U.S. patent application number 11/804299 was filed with the patent office on 2008-11-13 for consolidated amorphous carbon materials, their manufacture and use.
This patent application is currently assigned to Reticle, Inc.. Invention is credited to Carl C. Nesbitt, Xiaowei Sun.
Application Number | 20080277284 11/804299 |
Document ID | / |
Family ID | 27378461 |
Filed Date | 2008-11-13 |
United States Patent
Application |
20080277284 |
Kind Code |
A1 |
Nesbitt; Carl C. ; et
al. |
November 13, 2008 |
Consolidated amorphous carbon materials, their manufacture and
use
Abstract
A carbon based material produced from the consolidation of
amorphous carbon by elevated temperature compression. The material
having unique chemical and physical characteristics that lend
themselves to a broad range of applications such as in electrical,
electrochemical and structural fields.
Inventors: |
Nesbitt; Carl C.; (Hancock,
MI) ; Sun; Xiaowei; (Houghton, MI) |
Correspondence
Address: |
MICHAEL BEST & FRIEDRICH LLP
100 E WISCONSIN AVENUE, Suite 3300
MILWAUKEE
WI
53202
US
|
Assignee: |
Reticle, Inc.
|
Family ID: |
27378461 |
Appl. No.: |
11/804299 |
Filed: |
May 17, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10935014 |
Sep 7, 2004 |
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11804299 |
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10315747 |
Dec 10, 2002 |
6787235 |
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10935014 |
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09675031 |
Sep 28, 2000 |
6544648 |
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10315747 |
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09365642 |
Aug 2, 1999 |
6350520 |
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09675031 |
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60097862 |
Aug 26, 1998 |
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60097960 |
Aug 26, 1998 |
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Current U.S.
Class: |
205/80 ; 204/294;
205/687; 23/314 |
Current CPC
Class: |
Y02E 60/50 20130101;
C02F 2103/10 20130101; C25D 17/10 20130101; H01M 4/583 20130101;
Y10T 428/2918 20150115; C02F 2101/18 20130101; Y10T 428/30
20150115; H01G 11/34 20130101; Y02E 60/13 20130101; C02F 1/4691
20130101; H01M 4/0471 20130101; C04B 2235/6565 20130101; H01M
4/8663 20130101; C04B 35/52 20130101; C04B 2235/77 20130101; C02F
1/4696 20130101; Y10T 428/2913 20150115; C04B 35/6455 20130101;
H01M 4/043 20130101; C02F 2103/08 20130101; Y10T 428/2982 20150115;
C01B 32/05 20170801; C02F 2001/46133 20130101; C04B 2235/5409
20130101; C02F 1/46109 20130101; Y02E 60/10 20130101; C25C 7/02
20130101; B01J 20/20 20130101; H01G 9/145 20130101; H01G 11/42
20130101 |
Class at
Publication: |
205/80 ; 23/314;
204/294; 205/687 |
International
Class: |
C25D 5/00 20060101
C25D005/00; C09C 1/60 20060101 C09C001/60; C02F 1/461 20060101
C02F001/461; C01B 31/00 20060101 C01B031/00 |
Claims
1. An electrode formed of a processed carbon material comprising
amorphous carbon that has been consolidated under elevated
temperature and pressure, wherein the temperature is less than
1000.degree. C., and wherein the processed carbon material has a
surface area of over 800 m.sup.2/g.
2. The electrode set forth in claim 1, wherein processed carbon
material has been consolidated with a temperature in the range of
at least about 600.degree. C.
3. The electrode set forth in claim 1, wherein the processed carbon
material has been consolidated with a pressure in the range of
about 500-20,000 psi.
4. The electrode set forth in claim 1, wherein the processed carbon
material has been consolidated under elevated temperature and
pressure for a holding time in the range of about 0.5-10 hours.
5. The electrode set forth in claim 1, wherein the processed carbon
material has a surface area of over 931 m.sup.2/q.
6. The electrode set forth in claim 1, wherein the processed carbon
material has an electrical resistivity in the range of 0.040-0.150
.OMEGA.cm.
7. The electrode set forth in claim 1, wherein the processed carbon
material has a porosity in the range of 12-31%.
8. The electrode set forth in claim 1, wherein the electrode is
used in at least one of the following applications: an
ultracapacitor, a water treatment system, an electroplating
circuit, a desalination cell, a metal concentration system, a waste
treatment system, a solid-liquid separation system, a battery, a
fuel cell, and combinations thereof.
9. A method for performing electrochemistry, the method comprising:
providing a first electrode comprising activated amorphous carbon
consolidated under elevated temperature and pressure; placing the
first electrode in fluid communication with a liquid comprising at
least one of an electrolyte solution or a slurry; and applying a
potential difference between the first electrode and a reference
electrode.
10. The method set forth in claim 9, wherein the first electrode is
formed by consolidating activated amorphous carbon at temperature
of less than 1000.degree. C.
11. The method set forth in claim 9, wherein the reference
electrode includes a second electrode comprising activated
amorphous carbon consolidated under elevated temperature and
pressure.
12. The method set forth in claim 9, wherein the liquid comprises
an electrolyte solution, and further comprising removing ions from
the electrolyte solution by forming an electrical double layer
between the electrolyte solution and the first electrode.
13. The method set forth in claim 12, further comprising grounding
the first electrode to discharge the ions from the first electrode;
reapplying a potential difference between the first electrode and a
reference electrode to reuse the first electrode to perform
electrochemistry.
14. The method set forth in claim 12, wherein removing ions from
the electrolyte solution includes removing at least one of metal
ions and salt ions.
15. The method set forth in claim 12, wherein removing ions from
the electrolyte solution includes removing ions to perform at least
one of the following functions: desalinating water, deionizing
water, concentrating metal ions, and removing pollutants from at
least one of a waste stream and water.
16. The method set forth in claim 9, wherein applying a potential
difference between the first electrode and a reference electrode
includes establishing a charge separation between the first
electrode and the electrolyte solution to form an
ultracapacitor.
17. The method set forth in claim 9, wherein applying a potential
difference between the first electrode and a reference electrode
includes establishing an electroplating circuit.
18. The method set forth in claim 9, wherein the liquid comprises a
slurry having charged particles, and further comprising attracting
the charged particles to at least one of the first electrode and
the reference electrode.
19. A process for the production of an electrode formed of
consolidated amorphous carbon, the process comprising:
consolidating amorphous carbon using elevated temperature and
pressure, wherein the consolidating is performed at a temperature
of less than 1000.degree. C.
20. The process set forth in claim 19, wherein the consolidating is
performed at a temperature in the range of 600-1000.degree. C.
21. The process set forth in claim 19, wherein the consolidating is
performed at a pressure in the range of 500-20,000 psi.
22. The process set forth in claim 19, wherein the electrode has a
surface area of at least 800 m.sup.2/g.
23. The process set forth in claim 19, wherein the electrode has a
porosity of at least 12%.
24. The process set forth in claim 19, wherein the electrode has a
resistivity in the range of 0.040-0.150.OMEGA.cm.
25. The process set forth in claim 19, wherein the electrode is
used in at least one of the following applications: storing
electrical energy, desalinating water, deionizing water, recovering
metal ions, removing solid particles from a slurry, removing
pollutants from at least one of a waste stream and water,
electroplating metal ions, and combinations thereof.
