U.S. patent application number 10/857470 was filed with the patent office on 2005-01-06 for methods of oxidizing multiwalled carbon nanotubes.
This patent application is currently assigned to Hyperion Catalysis International, Inc.. Invention is credited to Chishti, Asif, Hoch, Robert, Moy, David, Niu, Chunming.
Application Number | 20050002850 10/857470 |
Document ID | / |
Family ID | 23410871 |
Filed Date | 2005-01-06 |
United States Patent
Application |
20050002850 |
Kind Code |
A1 |
Niu, Chunming ; et
al. |
January 6, 2005 |
Methods of oxidizing multiwalled carbon nanotubes
Abstract
Methods of oxidizing multiwalled carbon nanotubes are provided.
The multiwalled carbon nanotubes are oxidized by contacting the
carbon nanotubes with gas-phase oxidizing agents such as CO.sub.2,
O.sub.2, steam, N.sub.2O, NO, NO.sub.2, O.sub.3, and ClO.sub.2.
Near critical and supercritical water can also be used as oxidizing
agents. The multiwalled carbon nanotubes oxidized according to
methods of the invention can be used to prepare rigid porous
structures which can be utilized to form electrodes for fabrication
of improved electrochemical capacitors.
Inventors: |
Niu, Chunming; (Lexington,
MA) ; Moy, David; (Winchester, MA) ; Chishti,
Asif; (Lowell, MA) ; Hoch, Robert;
(Hensonville, NY) |
Correspondence
Address: |
KRAMER LEVIN NAFTALIS & FRANKEL LLP
INTELLECTUAL PROPERTY DEPARTMENT
919 THIRD AVENUE
NEW YORK
NY
10022
US
|
Assignee: |
Hyperion Catalysis International,
Inc.
|
Family ID: |
23410871 |
Appl. No.: |
10/857470 |
Filed: |
May 28, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10857470 |
May 28, 2004 |
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09358745 |
Jul 21, 1999 |
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Current U.S.
Class: |
423/447.1 |
Current CPC
Class: |
C01B 2202/10 20130101;
Y02E 60/50 20130101; C04B 35/83 20130101; H01G 11/36 20130101; Y10T
428/30 20150115; Y02E 60/10 20130101; B82Y 30/00 20130101; C01B
2202/06 20130101; C04B 2235/5252 20130101; C04B 35/6265 20130101;
H01M 4/96 20130101; Y02E 60/13 20130101; D01F 11/123 20130101; Y10S
977/847 20130101; C01B 32/168 20170801; Y10T 428/292 20150115; C01B
32/162 20170801; C04B 2235/5264 20130101; H01M 4/583 20130101; C01B
32/174 20170801; C01B 2202/36 20130101; D01F 11/12 20130101; B82Y
40/00 20130101; D01F 11/122 20130101; C01B 2202/34 20130101; C04B
2235/5288 20130101; Y02P 20/54 20151101; C01B 2202/28 20130101 |
Class at
Publication: |
423/447.1 |
International
Class: |
D01F 009/12 |
Claims
We claim:
1. A method of oxidizing multiwalled carbon nanotubes having a
diameter no greater than 1 micron, said method comprising
contacting said multiwalled carbon nanotubes with a gas-phase
oxidizing agent under conditions sufficient to form oxidized
nanotubes.
2. The method of claim 1, wherein said oxidation occurs on the
exterior side walls of said multiwalled carbon nanotubes.
3. The method of claim 1, wherein said oxidation occurs on the
surface of said multiwalled carbon nanotubes.
4. The method of claim 1, wherein said diameter of said carbon
nanotubes is from 2 to 100 nanometers.
5. The method of claim 1, wherein said diameter of said carbon
nanotubes is from 3.5 to 75 nanometers.
6. The method of claim 1, wherein said multiwalled carbon nanotubes
include at least a plurality of graphitic layers that are
substantially parallel to the axis of said nanotubes.
7. The method of claim 1, wherein said multiwalled carbon nanotubes
are substantially cylindrical, graphitic nanotubes having a length
to diameter ratio of greater than 5 and a diameter of less than 0.1
micron.
8. The method of claim 1, wherein said multiwalled carbon nanotubes
are substantially cylindrical, free of a continuous pyrolitically
deposited carbon overcoat, the projection of the graphite layers on
said nanotubes extending for a distance of at least two nanotube
diameters.
9. The method of claim 1, wherein said multiwalled carbon nanotube
is a fishbone fibril.
10. The method of claim 1, wherein said multiwalled carbon
nanotubes are grown on supported catalysts.
11. The method of claim 1, wherein said oxidized nanotubes comprise
moieties selected from the group consisting of carbonyl, carboxyl,
aldehyde, phenolic, hydroxy, esters, lactones and derivatives
thereof.
12. The method of claim 1, wherein said oxidized nanotubes exhibit
upon titration an acid titer of from 0.05 to about 0.6 meq/g.
13. The method of claim 1, wherein said oxidized nanotubes exhibit
upon titration an acid titer from 0.1 to 0.4 meq/g.
14. The method of claim 1, wherein said oxidized carbon nanotubes
exhibit a weight loss of from 1% to 60% by comparison to unoxidized
carbon nanotubes.
15. The method of claim 1, wherein said oxidized nanotubes exhibit
a weight loss from 2% to 15% by comparison with said unoxidized
carbon nanotube.
16. The method of claim 1, wherein said gas-phase oxidizing agent
is selected from the group consisting of CO.sub.2, O.sub.2, steam,
N.sub.2O, NO, NO.sub.2 ,O.sub.3, ClO.sub.2 and mixtures
thereof.
17. The method of claim 1, wherein said gas-phase oxidizing agent
is diluted with an inert diluant selected from the group consisting
of nitrogen, noble gases and mixtures thereof.
18. The method of claim 1, wherein said gas-phase oxidizing agent
is near critical or supercritical water.
19. The method of claim 1, wherein said oxidizing of said
multiwalled carbon nanotubes with said gas-phase oxidizing agent is
performed for a period of time from about 0.1 hours to about 24
hours.
20. The method of claim 1, wherein said oxidizing of said
multiwalled carbon nanotubes with said gas-phase oxidizing agent is
performed for a period of time from about 1 hour to about 8
hours.
21. The method of claim 1, wherein said oxidizing of said
multiwalled carbon nanotubes with said gas-phase oxidizing agent is
performed in a temperature range from about 200.degree. C. to about
600.degree. C. and in a range of partial pressure of said oxidizing
agent from about 1 torr to about 7600 torr whenever said gas-phase
oxidizing agent is selected from the group consisting of
O.sub.2,O.sub.3, N.sub.2O, NO, NO.sub.2, ClO.sub.2 and mixtures
thereof.
22. The method of claim 21, wherein the partial pressure range of
the gas-phase oxidizing agent is from 5 torr to 760 torr.
23. The method of claim 1, wherein said oxidizing of said
multiwalled carbon nanotubes with said gas-phase oxidizing agent is
performed in a temperature range from about 400.degree. C. to about
900.degree. C. and in a range of partial pressure of the oxidizing
agent from about 1 torr to about 7600 torr whenever said gas-phase
oxidizing agent is CO.sub.2 or steam.
24. The method of claim 23, wherein the partial pressure range of
the gas-phase oxidizing agent is from 5 torr to 760 torr.
25. The method of claim 1, further comprising a secondary treatment
step of said oxidized nanotubes with a reactant suitable to react
with moieties of said oxidized nanotubes thereby adding at least a
secondary group onto the surface of said oxidized nanotubes.
26. The method of claim 25, wherein said additional secondary group
is selected from the group consisting of an alkyl or aryl silane
wherein said alkyl has C.sub.1 to C.sub.18, said aryl has C.sub.1
to C.sub.18, an alkyl of C.sub.1 to C.sub.18 or an aralkyl group of
C.sub.1 to C.sub.18, a hydroxyl group of C.sub.1 to C.sub.18 and an
amine group of C.sub.1 to C.sub.18.
27. The method of claim 25, wherein said additional secondary group
is a fluorocarbon.
28. The method of claim 1 further comprising dispersing said
surface-oxidized nanotubes into a liquid medium.
29. The method of claim 28, wherein after being dispersed in said
liquid medium, said oxidized nanotubes are filtered and dried to
form a mat.
30. The method of claim 29, further comprising heating said mat
from 200.degree. C. to 900.degree. C.
31. The method of claim 29, further comprising forming said mat
into an electrode.
32. A method for producing a network of carbon nanotubes comprising
the steps of: (a) oxidizing said carbon nanotubes with a gas-phase
oxidizing agent under conditions sufficient to form oxidized
nanotubes; (b) subjecting said oxidized nanotubes to conditions
sufficient to cause crosslinking.
33. The method of claim 32, wherein said conditions include heating
said oxidized nanotubes in air in a temperature range from
200.degree. C. to 600.degree. C.
34. The method of claim 29, wherein said conditions include heating
said oxidized nanotubes in an inert atmosphere in a temperature
range from 200.degree. C. to 2000.degree. C.
35. A method for producing a network of oxidized carbon nanotubes
comprising the steps of: (a) oxidizing said carbon nanotubes with a
gas-phase oxidizing agent under conditions sufficient to form
oxidized nanotubes; (b) treating said oxidized nanotubes with a
reactant suitable to react with moieties of said oxidized nanotubes
thereby adding at least a secondary group onto the surface of said
oxidized nanotubes; (c) further contacting said nanotubes bearing
secondary groups with an effective amount of crosslinking
agent.
