METHOD OF TREATING NANOPARTICLES USING A PROTON EXCHANGE MEMBRANE AND LIQUID ELECTROLYTE CELL

Abstract

One embodiment of the invention includes an electrochemical cell including a proton exchange membrane and a method of treating nanoparticles using the same.

Description

TECHNICAL FIELD

The field to which the disclosure generally relates includes methods of treating nanoparticles.

BACKGROUND

The electrochemical treatment of large quantities of nanoparticles, including coating, stripping, oxidation, reduction, cleaning, dealloying of nanoparticles and so on, has long been a technical barrier for more extensive applications of this technique in many fields such as for fuel cells, batteries, and heterocatalysis. Heretofore, such electrochemical treatment has resulted in non-uniform treatment of the nanoparticles.

SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION

One embodiment of the invention includes a method of using an electrochemical cell including a liquid electrolyte, a working electrode with nanoparticles supported thereon, a counter electrode, and a polymer electrolyte membrane completely separating the liquid electrolyte at the working electrode side and liquid electrolyte at the counter electrode side.

Other exemplary embodiments of the invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while disclosing exemplary embodiments of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will become more fully understood from the detailed description and the accompanying drawings, wherein:

FIG. 1 illustrates an electrode chemical cell according to one embodiment of the invention.

FIG. 2 is a graph showing a comparison of the platinum supported on graphitized carbon pre-oxidation curve at 1.2V(RHE) obtained by this cell design versus by a conventional electrochemical cell.

FIG. 3 is a graph showing a comparison of fuel cell performance data for membrane electrode assemblies (MEAs) containing the 1.4V-pretreated Pt on graphitized carbon as cathode catalyst and for MEAs of non-treated Pt on graphitized carbon catalyst.

FIG. 4 illustrates a multi-cell according to one embodiment of the invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following description of the embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.

FIG. 1 illustrates an electrochemical cell 10 according to one embodiment of the invention. The electrochemical cell 10 may include a container 12 that holds a liquid electrolyte 14. The liquid electrolyte 14 may be an aqueous acid solution, for example including perchloric acid, sulfuric acid, or phosphoric acid. The liquid electrolyte may also be any salt solution, like copper sulfate, lead sulfate, copper nitrate; or combination of salt and acid solutions. The container 12 may be made from any of a variety of materials, for example PTFE, glass, or other acid resistant material. The electrochemical cell 10 may include a working electrode 16 and a counter electrode 22. Suitable material for the working electrode 16 and counter electrode 22 include, but are not limited to, metals such as Pt, Au, or graphite. The working electrode 16 and the counter electrode 22 may be in the form of gauze. The gauze material serves the function of increasing the contact area and decreasing the mass transport resistance. The electrochemical cell 10 may include nanoparticles 20 to be treated, which may be spread on a support material 18 such as a first carbon cloth. The nanoparticles 20 are electrically conductive and may be solid particle, shells with hollow cores, or strands of connected particles. For example, the nanoparticles 20 may include, but are not limited to carbon, Pt or Pt alloy, Ni or other metals, TiO₂, or electrically conductive shells. The function of the first carbon cloth 18 is increasing the contact area between the nanoparticles and the supporting material. The support material or first carbon cloth 18 is further supported by the working electrode 16 which may be a gauze material including, for example, platinum, gold, or graphite. A second platinum gauze and a second carbon cloth 24 are used as the counter electrode. The counter electrode may contain a layer of Pt/C nanoparticles or Pt black spread on the carbon cloth. The function of these Pt/C nanoparticles is to increase the active surface area of the counter electrode 22. Depending on the electrochemical reaction occurring on the counter electrode 22, the material of counter electrode may also be Cu, Pb, Ag, or other metals or metal alloys. The arrangement of the gauze working electrode 16, overlying first carbon cloth 18, and the gauze counter electrode 22 and underlying second carbon cloth 24 minimizes the in-cell electronic resistance which can cause non-uniform potential distribution in the working electrode 16 and counter electrode 22. The electronic resistance in the thickness direction is very small.

