CS208688B2 - Method of making the powder material impregnated against the humidity,determinated for production of electrodes by the diffusing gas - Google Patents

Abstract

1392341 Wetproofed carbon powder ZLEHIT PRI BAN 16 March 1972 [29 March 1971] 12445/72 Heading C1A [Also in Divisions D1 and H1] A method for producing powdered wetproofed materials, suitable for the production of gas diffusion electrodes, comprises mixing a "micropore-free" electroconductive powder, directly or as an aqueous suspension, with a wetproofing agent in the form of an aqueous dispersion or a solution in an organic solvent. "Micropore-free" means particles which are not thoroughly transpierced by the micropores. The mixing process proceeds at normal or elevated temperature, and after homogenization has been achieved the disperse medium is discarded and the solid phase (wetproofed material) dried. When using an aqueous dispersion of the wetproofing agent the dispersion stabilizers are removed, e.g. by heating at 250-350‹ C. for 30 to 90 min. or by washing and subsequently heating at 100-140‹ C., and the material is ground anew. The electroconductive powder is preferably carbon black and may comprise 3-5% by weight of the suspension. The wetproofing agent is a substance having a contact angle greater than 90 degrees and may comprise one or more hydrophobic polymers, e.g. polytetrafluoroethylene, polyethylene (high pressure), polyisobutylene, polypropylene, natural rubber or synthetic (butadiene-styrene) rubber. The polymer may be present in an amount of 10-60% by weight of the dispersion or solution. When using solu ions of the wetproofing agents the electroconductive powder may be introduced at the boiling temperature of the solvent. The powdered wetproofed materials may be made into gas diffusion electrodes using various catalysts, e.g. activated carbon.

Description

The invention relates to a process for the production of moisture-impregnated powder material for producing gas diffusing electrodes from an electrically conductive powder and moisture-impregnating agents.

The use of carbon black and other catalytically active substances together with moisture-impregnating β substances such as polytetrafluoroethylene, polyethylene, polypropylene, elastomers and the like for the production of gas diffusing electrodes 8 is already known.

A characteristic feature of the prior art methods of producing such electrodes is that catalytically active substances such as platinum black, silver, activated carbon and the like are impregnated against moisture, and this impregnation takes place during and after electrode processing (U.S. Pat. No. 3). , 348.9 ^ 4, No. 3, 386.859, No. 3,405.010, No. 3, 423.247, French Pat. No. 1, 529.016 and GDR Pat. No. 56.837).

However, these methods have numerous drawbacks. A large part of the active surface of the catalyst is blocked when the impregnating agent is applied to the electrode and is thus deactivated during the electrochemical reaction; as a result, the efficiency of the electrode decreases. For each type of catalytically active material, a special procedure and conditions for impregnating against moisture as well as suitable impregnating agents must be chosen.

The working gas diffusion in this type of electrode depends mainly on the structure of the catalytically active material. The gas-permeable layer of known gas diffusion electrodes is produced from a porous polytetrafluoroethylene film having a pore size of 0.1 to 0.5 (µm.). electrode function.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an efficient process for the production of moisture-impregnated powder material for producing gas diffusion electrodes. The powder material must be highly and uniformly hydrophobic, its structure must provide good electrochemical characteristics, the electrode made of such a powder material must have a long service life, the active catalyst surfaces must be maximally utilized and the unobstructed passage of gas must be ensured.

The essence of the process according to the invention consists in the addition of 65 to 70 wt. % of electrically conductive powder free of micropores such as carbon black is mixed in the form of an aqueous suspension or directly with 30 to 35 wt. % of a polymeric impregnating agent dispersed in an aqueous suspension or dissolved in an organic solvent in an amount of 10 to 60 wt. %, after which the mixture is homogenized, the dispersion medium is removed and the impregnated solid phase material is dried, milled and, after removal of the stabilizers, milled.

The aqueous suspension contains 3 to 5 wt. % electrically conductive powder. As the water-impregnating agent, the cell is used alone or in a combination of hydrophobic polymers such as polytetrafluoroethylene, polyethylene, polypropylene and natural or synthetic latex.

The impregnating agent content in the aqueous dispersions or in the organic solutions is suitably chosen to be 30% by weight. When organic impregnating solutions are used, an electrically conductive powder is introduced into them at a temperature close to the boiling point of the organic solvent. The aqueous polymer dispersion stabilizers are removed from the impregnated material by heating to 250 to 350 ° C for 30 to 90 min, or removed by washing followed by heating to 100 to 140 ° C.

By a micropore-free particle is meant particles that are not penetrated by micropores, and their surface can be rough. By the electrically conductive powder suspension is meant a system of two coarse-dispersed phases, i.e. a solid phase and a liquid. In this case, the liquid is a dispersion medium and the solid phase is a dispersed phase.

Moisture impregnants are substances with a dipping angle greater than 90 °, predominantly organic high molecular weight compounds. They will be used in the form of organic solutions, fine powders or aqueous dispersions. In order to prevent sedimentation of the aqueous dispersions, capillary active substances, so-called stabilizers, are usually incorporated therein.

The powder materials prepared by the process according to the invention make it possible to produce air electrodes with a long service life. The pores of the diffusion layer of these pulverulent materials are of an order of magnitude smaller than those of the known electrodes and the diffusion of the working gas - oxygen - does not depend on the presence of electrochemically inactive gases. The material has uniform hydrophobic properties, which ensures stable performance characteristics of the electrodes produced.