36. The method of claim 32, wherein said gas phase oxidizing agent
is selected from the group consisting of CO.sub.2, O.sub.2, steam,
N.sub.2O, NO, NO.sub.2, O.sub.3, ClO.sub.2 and mixtures
thereof.
37. The method of claim 35, wherein said crosslinking agent is
selected from the group consisting of a polyol or polyamine.
38. The method of claim 37, wherein said polyol is a diol and said
polyamine is a diamine.
39. The method of claim 32, wherein said oxidized nanotubes
comprise moieties selected from the group consisting of carbonyl,
carboxyl, aldehyde, ketone, hydroxy, phenolic, esters, lactones and
derivatives thereof.
40. A method of treating aggregates of carbon nanotubes which
comprises contacting said aggregates with an gas-phase oxidizing
agent under conditions sufficient to oxidize said carbon
nanotubes.
41. The method of claim 40, wherein said aggregates have a
macromorphology resembling a shape selected from the group
consisting of bird nests, combed yarn and open net aggregates.
42. The method of claim 40, wherein said aggregate particles have
an average diameter of less than 50 microns.
43. The method of claim 40, wherein said carbon nanotubes are
substantially cylindrical with a substantially constant diameter
are multiwalled having graphitic layers concentric with the
nanotube axis and are substantially free of pyrolitically deposited
carbon.
44. The method of claim 40, wherein said carbon nanotubes are a
fishbone fibril.
45. The method of claim 40, wherein said treated aggregates
resemble a weathered rope.
46. A method for preparing a rigid porous structure comprising the
steps of: (a) oxidizing a multiplicity of multiwalled carbon
nanotubes according to the method of claim 1 to oxidized nanotubes;
(b) dispersing said oxidized nanotubes in a medium to form a
suspension; (c) separating said medium from said suspension to form
a porous structure of entangled oxidized nanotubes wherein said
nanotubes are interconnected to form a rigid porous structures.
47. The method of claim 46, wherein said multiwalled carbon
nanotubes are uniformly and evenly distributed throughout said
structure.
48. The method of claim 46, wherein said carbon nanotubes are in
the form of aggregate particles selected from the groups consisting
of aggregate particles resembling a shape selected from the group
consisting of bird nest, combed yarn and open net.
49. The method of claim 46, wherein said oxidized nanotubes are in
the form of aggregate particles resembling a weathered rope.
50. The method of claim 46, further comprising heating said
suspension in air to a temperature in a range from about
200.degree. C. to about 600.degree. C. thereby forming said rigid
porous structure.
51. The method of claim 46, further comprising heating said
suspension in an inert gas to a temperature in a range from about
200.degree. C. to about 2000.degree. C. therebyforming said rigid
porous structure.
52. The method of claim 46, wherein said medium is water or organic
solvents.
53. The method of claim 46, wherein said medium comprises a
dispersant selected from the group consisting of alcohols,
glycerin, surfactants, polyethylene glycol, polyethylene imines and
polypropylene glycol.
54. The method of claim 46, wherein said suspension further
comprises gluing agents selected from the group consisting of
cellulose, carbohydrate, polyethylene, polystyrene, nylon,
polyurethane, polyester, polyamides and phenolic resins.
55. The method of claim 46, further comprising the steps of: (a)
forming said rigid porous structure into a mat; and (b) forming
said mat into an electrode.
56. An electrochemical capacitor having at least one electrode
comprising the oxidized carbon nanotubes prepared by the method of
claim 1.
57. An electrochemical capacitor having at least one electrode
prepared by a method which comprises the following steps: (a)
contacting aggregates of carbon nanotubes with a gas-phase
oxidizing agent under conditions sufficient to oxidize said carbon
nanotubes; (b) dispersing said aggregates of oxidized nanotubes
prepared in step (a) in a liquid medium to form a slurry; (c)
filtering and drying said slurry to form a mat of oxidized carbon
nanotubes; (d) subjecting said mat to conditions sufficient to
cause the crosslinking of said oxidized carbon nanotubes.
58. The electrochemical capacitor of claim 57, wherein said
conditions of step (d) include heating said mat from 180.degree. C.
to 350.degree. C.
59. An electrochemical capacitor having at least one electrode
formed by a method comprising the following steps: (a) dispersing
aggregates of carbon nanotubes in a liquid medium to form a slurry;
(b) filtering and drying said slurry to form a mat of carbon
nanotubes; (c) treating said mat according to the method of claim 1
under conditions sufficient to oxidize said carbon nanotubes.
60. The capacitor of claim 55, wherein said gas-phase oxidizing
agent is selected from the group consisting of CO.sub.2, O.sub.2,
steam, N.sub.2O, NO, NO.sub.2, O.sub.3, ClO.sub.2 and mixtures
thereof.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of Invention
[0002] The invention relates broadly to methods of oxidizing the
surface of multiwalled carbon nanotubes. The invention also
encompasses methods of making aggregates of surface-oxidized
nanotubes, and using the same. The invention also relates to
complex structures comprised of such surface-oxidized carbon
nanotubes linked to one another.
[0003] 2. Description of the Related Art
[0004] Carbon Nanotubes
[0005] This invention lies in the field of submicron graphitic
carbon fibrils, sometimes called vapor grown carbon fibers or
nanotubes. Carbon fibrils are vermicular carbon deposits having
diameters less than 1.0.mu., preferably less than 0.5.mu., and even
more preferably less than 0.2.mu.. They exist in a variety of forms
and have been prepared through the catalytic decomposition of
various carbon-containing gases at metal surfaces. Such vermicular
carbon deposits have been observed almost since the advent of
electron microscopy. (Baker and Harris, Chemistry and Physics of
Carbon, Walker and Thrower ed., Vol. 14, 1978, p. 83; Rodriguez,
N., J. Mater. Research, Vol. 8, p. 3233 (1993)).
[0006] In 1976, Endo et al. (see Obelin, A. and Endo, M., J. of
Crystal Growth, Vol. 32 (1976), pp. 335-349), hereby incorporated
by reference, elucidated the basic mechanism by which such carbon
fibrils grow. They were seen to originate from a metal catalyst
particle, which, in the presence of a hydrocarbon containing gas,
becomes supersaturated in carbon. A cylindrical ordered graphitic
core is extruded which immediately, according to Endo et al.,
becomes coated with an outer layer of pyrolytically deposited
graphite. These fibrils with a pyrolytic overcoat typically have
diameters in excess of 0.1.mu., more typically 0.2to 0.5.mu..
[0007] In 1983, Tennent, U.S. Pat. No. 4,663,230, hereby
incorporated by reference, describes carbon fibrils that are free
of a continuous thermal carbon overcoat and have multiple graphitic
outer layers that are substantially parallel to the fibril axis. As
such they may be characterized as having their c-axes, the axes
which are perpendicular to the tangents of the curved layers of
graphite, substantially perpendicular to their cylindrical axes.
They generally have diameters no greater than 0.1.mu. and length to
diameter ratios of at least 5. Desirably they are substantially
free of a continuous thermal carbon overcoat, i.e., pyrolytically
deposited carbon resulting from thermal cracking of the gas feed
used to prepare them. Thus, the Tennent invention provided access
to smaller diameter fibrils, typically 35 to 700 .ANG. (0.0035 to
0.070.mu.) and to an ordered, "as grown" graphitic surface.
Fibrillar carbons of less perfect structure, but also without a
pyrolytic carbon outer layer have also been grown.
[0008] The carbon nanotubes which can be oxidized as taught in this
application, are distinguishable from commercially available
continuous carbon fibers. In contrast to these fibers which have
aspect ratios (L/D) of at least 10.sup.4 and often 10.sup.6 or
more, carbon fibrils have desirably large, but unavoidably finite,
aspect ratios. The diameter of continuous fibers is also far larger
than that of fibrils, being always >1.0.mu. and typically 5 to
7.mu..
[0009] Tennent, et al., U.S. Pat. No. 5,171,560, hereby
incorporated by reference, describes carbon fibrils free of thermal
overcoat and having graphitic layers substantially parallel to the
fibril axes such that the projection of said layers on said fibril
axes extends for a distance of at least two fibril diameters.
Typically, such fibrils are substantially cylindrical, graphitic
nanotubes of substantially constant diameter and comprise
cylindrical graphitic sheets whose c-axes are substantially
perpendicular to their cylindrical axis. They are substantially
free of pyrolytically deposited carbon, have a diameter less than
0.1.mu. and length to diameter ratio of greater than 5. These
fibrils can be oxidized by the methods of the invention.
[0010] When the projection of the graphitic layers on the nanotube
axis extends for a distance of less than two nanotube diameters,
the carbon planes of the graphitic nanotube, in cross section, take
on a herring bone appearance. These are termed fishbone fibrils.
Geus, U.S. Pat. No. 4,855,091, hereby incorporated by reference,
provides a procedure for preparation of fishbone fibrils
substantially free of a pyrolytic overcoat. These carbon nanotubes
are also useful in the practice of the invention.
[0011] Carbon nanotubes of a morphology similar to the
catalytically grown fibrils described above have been grown in a
high temperature carbon arc (Iijima, Nature 354, 56, 1991). It is
now generally accepted (Weaver, Science 265, 1994) that these
arc-grown nanofibers have the same morphology as the earlier
catalytically grown fibrils of Tennent. Arc grown carbon nanofibers
after colloquially referred to as "bucky tubes", are also useful in
the invention.
[0012] Carbon nanotubes differ physically and chemically from
continuous carbon fibers which are commercially available as
reinforcement materials, and from other forms of carbon such as
standard graphite and carbon black. Standard graphite, because of
its structure, can undergo oxidation to almost complete saturation.
