DE102022115040A1 - Bifunctional gas diffusion electrode for alkaline electrochemical energy conversion systems and method for their production - Google Patents

Abstract

The invention relates to a bifunctional gas diffusion electrode for alkaline electrochemical energy conversion systems and a method for producing the same. According to the invention, a novel, long-lasting bifunctional gas diffusion electrode (GDE), which is material-stable in an alkaline medium, comprises a layered body (1) made of at least one metallic, carbon-free porous substrate (2), wherein the laminated body (1) is provided on one electrolyte side (3) for electrolyte contact and on an opposite air side (4) for air contact in the energy converter system and has a macropore size gradient falling from the electrolyte side (3) to the air side (4) and the at least one Substrate (2) comprises an oxygen oxidation and oxygen reduction active or hydrogen formation and hydrogen reduction active catalyst coating (5) and at least partially a hydrophobic coating (6), so that from the electrolyte side (3) to the air side (4) of the layered body (1) there is a transition from hydrophilic is too hydrophobic.

Description

The invention relates to a bifunctional gas diffusion electrode for alkaline electrochemical energy conversion systems with oxygen oxidation and reduction or hydrogen formation and reduction, which can be used in particular in regenerative fuel cells, metal-air batteries and electrolyzers, as well as a method for producing such a gas diffusion electrode (GDE).

Gas diffusion electrodes enable chemical reactions such as oxygen formation or oxygen oxidation (OER) or oxygen reduction (ORR). Depending on the application, either just one reaction or two reactions can take place at the GDEs. This depends on the catalysts used. If a GDE can be used for two reactions, for example oxygen formation (OER) and oxygen reduction (ORR) or hydrogen formation (HER) and hydrogen reduction (HRR) reactions, then this GDE is said to be bifunctional.

The WO 2007/065899 A1 shows an example of an OER and ORR gas diffusion electrode for metal-air batteries or metal hybrid-air batteries. The GDE includes a gas diffusion layer, an active layer, an oxygen generation layer and a current collector in electrical contact with the active layer, the active layer containing an oxygen reduction catalyst and a bifunctional catalyst selected from La ₂ O ₃ , Ag ₂ O and spinels.

Two different variants of gas diffusion electrodes are known from the prior art. On the one hand, gas diffusion electrodes with a layer design and on the other hand, gas diffusion electrodes with a porous electrode. The gas diffusion electrodes with a layer design consist of a catalyst layer, a hydrophobic microporous carbon layer and a hydrophobic macroporous carbon substrate. Gas diffusion electrodes with a porous electrode consist of a mixture of porous carbon, an electrocatalyst and polytetrafluoroethylene (PTFE). In both variants, the porous layers serve as a gas diffusion layer, which is hydrophobic or water-repellent thanks to polytetrafluoroethylene. The electrocatalytic reactions take place on the catalyst layer or on the outer surface of the gas diffusion electrode, which is in contact with an electrolyte.

The WO 2020/264415 A1 shows a GDE with a layered design in which the OER electrode is made of a metallic catalyst-coated porous material (e.g. nickel, nickel alloys, iron, stainless steel, carbon steel, titanium). The catalyst is selected from the materials manganese oxide, nickel and iron. In order to achieve bifunctionality, in addition to the OER electrode, there is also an ORR electrode that rests on the OER electrode and is in contact with air. The ORR electrode is made of a carbon material. This GDE, consisting of two electrodes, can only be installed horizontally in alkaline energy converters and must be switched electrically depending on the oxidation or reduction operation.

The problem with such GDEs made of carbon is that the carbon is particularly susceptible to corrosion. This leads to a shortened lifespan of the GDE and battery or electrolyzer failure. To counteract this, carbon-free and therefore corrosion-stable GDEs were developed that act bifunctionally, i.e. are capable of both reactions (OER and ORR).

The WO 2014/037846 A2 shows a carbon-free GDE with a layer design, where the carrier layer consists of electrically conductive metal oxides, carbides, nitrides, borides, silicides or organic semiconductors. A catalyst is applied to the carrier layer, made from powders with a particle size of 20 to 50 µm.

The object of the invention is to find a new way to realize a bifunctional gas diffusion electrode (GDE) that is materially stable and durable in an alkaline medium.

This object is achieved by a bifunctional gas diffusion electrode for alkaline electrochemical energy converter systems with oxygen oxidation and reduction or hydrogen formation and reduction according to the features of independent claim 1. Advantageous embodiments are the subject of the dependent claims.

Furthermore, the object is achieved by a method for producing a bifunctional gas diffusion electrode for alkaline electrochemical energy conversion systems with oxygen oxidation and reduction or hydrogen formation and reduction with the features of independent claim 6. Advantageous embodiments are described in the dependent claims.

