Title of Invention

A PROCESS FOR THE PRODUCTION OF A GAS DIFFUSION ELECTRODE

Abstract

With a method for production of a gas diffussion electrode made of a silver catalysator no PTFE substratum reproducible results should be obtained under avoidance of the dis-advantages of the state of the art technic, where this is achieved thereby, that the poring system of the stiver catalysator is filled up with a moistening liquidity a dimension stable solid body with a grain size above that of the silver catalysator under the silver catalysator is mixed this accordingly compression stable substance is formed in a calender for a homogeneous catalystator band and in a second calender step an electrically conductive deflecting material is pressed-in in the catalysator band.

Full Text

The present invention pertains to a method for production of porous gas diffusing electrodes of the class (sort) indicat- ed in the patent claim no. 1. Such a gas diffusing electrode can base on , for example , a catalytic-active silver or silver alloys for the application in electro-chemical cells , especially of the chlorine (gas) - alkali - electrolysis or on alkaline fuel cells. In electrochemical cells the reduction of oxygen is imple- mented at platinum, silver or also carbon. Platinum is appli- cable both in acidic as well as in alkaline surrounding , whereas silver and carbon are corrosion resistant only in .. alkalic electrolytes. However, in case of the silver catalyt- ic agent also in alkaline media a rash deactivisation occurs, which is explained by rearrangement of the oxldic surface of the silver. ( Texas Instruments, U.S. 35 05 120 ). It has been attempted on several occassions , to reduce the corro- sive attack of the silver by suitable alloy partners. There both alloys with precious materials like platinum, palladium gold and mercury ( quick silver) ( DE 20 21 009 ) , but also with non -precious materials (metals) , like nickel (DE 15 46 728), , copper and other metals are known. It has also been attempted to achieve a stabilisation of the silver both by purification as well by anodic corrossion -protection (local element), In case of the corrosion, there builds at first a silver- oxide-surface Since silver oxide is relatively good solu- ble in alloys , with that a rearrangement of the silver cr- ystals can take place. In the print 2 and 3 REM inclusion of stiver electrodes before and after the operation are represen -ted. It is possible to recognise very distinctly the shrink- age of the inner porous structure. The catalytic activities goes down. Besides the stabilisation the method for production of an active silver catalytic agent (catalyser) must also ensure, that the active surface of the silver is adequately large, that means, the grain size of the silver is as far as possi- ble small. Thus, for example, is known from the booklet (US 3 668 101), that the very active silver catalysers are achieved in case of particle diameter of 5 to 10 µm size. Further more methods are known, in case of which it is attem- pted, to produce smallest particles of stable silver alloys. Adequately small silver particles are generated during proced- ures, of precipitation. Besides the control of p-H value, of the temperature and of the supersaturation, there plays so called crystallsing embryo an extraordinary role, in order to produce smallest silver particles. Known is also a method (EP 0 115 8^5)t in case of which a mixture from silver nltr - ate and quick silver (mercury) nitrate is deposited on a PTFE dispersion by addition of potassium hydroxide. Herewith a silver amalgam is produced with smallest particle diameter . In order to produce so called gas diffussion electrode from this catalytic agents, as these are required in fuel cells or in the chloride -alkali-electrolysis, the powder must be processed for a homogeneous and laminar electrode. This elec- trode must be electrically conductive and must render possi- ble both the entrance of the electrolyte as well as also gas. Even the spheres of the electrode must be in a position to be moistened,whereas other spheres of moistening must be protected. A solution of this problem was represented with a bi-porous poring structure. The electrolyte can at first penetrate without any trouble both in the small as well as in the large pores. By en excess pressure of the gas or the gravitational force the electrolyte is distanced again from the larger pores.Such biporous structures operate only then satis- factorily, when a difference of pressure