POST-TREATMENT METHODS AND SYSTEMS FOR CORE-SHELL CATALYSTS

Abstract

Provided is a post-treatment method and system for a core-shell catalyst, which relate to the field of fuel cell materials. The post-treatment method of the present disclosure includes the following steps: a core-shell catalyst is added into an electrolyte solution containing citric acid or ethylenediamine tetraacetic acid, a gas containing oxygen is introduced into the electrolyte solution followed by stirring for a predetermined reaction time, the open circuit potential of the reactor base is recorded during the reaction time, and the open circuit potential should stabilize at 0.90˜1.0 V vs. RHE when the reaction is completed. The molar ratio of citric acid or ethylenediamine tetraacetic acid to platinum of the core-shell catalyst is 10 to 1000:1. A percentage of oxygen in the gas is 10 to 100% by volume. The post-treatment method of the present disclosure can significantly improve the platinum mass activity and PGM mass activity and durability of core-shell catalyst.

Claims

1-10. (canceled)

11. A post-treatment method for a core-shell catalyst, including steps of: adding a core-shell catalyst into an electrolyte solution containing citric acid or ethylenediamine tetraacetic acid; introducing a gas containing oxygen into the electrolyte solution followed by stirring and reacting for a predetermined reaction time; and recording an open circuit potential during the reaction time, wherein the open circuit potential is stable at 0.90 to 1.0 V vs. RHE when the reaction is completed; wherein a molar ratio of citric acid or ethylenediamine tetraacetic acid to platinum of the core-shell catalyst is 10 to 1000:1; and wherein a percentage of oxygen in the gas is 10% to 100% by volume.

12. The post-treatment method for the core-shell catalyst according to claim 11, wherein the core-shell catalyst is selected from any one of palladium-platinum core-shell catalysts, ruthenium-platinum core-shell catalysts, and palladium-alloy-platinum core-shell catalysts.

13. The post-treatment method for the core-shell catalyst according to claim 11, wherein the electrolyte solution is a copper sulfate solution, the gas containing oxygen is air or pure oxygen.

14. The post-treatment method for the core-shell catalyst according to claim 11, wherein the concentration of the citric acid or ethylenediamine tetraacetic acid is 5 to 50 mM.

15. The post-treatment method for the core-shell catalyst according to claim 11, wherein the predetermined reaction time is 6 to 12 hours.

16. The post-treatment method for the core-shell catalyst according to claim 11, wherein the post-treatment method further includes a purification step: after the reaction is completed, a catalyst suspension is filtered, and a solid is retained, washed, and dried to obtain a post-treated core-shell catalyst via core-dissolution.

17. The post-treatment method for the core-shell catalyst according to claim 11, wherein the core-shell catalyst is obtained by a copper-platinum replacement reaction.

18. The post-treatment method for the core-shell catalyst according to claim 12, wherein the core-shell catalyst is obtained by a copper-platinum replacement reaction.

19. The post-treatment method for the core-shell catalyst according to claim 13, wherein the core-shell catalyst is obtained by a copper-platinum replacement reaction.

20. The post-treatment method for the core-shell catalyst according to claim 14, wherein the core-shell catalyst is obtained by a copper-platinum replacement reaction.

21. The post-treatment method for the core-shell catalyst according to claim 15, wherein the core-shell catalyst is obtained by a copper-platinum replacement reaction.

22. The post-treatment method for the core-shell catalyst according to claim 16, wherein the core-shell catalyst is obtained by a copper-platinum replacement reaction.

