DE102011007297A1 - Gas diffusion electrode, process for its preparation and its use - Google Patents

Abstract

A method for producing a gas diffusion electrode and a method for producing a metal-air cell having a gas diffusion electrode as the air cathode are described. In the methods, the electrode is applied to a substrate in the form of a flat layer by means of a printing process, in particular by means of a screen printing process. The gas diffusion electrode produced in this way and metal-air cells with such a gas diffusion electrode are also described.

Description

The present invention relates to a plastic-bonded gas diffusion electrode, a process for producing such gas diffusion electrodes and metal-air cells having such gas diffusion electrodes.

Metal-air cells usually contain as electrochemical active components a metal-based anode and an air cathode, which are separated from each other by an ion-conductive electrolyte. During the discharge, oxygen is reduced under the electrode holder at the air cathode. Hydroxide ions are formed which can migrate via the electrolyte to the anode. There, a metal is oxidized with electron donation. The resulting metal ions react with the hydroxide ions.

There are both primary and secondary metal-air cells. A secondary metal-air cell is recharged by applying a voltage between the anode and cathode and reversing the described electrochemical reaction. This releases oxygen. The best known example of a metal-air cell is the zinc-air cell.

Metal-air cells have a relatively high energy density because the need for oxygen at the cathode can be met by atmospheric oxygen from the environment. Accordingly, the cathode must be supplied with oxygen during the unloading process. Conversely, when charging a metal-air cell at the air cathode resulting oxygen must be dissipated.

Gas diffusion electrodes are usually used as the air cathode in metal-air cells. Gas diffusion electrodes are electrodes in which the substances involved in the electrochemical reaction (usually a catalyst, an electrolyte and atmospheric oxygen) are present side by side in solid, liquid and gaseous form and can come into contact with each other. The catalyst catalyzes the reduction of the atmospheric oxygen during the discharge and optionally also the oxidation of hydroxide ions during the charge of the cells.

Plastic-bonded gas diffusion electrodes are most commonly used as air cathodes in metal-air cells. For example, in the DE 37 22 019 A1 Described electrodes in which a plastic binder (usually polytetrafluoroethylene, short PTFE) forms a porous matrix in which particles of an electrocatalytically active material (eg., From a noble metal such as platinum or palladium or a manganese oxide) are embedded. These must be able to catalyze the aforementioned reaction of atmospheric oxygen. The preparation of such electrodes is usually carried out by a dry mixture of the binder and the catalyst is rolled into a film. This can in turn be rolled into a metal net, for example of silver, nickel or silver-plated nickel. The metal network forms a discharge structure within the electrode and serves as a current collector.

Batteries can be produced not only by joining solid individual components, but in recent years reinforced batteries are gaining in importance, for their production at least individual functional parts, in particular the electrodes and / or required tracks, by pressure, ie from a solution and / or suspensionsmittelhaltigen Paste, to be prepared. As a rule, printed batteries have a multilayer structure. In a conventional construction, a printed battery usually comprises two current collector planes, two electrode planes and a separator plane in a stack-like arrangement. The separator plane is arranged between the two electrode planes while the current collectors form the top and the bottom of the battery. A battery with such a structure is for example in the US 4,119,770 described.

Significantly flatter batteries, in which the electrodes are next to each other on a flat, electrically non-conductive substrate, are the WO 2006/105966 refer to. The electrodes are connected to each other via an ion-conductive electrolyte, which may be, for example, a gelatinous zinc chloride paste. As a rule, the electrolyte is reinforced and stabilized by a non-woven or net-like material.

So far, only printed batteries with solid electrodes are known. These are on the cathode side, for example, brownstone electrodes in aqueous systems and electrodes of lithium cobalt oxide or lithium iron phosphate in systems with organic electrolyte. Batteries that combine printed functional parts with a gas diffusion electrode are still unknown.

The present invention has for its object to provide a gas diffusion electrode, which is suitable as an air cathode for printed batteries.

