursor; an amount of
H.sub.2O relative to a precursor; an amount of a solvent relative
to a precursor; varying an amount of a second acid relative to a
precursor; an amount of a second acid relative to an amount of HF;
and/or a reaction temperature. The relative amounts of the
precursor, H.sub.2O and solvent, if present, may be stoichiometric
or nonstoichiometric.
[0068] Further, the properties of the second acid, if used, may be
selected to control at least one of an average pore size and a pore
size distribution, and in some cases an average pore size and a
pore size distribution associated with that average pore size. For
example, a strong acid, e.g., an acid having a first pK.sub.a that
is lower about -1 or lower, e.g., HCl, H.sub.2SO.sub.4, HNO.sub.3,
or a combination thereof, may be used as a second acid in addition
to HF to catalyze the hydrolysis and/or the gelation processes. In
some variations, a weak acid, e.g., an acid having a first pK.sub.a
that is about 2 or greater, e.g., a first pK.sub.a of about 2 to
about 5, or about 2 to about 4. For example, citric acid, acetic
acid, formic acid, or combinations thereof, may be used as a second
acid in addition to HF as a catalyst. In some variations, the
second acid is an organic acid (e.g., citric acid, acetic acid,
formic acid) which can be removed or burned from the gelled sol
during the drying process. In certain variations, an intermediate
acid, e.g., an acid having a first pK.sub.a that is between -1 and
2, e.g., oxalic acid, mellitic acid, or ketomalonic acid, may be
used in combination with HF.
[0069] In general, narrow pore size distributions with a tunable
average pore size may be produced by hydrolyzing and polymerizing
the precursor in the presence of a relatively low amount of HF
compared to the precursor. For example, if a non-stoichiometric
amount of water relative to precursor is used, e.g., by using 2
moles of water relative to one mole of a precursor such as TEOS or
TMOS, the molar ratio of HF to the precursor used in the methods
described herein may be about 0.01:1 or less, e.g., about 0.01:1,
about 0.009:1, about 0.008:1, about 0.007:1, about 0.006:1, about
0.005:1, about 0.004:1, about 0.003:1, about 0.002:1, or about
0.001:1. In another example, if a non-stoichiometric amount of
water relative to a precursor is used, e.g., 2.25 moles of water
relative to one mole of a precursor such as TEOS or TMOS, the molar
ratio of HF to the precursor used in the methods may be about
0.1:1, about 0.09:1, about 0.085:1, about 0.08:1, about 0.075:1,
about 0.07:1, about 0.065:1, about 0.06:1, about 0.055:1, about
0.05:1, about 0.045:1, or about 0.4:1. For these non-stoichiometric
situations, a molar ratio of the second acid to the starting
material (e.g., the precursor) may be about 0.075:1, about 0.07:1,
about 0.065:1, about 0.06:1, about 0.055:1, about 0.05:1, about
0.04:1, about 0.03:1, about 0.02:1, about 0.018:1, about 0.015:1,
about 0.01:1, about 0.008:1, about 0.005:1, about 0.003:1, or about
0.001:1. The second acid in these instances may be any suitable
acid, e.g., a strong acid (such as HCl, H.sub.2SO.sub.4, HNO.sub.3,
or a combination thereof), a weak acid (such as citric acid, acetic
acid, formic acid, or a combination thereof), or an intermediate
acid.
[0070] If a stoichiometric amount of water relative to a precursor
is used, a molar ratio of HF to precursor that is about 0.01:1 or
less may be used, e.g., about 0.01:1, about 0.009:1, about 0.008:1,
about 0.007:1, about 0.006:1, about 0.005:1, about 0.004:1, about
0.003:1, about 0.002:1, about 0.001:1, about 0.0005:1, or even
less, and in some cases no HF may be used. In general, an amount of
HF used in a catalyst may be increased to increase an average pore
size. To achieve fine control of pore size and/or pore size
distribution, the amount of HF may be adjusted using fine
increments, e.g., by changing the molar ratio of HF relative to the
precursor in increments of about 0.005 or about 0.001. A molar
ratio of a second acid may be about 0.01:1 or less, e.g., about
0.01:1, about 0.009:1, about 0.008:1, about 0.007:1, about 0.006:1,
about 0.005:1, or even less, and in some cases, no second acid may
be used. Here again, the second acid may be any suitable acid,
e.g., a strong acid such as HCl, H.sub.2SO.sub.4, HNO.sub.3, or a
combination thereof, a weak acid, or an intermediate acid.
[0071] In certain variations, under either nonstoichiometric or
stoichiometric hydrolysis conditions, the second acid may be a weak
acid that has a first pK.sub.a of about 2 or higher, or about 3 or
higher. Some of these weak acids may be organic acids, or small
molecule acids. In some cases, the first pK.sub.a of the weak acid
may be about 2 to about 5, e.g., about 2, about 2.5, about 3, about
3.5, about 4, about 4.5, or about 5. In certain variations, the
first pK.sub.a of the weak acid may be about 2 to about 4.
Non-limiting examples of weak acids that may be used include citric
acid, acetic acid, formic acid, ascorbic acid, succinic acid,
benzoic acid, acetoacetic acid, malic acid, pyruvic acid, vinyl
acetic acid, tartartic acid, fumaric acid, phthalic acid,
isophthalic acid, terephthalic acid, itaconic acid, hemimellitic
acid, trimellitic acid, malonic acid, dicarboxylic acids such as
methyl dicarboxylic acid, ethyl dicarboxylic acid, n-propyl
dicarboxylic acid, isopropyl dicarboxylic acid, dimethyl
dicarboxylic acid, methylethyl dicarboxylic acid, ethyl-n-propyl
dicarboxylic acid, di-n-propyl dicarboxylic acid, glutaric acid,
adipic acid, pimelic acid, suberic acid, amino acids such as
alanine, aspartic acid and glutamic acid.
[0072] For any of the methods described here, a temperature or
temperature profile used in the hydrolysis and polymerization
process used in making the wet porous gel monoliths may be varied
to tune a reaction rate, which can in turn affect monolith
microstructure. Thus, different temperatures or temperature
profiles may be used, and may depend on a catalyst selected. In
some situations, a temperature or temperature ramp that includes
temperatures below ambient may be used for gelation, e.g., as
described in U.S. Pat. No. 6,884,822, which is incorporated herein
by reference in its entirety. In other instances, elevated reaction
temperatures may be used, which may be at least in part due to
exothermic hydrolysis reaction. Reaction temperatures may range
from about 0.degree. C. to about 80.degree. C., or from about
15.degree. C. to about 125.degree. C., or from about 45.degree. C.
to about 100.degree. C. In some cases, a reaction temperature may
be naturally ramped up during the hydrolysis process due to the
exothermic reaction, e.g., from about 0.degree. C. to about
100.degree. C. over a period of about 1 to 2 hours. For example, an
exothermic hydrolysis reaction solution may be mixed while the
reaction temperature ramped from about 0.degree. C. to about
70.degree. C. over a period of about 1 to 2 hours. The mixture may
then be cast into an appropriate mold and held at an appropriate
temperature, e.g., from about 0.degree. C. to about 70.degree. C.
(such as about 33.degree. C.) for an additional 1 to 30 hours to
allow further gelation. In some cases, the mixture may be held in a
mold at about 20.degree. C. for 1 to 2 hours to allow gelation,
held at about 20.degree. C. for an additional 12 to 24 hours to
allow the gelled sol to begin shrinkage (e.g., about 0.5% to about
5% volume shrinkage), and then removed from the mold, or remained
in the mold for further drying process.
[0073] As indicated above, the wet, porous monoliths as prepared by
any of the methods provided above may be formed in a mold so that
it may be dried in a desired shape and configuration. Any suitable
molding method or technique, and any suitable drying method or
technique as described herein, now known, or later developed, may
be used to form and dry the wet gels formed herein. A mold for
example may be formed of polyethylene, polystyrene,
polytetrafluoroethylene (Teflon.TM.), polymethylpentene (PMP),
glass, or any combination thereof. Further, a mold surface may be
treated or conditioned so as to impart a desired surface quality to
the molded monolith, e.g., hydrophobically treated. For example, a
mold surface may be chemically cleaned, physically cleaned, and/or
have static charges removed.
[0074] Suitable examples of molding and drying techniques and
methods are described in U.S. Pat. No. 6,884,822 entitled "Sol-Gel
Process Utilizing Reduced Mixing Temperature," U.S. Pat. No.
6,620,368 entitled, U.S. Pat. No. 5,264,197 entitled "Sol-Gel
Process for Providing a Tailored Gel Microstructure," U.S. Pat. No.
4,849,378 entitled "Ultraporous Gel Monoliths Having Predetermined
Pore Sizes and Their Production," U.S. Pat. No. 4,851,150 entitled
"Drying Control Additives for Rapid Production of Large Sol-Gel
Derived Silicon, Boron and Sodium Containing Monoliths," U.S. Pat.
No. 4,851,373 entitled "Large Sol-Gel SiO.sub.2 Monoliths
Containing Transition Metal and Their Production," U.S. Pat. No.
5,071,674 entitled "Method for Producing Large Silica Sol-Gel Doped
with Inorganic and Organic Compounds," U.S. Pat. No. 5,196,382
entitled "Method for Production of Large Sol-Gel SiO.sub.2
Containing Monoliths of Silica with and without Transition Metals,"
U.S. Pat. No. 5,023,208 entitled "Sol-Gel Process for Glass and
Ceramic Articles," U.S. Pat. No. 5,243,769 entitled "Process for
Rapidly Drying a Wet, Porous Gel Monolith," U.S. Pat. No.
7,000,885, entitled "Apparatus and Method for Forming a Sol-Gel
Monolith Utilizing Multiple Casting," U.S. Pat. No. 7,001,568,
entitled "Method of Removing Liquid from Pores of a Sol-Gel
Monolith," U.S. Pat. No. 7,026,362, entitled "Sol-Gel Process
Utilizing Reduced Mixing Temperatures," U.S. Pat. No. 7,125,912,
entitled "Doped Sol-Gel Materials and Method of Manufacture
Utilizing Reduced Mixing Temperatures", each of which is
incorporated herein by reference in its entirety.
[0075] In general, a wet, porous monolith that has been placed in a
mold may be held in a storage area under generally ambient
conditions for about one to three days. After this initial period,
the monolith may be removed from the mold or remained in the mold.
