EP4141145A1 - Electrolytic cell for polymer electrolyte membrane electrolysis and corrosion-resistant coating - Google Patents

Abstract

An electrolysis cell (1) for polymer electrolyte membrane electrolysis with a cathodic half-cell (5) and an anodic half-cell (7) is proposed. The cathodic half cell (5) and the anodic half cell (7) are separated from each other by a polymer electrolyte membrane (4). The anodic half-cell (7) has a channel structure (13a) comprising a base body (23) made of a metallic base material (25, 27), a corrosion-resistant coating (29) being applied to the base body (23).

A corrosion-resistant coating (29) for application as a protective layer to a metallic component of an electrolytic cell (1) is also specified. This has the components titanium niobium (TiNb) and titanium niobium nitride (TiNbN) as well as iridium carbide (IrC) and/or (Ir).

Description

The invention relates to an electrolytic cell for polymer electrolyte membrane electrolysis, a corrosion-resistant coating and its use.

• Anode
  
           2 H₂O → 4 H⁺ + O₂ + 4 e^-     (I)
  
  
• Kathode
  
           H⁺ + 2 e^- → H₂     (II)
  
  

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
  
  _2H2O → 4H ⁺ + _O2 + 4e ^- (I)
  
  
• cathode
  
  H ⁺ + 2 e ^- → H ₂ (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 as the 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 electrolytic plant typically comprises a plurality of PEM electrolytic cells, such as in FIG EP 3 489 394 A1 described.

A PEM electrolytic cell is for example in EP 2 957 659 A1 described. The PEM electrolytic cell shown there comprises an electrolyte made of 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 hydrogen product gas H ₂ .

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 electrolytic 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. As titanium, in particular for the gas diffusion layer. However, high requirements also apply to 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 oxygen content 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 there, 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 elementary oxygen is not necessary here, as this is provided by the dissociation of the surrounding water alone. 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 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 was proposed for the anode-side catalyst to introduce a larger amount of catalytically active species into the anodic catalyst layer in the catalyst layer, ie 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. About degradation effects on the anode side due to local electrical contact resistances and an associated inhomogeneous current distribution in the composite system To reduce gas diffusion layer and catalyst layer, the optimization of the contact pressure of the gas diffusion layer on the catalyst layer was proposed, among other things. However, this always harbors the risk of perforation of the membrane-electrode unit (MEA) due to an excessively high contact pressure when assembling and axially stacking the electrolysis cells to form an electrolysis 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.

In connection with the channel structure, for example, which spatially delimits an electrolytic cell and at the same time ensures contact and mass transport, hardly any measures have been proposed to date with a view to improving corrosion protection. 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 at the anode, sufficiently permanently and reliably for the operation of the electrolytic cell. At best, they represent temporary measures to partially reduce corrosion-related degradation effects 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 corrosion-resistant coating which satisfies these special requirements when operating 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. The anodic half-cell has a channel structure comprising a base body made of a metallic base material, with a corrosion-resistant coating being applied to the base body.

For the terms electrolysis and polymer electrolyte membrane as well as the reactions and (corrosion) processes taking place, please refer 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), e.g. B. run according to equation (II). The anodic half-cell forms the reaction space in which the anode reaction(s), e.g. B. run according to equation (I).

The invention is based on the finding that in an electrolytic cell, in particular on the side of the anodic half-cell, only inadequate solutions have been proposed to avoid or reduce technical problems and limitations due to corrosion-related degradation effects, which do not address the cause itself. For example, in the case of water electrolysis in an electrolytic cell, particularly metallic components and parts of the anode, ie components and parts on the oxygen side, are exposed to a high degree of oxidative attack, which jeopardizes long-term reliable and efficient operation. Although come due to the high Due to the oxygen content in the anode space of the electrolytic cell, high-quality metallic materials such as titanium and stainless steel of category 1.4404 or 1.4571 are used, but these materials also form oxides due to massive exposure to oxygen. Corrosion stability is insufficient.

These oxidative wear or degradation phenomena lead to material removal and also to a disadvantageous increase in the contact and transition resistances in the electrolytic cell. The gas diffusion layer and the bipolar plate are particularly affected by this on the anode side. In addition to the electrical contacting, the latter also forms a channel structure for the media transport of the educt flow and product flow. 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.

The observation of the increase in the local electrical contact resistance has an essential reason in the oxidation (passivation) of the material, which occurs quickly, during operation of the gas diffusion layer. The oxidation is predominantly at the surface of the gas diffusion layer and adjacent live electrical contact areas. 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. Loss of efficiency 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 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 from 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 simultaneous mass transport for educts and products in the anodic half-cell is still guaranteed.