This object is achieved by the method having the features of claim 1. Preferred embodiments of the method according to the invention are specified in the dependent claims 2 to 5. Furthermore, the method with the features of claim 6 and the gas diffusion electrode and the metal-air cell according to claims 10 and 11 of the present invention are also included. Preferred embodiments of the A method according to claim 6 are given in the dependent claims 7 to 9. The wording of all claims is hereby incorporated by reference into the content of this specification.

The inventive method is used to produce plastic-bonded gas diffusion electrodes with the functionality described above. Like these, the gas diffusion electrodes according to the invention have a porous plastic matrix in which particles of an electrocatalytically active material (in short: catalyst particles) are embedded. In particular, gas diffusion electrodes produced according to the method of the invention are suitable as air cathodes for metal-air cells.

In particular, the method according to the invention is characterized in that the electrodes are formed by means of a printing process, preferably applied to a substrate in the form of a flat layer.

In the present case, a printing process should generally be understood to mean that a paste, a solid-liquid mixture, is applied to a substrate.

Surprisingly, it has been found that functioning gas diffusion electrodes can be easily printed from a paste comprising a solvent and / or a suspending agent, particles of an electrocatalytically active material (the catalyst particles) and particles of a hydrophobic plastic (from which the porous plastic matrix is formed). As mentioned at the outset, the production of polymer-bonded gas diffusion electrodes takes place conventionally by pressing dry mixtures of a plastic binder and a catalyst. The fact that functioning gas diffusion electrodes can also be produced by a comparatively simple printing process from a solvent and / or a suspending agent-containing paste was not to be expected a priori.

The mentioned printing process is particularly preferably a screen printing process. Screen printing is known to be a printing process in which printing pastes are pressed by means of a doctor blade through a fine-meshed fabric onto the material to be printed. At the points of the fabric where no paste is to be printed according to the printed image, the mesh openings of the fabric are made impermeable by a template. On the other hand, the print paste should be able to penetrate the mesh openings easily. To prevent clogging of the mesh openings, the solids contained in the printing paste should not exceed a certain maximum size, which should be less than the mesh opening width.

The particles in the presently preferably used pastes have in particular an average diameter between 1 .mu.m and 50 .mu.m. The pastes preferably contain no particles with a diameter and / or a length> 120 μm, more preferably> 80 μm. These preferred size ranges apply to both the particles of the hydrophobic plastic and those of the electrocatalytically active material.

The solvent and / or the suspending agent is preferably a polar solvent, in particular water. Optionally, water-alcohol mixtures can also be used. The solvent or suspending agent is usually removed after application of the paste. You can simply let it evaporate at room temperature. Of course it is also possible to support the evaporation by active measures such as elevated temperatures or the application of negative pressure.

The particles of the electrocatalytically active material are preferably the catalyst materials already mentioned at the beginning, ie in particular particles of a noble metal such as palladium, platinum, silver or gold and / or a manganese oxide. With regard to applicable manganese oxides is in particular the already mentioned DE 37 22 019 A1 reference, the contents of which are hereby incorporated by reference in their entirety into the content of the present description.

The particles of the hydrophobic plastic are in particular particles of a fluoropolymer. Particularly suitable as a fluoropolymer is the already mentioned PTFE. This is particularly suitable because of its chemical resistance and its hydrophobic character. Mixed with the more hydrophilic electrocatalytically active particles, it forms an electrode structure with both hydrophilic and hydrophobic areas. In such a structure, both aqueous electrolyte and air can penetrate. Thus, the already mentioned aggregate states in the electrode can exist parallel to one another. That the production of such porous structures without hot pressing or sintering is possible is very surprising.

The paste used in a method according to the invention preferably contains at least one conductivity-improving additive, in particular a particulate conductivity-improving additive. This can in particular be selected from the group with carbon nanotubes (CNTs), carbon black and metal particles (eg from nickel).

