Catalyst powder, particularly suitable for the production of an anode catalyst for proton exchange membrane water electrolysis, and methods for producing such a catalyst powder

Abstract

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

Description

The present invention relates to a catalyst powder particularly suitable for the preparation of an anode catalyst for proton exchange membrane water electrolysis and to a process for the preparation of such a catalyst powder.One task of catalysts is, for example, the electrocatalytic action of oxygen evolution reaction (OER) in proton exchange membrane water electrolysis (PEMWE). In an electrolyser, water is broken down into its constituents hydrogen and oxygen, from which their molecular gases are subsequently produced.Climate conversion presents an ever greater challenge and its negative consequences can only be slowed down if human beings are able to generate energy from sources other than fossil fuels. This means, for example, the use of renewable energy sources. However, in order to effectively switch our power supply, a large-scale power storage method must be developed. Current research results suggest that this can be achieved through the parallel use of various technologies, such as batteries, electric fuels, and hydrogen-related technologies. It is to be understood that about 50% of the energy storage should be covered by hydrogen-based technologies. For the generation of hydrogen from water, proton exchange membrane water electrolysis is a promising candidate, also referred to briefly as "PEMWE", due to the high production rates. However, one of the major challenges with PEMWE is the high demand for noble metals, especially iridium with an annual world-wide promotion of only about 7 to 8 tons. Iridium is generally used as anode catalyst for the OER. Since research into noble metal-free catalysts for the anode side has not yet brought about 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 far as possible.Industrially, catalyst powders having an iridium content of between 75% by weight and 100% by weight are typically used for the PEMWE. 0-25 wt.% titanium dioxide (TiO 2) can be added, for example, as carrier material. These catalysts are normally used to produce anodic catalyst layers having an iridium loading of 1-2 mg Ir ·cm -2. However, since iridium is expensive and available in particular only in finite quantities, the iridium loading has to be drastically reduced for a large-surface expansion of this technology.In the reduction of the iridium loading, there is a limit value in commercially available catalysts below which the catalyst layers result in a marked loss of performance of the PEMWE. According to the current state of research, this limit value is approximately 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 for hydrogen generation.This loss in performance 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 is accordingly reduced.In order to reduce the iridium loading below 0.5 mg Ir ·cm -2 and at the same time to produce a catalyst layer having a sufficient layer thickness, the iridium content in the catalyst powder can be lowered, i.e. replaced by more support material. Catalysts with inorganic oxides such as TiO 2 as carrier material suffer from the poor conductivity of the carrier, however, since electrical percolation is no longer present 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 than 75-100 wt %.Proceeding from 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 anode catalyst in the PEMWE.To achieve this object, the present invention provides a catalyst powder which is suitable in particular for producing an anode catalyst for proton exchange membrane water electrolysis, having catalyst particles having a core made of a semiconducting material and a catalyst shell layer comprising iridium and / or iridium oxide, which shell layer completely covers the core, 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 permit high utilization of the iridium content, which leads to a high iridium-specific power density. This enables the iridium loading of a catalyst layer produced with the catalyst powder to be drastically reduced, while the performance losses, i.e. with a minimally lower or constant hydrogen generation, are kept as low as possible.The catalyst particles of the catalyst powder of the invention have a core-shell structure (core-shell structure). For this purpose, a synthesis process described in more detail below was developed, with which these catalyst particles can be produced from iridium (oxide) and titanium dioxide as base materials. The titanium dioxide forms the core (core) and serves merely as a support material, on the surface of which a thin enveloping layer (shell) of iridium (oxide) is deposited. A particular feature is the homogeneity of this cladding 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, for example.The cores are advantageously formed from spherical carrier particles, rod-shaped carrier particles or from two-dimensional carrier particles.According to one aspect of the present invention, the cores have a specific surface area in the range of 1 to 10 m 2 / g.The catalyst shell layers preferably have a layer thickness in the range from 1 to 20 nm. Since the electrochemical reaction takes place only on the surface, this thin cladding layer is sufficient to catalyze OER. Furthermore, however, the electrical conductivity of the powder must be ensured, which is likewise achieved by the closed enveloping layers of conductive iridium (oxide).The iridium content of the catalyst shell layers is advantageously in the range from 5 to 50% by weight.According to one embodiment of the present invention, further catalyst particles, in particular iridium and / or iridium oxide particles, are arranged on the catalyst shell layers in order 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 iridium active cladding layers.The present invention furthermore provides a process for preparing catalyst powder according to the invention, comprising the steps of: a) preparing an aqueous solution of Ir 3+ ions by dissolving IrCl 3 or other water-soluble Ir precursors / Ir salts in water; b) preparing a dispersion of carrier particles from a semiconducting material, in particular TiO 2- carrier particles, in water by adjusting the pH by means of a base, for example KOH, and then dispersing, for example by means of ultrasound; c) adding the solution prepared in step a) to the dispersion prepared in step b), adjusting the pH by means of a base and preferably adding a so-called hole scavenger ("hole scavenger"), for example in the form of isopropanol; d) depositing metallic or oxidic iridium on the surface of the carrier particles by irradiation of 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; e) filtering, washing, drying and grinding the powder prepared in step d); and f) oxidizing the deposited metallic or oxidic iridium at a temperature in the range from 100-600° C., preferably 350° C., under an oxygen-containing atmosphere.While the cladding layer is not yet closed after step d) is carried out, step f) produces a closed cladding layer of iridium oxide.The synthesis method according to the invention is based on the so-called photodeposition. In this process, semiconducting materials are electromagnetically irradiated in a solution of metal ions. 