Method for producing an electrically conductive contact layer on an oxidation-loaded component, and component of an electrochemical cell

Abstract

The invention relates to a method for producing an electrically conductive contact layer on an oxidation-loaded component, in which a component comprising a titanium-based alloy (27), which comprises at least one noble metal in a titanium matrix (33) as an alloy element, is provided. An etching agent is applied to a surface (35) of the component, material of the titanium matrix (33) being selectively removed. As a result, material of the alloy element is exposed such that an electrically conductive contact layer (29) comprising the noble metal is formed close to the surface. The contact layer (29) can optionally be configured as a porous layer having a porosity that can be adjusted by means of the etching process. The method can be used to produce a contact layer on oxidation-loaded components of an electrochemical cell, such as components of an electrolysis cell (1) or a fuel cell, in particular a bipolar plate (21a, 21b) or a gas diffusion layer (11a, 11b) having a titanium-based alloy (27).

Description

[0001] Description

[0002] Method for producing an electrically conductive contact layer on an oxidation-stressed component and component of an electrochemical cell

[0003] The invention relates to a method for producing an electrically conductive contact layer on a component subject to oxidation. The invention further relates to a component of an electrochemical cell, in particular an electrolysis cell or a fuel cell, in particular a bipolar plate of an electrolysis cell for polymer electrolyte membrane electrolysis.

[0004] Hydrogen can be produced from deionized water by electrolysis. This involves the electrochemical cell reactions of the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). In the case of acid electrolysis, the reactions at the anode and cathode can be defined as follows:

[0005] Anode 2 H20 -> 4 H+ + 02+ 4 e~ (I)

[0006] Cathode 2H+ + 2 e~ H2(II)

[0007] For example, in so-called polymer electrolyte membrane electrolysis (PEM electrolysis), the two partial reactions according to equations (I) and (II) are carried out spatially separated 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 a 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 hydrogen ions as positively charged particles. A PEM electrolysis system typically comprises a plurality of appropriately designed PEM electrolysis cells, as described, for example, in EP 3 489 394 A1. A PEM electrolysis cell is described, for example, in EP 2 957 659 A1.The PEM electrolysis 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 unit consisting of membrane and electrodes is called a membrane-electrode assembly (MEA). A gas diffusion layer is located on each electrode. The gas diffusion layers are contacted by so-called bipolar plates. Each bipolar plate can form or have a channel structure designed for the media transport of the reactant and product streams involved. A planar design of a bipolar plate without the formation of a channel structure is, in principle, also possible. At the same time, the bipolar plates separate the individual electrolysis cells from one another, which are stacked to form an electrolysis stack with a large number of electrolysis cells.The PEM electrolysis cell is fed with deionized water as the reactant, which is electrochemically decomposed at the anode into oxygen product gas and protons H+. The protons H+ migrate through the electrolyte membrane toward the cathode. On the cathode side, they recombine to form hydrogen product gas H2.

[0008] The cell reactions mentioned in 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 for this reason, among other things. In principle, electrochemical water splitting is possible from 1.23 V if sufficient thermal energy is available in addition to the electrical energy. The thermoneutral voltage for water splitting is 1.48 V. However, due to various losses such as activation losses, ohmic losses, mass transport losses, etc., water splitting is generally only possible from around 1.7 or 1.8 V. Due to degradation, the cell voltage then gradually rises to 2.1 to 2.3 V and possiblyDepending on the operating conditions, this may be even higher. In practice, PEM electrolysis is typically carried out at a cell voltage of approximately 1.8–2.1 V.

[0009] 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. The PEM electrolysis cell consists, from the outside inward, of two bipolar plates or half-plates, gas diffusion layers, catalyst layers, and a proton-conducting membrane. The latter separates the anodic from the cathodic half-cell.

[0010] Very high oxidative potentials arise at the anode due to the oxygen formation reaction, which is why high-quality materials with rapid passivation kinetics / oxide layer formation, e.g. titanium, are generally used at the anode, particularly for the gas diffusion layer. High demands also apply to the choice of material for the anode-side catalyst material and for the bipolar plate, which lies against the gas diffusion layer and makes electrical contact with it. This creates a respective channel structure for the media transport of the reactant and product streams involved. The protective oxide layers that form at high potentials lead to increased contact resistance between the individual metallic components and ultimately to increased voltage degradation of the cell. To date, this problem has been solved by coating corrosion-resistant materials such as titanium with a layer of precious metal.The precious metals are stable under the aforementioned oxidative conditions or at least form conductive oxides. Thus, the contact resistance at the interface remains comparatively low, at least with an intact layer. Various approaches have been described in the prior art to counteract the degradation effects at the anode and cathode described above. For example, for the anode-side catalyst, it was proposed to introduce a larger amount of catalytically active species into the anodic catalyst layer, i.e., to maintain a higher concentration of catalytically active species. This makes it possible, in particular, to temporarily compensate for degradation effects of the anode with regard to catalytic activity.To reduce degradation effects on the anode side caused by local electrical contact resistance and the associated inhomogeneous current distribution in the composite system of gas diffusion layer and catalyst layer, one of the proposed measures was to optimize the contact pressure of the gas diffusion layer against the catalyst layer. However, this always carries the risk of perforation of the membrane electrode assembly (MEA) due to excessive contact pressure when joining and axially stacking the electrolysis cells to form an electrolysis stack and its mechanical tensioning with a high axial force. This leads to short circuits, rendering the electrolysis cell unusable and significantly endangering or even preventing the operation of the electrolyzer.