The particles have preferred sizes in the areas already mentioned above for the particles of the hydrophobic plastic and of the electrocatalytically active material.

The paste may further comprise one or more additives, in particular for adjusting the processing properties of the paste. Correspondingly, all additives suitable for printing pastes can be used as additives, for example, auxiliaries with which the viscosity of the paste can be adjusted.

The paste used according to the invention preferably has a proportion of the solvent and / or of the suspending agent of between 20% by weight and 50% by weight. In other words, the solids content of the paste is preferably in the range between 50 wt .-% and 80 wt .-%.

• • zwischen 20 Gew.-% und 50 Gew.-% des Lösungsmittels und/oder des Suspensionsmittels,
• • zwischen 0 Gew.-% und 20 Gew.-% der Partikel aus dem elektrokatalytisch aktiven Material,
• • zwischen 0,5 Gew.-% und 5 Gew.-% der Binderpartikel aus dem hydrophoben Kunststoff und
• • zwischen 30 Gew.-% und 80 Gew.-% des mindestens einen leitfähigkeitsverbessernden Additivs.

Most preferably, the paste comprises the following components in the following proportions:

• Between 20% by weight and 50% by weight of the solvent and / or of the suspending agent,
• Between 0% by weight and 20% by weight of the particles of the electrocatalytically active material,
• Between 0.5 wt .-% and 5 wt .-% of the binder particles of the hydrophobic plastic and
• Between 30% by weight and 80% by weight of the at least one conductivity-improving additive.

The percentage proportions of said constituents preferably add up to 100% by weight.

The process according to the invention is particularly preferably a part or a partial step of a process for producing a metal-air cell which has the plastic-bonded gas diffusion electrode as the air cathode. Accordingly, a method for producing a metal-air cell with an air cathode, which is prepared as described above, subject of the present application and invention.

• – Bereitstellen eines Substrats und
• – Aufdrucken der Luftkathode bevorzugt in Form einer flächigen Schicht auf das Substrat.

The method according to the invention for producing a metal-air cell generally comprises the steps

• - Providing a substrate and
• - Printing the air cathode preferably in the form of a flat layer on the substrate.

In this case, the air cathode is produced in particular according to a method as described above.

Particularly preferably, the electrode is applied to a substrate having a surface which is provided with a preferably grid or grid-like arrester structure. Accordingly, in preferred embodiments, the method according to the invention is characterized in that such a diverter structure is applied to the substrate before the air cathode is printed.

The arrester structure is preferably composed of interconnects and serves primarily as a current collector. Such interconnects can be realized in various ways. On the one hand, it is possible to use electrical foils, in particular metal foils, as conductor tracks. It is also possible to use a mesh or grid made of a metal, for example nickel, silver or silver-plated nickel. On the other hand, the interconnects can also be thin metal layers which can be applied to a substrate by means of a customary metallization process (eg by deposition from the gas phase). Finally, the printed conductors can of course also be printed, for example using a paste containing silver particles.

As an alternative or in addition, it is also quite possible for a arrester structure such as that described to be applied to the printed air cathode.

The substrate is preferably an air-permeable substrate, in particular a planar substrate made of a microporous material such as a fleece, paper, felt or a microporous plastic.

When printing the air cathode on the substrate, optionally on the arrester structure, layer composites with the layer sequence "substrate - air cathode" or "substrate - arrester structure - air cathode". These can be combined with a separator and an anode to produce the metal-air cell.

In particularly preferred embodiments, a separator, in particular a separator, as required in a metal-air cell, in particular in the form of a sheet-like layer on the air cathode or optionally on the arrester structure on the air cathode can be printed.

That also separators can be produced by printing, is in the hitherto unpublished patent application with the file number DE 10 2010 018 071.8 described. This proposes, for printing separators, a separator printing paste comprising a solvent, at least one conducting salt dissolved in the solvent, and particles and / or fibers which are at least nearly, preferably completely, insoluble in the solvent at room temperature and electrically non-conductive at room temperature are included. Surprisingly namely found that, for example, separators formed from a microporous film or from a nonwoven can be functionally readily replaced by an obtainable from such a Separatordruckpaste electrolyte layer having the mentioned particles and / or fibers.

