DE102023117162A1 - Catalyst powder which is particularly suitable for producing an anode catalyst for proton exchange membrane water electrolysis, and process for producing such a catalyst powder - Google Patents

Abstract

The present invention relates to a catalyst powder which is suitable for producing an anode catalyst for proton exchange membrane water electrolysis, comprising catalyst particles (1) with a core (2) made of a semiconducting material and a catalyst coating layer (3) completely covering the core and comprising iridium and/or iridium oxide.

Description

The present invention relates to a catalyst powder which is particularly suitable for producing an anode catalyst for proton exchange membrane water electrolysis, and to a process for producing such a catalyst powder.

One task of catalysts, for example, is the electrocatalysis of the oxygen evolution reaction (OER) in proton exchange membrane water electrolysis (PEMWE). In an electrolyzer, water is split into its components hydrogen and oxygen, from which their molecular gases are then produced.

Climate change is an ever-increasing challenge, and its negative impacts can only be slowed if humanity is able to generate energy from sources other than fossil fuels. This means, for example, using renewable energy sources. However, to effectively transform our energy supply, a large-scale method of energy storage must be developed. Current research suggests that this can be achieved by using different technologies in parallel, such as batteries, e-fuels and hydrogen-related technologies. It is expected that around 50% of energy storage should be covered by hydrogen-based technologies. For the production of hydrogen from water, proton exchange membrane water electrolysis is a promising candidate due to its high production rates, also referred to as "PEMWE" for short. However, one of the biggest challenges in PEMWE is the high demand for precious metals, especially iridium, with an annual global production of only about 7 to 8 tons. Iridium is usually used as an anode catalyst for OER. Since research into precious metal-free catalysts for the anode side has not yet produced any industry-relevant innovations, the focus in the development of new catalysts for the near future should be on reducing the iridium loading in the catalyst layer as much as possible.

In industry, catalyst powders with an iridium content between 75 wt.% and 100 wt.% are typically used for PEMWE. 0-25 wt.% titanium dioxide (TiO ₂ ), for example, can be added as a carrier material. These catalysts are normally used to produce anodic catalyst layers with an iridium loading of 1-2 mg _Ir cm ^-2 . However, since iridium is expensive and, above all, only available in finite quantities, the iridium loading must be drastically reduced for large-scale expansion of this technology.

When reducing the iridium loading, there is a limit in commercially available catalysts below which the catalyst layers result in a significant loss of performance of the PEMWE. According to the current state of research, this limit is around 0.5 mg _Ir cm ^-2 . If the iridium loading in the layer is further reduced, this results in a significantly higher cell voltage and thus significantly higher losses when using electrical energy to generate hydrogen.

This loss of performance is attributed to the fact that the layers become too thin and inhomogeneous. This means that parts of the iridium present in the layer can no longer be electrically contacted and thus no (electro)catalytic reaction can take place there. The utilization of the catalyst decreases accordingly.

In order to reduce the iridium loading to below 0.5 mg _Ir cm ^-2 and at the same time produce a catalyst layer with sufficient layer thickness, the iridium content in the catalyst powder can be reduced, i.e. replaced by more carrier material. However, catalysts with inorganic oxides such as TiO ₂ as the carrier material suffer from the poor conductivity of the carrier, since electrical percolation is no longer possible with a low iridium content. This low conductivity reduces the composite conductivity of the catalyst (catalyst + carrier), which in turn leads to higher resistance-related energy losses. Therefore, the iridium content in the catalyst powder is typically at least 30 wt.%, but tends to be significantly higher at 75-100 wt.%.

Based on this prior art, it is an object of the present invention to provide a more efficient catalyst with preferably simultaneously low iridium content for use as an anode catalyst in PEMWE.

To achieve this object, the present invention provides a catalyst powder which is particularly suitable for producing an anode catalyst for proton exchange membrane water electrolysis comprising catalyst particles with a core made of a semiconducting material and a catalyst coating layer completely covering the core and comprising iridium and/or iridium oxide, wherein the catalyst powder consists in particular of such catalyst particles.

Such a catalyst powder is based on iridium and/or iridium oxide as the active material. It should have as low an iridium content as possible, but at the same time enable a high utilization of the iridium content, which leads to a high iridium-specific power density. This means that the iridium loading of a catalyst layer produced with the catalyst powder can be drastically reduced, while the performance losses, i.e. with minimally lower or constant hydrogen production, are kept as low as possible.