The invention relates to a bifunctional gas diffusion electrode for alkaline electrochemical energy conversion systems with oxygen oxidation and reduction or hydrogen formation and reduction. The bifunctional gas diffusion electrode comprises a laminate made of at least one metallic, carbon-free porous substrate, the laminate being on an electrolyte side for electrolyte contact and on an opposite air side for air contact in the energy converter system, has a macropore size gradient falling from the electrolyte side to the air side. The at least one substrate has an oxygen oxidation and oxygen reduction active or hydrogen formation and hydrogen reduction active catalyst coating and at least partially a hydrophobic coating. There is a transition from hydrophilic to hydrophobic from the electrolyte side to the air side of the layered body. This prevents the GDE from decomposing in the alkaline medium and ensures that the electrochemical bifunctionality of the GDE is retained over a long period of time.

An advantageous aspect provides that the laminated body comprises at least two metallic, porous substrates which have a first and a second macropore size and are connected and/or in contact with one another, so that the composite laminated body has the falling macropore size gradient and the transition from hydrophilic to hydrophobic having. In this way, a bifunctional gas diffusion electrode can be provided for alkaline energy converter systems.

It is particularly advantageous if the macropore size varies over the at least one substrate in a range between 600 μm and 5 μm. This makes it possible to provide a GDE with a large active surface on the electrolyte side, while the opposite side of the laminate can be designed to have small pores, which is conducive to a hydrophobic surface property on the air side.

According to a preferred aspect, the macropore size gradient of the layered body has a pore size difference between the electrolyte side and the air side of at least 95 μm. This enables the performance of the GDE to be improved through electrochemical alternating polarization.

According to an advantageous aspect, the macropore size of the at least second substrate on the air side of the laminated body is in a range from 20 μm to 5 μm.

By combining the substrates with different macropore sizes or placing them on top of each other, the macropore gradient is created.

It is particularly advantageous if the laminate comprises three substrates and the macropore size of the first substrate on the electrolyte side of the laminate is in a range of 600 µm to 300 µm, the macropore size of the second substrate is in a range of 300 µm to 50 µm and the macropore size of the third substrate is in a range from 50 µm to 5 µm. In this way, a stepwise transition in macropore size from large to small is created, which is essential for the bifunctionality of the GDE. A laminated body made of four or more substrates with more finely graduated macropore sizes can also be advantageous.

According to a preferred aspect, one of the connections between the first, second and third substrate in the laminated body is formed by material connection and another of the connections is formed as a contact by pressing. Depending on the area of application, a corresponding GDE can be produced.

1. a. Bereitstellung wenigstens eines metallischen, kohlenstofffreien, porösen Substrats mit unterschiedlichen Makroporengrößen zur Erzeugung eines Schichtkörpers, der an einer Elektrolytseite für einen Elektrolytkontakt und an einer gegenüberliegenden Luftseite für einen Luftkontakt im Energiewandlersystem vorgesehen ist und einen von der Elektrolytseite zur Luftseite fallenden Makroporengrößengradienten aufweist,
2. b. Entfetten des wenigstens einen Substrats mittels eines Lösungsmittels in einem Ultraschallbad,
3. c. Ätzen des wenigstens einen Substrates mittels einer Säure,
4. d. Ausheizen des wenigstens einen Substrats unter Luftatmosphäre bei 350-400°C,
5. e. Elektrochemische Katalysatorabscheidung eines sauerstoffoxidations- und sauerstoffreduktionsaktiven oder wasserstoffbildungs- und wasserstoffreduktionsaktiven Katalysators auf das wenigstens eine Substrat,
6. f. Ausheizen des wenigstens einen Substrats unter Luftatmosphäre bei 280-320°C, falls ein sauerstoffoxidations- und sauerstoffreduktionsaktiver Katalysator abgeschieden wurde,
7. g. Beschichten wenigstens eines Teils des wenigstens einen Substrats oder eines Anteils von mehreren Substraten, der als Luftseite des Schichtkörpers verwendet wird, mit einer hydrophobierenden Beschichtung.

The invention also relates to a method for producing a bifunctional gas diffusion electrode for alkaline electrochemical energy conversion systems with oxygen oxidation and reduction or hydrogen formation and reduction. The procedure includes the steps:

1. a. Providing at least one metallic, carbon-free, porous substrate with different macropore sizes for producing a layered body which is provided on an electrolyte side for electrolyte contact and on an opposite air side for air contact in the energy converter system and has a macropore size gradient falling from the electrolyte side to the air side,
2. b. degreasing the at least one substrate using a solvent in an ultrasonic bath,
3. c. etching the at least one substrate using an acid,
4. d. Baking out the at least one substrate under an air atmosphere at 350-400 ° C,
5. e. Electrochemical catalyst deposition of an oxygen oxidation and oxygen reduction active or hydrogen formation and hydrogen reduction active catalyst onto the at least one substrate,
6. f. baking the at least one substrate under an air atmosphere at 280-320 ° C if an oxygen oxidation and oxygen reduction-active catalyst has been deposited,
7. G. Coating at least a part of the at least one substrate or a portion of several substrates, which is used as the air side of the laminated body, with a hydrophobic coating.

With this process, a bifunctional GDE can be produced cost-effectively and in a resource-saving manner.

Step a) is advantageously preceded by a step in which the at least one substrate is pressed so that the macropore size is reduced. In step g), the air side of the laminated body is coated with the hydrophobic coating. This ensures that the coating material penetrates deeply into the macropores of the substrate.

