Electrodes for an electrochemical cell for an alkaline separation of water into hydrogen and oxygen, and method for producing same

Abstract

According to the invention, electrodes are arranged on two opposite surfaces of a separator. Each electrode consists of an open-pore metal structure, in particular a metal foam made of at least one of the chemical elements Ni, Al, Mo, Fe, Mn, Co, Zn, La, Ce, or an alloy of at least two of said chemical elements or an intermetallic compound of at least two of said chemical elements. A continuously decreasing catalytic activity is provided from the surface facing a separator or the respective other electrode of each electrochemical cell to the opposite surface of the respective electrode, and / or a continuously increasing porosity and / or pore size and / or a continuously decreasing specific surface area is provided from the surface facing a separator or the respective other electrode of each electrochemical cell to the opposite surface of the respective electrode.

Description

[0001] Alantum Europe GmbH, FRAUNHOFER SOCIETY...eV

[0002] 248 PCT 1150

[0003] Electrodes for an electrochemical cell for the alkaline splitting of water into hydrogen and oxygen and process for their production

[0004] The invention relates to electrodes for an electrochemical cell for the alkaline splitting of water into hydrogen and oxygen and to processes for their production.

[0005] Electrochemical water splitting has gained considerable interest over the past decade due to the enormous efforts of many countries to reduce their CO2 emissions to zero by 2050. Sustainable green hydrogen, produced using renewable electricity, is needed as an emission-free feedstock for refineries, the steel industry, and the chemical industry. It is also considered a promising alternative to fossil fuels, at least in certain industries, such as the steel industry. Furthermore, it is being discussed as a potential candidate for storing excess electrical energy, which can be generated, for example, in wind farms.The electrochemical production of hydrogen is carried out using so-called electrolyzers, in which the smallest functional unit, the cell, can be understood as two active metal electrodes in contact with an aqueous salt solution, which release hydrogen and oxygen upon application of an electrical potential. On a commercial scale, the technology is mainly dominated by two different types: AEL (alkaline electrolysis) and PEMEL (proton exchange membrane electrolysis). The following section addresses the problems that arise, particularly—but not exclusively—with alkaline technology.

[0006] During operation of an electrolyzer, the released gases must be transported from the electrode surface toward a gas collection system. In a conventional electrode, the gas bubbles form evenly distributed due to the 3D or, in some cases, even 2D structure of the material, blocking the path of gas transport from the areas close to the separator. The electrical overpotential therefore gradually increases as the active surface area decreases due to surface coverage with gas. Furthermore, the areas closer to or in contact with the separator, when covered with trapped gas bubbles, reduce the ionic conductivity throughout the entire system, thereby increasing the overall ohmic resistance and reducing energy efficiency. Current electrodes do not offer simple, large-scale solutions to this problem, leading to inefficient electricity use.

[0007] For example, WO 2019 / 057625 A1 discloses a process for producing an open-pore shaped body with a modified surface formed with a metal and a shaped body produced by the process.

[0008] US 2014 / 0221700 A1 discloses a modified metal molded body and a method for its production.

[0009] The object of the invention is therefore to increase the efficiency of electrolyzers through improved transport and removal of the released gases. According to the invention, this object is achieved with electrodes having the features of claim 1. Claim 5 defines a method for producing the electrodes. Advantageous embodiments and further developments of the invention can be realized with features defined in the dependent claims.

[0010] Each electrode consists of an open-pore metal structure, in particular a foam, a grid, network, flow, or a powder foil made of Ni, Al, Mo, Fe, Mn, Co, Cu, Zn, La, Ce, an alloy of at least one of these chemical elements, or an intermetallic compound thereof. Examples of such active alloys or surface coatings that can be used in this invention include NiMo, NiFe, NiMoAl, NiFeCo, NiRuCo, NiMoPt, NiPt, NiPd, NiMoPd, NiPtPd, NiRe, or NiMoRe, etc. A continuously decreasing catalytic activity is present, starting from the surface facing a separator or another electrode of the respective electrochemical cell up to the opposite surface of the respective electrode.

