IRIDIUM-CONTAINING CATALYST FOR WATER ELECTROLYSIS

Abstract

The invention relates to a particulate catalyst, containing: a support material, an iridium-containing coating which is provided on the support material and which contains iridium oxide, an iridium hydroxide, or an iridium hydroxide oxide, wherein the support material has a BET surface area ranging from 2 m.sup.2/g to 50 m.sup.2/g, and the iridium content of the catalyst satisfies the following condition: (1.505 (g/m.sup.2)BET)/(1+0.0176 (g/m.sup.2)BET)Ir-G(4.012 (g/m.sup.2)BET)/(1+0.0468 (g/m.sup.2)BET), where BET is the BET surface area of the support material, in m.sup.2/g, and Ir-G is the iridium content, in wt. %, of the catalyst.

Claims

1. A particulate catalyst, comprising: a support material; and, an iridium-containing coating which is provided on the support material and which contains an iridium oxide, an iridium hydroxide, or an iridium hydroxide oxide, wherein the support material comprises a BET surface area in the range from 2 m.sup.2/g to 50 m.sup.2/g; and, the iridium content of the catalyst satisfies the following condition:
(1.505 (g/m.sup.2)BET)/(1+0.0176 (g/m.sup.2)BET)Ir-G(4.012 (g/m.sup.2)BET)/(1+0.0468 (g/m.sup.2)(BET), where: BET is the BET surface area, in m.sup.2/g, of the support material; and, Ir-G is the iridium content, in % by weight, of the catalyst.

2. A particulate catalyst, comprising: a support material; and, an iridium-containing coating which is provided on the support material; and which: contains an iridium oxide, an iridium hydroxide or an iridium hydroxide oxide or a mixture of at least two of these iridium compounds; and, has an average layer thickness in the range from 1.5 nm to 5.0 nm, wherein the catalyst comprises an iridium content of at most 50% by weight.

3. The particulate catalyst according to claim 1, wherein the iridium content of the catalyst is at most 40% by weight, more preferably at most 35% by weight.

4. The particulate catalyst according to claim 1, wherein the BET surface area of the support material is 2 m.sup.2/g to 40 m.sup.2/g, more preferably 2 m.sup.2/g to <10 m.sup.2/g, even more preferably 2 m.sup.2/g to 9 m.sup.2/g.

5. A particulate catalyst, comprising: a support material that comprises a BET surface area in the range from 2 m.sup.2/g to <10 m.sup.2/g, more preferably 2 m.sup.2/g to 9 m.sup.2/g; and, an iridium-containing coating which is provided on the support material and which contains: an iridium oxide, an iridium hydroxide, or an iridium hydroxide oxide, or a mixture of at least two of these iridium compounds, wherein the catalyst comprises an iridium content of 5% by weight to 20% by weight, more preferably 5% by weight to 14% by weight.

6. The particulate catalyst according to claim 1, wherein the iridium content of the catalyst satisfies the following condition:
(1.705 (g/m.sup.2)BET)/(1+0.0199 (g/m.sup.2)BET)Ir-G(3.511 (g/m.sup.2)BET)/(1+0.0410 (g/m.sup.2)(BET); where: BET is the BET surface area, in m.sup.2/g, of the support material; and, Ir-G is the iridium content, in % by weight, of the catalyst.

7. The particulate catalyst according to claim 1, wherein the average layer thickness of the iridium-containing coating is 1.5 nm to 4.0 nm, more preferably 1.7 nm to 3.5 nm.

8. The particulate catalyst according to claim 1, wherein the catalyst particles comprise a core-shell structure in which the support material forms the core and the iridium-containing coating forms the shell.

9. The particulate catalyst according to claim 1, wherein the iridium is exclusively present as iridium in the +3 oxidation state (iridium(III)) and/or as iridium in the +4 oxidation state (iridium(IV)).

10. The particulate catalyst according to claim 1, wherein the support material is an oxide of a transition metal, an oxide of a main group metal, SiO.sub.2 or a mixture of two or more of the aforementioned support materials.

11. The particulate catalyst according to claim 10, wherein the support material is a titanium oxide.

12. The particulate catalyst according to claim 1, wherein the catalyst has been subjected to thermal treatment at a temperature of more than 250 C., preferably >250 C. to 550 C., more preferably 300-450 C., even more preferably 300-380 C.; or the iridium-containing coating has an atomic ratio of iridium(IV) to iridium(III), determined by means of X-ray photoelectron spectroscopy (XPS), in the range from 1.9/1.0 to 4.7/1.0.