Moreover, carbon black is amorphous carbon generally in the form of
spheroidal particles having a graphene structure, carbon layers
around a disordered nucleus. The differences make graphite and
carbon black poor predictors of nanotube chemistry.
[0013] Aggregates of Carbon Nanotubes and Assemblages
[0014] As produced carbon nanotubes may be in the form of discrete
nanotubes, aggregates of nanotubes or both.
[0015] Nanotubes are prepared as aggregates having various
morphologies (as determined by scanning electron microscopy) in
which they are randomly entangled with each other to form entangled
balls of nanotubes resembling bird nests ("BN"); or as aggregates
consisting of bundles of straight to slightly bent or kinked carbon
nanotubes having substantially the same relative orientation, and
having the appearance of combed yarn ("CY") e.g., the longitudinal
axis of each nanotube (despite individual bends or kinks) extends
in the same direction as that of the surrounding nanotubes in the
bundles; or, as, aggregates consisting of straight to slightly bent
or kinked nanotubes which are loosely entangled with each other to
form an "open net" ("ON") structure. In open net structures the
extent of nanotube entanglement is greater than observed in the
combed yarn aggregates (in which the individual nanotubes have
substantially the same relative orientation) but less than that of
bird nest.
[0016] The morphology of the aggregate is controlled by the choice
of catalyst support. Spherical supports grow nanotubes in all
directions leading to the formation of bird nest aggregates. Combed
yarn and open nest aggregates are prepared using supports having
one or more readily cleavable planar surfaces, e.g., an iron or
iron-containing metal catalyst particle deposited on a support
material having one or more readily cleavable surfaces and a
surface area of at least 1 square meters per gram. Moy et al., U.S.
application Ser. No. 08/469,430 entitled "Improved Methods and
Catalysts for the Manufacture of Carbon Fibrils", filed Jun. 6,
1995, hereby incorporated by reference, describes nanotubes
prepared as aggregates having various morphologies (as determined
by scanning electron microscopy).
[0017] Further details regarding the formation of carbon nanotube
or nanofiber aggregates may be found in the disclosure of U.S. Pat.
No. 5,165,909 to Tennent; U.S. Pat. No. 5,456,897 to Moy et al.;
Snyder et al., U.S. patent application Ser. No. 07/149,573, filed
Jan. 28, 1988, and PCT Application No. US89/00322, filed Jan. 28,
1989 ("Carbon Fibrils") WO 89/07163, and Moy et al., U.S. patent
application Ser. No. 413,837 filed Sep. 28, 1989 and PCT
Application No. US90/05498, filed Sep. 27, 1990 ("Battery") WO
91/05089, and U.S. application Ser. No. 08/479,864 to Mandeville et
al., filed Jun. 7, 1995 and U.S. application Ser. No. 08/284,917,
filed Aug. 2, 1994 and U.S. application Ser. No. 08/320,564, filed
Oct. 11, 1994 by Moy et al., all of which are assigned to the same
assignee as the invention here and are hereby incorporated by
reference.
[0018] Nanotube mats or assemblages have been prepared by
dispersing nanofibers in aqueous or organic mediums and then
filtering the nanofibers to form a mat or assemblage. The mats have
also been prepared by forming a gel or paste of nanotubes in a
fluid, e.g. an organic solvent such as propane and then heating the
gel or paste to a temperature above the critical temperature of the
medium, removing the supercritical fluid and finally removing the
resultant porous mat or plug from the vessel in which the process
has been carried out. See, U.S. patent application Ser. No.
08/428,496 entitled "Three-Dimensional Macroscopic Assemblages of
Randomly Oriented Carbon Fibrils and Composites Containing Same" by
Tennent et al., which has issued as U.S. Pat. No. 5,691,054 on Nov.
25, 1997, hereby incorporated by reference.
[0019] Oxidation of Fibrils
[0020] McCarthy et al., U.S. patent application Ser. No. 08/329,774
filed Oct. 27, 1994, hereby incorporated by reference, describes
processes for oxidizing the surface of carbon fibrils that include
contacting the fibrils with an oxidizing agent that includes
sulfuric acid (H.sub.2SO.sub.4) and potassium chlorate (KClO.sub.3)
under reaction conditions (e.g., time, temperature, and pressure)
sufficient to oxidize the surface of the fibril. The fibrils
oxidized according to the processes of McCarthy, et al. are
non-uniformly oxidized, that is, the carbon atoms are substituted
with a mixture of carboxyl, aldehyde, ketone, phenolic and other
carbonyl groups.
[0021] Fibrils have also been oxidized non-uniformly by treatment
with nitric acid. International Application PCT/US94/10168 filed on
Sep. 9, 1994 as WO95/07316. discloses the formation of oxidized
fibrils containing a mixture of functional groups. Hoogenvaad, M.
S., et al. ("Metal Catalysts supported on a Novel Carbon Support",
Presented at Sixth International Conference on Scientific Basis for
the Preparation of Heterogeneous Catalysts, Brussels, Belgium,
September 1994) also found it beneficial in the preparation of
fibril-supported precious metals to first oxidize the fibril
surface with nitric acid. Such pretreatment with acid is a standard
step in the preparation of carbon-supported noble metal catalysts,
where, given the usual sources of such carbon, it serves as much to
clean the surface of undesirable materials as to functionalize
it.
[0022] In published work, McCarthy and Bening (Polymer Preprints
ACS Div. of Polymer Chem. 30 (1)420(1990)) prepared derivatives of
oxidized fibrils in order to demonstrate that the surface comprised
a variety of oxidized groups. The compounds they prepared,
phenylhydrazones, haloaromaticesters, thallous salts, etc., were
selected because of their analytical utility, being, for example,
brightly colored, or exhibiting some other strong and easily
identified and differentiated signal. These compounds were not
isolated and are, unlike the derivatives described herein, of no
practical significance.
[0023] Fisher et al., U.S. Ser. No. 08/352,400 filed Dec. 8, 1994,
Fisher et al., U.S. Ser. No. 08/812,856 filed Mar. 6, 1997, Tennent
et al., U.S. Ser. No. 08/856,657 filed May 15, 1997, Tennent, et
al., U.S. Ser. No. 08/854,918 filed May 13, 1997 and Tennent et
al., U.S. Ser. No. 08/857,383 filed May 15, 1997 all hereby
incorporated by reference describe processes for oxidizing the
surface of carbon fibrils that include contacting the fibrils with
a strong oxidizing agent such as a solution of alkali metal
chlorate in a strong acid such as sulfuric acid.
[0024] Additionally, these applications also describe methods of
uniformly functionalizing carbon fibrils by sulfonation,
electrophilic addition to deoxygenated fibril surfaces or
metallation. Sulfonation of the fibrils can be accomplished with
sulfuric acid or SO.sub.3 in vapor phase which gives rise to carbon
fibrils bearing appreciable amounts of sulfones so much so that the
sulfone functionalized fibrils show a significant weight gain. U.S.
Pat. No. 5,346,683 to Green, et al. describes uncapped and thinned
carbon nanotubes produced by reaction with a flowing reactant gas
capable of reacting selectively with carbon atoms in the capped end
region of arc grown nanotubes.
[0025] U.S. Pat. No. 5,641,466 to Ebbesen et al. describes a
procedure for purifying a mixture of arc grown arbon nanotubes and
impurity carbon materials such as carbon nanoparticles and possibly
amorphous carbon by heating the mixture in the presence of an
oxidizing agent at a temperature in the range of 600.degree. C. to
1000.degree. C. until the impurity carbon materials are oxidized
and dissipated into gas phase.
[0026] In a published article Ajayan and Iijima (Nature 361, p.
334-337 (1993)) discuss annealing of carbon nanotubes by heating
them with oxygen in the presence of lead which results in opening
of the capped tube ends and subsequent filling of the tubes with
molten material through capillary action.
[0027] In other published work, Haddon and his associates
((Science, 282, 95 (1998) and J. Mater. Res., Vol. 13, No. 9, 2423
(1998)) describe treating single-walled carbon nanotube materials
(SWNTM) with dichlorocarbene and Birch reduction conditions in
order to incorporate chemical functionalities into SWNTM.
Derivatization of SWNT with thionyl chloride and octadecylamine
rendered the SWNT soluble in common organic solvents such as
chloroform, dichlororomethane, aromatic solvents and CS.sub.2.
[0028] Functionalized Nanotubes
[0029] Functionalized nanotubes have been generally discussed in
U.S. Ser. No. 08/352,400 filed on Dec. 8, 1994 and in U.S. Ser. No.
08/856,657 filed May 15, 1997, both incorporated herein by
reference. In these applications the nanotube surfaces are first
oxidized by reaction with strong oxidizing or other environmentally
unfriendly chemical agents. The nanotube surfaces may be further
modified by reaction with other functional groups. The nanotube
surfaces have been modified with a spectrum of functional groups so
that the nanotubes could be chemically reacted or physically bonded
to chemical groups in a variety of substrates.
[0030] Complex structures of nanotubes have been obtained by
linking functional groups on the tubes with one another by a range
of linker chemistries.
[0031] Representative functionalized nanotubes broadly have the
formula
[C.sub.nH.sub.L--]R.sub.m
[0032] where n is an integer, L is a number less than 0.1 n, m is a
number less than 0.5n,
[0033] each of R is the same and is selected from SO.sub.3H, COOH,
NH.sub.2, OH, O, CHO, CN, COCl, halide, COSH, SH, R', COOR', SR',
SiR'.sub.3, SiOR'.paren close-st.R'.sub.3-y, SiO--SiR'.sub.2.paren
close-st.OR', R", Li, AIR'.sub.2, Hg--X, TIZ.sub.2 and Mg--X,
[0034] y is an integer equal to or less than 3,
[0035] R' is alkyl, aryl, heteroaryl, cycloalkyl aralkyl or
heteroaralkyl,
[0036] R" is fluoroalkyl, fluoroaryl, fluorocycloalkyl,
fluoroaralkyl or cycloaryl,
[0037] X is halide, and
[0038] Z is carboxylate or trifluoroacetate.
