U.S. patent application number 11/727974 was filed with the patent office on 2007-08-09 for process for producing activated carbon for electrode of electric double-layer capacitor.
This patent application is currently assigned to Honda Giken Kogyo Kabushiki Kaisha. Invention is credited to Takeshi Fujino, Takahiro Haga, Yuji Kawabuchi, Takashi Maeda, Shushi Nishimura, Minoru Noguchi, Naohiko Oki, Shigeki Oyama, Kenji Sato.
Application Number | 20070183958 11/727974 |
Document ID | / |
Family ID | 27477237 |
Filed Date | 2007-08-09 |
United States Patent
Application |
20070183958 |
Kind Code |
A1 |
Fujino; Takeshi ; et
al. |
August 9, 2007 |
Process for producing activated carbon for electrode of electric
double-layer capacitor
Abstract
To produce activated carbon for an electrode of an electric
double-layer capacitor, the following steps are carried out
sequentially: a step of subjecting a massive mesophase pitch to a
pulverizing treatment to provide a pulverized powder; a step of
subjecting the pulverized powder to an infusibilizing treatment
under conditions of a temperature in a range of 300.degree. C.
(inclusive) to 450.degree. C. (inclusive) in the atmospheric air
current, a step of subjecting the pulverized powder to a
carbonizing treatment under conditions of a temperature in a range
of 600.degree. C. (inclusive) to 900.degree. C. (inclusive) in an
inert gas current to provide a carbonized powder, a step of
subjecting the carbonized powder to an alkali activating treatment
under conditions of a temperature in a range of 500.degree. C.
(inclusive) to 1,000.degree. C. (inclusive) in an inert gas
atmosphere, followed by the post treatments, thereby producing
alkali-activated carbon, and a step of subjecting the
alkali-activated carbon to a pulverizing treatment. If an electrode
is produced using the activated carbon, the electrode density can
be increased.
Inventors: |
Fujino; Takeshi; (Wako-shi,
JP) ; Oyama; Shigeki; (Wako-shi, JP) ; Oki;
Naohiko; (Wako-shi, JP) ; Noguchi; Minoru;
(Wako-shi, JP) ; Sato; Kenji; (Wako-shi, JP)
; Nishimura; Shushi; (Bizen-shi, JP) ; Maeda;
Takashi; (Kashima-gun, JP) ; Kawabuchi; Yuji;
(Kashima-gun, JP) ; Haga; Takahiro; (Kashima-gun,
JP) |
Correspondence
Address: |
ARENT FOX PLLC
1050 CONNECTICUT AVENUE, N.W.
SUITE 400
WASHINGTON
DC
20036
US
|
Assignee: |
Honda Giken Kogyo Kabushiki
Kaisha
Kuraray Chemical Co., Ltd.
Kashima Oil Co., Ltd.
|
Family ID: |
27477237 |
Appl. No.: |
11/727974 |
Filed: |
March 29, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10048470 |
Jun 7, 2002 |
7214646 |
|
|
PCT/JP00/05340 |
Aug 9, 2000 |
|
|
|
11727974 |
Mar 29, 2007 |
|
|
|
Current U.S.
Class: |
423/445R ;
423/460 |
Current CPC
Class: |
Y02E 60/13 20130101;
H01G 11/34 20130101; H01G 11/58 20130101; H01G 9/155 20130101 |
Class at
Publication: |
423/445.00R ;
423/460 |
International
Class: |
C01B 31/02 20060101
C01B031/02 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 10, 1999 |
JP |
11-226719 |
Jan 28, 2000 |
JP |
2000-24815 |
Jun 29, 2000 |
JP |
2000-195922 |
Aug 2, 2000 |
JP |
2000-234674 |
Jan 28, 2000 |
JP |
2000-024815 |
Claims
1. A process for producing activated carbon for an electrode of an
electric double-layer capacitor, comprising the steps of:
subjecting a massive mesophase pitch to a first pulverizing
treatment to provide a pulverized powder, subjecting said
pulverized powder to an infusibilizing treatment under conditions
of a temperature in a range of 300.degree. C. (inclusive) to
450.degree. C. (inclusive) in the atmospheric air current,
subjecting said pulverized powder to a carbonizing treatment under
conditions of a temperature in a range of 600.degree. C.
(inclusive) to 900.degree. C. (inclusive) in an inert gas current
to provide a carbonized powder, subjecting said carbonized powder
to an alkali activating treatment under conditions of a temperature
in a range of 500.degree. C. (inclusive) to 1,000.degree. C.
(inclusive) in an inert gas atmosphere, followed by post
treatments, thereby producing alkali-activated carbon, and
subjecting said alkali-activated carbon to a second pulverizing
treatment, wherein the above steps are carried out in the mentioned
order, and wherein the electric double-layer capacitor has an
electrode density of from greater than 0.9 q/cc to about 1
g/cc.
2-4. (canceled)
5. A process for producing activated carbon for an electrode of an
electric double-layer capacitor according to claim 1, further
comprising a step of subjecting a graphitizable carbon powder to an
alkali activating treatment to produce activated carbon for an
electrode of an electric double-layer capacitor, a mixed activating
agent comprising KOH and NaOH being used as an alkali activating
agent in said alkali activating treatment.
6. A process for producing activated carbon for an electrode of an
electric double-layer capacitor according to claim 5, wherein the
content of NaOH in said mixed activating agent is in a range of 10%
by weight.ltoreq.NaOH.ltoreq.90% by weight.
7-8. (canceled)
9. The process of claim 1 wherein said post treatments are at least
one of neutralization by hydrochloric acid, washing or drying.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a process for producing
activated carbon for an electrode of an electric double-layer
capacitor.
BACKGROUND ART
[0002] Such a conventionally known type of activated carbon for an
electrode includes alkali-activated carbon, which is a carbonized
powder made using a graphitizable carbon powder, e.g., mesophase
pitch as a starting material, aiming at the increase of the
electrostatic capacity.
[0003] This alkali-activated carbon is produced by a process
comprising a step of producing a fibrous material by spinning using
mesophase pitch, a step of subjecting the fibrous material to an
infusibilizing treatment and then to a carbonizing treatment, a
step of subjecting the carbonized material to an alkali activating
treatment and then to a pulverizing treatment, or to a pulverizing
treatment and then to an alkali activating treatment.
[0004] The alkali-activated carbon made in the conventional
process, however, suffers from the following problem: The
alkali-activated carbon is an activated carbon powder made by
pulverizing the fibrous material and hence, even if the length of
particles of the powder is shortened due to the pulverization, the
longitudinal breaking of the particles, namely, the breaking such
as to break end faces of the fiber is hard to occur, and the
over-pulverization brings about the degradation of the performance.
Thus, the alkali-activated carbon contains a large number of
columnar particles. If an electrode is produced using such
alkali-activated carbon, the columnar particles are dispersed at
random, whereby gaps are liable to be created between the columnar
particles. As a result, an electrode density (g/cc) is low and it
is impossible to increase the electrostatic capacity density (F/cc)
of an electric double-layer capacitor.
[0005] If the alkali acting treatment is utilized, it is possible
to produce activated carbon for an electrode, which has relatively
uniform pores made therein and a high electrostatic capacity
density. However, the activated carbon for the electrode having the
pores made by the alkali activation with such an electrostatic
capacity density being largely taken into account is accompanied by
a problem that it is difficult to ensure a pore diameter sufficient
for the diffusion of a liquid electrolyte and ions, and an electric
double-layer capacitor produced using the activated carbon and the
like has an increased internal resistance due to the foregoing.
[0006] Further, if a polarizing electrode is formed using the
activated carbon for an electrode produced in the conventional
process, the amount of polarizing electrode expanded during
charging is large. For this reason, for example, in a
laminated-type or rolled-type electric double-layer capacitor, it
is necessary to take a measure for providing a space corresponding
to the amount of polarizing electrode expanded within a case, or a
measure for increasing the strength of the case to receive a force
of expansion of the polarizing electrode. However, the former
brings about a disadvantage of a decrease in electrostatic capacity
per unit volume, and the latter brings about disadvantages of an
increase in cost of the case, an increase in weight of the case and
the like.