26. The process set forth in claim 19, wherein the electrode is
used in at least one of a battery and a fuel cell.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 10/935,014, filed on Sep. 7, 2004, which is a
continuation of U.S. patent application Ser. No. 10/315,747, filed
Dec. 10, 2002 and issued Sep. 7, 2004 as U.S. Pat. No. 6,787,235,
which is a continuation of U.S. patent application Ser. No.
09/675,031, filed Sep. 28, 2000 and issued Apr. 8, 2003 as U.S.
Pat. No. 6,544,648, which is a divisional of U.S. patent
application Ser. No. 09/365,642, filed Aug. 2, 1999 and issued Feb.
26, 2002 as U.S. Pat. No. 6,350,520, which claimed the benefit of
U.S. Provisional Patent Application No. 60/097,862, filed Aug. 26,
1998 and U.S. Provisional Patent Application No. 60/097,960, filed
Aug. 26, 1998.
FIELD OF THE INVENTION
[0002] This invention relates to a new carbon based material, its
manufacture and use. More particularly, the invention relates to a
carbon based material produced from the consolidation of amorphous
carbon under elevated temperature compression having a broad range
of applications, such as for example, as electrode material and as
structural material.
BACKGROUND OF THE INVENTION
[0003] Carbon is a solid element that exists in many forms. Solid
carbon can have a tetrahedral crystalline array (diamond) or
hexagonal graphine planes. If the graphine planes are arranged in
planar formations, the resulting solid is known as graphite. If the
graphine planes are more randomly arranged, the resulting form of
carbon is known as amorphous carbon. Activated carbon, carbon black
and charcoal are examples of amorphous carbon. With respect to
crystallinity, graphite has short range and long range order, while
amorphous carbon has only short range order in the graphine planes.
This difference is manifested in their surface properties with
amorphous carbon being more reactive than graphite. The difference
is also manifested in the spectral patterns generated when the
material is tested by x-ray diffraction--graphite spectra show
ordered crystal patterns, while the amorphous material pattern has
no discernible pattern.
[0004] One form of amorphous carbon, activated carbon, is
manufactured from an organic source material. Typically, activated
carbon is made through carbonization of organic materials, such as
wood, coal, pitch, coconut shells, petroleum, animal bones, etc.,
followed by an activation process. During the activation process,
some of the surface platelets are burned out leaving behind many
pores with different shapes and sizes, hence activated carbon with
an increased surface area and porosity is generated. In general,
the pore size plays a role in determining the properties of the
activated carbon for various applications. According to IUPAC
definitions, pores can be characterized as macropores with pore
diameters above 50 nm, mesopores with pore diameters between 2-50
nm, and micropores with pore diameters below 2 nm. In addition to
its porosity, activated carbon is conductive and usually inert in
many aqueous and organic systems.
[0005] Because of its porosity, activated carbon has been widely
used in various industries as an adsorbent. The most commonly seen
applications include deodorizing, decoloring of gas or liquid phase
substances, and removing of toxic organics/inorganics from air and
water. The mining industry uses activated carbon for the recovery
of precious metals like gold from leaching solutions. Typically,
activated carbon is packed into a column through which the gas or
liquid to be treated is percolated continuously. The adsorption
process takes place at the interface between the carbon phase and
the fluid phase.
[0006] Its large specific surface area, porosity, conductivity and
inert nature make it suited for use as an electrode in
electrochemical applications such as energy storage devices and
water deionization/desalination devices. The underlying principles
of these electrochemical electrodes are rooted in the way that
dissolved ions in water behave next to charged solids. Salt
dissolves in water forming an electrolyte solution which has no net
charge, that is, the net cationic charge will exactly equal the net
anionic charge. When a charged solid (i.e., a particle, plate,
etc.) is placed in such a solution, the ions of the electrolyte
distribute in a manner that will minimize the charge density
through a layer known as the electric double layer. Counter ions
will be more concentrated within layers nearest the charged
surface, but the concentration will gradually decay to equal ion
charge in the bulk. A capacitor is formed between the charged
surface and the net zero potential of the bulk. A typical value for
this capacitance is on the order of 10 .mu.F/cm.sup.2 of surface
area.
[0007] If two electrodes are placed in an electrolyte solution with
an applied potential, the ions will partition so that the cations
will migrate to the cathode to fill one double layer, and the
anions will migrate to the anode and fill the other double layer.
The separation of the cationic and anionic species in this manner
is a means to store energy (ultracapacitors) or a means to
desalinate water (capacitive deionization). Ultracapacitors have
been studied as a potential storage mechanism in applications that
require large energy storage devices capable of rapid energy
discharge. The primary interest of these devices has been in
electric automobiles and electronic devices. Capacitive
deionization technology is recently being used in treating brackish
water and seawater.
[0008] The basic operating principles of carbon electrodes are
readily understood, but the manufacturing techniques for producing
activated carbon electrode material have been limited. Three
processes are currently used, identified by the types of materials
they employ as feedstock: granular activated carbon, carbonization
of polymers, and carbon aerogels.
[0009] Early in the 1950's, researchers started to use granular
activated carbon to make electrodes for electrochemical studies.
Because carbon particles cannot consolidate under normal
conditions, it is thought necessary to either apply high pressure
or some kind of binder to keep the carbon particles in contact in
order to form an electrode. It is difficult to make such an
electrode that is maintained under constant high pressure, the
system would be unacceptably bulky and dangerous. Thus, most
studies have been carried out on carbon electrodes with an organic
or polymeric binder mixed together with the carbon powders. The
binders can be organic polymers, clays, or inorganic chemicals.
Disadvantages exist with the use of binders to form the electrodes.
Binders block a large portion of carbon surfaces, causing some
pores to be blinded, and occlusion therefore is inevitable, thus
lowering the available surface area of the carbon. Binders also
deteriorate the conductivity of the electrodes because most binders
are themselves nonconductive. The contamination from the binders
also hinders their uses in electroanalytical applications.
[0010] Modern carbon electrodes are manufactured from phenolic
resins or other types of resins by a process in which the resin is
preformed to a certain shape then subjected to high temperatures
for extended periods of time until complete carbonization occurs.
The resulting carbon has relatively large surface area, but the
manufacturing technique requires the use of toxic and
environmentally dangerous chemicals. Often, organic solvents and
aromatic compounds, such as benzene and toluene, are evolved during
the manufacturing process. The volume of carbon formed is
considerably smaller than the original resin size which leads to
low product yield. This is a significant problem if specific
geometric shapes or sizes are required. This manufacturing
technique also has the disadvantages of high material cost and weak
material strength due to the "shrinking" of the precursor carbon at
high carbonization temperatures.
[0011] Some specific carbon electrodes are manufactured from
aerogel compounds with sol-gel technology by similarly carbonizing
organic compounds. Resorcinol-formaldehyde, for instance, can be
infiltrated into a conductive substrate or formed into a solid.
Solvents may be rinsed through the material prior to pyrolization
in an inert atmosphere, such as in argon or nitrogen. The pyrolysis
process produces a vitreous carbon material which has a high
surface area and high electrical conductivity. However, this
manufacturing technique includes extremely high manufacturing costs
and leads to the release of organic solvents such as acetone,
formaldehyde and aromatic compounds as the substrate is thermally
changed to carbon. These can pose serious health hazards to workers
near the furnaces. The final shape of the carbon materials is much
smaller than the feed material. Additional processing would be
required to produce a specific geometric shape.