[0011] Alternatively or in addition to the changed activity, a continuously increasing porosity and pore size and / or a continuously decreasing specific surface area may also be present, starting from the surface facing a separator or another electrode of the respective electrochemical cell up to the oppositely arranged surface of the respective electrode.

[0012] Accordingly, a continuously decreasing catalytic activity should be present from the surface facing a separator or another electrode of the respective electrochemical cell to the opposite surface of the respective electrode, or a continuously increasing porosity and / or pore size should be present from the surface facing a separator or another electrode of the respective electrochemical cell to the opposite surface of the respective electrode. Continuous should be understood as an at least nearly uniform change within the open-pore metal structure from one surface to the opposite surface of the respective electrode.To evaluate this continuously changing property of the electrode, one can specifically consider three regions within the electrode morphology: near or at the surface facing the separator, in the center or interior of the electrode, and at or near the surface facing away from the separator. These three regions are characterized by a property, such as the catalytic activity in the first example, continuously decreasing or increasing from one side to the other.

[0013] Larger jumps, where deviations of more than 30%, better of 20% -10%, preferably less than 10% from the respective mean value of a proportional curve within the electrode material occur at "interfaces", should be avoided.

[0014] The specific surface area should be more than 1000 m 2 / l continuously at less than 100 m 2 / l, especially in the range of 8000 m 2 / l up to 80 m 2 / l and preferably between 16 000 m 2 / l and 50 m 2 / l, the pore size must be continuously increased from a minimum of 3 nm to at least 100 pm and the porosity from 20%-60% to at least 90% and / or the proportion of at least one catalytically active component on the surface of the metal foam must be reduced from 20% to at least 0%.

[0015] Hollow channels can be formed on the surface facing away from a separator or another electrode of the respective electrochemical cell. These improve the removal of the released gas through the respective electrode and from the electrode surface. Hollow channels can be formed, for example, by placeholders during manufacturing, screen printing, laser cutting, punching, pressing, or machining.

[0016] However, these placeholders mentioned here can also be used to optimize the porosity in the area facing a separator or another electrode of the respective electrochemical cell.

[0017] The density can also vary from 0.05 g / mm 3 to 5 g / mm 3 starting from the surface of the respective electrode facing away from the respective separator or the other electrode of the respective electrochemical cell, which can also be realized by the corresponding porosities, pore sizes and specific surfaces.

[0018] There are also several alternative manufacturing processes. For example, an open-pore Meta II structure consisting of one or more of the metals Ni, Fe, Mo, Co, Cu or Ag or an alloy or an intermetallic compound of at least one of these chemical elements can be coated on the surface facing the respective separator or the other electrode of a respective electrochemical cell with a metal powder selected from Ni, Fe, Mo, Co, Cu and Ag and alloys containing at least one of the metals. The particles of the metal powder can be bonded to the surface of the open-pore metal structure by sintering in such a way that the porosity and the specific surface area are changed, whereby the porosity and the specific surface area are continuously reduced starting from this surface towards the opposite surface.

[0019] However, the production can also be carried out in such a way that an open-pore metal structure consisting of an alloy of Fe, Co, Mo or Ni is coated on the surface facing away from the separator or the other electrode of the respective electrochemical cell with a metal powder with low catalytic activity on the electrode used as anode or cathode and the metal powder is then sintered on or no metal powder is applied to this surface.

[0020] However, it is also possible to bond an open-pored metal structure made of one or more catalytically active components at the cathode or the anode as the electrode of an electrochemical cell, which is formed from Ni and Fe, Ni and Co or Ni and Mo, and which has a smaller porosity, pore size and a greater density at the respective separator or the other electrode of the respective electrochemical cell to an open-pored Meta II structure, which preferably consists of Ni with high porosity, wherein the open-pored metal structure arranged facing the separator or the other electrode of an electrochemical cell advantageously also contains a metal or a metal alloy, which in particular contains Al, which can be at least partially removed by leaching to increase the specific surface area. A base, in particular KOH, can be used for the leaching.