between the gas cha- amber and the electrolyte room can prevail. Whether such di- fference of pressure can build automatically, when in the electrolyte a membrane separates the anode and cathode from one another, is questionable. Thereby, it is not possible that these electrodes be related to the chlorlde-alkaline-electr- lysis or in fuel cells with alkalic membrane. Therefore it was attempted to construct on the material pro- perties a bi-porous poring system. That means , one requires hydrophile and hydrophob materials. Suitable hydrophobmate- terials are some thermoplasts- asfor example , Polytetrafl- uorethylene. The mentioned catalytic agents and also the silver are always hydrophyl. If one mixes silver and PTFE together and forms therefrom a plane electrode, then this displays different spheres with hydrophil and hydrophob pro- perties. Mostly an additional metalic conductor is integrat _ ed, in order to achieve better electrical properties. Some methods are known, to produce mixtures from PTFE (Poly- tetrafluorethylene) and catalytic agent a so called gas di- ffussion electrode. Thus it is proposed, for example , in the write-up (EP 0 115 845) to float (suspend) on the mate- rials in the manner that one gets a paste, which in form can be poured, passed and dried up. Disadvantageous in case of such " pasted " electrodes is the homogeneous division of the material - It can easily result in small holes , through which gas or electrolyte can penetrate. In order to prevent this, the electrodes are produced at least 0.8 mm thick. As a result, there prevails a very high quantity of silver in the electrode (around 2 kg/m2), so that the ad- vantage in respect of the silver is again gone lost. Since these commercial oxygen electrodes with around 4-40 g/m2 Platinum/Carbon are known. Two methods are known, in case of which out of such hydroph- obes/ hydrophlle materials a thin , homogeneous gas dlffu- ssion electrode is rolled. According to the method (EP-0 144 002, US 4 696 872) in one special mixer the catalytic parti- cles and the PTFE are mixed with one another in such a manner, that a finely mixing hydrophobe network system condenses on the Catalysator. In a powder roller the loosely poured subs- stance is rolled together to a folio of around 0.2mm. thick. This method has proved Itself worthwhile for mixer of PTFE and Carbon, or PTFE and Raney-Nickel. It is similarly possi- ble, thereby as a result, to roll (rough) down a Ranny-Sil- ver- Alloy with 80% Allumlnium for a porous foil. Such s calender rolling mill is represented in the print no. 1. However, it is not possible to process the ductile Silver. In case of the necessary surface (contact)-pressure-around, 0.01 to 0.6 t/cm2 in such powder rollers PTFE and Silver are pressed to a compact, gas and electrolyte impermeable foil. The cirrent-voltage-characteristic line of such an electrode has to be seen in Print no.5. In order to be able to produce Silver electrodes, at first a Silver Oxide/ PTFE mixture is processes in the pulverising roller and subsequently reduced Electro-chemlcally/ (DE 37 10 168). The Silver oxide is stable enough, in order to resist the surface pressure of the roller. Besides that, the volume reduces itself during transition from Silver oxide to the Silver, so that additional pores develop in thegas diffussion electrode. By the parameter during reduction the grainsire of the particle can very well be adjusted. Disadvantageous in case of this method is that it is not yet known, how Siver Alloys with cata- lytic properties can be reduced electrochemlcally. Thereby it is also not possible to produce long time stable Silver elec- trodes by the electrochemical reduction. The task of the present invention exists therein , to make readily available a method of production of a gas diffusion electrode, with which not only the disadvantages of the state of art technique is avoided, but also specially reproducible results during methodical production adjusts itself. With a method of the nature Indicated in the beglning this prob lem is solved in accordance with the invention thereby , that the -the porous system of the Silver catalytic agent is filled up with a moistening liquidity, - a dimension stable rigid body is mixed with a grain size above that of the Silver catalytic agent below the silver catalytic agent - this consequent compression-stable substance is formed but in a calender homogeneous to a catalytic agent band and - ina «econd calender step an electrically conductive arresting material is