Chemistry: electrical current producing apparatus – product – and – Having earth feature
Reexamination Certificate
1999-04-12
2001-08-21
Bell, Bruce F. (Department: 1741)
Chemistry: electrical current producing apparatus, product, and
Having earth feature
C429S047000, C429S047000, C429S047000, C029S746000
Reexamination Certificate
active
06277513
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to electrodes for use in electrochemical cells.
BACKGROUND OF THE INVENTION
Electrochemical cells are desirable for various applications, particularly when operated as fuel cells. Fuel cells have been proposed for many applications including electrical vehicular power plants to replace internal combustion engines. One fuel cell design uses a solid polymer electrolyte (SPE) membrane or proton exchange membrane (PEM), to provide ion exchange between the cathode and anode. Gaseous and liquid fuels are useable within fuel cells. Examples include hydrogen and methanol, and hydrogen is favored. Hydrogen is supplied to the fuel cell's anode. Oxygen (as air) is the cell oxidant and is supplied to the cell's cathode. The electrodes are formed of porous conductive materials, such as woven graphite, graphitized sheets, or carbon paper to enable the fuel to disperse over the surface of the membrane facing the fuel supply electrode. A typical fuel cell is described in U.S. Pat. No. 5,272,017 and U.S. Pat. No. 5,316,871 (Swathirajan et al.).
Important aspects of a fuel cell include reaction surfaces where electrochemical reactions take place, catalysts which catalyze such reaction, ion conductive media, and mass transport media. The cost of power produced by a fuel cell is in part dependent on the cost of the catalyst. The cost of power produced by a fuel cell is significantly greater than competitive power generation alternatives, partly because of relatively poor utilization of precious metal catalysts in conventional electrodes. However, power produced from hydrogen-based fuel cells is desirable because hydrogen is environmentally acceptable and hydrogen fuel cells are efficient. Therefore, it is desirable to improve the catalyst utilization in fuel cell assemblies to render fuel cells more attractive for power generation. It is also desirable to improve reactant gas diffusion and movement of product water in the fuel cell.
SUMMARY OF THE INVENTION
In one aspect there is provided an electrode structure comprising a current collector sheet, a first electrode layer, and a second electrode layer. The first electrode layer is between the current collector sheet and the second electrode layer. The first layer comprises a first group of carbon particles and the second layer comprises a second group of carbon particles. The first layer is uncatalyzed or catalyzed with a first group of very finely divided catalytic particles; and the second layer is catalyzed with a second group of very finely divided catalytic particles. The weight ratio of catalytic particles to carbon particles of the first layer is less than that of the second layer.
In one embodiment, each one of the carbon particle groups comprises a plurality of the carbon particles having internal and external surfaces defining a plethora of pores within and between the carbon particles. The very finely divided catalytic particles are supported on the internal and the external surfaces of the carbon particles.
In another embodiment, the first layer is uncatalyzed and the second layer comprises the carbon particles having very finely divided catalytic particles supported on the internal and the external surfaces of the carbon particles.
Preferably, the first group of carbon particles is characterized by a density of 0.1 grams per cubic centimeter or less, corresponding to a volume per gram of at least 10 cubic centimeters per gram. Desirably, the second group of carbon particles is characterized by a pH which is in a range of about 6 to about 9. Preferably, each one of the carbon particle groups is characterized by a pH which is in a range of about 6 to about 9. Desirably, the second group of carbon particles is characterized by an average pore radius which is greater than 5 nanometers. Each one of the layers further comprises a proton conductive material intermingled with the carbon particles and the catalytic particles.
Desirably, the catalytic particle loading of the second layer is less than about 0.30 mg per cm
2
of electrode surface area. The catalytic loading of the first layer is less than that of the second layer, desirably is on the order of up to about 0.15 mg/cm
2
, and preferably is on the order of up to about 0.02 mg/cm
2
.
In one aspect, the second layer comprises catalytic particles and carbon particles in a weight ratio of about 20:80. The proton conductive material constitutes 30 to 35 percent by weight of said second layer, and catalytic and carbon particles constitute the balance.
In one embodiment there is provided a method of making the improved electrode structure described above for use in an electrochemical cell. The first layer of the electrode is produced by forming a mixture comprising proton-conductive material, a first group of carbon particles, and optimally catalytic particles. The mixture is applied to a current collector sheet to form a film. The second layer of the electrode is produced by forming a second layer over the first layer, where said second layer comprises proton-conductive material, a second group of carbon particles, and catalytic particles. The amount by weight of catalytic particles relative to carbon particles of the second layer is greater than that of the first layer. This method produces an electrode having significantly increased catalyst utilization, dramatic reduction of catalyst loading, and which is consequently less expensive to produce than electrodes produced by prior art methods.
There is also provided a method of making a combination electrolyte and electrode structure for an electrochemical cell having an electrolyte membrane of solid polymer proton-conductive material and first and second electrodes disposed on either side of the electrolyte membrane. At least one of the electrodes is formed by the method of the invention described above. The electrode produced in this method is then placed on a first surface of the electrolyte membrane such that the second layer faces the membrane. A second electrode is placed on the opposite surface of the membrane and the resulting structure is heated and compressed to adhere the electrodes to the membrane. In a preferred embodiment of the invention method the electrodes are adhered to the membrane by subjecting the assembly to a compressive load and an elevated temperature to result in some of the particles becoming at least partially embedded in the membrane, thereby providing a continuous path for protons to the catalyst site where reaction occurs.
The first and second groups of carbon particles are the same or different. That is, they may have the same characteristics or differ in at least one characteristic. In the case where both layers are catalyzed, the catalyst of the respective layers may be the same or different.
As can be seen from the description of the electrode, membrane electrode assembly, and the fuel cell system described above, the invention provides improved catalyst utilization and improved water management.
It is an object of the invention to provide new electrodes and new membrane electrode assemblies. Another object is to provide a method for preparing the electrodes and assemblies containing the improved electrodes. Advantageously, the membrane/electrode assembly of the invention provides relatively high power output with unexpectedly low catalyst loading.
These and other objects, features and advantages will become apparent from the following description of the preferred embodiments, claims, and accompanying drawings.
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patent: 4876115 (1989-10-01), Raistrick
patent: 5211984 (1993-05-01), Wilson
patent: 5234777 (1993-08-01), Wilson
patent: 5272017 (1993-12-01), Swathirajan et al.
patent: 5316871 (1994-05-01), Swathirajan et al.
patent: 5350643 (1994-09-01), Imahashi et al.
patent: 5431800 (1995-07-01), Kirchhoff et al.
patent: 5561000 (1996-10-01), Dirven et al.
patent: 5882810 (1999-03-01), Mussell et al.
patent: 6017650 (2000-01-01), Ramunni et al.
patent: 0569062A3 (1993-11-01), None
patent: 05
Mikhail Youssef M.
Swathirajan Swathy
Bell Bruce F.
General Motors Corporation
Plant Lawrence B.
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