[0021] This is a composite of two different metal foams. The continuous changes can be achieved, for example, by the low overall thickness and, particularly advantageously, by increasing the specific surface area through leaching. Instead of Al, another so-called Raney alloy can also be used for leaching.

[0022] The surface of the respective electrode, which is arranged facing the separator or the other electrode of the respective electrochemical cell, can also be coated with particles of at least one catalytically active component and these particles of the metal powder can then be sintered on.

[0023] In the production of electrodes according to the invention, the surface of an open-pore metal structure formed from Ni, Al, Mo, Fe, Mn, Co, Cu, Zn, La, Ce, an alloy of at least one of these chemical elements or an intermetallic compound can be coated on the surface facing the separator or the other electrode of the respective electrochemical cell with a metal powder selected from Co, Mo, Ag, Fe, Cu, Mn, La, and Ce. The particles of the metal powder are then bonded to surfaces of the open-pore metal structure by sintering. Aluminum powder is then applied to this surface and caused to react on the surface at a temperature between 650 °C and 730 °C under a nitrogen atmosphere, so that leaching aluminide phases are formed.Subsequently, a leaching process is carried out in which, in order to increase the specific surface area in the area of ​​the surface coated with aluminide phase, this aluminide phase is at least partially removed, whereby the specific surface area in this volume range is increased.

[0024] The sintering of the metal powder can advantageously be carried out at a temperature corresponding to 40% - 95% of the melting temperature of the metal powder used, with a holding time between 20 min and 90 min under a hydrogen atmosphere or in a high vacuum.

[0025] In all of the examples cited, a continuously changing structure within the electrode material is optimally utilized to simplify the overall system. Thus, the clever design of an electrode with activity differences can significantly reduce the number of components and the construction costs of the resulting electrolyzer technology. The electrode structure presented here can also be used for improved electrolyzer technologies and prevents membrane damage during assembly. Furthermore, the reduction of individual components leads to a reduction in interfaces between components, which can be a starting point for corrosion reactions and degradation, among other things. This can be expected to increase service life while simultaneously reducing costs.

[0026] In this way, both anodes and cathodes can be provided as electrodes for electrochemical cells, with which increased efficiency in the release of oxygen and hydrogen from water can be achieved.

[0027] Activity should not be limited to the catalytic activity of a material used as a coating or base material of the electrode, but should generally be referred to as the extent to which product gas (hydrogen or oxygen) is released in a particular region of the electrode, which correlates with the electric current density that can be achieved at a particular applied electric potential in the respective region, and may also include, but is not limited to, the kinetics of a surface coating or base material in the respective region of the electrode with respect to water oxidation or proton reduction.

[0028] Accordingly, the activity and thus the continuous activity difference compared to other areas of the electrode can be realized by:

[0029] - Density of the active electrode material / electrode base material - Porosity of the electrode material

[0030] - Catalytic activity of the electrode material (base metal and / or surface coating)

[0031] - specific surface area of ​​the electrode material or composite (surface coating and base metal)

[0032] Since the ions are conducted through the separator, the highest activity should be near or in contact with the aforementioned areas to avoid limitation by ion diffusion or charge transport. The activity should gradually decrease within the electrode, reaching almost zero on the side facing away from the separator. Thus, gas formation is minimized at greater distances from the separator, and the structure inherently opens one or more channels through the respective electrode. Thus, any gas bubbles formed can be transported away from the active areas near the separator, thereby maintaining the electrical current density or overpotential of the system. This inherent "suction effect" removes bubbles very efficiently. The active surface therefore remains unchanged during operation (no pore or surface blockage), and the electrical power consumption of the system is reduced.The innovative design also prevents membrane perforations on the separator and the resulting gas mixtures, as the pressure on the membrane side can be evenly absorbed due to the higher density and a flatter surface. Furthermore, an electrode according to the invention can exhibit properties of gas diffusion layers (GDLs), in some cases eliminating the need for an additional GDL.

