Bifunctional gas diffusion electrode for alkaline electrochemical energy conversion systems and method for producing said bifunctional gas diffusion electrode

Abstract

The invention relates to a bifunctional gas diffusion electrode for alkaline electrochemical energy conversion systems and to a method for producing said bifunctional gas diffusion electrode. According to the invention, a long-life bifunctional gas diffusion electrode (GDE) which is of a new type and which is material-stable in the alkaline medium comprises a layered body (1) made of at least one metal, carbon-free porous substrate (2), the layered body (1) being provided for electrolyte contact on an electrolyte side (3) and for air contact on an opposite air side (4) when in the energy conversion system and having a falling or rising macropore size gradient from the electrolyte side (3) to the air side (4), and the at least one substrate (2) comprising an oxygen-oxidation-active and oxygen-reduction-active or hydrogen-formation-active and hydrogen-reduction-active catalyst coating (5) and, at least in part, a hydrophobing 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 to hydrophobic.

Description

[0001] Bifunctional gas diffusion electrode for alkaline electrochemical energy conversion systems and process for its production

[0002] 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).

[0003] Gas diffusion electrodes enable chemical reactions such as oxygen formation or oxygen oxidation (OER) or oxygen reduction (ORR). Depending on the application, GDEs can perform either one or two reactions. This depends on the catalysts used. If a GDE can be used for two reactions, e.g., oxygen formation reactions (OER) and oxygen reduction reactions (ORR) or hydrogen formation reactions (HER) (more precisely: hydrogen oxidation reactions, HÖR for short) and hydrogen reduction reactions (HRR), then this GDE is referred to as bifunctional.

[0004] WO 2007 / 065899 A1 exemplifies an OER and ORR gas diffusion electrode for metal-air batteries or metal-hybrid-air batteries. The GDE comprises a gas diffusion layer, an active layer, an oxygen evolution layer, and a current collector in electrical contact with the active layer; the active layer contains an oxygen reduction catalyst and a bifunctional catalyst selected from La2O3, Ag2O3, and spinels.

[0005] Two different variants of gas diffusion electrodes are known from the state of the art. One is gas diffusion electrodes with a layered design, the other is gas diffusion electrodes with a porous electrode. Gas diffusion electrodes with a layered 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 the gas diffusion layer, which is hydrophobic or water-repellent thanks to polytetrafluoroethylene. The electrocatalytic reactions take place at the catalyst layer or at the outer surface of the gas diffusion electrode, which is in contact with an electrolyte.

[0006] WO 2020 / 264415 A1 shows a GDE with a layered design, in which the OER electrode consists 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. To achieve bifunctionality, in addition to the OER electrode, there is also an ORR electrode that rests on top of 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 therefore only be installed horizontally in alkaline energy converters and must be electrically switched depending on whether the system is operating in oxidation or reduction mode.

[0007] The problem with such carbon-based GDEs is that the carbon is particularly susceptible to corrosion. This leads to a shortened service life of the GDE and failure of the battery or electrolyzer. To counteract this, carbon-free and thus corrosion-resistant GDEs have been developed that are bifunctional, i.e., capable of both reactions (OER and ORR).

[0008] WO 2014 / 037846 A2 describes a carbon-free GDE with a layered 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 of powders with a particle size of 20 to 50 pm.

[0009] The object of the invention is to find a new way of realizing a bifunctional gas diffusion electrode (GDE) that is materially stable and durable in alkaline medium.

[0010] This object is achieved by a bifunctional gas diffusion electrode for alkaline electrochemical energy conversion 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.

[0011] 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 having the features of independent claim 8. Advantageous embodiments are described in the dependent subclaims.

[0012] 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 term "hydrogen formation" refers in particular to hydrogen oxidation (HÖR for short). The bifunctional gas diffusion electrode comprises a layered body made of at least one metallic, carbon-free porous substrate. The layered body, which is provided on one electrolyte side for electrolyte contact and on an opposite air side for air contact in the energy conversion system, has a decreasing or increasing macropore size gradient from the electrolyte side to the air side. The at least one substrate has a catalyst coating active toward oxygen oxidation and oxygen reduction or active toward hydrogen formation and hydrogen reduction, and at least partially a hydrophobic coating.This creates a hydrophilic-to-hydrophobic transition from the electrolyte side to the air side of the laminate. This prevents the GDE from decomposing in alkaline media and ensures that the electrochemical bifunctionality of the GDE is maintained over a long period of time.

