Chemistry of inorganic compounds – Carbon or compound thereof – Elemental carbon
Reexamination Certificate
2001-05-31
2004-08-24
Hendrickson, Stuart L. (Department: 1754)
Chemistry of inorganic compounds
Carbon or compound thereof
Elemental carbon
C423S449200, C429S231800
Reexamination Certificate
active
06780388
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an electrically conducting fine carbon composite powder. More specifically, the present invention relates to fine carbon composite powder useful as an electrically conducting material for an electrode material used particularly in a Lithium(Li) battery, electrical double-layer capacitor and the like, and fine carbon composite powder useful for supporting a catalyst for use in a fuel battery, and also relates to the method for producing the powder, a catalyst for polymer electrolyte fuel battery using the carbon composite powder, a polymer electrolyte fuel battery cell and battery using the catalyst.
BACKGROUND ART
In recent years, use of carbon powder materials for Li battery, electrical double-layer capacitor, fuel battery and the like is increasing. Particularly, fine carbon powder represented by carbon black has heretofore been used as an electrical conductivity-imparting material (for example, added to a resin) or a sliding member and in addition thereto, is being widely used in a battery as an electrode material, an additive or a support for supporting a catalyst.
For example, in a Li battery, the fine carbon powder is used as an additive for maintaining the electrical conductivity between graphite powder particles which are the main material of the negative electrode. In a fuel cell, the fine carbon powder in the state of supporting platinum is coated on a carbon substrate and used as an electrode catalyst for the anode electrode, cathode electrode or the like. In an electrical double-layer capacitor, the fine carbon powder is used as an additive for maintaining the electrical conductivity between fine activated carbon particles which are the main material of the electrode. The carbon powder used in these applications is so-called submicron order sized carbon powder smaller than normal carbon powder having a size of &mgr;m order obtained by the pulverization of coke or the like. By virtue of its small size, the carbon powder is useful as an electrical conducting material for improving the electrical conductivity between larger particles having a size of several &mgr;m to tens of &mgr;m.
This fine carbon power is required to have properties comparable to normal graphite powder, more specifically, good electrical conductivity as an electrode and in the case of a battery, electrical or chemical properties such that the carbonaceous member is resistant against a corrosion by an acid.
Carbon black is a material having properties satisfying these requirements to a certain extent and is used over a wide range. In general, carbon commonly obtained from coke is graphitized, for example, by heating at a high temperature with an attempt to stabilize chemically and improve the corrosion resistance. However, carbon black is a material difficult to graphitize and can be hardly graphitized by mere heating.
Therefore, for example, JP-A-62-246813 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses a technique of adding boric acid to carbon black and heating the obtained slurry at a temperature of 1,000 to 2,000° C. to reduce the d
002
of carbon crystal, which is an index of showing the graphitization, even to 3.41 Å (0.341 nm), thereby attaining the graphitization. However, according to the study by the present inventors, d
002
of carbon black cannot be lowered to less than 3.40 Å which is by far larger than the theoretical value for complete graphite (i.e. 3.354 A). Furthermore, mere heating for the graphitization fails in elevating the electrical conductivity as demanded.
Therefore the first object of the present invention is to obtain graphitized fine carbon powder having excellent crystallinity and thereby increased in the resistance against chemical corrosion and at the same time, improved in the electrical conductivity, and to provide a high performance catalyst for polymer electrolyte fuel battery and polymer electrolyte fuel battery using the catalyst.
In order to cope with recent environmental pollution issue due to exhaust gas from the internal combustion engine of an automobile or the like, an electric vehicle (EV) is being developed as an alternative in recent years. To keep up with this tendency, a fuel cell is increasingly expected to undertake the power source for EV and therefor a compact and high-performance fuel cell is demanded.
The fuel cell includes various types of fuel cells such as, according to the kind of electrolytic solution used, alkali type, phosphoric acid type, fused carbonate type and polymer electrolyte type. Among these, a polymer electrolyte fuel cell is attracting an attention as a power source for electric vehicle (EV) because of its operability at a lower temperature, easy handling and high output density.
For example,
FIG. 2
shows a cross-sectional structure of one example of a unit cell used in a polymer electrolyte fuel battery. The fundamental structure of a unit cell is such that an ion exchange membrane
14
having appropriate water content is disposed in the center and sandwiched by the electrode comprising an anode catalyst layer
13
and a cathode catalyst layer
15
. The anode catalyst layer
13
and the cathode catalyst layer
15
each is usually a sheet coated with a paste of carbon powder having supported thereon platinum or platinum alloy powder. The carbon powder is not particularly limited on the kind thereof as long as it has electrical conductivity, but those having a specific surface area large enough to support a catalyst are preferred and in general, carbon black is used.
In the outer side of the anode catalyst layer
13
and the cathode catalyst layer
15
, electrically conducting anode gas-diffusing porous sheet
12
and cathode gas-diffusing porous sheet
16
for passing water and gas generated at the reaction are disposed respectively and in the outermost side, a carbon-based separator plate with grooves
11
is disposed to provide reaction gas passages, thereby constructing a unit cell. By stacking the many unit cells (several hundreds of cells) to form a multilayer structure, a high-output fuel battery is constructed.
Since the reaction of a fuel battery takes place on the catalyst layers, the greatest factor determining the energy amount of a fuel cell is how effectively to use the catalyst. In order to use the platinum catalyst with highest efficiency, the characteristics of carbon as the support such as electrical conductivity, adhesion of platinum (supporting property), corrosion resistance against electrolytic solution (ion) and heat conductivity need to be improved.
Furthermore, adhesion as a constituent element of a cell, for example, plane pressure to the ion exchange membrane and the gas diffusion sheet must be maintained over a long period of time.
The fuel battery having a structure such that hundreds of unit cells are stacked and the whole is cramped up under a predetermined cramping pressure is operated over a long period of time, the separator plate, the gas diffusion sheet and the like undergo creeping (a phenomenon that the thickness decreases) and although this creeping amount is small per unit cell, the sum total in the creeping amount of hundreds of cells as a whole is fairly large.
In this meaning, simple carbon black currently used as a support is not only deficient in the electrical conductivity necessary for a high-performance battery but also, when the battery is operated for a long period of time and the plane pressure between respective parts decreases to cause increase in the contact resistance between respective parts, the internal resistance of the battery increases and the battery performance disadvantageously deteriorates. Specifically, in the durability test over a time period in excess of ten hundreds of hours, the output often lowers to the level of 70 to 80%.
Therefore the second object of the present invention is to develop a catalyst support capable of compensating for the deterioration in the long-term durability of elemental carbon conventionally used as a catalyst supp
Masuko Tsutomu
Nanba Yoichi
Hendrickson Stuart L.
Lish Peter J
Showa Denko K.K.
Sughrue & Mion, PLLC
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