DISCLOSURE OF THE INVENTION
[0007] Accordingly, it is an object of the present invention to
provide a producing process of the above-described type capable of
producing alkali-activated carbon, by employing a particular
measure, from which an electrode having an increased density can be
produced.
[0008] To achieve the above object, according to the present
invention, there is provided a process for producing activated
carbon for an electrode of an electric double-layer capacitor,
comprising a step of subjecting a massive mesophase pitch to a
pulverizing treatment to provide a pulverized powder, a step of
subjecting the pulverized powder to an infusibilizing treatment
under conditions of a temperature in a range of 300.degree. C.
(inclusive) to 450.degree. C. (inclusive) in the atmospheric air
current, a step of subjecting the pulverized powder to a
carbonizing treatment under conditions of a temperature in a range
of 600.degree. C. (inclusive) to 900.degree. C. (inclusive) in an
inert gas current to provide a carbonized powder, a step of
subjecting the carbonized powder to an alkali activating treatment
under conditions of a temperature in a range of 500.degree. C.
(inclusive) to 1,000.degree. C. (inclusive) in an inert gas
atmosphere, followed by the post treatments, thereby producing
alkali-activated carbon, and a step of subjecting the
alkali-activated carbon to a pulverizing treatment, wherein the
above steps are carried out in the mentioned order.
[0009] The alkali-activated carbon produced by the above-described
process comprises fine massive particles, because the powder
resulting from the pulverization of the massive mesophase pitch is
used as a starting material. Therefore, fine massive particles in
an electrode made using the alkali-activated carbon present a
structure near the closest filled structure and hence, the
electrode density can be enhanced largely.
[0010] If the temperature in the infusibilizing treatment is lower
than 300.degree. C., the infusibilization is insufficient and for
this reason, the pulverized powder is molten in the subsequent
carbonizing treatment. On the other hand, if the temperature in the
infusibilizing treatment exceeds 450.degree. C., the oxidization
advances excessively and hence, the temperature exceeding
450.degree. C. is not preferred. If the temperature in the
carbonizing treatment is lower than 600.degree. C., the density of
the alkali-activated carbon produced in the subsequent step is
decreased. On the other hand, if the temperature in the carbonizing
treatment exceeds 900.degree. C., the activation of the carbonized
powder is very hard to advance. Further, if the temperature in the
alkali activating treatment is lower than 500.degree. C., the
activation is hard to advance. On the other hand, if the
temperature in the alkali activating treatment exceeds
1,000.degree. C., the activation advances excessively, resulting in
a decreased density of the resulting alkali-activated carbon and a
reduced yield of the alkali-activated carbon.
[0011] It is another object of the present invention to provide a
producing process of the above-described type, capable of producing
activated carbon for an electrode, which has a pore diameter
facilitating the diffusion of a liquid electrolyte and ions, and an
electrostatic capacity density as hitherto.
[0012] To achieve the above object, according to the present
invention, there is provided a process for producing activating
carbon for an electrode of an electric double-layer capacitor,
comprising a step of subjecting a starting material for activated
carbon and containing a metal compound incorporated therein to a
carbonizing treatment and a subsequent activating treatment.
[0013] If the above-described means is employed, the metal compound
is decomposed and gasified in the carbonizing treatment to produce
a gas, and fine pores are formed by the gas. In the activating
treatment, the fine pores serves as a source, and the metal serves
as an assistant or a catalyst to promote the formation of the
pores. Therefore, the diameter of the pores in the activated carbon
for the electrode is enlarged. In addition, the conductivity of the
activated carbon itself for the electrode is enhanced by the
incorporation of the metal. Thus, the internal resistance of an
electric double-layer capacitor produced can be reduced. On the
other hand, if the enlargement of the pore diameter is to this
extent, the electrostatic capacity density of the activated carbon
for the electrode is not varied and hence, the activated carbon has
the electrostatic capacity density as hitherto.
[0014] It is a further object of the present invention to provide a
producing process of the above-described type capable of producing
activated carbon for an electrode, wherein when a polarizing
electrode is produced from the activated carbon, the amount of
polarizing electrode expanded during charging can be decreased.
[0015] To achieve the above object, according to the present
invention, there is provided a process for producing activated
carbon for an electrode of an electric double-layer capacitor,
comprising a step of subjecting a graphitizable carbon powder to an
alkali activating treatment to produce activated carbon for an
electrode of an electric double-layer capacitor, a mixed activating
agent comprising KOH and NaOH being used as an alkali activating
agent.
[0016] It can be considered that NaOH in the mixed activating agent
exhibits a function to decrease the diameters of pores in the
activated carbon and reduce the distance between carbon
crystallites to strengthen the cross-linking of --C--O--C--
(therefore, reduce the decomposition of the crosslinkage). Thus, it
is possible to reduce the amount of polarizing electrode expanded
during charging, as compared with a case where only KOH is
employed.
[0017] According to the present invention, there is also provided a
process for producing activated carbon for an electrode of an
electric double-layer capacitor, comprising a step of subjecting a
starting material for activated carbon, which is an aggregate of
solids, to an oxygen-adding treatment to provide an oxygen-added
material with oxygen dispersed in all the solids, a step of
subjecting the oxygen-added material to a carbonizing treatment to
provide a carbonized material, and a step of subjecting the
carbonized material to an activating treatment to produce activated
carbon.
[0018] If the oxygen-adding treatment as described above is carried
out, when a polarizing electrode is formed using the activated
carbon produced through the subsequent carbonizing and activating
treatments, the amount of polarizing electrode expanded during
charging can be reduced.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a broken-away perspective view of an essential
portion of a cylindrical electric double-layer capacitor;
[0020] FIG. 2 is a sectional view taken along a line 2-2 in FIG.
1;
[0021] FIG. 3 is a diagram showing a microscopic structure of a
surface of one example of a polarizing electrode;
[0022] FIG. 4 is a diagram showing a microscopic structure of a
surface of another example of a polarizing electrode;
[0023] FIG. 5 is a graph showing one example of the relationship
between the average particle size of KOH-activated carbon and the
electrode density;
[0024] FIG. 6 is a graph showing another example of the
relationship between the average particle size of KOH-activated
carbon and the electrode density;
[0025] FIG. 7 is a graph showing the relationship between the
number of cycles and the rate of deterioration of electrostatic
capacity;
[0026] FIG. 8 is a graph showing the relationship between the
number of cycles and the rate of increase in internal
resistance;
[0027] FIG. 9 is a broken-away front view of an essential portion
of a button-type electric double-layer capacitor;
[0028] FIG. 10 is a diagram of a primary power spectrum of
Al-incorporated activated carbon;
[0029] FIG. 11 is a diagram of a primary power spectrum of
Fe-incorporated activated carbon;
[0030] FIG. 12 is a diagram of a primary power spectrum of
Ni-incorporated activated carbon;
[0031] FIG. 13 is a diagram of a primary power spectrum of
Co-incorporated activated carbon;
[0032] FIG. 14 is a diagram of a primary power spectrum of
metal-free activated carbon;
[0033] FIG. 15 is a schematic illustration showing an apparatus for
measuring an expansion rate of a polarizing electrode;
[0034] FIG. 16 is a graph showing the relationship between the
contents of KOH and NaOH and the electric resistance value as well
as the expansion rate;
[0035] FIG. 17 is a graph showing the concentration of oxygen in
one example of carbon fiber;
[0036] FIG. 18 is a graph showing the concentration of oxygen in
another example of carbon fiber;
[0037] FIG. 19 is a graph showing the concentration of oxygen in a
further example of carbon fiber; and
[0038] FIG. 20 is a graph showing the relationship between the rate
of increase in weight of an oxygen-added material and the expansion
rate of a polarizing electrode.