[0039] The carbon atoms, C.sub.n, are surface carbons of the
nanofiber.
[0040] There are many drawbacks associated with the methods now
available to provide oxidized carbon nanotubes. For example, one
disadvantage of using strong acid treatment is the generation of
environmentally harmful wastes. Treating such wastes increases the
production costs of the products in which oxidized nanotubes can be
used, such as electrodes and capacitors.
[0041] It would, therefore, be desirable to provide methods of
oxidizing carbon nanotubes which do not use or generate
environmentally hazardous chemicals, and which can be scaled up
easily and inexpensively.
[0042] While many uses have been found for carbon nanotubes and
aggregates of carbon nanotubes, as described in the patents and
patent applications referred to above, many different and important
uses may still be developed if the nanotubes surfaces are oxidized.
Oxidation permits interaction of the oxidized nanotubes with
various substrates to form unique compositions of matter with
unique properties and permits structures of carbon nanotubes to be
created based on linkages between the functional sites on the
surfaces of the carbon nanotubes.
OBJECTS OF THE INVENTION
[0043] It is, therefore, a primary object of this invention to
provide methods of oxidizing multiwalled carbon nanotubes having a
diameter no greater than 1 micron.
[0044] It is a further and related object to provide methods of
oxidizing multiwalled carbon nanotubes by utilizing environmentally
benign oxidizing agents such as CO.sub.2, O.sub.2, steam, H.sub.2O,
No, NO.sub.2, O.sub.3 and ClO.sub.2.
[0045] It is a further object to provide methods of producing a
network of multiwalled carbon nanotubes oxidized by the methods of
the invention.
[0046] It is still a further object to provide methods for
preparing rigid porous structures from oxidized multiwalled
nanotubes.
[0047] It is still a further object to provide an electrochemical
capacitor having at least one electrode prepared from multiwalled
carbon nanotubes oxidized according to methods of the
invention.
SUMMARY OF THE INVENTION
[0048] The present invention, which addresses the needs of the
prior art provides methods of oxidizing multiwalled carbon
nanotubes having a diameter no greater than 1 micron.
[0049] More specifically, it has now been found that multiwalled
nanotubes can be oxidized by contacting them with a gas-phase
oxidizing agent at defined temperatures and pressures. The
gas-phase oxidizing agents of the invention include CO.sub.2,
O.sub.2, steam, N.sub.2O, NO, NO.sub.2, O.sub.3, ClO.sub.2 and
mixtures thereof. Near critical and supercritical water can also be
used as oxidizing agents. The oxidized multiwalled carbon nanotubes
prepared according to methods of the invention include carbon and
oxygen containing moieties, such as carbonyl, carboxyl, aldehyde,
ketone, hydroxy, phenolic, esters, lactones and derivatives
thereof.
[0050] The multiwalled carbon nanotubes oxidized according to
methods of the present invention can be subjected to a secondary
treatment step whereby the oxygen containing moieties of the
oxidized nanotubes react with suitable reactants to add at least a
secondary group onto the surface of the oxidized nanotubes.
[0051] As a result of the present invention multiwalled carbon
nanotubes oxidized according to methods of the invention are
provided which are also useful in preparing a network of carbon
nanotubes, a rigid porous structure or as starting material for
electrodes utilized in electrochemical capacitors.
[0052] Electrochemical capacitors assembled from electrodes made
from the oxidized multiwalled carbon nanotubes of the invention
exhibit enhanced electrochemical characteristics, such as specific
capacitance.
[0053] Other improvements which the present invention provides over
the prior art will be identified as a result of the following
description which sets forth the preferred embodiments of the
present invention. The description is not in any way intended to
limit the scope of the present invention, but rather only to
provide a working example of the present preferred embodiments. The
scope of the present invention will be pointed out in the appended
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0054] FIG. 1 is a schematic illustration of a quartz reactor used
to carry out gas phase oxidation.
[0055] FIG. 2 is an SEM micrograph illustrating aggregates of
multiwalled carbon nanotubes oxidized according to the invention at
.times.3000 magnification.
[0056] FIG. 3 is an SEM micrograph illustrating aggregates of
multiwalled carbon nanotubes oxidized according to the invention at
.times.50,000 magnification.
[0057] FIG. 4 is an SEM micrograph illustrating aggregates of
multiwalled carbon nanotubes oxidized according to the invention at
.times.10,000 magnification.
[0058] FIG. 5 is an SEM micrograph illustrating the tip portion of
an aggregate of multiwalled carbon nanotubes oxidized according to
the invention at .times.50,000 magnification.
[0059] FIGS. 6A to 6C are each a complex-plane impedance plot, a
Bode impedance plot, and a Bode angle plot, respectively, recorded
from an electrochemical capacitor fabricated from electrodes
prepared from multiwalled carbon nanotubes oxidized according to
methods of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0060] Definitions
[0061] The terms "nanotube", "nanofiber" and "fibril" are used
interchangeably. Each refers to an elongated hollow structure
having a cross section (e.g. angular fibers having edges) or a
diameter (e.g. rounded) less than 1 micron. The term "nanotube also
includes "buckytubes", and fishbone fibrils.
[0062] "Multiwalled nanotubes" as used herein refers to carbon
nanotubes which are substantially cylindrical, graphitic nanotubes
of substantially constant diameter and comprise cylindrical
graphitic sheets or layers whose c-axes are substantially
perpendicular to their cylindrical axis, as also described in U.S.
Pat. No. 5,171,560 to Tennent, et al..
[0063] The term "functional group" refers to groups of atoms that
give the compound or substance to which they are linked
characteristic chemical and physical properties.
[0064] A "functionalized" surface refers to a carbon surface on
which chemical groups are adsorbed or chemically attached.
[0065] "Graphenic" carbon is a form of carbon whose carbon atoms
are each linked to three other carbon atoms in an essentially
planar layer forming hexagonal fused rings. The layers are
platelets only a few rings in diameter or they may be ribbons, many
rings long but only a few rings wide.
[0066] "Graphenic analogue" refers to a structure which is
incorporated in a graphenic surface.
[0067] "Graphitic" carbon consists of grapheric layers which are
essentially parallel to one another and no more than 3.6 angstroms
apart.
[0068] The term "aggregate" refers to a dense, microscopic
particulate structure comprising entangled carbon nanotubes.
[0069] The term "micropore" refers to a pore which has a diameter
of less than 2 nanometers.
[0070] The term "mesopore" refers to pores having a cross section
greater than 2 nanometers and less than 50 nanometers.
[0071] The term "surface area" refers to the total surface area of
a substance measurable by the BET technique.
[0072] The term "accessible surface area" refers to that surface
area not attributed to micropores (i.e., pores having diameters or
cross-sections less than 2 ram).
[0073] The term "isotropic" means that all measurements of a
physical property within a plane or volume of the structure,
independent of the direction of the measurement, are of a constant
value. It is understood that measurements of such non-solid
compositions must be taken on a representative sample of the
structure so that the average value of the void spaces is taken
into account.
[0074] The term "physical property" means an inherent, measurable
property, e.g., surface area, resistivity, fluid flow
characteristics, density, porosity, and the like.
[0075] The term "relatively" means that ninety-five percent of the
values of the physical property when measured along an axis of, or
within a plane of or within a volume of the structure, as the case
may be, will be within plus or minus 20 percent of a mean
value.
[0076] The term "substantially" means that ninety-five percent of
the values of the physical property when measured along an axis of,
or within a plane of or within a volume of the structure, as the
case may be, will be within plus or minus ten percent of a mean
value.
[0077] The terms "substantially isotropic" or "relatively
isotropic" correspond to the ranges of variability in the values of
physical properties set forth above.
[0078] The term "predominantly" has the same meaning as the term
"substantially".
[0079] Methods of Oxidizing Carbon Nanotubes and Aggregates of
Carbon Nanotubes
[0080] The present invention provides methods of oxidizing the
surface of carbon nanotubes. The resulting oxidized nanotubes can
be easily dispersed in both organic and inorganic solvents, and
especially in water. The surface-oxidized nanotubes obtained by the
methods of the present invention can be placed in matrices of other
materials, such as plastics, or made into structures useful in
catalysis, chromatography, filtration systems, electrodes,
capacitors and the like.
[0081] The carbon nanotubes useful for the methods of the present
invention have been more specifically described above under the
heading "Carbon Nanotubes," and they are preferably prepared
according to U.S. application Ser. No. 08/459,534 filed Jun. 2,
1995 assigned to Hyperion Catalysis International, Inc. of
Cambridge, Mass., incorporated herein by reference.
[0082] The carbon nanotubes preferably have diameters no greater
than one micron, more preferably no greater than 0.2 micron. Even
more preferred are carbon nanotubes having diameters between 2 and
100 nanometers, inclusive. Most preferred are carbon nanotubes
having diameters between 3.5 and 75 nanometers, inclusive.
[0083] The nanotubes are substantially cylindrical, graphitic
carbon fibrils of substantially constant diameter and are
substantially free of pyrolytically deposited carbon. The nanotubes
include those having a length to diameter ratio of greater than 5
with the projection of the graphite layers on the nanotubes
extending for a distance of at least two nanotube diameters. Most
preferred are multiwalled nanotubes as described in U.S. Pat. No.