1. h. Auswahl wenigstens eines Substrats ohne hydrophobierende Beschichtung und wenigstens eines Substrats mit hydrophobierender Beschichtung derart, dass das Substrat mit hydrophobierender Beschichtung kleinere Makroporengrößen als das Substrat ohne hydrophobierende Beschichtung aufweist,
2. i. Kombination des Substrats ohne hydrophobierende Beschichtung mit dem Substrat mit hydrophobierender Beschichtung, sodass der Schichtkörper als ein zur Luftseite hin mit fallendem Makroporengrößengradient und einem Übergang von hydrophil zu hydrophob ausgestatteter Schichtkörper erzeugt wird.

It has proven to be advantageous if step a) includes the provision of at least two metallic, carbon-free, porous substrates with different macropore sizes, and the method further comprises the steps:

1. H. Selecting at least one substrate without a hydrophobic coating and at least one substrate with a hydrophobic coating such that the substrate with a hydrophobic coating has smaller macropore sizes than the substrate without a hydrophobic coating,
2. i. Combination of the substrate without a hydrophobic coating with the substrate with a hydrophobic coating, so that the laminated body is produced as a laminated body equipped towards the air side with a falling macropore size gradient and a transition from hydrophilic to hydrophobic.

This creates both a hydrophilic-to-hydrophobic transition and a large-to-small macropore size transition, which is essential for the bifunctionality of the carbon-free GDE.

An advantageous aspect provides that MnO _x structures are applied in step e). Catalysts that are active in oxygen oxidation and reduction or are active in hydrogen formation and reduction are thus deposited onto the substrate.

It is particularly advantageous if the macropore size of the at least one substrate is in a range between 600 μm and 5 μm. This allows the fabrication of a bifunctional, carbon-free GDE with a large active surface area.

In step i), a first substrate with a macropore size between 300 μm and 50 μm is advantageously combined with a second substrate with a macropore size between 50 μm and 5 μm. In this way, a GDE with a decreasing macropore size gradient can be produced.

According to an advantageous aspect, following step i), a metallic net is placed as a current collector on an outer surface of the substrate with the smaller macropore size and pressed into an alkaline cell. This allows the GDE to be used in an alkaline cell, such as a metal-air battery.

It is particularly advantageous if step a) is followed by a step in which the substrate is selected and/or combined with at least one second substrate and welded to produce the laminated body in such a way that the laminated body has a macropore size gradient falling from the electrolyte side to the air side. This enables subsequent coating of the laminated body without the coating being destroyed or damaged by welding.

An advantageous aspect provides that a bifunctional gas diffusion electrode is used in an alkaline electrochemical battery or a fuel cell or in an electrolyzer. This makes an important contribution to environmentally friendly and sustainable energy conversion.

The present invention opens up the novel possibility of realizing a bifunctional gas diffusion electrode (GDE) that is materially stable and durable in an alkaline medium. This GDE is corrosion-resistant and can be used for two different reaction pairings, namely oxygen formation (OER) and oxygen reduction (ORR) or hydrogen formation (HER) and hydrogen reduction reactions (HRR).

• 1 eine schematische Darstellung eines Schichtkörpers aus einem metallischen, kohlenstofffreien porösen Substrat zur Realisierung einer erfindungsgemäßen bifunktionalen Gasdiffusionselektrode für alkalische elektrochemische Energiewandlersysteme,
• 2 eine schematische Darstellung eines Schichtkörpers aus zwei metallischen, kohlenstofffreien porösen Substraten mit unterschiedlichen Makroporengrößen zur Erzeugung einer erfindungsgemäßen bifunktionalen Gasdiffusionselektrode und
• 3 eine schematische Darstellung eines Schichtkörpers aus drei metallischen, kohlenstofffreien porösen Substraten mit unterschiedlichen Makroporengrößen zur Herstellung einer erfindungsgemäßen bifunktionalen Gasdiffusionselektrode.

The invention will be explained in more detail below using exemplary embodiments with reference to drawings. Show this:

• 1 a schematic representation of a layered body made of a metallic, carbon-free porous substrate for realizing a bifunctional gas diffusion electrode according to the invention for alkaline electrochemical energy converter systems,
• 2 a schematic representation of a layered body made of two metallic, carbon-free porous substrates with different macropore sizes for producing a bifunctional gas diffusion electrode according to the invention and
• 3 a schematic representation of a layered body made of three metallic, carbon-free porous substrates with different macropore sizes for producing a bifunctional gas diffusion electrode according to the invention.

1 shows a schematic representation of a layered body 1 made of a metallic, carbon-free porous substrate 2 for use in a bifunctional gas diffusion electrode according to the invention for alkaline electrochemical energy conversion systems. A bifunctional gas diffusion electrode for alkaline electrochemical energy converter systems with oxygen oxidation and reduction or hydrogen formation and reduction comprises a layered body 1 made of at least one metallic, carbon-free porous substrate 2. The layered body 1, which has an electrolyte side 3 for electrolyte contact and an opposite air side 4 for Air contact is provided in the energy converter system, has a macropore size gradient falling from the electrolyte side 3 to the air side 4. The at least one substrate 2 has an oxygen oxidation and oxygen reduction active or hydrogen formation and hydrogen reduction active catalyst coating 5 and at least partially a hydrophobic coating 6.