BEST MODE FOR CARRYING OUT THE INVENTION
Embodiment-I
[0039] Referring to FIGS. 1 and 2, a cylindrical electric
double-layer capacitor 1 comprises a vessel 2 made of aluminum, an
electrode roll 3 accommodated in the vessel 2, and a liquid
electrolyte poured into the vessel 2. The vessel 2 comprises a
bottomed cylindrical body 4, and a terminal plate 5 which closes an
opening in one end of the cylindrical body 4. The terminal plate 5
is provided with positive and negative terminals 6 and 7 and a
safety valve 8.
[0040] The electrode roll 3 includes a positive-pole laminated band
9 and a negative-pole laminated band 10. The positive-polar
laminated band 9 comprises a band-shaped collector 11 formed of an
aluminum foil, band-shaped polarizing electrodes e adhered to
opposite sides of the band-shaped collector 11 with a conductive
adhesive, respectively, and a first separator 13 made of PTFE
(polytetrafluoroethylene) and superposed on one of the band-shaped
polarizing electrodes e. A band-shaped positive pole 12 is formed
by the pair of the polarizing electrodes e. The liquid electrolyte
is retained in an impregnated manner in the first separator 13. The
negative-polar laminated band 10 comprises a band-shaped collector
14 formed of an aluminum foil, band-shaped polarizing electrodes e
adhered to opposite sides of the band-shaped collector 14 with a
conductive adhesive, respectively, and a second separator 16 made
of PTFE and superposed on one of the band-shaped polarizable
electrodes e. A band-shaped negative pole 15 is formed by the pair
of the polarizable electrodes e. The liquid electrolyte is retained
in an impregnated manner in the second separator 16.
[0041] To produce the electrode roll 3, the second separator 16 of
the negative-polar laminated band 10 is superposed onto the exposed
polarizing electrode e of the positive-polar laminated band 9. This
superposed assembly is wound spirally, so that the first separator
13 of the positive-polar laminated band 9 is located on the
outermost side.
[0042] One example of the liquid electrolyte, which may be used, is
a solution of a quaternary ammonium borofluoride compound, e.g.,
TEMA.BF.sub.4[(C.sub.2H.sub.5).sub.3CH.sub.3N.BF.sub.4
(triethylmethyl ammonium borofluoride), (a solute)] in PC
(propylene carbonate, a solvent).
[0043] Alkali-activated carbon made from mesophase pitch used as a
starting material is used as activated carbon for the electrode.
The alkali-activated carbon is produced in the following
manner:
[0044] The alkali-activated carbon is produced through a step of
subjecting massive mesophase pitch to a pulverizing treatment to
produce a pulverized powder, a step of subjecting the pulverized
powder to an infusibilizing treatment under conditions of a
temperature in a range of 300.degree. C. (inclusive) to 450.degree.
C. (inclusive) in the atmospheric air current, a step of subjecting
the pulverized powder to a carbonizing treatment under conditions
of a temperature in a range of 600.degree. C. (inclusive) to
900.degree. C. (inclusive) in an inert gas current to provide a
carbonized powder, a step of subjecting the carbonized powder to an
alkali activating treatment under conditions of a temperature in a
range of 500.degree. C. (inclusive) to 1,000.degree. C. (inclusive)
in an inert gas atmosphere, followed by the post treatments,
thereby producing alkali-activated carbon, and a step of subjecting
the alkali-activated carbon to a pulverizing treatment.
[0045] It is desirable that the treating time be equal to or longer
than 0.5 hour and equal to or shorter than 10 hours in any of the
infusibilizing treatment, the carbonizing treatment and the alkali
activating treatment. If the treating time is shorter than the
lower limit value in any of the treatments, an intended object
cannot be achieved. On the other hand, if the treating time exceeds
the upper limit value, there is a possibility that the
characteristics of the treated material are detracted.
[0046] The alkali-activated carbon provided by the above process is
made using the powder resulting from the pulverization of the
massive mesophase pitch as a starting material and hence, comprises
fine massive particles. Therefore, in the band-shaped positive and
negative poles 12 and 15, the fine massive particles present a
structure near the closest filled structure and hence, the
electrode density can be enhanced largely.
[0047] Particular examples will be described below.
EXAMPLE-1
[0048] First, alkali-activated carbon using a starting material of
mesophase pitch, i.e., KOH-activated carbon in Example-1 was
produced in the following manner:
[0049] (a) Massive mesophase pitch was subjected to a pulverizing
treatment at room temperature to produce a pulverized powder having
an average particle size of 300 .mu.m. Then, the powder was
subjected to an infusibilizing treatment at 350.degree. C. for 2
hours in the atmospheric air current and then to a carbonizing
treatment at 700.degree. C. for 1 hour in the current of nitrogen
gas to provide a carbonized powder. (b) The carbonized powder and
an amount of KOH two times the weight of carbon in the powder were
mixed together, and the resulting mixture was subjected to a
potassium activating treatment as an alkali activating treatment at
800.degree. C. for 5 hours in the nitrogen atmosphere, followed by
the post treatments such as the neutralization by hydrochloric
acid, washing and drying, thereby providing KOH-activated carbon as
activated carbon. (c) The KOH-activated carbon was subjected to a
pulverizing treatment using a jetmill to produce fine KOH-activated
carbon having a predetermined average particle size. This fine
KOH-activated carbon is called simply KOH-activated carbon.
[0050] The KOH-activated carbon having the predetermined average
particle size, a graphite powder (a conductive filler) and PTFE (a
binder) were weighed, so that a weight ratio of 85:12.5:2.5 was
obtained. Then, the weighed materials were kneaded together and
then subjected to a rolling to fabricate an electrode sheet having
a thickness of 175 .mu.m. A plurality of band-shaped polarizing
electrodes e each having a width of 95 mm and a length of 1,500 mm
were cut from the electrode sheet. Then, a positive-polar laminated
band 9 was fabricated using the two band-shaped polarizing
electrodes e, a band-shaped collector 11 having a width of 105 mm,
a length of 1,500 mm, a thickness of 40 .mu.m and a conductive
adhesive and a first separator 13 made of PTFE and having a
thickness of 75 .mu.m. Further, a negative-polar laminated band 10
was fabricated using the two similar band-shaped polarizing
electrodes e, a band-shaped collector 14, a conductive adhesive and
a second separator 16 having a thickness of 75 .mu.m.
[0051] Then, the second separator 16 of the negative-polar
laminated band 10 was superposed onto the exposed band-shaped
polarizing electrode e of the positive-polar laminated band 9, and
the resulting superposed assembly was rolled spirally, so that the
first separator 13 of the positive-polar laminated band 9 was
located on the outermost side, thereby producing an electrode roll
3. This electrode roll 3 and a liquid electrolyte comprising 1.5
moles of TEMA.BF.sub.4 dissolved in a solution of PC were placed
into a bottomed cylindrical body 4 of a vessel 2 having an inside
diameter of 50 mm and a length of 130 mm, and an opening in the
bottomed cylindrical body 4 was closed using a terminal plate 5 to
provide a cylindrical electric double-layer capacitor 1. In the
closing of the opening, the collectors 11 of the positive-pole
laminated band 9 and the negative-polar laminated band 10 were
connected to the positive terminal 6 and the negative terminal 7 of
the terminal plate 5, respectively. Further, four cylindrical
electric double-layer capacitors 1 were produced in the same manner
as that described above. These capacitors 1 are called examples (1)
to (5).
EXAMPLE-II
[0052] For comparison, a fiber-shaped material having an average
diameter of 10 .mu.m was produced by subjecting mesophase pitch to
a spinning and then subjected sequentially to an infusibilizing
treatment, a carbonizing treatment, an alkali activating treatment
and a pulverizing treatment under the same conditions as in
Example-I to provide KOH-activated carbon having a predetermined
average particle size.
[0053] Then, five cylindrical electric double-layer capacitors 1
were produced in the same manner as in Example-I. These capacitors
1 are called examples (6) to (10).
