EP4370728A1

EP4370728A1 - Electrolysis cell for polymer electrolyte membrane electrolysis and coating

Info

Publication number: EP4370728A1
Application number: EP22734895.0A
Authority: EP
Inventors: Andre KLINGER; Yashar Musayev
Original assignee: Siemens Energy Global GmbH and Co KG
Current assignee: Siemens Energy Global GmbH and Co KG
Priority date: 2021-08-23
Filing date: 2022-06-08
Publication date: 2024-05-22
Also published as: EP4141145A1; CA3229870A1; US20240352601A1; WO2023025426A1; CN117858979A

Abstract

The invention relates to an electrolysis cell (1) for polymer electrolyte membrane electrolysis with a cathodic half-cell (5) and an anodic half-cell (7). The cathodic half-cell (5) and the anodic half-cell (7) are separated from one another by means of a polymer electrolyte membrane (4). At least one of the half-cells (7, 8) has a channel structure (13a, 13b) which is formed by a gas diffusion layer (11a, 11b) and by a bipolar plate (21a, 21b). The bipolar plate (21a, 21b) has a main body (23) made of a metal base material (25, 27), to which a coating (29) made of a coating material (31) is applied. The coating material (31) is present in a homogeneous mixed phase comprising titanium niobium (TiNb), titanium niobium nitride and also iridium and/or iridium carbide (IrC), in particular in a homogeneous single layer which is formed on the metal main body (23). A coating (29) to be applied as a protective layer to a metal component of an electrolysis cell (1) is also described. This coating comprises the constituents titanium niobium (TiNb) and titanium niobium nitride (TiNbN) and also admixtures of iridium carbide (IrC) and/or (Ir).

Description

Electrolytic cell for polymer electrolyte membrane electrolysis and coating

The invention relates to an electrolytic cell for polymer electrolyte membrane electrolysis, a coating for corrosion protection and the use thereof.

Hydrogen can be obtained from deionized water by electrolysis. The electrochemical cell reactions of the hydrogen formation reaction (HER) and oxygen formation reaction (OER) take place. In the case of acidic electrolysis, the reactions at the anode and cathode can be defined as follows:

Anode 2 H ₂ O -> 4 H+ + O ₂ + 4 e (I)

Cathode H ⁺ + 2e~ _H2 (II)

In what is known as polymer electrolyte membrane electrolysis (PEM electrolysis), the two partial reactions according to equations (I) and (II) are carried out spatially separately from one another in a respective half-cell for OER and HER. The reaction spaces are separated by means of a proton-conducting membrane, the polymer electrolyte membrane (PEM), also known under the term proton exchange membrane. The PEM ensures extensive separation of the product gases hydrogen and oxygen, the electrical insulation of the electrodes and the conduction of the hydrogen ions as positively charged particles. A PEM electrolysis system typically comprises a plurality of PEM electrolysis cells, as described in EP 3 489 394 A1.

A PEM electrolytic cell is described, for example, in EP 2 957 659 A1. The PEM electrolytic cell shown there comprises an electrolyte made from a proton-conducting membrane (proton exchange membrane, PEM), on which the electrodes, a cathode and an anode, are located on both sides. The assembly of membrane and electrodes is referred to as a membrane-electrode assembly (MEA). A gas diffusion layer lies against each of the electrodes. The gas diffusion layers are contacted by so-called bipolar plates. A respective bipolar plate forms a channel structure that is designed for the media transport of the reactant and product streams involved. At the same time, the bipolar plates separate the individual electrolysis cells, which are stacked to form an electrolysis stack with a large number of electrolysis cells. The PEM electrolytic cell is fed with water as a starting material, which is electrochemically broken down at the anode into oxygen product gas and protons H ⁺ . The protons H ⁺ migrate through the electrolyte membrane towards the cathode. On the cathode side, they recombine to form hydrogen product gas H2.

The cell reactions mentioned according to equations (I) and (II) are in equilibrium with their reverse reactions at a cell voltage of 1.48 V, taking into account the increase in entropy when changing from liquid water to gaseous hydrogen or oxygen. In order to achieve correspondingly high product flows in a reasonable time (production output) and thus a current flow, a higher voltage, the overvoltage, is necessary. The PEM electrolysis is therefore carried out at a cell voltage of approx. 1.8 - 2.1 V.

A PEM electrolysis cell is also described in Kumar, S, et al., Hydrogen production by PEM water electrolysis - A review, Materials Science for Energy Technologies, 2 (3) 2019, 442-454. https://doi.org/10.1016/j.mset.2019.03.002. Viewed from the outside in, the PEM electrolytic cell consists of two bipolar plates, gas diffusion layers, catalyst layers and the proton-conducting membrane.

Due to the formation reaction of the oxygen, high oxidative potentials arise at the anode, which is why high-quality materials with a rapid Passivation kinetics, e.g. B. Titanium, in particular for the Gasdi f fusion layer are used. However, there are also high requirements for the choice of material for the anode-side catalyst material and for the bipolar plate, which rests against the gas diffusion layer and makes electrical contact with it. As a result, a respective channel structure is formed for the media transport of the educt and product streams involved. However, due to the high proportion of oxygen at the anode itself, the degradation effects and corrosion at the anode cannot be completely avoided, but at best delayed.

On the other hand, the cathodic potential is less oxidative, so that the gas diffusion layers can be made of stainless steel, for example. However, these corrode, among other things, due to the acidic environment of the PEM electrolysis. This corrosion process is called acid corrosion. The presence of elemental oxygen is not necessary here, since this is provided solely by the dissociation of the surrounding water. The metal ions at the interface of the metal surface are oxidized by the hydroxide anion to form the respective hydroxide salt. This leads to degradation of the cell, which manifests itself in an increased internal resistance and in foreign ions entering the PEM.