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 which is capable of accepting the electron tails from the semiconductor, a so-called hole scavenger ("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, thereby depositing the metal 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 catcher is preferably isopropanol. This specific synthesis method also allows the use of carrier particles other than TiO 2, as long as they have a band gap, for example doped tin oxides such as ITO, ATO, FTO or others. The structure of the carrier particles can also vary. Thus, spherical carrier particles, rod-shaped carrier particles or so-called nanowires, 2-dimensional carrier particles and the like can be used. Also, other metals or metal oxides may be deposited on these support particles using other precursors, such as ruthenium salts.By utilizing 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 significantly lowered below 75% by weight, since the overall conductivity of the catalyst composite (catalyst+carrier) is sufficiently high as a result of the conductive shell layer made of iridium or iridium oxide. If, in addition, carrier particles having a low specific surface area in the range from about 1 to 10 m 2 / g are used, particles having a very low iridium content in the range from only about 5 to 50% by weight can be produced.After carrying out step d), a reduction of the residual iridium ions can take place as a second synthesis step, for example by heating the reaction solution, so that the hole catcher acts as a reducing agent.Alternatively or additionally, additional iridium and / or iridium oxide particles can be applied after carrying out step d).The present invention further proposes using catalyst powder according to the invention for preparing an anode catalyst for proton exchange membrane water electrolysis (PEMWE).Further features and advantages of the present invention will become apparent from the following description of several examples with reference to the accompanying drawings. It is therein FIG. 1 is a High-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) image showing the surface of a core-shell particle of a catalyst powder according to an embodiment of the present invention; FIG. 2 is a high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) 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; FIG. 3 is a scanning electron micrograph of a cross section (FIB-SEM) prepared by means of a focused ion beam through a catalyst layer produced with catalyst powder according to one 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 FIG. 4 shows a polarization curve of a 5 cm 2 full PEMWE cell with the core-shell layer particles of a catalyst powder according to one 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 of water inflow on the anode, cathode dry).EXAMPLE 1The synthesis of a catalyst powder having catalyst particles 1 with a core 2 made of a semiconducting material and a catalyst shell layer 3 comprising iridium and / or iridium oxide, which completely covers the core 2, is described below by way of example on a catalyst having 40 wt % iridium content in the powder:In a round bottom flask or other suitable vessel, 1.39 g of hydrated iridium chloride (IrCl 3* xH 2 O) is dissolved in 50 mL of deionized water. This takes place preferably under a nitrogen atmosphere and the solution is stirred for at least 2 hours, preferably for a longer time. In another round-bottom flask made of quartz glass, 1.12 g of TiO 2 particles (rutile crystal structure, particle size <5 μm, Sigma Aldrich) are placed in 100 ml of deionized water and 0.5 ml of single molar KOH (total pH ~10-11) and dispersed in an ultrasonic bath for at least 20 minutes. Subsequently, the solution with the iridium salt is likewise added to the quartz glass and the pH is adjusted again with one molar KOH, so that the pH is between 10 and 11. Furthermore, 30 ml of isopropanol are added. As the next step, the reaction solution is irradiated with UV-C radiation having a wavelength of 254 nm. The irradiation time is typically between 1 and 24 hours. As a second optional step, the reaction solution is subsequently heated to 100° C. in order thus to reduce the residual iridium from the solution and achieve the desired iridium content. The initially added isopropanol 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 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) photographs in FIGS. 1 and 2. The desired core-shell layer structure with additional iridium (oxide) particles 4 has been successfully synthesized.In order to produce catalyst powders having a different iridium content, the weighed mass of iridium trichloride can be adjusted accordingly.EXAMPLE 2This example demonstrates the electrochemical activity of the IrO x- TiO 2- catalyst of the present invention in a three electrode construction. Three different catalysts with different high iridium contents were tested: 10 wt %, 25 wt % and 40 wt % iridium. These were compared with the commercially available catalyst containing 75% by weight iridium.The catalysts were characterized in half-cell measurement with the aid of the so-called rotating disk electrode (RDE). For this, an ink was first prepared from the catalyst powders. The amount of catalyst in the ink was chosen to provide 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 the ultrasonic bath for 10 minutes. By coating the RDE electrode (gold, diameter 5 mm) with 20 μl ink, an iridium loading of 50 μg Ir· cm -2 was thus achieved for all catalysts. Testing was carried out in 0.1 M HClO 4, which was flushed 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 of the invention show a higher activity than the commercial reference catalyst, as additionally shown in Table 1. A high catalytic activity is distinguished 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 design. Table 1: The most important electrochemical parameters of three new IrOx@TiO2nuclear shell layer catalysts and a commercial catalyst measured in a three electrode construction.Voltage at 10 mA·cm-2 / V1,581,601,621,72Mass specific current density at 1.6 V vs RHE / mA·mg Ir-129418714530EXAMPLE 3This example demonstrates the electrochemical properties of the IrO x- TiO 2- catalyst of the present invention in a membrane-electrode assembly (MEA).Production of the Membrane Electrode Unit:An MEA for use in a laboratory scale PEM water electrolyser consists of two electrodes (anode and cathode) joined by hot pressing 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 an anode (iridium loading 0.52 mg Ir / cm 2). To prepare the cathode, 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 / cm 2 was used. As the membrane, Nafion™ N212 having a thickness of 50 μm was used.Electrode Fabrication:For electrode production, 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 thus prepared is coated as a thin layer on a PTFE film by a Mayer-rod coating method. 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 hours. To produce the membrane-electrode assembly, 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 having a homogeneous porosity is shown in Fig. 3. The catalyst particles according to the invention make it possible to produce layers with low iridium loading, while at the same time preventing the layer thickness from becoming too low. This can be clearly seen in FIG. 3. 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 here, whereas a catalyst layer of the same iridium loading with a commercial catalyst would have a significantly smaller layer thickness of only 3 μm. As a result, it is possible to produce layers having an iridium loading of less than 0.5 mg Ir ·cm -2 whose energy losses are significantly lower than in the case of a comparable layer based on commercially available catalysts.Construction of the Electrolysis Cell and Testing:Testing of the MEA was performed in a serpentine flow field electrolysis cell on a commercial test system (ETS 600, Scribner Associates, Inc.) with separate potentiostat (BioLogic Science Instruments). The cell compression is set to 18% with the aid of 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 to flow fields and end plates. For testing, the anode flow field is flushed with 100 ml / min of deionized water. The water is recirculated and passed through an ion exchanger for purification. The incoming water and the electrolysis cell are heated to 80° C. The measurement is carried out at ambient pressure. Characterization of the MEA is performed after successful conditioning. For this purpose, the current density is increased stepwise from 10 mA / cm 2 to 4 A / cm 2 and the cell potential which is established in the process is measured. The measured potential is plotted in FIG. 4 over the applied current densities. The novel catalyst exhibits 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 for the purpose of illustrating the invention and are not to be considered as limiting. Rather, modifications and changes are possible without departing from the scope of the appended claims.LIST OF REFERENCE NUMERALS1 Catalyst particles 2 Core 3 Catalyst shell layer 4 Iridium (oxide) particles 5 Catalyst layer