For example, noble metals or their oxidic form, metal oxides, phosphides or nitrides can be used as bifunctional catalysts for the oxygen reaction. Examples of a bifunctional catalyst for the hydrogen reaction are platinum-based nitrides, nickel, nickel-silver or cobalt-nickel nitrides.

From the electrolyte side 3 to the air side 4 of the layered body 1 there is a transition from hydrophilic to hydrophobic. The hydrophobic coating 6 consists, for example, of polytetrafluoroethylene (PTFE). A hydrophobic coating 6 made of polyvinylidene difluoride (PVDF) or fluoroethylene-propylene (FEP) can also be used.

In this first exemplary embodiment, the macropore size of the at least one substrate 2 is in a range between 600 μm and 5 μm, with a macropore size difference of at least 95 μm between the electrolyte side 3 and the air side 4 of the laminated body 1 being provided. The hydrophobic coating 6 in this first exemplary embodiment is distributed over the surface of the substrate 2 and largely covers the surface.

The hydrophobic coating 6 is evenly distributed at points over the entire surface. The frequency of the selective hydrophobic coating 6 determines the hydrophilic/hydrophobic transition, which is generally to be viewed as a smooth transition. The classification as to when the hydrophobic state is present, i.e. the hydrophobic coating 6 is complete, is carried out as follows.

A complete hydrophobic coating 6 should also be considered if there are isolated uncoated areas, but the hydrophobic coating 6 is evenly distributed at points over the entire surface.

A partially coated substrate 2 has more uncoated surface than surface coated with hydrophobic coating 6.

1. a. Bereitstellung wenigstens eines metallischen, kohlenstofffreien, porösen Substrats 2 mit unterschiedlichen Makroporengrößen zur Erzeugung eines Schichtkörpers 1, der an einer Elektrolytseite 3 für einen Elektrolytkontakt und an einer gegenüberliegenden Luftseite 4 für einen Luftkontakt im Energiewandlersystem vorgesehen ist und einen von der Elektrolytseite 3 zur Luftseite 4 fallenden Makroporengrößengradienten aufweist,
2. b. Entfetten des wenigstens einen Substrats 2 mittels eines Lösungsmittels in einem Ultraschallbad,
3. c. Ätzen des wenigstens einen Substrats 2 mittels einer Säure,
4. d. Ausheizen des wenigstens einen Substrats 2 unter Luftatmosphäre bei 350-400°C,
5. e. Elektrochemische Katalysatorabscheidung eines sauerstoffoxidations- und sauerstoffreduktionsaktiven oder wasserstoffbildungs- und wasserstoffreduktionsaktiven Katalysators auf das wenigstens eine Substrat 2,
6. f. Ausheizen des wenigstens einen Substrats 2 unter Luftatmosphäre bei 280-320°C, falls ein sauerstoffoxidations- und sauerstoffreduktionsaktiver Katalysator abgeschieden wurde,
7. g. Beschichten wenigstens eines Teils des wenigstens einen Substrats 2 oder eines Anteils von mehreren Substraten 2, der als Luftseite 4 des Schichtkörpers 1 verwendet wird, mit einer hydrophobierenden Beschichtung 6.

The process for producing a bifunctional gas diffusion electrode for alkaline electrochemical energy conversion systems with oxygen oxidation and reduction or hydrogen formation and reduction includes the steps:

1. a. Providing at least one metallic, carbon-free, porous substrate 2 with different macropore sizes for producing a layered body 1, which is provided on an electrolyte side 3 for electrolyte contact and on an opposite air side 4 for air contact in the energy converter system and one falling from the electrolyte side 3 to the air side 4 has macropore size gradients,
2. b. Degreasing the at least one substrate 2 using a solvent in an ultrasonic bath,
3. c. etching the at least one substrate 2 using an acid,
4. d. Baking out the at least one substrate 2 under an air atmosphere at 350-400 ° C,
5. e. Electrochemical catalyst deposition of an oxygen oxidation and oxygen reduction active or hydrogen formation and hydrogen reduction active catalyst onto the at least one substrate 2,
6. f. baking the at least one substrate 2 under an air atmosphere at 280-320 ° C, if an oxygen oxidation and oxygen reduction-active catalyst has been deposited,
7. G. Coating at least a part of the at least one substrate 2 or a portion of several substrates 2, which is used as the air side 4 of the laminated body 1, with a hydrophobic coating 6.

Complete coatings 6 are flat on the substrate 2 and largely cover the surface, so that only isolated uncoated areas are visible.

