Catalyst powder suitable in particular for production of an anode catalyst for proton exchange membrane water electrolysis, and process for producing such a catalyst powder

Abstract

The present invention relates to a catalyst powder suitable for production of an anode catalyst for proton exchange membrane water electrolysis, having catalyst particles (1) having a core (2) of a semiconductive material, and a catalyst shell layer (3) that fully covers the core and comprises iridium and / or iridium oxide.

Description

[0001] DESCRIPTION

[0002] Catalyst powder particularly suitable for producing an anode catalyst for proton exchange membrane water electrolysis, and process for producing such a catalyst powder

[0003] 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.

[0004] One function 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 subsequently produced.

[0005] Climate change is an ever-increasing challenge, and its negative consequences can only be slowed if humanity is able to generate energy from sources other than fossil fuels. This means, for example, the use of renewable energy sources. To effectively transition our energy supply, however, a large-scale method for energy storage must be developed. Current research suggests that this can be achieved through the parallel use of various technologies, 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, also referred to as "PEMWE" for short, is a promising candidate due to its high production rates.However, one of the biggest challenges facing PEMWE is the high demand for precious metals, especially iridium, with annual global production of only about 7 to 8 tons. Iridium is typically 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 developing new catalysts in the near future should be on reducing the iridium loading in the catalyst layer as much as possible.

[0006] Industrially, catalyst powders with an iridium content between 75 wt.% and 100 wt.% are typically used for PEMWE. 0-25 wt.% titanium dioxide (TiO2), for example, can be added as a support material. These catalysts are typically used to form anodic catalyst layers with an iridium loading of 1-2 mgi. r cm- 2However, since iridium is expensive and, above all, only available in finite quantities, the iridium loading must be drastically reduced for large-scale deployment of this technology.

[0007] When reducing the iridium loading, there is a limit for commercially available catalysts below which the catalyst layers result in a significant loss of performance in the PEMWE. According to current research, this limit is approximately 0.5 mgi. r cm- 2If 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 performance loss is attributed to the layers becoming too thin and inhomogeneous. As a result, 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.

[0008] To keep the iridium loading below 0.5 mgi r cm- 2To reduce the iridium content and simultaneously produce a catalyst layer with sufficient layer thickness, the iridium content in the catalyst powder can be lowered, i.e., replaced with more support material. However, catalysts with inorganic oxides such as TiO2 as the support material suffer from the poor conductivity of the support, since electrical percolation is no longer possible at a low iridium content. This low conductivity reduces the composite conductivity of the catalyst (catalyst + support), 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.%.

[0009] 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.

[0010] 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, comprising iridium and / or iridium oxide, wherein the catalyst powder consists in particular of such catalyst particles. Such a catalyst powder is thus based on iridium and / or iridium oxide as the active material. It should have the lowest possible iridium content, but at the same time enable a high utilization of the iridium content, which leads to a high iridium-specific power density. As a result, 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, as low as possible.

[0011] The catalyst particles of the catalyst powder according to the invention have a core-shell structure. A synthesis process, described in more detail below, was developed for this purpose. This process allows these catalyst particles to be produced from iridium oxide and titanium dioxide as base materials. The titanium dioxide forms the core and serves merely as a support material, on whose surface a thin shell layer of iridium oxide is deposited. A special feature is the homogeneity of this shell layer, which completely covers the core.

[0012] The semiconducting material is preferably titanium oxide (TiO2) or doped tin oxides, such as ITO, ATO or FTO.

[0013] The cores are advantageously formed from spherical carrier particles, rod-shaped carrier particles or two-dimensional carrier particles.

[0014] 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 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. Furthermore, the electrical conductivity of the powder must be ensured, which is also achieved by the closed coating layers of conductive iridium oxide.

[0015] Advantageously, the iridium content of the catalyst coating layers is in the range of 5 to 50 wt.%.

[0016] According to one embodiment of the present invention, additional catalyst particles, in particular iridium and / or iridium oxide particles, are arranged on the catalyst coating layers to further increase the electrocatalytically active surface area. These additional particles should be significantly smaller than the carrier particles to further increase the surface area of ​​the active iridium coating layers.

[0017] Furthermore, the present invention provides a process for producing catalyst powder according to the invention, comprising the steps: a) preparing an aqueous solution of lr 3+Ions by dissolving IrCh or other water-soluble Ir precursors / Ir salts in water; b) producing a dispersion of carrier particles made of a semiconducting material, in particular TiO2 carrier particles, in water by adjusting the pH using a base, for example KOH, and subsequent dispersing, 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 in the form of isopropanol; 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 using a mercury vapor lamp; e) filtering, washing, drying and grinding the powder produced in step d);and f) oxidation of the deposited metallic or oxide iridium at a temperature in the range of 100-600 °C, preferably 350 °C, in an oxygen-containing atmosphere.;

[0018] While the shell layer is not yet closed after step d), step f) produces a closed shell layer of iridium oxide.