A polymer electrolyte membrane 26 is interposed between the support material 18 and the counter electrode 22 so that the polymer electrolyte membrane serves to separate a working electrode compartment 7 and a counter electrode compartment 9 of the cell 10 wherein the polymer electrolyte membrane 26 completely separates the liquid electrolyte 14 in the working electrode compartment 7 from the liquid electrolyte 14 in the counter electrode compartment 9 of the cell 10. The second carbon cloth 24 may be interposed between the counter electrode 22 and the polymer electrolyte membrane 26. The function of the second carbon cloth 24 is to reduce the stress that the Pt gauze applies on the membrane. The second carbon cloth may also function as a support for Pt/C nanoparticles, in case a layer of Pt/C nanoparticles or Pt black is included as a part of the counter electrode 22.

In one embodiment of the invention, the working electrode 16, first carbon cloth 18, nanoparticles 20, membrane 26, and optionally the second carbon cloth 24 and counter electrode 22 are all supported by the container 12. This prevent damage to materials such as the membrane 26.

A reference electrode 28 may be provided immersed in the liquid electrolyte 14 on the working electrode side of the cell 10. Suitable reference electrodes 28 include, but are not limited to a Ag/AgCl electrode, a Calomel electrode, or a reversible hydrogen electrode. A gas purge tube 30 may be provided immersed in the liquid electrolyte 14 in the working electrode compartment 7 of the cell 10. A cover 32 may be placed over the container 12 with a seal or gasket 34 interposed between the cover 32 and the container 12. Both the cover 32 and the container 12 may be made from a material including, but not limited to, polytetrafluororethylene, glass, or other acid-resistant material. A potential is applied across the electrodes to treat the nanoparticles 20, using an energy source such as a battery. This arrangement may be utilized for coating, stripping, oxidation, reduction, cleaning, or dealloying the nanoparticles 20.

This design ensures uniform potential and uniform current density distribution throughout the working electrode 16 and counter electrode 22 even at high current conditions and consequently ensures a uniform and highly efficient electrochemical treatment of the nanoparticles. The cell design combines some advantages of the polymer electrolyte membrane fuel cell and some of the conventional liquid electrolyte electrochemical cell. In the case where the electrochemical reaction at the counter electrode 22 is not the reverse reaction of the working electrode 16 (for example when H₂ or O₂ evolution occurs at the counter electrode), the design can easily prevent the reaction products (H₂ or O₂) from diffusing into the working electrode 16. As the nanoparticles 20 are immersed in the liquid electrolyte 14, the utilization of the nanoparticles 20 approaches 100%, i.e. all of the nanoparticles 20 can be treated and can be easily washed out after the treatment. Neither of these features can be achieved for the catalyst layer in a polymer electrolyte membrane fuel cell, in which the catalyst layer is mixed with a solid ionomer phase.

As an example, FIG. 2 shows a comparison of the platinum supported on graphitized carbon (Pt/GrC) pre-oxidation current at 1.2V(RHE) by using an electrochemical cell according to the present invention versus the same process in a conventional electrochemical cell. The much higher current for the conventional cell is ascribed to the oxidation of H₂ diffusing from the counter electrode, which is not a desirable process and prevents monitoring the progress of the desired treatment of the nanoparticles through a simple current measurement. The actual Pt/GrC pre-oxidation current is achieved with the electrochemical cell according to one embodiment of the invention, with the current dropping down to less than 10 mA/g(Pt/GrC) in the initial 10 minutes. As such, the electrochemical cell shown in FIG. 1 can be utilized to electrochemically treat large quantities of nanoparticles with uniformity, high efficiency, and facile monitoring of the state of progress of the treatment.

As an example of an application of this cell, FIG. 3 shows that pre-oxidized Pt nanoparticles supported on graphitized carbon by using the present invention give higher fuel cell performance than non-treated Pt nanoparticles supported on graphitized carbon. FIG. 3 shows a comparison of fuel cell performance data at the conditions indicated in the graph for various membrane electrode assemblies (MEAs), which refers to the combination of the anode catalyst, cathode catalyst, and the membrane. The solid curves are for MEAs containing the 1.4V-pretreated Pt on graphitized carbon as cathode catalyst. The dashed curves are for MEAs of non-treated Pt on graphitized carbon catalyst. At 1.5 A/cm², the improvement is 25 mV. At 0.6 A/cm², the improvement is as much as 50 mV. In one embodiment, the nanoparticles 20 used in a H₂/air proton exchange membrane (PEM) fuel cell operated at high current densities can achieve higher voltage.

