Methods for manufacturing supported iridium oxygen evolution reaction catalysts, their products, and their uses

Abstract

A method is provided for preparing an oxygen evolution reaction (OER) catalyst comprising an iridium oxide component supported on a particulate solid support, the method comprising the steps of: (i) forming an aqueous mixture comprising a solution of a particulate solid support and a halide-free metal iridium salt; (ii) lowering the pH of the aqueous mixture to ≤5.0 to precipitate the iridium oxide component onto the particulate solid support; and (iii) separating the product of step (ii).

Description

Technical Field

[0001] This invention relates to a method for preparing a supported iridium oxygen evolution reaction (OER) catalyst, a supported iridium OER catalyst that can be obtained or is obtainable by the method, a catalyst-coated membrane (CCM) comprising the supported iridium OER catalyst, and a fuel cell or water electrolyzer comprising the CCM. Background Technology

[0002] Oxygen evolution reaction (OER) catalysts are important components in fuel cells and water electrolyzers. The OER reaction under acidic conditions is approximated by the following equation:

[0003] 2H₂O→O₂+4H + +4e -

[0004] Iridium-containing catalysts for promoting this reaction and methods for their preparation are known, including wet chemical precipitation of amorphous iridium-containing precursor compounds from an alkaline aqueous medium. It is said that drying and calcining the precipitate produces more conductive crystalline IrO2 (see, for example, a comparative example in U.S. Patent Publication 2007 / 292744 A1).

[0005] However, supported iridium catalysts are generally more useful in the preparation of catalyst-coated films (CCMs), for example when formulated with suitable polymers to prepare printable inks, because they can produce more efficient OER catalysts, for example by promoting higher activity at lower iridium loadings while maintaining catalyst layer thickness.

[0006] In this regard, EP2608297A1 discloses an iridium oxide-based catalyst for use as an anode catalyst in proton exchange membrane (PEM) water electrolysis and other applications. The disclosed composite catalyst material comprises iridium oxide (IrO2) combined with inorganic oxides (e.g., TiO2, Al2O3, ZrO2, and mixtures thereof) and optionally ruthenium oxide (RuO2). The inorganic oxides have a 30m... 2 / g to 200m 2 The BET surface area is in the range of / g, and is present in amounts of 25% to 70% by weight based on the total weight of the catalyst. The catalyst material is said to be characterized by an electrical conductivity >0.01 S / cm and a high current density.

[0007] US Patent Publication 2022 / 0259750 A1 discloses a method for preparing a catalyst composition, wherein an iridium-containing solid is deposited on a support material in an aqueous medium containing an iridium compound at pH 9, and the support material loaded with the iridium-containing solid is separated from the aqueous medium and dried, wherein in this method, the support material loaded with the iridium-containing solid is not subjected to heat treatment at a temperature above 250°C for a period of time longer than 1 hour.

[0008] Existing methods for preparing supported iridium catalyst materials typically utilize iridium chlorides as precursors, such as IrCl3, IrCl4, and H2IrCl6. For example, the embodiment in U.S. Patent Publication 2022 / 0259750 A1 uses iridium chloride (IV) as a precursor. In the comparative example of this application, the applicant's inventors repeated this prior art embodiment, and despite extensive washing, a considerable amount of chloride was found.

[0009] It is known that halides (such as chlorides) negatively impact the performance and durability of proton exchange membrane (PEM) fuel cells due to enhanced Pt dissolution from Pt-C catalysts (see H. Li et al., Journal of PowerSources 196 (2011) 6249-6255). This literature also includes reports of membrane system durability and integrity being affected by contact with chlorides. For example, chlorides can accelerate corrosion by forming ligands with platinum group metals, thereby stabilizing them in solution. Furthermore, chlorides can evolve into hydrochloric acid, which can dilute the purity of reactant gases and corrode internal components of the electrolyzer. Additionally, chlorides act as catalyst site blockers in the cathode catalyst. Specific examples can be found in the paper “Impact of impurities on water electrolysis: a review,” H. Becker et al., Sustainable Energy & Fuels, 2023, 7, 1565-1603. It is also known that halides (such as chlorides) can cause corrosion of metal parts (such as those present in an electrolytic cell stack).

[0010] Therefore, it is desirable in the art to reduce or prevent contact between membrane-supported catalysts and halides (including chlorides). The applicant has now developed a method for preparing OER active supported iridium catalyst components, which reduces or avoids problems associated with the prior art. Summary of the Invention

[0011] According to a first aspect, the present invention provides a method for preparing an oxygen evolution reaction (OER) catalyst comprising an iridium oxide component supported on a particulate solid support, the method comprising the steps of: (i) forming an aqueous mixture comprising a solution of a particulate solid support and a halide-free metal iridium salt; (ii) lowering the pH of the aqueous mixture to ≤5.0 to precipitate the iridium oxide component onto the particulate solid support; and (iii) separating the product of step (ii).

