Caged nanoparticle electrocatalyst with high stability and gas transport property

Abstract

A method for forming a caged electrocatalyst particles for fuel cell applications include a step of forming modified particles having a porous SiO.sub.2 shell on a surface of platinum-containing particles. The modified particles are subjected to acid treatment or electrochemical oxidation to remove a portion of the platinum-containing particle thereby creating caged electrocatalyst particles having a gap between the platinum-containing particles and their SiO.sub.2 shell.

Claims

1. A method for forming caged electrocatalyst particles for a fuel cell catalyst layer, the method comprising: a) reacting platinum group metal-containing particles, which are supported on carbon particles, with an absorption solution, the absorption solution including a compound having formula 1 and a compound having formula 2:
X.sub.1—R.sub.2-M-(OR.sub.1).sub.n  1
X.sub.2—R.sub.3—Y  2 wherein: X.sub.1 and X.sub.2 are each independently SH or NH.sub.2; R.sub.1 are each independently C.sub.1-6 alkyl; M is a metal that can form a metal oxide; Y is a moiety that does not react with the compound having formula 1; n is an integer represent the number of OR.sub.1 groups attached to M; R.sub.2, R.sub.3 are each independently is a C.sub.1-6 alkylenyl; the compounds having formula 1 and formula 2 being adsorbed onto the platinum group metal-containing particles to form an adsorbed layer over the platinum group metal-containing particles; b) allowing M in the compound having formula 1 to hydrolyze to form modified particles on the carbon particles, each of the modified particles having a porous metal oxide shell on surfaces of the platinum group metal-containing particles; and then c) subjecting the modified particles to acid treatment or electrochemical oxidation to remove a portion of the platinum group metal-containing particles from the modified particles and thereby form a gap between the platinum group-metal containing particles and each corresponding porous metal oxide shell that surrounds the platinum group metal-containing particles, thus creating caged electrocatalyst particles in which each of the corresponding metal oxide shells is attached to each of the corresponding carbon particles and the platinum group metal-containing particles are disposed within a central cavity defined by each of the corresponding metal oxide shells such that each of the corresponding metal oxide shells and the platinum group metal-containing particles are separated by the gap inside each of the corresponding metal oxide shells and do not contact one another.

2. The method of claim 1 wherein M is Si, Al, Ti, or W.

3. The method of claim 1 wherein when M is Si or Ti, n is 3 and when M is Al, n is 2.

4. The method of claim 1 wherein M is Si.

5. The method of claim 1 wherein Y is CO.sub.2H, CH.sub.3, NH.sub.2, or halo.

6. The method of claim 1 wherein X.sub.1 and X.sub.2 are each SH.

7. The method of claim 1 wherein the platinum group metal-containing particles are each nanoparticles.

8. The method of claim 1 wherein the platinum group metal-containing particles are platinum alloy particles.

9. The method of claim 8 wherein the platinum group metal-containing particles are PtNi.sub.3 particles, PtCo particles, PtCo.sub.3 particles, PtCu.sub.3 particles, PtFe.sub.3 particles, PdNi.sub.3 particles, PdFe.sub.3 particles, or PdRhFe.sub.3 particles.

10. The method of claim 9 wherein in step c) a portion of the Ni is removed from PtNi.sub.3 particles.

11. The method of claim 1 wherein the compound having formula 1 is mercaptopropyltrimethoxysilane (MPTS).

12. The method of claim 1 wherein the compound having formula 2 is mercaptopropionic acid (MPA).

13. The method of claim 1 wherein step b) comprises dispersing the platinum group metal-containing particles in a mixture comprising water and tetraethoxysilane (TEOS).

14. The method of claim 1 wherein in step c) all of the compound having formula 2 is removed from the surface of the platinum group metal-containing particle.

15. The method of claim 1 wherein step c) comprises stirring the modified particles in a sulfuric acid solution.

16. The method of claim 1 comprising, after step c), converting remaining R—SH on the porous metal oxide shell to R—SO.sub.3H, R—SO.sub.2NHSO.sub.2—R, or R—R—NH.sub.2, wherein R is a C.sub.1-6 alkyl or a C.sub.1-6 alkylenyl.

