Method for producing an electrode having a noble metal catalyst for alkaline water electrolysis

Abstract

A method is disclosed for producing an electrode (4) having a noble metal catalyst for alkaline water electrolysis. The method comprises: (S1) providing the electrode substrate (1); (S2) providing a matrix material (2) and a catalyst material (3) as starting materials for the coating; (S3) mixing the matrix material (2) and the catalyst material (3); and, (S4) coating the substrate (1) with the mixture of matrix material (2) and catalyst material (3) by means of high-velocity oxygen fuel spraying (HVOF). A correspondingly produced electrode (4), an electrochemical cell (10) comprising said electrode, and an electrolyser (20) are also specified.

Description

[0001] Description

[0002] Process for producing an electrode with a noble metal catalyst for alkaline water electrolysis

[0003] The present invention relates to a method for producing an electrode with a noble metal catalyst and a method for coating an electrode substrate. The novel method is particularly intended for producing electrodes or electrochemical cells for alkaline water electrolysis.

[0004] If an electrolysis reaction takes place in the alkaline range, a diaphragm (also called a separator) can be used, or specifically an anion exchange membrane (AEM). If, however, the electrolysis reaction takes place in the acidic range, a proton exchange membrane (PEM) is used instead.

[0005] The production of green hydrogen from water is now largely electrolytic, using the processes mentioned above. Electrolysis is an electrochemical process in which water is separated into its chemical components, oxygen and hydrogen. The electrochemical cell reactions, or operating mode, of alkaline electrolysis can be summarized as follows:

[0006] To achieve the political goals, particularly the US Department of Energy's goal of hydrogen production costs below $2 / kg, technical advances in electrolysis systems are required. In addition to material improvements, significant cost reductions and efficiency gains can be achieved through improvements in manufacturing processes.

[0007] Alkaline water electrolysis (AEL) is an industrially established technology that requires low investment costs, resulting from its market maturity. AEL is also characterized by high long-term stability and the fact that virtually no critical raw materials are required.

[0008] Water electrolyzers are primarily measured by their efficiency and current density. The process described here primarily describes the catalyst coating of large-area metal substrates.

[0009] A variety of technical coating methods for electrodes for alkaline water electrolysis are known from the literature. Most are based on the galvanic co-deposition of Ni with Zn or Al. Due to their amphoteric behavior, the elements Zn and Al dissolve in alkali, creating, for example, an amorphous Raney Ni structure.

[0010] Electroplating is highly surface-sensitive and usually requires complex pretreatment or activation of the substrate. In total, many steps are required to produce a highly active and mechanically stable electrode. Thermal spraying processes are also known in the literature, in which Ni / Al powder (50:50) is applied, for example, by plasma spraying (APS). Leaching of the aluminum is also required to obtain the active catalyst. This method is also suitable for the direct deposition of precious metal layers. However, due to the high transfer rates, very large loadings of precious metals are achieved, which are not economical.

[0011] In addition to their chemical stability, precious metals such as Pt exhibit low electrochemical overpotentials for the hydrogen evolution reaction and are therefore particularly suitable for use as catalysts.

[0012] Thermal plasma spray (APS) processes for Raney NiMO coatings (e.g., NiAlMo in (44 / 39 / 17 wt.%)) are also known. However, these also require post-treatment with concentrated alkali to dissolve the aluminum. Precious metal electrodes often exhibit increased deposit stability compared to Raney nickel, as they are advantageously not decomposed by reverse currents.

[0013] So-called "Raney nickel" is a well-known solid catalyst consisting of fine grains of a nickel-aluminum alloy and used in many industrial processes. Raney nickel, for example, is produced by leaching an aluminum-nickel block using concentrated caustic soda. This "activation" dissolves a large portion of the aluminum from the alloy, leaving behind a porous structure that, due to its large surface area, can strongly influence chemical reactions. The disadvantage, however, is that leaching within the cell assembly leads to damage to the diaphragm and can therefore only be performed separately (ex situ).

[0014] The object of the present invention is to provide a novel method for producing a significantly improved electrode structure. These methods according to the invention advantageously make it possible to compensate for the above-described deficiencies of known methods, to achieve significantly improved mechanical strength, to reduce electrical and / or ionic contact resistance at the electrodes, and to reduce surface loading with precious metal in a resource-efficient manner.

[0015] This object is achieved by the subject matter of the independent patent claims. Advantageous embodiments are the subject matter of the dependent patent claims. One aspect of the present invention relates to a method for producing an electrode with a noble metal catalyst for alkaline water electrolysis.

[0016] The method comprises providing the electrode substrate, which can be made of steel, nickel, or nickel-plated metal, for example, as a metal fiber fleece, metal foam, metal mesh, or expanded metal. Particularly when the electrode is manufactured as the cathode of an electrochemical cell, the electrode substrate can be provided, for example, as a nickel mesh.