Various approaches are described in the prior art in order to counteract the degradation effects at the anode and at the cathode explained above. For example, it has been proposed for the anode-side catalyst to introduce a larger amount of catalytically active species into the anodic catalyst layer in the catalyst layer, i. H . to provide a higher concentration of catalytically active species. In this way, in particular, degradation effects of the anode with regard to the catalytic activity can be compensated for at least temporarily. In order to avoid degradation effects on the anode side due to local electrical contact resistances and an associated inhomogeneous current distribution in the To reduce the bonded system of gas di f fusion layer and catalyst layer, optimization of the contact pressure of the gas di f fusion layer on the catalyst layer was suggested, among other things. However, this always harbors the risk of perforation of the membrane electrode assembly (MEA) due to an excessively high contact pressure when assembling and axially stacking the electrolytic cells to form an electrolytic stack and its mechanical bracing with a high axial force. This leads to short circuits, making the electrolytic cell unusable and the operation of the electrolyzer is significantly endangered or even impossible.

DE 10 2017 118 320 A1 describes a method for producing components, in particular components for energy systems such as fuel cells or electrolyzers. A current-collecting metallic bipolar plate is disclosed therein, the base body of which—a metal sheet—is coated with a three-layer system, with a first underlying layer being applied to the metal sheet. A second backing layer is applied to the first backing layer and finally a cover layer is applied to the second backing layer as a third layer. The first underlying layer is in the form of a metallic alloy layer comprising the metals titanium and niobium with a layer thickness of 0.1 μm. The second underlying layer with a layer thickness of 0.4 μm has the alloy metals titanium, niobium and also nitrogen. A top layer of iridium carbide (IrC) with a layer thickness of 10 to 20 nm is applied to the second underlayer. In the multilayer system of DE 10 2017 118 320 A1, a layer thickness of more than 0.5 μm must therefore be set in order to achieve corrosion protection for a bipolar plate.

In connection with the channel structure, for example, which spatially delimits an electrolytic cell and at the same time ensures contact and material transport, so far hardly any measures have been taken with a view to improving the corrosion Protection proposed. There is also a considerable need for improvement in terms of corrosion protection for the other metallic components of the electrolytic cell, for example the gas diffusion layer, especially on the anode side.

The approaches mentioned above therefore do not solve the actual problems of degradation caused by corrosion, in particular on the anode, sufficiently permanently and reliably for the operation of the electrolytic cell. At best, they represent temporary measures to partially reduce degradation effects caused by corrosion and extend them over a longer period of time, but they do not address the cause.

Against this background, it is the object of the invention to provide an electrolytic cell with which the corrosion problems mentioned above can be reduced or, if possible, avoided entirely. A further object of the invention is to specify a coating for corrosion protection which satisfies these special requirements in the operation of an electrolytic cell.

This object is achieved according to the invention by the subject matter of the independent claims. The dependent claims relate to preferred configurations of the solutions according to the invention.

A first aspect of the invention relates to an electrolytic cell for polymer electrolyte membrane electrolysis with a cathodic half-cell and with an anodic half-cell, the cathodic half-cell and the anodic half-cell being separated from one another by a polymer-electrolyte membrane. At least one of the half-cells has a channel structure formed by a gas diffusion layer and a bipolar plate, the bipolar plate having a base body made of a metallic base material on which a coating made of a coating material is applied, wherein the coating material is present in a homogeneous mixed phase comprising titanium niobium (TiNb), titanium niobium nitride and iridium and/or iridium carbide (IrC).

The anodic half-cell and the cathodic half-cell thus optionally each have a channel structure comprising a base body made of a metallic base material, with an advantageously thin mixed-crystal coating being applied to the base body of the bipolar plate. This homogeneous mixed phase of binary and ternary titanium compound with an admixture of iridium black or iridium carbide leads to a significant increase in corrosion protection for bipolar plates in electrolytic cells and electrolysers. At the same time, the use of expensive iridium compounds is reduced by the homogeneous mixture, so that correspondingly thinner layer thicknesses and simpler structures are possible compared to known multi-layer systems. In this way, a closed, corrosion-resistant coating can be specified.

For the terms electrolysis and polymer electrolyte membrane as well as the reactions and (corrosion) processes taking place, reference is made to the introductory explanations. The polymer electrolyte membrane can be formed, for example, from a tetrafluoroethylene-based polymer with sulfonated side groups. The cathodic half-cell forms the reaction space in which the cathode reaction (s), z. B. proceed according to equation (II). The anodic half-cell forms the reaction space in which the anode reaction (s), z. B. proceed according to equation (I).

The invention is based on the knowledge that in an electrolytic cell, in particular on the side of the anodic half-cell, only inadequate approaches to avoiding or reducing technical problems and limitations due to corrosion-related degradation effects have been proposed that do not address the cause itself. For example, in the case of water rolysis in an electrolytic cell, in particular metallic components and parts of the anode, d. H . Components and parts on the oxygen side are subject to high levels of oxidative attack, jeopardizing long-term reliable and effi cient operation. Due to the high proportion of oxygen in the anode chamber of the electrolytic cell, high-quality metallic materials such as titanium and category 1 stainless steel are used. 4404 or 1 . 4571 , but these materials also form oxides due to massive exposure to oxygen . Corrosion stability is insufficient.

These oxidative wear or Degradation phenomena lead to the removal of material and, moreover, to a disadvantageous increase in the contact and transition resistances in the electrolytic cell. On the anode side, the gas diffusion layer and the bipolar plate are particularly affected by this. In addition to the electrical contacting, the latter also forms a channel structure for the media transport of the educt stream and product stream. On the one hand, this worsens the cell voltage; on the other hand, the problem due to the limited corrosion stability of the stainless steel used is that metal cations, such as Fe ³⁺ or Fe ²⁺ cations, enter the polymer-electrolyte membrane and subsequent damaging reactions occur. This also leads to a deterioration in cell voltage due to reduced ionic conductivity of the membrane.