[Performance of Electric Double-Layer Capacitor]
[0054] Table 1 shows the average particle size of the KOH-activated
carbon, the electrode density of the band-shaped positive and
negative poles 12 and 15 and thus polarizing electrode e, and the
electrostatic capacity density (F/cc) for the examples (1) to (10)
of the electric double-layer capacitors 1. The average particle
size of the KOH-activated carbon was measured using a particle size
distribution-measuring device, and the charging was carried out at
2.5 V in an atmosphere of 45.degree. C. TABLE-US-00001 TABLE 1
Electric Electrostatic double-layer Average particle Electrode
density capacity density capacitor size (.mu.m) (g/cc) (F/cc)
Example (1) 2.36 0.924 33.6 Example (2) 8.64 0.987 36.0 Example (3)
9.8 0.983 35.7 Example (4) 13.23 0.956 34.8 Example (5) 26.28 0.913
33.0 Example (6) 1.03 0.832 29.9 Example (7) 7.67 0.865 31.1
Example (8) 9.7 0.863 31.4 Example (9) 14.45 0.845 30.7 Example
(10) 28 0.779 28.4
[0055] FIG. 3 shows a photomicrograph of a surface of the
polarizing electrode e in example (2). It can be seen from FIG. 3
that fine massive particles present a structure near the closest
filled structure.
[0056] FIG. 4 shows a photomicrograph of a surface of the
polarizing electrode e in example (7). It can be seen from FIG. 4
that columnar particles are dispersed at random to create gaps
between them.
[0057] FIGS. 5 and 6 are graphs taken based on Table 1 and showing
the relationship between the average particle size and the
electrode density for examples (1) to (5) and examples (6) to (10).
As shown in FIG. 5, if the KOH-activated carbon according to the
embodiment of the present invention is used, the electrode density
can be increased to about 1 g/cc as in examples (2) and (3). In the
case of the comparative example, however, the electrode density is
confined to about 0.9 g/cc as in examples (7) and (8).
[0058] As is apparent from Table 1 and FIGS. 5 and 6, in examples
(1) to (5) made using the KOH-activated carbon according to the
embodiment of the present invention, the electrode density is high,
as compared with examples (6) to (10) made using the KOH-activated
carbon according to the comparative example, and correspondingly,
the electrostatic capacity density is high. This is attributable to
a difference between the particle shapes of the KOH-activated
carbons, as also shown in FIGS. 3 and 4.
[0059] Then, one of examples (1) to (5) having the highest
electrode density, i.e., example (2) and one of examples (6) to
(10) having the highest electrode density, i.e., example (7) were
subjected to a cycle test to examine a rate of deterioration of the
electrostatic capacity and a rate of increase in internal
resistance, thereby providing results shown in FIGS. 7 and 8. In
the cycle test, each of the examples was charged for 20 minutes at
2.5 V and 30 A in an atmosphere of 45.degree. C. and then
discharged to zero farad. The charging and discharging were defined
as one cycle and repeated 200 cycles. A rate A of deterioration of
the electrostatic capacity was determined according to
A=(C/B).times.100 (%), wherein B represents an initial
electrostatic capacity density, and C represents an electrostatic
capacity density after a predetermined number of cycles. A rate D
of increase in internal resistance was determined according to
D=(F/E).times.100 (%), wherein E represents an initial internal
resistance, and F represents an internal resistance after a
predetermined number of cycles.
[0060] As is apparent from FIGS. 7 and 8, it can be seen that both
of the rate of deterioration of the electrostatic capacity and the
rate of increase in internal resistance of example (2) are low, as
compared with those of example (7). This is considered to be
attributable to the following reason: The KOH-activated carbon made
from the mesophase pitch used as the starting material has a nature
that it is expanded at an initial stage of repetition of the
charging and discharging. Therefore, in example (2), the electrode
density is further increased due to such expansion and decreased
extremely slowly with the passage of time. On the other hand, in
example (7), the separation of the columnar particles due to the
expansion and the enlargement of gaps attendant on such separation
advance rapidly with the passage of time.
[0061] According to Embodiment-I, the alkali-activated carbon
comprising fine massive particles can be produced, whereby the
electrode density can be increased to enhance the characteristics
of the electric double-layer capacitor.
Embodiment-II
[0062] Referring to FIG. 9, a button-type electric double-layer
capacitor 17 includes a case 18, a pair of polarizing electrodes 19
and 20 accommodated in the case 18, a spacer 21 sandwiched between
the polarizing electrodes 19 and 20, and a liquid electrolyte
filled in the case 18. The case 18 comprises a vessel body 23 made
of aluminum and having an opening 22, and a lid plate 24 made of
aluminum and closing the opening 22. An outer periphery of the lid
plate 24 and an inner periphery of the vessel body 23 are sealed
from each other by a seal material 25. Each of the polarizing
electrodes 19 and 20 is made of a mixture comprising activated
carbon, a conductive filler and a binder.
[0063] Activated carbon for the electrode is produced by subjecting
a starting material having a metal compound incorporated therein to
a carbonizing treatment and then to an activating treatment.
Examples of the starting material for the activated carbon, which
may be used, are various synthetic resins, petroleum pitch, coal
pitch, coal tar and the like. At least one compound selected from
the group consisting of a metal chloride, a metal oxide and an
organic metal compound is used as the metal compound, and at least
one metal selected from the group consisting of Al, Ni, Fe and Co
corresponds to a metal in the metal compound. More specifically,
the chlorides include AlCl.sub.3, NiCl.sub.2, FeCl.sub.3,
CoCl.sub.2 and the like; the oxides include AlO.sub.3, NiO.sub.2,
Fe.sub.2O.sub.3, CoO.sub.2 and the like; the organic metal
compounds include aluminum acetyl acetate, alkoxide and the
like.
[0064] The amount of metal compound incorporated is set so that a
relation, 0.1% by weight.ltoreq.J.ltoreq.10% by weight is
established in {H/(G+H)}.times.100=J (% by weight), wherein G
represents an amount of starting material used for activated
carbon; H represents an amount of metal incorporated; and J
represents a rate of metal incorporated.
[0065] The carbonizing treatment is carried out under conditions of
a temperature in a range of 500 to 1,000.degree. C. and a time in a
range of 0.5 to 10 hours in an inert atmosphere. An alkali
activating treatment is utilized as the activating treatment and
carried out under conditions of a temperature in a range of 500 to
1,000.degree. C. and a time in a range of 0.5 to 10 hours in the
presence of KOH in an inert atmosphere. As required, an
infusibilizing treatment may be carried out prior to the
carbonizing treatment under conditions of a temperature in a range
of 250 to 500.degree. C. and a time in a range of 30 to 120 minutes
in an atmospheric air current.
[0066] If the above-described measure is employed, the metal
compound is decomposed and gasified during the carbonizing
treatment, whereby fine pores are created by the resulting gas. In
the activating treatment, the fine pores serve as a source, and the
metal serves as an assistant or a catalyst to promote the creation
of the pores. Therefore, the diameter of each of pores in the
activated carbon for the electrode is enlarged. In addition, the
electric conductivity of the activated carbon itself is increased
by the incorporation of the metal. Thus, it is possible to reduce
the internal resistance of the electric double-layer capacitor. On
the other hand, the electrostatic capacity density of the activated
carbon for the electrode is not varied by the enlargement of the
pores to this extent and hence, the activated carbon has an
electrostatic capacity density as hitherto.
[0067] However, if the rate J of metal incorporated is lower than
0.1% by weight, an effect of incorporation of the metal is not
provided. On the other hand, if the rate J of metal incorporated is
higher than 10% by weight, the creation of the pores advances
excessively, thereby bringing about a reduction in electrode
density and lowering the electrostatic capacity density just
slightly. The rate J of metal incorporated is preferably in a range
of 0.5% by weight.ltoreq.J.ltoreq.5.0% by weight.