In various embodiments, the polymer electrolyte membrane 26 may include a variety of different types of membranes. The polymer electrolyte membrane 26 useful in various embodiments of the invention may be an ion-conductive material. Examples of suitable membranes are disclosed in U.S. Pat. Nos. 4,272,353 and 3,134,689, and in the Journal of Power Sources, Volume 28 (1990), pages 367-387. Such membranes are also known as ion exchange resin membranes. The resins include ionic groups in their polymeric structure; one ionic component for which is fixed or retained by the polymeric matrix and at least one other ionic component being a mobile replaceable ion electrostatically associated with the fixed component. The ability of the mobile ion to be replaced under appropriate conditions with other ions imparts ion exchange characteristics to these materials.

The ion exchange resins can be prepared by polymerizing a mixture of ingredients, one of which contains an ionic constituent. One broad class of cationic exchange, proton conductive resins is the so-called sulfonic acid cationic exchange resin. In the sulfonic acid membranes, the cationic exchange groups are sulfonic acid groups which are attached to the polymer backbone.

The formation of these ion exchange resins into membranes or chutes is well-known to those skilled in the art. The preferred type is perfluorinated sulfonic acid polymer electrolyte in which the entire membrane structure has ionic exchange characteristics. These membranes are commercially available, and a typical example of a commercial sulfonic perfluorocarbon proton conductive membrane is sold by E. I. DuPont D Nemours & Company under the trade designation NAFION. Other such membranes are available from Asahi Glass and Asahi Chemical Company.

The use of other types of membranes, such as, but not limited to, perfluorinated cation-exchange membranes, hydrocarbon based cation-exchange membranes as well as anion-exchange membranes are also within the scope of the invention.

The electrochemical cell 10 may be used to coat nanoparticles 20 with a catalyst such as platinum to provide a plurality of supported catalyst particles. The supported catalyst particles may be combined with an ionomer which may be the same as the material for the above described membrane material. The supported catalyst particles and ionomer may be applied to both faces of a polymer electrolyte membrane of a fuel cell. The supported catalyst particles and ionomer may alternatively be applied to a fuel cell gas diffusion media layer or onto a decal backing for later application as desired.

The above description is for a single cell design. Another embodiment of the invention includes a multi-cell design or electrochemical multi-cell 38. A schematic drawing of one embodiment is shown in FIG. 4, wherein 40, 44, 46, 50, 52, and 56 are working electrodes similar to the working electrode 16 described above. The working electrodes 40, 44, 46, 50, 52, and 56 contain nanoparticles 20 to be treated, supported on Pt or Au gauze or on other highly electronically conductive and acid-resistant materials. These working electrodes may be supported or sandwiched by backing material. The appropriate types of backing materials include but are not limited to perforated PTFE board. The multi-cell design 38 also includes counter electrodes 42, 48, and 54. Depending on the electrochemical reaction occurring on the counter electrode, the material of counter electrodes 42, 48, and 54 may include Pt, Cu, Pb, Ag, or other metals or metal alloys. An electrolyte 60 fills each of working electrode compartment 64 and counter electrode compartment 66. Membranes 62 separate the electrolyte in the working electrode compartments 64 from that in the counter electrode compartments 66. The multi-cell design 38 may include a container 58, which may be glass, PTFE or other acid-resistant material. In one embodiment, the multi-cell may have a cover made of acid resistant material (not shown). Gas may be purged into each compartment. A reference electrode (not shown) may be placed close to any of the working electrodes. One counter electrode may be shared by multiple working electrodes.

When the terms “over”, “overlying”, “overlies” or “under”, underlying” or “underlies” or the like are used herein with respect to the relative position of layers or components to each other such shall mean that the layers or components are in direct contact with each other or that another layer, layers, component or components may be interposed between the layers components.

The above description of embodiments of the invention is merely exemplary in nature and, thus, variations thereof are not to be regarded as a departure from the spirit and scope of the invention.