The catalyst particles of the catalyst powder according to the invention have a core-shell structure. For this purpose, a synthesis process was developed, described in more detail below, with which these catalyst particles can be produced from iridium (oxide) and titanium dioxide as base materials. The titanium dioxide forms the core and serves only as a carrier material, on the surface of which a thin shell layer of iridium (oxide) is deposited. A special feature is the homogeneity of this shell layer, which completely covers the core.

The semiconducting material is preferably titanium oxide (TiO ₂ ) or doped tin oxides, such as ITO, ATO or FTO.

The cores are advantageously formed from spherical carrier particles, rod-shaped carrier particles or two-dimensional carrier particles.

According to one embodiment of the present invention, the cores have a specific surface area in the range of 1 to 10 m ² /g.

The catalyst coating layers preferably have a layer thickness in the range of 1 to 20 nm. Since the electrochemical reaction only takes place on the surface, this thin coating layer is sufficient to catalyze the OER. However, the electrical conductivity of the powder must also be ensured, which is also achieved by the closed coating layers made of conductive iridium (oxide).

Advantageously, the iridium content of the catalyst coating layers is in the range of 5 to 50 wt.%.

According to one embodiment of the present invention, further catalyst particles, in particular iridium and/or iridium oxide particles, are arranged on the catalyst coating layers in order to further increase the electrocatalytically active surface. These additional particles should be significantly smaller than the carrier particles in order to further increase the surface area of the active iridium coating layers.

1. a) Herstellung einer wässrigen Lösung aus Ir³⁺ Ionen durch das Auflösen von IrCl₃ oder anderen wasserlöslichen Ir-Präkursoren/Ir-Salzen in Wasser;
2. b) Herstellung einer Dispersion von Trägerteilchen aus einem halbleitenden Material, insbesondere TiO₂-Trägerteilchen, in Wasser durch die Einstellung des pH-Werts mittels einer Base, beispielsweise KOH, und eine anschließende Dispergierung, beispielsweise mittels Ultraschalls;
3. c) Zugeben der in Schritt a) hergestellten Lösung zur in Schritt b) erzeugten Dispersion, Einstellen des pH-Werts mittels einer Base und bevorzugt Hinzugabe eines sogenannten Lochfängers („hole scavenger"), beispielsweise in Form von Isopropanol;
4. d) Abscheidung von metallischem oder oxidischem Iridium auf der Oberfläche der Trägerteilchen durch die Bestrahlung der in Schritt c) hergestellten Reaktionslösung mit UV-Strahlung, bevorzugt mit einer Wellenlänge von 254 nm, beispielsweise mit Hilfe einer Quecksilberdampflampe;
5. e) Filtern, Waschen, Trocknen und Mahlen des in Schritt d) hergestellten Pulvers; und
6. f) Oxidation des abgeschiedenen metallischen oder oxidischen Iridiums bei einer Temperatur im Bereich von 100-600 °C, bevorzugt 350 °C, unter sauerstoffhaltiger Atmosphäre.

Furthermore, the present invention provides a process for producing catalyst powder according to the invention, comprising the steps:

1. a) Preparation of an aqueous solution of Ir ³⁺ ions by dissolving IrCl ₃ or other water-soluble Ir precursors/Ir salts in water;
2. b) producing a dispersion of carrier particles made of a semiconducting material, in particular TiO ₂ carrier particles, in water by adjusting the pH value using a base, for example KOH, and subsequent dispersion, for example by means of ultrasound;
3. c) adding the solution prepared in step a) to the dispersion produced in step b), adjusting the pH using a base and preferably adding a so-called hole scavenger, for example in the form of isopropanol;
4. d) deposition of metallic or oxidic iridium on the surface of the carrier particles by irradiating the reaction solution prepared in step c) with UV radiation, preferably with a wavelength of 254 nm, for example with the aid of a mercury vapor lamp;
5. e) filtering, washing, drying and grinding the powder produced in step d); and
6. f) oxidation of the deposited metallic or oxide iridium at a temperature in the range of 100-600 °C, preferably 350 °C, under an oxygen-containing atmosphere.

While the shell layer is not yet closed after step d), step f) produces a closed shell layer of iridium oxide.