[0012] Thus, there exists a need for a more efficient, less
expensive, more environmentally friendly process to manufacture
activated carbon electrodes.
[0013] With respect to ultracapacitors, in the early 1980s,
technology was developed to make an ultracapacitor of very large
capacitance, on the order of Farads. Normal capacitors have a pico-
to micro Farad capacity. As high-energy storage devices,
ultracapacitors can be used as load leveling devices for electric
and hybrid vehicles, memory backup for computers, as well as
applications in areas such as portable communications, pulse energy
systems and actuators. With the development of electrical and
electronic technology, demands for high-performance energy storage
devices have emerged and have kept growing.
[0014] The idea of ultracapacitors is based on the theory of the
electrical double layer. An electrical double layer is the ionic
layer developed at the interface between a charged solid and an
electrolyte. When a potential is applied over two electrodes in an
electrolyte solution, electrical double layers are developed and a
charge separation is obtained by building up of ions of opposite
signs with the electrode. If electrodes are polarizable, a final
charge state will be reached at equilibrium. Since an electrical
double layer is essentially a charge separation layer, it behaves
as an electrical capacitor. Accordingly to the double layer theory,
the capacitance of an electrical double layer depends on the
charges stored in the double layer and the permitivity of the
solvent within the double layer region. Typically, the specific
capacitance of a double layer is on the order of 10 .mu.F/cm.sup.2.
Much effort has been made to make ultracapacitors with various
forms of activated carbon. Although prototype and commercial
ultracapacitors have been made with activated carbon, overall
performance has not been satisfactory mainly due to the inevitable
problem of occlusion from binders used or the high cost of material
manufacturing.
[0015] With respect to capacitive deionization, by taking advantage
of the very high surface area of activated carbon, ions can be
"stored" in electrical double layers when a potential is applied
across two activated carbon electrodes, even though these ion
species have no affinity to activated carbon in the absence of the
applied potential. Once the electrodes are grounded or the polarity
is reversed, the double layers are relaxed/reversed, then the
stored ions are released back to the bulk solution. Therefore, a
coupled deionization and regeneration process can be achieved.
Previously, either an inert polymeric binder was used to form a
block electrode or a membrane was used to constrain the carbon
particles. As a consequence, the electrical and mass transfer
resistance is very high and the overall performance is poor. It is
clear that a block type electrode without a binder is greatly
desired if activated carbon is going to be used for such
electrochemical applications. It is obvious that a highly
conductive monolithic activated carbon material with high surface
area, larger macropore size and of lower cost is greatly desired
for effective desalination/deionization.
[0016] Turning now to the use of amorphous carbon in producing
structural materials, in the materials industry, few forms of
carbon are useful for fabricating parts. Graphite is most commonly
used in applications requiring conductive materials with high
strength and low density, such as in various high temperature
casting molds or electrode materials. Graphite can also be an
admixture to improve the properties of other materials. Carbon
reinforced with graphite fibers is a relatively new material that
has found broad uses in lightweight structural material, sporting
equipment, such as bicycle frames, golf clubs and tennis racquets,
and by NASA for use in space vehicles such as the shuttle. These
materials have unique high temperature strength properties which
retain stiffness and strength even at temperatures exceeding
1650.degree. C. These are very expensive materials because of the
complex manufacturing process. Carbon fibers are mixed within
resins, then pyrolyzed to generate the carbon-matrix materials
around the carbon fiber reinforcement. These materials are then
subjected to a long and complicated densification process known as
chemical vapor deposition to produce the final product.
[0017] Therefore, there exists a need for a more efficient, less
expensive process to manufacture carbon structural materials.
[0018] In the 1950's, a metallurgical process called hot isostatic
pressing (HIPing) was introduced into the area of metallurgy.
HIPing involves the isostatic application of a high pressure gas at
an elevated temperature in a specifically constructed vessel. Under
these conditions of heat and pressure, internal pores or defects
within a solid body collapse and weld up in a process known as
sintering. Encapsulated powder and sintered components alike are
densified and consolidated. It is typical to operate a HIP at
temperatures of 1000-3000.degree. C. and pressures of 25,000-60,000
psi. Cold isostatic presses (CIPs) have also been developed which
typically apply an isostatic pressure to a material at or near room
temperature.
SUMMARY OF THE INVENTION
[0019] The present invention is a novel carbon based material and
process for its production which takes advantage of the properties
of amorphous carbon to produce a vastly improved material which has
broad applications. The process incorporates consolidation of
amorphous carbon under elevated temperature compression. The
products of the process have unique chemical, electrical and
physical characteristics.
[0020] The novel carbon based material of the present invention is
versatile so as to be used in a broad range of applications such as
in the manufacture of structural materials and of electrode
materials. The process of the present invention is an inexpensive
manufacturing method that produces materials that are near net
shape or are readily machinable to specifications and the process
is effective at generating monolithic carbon material without the
use of binders, or any noxious or toxic chemicals. Carbon source
material can be selected based on any combination of properties
such as available surface area, particle size distribution, and
conductivity to produce material with optimal properties for the
specific application desired. Additionally, the process parameters
can be optimized to produce specific material properties, such as
degree of densification, internal porosity, available surface area,
or other property that the end user may require. The process of the
present invention provides for the making of large billets of
activated carbon so that production costs could be reduced.
[0021] After consolidation at elevated pressures and temperatures,
novel carbon material can be produced with desired surface areas,
porosity, density, strength and resistivity. Cyclic voltammetry
(CV) curves demonstrate that the novel material is stable over a
wide potential range in aqueous solution and therefore suitable for
electrochemical applications. A capacitive feature of the CV curves
indicates that the novel material is capable of storing a great
amount of charge. The novel material is suitable for application of
ultracapacitors. For example, test cells using electrodes of the
novel material demonstrated that the capacitor had a specific
capacitance of 53 F/g in an aqueous electrolyte and 23 F/g in an
organic electrolyte, based upon the electrode material only.
Electrodes of the novel material can be used for deionization, such
as desalination. Such electrodes are effective at removing ions at
a low energy consumption rate.
[0022] It is a feature of the present invention to provide a novel
material made of amorphous carbon consolidated under elevated
temperature and pressure.
[0023] It is another feature of the present invention to provide a
manufacturing process for the production of said novel carbon based
material.
[0024] It is another feature of the present invention to provide a
manufacturing process whose parameters can be altered to obtain the
novel material having optimized characteristics for a particular
application.
[0025] It is another feature of the present invention to provide a
said novel material for a broad range of applications.
[0026] It is another feature of the present invention to provide an
electrode made from the novel material.
[0027] It is another feature of the present invention to provide an
activated carbon electrode made from the novel material.
[0028] It is another feature of the present invention to provide
for the application of this novel material for use in the
desalination of water.
[0029] It is another feature of the present invention to provide
for the application of this novel material for use in
ultracapacitors.
[0030] It is another feature of the present invention to provide
the application of this novel material for use in the removal of
solids from water in a manner of dewatering slurries or separating
different solids in suspensions.
[0031] It is another feature of the present invention to provide
the application of this novel material for use in the direct
electroplating of metal from aqueous and non-aqueous electrolyte
solutions.
[0032] It is another feature of the present invention to provide
the application of this novel material in the deionization of
water.
[0033] It is another feature of the present invention to provide
the application of this novel material in environmental processing
for the direct electrochemical destruction of pollutants and
contaminants such as from water.