[0013] An advantageous aspect provides that the composite body comprises at least two metallic, porous substrates having a first and a second macropore size and being connected and / or in contact with each other, such that the composite composite body exhibits the decreasing or increasing macropore size gradient and the transition from hydrophilic to hydrophobic. In this way, a bifunctional gas diffusion electrode for alkaline energy conversion systems can be provided.

[0014] It is particularly advantageous if the macropore size varies across the at least one substrate in a range between 600 pm and 5 pm. This allows for a GDE with a large active surface area on the electrolyte side, while the opposite side of the laminate can be designed with small pores, which promotes a hydrophobic surface property on the air side.

[0015] According to a preferred aspect, the macropore size gradient of the laminated body has a pore size difference between the electrolyte side and the air side of at least 95 pm. This enables an improvement in the performance of the GDE through electrochemical alternating polarization.

[0016] 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 of 20 pm to 5 pm. By combining or stacking the substrates with different macropore sizes, the macropore gradient is created.

[0017] It is particularly advantageous if the laminated body comprises three substrates and the macropore size of the first substrate on the electrolyte side of the laminated body is in a range from 600 μm to 300 μm, the macropore size of the second substrate is in a range from 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 defined, graduated transition in the macropore size from large to small is created, which is essential for the bifunctionality of the GDE. A laminated body comprising four or more substrates with more finely graduated macropore sizes can also be advantageous.

[0018] According to a preferred aspect, one of the connections between the first, second, and third substrates in the laminated body is formed by a physical bond, and another of the connections is formed as a contact by pressing. This allows a corresponding GDE to be produced depending on the application.

[0019] 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 method comprises the steps: a) providing at least one metallic, carbon-free, porous substrate with different macropore sizes to produce a laminated body, which is provided on one electrolyte side for electrolyte contact and on an opposite air side for air contact in the energy conversion system and has a decreasing or increasing macropore size gradient from the electrolyte side to the air side, b) degreasing the at least one substrate using a solvent in an ultrasonic bath, c) etching the at least one substrate using an acid, d) annealing the at least one substrate in 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, f) Heating 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, g) Coating at least a portion of the at least one substrate or a portion of several substrates used as the air side of the laminated body with a hydrophobic coating.

[0020] This process allows for the cost-effective and resource-efficient production of a bifunctional gas diffusion electrode. In a preferred embodiment, the process according to the invention is used to produce a bifunctional gas diffusion electrode according to the invention.

[0021] Advantageously, step a) is preceded by a step in which the at least one substrate is compressed to reduce the macropore size. In step g), the air side of the laminate is coated with the hydrophobic coating. This ensures that the coating material penetrates deeply into the macropores of the substrate.

[0022] It has been found to be advantageous if step a) comprises providing at least two metallic, carbon-free, porous substrates with mutually different macropore sizes, and the method further comprises the steps of: 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, i) combining the substrate without a hydrophobic coating with the substrate with a hydrophobic coating, so that the laminated body is produced as a laminated body with a decreasing macropore size gradient towards the air side and a transition from hydrophilic to hydrophobic.

[0023] 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.

[0024] An advantageous aspect provides that in step e) MnO x -structures are applied. This allows oxygen oxidation and oxygen reduction active or hydrogen formation and hydrogen reduction active catalysts to be deposited onto the substrate.

[0025] It is particularly advantageous if the macropore size of the at least one substrate is in a range between 600 pm and 5 pm. This allows the production of a bifunctional, carbon-free GDE with a large active surface area.

[0026] Advantageously, in step i), a first substrate with a macropore size between 300 pm and 50 pm is combined with a second substrate with a macropore size between 50 pm and 5 pm. This allows a GDE with a decreasing or increasing macropore size gradient to be produced. According to an advantageous aspect, following step i), a metallic mesh is applied as a current collector to an outer surface of the substrate with the smaller macropore size and pressed to form an alkaline cell. This allows the GDE to be used in an alkaline cell, such as a metal-air battery.

[0027] 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 decreasing or increasing macropore size gradient from the electrolyte side to the air side. This enables subsequent coating of the laminated body without the coating being destroyed or damaged by the welding.

[0028] One advantageous aspect involves the use of a bifunctional gas diffusion electrode in an alkaline electrochemical battery, a fuel cell, or an electrolyzer. This can make an important contribution to environmentally friendly and sustainable energy conversion.