pressed-in in the catalytic agent band The speciality of the method in accordance with the invention lies therein, that inner porous system of the ductile mate- rials has to be filled up with a liquidity. Since this liqui- dity cannot be condensed and on the other hand is connected through the capillar forces rigidly in the poring system, it ls possible also in case of the present pressure of maximum i 600 kg/ cm2 to distance the liquidity from the micro pores. A further mixture of some Carbon powder or of the volatile ammonium carbonate can furthermore receive (hold) the mecha- nical pressure of the powder rolls. By means of these coarse grainedjadditions of typical 10 - 100 µm grain diameter the pouring system with larger pore diameter is protected from the condensation. By means of a subsequent temperature pace it is possible that both the liquidity as well as the solid matter ammonium carbonate are operated from the electrode. In this manner it is possible to receive large pores in the gas diffusion electrode,which take care of the quick transport of gas and smaller pores in a catalytic agent which permits a homogeneous utlllisation of the catalysator ( catalytic agent). A prefered implementation of this method is represented as follows : at first Silver or a Silver alloy is produced by a precipitation process. There it is advantageous,to imple- ment the precipitation on a PTPE dispersion. The best expe- riences are made with a mixture of 15% Teflon and 85% Silver. With the help of addition of formaldehyde during precipitation there converts itself in the alkalic surrounding the Silver hydroxide immediately in a Silver crystal. The precipitation substance is washed and dried up. A subsequent temperature at 200 C improves the electrical contact between the Silver particles and drives the residual liquidities out. To this powder a volume of around 5% - 40 %t preferably however 8 %% of a liquidity is given, which can penetrate in the poring system of the PTPE and of the Silver. Because of the hydrophobe character of the PTPE there come thereto only Isopropanol Ethanol and Methanol in question. If the powder is moistened and filled up with such solution agent, then an exchange of liquidities can be connected at this . If one brings, for example, a powder impregnated in Iso- propanol in a water bath, or Glycerine, then there exchange within hours the liquidities by diffussion. In this manner there arrives a liquidity in the poring system of the PTFE, which is usually knocked off (planned off) by the PTFE. This thus moistened ( dampened) material behaves ltsdlf taa in this manner outwards as a powder, since the liquidity prevails only in the inner poring system. Another class of the moistening medium are the so called tenside. These penetrate both in the poring system, cover also the surface of the catalytic agent and reduce thus its surface roughness. This small surface roughness leads in the rolling process thereto, that the Silver Catalytic agent can deviate from the compression *one, whereas other powder components, which have not been treated, remain in the compression tone and thus then generate the electrode connection level, in which the Silver Catalytic agent is embedded (print 4). As such a powder there proffer (offer) Ammonium carbonate or active coal, which is now mixed with the Silver Catalytic agent in a crusher to a homogeneous substance, as it is described in EP 0 144 002. Subsequently the loose vibrated (shaken) substance is cogged (rolled out) with the help of powder roll to approximately 0.2 mm thick foil. In a second pair of rolls a metallic supporting frame in the form of a woven network or expanded metal can be rolled in, and thereby the mechanical stability and the electri- cal conductivity can improve. According to this procedure the gas diffussion electrode is dried up. According to that the electrode has a silver coating between 0.2 kg/m2 and 1.5 kg/m2. Conventionally a weight of around 0.5 kg/m2 is attem- pted . Therewith upto 75% of the uptil now necessary silver can be saved (spared). Despite the reduced silver weight there follows with such electrodes current-voltage-charac- teristic line as are to be seen from the print no.6. This method may be obviously combined also with other methods. Thus it is possible to ignore the environmentally harmful formaldehyde during the precipitation and for this purpose the reduction after the production of GDE can be completed in electrochemical way. It may also similarly produce alloys in which a