[0033] The invention relates to the continuously changing catalytic activity of an open-pore metal structure electrode and similar open-pore 3D structures, which can be used in particular in alkaline water electrolysis, as well as their structure and mode of operation. Such an open-pore metal structure electrode is characterized in that, in an alkaline-operated water electrolyzer, it has an increased catalytic activity for both a cathode (HER) and an anode (OER) on the active side, i.e., the side facing the separator (diaphragm or membrane), which, for example, at 0.5 A / cm 2 results in an electrical overvoltage of less than 150 mV for a cathode (HER) and 300 mV for an anode (OER).

[0034] Furthermore, such an electrode is characterized in that it has a reduced catalytic activity on the back, ie the side facing away from the separator (diaphragm or membrane), which at an electrical current density of 0.5 A / cm 2 This results in an electrical overvoltage of more than 150 mV for a cathode and more than 300 mV for an anode. The activity decreases continuously across the cross-section from the active side to the back.

[0035] This can be achieved, for example, by increasing the specific inner surface of the electrode on the active side to more than 5000 m 2 / l (for e.g. Raney-Ni; Ni powder bed approx. 500 m 2 / l), but on the back only less than 50 m 2 / l and the required transfer occurs continuously across the electrode cross-section. The specific surface area can be determined using the BET method.

[0036] This can result in a double layer capacity of more than 0.01 F / cm 2 on the active side to less than 0.001 F / cm 2 on the back side. Likewise, the pore size can be increased from less than 10 pm on the more active side of the electrode, which faces a separator, to greater than 100 pm on the back side.

[0037] The thickness of an electrode is between a minimum of 0.1 mm and a maximum of 10 mm. An electrode can be made of the following materials and / or composites. It can be an open-pore metal structure with continuously varying porosity due to the electrode material, which has low porosity near the separator and high porosity further away from the separator. An electrode according to the invention can also be formed from an open-pore metal structure in which a surface is coated with a metal powder and the particles of this powder are sintered in order to increase the material density and specific surface area on one side of the respective electrode. In this way, the internal electrical resistance in the region of the electrode near the separator can be reduced, which leads to a higher electrical current density and consequently to higher electrochemical activity.

[0038] It can also be an open-pore metal structure, with one side made of a more catalytically active metal (OER or HER) or alloy, and the other side of the electrode made of a metal with lower or no catalytic activity (OER or HER). It can therefore be a metallic composite in which an open-pore metal structure made of a catalytically active material for OER or HER, which has a lower porosity, is bonded to an open-pore structure made of a material with lower activity and a higher porosity, or an open-pore metal structure with a surface coating made of an active material for OER or HER, whereby the surface loading with said material decreases within the electrode away from the separator. Thus, the most suitable catalytically active materials for a cathode are Mo, Re, Ru, Ag, Pd, La, and Pt, and for an anode, the most suitable materials are Co, Ru, Fe, Cu, Mn, Cr, and Ce.

[0039] A porous metal layer can also be formed with a continuously varying density of at least one catalytically active component extending from one surface to the opposite surface through the electrode material perpendicular to the separator, with a high density in the immediate vicinity of the separator and a very low density further away. The activity difference between two surfaces of an electrode behaves analogously to the material density, enabling optimal gas transport.

[0040] To produce electrodes according to the invention, an open-pore metal structure can also be used, in which one side is treated by a process or the application of nanoparticles, e.g. of the metals Ni, Co, Fe, Cu, Ag, Mo, Mn, Pt, Pd, Re, Ru and Ir (but not limited to these), by chemical processes (precipitation, etc.) or plating (dendritic structures) in order to increase the surface area of ​​the material. This process can involve the sintering of leachable metal powders on one side or the etching of the open-pore metal structure on one side to create defects and increase the surface area.