[0029] The present invention opens up the novel possibility of creating a bifunctional gas diffusion electrode (GDE) that is materially stable and durable in alkaline media. This GDE is corrosion-resistant and can be used for two different reaction pairs: oxygen formation (OER) and oxygen reduction (ORR) or hydrogen formation (HER) and hydrogen reduction (HRR).

[0030] The invention will be explained in more detail below by means of exemplary embodiments based on drawings. These show:

[0031] Fig. 1 is a schematic representation of a laminated body made of a metallic, carbon-free porous substrate for the realization of a bifunctional gas diffusion electrode according to the invention for alkaline electrochemical energy conversion systems,

[0032] Fig. 2 is 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

[0033] Fig. 3 is 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.

[0034] Fig. 1 shows a schematic representation of a laminated body 1 made of a metallic, carbon-free porous substrate 2 for use in a bifunctional gas diffusion electrode for alkaline electrochemical energy conversion systems according to the invention. A bifunctional gas diffusion electrode for alkaline electrochemical energy conversion systems with oxygen oxidation and reduction or hydrogen formation and reduction comprises a laminated body 1 made of at least one metallic, carbon-free porous substrate 2. The laminated 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 conversion system, has a decreasing or increasing macropore size gradient 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.

[0035] As a bifunctional catalyst for the oxygen reaction, for example, noble metals or their oxidic form, metal oxides (e.g. MnO x), phosphides, or nitrides can be used. Examples of a bifunctional catalyst for the hydrogen reaction are platinum-based nitrides, nickel, nickel-silver, or cobalt-nickel nitrides. In a preferred embodiment, the bifunctional catalyst contains or consists of a catalyst selected from the group consisting of metal alloys (e.g., selected from the group consisting of NiFe, Raney nickel, NiMoAl, and combinations thereof), spinels, perovskites, and combinations thereof. The bifunctional catalyst can (further) contain layered double hydroxides (LDHs; e.g., containing or consisting of nickel-iron oxide).

[0036] From the electrolyte side 3 to the air side 4 of the laminated 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.

[0037] In this first embodiment, the macropore size of the at least one substrate 2 is in a range between 600 pm and 5 pm, with a macropore size difference of at least 95 pm being provided between the electrolyte side 3 and the air side 4 of the laminated body 1. The hydrophobic coating 6 in this first embodiment is distributed over the entire surface of the substrate 2 and covers most of the surface.

[0038] The hydrophobic coating 6 is evenly distributed in spots over the entire surface. The frequency of the spot hydrophobic coating 6 determines the hydrophilic / hydrophobic transition, which is generally considered a smooth transition. The classification of when the hydrophobic state is present, i.e., when the hydrophobic coating 6 is complete, is made as follows.

[0039] A complete hydrophobic coating 6 shall also be considered if there are isolated uncoated areas, but the hydrophobic coating 6 is evenly distributed over the entire surface.

[0040] A partially coated substrate 2 has more uncoated surface than surface coated with hydrophobic coating 6.

[0041] The method for producing a bifunctional gas diffusion electrode for alkaline electrochemical energy conversion systems with oxygen oxidation and reduction or hydrogen formation and reduction comprises the steps: a. Providing at least one metallic, carbon-free, porous substrate 2 with different macropore sizes to produce a laminated 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 conversion system and has a macropore size gradient falling or rising 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 the at least one substrate 2 in 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. Heating the at least one substrate 2 in an air atmosphere at 280-320°C if an oxygen oxidation- and oxygen reduction-active catalyst was deposited; g. Coating at least a portion 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.

[0042] Complete coatings 6 are flat on the substrate 2 and cover most of the surface, so that only isolated uncoated areas are visible.

[0043] Partially coated substrates 2 have more uncoated than coated surface. The PTFE patches, in the exemplary PTFE coating chosen as the hydrophobic coating 6, are evenly distributed across the entire surface. The increasing frequency of the PTFE patches determines the hydrophilic / hydrophobic transition. A state exists where the frequency is so high that the coating can be described as fully coated, with only a few uncoated patches remaining. The completeness of the hydrophobic coating 6 is assessed by expressing it as a mass percentage of PTFE relative to the total mass of the substrate 2.

[0044] The hydrophobic coating 6, which in this example is a polytetrafluoroethylene emulsion (PTFE coating), is activated by annealing. The substrate 2 is dried at 80 to 120°C and then annealed for a final time at 280 to 310°C for 30 minutes to evaporate any residual water and emulsifiers contained in the hydrophobic coating 6 adhering to the substrate 2.

