Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
2001-09-28
2004-11-02
Bell, Bruce F. (Department: 1746)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S006000, C429S006000, C429S006000, C429S006000
Reexamination Certificate
active
06811915
ABSTRACT:
BACKGROUND
This disclosure relates to electrochemical cells, and, more particularly, to an apparatus and methods for improving cell operation.
Electrochemical cells are energy conversion devices that are usually classified as either electrolysis cells or fuel cells. Proton exchange membrane electrolysis cells can function as hydrogen generators by electrolytically decomposing water to produce hydrogen and oxygen gases. Referring to
FIG. 1
, a section of an anode feed electrolysis cell of the related art is shown at
10
and is hereinafter referred to as “cell 10.” Reactant water
12
is fed to cell
10
at an oxygen electrode (e.g., an anode)
14
where a chemical reaction occurs to form oxygen gas
16
, electrons, and hydrogen ions (protons). The chemical reaction is facilitated by the positive terminal of a power source
18
connected to anode
14
and a negative terminal of power source
18
connected to a hydrogen electrode (e.g., a cathode)
20
. Oxygen gas
16
and a first portion
22
of the water are discharged from cell
10
, while the protons and a second portion
24
of the water migrate across a proton exchange membrane
26
to cathode
20
. At cathode
20
, hydrogen gas
28
is formed and is removed for use as a fuel or a process gas. Second portion
24
of water, which is entrained with hydrogen gas, is also removed from cathode
20
.
Another type of water electrolysis cell that utilizes the same configuration as is shown in
FIG. 1
is a cathode feed cell. In the cathode feed cell, process water is fed on the side of the hydrogen electrode. A portion of the water migrates from the cathode across the membrane to the anode. A power source connected across the anode and the cathode facilitates a chemical reaction that generates hydrogen ions and oxygen gas. Excess process water exits the cell at the cathode side without passing through the membrane.
A typical fuel cell also utilizes the same general configuration as is shown in FIG.
1
. Hydrogen gas is introduced to the hydrogen electrode (the anode in the fuel cell), while oxygen, or an oxygen-containing gas such as air, is introduced to the oxygen electrode (the cathode in the fuel cell). The hydrogen gas for fuel cell operation can originate from a pure hydrogen source, a hydrocarbon, methanol, an electrolysis cell, or any other source that supplies hydrogen at a purity level suitable for fuel cell operation. Hydrogen gas electrochemically reacts at the anode to produce protons and electrons, the electrons flow from the anode through an electrically connected external load, and the protons migrate through the membrane to the cathode. At the cathode, the protons and electrons react with oxygen to form water.
Conventional electrochemical cell systems generally include one or more individual cells arranged in a stack, with the working fluids directed through the cells via input and output conduits formed within the stack structure. The cells within the stack are sequentially arranged, each including a membrane electrode assembly (hereinafter “MEA”) defined by the cathode, the proton exchange membrane, and the anode. Each cell typically further comprises a first flow field in fluid communication with the cathode and a second flow field in fluid communication with the anode. The MEA may be supported on either or both sides by flow field support members such as screen packs or bipolar plates disposed within the flow fields, and which may be configured to facilitate membrane hydration and/or fluid movement to and from the MEA. Because a differential pressure often exists across the MEA during operation of the cell, pressure pads or other compression means are employed to maintain uniform compression of the cell components, thereby maintaining intimate contact between flow fields and cell electrodes over long time periods.
Referring now to
FIG. 2
, a conventional electrochemical cell system illustrating the spatial relationship between the active area (defined by the electrodes and the space therebetween) and cell frames is shown at
20
. In cell system
20
, the MEA
22
is typically supported by the flow field support members
24
and clamped between cell frames
26
. Limitations inherent in the precision manufacture of flow field support members
24
and cell frames
26
result in the presence of a first gap
30
of dimension l
1
between a peripheral outer surface of flow field support member
24
and an inner boundary surface of cell frame
26
during the assembly of the cell. When the cell is fully assembled and MEA
22
is supported within cell frames
26
, the pressure differential is such that the pressure on one side of MEA
22
is higher than the pressure on the other side of MEA
22
. During operation of the cell, MEA
22
must be capable of supporting this pressure differential. First gap
30
between cell frame
26
and flow field support member
24
oftentimes exceeds a width beyond which MEA
22
can span and support the pressure differential without deforming. Deforming of MEA
22
may result in a compromise of the structural integrity of cell system
20
.
One manner of accommodating the presence of first gap
30
and the problems associated with pressure differentials involves incorporating a thin metal or polymer protector ring
32
into the electrochemical cell. Protector ring
32
supports the pressure load imposed on MEA
22
over first gap
30
. At high cell operating pressures, however, internal cell dynamics associated with repeated pressure cycles may cause relative motion between cell components, which may dislocate protector ring
32
even after successful cell assembly and cause the presence of a second gap
34
of dimension l
2
between protector ring
32
and cell frame
26
. The dislocation of protector ring
32
may result in the exposure of MEA
22
to gaps
30
,
34
, which may cause less than optimum performance of the cell to be realized.
The maintaining of compression within the cell and the containment of the various electrochemical reactants and by-products generated in the cell is achieved by the use of thin, non-resilient gaskets, which are typically fabricated from polytetrafluoroethylene. When placed under the clamping loads encountered within the electrochemical cell, these non-resilient gaskets creep or deform to fill any imperfections in the surfaces of the components that they are intended to seal. The internal pressures that are effectively contained using such clamping methods may be considerably less than the pressure load exerted on the gaskets prior to any internal pressure being generated. As a result, the containment of high pressures using the non-resilient gasket approach requires very high clamping loads, which may, over the lifetime of the cell, become impractical. Furthermore, since such gaskets are non-resilient, they are ineffective at accommodating any creep that may occur as a result of a lessening of the clamping load. As such, they are likely to develop leaks over time as creep effects cause the clamping load to be relaxed. Moreover, the non-resilient gaskets may require a time consuming creep-inducing “heat soak” procedure to initiate the sealing of components.
While existing protector rings and gaskets are suitable for their intended purposes, there still remains a need for an improved apparatus and method of maintaining the compression of the cell and of protecting the MEA, particularly regarding the bridging of the gap between the flow field support member and the cell frame and the retaining of the protector ring thereacross during both assembly and operation of the cell. Therefore, a need exists for an integrally structured cell frame/flow field support member that allows cell compression to be maintained while protecting and supporting the MEA.
SUMMARY
The above-described drawbacks and disadvantages are alleviated by an electrochemical cell system in which a cell frame is integrated with a flow field support member. The cell system includes an electrode, a proton exchange membrane and the flow field support member disposed at the electrode to s
Dristy Mark E.
Hanlon Greg A.
Bell Bruce F.
Cantor & Colburn LLP
Proton Energy Systems, Inc.
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