5,171,560 to Tennent, et al, incorporated herein by reference.
[0084] The methods of the invention include contacting the carbon
nanotubes with a gas-phase oxidizing agent under conditions
sufficient to oxidize the surface of the carbon nanotubes, and
especially the external side walls of the carbon nanotubes.
[0085] Compounds useful as gas-phase oxidizing agents are
commercially readily available and include carbon dioxide, oxygen,
steam, N.sub.2O, NO, NO.sub.2, ozone, ClO.sub.2 and mixtures
thereof In a preferred embodiment the gas-phase oxidizing agents
can be diluted with inert gases such as nitrogen, noble gases and
mixtures thereof. The dilution reduces the partial pressure of the
oxidant to the range of 1 to 760 torr.
[0086] Suitable conditions for oxidizing the carbon nanotubes of
the invention include a temperature range from about 200.degree. C.
to about 600.degree. C. whenever the oxidizing agent is oxygen,
ozone, N.sub.2O, NO, NO.sub.2, ClO.sub.2 or mixtures thereof. The
mass molecular weight of the oxidizing agents of the present
invention does not exceed 70 g/mole. When the oxidizing agent is
carbon dioxide or steam, the treatment of the carbon nanotubes with
the gas-phase oxidizing agent is preferably accomplished in a
temperature range from about 400.degree. C. to about 900.degree. C.
Useful partial pressures of the oxidizing agent for the methods of
the present invention include contacting of the carbon nanotubes
with the gas-phase oxidizing agents of the invention in a range
from about 1 torr to about 10 atm or 7600 torr, preferably 5 torr
to 760 torr.
[0087] In one aspect of the invention the gas-phase oxidizing agent
is near critical or supercritical water. Supercritical water refers
to water above its critical temperature of 374.degree. C. At this
temperature, retention of a condensed phase requires a pressure in
excess of 3200 psia. It is well known that supercritical water
exhibits anomalously low viscosity, thus enabling it to penetrate
aggregates.
[0088] In the vicinity of the critical point, viscosity, and, in
fact, most of the thermodynamic and transport properties of a
compound, correlate with specific volume. Viscosities useful in
practicing the invention can also be achieved in near critical
water having a specific volume up to twice its critical specific
volume of 0.05 ft3/lb or up to 0.10 ft3/lb. While this range of
specific volumes can be achieved by various combinations of near
critical temperature and pressure, at saturation, this corresponds
to a temperature of 363.degree. C. and a pressure of 3800 psia.
[0089] A useful period of time for contacting of the carbon
nanotubes or aggregates of carbon nanotubes, with the gas-phase
oxidizing agents of the invention is from 0.1 hours to about 24
hours, preferably from about 1 hour to about 8 hours, and most
preferably for about 24 hours.
[0090] The present invention provides economical, environmentally
benign methods to oxidize the surface of the multiwalled carbon
nanotubes. While not wishing to be bound by theory, it is believed
that when treating the carbon nanotubes with the oxidizing agents
of the invention oxygen-containing moieties are introduced onto the
surface side walls of the carbon nanotubes. The oxidized nanotubes
include moieties such as carbonyl, carboxyl, aldehyde, phenol,
hydroxy, esters, lactones and mixtures thereof. Specifically
excluded are moieties in which oxygen is not directly bonded to
carbon. For example, the use of SO.sub.3 vapor results in the
sulfonation of the carbon nanotubes whereby sulfur containing
moieties are introduced onto the surface of the nanotubes.
Sulfonated nanotubes exhibit a significant weight gain by
comparison to non-sulfonated carbon nanotubes.
[0091] It has been unexpectedly found that upon treatment with the
oxidizing agents of the present invention, the oxidized nanotubes
have experienced a weight loss rather than gain. For example, it
has been found that the carbon nanotubes experienced a weight loss
from about 1% to about 60% by weight and preferably from about 2%
to about 15% by weight by comparison to the unoxidized carbon
nanotubes.
[0092] While it is not intended to be bound by theory, it has been
well established that the edge carbon of a graphite sheet is much
more susceptible to chemical reaction than the basal plane carbon.
The carbon nanotubes useful in the present invention have a tubular
structure resembling buckytubes. On the surface along the axis, the
carbon atoms have the characteristics of basal plane graphite
except for those associated with defect sites. However, the carbon
atoms at the end of a nanotube are either edge carbons or carbons
associated with high-energy bonds, like members of a five-carbon
ring or atoms attached to a catalyst particles. All of these
carbons are much more susceptible to chemical attack. Thus, upon
treatment with the oxidizing agents of the invention the nanotubes
may become shortened and surface carbon layers may be partially
stripped.
[0093] The oxidized nanotubes produced by the methods of the
invention exhibit upon titration an acid titer of from about 0.05
meq/g to about 0.6 meq/g and preferably from about 0.1 meq/g to
about 0.4 meq/g. For example, the content of carboxylic acid is
determined by reacting an amount of 0.1 N NaOH in excess of the
anticipated titer with the sample and then back titrating the
resulting slurry with 0.1N HCl to an end point determined
potentiometrically at pH7.
[0094] Another aspect of the invention relates to treating
aggregates of carbon nanotubes with the gas-phase oxidizing agents.
The aggregates treated according to the invention display a
macromorphology which can be described as "loose bundles" having
the appearance of a severely weathered rope. The nanotubes
themselves retain a morphology similar to the as-synthesized
nanotubes, however, with oxygen-containing moieties attached to the
nanotube surfaces. While it is not intended to be bound by theory,
it is believed that in the case of aggregates, the chemical bonding
between the catalyst plate which defines the size of the bundles
and the nanotubes is eliminated. In addition, the nanotubes may
exhibit shortening and carbon layers are believed to become
partially stripped. An increase in specific surface area has also
been observed. For example, untreated aggregates have a specific
surface area of about 250 m.sup.2/gm, while oxidized aggregates
display a specific surface area up to 400 m.sup.2/gm. The foregoing
changes which take place upon oxidiation with the oxidizing agents
of the invention have been observed by scanning electron microscopy
(SEM), transmission electron microscopy (TEM) and surface area
measurement. SEM photographs shown in FIGS. 2-5 taken after
treatment of aggregates with the oxidizing agents of the invention
support the structured changes of the nanotubes discussed above.
Specifically, FIGS. 3 and 5 show many shortened and separated
nanotube ends which can be seen at the ends of and on the surface
of the "loose bundles" of oxidized nanotube aggregates. More
importantly, FIGS. 3 and 5 also show that the macrostructure or
macromorphology and average diameters of the aggregates (combed
yarn in FIG. 5) remain almost unchanged by comparison to those of
the unoxidized aggregates. Similarly, the oxidized aggregates
retain the original loose powder form of the unoxidized aggregates.
As a result of the addition of oxygen containing moieties on the
surface of oxidized carbon nanotubes and the retention of their
macrostructure, the gas-phase oxidized nanotubes obtained by
methods of the invention have shown increased dispersion in polar
solvents.
[0095] Gas phase oxidized nanotubes can also be used in the
production of high quality extrudates which can be formed by using
a small amount of water soluble binder. In the preparation of
extrudates, the oxidized surface of the nanotubes allows for
improved binder dispersion during the mixing stage and minimizes
the segregation of binder in the subsequent heating step.
[0096] Secondary Derivatives of Oxidized Nanotubes
[0097] Advantageously, the oxidized nanotubes obtained by the
oxidizing methods of the invention can be further treated. In one
embodiment of the invention after the oxidized nanotubes are
formed, they may be further treated in a secondary treatment step,
by contacting with a reactant suitable to react with moieties of
the oxidized nanotubes thereby adding at least another secondary
functional group. Secondary derivatives of the oxidized nanotubes
are essentially limitless. For example, oxidized nanotubes bearing
acidic groups like --COOH are convertible by conventional organic
reactions to virtually any desired secondary group, thereby
providing a wide range of surface hydrophilicity or
hydrophobicity.
[0098] The secondary group that can be added by reacting with the
moieties of the oxidized nanotubes include but are not limited to
alkyl/aralkyl groups having from 1 to 18 carbons, a hydroxyl group
having from 1 to 18 carbons, an amine group having from 1 to 18
carbons, alkyl aryl silanes having from 1 to 18 carbons and
fluorocarbons having from 1 to 18 carbons. Other appropriate
secondary groups that can be attached to the moieties present on
the oxidized nanotubes include a protein, a peptide, an enzyme, an
antibody, a nucleotide peptide, an oligonucleotide, an antigen or
an enzyme substrate, enzyme inhibitor or the transition state
analog of an enzyme substrate.
[0099] Other Structures
[0100] The invention is also in methods for producing a network of
carbon nanotubes comprising treating carbon nanotubes with a gas
phase oxidizing agent of the invention for a period of time
sufficient to oxidize the surface of the carbon nanotubes,
contacting the oxidized carbon nanotubes with a reactant suitable
for adding a secondary functional group to the surface of the
carbon nanotube, and further contacting the secondarily treated
nanotubes with a cross-linking agent effective for producing a
network of carbon nanotubes. A preferred cross-linking agent is a
polyol, polyamine or polycarboxylic acid. A useful polyol is a diol
and a useful polyamine is a diamine.