[0045] The macropore size of the at least one substrate 2 is in a range between 600 pm and 5 pm. It should have a pore size difference between the electrolyte side 3 and the air side 4 of at least 95 pm.

[0046] In step e) MnO xStructures are applied that act as oxygen oxidation and oxygen reduction active catalysts. Other metals or metal oxides or metal alloys (e.g., selected from the group consisting of NiFe, Raney nickel, NiMoAl, and combinations thereof), spinels, and / or perovskites can also be used as catalysts (or applied in step e)), which are either oxygen oxidation and oxygen reduction active or hydrogen formation and hydrogen reduction active. Furthermore, layered double hydroxides (LDHs for short; e.g., containing or consisting of nickel iron oxide) can be used as (additional) catalysts (or applied in step e)).

[0047] It is conceivable that step a) is preceded by a step in which the at least one substrate 2 is pressed to reduce the macropore size. In step g), the air side 4 of the laminate 1 is coated with the hydrophobic coating 6.

[0048] Fig. 2 is a schematic representation of a laminated 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, such that the assembled laminated body 1 has the falling or rising macropore size gradient and the transition from hydrophilic to hydrophobic. The macropore size of the first substrate 2 is in a range from 600 pm to 300 pm. The macropore size of the second substrate 2 is in a range from 300 pm to 50 pm. It is also conceivable that the macropore size of the second substrate 2 is in a range from 50 pm to 5 pm.

[0049] The method for producing a laminated 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:

[0050] A. Providing at least two metallic, carbon-free, porous substrates 2 with different macropore sizes.

[0051] The following steps then follow:

[0052] B. Degreasing the at least two substrates 2 by means of a solvent in an ultrasonic bath,

[0053] C. Etching the substrates 2 using an acid,

[0054] D. Baking the substrates 2 under air atmosphere at 350-400°C,

[0055] E. electrochemical catalyst deposition of an oxygen oxidation and oxygen reduction active or hydrogen formation and hydrogen reduction active catalyst onto at least one of the substrates 2,

[0056] F. Baking the substrates 2 under air atmosphere at 280-320°C, if an oxygen oxidation and oxygen reduction active catalyst has been deposited,

[0057] G. Coating at least a portion of the at least two substrates 2 used as the air side 4 of the laminated body 1 with a hydrophobic coating 6.

[0058] H. Selecting at least one of the substrates 2 without hydrophobic coating 6 and at least one further substrate 2 from the portion of the substrates 2 with hydrophobic coating 6 such that the at least one further substrate 2 with hydrophobic coating 6 has smaller macropore sizes than the at least one substrate 2 without hydrophobic coating 6,

[0059] I. Combination of selected substrates 2 of the at least one substrate 2 without hydrophobic coating 6 with the at least one further substrate 2 with hydrophobic coating 6, so that the laminated body 1 is produced as a laminated body 1 equipped with a decreasing macropore size gradient towards the air side 4 and a transition from hydrophilic to hydrophobic.

[0060] An alcohol or ketone solution, such as acetone, isopropanol, or ethanol, is used to degrease substrate 2. Degreasing takes between 15 and 30 minutes in an ultrasonic bath.

[0061] For example, 2-4 molar hydrochloric acid (HCl) is used to etch substrate 2. The etching takes between 20 and 45 minutes. After etching, the annealing takes place for approximately 10 to 20 minutes.

[0062] After catalyst deposition, annealing is performed at 280-320 °C for approximately 2 hours to oxidize the catalyst structure and thereby increase its electrochemical activity if an oxygen oxidation and oxygen reduction active catalyst was deposited. If a hydrogen formation and hydrogen reduction active catalyst was deposited, annealing is not necessary.

[0063] The coating of the substrate 2 with the hydrophobic coating 6 can be done by spraying, brushing, or dipping. In the dipping method, the substrate 2 to be coated is immersed in 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.

[0064] For substrates 2 with a macropore size between 200 pm and 600 pm, a mass fraction of 5 to 10% PTFE can be deposited. For substrates 2 with a macropore size smaller than 20 pm, a mass fraction of 15 to 24% PTFE can be deposited.

[0065] Subsequently, a metallic mesh is 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).

[0066] 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 decreasing or increasing macropore size gradient 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 to be welded must be applied 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 welds.

[0067] In general, the macropore gradient is created by combining or stacking substrates 2 with different macropore sizes, which are either pressed after coating or welded before coating.