[0041] For example, an etching process with an inorganic acid, in particular HNO3, and a deposition of metal particles of at least one of the metals selected from Ni, Co, Fe, Mo, Mn, Re, Pt, Pd, Ru, and Ir on an open-pore metal structure can be carried out to increase the catalytic activity of the side of a respective electrode facing the separator. First, an aqueous nitric acid solution of one or more of the aforementioned metals is prepared, with the acid concentration being between 65% and 30%, ideally 32.5%, and the concentration of the resulting metal ions being between 0.1% and 5% by mass. This can be achieved either by using one or more of the aforementioned metals in salt form, ideally as nitrate, or by directly dissolving the metal(s) in the acid. The use of metal salts is preferred for noble metals such as Pt, Pd, Ru, Ir, and Re, although both approaches are possible for base metals.The etching process of the open-pore metal structure can then be carried out in a single step by applying the resulting nitric acid aqueous solution of one or more of the aforementioned metals in a mass ratio of metal cations to the open-pore metal structure of 0.1% to 5%. Application can be carried out, for example, by dipping, spraying, or brushing.

[0042] An open-pore metal structure in which the area further away from the separator has been hollowed out / structured, e.g. by laser cutting, to create hollow channels, can also be used for an electrode.

[0043] Technical examples of materials with continuously changing activity within the scope of this invention include: A, more precisely an open-pore metal structure made of one or more of the metals Ni, Fe, Mo, Co, Cu, or Ag, wherein a metal powder, in particular a metal powder made of Ni, Fe, Mo, Co, Cu, or Ag, is sintered onto one side of the open-pore metal structure (foam and powder material are identical) in order to increase the active surface / material density on one side of the open-pore metal structure. The region with high density / high activity is attached to the separator in a water electrolyzer, preferably in an alkaline electrolyzer, in particular in a system with a separator.Such an open-pore metal structure can be manufactured in such a way that one side is manufactured according to 1, but the sintered metals only comprise alloys with Fe, Co, Mo or Ni thereof and the other side is sintered with a metal powder with low catalytic activity towards OER (anode) or HER (cathode), preferably Ni, or no metal powder is applied to this side, whereby the side of the electrode facing away from the separator remains untreated according to point 1. An open-pore metal structure in which one side of an open-pore metal structure consists of one or more catalytically active (HER or OER) and high-surface area components with low porosity / high density, preferably Ni and Fe or Ni and Co or Ni and Mo, wherein this surface area can also contain Al for leaching, and the other side of an open-pore metal structure, which preferably consists of Ni with high porosity.In a water electrolyzer, preferably an alkaline electrolyzer, especially in a system that uses a separator, the side of the electrode with the larger specific surface area is in contact with the separator, and the material with the highest porosity is located farther away from the separator. Adding post-treatment processes to increase local activity (e.g., nanoparticle deposition, precious metal deposition, etching, dendrites, etc.) is possible. The invention will be further explained below using examples.

[0044] Example 1

[0045] For the production of the electrodes according to the invention with continuously changing catalytic activity, a commercial metal foam with a pore size of 450 pm to 5000 pm, a porosity of 80% - 95% and a surface weight between 250 g / m 2 up to 1500 g / m 2, a thickness in the range of 1 - 10 mm, whereby the thickness can be reduced to 0.1 mm by calendering. The metal foam can consist of Ni, Fe, Mo, Co, Cu, or Ag or an alloy or intermetallic compound of these metals. To increase the specific surface area and reduce the porosity on the side facing the separator, the foam is coated on one side with a metal powder of the same chemical element, alloy or compound from which the metal foam is made. The metal powder can contain alloyed nickel powder with up to 60 mass% Fe, Co or Mo. The particle size of the metal powder is in the range of 0.1 pm - 100 pm.