Claims

Catalyst powder suitable for producing an anode catalyst for proton exchange membrane water electrolysis, comprising catalyst particles (1) having a core (2) made of a semiconducting material and a catalyst shell layer (3) comprising iridium and / or iridium oxide, which completely covers the core.Catalyst powder according to Claim 1, characterized in that the semiconducting material is titanium oxide (TiO2) or doped tin oxides.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 from 1 to 10 m 2 / g.Catalyst powder according to one of the preceding claims, characterized in that the catalyst shell layers (3) have a layer thickness in the range from 1 to 20 nm.Catalyst powder according to one of the preceding claims, characterized in that the iridium content of the catalyst shell layers (3) is in the range from 5 to 50% by weight.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 shell layers (3).A process for preparing catalyst powder according to any of the preceding claims, comprising the steps of: 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 prepared in step b), adjusting the pH by means of a base and preferably adding a so-called hole scavenger ("hole scavenger"), for example isopropanol; d) Deposition of metallic or oxidic iridium on the surface of the carrier particles by irradiation of the reaction solution prepared in step c) with UV radiation, preferably having a wavelength of 254 nm; e) filtering, washing, drying and grinding of the powder prepared in step d); and f) oxidation of the deposited metallic or oxidic iridium at a temperature in the range from 100-600° C., preferably 350° C., under an oxygen-containing atmosphere.Process according to Claim 8, characterized in that, after carrying out step d), a second synthesis step is followed by a reduction of the residual iridium ions, for example by heating the reaction solution, with the result that the hole catcher acts as reducing agent.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.Use of catalyst powder according to any of claims 1 to 7 for the preparation of an anode catalyst for proton exchange membrane water electrolysis (PEMWE).

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