With this configuration provided by the invention, the causes of the various corrosion phenomena in the channel structure can be counteracted for the first time, so that the anodic half-cell can withstand 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 delimit the channel wall of the channel structure for this purpose. 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 from a metallic base material is significant are, such as nonwovens, expanded metal and gas diffusion layers that have a 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 also other components with a base body made of a metallic base material, depending on the structure of the electrolytic cell, which 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 embodiment of the invention, the corrosion-resistant coating has a coating material with a high oxidation potential.

Noble metals or noble 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 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 a first 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 accept 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 is significantly more positive at +0.35 V than that of des Zn/Zn ²⁺ pair with -0.76 V. And that in turn means that Zn is one of the baser metals and is a stronger reducing agent, i.e. its reactant is reduced and itself oxidized and emits 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 can be present together as components in the coating material in the coating material. It has been shown that iridium or iridium carbide are particularly suitable as corrosion protection in an electrolytic cell. Other components in the coating material are not excluded 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 preferably 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 and proportions of iridium and/or iridium carbide. The coating system can then be present, for example, in a mixed phase or as a mixed crystal with the appropriate number of components. In thermodynamics, a mixed phase is a homogeneous phase consisting 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 mole fraction x. The amount of substance 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.

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

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 metal. Depending on the component, the layer thickness range is preferably 0.02 - 0.5 microns and can be adjusted by the selected coating process.

In a further preferred embodiment, the corrosion-resistant coating is implemented 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. Here, for example, the selected layer components are present in the titanium-niobium, titanium-niobium-nitride system with the iridium and any other additives in a homogeneous mixed phase. 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 composition of the layer, or as a graded multi-layer coating with a continuous gradient in the chemical composition of the layer. 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.

Unlike chemical vapor deposition processes, physical processes are used to convert the starting material into the gas phase. 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 at the substrate to be coated (direct plasma method) or in a separate chamber (remote plasma method).

Complex coatings of different compositions and of high quality can be applied by using these processes. For example, to apply a TiNbN coating using plasma technology, titanium or niobium atoms are released from the target by the energy input of an arc into the target, these are ionized and accelerated by the applied voltage in the direction of the substrate, i.e. the base body made of the metallic base material. The titanium or niobium atoms then combine with the introduced nitrogen atoms to form the desired TiNbN layer. The entry 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 chosen 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 on the other hand for mass transport during electrolysis with a equipped with effective corrosion protection. The choice of substrate and coating material also enables good process control in the respectively selected 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 microns can be achieved, which is of great advantage for good and, above all, uniform electrical contacting in an electrolytic cell. Furthermore, a high level of adhesion 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.

It is thus 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.

The reactions according to equations (I) and (II) can thus advantageously be carried out in the cathodic or anodic half-cell 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.

One of the above-described electrolytic cells for polymer electrolyte membrane electrolysis can be coated with the corrosion-resistant coating according to the invention, or the corrosion-resistant coating is applied to components of an electrolytic cell that are particularly susceptible to corrosion on their base body made of a metallic base material. The corrosion-resistant coating is therefore preferably used on 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 preferred here 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 use of expensive iridium material or, compared to a monolayer, one made of an iridium-based protective layer, without impairing the corrosion protection.

In a particularly preferred embodiment, the corrosion-resistant coating is designed as a homogeneous single-layer coat, a graded single-layer coat or a graded multi-layer coat.

In the simplest case, the corrosion-resistant coating can therefore be designed as a homogeneous single layer layer. Here, for example, the selected layer components in the system titanium-niobium, titanium-niobium-nitride with iridium and any other additives are present 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 in the form of a graded single-layer coating with a continuous gradient in the chemical composition of the layer or as a graded multi-layer coating with a continuous gradient in the chemical composition of the layer. The configuration as a nanostructured layer system for the corrosion-resistant coating is also conceivable.

The corrosion-resistant coating is preferably designed as a multilayer coating with iridium carbide and/or iridium as a component of the top layer.

In this way, a material saving can advantageously be achieved for the use of expensive iridium and iridium carbide. 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. Preferably these are the essential components of the top layer, or it is possible for the top layer to comprise essentially iridium or iridium oxide as the main component. In a multilayer then further layers are provided below the cover layer, which have further 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 set concentration gradient, preferably decreasing in the direction of the substrate or base body . As a result, appropriate 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 that is 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.e. the reactant flow and product flow are guided through the channel structure. Advantageously, therefore, the surfaces of the channel structure that delimit the channel structure and that are exposed to the oxygen during operation and are exposed to corrosive attack 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 made of titanium as the base material with a porous base body, such as a titanium-based expanded metal or wire mesh, and provided with the corrosion-resistant coating. As a result, the service life or service life of the gas diffusion 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 the current density over the cell area is therefore more homogeneous.

