HIGH-STABILITY CATALYST FOR AN ELECTROCHEMICAL CELL

Abstract

The present invention relates to a method for producing a catalyst for an electrochemical cell, wherein: a graphited porous carbon material is treated with an oxygen-containing plasma or an aqueous medium containing an oxidising agent, at least one noble metal compound is deposited on the treated carbon material, the impregnated carbon material is brought into contact with a reducing agent such that the noble metal compound is reduced to a metallic noble metal.

Claims

1. A method for producing a catalyst for an electrochemical cell, the method comprising: treating a graphitized, porous carbon material with an oxygen-containing plasma or an aqueous medium containing an oxidizing agent, depositing at least one noble metal compound on the treated carbon material such that an impregnated carbon material is obtained, and bringing the impregnated carbon material into contact with a reducing agent such that the noble metal compound is reduced to a metallic noble metal and a noble-metal-loaded carbon material is obtained.

2. The method of claim 1, wherein the graphitized, porous carbon material has a degree of graphitization of at least 60%, wherein the degree of graphitization g is determined based upon the following formula (1):
g=[(344 pm−d.sub.002)/(344 pm−335.4 pm)]×100  (1) where d.sub.002 is the graphite-basal plane distance determined by powder diffractometry on the basis of the diffraction reflection of the (002) plane; and/or the graphitized, porous carbon material has an La/Lc ratio of at least 0.15, determined by powder diffractometry, wherein La is the average crystallite size in the parallel direction and Lc is the average crystallite size in the perpendicular direction to the basal planes of the graphite structure.

3. The method of claim 1, wherein the graphitized, porous carbon material has a specific BET surface area, determined with nitrogen at 77 K in accordance with ISO 9277:2010, in the range of 5 m.sup.2/g to 200 m.sup.2/g and/or a pore volume, determined by mercury porosimetry in accordance with ISO 15901-1:2016, in the range of 0.7 cm.sup.3/g to 3.5 cm.sup.3/g.

4. The method of claim 1, wherein the graphitized, porous carbon material is obtained by converting a polyhydroxy compound into a carbonized carbon material via a carbonization and by graphitizing the carbonized carbon material.

5. The method of claim 4, wherein the polyhydroxy compound is a saccharide, a polyvinyl alcohol, or a phenolic resin.

6. The method of claim 4, wherein the graphitized, porous carbon material is obtained by a method comprising: impregnating a porous inorganic solid with the polyhydroxy compound, carbonizing the polyhydroxy compound present in the porous inorganic solid so that a carbonized carbon material is obtained, removing the inorganic solid, and graphitizing the carbonized carbon material exposed by the removal of the inorganic solid.

7. The method of claim 1, wherein a gas having an oxygen content of at least 20 vol % is used for the generation of the oxygen-containing plasma.

8. The method of claim 1, wherein the oxidizing agent present in the aqueous medium is HNO.sub.3, H.sub.2SO.sub.4, a permanganate, H.sub.2O.sub.2, or a mixture of at least two of the aforementioned oxidizing agents.

9. The method of claim 1, wherein the deposition of the noble metal compound on the treated carbon material occurs in an aqueous medium.

10. The method of claim 1, wherein the metallic noble metal is platinum, and the noble metal compound is a platinum(II) or platinum(IV) compound.

11. The method of claim 1, wherein the impregnated carbon material is brought into contact with the reducing agent in an aqueous medium; and/or wherein the reducing agent is formic acid, a metal borohydride, an alkali metal hydride, hydrogen (H.sub.2), a metal thiosulfate, an aldehyde, an alcohol, hydrazine, hydrazine hydrate, hydrazine hydrochloride, or ascorbic acid, or a mixture of at least two of these reducing agents.

12. The method of claim 1, wherein the noble-metal-loaded carbon material contains the noble metal in a quantity of 5-60 wt %.

13. Catalyst obtainable by the A catalyst produced by a method according to claim 1.

14. An electrochemical cell containing the catalyst of claim 13.

15. The electrochemical cell according of claim 14, wherein the electrochemical cell is a PEM fuel cell or a PEM electrolytic cell.

16. The method of claim 1, wherein a gas having an oxygen content of at least 95 vol % is used for the generation of the oxygen-containing plasma.

Description

EXAMPLES

Example 1 According to the Invention (EB1)

[0113] In EB1, a graphitized, porous carbon material was produced via nanocasting by a porous SiO.sub.2 template being impregnated with saccharose, the saccharose being carbonized, the SiO.sub.2 template being removed, and the carbonized carbon material being graphitized.

[0114] Treatment with Oxygen-Containing Plasma

[0115] The graphitized, porous carbon material was treated with an oxygen-containing plasma. For the plasma generation, was pure oxygen, pressure: 0.3 mbar (low pressure plasma); power: 200 W. Plasma treatment took 30 min.