[0034] It is another feature of the present invention to provide
the application of this novel material in water treatment, such as
water softening and pH control.
[0035] It is another feature of the present invention to provide
said novel material for use as a carbon structural material in a
broad range of applications.
[0036] It is another feature of the present invention to provide
the parameters of the manufacturing process for production of the
novel material that could be used as a carbon-based composite or
carbonaceous structural material.
[0037] It is another feature of the present invention to provide
the application of this novel material for uses in highly corrosive
or chemically active environments.
[0038] It is another feature of the present invention to provide
the application of this novel material for uses in high temperature
applications.
[0039] It is another feature of the present invention to provide
the application of this novel material in uses in applications
requiring materials of high strength, low density and/or specific
porosity.
[0040] Other features and advantages of the invention will become
apparent to those of ordinary skill in the art upon review of the
following drawings, detailed description and claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0041] FIG. 1 is a flowchart of one embodiment of the process
embodying the invention;
[0042] FIG. 2(a) is a graph depicting exemplary pressure and
temperature profiles for the process for manufacturing an electrode
material;
[0043] FIG. 2(b) is a graph depicting exemplary pressure and
temperature profiles for the process for manufacturing a structural
material;
[0044] FIG. 3 is a front view of a capsule used in the process;
[0045] FIG. 4 is a graph of a temperature and pressure profile used
in one embodiment of the process;
[0046] FIG. 5 is a graph of the relative pore sizes for the novel
material manufactured under various pressures;
[0047] FIG. 6 is a schematic of the set up for the study of
electrochemical properties of the novel material;
[0048] FIG. 7 is a graph of cyclic voltammetry (CV) curves of the
novel material;
[0049] FIG. 8 is a graph of CV curves of the novel material;
[0050] FIG. 9 is a graph of CV curves of the novel material;
[0051] FIG. 10 is a graph of CV curves of the novel material;
[0052] FIG. 11 is a schematic depicting the use of the novel
material in an ultracapacitor;
[0053] FIG. 12 is a schematic depicting a desalination/deionization
cell utilizing the novel material as an activated carbon electrode;
and
[0054] FIG. 13 is a graph showing the performance of a desalination
device utilizing the novel material as an electrode to remove salt
from water, with the initial part of the plot showing how the
capacity of the device is loaded, the peak of the plot showing how
the device is regenerated by shorting the charge causing the salt
to return into the bulk solution.
[0055] Before one embodiment of the invention is explained in
detail, it is to be understood that the invention is not limited in
its application to the details of construction and the arrangement
of components set forth in the following description or illustrated
in the drawings. The invention is capable of other embodiments and
of being practiced or being carried out in various ways. Also, it
is to be understood that the phraseology and terminology used
herein is for the purpose of description and should not be regarded
as limiting.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0056] The present invention involves the consolidation of
amorphous carbon using heat and pressure for a prescribed time to
produce a novel material, termed herein consolidated amorphous
carbon (CAC) material, that is still amorphous and that has
superior properties over currently available carbon materials. The
properties of the CAC material can be altered by choosing different
source materials, by controlling the process parameters of the
manufacturing process, or by blending specific materials prior to
processing. The properties of the CAC material that can be varied
include, for example, densification, strength, porosity,
conductivity and adsorptive surface area. By selecting materials
and process parameters to achieve desired properties, CAC materials
can be tailored for their use in a specific application such as
electrochemical applications (i.e., water treatment, desalination,
energy storage devices) or structural materials (i.e.,
carbon-carbon composites, low density/high strength members). The
resulting CAC material is strong enough for handling and is able to
be machined, ground or cut into the desired shape. Grinding or
cutting tools such as a diamond cutting saw can be used to bring
the CAC material to the final specifications for the specific
application. The CAC material visually looks like non-shiny
graphite.
[0057] With respect to source material, preferably, the form of
amorphous carbon that is used in the present process is powder
activated carbon. The examples set forth herein utilize this form
of amorphous carbon, however, it should be noted that the invention
is not limited to the activated carbon form of amorphous carbon.
The principle characteristic of the CAC material that may be
altered by using different amorphous carbon source material is the
adsorptive surface area. Carbon particles that have high specific
surface areas (as measured by the BET isotherm or other analytical
techniques) can be selected to increase the net surface area of the
CAC material after processing. Carbon source material can be
selected based upon surface area, hardness, density and grain
size.
[0058] For example, activated granular carbon with an active
surface area of 1400 m.sup.2/g was used to manufacture bulk CAC
material that had a net surface area of about 1200 m.sup.2/g. CAC
material was observed to have surface areas approximately 10% less
than the original source material depending on the processing
parameters. Activated carbon is currently commercially available
with adsorptive surface areas as high as 3000 m.sup.2/g and it is
believed that the process of the present invention could be used to
generate CAC materials with 2800 m.sup.2/g of surface area.
Preferably, the device that carries out the elevated temperature
compression of the amorphous carbon is a hot isostatic press (HIP)
such as the MINI HIPer manufactured by ABB Autoclave Systems Inc.
An advantage of using isostatic pressure is that the consolidation
of the carbon is uniform throughout the material. However, it
should be noted that other devices in addition to HIPs can be
utilized for the consolidation under heat and pressure of the
amorphous carbon.
[0059] With respect to the process parameters in the manufacture of
the CAC materials, the process parameters of temperature, pressure
and time can be varied to alter the specific characteristics and
properties of the produced CAC material. Preferably, the
temperature can range from 200.degree. C. to 2700.degree. C., the
pressure can range from 500 to 50,000 psi and the holding time or
time at temperature and pressure may vary from 0.5 to 20 hours.
Preferably, the target pressure is obtained and the temperature is
thereafter ramped up to the target value in a period of time such
as one hour. It will be appreciated that all of these parameters
interact and that one could use a condition outside these cited
ranges by compensating changes in other parameters.
[0060] The specific combination of parameters that may be applied
is determined for the specific material properties desired. For
example, powder activated carbon consolidated at a temperature of
800.degree. C. and a pressure of 3 ksi for one hour is more porous
and more brittle than carbon consolidated at a temperature of
900.degree. C. and a pressure of 25 ksi for one hour. The first CAC
material is best used in an application such as an electrode, while
the second CAC material could be used as a structural material.
Generally, changes to the process parameters of temperature,
pressure and time directly effect the properties of final density,
strength, and porosity of the CAC material while the properties of
conductivity, strength and adsorptive surface area are altered to
lesser degree.
[0061] With respect to temperature in particular, the temperature
range of 600.degree. C. to 1400.degree. C. is most preferred for
most applications. Most preferred temperatures for forming
electrodes from CAC material is in the lower end of the range, from
about 600.degree. C. to about 1000.degree. C. Most preferred
temperatures for forming structural products from CAC material is
in the higher end of the temperature range, from about 800.degree.
C. to about 1400.degree. C.
[0062] With respect to pressure in particular, most preferably, the
pressure ranges from 500 psi to 25,000 psi. Pressures in the lower
end of these ranges, for example 500 psi to 20,000 psi are
typically preferred for making electrode material. Pressures in the
higher end of the range, from 2000 psi to 25,000 psi are typically
preferred for making structural products. Pressure has an influence
on the capacitance of any electrode made from CAC material. With
higher pressures, more dense materials are produced, macropore size
shifts and therefore a CAC material with a lower capacitance is
obtained.
[0063] With respect to holding time, holding times of from about
0.75 hours to about 10 hours are typical for the present process.