[0017] The method further comprises providing a matrix material, in particular matrix powder, and a catalyst material, in particular precious metal powder, as starting materials for the coating.

[0018] In one embodiment, the matrix material is preferably a nickel aluminide, in particular Ni3Al. Alternatively, the matrix material can consist of this nickel aluminide, in particular Ni3Al.

[0019] According to a preferred embodiment, the matrix material and the catalyst material are in powder form.

[0020] In one embodiment, the catalyst material is present with a primary particle size of 3 pm to 10 pm or is provided accordingly.

[0021] In one embodiment, the catalyst material comprises nanoparticles of Pt, Ru, Ni, Rh, Re, and / or Pd. The nanoparticles can accordingly be secondary particles which are part of the primary particles or are embedded in them.

[0022] The method further comprises mixing the matrix material and the catalyst material. In one embodiment, the matrix material is mixed with a precious metal powder as catalyst material in a weight proportion of between 10% and 20%.

[0023] The process further comprises coating the substrate with the mixture of matrix material and catalyst material by means of high-velocity oxygen fuel (HVOF) spraying.

[0024] By means of the method steps according to the invention, a surprisingly advantageous electrode morphology can be achieved which achieves all of the objects according to the invention.

[0025] This method makes it particularly possible to achieve a reduction in the loading of the valuable precious metal. The metallically conductive matrix creates a particularly stable mechanical bond between the active nanoparticles and the metallic substrate. The coating's morphology is rough and porous. Further special features include high material compatibility and very good mechanical strength. From an economic perspective, complex and costly leaching procedures or thermal post-treatment processes are also eliminated, unlike conventional Raney Ni coatings. As indicated, leaching leads to very poor bonding of the coating, which is sometimes even washed out again at the same time. Furthermore, leaching requires large quantities of (excess) lye, which is why this known approach is expensive and material-inefficient.

[0026] In contrast, the electrode obtained using the process described here is characterized by particularly strong adhesion of the coating to the substrate. In other words, a reliable, liquid-to-material bond is formed, in which the coating is virtually "welded on." The present invention further enables a geometrically fully scalable and very simple coating of large-area (large-format) electrodes and the reduction of the precious metal loading through a dispersion effect in the matrix component.

[0027] Furthermore, the method according to the invention makes it possible to achieve a metallically conductive embedding of the active species in the form of nanoparticles (for example Pt, Ru, Ni, Rh, Re, Pd) in the described matrix and to produce a corresponding functional and also mechanically strong material composite.

[0028] The process also impresses with its particularly fast processability and material freedom (in principle applicable for a large number of electrode materials), low costs compared to the so-called Raney processing, a simple process chain without special process technology, very good mechanical stability of the electrode, as well as highly efficient raw material utilization, which advantageously enables emissions to be reduced and the availability of rare catalyst metals to be largely maintained.

[0029] In one embodiment, the electrode is a cathode of an electrochemical cell, in particular an alkaline electrolysis cell, wherein the catalyst material comprises so-called "Pt black", a "RuPt" black and / or "Ru" black.

[0030] In one embodiment, the substrate is preheated to over 180°C, in particular about 200°C.

[0031] A further aspect of the present invention relates to an electrode which, for example, is characterized by improved mechanical strength as well as increased porosity and surface grain size or roughness compared to known electrodes. Furthermore, according to one embodiment, the surface loading of the electrode surface is significantly reduced, for example, due to a dispersion effect (compared to conventional concepts), and amounts in particular to only 30 mg / cm 2 , preferably 20 mg / cm 2 or less precious metal.

[0032] A further aspect of the present invention relates to an electrochemical cell and / or an electrolysis stack comprising the electrode produced as described.

[0033] Yet another aspect of the present invention relates to an electrolyzer having the electrochemical cell.

[0034] Embodiments, features and / or advantages relating to the method in the present case also relate to the electrode directly or to the electrochemical cell, and vice versa.

[0035] The term "and / or" or "respectively," when used in a series of two or more elements, means that any one of the listed elements may be used alone, or any combination of two or more of the listed elements may be used.

[0036] Further details of the invention are described below with reference to the figures.

[0037] Figure 1 shows a schematic flow diagram with process steps according to the invention as well as a pictogram for the process step of high-speed spraying.

[0038] Figure 2 shows a qualitative diagram of the electrocatalytic activity of a hydrogen cathode made of so-called Raney nickel, plotted against the metal-hydrogen bond strength.

[0039] Figure 3 shows an aluminum-nickel phase diagram, particularly the AlNi3 phase, of the matrix material according to the invention. Figure 4 schematically shows an electrolyzer using alkaline electrolysis for hydrogen production, comprising an electrochemical cell incorporating the electrode produced according to the invention.