co-precipitation of Silver and Quick silver (mer- cury) , Titan, Nickel, Copper, Cobalt or Bismuth be imple- mented. Specially for the Chlorine alkali-electrolysis it is possi- ble to under-talie changes at the finished gas diffussion electrode, which render possible a better transport of the developing caustic-sodat For this purpose, for example, the induction of a coarse deflecting system would be advisable. This is possible, if on the finished electrode a network is pressed and subsequently is drawn out again. The negative pressure of the network builds channels, in which the elec- trolyte can flow off later parallel to the electrode surface. Further features, details and advantages of the invention result on the ground of the following prints. These show in Pig. 1 j (Print 1) a functional diagram of a plant in accordance with the Invention Print 2 : a microscopic absorption of a silver electrode before the application (use), Print 3 * *n same kind of representation a silver electrode after the application (use) Print k t a PTPE-frame embedded in a Silver catalytic agent Print 5 t a current/voltage diagram of a Chlorine alkali- electrolysis as well as in Print 6 $ the same curve according to the measure of the Invention WE CLAIM 1. A process for the production of a gas diffusion electrode from a silver catalyst on PTFE substrate, wherein that. - the poring system of the silver catalyst is filled with a wetting liquid, - a dimensionally stable solid body with a grain size above that of the silver catalyst is mixed among the silver catalyst, - that mass which is thus stable in respect of compression is shaped in a calendar to form a homogeneous catalyst strip, and - in a second calendering step an electrically conductive discharge conductor material is impressed into the catalyst strip. 2. A process as claimed in claim 1 wherein that 5% isopropanol is used as the wetting liquid and 30% ammonium carbonate or ammonium hydrogen carbonate is used as the solid and those two fillers are expelled after electrode production by a heat treatment step at preferably 110°C. 3. A process as claimed in claim 1 wherein a tenside - preferably 5% Triton X 100 - is used as the wetting liquid, which both penetrates into the pore system of the catalyst but also reduces surface friction so that the silver catalyst can slide out of the compacting zone and the dimensionally stable am-monium carbonate and the PTFE binder absorb the rolling pressure. 4. A process in particular as claimed in claim 1 wherein in the first calender step a homogeneous catalyst strip of a thickness of 0.2 - 0.5 mm is produced. 5. A process as claimed in one of the preceding claims wherein the rolling gap is set to 350 pm and the rolling advance is set to about 2 meters per minute. 6. A process as claimed in one of the preceding claims wherein a silver - plated nickel wire mesh of a wire thickness of 0.15 mm and a mesh width of 0.45 mm with a silver covering of about 10 µm in thickness is used as the electrical discharge conductor material. Dated 13th DAY OF November 2003 With a method for production of a gas diffussion electrode made of a silver catalysator no PTFE substratum reproducible results should be obtained under avoidance of the dis-advantages of the state of the art technic, where this is achieved thereby, that the poring system of the stiver catalysator is filled up with a moistening liquidity a dimension stable solid body with a grain size above that of the silver catalysator under the silver catalysator is mixed this accordingly compression stable substance is formed in a calender for a homogeneous catalystator band and in a second calender step an electrically conductive deflecting material is pressed-in in the catalysator band.

Documents:

1482-KOLNP-2003-CORRESPONDENCE.pdf

1482-KOLNP-2003-FOR ALTERATION OF ENTRY.pdf

1482-KOLNP-2003-FORM 27.pdf

1482-KOLNP-2003-FORM-27.pdf

1482-kolnp-2003-granted-abstract.pdf

1482-kolnp-2003-granted-claims.pdf

1482-kolnp-2003-granted-correspondence.pdf

1482-kolnp-2003-granted-description (complete).pdf

1482-kolnp-2003-granted-drawings.pdf

1482-kolnp-2003-granted-examination report.pdf

1482-kolnp-2003-granted-form 1.pdf

1482-kolnp-2003-granted-form 18.pdf

1482-kolnp-2003-granted-form 2.pdf

1482-kolnp-2003-granted-form 26.pdf

1482-kolnp-2003-granted-form 3.pdf

1482-kolnp-2003-granted-form 5.pdf

1482-kolnp-2003-granted-reply to examination report.pdf

1482-kolnp-2003-granted-specification.pdf

1482-kolnp-2003-granted-translated copy of priority document.pdf