Preferably, for electrode material, the holding times are shorter
due to the desired surface area and porosity. It is generally
beneficial to cool the CAC material products gradually after
processing. Gradual cooling rates of from about 200.degree. C./hr
to about 1000.degree. C./hr are typically, with ranges of
300.degree. C./hr to 800.degree. C./hr being most preferred.
[0064] Mixing fibers or other particles with the carbon source
material prior to processing can dramatically increase the tensile
and compressive strength of the CAC material. Long graphite fibers,
for example, can be blended to improve the directional strength.
Short whiskers could be added to improve the strength
isotropically. Depending on the processing parameters, the carbon
particles in the source material will interact with any added
carbon fibers in much the same way that they interact with each
other, reducing the amount of pull-out or crack propagation. The
weight proportion of added material in the final CAC product can
range from 0% to 40% and higher.
[0065] Referring now to FIG. 1, one specific embodiment of the
present process is illustrated for the efficient and
environmentally benign production of CAC materials. The illustrated
process utilizes granular amorphous carbon that has been prepared
such as by grinding or drying. The carbon particles are loaded into
a capsule such as a metal can made from copper or stainless steel,
and thereafter subjected to isostatic pressure and temperature for
a period of time in a HIP. The process parameters are varied in a
manner commensurate with the desired end use of the material
produced. As shown in FIG. 1, typical process parameter values
include 3 ksi at 800.degree. C. for 1 hour. Further examples of
pressure, temperature and time profiles are shown in FIG. 2(a) for
exemplary CAC electrode material and FIG. 2(b) for exemplary
structural CAC material.
[0066] The process of the present invention as shown in FIG. 1
yields a monolithic type material of consolidated activated carbon
without binders. The mechanism of consolidation under elevated
temperature and pressure is believed to be related to the limited
diffusion taking place in the region where the activated carbon
powders are in contact. From a powder sintering point of view, the
curvature of the particle surface provides the driving force for
consolidation as the system tends to reduce the surface energy by
reducing the curvature of the particle surface. Carbon is a
material with an extremely high melting (softening) point, about
3650.degree. C. for graphite. Therefore, sintering of carbon is
almost impossible under normal conditions without the addition of
binders or fluxes. According to the present invention, sintering of
activated carbon is made possible by the application of certain
pressure.
[0067] A more specific example of the novel process is set forth as
follows. Activated carbon granules, CX0648-1 available from EM
Science, size 0.5-0.85 mm, BET specific surface area 1400
m.sup.2/g, were washed with distilled water and dried at 70.degree.
C. for 24 hours. A rod mill was used to grind the dried granules
into fine powders. The grinding process lasted about 15 minutes at
room temperature. A copper can with a design as illustrated in FIG.
3 was used as the capsule. The activated carbon powders were filled
through the stems of the capsule which was sealed right after
filling was finished. The filled capsule was then degassed under
vacuum for 12 hours at a temperature of 150.degree. C. Copper wool
and porous alumina were used as a filter to avoid the carbon powder
being drawn out. After degassing, the stem was sealed with argon
weld and the package was ready for elevated temperature
compression. The HIP technique was employed and carried out using
an ABB Autoclave Systems Inc. MINI HIPer with argon as the medium.
A temperature of 800.degree. C. was used. In order to consolidate
the carbon powders while maintaining the large surface area and
high porosity, a low pressure range was used, which specifically
ranged from 3 ksi (21 MPa) up to 25 ksi (172 MPa). The holding time
was one hour to ensure good consolidation. The time scheme used is
illustrated in FIG. 4. After cooling to room temperature, the
capsule was cut open and the hockey puck like CAC material was
removed.
[0068] The monolithic CAC material manufactured by this novel
process can be characterized by any of several properties including
adsorptive surface area, porosity, density, strength, conductivity,
surface morphology, x-ray diffraction and electrochemical
properties. Each of these properties will be discussed in detail
below.
[0069] With respect to surface area, this property can be measured
for example using a BET surface area analyzer, model ASAP 2000 from
Micromeritics Instrument Corporation. A sample of the CAC material
is prepared by crushing the material into particles with a nominal
size of about 2 mm. Before the BET measurement, samples are
degassed at 250.degree. C. under a flow of helium gas to remove
moisture, and then weighed. Low temperature nitrogen gas (77 K) is
used for BET analysis. In this analysis, the nitrogen gas is
adsorbed on the clean solid surface to form a single molecular
layer. The total amount of gas adsorbed is then determined by
measuring the pressure change before and after an equilibrium
state. The solid surface area is then calculated.
[0070] The BET measured surface area for CAC material using powder
activated carbon processed at different pressures at 800.degree. C.
for 1 hour in a HIP are set forth in Table 1 as follows.
TABLE-US-00001 TABLE 1 Results of BET Analysis For CAC Material
Processed with Different Pressures Micropore HIP Pressure Volume
(ksi)/(MPa) BET Specific Total Pore (d < 20 .ANG.) at
800.degree. C. Surface Area (m.sup.2/g) Volume (cm.sup.3/g)
(cm.sup.3/g) 25/172 931 .+-. 15 0.4599 0.2067 10/69 1026 .+-. 20
0.5068 0.2064 3/21 1238 .+-. 21 0.7159 0.2149 raw activated 1400
.+-. 22 0.6239 0.2077 carbon
[0071] From Table 1 it can be seen that the surface area of the
carbon decreased by only approximately 10% with consolidation at 3
ksi. This can be explained by the fact that higher pressures
promote densification of the source material and tend to close the
pores of the carbon. In comparison, with other activated carbon
materials using binders, the surface area is reduced greatly
(>50%) because of the occlusion effect. From the pore volume
data set forth in Table 1, one can see that the micropore volumes
of pores less than 20 .ANG. did not change with the pressure.
However, the total pore volume decreased significantly with the
increases in pressure.
[0072] Accordingly, by changing the process parameters and by the
selection of the source material, CAC material with varying surface
areas, for example, between 400 m.sup.2/g and 3000 m.sup.2/g, can
be produced.
[0073] Turning now to porosity, macroporosity and mesoporosity can
be analyzed using a conventional mercury penetration method and a
PORESIZER 9320 available from Micromeritics Instrument Corporation.
During the analysis, mercury is intruded under certain pressure
into the pores of the specimen. When an equilibrium state is
reached, the applied pressure balances with the surface tension of
mercury inside the pores. By measuring the volume intruded into the
specimen, the pore volumes of correspondent pore diameters can be
determined. Mercury pressure can range from atmospheric pressure to
30,000 psi (210 MPa) corresponding to a minimum pore diameter of
about 6 mm. The pore diameters with largest volume and total
porosity of the CAC materials analyzed are set forth in Table
2.
TABLE-US-00002 TABLE 2 Results of Mercury Porosimetry Tests on CAC
Materials Micropore HIP Pressure Pore Diameter Volume (ksi)/(MPa)
of Max. Porosity (d < 20 .ANG.) Skeletal Density at 800.degree.
C. Volume (nm) (%) (cm.sup.3/g) (g/cm.sup.3) 25/172 61 11.94 0.7517
1.1918 10/69 330 16.83 0.9387 1.1287 3/21 720 31.02 1.0495 1.0897
raw activated 909 19.76 0.6606 0.8233 carbon
[0074] Pore size distribution of CAC material with differing
process pressures at 800.degree. C. is shown in FIG. 5. It can be
seen from Table 2 and FIG. 5 that the pore size distribution of the
CAC material is related to the process pressure. For example, the
pore diameter of maximum pore volume is the largest when activated
carbon powders were consolidated under a process pressure of 3 ksi.
