Compositions – Electrically conductive or emissive compositions – Elemental carbon containing
Reexamination Certificate
2000-07-12
2002-08-06
Barr, Michael (Department: 1762)
Compositions
Electrically conductive or emissive compositions
Elemental carbon containing
C427S115000, C427S426000, C423S449200, C429S047000
Reexamination Certificate
active
06428722
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a material for a gas diffusion electrode for use in electrolysis of alkali chloride solutions or in fuel cells, etc., a process for producing the same, and a process for producing a gas diffusion electrode.
BACKGROUND ART
A gas diffusion electrode generally has a structure comprising a gas feed layer and a reaction layer, and applications thereof include fuel cells, the electrolysis of alkali chloride solutions, etc.
In the electrolysis of an alkali chloride solution by the ion-exchange membrane method, a gas diffusion electrode is used as an oxygen cathode.
Electrolysis by the ion-exchange membrane method is usually conducted in an electrolytic cell partitioned into an anode chamber and a cathode chamber with an ion-exchange membrane which is a cation-exchange membrane. In this electrolytic cell, the anode chamber having an anode contains an aqueous sodium chloride solution, while the cathode chamber having a cathode contains an aqueous caustic soda solution. Among such cells, electrolytic cells of the type employing a gas diffusion electrode as the cathode have a structure whose cathode part comprises: a catholyte chamber ranging from an ion-exchange membrane to the gas diffusion electrode and containing an aqueous caustic soda solution; the gas diffusion electrode, which comprises a reaction layer and a gas feed layer; and an oxygen gas chamber.
Electrolytic cells having such constitution have an advantage that when a current is permitted to flow through the two electrodes, i.e., the anode and the gas diffusion electrode of the cathode part, to conduct electrolysis, then an oxygen-reducing reaction takes place on the gas diffusion electrode to elevate the cathode potential and, as a result, the electrolytic voltage is considerably reduced.
Gas diffusion electrodes generally have a structure comprising a reaction layer and a gas feed layer. In the case of producing a gas diffusion electrode having a reaction layer comprising fine silver particles, a hydrophobic carbon black, and polytetrafluoroethylene (hereinafter sometimes referred to as “PTFE”), the hydrophobic carbon black used for this gas diffusion electrode has particle sizes of from 20 to 300 &mgr;m due to secondary aggregation, although the diameters of the primary particles are about 0.05 &mgr;m.
In producing a high-performance electrode, a technique is used in which a carbon black having a reduced particle size is employed to thereby form a larger amount of three-phase zone interfaces.
Incidently, in a conventional process, the reaction layer of a gas diffusion electrode has been produced by dispersing a hydrophilic carbon black with the aid of a surfactant, mixing a PTFE dispersion therewith, freezing the resultant mixture, dispersing a hydrophobic carbon black with the aid of a surfactant, mixing a PTFE dispersion therewith, freezing the resultant mixture, thawing the two frozen mixtures, and mixing the thawed mixtures together. In this process, the carbon blacks and the PTFE dispersion undergo aggregation through the freezing and thawing operations to thereby form hydrophilic aggregates and hydrophobic aggregates. The two kinds of aggregates are mixed together to obtain a mixture of fine hydrophilic parts and fine hydrophobic parts which serves as a reaction layer material.
However, since such a conventional process for producing the reaction layer of a gas diffusion electrode requires a prolonged time period for the freezing and thawing operations for aggregating a PTFE dispersion, etc., the electrode production cost is high. In addition, the mixture of hydrophilic parts and hydrophobic parts is not always in a desirable state from the standpoint of obtaining electrodes having satisfactory performance. There has further been a problem that control is difficult.
In this process for producing a gas diffusion electrode, the hydrophobic carbon black has been dispersed to about several micrometers in the aqueous solution containing a surfactant even though it has undergone a dispersion treatment. With this dispersion is mixed a PTFE dispersion having an average particle diameter of 0.3 &mgr;m. From the resultant mixture, a reaction layer is produced through aggregation, filtration, sheet formation, etc. As long as the hydrophobic carbon black has been dispersed to an average particle diameter of about several micrometers, practically sufficient performance and reproducibility are secured. Although ultrasonic dispersion is the simplest and excellent technique for dispersing the hydrophobic carbon black, it has been difficult to obtain an average particle diameter of 1 &mgr;m or smaller because of the operation.
Consequently, there has been a desire for a gas diffusion electrode production process in which the production is accomplished through simplified steps in a reduced time period and by which the reaction layer of a gas diffusion electrode having a high oxygen-reducing ability can be produced.
In producing a gas diffusion electrode, a hydrophobic carbon black is first dispersed in an aqueous solution containing a surfactant as described above. Although ultrasonic dispersion generally employed as a dispersion technique has been used for dispersing the hydrophobic carbon black in the aqueous solution containing a surfactant so as to reduce the carbon black to primary particles, this technique has a drawback that the result is a widened particle size distribution of from 0.5 to 50 &mgr;m.
Another drawback of the technique described above is that the dispersion cost is high because the horn of the ultrasonic disperser, which is made of titanium, readily wears out due to contact with the hydrophobic carbon black. Fine silver particles in a dry state are less apt to be dispersed in the solution and necessitate a prolonged period for ultrasonic dispersion operation. However, it has been found that when an ultrasonic dispersion operation is conducted over a prolonged period, fine particles become larger far from being reduced. There has been a further problem that since fine silver particles undergo sintering and become larger in a pressing step, it is necessary to use a large amount of silver for securing performance.
When a dispersion containing a hydrophobic carbon black and fine PTFE particles wherein the two kinds of particles have different particle diameters as shown above, i.e., from 20 to 300 &mgr;m and 0.3 &mgr;m, self-organizes to form a reaction layer, this layer is in such a state that large secondary particles of the hydrophobic carbon black each is surrounded by fine PTFE particles having a size about one-tenth the size of the secondary particle to constitute hydrophobic parts. Since the regions of these hydrophobic parts are large, the hydrophilic parts formed by the self-organization of hydrophilic fine particles also are large. As a result, the three-phase zone interfaces, which participate in reactions, are small. Namely, this means that the electrode performance possessed by each material is not exhibited to the ultimate degree.
DISCLOSURE OF THE INVENTION
The invention has been achieved in view of such conventional problems. An object of the invention is to provide a reaction layer material for a gas diffusion electrode which has high performance, is inexpensive, has a long life, and can be produced through simple operations, and to provide a process for producing the same.
Another object of the invention is to provide a reaction layer material for a high-performance gas diffusion electrode by using a PTFE dispersion in combination with a hydrophobic carbon black finely dispersed so as to have nearly the same average particle diameter as the PTFE dispersion to thereby form a larger amount of three-phase zone interfaces, and to provide a process for producing the same.
Still another object of the invention is to provide a process for producing a reaction layer material for a gas diffusion electrode in which fine silver particles as a material for a reaction layer are prevented from sintering and becoming larger during a pressing step.
The present inv
Barr Michael
Furuya Nagakazu
Young & Thompson
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