[0012] In initial results, the applicant has found that the OER catalyst prepared by the method according to the first aspect of the invention contains significantly less chloride than an equivalent catalyst prepared using an IrCl3 (e.g., IrCl4·H2O) precursor, even after extensive washing, for example, with a conductivity of <50 μS / cm in the washed water. The applicant believes that chloride remains in prior art supported iridium OER catalysts prepared using iridium chloride precursors, and that this chloride can still be leached during use. The increased chloride level in the supported iridium OER catalyst is expected to present similar problems in terms of catalyst and CCM stability during use as those observed with chloride impurities in Pt / C catalysts in PEM fuel cell applications.

[0013] Furthermore, initial analytical results indicate that the catalyst product of the first aspect of the invention exhibits beneficial physical differences compared to the catalyst prepared using the IrCl3 precursor (see Example 6 below), showing reduced chloride content and lower iridium dissolution during initial electrochemical testing. According to a second aspect, an oxygen evolution reaction (OER) catalyst is provided comprising an iridium oxide component supported on a particulate solid support, wherein the OER catalyst as a whole has a halide content of less than 500 ppm, as determined by ICP-OES. Preferably, the halide content is less than 400 ppm, less than 300 ppm, or less than 250 ppm.

[0014] Preferably, when the particulate solid support is a transition metal (TM) oxide, the ratio of iridium atoms to transition metal atoms (Ir:TM) at the surface of the catalyst, as determined by XPS analysis, is at least 1.0, such as in the range of 1.0 to 3.0 and including 1.0 and 3.0. An atomic ratio greater than 1.0 indicates that the iridium oxide component is uniformly distributed on the surface of the particulate solid support in the catalyst.

[0015] Such materials can be obtained or are available through the method of the first aspect. According to the third aspect, the applicant provides an oxygen evolution reaction catalyst that can be obtained or is available through the method of the first aspect of the invention.

[0016] The OER catalyst product of the first aspect of the present invention, including the products of the second and third aspects of the present invention, generally does not contain iridium metal (Ir(0)) that reaches the detection limit using X-ray diffraction (XRD).

[0017] In a fourth aspect, the present invention provides a catalyst-coated membrane comprising an oxygen evolution reaction catalyst according to a second or third aspect of the present invention.

[0018] According to a fifth aspect, the present invention provides a fuel cell or water electrolyzer comprising a catalyst-coated membrane according to a fourth aspect of the present invention.

[0019] In a sixth aspect, an oxygen evolution reaction catalyst according to the second aspect of the invention is also provided for catalyzing the reaction 2H₂O → O₂ + 4H₂O. + + 4e - Its uses. Detailed Implementation

[0020] According to a preferred embodiment of the first aspect of the invention, the halide-free metal iridium salt used in step (i) is obtained by the following steps: (a) combining halide-free iridium powder and halide-free peroxide salt to produce a powder mixture; and (b) heat-treating the powder mixture. The advantage of this method is that it uses iridium powder as a raw material, rather than iridium salts used in the synthesis of many previously described OER catalysts. Iridium powder is generally cheaper than iridium salts and allows for the formation of products with very low chloride content.

[0021] The peroxide salt is used to oxidize iridium powder. Preferably, the peroxide salt is a Group I or Group II peroxide salt, and most preferably a Group I peroxide salt. A commercially available sodium peroxide is a preferred peroxide salt.

[0022] The molar ratio of Ir to peroxide salt is preferably 1:3 or higher. The applicant has found that when the molar ratio is less than 1:3, the conversion rate is lower than desired, but not necessary. Although there is no particular upper limit to the equivalent amount of peroxide salt, for process safety and cost considerations, and to avoid product contamination by metal ions in the peroxide salt, an excess of peroxide salt is preferably avoided. A preferred molar ratio of Ir to peroxide salt is 1:4 to 1:10, more preferably 1:6 to 1:9.

[0023] The thermal reaction of iridium powder with sodium peroxide is known and described in the article "Chemical Dissolution of Iridium Powder Using Alkali Fusion Followed by High-Temperature Leaching" (Materials Transactions, Vol. 52, No. 11 (2011), pp. 2067-2070). In that reference, iridium powder and sodium peroxide were combined using a planetary ball mill containing zirconia balls at an Ir:Na₂O₂ molar ratio ranging from 1:0.8 to 1:2.0. The milled material was then transferred to a nickel crucible and heated in an electric furnace at 500°C or 600°C in air for 4 hours or 24 hours. Although the reference describes the melting of the iridium powder and sodium peroxide, and the leaching of iridium from the resulting melt, it does not describe the separation of the OER catalyst.

[0024] Various heating techniques can be used for heat treatment steps. In one method, the heat treatment step is carried out in a static oven or static furnace. In this technique, the powder mixture is held in a container (e.g., a tray) within the oven or furnace.