Subsequently, a monolith may be dried under controlled, but not
necessarily supercritical, drying conditions. The drying conditions
can remove liquid, e.g., water and/or a solvent such as an alcohol
from the interior of the porous network under controlled conditions
such that the monolith does not crack and the integrity of the
monolith remains intact. During drying, the monolith shrinks, and
capillary forces in the pores increase as liquid is drawn out.
Thus, any suitable drying temperature profile and/or drying
atmosphere may be used with the monoliths formed such to avoid
cracking of the monoliths, e.g., by keeping capillary forces due to
the liquid being extracted below the limit of the pore walls to
withstand such forces. The temperature profile used for drying can
be adjusted so that the evaporation rate of liquid from the pores
is approximately the same as or less than the diffusion rate of the
liquid through the pores. In some cases, a modulated temperature
profile (temperature cycling) may be used. Temperature cycling in
some instances may reduce a drying time. Drying profiles may be
used that allow drying of a monolith over a time period of a few
days or less, e.g., within a week, or within 5 days, or within 3
days, or within 2 days, or within 1 day. As is described in more
detail below, the extent of reaction (e.g., shrinkage) and drying
may be monitored by weight loss, vapor pressure and/or physical
(e.g., microscopic) inspection.
[0076] For example, monoliths as described here may be dried using
the methods similar to those described in U.S. Pat. No. 6,620,368
which has already been incorporated herein by reference in its
entirety. That is, a portion of the liquid (e.g., water and/or
alcohol) in the pores of the wet monolith may be removed while the
gel remains wet at least in an outer circumferential outer region
of the monolith. Thus, the gel can dry more or less from the inside
out, e.g., the outer peripheral region of the monolith may dry
after an inner core region of the monolith has substantially
dried.
[0077] In some cases, drying methods and techniques may be used
that are similar to those described in U.S. Pat. No. 7,001,568,
which has already been incorporated herein by reference in its
entirety. That is, the monoliths may be dried by removing a portion
of liquid, e.g., water and/or an alcohol such as ethanol, from
pores of a body of a gel monolith while both an inner core region
and an outer peripheral region of the gel remain wet. The gel may
be allowed to shrink and become denser while the inner core region
and the outer peripheral region remain wet. After this initial
partial drying procedure, the remainder of the liquid may be
removed from the monolith by applying a modulated temperature
gradient between the outer peripheral region and the inner core
region.
[0078] As stated above, any suitable method, technique, instrument,
or combination thereof may be used for monitoring the extent of
reaction and corresponding monolith shrinkage, e.g., mass loss,
vapor pressure, and/or physical inspection. It may be desired to
monitor shrinkage using a relatively precise technique, as
incomplete or nonuniform reaction or drying may lead to cracking,
or may lead to broadened distributions of pore sizes. For example,
shrinkage of the monolith may be monitored locally and
microscopically over its body to gauge an extent and uniformity of
shrinkage. Such microscopic monitoring may be conducted using any
suitable tools or technique. Any technique that is capable of
detecting and resolving micron sized or submicron sized distance
changes may be suitable. For example, any type of displacement
sensor that is capable of about 1 .mu.m, about 0.5 .mu.m, about 0.1
.mu.m, or even finer resolution may be used. Contact or non-contact
techniques may be used to monitor the drying of a monolith.
Physical shrinkage measurements may be made on a continuous basis,
or may be made at selected time intervals. Multiple displacement
sensors may used, e.g., to measure displacement along different
dimension such as a cross-sectional dimension (e.g., a width,
diameter or radius) or a longitudinal dimension (e.g., a height or
length). In some cases, multiple displacement sensors may be used
to monitor shrinkage in different regions of a monolith. Linear,
two-dimensional, or three-dimensional displacement sensing tools
may be used. The monolith may be placed on a vibration-controlled
support, e.g., an optical table, to improve accuracy and precision
of displacement measurements.
[0079] Non-limiting examples of contact-type displacement sensors
that may be used to monitor shrinkage of a monolith include dial
indicators, linear variable differential transformers (LVDT), and
differential variable reluctance transformers (DVRT). Non-limiting
examples of non-contact displacement sensors that may be used
include eddy-current (inductive) type magnetic field displacement
sensors and optical displacement sensors. For example, any
commercially-available laser displacement sensor that is capable of
1 .mu.m or less resolution may be used. Laser displacement sensors
may be scanning, e.g., to monitor a surface, or non-scanning
varieties, e.g., to monitor a targeted position. Non-limiting
examples of suitable vendors for contact and/or non-contact
displacement sensors include Keyence, Inc. (www.sensorcentral.com),
Acuity, Inc. (www.acuity.com), Micro-Epsilon, Inc.
(www.micro-epsilon.com), MTI Instruments, Inc.
(www.mtiinstruments.com), Honeywell, Inc.
(www.honeywell.com/sensing), Baumer, Ltd. (www.baumerelectric.com),
Banner Engineering, Inc. (www.bannerengineering.com), and
Microstrain, Inc. (www.microstrain.com). It may be desired to use a
displacement measurement technique that is substantially
temperature-sensitive or allows for temperature compensation, e.g.,
an optical displacement t sensor or a DVRT. Combinations of
displacement sensor technologies may be used, e.g., one type may be
used to monitor a longitudinal dimension, whereas another type of
sensor may be used to monitor a cross-sectional dimension.
[0080] Thus, the shrinkage of one or more dimensions and/or one or
more regions of a monolith may be monitored to detect a plateau in
the shrinkage process. In many cases, shrinkage may be monitored at
multiple positions to detect a plateau has been reached throughout
the body of the monolith, instead of only in portions of the
monolith. A plateau may be reached when shrinkage is less than
about 100 ppm, less than about 5 ppm, less than about 1 ppm, or
even less. For example, a plateau may be reached when dimensional
changes are about 1 .mu.m or less. In some cases, a relatively
imprecise measurement technique such as mass loss may be used for
monitoring an initial shrinkage phase, whereas a more precise
monitoring technique as described above may be used for monitoring
final shrinkage. The shrinkage may be carried out at a temperature
in the range of about 70.degree. C. to about 90.degree. C.
Generally, before reaching shrinkage plateau, no gas is used to
purge the vapor out of the monolith.
[0081] Thus, the shrinkage and drying of the gels can be described
in terms of a two phase treatment. In a first phase, the wet gel
structure has reached its final shrinkage (e.g., monitored b
physical displacement as described above) with its internal open
pores still filled with its own pore liquid (e.g., molecular
species such as water molecules and alcohol molecules). Thereafter,
in a second phase, the monolith can be heated to remove any
residual liquid in the pores. The heat treatment itself can include
multiple heat treatment stages. In a first heat treatment stage,
temperatures greater than the boiling point of the molecular
species inside the porous gel structure may be used, e.g., to
overcome capillary forces within the pores. In some cases, in this
stage, a temperature ramp from about 80.degree. C. to about
200.degree. C., from about 90.degree. C. to about 180.degree. C.,
or from about 90.degree. C. to about 120.degree. C. may be used to
drive off molecular water, alcohol (e.g., ethanol), and catalysts
remaining in the pores. During the first heat treatment stage, the
heating condition is sufficient to evaporate the molecular species
from inside the pores, but is insufficient cause removal of
chemisorbed molecular species. Thus, a temperature ramp from about
200.degree. C. to about 450.degree. C., or from about 180.degree.
C. to about 400.degree. C. may be used to burn off molecular water,
alcohol (e.g., ethanol), remaining in the pores in a second heat
treatment stage, as is described in more detail below. No purging
gas is used before the pore liquid are totally evaporated to become
vapor phase. A temperature ramp used any of the heat treatment
stages may depend on the dimensions, especially a thickness, of a
monolith, but for a rod-shaped or brick-shaped monolith having a
thickness of several cm, this temperature ramp may occur over about
1 hour to about 2 hours. After all the pore liquid becomes vapor
phase, a nitrogen atmosphere (or inert gas such as helium or
argon), or air atmosphere may be used to purge or exchange
vaporized pore liquid out. At this stage, the temperature is
increased from 120.degree. C. to 180.degree. C. to get rid of all
and any molecular water and alcohol inside the pore of gel.
[0082] As stated above, in a second heat treatment stage,
chemisorbed species that still remain in the pores may be burned
off in an air atmosphere. Thus, the temperatures used in this stage
of the heat treatment may be sufficient to burn off chemisorbed
alcohol or other organic species such as higher molecular weight
alcohols that are still present in the pores, but insufficient to
cause the pores to close. For the this stage, air or
N.sub.2/O.sub.2 combination may be introduced at about 140.degree.
C. to about 200.degree. C. or about 180.degree. C. to about
200.degree. C. and the temperature increased to a baking
temperature in a range from about 400.degree. C. to about
800.degree. C., e.g., about 400.degree. C., about 450.degree. C.,
about 500.degree. C., about 600.degree. C., or about 700.degree. C.
The baking time for the second stage of the heat treatment may be
varied based on a thickness of a monolith. For example, a
block-shaped or rod-like monolith having a thickness of several
centimeters may be baked for about 2 to about 5 hours. In some
cases, the second stage of the heat treatment can form hydroxyl
reaction sites, e.g., at a density of about 4 to about 6 hydroxyl
groups per nm.sup.2. Such hydroxyl reaction sites may be used for
reactively coating a surface of the pores, e.g., applying
conductive coating so that the monolith may be used as a substrate
for a high surface area electrode, as is described in more detail
below.
[0083] In some variations, the invention provides a method of
drying the gels described herein, the method comprising heating the
gel that has reached shrinkage plateau at a temperature that the
molecular species (including alcohol, water, and catalysts) are in
vapor phase (e.g., at a temperature in the range of 90.degree. C.
to 120.degree. C.); introducing a gas (such as nitrogen) to purge
or exchange the vaporized molecular species out of the pores (e.g.,
at a temperature in the range of about 120.degree. C. to about
180.degree. C.); and burning the gel to remove chemisorbed species
out the pores in the presence of N.sub.2/O.sub.2 combination or air
(e.g., at a temperature in the range of about 140.degree. C. to
about 450.degree. C.).