Particles and / or fibers contained in the separator printing paste can form a three-dimensional matrix in the printing process, which gives the resulting separator a solid structure and a sufficiently high mechanical strength to prevent contacts between oppositely poled electrodes. Prerequisite, as already stated, that the particles and / or fibers are electrically non-conductive. Furthermore, they should be chemically stable with respect to the solution of the at least one conducting salt and the solvent, at least at room temperature, especially not or only very slightly dissolve. The particles and / or the fibers are preferably present in the separator printing paste in a proportion of between 1% by weight and 75% by weight, in particular between 10% by weight and 50% by weight. It is irrelevant whether only particles or fibers or a mixture of particles and fibers is used.

The particles and / or the fibers preferably have an average diameter or, in the case of the fibers, an average length of between 1 μm and 50 μm. In this case, the separator printing paste is particularly preferably free of particles and / or fibers which have a diameter and / or a length of more than 120 μm. Ideally, the maximum diameter and / or the maximum length of the particles and / or fibers contained in the separator printing paste is 80 μm. This is due to the fact that the separator printing paste is also provided in particular for processing by screen printing.

The particles and / or fibers in the separator printing paste can basically consist of a wide variety of materials, provided that the above-mentioned requirements (electrically non-conductive properties and insolubility in or chemical resistance to the electrolyte solution) are met. Accordingly, the particles and / or fibers may consist of both an organic and an inorganic solid. For example, it is possible to mix fibers of organic materials with inorganic particles or vice versa.

The inorganic solid particularly preferably comprises at least one component from the group consisting of ceramic solids, salts which are almost or completely insoluble in water, glass, basalt or carbon. The term "ceramic solids" is intended to include all solids which may be used to make ceramic products, including siliceous materials such as aluminum silicates, glasses and clay minerals, oxidic raw materials such as titanium dioxide and alumina, and non-oxide materials such as silicon carbide or silicon nitride.

The organic solid preferably has at least one component from the group of synthetic plastics, semisynthetic plastics and natural substances.

The term "almost or completely insoluble at room temperature" means that at room temperature in a corresponding solvent there is at most a low, preferably no, solubility. The solubility of particles and / or fibers which can be used according to the invention, in particular the salts mentioned which are virtually or completely insoluble in water, should ideally not exceed the solubility of calcium carbonate in water at room temperature (25 ° C.). Incidentally, calcium carbonate is a particularly preferred example of an inorganic solid which may be contained as a component having a spacer function, especially in particulate form in a separator printing paste.

The term "fiber" is to be interpreted in this case very broad. It should be understood in particular elongated structures that are very thin in relation to their length. For example, fibers of synthetic polymers such as polyamide fibers or polypropylene fibers can be used well. Alternatively, fibers of inorganic or organic origin such as glass fibers, ceramic fibers, carbon fibers or cellulose fibers can be used.

The solvent in the separator printing paste is preferably a polar solvent, for example water. In principle, however, it is also possible to use nonaqueous aprotic solvents, as known from the field of lithium-ion batteries.

The conductive salt in a separator printing paste is preferably at least one compound which is soluble at room temperature in the solvent contained in the printing paste or which is present in this form in the form of solvated ions. It preferably comprises at least one component from the group with zinc chloride, potassium hydroxide and sodium hydroxide. In addition, conductive salts, such as lithium tetrafluoroborate, which are also known in particular from the field of lithium-ion batteries, may optionally also be used as conductive salt.