23. The post-treatment method for the core-shell catalyst according to claim 17, wherein the copper-platinum replacement reaction specifically includes steps of: STEP 1: placing a core material into a reactor, and adding water to prepare a suspension, and then adding sulfuric acid solution under stirring; subsequently, introducing an inert gas into the solution to remove oxygen in the reactor, and then introducing hydrogen to remove impurities adsorbed on the surface of the core material, after that, introducing an inert gas to remove hydrogen; introducing oxygen or air to remove the hydrogen embedded in the crystal lattice, and finally introducing the inert gas to remove oxygen dissolved in the solution; STEP 2: continuously introducing the inert gas and then stopping stirring; after the core material settles, applying cyclic potential scans to the reactor base; stirring the suspension for 10 to 70 seconds every 20 to 40 minutes of settling; the above process is repeated until a cyclic voltammetric curve becomes stable; STEP 3: adding a copper sulfate solution to the reactor, and meanwhile recording the open circuit potential; after that, stopping stirring to allow the material to settle, and applying a constant potential hold to the reactor base; stirring the suspension for 10 to 70 seconds every 20 to 40 minutes of settling; the above is repeated until a current recorded is stable; and STEP 4: preparing a precursor solution containing platinum ions, citric acid, and sulfuric acid, and introducing an inert gas to obtain a platinum precursor solution; at the end of the constant potential hold, stopping potential control, starting stirring, and adding the platinum precursor solution in a drop-wise manner to allow a copper-platinum replacement reaction; when the copper-platinum replacement reaction is completed, the suspension is filtered, and a solid is retained, washed, and dried to obtain a core-shell catalyst without core-dissolution post-treatment.

24. The post-treatment method for the core-shell catalyst according to claim 18, wherein the copper-platinum replacement reaction specifically includes steps of: STEP 1: placing a core material into a reactor, and adding water to prepare a suspension, and then adding sulfuric acid solution under stirring; subsequently, introducing an inert gas into the solution to remove oxygen in the reactor, and then introducing hydrogen to remove impurities adsorbed on the surface of the core material, after that, introducing an inert gas to remove hydrogen; introducing oxygen or air to remove the hydrogen embedded in the crystal lattice, and finally introducing the inert gas to remove oxygen dissolved in the solution; STEP 2: continuously introducing the inert gas and then stopping stirring; after the core material settles, applying cyclic potential scans to the reactor base; stirring the suspension for 10 to 70 seconds every 20 to 40 minutes of settling; the above process is repeated until a cyclic voltammetric curve becomes stable; STEP 3: adding a copper sulfate solution to the reactor, and meanwhile recording the open circuit potential; after that, stopping stirring to allow the material to settle, and applying a constant potential hold to the reactor base; stirring the suspension for 10 to 70 seconds every 20 to 40 minutes of settling; the above is repeated until a current recorded is stable; and STEP 4: preparing a precursor solution containing platinum ions, citric acid, and sulfuric acid, and introducing an inert gas to obtain a platinum precursor solution; at the end of the constant potential hold, stopping potential control, starting stirring, and adding the platinum precursor solution in a drop-wise manner to allow a copper-platinum replacement reaction; when the copper-platinum replacement reaction is completed, the suspension is filtered, and a solid is retained, washed, and dried to obtain a core-shell catalyst without core-dissolution post-treatment.

25. The post-treatment method for the core-shell catalyst according to claim 19, wherein the copper-platinum replacement reaction specifically includes steps of: STEP 1: placing a core material into a reactor, and adding water to prepare a suspension, and then adding sulfuric acid solution under stirring; subsequently, introducing an inert gas into the solution to remove oxygen in the reactor, and then introducing hydrogen to remove impurities adsorbed on the surface of the core material, after that, introducing an inert gas to remove hydrogen; introducing oxygen or air to remove the hydrogen embedded in the crystal lattice, and finally introducing the inert gas to remove oxygen dissolved in the solution; STEP 2: continuously introducing the inert gas and then stopping stirring; after the core material settles, applying cyclic potential scans to the reactor base; stirring the suspension for 10 to 70 seconds every 20 to 40 minutes of settling; the above process is repeated until a cyclic voltammetric curve becomes stable; STEP 3: adding a copper sulfate solution to the reactor, and meanwhile recording the open circuit potential; after that, stopping stirring to allow the material to settle, and applying a constant potential hold to the reactor base; stirring the suspension for 10 to 70 seconds every 20 to 40 minutes of settling; the above is repeated until a current recorded is stable; and STEP 4: preparing a precursor solution containing platinum ions, citric acid, and sulfuric acid, and introducing an inert gas to obtain a platinum precursor solution; at the end of the constant potential hold, stopping potential control, starting stirring, and adding the platinum precursor solution in a drop-wise manner to allow a copper-platinum replacement reaction; when the copper-platinum replacement reaction is completed, the suspension is filtered, and a solid is retained, washed, and dried to obtain a core-shell catalyst without core-dissolution post-treatment.