[0084] In some cases, a monolith may be sliced into thin wafers,
e.g., wafers having a thickness of about 1 mm, about 0.9 mm, about
0.8 mm, about 0.7 mm, about 0.6 mm, about 0.5 mm, about 0.4 mm,
about 0.3 mm. about 0.2 mm, about 0.15 mm, or about 0.1 mm. Any
suitable slicing or cutting technique or tool may be used to form
such wafers, e.g., a saw or any other kind of cutting tool, e.g., a
wire saw, a diamond saw, or a water jet cutting tool. For thin
wafers, the times for the heat treatment to drive off molecular
species and/or burn chemisorbed species as described above may be
dramatically reduced. For example, a 200 .mu.m thick wafers may be
baked at about 400.degree. C., about 450.degree. C., or about
500.degree. C. for less than an hour, e.g., about 10 minutes, to
burn off chemisorbed species.
[0085] In certain variations, thin wafers may be molded. For
example, wafers may be formed by casting the gel formulation into a
mold or container comprising multiple slots, each slot comprising
opposing, closely-spaced parallel sidewalls and hydrophobic inner
surfaces. The gel can then undergo shrinkage in the slots in the
mold, as described above. The spacing between the parallel
sidewalls in the slots can be set so that the resulting sol-gel
wafer has a desired thickness, and a cross-sectional dimension of
the slots can be selected so that the resulting sol-gel wafer has a
desired cross-sectional area. After molding such sol-gel wafers,
they can undergo further shrinkage and drying as described above.
In some cases, the molded sol-gel wafers may undergo further
shrinkage and drying while still in the mold, whereas in certain
circumstances, the molded wafers may be removed from the mold to
undergo one or more shrinkage or drying steps. As stated above, for
thin wafers, the times for heat treatment for thin wafers may be
shortened compared to those for block-like monoliths. Wafers may
have a thickness of about 1 mm, about 0.9 mm, about 0.8 mm, about
0.7 mm, about 0.6 mm, about 0.5 mm, about 0.4 mm, about 0.3 mm.
about 0.2 mm, about 0.15 mm, or about 0.1 mm.
[0086] In certain variations, one or more additional components may
be added during the hydrolysis and polymerization of the gels
described herein. For example, one or more drying control agents
may be used, such as those described in U.S. Pat. No. 4,851,150,
which has already been incorporated herein by reference in its
entirety. Further, one or more porogens may be added, such as those
described in International Patent Publication No. WO 2006/068797,
which has already been incorporated herein by reference in its
entirety.
[0087] The monoliths made according to the methods described herein
may have microstructure having a desired microstructure and a
desired surface area for the open network of pores. As stated
above, the total pore volume of a monolith may be determined using
a pore size analyzer such as a Quantachrome Quadrasorb.TM. SI
Krypton/Micropore analyzer, and the bulk density of a monolith may
then be calculated using the total pore volume and the density of
the material making up the framework in the monolith. The monoliths
according to the methods described here may have a total pore
volume of at least about at least about 0.1 cm.sup.3/g, at least
about 0.2 cm.sup.3/g, at least about 0.3 cm.sup.3/g, at least about
0.4 cm.sup.3/g, at least about 0.5 cm.sup.3/g, at least about 0.6
cm.sup.3/g, at least about 0.7 cm.sup.3/g, at least about 0.8
cm.sup.3/g, at least about 0.9 cm.sup.3/g, at least about 1
cm.sup.3/g, at least about 1.1 cm.sup.3/g, at least about 1.2
cm.sup.3/g, at least about 1.3 cm.sup.3/g, at least about 1.4
cm.sup.3/g, at least about 1.5 cm.sup.3/g, at least about 1.6
cm.sup.3/g, at least about 1.7 cm.sup.3/g, at least about 1.8
cm.sup.3/g, at least about 1.9 cm.sup.3/g, at least about 2.0
cm.sup.3/g, or even higher. Thus, some monoliths may have a total
pore volume in a range from about 0.3 cm.sup.3/g to about 2
cm.sup.3/g, or from about 0.5 cm.sup.3/g to about 2 cm.sup.3/g, or
from about 0.5 cm.sup.3/to about 1 cm.sup.3/g, or from about 1
cm.sup.3/g to about 2 cm.sup.3/g. A porosity of the monoliths may
be about 30% to about 90% by volume, e.g., about 30% to about 80%,
about 40% to about 80%, or about 45% to about 75%. In some
variations, the porosity may be lower than about 30% by volume or
higher than about 90% by volume, e.g., up to about 95% by
volume.
[0088] An average pore size (such as average pore diameter) of the
pores in the open pore network formed in the monoliths described
herein may be tunable of a range from about 0.3 nm to about 300 nm,
about 0.3 nm to about 100 nm, about 0.3 nm to about 50 nm, about
0.3 nm to about 30 nm, or about 0.3 nm to about 10 nm. For example
average pore sizes of about 0.3 nm, about 0.5 nm, about 0.8 nm,
about 1 nm, about 2 nm, about 3 nm, about 4 nm, about 5 nm, about 6
nm, about 7 nm, about 8 nm, about 9 nm, or about 10 nm may be
preselected and achieved using the methods described herein. For
any preselected average pore size achieved in the monoliths
described herein, a relatively narrow distribution around that
average may be achieved. For example, at least about 50%, at least
about 60%, at least about 70%, or at least of about 75% of the
pores may be within about 40%, within about 30%, within about 20%,
or within about 10% of an average size. In certain variations, at
least about 50% of the pores may be within about 1 nm, within about
0.5 nm, within about 0.2 nm, or within about 0.1 nm of an average
pore size. As used herein "within" a designated percentage or
designated amount of an average pore size is meant to encompass
that percentage deviation or a lesser percentage deviation, or that
amount of deviation or a lesser amount of deviation to either the
higher side or a lower side of the average pore size. That is, a
pore size distribution that is within about 20% of an average pore
size is meant to encompass pore sizes in a range from the average
pore size minus 20% of that average pore size to the average pore
size plus 20% of that average pore size, inclusive.
[0089] Thus, some variations of monoliths may have an average pore
size that can be selected in a range from about 0.3 nm to about 300
nm, or in a range from about 0.3 nm to about 100 nm, or in a range
from about 0.3 nm to about 30 nm, or in a range from about 0.3 nm
to about 10 nm, and a distribution such that at least about 50% or
at least about 60% of the pores are within about 20% of the average
pore size, or within about 10% of the average pore size. Certain
variations may have even tighter pore size distributions, e.g.,
monoliths may have an average pore size selectable in a range from
about 0.3 nm to about 30 nm or in a range from about 0.3 nm to
about 10 nm, and have a distribution such that at least about 50%
of pores are within about 10% of the average. For monoliths having
relatively small average pore sizes, e.g., 5 nm or smaller, e.g.,
about 5 nm, about 4 nm, about 3 nm, about 2 nm, about 1 nm, about
0.5 nm, or about 0.3 nm, at least 50% of the pores may be within
about 1 nm, about 0.5 nm, about 0.2 nm, or about 0.1 nm of the
average.
[0090] In general, the surface area of a monolith increases for
smaller particles sizes, and in particular when a pore size
decreases below about 3 nm, the corresponding surface area
increases rapidly, e.g., exponentially or approximately
exponentially. The surface area of a monolith may be measured by
using the B.E.T. surface area method, or may be calculated using an
average pore size as described above (Eq. 1). In general, the
surface area of a monolith increases for smaller particles sizes,
and in particular when a pore size decreases below about 3 nm, the
corresponding surface area increases rapidly in a nonlinear manner,
e.g., exponentially or approximately exponentially. This
relationship is illustrated graphically in FIG. 2. There, a bulk
surface area (SA) in m.sup.2/g has been calculated for versus
average pore diameter (D) as described above in connection with Eq.
1. Data point symbols indicate bulk surface areas measured by
B.E.T. analysis. Monoliths with dramatically increased surface
areas may be prepared by the methods described herein, e.g., where
the average pore size may be controlled to be about 3 nm or
smaller.
[0091] As shown, as a pore size decreases from about 3 nm to about
0.6 nm, the corresponding surface area increases from about 1000
m.sup.2/g to about 5000 m.sup.2/g, e.g., a five-fold increase.
Monoliths with dramatically increased surface areas may be used for
the high surface area electrodes described herein, where the
average pore size may be controlled to be about 5 nm or smaller, or
about 3 nm or smaller.
[0092] Thus, a surface area of the open pore network in the
monoliths used in the high surface area electrodes described herein
may be about 50 m.sup.2/g to about 5000 m.sup.2/g, or even higher,
e.g., at least about 50 m.sup.2/g, at least about 100 m.sup.2/g, at
least about 150 m.sup.2/g, at least about 200 m.sup.2/g, at least
about 300 m.sup.2/g, at least about 400 m.sup.2/g, at least about
500 m.sup.2/g, at least about 600 m.sup.2/g, at least about 700
m.sup.2/g, at least about 800 m.sup.2/g, at least about 1000
m.sup.2/g, at least about 1200 m.sup.2/g, at least about 1400
m.sup.2/g, at least about 1600 m.sup.2/g, at least about 1800
m.sup.2/g, at least about 2000 m.sup.2/g, at least about 2200
m.sup.2/g, at least about 2400 m.sup.2/g, at least about 2600
m.sup.2/g, at least about 2800 m.sup.2/g, at least about 3000
m.sup.2/g, at least about 3500 m.sup.2/g, at least about 4000
m.sup.2/g, at least about 4500 m.sup.2/g, or at least about 5000
m.sup.2/g.
[0093] A molded, dried monolith may be further processed to make a
substrate or template for a high surface area electrode. For
example, a monolithic rod or brick can be sliced into wafers, e.g.,
using a saw or any kind of cutting tool, e.g., a wire saw, a
diamond saw, or a water jet cutting tool. Wafers so formed can have
any suitable thickness, e.g., about 1 mm or less, about 0.5 mm or
less, about 0.25 mm or less, about 0.2 mm or less, about 0.15 mm or
less, or 0.1 mm or less. Referring now to FIG. 3, a monolithic
sol-gel silica rod 300 having a cross-sectional diameter of about 3
inches and a length of about 20 inches can be sliced into over one
thousand wafers 301 each having a thickness of about 250 microns or
less. In certain variations, wafers may be molded, as described
above.