13. A method for producing the particulate catalyst according to claim 1, wherein an iridium-containing coating containing an iridium oxide, an iridium hydroxide or an iridium hydroxide oxide is deposited on a support material.

14. The method according to claim 13, wherein the coated support material is subjected to thermal treatment at a temperature of more than 250 C., preferably >250 C. to 550 C., more preferably 300-450 C., even more preferably 300-380 C.

15. A composition, containing the particulate catalyst according to claim 1; and, an ionomer, in particular a sulfonic acid group-containing ionomer.

16. A use of the particulate catalyst according to claim 1 as an anode for water electrolysis.

17. The particulate catalyst according to claim 2, wherein the iridium content of the catalyst is at most 40% by weight, more preferably at most 35% by weight.

18. A use of the particulate catalyst of the composition according to claim 15 as an anode for water electrolysis.

Description

EXAMPLES

Invention Example IE1

[0129] 27.80 g of iridium(IV) chloride (IrCl.sub.4 hydrate, Heraeus Deutschland GmbH & Co. KG) were dissolved in 4000 mL of water at room temperature. Next, 29.94 g of TiO.sub.2 (DT20, Tronox, BET surface area: 20 m.sup.2/g) were added. The pH was adjusted to 10.3 by addition of NaOH. The aqueous medium was heated to 70 C. and the pH was adjusted to 11. It was stirred overnight at 70 C. The pH was maintained at >11.0. The TiO.sub.2 support material loaded with the iridium-containing solid was filtered off, washed and dried. A one-hour thermal post-treatment was carried out at 350 C. in an oxygen-containing atmosphere. The catalyst comprises a core-shell structure. The XPS analysis showed that the iridium-containing coating present on the support material is an iridium hydroxide oxide. Ir(IV)/Ir(III) ratio: 4.0:1.0.

Invention Example IE2

[0130] 64.59 g of iridium(IV) chloride (IrCl.sub.4 hydrate, Heraeus Deutschland GmbH & Co. KG) were dissolved in 2500 mL of water at room temperature. Next, 53.15 g of TiO.sub.2 (DT30, Tronox, BET surface area: 30 m.sup.2/g) were added. The pH was adjusted to 9.0 by addition of NaOH. The aqueous medium was heated to 70 C. and the pH was adjusted to 9.2. It was stirred overnight at 70 C. The pH was maintained at >9.0. The TiO.sub.2 support material loaded with the iridium-containing solid was filtered off, washed and dried. A one-hour thermal post-treatment was carried out at 350 C. in an oxygen-containing atmosphere. The catalyst comprises a core-shell structure. The XPS analysis showed that the iridium-containing coating present on the support material is an iridium hydroxide oxide. Ir(IV)/Ir(III) ratio: 3.9:1.0.

Invention Example IE3

[0131] 9.23 g of iridium(IV) chloride (IrCl.sub.4 hydrate, Heraeus Deutschland GmbH & Co. KG) were dissolved in 1000 mL of water at room temperature. Next, 44.85 g of TiO.sub.2 (DT-X5, Tronox, BET surface area: 5 m.sup.2/g) were added. The pH was adjusted to 9.0 by addition of NaOH. The aqueous medium was heated to 70 C. and the pH was adjusted to 9.2. It was stirred overnight at 70 C. The pH was maintained at >9.0. The TiO.sub.2 support material loaded with the iridium-containing solid was filtered off, washed and dried. A one-hour thermal post-treatment was carried out at 350 C. in an oxygen-containing atmosphere. The catalyst comprises a core-shell structure. The XPS analysis showed that the iridium-containing coating present on the support material is an iridium hydroxide oxide. Ir(IV)/Ir(III) ratio: 4.6:1.0.

Comparative Example CE1

[0132] 48.35 g of iridium(IV) chloride (IrCI.sub.4 hydrate, Heraeus Deutschland GmbH & Co. KG) were dissolved in 4000 mL of water at room temperature. Next, 51.9 g of TiO.sub.2 (Activ G5, Evonik, BET surface area: 150 m.sup.2/g) were added. The pH was adjusted to 11.2 by addition of NaOH. The aqueous medium was heated to 70 C. and the pH was adjusted to >11. It was stirred overnight at 70 C. The pH was maintained at >11. The TiO.sub.2 support material loaded with the iridium-containing solid was filtered off, washed and dried. A one-hour thermal post-treatment was carried out at 350 C. in an oxygen-containing atmosphere. Isolated iridium-containing islands are present on the support material. The XPS analysis showed that the isolated iridium-containing islands present on the support material contain an iridium hydroxide oxide.