[0011] In connection with, for example, the channel structure which spatially delimits an electrolysis cell and at the same time ensures electrical contact and material transport, hardly any measures have been proposed to date with a view to improving corrosion protection and improved electrical contact with long service life. There is also a considerable need for improvement for the other metallic components of the electrolysis cell, for example the gas diffusion layer, in order to provide a corrosion-stable, electrically conductive contact layer or coating, especially on the anode side of an electrochemical cell. The approaches mentioned above therefore do not solve the actual problems and causes of corrosion-induced degradation and sometimes inadequate electrical contact, particularly at the anode, in a sufficiently long-lasting and reliable manner for the operation of the electrolysis cell.At best, they represent temporary measures to partially reduce corrosion-related degradation effects and to extend them for longer operation. At the same time, particularly during longer operating times of an electrolytic cell, the electrical contact properties can be reduced by associated oxidation effects. Thus, despite the application of sometimes expensive metal alloys, the electrical conductivity of the contacts can decrease, leading to increased contact resistance and corresponding ohmic losses.

[0012] The precious metal coating method previously used to reduce contact resistance between corrosion-resistant components has the disadvantage that the layers are very thin and can therefore be easily damaged during assembly. Furthermore, the coating process is costly and time-consuming, usually performed under vacuum. Furthermore, thin precious metal layers (e.g., Pt) can chip off, oxidize, or suffer mechanical, chemical, or electrochemical damage due to the high contact pressure during assembly of the electrolysis stacks or during electrolysis.

[0013] Against this background, the object of the invention is to provide a method for producing an electrically conductive contact layer on a component subject to oxidation, which is characterized by reduced ohmic losses and a simple and at the same time reliable application of an electrically conductive contact layer. The component created in this way is intended to be particularly suitable for use in a highly oxidative environment of an electrochemical cell, such as an electrolysis cell or a fuel cell. This object is achieved according to the invention by a method for producing an electrically conductive contact layer on a component subject to oxidation, which method comprises the following process steps:

[0014] ( 51 ) Providing a component comprising a titanium - based alloy which has at least one noble metal in a titanium matrix as an alloying element ,

[0015] ( 52 ) applying an etchant to a surface of the component, whereby material of the titanium matrix is ​​selectively removed, and

[0016] ( 53 ) Exposing material of the alloying element in such a way that an electrically conductive contact layer comprising the precious metal is formed near the surface.

[0017] The targeted exposure of material creates an electrically conductive contact layer rich in precious metals, which specifically reduces the contact resistance of the alloy close to the surface. The manufacturing process therefore makes it possible in a very advantageous way to provide a metallic and electrically conductive component, particularly for use in an electrolysis cell, in which the above-mentioned operational degradation problems are considerably reduced and the ohmic losses are reduced or completely avoided. The proposed etching process causes material to be removed from the component surface. Titanium, as an alloy component, is therefore etched out of the titanium matrix in a targeted manner and close to the surface, i.e. it is dissolved out of the alloy. Titanium is dissolved by exposure to the etchant.This results in, for example, titanium, as a titanium salt in an aqueous solution, being selectively removed from a thin layer of the component's surface. The removal of titanium results in precious metal-containing alloy elements remaining in a locally coherent structure close to the surface, which is not attacked by the etching agent used. The precious metal-rich contact layer is corrosion-resistant and still as oxidation-resistant as the previously untreated titanium-based alloy.

[0018] In a preferred embodiment of the process, it is also possible that, depending on the application of the etching process, the formation of a porous structure containing precious metals is specifically brought about near the surface.

[0019] The choice of precious metal, through the targeted dissolution of the titanium matrix, creates a contact layer with high electrical conductivity and a long service life. The contact layer thus created allows for longer service life of the contact layer and thus of the oxidation-stressed component during operation.

[0020] The application of the etching agent to the surface of the component can be carried out, for example, by partially immersing the component in a bath containing a liquid etching agent or by spraying the liquid etching agent onto the surface of the component. The etching causes material from the titanium matrix to be removed in the form of depressions on the surface of the component by the application of the etching substance. It is conceivable that specifically selected areas of the surface of the component that are not to be etched or deepened are prepared beforehand or masked and protected against the etching attack. In principle, it is therefore possible for these areas to be protected beforehand with a covering varnish before exposure to the etching agent.

[0021] The process advantageously makes it possible, even on an industrial scale, to etch oxidation-stressed components flexibly and, if required, as so-called "contour etched parts", so that even non-flat surface contours with complex surface geometries are possible. In addition to component outlines, the surface of the component can be structured in a defined manner or cross profiles (topographies) can be individually produced through a special application of the etching process. This means, for example, that in addition to pure deep etching, extremely fine channels can also be etched into the titanium alloy and the titanium can be selectively removed from the titanium matrix close to the surface.