With the increase in pressure, the pore diameter of maximum pore
volume shifts to smaller sizes and the total pore volume decreases.
Since macropores as large as several hundred nanometers will
greatly facilitate the process of mass transfer of
electrochemically active species in the electrolyte, they are an
important characteristic for activated carbon electrodes. From
Table 2 it can also be seen that the skeletal density of
consolidated activated carbon is less than 1.5 g/cm.sup.3, which
indicates that the microporosity is still large after consolidation
using the present invention.
[0075] The degree of porosity (pore volume remaining between the
particles after processing) of the CAC material can vary from about
55% (voids by volume) to less than 1% depending of the process
parameters employed. Typically, the higher porosity CAC material
(less consolidation) is ideal for electrochemical applications
while more consolidated CAC material (less porosity) will have
better structural properties. Porosity has an influence on the CAC
material's capacity for ion storage such as in desalination units
and also affects regeneration time when CAC electrodes are
discharged. The voids between the carbon particles tend to shrink
as the CAC process continues and they could be completely
eliminated provided that a sufficiently high temperature and
pressure are achieved. These voids, however, are found to be useful
in the electrochemical applications since they would allow
electrolytes to reach the inner part of the electrode. Thus, to
retain some porosity, a lower pressure is preferred in this case
while it is necessary to maintain high temperatures in order to
facilitate the diffusion process (i.e. consolidation).
[0076] With respect to density, the process parameters can be
controlled to increase densification so that the CAC material will
have better mechanical properties and be cheaper to produce than
current carbon-carbon composites or structural materials which are
manufactured by a resin pyrolysis process. Because the CAC material
is principally particles of carbon that have been sintered by the
manufacturing process, the degree of densification will determine
the density of the material. Consolidation of amorphous carbon
particles at high temperatures and pressures causes the particles
to bond together resulting in a monolithic material that has
excellent thermal properties and high strength.
[0077] With respect to strength of the CAC material, the
consolidation process of the present invention results in a
material that has high strength and excellent thermal properties.
Strength can be determined by standard tensile and compression
tests and will vary with the degree of densification. CAC material
that has little or no void spacing will have higher strength than
CAC material with more void volume. The strength can be increased
if fibers are admixed with the carbon prior to consolidation.
Carbon fibers, for instance, will bond with the carbon powders.
This bonding will give the CAC material greater strength by
dramatically halting crack propagation.
[0078] With respect to the property of conductivity, conductivity
is an important property of electrode material. To be effective,
electrode material has to be highly conductive. Typically,
activated carbon particles mixed with binder materials to form
solid electrodes have a high resistivity of more than 15 .OMEGA.cm.
CAC materials can be produced having much lower conductivity
values, on the order of 0.04 .OMEGA.cm to 1.5 .OMEGA.cm, for
example. The electrical resistivity of the CAC material can be
measured with a conventional four-point probe resistivity
instrument. The resistivity of the specimen is calculated by
dividing the supplied current from the voltage measured, with the
results further corrected for the specimen shape factor. Exemplary
results are as follows.
TABLE-US-00003 TABLE 3 Resistivity of CAC Material HIP Pressure at
800.degree. C. for 1 hour (ksi/MPa) Resistivity (.OMEGA. cm) 25/172
0.047 10/69 0.060 3/21 0.134
[0079] As shown in Table 3, the higher the process pressure, the
lower the resistivity of the CAC electrode material. However, even
at lower process pressures of 3 ksi, the resistivity of the CAC
material is still low as compared with those activated carbon
materials using binders. The reason for this is that the carbon
particles in the CAC material are interconnected rather than merely
in contact with each other.
[0080] With respect to surface morphology, this property of CAC
material can be investigated using scanning electron microscopy
(SEM). SEM pictures of the fractured surface of CAC materials
processed at different parameters were obtained. From these SEM
pictures, it can be seen that the carbon particles formed
agglomerates during processing even though no binder was used. At
lower temperatures or lower pressures, the carbon particles still
kept their shape, with large voids between them. For higher
temperatures and pressures, the carbon particles tended to mingle
and form a continuous matrix structure. Neck formations between
carbon particles are seen indicating that the particles are
interconnected after processing rather than loosely bonded as in
the case where binders are used. Such an interconnected particle
structure provides the material with strength and conductivity.
[0081] With respect to x-ray diffraction, because the source
material is amorphous carbon, and the parameters of the process are
not stringent enough to recrystallize the carbon to form graphite,
x-ray diffraction patterns of the CAC material show little or no
crystallization. The process of the present invention is not
intended to crystallize the carbon source material but rather makes
the random pattern of graphine planes less random.
[0082] Turning now to electrochemical properties of the CAC
material, these properties can be studied with a computerized
potentiostat and the set up as shown in FIG. 6. For example, CAC
material was cut into pieces of electrode material dimension as
15.times.15.times.1 mm. A graphite block was used as a current
collector and supporting material. The CAC electrode material was
adhered to the graphite with graphite powder filled epoxy. Other
exposed surfaces of the graphite were encapsulated with epoxy to
avoid contact with electrolytes. The assembled electrode was then
mounted on one end of a 20 cm glass tube through which a copper
wire was directed as the lead.
[0083] One electrochemical property investigated was cyclic
voltammetry (CV). CV is an electrochemical method used for studies
of redox couples of a system. In a CV study, the applied voltage
over the working and counter electrode (or reference electrode)
ramps up and down. During this process, if a redox couple exists in
the system, a current peak will be depicted in the current profile
in both scan directions. These peaks represent the generation and
consumption of a reduction or oxidation species brought by the
variation of the applied potential. If there is no significant
redox reaction taking place in the system, the current vs.
potential curve will be flat indicating that the electrode is
stable within the scan range.
[0084] To investigate the behavior of the CAC material as an
electrode in aqueous systems, CV experiments were carried out. A
CAC material electrode was used in the set up of FIG. 6 with a
platinum basket used as the counter electrode and a saturated
calomel electrode used as the reference electrode. FIG. 7 is a
graph showing the cyclic voltammogram of the CAC material
(processed at 3 ksi pressure at 800.degree. C. for 1 hour) in a 1 M
KCl solution with different scan rates. It can be seen from FIG. 7
that the CV curves show a featureless polarization in both scan
directions, while a capacitive nature of the electrode is clearly
seen by noticing that the current increases with the scanning rate.
The potential window is wide enough (-1.0 to 1.0 vs. SCE) to allow
the CAC material electrode to be used for general applications in
aqueous solutions without significant oxidation/reduction reactions
between the electrode material and the solvent.
[0085] FIG. 8 shows the CV curves of a CAC material electrode which
was processed at 3 ksi pressure at 800.degree. C. for 1 hour. The
electrolyte used was a 30% wt sulfuric acid solution, which is
widely used for ultracapacitors. The featureless curves indicate
that the CAC material is suitable to be used as electrodes in
ultracapacitors. The specific capacitance of the electrode is
estimated at about 21.degree. F./g. FIGS. 9 and 10 illustrate the
CV curves for CAC material electrodes processed under higher
pressures. One can see that with higher pressures, the double layer
charging current decreases.
