Compositions – Electrically conductive or emissive compositions – Elemental carbon containing
Reexamination Certificate
2000-06-30
2003-10-07
Gupta, Yogendra N. (Department: 1751)
Compositions
Electrically conductive or emissive compositions
Elemental carbon containing
C252S500000, C252S502000, C252S511000, C106S031920, C204S280000, C204S282000, C204S283000, C429S047000, C429S047000, C429S047000, C429S047000
Reexamination Certificate
active
06630081
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for producing a material, e.g., a mixture of a carbon black and polytetrafluoroethylene (hereinafter referred to as “PTFE”) or a mixture of silver and PTFE, for use forming a reaction layer or gas feed layer of a gas diffusion electrode. Specifically, the invention relates to a process for producing the above-described mixture while minimizing the use of a surfactant.
DESCRIPTION OF THE RELATED ART
In a conventional process for producing a reaction layer of a gas diffusion electrode, a reaction layer material or a gas feed layer material has been obtained in the following manner. First, a hydrophilic carbon black is dispersed into water containing a surfactant, and a finely particulate catalyst, e.g., silver, is deposited on or added to the dispersed carbon black. Thereafter, a hydrophobic carbon black is dispersed into the dispersion, and a PTFE dispersion is then mixed therewith to prepare a dispersion.
Subsequently, an alcohol is added to the resultant dispersion to self-organize it, and the self-organized dispersion is filtered and dried to obtain a reaction layer material powder or gas feed layer material powder.
In preparing the carbon black dispersion in the conventional process described above, water is added to the carbon blacks in an amount as large as at least 30 times by weight the amount of the carbon blacks and a surfactant is added in an amount at least 2%. After this dispersion is mixed with a PTFE dispersion, an alcohol is added thereto in the same amount as the amount of the resultant dispersion to cause coagulation, and the resultant mixture is filtered and dried to produce the reaction layer material powder or gas feed layer material powder. For inexpensively producing the material powder, it is necessary to reduce the amounts of the water, surfactant, and alcohol used as sub-raw materials.
When the use amounts of these ingredients are reduced, this results in a decrease in the amount of all the raw materials to be treated. As a result, a smaller equipment size is usable and a cost reduction is possible accordingly. Furthermore, in the case where the dispersion containing carbon blacks and PTFE particles is used by any of a coating method in which the dispersion is applied as it is to form a film, a spraying method in which the dispersion is applied by spraying to form a film and a spray drying method in which the dispersion is spray-dried to form a powder, this technique has had a drawback that a large amount of energy is necessary for removing the solution because of too large an amount thereof. It has hence been desired to diminish to the lowest possible level the water and surfactant to be used. In particular, there has been a strong desire for a reduction of the surfactant amount because use of a large amount of a surfactant raises difficulties in removing the surfactant in a later step and in discarding the surfactant as a waste.
SUMMARY OF THE INVENTION
The invention has been achieved in view of the above-described problems in the conventional techniques.
Accordingly, an object of the invention is to provide a process for producing gas diffusion electrode materials which is effective in minimizing the use amounts of water and a surfactant and attaining a cost reduction brought about by equipment size reduction, contributes to energy saving, and is effective in shortening the production time.
Other objects and effects of the present invention will become apparent from the following description.
The conventional process requires a surfactant for dispersing carbon blacks into water. In order to overcome this problem, the present inventor made extensive investigations on means for mixing which are sufficient to disperse raw materials into solvents such as, e.g., alcohol solvents, hydrocarbon solvents, and ester solvents, without using a surfactant. Even a hydrophobic carbon black which is not wetted by water is satisfactorily wetted by those organic solvents. It was found that when a PTFE dispersion is added to and mixed with a dispersion prepared by wetting a carbon black by a water-miscible organic solvent, e.g., an alcohol, and dispersing the carbon black thereinto, then the alcohol diffuses into the water that serves as the dispersion medium of the PTFE dispersion and, simultaneously therewith, particles of the carbon black come into spaces among the fine PTFE particles to thereby form a dispersion structure desirable for gas diffusion electrodes. Namely, it was found that when a PTFE dispersion is mixed with a carbon black dispersed in an alcohol, then the PTFE dispersion self-organizes to form a satisfactory reaction layer structure. The invention has thus been completed.
It was further found that a dispersion structure desirable for gas diffusion electrodes is obtained also by wetting a carbon black by an alcohol and dispersing the carbon black thereinto, adding a fine PTFE powder to the dispersion to allow the PTFE particles to sufficiently absorb the solvent, and then subjecting the resultant mixture to an ultrasonic dispersion treatment.
Namely, it has been found that when a carbon black dispersed in an alcohol is mixed with a fine PTFE powder and the solvent is irradiated with ultrasonic, then the fine powder is disaggregated and mixed with the carbon black to give a satisfactory reaction layer structure. The invention has thus been completed.
On the other hand, since a PTFE dispersion is an aqueous dispersion, addition thereof to a water-immiscible solvent, e.g., solvent naphtha, results in separation into two layers. Consequently, a carbon black does not mix with the PTFE in solvent naphtha.
It was, however, found that when the system composed of the oily phase and the aqueous phase separated therefrom is vigorously mixed, then the fine PTFE particles contained in the PTFE dispersion as the aqueous phase move to the oily phase because these particles in themselves are originally hydrophobic, and the carbon black and the PTFE self-organize in the oily phase.
Namely, it was found that when solvent naphtha containing a hydrophilic carbon black and a hydrophobic carbon black both dispersed therein is vigorously shaken together with a PTFE dispersion, then the fine PTFE particles move from the aqueous phase to the oily phase and self-organize together with the carbon blacks in the solvent naphtha to thereby form a satisfactory reaction layer structure. The invention has thus been completed.
It was furthermore found that according to this means, a gas feed layer structure can be similarly formed when solvent naphtha containing a hydrophobic carbon black dispersed therein is vigorously shaken together with a PTFE dispersion.
Specifically, the above-described objects of the present invention have been achieved by providing the following processes.
(1) A process for producing a reaction layer material or gas feed layer material for a gas diffusion electrode, which comprises the steps of:
dispersing a gas diffusion electrode raw material excluding polytetrafluoroethylene in to an organic solvent to prepare a dispersion;
adding polytetrafluoroethylene to the dispersion to prepare a mixture; and
mixing the mixture.
(2) The process according to the above process (1), wherein polytetrafluoroethylene is added in the form of dispersion.
(3) The process according to the above process (1), wherein polytetrafluoroethylene is added in the form of fine powder.
(4) The process according to the above process (1), wherein the mixing is conducted by or with the aid of ultrasonic irradiation or vigorous agitation or shaking.
(5) The process according to the above process (1), wherein the organic solvent comprises a solvent miscible with water.
(6) The process according to the above process (5), wherein the organic solvent comprises at least one member selected from the group consisting of methanol, ethanol, isopropyl alcohol, ethylene glycol, glycerol, and acetone.
(7) The process according to the above process (1), wherein the organic solvent is a solvent immiscible with water.
(8) The process accord
Furuya Nagakazu
Gupta Yogendra N.
Hamlin D G
Young & Thompson
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