[0022] In a particularly preferred embodiment of the method, application of the etchant causes selective corrosion of titanium in the titanium matrix, so that a porous and crystalline structure with precious metal crystallites remains.

[0023] The titanium dissolves into an aqueous solution due to the action of the etching agent. Dissolved titanium salt can be removed and discharged after the exposure time, whereby precious metal crystallites are formed or revealed through exposure. These precious metal crystallites are particularly advantageous with regard to the required contacting properties of the component surface in terms of particularly low and uniform contact resistance across the surface and the mechanical stability of the contact layer. It is also optionally possible to carry out the etching process in such a way that a precious metal-rich surface layer remains in a porous structure.

[0024] With conventional contact layers, the operationally negative effect of an increase in electrical resistance caused by titanium oxide formation is counteracted by coating the surface of the titanium component with a continuous layer of a precious metal, e.g. a thin layer of platinum as a corrosion protection layer against oxidative attack. However, these layers are relatively thin and can be damaged during assembly of the components and during operation, so that their functionality and long-term stability is impaired or limited, especially when used in an electrolysis cell. Another disadvantage is that the application of the precious metal coating to the titanium substrate has to take place in a separate process step, which implies additional expenditure of time and requires special coating equipment.

[0025] In contrast, in a preferred embodiment of the method, a plurality of individual islands of the alloying element are formed on the surface of the component, which remain after the application of the etchant.

[0026] The number, size, shape, and distribution of these contact islands over the component surface can be adjusted within certain limits by the etching process and the alloy. For a locally flat component surface, it is advantageous to provide the most even distribution possible for the precious metal-rich contact islands. It is also possible to carry out the etching process in such a way that pairs or groups of connected islands are formed, thus forming clusters of islands.

[0027] In a preferred embodiment of the method, the alloying element is optionally exposed in such a way that a porous, noble metal-rich, conductive contact layer is formed near the surface.

[0028] Initial estimates have shown that setting a porosity of 50% to 90%, preferably between 70% and 80%, in the contact layer is particularly useful in terms of the desired electrical contact properties on the one hand and good transport properties for the media on the other. Porosity is defined as a dimensionless measurement and represents the ratio of void volume to the total volume of a material or mixture of materials, in this case the porous precious metal layer. It serves as a classifying measure for the voids actually present. This parameter is used in the areas of materials and construction engineering as well as in the geosciences. Porosity has a major influence on the density of a material and on the resistance when flowing through a fill according to Darcy's law.In this case, the etching process and the dissolution of the titanium matrix create a macroporous precious metal structure with an average pore size of greater than about 50 nm.

[0029] Furthermore, the porous, precious-metal-rich support structure provides additional mechanical elasticity. When installed in a component exposed to oxidation, such as in the electrolysis cell of a PEM electrolyzer, this ensures particularly uniform and, to a certain extent, even spring-elastic surface pressure and contact.

[0030] The thickness of the noble metal-containing conductive contact layer produced on the component is preferably set between 5 nm and 50 pm, so that a wide range of layer thicknesses is accessible for layer formation using the etching process, which can be used advantageously. Particularly preferred for the etching process and quality is the formation of a layer thickness for the noble metal-rich contact layer between approximately 50 nm and 5 pm.

[0031] In addition to the formation of islands containing precious metals, it is also possible, in a particularly advantageous embodiment of the process, to form a thin, homogeneous and coherent layer area from the alloying element.

[0032] One or more precious metals are embedded in the titanium matrix as alloying elements. Targeted etching enables the formation of precious metal-rich islands or a continuous, homogeneous contact layer on titanium made from a titanium alloy.

[0033] In a particularly preferred embodiment of the method, the layer region is designed such that a closed, electrically conductive contact layer is formed on the oxidation-stressed surface of the component. This embodiment allows for a particularly improved electrical connection on the oxidation-stressed component surface.

[0034] The formation of precious metal-rich islands or a continuous layer is to be achieved by the targeted etching of the surface of titanium alloys that contain platinum group metals (such as platinum, palladium, rhodium, ruthenium) as alloying elements. By etching titanium alloys, only the titanium matrix is ​​specifically etched, leaving behind individual islands or a thin, homogeneous layer of one or more precious metals that has very high corrosion resistance and very good electrical conductivity. The process can be used for components with flat or planar surfaces as well as for components with 3D surface geometries, e.g. for so-called bipolar plates or gas diffusion layers in electrolysis cells or fuel cells.In principle, all oxidation-stressed components in which titanium grade 1 or grade 2 is integrated into an electrochemical cell on the anode side are suitable for the etching process. This also includes, for example, parts and components of so-called porous transport layers, PTL or MPL, which can be made from fibers or particles of titanium alloys (grade 7) as the base material.

[0035] In a particularly preferred embodiment of the process, at least one alloying element is selected from the elements of the platinum group, comprising ruthenium, rhodium, palladium, osmium, iridium or platinum.

[0036] Particularly preferred alloys for this purpose are titanium grade 7 and titanium grade 11 with 0.12-0.25% palladium, but other titanium alloys are also used. Other titanium alloys that are preferred are, for example, titanium-platinum, titanium-ruthenium or titanium-rhodium alloys, each with 0.1-10% precious metal content. Combinations with several alloying elements and precious metal components are possible. In a preferred embodiment of the process, an acidic etchant containing hydrofluoric acid HF or nitric acid HNO3 or a mixture thereof is used as the selective etchant.