[0094] A conductive material or a material that can be converted
into a conductive material by subsequent processing may be applied
to (e.g., by coating or impregnating) any suitably formed sol-gel
derived monolith, e.g., a monolith as molded and dried, or a
monolith that has been molded, dried, and subsequently processed,
e.g., by slicing into wafers as described above in connection with
FIG. 3. The material used for coating and/or impregnating may
comprise any suitable material, e.g., graphite, graphite-like
conductive carbon carbide, carbon, activated carbon, conductive
carbons derived from the polymerization and carbonization of carbon
precursor materials like, furfural, furfuryl alcohol
(2-furylmethanol), polyfurfuryl alcohol, resorcinol formaldehyde,
sucrose, and glucose, a metal such as platinum, nickel, gold,
palladium, molybdenum, a metal oxide such as tin oxide, indium tin
oxide, zinc oxide, molybdenum oxide, ruthenium oxide, tungsten
oxide, manganese dioxide, silver oxide, nickel oxyhydroxide,
aluminum doped zinc oxide, titanium oxide, vanadium pentoxide,
sulfides such as molybdenum sulfide, tungsten sulfide, iron
sulfide, nitrides such as tungsten nitride, molybdenum nitride or
combinations thereof, conductive polymers such as
poly(3-methylthiophene). As used herein, a "graphite-like" material
is a carbon-based material that is similar to graphite but has a
conductivity and a density approaching that of graphite. As the
density of a graphite-like material is increased toward that of
graphite, the conductivity of a graphite-like material
correspondingly approaches that of graphite. A graphite-like
material may contain more defects than graphite, or may contain
impurities. As stated above, the electrodes may be used in a
capacitor, an ultracapacitor, a fuel cell, or a battery and the
electrodes may, based in part on choice of conductive coating
material, be capable of being charged and discharged both or
individually electrostatically or faradiacally.
[0095] Of course, the composition of a conductive coating formed on
the open network of pores or the conductive composition filled into
the open network of pores used in the electrodes can affect
electrical properties of the electrodes described herein. For
example, conductive material selection can affect the electrical
conductivity of the electrode, as well as energy-handling
capabilities, e.g., whether the electrode is suitable for
relatively high voltages and/or relatively high currents. Further,
conductive material selection can affect contact resistance of the
electrode, which can contribute to ESR.
[0096] In general, the conductive coating on the open network of
pores may be relatively uniform, e.g., to prevent high resistance
regions that can lead to hot spots and subsequent failure. In some
cases, the conductive coating may be uniform throughout an entire
open network of pores of a sol-gel derived monolith. In other
instances, an open network of pores is only partially coated with a
conductive coating. However, in these cases, the partial coating
may still be generally continuous over a relatively large surface
area of the monolith, rather than forming isolated islands of
conductive coating, so as to provide a substantial conductive
surface area. In the instance where the conductive materials are
filled into the pores the same considerations apply; and the
conductive network within open pore network of pores may be uniform
or in other instances non-uniform but still connected and
conductive forming a conductive monolith network within the open
network of pores.
[0097] The thickness of the coating can also affect the electrical
properties of the electrodes. For example, if a relatively thin
conformal conductive coating is used to coat the open pore network,
the conductive surface area may be close to that of the underlying
open pore network. An example of such an electrode is depicted in
FIG. 4A. There, electrode 450 comprises an open network of pores
451. In this variation, the network 451 is lined with a relatively
conformal conductive coating 452. In other instances, e.g., for
high current or voltage applications, the open pore networks may be
substantially filled with the conductive coating. An example of
such an electrode is illustrated in FIG. 4B. There, electrode 400
comprises an open network of pores 401 that is substantially filled
with a conductive material 402. In this variation, the conductive
surface area of the electrode 401 is less than the surface area of
the open network of pores as it existed in monolith 401 before
coating.
[0098] It should be noted that the conductive pathways formed by
making the open pore network in sol-gel derived monoliths
conductive are generally continuous conductive pathways, rather
than conductive pathways that are formed from point-point contacts
between discrete conductive particles. Therefore, the high internal
resistance that can result from particle-particle contacts, e.g.,
carbon-carbon contacts, may not be present or dominant in the
electrodes described here. Therefore, the electrodes may have
overall lower internal resistance, lower ESR, and higher
conductivity. Besides reducing power losses due to resistance
between conductive particles, the increased conductivity may allow
thicker electrodes to be made, which may increase surface area even
more, and increase physical robustness of the electrodes.
[0099] Any suitable method or technique may be used to make the
open pore network in the sol-gel derived monoliths conductive. For
example, the surface of the open pore network may be coated or the
open network of pores may be partially filled with graphite, a
graphite-like material, or a conducting carbon derived from the
polymerization and carbonization of carbon precursor materials like
but not limited to, furfural, furfuryl alcohol (2-furylmethanol),
polyfurfuryl alcohol, resorcinol formaldehyde, sucrose, and
glucose. For example, coating may be achieved by synthesizing a
layer of graphite or a layer of a graphite-like material on the
surface. In other instances, a layer of carbide may be formed on
the surface of the open pore network. In still other cases, metal
or conductive metal oxides may be deposited in the open pore
network, e.g., by chemical vapor deposition techniques such as
atomic layer deposition. In some situations, a colloidal solution
containing a metal and/or conductive metal oxide may be impregnated
into the porous network, the liquid of the colloidal solution
removed, and the metal and/or metal oxide particles allowed to
coalesce, e.g., by melting or rapid thermal processing.
[0100] Any method for in situ synthesis of a conductive carbon
coating, e.g., graphite, graphite-like conducting carbon and/or
carbide, in an open pore network in a silica sol-gel derived
monolith described herein, now known, or later developed, may be
used. For example, suitable methods for forming carbon-based
conductive coatings a silica surface are provided in C. Lin et al.,
J. Electrochem. Soc. 146 (1999) 3639, C. Lin and J. A. Ritter,
Carbon 35 (1997) 1271, and C. Lin et al., Carbon 38 (2000) 849,
each of which is incorporated by reference herein in its
entirety.
[0101] An illustrative example of an in situ synthesis of a
carbon-based, graphite-like conductive coating (e.g., conductive
carbon) on an open pore network in a silica sol-gel derived
monolith is shown in FIGS. 5A-5C. A liquid phase comprising
resorcinol (R) and formaldehyde (F) can be introduced into the
sol-gel and then polymerized in the presence of a base catalyst
(e.g., KOH, Na.sub.2CO.sub.3, NaOH: potassium hydroxide, sodium
carbonate, and sodium hydroxide) and heat to form a polymer layer
threaded through the open pore network. FIG. 5A illustrates pores
immersed in an aqueous solution comprising monomer and catalyst,
and FIG. 5B shows a polymer fragment resulting from the
polymerization. Unreacted residual materials such as resorcinol
and/or formaldehyde that may be adsorbed onto the nanoporous
surface may be removed by washing with organic solvents like
acetone, drying and evaporation, e.g., at a temperature from about
40.degree. C. to about 120.degree. C. Following drying, the
modified sol-gel can be exposed to carbonizing conditions in an
inert atmosphere (e.g., nitrogen atmosphere) and may be followed by
thermal treatment with carbon dioxide from about 200.degree. C. to
about 1200.degree. C. to form a graphite-like layer on the silica
nanoporous surface. FIG. 5C illustrates a conception of a
graphite-like material resulting from the thermal processing in
nitrogen and carbon dioxide as described above in connection with
FIG. 5B. In the polymeric layer comprising graphite-like sections
as illustrated conceptually in FIG. 5C, the density approaches that
of graphite.
[0102] In some variations, atomic layer deposition (ALD) or
chemical vapor deposition (CVD) may be used to apply metal and/or
conductive metal oxide particles to the open pore network in a
sol-gel derived monolith. ALD may be used to build up a conformal
and relatively uniform conducting film on the open pore network. As
is known, ALD can be used to deposit such conformal coatings of
conductive oxides, nitrides, sulfides and metals. For example, as
described in Jeffrey W. Elam et al., J. Nanomaterials 2006 (2006)
1-5, which is hereby incorporated by reference in its entirety,
titanium tetrachloride may be reacted with H.sub.2O at a deposition
temperature of 100.degree. C. to produce a conformal layer of
titanium dioxide on nanoporous materials, diethyl zinc may be
reacted with H.sub.2O at a deposition temperature of 177.degree. C.
to produce a conformal coating of ZnO on nanoporous materials,
vanadyl oxytriisopropoxide may be reacted with H.sub.2O.sub.2 to
produce a conformal coating of V.sub.2O.sub.5 on nanoporous
materials, and Pd hexafluoroacetylacetonate may be reacted with
formaldehyde at a reaction temperature of 200.degree. C. to produce
a conformal coating of palladium on nanoporous materials.
[0103] Any form of chemical vapor deposition now known or later
developed may be used to form a conductive coating on an open pore
network of the monoliths used in the electrodes. For example,
thermal chemical vapor deposition, electron beam chemical vapor
deposition, and/or sputtering may be used to form a metal layer or
conductive metal oxide layer. The conductive coating itself may
comprise any suitable material, e.g., graphite, graphite-like
conductive carbon carbide, carbon, a metal such as platinum,
nickel, gold, palladium, molybdenum, a metal oxide such as tin
oxide, indium tin oxide, zinc oxide, molybdenum oxide, ruthenium
oxide, tungsten oxide, manganese dioxide, silver oxide, nickel
oxyhydroxide, aluminum doped zinc oxide, titanium oxide, vanadium
pentoxide, sulfides such as molybdenum sulfide, tungsten sulfide,
iron sulfide, nitrides such as tungsten nitride, molybdenum nitride
or combinations thereof.
[0104] In some cases, aluminum doped zinc oxide (ZnO:Al) may be
used as a conductive coating. ZnO:Al also may be used in
combination with zinc oxide, titatanium dioxide, indium tin oxide,
tin oxide, platinum, nickel, gold, palladium or vanadium pentoxide.
A conductive coating having a tunable resistivity may be produced
by varying an amount of aluminum oxide doped into zinc oxide.