Furthermore, the anodic half-cell preferably has a gas diffusion layer formed from titanium as the base material. Here, preference is given to using a configuration as a fine-meshed titanium material as the base material for the gas diffusion layer, for example titanium nonwovens, titanium foams, titanium fabrics, titanium-based expanded gratings or combinations thereof. As a result, the local contact points to the electrode are also increased and the electrical resistance in the contact area is particularly uniform. In the present context, the term “grid” designates a fine-meshed network. The carrier materials mentioned are characterized by high corrosion resistance. The terms "grid" 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 functional due to 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 a bipolar plate or be designed as such. 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.

FIG 1

eine schematische Darstellung einer Elektrolysezel-le zur Polymerelektrolytmembran-Elektrolyse gemäß dem Stand der Technik;

FIG 2

eine beispielhafte anodische Halbzelle mit Kanal- struktur und korrosionsfester Beschichtung gemäß der Erfindung;

FIG 3

eine korrosionsfeste Beschichtung in einer Ausgestaltung als homogene Einlagenschicht;

FIG 4

eine korrosionsfeste Beschichtung in einer Ausgestaltung als gradierte Einlagenschicht;

FIG 5

eine korrosionsfeste Beschichtung in einer Ausgestaltung als Mehrlagenschicht;

FIG 6

eine korrosionsfeste Beschichtung in einer Ausgestaltung als gradierte Mehrlagenschicht.

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:

FIG 1

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

FIG 2

an exemplary anodic half-cell with channel structure and corrosion-resistant coating according to the invention;

3

a corrosion-resistant coating configured as a homogeneous single layer;

FIG 4

a corrosion resistant coating in a graded single layer configuration;

5

a corrosion-resistant coating in a configuration as a multi-layer coating;

6

a corrosion-resistant coating configured as a graded multi-layer coating.

FIG 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 . On one side of the polymer electrolyte membrane 3, according to the illustration figure 1 , left, the cathodic half-cell 5 of the electrolytic cell 1 is arranged on the other side of the polymer electrolyte membrane 3, in the illustration according to figure 1 , right, the anodic half-cell 7 of the electrolytic cell 1 is 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 formed for fluid transport is. The anodic catalyst layer 9 has an anodic catalyst material 15 and catalyzes the anode reaction according to equation (I). Iridium or iridium oxide is selected as the catalytically active species as the anodic catalyst material 15 and 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 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 readily 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 stainless 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 costs 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 is. 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 mass transport of the reactants 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 which have a very disadvantageous effect on the service life of the electrolytic cell 1 .

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 applies, for example, to a particular extent to the stainless steels with the material number 1.4404 or 1.4571, which are used as the material for the bipolar plate which forms the channel structure 13a. But even the more resistant Titan is subject to oxidative attacks subject. 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 areas. These local contact surfaces, which are then poorly electrically 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. Loss of efficiency and degradation of the anodic half-cell 7 are the result of inhomogeneous current distribution with disadvantageous local current peaks.

Certain disadvantageous effects can also be observed on the part of the cathodic half-cell 5, above all with regard to the acid corrosion promoted by elemental oxygen, but these are not discussed in detail here.

To eliminate 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 during operation is inhibited or at best even largely prevented. Such an advantageously modified and further developed electrolytic cell 1 is exemplified in FIG 2 shown schematically, but in a opposite FIG 1 greater detailing of the essential components. The anodic half-cell 7 of the in FIG 2 shown embodiment of an electrolytic cell 1 is analogous to the electrolytic cell 1 according to the basic structure FIG 1 composed, so that reference can be made to the relevant statements.

The anodic half-cell 7 has, also in analogy to the electrolytic cell according to FIG 1 , 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. A large number of grooves or channels are milled into the base body 23 of the bipolar plate 21a in order to promote fluid transport and at the same time uniform electrical contacting and voltage supply of the anodic half-cell 7.

In this case, the gas diffusion layer 11a and the bipolar plate 21a are configured and arranged adjacent to one another in such a way that a channel structure 13a is formed, which comprises the metallic base body 23 made of the metallic 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 mixed phase, with iridium and/or iridium carbide being added to the mixed phase as further components in a flexibly adjustable amount.

As a result, the need for iridium required can be limited to a minimum or significantly reduced compared to an iridium layer or iridium carbide layer. 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 base body 23 over the entire surface of the bipolar plate 21a in the form of a closed protective layer. Alternatively, the coating measure can also be locally limited to the particularly critical surface areas of the bipolar plate 21a 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 sensible. 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 on components or parts with very complex surface structures 29 applied 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.

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 below with reference to the 3 to 5 embodiments shown explained in more detail. What these exemplary embodiments have in common is that titanium or stainless steel is used as the metallic base material 25 , 27 , as is typically provided for an anodic half-cell 7 . The metallic base material 25 of the bipolar plate 21a is a high-grade steel, for example material number 1.4404 or 1.4571, and the metallic base material 27 of the gas diffusion layer 11b is titanium.