[0025] In an alternative method, a belt furnace is used for the heat treatment step. In this technique, the powder mixture is held in a container (e.g., a tray) and passed through the furnace by means of a belt. The furnace can be designed for single-zone or multi-zone operation. Belt furnaces are commercially available.

[0026] In alternative methods, a rotary calcining furnace is used for the heat treatment step. A rotary calcining furnace typically comprises an externally heated rotating drum. Rotary calcination is superior to static ovens, static furnaces, or belt furnaces because it mixes the powder, thus helping to ensure a uniform distribution of heat, which is considered beneficial for achieving high iridium conversion rates. If the method is operated continuously, the drum can be tilted to control the residence time of the powder within it. Rotary calcining furnaces are commercially available.

[0027] The powder mixture is heated at a temperature and duration suitable for achieving the desired conversion of iridium metal to its oxide form. It should be understood that the temperature and duration can vary depending on the choice and scale of the equipment used. A skilled technician will be able to determine the appropriate conditions for a given piece of equipment and scale.

[0028] According to a preferred embodiment, step (i) includes the following sub-steps: (i)(a) dissolving a halide-free metal iridium salt in water to produce a solution; and (i)(b) adding a particulate solid carrier to the solution.

[0029] The separation product from step (iii) of the method of the first aspect of the invention can be washed to remove the reagent liquid.

[0030] In one embodiment, the product of step (iii) can be directly combined with a suitable liquid medium in the form of a “wet filter cake” to prepare a catalyst-containing printable ink. Suitable liquid media that serve as ink base materials for supported catalysts and can be used to produce catalyst-coated electrodes in electrolyzers (e.g., PEM electrolyzers for water electrolysis) or PEM fuel cells are known to those skilled in the art. For example, the ink base medium may contain an ionomer (e.g., a polymer containing monomers containing sulfonic acid groups) and one or more short-chain alcohols (e.g., methanol, ethanol, or n-propanol, or a mixture of at least two of these alcohols).

[0031] Alternatively, the method of the first aspect of the invention may include step (iv) of drying the product of step (ii) separated in step (iii) to remove water therefrom (e.g., to produce a dried powder). The resulting dried product may be combined with a suitable liquid medium to produce a catalyst-containing printable ink. The drying step may, for example, be carried out at a temperature of 50°C to 120°C.

[0032] Alternatively, the method according to the first aspect of the invention preferably includes a step of heat treatment of the product of step (iii) or step (iv). In the case of heat treatment of the product of step (iii) (e.g., "wet filter cake"), the heat treatment step performs the dual function of removing excess water from the product and heat treating the product.

[0033] The applicant has discovered that a heat treatment step can improve the durability of OER catalysts with a moderate reduction in electrochemical activity. In this regard, the preferred temperature range for heat treatment is 150°C to 400°C. In tests conducted using a particulate TiO2 support (doped), the applicant found that when the heat treatment temperature increased above 400°C, for example, 500°C, the electrical conductivity of the OER powder and the corresponding electrocatalytic catalyst activity decreased. This observation appears to be associated with a significant decrease in the atomic percentage ratio of surface Ir:Ti, as determined by X-ray photoelectron spectroscopy (XPS), an increase in the crystallinity of iridium oxide, as determined by XRD, and an increase in iridium aggregation, as observed by transmission electron microscopy (TEM)-energy dispersive X-ray (EDX) spectroscopy.

[0034] Unwilling to be bound by theory, the applicant's initial analytical tests indicated that the supported amorphous iridium oxide material was more electrocatalytically active than the corresponding crystalline material. However, the crystalline material was more durable. Therefore, depending on the catalyst's end application, consumers may want to specify a higher level of catalyst durability at the expense of moderately lower "fresh" (i.e., at manufacturing) electrocatalytic activity, and vice versa.

[0035] Therefore, if a higher level of durability is specified – and thus a relatively higher level of crystallinity is specified, a higher initial loading of ionic iridium oxide is required to compensate for the moderately lower “fresh” electrocatalytic activity of the crystalline material relative to the more amorphous catalyst, for the same level of electrocatalytic activity.

[0036] In this regard, in order to achieve the selected level of electrocatalytic activity in the final product, the total iridium content in the aqueous mixture formed in step (ii) is 1 g. Ir / L to 22g Ir / L, preferably 1g Ir / L to 20g Ir / L or most preferably 1g Ir / L to 10g Ir / L. A diluter system is preferred because the expensive iridium component can be used more sustainably and efficiently to achieve similar or better activity.

[0037] The iridium loading in the final product of the method of the first aspect of the invention can be defined by the weight ratio of the particulate solid support to the iridium oxide material. In this respect, the weight percentage of iridium can be from 5% to 70% by weight of the total weight of the OER catalyst, preferably from 8% to 50% by weight.