ZnO:Al coatings may be produced by any suitable method, e.g., ALD,
CVD, magnetron sputtering, electron beam evaporation, and pulsed
laser deposition. Methods for making ZnO:Al conductive coatings are
provided in S.-H. K. Park et al., Japanese Journal of Applied
Physics 44 (2005), L242-L245, which is hereby incorporated by
reference herein in its entirety. For example, if ALD is used to
build up the films, diethyl zinc (DEZ) (zinc precursor) and
H.sub.2O (oxygen precursor) can be alternately injected into a
reactor in which a substrate is held at 180.degree. C., using
nitrogen as a carrier gas with a flow rate of 100 sccm, and using
pulsing times of 1.65 seconds for DEZ and H.sub.2O, and a nitrogen
purge time of 4.4 seconds. Aluminum can be doped into the films by
introducing an aluminum precursor such as trimethylaluminum into
the reaction chamber with certain DEZ deposition cycles, e.g.,
about one injection of trimethylaluminum per 19 injections of DEZ.
The injections of DEZ, H.sub.2O, and trimethylaluminum can be
repeated any number of cycles to build up a desired layer
thickness. In some variations, a layer of Al.sub.2O.sub.3 may be
built up on top of the ZnO:Al layer. Methods for making ZnO:Al
films using magnetron sputtering are described in K. Elmer et al,
Thin Solid Films, 247 (1994), 15-23, and using pulsed laser
deposition are described in Z. Y. Ning et al., Thin Solid Films,
307 (1997), 50-53, each of which is incorporated herein by
reference in its entirety. ZnO:Al films may have a resistivity in a
range from about 5.times.10.sup.-4 ohm-cm to about
1.times.10.sup.-2 ohm-cm, or from about 5.times.10.sup.-4 ohm-cm to
about 1.times.10.sup.-3 ohm-cm.
[0105] As stated above, metal and/or metal oxide particles in an
aqueous colloidal solution may be allowed to ingress into an open
pore network in the monoliths used to make the electrodes described
herein. The solvent may then be drawn off, leaving metal and/or
conductive metal oxide particles lining the pores. Any technique
may then be used to allow the metal and/or conductive metal oxide
particles to coalesce to form a continuous conductive network. For
example, the monolith may be subjected to rapid high temperature
excursions to melt or coalesce the particles, similar to rapid
thermal processing (RTP) that is used in the semiconductor
industry. If desired, this process can be repeated to build up a
conductive network. Any suitable metal and/or conductive metal
oxide particles may be used build up a conductive network. RTP may
be used in this instance to reduce the thermal budget and/or to
prevent thermal equilibrium from reducing or restricting the
ability of the conductive material to diffuse during thermal
processing. During RTP, conductive particles may be subject to
temperatures ranging from about 200.degree. C. to about
1200.degree. C. with ramp rates varying from about 20.degree.
C./sec to about 250.degree. C./sec. Typical processing times for
RTP are less than about 1 minute. In general, the particles may be
nanoparticles having an outer dimension smaller than that of the
nanopores present in the monolith. For example, gold nanoparticles
may be deposited in the open pore network as described above, and
then subjected to RTP conditions to form a continuous gold network
threading its way through the monolith.
[0106] In still other variations, reaction precursors to conducting
polymer films may be impregnated into a nanoporous monolith.
Concentrations of the reaction precursors can be adjusted so as to
coat the open pore network of the monolith with the precursor. In
situ polymerization reaction conditions such as the presence of
catalysts, pH, temperature profile, and reaction time, can be
adjusted to result in a polymer coating on the open pore network.
Following formation of a continuous polymer film threading its way
through the open pore network, post reaction processing, e.g.,
carbonization, may be used to convert the polymer film into a
conducting film on the open pore network or convert the polymer to
fill. For example, as described above, furfuryl alcohol may be
introduced and polymerized in the open network of pores of the
silica sol-gel derived monolith. The sol-gel derived monolith can
be fabricated as a thin free standing wafer. A 100% solution of
furfuryl alcohol is then introduced into the pore network of the
water at room temperature. The wafer containing the furfuryl
alcohol is then heated between 80 and 135.degree. C. for from
between 5 and 25 hours to initiate polymerization. This process can
be repeated several times to maximize pore impregnation by the
furfural. The wafer containing the polymerized furfuryl alcohol is
then removed to a furnace to be heat to a final temperature of
between 600 and 1100.degree. C. under an inert atmosphere to
complete the carbonization process and generate a conducting carbon
material. The template can then be removed partially or completely.
In some cases, the wafer containing the polymerized furfuryl
alcohol is removed at lower temperatures from between 300 and
600.degree. C. and the template is removed partially or completely.
The free standing or partially free standing polymerized furfuryl
alcohol is returned to the furnace for additional thermal treatment
to complete the carbonization process and generate a conducting
carbon monolith. Additionally, in the case of monomers like
furfuryl alcohol which polymerize in a linear chain, cross-linking
agents can be added to the reaction mixture prior to heating to
increase the connectivity of the polymer. For example, lysine 5% by
weight can be used. Another example, as described above, resorcinol
and formaldehyde in aqueous solution may be polymerized in situ in
the open pore network of a silica sol-gel derived monolith, wherein
small amounts of base may be added to catalyze the polymerization.
Examples of suitable reaction conditions are provided in C. Lin and
J. A. Ritter, Carbon 35 (1997) 1271, which has already been
incorporated herein by reference in its entirety. In both the above
examples, post-polymerization processing in the presence of an
oxidizing gas, e.g., CO.sub.2, at elevated temperature may be used
to increase the density of the film formed from the polymer film.
As the density of the film increases, the film becomes more
graphite-like, leading to increasing conductivity with increasing
density. Post-polymerization process in the presence of an
oxidizing gas may be completed using processing times up to about
10 hours, and at processing temperatures in a range from about
200.degree. C. to about 1200.degree. C.
[0107] An indicated above, the addition of a conductive layer to an
open pore network in a sol-gel derived monolith may affect the
surface area. Thus, the conductive surface area may be relatively
close to that of the underlying monolith if a relatively thin
conformal layer is applied, or the conductive surface area may be
somewhat less, e.g., a factor of 2, or a factor of 3, or an even
higher factor, than that of the underlying monolith. Any of the
electrodes described herein may have a conductive surface area that
is about 20 m.sup.2/g to about 5000 m.sup.2/g, or even higher,
e.g., at least about 20 m.sup.2/g, at least about 30 m.sup.2/g, at
least about 40 m.sup.2/g, at least about 50 m.sup.2/g, at least
about 60 m.sup.2/g, at least about 70 m.sup.2/g, at least about 80
m.sup.2/g, at least about 90 m.sup.2/g, at least about 100
m.sup.2/g, at least about 120 m.sup.2/g, at least about 150
m.sup.2/g, at least about 180 m.sup.2/g, at least about 200
m.sup.2/g, at least about 220 m.sup.2/g, at least about 250
m.sup.2/g, at least about 280 m.sup.2/g, at least about 300
m.sup.2/g, at least about 320 m.sup.2/g, at least about 350
m.sup.2/g, at least about 380 m.sup.2/g, at least about 400
m.sup.2/g, at least about 420 m.sup.2/g, at least about 450
m.sup.2/g, at least about 480 m.sup.2/g, at least about 500
m.sup.2/g, at least about 550 m.sup.2/g, at least about 600
m.sup.2/g, at least about 650 m.sup.2/g, at least about 700
m.sup.2/g, at least about 750 m.sup.2/g, at least about 800
m.sup.2/g, at least about 850 m.sup.2/g, at least about 900
m.sup.2/g, at least about 950 m.sup.2/g, at least about 1000
m.sup.2/g, at least about 1100 m.sup.2/g, at least about 1200
m.sup.2/g, at least about 1300 m.sup.2/g, at least about 1400
m.sup.2/g, at least about 1500 m.sup.2/g, at least about 1600
m.sup.2/g, at least about 1700 m.sup.2/g, at least about 1800
m.sup.2/g, at least about 1900 m.sup.2/g, at least about 2000
m.sup.2/g, at least about 2200 m.sup.2/g, at least about 2400
m.sup.2/g, at least about 2600 m.sup.2/g, at least about 2800
m.sup.2/g, at least about 3000 m.sup.2/g, at least about 3500
m.sup.2/g, at least about 4000 m.sup.2/g, at least about 4500
m.sup.2/g, or at least about 5000 m.sup.2/g. These electrodes may
have an average pore size in a range from about 0.3 nm to about 100
nm, from about 0.3 nm to about 30 nm, from about 0.3 nm to about 10
nm. In some variations, the electrodes have a pore size
distribution wherein at least about 50% of the pores are within
about 30% of an average pore size, within about 20% of an average
pore size, or within about 10% of an average pore size.
[0108] Additional variations of high surface area conductive
electrodes are provided herein. These electrodes comprise a
continuous skeletal framework formed of a conductive material that
is formed using a sol-gel derived monolith as a template. Thus,
these electrodes may be formed by substantially filling an open
pore network with a conductive material, as described herein, e.g.,
as illustrated and discussed in connection with FIG. 4B. After a
three-dimensional conductive network is formed in the monolith, the
sol-gel template may be selectively removed, or at least partially
removed leaving behind a free-standing or substantially
free-standing conductive framework. The sol-gel template may be
removed using any suitable technique, e.g., by dissolving using a
suitable solvent. For example, for silica sol-gel monoliths,
hydrofluoric acid (HF) may be used to dissolve the monolith and
leave behind a conductive framework. The dissolution temperatures
will range near room temperature and may be controlled from about
20.degree. C. to about 60.degree. C. The dissolution concentrations
of HF used can vary from about 1% to about 48% (e.g., from about 1%
to about 30%) HF by weight. In some cases, it may be desired to
leave behind a residual amount of the monolith to impart structural
strength to the conducting framework. Another method used for
template removal is the dissolution of the template using a 0.1 M
to 5 M solution of NaOH at 30.degree. C. to 75.degree. C. to
dissolve the silica and leave a free standing or substantially free
standing conductive framework in place. As with the other electrode
variations described herein, these free-standing high surface area
electrodes may be used as electrodes in any device requiring high
surface area electrodes, e.g., energy storage devices such as
capacitors, e.g., ultracapacitors, batteries, and fuel cells.