[0037] In principle, different solutions can be used as etching agents which can selectively dissolve titanium as a metal in the alloy without attacking the precious metal as a component of the titanium alloy, e.g. hydrofluoric acid HF in an aqueous solution with a concentration of 1%-50%. The etching agent can advantageously contain other acids and organic components in a mixture, such as nitric acid HNO3 in a concentration of 30-60%, sulfuric acid H2SO4 in a concentration of 10-50%, hydrochloric acid HCl in a concentration of 20-50%, glycerol in a concentration of 40-50%, hydrogen peroxide H2O2 in a concentration of about 4%. Furthermore, anhydrous solutions such as bromoethanol can also be used.In principle, all known methods for dissolving titanium as a metal can be used as an etching process that acts selectively on the titanium component in the alloy, for example immersion, spraying or electrolytic etching / pickling. Depending on the process selected, the temperature can be set between -25 ° C - in anhydrous electrolytes - up to 100 ° C - in water-based electrolytes. During electrolytic etching, the potential can be between 3 - 75 V. The exposure time can be from a few seconds to several minutes. After etching / pickling, the surface of the component is passivated and dried under defined conditions in a protective atmosphere or in air. This causes a thin passivation layer of titanium oxide to form on the surface, from which the precious metal structure protrudes.The precious metal structure exposed in this way can optionally be formed as a porous electrical contact layer. An aqueous solution of the highly concentrated acids mentioned above can be applied particularly easily to the component surface of the titanium alloy to be etched and can be targeted accordingly. In principle, it is also possible for the titanium alloy to be etched using both hot and cold etching. Hot etching is an advantageous and recommendable method as it delivers particularly reliable, consistent, and successful results. The etchant is heated before use. Depending on the etchant, it may first have to be prepared as a solution and activated accordingly before use, for example to convert a titanium etch solution from a crystalline starting substance to an aqueous etchant solution. For this purpose, a crystalline starting substance is dissolved in distilled water.Sufficient time, heat, and homogeneous stirring are applied so that the crystals dissolve completely and become active. The etchant is added to water, and the aqueous solution is heated and kept warm, for example, at between 60°C and 70°C, stirring regularly for about 30 minutes. Once completely dissolved, the etchant is active and ready to use. The etchant can then be used directly for etching, or it can be cooled and stored for later use. Cold etching, on the other hand, advantageously requires no heating equipment, but the results can sometimes be less consistent. The reaction time, i.e. the time the component is exposed to the etchant, ranges from 10 seconds to several minutes.

[0038] In a particularly preferred embodiment of the method, a component of an electrolysis cell or a fuel cell is provided as the oxidation-stressed component, on which the electrically conductive contact layer is produced.

[0039] Preferably, the oxidation-stressed

[0040] A bipolar plate is provided on the component, on which the electrically conductive contact layer is produced. The bipolar plate is preferably prepared and provided accordingly for use in an electrolysis cell for carrying out PEM electrolysis or, alternatively, is provided as a bipolar plate for use in a fuel cell.

[0041] In a preferred embodiment of the method, it is also possible for a gas diffusion layer of an electrolysis cell, in particular a PEM electrolysis cell, to be provided as the oxidation-stressed component, on which an electrically conductive contact layer is produced.

[0042] Compared to line-of-sight processes (e.g., PVD sputter coating processes), the material-removing process of the invention, through the use of an etchant, enables the formation of the precious metal layer even on complex 3D surfaces. A significant advantage over the flame-spraying coating process is that little or no heat is introduced into the component, allowing a contact layer to be formed even on thinner titanium components without thermally induced deformation.

[0043] Furthermore, the bonding of the precious metal-rich contact layer to the base material of the titanium alloy is much better than with conventional coating processes because the formation of the layer from the base material takes place through the etching process.

[0044] A further aspect of the invention relates to a component of an electrochemical cell, for example an electrolysis cell or a fuel cell, which has a contact layer produced according to the method. Components for electrolysis cells or fuel cells that are subject to oxidation are, for example, bipolar plates and gas diffusion layers, which are introduced and operated in particular as components in the highly corrosive anodic half-cell. Transport layers of the PTL and MPL type - as described above - are also possible. A particularly preferred use of the mechanically and electrochemically stable, highly conductive contact layer is in a channel structure or in a gas diffusion layer, which are used or formed as a metallic component or component of an electrolysis cell, in particular in an anodic half-cell.In this case, a channel structure is preferably formed by the immediately adjacent arrangement of a bipolar plate with the fluid-permeable gas diffusion layer consisting of a number of gas diffusion layers. Due to the required electrical conductivity, both the bipolar plate and the gas diffusion layer form metallic components or oxidation-stressed components of the electrolysis 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 corresponding channel structure. In this case, an anodic and a cathodic channel structure are formed in an electrolysis cell. Advantageously, therefore, the surfaces of the channel structure delimiting the channel structure, which surfaces are exposed to oxygen on the anode side during operation and are exposed to particularly high levels of corrosion, are provided with the electrically conductive and corrosion-resistant contact layer.The channel structure can generally also simply comprise a bipolar plate made of a flat metal sheet based on a titanium alloy, or be designed as such. Therefore, in the context of the present invention, a channel structure does not necessarily always comprise structured, separate channels for fluid transport. In this case, the contacting properties and the electrical function of the component are paramount, regardless of the component geometry.