Observing the increase in the local electrical contact resistance has an essential cause in a rapidly emerging oxidation (passivation) of the material during operation of the gas diffusion layer. Oxidation is predominantly at the surface of the gas diffusion layer and adjacent live electrical contact surfaces. These local contact surfaces, which are then poorly electrically conductive due to the oxidation, lead to high ohmic losses in the electrolytic cell and to a necessary increase in the cell voltage at a constant current density. Ef fi ciency losses and degradation of the anode are the result of inhomogeneous current distribution with disadvantageous local current peaks.

These disadvantageous effects are largely avoided or even overcome with the invention.

The invention addresses the problems described in an advantageous manner in that the channel structure of the anodic half-cell or the cathodic half-cell has a corrosion-resistant coating. The corrosion-resistant coating of the channel structure is applied to the metallic base material as a protective layer. The channel structure has a base body of suitable geometry made of a metallic base material, for example titanium or stainless steel, to which the corrosion-resistant coating is applied. The corrosion-resistant coating itself is designed to be electrically conductive, so that electrical contact with the simultaneous material transport for educts and products in the anodic half-cell is still guaranteed.

This configuration provided by the invention makes it possible for the first time to counteract the causes of the various corrosion phenomena in the channel structure, so that the anodic half-cell withstands the oxidative environment for longer during water electrolysis. In the coating concept, the corrosion-resistant coating is preferably applied to the channel structure, either as a full-surface, closed and corrosion-resistant protective layer or adapted to the local environment with at least one area-specific coating of the surface of the metallic base body at the points particularly at risk of oxidation.

Functionally, depending on the design of the anodic half-cell, the channel structure can also include the adjoining gas diffusion layer or functional parts of the gas diffusion layer, which in any case also enable media transport in the half-cell and the channel wall of the channel structure for this purpose. limit . Thus, the corrosion-resistant coating can be applied as required and flexibly to the surface of several functionally interacting components that form the channel structure in the anodic half-cell. Thus, in addition to the bipolar plate itself, other components for which a functional formation of a channel structure of the anodic half-cell made of a metallic base material is important, such as fleece, expanded metal and gas diffusion layers, have the corrosion-resistant coating. The term channel structure is therefore to be understood comprehensively and functionally for the invention. In particular, the channel structure therefore includes not only the configuration or Realization in the form of a bipolar plate made of a metallic base body, but depending on the structure of the electrolytic cell also other components with a base body made of a metallic base material that form the channel structure. The channel structure can therefore generally be formed by the interaction of several components, for example by an interaction of the bipolar plate and a gas diffusion layer arranged immediately adjacent, so that a fluid channel is formed for the transport of the educt and product streams.

In a particularly preferred configuration of the electrolytic cell, the coating is applied to the base body as a homogeneous single layer.

The coating preferably has titanium niobium (TiNb) and titanium niobium nitride (TiNbN) as basic components, to which iridium (Ir) and/or iridium carbide (IrC) is added. In the homogeneous mixed phase or in the mixed crystal, the admixture of, for example, 5 wt%-25 wt%, preferably 8 wt% to 15 wt%, of iridium black or iridium carbide allows a reduction in the use of expensive iridium material, with the desired anti-corrosion effect being achieved. In this case, small layer thicknesses are possible with good adhesion properties of the mixed crystal on the base body. In a particularly preferred embodiment of the invention, the corrosion-resistant coating has a coating material with a high oxidation potential.

Precious metals or precious metal alloys, which are largely resistant to oxygen corrosion, generally have a high oxidation potential. These coating materials can exist in the environment of the high oxygen concentration in the channel structure of the anodic half-cell and are in principle suitable for the corrosion-resistant protective layer. A closed corrosion-resistant or corrosion-preventing protective layer on the metallic base body is preferable here in order to shield the anodic oxygen concentration and corresponding oxidative attacks on the metallic base material.

For an initial selection of coating material for the corrosion-resistant coating of the channel structure, the redox potential can serve as a measure of the readiness of the ions to absorb the electrons. The ions of noble metals accept electrons more readily than the ions of base metals, which is why the redox potential of the Cu/Cu ²⁺ pair with +0.35 V under standard conditions is significantly more positive than that of the Zn/Zn ²⁺ pair with -0. 76v. And that in turn means that Zn is one of the baser metals and is a stronger reducing agent, so it reduces its reactant and is itself oxidized and gives up electrons. This can be adjusted under cost-benefit considerations, process management for the coating process and the achievable quality of the corrosion-resistant coating.

In a particularly preferred embodiment of the invention, the corrosion-resistant coating has iridium and/or iridium carbide as a component. In this case, either iridium or iridium carbide or, alternatively, both iridium and iridium carbide together can be present in the coating material as components in the coating material. It has been shown that particularly iridium or iridium carbide itself Particularly suitable as corrosion protection in an electrolytic cell. Other components in the coating material are not ruled out here, so that the material requirement for expensive iridium can be limited to a functionally small amount required in the corrosion-resistant coating for a corrosion protection effect to be achieved. The proportion required for the effect can be set accordingly depending on the requirement. It is therefore generally provided that, in addition to iridium and/or iridium carbide, further components are present in the corrosion-resistant coating.

The corrosion-resistant coating has a binary and/or ternary compound containing titanium as a component. A mixture of binary and ternary titanium compounds can also be flexibly used here. It is therefore preferably provided that the corrosion-resistant coating contains a mixture of binary and ternary titanium compounds as well as proportions of iridium and/or iridium carbide as an admixture. The coating system can then be present, for example, in a mixed phase or as a mixed crystal with the appropriate number of components, which is particularly advantageous. In thermodynamics, a mixed phase is a homogeneous phase that consists of two or more substances. Solid mixed phases are also referred to as solid solutions or mixed crystals. The proportions of the individual components of a mixed phase can be specified as partial variables using the proportion x. The amount of material can advantageously be adjusted via the thickness of the corrosion-resistant coating in order to adapt the anti-corrosion effect and to optimize the use of materials.