[0038] One variable for appropriately selecting the total iridium content in the final product can be the specific surface area (SSA) of the particulate solid support prior to the deposition of the iridium material, as determined by nitrogen BET (Brunauer, Emmett, and Teller) methods. In this regard, the applicant's preliminary development review indicates that support materials with relatively low specific surface areas are preferred, as this helps to promote contact with the dispersed iridium material and the conductivity between the material and the dispersed iridium material at lower total iridium loadings, thereby enhancing electrocatalytic activity.

[0039] Therefore, the N2 BET specific surface area of ​​the particulate solid carrier used in conjunction with this application in this invention is preferably <10 m². 2 / g.

[0040] Depending on the application of the OER catalyst end product, the particulate solid support can be a transition metal oxide; or carbon, optionally having the appropriate N2 BET specific surface area mentioned above. When the particulate solid support is a transition metal oxide, it is selected from optionally doped TiO2, optionally doped ZrO2, CeO2 / ZrO2 mixed oxides, Ta2O5, Nb2O5, Al2O3, SnO2, SnO2 optionally doped with antimony or fluorine, and mixtures of any two or more of these. The application environment for OER catalysts is typically strongly acidic, for example, pH 1.0. Therefore, a support with greater acid stability is preferred. In this regard, Al2O3 is less preferred even when doped with, for example, silica, because it lacks long-term durability in strongly acidic aqueous OER environments. More preferred support materials are optionally doped TiO2, Nb2O5, optionally doped ZrO2, and SnO2, and even more preferred support materials are optionally doped TiO2, Nb2O5, and optionally doped ZrO2, with optionally doped TiO2 being the most preferred. Optional dopants for TiO2 include silicon dioxide and / or tungsten.

[0041] It should be understood that halide determination methods include inductively coupled plasma (ICP)-optical emission spectrometry (OES) and ICP-mass spectrometry (MS), which report the halide (e.g., chloride) content of both the supported iridium material and the carrier material. While the reagents chosen for steps (i) and (ii) can be halide-free, such that the precipitated iridium oxide is also halide-free, acid-free particulate carriers can use halide-containing compounds in their manufacture. For example, preferred titanium dioxide carriers can be manufactured via a chloride process, wherein titanium compounds in titanium-containing ore are converted to titanium tetrachloride (TiCl4), which is readily purified and converted to dioxide. Although any residual chloride in the carrier is understood to be bound and not to interact with the supported iridium oxide, preferred carriers have low chloride content such that the upper limit of chloride content detected by ICP-OES for the product of the invention itself, whether obtained via the methods of the first and second aspects of the invention, is a maximum of 0.05 ppm by weight.

[0042] In this regard, to avoid introducing halides into the supported iridium oxide material, an inorganic acid free of halides is used, preferably nitric acid or sulfuric acid, to appropriately lower the pH in step (ii). It is further preferred that nitric acid is used to lower the pH in step (ii).

[0043] The method according to the first aspect of the invention may include a subsequent step of incorporating a catalyst into a catalyst layer or a catalyst-coated membrane (CCM), such as a catalyst-coated proton exchange membrane (PEM) or a catalyst-coated anion exchange membrane (AEM), for example, in the form of a printable ink as described above. When the final application is as a CCM, the method may include a subsequent step of incorporating the CCM into a fuel cell or a water electrolyzer.

[0044] Depending on the application, the catalyst-coated membrane (CCM) may contain an OER catalyst according to the second or third aspect of the invention. In a preferred application, a fuel cell or water electrolyzer contains a CCM.

[0045] The applicant believes that the physical differences between the OER catalyst comprising the product of the first aspect of the present invention and the catalyst of the second aspect, and the prior art prepared using a chloride precursor at alkaline pH, may lie in one or more of the following definitions:

[0046] (i) Where the particulate solid support is a transition metal (TM) oxide (such as TiO2), the ratio of iridium atoms to transition metal atoms (Ir:TM) at the surface of the catalyst, such as the ratio of iridium atoms to titanium atoms, is at least 1.0, such as in the range of 1.0 to 3.0, or 1.0 to 2.0, and including 1.0 and 3.0 or 1.0 and 2.0, as determined by XPS analysis (especially as defined in Example 5 below). An atomic percentage ratio greater than 1.0 indicates that the method provides a catalyst in which the iridium oxide component is uniformly distributed on the surface of the particulate solid support;

[0047] (ii) The OER catalyst as a whole has a halide content of less than 500 ppm as determined by ICP-OES (see Example 5 below), preferably less than 400 ppm, less than 300 ppm or less than 250 ppm;

[0048] (iii) The mass electrocatalytic activity (j mass) at 1.47 volts in the button cell test (especially as defined in Example 5 below) is >80 A / g IrO. x ;and

[0049] (iv) The powder conductivity of the catalyst (especially as defined in Example 5 below) is >0.1 S / cm at 0.3 MPa, and optionally >10.0 S / cm at 44.5 MPa.