The invention is illustrated by the following non-limiting examples.

Example 1 1 g of carbon black is suspended in 350 cm @ ^{3 of} water. 150 cm ^{3 of an} aqueous dispersion of polytetrafluoroethylene having a dry matter content of, for example, 3 wt. % at a temperature of 15 to 45 ° C.

After homogenization of the mixture, the dispersion medium is removed by filtration and drying of the precipitate. The dry material is milled and heated at 250 to 350 ° C for 30 to 90 minutes, and then milled again. The powdered material obtained contains, for example, 70 wt. % carbon black of polytetrefluoroethylene is used for the production of electrodes with diffusing gas, in particular air oxygen, for metal-air sources of energy and fuel cells.

Example 2

The procedure is as in Example 1, except that an appropriate amount of natural rubber in the form of an aqueous dispersion (latex) with a dry matter content of 5 wt. %. Stabilizers are removed from the end product by washing with hot water followed by drying at 100-140 ° C.

Example 3

The procedure was as in Example 2, except that butadiene-styrene rubber in the form of an aqueous dispersion or latex was used as the moisture-impregnating agent.

EXAMPLE 4 g of high pressure polyethylene are dissolved in 200 cm @ 3 of toluene with heating to 100 DEG-140 DEG. 8 g of carbon black are introduced into the solution obtained and 300 cm @ 3 of toluene are added. After cooling the suspension, the organic solvent is filtered off and the precipitate is dried and milled. The moisture-impregnated powder material obtained is used to produce electrodes with diffusing gas, in particular air oxygen.

Example 5

The procedure was as in Example 4, except that polyieobutylene was used as moisture.

Impregnating agents against

Example 6

The procedure is as in Example 4, except that polypropylene is used as the moisture-impregnating agent.

The moisture impregnated powder materials according to the procedures of Examples 1 to 6 have all the necessary properties to form a hydrophobic porous structure of diffusing gas electrodes. The physical properties of hydrophobic powdered materials using various waterproofing agents are compiled in the following comparative table.

Table

Moisture impregnants Porovitost% Specific surface area (BET method) m ² .g “' Gas permeability coefficient 7 -1 -1 cm .s .Pa Specific electric resistance Hcm Polytetrafluor- ethylene 54.7 21.8 3,76.10 ¹ 0.63 Polyethylene high pressure 64.0 22.5 4,50.10 ⁷ 0.56 Polyisobutylene 63.0 25.2 5.25.Ю ⁷ 0.68

Content of impregnating substance

P = 20 MPa against moisture-immutable

These hydrophobic powder materials can be stored in large quantities for long periods without altering the quality. As a result, they are a suitable material for mass production of diffusing gas electrodes.

Using the moisture-impregnated powder materials obtained by the process of the invention, diffusing gas electrodes can be produced with different catalysts, capable of operating in different systems, eg metal-air systems or fuel cells 8, with gaseous or liquid fuel using acidic or liquid fuel. alkaline electrolytes.

Air oxygen diffusion electrodes made of said powdered materials using activated carbon catalyst without noble metal additive allow a current density of 150 niA.cm &^lt; ² &^gt; with respect to the reference electrode Hg / HgO and at the gas is oxygen.

the lifetime of said electrodes during continuous operation varies and depends on both the type of moisture-impregnating substance and the operating current density.

For example, the lifetime of an electrode working with air as a process gas and a polyethylene impregnating binder at a current density of 50 mA.cm ² is 2000 hours.

If the diffusion electrode contains polyisobutylene as the impregnating and binding agent, it has a double lifetime at a current density of 50 mA.cm &^lt; ² &^gt;, ie 4,000 hours.

Best results are obtained when polytetrafluoroethylene is used as the impregnating agent and binder. At a current density of 50 mA.cm ” ² , the life of the electrode is 10,000 hours, and at a current density of 30 mA.cm” ² , the life of the electrode is 15,000 hours.

Claims (7)

SUBJECT OF THE INVENTION Process for producing a moisture-impregnated powder material for producing diffusing gas electrodes from an electrically conductive powder and a moisture-impregnating agent, characterized in that 65 to 70 wt. % of the electrically conductive powder free of micropores such as carbon black is mixed in the form of an aqueous suspension or directly in 30 to 35 wt. % polymeric or impregnating agent, dispersed in an aqueous suspension or dissolved in an organic solvent in an amount of 10 to 60 wt. %, after which the mixture is homogenized, the dispersion medium is removed and the impregnated solid phase material is dried, milled and, after removal of the stabilizers, milled. 2. A process according to claim 1 wherein the aqueous suspension comprises 3-5 wt. % electrically conductive powder. Method according to claim 1, characterized in that the polymeric water-impregnating agent is polytetrafluoroethylene and / or polyethylene and / or polyisobutylene and / or propylene and / or natural and / or synthetic latex. 4. The process according to claim 1, wherein the aqueous dispersion or organic solution comprises 30 wt. % moisture impregnating agent. 5. The method of claim 1, wherein the electrically conductive powder and the moisture impregnating agent are mixed and homogenized at room temperature or near the boiling point of the organic solvent, whereupon the resulting mixture is cooled. 206688 6. A process according to claim 1, wherein the stabilizers are removed from the impregnated material by heating to 250 to 350 [deg.] C. for 30 to 90 minutes. 7. The method of claim 1, wherein the stabilizers are removed from the moisture-impregnated material by washing and subsequent heating to a temperature of 100 to 140 ° C.