[0086] The CAC material of the present invention has superior
properties over current carbon based materials. Carbon electrode
materials require good electrical conductivity, and, for most
applications, require large surface areas. As previously described,
actuated carbons are very conductive, consolidation of the
particles will ensure that all particles are connected making the
monolithic solid conductive. By selecting high surface area carbon
for the processing accordingly to the invention, for example,
2000-3000 m.sup.2/g, the CAC electrode material generated by the
process will have considerably larger available surface areas than
current materials. The CAC material described herein has a
substantially higher net capacity for ions or charge than currently
available materials. The CAC material of the present invention has
excellent electrical conductivity and very high specific surface
area (>1200 m.sup.2/g) depending on the source material
used.
[0087] Some of the specific applications potentially available for
this CAC material include, but are not limited to, the following
two areas, activated carbon electrode material and structural
carbon materials. Other applications not specifically stated for
the novel CAC material are assumed to be part of the invention.
[0088] The novel CAC material has exceptional properties for use as
electrode material. The process parameters for producing CAC
electrode material should be maintained to only partially densify
the carbon materials, thus keeping larger pores between the
particles, but still maintaining good particle-particle contact.
The macroporosity, controlled by the degree of consolidation
attained while processing, enables the CAC material to be better
for electric double layer storage materials. The following list
describes some of the potential applications for the novel CAC
material as an activated carbon electrode, however, this list is
not intended to limit the potential application of the CAC
material: desalination of brackish or sea water; deionization of
water; water treatment including softening or pH control;
solid-liquid separation including removal of fine, solid particles
from water streams or slurries; metal concentration or direct
recovery by electroplating; environmental processing including
direct electrochemical destruction of pollutants and contaminants
from water; ultracapacitor; energy storage devices for electric
cars, electronic devices, etc.; batteries and fuel cells.
[0089] The novel CAC material has exceptional properties for use as
structural materials. Carbon structural materials require little or
no macroporosity to be effective. Accordingly, the process
parameters for CAC material intended for structural use should be
chosen to more fully densify the carbon materials, thus reducing
the net amount of large porosity. Fibers such as those of graphite,
silicon carbide, etc. could be blended with the carbon source
material in varying amounts to provide structural reinforcement in
the CAC material. The following list describes some of the
potential applications for the CAC material as a structural
material, however, this list is not intended to limit the potential
application of the CAC material: applications in corrosive or
chemically active environments; applications requiring
high-temperature strength; applications of materials with high
strength/weight ratio and applications of materials with low
density.
[0090] The following are examples of the use of the novel CAC
material in varying applications. The examples are intended to be
illustrative of potential uses of the CAC material and are not
intended to limit the application of the CAC material.
Example 1
Ultracapacitor
[0091] If an electrolyte solution is placed between two electrodes
made of the CAC material, an applied voltage will separate the
various ions of the electrolyte into the respective double layers
that form. The result is a device that can store electrical energy,
which can be quickly recovered. When a battery is discharged
quickly, its voltage will drop substantially. Net result of
periodic discharges is a shorter battery life. But, if a storage
device is available that could take the burden of fast discharges,
then it could be used in combination with a battery, and thus
extend the battery's life through a process known as load leveling.
Such applications could be incorporated into modern electric cars,
electric toys, etc.
[0092] With reference to FIG. 11, an ultracapacitor was constructed
using CAC material as electrodes. Two pieces of the electrodes were
sandwiched between two graphite current collectors which have been
impregnated with wax to make them leak-proof and were polished
before use. The electrodes were dried in a vacuum oven for at least
12 hours and subsequently back-filled with desired electrolytes to
ensure good impregnation. The electrodes were further
ultrasonically treated for 15 minutes to remove loose particles on
the outer surfaces. A glass fiber or non-woven cloth was placed
between the electrodes to function as an insulating separator. A
thermal shrinkable tube was used as a casing material. Assembled
ultracapacitors were tested under different charging and
discharging conditions. Differential capacitance was measured by a
constant current discharge method. The maximum discharge current
was estimated with a potential step method.
[0093] Two electrolyte systems were used for the ultracapacitors,
an inorganic aqueous system and an organic non-aqueous system. For
the inorganic system, 30% wt of sulfuric acid (reagent grade) in
deionized water was used as the electrolyte. The results are set
forth in Table 4 below.
TABLE-US-00004 TABLE 4 Specific Capacitance of Ultracapacitors made
from CAC material Electrodes at 1.0 V Potential Mass Specific
Capacitance Cell Specific of the Volume Specific HIP Pressure
Capacitance Electrode Capacitance of the (ksi/MPa) (F/g) (F/g)
Electrode (F/cm3) 3 53 212 160 10 45 160 152 21 20 80 84
[0094] The capacitance per unit area is calculated by dividing the
mass specific capacitance by the value of the surface area (see
Table 1) with the results as follows in Table 5.
TABLE-US-00005 TABLE 5 Double Layer Capacitance Per Unit Area HIP
Pressure (ksi/MPa) Capacitance Per Unit Area (.mu.F/cm2) 3/21 17.0
10/69 15.6 25/172 8.6
[0095] In order to inspect the ultracapacitor's ability to quickly
discharge its stored energy, a CAC material (processed at the
800.degree. C. and 3 ksi for 1 hour) capacitor was subjected to
discharge at various current densities. The capacitance measured
for each discharging condition is listed in Table 6.
TABLE-US-00006 TABLE 6 Capacitance at Different Discharging
Conditions Discharging Current Measured Cell Specific Density
(mA/cm2) Capacitance (F/g) 3 51 30 53 100 48
[0096] Table 6 demonstrates that the novel CAC material electrodes
are capable of undergoing rapid charging and discharging.
[0097] The energy density of the ultracapacitors was calculated to
be higher than 7 Wh/kg based on electrode materials only, and 3.5
Wh/kg if one takes into account the weight of the electrolyte,
separator and current collector.
[0098] The peak power density of the capacitors was estimated using
a transient method. For material with a density of 0.75 g/cm.sup.3,
a 2 cm.sup.2.times.0.1 cm electrode weighs 0.15 g and a total of
0.3 g of electrode material was used for the cell. A power density
based on the CAC electrode material is estimated to be 23 kW/kg.
This performance is made possible by the unique pore size
distribution and the high conductivity of the CAC material.
[0099] Turning now to the organic system, since the break down
voltage of an organic electrolyte is much higher than an aqueous
electrolyte, a higher operating cell voltage can be achieved by
using an organic electrolyte. For the organic system, propylene
carbonate (PC, Alfa AESAR, 99%) was used as the solvent with
tetraammoniumethylene tetrafluoroborate (Et.sub.4BF.sub.4) as the
salt, at a concentration of 1 M. Since PC is very sensitive to
moisture, all tests with the organic system were carried out in a
glove box under a dry nitrogen atmosphere. The measured capacitance
of the CAC material (processed at 800.degree. C. and 3 ksi for 1
hour) at 3 V potential was 22.5 F/g, corresponding to an energy
density of 28 Wh/kg of electrode material. If the cell voltage is
2.8 V, an energy density of 24.6 Wh/kg is estimated.
Example 2
Desalination Unit
[0100] A capacitor of CAC material could be used to remove the salt
from water, in much the same manor as an energy storage device
stores energy. As the charged units "load up" on the salts from the
water, the units will charge. Once filled, the unit energy could be
used to drive a second unit. As the first discharges, the salt ions
fixed on the surface will be discharged, thus regenerating the
electrode for reuse. The net energy savings of desalination could
be large, as compared to current techniques, such as reverse
osmosis, distillation, etc., which are processes that require high
pressures and/or high temperatures. In addition, given the larger
capacity of the CAC material, the size of the units would be
greatly reduced compared to conventional desalination units with
currently available carbon.
