CATALYST FOR OXYGEN GENERATION REACTION DURING WATER ELECTROLYSIS

Abstract

The invention relates to a method for preparing a catalyst composition, wherein in an aqueous medium containing an iridium compound, at a pH 9, an iridium-containing solid is deposited on a support material, and the support material loaded with the iridium-containing solid is separated from the aqueous medium and dried, wherein, in the method, the support material loaded with the iridium-containing solid is not subjected to a thermal treatment at a temperature of more than 250° C. for a period of time of longer than 1 hour.

Claims

1. A method for preparing a catalyst composition, the method comprising: using an aqueous medium containing an iridium compound, depositing an iridium-containing solid on a support material at a pH ≥9; separating the support material loaded with the iridium-containing solid from the aqueous medium; and drying the support material loaded with the iridium-containing solid, wherein, in the method, the support material loaded with the iridium-containing solid is not subjected to any thermal treatment at a temperature of more than 250° C. for a period of more than 1 hour.

2. The method 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 carbon material; and/or the support material has a specific BET surface area of less than 100 m.sup.2/g.

3. The method of claim 2, wherein the transition metal oxide is a titanium oxide, a zirconium oxide, a niobium oxide, a tantalum oxide, or a cerium oxide.

4. The method of claim 2, wherein the carbon material functioning as support material has a degree of graphitization of at least 60%.

5. The method of claim 1, wherein the iridium compound present in the aqueous medium is an iridium(IV) compound or an iridium(III) compound.

6. The method of claim 5, wherein the iridium(IV) or iridium(III) compound is a salt.

7. The method of claim 1, wherein the aqueous medium for the deposition of the iridium-containing solid on the support material has a pH ≥10, more preferably ≥11; and/or the temperature of the aqueous medium for the deposition of the iridium-containing solid on the support material is from 40° C. to 100° C.

8. The method of claim 1, wherein, in the method, the support material loaded with the iridium-containing solid is not subjected to any thermal treatment at a temperature of more than 200° C. for a time period of more than 30 minutes.

9. The method of claim 1, wherein the support material loaded with the iridium-containing solid is dried at a temperature of at most 200° C. and is not subjected to any further thermal treatment after drying.

10. The method of claim 1, wherein the iridium-containing solid obtained after drying is an iridium hydroxide oxide.

11. The method of claim 10, wherein the iridium hydroxide oxide-loaded support material contains iridium in a proportion of at most 50 wt. %; and/or the iridium hydroxide oxide present on the support material has an atomic ratio of iridium(IV) to iridium(III), as determined by X-ray photoelectron spectroscopy (XPS), of at most 1.7/1.0.

12. The method of claim 1, wherein, after drying, the support material loaded with the iridium-containing solid is dispersed in a liquid medium to obtain a catalyst-containing ink.

13. A catalyst composition prepared by a method according to claim 1.

14. An electrolysis cell comprising a catalyst composition according to claim 13.

15. A fuel cell comprising a catalyst composition according to claim 13.

16. A method of preparing oxygen, the method comprising using a catalyst composition according to claim 13 as a catalyst for the an oxygen evolution reaction in water electrolysis.

17. The method of claim 6, wherein the salt is an iridium halide, a salt whose anion is a chloro complex IrCl.sub.6.sup.2−, an iridium nitrate, an iridium acetate, or an acid containing iridium.

18. The electrolysis cell of claim 14, wherein the electrolysis cell is a polymer electrolyte membrane (PEM) electrolysis cell.

19. The fuel cell of claim 15, wherein the fuel cell is a polymer electrolyte membrane (PEM) fuel cell.

Description

EXAMPLES

Example 1 According to the Invention (EB1) and Comparative Examples 1-3 (VB1-VB3)

[0079] In EB1 and VB1-VB3, the same support material was used, namely a TiO.sub.2 having a specific BET surface area of 60 m.sup.2/g.