The binary compound preferably has titanium niobium (TiNb) and the ternary compound has titanium niobium nitride (TiNbN). Surprisingly, it has been shown that this class of materials based on titanium-niobium is particularly effective under the given requirements for the coating of the channel structure for corrosion protection in connection with the iridium or iridium carbide proves . Thus, a coating system is advantageously created for corrosion protection that allows various adaptations and allows for different configurations in terms of material proportions and number of layers and applications in the coating of components and surface areas of the electrolytic cell that are particularly at risk from oxidation, such as the bipolar plate, the gas diffusion layer, in nonwovens and in gas di f fusion layers and in general the channel structure designed for material transport and electrical contacting.

Preferably, at least in the region of the channel structure, the coating is applied over the entire area of the bipolar plate in the form of a closed protective layer on its base body.

The base body of the bipolar plate preferably has a large number of grooves or Channels, so that during operation of the electrolytic cell in the channel structure favors a fluid transport and a uniform electrical contact and voltage supply of the half-cells can be brought about.

In a particularly preferred embodiment, the corrosion-resistant coating has a layer thickness of approximately 0.02 to 0.5 microns, in particular approximately 0.08 to 0.3 microns. Depending on the component to be coated for corrosion protection, the layer thickness and layer composition can be selected and adjusted. Thus, in the electrolytic cell, the components at risk of corrosion and areas with metallic base material can be provided with the coating. In this case, several components can have the corrosion-resistant coating with a respective layer thickness, in particular the components forming the channel structure of the anodic half-cell, such as bipolar plates, gas diffusion layers, fleece and expanded mesh. The layer thickness range is preferably 0, depending on the component. 02 - 0 . 5 microns and can be adjusted in each case by the selected coating process.

In a further preferred embodiment, the corrosion-resistant coating is designed in one layer, in multiple layers or in graded layers.

In the simplest case, the corrosion-resistant coating can therefore be applied as a homogeneous single-layer layer on the substrate, ie, for example, on the metallic base material of the channel structure. The selected layer components are particularly preferably present in the titanium-niobium, titanium-niobium-nitride system with the iridium and any other additives in a homogeneous mixed phase or as a mixed crystal. However, it is also possible for the corrosion-resistant coating to be designed as a graded multi-layer coating, as a graded single-layer coating with a continuous gradient in the chemical layer composition, or as a graded multi-layer coating with a continuous gradient in the chemical layer composition. The configuration as a nanostructured layer system for the corrosion-resistant coating is also conceivable.

Coating processes such as physical vapor deposition (PVD) or plasma-assisted chemical vapor deposition (PACVD) or general coating processes of physical thin-film technology are preferably used for applying the corrosion-resistant coating to the metallic base material of the component.

In the PVD (physical vapor deposition) process, an ionized metal vapor is generated, which reacts with various gases in the plasma and deposits a thin layer on the workpiece surface. The most widely used PVD methods today are arc deposition and sputtering. Both methods are carried out under high vacuum conditions in a coating chamber. In contrast to chemical vapor deposition processes, the starting material is converted into the gas phase with the help of physical processes. The gaseous material is then fed to the substrate to be coated, where it condenses and forms the target layer.

The plasma-enhanced chemical vapor deposition PECVD (plasma-enhanced chemical vapor deposition) or also called PACVD (plasma-assisted chemical vapor deposition) is a special form of chemical vapor deposition (CVD), in which the chemical deposition is supported by a plasma. The plasma can burn directly on the substrate to be coated (direct plasma method) or in a separate chamber (remote plasma method).

Complex coatings of different composition and of high quality can be applied by using these processes. For example, to apply a TiNbN coating using plasma technology, titanium or Dissolved niobium atoms , these ionized and by the applied voltage in the direction of the substrate , ie . H . of the base body, made of the metallic base material, is accelerated. The titanium and Niobium atoms with the introduced nitrogen atoms to form the desired TiNbN layer. The introduction of further components into the coating or layer takes place accordingly.

In a preferred embodiment, the base material is stainless steel or titanium. This is a particularly advantageous choice of material for gas diffusion layers, non-woven fabrics, bipolar plates and other components of the electrolytic cell, in particular for the anodic half-cell. For example, titanium can be selected for the gas diffusion layer and stainless steel for the bipolar plate. Thus, the channel structure formed on the anodic half-cell is electrically conductive on the one hand and is equipped with effective corrosion protection for material transport during electrolysis on the other. through the material Choosing the substrate and coating material also allows for good process control in the chosen coating process and good layer adhesion of the corrosion-resistant coating and thus layer quality, which increases the service life due to the high adhesive strength of the titanium-based coating.

In particular, a desired layer surface with low surface roughness of, for example, an arithmetic mean roughness value of R _a <0.05 micrometers can be achieved, which is of great advantage for good electrical contacting in an electrolytic cell, which is particularly uniform over the area. Furthermore, a high adhesive strength of the corrosion-resistant coating and mechanical wear resistance can be achieved.

In connection with the present invention, a layer or coating can be understood to mean a flat structure whose dimensions in the plane of the layer, length and width, are significantly larger than the dimension in the third dimension, which characterizes the layer thickness.

The introduction of material in layers, for example a corrosion-resistant coating material or other materials, makes it possible in a particularly simple manner to implement a predeterminable distribution of the materials in the anodic half-cell. In addition, the handling of the materials can be facilitated.

Thus, it is preferably possible for the gas diffusion layer, which is formed from a base body made of a base material, to which a corrosion-resistant coating is applied over the entire surface. Thus, in this sense, in the case of the gas diffusion layer, the base body with the base material forms a first layer and the corrosion-resistant coating forms a second layer of the gas diffusion layer. In this case, the second layer can in turn be a layer system with several layers. The same applies to the other components of the Electrolytic cell, such as the bipolar plate or all other components forming a channel structure, to which a corrosion-resistant coating is applied.

A further aspect of the invention relates to the use of an electrolytic cell for the electrolytic production of hydrogen.

Thus, advantageously in the cathodic or. anodic half-cell, the reactions according to equations (I) and (II) are performed when the electrolytic cell is traversed by electric current.