[0019] The synthesis process according to the invention is based on photodeposition. Semiconducting materials in a solution of metal ions are electromagnetically irradiated. If the photons have sufficiently high energy, charge carriers (electrons) are lifted into the conduction band of the material. If a substance capable of absorbing the electron vacancies from the semiconductor, a so-called hole scavenger, is present, an electron is available for a chemical reaction on the surface of the semiconductor. This electron can reduce the positively charged metal ions, thereby depositing the metal onto the surface of the semiconductor. In the present invention, the semiconducting material is, in particular, TiO2, and the metal ions are Ir. 3+and, for the hole trap, isopropanol is preferred. This special synthesis process also allows the use of support particles other than TiO2, as long as they have a band gap, e.g., doped tin oxides such as ITO, ATO, FTO, or others. The structure of the support particles can also vary. Spherical support particles, rod-shaped support particles or so-called nanowires, 2-dimensional support particles, and the like can be used. Other metals or metal oxides can also be applied to these support particles using other precursors, such as ruthenium salts.

[0020] By utilizing the core-shell structure of the particles of the catalyst powder according to the invention, the iridium content in the catalyst powder can be reduced significantly below 75 wt.%, since the conductive shell layer of iridium or iridium oxide ensures that the overall conductivity of the catalyst composite (catalyst + support) is sufficiently high. In addition, if support particles with a low specific surface area in the range of approximately 1 to 10 m 2 / g is used, particles with a very low iridium content in the range of only about 5 to 50 wt.% can be produced.

[0021] After carrying out step d), a second synthesis step can be a reduction of the remaining iridium ions, for example by heating the reaction solution so that the hole scavenger acts as a reducing agent.

[0022] Alternatively or additionally, additional iridium and / or iridium oxide particles may be applied after step d) has been carried out.

[0023] Furthermore, the present invention proposes the use of catalyst powder according to the invention for producing an anode catalyst for proton exchange membrane water electrolysis (PEMWE). Further features and advantages of the present invention will become clear from the following description of several examples with reference to the accompanying drawing.

[0024] Figure 1 is 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;

[0025] Figure 2 is 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;

[0026] Figure 3 is a scanning electron micrograph of a focused ion beam (F1 B-SEM) cross-section through a catalyst layer produced with catalyst powder according to an embodiment of the present invention, the layer thickness of which is approximately 10 pm and the iridium loading of which is 0.7 mgir cm 2 is; and

[0027] Figure 4 shows a polarization curve of a 5 cm 2large 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 mgpt / cm 2 ; Membrane: Nafion N212; Measurement conditions: 80 °C, ambient pressure, 100 ml / min water flow on the anode, cathode dry).

[0028] EXAMPLE 1 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 wt.% iridium content in the powder as an example:

[0029] In a round-bottomed flask or other suitable container, 1.39 g of hydrated iridium chloride (IrCh*xH2O) is 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 TiO2 particles (rutile crystal structure, particle size < 5 pm, Sigma Aldrich) are added to 100 ml of deionized water and 0.5 ml of 1M KOH (total pH ~ 10-11) and dispersed in an ultrasonic bath for at least 20 minutes. The solution containing the iridium salt is then also poured into the quartz glass, and the pH is again adjusted with 1M KOH to a pH between 10 and 11. 30 ml of isopropanol is also added. In the next step, the reaction solution is irradiated with UV-C radiation at a wavelength of 254 nm. The irradiation duration is typically between 1 and 24 hours.As a second optional step, the reaction solution is subsequently heated to 100 °C to reduce the remaining iridium from the solution and achieve the desired iridium content. The isopropanol added initially acts as a reducing agent for the iridium ions. The reaction solution is then filtered, and the colorless filtrate is discarded. The powder is washed with one liter of DI water. After drying the powder at 60 °C for at least 4 h, the powder is ground in a hand mortar. This is followed by the final step of the synthesis. The powder is oxidized in a tube furnace 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 by transmission electron microscopy (TEM) images in Figures 1 and 2. The desired core-shell structure with additional iridium (oxide) particles 4 was successfully synthesized.

[0030] To produce catalyst powder with a different iridium content, the weighed mass of iridium trichloride can be adjusted accordingly.

[0031] EXAMPLE 2

[0032] This example shows the electrochemical activity of the inventive lrO x -TiO2 catalyst 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.

[0033] The catalysts were characterized in half-cell measurements using a rotating disk electrode (RDE). First, an ink was prepared from the catalyst powders. The amount of catalyst in the ink was selected to contain 1.96 mg of iridium. Subsequently, 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 μg / cm³ was achieved for all catalysts. 2The test was carried out in 0.1 M HCIO4, which was purged with Ar for at least 30 min prior to 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 exhibit higher activity than the commercial reference catalyst, as additionally shown in Table 1. High catalytic activity is characterized by a low potential at a given current density or by a high mass-specific current density at a given potential.