17. The method of claim 1 comprising dispersing the caged electrocatalyst particles in isopropanol with a hydrophobic ionic liquid to incorporate the ionic liquid into the gap between the platinum group metal-containing particles and each corresponding metal oxide shell that surrounds the platinum group metal-containing particles.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 provides a schematic of a fuel cell system including an embodiment of a carbon coated bipolar plate;

(2) FIG. 2 is a schematic cross-section of a caged electrocatalyst particle that can be incorporated into a fuel cell catalyst layer;

(3) FIG. 3A provides a schematic flowchart showing the formation of caged electrocatalyst particles with high stability; and

(4) FIG. 3B is a continuation of FIG. 3.

DETAILED DESCRIPTION

(5) Reference will now be made in detail to presently preferred compositions, embodiments and methods of the present invention which constitute the best modes of practicing the invention presently known to the inventors. The Figures are not necessarily to scale. However, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. Therefore, specific details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for any aspect of the invention and/or as a representative basis for teaching one skilled in the art to variously employ the present invention.

(6) Except in the examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word “about” in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred. Also, unless expressly stated to the contrary: percent, “parts of,” and ratio values are by weight; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description and does not necessarily preclude chemical interactions among the constituents of a mixture once mixed; the first definition of an acronym or other abbreviation applies to all subsequent uses herein of the same abbreviation and applies mutatis mutandis to normal grammatical variations of the initially defined abbreviation; and, unless expressly stated to the contrary, measurement of a property is determined by the same technique as previously or later referenced for the same property.

(7) It is also to be understood that this invention is not limited to the specific embodiments and methods described below, as specific components and/or conditions may, of course, vary. Furthermore, the terminology used herein is used only for the purpose of describing particular embodiments of the present invention and is not intended to be limiting in any way.

(8) It must also be noted that, as used in the specification and the appended claims, the singular form “a,” “an,” and “the” comprise plural referents unless the context clearly indicates otherwise. For example, reference to a component in the singular is intended to comprise a plurality of components.

(9) Throughout this application, where publications are referenced, the disclosures of these publications in their entireties are hereby incorporated by reference into this application to more fully describe the state of the art to which this invention pertains.

(10) The term “alkylenyl” means bridging divalent alkyl radicals such as methylenyl and ethylenyl. In one refinement, alkylenyl is —(CH.sub.2)— where n is 1 to 10 or 1 to 4.

(11) The term “nanoparticle” means a particle having at least one dimension less than 100 nanometers.

(12) The term “platinum group metal” means ruthenium, rhodium, palladium, osmium, iridium, or platinum.

(13) The term “non-noble metal” means a metal that is not a platinum group metal.

(14) With reference to FIG. 1, a schematic cross section of a fuel cell that incorporates an embodiment of a grafted porous membrane is provided. Proton exchange membrane fuel cell 10 includes polymeric ion conducting membrane 12 disposed between cathode catalyst layer 14 and anode catalyst layer 16. Advantageously, one or both of cathode catalyst layer 14 and anode catalyst layer 16 include the caged electrocatalyst particles set forth below. Collectively, the combination of the ion conducting membrane 12, cathode catalyst layer 14 and anode catalyst layer 16 are a metal electrode assembly. Fuel cell 10 also includes flow field plates 18, 20, gas channels 22 and 24, and gas diffusion layers 26 and 28. Typically, gas diffusion layers 26 and 28 respectively include microporous layers 30, 30′ disposed over a face of diffusion material in the gas diffusion layers. The microporous layers 30, 30′ respectively contact cathode catalyst layer 14 and anode catalyst layer 16. In a refinement, flow field plates 18, 20 are bipolar plates. Typically, flow field plates are electrically conductive and are therefore formed from a metal such as stainless steel. In other refinements, the flow field plates formed from an electrically conductive polymer. Hydrogen ions are generated by anode catalyst layer 16 migrate through polymeric ion conducting membrane 12 were they react at cathode catalyst layer 14 to form water. This electrochemical process generates an electric current through a load connected to flow field plates 18 and 20.

(15) With reference to FIG. 2, a schematic cross section of a cage catalyst particle formed by the methods set forth below is provided. Caged electrocatalyst particles 32 include metal oxide shell 34 which defines a central cavity 36. Platinum group metal-containing particles 38 are disposed with central cavity 36. Metal oxide shell 34 defines a gap 39 between the metal oxide shell and the platinum group metal-containing particle. In a refinement, the metal oxide shell 34 is formed from silicon oxide (e.g., SiO.sub.2), aluminum oxide or combinations thereof. Typically, metal oxide shell 34 has an average spatial diameter (or largest average dimension) less than 100 nm. In a refinement, metal oxide shell 34 has an average spatial diameter (or largest average dimension) from about 1 to 15 nm. In another refinement, metal oxide shell 34 has an average spatial diameter (or largest average dimension) from about 5 to 10 nm.