A further aspect of the invention relates to a corrosion-resistant coating for application as a protective layer on a metallic component of an electrolytic cell, comprising the components titanium niobium and titanium niobium nitride (TiNbN) and iridium carbide and/or iridium.

Use in a bipolar plate forming a channel structure and/or a gas diffusion layer as a metallic component of an electrolysis cell, in particular in an anodic half-cell ( 7 ) of an electrolysis cell ( 1 ), is particularly preferred.

By means of the corrosion-resistant coating according to the invention, one of the electrolytic cells described above for polymer electrolyte membrane electrolysis can be coated or. the corrosion-resistant coating is applied to components of an electrolytic cell that are particularly at risk of corrosion on their base body made of a metallic base material. The corrosion-resistant coating is therefore preferably used in the anodic half-cell to counteract the predominant oxygen-induced corrosion there. Correspondingly, in the case of the corrosion-resistant coating, reference is made to the above explanations and the advantages of these electrolytic cells. It is provided according to the invention that the corrosion-resistant coating has titanium niobium and titanium niobium nitride and an admixture of iridium and/or iridium carbide as basic components. The admixture limits the material use of expensive iridium or compared to a monolayer of an iridium-based protective layer, without impairing the corrosion protection.

In a particularly preferred embodiment, the corrosion-resistant coating is in the form of a homogeneous single-layer coating, a graded single-layer coating or a graded multi-layer coating.

In the simplest case, the corrosion-resistant coating can therefore be in the form of a homogeneous single-layer coating. Here, for example, the selected layer components are in the system titanium-niobium, titanium-niobium-nitride with the iridium and any other additives in a homogeneous mixed phase, which preferably forms a closed corrosion-resistant protective layer or a coating on the base body. This is a particularly cost-effective form of corrosion protection in the oxidative environment.

However, it is also possible for the corrosion-resistant coating to be designed as a graded single-layer coating with a continuous gradient in the chemical layer composition or as a graded multi-layer coating with a continuous gradient in the chemical layer composition. The configuration as a nanostructured layer system for the corrosion-resistant coating is also conceivable.

In an alternative configuration, the corrosion-resistant coating can preferably be implemented as a multilayer coating with iridium carbide and/or iridium as a component of the top layer.

This advantageously also saves material for the use of expensive iridium and iridium carbide achievable . It is essentially sufficient to provide iridium and/or iridium oxide as components in a sufficient concentration in the top layer of a multilayer coating that is directly exposed to the oxidative environment. These are preferably the essential components of the top layer, or it is possible for the top layer to have iridium or iridium oxide as its main component. In a multi-layer coating, further layers are then provided below the top layer, which have other materials such as titanium niobium and titanium niobium nitride, optionally with proportions of iridium and/or iridium oxide as an admixture or in a correspondingly adjusted manner, preferably in the direction of the substrate or Basic body decreasing concentration gradient. As a result, corresponding graded multi-layer mixed phases are optionally formed in the corrosion-resistant coating.

Iridium and/or iridium carbide is preferably used as part of a corrosion-resistant coating of a metallic component of an electrolytic cell.

More preferably, titanium niobium and/or titanium niobium nitride is used as a further component of the corrosion-resistant coating.

A particularly preferred use of the corrosion-resistant coating is in a channel structure or in a gas diffusion layer used as metallic components of an electrolytic cell, in particular in an anodic half-cell. In this case, the channel structure is preferably formed by the immediately adjacent arrangement of a bipolar plate with the fluid-permeable gas diffusion layer. Due to the required electrical conductivity, both the bipolar plate and the gas diffusion layer form metallic components or components of the electrolytic cell and at the same time form the flow channel for the transport of the fluids, i. H . Educt flow and product flow are guided through the channel structure. Advantageously, therefore, are the Surfaces of the channel structure that limit the channel structure and that are exposed to the oxygen during operation and are exposed to corrosion attacks are provided with the corrosion-resistant coating.

The gas diffusion layer of the anodic half-cell has a porous material to ensure adequate gas permeability. The gas diffusion layer can, for example, be made of titanium as the base material with a porous base body, such as a titanium-based expanded metal or wire mesh, and be provided with the corrosion-resistant coating. As a result, the useful life or The service life of the gas di f fusion layer can be increased, and the disadvantageous degradation effects caused by oxidation are reduced. It is therefore advantageous to apply the corrosion-resistant coating according to the invention as a protective layer on the metallic base body of the gas diffusion layer. The corrosion-resistant coating can be used as corrosion protection on other components of the anodic half-cell with a metallic base body, in particular titanium or stainless steel.

The coating concept of the invention also means that the local electrical contacts are significantly improved and thus the current density over the cell area is more homogeneous. In addition to protection against corrosion, this leads to better and, above all, more uniform current distribution during operation of the electrolytic cell.

Furthermore, the anodic half-cell preferably has a gas diffusion layer formed from titanium as the base material. An embodiment of fine-meshed titanium material is preferably used here as the base material for the gas diffusion layer, for example titanium nonwovens, titanium foams, titanium fabrics, titanium-based expanded gratings or combinations thereof. This also increases the local contact points to the electrode and the electrical resistance in the contact area is particularly uniform. The term "lattice" refers to present context a fine-meshed network. The carrier materials mentioned are characterized by high corrosion resistance. The terms "lattice" and "fabric" describe a directional structure, the term "fleece" a non-directional structure.

A channel structure can preferably be arranged adjacent, in particular directly adjacent, to the gas diffusion layer or a channel structure is functionally formed by the adjacent arrangement of gas diffusion layer and a component, in particular a bipolar plate. The channel structure is used to collect and discharge the gaseous reaction product of the electrolysis in the anodic half-cell, ie z. B. Oxygen according to equation (I). The channel structure can, for example, comprise or be designed as a bipolar plate. Bipolar plates allow several electrolytic cells to be stacked to form an electrolytic cell module by electrically conductively connecting the anode of one electrolytic cell to the cathode of an adjacent electrolytic cell. In addition, the bipolar plate enables gas separation between adjacent electrolytic cells.