[0068] When the metal compound is incorporated into the starting
material for the activated carbon, if the melting points of them
are near to each other, a high dispersibility of the metal compound
is provided, even if the metal compound and the starting material
are mixed together and molten in their powdery states. On the other
hand, if a difference between the melting points of the metal
compound and the starting material is large, it is preferable that
they be dissolved and mixed in an organic solvent such as quinoline
from the viewpoint of the enhancement of the dispersibility of the
metal compound.
[0069] Particular examples will be described below.
EXAMPLE-I
[0070] (a) Aluminum acetyl acetate was incorporated in amount of
3.65 g into 30 g of petroleum mesophase pitch, and they were
pulverized and mixed together sufficiently using a mortar. In this
case, a rate J of aluminum incorporated was equal to 3% by weight.
(b) The mixture was subjected to an infusibilizing treatment for 90
minutes at 480.degree. C. in an atmospheric air current. The
mixture was molten and cooled to assume a massive state. The
resulting massive material was pulverized to provide a powder. (c)
The powder was subjected to a carbonizing treatment for one hour at
700.degree. C. in a current of nitrogen gas to provide a carbonized
powder. (d) An amount of KOH twice the weight of the carbonized
powder was mixed to the carbonized powder, and the resulting
mixture was subjected to an alkali activating treatment (a
potassium activating treatment) for 5 hours at 800.degree. C. in a
current of nitrogen gas to provide activated carbon. Then, the
activated carbon was subjected sequentially to an acid washing, a
water washing, a filtration and a drying. This activated carbon for
the electrode is called Al-incorporated activated carbon.
EXAMPLE-II
[0071] Activated carbon was produced in the same manner as in
Example-I, except that the aluminum acetyl acetate was replaced by
iron acetyl acetate. A rate J of iron incorporated in this case is
equal to 3% by weight as in Example-I. This activated carbon for
the electrode is called Fe-incorporated activated carbon.
EXAMPLE-III
[0072] Activated carbon was produced in the same manner as in
Example-I, except that the aluminum acetyl acetate was replaced by
nickel acetyl acetate. A rate J of nickel incorporated in this case
is equal to 3% by weight as in Example-I. This activated carbon for
the electrode is called Ni-incorporated activated carbon.
EXAMPLE-IV
[0073] Activated carbon was produced in the same manner as in
Example-I, except that the aluminum acetylacetonate was replaced by
cobalt acetylacetonate. A rate J of cobalt incorporated in this
case is equal to 3% by weight as in Example-I. This activated
carbon for the electrode is called Co-incorporated activated
carbon.
EXAMPLE-V
[0074] Activated carbon was produced through steps (b) to (d) in
Example-I using a mesophase pitch precursor similar to that in
Example-I, but containing no organic metal compound incorporated
therein. This activated carbon for the electrode is called
metal-free activated carbon.
[0075] The Al-incorporated activated carbon, the Fe-incorporated
activated carbon, the Ni-incorporated activated carbon, the
Co-incorporated activated carbon and the metal-free activated
carbon produced in Example-I to Example-IV and Example-V were
subjected to a TEM shooting, and a distribution of pore diameters
and a fractal dimension were determined by an image picture
analysis. Shooting conditions are as follows: Phillips CM120 was
used; an acceleration voltage was set at 120 kV; and a shooting
magnification was set at 200,000. Conditions for the analysis of
the distribution of pore diameters were set as follows: The TEM
negative image was digitalized at a resolution of 600 dpi and at
512.times.512 pixels with 256 levels of gray; a two-dimensional FFT
was carried out and then, a periodic integration was carried out,
thereby providing a one-dimensional power spectrum.
[0076] FIGS. 10 to 14 show one-dimensional power spectra for the
Al-incorporated activated carbon and the like. Most-frequent values
of pore diameters of the Al-incorporated activated carbon and the
like obtained from the one-dimensional power spectra are as shown
in Table 2. TABLE-US-00002 TABLE 2 Most-frequent value of pore
diameter (nm) Al-incorporated activated carbon 1.29 Fe-incorporated
activated carbon 1.29 Ni-incorporated activated carbon 1.35
Co-incorporated activated carbon 1.35 Metal-free activated carbon
1.18
[0077] It can be seen from Table 2 that the diameters of the pores
in the Al-incorporated activated carbon and the like were
increased, as compared with those in the metal-free activated
carbon.
[0078] Then, a mixture comprising the Al-incorporated activated
carbon produced in Example-I, a graphite powder (a conductive
filler) and PTFE (a binder) blended at a ratio of 90:5:5 by weight
was kneaded sufficiently. The kneaded mixture was subjected to a
rolling, thereby producing two polarizing electrodes 19 and 20
having a diameter of 20 mm and a thickness of 185 .mu.m. A
button-type electric double-layer capacitor 17 shown in FIG. 9 was
assembled using the polarizing electrodes 19 and 20, a spacer 21
made of PTFE and having a thickness of 75 .mu.m and a solution of
1.4 mol/L of TEMA.BF.sub.4
[(C.sub.2H.sub.5).sub.3CH.sub.3N.BF.sub.4(triethylmethyl ammonium
borofluoride)] in PC (propylene carbonate) as a liquid electrolyte.
A button-type electric double-layer capacitor 17 was also assembled
in a manner similar to that described above, using the
Fe-incorporated activated carbon, the Ni-incorporated activated
carbon, the Co-incorporated activated carbon produced in Example-II
to Example-IV and the metal-free activated carbon produced in
Example-V.
[0079] For each of the capacitors 17, an electrical resistivity was
measured under conditions of a charging voltage of 2.5 V and a
charging current of 5 mA, and an electrostatic capacity density per
unit weight and an electrostatic capacity density per unit volume
of the activated carbon were calculated, thereby providing results
shown in Table 3. The electrode density is also given in Table 2.
TABLE-US-00003 TABLE 3 Electric double-layer capacitor Activated
carbon Electrode Electrical Electrostatic Electrostatic density
resistivity capacity capacity (g/cc) (.OMEGA.cm.sup.2) density
(F/g) density (F/cc) Al-incorporated 0.956 6.5 34.1 32.6
activatedcarbon Fe-incorporated 0.976 6.8 33.9 33.0 activatedcarbon
Ni-incorporated 0.973 6.3 34.3 33.4 activatedcarbon Co-incorporated
0.951 6.4 34.4 32.7 activatedcarbon Metal-free 0.984 9.2 33.8 33.3
activatedcarbon
[0080] As is apparent from Table 3, it can be seen that the
electrical resistivity of the electric double-layer capacitor 17
made using the polarizing electrodes 19 and 20 produced from the
Al-incorporated activated carbon and the like is low, as compared
with that of the electric double-layer capacitor 17 made using the
polarizing electrode produced from the metal-free activated carbon.
On the other hand, it can be seen that the electrostatic capacity
densities of the activated carbons are equivalent, irrespective of
the presence and absence of the incorporated metal.
[0081] According to Embodiment II, it is possible to produce
activated carbon for an electrode, which has a pore diameter such
that the diffusion of a liquid electrolyte and ions is liable to
occur, a relatively high conductivity and an electrostatic capacity
density as hitherto. Such activated carbon for the electrode is
effective for reducing the internal resistance of the electric
double-layer capacitor.
Embodiment III
[0082] Activated carbon for an electrode is produced by subjecting
a graphitizable carbon powder to an alkali activating treatment.
Examples of the graphitizable carbon, which may be used, are
carbonized powders, carbon fibers, carbonized pitches and the like
made using coke, petroleum pitch, mesophase pitch, polyvinyl
chloride, polyamide, PAN and the like as a starting material.
[0083] A mixed activating agent comprising KOH and NaOH is used as
an alkali-activating agent. It can be considered that NaOH in the
mixed activating agent exhibits a function to decrease the diameter
of pores in the activated carbon and reduce the distance between
the carbon crystallites to strengthen the crosslinkage bond of
--C--O--C-- (therefore, reduce the decomposition of the
crosslinkage). Thus, it is possible to reduce the amount of
polarizing electrodes 3 and 4 expanded during charging, as compared
with the case where only KOH is used.