[0068] It is also conceivable to produce the layered body 1 with macropore gradients as follows: a) layering a powder with different grain sizes on top of each other and then sintering it or b) using 3D-printed substrates 2 for which the macropore gradient was previously designed on the PC.

[0069] In one embodiment of the method, a first substrate 2 having a macropore size between 300 pm and 50 pm is combined with a second substrate 2 having a macropore size between 50 pm and 5 pm in order to produce a macropore gradient of at least 95 pm, preferably over 200 pm, particularly preferably over 400 pm, in the resulting layered body 1.

[0070] Fig. 3 shows a schematic representation of a laminated body 1 composed 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 pm to 300 pm. The macropore size of the second substrate 2 is in a range from 300 pm to 50 pm. The macropore size of the third substrate 2 is in a range from 50 pm to 5 pm.

[0071] Such a laminated body 1 can consist of three or more different metallic porous substrates 2, which are welded together without any coatings. This connection creates a laminated body 1 from the different metallic porous substrates 2. Subsequently, the individual treatment and coating steps for applying the catalyst coating 5 and the hydrophobic coating 6 follow. If these treatment and coating steps were performed beforehand, the respective treated and / or applied surface layer would be damaged by the welding. Alternatively, the laminated body 1 can be produced from two, three, or more different metallic porous substrates 2, which are bonded together by pressing.The individual treatment and coating steps for applying the catalyst coating 5 and the hydrophobic coating 6 are carried out before pressing.

[0072] Stainless steel (Cr-Ni steel with Cr < 16.5), nickel (99.2 - 99.6) or nickel alloys (e.g. Incoloy 825*(NiCr21Mo) / Hastelloy) can be used as materials for the metallic porous substrates 2.

[0073] The macropore size of the first substrate 2 is in a range between 600 pm and 350 pm, the macropore size of the second substrate 2 is in a range between 350 pm and 50 pm. For the third substrate 2, the macropore size is in a range between 50 pm and 5 pm. For potentially more than three substrates 2, appropriately graduated macropore sizes must be selected.

[0074] The macropore size gradient of the first and second substrates 2 is + / - 100 pm each. The macropore size gradient of the third substrate 2 is + / - 10 pm.

[0075] The catalyst coating 5 is applied, for example, galvanically, chemically, by chemical vapor deposition, or by thermal spraying. Important for all coating methods is that the catalyst coating 5 can penetrate deep into the structures.

[0076] The hydrophobic coating 6 is applied, for example, by dipping, spraying, brushing, or partial dipping. This is followed by rinsing with alcohol, e.g., ethanol. PTFE, for example, is used for hydrophobicization. It is also conceivable to use PVDF or FEP for hydrophobicization.

[0077] There are four ways to connect the three substrates 2 together:

[0078] 1. The three substrates 2 are completely welded together.

[0079] 2. The three substrates 2 are completely pressed together.

[0080] 3. Two of the three substrates 2 are welded together and the third substrate 2 is pressed with the two welded substrates 2, whereby the two welded substrates 2 with partial hydrophobic coating 6 form the electrolyte side 3 of the laminated body 1, or

[0081] 4. two of the three substrates 2 are welded together and the third substrate 2 is pressed with the two welded substrates 2, wherein the two welded substrates 2 form the air side 4 of the laminated body 1 with increasing frequency until the hydrophobic coating 6 is completely applied.

[0082] In the case of substrates 2 being welded together, the catalyst coating 5 and the hydrophobic coating 6 of the substrates 2 being welded must be applied after welding. This means that step I (combination) is performed before step E (catalyst deposition) to avoid damage to the catalyst coating 5 and the hydrophobic coating 6 (PTFE layer), as well as irregular welds.

[0083] In the case of pressing the plurality of substrates 2 together, steps A to I are carried out successively as described above.

[0084] 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 applied, these welded, coated substrates 2 can then either be pressed together with the third, hydrophobic coated substrate 2, which forms the air side of the laminated body 1, or can be covered with increasing to complete hydrophobic coating 6 (e.g. by partial dipping, spraying or brushing) and then pressed together with a third substrate 2 without hydrophobic coating 6, which forms the electrolyte side 3 of the laminated body 1.

[0085] The completeness of the hydrophobic coating 6 is assessed by relating it as a mass percentage of PTFE (or alternatively, PVDF or FEP) to the total mass of the substrate 2. For example, on the second substrate 2 with a pore size of 200 pm, the PTFE could have a mass fraction of approximately 8%, and on the third substrate 2 with a pore size of 20 pm, the PTFE could have a mass fraction of approximately 20%. Although these values ​​could be further optimized, the general trend is that the third substrate 2 has a higher PTFE mass fraction than the second substrate 2 and is thus more hydrophobic and better equipped for the air side 4 of the laminate 1.