The synthesis process according to the invention is based on so-called photodeposition. Semiconducting materials in a solution of metal ions are electromagnetically irradiated. If the photons have a sufficiently high energy, charge carriers (electrons) are raised into the conduction band of the material. If a substance is present that is able to absorb the electron vacancies from the semiconductor, a so-called hole scavenger, an electron is available for a chemical reaction on the surface of the semiconductor. This electron can reduce the positively charged metal ions, whereby the metal is deposited on the surface of the semiconductor. In the present invention, the semiconducting material is in particular TiO ₂ , the metal ions are Ir ³⁺ and the hole scavenger is preferably isopropanol. This special synthesis process also enables the use of carrier particles other than TiO ₂ , as long as they have a band gap, e.g. doped tin oxides such as ITO, ATO, FTO or others. The structure of the carrier particles can also vary. For example, spherical carrier particles, rod-shaped carrier particles or so-called nanowires, 2-dimensional carrier particles and the like can be used. Other metals or metal oxides can also be applied to these carrier particles by using other precursors, such as ruthenium salts.

By using the core-shell layer structure of the particles of the catalyst powder according to the invention, the iridium content in the catalyst powder can be reduced to well below 75% by weight, since the conductive shell layer made of iridium or iridium oxide means that the overall conductivity of the catalyst composite (catalyst + carrier) is sufficiently high. If carrier particles with a low specific surface area in the range of about 1 to 10 m ² /g are also used, particles with a very low iridium content in the range of only about 5 to 50% by weight can be produced.

After carrying out step d), a reduction of the remaining iridium ions can be carried out as a second synthesis step, for example by heating the reaction solution so that the hole scavenger acts as a reducing agent.

Alternatively or additionally, additional iridium and/or iridium oxide particles may be applied after step d) has been carried out.

Furthermore, the present invention proposes using catalyst powder according to the invention for producing an anode catalyst for proton exchange membrane water electrolysis (PEMWE).

• 1 ein Rastertransmissionselektronenmikroskopie-Dunkelfeldbild („High-angle annular dark-field scanning transmission electron microscope"; HAADF-STEM), welches die Oberfläche eines Kern-Hüllschicht-Partikels eines Katalysatorpulvers gemäß einer Ausführungsform der vorliegenden Erfindung zeigt;
• 2 ein Rastertransmissionselektronenmikroskopie-Dunkelfeldbild („High-angle annular dark-field scanning transmission electron microscope"; HAADF-STEM), welches einen Kern-Hüllschicht-Partikel eines Katalysatorpulvers gemäß einer weiteren Ausführungsform der vorliegenden Erfindung mit zusätzlichen Iridium(oxid)-Partikeln auf der Oberfläche zeigt;
• 3 eine Rasterelektronenmikroskopie-Aufnahme eines mittels fokussiertem Ionenstrahl präparierten (FIB-SEM) Querschnitts durch eine mit Katalysatorpulver gemäß einer Ausführungsform der vorliegenden Erfindung hergestellte Katalysatorschicht, deren Schichtdicke etwa 10 µm und deren Iridiumbeladung 0,7 mg_Ir·cm^-2 beträgt; und
• 4 eine Polarisationskurve einer 5 cm² großen PEMWE-Vollzelle mit den Kern-Hüllschicht-Partikeln eines Katalysatorpulvers gemäß einer Ausführungsform der vorliegenden Erfindung als anodischer Katalysator (Kathode: Pt/C (40 Gew.% Pt on Vulcan XC72 carbon), 0,2 ± 0,08 mg_Pt/cm²; Membran: Nafion N212; Messbedingungen: 80 °C, Umgebungsdruck, 100 ml/min Wasserzufluss auf der Anode, Kathode trocken).

Further features and advantages of the present invention will become apparent from the following description of several examples with reference to the accompanying drawing.

• 1 a high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) dark-field image showing the surface of a core-shell layer particle of a catalyst powder according to an embodiment of the present invention;
• 2 a high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) dark-field image showing a core-shell layer particle of a catalyst powder according to another embodiment of the present invention with additional iridium (oxide) particles on the surface;
• 3 a scanning electron microscopy image of a cross-section prepared by means of a focused ion beam (FIB-SEM) through a catalyst layer produced with catalyst powder according to an embodiment of the present invention, the layer thickness of which is about 10 µm and the iridium loading of which is 0.7 mg _Ir ·cm ^-2 ; and
• 4 a polarization curve of a 5 cm ² PEMWE full cell with the core-shell layer particles of a catalyst powder according to an embodiment of the present invention as anodic catalyst (cathode: Pt/C (40 wt.% Pt on Vulcan XC72 carbon), 0.2 ± 0.08 mg _Pt /cm ² ; membrane: Nafion N212; measurement conditions: 80 °C, ambient pressure, 100 ml/min water inflow on the anode, cathode dry).