[0084] If the content of NaOH in the mixed activating agent is
increased, the amount of polarizing electrodes 3 and 4 expanded is
decreased gradually, but if the content of NaOH reaches 100% by
weight or near 100% by weight, the resistance value of the
polarizing electrodes 3 and 4 is increased suddenly. This is
because an effect of KOH for suppressing the function of NaOH for
reducing the diameter of pores is reduced with a reduction in
content of NaOH.
[0085] Therefore, the content of NaOH in the mixed activating agent
is set in a range of 10% by weight.ltoreq.NaOH.ltoreq.90% by
weight. If the content of NaOH is lower than 10% by weight, the
NaOH addition effect is small and hence, the amount of polarizing
electrodes 3 and 4 expanded during charging is increased. On the
other hand, a disadvantage due to the content of NaOH higher than
90% by weight is as described above.
[0086] In the alkali activating treatment, a treating temperature T
is set in a range of 500.degree. C..ltoreq.T.ltoreq.1,000.degree.
C., and a treating time t is set in a range of 10
minutes.ltoreq.t.ltoreq.10 hours. If the treating temperature T is
lower than 500.degree. C., or if the treating time t is shorter
than 10 minutes, the activation is hard to advance. On the other
hand, if T>1,000.degree. C. or if t>10 hours, the excessive
activation occurs to reduce the density of the activated carbon and
the yield of the activated carbon. If the heated state to 350 to
500.degree. C. is maintained for 10 minutes to 3 hours in the
course of raising the temperature in the alkali activating
treatment, the graphitizable carbon powder and the mixed activating
agent are soaked and dehydrated sufficiently, whereby the
activation can be allowed to advance smoothly.
[0087] Particular examples will be described below.
[0088] I-[1] Production of activated carbon
EXAMPLE-I
[0089] A. Production of Graphitizable carbon Powder
[0090] (a) Granulatable mesophase pitch was subjected to an
infusibilizing treatment at 320.degree. C. for 10 minutes in an
atmospheric air current and then, the treated powder was subjected
to a carbonizing treatment at 700.degree. C. for 1 hour in a
current of nitrogen gas to provide a carbonized powder. (b) The
carbonized powder was subjected to a pulverizing treatment to
regulate the particle size. The density of the carbonized powder
was 1.55 g/cc.
[0091] B. Alkali Activating Treatment
[0092] (a) 2.5 grams of the carbonized powder and 5.0 grams of a
mixed activating agent, namely, a pellet mixture comprising a KOH
pellet and an NaOH pellet mixed at a predetermined ratio were
placed into a mortar, where the pellet mixture was mixed
sufficiently with the carbonized powder, while being pulverized.
Then, the resulting mixture was packed into a boat made of INCONEL.
(b) The boat was placed into a tubular furnace and heated to a
predetermined temperature at a temperature-raising rate of
200.degree. C./hr in a current of nitrogen gas of 60 ml/min and
maintained at such temperature for a predetermined time. Then, the
boat was removed from the tubular furnace and placed into an
ultrasonic wave-washing vessel, where the treated powder was
subjected to a washing treatment with pure water for 15 minutes to
remove KOH. Thereafter, the treated powder was subjected to the
removal of NaOH by HCl washing, the washing with about 3 liters of
warm water, the filtration and the drying, thereby producing
activated carbon.
EXAMPLE-II
[0093] For comparison, activated carbon was produced in a manner
similar to that in Example-I, except that an activating agent
comprising only KOH or only NaOH was used.
[0094] I-[2] Conditions for Activating Treatment
[0095] Conditions for the activating treatment for examples 1 to 6
of activated carbon produced in Example-I and examples 7 to 11 of
activated carbon produced in Example-II are as shown in Table 4.
TABLE-US-00004 TABLE 4 Example of Mixed activating agent Activating
treatment activated KOH (% by NaOH (% by Temperature carbon weight)
weight) (.degree. C.) Time (hr) 1 10 90 600 1 2 20 80 550 5 3 50 50
600 1 4 80 20 700 3 5 80 20 800 3 6 90 10 700 3 7 100 0 800 5 8 100
0 700 5 9 100 0 650 5 10 100 0 600 5 11 0 100 600 1
[0096] II-[1] Fabrication of Button-Type Electric Double-Layer
Capacitor (see FIG. 9)
[0097] Example 1 of the activated carbon, carbon black (a
conductive filler) and PTFE (a binder) were weighed so that a
weight ratio of 85.6:9.4:5 was provided. Then, the weighed
materials were added together and thereafter, the kneaded mixture
was subjected to a rolling, thereby fabricating an electrode sheet
having a thickness of 185 .mu.m. Two polarizing electrodes 19 and
20 having a diameter of 20 mm were cut from the electrode sheet,
and a button-type electric double-layer capacitor 17 was fabricated
using the two polarizing electrodes 19 and 20, a spacer 21 made of
glass fiber and having a diameter of 20 mm and a thickness of 0.35
mm, a liquid electrolyte and the like. A solution of 0.1 1.4 M of
triethylmethyl ammonium tetrafluoroborate
[(C.sub.2H.sub.5).sub.3CH.sub.3NBF.sub.4] in propylene carbonate
was used as the liquid electrolyte.
[0098] Such button-type electric double-layer capacitor 17 made
using example 1 of the activated carbon is called a sample 1.
Samples 2 to 11 of button-type electric double-layer capacitors 17
were fabricated in the same manner as described above using
examples 2 to 11 of the activated carbons.
[0099] II-[2] Electrostatic Capacity Density of Activated Carbon
and Electric Resistance Value of Polarizing Electrode
[0100] Sample 1 was subjected to a charging/discharging cycle
treatment which will be described below. Then, electrostatic
capacity densities (F/g, F/cc) of example 1 of the activated carbon
were determined in an energy conversion process, and an expedient
electric resistance value (.OMEGA.cm.sup.2) of the polarizing
electrode was calculated.
[0101] In the charging/discharging cycle treatment, such a process
was employed that the charging for 90 minutes and the discharging
for 90 minutes were conducted two times at 2.7 V, two times at 2.8
V, two times at 3.0 V and two times at 2.7V.
[0102] Samples 2 to 11 were also subjected to a similar
charging/discharging cycle treatment, whereby electrostatic
capacity densities of examples 2 to 11 of the activated carbons and
the like were determined.
[0103] II-[3] Measurement of Expansion Rate of Polarizing Electrode
(see FIG. 9)
[0104] As is shown in FIG. 15, a laminate 26 comprising two
polarizing electrodes 19 and 20 similar to those described above
and made using example 1 of the activated carbon and a glass fiber
spacer 21 having a diameter of 30 mm and a thickness of 0.24 mm and
interposed between the polarizing electrodes 19 and 20 was placed
as sample 1 onto an internal bottom surface of a liquid bath 27
made of aluminum. A liquid electrolyte 28 similar to that described
above was poured into the liquid bath 27 and then, a lower end face
of a copper-made vertical portion 30 of a pressing member 29 was
put on an upper surface of the upper polarizing electrode 19.
Further, the vertical portion 30 and the liquid bath 27 were
connected to a charging/discharging circuit 33 through connecting
wires 31 and 32. In a state in which a load applied to the
polarizing electrodes 19 and 20 by the pressing member 29 was set
at 3 kg, the charging and discharging were conducted repeatedly at
5 mA and 2.7V using the charging/discharging circuit 33. A total
amount of polarizing electrodes 19 and 20 expanded in a
thickness-wise direction during the charging was measured in terms
of an amount of pressing member 29 displaced, using a laser
displacement meter.
[0105] Samples 2 to 11 of laminates 26 having the polarizing
electrodes 19 and 20 made using examples 2 to 11 of the activated
carbon were also subjected to a similar measurement.
