Hybrid membrane electrode assembly

Details Hybrid membrane electrode assembly Hybrid membrane electrode assembly

1999-05-14

2001-05-29

Tung, T. (Department: 1743)

Chemistry: electrical and wave energy

Apparatus

Electrolytic

C204S415000, C204S280000, C204S291000, C204S292000, C429S047000, C429S047000

Reexamination Certificate

active

06238534

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a hybrid membrane electrode assembly (MEA) having an anode comprising a dense distribution of catalyst that may be borne on small, high-aspect ratio supports, such as nanostructured elements, and a cathode comprising a less dense distribution of catalyst that may be borne on lower-aspect ratio supports, such as carbon particle supported catalyst.
BACKGROUND OF THE INVENTION
A membrane electrode assembly (MEA) may be the central element of electrochemical devices such as proton exchange membrane fuel cells, sensors, electrolyzers, chlor-alkali cells, and the like. Such MEAs typically comprise an ion conductive membrane (ICM), which functions as a solid electrolyte, in contact with electrode layers that include catalytic electrode material such as platinum. In a typical electrochemical cell, an ICM is in contact with a cathode layer and an anode layer, and transports ions that are formed at the anode to the cathode, allowing electrical current to flow in an external circuit connecting the electrodes.
One form of catalyst used in MEAs consists of Pt or Pt alloys coated onto carbon particles by wet chemical methods, such as the reduction of chloroplatinic acid. This conventional form of catalyst is dispersed with ionomeric binders, solvents and often polytetrafluoroethylene (PTFE) particles, to form an ink, paste or dispersion that is applied to either the ICM or to an electrode backing material to be placed adjacent to the ICM. In addition to providing mechanical support, it is generally believed in the art that carbon support particles provide necessary electrical conductivity within the electrode layer.
In another variation, Pt fines can be mixed directly with a solution of solvents and polymer electrolyte or Teflon™ and coated onto the electrode backing layer or membrane ICM. However, because of limitations on how small the fines can be made, this approach typically results in very high loading of the catalyst with resulting increase in expense.
Nanostructured composite articles are disclosed in U.S. Pat. Nos. 4,812,352, 5,039,561, 5,176,786, 5,336,558, 5,338,430, and 5,238,729. U.S. Pat. No. 5,338,430 discloses that nanostructured electrodes embedded in solid polymer electrolyte offer superior properties over conventional electrodes employing metal fines or carbon supported metal catalysts, including more efficient use of the electrode material and enhanced catalytic activity per unit mass of Pt.
U.S. Pat. No. 5,879,828 concerns MEAs having electrode layers comprising nanostructured elements. U.S. Pat. No. 5,879,827 concerns nanostructured elements bearing nanoscopic catalyst particles which may be suitable for use in MEAs.
SUMMARY OF THE INVENTION
Briefly, the present invention provides a hybrid membrane electrode assembly (MEA) having an anode layer and a cathode layer wherein catalyst material is borne on support particles, wherein the average density of the first catalyst material in the anode layer is greater than 1.0 mg/mm
3
and average density of the second catalyst material in the cathode layer is less than 1.0 mg/mm
3
.
In another aspect, the present invention provides a hybrid MEA wherein the electrochemical surface area/volume ratio of the catalyst material in the anode layer is greater than 200 cm
2
/mm
3
and wherein the electrochemical surface area/volume ratio of the catalyst material in the cathode layer is less than 200 cm
2
/mm
3
.
In another aspect, the present invention provides a hybrid MEA having an anode layer comprising a catalyst material borne on support particles having an average aspect ratio of greater than 3 and a cathode layer comprising a catalyst material borne on support particles having an average aspect ratio of less than 3.
What has not been described in the art, and is provided by the present invention is a hybrid MEA showing improved performance by the use of a dense distribution of catalyst in the anode layer, preferably by use of nanostructured elements, and a less dense distribution of catalyst in the cathode layer, which may be achieved by the use of carbon-supported catalyst.
In this application:
“electrochemical surface area” means the surface area available for participation in an electrochemical reaction as determined by H
2
adsorption/desorption;
“membrane electrode assembly” means a structure comprising a membrane that includes an electrolyte and at least one but preferably two or more electrodes adjoining the membrane;
“microtextures” means surface structures, features or convolutions made by any process, including impression, molding or etching, whose average depth is between 1 and 100 micrometers;
“nanostructured element” means an acicular, discrete, microscopic structure comprising a catalytic material on at least a portion of its surface;
“microstructure” means an acicular, discrete, microscopic structure;
“nanoscopic catalyst particle” means a particle of catalyst material having at least one dimension of about 10 nm or less or having a crystallite size of about 10 nm or less, measured as diffraction peak half widths in standard 2-theta x-ray diffraction scans;
“acicular” means having a ratio of length to average cross-sectional width of greater than or equal to 3;
“discrete” refers to distinct elements, having a separate identity, but does not preclude elements from being in contact with one another;
“microscopic” means having at least one dimension equal to or smaller than about a micrometer; and
“substituted” means, for a chemical species, substituted by conventional substituents which do not interfere with the desired product or process, e.g., substituents can be alkyl, alkoxy, aryl, phenyl, halo (F, Cl, Br, I), cyano, nitro, etc.
It is an advantage of the present invention to provide MEAs having improved performance characteristics for use in electrochemical cells including fuel cells.


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Liu L et al.; “Carbon Supported And Unsupported Pt-Ru-Anodes For Liquid Feed Direct Methanol Fuel Cells”,Electrochimica Acta, Gb. Elsevier Science Publishers, Barking, vol. 43, No. 24, Aug. 21, 1998, pp. 3657-3663.

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