26. The post-treatment method for the core-shell catalyst according to claim 20, wherein the copper-platinum replacement reaction specifically includes steps of: STEP 1: placing a core material into a reactor, and adding water to prepare a suspension, and then adding sulfuric acid solution under stirring; subsequently, introducing an inert gas into the solution to remove oxygen in the reactor, and then introducing hydrogen to remove impurities adsorbed on the surface of the core material, after that, introducing an inert gas to remove hydrogen; introducing oxygen or air to remove the hydrogen embedded in the crystal lattice, and finally introducing the inert gas to remove oxygen dissolved in the solution; STEP 2: continuously introducing the inert gas and then stopping stirring; after the core material settles, applying cyclic potential scans to the reactor base; stirring the suspension for 10 to 70 seconds every 20 to 40 minutes of settling; the above process is repeated until a cyclic voltammetric curve becomes stable; STEP 3: adding a copper sulfate solution to the reactor, and meanwhile recording the open circuit potential; after that, stopping stirring to allow the material to settle, and applying a constant potential hold to the reactor base; stirring the suspension for 10 to 70 seconds every 20 to 40 minutes of settling; the above is repeated until a current recorded is stable; and STEP 4: preparing a precursor solution containing platinum ions, citric acid, and sulfuric acid, and introducing an inert gas to obtain a platinum precursor solution; at the end of the constant potential hold, stopping potential control, starting stirring, and adding the platinum precursor solution in a drop-wise manner to allow a copper-platinum replacement reaction; when the copper-platinum replacement reaction is completed, the suspension is filtered, and a solid is retained, washed, and dried to obtain a core-shell catalyst without core-dissolution post-treatment.

27. The post-treatment method for the core-shell catalyst according to claim 21, wherein the copper-platinum replacement reaction specifically includes steps of: STEP 1: placing a core material into a reactor, and adding water to prepare a suspension, and then adding sulfuric acid solution under stirring; subsequently, introducing an inert gas into the solution to remove oxygen in the reactor, and then introducing hydrogen to remove impurities adsorbed on the surface of the core material, after that, introducing an inert gas to remove hydrogen; introducing oxygen or air to remove the hydrogen embedded in the crystal lattice, and finally introducing the inert gas to remove oxygen dissolved in the solution; STEP 2: continuously introducing the inert gas and then stopping stirring; after the core material settles, applying cyclic potential scans to the reactor base; stirring the suspension for 10 to 70 seconds every 20 to 40 minutes of settling; the above process is repeated until a cyclic voltammetric curve becomes stable; STEP 3: adding a copper sulfate solution to the reactor, and meanwhile recording the open circuit potential; after that, stopping stirring to allow the material to settle, and applying a constant potential hold to the reactor base; stirring the suspension for 10 to 70 seconds every 20 to 40 minutes of settling; the above is repeated until a current recorded is stable; and STEP 4: preparing a precursor solution containing platinum ions, citric acid, and sulfuric acid, and introducing an inert gas to obtain a platinum precursor solution; at the end of the constant potential hold, stopping potential control, starting stirring, and adding the platinum precursor solution in a drop-wise manner to allow a copper-platinum replacement reaction; when the copper-platinum replacement reaction is completed, the suspension is filtered, and a solid is retained, washed, and dried to obtain a core-shell catalyst without core-dissolution post-treatment.