[0133] The iridium content of the catalysts and the average layer thicknesses of the iridium-containing coating present on the support material are summarized in table 1 below.

[0134] Table 2 summarizes the BET surface areas of the support materials. In addition, table 2 calculates, for each of the samples and based on the relevant BET surface area of the support material and using the relationship

(1.505 (g/m.sup.2)BET)/(1+0.0176 (g/m.sup.2)BET)Ir-G(4.012 (g/m.sup.2)BET)/(1+0.0468 (g/m.sup.2)BET), [0135] the iridium content range according to the claims. In the inventive samples IE1, IE2 and IE3, the BET surface areas of the support material and the iridium content of the catalyst are matched to one another such that the inventive relationship is satisfied. In the comparative material CE1, an iridium content is used which is too low with respect to the BET surface area of the support material.

TABLE-US-00001 TABLE 1 Iridium content of the catalysts and average thickness of the Ir-containing coatings Iridium content of the catalyst Average thickness of the Ir- Sample [% by weight] containing coating [nm] IE1 30 2.7 IE2 35 3.0 IE3 10 2.8 CE1 30 No coating, but only isolated iridium hydroxide oxide islands dispersed on the surface of the support material.

[0136] In samples IE1 to IE3, the relative standard deviation of the average thickness of the iridium-containing coating is at most 20%.

TABLE-US-00002 TABLE 2 BET surface area of the support materials and iridium content of the catalysts Range for iridium BET surface area content (in % by of the support Iridium content weight) given by the material of the catalyst relationship.sup.(1) Sample [m.sup.2/g] [% by weight] according to the claims. IE1 20 30 22-41 IE2 30 35 30-50 IE3 5 10 7-16 CE1 150 30 62-75 .sup.(1)(1.505 (g/m.sup.2) BET)/(1 + 0.0176 (g/m.sup.2) BET) Ir-G (4.012 (g/m.sup.2) BET)/(1 + 0.0468 (g/m.sup.2) BET)

[0137] Production of Coated Membrane and Determination of Activity

[0138] The catalyst materials produced in examples IE1, IE2, IE3 and CE1 were used for the production of coated membranes. To this end, the catalyst materials of examples IE1, IE2, IE3 and CE1 were dispersed in an ink and applied to a membrane containing a sulfonic acid group-containing fluorinated polymer, in order to form the anode.

[0139] The coating was achieved by what is referred to as a decal method of transferring PTFE transfer films onto the polymer membrane (Nafion 117, 178 m, Chemours). The coating of the PTFE film was carried out using a Mayer Bar coating machine. 5 cm.sup.2 decals were punched out of the dried layers and pressed onto the polymer membrane under pressure (2.5 MPa) and temperature (155 C.). The loading was determined by weighing the PTFEs before and after the transfer process.

[0140] For each of the coated membranes, the cell voltage was determined as a function of the current density.

[0141] The test procedures for IE1, IE2, IE3 and CE1 are identical and were carried out in an automated manner in an on-site measurement setup. The current-voltage management was controlled with a potentiostat and booster (Autolab PGSTAT302N and Booster 10A from Metrohm). After a warm-up phase and a conditioning step, galvanostatic polarization curves were recorded in the current density range of 0.01-2.00 A/cm.sup.2 at a cell temperature of 80 C.

[0142] At each current point, a cell voltage was determined which corresponds to an averaged value over a period of 10 seconds after equilibrium was set. FIG. 1 shows the measurement curves (cell voltage as a function of the current density for IE1, IE2, IE3 and CE1). FIG. 2 also shows an increase in the relevant range for IE1, IE2 and IE3 from FIG. 1. In parallel, the high-frequency resistance was determined by electrochemical impedance spectroscopy measurements at the specified current points, so that the cell resistance could be corrected (IR-free). These curves are not shown.

[0143] The results are summarized in table 3.

TABLE-US-00003 TABLE 3 Properties of the coated membranes Degree of iridium Sample used for the loading of the Activity at Activity at production of the anode [mg Ir/cm.sup.2 1.50 V 1.45 V.sub.IR-free coated membrane of membrane] [A/g of Ir] [A/g of Ir] IE1 0.25 764 399 IE2 0.25 764 440 IE3 0.29 641 345 CE1 0.27 Performance Performance not high not high enough to be enough to be measurable. measurable.

[0144] The results show that the catalyst according to the invention makes it possible to produce an anode which has a very low surface-based iridium loading (less than 0.30 mg of iridium per cm.sup.2 of coated membrane surface) and nevertheless has very high electrochemical activity.