[0116] Deposition of a Platinum Compound on the Plasma-Treated Carbon Material

[0117] 6 g of the plasma-treated carbon material were slurried with 100 mL of water, added to a double-shell reactor, and filled to 2 L with water. The mixture was stirred and the suspension heated to 70° C. After a holding time of 1 hour, 40 g of a nitric acid Pt-nitrate solution (10 wt % of Pt) were metered in and then kept for 1 hour under constant mixing and temperature.

[0118] Reduction to Metallic Platinum

[0119] By addition of Na.sub.2CO.sub.3, the pH of the aqueous medium was adjusted to a value of 5.6. This was followed by the addition of formic acid, which served as a reducing agent. The mixture was stirred, and the temperature of the aqueous medium was 70° C. During the reduction, the platinum compound present on the carbon material was reduced to metallic platinum. This yielded a carbon material laden with metallic platinum. After 0.5 hours, the catalyst composition was filtered off from the aqueous medium and washed with water, and dried at 110° C. under a nitrogen atmosphere. The platinum content of the catalyst composition was 40 wt %.

Example 2 According to the Invention (EB2)

[0120] The substrate used in EB2 is likewise a graphitized, porous carbon material produced by nanocasting by a porous SiO.sub.2 template having been impregnated with saccharose, the saccharose having been carbonized, the SiO.sub.2 template having been removed, and the carbonized carbon material having been graphitized.

[0121] Treatment with an Aqueous Medium Containing Oxidizing Agents

[0122] The graphitized, porous carbon material was dispersed at 70° C. for 180 min in 69 wt % nitric acid while being stirred. After removal from the nitric acid, the treated carbon material was washed with water and dried.

[0123] Deposition of a Platinum Compound on the Treated Carbon Material

[0124] 6 g of the carbon material treated in the oxidizing-agent-containing aqueous medium were impregnated with a platinum compound under the same conditions as in example EB1.

[0125] Reduction to Metallic Platinum

[0126] The reduction to metallic platinum took place under the same conditions as in example EB1. The platinum content of the catalyst composition was 40 wt %.

Comparative Example 1 (VB1)

[0127] As in examples EB1 and EB2 according to the invention, the graphitized, porous carbon material used in comparative example VB1 was produced by nanocasting. A porous SiO.sub.2 template was impregnated with pitch P15 from Rain Carbon, Inc., as an organic precursor compound, the precursor compound was carbonized, the SiO.sub.2 template was removed, and the carbonized carbon material was graphitized.

[0128] Thermal Treatment in Air

[0129] The graphitized, porous carbon material was subjected to thermal treatment in air at 430° C. for 13 hours.

[0130] Deposition of a Platinum Compound on the Treated Carbon Material

[0131] 6 g of the carbon material thermally treated in air were impregnated with a platinum compound under the same conditions as in example EB1.

[0132] Reduction to Metallic Platinum

[0133] The reduction to metallic platinum took place under the same conditions as in example EB1. The platinum content of the catalyst composition was 40 wt %.

[0134] Investigation of the Start-Up/Shut-Down (SUSD) Cycle Stability of the Catalyst Compositions Prepared in EB1, EB2, and VB1:

[0135] Test cells were prepared with the catalyst compositions produced in EB1, EB2, and VB1, and the cell voltage was determined as a function of the number of start-up/shut-down cycles. The SUSD tests were carried out as described by Gasteiger et al. in Journal of the Electrochemical Society, 165 (16) F1349-F1357 (2018).

[0136] The results were as follows:

EB1: 90% of the original voltage after 220 SUSD cycles
EB2: 90% of the original voltage after 240 SUSD cycles
VB1: 90% of the original voltage after 70 SUSD cycles

[0137] Compared to the test cell with the catalyst composition produced in VB1, the test cells with the catalyst compositions produced in EB1 and EB2 show a significantly smaller decrease in the cell voltage as a function of the SUSD cycle number. This is due to the higher corrosion stability of the catalyst.

TABLE-US-00001 TABLE 1 Results of the SUSD tests Number of SUSD cycles at 90% Activation treatment of the original voltage EB1 Plasma 220 EB2 Oxidative aqueous 240 medium VB1 Air, 70 400-500° C.

[0138] Electrochemically-Active Surface Area (EASA)

[0139] The electrochemically-active surface area was determined in each case for the catalyst compositions prepared in EB1 and VB1. The results are listed in Table 2.

TABLE-US-00002 TABLE 2 Electrochemically-active surface areas EASA Activation EASA treatment [m.sup.2 Pt/g Pt] EB1 Plasma 49.7 VB1 Air, 33.5 400-500° C.

[0140] The catalyst composition prepared with the method according to the invention has a significantly higher electrochemically-active surface area than does a catalyst composition whose carbon substrate was thermally treated in air before noble metal deposition.