EXAMPLE 1

• In einem Rundkolben oder anderem geeigneten Gefäß werden 1,39 g hydratisiertes Iridiumchlorid (IrCl₃*xH₂O) in 50 ml deionisiertem Wasser gelöst. Dies findet bevorzugt unter Stickstoffatmosphäre statt und die Lösung wird mindestens 2 Stunden, bevorzugt länger, gerührt. In einem anderen Rundkolben aus Quarzglas werden 1,12 g TiO₂ Partikel (Rutil-Kristallstruktur, Partikelgröße < 5 µm, Sigma Aldrich) in 100 ml deionisiertem Wasser und 0,5 ml einmolarer KOH (Gesamt-pH ~ 10-11) gegeben und in einem Ultraschallbad für mindestens 20 Minuten dispergiert. Anschließend wird die Lösung mit dem Iridiumsalz ebenfalls in das Quarzglas gegeben und der pH-Wert erneut mit einmolarer KOH angepasst, sodass der pH-Wert zwischen 10 und 11 liegt. Außerdem werden 30 ml Isopropanol hinzugegeben. Als nächster Schritt wird die Reaktionslösung mit UV-C Strahlung mit einer Wellenlänge von 254 nm bestrahlt. Die Dauer der Bestrahlung beträgt typischerweise zwischen 1 bis 24 Stunden. Als zweiter optionaler Schritt wird die Reaktionslösung im Anschluss auf 100 °C erhitzt, um so das restliche Iridium aus der Lösung zu reduzieren und den gewünschten Iridium-Gehalt zu erreichen. Dabei wirkt das anfangs hinzugefügte Isopropanol als Reduktionsmittel für die Iridium-Ionen. Anschließend wird die Reaktionslösung gefiltert und das farblose Filtrat entsorgt. Dabei wird das Pulver mit einem Liter DI-Wasser gewaschen. Nach der Trocknung des Pulvers bei 60 °C für mindestens 4 h wird das Pulver in einem Handmörser gemahlen. Daraufhin folgt der letzte Schritt der Synthese. Das Pulver wird in einem Röhrenofen bei 350 °C für 30 Minuten unter einem Fluss von synthetischer Luft oxidiert. Die resultierende Struktur der Katalysatorpartikel 1 des Katalysatorpulvers ist durch Transmissionselektronenmikroskopie (TEM)-Aufnahmen in den 1 und 2 gezeigt. Die gewünschte Kern-Hüllschicht-Struktur mit zusätzlichen Iridium(oxid)-Teilchen 4 wurde erfolgreich synthetisiert.

The synthesis of a catalyst powder comprising catalyst particles 1 with a core 2 made of a semiconducting material and a catalyst coating layer 3 completely covering the core 2 and comprising iridium and/or iridium oxide is described below using a catalyst with 40% by weight iridium content in the powder as an example:

• In a round-bottomed flask or other suitable vessel, 1.39 g of hydrated iridium chloride (IrCl ₃ *xH ₂ O) are dissolved in 50 ml of deionized water. This is preferably done under a nitrogen atmosphere and the solution is stirred for at least 2 hours, preferably longer. In another round-bottomed flask made of quartz glass, 1.12 g of TiO ₂ particles (rutile crystal structure, particle size < 5 µm, Sigma Aldrich) are added to 100 ml of deionized water and 0.5 ml of 1 molar KOH (total pH ~ 10-11) and dispersed in an ultrasonic bath for at least 20 minutes. The solution with the iridium salt is then also added to the quartz glass and the pH value is again adjusted with 1 molar KOH so that the pH value is between 10 and 11. In addition, 30 ml of isopropanol are added. As a next step, the reaction solution is irradiated with UV-C radiation with a wavelength of 254 nm. The duration of the irradiation is typically between 1 and 24 hours. As a second optional step, the reaction solution is then heated to 100 °C in order to reduce the remaining iridium from the solution and to achieve the desired iridium content. The isopropanol added at the beginning acts as a reducing agent for the iridium ions. The reaction solution is then filtered and the colorless filtrate is disposed of. The powder is washed with one liter of DI water. After drying the powder at 60 °C for at least 4 hours, the powder is ground in a hand mortar. This is followed by the last step of the synthesis. The powder is oxidized in a tube oven at 350 °C for 30 minutes under a flow of synthetic air. The resulting structure of the catalyst particles 1 of the catalyst powder is shown in the figures by transmission electron microscopy (TEM) images. 1 and 2 The desired core-shell structure with additional iridium (oxide) particles 4 was successfully synthesized.