Claims

1. A method comprising: supporting electrically conductive nanoparticles to be electrochemically treated over a working electrode, immersing the working electrode with the supported nanoparticles in a liquid electrolyte solution, immersing a counter electrode in the electrolyte solution, and immersing a polymer electrolyte membrane in the electrolyte solution between the working electrode with nanoparticles supported thereon and the counter electrode to define a working electrode compartment and a counter electrode compartment of the cell,applying a potential or a current across the electrodes to treat the nanoparticles.

2. A method as set forth in claim 1 further comprising immersing a reference electrode on the working electrode side.

3. A method as set forth in claim 2 further comprising providing a gas purge tube in the liquid electrolyte on the working electrode side of the cell.

4. A method as set forth in claim 3 further comprising providing a container for holding the electrolyte solution and a cover over the container.

5. A method as set forth in claim 1 wherein the working electrode comprises a first carbon cloth and a gauze comprising a metal supporting the carbon cloth, and wherein the particles are spread on the first carbon cloth.

6. A method as set forth in claim 5 wherein the gauze comprises platinum or gold or graphite.

7. A method as set forth in claim 5 wherein the counter electrode comprises a second carbon cloth and a gauze material comprising a metal supporting the carbon cloth.

8. A method as set forth as claim 5 wherein the counter electrode comprises a second carbon cloth supported by a gauze comprising platinum or supported Pt nanoparticles.

9. An electrochemical cell comprising: a container and a liquid electrolyte received in the container;a working electrode, and nanoparticles supported by the working electrode;a counter electrode; anda polymer electrolyte membrane separating liquid electrolyte on the counter electrode side from liquid electrolyte on the working electrode side of the cell.

10. An electrochemical cell as set forth in claim 9 wherein the nanoparticles comprise at least one of Pt, Pt alloy, Ni, or other noble metals or metal alloys.

11. An electrochemical cell as set forth in claim 9 wherein the working electrode comprises a first carbon cloth supporting the nanoparticles, and a gauze comprising a metal supporting the carbon cloth.

12. An electrochemical cell as set forth in claim 11 wherein the gauze comprises at least one of platinum or gold or graphite.

13. An electrochemical cell as set forth in claim 9 wherein the counter electrode comprises a second carbon cloth supported by a gauze comprising a metal.

14. An electrochemical cell as set forth in claim 13 wherein the gauze comprises at least one of platinum or gold or graphite.

15. An electrochemical cell as set forth in claim 9 further comprising a reference electrode immersed in the liquid electrolyte on the working electrode side of the cell.

16. An electrochemical cell as set forth in claim 9 further comprising a gas purge tube immersed in the liquid electrolyte on the working electrode side of the cell.

17. An electrochemical cell as set forth in claim 16 further comprising a cover over the container.

18. A method as set forth in claim 1 wherein the nanoparticles comprise Pt/C, and the electrochemical treatment of the nanoparticles comprises an electrochemical oxidation step.

19. A method as set forth in claim 18 further comprising using the nanoparticles in a H2/air proton exchange membrane (PEM) fuel cell operated at high current densities to achieve higher voltage.

20. An electrochemical multi-cell comprising: a container and a liquid electrolyte received in the container;at least two working electrodes, and nanoparticles supported by the working electrodes;at least one counter electrode; andat least two polymer electrolyte membranes separating liquid electrolyte in counter electrode compartments from liquid electrolyte in working electrode compartments in the multi-cell.

21. An electrochemical multi-cell as set forth in claim 20 wherein the nanoparticles comprise at least one of Pt, Pt alloy, Ni, or other noble metals or metal alloys.

22. An electrochemical multi-cell as set forth in claim 20 wherein the working electrodes comprise a first carbon cloth supporting the nanoparticles, and a gauze comprising a metal supporting the carbon cloth.

23. An electrochemical multi-cell as set forth in claim 22 wherein the gauze comprises at least one of platinum or gold or graphite.

24. An electrochemical multi-cell as set forth in claim 20 wherein the counter electrodes comprise a second carbon cloth supported by a gauze comprising a metal.

25. An electrochemical multi-cell as set forth in claim 24 wherein the gauze comprises at least one of platinum or gold or graphite.