[0040] In the exemplary embodiments and figures, identical or equivalent elements may be provided with the same reference numerals. The illustrated elements and their relative sizes are generally not to scale; rather, individual elements may be exaggeratedly thick or oversized for clarity and / or clarity.

[0041] Figure 1 shows a schematic flow diagram with process steps according to the invention. The process according to the invention is, in particular, a process for coating an electrode substrate or a corresponding process for producing an electrode 4 with a noble metal catalyst for alkaline water electrolysis.

[0042] The method comprises in S1 the provision of the electrode substrate 1.

[0043] The method further comprises in S2 the provision of a matrix material 2 and a catalyst material 3 as starting materials for the coating.

[0044] The method further comprises in S3 the mixing of the matrix material 2 and the catalyst material 3 and, in S4, the coating of the substrate 1 with the mixture of matrix material 2 and catalyst material 3 by means of high velocity flame spraying HVOF.

[0045] High-velocity flame spraying typically involves largely continuous fuel combustion under high pressure within a water- or air-cooled combustion chamber. Combustion gases (such as propane, ethylene, propene, butane, acetylene, hydrogen), liquid fuels (e.g., diesel, kerosene), and combinations of these are used as fuels. The oxidizing agent is usually oxygen, but air can also be used (common process name: HVAF, derived from high-velocity air fuel). The high pressure of the burning fuel-oxygen mixture generated in the combustion chamber and the usually downstream expansion nozzle generate the high velocity of the gas jet required for coating.

[0046] Powdered spray materials with grain sizes between 1 pm and 150 pm are commonly used, but rods or wires are also used. These are fed axially into the combustion chamber or radially into the expansion nozzle area. This accelerates the spray particles to high speeds, resulting in very dense spray layers with excellent adhesion properties. The controllable and just sufficient heat input advantageously ensures only minimal metallurgical changes to the spray material.

[0047] The following spray parameters are recommended for the described method according to the invention: Preheating of the substrate 1 to over 180 °C, in particular about 200 °C, at an oxygen pressure of 4.5 bar, a fuel gas or acetylene pressure of 1 bar, a volume flow rate of about 50 ml / min kerosene, a distance from the nozzle to the sample substrate of about 20 cm, a surface temperature of 700 °C and a feed rate of 10 m / min.

[0048] A metal fiber fleece, a metal foam, a metal mesh, or an expanded metal is preferably used as the electrode or coating substrate 1.

[0049] The matrix material 2 is preferably in powder form and is mixed with a precious metal powder as catalyst material 3 in weight proportions between 10% and 20%. The matrix material 2 is particularly preferably a nickel aluminide, in particular Ni3Al (see Figure 3 below).

[0050] The catalyst material 3 is preferably in powder form and has a primary particle size of 3 pm to 10 pm. Subsequent treatment or activation is normally not required. In particular, the catalyst material 3 can comprise nanoparticles of Pt (platinum), Ru (ruthenium), Ni (nickel), Rh (rhodium), Re (rhenium), and / or Pd (palladium).

[0051] Furthermore, as catalyst material 3, which is provided in particular on a cathode of a corresponding electrochemical cell 10, such as an alkaline electrolysis cell, so-called "Pt-black", a "Ru / Pt-black" and / or "Ru-black" can be used (alternatively or additionally).

[0052] The special feature of the present invention compared to known processes can be seen, for example, in the combination or embedding of highly active noble metal nanoparticles (secondary particles) in a binder matrix made of Ni3Al.

[0053] The process described here is applicable to virtually all metallic substrates. The preferred matrix component is the spray powder Proxon 21021 (NiMo5A15) (Castolin Eutectic) or Amperit (281.002; 281.003; 291.059, etc.) (Höganäs). Platinum black, alternatively Ru / Pt black, or Ru black, is used as the cathode catalyst.

[0054] As described, the catalyst powders should preferably have a primary particle size of 3 pm to 10 pm.

[0055] As electrode substrates for the cathode electrode, a nickel mesh with the following specifications is recommended: Haver-Boecker, Ni 2.4066, w0.500, dO, 140mm or Haver-Boecker Ni 2.4066, w0.224, dO, 100mm. According to a particularly preferred embodiment of the present invention, the electrode fabrication involves a Pt cathode or the coating of a stainless steel metal fleece substrate with 10 wt% Pt (weight percent) and 90 wt% of the matrix component. This coating results in a total loading of the electrode with precious metal of only 20 mg / cm 2 The so-called electrochemical activity is 2.2 V at a current density of about 2 A / cm 2 .

[0056] According to another particularly preferred embodiment of the present invention, the production relates to a cathode electrode, wherein a platinum-containing catalyst is applied to a nickel mesh. As in the above-described embodiment, the substrate is preferably preheated to approximately 200°C using the HVOF parameters described above.