Partially coated substrates 2 have more uncoated than coated surface. The PTFE points with the PTFE coating selected as an example as a hydrophobic coating 6 are distributed evenly across the entire surface. The increasing frequency of PTFE punctual sites determines the hydrophilic/hydrophobic transition. There is then a state where the frequency is so high that it can be described as completely coated when there are only isolated uncoated areas. The completeness of the hydrophobic coating 6 is assessed by relating it as a mass percent of PTFE to the total mass of the substrate 2.

The hydrophobic coating 6, which in this example is a polytetrafluoroethylene emulsion (PTFE coating), is activated by baking. The substrate 2 is dried at 80 to 120 ° C and finally baked for the last time at 280 to 310 ° C for 30 minutes in order to evaporate residual water and emulsifiers contained in the hydrophobic coating 6 adhering to the substrate 2 through the baking .

The macropore size of the at least one substrate 2 is in a range between 600 μm and 5 μm. It should have a pore size difference between the electrolyte side 3 and the air side 4 of at least 95 μm.

In step e), MnO _x structures are applied which act as oxygen oxidation and oxygen reduction active catalysts. Other metals or metal oxides or spinels, perovskites can also be used as catalysts, which are either active in oxygen oxidation and oxygen reduction or are active in hydrogen formation and hydrogen reduction.

It is conceivable that step a) is preceded by a step in which the at least one substrate 2 is pressed so that the macropore size is reduced. In step g), the air side 4 of the laminated body 1 is coated with the hydrophobic coating 6.

In 2 is a schematic representation of a layered body 1 made of two metallic, carbon-free porous substrates 2 for use in a bifunctional gas diffusion electrode according to the invention for alkaline electrochemical energy conversion systems. In this second exemplary embodiment, the laminated body 1 comprises at least two metallic, porous substrates 2, which have a first and a second macropore size and are in material connection or contact with one another, so that the composite laminated body 1 has the falling macropore size gradient and the transition from hydrophilic to hydrophobic . The macropore size of the first substrate 2 is in a range from 600 μm to 300 μm. The macropore size of the second substrate 2 is in a range from 300 μm to 50 μm. It is also conceivable that the macropore size of the second substrate 2 is in a range from 50 μm to 5 μm.

1. A. Bereitstellung wenigstens zweier metallischer, kohlenstofffreier, poröser Substrate 2 mit voneinander verschiedenen Makroporengrößen.

The method for producing a layered body 1 from several metallic, carbon-free porous substrates 2 for use in a bifunctional gas diffusion electrode according to the invention for alkaline electrochemical energy conversion systems comprises the steps:

1. A. Provision of at least two metallic, carbon-free, porous substrates 2 with different macropore sizes.

1. B. Entfetten der mindestens zwei Substrate 2 mittels eines Lösungsmittels in einem Ultraschallbad,
2. C. Ätzen der Substrate 2 mittels einer Säure,
3. D. Ausheizen der Substrate 2 unter Luftatmosphäre bei 350-400°C,
4. E. elektrochemische Katalysatorabscheidung eines sauerstoffoxidations- und sauerstoffreduktionsaktiven oder wasserstoffbildungs- und wasserstoffreduktionsaktiven Katalysators auf wenigstens eines der Substrate 2,
5. F. Ausheizen der Substrate 2 unter Luftatmosphäre bei 280-320°C, falls ein sauerstoffoxidations- und sauerstoffreduktionsaktiver Katalysator abgeschieden wurde,
6. G. Beschichten wenigstens eines Anteils der mindestens zwei Substrate 2, der als Luftseite 4 des Schichtkörpers 1 verwendet wird, mit einer hydrophobierenden Beschichtung 6.
7. H. Auswahl wenigstens eines der Substrate 2 ohne hydrophobierende Beschichtung 6 und wenigstens eines weiteren Substrats 2 aus dem Anteil der Substrate 2 mit hydrophobierender Beschichtung 6 derart, dass das wenigstens eine weitere Substrat 2 mit hydrophobierender Beschichtung 6 kleinere Makroporengrößen als das wenigstens eine Substrat 2 ohne hydrophobierende Beschichtung 6 aufweist,
8. I. Kombination von ausgewählten Substraten 2 von dem wenigstens einen Substrate 2 ohne hydrophobierende Beschichtung 6 mit dem wenigstens einen weiteren Substrat 2 mit hydrophobierender Beschichtung 6, sodass der Schichtkörper 1 als ein zur Luftseite 4 hin mit fallendem Makroporengrößengradient und einem Übergang von hydrophil zu hydrophob ausgestatteter Schichtkörper 1 erzeugt wird.