In the following, the invention is explained by way of example with reference to the attached figures using preferred embodiments, with the features presented below being able to represent an aspect of the invention both individually and in various combinations with one another. Show it :

1 shows a schematic representation of an electrolysis cell for polymer electrolyte membrane electrolysis according to the prior art;

2 shows an exemplary anodic half-cell with channel structure and corrosion-resistant coating according to the invention; 3 shows a corrosion-resistant coating in an embodiment as a homogeneous single-layer coating;

4 shows a corrosion-resistant coating in an embodiment as a graded single-layer coating;

5 shows a corrosion-resistant coating in an embodiment as a multi-layer coating;

6 shows a corrosion-resistant coating in an embodiment as a graded multi-layer coating.

1 shows an electrolytic cell 1 for polymer electrolyte membrane electrolysis according to the prior art in a schematic representation. The electrolytic cell 1 is used for the electrolytic generation of hydrogen.

The electrolytic cell 1 has a polymer electrolyte membrane 3 . The cathodic half-cell 5 of the electrolytic cell 1 is arranged on one side of the polymer electrolyte membrane 3, in the representation according to FIG anodic half-cell 7 of the electrolytic cell 1 arranged.

The anodic half cell 7 comprises an anodic catalyst layer 9 arranged directly adjacent to the polymer electrolyte membrane 3, a gas diffusion layer 11a arranged directly adjacent to the anodic catalyst layer 9 and a bipolar plate 21a arranged directly adjacent to the gas diffusion layer 11a, so that a channel structure 13a is designed for fluid transport. The anodic catalyst layer 9 has an anodic catalyst material 15 and catalyzes the anode reaction according to equation (I). As the anodic catalyst material 15, iridium or Iridium oxide is selected as the catalytically active species which is introduced into the anodic catalyst layer 9 . iridium or . Iridium oxide have a high oxidation and solution stability and are therefore well suited to be used as an anodic catalyst material 9 . In order to reduce corrosion, the gas diffusion layer 11a is made of a material on the surface of which a passivation layer is quickly formed, e.g. B. made of titanium . Titanium dioxide is formed as a result of the passivation of the titanium, although this has a lower electrical conductivity than titanium. The channel structure 13a is formed by the bipolar plate 21a, so that a stacking of several electrolytic cells 1 is made possible.

The cathodic half cell 5 comprises a cathodic catalyst layer 17 with a cathodic catalyst material 19 which is arranged directly adjacent to the polymer electrolyte membrane 3 . The cathodic catalyst material 19 is designed to catalyze a reduction of hydrogen ions (protons), in particular according to equation (II) to form molecular hydrogen. A gas diffusion layer 11b is also arranged on the cathodic catalyst layer 19 . In contrast to the gas diffusion layer 11a of the anodic half-cell 7, the gas diffusion layer 11b of the cathodic half-cell 5 is made of high-grade steel. This is possible due to the lower oxidation potential in the cathodic half-cell 5 compared to the anodic half-cell 7 and reduces the cost of the electrolytic cell 1 . A channel structure 13b is also arranged immediately adjacent to the gas diffusion layer 11b, which, analogously to the anodic half-cell 7, is designed as a bipolar plate 21b. The gas diffusion layers 11a, 11b, in cooperation with the respective bipolar plates 21a, 21b arranged immediately adjacent, functionally form a respective channel structure 13a, 13b, ie a fluid-tight flow channel for the material transport of the starting materials and the products during the electrolysis.

A disadvantage of this electrolytic cell 1 known from the prior art, as explained at the outset, is generally the susceptibility to corrosion of the materials. Especially in the anodic half-cell 7 there are significant degradation effects record, which affect the service life of the electrolytic cell 1 very adversely.

On the one hand, due to the high oxygen concentration in the anode-side half-cell 7 and the high oxidative potential during operation, damaging corrosion of the materials used can be observed. This is especially true for the stainless steels with material number 1, for example. 4404 or 1 . 4571, which are used as materials for the bipolar plate 21a, which form the channel structure 13a. But even the more resistant titanium is subject to oxidative attack. As a result, an increase in the local electrical contact resistance can be observed during operation. This has an essential cause in a rapidly occurring oxidation (passivation) of the titanium during operation of the gas diffusion layer 11a. The oxidation is primarily recorded on the surface of the gas diffusion layer 11a and adjacent current-carrying electrical contact surfaces.

These local contact surfaces, which are then electrically poorly conductive due to the oxidation, lead to high ohmic losses in the electrolytic cell 1 and to a then necessary increase in the cell voltage at a constant current density. Ef fi ciency losses and degradation of the anodic half-cell 7 are the result of inhomogeneous current distribution with disadvantageous local current peaks.

On the side of the cathodic half-cell 5, certain disadvantageous effects can also be observed, especially with regard to the acid corrosion promoted by elemental oxygen, but these are not discussed in detail here.

To remedy the disadvantages of the anodic half-cell 7, it is proposed to provide the channel structure 13a in the anodic half-cell 7 with a corrosion-resistant coating 29, so that the disadvantageous and ongoing oxidative attack is inhibited or, at best, largely prevented. Such an advantageous way modi fi- The electrolytic cell 1 which has been decorated and further developed is shown schematically by way of example in FIG. The anodic half-cell 7 of the exemplary embodiment of an electrolytic cell 1 shown in FIG. 2 is composed in terms of its basic structure analogously to the electrolytic cell 1 according to FIG. 1, so that reference can be made to the relevant statements.

Similarly to the electrolytic cell according to FIG. 1, the anodic half-cell 7 has a gas diffusion layer 11a and a bipolar plate 21a. The gas diffusion layer 11a has a base body 23 made of a metallic base material 27, which is titanium in the present case. The gas diffusion layer 11a is in this case designed as a titanium-based expanded metal, so that fluid transport is made possible. Analogously, the bipolar plate 21a has a base body 23 made of a metallic base material 25 , which is stainless steel in the present case. In the base body 23 of the bipolar plate 21a is a multiplicity of grooves or Milled channels to promote fluid transport and at the same time a uniform electrical contact and voltage supply of the anodic half-cell 7 .