In addition to conductive salts, a solvent and the described particles and / or fibers, the separator printing paste may additionally comprise a binder and / or one or more additives. While the binder serves in particular to impart better mechanical stability, ideally better mechanical strength and flexibility, to the separator which can be produced from the separator printing paste, the additives serve, in particular, to vary the processing properties of the separator printing paste. Correspondingly, all additives suitable for printing pastes can in principle be used as additives, for example, auxiliaries for the formulation, with which the viscosity of the separator printing paste can be adjusted. The binder may be, for example, an organic binder such as carboxymethylcellulose. Other, possibly also inorganic components such as silicon dioxide are suitable as additives with binding properties.

The separator is preferably printed in a thickness between 10 .mu.m and 500 .mu.m, in particular between 10 .mu.m and 100 .mu.m. In this range it has sufficiently good separating properties to prevent a short circuit between oppositely poled electrodes.

By printing the separator onto the air cathode, layer composites with the layer sequence "substrate - air cathode separator", "substrate - arrester structure - air cathode - separator", "substrate - air cathode - arrester structure - separator" or "substrate - arrester structure - air cathode - arrester structure - separator ". These can be combined with an anode to form the metal-air cell.

In further preferred embodiments, an anode may also be printed on the separator layer. It is thus possible to produce cells by means of the method according to the invention in which all functional parts (the anode, the air cathode, optionally the arrester structures and the separator) are printed.

• • Bereitstellen eines flächigen Separators als Substrat,
• • gegebenenfalls Aufbringen einer Ableiterstruktur auf den Separator und
• • Aufdrucken der Luftkathode in Form einer flächigen Schicht auf den Separator, gegebenenfalls auf die Ableiterstruktur.

In an alternative embodiment of the method according to the invention, this comprises the steps

• Providing a planar separator as a substrate,
• Optionally applying a trap structure to the separator and
• • Printing the air cathode in the form of a flat layer on the separator, optionally on the arrester structure.

As separators for this purpose, in particular the already mentioned printed separators are suitable. In preferred embodiments, a separator printed on a sheet anode is used as the substrate. Of course, the anode may also be a printed electrode, for example, an anode printed on an electrically non-conductive substrate provided with a drain structure.

As already mentioned, printed plastic-bonded gas diffusion electrodes, which are produced or can be produced by a process according to the invention, are also the subject of the present invention. With particular preference these gas diffusion electrodes are air cathodes for metal-air cells.

Preferably, these are formed as a sheet-like layer with a thickness between 60 microns and 300 microns.

Metal-air cells with such a gas diffusion electrode are also included in the present invention. These are particularly preferably zinc-air cells, ie cells which have a zinc-containing anode. The metal-air cells according to the invention are present in particular as printed batteries.

These preferably have a one-part layer composite comprising a planar substrate and an air cathode applied thereto.

Particularly preferably, the one-part layer composite has one of the following layer sequences:

• (1) air-permeable, flat substrate - air cathode
• (2) air-permeable, flat substrate - arrester structure - air cathode
• (3) air-permeable, flat substrate - air cathode - arrester structure
• (4) air-permeable, flat substrate - arrester structure - air cathode - arrester structure
• (5) air-permeable, flat substrate - arrester structure - air cathode - arrester structure - separator
• (6) Arrester structure - air cathode - arrester structure - separator
• (7) Air cathode - arrester structure - separator
• (8) Arrester structure - air cathode - separator
• (9) Air Cathode Separator

Further features of the invention will become apparent from the following description of preferred embodiments. It should be emphasized at this point that all facultative aspects of the method according to the invention or of the products according to the invention described in the present application in each case alone or in combination with one or more of the further described optional aspects in a Embodiment of the invention can be verwirktlicht. The following description of preferred embodiments is merely for the purpose of illustrating and understanding the invention and is in no way limiting.

embodiments

(1) Production of a Laminate with the Layer Sequence "Substrate - Arrester Structure - Air Cathode"