To produce catalyst powder with a different iridium content, the weighed mass of iridium trichloride can be adjusted accordingly.

EXAMPLE 2

This example shows the electrochemical activity of the IrO _x -TiO ₂ catalyst according to the invention in a three-electrode setup. Three different catalysts with different iridium contents were tested: 10 wt.%, 25 wt.% and 40 wt.% iridium. These were compared with the commercially available catalyst with 75 wt.% iridium.

The catalysts were characterized in the half-cell measurement using the so-called Rotating Disk Electrode (RDE). For this purpose, an ink was first produced from the catalyst powders. The amount of catalyst in the ink was selected so that 1.96 mg of iridium was present. Then 3 ml of Milli-Q water, 1 ml of isopropanol and 40 µl of Nafion D520 were added and the ink was dispersed by sonication in an ultrasonic bath for 10 minutes. By coating the RDE electrode (gold, diameter 5 mm) with 20 µl of ink, an iridium loading of 50 pg _Ir cm ^-2 was achieved for all catalysts. The test was carried out in 0.1 M HClO ₄ , which was rinsed with Ar for at least 30 min before the measurement. A platinum wire was used as the counter electrode and a reversible hydrogen electrode (RHE) as the reference electrode.

All three variations of the catalyst according to the invention show a higher activity than the commercial reference catalyst, as additionally shown in Table 1. A high catalytic activity is characterized by a low voltage at a given current density or by a high mass-specific current density at a given voltage. Table 1: The most important electrochemical parameters of three new IrO _x @TiO ₂ core-shell catalysts and a commercial catalyst, measured in a three-electrode setup. parameter IrO _x @TiO ₂ (10 wt.% Ir) IrO _x @TiO ₂ (25 wt.% Ir) IrO _x @TiO ₂ (40 wt.% Ir) Commercial catalyst Voltage at 10 mA cm ^-2 / V 1.58 1.60 1.62 1.72 Mass-specific current density at 1.6 V vs RHE / mA·mg _Ir ^-1 294 187 145 30

EXAMPLE 3

This example shows the electrochemical properties of the inventive IrO _x -TiO ₂ catalyst in a membrane-electrode assembly (MEA).

Manufacturing of the membrane electrode assembly:

A MEA for use in a laboratory-scale PEM water electrolyzer consists of two electrodes (anode and cathode) which are hot-pressed together with a proton-conductive membrane. The core-shell catalyst of this invention containing 40 wt% iridium was used to prepare the membrane electrode assembly as anode (iridium loading 0.52 mg _Ir / ^cm2 ). A commercial catalyst consisting of platinum on carbon (Pt/C, 40 wt% Pt on Vulcan XC72 carbon) with a platinum loading of 0.2 ± 0.08 mg _Pt / ^cm2 was used to prepare the cathode. Nafion™ N212 with a thickness of 50 µm was used as the membrane.

Electrode production:

To produce the electrodes, the respective catalyst powder is dispersed with solvent (deionized water and 1-propanol) and an ionomer dispersion (D2021 Nafion™ dispersion) in an ultrasonic bath. The ink produced in this way is applied as a thin layer to a PTFE film using a Mayer rod coating process. The same method is used for both the anode and the cathode. Only the ink recipe is adapted to the respective catalyst. The electrodes are then dried at 70 °C for 2 h. To produce the membrane electrode unit, a Nafion™ 212 membrane is placed between the electrodes and combined with a hot press at 155 °C and 2.5 MPa. In a final step, the PTFE film is removed. The structure of the catalyst layer 5 with a homogeneous porosity is in 3 The catalyst particles according to the invention enable the production of layers with a low iridium loading, while at the same time preventing the layer thickness from becoming too small. This is shown in 3 can be clearly seen. A catalyst layer 5 made of the catalyst particles according to the invention with an iridium loading of 0.7 mg _Ir ·cm ^-2 has a thickness of 10 µm, whereas a catalyst layer with the same iridium loading using a commercial catalyst would have a significantly lower layer thickness of only 3 µm. This makes it possible to produce layers with an iridium loading of less than 0.5 mg _Ir ·cm ^-2 , the energy losses of which are significantly lower than with a comparable layer based on commercially available catalysts.