[0057] Figure 2 shows a qualitative diagram of the electrocatalytic activity A of some metals or noble metals, plotted against the so-called metal-hydrogen bond strength MH. The data originate in particular from Borucihski T, Rausch S, and Wendt H. based on a Raney nickel-activated hydrogen cathode (see also "Part II: Correlation of morphology and effective catalytic activity of Raney-nickel coated cathodes. J Appl Electrochem 1992;22:1031-8"). The diagram shows that noble metals, such as Pt, in addition to their particularly advantageous chemical stability, advantageously exhibit low overpotentials for the cathode-side hydrogen evolution reaction (HER). The same applies to the noble metals rhodium and rhenium.

[0058] Figure 3 shows a nickel-aluminium phase diagram, where the temperature is plotted against the nickel content (bottom in atomic percent and top in weight percent) (cf.

[0059] DOI: 10.1361 / 15477030420232 , Al-Ni (aluminum-nickel ) (springer, com) ). With reference to the matrix component of the starting material, which preferably consists of or comprises nickel aluminide, Figure 3 shows the relevant conditions or phases that develop depending on temperature and nickel content, particularly on the right, for the generation of the Ni3Al phase. Such a matrix component can be achieved, for example, by the spontaneous cooling of a nickel-rich melt.

[0060] Figure 4 shows a simplified diagram of an electrolyzer 20 comprising an electrochemical cell 10. Such electrolyzers, especially alkaline ones, typically comprise a plurality of so-called cells in corresponding stack arrangements (not explicitly marked here). This is particularly true for the large-scale, industrial application of water electrolyzers for the production of green hydrogen from renewable energy, which is becoming increasingly important in terms of energy technology and climate policy.

[0061] The electrochemical cell 10 comprises two electrodes 4 produced by the method described here. Without limiting the generality, the cathode of the cell 10 is indicated in the left-hand part of the illustration, and the anode of the cell 10 is indicated in the right-hand part of the illustration.

[0062] The novel electrode 4 that can be achieved by the process according to the invention is clearly distinguished from known concepts by the advantages described above, both structurally and with regard to its electrochemical and electrocatalytic advantages. In particular, the HVOF process allows a significantly improved mechanical strength of the electrode structure as well as increased porosity and an advantageously rough surface, whereby the fluid kinetics of an electrolysis cell 10 comprising the electrode can be significantly improved. Likewise, a reduction in overvoltages and an improvement in ionic and electrical conductivity can be achieved in a technically advantageous manner. Furthermore, the electrode produced according to the invention is characterized by an advantageously low surface loading of the electrode surface with precious metal. In particular, the surface loading can be less than 30 mg / cm 2, preferably less than 20 mg / cm 2 , such as 10 mg / cm 2 , amount to.

Claims

Patent claims 1. A method for producing an electrode (4) with a noble metal catalyst for alkaline water electrolysis, comprising the following steps: - (Sl) providing the electrode substrate (1), - (S2) providing a matrix material (2) and a catalyst material (3) as starting materials for the coating, - (S3) mixing the matrix material (2) and the catalyst material (3), wherein the matrix material (2) is in powder form and is mixed in weight proportions between 10% and 20% with a precious metal powder as catalyst material (3), and - (S4) Coating the substrate (1) with the mixture of matrix material (2) and catalyst material (3) by means of high-velocity oxyfuel spraying (HVOF).

2. Method according to claim 1, wherein a metal fiber fleece, a metal foam, a metal mesh, or an expanded metal is used as the electrode substrate (1).

3. Method according to one of the preceding claims, wherein the matrix material (2) is a nickel aluminide, in particular Ni3Al.

4. Process according to one of the preceding claims, wherein the catalyst material (3) is in powder form and has a primary particle size of 3 pm to 10 pm.

5. The method according to any one of the preceding claims, wherein the catalyst material (3) comprises nanoparticles of Pt, Ru, Ni, Rh, Re, and / or Pd.

6. Method according to one of the preceding claims, wherein the catalyst material (3) which is applied to a cathode of a corresponding electrochemical cell (10), such as an alkaline chemical electrolysis cell, comprises so-called "Pt-black", a "RuPt"-black and / or "Ru"-black.

7. Method according to one of the preceding claims, wherein the substrate (1) is preheated to above 180°C, in particular about 200°C.

8. Electrode (4) which is produced according to the method according to one of the preceding claims, wherein the electrode (4) is distinguished - compared to known electrodes - by an improved mechanical strength as well as by an increased porosity, wherein a surface loading of the electrode surface with precious metal is in particular less than 30 mg / cm 2 amounts.

9. Electrochemical cell (10) with an electrode (4) according to claim 8.

10. Electrolyzer (20) with an electrochemical cell (10) according to claim 9.

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