[0046] For the coating, the metal foam is first sprayed with an aqueous binder solution and then coated with the metal powder using a scattering method, preferably on one side. Alternatively, the powder can be applied by doctoring in a dry state or as a suspension using doctoring, wet powder spraying, metal powder foil drawing, screen printing, or dispensing. The powder penetrates up to 50% of the total metal foam thickness, with the porosity decreasing from the surface to the center of the metal foam.This is followed by a thermal treatment, in which the binder is decomposed under an inert gas or hydrogen atmosphere at 300°C - 600°C with a holding time of 0 to 180 minutes. Sintering takes place under hydrogen or high vacuum at 40% - 95% of the melting temperature of the respective metal powder used with a holding time of between 20 and 90 minutes, creating a solid bond between the porous powder layer and the metal foam structure. This results in a porosity of 40% to 90% on the surface coated with metal powder, with a specific surface area of ​​> 100 m². 2 / l and the uncoated opposite side between 80% and 95% with a specific surface area of ​​< 50 m 2 / l received.

[0047] The subsequent process step serves to form a leaching Raney alloy on the side of the respective electrode coated with the metal powder. For this purpose, an aluminum powder with a particle size between 5 μm and 100 μm is applied using one of the previously mentioned methods and reacted on the surface at a temperature between 650 °C and 730 °C under a nitrogen atmosphere, so that leaching aluminide phases form. This is followed by a leaching process with a 30% KOH solution + 6.6% K-Na tartrate addition. The thus-treated electrode material is first leached for 24 hours at room temperature, then the temperature is raised to 80 °C - 95 °C for a period of 8 hours. This gives the metal foam on the active powder bed side a specific surface area of ​​> 5000 m². 2 / l.

[0048] Example 1:

[0049] A nickel foam with 450 pm pore size, a basis weight of 1000 g / m2 , measuring 7 cm x 7 cm and weighing approximately 5 g, was coated on one side with a nickel powder suspension consisting of 60 g nickel powder with a particle size < 5 pm and 16 g 10% aqueous polyvinylpyrrolidone solution using a doctor blade device.

[0050] Subsequently, debinding was carried out under hydrogen for 30 minutes at 600°C, during which the organic components were removed. Sintering took place for 60 minutes at 700°C. The resulting mass was approximately 14 g, meaning approximately 9 g of nickel powder were sintered on.

[0051] A 2.5% aqueous polyethyleneimine solution was then sprayed onto the powder bed, and approximately 2 g of aluminum powder with a particle size of < 56 pm was evenly scattered. This was followed by thermal treatment in a nitrogen atmosphere. The material was first debindered at 500 °C for 30 minutes and then sintered at 670 °C for 1 minute. In the subsequent leaching process, the prepared semi-finished product was aged for 24 hours at room temperature, then for 8 hours at 92 °C in a 30% KOH solution with 6.6% potassium hydroxide tartrate.

[0052] The following properties were determined.

[0053] Uncoated Ni foam side:

[0054] Porosity: 92.5%

[0055] Pore ​​size: 450 um

[0056] - Specific surface area: 12.6 m 2 / l

[0057] Double layer capacitance: 0.27 mF / cm 2 Overvoltage OER (at 500 mA / cm 2): 375 mV overvoltage HER (at -500 mA / cm 2 ): -413 mV

[0058] Ni powder bed:

[0059] Porosity: 67.3%

[0060] Pore ​​size: 12 pm

[0061] - Specific surface area: 308 m 2 / l

[0062] Double layer capacitance: 3.81 mF / cm 2 Overvoltage OER (at 500 mA / cm 2 ): 344 mV overvoltage HER (at -500 mA / cm 2 ): -337 mV

[0063] Raney nickel layer in leached state:

[0064] Pore ​​size: 4 nm

[0065] - Specific surface area: 21040 m 2 / l double layer capacitance: 327 mF / cm 2 Overvoltage OER (at 500 mA / cm 2 ): 262 mV overvoltage HER (at -500 mA / cm 2 ): -112 mV

[0066] Example 2

[0067] A nickel foam with 450 pm pore size, a basis weight of 1000 g / m 2Measuring 18 cm x 7 cm and weighing approximately 13 g, the foam was coated on one side with NiMo45.2 powder with an average particle size of 30 pm using a scattering technique. The foam is first sprayed with a 2.5% aqueous polyethyleneimine solution and then sprinkled with the powder.