[0034] Table 1:

[0035] The most important electrochemical parameters of three new IrO x @TiC>2 core-shell catalysts and a commercial catalyst, measured in a three-electrode setup.

[0036] Parameters IrO x @TiC>2 IrO x @TiC>2 IrO x @TiC>2 Commercial

[0037] (10 wt% Ir) (25 wt% Ir) (40 wt% Ir) Catalyst

[0038] Voltage at 1.58 1.60 1.62 1.72

[0039] 10 mA-cm' 2 / V

[0040] Mass-specific

[0041] Current density at 294 187 145 30

[0042] 1.6 V vs RHE / mA-mgir -1

[0043] EXAMPLE S

[0044] This example shows the electrochemical properties of the inventive lrO x -TiO2 catalyst in a membrane-electrode assembly (MEA).

[0045] Manufacturing of the membrane electrode assembly:

[0046] An MEA for use in a laboratory-scale PEM water electrolyzer consists of two electrodes (anode and cathode) joined by hot pressing to a proton-conducting membrane. The core-shell catalyst of this invention containing 40 wt% iridium was used to prepare the membrane electrode assembly as the anode (iridium loading 0.52 mgir / cm 2 ) was used. 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 mgpt / cm 2 The membrane used was Nafion™ N212 with a thickness of 50 μm.

[0047] Electrode production:

[0048] 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. This ink is then applied as a thin film to a PTFE foil using a Mayer rod coating process. The same method is used for both the anode and the cathode. Only the ink formulation is adapted to the respective catalyst. The electrodes are then dried at 70 °C for 2 h. To produce the membrane-electrode assembly, a Nafion™ 212 membrane is placed between the electrodes and combined using a hot press at 155 °C and 2.5 MPa. In a final step, the PTFE foil is removed. The structure of the catalyst layer 5 with a homogeneous porosity is shown in FIG. 3.The catalyst particles according to the invention enable the production of layers with a low iridium loading while simultaneously preventing the layer thickness from becoming too thin. This is clearly visible in FIG. 3. A catalyst layer 5 made of the catalyst particles according to the invention with an iridium loading of 0.7 mgi. r cm- 2 has a thickness of 10 pm, whereas a catalyst layer with the same iridium loading using a commercial catalyst would have a significantly lower layer thickness of only 3 pm. This allows layers with an iridium loading of less than 0.5 mg / cm³ 2 whose energy losses are significantly lower than those of a comparable layer based on commercially available catalysts.

[0049] Construction of the electrolysis cell and testing:

[0050] 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 (Biologie Science Instruments). The cell compression is set to 18% using a torque wrench and gaskets of defined thickness. The electrolysis cell is constructed as follows: The MEA is placed 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 MEA is characterized after successful conditioning. For this purpose, a current density of 10 mA / cm is used. 2 gradually to 4 A / cm 2 increased and the resulting cell potential was measured. The measured potential is plotted against the applied current density in FIG. 4. The novel catalyst also shows high performance at a low iridium loading of 0.52 mgi r / cm 2 good performance with low energy losses.

[0051] It should be understood that the above-described examples are for illustrative purposes only and are not to be construed as limiting the invention. Rather, modifications and changes are possible without departing from the scope of protection defined by the appended claims.

[0052] 1 Catalyst particle 2 Core

[0053] 3 Catalyst coating layer

[0054] 4 iridium (oxide) particles

[0055] 5 Catalyst layer

Claims

CLAIMS 1. 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.

2. Catalyst powder according to claim 1, characterized in that the semiconducting material is titanium oxide (TiO2) or doped tin oxides.

3. 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 two-dimensional carrier particles.

4. Catalyst powder according to one of the preceding claims, characterized in that the cores (2) have a specific surface in the range of 1 to 10 m 2 / g.

5. 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.

6. 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.%.

7. 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).

8. A process for the preparation of catalyst powder according to any one of the preceding claims, comprising the steps of: a) preparing an aqueous solution of lr 3+Ions by dissolving IrCh or other water-soluble iridium precursors / iridium salts in water; b) producing a dispersion of carrier particles, in particular TiC carrier particles, in water by adjusting the pH using a base, for example KOH, and subsequent dispersing, 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) 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 having a wavelength of 254 nm; e) filtering, washing, drying and grinding the powder produced in step d);and f) oxidation of the deposited metallic or oxide iridium at a temperature in the range of 100-600 °C, preferably 350 °C, in an oxygen-containing atmosphere.; 9. The method according to claim 8, characterized in that after carrying out step d), a reduction of the remaining iridium ions takes place as a second synthesis step, for example by heating the reaction solution, so that the hole scavenger acts as a reducing agent.

10. The method according to claim 8 or 9, characterized in that after carrying out step d) additional iridium and / or iridium oxide particles (4) are applied.

11. Use of catalyst powder according to one of claims 1 to 7 for producing an anode catalyst for proton exchange membrane water electrolysis (PEMWE).

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