(16) Since the metal oxide shell typically has an average spatial diameter less than 100 nm, the caged electrocatalyst particles 32 are also usually nanoparticles having an average diameter (or largest average dimension) less than 100 nm. In a refinement, caged electrocatalyst particles 32 are nanoparticles having an average diameter (or largest average dimension) from about 1 to 15 nm. In still another refinement, platinum group metal-containing particles 38 are nanoparticles having an average spatial diameter (or largest average dimension) from about 2 to 8 nm. In a further refinement, gap 39 has an average distance between shell 34 and electrocatalyst particles 38 from about 0.1 to 4 nm. In other refinement, gap 39 has an average distance between shell 34 and platinum group metal-containing particles 38 from about 0.5 to 2 nm. These electrocatalyst particles can then be incorporating into fuel cell catalyst layers set forth above by methods known to those skilled in the art of fuel cell technology. In many applications such as in fuel cells, caged electrocatalyst particles 32 are supported on substrates 42 such as carbon particles (e.g. carbon black).

(17) In one variation, platinum group metal-containing particles 38 are platinum alloys particles. In a refinement, such platinum alloys include a non-noble metal such as Fe, Ni, Co, Cu, and the like. In a refinement, the platinum-containing particles have formula P.sup.OM.sup.O.sub.y, where P.sup.O is a platinum-group metal (e.g., ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), and platinum (Pt)), M.sup.O is a non-noble metal, and y is the atomic ratio between the P.sup.O is and the M.sup.O. In a refinement, y ranges from 1 to 18. In a further refinement, P.sup.O is Ru, Rh, Pd, Ir, or Pt. In a refinement, M.sup.O is Fe, Ni, Co, Cu, and the like. An example of a particularly useful platinum alloy is platinum-nickel alloy such as PtNi.sub.3 particles. Other examples of platinum group metal particles are PtCo particles, PtCo.sub.3 particles, PtCu.sub.3 particles, PtZn.sub.3 particles, PtFe.sub.3 particles, PdNi.sub.3 particles, PdFe.sub.3 particles, PdRhFe.sub.3 particles, PtPdRhNi.sub.2 and the like.

(18) With reference to FIGS. 3A and 3B, a method for making caged electrocatalyst particles is schematically illustrated. In step a), platinum group metal-containing particles 40 are reacted with absorption solution. In a refinement, platinum group metal-containing particles 40 are nanoparticles. In a further refinement, platinum group metal-containing particles 40 are nanoparticles having an average diameter (or largest average dimension) less than or equal to 100 nm. In still another refinement, platinum group metal-containing particles 40 are nanoparticles having an average diameter (or largest average dimension) from about 1 to 15 nm. In still another refinement, platinum group metal-containing particles 40 are nanoparticles having an average diameter (or largest average dimension) from about 2 to 10 nm. Examples of solvents include, but are not limited to, alcohols, toluene, tetrahydrofuran, and combinations thereof. In a refinement, the absorption solution includes a compound having formula 1 and a compound having formula 2:
X.sub.1—R.sub.2-M-(OR.sub.1).sub.n  1
X.sub.2—R.sub.3—Y  2
wherein: X.sub.1 and X.sub.2 are each independently SH or NH.sub.2; R.sub.1 are each independently C.sub.1-6 alkyl; M is a metal that forms a metal oxide such as Si, Al, Ti, W and the like; Y is a moiety that does not react with the compound having formula 1 such as CO.sub.2H, CH.sub.3, NH.sub.2, halo, and the like; n is an integer represent the number of OR.sub.1 groups attached to M. In a refinement, n is 1, 2, 3, 4, 5, or 6. In an example, when M is Si or Ti, n is 3 and when M is Al, n is 2; and R.sub.2, R.sub.3 are each independently is a C.sub.1-6 alkylenyl. In a particularly useful refinement, X.sub.1 and/or X.sub.2 is SH. In another refinement, M is not a platinum group metal.