[0109] As these electrodes are made using the open pore network of
a sol-gel derived monolith as a template, their conductive surface
may be similar to that of the template. However, since the
conductive material has been used to fill the open pore network,
the conductive surface area of the resulting free-standing
electrodes may be lower than that of the sol-gel template. These
electrodes may have a conductive surface area that is about 50
m.sup.2/g to about 5000 m.sup.2/g, or even higher, e.g., at least
about 50 m.sup.2/g, at least about 60 m.sup.2/g, at least about 70
m.sup.2/g, at least about 80 m.sup.2/g, at least about 90
m.sup.2/g, at least about 100 m.sup.2/g, at least about 120
m.sup.2/g, at least about 150 m.sup.2/g, at least about 180
m.sup.2/g, at least about 200 m.sup.2/g, at least about 220
m.sup.2/g, at least about 250 m.sup.2/g, at least about 280
m.sup.2/g, at least about 300 m.sup.2/g, at least about 320
m.sup.2/g, at least about 350 m.sup.2/g, at least about 380
m.sup.2/g, at least about 400 m.sup.2/g, at least about 420
m.sup.2/g, at least about 450 m.sup.2/g, at least about 480
m.sup.2/g, at least about 500 m.sup.2/g, at least about 550
m.sup.2/g, at least about 600 m.sup.2/g, at least about 650
m.sup.2/g, at least about 700 m.sup.2/g, at least about 750
m.sup.2/g, at least about 800 m.sup.2/g, at least about 850
m.sup.2/g, at least about 900 m.sup.2/g, at least about 950
m.sup.2/g, at least about 1000 m.sup.2/g, at least about 1100
m.sup.2/g, at least about 1200 m.sup.2/g, at least about 1300
m.sup.2/g, at least about 1400 m.sup.2/g, at least about 1500
m.sup.2/g, at least about 1600 m.sup.2/g, at least about 1700
m.sup.2/g, at least about 1800 m.sup.2/g, at least about 1900
m.sup.2/g, at least about 2000 m.sup.2/g, at least about 2200
m.sup.2/g, at least about 2400 m.sup.2/g, at least about 2600
m.sup.2/g, at least about 2800 m.sup.2/g, at least about 3000
m.sup.2/g, at least about 3500 m.sup.2/g, at least about 4000
m.sup.2/g, at least about 4500 m.sup.2/g, or at least about 5000
m.sup.2/g. These electrodes may have an average pore size in a
range from about 0.3 nm to about 300 nm, from about 0.3 nm to about
100 nm, from about 0.3 nm to about 30 nm, from about 0.3 nm to
about 10 nm. In some variations, the electrodes have a pore size
distribution wherein at least about 50% of the pores are within
about 30% of an average pore size, within about 20% of an average
pore size, or within about 10% of an average pore size.
[0110] The electrodes described herein made of carbon material may
has a resistivity between about 0.001 .OMEGA.-cm to about 0.1
.OMEGA.-cm, such as from about 0.001 .OMEGA.-cm to about 0.05
.OMEGA.-cm, about 0.001 .OMEGA.-cm to about 0.01 .OMEGA.-cm, or
about 0.001 .OMEGA.-cm to about 0.005 .OMEGA.-cm.
[0111] The electrode material described herein can be made into
powders using methods known in the art, such as by grinding, using
ball mill or jet milling techniques. The electrode can be made from
the conductive powder using methods known in the art. See Bonnefoi,
L. et al J. Power Sources 79 (1999), 37-42; and U.S. Pat. No.
6,187,061. In one variation, for example, a suspension of the
conductive powder and a binder in a appropriate solvent can be cast
upon or spread out on a current collector; the wet film is then
allowed to dry at room temperature or dried by thermal treatment in
vacuum. In another variation, a dried film can be prepared first
from the conductive powder and a binder; and is then attached or
plated on to the current collector. The thickness of the film is
from about 50 to about 1000, about 50 to about 500, about 50 to
about 200, about 75 to about 200, about 100 to about 200, about 75
to about 125, about 125 to about 175, about 90 to about 120, about
120 to about 150, about 150 to about 180, about 180 to about 210,
or about 140 to about 160 microns. The conductive powder is a
powder of any one or a mixture of more than one of the electrode
materials described herein. The binder is a suitable polymeric
material such as polytetrafluoroethylene (PTFE),
carboxymethylcellulose (CMC), polyvynilidiene chloride (PVDC), and
the like. The amount of the binder material used is less than or
about any of 20%, 15%, 10%, 8%, 5%, 3% or 1% by weight relative to
the weight of the conductive powder material. A suitable solvent
(e.g. acetone, water, 1-methyl-2-pyrrolidine (NMP),
tetrahydrofurane (THF)) is used for suspending the conductive
powder and the binder. For example, water can be used when CMC is
used as a binder, NMP can be used when PTFE is used as a binder, or
THF can be used when PVDC is used as a binder.
II. Energy Storage Devices
[0112] Energy storage devices are described herein. In general, the
devices comprise a cell that has first and second electrodes across
which a potential may be applied. An electrolyte is provided in the
cell between the first and second electrodes. A separator is
provided in the electrolyte to prevent the cell from shorting. For
example, if the device is a capacitor, an ultracapacitor or
battery, the separator is insulating and permeable to ions of the
electrolyte so that ions can diffuse to the electrodes to build up
the electric double layers at the electrode surfaces, but does not
allow substantial current to flow in the electrolyte between the
electrodes. If the device is a fuel cell, the separator may
comprise a proton exchange membrane. Any electrodes described
herein may be used as the first and/or the second electrode in the
energy storage device. For example, in the energy storage devices,
the first electrode comprises a first conductive network formed by
coating an open pore network of a sol-gel derived monolith, e.g., a
silica sol-gel derived monolith, with a conductive coating. In some
variations of the devices, the first conductive network may remain
in the monolith in the first electrode, as described above. In
other variations, the first conductive network may be a stand-alone
or substantially stand-alone conductive network. These electrodes
are described above, and are formed by selectively removing the
monolith template after filling an open pore network of the
monolith with a conductive material to form the first conductive
network. In some energy storage devices, the second electrode also
comprises a conductive network formed by coating an open pore
network of a sol-gel derived monolith with a conductive
coating.
[0113] In certain variations, at least one electrode in the energy
storage devices (e.g., an ultracapacitor) may comprise a sol-gel
derived monolith comprising an open network of pores, and a
conductive network formed by coating a surface of the open network
of pores with a material that lends itself to faradaic charge
storage and discharge, e.g., a material selected from the group
consisting of ruthenium oxide, molybdenum oxide, molybdenum
nitride, molybdenum sulfide, tungsten oxide, tungsten nitride,
tungsten sulfide, manganese dioxide, iron sulfide, silver oxide,
nickel oxyhydroxide, and combinations thereof, and
poly(3-methylthiophene) leading to the formation of an asymmetric
capacitor.
[0114] Any of the high surface area electrodes derived from a
nanoporous sol-gel derived monolith as described herein may be used
in the devices. Further, the first electrode and the second
electrode may be the same or different. For example, if the energy
storage device is a capacitor, both electrodes may be high surface
area electrodes formed from sol-gel derived monoliths as described
herein. In other variations, the first and second electrodes may be
different. For example, if the energy storage device is a battery,
an anode may be a high surface area electrode formed from sol-gel
derived nanoporous electrode that comprises a reactive species.
[0115] In the energy storage devices, an average pore size and/or a
pore size distribution may be selected to accommodate an ionic
species of the electrolyte. For example, in an ultracapacitor, an
ion of the electrolyte must be able to access the pores of a high
surface area electrode as described herein to take advantage of
that surface area. If the topography of the conductive surface
contains features of a scale too small to accommodate the ionic
species of the electrolyte, the effective conductive surface area
is reduced. For example, a substantial fraction of the pores in an
electrode may have a dimension of about 1 nm to about 2 nm.
Further, in some cases, the pore sizes may be adjusted to be
generally smaller than a solvation shell of an ionic species in the
electrolyte. This may allow the ionic species to move even closer
to an electrode surface. Creating a broad distribution of pore
sizes may lead to underutilized volume in the electrodes due to
relatively low surface area sections and underutilized surface area
in the electrodes due to pore sizes that are too small to
accommodate an ionic species. Thus, an electrode may have its pore
size finely tuned as described herein to increase
electrolyte-electrode interactions, and to increase effective
utilization of the conductive surface area of the electrodes. In
some instances, the electrodes may be customized or selected for
use with particular electrolytes.
[0116] In some variations, the energy storage device may be an
ultracapacitor. An example of an ultracapacitor is provided in FIG.
6A. There, ultracapacitor 600 comprises a first nanoporous
electrode 601 and a second nanoporous electrode 602 separated by an
insulating separator 603. Current spreading plates 604 and 605 are
placed in electrical contact with the nanoporous electrodes 601 and
602, respectively. An electrolyte 606 is present between the first
and second electrode and ionic species of the electrolyte
interpenetrates the nanopores in each of the electrodes 601 and
602. When a potential (e.g., from a battery 607) is applied between
the two electrodes 601 and 602 via electrical leads 609 and 608,
respectively, ions of the electrolyte migrate to the electrode
having the opposite charge, including through the separator 603
that is permeable to ions of the electrolyte, to store charge. The
ultracapacitor illustrated in FIG. 6A may be sealed in a container
(not shown). Any suitable type of container now known or later
developed may be used, e.g., a barrel-shaped can, or a flat
coin-shaped can.
[0117] The current spreading plates may be formed of any metal, and
have any thickness selected to withstand the voltage and or current
levels to which the ultracapacitor will be exposed. For example, in
variations, the current spreading plates may comprise copper,
nickel, or aluminum.
[0118] The electrodes may comprise any of the high surface area
nanoporous electrodes described herein. Although the example shown
in FIG. 6A includes two nanoporous electrodes, variations are
contemplated in which only one of the electrodes is a nanoporous
electrode, and the other electrode is a standard electrode, e.g., a
carbon-based electrode. As described above, the nanoporous
electrodes may have an average pore size and pore size distribution
selected to allow ingress of an ionic species of electrolyte into
the pores, so that the ion can take advantage of the high surface
area. In addition, also as described above, the size of the pores
may be finely tuned so as to be sized smaller than a solvation
sphere of the ionic species to allow that ion to approach even
closer to the conductive electrode surface.
[0119] A thickness of an electrode (shown as dimension 610 for
electrode 610 and as dimension 612 for electrode 612 in FIG. 6A)
may be any suitable thickness. Factors that may be considered in
selecting an electrode thickness include an electrode porosity, a
device voltage rating, a surface area of an electrode, a
composition of an electrode, and a conductivity of an electrode. In
some cases, an electrode thickness may be about 100 microns, about
150 microns, about 200 microns, about 250 microns, about 300
microns, about 350 microns, about 400 microns, about 450 microns,
about 500 microns, about 600 microns, about 700 microns, about 800
microns, about 900 microns, or about 1 mm. Electrodes may be formed
from sliced sol-gel derived monoliths, e.g., as illustrated and
described in connection with FIG. 3.
