Encapsulating seals for electrochemical cell stacks and...

Details Encapsulating seals for electrochemical cell stacks and... Encapsulating seals for electrochemical cell stacks and...

2000-11-06

2003-07-22

Gulakowski, Randy (Department: 1746)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S006000, C429S006000, C429S006000, C429S006000, C029S623200, C427S115000

Reexamination Certificate

active

06596427

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to electrochemical cell stacks comprising encapsulating seals in addition to individual cell seals. The present invention also relates to improved methods of manufacturing and sealing electrochemical cell stacks through the use of individual cell seals and an encapsulating seal. The encapsulating seal is preferably be formed by injection molding or other suitable methods.
BACKGROUND OF THE INVENTION
Electrochemical cells comprising polymer electrolyte membranes (PEMs) may be operated as fuel cells. In such fuel cells, a fuel and an oxidant are electrochemically converted at the cell electrodes to form a reaction product, and producing electrical power in the process. Electrochemical cells comprising PEMs may also be operated as electrolyzers, in which 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, prior art electrochemical cell comprising a proton exchange membrane, 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
. Each MEA
5
comprises an ion-conducting proton exchange membrane
2
interposed between two electrode layers
1
,
3
which are typically porous and electrically conductive. Each electrode comprises an electrocatalyst at the interface with the adjacent PEM
2
for promoting the desired electrochemical reaction. The electrocatalyst generally defines the electrochemically active area of the cell. The membrane electrode assembly may be consolidated as a bonded laminated assembly.
In an individual cell
10
, illustrated in an exploded view in
FIG. 1
b
, a membrane electrode assembly is interposed between a pair of separator plates
11
,
12
, which are typically fluid impermeable and electrically conductive. Fluid flow spaces, such as passages or chambers, are provided between each 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 (or reactant) flow field plates.
Electrochemical cells with an ion-conductive PEM layer, hereinafter referred to as PEM cells, are advantageously arranged 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, to 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
with MEA
5
disposed between them. 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 membrane electrode assemblies. Such a bipolar separator plate contacts the cathode of one cell and the anode of the adjacent cell, thus resulting in only one separator plate per membrane electrode assembly in the stack (except for the end cell). In some arrangements, the stack 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. Seals are typically provided between the faces of the membrane electrode assembly
5
and between each separator plate
11
,
12
around the perimeter of the fluid manifold openings to prevent leakage and intermixing of fluid streams in the operating stack.
Sealing and construction of seals for electrochemical cell stacks is an important practical consideration. In some conventional cell stacks, resilient gaskets or seals are provided between the faces of the membrane electrode assembly
5
and each separator plate
11
,
12
around the perimeter or at the edge to prevent leakage of fluid reactant and product streams. Such resilient gaskets are typically formed from elastomeric materials, and are typically disposed within grooves in the separator plates or membrane electrode assemblies, as described in, for example, U.S. Pat. Nos. 5,176,966 and 5,284,718. Over the course of the service life of an electrochemical cell, the elastomeric gaskets are subjected to prolonged deformation and sometimes a harsh operating environment. Over time, the resiliency of such gaskets tends to decrease due to, for example, compression set and chemical degradation, and the gaskets may become permanently deformed. This deformation impacts negatively on the sealing function and can ultimately lead to an increased incidence of leaks. Prevention of leakage and intermixing of reactants and/or coolant is an important consideration for cell stack design and manufacture. The present sealing technique overcomes problems caused by leakage of reactant and/or coolant streams from around and between gaskets, thereby improving cell stack performance.
In PEM electrochemical cells, the proton exchange membrane may project beyond the edges of the electrodes and cell separator plates around the perimeter and around manifold openings. The projecting portion of the proton exchange membrane may serve to avoid short circuits between plates, and it typically contacts and cooperates with the gaskets to form the fluid seal between the membrane electrode assembly and separator plates. Such designs tend to leave the edge of the proton exchange membrane exposed to air and/or reactant or coolant streams, however. Exposure to air or other dry gas streams can cause drying of the proton exchange membrane beginning from the edge and moving towards the center. Drying of the membrane can lead to permanent damage to the membrane, reduced cell performance and ultimately malfunction of the PEM cells. Exposure of the edge of the proton exchange membrane to some coolants and other streams can result in physical and/or chemical damage to the membrane or electrodes.
Another approach to sealing the membrane electrode assembly involves the use of an adhesive bond between each separator plate and the MEA in those regions of the cell where sealing is necessary or desirable. The adhesive bond must be substantially gas and liquid impermeable. Adhesive materials (otherwise commonly referred to as adhesives, bonding agents, glues or cements) are typically employed to form a seal and bond, for example, around the perimeter of the electrochemically active area of the MEA and around fluid manifold openings formed in the cell elements, while consolidating individual components of the PEM cell into a unitary structural unit. The MEA is preferably firmly bonded or adhered to the separator plates such that force would be required to separate the components.
In the design and manufacture of PEM cells, it is desirable to make the individual cell elements thinner. Due to the increasing demands on seals as cell elements become progressively thinner, providing for reliable sealing of fuel cell stacks will be an important part of increasing service life and decreasing maintenance costs. As cell thickness decreases, the seals between the membrane electrode assembly and separator plates have become thinner. As cells have become thinner, the cells have become more difficult to make reliably leak-proof. Further, they have become more vulnerable to electrical shor

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