[0045] The gas diffusion layer of the anodic half-cell comprises a material with a porous structure to ensure sufficient 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 can be provided with the conductive contact layer to permanently reduce the electrical contact resistance. This can increase the useful life or service life of the gas diffusion layer, and the described disadvantageous, oxidatively induced degradation effects are reduced. It is therefore advantageous to apply the electrically conductive contact layer according to the invention as a protective layer on the metallic base body of the gas diffusion layer. The electrically conductive contact layer formed in this way is stable against degradation and corrosion effects.It can be formed and applied as a contact layer on other parts and components of the anodic half-cell with a metallic base body, in particular made of a titanium-based alloy.

[0046] The surface treatment with the associated formation of a contact layer according to the invention also leads to a significant improvement in local electrical contacts, thus making the current density more homogeneous across the cell surface. In addition to preventing or slowing down degradation processes, this leads to a better and, above all, more uniform current distribution during operation of an electrochemical cell, such as an electrolysis cell or a fuel cell.

[0047] Furthermore, the anodic half-cell preferably has a gas diffusion layer formed from titanium as the base material. A design as a fine-mesh titanium material is preferably used here as the base material for the gas diffusion layer, for example titanium fleeces, titanium foams, titanium fabrics, titanium-based expanded metals or combinations thereof. This also increases the local contact points to the electrode and the electrical resistance in the contact surface is particularly uniform. The term "grid" in the present context refers to a fine-mesh network. The aforementioned carrier materials are characterized by high corrosion resistance. The terms "grid" and "fabric" describe a directed structure, the term "fleece" a non-directed structure.Preferably, a channel structure can be arranged adjacent to, in particular directly adjacent to, the gas diffusion layer, or a channel structure is functionally formed by the adjacent arrangement of the gas diffusion layer and a component, in particular a bipolar plate. The channel structure serves to collect and discharge the gaseous reaction product of the electrolysis in the anodic half-cell, i.e., for example, oxygen according to equation (I). The channel structure can, for example, comprise a bipolar plate or be designed as such. Bipolar plates enable the stacking of several electrolysis cells to form an electrolysis cell module by electrically connecting the anode of one electrolysis cell to the cathode of an adjacent electrolysis cell. In addition, the bipolar plate enables gas separation between adjacent electrolysis cells.

[0048] A further aspect of the invention relates to an electrochemical cell, in particular an electrolysis cell or a fuel cell, which has such a component with a contact layer produced in this way.

[0049] A further aspect of the invention relates to an electrolyzer, in particular a PEM electrolyzer, having a plurality of electrochemical cells designed in this way, which are designed as electrolysis cells.

[0050] A further aspect of the invention relates to a fuel cell unit having a plurality of electrochemical cells designed in this way, which are designed as fuel cells.

[0051] The invention is explained below by way of example with reference to the accompanying figures using preferred embodiments, wherein the features presented below can represent an aspect of the invention both individually and in various combinations with one another. They show: FIG. 1 shows a schematic representation of an electrochemical cell using the example of an electrolysis cell for polymer electrolyte membrane electrolysis;

[0052] FIG 2 shows an exemplary anodic half-cell with a channel structure and an electrically conductive contact layer according to the invention;

[0053] FIG 3 an electrically conductive contact layer formed on a titanium-platinum alloy;

[0054] FIG 4A a near-surface sectional view through a component exposed to oxidation before application of the etching agent;

[0055] FIG 4B is a view of the oxidation-stressed component from FIG 4A after the application of the etchant.

[0056] FIG. 1 shows a sectional view of the basic structure of an electrolysis cell 1 for polymer electrolyte membrane electrolysis (PEM electrolysis) in a schematic representation. The electrolysis cell 1 serves for the electrolytic production of hydrogen and is one application of an electrochemical cell. Another application of an electrochemical cell is a fuel cell for electricity generation.

[0057] The electrolysis cell 1 has a polymer electrolyte membrane 3. On one side of the polymer electrolyte membrane 3, on the left in the illustration according to Figure 1, the cathodic half-cell 5 of the electrolysis cell 1 is arranged, and on the other side of the polymer electrolyte membrane 3, on the right in the illustration according to Figure 1, the anodic half-cell 7 of the electrolysis cell 1 is arranged.

[0058] 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 for fluid transport is formed at the same time. 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 for use as the anodic catalyst material 9.To reduce corrosion, the gas diffusion layer 11a is made of a material on whose surface a passivation layer quickly forms, e.g. titanium. The passivation of the titanium forms titanium dioxide, which, however, has a lower electrical conductivity than metallic titanium. The channel structure 13a is formed by the bipolar plate 21a, so that axial stacking and contacting of a large number of electrolysis cells 1 is possible. The comparatively low metal dissolution for titanium-based metallic parts and components such as the bipolar plate 21a or the gas diffusion layer 11a in an oxidative environment during operation of the electrolysis cell 1 can be explained by the formation of this near-surface titanium oxide passive layer. However, titanium oxide behaves like a semiconductor, i.e. the electrical conductivity is much lower than for pure titanium.This has the disadvantage that over longer operating times the ohmic losses caused by the formation of titanium oxide significantly affect the performance and economic life of the electrolytic cell 1.