[0101] In one aspect of the invention a network of carbon nanotubes
is obtained by first oxidizing the as-produced carbon nanotubes
with the gas-phase oxidizing agents of the invention, followed by
subjecting the oxidized nanotubes to conditions which foster
crosslinking. For example, heating the oxidized nanotubes in a
temperature range from 180.degree. C. to 450.degree. C. resulted in
crosslinking the oxidized nanotubes together with elimination of
the oxygen containing moieties of the oxidized nanotubes.
[0102] The invention also includes three-dimensional networks
formed by linking the surface-modified nanotubes of the invention.
These complexes include at least two surface-modified nanotubes
linked by one or more linkers comprising a direct bond or chemical
moiety. These networks comprise porous media of remarkably uniform
equivalent pore size. They are useful as adsorbents, catalyst
supports and separation media.
[0103] Three Dimensional Structures
[0104] The oxidized nanotubes of the invention are more easily
dispersed in aqueous media than unoxidized nanotubes. Stable,
porous 3-dimensional structures with meso- and macropores
(pores>2 nm) are very useful as catalysts or chromatography
supports. Since nanotubes can be dispersed on an individualized
basis, a well-dispersed sample which is stabilized by cross-links
allows one to construct such a support. Surface-oxidized nanotubes
are ideal for this application since they are easily dispersed in
aqueous or polar media and the oxygen-containing moieties present
on the oxidized nanotubes provide cross-link points. Additionally,
the oxygen-containing moieties also provide points to support the
catalytic or chromatographic sites. The end result is a rigid,
3-dimensional structure with its total surface area accessible with
secondary group sites on which to support the active agent.
[0105] Although the interstices between these nanotubes are
irregular in both size and shape, they can be thought of as pores
and characterized by the methods used to characterize porous media.
The size of the interstices in such networks can be controlled by
the concentration and level of dispersion of nanotubes, and the
concentration and chain lengths of the cross-linking agents. Such
materials can act as structured catalyst supports and may be
tailored to exclude or include molecules of a certain size. Aside
from conventional industrial catalysis, they have special
applications as large pore supports for biocatalysts.
[0106] Typical applications for these supports in catalysis include
their use as a highly porous support for metal catalysts laid down
by impregnation, e.g., precious metal hydrogenation catalysts.
Moreover, the ability to anchor molecular catalysts by tether to
the support via the secondary groups combined with the very high
porosity of the structure allows one to carry out homogeneous
reactions in a heterogeneous manner. The tethered molecular
catalyst is essentially dangling in a continuous liquid phase,
similar to a homogeneous reactor, in which it can make use of the
advantages in selectivities and rates that go along with
homogeneous reactions. However, being tethered to the solid support
allows easy separation and recovery of the active, and in many
cases, very expensive catalyst.
[0107] These stable, rigid structures also permits carrying out
heretofore very difficult reactions, such as asymmetric syntheses
or affinity chromatography by attaching a suitable enantiomeric
catalyst or selective substrate to the support. The rigid networks
can also serve as the backbone in biomimetic systems for molecular
recognition. Such systems have been described in U.S. Pat. No.
5,110,833 and International Patent Publication No. WO93/19844. The
appropriate choices for cross-linkers and complexing agents allow
for stabilization of specific molecular frameworks.
[0108] Methods of Preparing Rigid Porous Structures
[0109] In one aspect of the invention rigid porous structures are
prepared by first preparing surface-oxidized nanotubes as described
above, dispersing them in a medium to form a suspension, separating
the medium from the suspension to form a porous structure, wherein
the surface-oxidized nanotubes are further interconnected to form a
rigid porous structure, all in accordance with methods more
particularly described in U.S. application Ser. No. 08/857,383
(WBAM Docket No. 0064734-0080) entitled "Rigid Porous Carbon
Structures, Methods of Making, Methods of Using and Products
Containing Same" filed on May 15, 1997, hereby incorporated by
reference.
[0110] The hard, high porosity structures can be formed from
regular carbon nanotubes or nanotube aggregates, either with or
without surface modified nanofibers (i.e., surface oxidized
nanofibers). In order to increase the stability of the nanotube
structures, it is also possible to deposit polymer at the
intersections of the structure. This may be achieved by
infiltrating the assemblage with a dilute solution of low molecular
weight polymer cement kie., less than about 1,000 MW) and allowing
the solvent to evaporate. Capillary forces will concentrate the
polymer at nanotube intersections. It is understood that in order
to substantially improve the stiffness and integrity of the
structure, only a small fraction of the nanotube intersections need
be cemented. One embodiment of the invention relates to a method of
preparing a rigid porous carbon structure having a surface area
greater than at least 100 m.sup.2/gm, comprising the steps of:
[0111] (a) dispersing a plurality of nanofibers in a medium to form
a suspension; and
[0112] (b) separating said medium from said suspension to form said
structure,
[0113] wherein said nanotubes are interconnected to form said rigid
structure of intertwined nanotubes bonded at nanotube intersections
within the structure.
[0114] The nanotubes may be uniformly and evenly distributed
throughout the structure or in the form of aggregate particles
interconnected to form the structure. When the former is desired,
the nanotubes are dispersed thoroughly in the medium to form a
dispersion of individual nanotubes. When the latter is desired,
nanotube aggregates are dispersed in the medium to form a slurry
and said aggregate particles are connected together with a gluing
agent to form said structure.
[0115] The medium used may be selected from the group consisting of
water and organic solvents. Preferably, the medium comprises a
dispersant selected from the group consisting of alcohols,
glycerin, surfactants, polyethylene glycol, polyethylene imines and
polypropylene glycol.
[0116] The medium should be selected which: (1) allows for fine
dispersion of the gluing agent in the aggregates; and (2) also acts
as a templating agent to keep the internal structure of the
aggregates from collapsing as the mix dries down.
[0117] One preferred embodiment employs a combination of
polyethylene glycol (PEG) and glycerol dissolved in water or
alcohol as the dispersing medium, and a carbonizable material such
as low MW phenol-formaldehyde resins or other carbonizable polymers
or carbohydrates (starch or sugar). Once the rigid porous structure
has been prepared, it can then be oxidized with the oxidizing
agents of the invention in preparation for use in electrochemical
capacitors, for example. The oxidation occurs in the same pressure
and temperature ranges as are used to oxidize nanotubes, aggregates
or assemblages of carbon nanotubes.
[0118] In another embodiment, if surface oxidized nanotubes are
employed, the nanotubes are oxidized prior to dispersing in the
medium and are self-adhering forming the rigid structure by binding
at the nanotube intersections. The structure may be subsequently
pyrolized to remove oxygen. A useful temperature range is from
about 200.degree. C. to about 2000.degree. C. and preferably from
about 200.degree. C. to about 900.degree. C.
[0119] According to another embodiment, the nanotubes are dispersed
in said suspension with gluing agents and the gluing agents bond
said nanotubes to form said rigid structure. Preferably, the gluing
agent comprises carbon, even more preferably the gluing agent is
selected from a material that, when pyrolized, leaves only carbon.
Accordingly, the structure formed with such a gluing may be
subsequently pyrolized to convert the gluing agent to carbon.
[0120] Preferably, the gluing agents are selected from the group
consisting of cellulose, carbohydrates, polyethylene, polystyrene,
nylon, polyurethane, polyester, polyamides and phenolic resins.
[0121] According to further embodiments of the invention, the step
of separating comprises filtering the suspension or evaporating the
medium from said suspension.
[0122] According to yet another embodiment, the suspension is a gel
or paste comprising the nanotubes in a fluid and the separating
comprises the steps of:
[0123] (a) heating the gel or paste in a pressure vessel tot
temperature above the critical temperature of the fluid;
[0124] (b) removing supercritical fluid from the pressure vessel;
and
[0125] (c) removing the structure from the pressure vessel.
[0126] Isotropic slurry dispersions of nanotube aggregates in
solvent/dispersant mixtures containing gluing agent can be
accomplished using a Waring blender or a kneader without disrupting
the aggregates. The nanotube aggregates trap the resin particles
and keep them distributed.
[0127] These mixtures can be used as is, or can be filtered to
remove sufficient solvent to obtain cakes with high nanotube
contents (5-20% dry weight basis). The cake can be molded, extruded
or pelletized. The molded shapes are sufficiently stable so that
further drying occurs without collapse of the form. On removing
solvent, disperant molecules, along with particles of gluing agent
are concentrated and will collect at nanotube crossing points both
within the nanotube aggregates, and at the outer edges of the
aggregates. As the mixture is further dried down and eventually
carbonized, nanotube strands within the aggregates and the
aggregates themselves are glued together at contact points. Since
the aggregate structures do not collapse, a relatively hard, very
porous, low density particle is formed.
[0128] As set forth above, the rigid, porous structures may also be
formed using oxidized nanotubes with or without a gluing agent.
Carbon nanotubes become self-adhering after oxidation. Very hard,
dense mats are formed by highly dispersing the oxidized nanotubes
(as individualized strands), filtering and drying. The dried mats
have densities between 1-1.2 g/cc, depending on oxygen content, and
are hard enough to be ground and sized by-sieving. Measured surface
areas are about 275 m.sup.2/g.
[0129] Substantially all the oxygen within the resulting rigid
structure can be removed by pyrolizing the particles at about
600.degree. C. in flowing gas, for example argon. Densities
decrease to about 0.7-0.9 g/cc and the surface areas increase to
about 400 m.sup.2/g. Pore volumes for the calcined particles are
about 0.9-0.6 cc/g, measured by water absorbtion.
[0130] The oxidized nanotubes may also be used in conjunction with
a gluing agent. Oxidized nanotubes are good starting materials
since they have attachment points to stick both gluing agents and
templating agents. The latter serve to retain the internal
structure of the particles or mats as they dry, thus preserving the
high porosity and low density of the original nanotube aggregates.