The steps then follow:

1. B. degreasing the at least two substrates 2 using a solvent in an ultrasonic bath,
2. C. etching the substrates 2 using an acid,
3. D. baking the substrates 2 under an air atmosphere at 350-400 ° C,
4. E. electrochemical catalyst deposition of an oxygen oxidation and oxygen reduction active or hydrogen formation and hydrogen reduction active catalyst on at least one of the substrates 2,
5. F. Baking the substrates 2 under an air atmosphere at 280-320 ° C, if an oxygen oxidation and oxygen reduction-active catalyst has been deposited,
6. G. Coating at least a portion of the at least two substrates 2, which is used as the air side 4 of the laminated body 1, with a hydrophobic coating 6.
7. H. Selection of at least one of the substrates 2 without a hydrophobic coating 6 and at least one further substrate 2 from the proportion of substrates 2 with a hydrophobic coating 6 such that the at least one further substrate 2 with a hydrophobic coating 6 has smaller macropore sizes than the at least one substrate 2 without has hydrophobic coating 6,
8. I. Combination of selected substrates 2 from the at least one substrate 2 without a hydrophobic coating 6 with the at least one further substrate 2 with a hydrophobic coating 6, so that the layered body 1 is equipped as a macropore size gradient towards the air side 4 with a falling macropore size gradient and a transition from hydrophilic to hydrophobic Layer body 1 is produced.

To degrease the substrate 2, an alcohol or ketone solution, such as acetone, isopropanol or ethanol, is used. Degreasing takes between 15 and 30 minutes in the ultrasonic bath.

For example, 2-4 molar hydrochloric acid (HCl) is used to etch the substrate 2. The etching takes between 20 and 45 minutes. After etching, baking takes place for around 10 to 20 minutes.

After the catalyst deposition, baking takes place at 280-320 ° C for about 2 hours in order to oxidize the catalyst structure and thereby make it electrochemically more active if an oxygen oxidation and oxygen reduction active catalyst has been deposited. If a hydrogen formation and hydrogen reduction active catalyst has been deposited, baking is not necessary.

The substrate 2 can be coated with the hydrophobic coating 6 by spraying, brushing or dipping. In the dipping method, the substrate 2 to be coated is dipped into a Teflon-PTFE-water emulsion for 10 seconds (“dip-coating process”) and then rinsed with ethanol or other alcohols. The substrate 2 is then dried at 80 to 120 ° C and finally baked at 280 to 310 ° C for 30 minutes.

For substrates 2 with a macropore size between 200 µm and 600 µm, a mass fraction of 5 to 10% PTFE can be deposited. For substrates 2 with a macropore size smaller than 20 µm, a mass fraction of 15 to 24% PTFE can be deposited.

A metallic net is then placed as a current collector on an outer surface of the substrate 2 with the smaller macropore size and pressed in an alkaline cell (not shown).

It is also conceivable that the substrate 2 is selected and/or combined with at least one second substrate 2 and welded to produce the laminated body 1 in such a way that the laminated body 1 has a macropore size gradient falling from the electrolyte side 3 to the air side 4. In this case, the catalyst coating 5 and the hydrophobic coating 6 of the substrates 2 that are welded must be carried out after welding. This means that step I (combination) is carried out before step E (catalyst deposition) in order to avoid damage to the catalyst coating 5 and the hydrophobic coating 6 as well as irregular welded connections.

In general, the macropore gradient is created by combining or placing substrates 2 with different macropore sizes on top of each other, which are either pressed after coating or welded before coating.

1. a) ein Pulver mit unterschiedlichen Korngrößen schichtweise aufeinanderschichten und anschließend versintern oder
2. b) 3D-gedruckte Substrate 2 zu verwenden, bei denen der Makroporengradient vorher am PC designt wurde.

It is also conceivable to produce the layered body 1 with macropore gradients as follows:

1. a) layer a powder with different grain sizes on top of each other and then sinter it or
2. b) to use 3D printed substrates 2 in which the macropore gradient was previously designed on the PC.

In one embodiment of the method, a first substrate 2 with a macropore size between 300 μm and 50 μm is combined with a second substrate 2 with a macropore size between 50 μm and 5 μm in order to create a macropore gradient of at least 95 μm, preferably over 200, in the resulting layered body 1 µm, particularly preferably over 400 µm.

3 shows a schematic representation of a layered body 1 made of three metallic, carbon-free porous substrates 2 for use in a bifunctional gas diffusion electrode according to the invention for alkaline electrochemical energy conversion systems. In this third exemplary embodiment, the laminated body 1 comprises three substrates 2. The macropore size of the first substrate 2 on the electrolyte side 3 of the laminated body 1 is in a range from 600 μm to 300 μm. The macropore size of the second substrate 2 is in a range from 300 μm to 50 µm. The macropore size of the third substrate 2 is in a range from 50 μm to 5 μm.

Such a layered body 1 can consist of three or more different metallic porous substrates 2, which are welded together without any coatings. This connection creates a layered body 1 from the different metallic porous substrates 2. The individual treatment and coating steps are then carried out to apply the catalyst coating 5 and the hydrophobic coating 6. If these treatment and coating steps were to take place beforehand, damage would occur to the respective treated and/or applied surface layer by welding.

Alternatively, it is possible to produce the layered body 1 from two, three or more different metallic porous substrates 2, which are connected to one another by pressing. The individual treatment and coating steps for applying the catalyst coating 5 and the hydrophobic coating 6 are carried out before pressing.

Stainless steel (Cr-Ni steel with Cr <16.5), nickel (99.2 - 99.6) or nickel alloys (e.g. Incoloy825*(NiCr21 Mo) / Hastelloy) can be used as materials for the metallic porous substrates 2 become.

