Polymer electrolyte membrane electrochemical fuel cells and...

Details Polymer electrolyte membrane electrochemical fuel cells and... Polymer electrolyte membrane electrochemical fuel cells and...

2000-03-10

2002-12-17

Brouillette, Gabrielle (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S047000

Reexamination Certificate

active

06495278

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to electrochemical energy converters with a polymer electrolyte membrane (PEM), such,as fuel cells or electrolyzer cells or stacks of such cells, wherein the cells or stacks comprise adhesively bonded layers.
BACKGROUND OF THE INVENTION
Electrochemical cells comprising polymer electrolyte membranes (PEMs) may be operated as fuel cells wherein a fuel and an oxidant are electrochemically converted at the cell electrodes to produce electrical power, or as electrolyzers wherein an external electrical current is passed between the cell electrodes, typically through water, resulting in generation of hydrogen and oxygen at the respective electrodes of the cell.
FIG. 1
illustrates a typical design of a conventional electrochemical cell comprising a PEM, and a stack of such cells. Each cell comprises a membrane electrode assembly (MEA)
5
such as that illustrated in an exploded view in
FIG. 1
a
. MEA
5
comprises an ion-conducting PEM layer
2
interposed between two electrode layers
1
,
3
which are typically porous and electrically conductive, and comprise an electrocatalyst at the interface with the adjacent PEM layer
2
for promoting the desired electrochemical reaction. The electrocatalyst generally defines the electrochemically active area of the cell. The MEA is typically consolidated as a bonded laminated assembly. In an individual cell
10
, illustrated in an exploded view in
FIG. 1
b
, an MEA is interposed between a pair of separator plates
11
,
12
, which are typically fluid impermeable and electrically conductive. The cell separator plates are typically manufactured from non-metals such as graphite or from metals, such as certain grades of steel or surface treated metals, or from electrically conductive plastic composite materials. Fluid flow spaces, such as passages or chambers, are provided between the plate and the adjacent electrode to facilitate access of reactants to the electrodes and removal of products. Such spaces may, for example, be provided by means of spacers between separator plates
11
,
12
and corresponding electrodes
1
,
3
, or by provision of a mesh or porous fluid flow layer between separator plates
11
,
12
and corresponding electrodes
1
,
3
. More commonly channels (not shown) are formed in the face of the separator plate facing the electrode. Separator plates comprising such channels are commonly referred to as fluid flow field plates. In conventional PEM cells, resilient gaskets or seals are typically provided between the faces of the MEA
5
and each separator plate
11
,
12
around the perimeter to prevent leakage of fluid reactant and product streams.
Electrochemical cells with a ion-conductive PEM layer, hereinafter called PEM cells, are advantageously stacked to form a stack
100
(see
FIG. 1
d
) comprising a plurality of cells disposed between a pair of end plates
17
,
18
. A compression mechanism (not shown) is typically employed to hold the cells tightly together, maintain good electrical contact between components and to compress the seals. In the embodiment illustrated in
FIG. 1
c
, each cell
10
comprises a pair of separator plates
11
,
12
in a configuration with two separator plates per MEA. Cooling spaces or layers may be provided between some or all of the adjacent pairs of separator plates in the stack assembly. An alternative configuration has a single separator plate or “bipolar plate” interposed between pairs of MEAs, contacting the cathode of one cell and the anode of the adjacent cell, thus resulting in only one separator plate per MEA in the stack (except for the end cell). The stack may comprises a cooling layer interposed between every few cells of the stack, rather than between each adjacent pair of cells.
The cell elements described have openings
30
formed therein which, in the stacked assembly, align to form fluid manifolds for supply and exhaust of reactants and products and, if cooling spaces are provided, for a cooling medium. Again, resilient gaskets or seals are typically provided between the faces of the MEA
5
and each separator plate
11
,
12
around the perimeter of these fluid manifold openings to prevent leakage and intermixing of fluid streams in the operating stack.
In the future it is anticipated that a major area of application for PEM fuel cells will be for electrical power generation in stationary power plants and portable power generation systems, and for propulsion in motor vehicles. For these applications, a PEM fuel cell service life of at least 10 years is desirable. Production costs are important and will play a central role in the successful commercialization of PEM fuel cells for these applications. Other important considerations when designing a PEM cell are simplicity and cost-effectiveness of maintenance and repair.
The present invention relates to improved sealing and construction of individual PEM cells and stacks of such cells. Conventional PEM cell sealing mechanisms generally employ resilient gaskets made of elastomeric materials, which are typically disposed in grooves in the separator plates or MEAs, for example, as described in U.S. Pat. Nos. 5,176,966 and 5,284,718. Over the course of an electrochemical cell's service life the elastomeric gaskets are subjected to prolonged deformation and sometimes a harsh operating environment. Over time such gaskets tend to decrease in resilience, for example due to compression set and chemical degradation, and may become permanently deformed. This impacts negatively on the sealing function and can ultimately lead to an increased incidence of leaks.
With such gasketed plates, the plastic deformation of the plates increases as the full force of pressure on the sealing area of the plate is continuously applied. Moreover, an uneven gasket pressure force distribution along the length of the stack, with a minimum in the center, can be observed in stacks using such a sealing mechanism. Thus, the sealing elements of the cells are typically exposed to higher pressure in the end plate areas in order to guarantee adequate sealing performance in the center cells of the stack. Increased sealing pressure applied to the cells in the end plate areas may then lead to increased plastic deformations and a shorter time to gasket failure.
The assembly of a PEM cell stack which comprises a plurality of PEM cells each having many separate gaskets which must be fitted to or formed on the various components is labor-intensive, costly and generally unsuited to high-volume manufacture due to the multitude of parts and assembly steps required. Further, in the design and manufacture of PEM cells, in order to achieve the desired specifications, such as increased power density, there is a desire to make the individual cell elements thinner. Accordingly, there will be finer dimensional tolerances required for such thin cell elements and it will become more difficult to design gaskets which will maintain high dimensional tolerances, despite the use of highly elastic materials, as even highly elastic materials have a limited elastic deformation range.
With conventional PEM cell designs, it is sometimes difficult to remove and repair an individual cell or to identify or test which cells in a stack may require repair. Furthermore, disassembly of a stack consisting of multiple cells each comprising separate cell components can be very costly as in many instances, after the removal of one cell, the gaskets of all the remaining cells may need to be replaced before the stack can be reassembled and reused.
Another disadvantage of conventional PEM cells arises because the PEM typically projects beyond the edges of the electrodes and cell separator plates around the perimeter and around manifold openings. The projecting portion of the PEM may serve to avoid short circuits between plates, and it typically contacts and cooperates with the gaskets to form the fluid seal between the MEA and separator plates. However, such designs tend to leave the PEM edge exposed to air and/or reactant or coolant streams. Expo

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