[0120] As stated above, the electrodes in an ultracapacitor may be
the same or different. In some cases, the electrodes described
herein may be used in asymmetric ultracapacitors, i.e., a capacitor
in which one electrode comprises a capacitive material that stores
charge electrostatically, and one electrode that comprises a
material that stores charge via a fast, reversible faradaic process
(electron transfer) at a certain electrode potential. Non-limiting
examples of capacitive materials include carbonaceous materials,
conducting metals and metalloids, and conducting metal oxides.
Non-limiting examples of materials that lend themselves to faradaic
charge storage include inorganic oxides, sulfides, or nitrides such
as oxides, sulfides or nitrides of molybdenum and tungsten,
ruthenium oxide, manganese dioxide, iron sulfide, silver oxide,
nickel oxyhydroxide, and conducting polymers such as polythiophenes
(e.g., poly(3-methylthiophene). Thus, in an asymmetric
ultracapacitor, the anode may comprise a capacitive material that
has a capacity for electrostatic charge storage, and the cathode
may comprise a material that has high faradaic charge storage
capacity.
[0121] In an asymmetric ultracapacitor, the anode and/or the
cathode may be formed from a nanoporous sol-gel monolith as
described herein. For example, the cathode of an asymmetric
ultracapacitor may be derived from a sol-gel monolith in which the
open pore network has been coated with a carbonaceous material such
as graphite, or a graphite-like conducting carbon, a metal or
metalloid, or a conducting metal oxide as described herein. As
described above, the sol-gel monolith may remain as a support for
the electrode, or may function as a template that is substantially
removed to result in a high surface area conductive framework that
had been formed within the open pore network. The anode of an
asymmetric ultracapacitor may be derived from a sol-gel monolith in
which the open pore network has been coated with a material that
lends itself to faradaic charge storage as described above. Here
again, the sol-gel monolith may remain as a support for the anode,
or may function as a template that is substantially removed to
result in a high surface area conductive framework that had been
formed within the open pore network. In some variations of
asymmetric ultracapacitors, a sol-gel derived cathode may be used
in combination with another type of anode, and a sol-gel derived
anode may be used in combination with another type of cathode.
Non-limiting examples of alternative cathodes and anodes that may
be used in any combination with any of the sol-gel derived
electrodes are for example described in U.S. Pat. No. 7,199,997,
and A. Balducci et al., Applied Physics A 82 (2006), 627-632, each
of which is incorporated herein by reference in its entirety.
[0122] An example of an asymmetric ultracapacitor is shown in FIG.
6B. There, asymmetric ultracapacitor 650 comprises current
spreading plates 654 and 656. The anode 651 is in electrical
contact with current spreading plate 654 and the cathode 652 is in
electrical contact with current spreading plate 656. The anode 651
may comprise a material that has high faradaic charge storage
capacity, and the cathode 652 may comprise a capacitive material
that has high electrostatic storage capacity. An insulating
separator 657 that is porous to the electrolyte (not shown) is
disposed between the anode 651 and cathode 652. The anode 651
and/or the cathode 652 may be nanoporous electrodes derived from
sol-gel monoliths as disclosed herein. The asymmetric
ultracapacitor illustrated in FIG. 6B may be sealed in a container
(not shown). Any suitable type of container now known or later
developed may be used, e.g., a barrel-shaped can, or a flat
coin-shaped can.
[0123] The electrolyte used in the ultracapacitors (symmetric or
asymmetric) may be any suitable electrolyte described herein,
otherwise known, or later discovered. For example, the electrolyte
may be organic or inorganic. The electrolyte may be selected based
on a voltage rating of the ultracapacitor. Organic electrolytes may
be selected for ultracapacitors designed to have a voltage rating
of about 1.5V to about 3V, or from about 2V to about 3V. Such
organic electrolytes may have dielectric constant of about 40. An
ionic liquid may be used for devices designed to have a higher
voltage rating, e.g., about 5V to about 6V, or about 5V to about
7V. The dielectric constant of ionic liquid electrolytes may be
about 30. For low voltage ratings, e.g., voltage ratings below
about 2.5V or about 3V, an aqueous electrolyte may be used. Such
aqueous electrolytes may have a dielectric constant of about
10.
[0124] Non-limiting examples of suitable organic electrolytes
include carbonates such as propylene carbonate, ethylene carbonate,
butylene carbonate, dimethyl carbonate, diethyl carbonate, dipropyl
carbonate, methyl ethyl carbonate, methyl butyl carbonate, methyl
propyl carbonate, ethyl propyl carbonate, butyl propyl carbonate,
1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentene
carbonate, 2,3-pentene carbonate; nitriles such as acetonitrile,
acrylonitrile, propionitrile; sulfoxides such as dimethyl
sulfoxide, diethyl sulfoxide, ethyl methyl sulfoxide, benzylmethyl
sulfoxide; amides such as formamide, dimethyl formamide;
pyrrolidones such as N-methylpyrrolidone; esters such as
p-butyrolactone, .gamma.-butyrolactone, .gamma.-valerolactone,
.beta.-valerolactone, .gamma.-butyrolactone,
2-methyl-.gamma.-butyrolactone, acetyl-.gamma.-butyrolactone,
phosphate triesters; and ethers such as 1,2-dimethoxyethane,
1,2-ethoxyethane, diethoxyethane, methoxyethoxyethane,
dibutoxyethane, nitromethane, dimethoxypropane, diethyoxypropane,
methoxyethoxypropane, tetrahydrofuran, 2-methyl-tetrahydrofuran,
alkyltetrahydrofurans, dialkyltetrahydrofurans,
alkoxytetrahydrofurans, dialkoxytetrahydrofurans,
2-methyltetrahydrofuran, 1,2-dioxolane, 1,3-dioxolane,
1,4-dioxolane, 2-methyldioxolane, 4-methyl-dioxolane,
alkyl-1,3-dioxolanes, sulfolane, 3-methylsulfolane, diethyl ether,
diethylene glycol dialkyl ethers, diethylene glycol dialkyl ethers,
triethylene glycol dialkyl ethers, tetraethylene glycol dialkyl
ethers, alkylpropionates, dialkyl malonates, alkyl acetates, methyl
formate, methyl acetate, methyl propionate, ethyl propionate, and
maleic anhydride.
[0125] In some variations of the ultracapacitors, e.g., asymmetric
ultracapacitors, solvent-free ionic liquids may be used as
electrolytes, e.g., 1-butyl-3-methyl-imidazolium tetrafluoroborate,
1-butyl-3-methyl-imidazolium hexafluorophosphate, and
N-butyl-N-methylpyrrolidinium
bis(trifluoromethanesulfonyl)-imide.
[0126] The separator used in the ultracapacitors (symmetric or
asymmetric) may be any suitable separator, and as described above,
is electrically insulating and permeable to ions of the electrolyte
so that the ions can migrate to the electrodes, but prevent
shorting of the capacitor. Non-limiting examples of separators
include thin paper films, or Celgard.TM. separator films. In some
variations, a sol-gel derived separator may be used, where the
sol-gel derived separator is designed to have an appropriate
permeability for ions in the electrolyte.
[0127] FIG. 7 illustrates various combinations of specific powers
(W/kg) and specific energy (W-h/kg) that may be achieved with the
ultracapacitors (symmetric or asymmetric) described here. There,
the cross-hatched region representing specific powers greater than
about 10, and specific energies of about 0.1 W-h/kg to about 1000
W-h/kg indicates the range of specific powers and specific energies
that may be delivered with the ultracapacitors described here.
Thus, some of the ultracapacitors may have a specific energy of at
least about 0.1 W-h/kg, at least about 1 W-h/kg, at least about 10
W-h/kg, at least about 30 W-h/kg, at least about 50 W-h/kg, at
least about 100 W-h/kg, at least about 120 W-h/kg, at least about
150 W-h/kg, at least about 170 W-h/kg, at least about 190 W-h/kg,
at least about 200 W-h/kg, at least about 300 W-h/kg, at least
about 400 W-h/kg, at least about 500 W-h/kg, or as high as about
1000 W-h/kg. Ultracapacitors described here may have a specific
power of about 10 W/kg or higher, e.g., about 10 W/kg, about 50
W/kg, about 100 W/kg, about 500 W/kg, about 1 kW/kg, about 5 kW/kg,
about 10 kW/kg, about 50 kW/kg, about 100 kW/kg, or even
higher.
[0128] Ultracapacitors (symmetric or asymmetric) as described
herein may be capable of many charge/discharge cycles, e.g.,
greater than about 3.times.10.sup.5 cycles. Further, the
ultracapacitors may have very high charge and discharge
efficiencies, e.g., about 90% or greater, about 95% or greater,
about 98% or greater, about 99% or greater, about 99.5% or greater,
about 99.8% or greater, about 99.9% or greater, or very close to
100%.
[0129] In an ultracapacitor described herein where the conductive
material in the electrode is made of conductive carbon from any
source, the capacitance of the carbon material may be in a range
from about 15 F/g to about 500 F/g. In an ultracapacitor device
described herein, the capacitance may be in the range of about 3
F/g to about 125 F/g for electrodes made with conductive carbon. In
an asymmetric ultracapacitor device described herein, the
capacitance may be in the range about 6 F/g to about 250 F/g.
[0130] Ultracapacitors (asymmetric or symmetric) having alternate
electrode configurations are possible. For example, the electrodes
described herein may form a portion of an interdigitated electrode
assembly. Such an interdigitated electrode assembly may be used to
increase an energy storage capacity or voltage rating of an
ultracapacitor. Referring now to FIG. 8, ultracapacitor 800
comprises a first electrode assembly 801, and a second electrode
assembly 802. A potential (not shown) may be placed between the
first and second electrode assemblies with electrical leads 803 and
804, respectively. The first electrode assembly 801 comprises a
first metal or metallized current spreading plate 805 with which
electrical lead 803 makes electrical contact, and the second
electrode assembly 804 comprises a second metal or metallized
current spreading plate 806 with which electrical lead 804 makes
electrical contact. Extending generally perpendicularly from first
current spreading plate 805 of the first electrode assembly 801 is
a series of spaced-apart electrodes 807. Any one or any combination
of the electrodes 807 may comprise a high surface area electrode as
described herein. For example, any single one, any subset, or all
of the electrodes 807 may comprise a high surface area silica
sol-gel derived electrode having a thickness of about 150 microns.