[0050] Specifically, the results presented in Example 6 below indicate that the ratio of iridium atom % to transition metal atom % at the surface of the catalyst and the halide content at (ii) appear to be important and define the characteristics of the product itself. This indicates that the iridium oxide component is uniformly distributed and the low chloride content provides stability benefits.

[0051] Definitions

[0052] As used herein, we mean by "halide-free" a compound without intentionally added halides, i.e., a compound that does not contain halides in its chemical formula, and we specifically aim to exclude chlorides. Defining halide-free does not exclude compounds that may contain trace halide impurities.

[0053] As used herein, we mean by "halide-free alkali iridate" a compound that contains iridium-containing oxygen ions and metal counterions and does not contain intentionally added halides. Preferably, the halide-free alkali iridate is sodium iridate. Preferably, the halide content of the halide-free alkali iridate is less than 100 ppm, or more preferably, less than 50 ppm or less than 10 ppm. Such materials are suitably provided by heat treatment of a mixture of halide-free iridium powder and halide-free peroxide salts (such as Group 1 peroxide salts, preferably sodium peroxide).

[0054] The term "powder" as used in connection with iridium powder is intended to cover both spherical powders and irregular powders, such as iridium sponges.

[0055] The term "iridium oxide material" as used herein is intended to include a range of amorphous and crystalline oxidation compositions, including but not limited to iridium(IV) oxide (IrO2), iridium(III) oxide (Ir2O3), and hydroxyiridium oxide. Hydroxyiridium oxide is an iridium compound having both oxo (Ir=O) and hydroxy (Ir-OH) functional groups and may have a composition, for example, that can be represented by the formula: IrOx(OH)y, where 1 ≤ x < 2 and 0 < y ≤ 2, and 3 ≤ 2x + y < 4. The catalyst produced by the method of the present invention has an oxygen content higher than that expected for pure IrO2. There are some indications that in addition to IrO2 and / or Ir2O3, the material also contains at least some IrO(OH)2. However, it should be understood that due to its preferred amorphous state, the applicant cannot more specifically define the iridium material present without unduly limiting the scope of the applicant's claims.

[0056] Examples

[0057] The following examples are provided by way of illustration only in order that the invention may be more fully understood.

[0058] Example 1 (According to the Invention)

[0059] 10 g of iridium sponge was mixed with 30 g of Na₂O₂ beads at 60 Hz for 30 seconds using an acoustic resonance mixer. The resulting powder mixture was transferred to an alumina crucible and calcined at 500 °C at a heating rate of 5 °C / min for 1 hour. The molten product was dissolved in 1 L of deionized (DI) water by stirring to form an iridium salt solution. Separately, 10 g of aeroxide was mixed using a Silverson mixer. ® P25 TiO2 (Evonik) was suspended in 0.5 L DI H2O. The TiO2 slurry was stirred into the iridium salt solution. The combined mixture was then stirred for another 30 minutes.

[0060] While stirring, concentrated nitric acid (HNO3) was added dropwise, and the target pH of <4.0 was maintained for 1 hour. The resulting solid was then collected by filtration and washed with DI H2O to achieve a conductivity of less than 50 µS / cm in the wash water. The separated solid was then air-dried at 150°C.

[0061] Example 2 (according to the present invention)

[0062] Using a Silverson mixer equipped with a 3 / 8-inch mixing head and at 10,000 rpm, mix 10.42g of solid particles of Aeroxide. ® P25 TiO2 (Evonik) was dispersed in 250 ml of DI water. The head and beaker of the Silverson mixer were washed with 183.3 ml of DI water, and the mixture and washings were transferred to the reaction vessel.

[0063] A Na₂O₂ iridium melt was prepared by combining iridium powder (300 g, 400 mesh, corresponding to a particle size below 23 μm) with 900 g of sodium peroxide and coarsely grinding the mixture until thoroughly mixed. The mixture was transferred to a nickel crucible and heated by a natural gas flame (temperature approximately 500 °C) while stirring with a nickel rod to prevent agglomeration. Heating was stopped once the mixture showed obvious signs of red-hotness, and the mixture was allowed to cool for 60 to 90 minutes.

[0064] 40 g of molten Na₂O₂ iridium product was added to the TiO₂ suspension while stirring (500 RPM) over a 10-minute period. The mixture was stirred for 30 minutes to ensure complete mixing. Concentrated nitric acid (HNO₃) was added dropwise to the stirred mixture. Once the mixture reached pH 3.25, the solution was maintained at pH 3.25 for 1 hour, with additional nitric acid added if necessary. The total mass of concentrated HNO₃ added was 61.76 g. After one hour, 1.5 L of DI water was added to the reaction mixture while stirring at 850 RPM for 5 minutes, followed by sedimentation. The blue solid product was collected by filtration using a three-piece funnel (100 mm) and 542 Whatman filter paper. The collected product was washed with DI water until the conductivity of the filtrate was measured to be less than 50 µS / cm. The precipitate was air-dried in an oven at 120 °C for 16 hours.

