Unitized barrier and flow control device for electrochemical...

Details Unitized barrier and flow control device for electrochemical... Unitized barrier and flow control device for electrochemical...

2000-10-05

2003-05-13

Kalafut, Stephen (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Having earth feature

C429S047000, C429S047000, C502S101000

Reexamination Certificate

active

06562507

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to separators between adjacent electrochemical cells. More particularly, the invention relates to lightweight bipolar plates and methods for their construction.
BACKGROUND OF THE INVENTION
Most of the components currently used in proton exchange membrane (PEM) fuel cells are derived from designs originally developed for use in phosphoric acid fuel cells (PAFC), and are not optimal for the higher performance of PEM fuel cells.
By the mid-1970s, components consisting entirely of carbon were made for use in PAFC's operating at temperatures in the 165-185° C. range. In the case of one manufacturer (Energy Research Corporation, Danbury, Connecticut) bipolar plates were molded from a mixture of graphite powder (approximately 67 wt %) and phenolic resin (approximately 33 wt %) and were carefully heat-treated to carbonize the resin without introducing excessive porosity by rapid degassing. Typically, heat treatment to 900° C. was sufficient to give the required chemical, physical and mechanical properties. Initially bipolar plates were molded flat and were machined to produce the required reactant gas distribution grooves (or cooling grooves for the bipolar plate). Later, grooved plates were molded in a die (which was slightly oversized to compensate for shrinkage during baking) to produce the glassy graphitic, carbon-composite plate. In work performed at Westinghouse in the late 1970s/early 1980s the “straight through” gas distribution grooves on the bipolar plate were redesigned to yield an arrangement which has become known as the Z-plate.
The bipolar/separator plate in United Technologies Corporation's (UTC's) 1 MW demonstration stack (ca. 1975) was molded from graphite powder and polyphenylene sulfide resin. The corrosion resistance of this plate was shown to be only marginally acceptable in the finished demonstrator. Therefore, as shown in
FIG. 1A
, the plates in the 4.5 MW New York demonstrator
10
(ca. 1978) and also in subsequent UTC stacks were prepared by molding from graphite powder and inexpensive resins, followed by baking and graphitization at about 2700° C. The surfaces of the molded ribs were then finished by sanding. In the 4.5 MW demonstrator
10
, separator plates
12
lying between the anodes
14
and cathodes
16
of adjacent cells were ribbed on both sides to provide gas channels arranged perpendicular to each other.
As shown in
FIG. 1B
, the later 40 kW on-site units fabricated by UTC (ca. 1983) used a new ribbed substrate stack
20
. This system placed the gas distribution channels
22
in the porous electrode substrate
24
itself, rather than in the flat bipolar plate
26
, which was about 1 mm thick. The ribbed sides of the substrate
24
contacted the surface of this flat bipolar plate. The catalytic electrode mix
28
was applied to the opposite sides. The initially perceived advantage for this technology was reduced cost, since it offered the possibility of molded bipolar plates requiring a minimum of surface finishing, together with ribbed substrates of relatively low porosity that would be easy to machine. Pressurized PAFCs require the use of fully, or at least partially, graphitized bipolar plates and electrocatalyst substrates with heat treatment temperatures of at least 1800° C. and preferably 2700° C., or alternatively glassy carbons produced at high temperature.
The technology for making carbon/graphite bipolar plates for PAFCs has been used in PEM fuel cells by all the major PEM fuel cell developers (International Fuel Cells, Inc., Ballard Power Systems, H-Power Corp., Energy Partners, Fuji, and Siemens). While it is effective, it is expensive, and it is difficult to produce thin carbon based bipolar plates, and consequently stacks built with these plates tend to be heavy and bulky.
An obvious approach to overcoming these limitations is to use a moldable graphite-based composite that does not have to be carbonized. In this type of material graphite powder, which serves as the conductor, is bonded into a rigid piece with a polymer matrix. The graphite retains its conductivity and corrosion resistance, and the polymer binder, which must also be stable under PEM operating conditions, allows it to be formed by conventional polymer forming processes.
This approach was examined by General Electric in the early 1980's, and has been used successfully by Energy Partners for the 7 kW stack that they built for the Ford Motor Company as a vehicle prototype.
This approach has distinct limitations. When the graphite is diluted with the polymer, its conductivity, already lower than any metal useful in this application, is reduced even further. A seven kilowatt stack with pure graphite bipolar plates would be expected to have a 16 Watt internal resistive loss. When the graphite is dispersed in a polymer matrix, this loss will be larger. This is clearly shown by the data in Table I, which contains the properties of a number of materials potentially useful for fabricating bipolar plates for PEM fuel cell stacks. This table gives the specific resistivity and density of each element. Also included in the table are the mass and through plate resistance for the flow field region of a bipolar plate made from each material. This hypothetical plate was modeled as being 3.75 mm thick, with an active area of 125 cm
2
and a serpentine flow field having channels 1.5 mm deep occupying 50% of the active area.
TABLE I
Properties of Materials Useful for Bipolar Plates
Specific
Resistivity
Density
Mass
R
Element
(&mgr;&OHgr;-cm)
(g/mL)
(g)
(&mgr;&OHgr;)
Cu
1.673
8.89
250
0.009
Al
2.655
2.70
75.9
0.014
Mg
4.450
1.74
48.9
0.024
Ti
42.0
4.50
127
0.23 
C
a
1375
2.25
63.3
7.4 
a
Graphite
The weight comparisons in Table I are based on plates which have identical dimensions and different compositions. In all cases, the thickness of the gas barrier is 0.75 mm at the thinnest point. For a graphite plate, this is very thin. Given the porosity of most graphitic materials, using a barrier this thin would require filling the pores with a sealant to produce a reliably gas-tight barrier. With any of the metals shown, this barrier could be made even thinner, with further gains in both size and weight.
Replacing graphite with any of the metals would increase both electrical and thermal conductivity significantly. Cu has the greatest conductivity of the metals listed in Table I, but it also has a high density. A solid metal stack would be quite heavy. Other strategies for bipolar plate construction have been developed to overcome some of the aforementioned limitations. As an example, recent efforts at Los Alamos National Laboratory have focused on the application of various expanded metal screens as flow fields. The screens are backed by a thin metal plate of the same material to create a bipolar plate configuration for use in a stack. An advantage of this system is the low material and manufacturing costs. However, one major disadvantage is the poor conductivity that results from multiple interfaces and the screen/plate point contacts.
Like much other PEM fuel cell technology, the basic electrode structures used in most PEM fuel cells are derived from phosphoric acid fuel cell (PAFC) technology. A conventional electrode structure
30
, as shown in
FIG. 2
, has a thin layer of Pt
32
supported on high surface area carbon
34
as the active electrocatalyst. This is supported on a much thicker gas diffusion layer
36
typically consisting of an open matrix PTFE bonded carbon powder composite impregnated into a conductive carbon cloth support. The carbon support is in contact with the graphitic or metallic flow fields
38
on the bipolar plate. An alternative design uses conductive carbon paper to serve both the gas diffusion and support functions. A more recent variation in this design has an even thinner electrode, (described as either a thin layer electrode or an ink electrode) fabricated directly on the membrane
39
. This electrode, while using less Pt, still uses the same gas diffusion structure as the co

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