[0068] Binding was then removed under hydrogen for 30 minutes at 600°C, followed by sintering for 30 minutes at 1200°C. The resulting mass was approximately 20 g, meaning approximately 7 g of nickel powder were sintered onto the powder bed. A 2.5% aqueous polyethyleneimine solution was then sprayed onto the powder bed, and then 3 g of aluminum powder with a particle size of < 63 pm was evenly scattered. This was followed by thermal treatment at 680°C for 1 minute under nitrogen. In the subsequent leaching process, the sample was leached at room temperature for 24 hours and then at 90°C for 8 hours in a KOH solution (30% KOH + 6.6% K-Na tartrate).

[0069] The following properties were determined.

[0070] Uncoated Ni foam side:

[0071] Porosity: 92.5%

[0072] Pore ​​size: 450 pm

[0073] - Specific surface area: 12.6 m 2 / l double layer capacitance: 0.27 mF / cm 2 Overvoltage OER (at 500 mA / cm 2 ): 375 mV overvoltage HER (at -500 mA / cm 2 ): -413 mV

[0074] NiMo-alloyed foam:

[0075] Porosity: 89%

[0076] Pore ​​size: 400 pm (foam), 10 pm (sintered powder particles)

[0077] - Specific surface area: 25.5 m 2 / l double layer capacitance: 3.5 mF / cm 2 Overvoltage OER (at 500 mA / cm 2 ): 371 mV overvoltage HER (at -500 mA / cm 2 ): -337 mV

[0078] Raney NiMo layer in leached state:

[0079] Pore ​​size: 4 nm

[0080] - Specific surface area: 25000 m2 / l double layer capacitance: 120 mF / cm 2 Overvoltage OER (at 500 mA / cm 2 ): 267 mV overvoltage HER (at -500 mA / cm 2 ): -78 mV

[0081] The specific surface area was determined using the BET method.

Claims

Patent claims 1. Electrodes for an electrochemical cell for the alkaline splitting of water into hydrogen and oxygen, which are arranged on two opposite surfaces of a separator, wherein each electrode consists of an open-pore Meta II structure, in particular a metal foam made of at least one of the chemical elements Ni, Al, Mo, Fe, Mn, Co, Cu, Zn, La, Ce, or an alloy of at least two of these chemical elements or an intermetallic compound of at least two of these chemical elements, and a continuously decreasing catalytic activity is present starting from the surface facing a separator or another electrode of the respective electrochemical cell up to the oppositely arranged surface of the respective electrode, and / or starting from the surface,which is arranged facing a separator or another electrode of the respective electrochemical cell, up to the oppositely arranged surface of the respective electrode, there is / are a continuously increasing porosity and / or pore size and / or a continuously decreasing specific surface area.

2. Electrodes according to claim 1, characterized in that the catalytically active activity decreases or increases continuously from one side to the other and interfaces with deviations of more than 30%, better of 20% -10%, preferably less than 10% from the respective mean value of a proportional curve within the electrode material are avoided.

3. Electrode according to one of claims 1 or 2, characterized in that the specific surface area starting from more than 1000 m 2 / L at less than 100 m 2 / L, preferably from more than 8000 m 2 / L at less than 80 m 2 / L and particularly preferably from more than 16000 m 2 / / L to less than 50 m 2 / L is continuously reduced and / or the pore size is continuously increased from a minimum of 3 nm to at least 400 pm and / or the porosity is continuously increased from 20% - 60% to at least 90% and / or the proportion of at least one catalytically active component on the surfaces of the open-pore metal structure is reduced from 20% to 0% by mass.