Similarly, extending perpendicularly from the second current
spreading plate 806 of the second electrode assembly 802 is a
series of spaced-apart electrodes 808. Here again, any single
electrode 808, any subset of the electrodes 808, or all of the
electrodes 808 may comprise a high surface area electrode as
described herein. Any one of the electrodes 808 may comprise a high
surface area silica sol-gel derived electrode having a thickness of
about 150 microns. The electrodes 807 and 808 may be interdigitated
with each other as shown in FIG. 8. Separating the electrodes 807
and 808 are separators 810. An electrolyte (not shown) is dispersed
in the cell in the volume 811 between the electrode assemblies 801
and 802, filling the volume between the interdigitated electrodes
807 and 808. The electrolyte may be any electrolyte described
herein, otherwise known or later developed. The separators 810 are
porous to ionic species in the electrolyte, and may be any
separators described herein, otherwise known, or later
developed.
[0131] Electric double layer capacitors (EDLCs) have applications
in a variety of technology areas which require energy storage and
energy delivery rapidly and repetitively with relatively high
power. For example, EDLC may be used in the automobile sector like
hybrid-electric vehicles of various types where the EDLC could be
used to augment the vehicle battery, leveling the load on the
battery by powering acceleration and recovering energy during
braking, thereby increasing battery life and reducing battery size
and weight. Another general application area would be in the motion
capture of energy that would otherwise be wasted; for example, the
capture of energy in the repetitious up and down movement of heavy
shipping containers (Miller and Burke The Electrohemical Society
Interface Spring 2008, 53-57). Additionally, applications could be
found for bulk energy storage by electric utilities by storing
off-peak electricity at night for use during the day or other grid
applications like load leveling solar and wind electric generating
farms. Many additional applications could be found in consumer
electronics and power tools.
III. Systems
[0132] Energy storage systems are also provided here. These systems
comprise multiple energy storage cells, at least some of which may
be connected in series or in parallel. In these systems, each
energy storage cell comprises two electrodes configured to be
oppositely charged and an electrolyte disposed between the two
electrodes. At least one electrode in at least one of the cells
comprises an electrode that has been derived from a sol-gel
monolith as described herein. For example, at least one of the
electrodes in the multi-cell energy storage systems may comprise a
sol-gel monolith having an open pore network that has been coated
with a conductive coating, or a conductive network formed from a
sol-gel monolith by coating or filling its open pore network with a
conductive material, and substantially removing the sol-gel
template to provide a stand-alone conductive framework. In certain
systems, more than one or all electrodes may be derived from
sol-gel monoliths. In these energy storage systems, the monolith
used may be derived from any suitable sol gel, but in some cases,
it is derived from a silica sol gel. Such multiple cell energy
storage systems may be used in applications requiring increased
energy storage capacity and/or increased voltage ratings.
[0133] Examples of energy storage systems are provided in FIGS.
9A-9B. Referring first to FIG. 9A, system 950 comprises multiple
energy storage cells 951 connected in series. An external potential
(not shown is applied across the series arrangement using
electrical leads 952 and 953. Each cell 951 comprises an
ultracapacitor (symmetric or asymmetric). At least one of cells 951
may comprise an ultracapacitor as described herein, e.g., similar
to that illustrated in FIG. 6A or 6B. Thus, each cell comprises a
first electrode 955 and an opposite polarity second electrode 956,
and a separator 957 that is permeable to ionic species in an
electrolyte (not shown) that is present in each cell. Each positive
polarity electrode of a cell is connected to a negative polarity
electrode of an adjacent cell so that the cells are arranged in
electrical series. The entire system may be sealed in any suitable
container, e.g., in a housing or barrel shaped can, or a lower
profile relatively flat container.
[0134] Referring next to FIG. 9B, system 900 comprises multiple
energy storage cells 901 connected in series. An external potential
(not shown) is applied across the entire series, using electrical
leads 902 and 903. Each cell 901 comprises an ultracapacitor
(symmetric or asymmetric). At least one of the cells 901 may
comprise an ultracapacitor as described herein, e.g., similar to
that illustrated in FIG. 6A or 6B. Thus each cell 901 comprises a
first electrode 905 and an opposite polarity second electrode 906.
Separating the first and second electrodes is a separator 907 that
is permeable to ionic species in the electrolyte (not shown) that
is present in each cell. Each positive polarity electrode of a cell
is connected to a negative polarity electrode of an adjacent cell
so that the cells are arranged in electrical series. In this
particular variation, adjacent electrodes are configured as bipolar
electrode structures, where electrodes of opposite polarity are
separated by electrolyte and a solid layer 908 that is nonporous to
the electrolyte. As illustrated, opposite polarity electrodes may
be arranged in an interdigitated manner relative to each other. The
entire system may be sealed in any suitable container, e.g., a box
or barrel shaped can, or a lower profile flatter container.
[0135] For energy storage systems comprising series-connected
cells, such as those illustrated in FIG. 9A or 9B, the electrodes
used may be any electrodes described herein. At least one of the
electrodes in the system 900 or system 950, which may be either a
positive electrode or a negative electrode, is derived from a
sol-gel as described herein.
[0136] Another example of an energy storage system is provided in
FIG. 10. There, system 1000 comprises multiple energy storage cells
1001 connected in parallel. An external potential (not shown) is
applied across the parallel arrangement using electrical leads 1002
and 1003. Any one of or all of the cells 1001 may comprise an
ultracapacitor similar to that illustrated in FIG. 6A or 6B. Thus,
each cell 1001 comprises a first electrode 1005 and an opposite
polarity second electrode 1006. Separating the first and second
electrodes is an insulating separator 1007 that is permeable to
ionic species in the electrolyte (not shown) that is present in
each cell. Each positive polarity electrode of a cell is connected
to the positive polarity electrodes of other cells in the circuit,
and each negative polarity electrode of a cell is connected to the
negative polarity electrodes of other cells in the circuit so that
the cells are connected in parallel. At least one of the electrodes
in the in one of the cells 1001 in system 1000 is a high surface
area electrode derived from a sol-gel as described herein.
[0137] The ultracapacitors described herein may be used in a
variety of applications. For example, they may be used to replace
batteries in certain applications, e.g., for handheld electronic
applications. In other situations, they may provide backup power
for electronic devices, e.g., for computers. The ultracapacitors
may be used in combination, e.g., in a parallel circuit, with a
battery to augment peak power delivery of that battery. The
ultracapacitors may be used as energy storage devices in a
hybrid-electric engine such as a hybrid-electric engine used to
power vehicles.
[0138] The following Example is provided to illustrate but not
limit the invention.
Example 1
Process for Preparing a Monolithic Carbon Electrode Material Using
a Nanoporous Monolith as a Template
[0139] The template used to form the monolithic carbon electrode
material was made from a thin sol-gel wafer. The wafer was
fabricated by casting a silica sol-gel solution into a mold. The
chemical composition and molar ratio of the components of the
sol-gel solution were 1 TEOS (tetraethyl orthosilicate), 2.25
H.sub.2O (water), 0.075 HF (hydrofluoric acid) and 0.01 HCl
(hydrochloric acid). These chemicals were then mixed, cast into a
mold and left sitting at room temperature for up to 3 hours. The
sample (sol-gel plus mold) was then placed in an incubator at
33.degree. C. to age for up to 72 hours. The sample was then
removed to an oven and baked at 160.degree. C. under nitrogen for
up to 24 hours (the ramp to 160.degree. C. was done in air and the
nitrogen turned on at 160.degree. C.). After the drying step at
160.degree. C. the sample was then sintered in a furnace at
720.degree. C. for 2 hours in air. The resulting silica wafer had a
surface area of 570 m.sup.2/g with an average pore diameter of 5
nm; and 69.5% of the pores in the resulting silica wafer were
within 20% of the average pore diameter of 5 nm. A wafer having 1
mm in thickness and 25 mm in diameter was generated.
[0140] The first step in the formation of a monolithic carbon
electrode material was to take the silica sol-gel derived wafer
having 1 mm in thickness and 25 mm in diameter and impregnate it
with furfuryl alcohol. This was accomplished by placing the wafer
in a shallow container that was filled with enough furfuryl alcohol
to cover the wafer and allowing it to soak for up to five hours.
The furfuryl alcohol saturated wafer was then removed from the
container and excess furfuryl alcohol was cleaned from the surface
of the wafer. The furfuryl alcohol saturated wafer was then heated
at 123.degree. C. in air for 16 hours to create polyfurfuryl
alcohol throughout the porous silica network of the wafer. This
composite material was then heated under nitrogen at 365.degree. C.
for five hours.
[0141] The sample was cooled to room temperature. The composite
wafer was then soaked in concentrated HF and sonicated for one hour
to remove the silica template leaving a monolith carbon wafer in
its place. The carbon monolith was heated to 1000.degree. C. for 3
hours in a 5% CO.sub.2, 95% N.sub.2 atmosphere. The sample was then
cooled to room temperature. The resulting material was tested for
surface area, resistivity, and capacitance. This material had a
measured BET surface area of 700 m.sup.2/g and average pore
diameter of 9 nm; and 46.4% of the pores were within 30% of the
average pore diameter of 9 nm. The resistivity of the material was
5 .OMEGA.-cm and had a measured capacitance of 50 F/g. Without
wishing to be bound by theory, the relatively high resistivity may
be due to defects in the linear chains formed during the furfuryl
alcohol polymerization process. Resistivity may be lowered by
adding cross-linking agents (such as lysine) to improve the
connectivity of the carbon framework as well as by optimizing the
carbonization process to maximize the graphite-like nature of the
final material.
[0142] This disclosure is illustrative and not limiting. Further
modifications will be apparent to one skilled in the art in light
of this disclosure and such modifications are intended to fall
within the scope of the appended claims. Each publication and
patent application cited in the specification is incorporated
herein by reference in its entirety as if each individual
publication or patent application were specifically and
individually put forth herein.
* * * * *
References