[0106] III Consideration
[0107] Table 5 shows the composition of the mixed activating agent,
the electrode density, the electrostatic capacity density (F/g) of
the activated carbon per unit weight, the electrostatic capacity
density (F/cc) of the activated carbon per unit volume and the
expansion rate of a polarizing electrode during the charging and
the expedient electric resistance (.OMEGA.cm.sup.2), for samples
(including the button-type electric double-layer capacitor 17 and
the laminate 26) 1 to 11. TABLE-US-00005 TABLE 5 Mixed activating
agent Polarizing electrode KOH Electrostatic Electric (% NaOH
Electrode capacity Expan- resistance Sam- by (% by density density
sion value ple weight) weight) (g/cc) (F/g) F/cc) rate (%) (.OMEGA.
cm.sup.2) 1 10 90 0.90 32.7 29.4 121 25.8 2 20 80 0.84 37.0 31.1
123 19.5 3 50 50 0.89 34.0 30.3 130 15.0 4 80 20 0.79 38.0 30.0 131
12.1 5 80 20 0.81 38.0 30.9 131 11.7 6 90 10 0.82 38.6 31.6 132
11.9 7 100 0 0.88 35.4 31.2 176 8.1 8 100 0 0.87 38.2 33.2 165 8.0
9 100 0 0.91 36.0 32.8 153 8.5 10 100 0 0.91 33.0 30.0 152 10.0 11
0 100 0.91 33.8 30.7 -- 69.7
[0108] FIG. 16 is a graph taken based on Table 5 and showing the
relationship between the content of KOH and the expansion rate of
the polarizing electrode as well as the electric resistance value
for samples 1 to 11. In FIG. 16, (1) to (11) correspond to samples
1 to 11.
[0109] As is apparent from FIG. 16, if the content of KOH in the
mixed activating agent is decreased and the content of NaOH in the
mixed activating agent is increased, the expansion rate of the
polarizing electrodes 19 and 20 during the charging is decreased.
Particularly, if the content of NaOH in the mixed activating agent
is set in a range of 10% by weight.ltoreq.NaOH.ltoreq.90% by weight
and hence, the content of KOH in the mixed activating agent is set
in a range of 10% by weight.ltoreq.KOH.ltoreq.90% by weight, it is
possible to produce activated carbon, from which a polarizing
electrode having an expansion rate during charging lower than 150%
and a low electric resistance value can be produced.
[0110] According to Embodiment III, it is possible to produce
activated carbon by employing the above-described measure, from
which a polarizing electrode whose expansion rate during charging
can be reduced can be produced, as well as activated carbon, from
which a polarizing electrode whose expansion rate and electric
resistance value during charging can be reduced can be
produced.
Embodiment IV
[0111] To produce activated carbon for an electrode, the following
steps are used: a step of subjecting a starting material for
activated carbon, which is an aggregate of solids, to an
oxygen-adding treatment to provide an oxygen-added material with
oxygen dispersed in the entire insides of the solids; a step of
subjecting the oxygen-added material to a carbonizing treatment to
provide a carbonized material, and a step of subjecting the
carbonized material to an activating treatment to provide activated
carbon.
[0112] If the oxygen-adding treatment described above has been
carried out to disperse oxygen in the entire insides of the solids,
when a polarizing electrode is formed using the activated carbon
produced through the subsequent carbonating and activating
treatments of the oxygen-added material, the amount of polarizing
electrode expanded during charging can be decreased.
[0113] Examples of the starting material for the activated carbon,
which may be used, are a powder, an aggregate of fibers (including
an aggregate of fibrous materials made by spinning) and the like
produced from petroleum pitch, mesophase pitch, polyvinylchloride,
polyimide, PAN and the like. Therefore, solids in the powder are
individual particles, and solids in the aggregate of fibers are
individual fibers or fibrous materials.
[0114] The oxygen-adding treatment is carried out by heating the
starting material for the activated carbon to a predetermined
temperature at a predetermined temperature increasing rate in the
air, or maintaining the starting material after reaching a
predetermined temperature, at such temperature for a predetermined
time.
[0115] In this case, if the weight of the starting material for the
activated carbon is represented by W, and the weight of the
oxygen-added material, namely, W+amount of oxygen is represented by
X, an increasing rate Y in weight upon the oxygen-adding treatment
is represented by Y={(X-W)/W}.times.100 (%) and set in a range of
2%.ltoreq.Y.ltoreq.20%. If the increasing rate Y in weight is lower
than 2%, the effect of suppressing the amount of polarizing
electrode expanded is insufficient. On the other hand, if Y>20%,
carbon is burned during the next step of the carbonizing treatment,
resulting in a decreased yield of a carbonized material.
[0116] In order to ensure that the increasing rate Y in weight is
fallen into the above-described range, the temperature increasing
rate V in the oxygen-adding treatment is set in a range of
5.degree. C./min.ltoreq.V.ltoreq.20.degree. C./min; the heating
time T1 is set in a range of 250.degree.
C..ltoreq.T1.ltoreq.350.degree. C.; and the maintaining time (hr)
t1 is set in a range of 1 hour.ltoreq.t1.ltoreq.10 hours.
[0117] In order to promote the oxygen-adding treatment, any of
P.sub.2O.sub.5, quinone, hydroxyquinone and the like, or any of
derivatives of them may be used.
[0118] The carbonizing treatment is carried out under known
conditions conventionally employed in a producing process of such a
type. Specifically, the carbonizing treatment is carried out in an
atmosphere of an inert gas at a heating temperature T2 set in a
range of 600.degree. C..ltoreq.T2.ltoreq.1,000.degree. C. and for a
heating time t2 set in a range of 1 hour.ltoreq.t2.ltoreq.10
hours.
[0119] An alkali activating treatment using KOH is utilized as the
activating treatment and carried out under known conditions
conventionally employed in a producing process of such a type.
Specifically, the activating treatment is carried out in an
atmosphere of an inert gas at a heating temperature T3 set in a
range of 500.degree. C..ltoreq.T3.ltoreq.1,000.degree. C. and for a
heating time t3 set in a range of 1 hour.ltoreq.t3.ltoreq.10
hour.
[0120] Particular examples will be described below.
[0121] A. Production of Fibrous Activated Carbon
[0122] 1. Oxygen-Adding Treatment
[0123] (a) An aggregate of fibrous materials having a diameter of
13 .mu.m was produced using mesophase pitch by spinning. (b) The
aggregate was subjected to an oxygen-adding treatment under
different conditions, thereby producing examples 1 to 9 of
oxygen-added materials. (c) An increasing rate Y in weight of each
of examples 1 to 9 was determined.
[0124] Table 6 shows the condition for the oxygen-adding treatment
and the increasing rate Y in weight for examples 1 to 9.
TABLE-US-00006 TABLE 6 Condition for oxygen-adding treatment
Temperature Oxygen- increasing Heating Maintaining added rate V
temperature time t1 Increasing rate material (.degree. C./min) T1
(.degree. C.) (.degree. C.) Y in weight (%) Example 1 28 280 -- 1.0
Example 2 5 280 -- 3.5 Example 3 5 300 -- 5.9 Example 4 5 320 --
6.7 Example 5 5 320 0.5 9.7 Example 6 3 280 -- 4.3 Example 7 3 300
-- 5.9 Example 8 3 320 -- 7.4 Example 9 3 320 2.0 10.6
[0125] No description in the column of maintaining time in Table 6
means that the oxygen-added material was removed from a treating
furnace immediately when the temperature within the furnace reached
the heating temperature.
[0126] 2. Carbonizing Treatment
[0127] Examples 1 to 9 of the oxygen-added material were subjected
to a carbonizing treatment at 700.degree. C. for one hour in a
current of nitrogen gas to provide examples 1 to 9 of a
graphitizable carbon fiber corresponding to examples 1 to 9 of the
oxygen-added material.
[0128] True densities of examples 1 to 9 are as shown in Table 7.