[0059] 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 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 electrolysis cell 1.Immediately adjacent to the gas diffusion layer 11b is also a channel structure 13b, which, analogous to the anodic half-cell 7, is designed as a bipolar plate 21b. The gas diffusion layers 11a, 11b, in cooperation with the immediately adjacent bipolar plates 21a, 21b, functionally form a respective channel structure 13a, 13b, i.e., a fluid-tight flow channel for the transport of the reactants and products during electrolysis.

[0060] As explained above, there is potential for improvement in this electrolysis cell 1, whose basic design is already known from the state of the art, generally with regard to the corrosion susceptibility of the materials and ensuring the lowest possible resistance and most durable electrical contact. Significant operational degradation effects can be observed, particularly in the anodic half-cell 7, which limit the service life of the electrolysis cell 1.

[0061] Here, 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 is observed. However, even the more resistant titanium or a titanium alloy containing a precious metal is subject to oxidative attack. As a result, an increase in the local electrical contact resistance can be observed during operation. A key cause of this is the rapid oxidation (passivation) of the titanium during operation of the gas diffusion layer 11a or the anodic bipolar plate 21a, which serves, among other things, for electrical contact and current conduction. The oxidation is predominantly observed on the surface of the gas diffusion layer 11a and adjacent current-carrying electrical contact surfaces.These local contact surfaces, which then become poorly electrically conductive due to oxidation, lead to high ohmic losses in the electrolytic cell 1 and to a necessary increase in the cell voltage at a constant current density. Efficiency losses and degradation, especially of the anodic half-cell 7, are the result of inhomogeneous current distribution with disadvantageous local current peaks.

[0062] Certain adverse effects can also be observed on the cathodic half-cell 5, particularly with regard to acid corrosion promoted by elemental oxygen, but these will not be discussed in detail here.

[0063] To eliminate these disadvantages, at least in the anodic half-cell 7, it is proposed to provide the channel structure 13a in the anodic half-cell 7 with a porous, precious metal-rich corrosion protection layer 29 close to the surface, so that the disadvantageous oxidative attack which continues during operation is inhibited or, at best, even largely prevented. Such an advantageously modified and further developed electrolysis cell 1 is shown schematically as an example in FIG. 2, but with greater detail of the essential components than in FIG. 1. The anodic half-cell 7 of the exemplary embodiment of an electrolysis cell 1 shown in FIG. 2 is, in terms of its basic structure, analogous to the electrolysis cell 1 according to FIG. 1, so that reference can be made to the relevant explanations.FIG. 2 therefore shows, by way of example, an anodic half-cell 7 with a channel structure 13a and a porous, corrosion-resistant and at the same time electrically conductive contact layer 29 according to the invention. The anodic half-cell 7 has, likewise analogously to the electrolysis 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, which in the present case is selected as a titanium-based alloy 27 which has at least one noble metal from the platinum group as an alloying element. Thus, ruthenium, rhodium, palladium, osmium, iridium or platinum is optionally embedded in the titanium base material as the noble metal component. The gas diffusion layer 11a is designed as a titanium-based expanded metal mesh so that fluid transport is possible, in this case oxygen O2 as anode product of the electrolysis in a mixture with water.Similarly, the bipolar plate 21a has a base body 23 made of a metallic base material 25, which in this case is also selected as a titanium-based alloy. A plurality of grooves or channels are milled into the base body 23 of the bipolar plate 21a to promote fluid transport and, at the same time, to ensure uniform electrical contact and voltage supply to the anodic half-cell 7. However, it is also possible for the bipolar plate 21a to be designed essentially as a planar contact plate that has no grooves or channels.

[0064] In the present case, for example, 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 comprises the metallic base body 23 on a base material 25, 27 based on a titanium alloy. In contrast to the simplified representation of the electrolysis cell 1 according to FIG. 1, it can be seen in FIG. 2 that the channel structure 13a of the anodic half-cell 7 is designed for effective corrosion protection against oxidative attack by oxygen O2 and, at the same time, high electrical conductivity. For this purpose, the channel structure 13a has, close to the surface, an electrically conductive contact layer 29 rich in precious metals, which is also designed as a protective layer against degradation effects on the base body 23 comprising the titanium alloy 27.The electrically conductive contact layer 29 can have or form a porous structure with a predetermined porosity. A permanently high electrical conductivity of the contact layer 29 during operation under oxidative stress is brought about by the noble metal-enriched and optionally porous metallic structure of the contact layer 29, which is formed on the titanium alloy 27. The layer material 31 comprises one or more elements of the platinum group as alloying elements, i.e. ruthenium, rhodium, palladium, osmium, iridium or platinum is optionally provided as the noble metal component. This layer material 31 was previously specifically exposed from the titanium alloy 27 of the base body 23 during production of the component by means of an etching process, so that a thin, noble metal-rich contact layer 29 is formed on the base body 23.The layer material 31 is selected such that it has a high oxidation potential and at the same time is a good electrical conductor.