[0065] Example 3 (Comparison - corresponding to the method of Example 1 in US Patent Publication US 2022 / 0259750 A1)

[0066] At room temperature, 10 g of IrCl4·H2O (calculated as 56.5% by weight of Ir metal, i.e., Ir(0)) was dissolved in 221 ml of DI water with stirring at 500 RPM. 4.83 g of solid particles of Aeroxide were dissolved using a Silverson mixer equipped with a 3 / 8-inch mixing head and stirred at 10000 RPM for 15 minutes. ® P25 TiO2 (Evonik) was dispersed in 50 mL of DI water. The TiO2 suspension was added to the IrCl4 solution in a Silverson mixer, and then the contents of the Silverson mixer were transferred to a reaction vessel. The Silverson head and beaker were washed with 50 mL of DI water, and the washings were added to the reaction vessel. The transferred mixture was stirred in the reaction vessel for 30 minutes, and then the pH of the mixture was adjusted to 9.7 by dropwise addition of 1 M NaOH solution and maintained at this pH for 30 minutes with stirring. The aqueous medium was then heated to 70 °C, and the pH was further adjusted to 11 by dropwise addition of 1 M NaOH solution. The mixture was maintained at 70 °C with stirring overnight and then allowed to cool. The solid product was collected by filtration using a three-piece funnel apparatus (100 mm) and 542 Whatman filter paper. The collected product was washed with DI water until the conductivity of the filtrate was measured to be less than 50 µS / cm. The precipitate was air-dried in an oven at 120 °C for 16 hours.

[0067] Example 4 (Comparison)

[0068] Unsupported, i.e. TiO2-free electrochemical-grade iridium oxide purchased from J&J Materials Inc. was used as a comparative catalyst.

[0069] Example 5 - Details of Product Characterization

[0070] The products of Examples 2, 3, and 4 are characterized below using various methods:

[0071] Iridium and Titanium Determination

[0072] The weight percent (%) of iridium and titanium in the sample was determined using a known inductively coupled plasma optical emission spectrometry (ICP-OES) technique (see, for example...). https: / / en.wikipedia.org / wiki / Inductively_coupled_ plasma_atomic_emission_spectroscopy ;and https: / / www.thermofisher.com / uk / en / home / industrial / spectroscopy-elemental-isotope-analysis / spectroscopy- elemental-isotope-analysis-learning-center / trace-elemental-analysis-tea- information / icp-oes-information.html Assume that all reported iridium weight % values ​​are “IrOx” and that the IrOx composition is 80 wt% iridium metal (Ir(0)).

[0073] Chloride determination

[0074] The elemental chlorine content (ppm) of a sample was determined using known liquid ion chromatography techniques (see, for example...). https: / / en.wikipedia.org / wiki / Ion_chromatography Ion chromatography is coupled with a "digestion system," namely an "automated rapid furnace" (AQF), to separate halogens, and the separated halogens are determined using ion chromatography techniques.

[0075] XPS

[0076] X-ray photoelectron spectroscopy (XPS) analysis was performed using a Thermo Fischer Scientific NEXSA spectrometer (Al Ka ​​X-ray source), and the spectra were analyzed using Thermo Fischer Scientific's proprietary Thermo Avantage software.

[0077] The surface ratio of iridium atoms to titanium atoms was calculated from iridium atoms and titanium atoms. A lower surface ratio indicates a greater dispersion of iridium on the catalyst support. The atomic percentage values ​​were determined by XPS analysis using Thermo Avantage software on titanium 2p spectra (background range: 475 eV to 450 eV, dominant Ti2p3 / 2 signal at 459 eV) and iridium 4d3 / 2 spectra (background range: 305 eV to 325 eV, signal at 313.7 eV) and the Schofield factor.

[0078] The surface ratio of iridium atoms (%) to zirconium or niobium atoms (%) can be calculated from iridium atoms (%) and zirconium or niobium atoms (%). This was achieved using Thermo Avantage software for zirconium 3p... 3 / 2Spectrum (background range: 325 eV to 340 eV, mainly Zr3p) 3 / 2 The signal is at 332.7 eV) or the niobium 3d spectrum (background range: 202 eV to 214 eV, mainly Nb3d). 5 / 2 The atomic percentage (%) was determined by XPS analysis of the iridium 4d3 / 2 spectrum (background range: 305 eV to 325 eV, signal at 313.7 eV) and the Schofield factor.