(19) In a refinement, platinum group metal-containing particles 40 are supported on substrates 42 such as carbon particles (e.g. carbon black). In still another refinement, platinum group metal-containing particles are platinum alloys particles. In a refinement, the platinum-containing particles have formula P.sup.OM.sup.O.sub.x, where P.sup.O is a platinum-group metal, M.sup.O is a non-noble metal, and x is the atomic ratio between the P.sup.O is and the M.sup.O. In a refinement, x ranges from 2 to 20. In a further refinement, P.sup.O is Ru, Rh, Pd, Ir, or Pt. In a refinement, M.sup.O is nickel (Ni) or cobalt (Co). An example of a particularly useful platinum alloy is platinum-nickel alloy such as PtNi.sub.3 particles. Other examples of platinum group metal particles are PtCo particles, PtCo.sub.3 particles, PtCu.sub.3 particles, PtZn.sub.3 particles, PtFe.sub.3 particles, PdNi.sub.3 particles, PdFe.sub.3 particles, PdRhFe.sub.3 particles, and the like. An example of a compound having formula 1 is mercaptopropyltrimethoxysilane (MPTS) while an example of a compound having formula 2 is and mercaptopropionic acid (MPA). The compounds having formula 1 and 2 adsorb on the surface of platinum group metal-containing particles 40 to form adsorbed layer 44 due to their strong affinity. FIG. 3A depicts this phenomenon for MPTS and MPA adsorbing on a PtNi.sub.3 surface. In step b), the metal M (e.g. silane) in the compound having formula 1 is allowed to hydrolyze to form modified particles 49 having porous shell 34 of metal oxide (SiO.sub.2) on surfaces of the platinum group metal-containing particles. In step c), the modified particles are then subjected to acid treatment or electrochemical oxidation to remove a portion of the platinum group metal-containing particles thereby creating caged electrocatalyst particles 32 having a gap 39 between the platinum group metal-containing particles and their metal oxide (e.g., SiO.sub.2) shell 34. For example, a fraction of the metal M.sup.O is removed from the P.sup.OM.sup.O.sub.x, particles (e.g., Ni is removed from PtNi.sub.3 particles). In a refinement, the caged electrocatalyst particles 32 are nanoparticles. These electrocatalyst particles can then be incorporating into fuel cell catalyst layers by methods known to those skilled in the art of fuel cell technology.

(20) In a refinement, one can micro-engineer functional groups on the shell to provide additional functions to the catalysts such as increased proton conductivity and increased tolerant to anion contamination. For example, R—SH remaining on the shell could be converted to a R—SO.sub.3H or R—SO.sub.2NHSO.sub.2—R. The R—SH can be converted to R—R—NH.sub.2. In these refinement, R can be any of the R.sub.1, R.sub.2, and R.sub.3 set forth above. In another refinement, the powder can be redispersed in isopropanol with a hydrophobic ionic liquid (e.g., about 65 mg) such as (7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene) (bis(perfluoroethylsulfonyl)imide). (“[mtbd][beti]”). Sonicating this solution incorporates the ionic liquid into the void between PtNi.sub.3 and SiO.sub.2 shell. This type of catalyst was shown to exhibit higher oxygen activity than one without ionic liquid.

(21) The following examples illustrate the various embodiments of the present invention. Those skilled in the art will recognize many variations that are within the spirit of the present invention and scope of the claims.

Preparation of Pt Alloy Nanoparticles Encapsulated with Porous Shell

(22) A transition metal-rich alloy nanoparticle supporting on carbon black such as PtNi.sub.3/CB (20 wt % Pt, 20 wt % Ni, 60 wt % carbon black) is used for this preparation. About 5 g of PtNi.sub.3/CB, 1M of mercaptopropyltrimethoxysilane (MPTS) and 1M mercaptopropionic acid (MPA) are mixed in a 200 ml of toluene solution. The solution is stir vigorously overnight. During this process the thiols will form a self-assemble adsorption on the PtNi.sub.3 surface. The suspension is filtered and washed several times with ethanol. Then the powder was redispersed in ethanol-water (5:1) mixture to allow hydrolyzation of silane to form a silicon dioxide porous shell on the PtNi.sub.3 particle. Optionally, 0.5M tetraethoxysilane (TEOS) was added to the mixture to increase the thickness of the SiO.sub.2 shell. The suspension is filtered and washed several times with ethanol and water. The powder is stirred in 0.5M H.sub.2SO.sub.4 solution for 1 day. During this process about 90% of Ni and all of the MPA were removed from the sample. This creates a void between the now Pt.sub.3Ni particles and the porous SiO.sub.2 shell. After being thoroughly washed and dried, the powder is ready for ink and electrode preparation in a conventional fashion.

(23) While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.