Good dispersions are obtained by slurrying oxidized nanotubes with
materials such as polyethyleneimine cellulose (PEI Cell), where the
basic imine functions form strong electrostatic interactions with
carboxylic acid functionalized fibrils. The mix is filtered to form
mats. Pyrolizing the mats at temperatures greater than 650.degree.
C. in an inert atmosphere converts the PEI Cell to carbon which
acts to fuse the nanotube aggregates together into hard structures.
The result is a rigid, substantially pure carbon structure, which
can then be oxidized with the oxidizing agents of the present
invention.
[0131] Solid ingredients can also be incorporated within the
structure by mixing the additives with the nanotube dispersion
prior to formation of the structure. The content of other solids in
the dry structure may be made as high as fifty parts solids per
part of nanotubes.
[0132] According to one preferred embodiment, nanotubes are
dispersed at high shear in a high-shear mixer, e.g. a Waring
Blender. The dispersion may contain broadly from 0.01 to 10%
nanotubes in water, ethanol, mineral spirits, etc.. This procedure
adequately opens nanotube bundles, i.e. tightly wound bundles of
nanotubes, and disperses the nanotubes to form self-supporting mats
after filtration and drying. The application of high shear mixing
may take up to several hours. Mats prepared by this method,
however, are not free of aggregates.
[0133] If the high shear procedure is followed by ultrasonication,
dispersion is improved. Dilution to 0.1% or less aids
ultrasonication. Thus, 200 cc of 0.1% fibrils, for example, may be
sonified by a Bronson Sonifier Probe (450 watt power supply) for 5
minutes or more to further improve the dispersion.
[0134] To achieve the highest degrees of dispersion, i.e. a
dispersion which is free or virtually free of nanotube aggregates,
sonication must take place either at very low concentration in a
compatible liquid, e.g. at 0.001% to 0.01% concentration in ethanol
or at higher concentration e.g. 0.1% in water to which a
surfactant, e.g. Triton X-100, has been added in a concentration of
about 0.5%. The mat which is subsequently formed may be rinsed free
or substantially free of surfactant by sequential additions of
water followed by vacuum filtration. The mat thus formed can then
be oxidized with the oxidizing agents of the invention under
conditions sufficient to form oxidized nanotubes within the
mat.
[0135] Particulate solids such as MnO.sub.2 (for batteries) and
Al.sub.2O.sub.3 (for high temperature gaskets) may be added to the
oxidized nanotube dispersion prior to mat formation at up to 50
parts added solids per part of nanotubes.
[0136] Reinforcing webs and scrims may be incorporated on or in the
mats during formation. Examples are polypropylene mesh and expanded
nickel screen.
[0137] Electrochemical Capacitors
[0138] Carbon nanotubes are electrically conductive. Electrodes and
their use in electrochemical capacitors comprising carbon nanotubes
and/or functionalized carbon nanotubes which have been described in
U.S. application Ser. No. 08/856,657 (WBAM Docket No. 0064736-0000)
entitled "Graphitic Nanofibers in Electrochemical Capacitors,"
filed on May 15, 1997 incorporated herein by reference.
[0139] Further details about electrochemical capacitors based on
catalytically grown carbon nanotubes are disclosed in Chumming Niu
et al., "High Power Electrochemical Capacitors based on Carbon
Nanotube Electrodes," in Applied Physics Letters 70(11), pp.
1480-1482, Mar. 17, 1997 incorporated herein by reference.
[0140] The quality of sheet electrode depends on the microstructure
of the electrode, the density of the electrode, the functionality
of the electrode surface and mechanical integrity of the electrode
structure.
[0141] The microstructures of the electrode, namely, pore size and
size distribution determines the ionic resistance of electrolyte in
the electrode. The surface area residing in micropores (pore
diameter <2 nm) is considered inaccessible for the formation of
a double layer (2). On the other hand, distributed pore sizes,
multiple-pore geometries (dead end pores, slit pores, cylindrical
pores, etc.) and surface properties usually give rise to a
distributed time constant. The energy stored in an electrode with a
distributed time constant can be accessed only with different
rates. The rapid discharge needed for pulsed power is not feasible
with such an electrode.
[0142] The density of the electrode determines its volumetric
capacitance. An electrode with density less than 0.4 g/cc is not
practical for real devices. Simply, the low-density electrode will
take up too much electrolyte, which will decrease both volumetric
and gravimetric capacitance of the device.
[0143] The surface of the carbon nanotubes is related to the
wetting properties of electrodes towards electrolytes. The surface
of as-produced, catalytically grown carbon nanotubes is
hydrophobic. It has been unexpectedly found that the hydrophobic
surface properties of the as-produced carbon nanotubes can be
changed to hydrophilic by treatment of the as-produced carbon
nanotubes or aggregates of carbon nanotubes with the oxidizing
agents of the present invention. It has also been unexpectedly
found that the dispersing properties in water of surface-oxidized
carbon nanotubes are related to weight loss during treatment with
such gas-phase oxidizing agents as CO.sub.2, O.sub.2, steam,
H.sub.2O, NO.sub.2, O.sub.3, ClO.sub.2 and mixtures thereof. For
example, oxidized nanotubes exhibiting a weight loss of about 10%
by weight can be easily dispersed in water. It is necessary to
oxidize on the surface of the carbon nanotubes to improve their
wetting properties for aqueous electrolytes. Furthermore, the
capacitance can be increased by further attaching redox groups on
the surface of the carbon nanotubes.
[0144] Finally, the structural integrity of the electrodes is
critical to reproducibility and long term stability of the device.
Mechanical strength of electrodes incorporating carbon nanotubes is
determined by the degree of entanglement of the carbon nanotube and
bonding between carbon nanotubes in the electrode. A high degree of
entanglement and carbon nanotube bonding can also improve the
conductivity, which is critical to the power performance of an
electrode. The specific capacitance (D.C. capacitance) of the
electrodes made from gas-phase treated fibrils was about 40
F/g.
[0145] One aspect of the present invention relates to preparing
electrodes and electrochemical capacitors from surface-oxidized
carbon nanotubes. Broadly, as prepared carbon nanotubes have been
treated with gas-phase oxidizing agents of the invention to provide
surface oxidized, multiwalled carbon nanotubes which can be used to
prepare the electrodes of the invention.
[0146] In another aspect of the invention, the oxidized nanotubes
can be further treated with a reactant suitable to react with
moieties present on the oxidized nanotubes to form nanotubes having
secondary groups on its surface which are also useful in preparing
the electrodes of the present invention.
[0147] Electrodes are assembled by simple filtration of slurries of
the treated nanotubes. Thickness is controlled by the quantity of
material used and the geometry, assuming the density has been
anticipated based on experience. It may be necessary to adjust
thickness to get self-supporting felts.
[0148] The electrodes are advantageously characterized by cyclic
voltammetry, conductivity and DC capacitance measurement.
EXAMPLES
[0149] The following examples serve to provide further apprection
of the invention but are not meant in any way to restrict the
effective scope of the invention.
Example 1
Oxidation of Carbon Nanotubes With Gas Phase CO.sub.2
[0150] Oxidized carbon nanotubes were prepared by using CO.sub.2 in
the gaseous phase. About 10 grams of carbon nanotubes were placed
into a reactor as shown in FIG. 1. The reactor was a heated quartz
tube having a reacting chamber connected at each end to a side
tube. The reacting chamber had an outside diameter of about 3
inches and each side tube has an outside diameter of about 1 inch.
Between the side tube at the bottom side and the reacting chamber
there was a gas permeable porous quartz plate, which supports a bed
of carbon nanotubes prepared as described in U.S. application Ser.
No. 08/459,534 filed on Jun. 2, 1995.
[0151] A stream of gaseous CO.sub.2 was continuously passed down
through the bed of carbon nanotubes at a rate of about 120 cc/min
for 2 hours at about 800.degree. C.
[0152] The degree of oxidation was measured by the weight loss
exhibited by the carbon nanotubes; a weight loss of about 10% was
recorded. The carbon nanotubes oxidized in this manner dispersed in
water quite easily whereas they hardly did so prior to treatment
with gaseous CO.sub.2.
Example 2
Oxidation of Carbon Nanotubes With Wet-air
[0153] Carbon nanotubes were oxidized by using wet air. About 10
grams of carbon nanotubes prepared according to U.S. application
Ser. No. 08/459,534 filed on Jun. 2, 1995 were charged into the
reactor described in Example 1.
[0154] Air saturated with water vapor at room temperature was
continuously passed down through the bed of carbon nanotubes at a
rate of about 120 cc/min. The temperature of the reactor, measured
by a k-type thermocouple positioned inside the bed of carbon
nanotubes, was set at 530.degree. C. The degree of oxidation was
controlled by variation of the reaction duration and monitored by
weight loss, compared to the initial weighted unoxidized carbon
nanotubes. Three samples with weight losses of 7.1, 12.4, and 68%
corresponding to 4, 5, and 8 hr oxidation, respectively, were
prepared.
Example 3
Oxidation of Carbon Nanotubes With Oxygen
[0155] Carbon nanotubes are oxidized by using oxygen in the gas
phase. About 10 grams of carbon nanotubes prepared according to
U.S. Ser. No. 08/459,534 filed on Jun. 2, 1995 are charged into a
reactor as described in Example 1.