[0079] Powder conductivity

[0080] The electrical conductivity of powdered samples was measured at ambient temperature using an NH Instruments PD 600 powder measurement system. For measurement, the powdered sample was forced into the cylindrical chamber of the measuring head at a predetermined pressure. The conductivity was then measured as the applied pressure increased until a substantially constant value was obtained. The maximum applied pressure for each sample was 45 MPa.

[0081] Button battery test

[0082] Ink was prepared by combining 0.1 g of the sample from Example 2, Comparative Example 3, or Comparative Example 4 with an aqueous solution of Nafion (11.92 wt% solids, 1 mL) to produce an ink containing 110 wt% Nafion relative to the sample. The ink was then shear-mixed for 5 minutes at 3000 rpm using 5 mm diameter yttrium-stabilized zirconia beads in a planetary mixer. The ink was manually stirred to break up any deposits, at which point it was mixed again in the planetary mixer for 5 minutes, and these steps were repeated for a total grinding time of 15 minutes.

[0083] The prepared ink was prepared at 0.2 mg cm -2 The loading was sprayed onto Toray paper (hydrophobic gas diffusion layer 60) and verified using X-ray fluorescence (XRF) measurements. The coin cells were then immersed overnight in a solution (1M H₂SO₄) equivalent to the test solution under vacuum to allow the solution to penetrate the gas diffusion layer, ensuring all catalysts were in contact with the electrolyte.

[0084] The button cell was then placed in an electrochemical cell containing 1M H₂SO₄, purged with nitrogen (liquid nitrogen exhaust gas), and maintained at 60°C. A reversible hydrogen electrode (RHE) (hydrogen bubbling over a Pt / C catalyst) and a Pt wire were used as the reference and counter electrodes, respectively. First, scans were performed at different rates (5 mV s⁻¹). -1 Up to 300mV s -1 The battery was cycled between 0V and 1.35V relative to RHE, and then at 1mV s. -1An activity scan was performed between 1V and 1.55V relative to RHE.

[0085] The activity data were corrected for iR by taking the high-frequency intercept of the impedance scan measured relative to RHE at 1.45 V; typical values ​​are between 0.2 Ω and 0.35 Ω. The ruthenium loading was measured by XRF, and the activity data were normalized to mass activity by assuming a ratio to the EDXA results. Degradation was monitored by taking a 1 mL sample of electrolyte solution for inductively coupled plasma mass spectrometry (ICP-MS) analysis at the start of the test and then after the early-life (BOL) activity test (referred to as “First Activity Test” in Table 1 below). 1 mL of sample was diluted with water to 1 v / v H₂SO₄, and then 1 v / v HCl was added. These samples were injected directly into the ICP-MS to obtain the concentration of the metal leached into the solution.

[0086] Reported mass electrocatalytic activity (j) at 1.47 volts 质量 Results (Ampere / g IrO) x ).

[0087] Example 6 - Test Results

[0088] The test results of the characterization method in Example 5 are listed in Table 1 below.

[0089] Table 1

[0090]

[0091] solid particles Aeroxide ® The chloride content of the P25 TiO2 (Evonik) feedstock was determined to be 1100 ppm and 58.5% by weight of elemental titanium. The chloride was produced by the TiO2 manufacturing process (chlorination method).

[0092] *Theoretical weight % loading based on the amount of iridium added. Calculations assume all Ir is present as IrOx, and that the IrOx composition is 80 weight % Ir.

[0093] †The chloride content of the samples is due to the dilution effect of the presence of IrOx relative to TiO2 alone. The chloride content of the product of Example 2 is significantly lower than that of the product of Example 3. It will be appreciated that the presence of chloride in the sample of Example 3, despite extensive washing, i.e., conductivity <50 µS / cm in the washed water, is likely due to the combination of IrCl4·H2O and TiO2 used, while the chloride present in the sample of Example 2 is entirely contributed by TiO2.

[0094] The results in Table 1 show that the products of the method according to the present invention have lower chloride content and lower iridium dissolution compared to the products of methods from the prior art.

[0095] It can be seen that the product of Example 2 (according to the present invention) has higher electrochemical activity than the unsupported iridium oxide (comparative example) of Example 4, but lower activity than the supported product prepared using the IrCl4·H2O precursor (Example 3 (comparative example)). While the method for improving the electrochemical activity of the supported product of the present invention continues, it is noted that the product of Example 3 (comparative example) contains significantly more chloride than the product of Example 2 (according to the present invention). Dissolution data show that the product of Example 2 is more stable.

[0096] The XRD pattern of materials produced by this method and dried at 150°C (such as in Example 2) indicates the formation of amorphous iridium oxide (believed to include some hydroxyl and oxide functional groups). The XRD pattern of materials produced by this method and heat-treated at 400°C indicates the formation of iridium oxide (IrO2).

[0097] To avoid any doubt, the full contents of all references identified in this specification are incorporated herein by reference.