The macropore size of the first substrate 2 is in a range between 600 μm and 350 μm, and the macropore size of the second substrate 2 is in a range between 350 μm and 50 μm. In the third substrate 2, the macropore size is in a range between 50 μm and 5 μm. For possibly more than three substrates 2, suitably graduated macropore sizes must be selected.

The macropore size gradient of the first and second substrate 2 is +/- 100 μm each. The macropore size gradient of the third substrate 2 is +/- 10 μm.

The catalyst coating 5 is carried out, for example, galvanically, chemically, by means of “chemical vapor deposition” or by thermal spraying. What is important for all coating methods is that the catalyst coating 5 can penetrate deeply into the structures.

The hydrophobic coating 6 is applied, for example, by dipping, spraying, brushing or partial dipping. Then rinse with alcohol, e.g. B. Ethanol. PTFE, for example, is used for hydrophobization. It is also conceivable to use PVDF or FEP for hydrophobization.

1. 1. Die drei Substrate 2 werden vollständig miteinander verschweißt.
2. 2. Die drei Substrate 2 werden vollständig miteinander verpresst.
3. 3. Zwei der drei Substrate 2 werden miteinander verschweißt und das dritte Substrat 2 wird mit den beiden verschweißten Substraten 2 verpresst, wobei die beiden verschweißten Substrate 2 mit teilweiser hydrophobierender Beschichtung 6 die Elektrolytseite 3 des Schichtkörpers 1 bilden, oder
4. 4. zwei der drei Substrate 2 werden miteinander verschweißt und das dritte Substrat 2 wird mit den beiden verschweißten Substraten 2 verpresst, wobei die beiden verschweißten Substrate 2 mit zunehmender Häufigkeit bis vollständiger hydrophobierender Beschichtung 6 die Luftseite 4 des Schichtkörpers 1 bilden.

There are four ways to connect the three substrates 2 together:

1. 1. The three substrates 2 are completely welded together.
2. 2. The three substrates 2 are completely pressed together.
3. 3. Two of the three substrates 2 are welded together and the third substrate 2 is pressed with the two welded substrates 2, the two welded substrates 2 with a partial hydrophobic coating 6 forming the electrolyte side 3 of the laminated body 1, or
4. 4. two of the three substrates 2 are welded together and the third substrate 2 is pressed with the two welded substrates 2, the two welded substrates 2 forming the air side 4 of the laminated body 1 with increasing frequency until the hydrophobic coating 6 is complete.

In the event that substrates 2 are welded together, the catalyst coating 5 and the hydrophobic coating 6 of the substrates 2 that are welded must be carried out after welding. This means that step I (combination) is carried out before step E (catalyst deposition) in order to avoid damage to the catalyst coating 5 and the hydrophobic coating 6 (PTFE layer) as well as irregular welded connections.

In the case of pressing the multiple substrates 2 together, steps A to I are carried out one after the other as described above.

For other conceivable cases in which two of the three substrates 2 are first welded and then the catalyst coating 5 and the hydrophobic coating 6 of the substrates 2 are carried out, these welded, coated substrates 2 can then either be coated with the third, hydrophobically coated substrate 2, which is the Air side of the laminated body 1 forms, can be pressed or can be covered with increasing to complete hydrophobic coating 6 (e.g. by partial dipping, spraying or brushing) and then pressed with a third substrate 2 without hydrophobic coating 6, which forms the electrolyte side 3 of the laminated body 1 become.

The completeness of the hydrophobic coating 6 is assessed by relating it as a mass percent of PTFE (or alternatively PVDF or FEP) to the total mass of the substrate 2. For example, on the second substrate 2 with a 200 μm pore size, the PTFE could have a mass fraction of approximately 8% and on the third substrate 2 with a 20 μm pore size, the PTFE could have a mass fraction of approximately 20%. Although these values could be further optimized, the trend is that the third substrate 2 has a higher PTFE mass fraction than the second substrate 2 and is therefore more hydrophobic and better equipped for the air side 4 of the laminated body 1.

All versions of the bifunctional gas diffusion electrode described above can be used in an alkaline electrochemical battery or a fuel cell or in an electrolyzer.

Reference symbol list

1

Layered body

2

(porous) substrate

3

Electrolyte side

4

Airside

5

Catalyst coating

6

hydrophobic coating

QUOTES INCLUDED IN THE DESCRIPTION

This list of documents listed by the applicant was 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

• WO 2007065899 A1 [0003]
• WO 2020264415 A1 [0005]
• WO 2014037846 A2 [0007]

Claims (17)