[0156] A stream of gaseous oxygen is continuously passed down
through the bed of carbon nanotubes at a rate of about 120 cc/min
for 2 hours at 600.degree. C. The temperature of the reactor is
measured by a k-type thermocouple positioned inside the bed of
carbon nanotubes. The degree of oxidation is controlled by
variation of the reaction duration and monitored by weight loss as
compared to the initial weight of unoxidized carbon nanotubes. The
resulting weight loss is about 10%. The carbon nanotubes oxidized
in this manner disperse in water quite easily whereas they hardly
do so prior to treatment with gaseous oxygen.
Example 4
Oxidation of Carbon Nanotubes With N.sub.2O
[0157] Carbon nanotubes are oxidized by using N.sub.2O in the gas
phase. About 10 grams of carbon nanotubes prepared according to
U.S. Ser. No.08/459,534 filed on Jun. 2, 1995 are charged into a
reactor as described in Example 1.
[0158] A stream of gaseous N.sub.2O is continuously passed down
through the bed of carbon nanotubes at a rate of about 120 cc/min
for 2 hours at 600.degree. C. The temperature of the reactor is
measured by a k-type thermocouple positioned inside the bed of
carbon nanotubes. The degree of oxidation is controlled by
variation of the reaction duration and monitored by weight loss as
compared to the initial weight of unoxidized carbon nanotubes. The
resulting weight loss is about 10%. The carbon nanotubes oxidized
in this manner disperse in water quite easily whereas they hardly
do so prior to treatment with gaseous N.sub.2O.
Example 5
Oxidation of Carbon Nanotubes With NO
[0159] Carbon nanotubes are oxidized by using NO in the gas phase.
About 10 grams of carbon nanotubes prepared according to U.S. Ser.
No.08/459,534 filed on June 2, 1995 are charged into a reactor as
described in Example 1.
[0160] A stream of gaseous NO is continuously passed down through
the bed of carbon nanotubes at a rate of about 120 cc/min for 2
hours at 600.degree. C. The temperature of the reactor is measured
by a k-type thermocouple positioned inside the bed of carbon
nanotubes. The degree of oxidation is controlled by variation of
the reaction duration and monitored by weight loss as compared to
the initial weight of unoxidized carbon nanotubes. The resulting
weight loss is about 10%. The carbon nanotubes oxidized in this
manner disperse in water quite easily whereas they hardly do so
prior to treatment with gaseous NO.
Example 6
Oxidation of Carbon Nanotubes With NO.sub.2
[0161] Carbon nanotubes are oxidized by using NO.sub.2 in the gas
phase. About 10 grams of carbon nanotubes prepared according to
U.S. Ser. No.08/459,534 filed on June 2, 1995 are charged into a
reactor as described in Example 1.
[0162] A stream of gaseous oxygen is continuously passed down
through the bed of carbon nanotubes at a rate of about 120 cc/min
for 2 hours at 600.degree. C. The temperature of the reactor is
measured by a k-type thermocouple positioned inside the bed of
carbon nanotubes. The degree of oxidation is controlled by
variation of the reaction duration and monitored by weight loss as
compared to the initial weight of unoxidized carbon nanotubes. The
resulting weight loss is about 10%. The carbon nanotubes oxidized
in this manner disperse in water quite easily whereas they hardly
do so prior to treatment with gaseous NO.sub.2.
Example 7
Oxidation of Carbon Nanotubes With Ozone
[0163] Carbon nanotubes are oxidized by using ozone in the gas
phase. About 10 grams of carbon nanotubes prepared according to
U.S. Ser. No.08/459,534 filed on June 2, 1995 are charged into a
reactor as described in Example 1.
[0164] A stream of gaseous ozone is continuously passed down
through the bed of carbon nanotubes at a rate of about 120 cc/min
for 2 hours at 600.degree. C. The temperature of the reactor is
measured by a k-type thermocouple positioned inside the bed of
carbon nanotubes. The degree of oxidation is controlled by
variation of the reaction duration and monitored by weight loss as
compared to the initial weight of unoxidized carbon nanotubes. The
resulting weight loss is about 10%. The carbon nanotubes oxidized
in this manner disperse in water quite easily whereas they hardly
do so prior to treatment with gaseous ozone.
Example 8
Oxidation of Carbon Nanotubes With ClO.sub.2
[0165] Carbon nanotubes are oxidized by using ClO.sub.2 in the gas
phase. About 10 grams of carbon nanotubes prepared according to
U.S. Ser. No.08/459,534 filed on Jun. 2, 1995 are charged into a
reactor as described in Example 1.
[0166] A stream of gaseous ClO.sub.2 is continuously passed down
through the bed of carbon nanotubes at a rate of about 120 cc/min
for 2 hours at 600.degree. C. The temperature of the reactor is
measured by a k-type thermocouple positioned inside the bed of
carbon nanotubes. The degree of oxidation is controlled by
variation of the reaction duration and monitored by weight loss as
compared to the initial weight of unoxidized carbon nanotubes. The
resulting weight loss is about 10%. The carbon nanotubes oxidized
in this manner disperse in water quite easily whereas they hardly
do so prior to treatment with gaseous ClO.sub.2.
Example 9
Electrochemical Capacitors Prepared From Carbon Nanotubes Oxidized
With CO.sub.2
[0167] 0.1 g of oxidized nanotubes as prepared in Example 1 were
dispersed in deionized water to form a slurry which was then
filtered on a 3.5" diameter filter membrane to form a mat with
diameter of about 3.3". The mat was dried at 120.degree. C. for
approximately one hour and heated at 350.degree. C. in air for 4
hr. The final weight was 0.095 g. The disk electrodes with diameter
of 0.5" were made from the mat and soaked overnight in 38%
sulfuiric acid held at approximately 85.degree. C. and then kept in
the acid solution at 25.degree. C. until cell assembly. The
electrodes were wetted easily by the electrolyte. Single cell test
devices were fabricated with two 38% sulfuric acid saturated
electrodes separated by a 0.001" thick polymer separator which was
also wetted with 38% sulfuric acid. The equivalent series
resistance (E.S.R.) of the test device measured at 1 kHz using a
fixed frequency meter was 0.0430. The capacitance of the device was
measured by a constant current discharging method. The calculated
specific capacitance for the electrode was 40 F/g. The frequency
response analysis was carried out at d.c. biases of 0V, 0.5V and 1V
with a 10 mV amplitude sinusoidal signal using a Solartron model
1250B frequency analyzer driving an EG&G PAR model 273
potensiostat/galvonostat.
Example 10
Electrochemical Capacitors Prepared From Wet-air Oxidized
Nanotubes
[0168] Nanotubes oxidized as in example 2 were prepared into an
electrode according to the process described in example 3. Three
single-cell test electrochemical capacitors were fabricated from
electrodes made from the nanotubes with weight losses of 7.1, 12.4,
and 68%, respectively. Table I summarizes properties of these
electrodes and test results of the capacitors made from them. The
resistivity of the electrodes was measured using the van der Pauw
method, on samples with dimensions of 0.5 cm.times.0.5 cm having
four leads attached to their comer edges. Ohmic contact of the
leads to the samples was tested by measuring a linear I-V
curve.
[0169] Scanning Electron Microscope (SEM) studies were carried out
with a LEO 982 scanning electron microscope equipped with a
Schottky field-emission gun.
[0170] Cyclic voltammograms were recorded using an EG&G PAR
Model 273 Potentiostat/Galvonostar connected to a three-electrode
cell consisting of a fibril working electrode, a platinum gauze
counter electrode and a standard Ag/AgC1 reference electrode. The
electrolyte was 38% sulfuric acid.
[0171] The equivalent series resistance (E.S.R.) of the test
devices was measured using a fixed frequency RCL meter (Fluke
PM6303) at 1 kHz. The specific capacitance was measured by a d.c.
constant current discharging method.
[0172] Impedance analysis was carried out with a Solartron 11250
frequency response analyzer driving an EG&G PAR model 273
Potentiostat/Galvonostat at a dc bias of 0, 0,5 and 1V with 10 mV
amplitude sinusoidal signal).
[0173] Certain characteristics of the three electrodes made from
oxidized nanotubes have been summarized in Table I below.
1TABLE I Sam- Weight Thick- Density Resistivity C.sub.p ples
Treatment loss (%) ness E.S.R. (g/cc) (.OMEGA.-cm) (F/g) C.sub.p, 1
khz A Wet air, 4 hr 7.1 0.0016" 0.074 0.48 1.8 .times. 10.sup.-2
33.14 26.53 B Wet air, 5 hr 12.4 0.0015" 0.041 0.51 1.5 .times.
10.sup.-2 35.85 22 C Wet air, 8 hr 68 0.0016" 0.036 0.52 2.5
.times. 10.sup.-2 35.44 24.25 E.S.R.-Equivalent series resistance
C.sub.p-Specific capacitance C.sub.p, 1 kHz-specific capacitance at
1 kHz.
[0174] For all three devices, more than 61% of the stored energy
was available for use at a frequency of one kHz. The frequency
responses of the three devices were almost identical. FIGS. 6A-C
show frequency response analysis result of the test device
fabricated from sample 1 (Table I). The electrodes functioned like
a non-porous, planar electrode. This was evidenced (FIGS. 6A,-6C)
in the complex-plane impedance plots in which no clear "knee" point
was present, and further in the Bode angle plot, up to 10 Hz,
showing a near -90.degree. phase angle for an ideal capacitor.
[0175] As illustrated by the foregoing description and examples,
the invention has application in the formulation of a wide variety
of oxidized nanofibers.
[0176] The terms and expressions which have been employed are used
as terms of description and not of limitations, and there is no
intention in the use of such terms or expressions of excluding any
equivalents of the features shown and described as portions
thereof, it being recognized that various modifications are
possible within the scope of the invention.
* * * * *