28. The post-treatment method for the core-shell catalyst according to claim 22, wherein the copper-platinum replacement reaction specifically includes steps of: STEP 1: placing a core material into a reactor, and adding water to prepare a suspension, and then adding sulfuric acid solution under stirring; subsequently, introducing an inert gas into the solution to remove oxygen in the reactor, and then introducing hydrogen to remove impurities adsorbed on the surface of the core material, after that, introducing an inert gas to remove hydrogen; introducing oxygen or air to remove the hydrogen embedded in the crystal lattice, and finally introducing the inert gas to remove oxygen dissolved in the solution; STEP 2: continuously introducing the inert gas and then stopping stirring; after the core material settles, applying cyclic potential scans to the reactor base; stirring the suspension for 10 to 70 seconds every 20 to 40 minutes of settling; the above process is repeated until a cyclic voltammetric curve becomes stable; STEP 3: adding a copper sulfate solution to the reactor, and meanwhile recording the open circuit potential; after that, stopping stirring to allow the material to settle, and applying a constant potential hold to the reactor base; stirring the suspension for 10 to 70 seconds every 20 to 40 minutes of settling; the above is repeated until a current recorded is stable; and STEP 4: preparing a precursor solution containing platinum ions, citric acid, and sulfuric acid, and introducing an inert gas to obtain a platinum precursor solution; at the end of the constant potential hold, stopping potential control, starting stirring, and adding the platinum precursor solution in a drop-wise manner to allow a copper-platinum replacement reaction; when the copper-platinum replacement reaction is completed, the suspension is filtered, and a solid is retained, washed, and dried to obtain a core-shell catalyst without core-dissolution post-treatment.

29. The post-treatment method for the core-shell catalyst according to claim 23, wherein the core material is carbon-supported palladium nanoparticles.

30. A post-treatment system for a core-shell catalyst, including: a reactor, providing a compartment for post-treatment reactions to take place, wherein a stirrer is placed in the reactor; an air supplying device, providing oxygen or pure oxygen to the reactor; and an electrochemical workstation, for recording an open circuit potential of a reactor base.

Description

BRIEF DESCRIPTION

[0038] Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein:

[0039] FIG. 1 is a schematic diagram of the synthesis and post-treatment reaction for the core-shell catalyst; and

[0040] FIG. 2 shows the results of the mass activity tests and accelerated stability tests for post-treated core-shell catalysts.

DETAILED DESCRIPTION

[0041] Embodiments of the invention will be described hereinafter with reference to the preferred examples for the sake of clarity. However, the present disclosure can be implemented in many different forms and is not limited to the examples described herein. On the contrary, the objective of providing these examples is to understand the present disclosure more thoroughly and comprehensively.

[0042] All technical and scientific terms used herein have the general meanings well understood by those skilled in the art unless otherwise defined. The terms in the descriptions of the present disclosure have been presented for purposes of illustrating the examples, but not limited to the present disclosure. The term “and/or” used herein includes any and all combinations of one or more related listed items.

[0043] In the following examples, copper sulfate, potassium chloroplatinate, and citric acid are all purchased from Sigma-Aldrich™, and carbon-supported palladium nanoparticles are provided by Tanaka Precious Metals Co. Ltd.

Example 1

[0044] 1. Preparation of Core-Shell Catalyst

[0045] (1) 1000 mg carbon-supported palladium nanoparticles are placed into a glass bottle, and an appropriate amount of ultra-pure water is added to prepare a well-dispersed suspension. The mixture is poured into the reactor, and the glass bottle is rinsed with sulfuric acid solution, which is also poured into the reactor until the concentration of the sulfuric acid solution in the reactor is 50 mM with a total volume of 600 mL. Argon gas is introduced into the reactor for 30 minutes to remove oxygen, and then hydrogen gas is introduced for about 40 minutes to remove the impurities adsorbed on the surface of the palladium. Subsequently, argon gas is introduced for 30 minutes to remove hydrogen dissolved in the solution. In each step of introducing the gas, the suspension is stirred at 300 rpm. After this step is completed, argon or nitrogen gas is supplied to the reactor until the copper-platinum replacement reaction is completed.