Bifunctional gas diffusion electrode for alkaline electrochemical energy converter systems with oxygen oxidation and reduction or hydrogen formation and reduction, comprising a layered body (1) made of at least one metallic, carbon-free porous substrate (2), the layered body (1) being on an electrolyte side (3) for an electrolyte contact and on an opposite air side (4) is provided for air contact in the energy converter system, has a macropore size gradient falling from the electrolyte side (3) to the air side (4), and the at least one substrate (2) has an oxygen oxidation and oxygen reduction active or hydrogen formation and hydrogen reduction-active catalyst coating (5) and at least partially a hydrophobic coating (6), so that there is a transition from hydrophilic to hydrophobic from the electrolyte side (3) to the air side (4) of the layered body (1). Bifunctional gas diffusion electrode Claim 1 , wherein the laminated body (1) comprises at least two metallic, porous substrates (2) which have a first and a second macropore size and are in material connection and/or contact with one another, so that the composite laminated body (1) has the falling macropore size gradient and the Transition from hydrophilic to hydrophobic. Bifunctional gas diffusion electrode according to one of the preceding claims, wherein the macropore size varies over the at least one substrate (2) in a range between 600 µm and 5 µm. Bifunctional gas diffusion electrode according to one of the preceding claims, wherein the macropore size gradient of the layered body (1) has a pore size difference between the electrolyte side (3) and the air side (4) of at least 95 µm. Bifunctional gas diffusion electrode Claim 2 , wherein the macropore size of the at least second substrate (2) on the air side (4) of the layered body (1) is in a range from 20 μm to 5 μm. Bifunctional gas diffusion electrode according to one of the preceding claims, wherein the laminate (1) comprises three substrates (2) and the macropore size of the first substrate (2) on the electrolyte side (3) of the laminate (1) is in a range from 600 µm to 300 µm , the macropore size of the second substrate (2) is in a range of 300 µm to 50 µm and the macropore size of the third substrate (2) is in a range of 50 µm to 5 µm. Bifunctional gas diffusion electrode Claim 6 , wherein one of the connections between the first, second and third substrate (2) in the laminated body (1) is formed as a material connection by welding and another of the connections is formed as a contact by pressing. Method for producing a bifunctional gas diffusion electrode for alkaline electrochemical energy conversion systems with oxygen oxidation and reduction or with hydrogen formation and reduction, comprising the steps: a. Providing at least one metallic, carbon-free, porous substrate (2) with different macropore sizes for producing a layered body (1), which is provided on one electrolyte side (3) for electrolyte contact and on an opposite air side (4) for air contact in the energy converter system and one has a macropore size gradient falling from the electrolyte side (3) to the air side (4), b. Degreasing the at least one substrate (2) using a solvent in an ultrasonic bath, c. etching the at least one substrate (2) using an acid, d. Baking out the at least one substrate (2) under an air atmosphere at 350-400 ° C, e. Electrochemical catalyst deposition of an oxygen oxidation and oxygen reduction active or hydrogen formation and hydrogen reduction active catalyst onto the at least one substrate (2), f. baking the at least one substrate (2) under an air atmosphere at 280 to 320 ° C if an oxygen oxidation and oxygen reduction-active catalyst has been deposited, G. Coating at least a part of the at least one substrate (2) or a portion of several substrates (2), which is used as the air side (4) of the laminated body (1), with a hydrophobic coating (6). Procedure according to Claim 8 , wherein step a) is preceded by a step in which the at least one substrate (2) is pressed so that the macropore size is reduced, and in step g) the air side (4) of the layered body (1) with the hydrophobic coating (6 ) is coated. Procedure according to Claim 8 or 9 , wherein in step g) after the hydrophobic coating (6) has been applied, the substrate (2) is baked to activate the hydrophobic coating (6). Procedure according to Claim 8 , wherein step a) comprises the provision of at least two metallic, carbon-free, porous substrates (2) with different macropore sizes and the method further comprises the steps: h. Selecting at least one substrate (2) without a hydrophobic coating (6) and at least one substrate (2) with a hydrophobic coating (6) such that the substrate (2) with a hydrophobic coating (6) has smaller macropore sizes than the substrate (2) without a hydrophobic coating Coating (6), i. Combination of the substrate (2) without a hydrophobic coating (6) with the substrate (2) with a hydrophobic coating (6), so that the layered body (1) is equipped as a macropore size gradient towards the air side (4) and a transition from hydrophilic to hydrophobic Laminated body (1) is produced. Method according to one of the preceding Claims 8 until 10 , whereby MnO _x structures are applied in step e). Method according to one of the preceding Claims 8 until 11 , wherein the macropore size of the at least one substrate (2) is in a range between 600 μm and 5 μm. Method according to one of the preceding Claims 8 , 10 and 11 , wherein in step i) a first substrate (2) with a macropore size between 300 µm and 50 µm is combined with a second substrate (2) with a macropore size between 50 µm and 5 µm. Method according to one of the preceding Claims 8 until 13 , whereby, following step i), a metallic net is placed as a current collector on an outer surface of the substrate with the smaller macropore size and pressed into an alkaline cell. Procedure according to Claim 8 , wherein step a) is followed by a step in which the substrate (2) is selected and/or combined with at least one second substrate (2) and welded to produce the laminated body (1), so that the laminated body (1) has one of The electrolyte side (3) has a falling macropore size gradient towards the air side (4). Use of a bifunctional gas diffusion electrode according to one of the Claims 1 until 7 in an alkaline electrochemical battery or a fuel cell or in an electrolyzer.

Images