In 3 a schematic representation of a corrosion-resistant coating 29 is shown 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. FIG. 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, shows FIG 4 an alternative embodiment of the corrosion-resistant coating 29 as a graded single layer layer. 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 increases towards the surface. The concentration of iridium or iridium carbide on the surface of the corrosion-resistant coating 29 can be up to 100%, so that a closed protective layer of iridium or iridium carbide is formed on the surface. The concentration of titanium niobium and titanium niobium nitride decreases towards the surface accordingly. The concentration of iridium or iridium carbide at the interface between the monolayer 37a and the substrate is negligibly small or equal to zero.

In the 5 a corrosion-resistant coating 29 is shown, which is designed as a multilayer coating comprising two layered monolayers 37a, 37b. It has a first layer 37a, which is applied to the substrate, and a second layer 37b, the second layer being applied to the first layer. 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 first layer is designed as a thin homogeneous monolayer with iridium or iridium carbide 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 connection of the corrosion-resistant coating 29 on the base body 23, which is formed by the binary and/or ternary titanium-based layer material 31 in the intermediate layer is favoured. It is also possible that according to the embodiment in FIG 4 both layers 37a, 37b or one of the layers 37a, 37b is designed as a graded monolayer.

In 6 is a corrosion-resistant coating 29 designed as a complex multi-layer system. This layer system is applied to the base body 23 from a metallic base material 35, in this case a stainless steel substrate. The three stacked layers 37a, 37b, 37c form an intermediate layer 35 to which layer 37d is applied as cover layer 33. The coating is a graded multilayer coating of layer thickness D, so that at least within the layers 27a, 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 Growth direction of the layers 37a to 37d varies and is specifically set with regard to the requirements for corrosion protection. The cover layer 33 is designed as a homogeneous single-layer layer in the layer 37d 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 particularly advantageous use for coating metallic components or functional parts of an electrolytic cell 1, preferably in the anodic half-cell 7. In a special way is at a anodic half-cell 7, use of the corrosion-resistant coating 29 is provided 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 are suitable 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 embodiments is, therefore, not to be taken in a limiting sense, and the scope 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 (16)

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 a polymer electrolyte membrane (4), the anodic half-cell (7) having: - A channel structure (13a) comprising a base body (23) made of a metallic base material (25, 27), wherein a corrosion-resistant coating (29) is applied to the base body (23). Electrolytic cell (1) according to claim 1, wherein the corrosion-resistant coating (29) comprises a coating material (31) with a high oxidation potential. Electrolytic cell (1) according to Claim 1 or 2, in which the corrosion-resistant coating (29) has iridium (Ir) and/or iridium carbide (IrC) as a component. Electrolytic cell (1) according to one of the preceding claims, in which the corrosion-resistant coating (29) has as a component a binary and/or ternary compound containing titanium. Electrolytic cell (1) according to Claim 4, in which the binary compound comprises titanium niobium (TiNb) and the ternary compound comprises titanium niobium nitride (TiNbN). Electrolytic cell (1) according to one of the preceding claims, in which the corrosion-resistant coating (29) has a layer thickness (D) of approximately 0.02 to 0.5 microns, in particular approximately 0.08 to 0.3 microns. Electrolytic cell (1) according to one of the preceding claims, in which the corrosion-resistant coating (29) is designed in one layer, in multiple layers or in graded layers. Electrolytic cell (1) according to one of the preceding claims, in which the corrosion-resistant coating (29) is applied by means of physical vapor deposition (PVD) or plasma-assisted chemical vapor deposition (PACVD). Electrolytic cell (1) according to one of the preceding claims, in which the metallic base material (25, 27) is stainless steel or titanium. Use of an electrolytic cell (1) according to one of the preceding claims for the electrolytic production of hydrogen. Corrosion-resistant coating (29) for application as a protective layer to a metallic component of an electrolytic cell (1), having the components titanium niobium (TiNb) and titanium niobium nitride (TiNbN) and iridium carbide (IrC) and/or iridium (Ir). Corrosion-resistant coating (29) according to Claim 11, designed as a homogeneous single-layer coating, graded single-layer coating or graded multi-layer coating. Corrosion-resistant coating (29) according to one of Claims 11 or 12, designed as a multilayer coating with iridium carbide (IrC) and/or iridium (Ir) as the top layer (33). Use of iridium (Ir) and/or iridium carbide (IrC) as part of a corrosion-resistant coating (29) on a metallic component of an electrolytic cell (1). Use according to Claim 14, comprising titanium niobium (TiNb) and/or titanium niobium nitride (TiNbN) as a further component of the corrosion-resistant coating (29). Use according to claim 14 or 15 in a bipolar plate (21a, 21b) forming a channel structure (13a, 13b) and/or a gas diffusion layer (11a, 11b) as metallic layer Component of an electrolytic cell (1), in particular in an anodic half-cell (7).

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