In this case, the gas diffusion layer 11a and the bipolar plate 21a are designed and arranged adjacent to one another in such a way that a channel structure 13a is formed, which encompasses the metal base body 23 made of the metal base material 25 , 27 . In contrast to the electrolytic cell 1 according to FIG. 1, the channel structure 13a of the anodic half-cell 7 is designed for effective protection against corrosion. For this purpose, the channel structure 13a has a corrosion-resistant coating 29 made of a coating material 31 . The coating material 31 is selected in such a way that it has a high oxidation potential and is electrically conductive at the same time. For this purpose, the coating material 31 has a binary and a ternary titanium compound, in this case titanium niobium (TiNb) and titanium niobium nitride in a homogeneous mixed phase, wherein the mixed phase iridium and / or iridium carbide is added as further components in a flexibly adjustable amount.

As a result, the need for iridium required can be reduced to a minimum or compared to an iridium layer or Iridium carbide layer can be significantly reduced. At the same time, the interaction of the binary and ternary titanium components in the coating material 31 achieves effective and long-term stable corrosion protection and promotes good layer adhesion on the metallic base material 25 , 27 . In the exemplary embodiment, the corrosion-resistant coating 29 is applied to the full surface of the bipolar plate 21a in the form of a closed protective layer on the base body 23, at least as shown in the area of surfaces that delimit the channel structure 13a. The coating measure can alternatively also be applied to the particularly critical ones Surface areas of the bipolar plate 21a are locally limited with regard to oxidation. The gas diffusion layer 11a is also provided with the corrosion-resistant coating 31 over its entire surface, at least on its side facing the bipolar plate 21a, so that a closed and effective corrosion protection is applied overall to the channel structure 13a. It is of great advantage that essentially the same coating material 31 with regard to the components can be used for the corrosion-resistant coating 31 for the gas diffusion layer 11a as well as for the bipolar plate 21a. However, adjustments with regard to the specific composition, such as the concentration of the respective component of the coating material 31 in the mixed phase, are possible and useful. Furthermore, adaptations can be made with a view to the specific layer structure of the corrosion-resistant coating 29, taking into account the respective component, its geometry and the oxidative environment, in particular with regard to the selection of the surface areas of the metallic base body 23 of the component to be coated. For example, the gas diffusion layer 11a for a functional formation of a channel structure 13a of the anodic half-cell made of a titanium base material is typically formed from a porous structure with a relatively large surface, for example from a fleece, an expanded metal and/or from several layered gas diffusion layers or combinations thereof. For effective protection against corrosion, such a porous structure with a large surface of the gas diffusion layer 11a is then preferably provided over the entire surface with a closed protective layer of coating material 31 containing iridium and/or iridium carbide. Consequently, the corrosion-resistant coating 29 is also applied to components or components with very complex surface structures 29 in order to achieve the most complete possible protection against corrosion. This then also influences the use of the coating process. The corrosion-resistant coating 31 is optionally applied using methods of physical thin-film technology, preferably coating methods such as physical vapor deposition (PVD) or plasma-assisted chemical vapor deposition. The corrosion-resistant coating 29 can be used flexibly for different components of an electrolytic cell 1, typically fleece, bipolar plates or gas diffusion layers. Depending on the respective component, the layer thickness range is set at 0.02-0.5 microns, for example also between 0.08 and 0.3 microns, and can therefore be selected to be comparatively thin for the application.

Within the scope of the invention, there are therefore various possibilities for the respective specific configuration of the corrosion-resistant coating 29, so that a large number of coating systems can be implemented within the framework of the corrosion protection concept.

This is explained in more detail below with reference to the exemplary embodiments shown in FIG. 3 to FIG. These exemplary embodiments have in common that titanium or stainless steel are used as the metallic base material 25, 27, such as this is typically provided for an anodic half cell 7 . The metallic base material 25 of the bipolar plate 21a is a high-grade steel, eg. B. the material number 1 . 4404 or 1 . 4571, and the metal base material 27 of the gas diffusion layer 11b is titanium.

FIG. 3 shows a schematic representation of a corrosion-resistant coating 29 in an embodiment as a homogeneous single-layer layer. The base body 23 forms the substrate made of a metallic base material 25, here a high-grade steel. A corrosion-resistant coating 29 is applied to the base body 23 in a single layer 37a—as a so-called monolayer—with a layer thickness D. The coating material 31 is present in a homogeneous mixed phase and has iridium as a layer component in a mixture with other components comprising titanium niobium and titanium niobium nitride. The concentrations of the components in the homogeneous single-layer layer are approximately constant over the layer thickness D.

In contrast, FIG. 4 shows an alternative configuration of the corrosion-resistant coating 29 as a graded single-layer coating. The graded single-layer layer is applied as a monolayer in a single layer 37a to the metallic base body 23 made of titanium 27 as the substrate. Due to the grading, the concentration of the components of the layer material changes in a targeted manner over the layer thickness D, so that a concentration gradient is formed. In the present case, the concentration of iridium or optionally also iridium carbide in layer 37a towards the surface. At the surface of the corrosion-resistant coating 29, the concentration of iridium or iridium carbide can be up to 100%, so that a closed protective layer of iridium or iridium carbide is formed. The concentration of titanium niobium and titanium niobium nitride decreases accordingly towards the surface. The concentration of iridium or Iridium carbide at the interface of the monolayer 37a to the substrate is vanishingly small or zero. FIG. 5 shows a corrosion-resistant coating 29 which is designed as a multi-layer coating comprising two layered mono-layers 37a, 37b. It has a first layer 37a, which is applied to the substrate, and a second layer 37b, the second layer 37b being applied to the first layer 37a. The substrate is formed from a metallic base body 23 with a metallic base material 27 , in this case titanium. The second layer 37b forms the cover layer 33 and thus the surface of the corrosion-resistant coating 29 . The second layer 37b is a thin, homogeneous monolayer with iridium or Iridium carbide designed as the essential coating material 31 . Admixtures of titanium niobium and/or titanium niobium nitride are possible. The first layer 37a forms an intermediate layer 35 between the base body 23 made of titanium and the cover layer 33 . The intermediate layer 35 promotes adhesion and ensures good adhesion and permanent attachment of the corrosion-resistant coating 29 on the base body 23 , which is promoted by the binary and/or ternary titanium-based layer material 31 in the intermediate layer 35 . It is also possible for both layers 37a, 37b or one of the layers 37a, 37b to be in the form of a graded monolayer in accordance with the exemplary embodiment in FIG.