[0065] In the exemplary embodiment, the contact layer 29 is applied over the entire surface of the bipolar plate 21a in the form of a closed, precious-metal-rich protective layer on the base body 23, at least as shown in the region of surfaces that delimit the channel structure 13a. A certain porosity of, for example, 50% to 70% can be adjusted by the corresponding material removal or titanium dissolution as a result of the etching process. Alternatively, the formation of the contact layer 29 can also be locally limited to the particularly critical surface areas of the bipolar plate 21a with regard to oxidation, so that precious-metal-rich islands are formed locally on the base body and protrude therefrom.In the example, the gas diffusion layer 11a is also fully covered, at least on its side facing the bipolar plate 21a, with the corrosion-resistant, noble metal-rich layer material 31, which optionally has a predetermined porosity, so that a closed and effective corrosion protection is applied to the channel structure 13a as a whole. It is of great advantage that essentially the same coating material 31 is used for the electrically conductive contact layer 29 for the gas diffusion layer 11a as well as for the bipolar plate 21a with regard to the noble metal components. However, adaptations 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 regard to the specific layer structure of the contact layer 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, as well as a specific porosity that may be adjusted, which is formed on the base body 23 by titanium dissolution, of the noble metal-rich contact layer 29.

[0066] For example, the gas diffusion layer 11a for the functional formation of a channel structure 13a of the anodic half-cell made of a titanium base material is typically made of a porous structure with a comparatively large surface area, for example of a fleece, an expanded metal and / or of several gas diffusion layers, layered or combinations thereof. For effective protection against degradation while at the same time having good electrical contact properties, such a porous structure with a large surface area of ​​the gas diffusion layer 11a is then provided on the surface, preferably over the entire area, with a closed protective layer made of coating material 31. This contains a precious metal from the platinum group, for example platinum from a titanium-platinum alloy, which has been exposed from the titanium alloy 27 close to the surface by a targeted etching process.An exposed, precious-metal-rich contact layer 29 can also be formed on oxidation-stressed components or parts with a very complex surface geometry in order to achieve the most complete and effective corrosion protection possible with high and permanent electrical conductivity. This allows the contact layer 29 to function as an electrically conductive contact layer 29 for long operating times.

[0067] The production or targeted formation of the electrically conductive layer 29 on the base body 23 of a bipolar plate 21a or a gas diffusion layer 11a as an oxidation-stressed component of the electrolysis cell 1 as well as the structure of the contact layer 29 is illustrated and explained below with reference to FIG 4A and FIG 4B.

[0068] 3 shows a schematic representation of an electrically conductive and corrosion-resistant contact layer 29 formed on a titanium-palladium alloy in order to illustrate the mechanism of surface preparation by the etching process. The base body 23 in the present case consists, for example, of a titanium-palladium alloy 27 as the base material. Palladium, as a noble metal and alloying element, is homogeneously embedded or correspondingly enclosed in a titanium matrix 33. The individual inclusions made of the palladium noble metal are not shown in detail in FIG. 3. The surface 35 is prepared by deliberately inducing selective initial corrosion by applying an etchant to the surface 35 of the base body 23. An acidic etchant containing a strong acid, such as nitric acid or hydrofluoric acid, is applied to the titanium matrix 33, as a result of which titanium is dissolved from the titanium matrix 33 on the surface 35.Due to the targeted action of acid close to the surface and the local removal or dissolution of titanium in the area of ​​the surface 35, a porous layer of palladium crystallites 37 initially embedded in the titanium matrix 33 remains on the surface 35, with crystallites 37 protruding from the base body 23. It is possible that, due to operational reasons, an oxidation reaction with an oxidizing agent additionally forms a passivation layer 39 made of titanium dioxide HO2 on the surface 35. However, the preparation of the surface 35 is always carried out and adjusted so that the crystallites 37 made of the precious metal of the alloy 27 protrude beyond the passivation layer 39, for example essentially perpendicular to the surface normal. Other orientations of the crystallites 37 relative to the surface normal are conceivable.It is also possible for crystallites 37 to be formed from other platinum group alloying elements or mixed crystals and to protrude from the surface 35 in a corresponding manner. This can be flexibly adjusted depending on the choice of the precious metal embedded in the titanium matrix 33 as an alloying element and adapted to the requirements of a particular oxidation-stressed component.