On a microporous Teflon film (as a substrate), a reticular structure of current conductors (the arrester structure) was printed from a silver paste. On this an air cathode was printed by means of a screen printing process. The paste for the air cathode contained a mixture of 5 parts by weight of Teflon particles (as particles of the electrocatalytically active material) with an average particle size of 10 .mu.m, 10 parts by weight of manganese oxide particles with an average particle size of 20 .mu.m (as particles of the electrocatalytically active material) and 50 Parts by weight of activated carbon (as a conductivity-improving additive) with a particle size of 50 μm. As a liquid component, the paste contained 35 parts by weight of butanol (as a suspension and / or solvent). The air cathode was printed in a layer thickness of about 100 microns on the Teflon film. After removal of the solvent or the suspending agent, the layer thickness of the resulting planar air cathode on the film was about 50 microns. The resulting layer composite with the layer sequence "substrate - arrester structure - air cathode" had a total thickness of about 150 microns.

(2) Production of a Laminate with the Layer Sequence "Substrate - Conductor Structure - Air Cathode - Separator"

A separator was printed on the laminate produced according to (1). To this was mixed 77.8 parts by weight of a 50% zinc chloride solution with 3.4 parts by weight of amorphous silica and 18.8 parts by weight of a calcium carbonate powder. The dissolved zinc chloride should ensure the required ion conductivity of the electrolyte in the battery to be manufactured. The calcium carbonate powder used consisted of about 50% of a powder having a mean particle size <11 microns and about another 50% of a powder having a mean particle size <23 microns. So it had a bimodal distribution. The silica was used in particular for adjusting the viscosity of the paste according to the invention.

With such a paste, the air cathode was overprinted. The resulting electrolyte or separator layer had a thickness of about 50 microns.

(3) Preparation of printed zinc air cell

To produce a printed zinc-air cell, the electrolyte or the separator layer of the layer composite produced according to (2) was overprinted with a zinc-containing anode paste. In this way, a cell was formed on a substrate such as the Teflon film used here, which has exclusively printed functional parts.

For electrical contacting of the zinc anode can be applied to the anode, for example, a further plastic film, which is provided with a corresponding arrester structure. This additional plastic film can form a housing with the Teflon film, which protects the printed zinc-air cell from disturbing environmental influences.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 3722019 A1 [0006, 0019]
• US 4119770 [0007]
• WO 2006/105966 [0008]
• DE 102010018071 [0036]

Claims (11)

Method for producing a plastic-bonded gas diffusion electrode, in particular an air cathode for metal-air cells, wherein the electrode is applied by means of a printing process, in particular a screen printing process, in the form of a sheet-like layer on a substrate. A method according to claim 1, characterized in that the electrode is printed from a paste comprising a solvent and / or a suspending agent, particles of an electrocatalytically active material and binder particles of a hydrophobic plastic. A method according to claim 2, characterized in that it is the solvent and / or the suspending agent is water. Method according to one of claims 2 or 3, characterized in that it is the particles of the electrocatalytically active material to particles of a noble metal and / or manganese oxide. Method according to one of claims 2 to 4, characterized in that it is the particles of the hydrophobic plastic to particles of a fluoropolymer, in particular of PTFE. A method of manufacturing a metal-air cell with an air cathode comprising the steps - Providing a substrate and Printing the air cathode in the form of a flat layer on the substrate, wherein the air cathode is produced in particular according to a method according to one of claims 1 to 5. A method according to claim 6, characterized in that an arrester structure on the air cathode and / or on the substrate (before the printing of the air cathode) applied, in particular imprinted, is. A method according to claim 6 or claim 7, characterized in that it is the substrate to an air-permeable substrate, in particular a flat substrate of a microporous material such as a non-woven, paper, felt or a microporous plastic. Method according to one of claims 6 to 8, characterized in that a separator in the form of a sheet-like layer is printed on the air cathode or optionally on the arrester structure on the air cathode. Printed, plastic-bonded gas diffusion electrode, in particular air cathode for metal-air cells, produced or producible by a process according to one of claims 1 to 5. Metal-air cell with a gas diffusion electrode according to claim 10 as an air cathode.