[0046] (2) The stirring is stopped to allow the core material to settle naturally, and an electrochemical workstation is used to apply cyclic potential scans (0.36 to 0.45V vs. RHE reversible hydrogen electrode, 5 mV/s of scan rate, all the potentials are referred to RHE hereinafter) to the reactor base. During the potential scans, the solution is stirred at 300 rpm for 1 minute every 30 minutes, until the CV curve recorded by the workstation is stable. This step removes impurities and oxides on the surface of the core material by applying external power, and it usually takes 2 hours and the solution is stirred for more than 4 times. One hour before finishing the potential scans, a copper sulfate solution is prepared and its concentration is calculated according to the requirement that the concentration of copper ion is 50 mM after the addition of which. The copper sulfate solution is quickly added to the suspension with a peristaltic pump, and meanwhile the open circuit potential of the reactor base is recorded with an electrochemical workstation. After completion of addition of copper sulfate solution (open circuit potential normally stabilizes at about 0.64V), the stirring is stopped to allow the core material to settle naturally, and the electrochemical workstation is used to apply a constant potential hold at 0.36V, during which stirring is performed at 300 rpm for 1 minute every 30 minutes, until a current recorded by the workstation is stable.

[0047] (3) A precursor solution of platinum ions at a concentration of about 4 to 10 mM, citric acid at a concentration of about 0.2 M, and sulfuric acid at a concentration of 50 mM, is prepared, and argon gas is introduced in it for 30 minutes. When the constant potential hold step is completed, the potential control is stopped and stirring at 400 rpm commences, the platinum precursor solution is added slowly into the suspension in a drop-wise manner using a peristaltic pump to allow copper-platinum replacement reaction. The open circuit potential is recorded using an electrochemical workstation. The open circuit potential gradually increases with the addition of platinum ions. After completion of the addition of platinum precursor solution, the replacement reaction is allowed to proceed for an extra 40 minutes to ensure equilibrium.

[0048] (4) After the replacement reaction is completed, the catalyst is collected by filtration. The filtrate is the blue-colored copper sulfate aqueous solution. The catalyst is washed with ultra-pure water several times, and dried in vacuum to obtain the non-post-treated core-shell catalyst without core-dissolution.

[0049] 2. Post-Treatment of Core-Shell Catalyst

[0050] (1) After the copper-platinum replacement reaction is completed, air is introduced into an electrolyte solution containing citric acid (the molar ratio of citrate:platinum is about 60:1), wherein the concentration of citric acid is 40 mM. The solution is stirred for 12 h, during which the open circuit potential is recorded using an electrochemical station, and it normally stabilizes at 0.97V vs. RHE at the end of core-dissolution reaction.

[0051] (2) After the reaction is completed, the catalyst is collected via filtration, and the filtrate is yellow-green. The catalyst is washed several times with ultra-pure water, and dried in vacuum to obtain a post-treated core-shell catalyst via core-dissolution.

[0052] In this example, the palladium/platinum mass ratio of the non-post-treated core-shell catalyst (Pd@Pt) is 1.80, while the palladium/platinum mass ratio of the post-treated core-shell catalyst is 1.30.

Example 2

[0053] A preparation method and post-treatment method of the core-shell catalyst are the same as those in Example 1, except that in the post-treatment step, pure oxygen is introduced (the oxygen content is 99.9992%).

[0054] In this example, the palladium/platinum mass ratio of the non-post-treated core-shell catalyst (Pd@Pt) is 1.80, while the palladium/platinum mass ratio of the post-treated core-shell catalyst is 1.10.

Comparative Example 1

[0055] Commercial carbon-supported platinum nanoparticle catalyst is provided by Tanaka Precious Metals Co., Ltd., a renowned research and development manufacturer for fuel cell catalysts.