[0080] Furthermore, in EB1 and VB1-VB3, a wet-chemical deposition of an iridium-containing solid precipitated at alkaline pH value onto the TiO.sub.2 support material took place in the same way. This wet-chemical deposition was carried out as follows:

[0081] 124.56 g of iridium(IV) chloride (IrCl.sub.4 hydrate, Heraeus Deutschland GmbH & Co. KG) was dissolved in 4000 mL of water at room temperature. Next, 60.17 g TiO.sub.2 (P25, Evonik, BET surface area: 60 m2/g) was added. The pH was adjusted to 9.7 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 and washed.

[0082] After separation of the support material loaded with the iridium-containing solid from the aqueous deposition medium, different thermal treatments took place in EB1 and VB1-VB3:

[0083] The loaded support materials of examples EB1 and VB1-VB3 each had an iridium content of about 45 wt %.

[0084] EB1: The loaded support material was dried overnight at 120° C. The XPS analysis showed that the dried iridium-containing solid present on the carrier is an iridium hydroxide oxide.

[0085] VB1: The loaded support material was heated at 300° C. for 4 hours.

[0086] VB2: The loaded support material was heated at 360° C. for 4 hours.

[0087] VB3: The loaded support material was heated at 400° C. for 4 hours.

[0088] For the catalyst compositions obtained in EB1 and VB1-VB3, the electrochemical activity (in mA per mg of iridium) was determined with regard to the oxygen evolution reaction during water electrolysis.

[0089] The activity was determined in electrochemical measurements of the rotating disk electrode.

[0090] This electrochemical characterization took place in a three-electrode set-up with a graphite rod as counter-electrode, a saturated calomel electrode as reference electrode (all potentials were converted to the RHE) and the catalyst materials in examples EB1 and VB1-VB3, which were each present as a thin film on a glassy carbon substrate as working electrode (loading level: 100 μg/cm2). The measurements took place at 60° in 0.5 M H2SO4 in air at a rotational speed of 1600 rpm. Taking hysteresis into account, the activity values were determined from anodic and cathodic potential run at 1.5 V.sub.RHE.

[0091] Table 1 shows the results of these measurements.

TABLE-US-00001 TABLE 1 Electrochemical activity Thermal treatment of the Electrochemical loaded support material activity [mA/g Ir] Inventive Example EB1 120° C. 660 Comparative Example VB1 300° C. 470 Comparative Example VB2 360° C. 420 Comparative Example VB3 400° C. 310

[0092] The results show that the support material loaded with the iridium-containing solid has a very high level of electrochemical activity when thermal treatment at high temperature is avoided. Thus, if the loaded support material is dried at a moderate temperature and subsequent calcination of the material at a high temperature is dispensed with, this material will exhibit a high level of catalytic activity in the oxygen evolution reaction under acidic conditions and will therefore be very well suited as a catalyst in the anodic side of a PEM water electrolysis cell or a PEM fuel cell.

Inventive Example 2 (EB2)

[0093] In EB2, a porous carbon material (Porocarb®, Heraeus) was used as support material instead of TiO.sub.2. The ratios between the iridium starting compound and the support material were selected so as to result in a loaded support material having an iridium content of approximately 30 wt. %. Apart from that, the iridium-containing solid was deposited on the support material under the same conditions as in EB1.

[0094] After separation from the aqueous deposition medium, the loaded support material was dried at 120° C. There was subsequent thermal treatment at higher temperatures. XPS analysis shows that the dried iridium-containing solid present on the carrier is an iridium hydroxide oxide.

[0095] The carbon material, loaded with iridium hydroxide oxide, and dried at 120° C. was subjected to the electrochemical activity measurement described above. An activity of 625 mA/g iridium was measured. Thus, even when using a carbon-based support material instead of an oxidic support material, a very high level of electrochemical activity with regard to the oxygen evolution reaction is exhibited if a thermal aftertreatment at high temperature is dispensed with.