Each of the true densities was evaluated by a specific gravity
conversion process using methanol. TABLE-US-00007 TABLE 7 Carbon
fiber True density (g/cc) Example 1 1.55 Example 2 1.57 Example 3
1.56 Example 4 1.58 Example 5 1.57 Example 6 1.69 Example 7 1.59
Example 8 1.61 Example 9 1.72
[0129] A concentration of oxygen in a diametrical portion of each
of the carbon fibers in each of examples 1, 6 and 9 was examined by
an electron ray step scanning of TEM-EDX, thereby providing results
shown in FIGS. 17 to 19.
[0130] FIG. 17 shows the result for example 1, and it can be seen
from FIG. 17 that added oxygen exists only in an outer periphery of
the carbon fiber and does not exist in an inner area of the carbon
fiber. This existence state of the added oxygen was substantially
the same as the carbon fibrils forming example 1. Example 1
corresponds to a conventional infusibilized material, as is
apparent from the conditions for the oxygen-adding treatment in
Table 1.
[0131] FIG. 18 shows the result for example 6, and FIG. 19 shows
the result for example 9. It can be seen from these figures that
added oxygen is dispersed in the entire insides of the carbon
fibrils in each of examples 6 and 9. In example 6 shown in FIG. 3,
the amount of added oxygen is smaller than that in example 9 shown
in FIG. 4 and for this reason, the amount of oxygen added at and
near the center portion is smaller than that in the outer
periphery. In the case of example 9, the amount of added oxygen is
large and correspondingly, the added oxygen is dispersed
substantially uniformly in the carbon fiber. The dispersed state of
the added oxygen in FIG. 18 was substantially the same as the
carbon fibrils forming example 6, and the dispersed state of the
added oxygen in FIG. 19 was substantially the same as the carbon
fibrils forming the example 9. It can be said from these facts that
the dispersed state of the added oxygen in each of the remaining
examples was substantially the same as the carbon fibrils forming
each of the examples.
[0132] 3. Alkali Activating Treatment
[0133] Examples 1 to 9 of the carbon fibers were subjected to an
alkali activating treatment comprising a primary stage conducted at
400.degree. C. for 1 hour and a secondary stage conducted at
800.degree. C. for 5 hours both in a current of nitrogen gas using
KOH, thereby producing examples 1 to 9 of fibrous activated carbons
corresponding to examples 1 to 9 and having an average diameter of
20 .mu.m.
[0134] B. Fabrication of Button-Type Electric Double-Layer
Capacitor (see FIG. 9)
[0135] Example 1 of the fibrous activated carbon, carbon black (a
conductive filler) and PTFE (a binder) were weighed so that a
weight ratio of 85.6:9.4:5 was provided. Then, the weighed
materials were kneaded together and there after, the kneaded
mixture was subjected to a rolling to fabricate an electrode sheet
having a thickness of 185 .mu.m. Two polarizing electrodes 19 and
20 having a diameter of 20 mm were cut from the electrode sheet,
and a button-type electric double-layer capacitor 17 was fabricated
using the two polarizing electrodes 19 and 20, a spacer 21 made of
glass fiber and having a diameter of 25 mm and a thickness of 0.35
mm, a liquid electrolyte and the like. A liquid electrolyte used
was a solution of 2.0 M of triethylmethyl ammonium
tetrafluoroborate [(C.sub.2H.sub.5).sub.3CH.sub.3N.BF.sub.4] in
propylene carbonate.
[0136] The button-type electric double-layer capacitor made using
example 1 of the activated carbon as described above is called
sample 1. Samples 2 to 9 of button-type electric double-layer
capacitors were also fabricated in a similar manner using examples
2 to 9 of the activated carbons having a fiber length similar to
that in example 1.
[0137] C. Electrostatic Capacity Density of Activated Carbon and
Electric Resistance Value of Polarizing Electrode
[0138] Sample 1 was subjected to a charging/discharging test which
will be described below, and electrostatic capacity densities (F/g,
F/cc) of example 1 of the activated carbon were determined in an
energy conversion process. In the charging/discharging test, such a
process was employed that the charging was conducted at 2.7 V for
90 minutes, and the discharging was conducted at 2.7 V for 90
minutes. Samples 2 to 9 were also subjected to a similar
charging/discharging test, and electrostatic capacity densities of
examples 2 to 9 of the activated carbons were determined.
[0139] D. Measurement of Expansion Rate of Polarizing Electrode
[0140] As shown in FIG. 15, a laminate 26 comprising two polarizing
electrodes 19 and 20 using example 1 of the activated carbon,
similar to those described above and a spacer 21 of a non-woven
fabric having a length of 40 mm, a width of 40 mm and a thickness
of 0.24 mm and interposed between the polarizing electrodes 19 and
20 was placed as sample 1 onto an internal bottom surface of a
liquid bath 27 made of aluminum. A liquid electrolyte 28 similar to
that described above was poured into the liquid bath 27 and then, a
lower end face of a copper-made vertical portion 30 of a pressing
member 29 was put on an upper surface of the upper polarizing
electrode 19. Further, the vertical portion 30 and the liquid bath
27 were connected to a charging/discharging circuit 33 through
connecting wires 31 and 32. In a state in which a load applied to
the polarizing electrodes 19 and 20 by the pressing member 29 was
set at 3 kg, the charging and discharging were conducted repeatedly
at 5 mA and 2.7 V using the charging/discharging circuit 33. A
total amount of polarizing electrodes 19 and 20 expanded in a
thickness-wise direction during the charging was measured in terms
of an amount of pressing member 29 displaced, using a laser
displacement meter.
[0141] Samples 2 to 9 of laminates 28 having the polarizing
electrodes 19 and 20 made using examples 2 to 9 of the activated
carbon were also subjected to a similar measurement.
[0142] E. Consideration
[0143] Table 8 shows the electrode density, the electrostatic
capacity density (F/g) of the activated carbon per unit weight, the
electrostatic capacity density (F/cc) of the activated carbon per
unit volume and the expansion rate of the polarizing electrode for
samples (including the button-type electric double-layer capacitors
17 and the laminates 28) 1 to 9. This expansion rate is one for the
polarizing electrode at the time when the charging was completed,
i.e., when the charging voltage reached 2.7 V. For convenience, the
increasing rate Y in weight of the oxygen-added material in Table 6
is also given in Table 8. TABLE-US-00008 TABLE 8 Increasing rate Y
in Electrode Electrostatic Expansion rate weight of density
capacity density of polarizing oxygen-added Sample (g/cc) (F/g)
(F/cc) electrode (%) material (%) 1 0.88 35.36 31.19 170.3 1.0 2
0.80 38.17 30.38 147.0 3.5 3 0.79 36.70 29.00 138.0 5.9 4 0.80
37.56 30.00 139.5 6.7 5 0.80 38.23 30.47 140.3 9.7 6 0.84 35.88
30.00 143.0 4.3 7 0.87 35.83 31.10 142.0 5.9 8 0.85 36.06 30.62
138.0 7.4 9 0.86 36.30 31.18 137.0 10.6
[0144] FIG. 20 is a graph taken based on Table 3 and showing the
relationship between the increasing rate Y in weight of the
oxygen-added material and the expansion rate of the polarizing
electrode for samples 1 to 9. In FIG. 20, (1) to (9) correspond to
samples 1 to 9, respectively.
[0145] As is apparent from FIG. 20, if the increasing rate Y in
weight of the oxygen-added material is increased, the expansion
rate of the polarizing electrode 19, 20 is decreased. In this case,
if the increasing rate Y in weight of the oxygen-added material is
equal to 1.0% as in sample 1, the expansion rate of the polarizing
electrode 19, 20 exceeds 150%.
[0146] According to Embodiment IV, when a polarizing electrode is
formed by employing a means as described above, it is possible to
produce activated carbon ensuring that the amount of polarizing
electrode expanded during charging can be decreased.
* * * * *