Comparative Example 2

[0056] A preparation method and post-treatment method of the core-shell catalyst are the same as those in Example 1, except that in the post-treatment step of the core-shell catalyst, citric acid is not added to the electrolyte solution.

[0057] Due to the lack of citric acid in the reaction system, the platinum shell is not protected by citrate and the obtained catalyst has its core exposed, ultimately resulting in a substantial decrease in ORR activity.

Comparative Example 3

[0058] A preparation method and post-treatment method of the core-shell catalyst are the same as those in Example 1, except that in the post-treatment step of the core-shell catalyst, the gas containing oxygen is not introduced, argon gas is introduced instead.

[0059] Under these reaction conditions, the platinum atoms does not rearrange and the pinhole defects on the shell is not repaired, resulting in an incomplete coating of the platinum shell and a part of the core to be exposed; the stability of the catalyst is not as good as that of the post-treated catalyst in an oxygen-containing atmosphere. This is reflected in the fact that the voltage loss of the MEAs prepared using the above catalyst is larger than that using the oxygen-containing gas post-treated catalyst in their lifespans. In addition, as more core palladium atoms are retained in an oxygen-free atmosphere and most of them are not involved in enhancing the ORR activity of the platinum shell, the overall PGM mass activity of the catalyst is not as good as that post-treated ones in oxygen-containing atmosphere.

Test Example 1

[0060] The catalysts of Example 1 and that of Comparative Example 1 are selected to test the activity of these catalysts under the same platinum loading in MEAs (0.05 mg/cm.sup.2 in anode, 0.11 mg/cm.sup.2 in cathode) via single cell test. Test conditions: hydrogen/oxygen, 80° C., 100% relative humidity, 5 cm.sup.2 active area, 1.5 atm back pressure.

[0061] The platinum mass activity of the post-treated core-shell catalyst (d-Pd@Pt/C) reaches 1.01 A/mg Pt, and the PGM mass activity reaches 0.48 A/mg. The platinum mass activity of d-Pd@Pt/C is 5 times that of commercial carbon-supported platinum nanoparticles. The PGM mass activity of d-Pd@Pt/C has reached the Year 2020 performance target, set by the US Department of Energy (0.44 A/mg for PGM mass activity, using MEA single cell test method).

Test Example 2

[0062] The same accelerated stability test, established by the US Department of Energy, is conducted for MEAs using the non-post-treated catalyst (Pd@Pt/C) in Example 1, the post-treated catalyst in Example 2 (d-Pd@Pt/C) and the commercial carbon-supported platinum nanoparticle catalyst (Pt/C) in Comparative Example 1. The test conditions are described briefly as follows, hydrogen/nitrogen, 80° C., 100% relative humidity, 5 cm.sup.2 active area, 1.5 atm back pressure, 0.1 mg/cm.sup.2 of platinum loading, 30,000 square wave cycles, constant voltage at 0.60V and at 0.95V, 3-second hold time at each voltage. The test results are shown in FIG. 2. The decay rate of mass activity is 50.0% for Pd@Pt/C, 55.7% for Pt/C, while that for d-Pd@Pt/C, using the post-treatment method of the present disclosure, is only 22.3%. Moreover, the PGM mass activity of d-Pd@Pt/C at any stage of the accelerated stability test is significantly higher than that of the commercial carbon-supported platinum nanoparticle catalyst.

[0063] In summary, the post-treatment method of the core-shell catalyst in the present disclosure can achieve a simple, reliable and effective production of core-shell catalyst in gram-batch-size.

[0064] The technical features of the above-mentioned embodiments can be combined arbitrarily. In order to make the description concise, not all possible combinations of the various technical features in the above-mentioned embodiments are described. However, all combinations of these technical features should be considered to fall within the scope of this specification as long as they have no contradiction.

[0065] Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention.

[0066] For the sake of clarity, it is to be understood that the use of ‘a’ or ‘an’ throughout this application does not exclude a plurality, and ‘comprising’ does not exclude other steps or elements.