In FIG. 6, a corrosion-resistant coating 29 is designed as a complex multi-layer system. This layer system is applied to the base body 23 from a metallic base material 35 , in the present case a stainless steel substrate. The three layered layers 37a, 37b, 37c form an intermediate layer 35 to which the layer 37d is applied as a cover layer 33. The coating is a graded multilayer coating of layer thickness D, so that at least within the layers 37a, 37b, 37c forming the intermediate layer 35, the concentration of the components titanium niobium, titanium niobium nitride and iridium and/or iridium carbide in the The growth direction of the layers 37a to 37d varies and is specifically adjusted with regard to the requirements for corrosion protection. The cover layer 33 is a homogeneous insert gene layer in the position 37d performed with a high proportion of iridium and / or iridium carbide of up to 100%. However, it is also possible for the cover layer 33 to be designed as a graded single layer, or for individual layers 37a, 37b, 37c of the intermediate layer 35 to be designed as a homogeneous single layer. During operation in an electrolytic cell 1 , the surface of the cover layer 33 is directly exposed to the corrosive attack due to the high oxygen concentration in an anodic half-cell 7

The corrosion-resistant coating 29 is used particularly advantageously for coating metallic components or functional parts of an electrolytic cell 1 , preferably in the anodic half-cell 7 . In the case of an anodic half-cell 7, use of the corrosion-resistant coating 29 is provided in a special way for a bipolar plate 13a or a gas diffusion layer 11a. In general, all components, parts or functional parts that form a channel structure 13a in the anodic half-cell 7 and whose surfaces are exposed to damaging, corrosive exposure to oxygen during operation can be considered for applying the corrosion-resistant coating 29 .

It is understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. Thus, features of the exemplary embodiments described herein can be combined with one another unless specifically stated otherwise. The description of the exemplary embodiments is therefore not to be taken in a limiting sense, and the scope of protection of the present invention is defined by the appended claims.

As used herein, the term "and/or" when used in a set of two or more items means that each of the listed items can be used alone, or any combination of two or more of the listed items can be used.

Claims

patent claims

1. Electrolytic cell (1) for polymer electrolyte membrane electrolysis with a cathodic half-cell (5) and an anodic half-cell (7), the cathodic half-cell (5) and the anodic half-cell (7) being separated from one another by means of a polymer electrolyte membrane (4), at least one of the half-cells (5, 7) having:

- A channel structure (13a, 13b), which is formed by a gas diffusion layer (11a, 11b) and by a bipolar plate (21a, 21b), wherein

- The bipolar plate (21a, 21b) has a base body (23) made of a metallic base material (25, 27) to which a coating (29) made of a coating material (31) is applied, wherein

- The coating material (31) is present in a homogeneous mixed phase comprising titanium niobium (TiNb), titanium niobium nitride and iridium and/or iridium carbide (IrC).

2. Electrolytic cell (1) according to claim 1, in which the coating (29) is applied as a homogeneous single-layer layer on the base body (23).

3. Electrolytic cell (1) according to claim 1 or 2, in which the coating (29) has titanium niobium (TiNb) and titanium niobium nitride (TiNbN) as basic components, to which iridium (Ir) and/or iridium carbide (IrC) is added.

4. Electrolytic cell (1) according to one of the preceding claims, in which at least in the region of the channel structure (13a) the coating (29) is applied over the entire surface of the bipolar plate (21a) in the form of a closed protective layer on the base body (23).

5. Electrolytic cell (1) according to one of the preceding claims, in which the base body (23) of the bipolar plate (21a, 21b) has a multiplicity of grooves or channels, so that during operation in the channel structure (13a) fluid transport is encouraged. taken and a uniform electrical contact and voltage supply of the half-cells (5, 7) can be brought about.

6. Electrolytic cell (1) according to one of the preceding claims, in which the coating (29) as a thin monolayer with a layer thickness (D) of 0.08 to 0.3 microns, in particular from about 0.02 to 0.5 microns, having.

7. electrolytic cell (1) according to any one of the preceding claims, wherein the coating (29) by means of physical vapor deposition (PVD) or plasma-assisted chemical vapor deposition (PACVD) is applied.

8. Electrolytic cell (1) according to one of the preceding claims, in which the metallic base material (25, 27) is stainless steel or titanium.

9. Coating (29) for application as a protective layer on a metallic component of an electrolytic cell (1), comprising the components titanium niobium (TiNb) and titanium niobium nitride (TiNbN) and optionally either iridium carbide (IrC) or iridium (Ir) , or iridium carbide (IrC) and iridium as an admixture, with a homogeneous mixed phase of the components being formed.

10. Coating (29) according to claim 9, designed as a homogeneous single-layer layer applied to a metallic base material (25, 27).

11. Use of a coating (29) according to any one of claims 9 or 10, for corrosion protection of a metallic component of an electrolytic cell (1).

12. Use according to claim 11, in a channel structure (13a, 13b) forming bipolar plate (21a, 21b) and / or a gas diffusion layer (11a, 11b) as a metallic component components of an electrolytic cell (1), in particular in an anodic half-cell (7) of an electrolytic cell (1).