[0069] FIG 4A shows a sectional view close to the surface through a component subject to oxidation before application of the etching agent. The method can also be illustrated with reference to FIG 4A and FIG 4B, with the indicated method steps S1, S2 and S3 for preparing the surface 35. The component has a curved surface 35. Inclusions of one or more precious metals from the platinum group, such as palladium or platinum, are embedded in the titanium matrix 33, so that in a first method step S1 a component made of titanium alloyed with precious metals from the platinum group is provided. In a further method step S2, the surface 35 is then exposed to the etching agent and the titanium matrix 33 is specifically etched or partially etched. As a result, titanium is dissolved out of the titanium matrix 33 close to the surface and selectively removed.As a result, the material of the embedded alloy element is exposed, so that finally, in a method step S3, an electrically conductive contact layer 29 with noble metal as layer material 31 is formed close to the surface. The electrically conductive contact layer 29 can be formed as a porous layer and have a porosity that can be adjusted via the etching process through targeted material exposure. In the example shown, the electrically conductive contact layer 29 shown in FIG 4B is designed as a thin, homogeneous, continuous and possibly porous layer with a layer thickness D. It is also possible to form several noble metal-rich islands, which can be connected or separate. The layer thickness D resulting from the etching process through material dissolution can be adjusted within a wide range and can be between approximately 5 nm and 50 pm; typically, a layer thickness D of approximately 50 nm to 5 pm is set.

[0070] Compared to conventional PVD coating processes, the material-removing process of the invention uses an etching agent to form a precious metal layer even on complex 3D surfaces. A key advantage over flame spraying coating processes is that little or no heat is introduced into the component, meaning a contact layer can be formed even on thinner titanium components without thermally induced deformation. Furthermore, the bond between the precious metal-rich contact layer and the titanium alloy base material is much better than with conventional coating processes because the layer is formed from the base material by the etching process selectively applied to the surface 35 of the component.

[0071] Depending on the particular part or component, the layer thickness D is selected to be between 5 nm and 50 pm, preferably between 50 nm and 5 pm, and can therefore be selected to be comparatively thin for many applications. The layer thickness D is set via the etching depth, which in turn can be set via the exposure time of the etchant applied to and acting on the surface of the component. Greater layer thicknesses D are, however, easily possible with a corresponding exposure time and deep etching into the titanium matrix 33. In this way, a greater layer thickness D with mechanical elasticity can be provided, which is provided with an optionally porous precious metal structure and which, if required, also has the necessary transport properties for the fluids. In this way, a certain spring elasticity can be brought about for the installation situation and operation through the morphology of the contact layer 29.

Claims

Patent claims 1. A method for producing an electrically conductive contact layer (29) on an oxidation-stressed component, comprising the steps: (51) Provision of a component comprising an alloy (27) based on titanium, which has as alloying element at least one precious metal in a titanium matrix (33), (52) applying an etchant to a surface (35) of the component, whereby material of the titanium matrix (33) is selectively removed, and (53) Exposing material of the alloying element in such a way that an electrically conductive contact layer (29) comprising the precious metal is formed near the surface.

2. The method according to claim 1, wherein the application of the etchant results in a selective dissolution of titanium from the titanium matrix (33) is effected, leaving a crystalline structure with noble metal crystallites (37).

3. Method according to claim 1 or 2, wherein the material of the alloying element is exposed from the titanium matrix (33) in such a way that a porous, noble metal-rich conductive contact layer is formed near the surface.

4. A method according to claim 1, 2 or 3, wherein a plurality of individual islands of the alloying element are formed on the surface (35) and remain after application of the etchant.

5. A method according to any one of the preceding claims, wherein a thin, homogeneous and continuous layer region is formed from the alloying element.

6. Method according to one of the preceding claims, in which the layer region is designed in such a way that a closed contact layer (29) is formed on the oxidation-stressed surface (35) of the component.

7. A method according to any one of the preceding claims, wherein an alloying element is selected from the elements of the platinum group, comprising ruthenium, rhodium, palladium, osmium, iridium or platinum.

8. The method according to claim 7, wherein palladium is provided as an alloying element in a concentration of 0.12 to 0.25% in the titanium matrix (33).

9. A method according to claim 7 or 8, wherein the titanium-based alloy provided is titanium-platinum, titanium-ruthenium or titanium-rhodium with a precious metal content of 0.1-10% each.

10. Method according to one of the preceding claims, wherein an acidic etchant containing hydrofluoric acid (HF) or nitric acid (HNO3) or a mixture thereof is used as the selective etchant.

11. Method according to one of the preceding claims, wherein a component of an electrochemical cell, in particular an electrolysis cell (1) or a fuel cell, is provided as the oxidation-stressed component, on which the electrically conductive contact layer (29) is produced.

12. The method according to claim 11, wherein a bipolar plate (21a, 21b), in particular a bipolar plate (21a, 21b) of an electrolysis cell (1) for PEM electrolysis, on which the electrically conductive contact layer (29) is produced, is provided as the oxidation-stressed component.

13. The method according to claim 12, wherein the oxidation-stressed component is a gas diffusion layer (11a, 11b) of an electrolysis cell (1), in particular a PEM electrolysis cell. is provided on which the electrically conductive contact layer (29) is produced.

14. Component of an electrochemical cell, in particular an electrolysis cell (1), which has an electrically conductive contact layer (29) produced by the method according to one of claims 1 to 13.

15. Electrochemical cell, especially electrolytic cell (1) or fuel cell, with a component according to claim 14.

16. Electrolyzer, in particular PEM electrolyzer, with a plurality of electrochemical cells according to claim 15, which are designed as electrolysis cells (1).

17. A fuel cell assembly comprising a plurality of electrochemical cells according to claim 15, which are designed as fuel cells.

Images