4. Electrodes according to one of claims 1 to 3, characterized in that hollow channels, e.g. by placeholders, are formed on the surface which is arranged facing away from a separator or another electrode of the respective electrochemical cell.

5. Electrodes according to one of claims 1 to 4, characterized in that the density of 0.05 g / mm 3 to 5 g / mm 3starting from the surface of the respective electrode facing away from the respective separator or the other electrode of the respective electrochemical cell.

6. A method for producing electrodes according to one of claims 1 to 5, characterized in that an open-pore metal structure consisting of one or more of the metals Ni, Fe, Mo, Co, Cu or Ag or an alloy or an intermetallic compound of at least one of these chemical elements is coated on the surface facing the respective separator or the respective other electrode of a respective electrochemical cell with a metal powder selected from Ni, Fe, Mo, Co, Cu and Ag or alloys of these metals and the particles of the metal- metal powder is bonded to the surface of the open-pore metal structure by sintering in a material-tight manner, so that the porosity and the specific surface area are changed, wherein the porosity and the specific surface area are continuously reduced starting from this surface in the direction of the oppositely arranged surface, or an open-pore metal structure consisting of an alloy of Fe, Co, Mo or Ni is coated on the surface facing away from the separator or the respective other electrode of the respective electrochemical cell with a metal powder with low catalytic activity on the electrode used as anode or cathode and the metal powder is then sintered on or no metal powder is applied to this surface, or an open-pore metal structure consisting of one or more catalytically active components on the cathode or the anode of an electrochemical cell, which are coated with Ni and Fe,Ni and Co or Ni and Mo, and has a small porosity, pore size and a higher density, is materially bonded on the side facing the respective separator or the respective other electrode of the respective electrochemical cell to an open-pore metal structure, which preferably consists of Ni with high porosity, wherein the metal structure arranged facing the separator or the respective other electrode of an electrochemical cell advantageously also contains a metal, in particular Al, which is at least partially removed by leaching to increase the specific surface area.

7. The method according to claim 6, characterized in that the surface of the respective electrode which is arranged facing the separator or the respective other electrode of the respective electrochemical cell is coated with particles of at least one catalytically active component and these particles are sintered on.

8. Method according to one of the preceding claims 6 or 7, characterized in that the surface of an open-pore metal metal structure formed with Ni, Al, Mo, Fe, Mn, Co, Cu, Zn, La, Ce, an alloy or an intermetallic compound of at least one of these chemical elements, coated on the surface facing the separator or the respective other electrode of the respective electrochemical cell with a metal powder selected from Co, Mo, Ag, Fe, Cu, Mn, La, and Ce, and the particles of the metal powder are bonded to surfaces of the open-pore metal structure by sintering, and then aluminum powder is applied to this surface and brought to a surface reaction at a temperature between 650 °C and 730 °C under a nitrogen atmosphere, so that leaching aluminide phases are formed, and then a leaching process is carried out in order to increase the specific surface area in the area of the surface coated with the aluminide phase,this aluminide phase is at least partially removed.

9. Method according to one of claims 6 to 8, characterized in that the sintering of the metal powder is carried out at a temperature corresponding to 40% - 95% of the melting temperature of the metal powder used in each case, with a holding time between 20 minutes and 90 minutes under a hydrogen atmosphere or in a high vacuum.

10. The method according to any one of claims 6 to 9, characterized in that an etching process with an inorganic acid, in particular HNO3 in a concentration of 30% - 60%, preferably 32.5%, and a deposition of metal particles of at least one of the metals selected from Ni, Co, Fe, Mo, Mn, Re, Pt, Pd, Ru and Ir on an open-pore metal structure is carried out in order to increase the catalytic activity of the side of a respective electrode facing the separator, wherein the etching process is carried out in one step by applying a nitric acid solution of one or more of the said metals in a mass ratio of the cations to the open-pore metal structure of 0.1% - 5%.