Construction of the electrolysis cell and testing:

The MEA was tested in an electrolysis cell with a serpentine flow field on a commercial test system (ETS 600, Scribner Associates Inc.) with a separate potentiostat (BioLogic Science Instruments). The cell compression is set to 18% using a torque wrench and seals with a defined thickness. The structure of the electrolysis cell is as follows: The MEA is arranged between porous transport layers (anode: sintered titanium fibers, 250 µm, NV Bekaert SA, cathode: carbon fiber, Freudenberg H24C5, Freudenberg Performance Materials SE & Co. KG). This arrangement is fixed and screwed together with flow fields and end plates. For testing, 100 ml/min of deionized water is flowed through the anode flow field. The water is recirculated and passed through an ion exchanger for purification. The inflowing water and the electrolysis cell are heated to 80 °C. The measurement is carried out at ambient pressure. The characterization of the MEA is carried out after successful conditioning. For this purpose, the current density is gradually increased from 10 mA/cm ² to 4 A/cm ² and the resulting cell potential is measured. The measured potential is shown in 4 plotted against the applied current densities. The novel catalyst shows good performance with low energy losses even at a low iridium loading of 0.52 mg _Ir /cm ² .

It should be understood that the examples described above are only illustrative of the invention and are not to be understood as limiting it. Rather, modifications and changes are possible without departing from the scope of protection defined by the appended claims.

REFERENCE NUMBER LIST

1

catalyst particles

2

core

3

catalyst coating layer

4

iridium (oxide) particles

5

catalyst layer

Claims (11)

Catalyst powder suitable for producing an anode catalyst for proton exchange membrane water electrolysis, comprising catalyst particles (1) with a core (2) made of a semiconducting material and a catalyst coating layer (3) completely covering the core and comprising iridium and/or iridium oxide. Catalyst powder according to claim 1 , characterized in that the semiconducting material is titanium oxide (TiO2) or doped tin oxide. Catalyst powder according to claim 1 or 2 , characterized in that the cores (2) are formed from spherical carrier particles, rod-shaped carrier particles or from two-dimensional carrier particles. Catalyst powder according to one of the preceding claims, characterized in that the cores (2) have a specific surface area in the range of 1 to 10 m ² /g. Catalyst powder according to one of the preceding claims, characterized in that the catalyst coating layers (3) have a layer thickness in the range of 1 to 20 nm. Catalyst powder according to one of the preceding claims, characterized in that the iridium content of the catalyst coating layers (3) is in the range from 5 to 50 wt.%. Catalyst powder according to one of the preceding claims, characterized in that further iridium and/or iridium oxide particles (4) are arranged on the catalyst coating layers (3). Process for producing catalyst powder according to one of the preceding claims, comprising the steps: a) preparing an aqueous solution of Ir ³⁺ ions by dissolving IrCl ₃ or other water-soluble iridium precursors/iridium salts in water; b) preparing a dispersion of carrier particles, in particular TiO ₂ carrier particles, in water by adjusting the pH by means of a base, for example KOH, and subsequent dispersion, for example by means of ultrasound; c) adding the solution prepared in step a) to the dispersion produced in step b), adjusting the pH using a base and preferably adding a so-called hole scavenger, for example isopropanol; d) depositing metallic or oxidic iridium on the surface of the carrier particles by irradiating the reaction solution prepared in step c) with UV radiation, preferably having a wavelength of 254 nm; e) filtering, washing, drying and grinding the powder produced in step d); and f) oxidizing the deposited metallic or oxidic iridium at a temperature in the range of 100-600 °C, preferably 350 °C, in an oxygen-containing atmosphere. procedure according to claim 8 , characterized in that after carrying out step d), a reduction of the remaining iridium ions is carried out as a second synthesis step, for example by heating the reaction solution, so that the hole scavenger acts as a reducing agent. procedure according to claim 8 or 9 , characterized in that after carrying out step d), additional iridium and/or iridium oxide particles (4) are applied. Use of catalyst powder according to one of the Claims 1 until 7 for the production of an anode catalyst for proton exchange membrane water electrolysis (PEMWE).

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