[0086] All the above-described versions of the bifunctional gas diffusion electrode can be used in an alkaline electrochemical battery or a fuel cell or in an electrolyzer.

[0087] List of reference symbols

[0088] 1 layered body

[0089] 2 (porous) substrate electrolyte side air side catalyst coating hydrophobic coating

Claims

Claims: Bifunctional gas diffusion electrode for alkaline electrochemical energy conversion 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), wherein the layered body (1), which is provided on an electrolyte side (3) for an electrolyte contact and on an opposite air side (4) for an air contact in the energy conversion system, has a macropore size gradient falling or rising from the electrolyte side (3) to the air side (4), and wherein 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), such that a transition from hydrophilic to hydrophobic is present from the electrolyte side (3) to the air side (4) of the layered body (1).A bifunctional gas diffusion electrode according to claim 1, wherein the layer body (1) comprises at least two metallic, porous substrates (2) having a first and a second macropore size and being materially bonded and / or in contact with each other, such that the composite layer body (1) exhibits a decreasing or increasing macropore size gradient and a transition from hydrophilic to hydrophobic. A bifunctional gas diffusion electrode according to any of the preceding claims, wherein the macropore size varies across the at least one substrate (2) in a range between 600 pm and 5 pm.

4. Bifunctional gas diffusion electrode according to one of the preceding claims, wherein the macropore size gradient of the layer body (1) has a pore size difference between electrolyte side (3) and air side (4) of at least 95 pm.

5. Bifunctional gas diffusion electrode according to claim 2, wherein the macropore size of the at least second substrate (2) on the air side (4) of the layer body (1) is in a range of 20 pm to 5 pm.

6. Bifunctional gas diffusion electrode according to one of the preceding claims, wherein the layer body (1) comprises three substrates (2) and the macropore size of the first substrate (2) on the electrolyte side (3) of the layer body (1) is in a range of 600 pm to 300 pm, the macropore size of the second substrate (2) is in a range of 300 pm to 50 pm and the macropore size of the third substrate (2) is in a range of 50 pm to 5 pm.

7. Bifunctional gas diffusion electrode according to claim 6, wherein one of the connections between first, second and third substrate (2) in the layer body (1) is formed as a material connection by welding and another of the connections is formed as a contact by pressing.

8. 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 of: a) providing at least one metallic, carbon-free, porous substrate (2) with different macropore sizes to produce 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 conversion system and has a macropore size gradient decreasing or increasing 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) Heating 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) Heating 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 layer body (1), with a hydrophobizing coating (6).A method 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 layer body (1) is coated with the hydrophobic coating (6). A method according to claim 8 or 9, wherein in step g) after the application of the hydrophobic coating (6) the substrate (2) is heated to activate the hydrophobic coating (6). A method according to claim 8, wherein step a) comprises providing 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. coating (6) and at least one substrate (2) with hydrophobic- i) coating (6) such that the substrate (2) with hydrophobic coating (6) has smaller macropore sizes than the substrate (2) without hydrophobic coating (6), i) combination of the substrate (2) without hydrophobic coating (6) with the substrate (2) with hydrophobic coating (6) such that the layer body (1) is produced as a layer body (1) equipped with a macropore size gradient decreasing towards the air side (4) and a transition from hydrophilic to hydrophobic. Method according to any one of the preceding claims 8 to 11, wherein in step e) MnO x-structures are applied. A method according to any one of the preceding claims 8 to 12, wherein the macropore size of the at least one substrate (2) is in a range between 600 pm and 5 pm. A method according to any one of the preceding claims 11 to 13, wherein in step i) a first substrate (2) with a macropore size between 300 pm and 50 pm is combined with a second substrate (2) with a macropore size between 50 pm and 5 pm. A method according to any one of the preceding claims 11 to 14, wherein, following step i), a metallic mesh is applied as a current collector to an outer surface of the substrate with the smaller macropore size and compressed to form an alkaline cell.The method of 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 layered body (1), such that the layered body (1) has a macropore size gradient decreasing or increasing from the electrolyte side (3) to the air side (4). Use of a bifunctional gas diffusion electrode according to any one of claims 1 to 7 in an alkaline electrochemical battery or a fuel cell or in an electrolyzer.

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