Chemistry: electrical current producing apparatus – product – and – Having magnetic field feature
Reexamination Certificate
2001-01-25
2003-02-04
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Having magnetic field feature
Reexamination Certificate
active
06514635
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to fuel cell systems and, more particularly, to procedures for shutting down an operating fuel cell system.
2. Background Information
It is well known in the fuel cell art that, when the electrical circuit is opened and there is no longer a load across the cell, such as upon and during shut-down of the cell, the presence of air on the cathode, coupled with hydrogen fuel remaining on the anode, often cause unacceptable anode and cathode potentials, resulting in catalyst and catalyst support oxidation and corrosion and attendant cell performance degradation. It was thought that inert gas needed to be used to purge both the anode flow field and the cathode flow field immediately upon cell shut-down to passivate the anode and cathode so as to minimize or prevent such cell performance degradation. Further, the use of an inert gas purge avoided the possible occurrence of a flammable mixture of hydrogen and air, which is a safety issue. While the use of 100% inert gas as the purge gas is most common in the prior art, commonly owned U.S. Pat. Nos. 5,013,617 and 5,045,414 describe using 100% nitrogen as the anode side purge gas, and a cathode side purging mixture comprising a very small percentage of oxygen (e.g. less than 1%) with a balance of nitrogen. Both of these patents also discuss the option of connecting a dummy electrical load across the cell during the start of purge to lower the cathode potential rapidly to between the acceptable limits of 0.3-0.7 volt.
It is undesirable to use nitrogen or other inert gas as a shut-down or start-up purge gas for fuel cells where compactness and service interval of the fuel cell powerplant is important, such as for automotive applications. Additionally, it is desired to avoid the costs associated with storing and delivering inert gas to the cells. Therefore, safe, cost effective shut-down and start-up procedures are needed that do not cause significant performance degradation and do not require the use of inert gases, or any other gases not otherwise required for normal fuel cell operation.
BRIEF SUMMARY OF THE INVENTION
In accordance with the present invention, a fuel cell system, which recirculates a portion of the anode flow field exhaust through the anode flow field in a recycle loop during operation, is shut-down by disconnecting the primary load from the external circuit and thereafter stopping the flow of fresh hydrogen containing fuel into the anode flow field and catalytically reacting hydrogen in the recirculating anode exhaust by recirculating such gases within the recycle loop into contact with a catalyst until substantially all the hydrogen in such gases is removed. Preferably, before the flow of fuel to the anode is stopped, upon disconnecting the primary load the oxidant flow to the cathode flow field is halted and a small auxiliary load is connected across the cell for a period of time to lower the cell voltage and cathode potential. An inert gas purge of the cell is not used or required as part of the shut-down procedure.
In one experiment using a stack of PEM fuel cells of the general type described in commonly owned U.S. Pat. No. 5,503,944, the primary electricity using device was disconnected, and the flow of fuel (hydrogen) to the anode and the flow of air to the cathode were shut off. No attempt was made to purge the anode flow field of residual fuel or to purge the cathode flow field of air, such as by using an inert gas purge. To restart the cell, fuel and oxidant were flowed directly into their respective flow fields. (The foregoing procedure is hereinafter referred to as an “uncontrolled” start/stop cycle.) It was found that a cell stack assembly operated in this manner experienced rapid performance decay which had not previously been observed. Further, it was discovered that a large number of start/stop cycles were more detrimental to cell performance than were a large number of normal operating hours under load. It was eventually determined, through experimentation, that both the shut-down and start-up procedures were contributing to the rapid performance decay being experienced by the cell; and it was known that such rapid decay did not occur when, in accordance with prior art techniques, inert gas was used to passivate the cell at each shut down. Examination of used cells that experienced only a few dozen uncontrolled start/stop cycles showed that 25% to 50% of the high surface area carbon black cathode catalyst support was corroded away, which had not previously been reported in the prior art.
Further testing and analysis of results led to the belief that the following mechanism caused the performance decay experienced in the foregoing experiment: With reference to
FIG. 2
, a diagrammatic depiction of a PEM fuel cell is shown. (Note that the mechanism to be described is also applicable to cells using other electrolytes, such as phosphoric acid or potassium hydroxide with appropriate changes in ion fluxes.) In
FIG. 2
, M represents a proton exchange membrane (PEM) having a cathode catalyst layer C on one side and an anode catalyst layer A on the other side. The cathode air flow field carrying air to the cathode catalyst is divided into air zones
1
and
2
by a vertical dotted line that represents the location of a moving hydrogen front through the anode flow field, as further described below. The anode fuel flow field that normally carries hydrogen over the anode catalyst from an inlet I to an exit E is also divided into two zones by the same dotted line. The zone to the left of the dotted line and adjacent the inlet I is filled with hydrogen and labeled with the symbol H
2
. The zone to the right of the dotted line and adjacent the exit E is zone
3
and is filled with air.
Upon an uncontrolled shut-down (i.e. a shut-down without taking any special steps to limit performance decay) some of the residual hydrogen and some of the oxygen in their respective anode and cathode flow fields diffuse across the PEM (each to the opposite side of the cell) and react on the catalyst (with either oxygen or hydrogen, as the case may be) to form water. The consumption of hydrogen on the anode lowers the pressure in the anode flow field to below ambient pressure, resulting in external air being drawn into the anode flow field at exit E creating a hydrogen/air front (the dotted line in
FIG. 2
) that moves slowly through the anode flow field from the fuel exit E to the fuel inlet I. Eventually the anode flow field (and the cathode flow field) fills entirely with air. Upon start-up of the cell, a flow of air is directed into and through the cathode flow field and a flow of hydrogen is introduced into the anode flow field inlet I. On the anode side of the cell this results in the creation of a hydrogen/air front (which is also represented by the dotted line in
FIG. 2
) that moves across the anode through the anode flow field, displacing the air in front of it, which is pushed out of the cell. In either case, (i.e. upon shut-down and upon start-up) a hydrogen/air front moves through the cell. On one side of the moving front (in the zone H
2
in
FIG. 2
) the anode is exposed substantially only to fuel (i.e. hydrogen); and in zone
1
of the cathode flow field, opposite zone H
2
, the cathode is exposed only to air. That region of the cell is hereinafter referred to as the H
2
/air region: i.e. hydrogen on the anode and air on the cathode. On the other side of the moving front the anode is exposed essentially only to air; and zone
2
of the cathode flow field, opposite zone
3
, is also exposed to air. That region of the cell is hereinafter referred to as the air/air region: i.e. air on both the anode and cathode.
The presence of both hydrogen and air within the anode flow field results in a shorted cell between the portion of the anode that sees hydrogen and the portion of the anode that sees air. This results in small in-plane flow of protons (H
+
) within the membrane M and a more significant through-plane flow of protons across the membrane, in the
Reiser Carl A.
Scheffler Glenn W.
Steinbugler Margaret M.
Van Dine Leslie L.
Kalafut Stephen
UTC Fuel Cells LLC
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