Claims

1. A method for preparing an oxygen evolution reaction (OER) catalyst, said catalyst comprising an iridium oxide component supported on a particulate solid support, said method comprising the following steps: (i) Forming an aqueous mixture comprising a particulate solid support and a solution of a metal iridium salt without halides; (ii) The pH of the aqueous mixture is lowered to ≤5.0 to allow the iridium oxide component to precipitate onto the particulate solid carrier; as well as (iii) Separate the product of step (ii).

2. The method of claim 1, wherein the halide-free metal iridium salt used in step (i) is obtained by the following steps: (a) combining halide-free iridium powder and halide-free peroxide salt to produce a powder mixture; and (b) heat-treating the powder mixture.

3. The method according to claim 1 or claim 2, wherein step (i) comprises the following sub-steps: (i)(a) dissolving a solid, halide-free metal iridium salt in water to produce a solution; and (i)(b) Add the particulate solid carrier to the solution.

4. The method according to claim 1, 2 or 3, wherein the method comprises (iv) drying the product of step (ii) separated in step (iii) to remove water therefrom.

5. The method according to any one of the preceding claims, the method comprising the step of heat treating the product of step (iii) or step (iv).

6. The method of claim 5, wherein the heat treatment step is performed in the range of 150°C to 400°C and includes temperatures of 150°C and 400°C.

7. The method according to claim 2 and any one of claims 3 to 6 when dependent on claim 2, wherein the halide-free peroxide salt is sodium peroxide.

8. The method according to claim 2 or any one of claims 3 to 7 when dependent on claim 2, wherein the halide-free iridium powder and the halide-free peroxide salt are combined in a molar ratio of 1:4 to 1:

10.

9. The method according to any one of the preceding claims, wherein the total iridium content in the aqueous mixture formed in step (i) is 1g. Ir / L to 22g Ir / L.

10. The method according to any one of the preceding claims, wherein the weight percentage of iridium is from 5% to 70% by weight, preferably from 8% to 50% by weight, of the total weight of the OER catalyst.

11. The method according to any one of the preceding claims, wherein nitric acid or sulfuric acid is used in step (ii) to lower the pH.

12. The method according to any one of the preceding claims, wherein in step (ii), the pH is reduced to less than 4.

13. The method according to any one of the preceding claims, wherein the particulate solid support is a transition metal oxide.

14. The method according to any one of the preceding claims, wherein the particulate solid support is selected from optionally doped TiO2, optionally doped ZrO2, CeO2 / ZrO2 mixed oxide, Ta2O5, Nb2O5, Al2O3, SnO2, SnO2 optionally doped with antimony or fluorine, and mixtures of any two or more thereof, preferably TiO2.

15. The method according to any one of claims 1 to 12, wherein the particulate carrier is carbon.

16. The method according to claim 13, 14 or 15, wherein the N2 BET specific surface area of ​​the particulate solid carrier added in step (i) is <10 m². 2 / g.

17. The method according to any one of the preceding claims, the method comprising the subsequent step of incorporating the catalyst into a catalyst layer.

18. The method according to any one of claims 1 to 16, the method comprising the subsequent step of incorporating the catalyst into a catalyst-coated membrane (CCM).

19. The method of claim 18, the method comprising the subsequent step of incorporating the catalyst-coated membrane into a fuel cell or a water electrolyzer.

20. An oxygen evolution reaction (OER) catalyst, said catalyst comprising an iridium oxide component supported on a particulate solid support, wherein: (i) The OER catalyst as a whole has a halide content of less than 500 ppm as determined by ICP-OES; (ii) The particulate solid support is a transition metal (TM) oxide, and as determined by XPS analysis, the ratio of iridium atoms to transition metal atoms (Ir:TM) at the surface of the catalyst is at least 1.0, such as in the range of 1.0 to 3.0 and including 1.0 and 3.

0.

21. An oxygen evolution reaction (OER) catalyst that is obtained or can be obtained by the method according to any one of the preceding claims.

22. The oxygen evolution reaction (OER) catalyst according to claim 21, wherein the OER catalyst as a whole has a halide content of less than 500 ppm, preferably less than 400 ppm, less than 300 ppm or less than 250 ppm as determined by ICP-OES.

23. The oxygen evolution reaction (OER) catalyst according to claim 21 or claim 22, wherein the particulate solid support is a transition metal (TM) oxide, and the ratio of iridium atoms to transition metal atoms (Ir:TM) at the surface of the catalyst, as determined by XPS analysis, is at least 1.0, such as in the range of 1.0 to 3.0 and including 1.0 and 3.

0.

24. A catalyst-coated membrane comprising an oxygen evolution reaction catalyst according to any one of claims 20 to 23.

25. A fuel cell or water electrolyzer comprising a catalyst-coated membrane according to claim 24.

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