[0101] With reference to FIG. 12, a desalination cell was built
with CAC material as a carbon electrode. The carbon electrodes of
55.times.15.times.0.8 mm were attached to graphite foil current
collectors with a thin layer of graphite powder filled epoxy in a
bipolar configuration. A rubber gasket between the current feeders
creates a channel between the two facing carbon electrode plates. A
peristaltic pump was used to keep a constant flow of the solution.
The concentration change of the salt at the outlet was monitored by
a specific conductance meter. The applied voltages ranged from 0.8
to 1.2 V. Solutions with a conductivity ranging from 100 to 1000
.mu.S were tested. All experiments were carried out at room
temperature.
[0102] The results demonstrated the significant desalting effects
were apparent with CAC material as the electrodes, considering the
fact that activated carbon has no affinity to NaCl if there was no
electric potential applied. The desalting effectiveness was more
significant with the increase of the applied potential, about 80%
removal of salt was achieved at 1.2 V. It was observed that the
regeneration process was faster than the desalting process.
[0103] Upon application of 1.2 V potential across the CAC material
electrodes, a significant decrease in dissolved salt concentration
can be achieved, as illustrated in FIG. 13. When the electrodes
were shorted or grounded, the "absorbed" ions were released back
into solution, and a peak salt concentration can be observed at the
outlet. By doing so, a desalination and regeneration cycle is
completed and the cell is ready for the next cycle. Since there is
no large resistance to the flow of treatment solution, the pressure
head is very low compared with a reverse osmosis process.
Significant energy reduction using the CAC material electrodes is
achieved over the distillation and RO processes of water
desalination. Experiments showed that a very low current is
required for desalting. In comparison with aerogel carbon
electrodes, the CAC material electrodes have the advantage of rapid
discharging rate because of the large macropores, and a relatively
low cost to produce.
[0104] The desalting cells can be used for removing various ionic
species in addition to NaCl. As long as a sufficient potential is
applied across the CAC material electrode, ions will be removed
from the solution and stored in the double layer. After the
electrodes are shorted, the ions are put back into the solution. An
example with respect to softening water is set forth immediately
below.
Example 3
Deionization/Water Softening
[0105] A stream of Houghton, Mich. drinking water was pumped
through a desalination cell constructed using CAC material as
illustrated in FIG. 12 under a 1.2 V potential. All concentrations
were determined using an inductively coupled plasma
spectrophotometer. Significant removal of Ca.sup.+2, Mg.sup.-2, and
Na.sup.+ ions was observed, as set forth in Table 7. The table
shows the specific ion concentrations in the feed water, the
product water, and in the water that was in the cell when the
voltage was shorted for regeneration.
TABLE-US-00007 TABLE 7 Deionization of Houghton, MI Tap Water (ICP
Results) Houghton MI Tap Metal Water Concentration Deionized Water
Regeneration Waste Ions (ppm) Concentration (ppm) Concentration
(ppm) Ca.sup.+2 60.00 none detected 160.0 Mg.sup.+2 11.88 none
detected 31.11 Na.sup.+ 16.16 none detected 44.82
[0106] Because of the fact that anions and cations are adsorbed on
anodes and cathodes separately, scaling problems are reduced to a
minimum. Deionization using CAC material electrodes is applicable
to both industrial and household uses. For example, it can be used
as a water softening treatment system for drinking water or for
feed water to boilers. Electroplating and mining industries produce
large amounts of waste discharges which could be treated with
application of the present invention.
Example 4
Copper Plating
[0107] CAC material was attached to a Pb current feeder and used as
an anode in an electroplating circuit. Copper was plated on the
cathode from solutions containing 400 mg Cu.sup.+2/L at a voltage
of 1.2 V. The large surface area of the anode improved the plating
efficiency for even low concentration solutions. The charging
current at the anode eliminates the necessity for a large
overpotential as in conventional plating arrangements. The
significance of this is that the CAC electrode material could be
used effectively to directly recover copper from very low
concentration solutions with very high current efficiency. Current
technology for recovering copper from low concentration solutions
uses solvents to concentrate copper to 30-40 g/L (300-400 times
more concentrated than was used in this test of the CAC electrode
material) to ensure high plating efficiencies. However, comparable
plating efficiencies were noted with CAC material and a lower
potential was required for plating as well.
Example 5
Metal Concentration
[0108] Metal ions in aqueous solutions can be concentrated in much
the same manner as demonstrated in Example 3. Low-grade gold ores
are economically processed by dissolving the gold into cyanide
solutions. The gold concentration in the solution is usually too
low concentrations (as low as 1 mg Au/L) without further treatment.
Typically, granular activated carbon is used to adsorb the
dissolved gold from leach solutions. Once loaded with Au, the ions
are stripped into solutions at concentrations of 10 to 1000 times
higher than the feed concentrations. The stripped solutions can
then be processed to recover the metallic gold.
[0109] Cathode and anode electrodes made of the CAC material were
placed in a solution of gold cyanide. When no voltage was applied,
both electrodes adsorbed 3 mg of Au/g of carbon. When a 1.2 V
potential was applied, the anode adsorbed over 5.4 mg of Au/g. In a
concentrating apparatus similar to the desalination unit, dilute Au
solutions can be treated to remove the gold, and regeneration (by
shorting of the potential) will result in higher gold
concentrations. The advantage to this technique would be in the
complete regenerative properties of the CAC material.
Example 6
Destruction and Removal of Cyanide
[0110] Sodium and potassium cyanide are important reagents in the
metal plating industry, precious metal mining, and in dye
manufacturing, and are extremely poisonous. Very small amounts of
free CN are allowed in waste streams from these industries. The CAC
material electrodes are effective at removing the CN in water by
electrochemical oxidation.
[0111] Solutions containing low amounts of NaCN were pumped through
the desalination units described in Example 3. It was observed that
the potential on the units was sufficient to oxidize the free
cyanide directly to cyanate (OCN). While cyanide is a regulated
toxin, cyanate is not toxic or controlled, and is free to be
discharged to waste locations. This result indicates the power of
the process to destroy environmentally troublesome matter in
water.
Example 7
Particle Slurry Separation
[0112] Particles generate a small electrochemical charge when
placed in water. The magnitude and sign of the charge is determined
by the solid composition and the electrolyte concentration in the
water. Removal of fine particles (such as clays, phosphates,
potash) from water is difficult because the like charges on the
particles tend to repel each other. This action tends to stabilize
the fine particles in the water; that is, the fine particles will
not flocculate, and settle.
[0113] When the CAC material electrode was placed in a slurry, and
a 1.2 V potential was applied between it and a graphite electrode
(anode), the result showed that fine, negatively charged particles
migrated and attached to the anode, much like anions in an
electrolyte solution. The anode could be taken out of the slurry,
the solids removed, and be ready for reloading. Placing the cathode
electrode in a solution of ferric and ferrous iron readily
discharged it. The electrode discharge reaction resulted in the
reduction of Fe.sup.+3 to Fe.sup.+2. After discharging, it was
ready to remove more solids.
* * * * *