U.S. patent application number 10/878188 was filed with the patent office on 2005-12-29 for fuel cell system and method for removal of impurities from fuel cell electrodes.
This patent application is currently assigned to NISSAN TECHNICAL CENTER N.A. INC.. Invention is credited to Takahashi, Shinichi.
Application Number | 20050287404 10/878188 |
Document ID | / |
Family ID | 35506192 |
Filed Date | 2005-12-29 |
United States Patent
Application |
20050287404 |
Kind Code |
A1 |
Takahashi, Shinichi |
December 29, 2005 |
Fuel cell system and method for removal of impurities from fuel
cell electrodes
Abstract
A fuel cell system and method of removing impurities from a
catalyst are provided. The fuel cell system comprises a fuel cell
stack comprising a pair of end plates and at least one unit cell.
The unit cell contains a gas diffusion layer in contact with a
membrane electrode assembly which is constructed of a polymer
electrolyte membrane enclosed between two electrodes. The at least
one unit cell is stacked between the end plates. The fuel cell
system further comprises a voltage supply means and a means of
impressing a cyclically varying voltage from the voltage supply
means on the fuel cell stack. The cyclically varying voltage
removes impurities that adhere to catalysts on the electrode
surfaces in the fuel cell stack.
Inventors: |
Takahashi, Shinichi;
(Kanagawa, JP) |
Correspondence
Address: |
McDERMOTT, WILL & EMERY
600 13th Street, N.W.
Washington
DC
20005-3096
US
|
Assignee: |
NISSAN TECHNICAL CENTER N.A.
INC.
|
Family ID: |
35506192 |
Appl. No.: |
10/878188 |
Filed: |
June 29, 2004 |
Current U.S.
Class: |
429/431 ;
180/68.5; 429/432; 429/454; 429/483; 429/492; 429/534 |
Current CPC
Class: |
Y02T 90/40 20130101;
B60L 58/30 20190201; H01M 8/04589 20130101; H01M 16/006 20130101;
Y02E 60/10 20130101; H01M 2250/20 20130101; H01M 8/04238 20130101;
H01M 2008/1095 20130101; Y02E 60/50 20130101 |
Class at
Publication: |
429/021 ;
180/068.5; 429/017 |
International
Class: |
H01M 008/18; B60L
011/18 |
Claims
What is claimed is:
1. A fuel cell system which generates electricity by supplying fuel
gas and oxidant gas to a fuel cell stack comprising: a fuel cell
stack comprising a pair of end plates and at least one unit cell
containing a gas diffusion layer in contact with a membrane
electrode assembly which is constructed of a polymer electrolyte
membrane enclosed between two electrodes, wherein said at least one
unit cell is stacked between the end plates; a voltage supply
means; and a means of impressing a cyclically varying voltage from
the voltage supply means on said fuel cell stack.
2. The fuel cell system according to claim 1, wherein said two
electrodes comprise one each of a fuel electrode and an oxidant
electrode.
3. The fuel cell system according to claim 1, further comprising
means of supplying fuel gas and oxidant gas to said fuel cell
stack.
4. The fuel cell system according to claim 1, wherein the voltage
supply means is selected from the group consisting of a battery and
a generator.
5. The fuel cell system according to claim 4, wherein said battery
is a secondary battery charged by said fuel cell stack or a
generator.
6. The fuel cell system according to claim 1, wherein said means of
impressing a cyclically varying voltage varies the voltage per unit
cell of the fuel cell stack between the range of from about -1.5 V
to about 1.5 V.
7. The fuel cell system according to claim 1, wherein said means of
impressing a cyclically varying voltage varies the voltage at a
rate of from about 1 mV/s to about 1000 mV/s.
8. The fuel cell system according to claim 1, further comprising a
means for measuring the current flowing in said fuel cell stack
when the cyclically varying voltage is impressed on the fuel cell
stack.
9. The fuel cell system according to claim 1, further comprising a
means for measuring the time for which the cyclically varying
voltage is impressed on the fuel cell stack.
10. The fuel cell system according to claim 1, further comprising a
means for measuring the number of cycles for which the cyclically
varying voltage is impressed on the fuel cell stack.
11. A motor vehicle comprising the fuel cell system of claim 1.
12. The motor vehicle according to claim 11, wherein the motor
vehicle is an automobile.
13. A method of impressing a cyclically varying voltage on a fuel
cell stack comprising: providing a fuel cell stack comprising a
pair of end plates and at least one unit cell containing a gas
diffusion layer in contact with a membrane electrode assembly which
is constructed of a polymer electrolyte membrane enclosed between
two electrodes, wherein said at least one unit cell is stacked
between the end plates; and applying a cyclically varying voltage
across said fuel cell stack using voltage supplied by a voltage
supply means.
14. The method of impressing a cyclically varying voltage on a fuel
cell stack according to claim 13, wherein the cyclically varying
voltage is impressed on the fuel cell stack before the fuel cell
stack starts to generate electricity or after the fuel cell stack
stops generating electricity.
15. The method of impressing a cyclically varying voltage on a fuel
cell stack according to claim 14, wherein the voltage supply means
is a battery or a generator.
16. The method of impressing a cyclically varying voltage on a fuel
cell stack according to claim 15, further comprising a step of
charging the battery with electricity generated by the fuel cell
stack.
17. The method of impressing a cyclically varying voltage on a fuel
cell stack according to claim 13, further comprising controlling
the voltage so that the voltage per unit cell is cyclically varied
between about -1.5 V and about 1.5 V.
18. The method of impressing a cyclically varying voltage on a fuel
cell stack according to claim 13, further comprising controlling
the voltage so that the voltage cyclically varies at a rate of
between about 1 mV/s and about 1000 mV/s.
19. The method of impressing a cyclically varying voltage on a fuel
cell stack according to claim 13, further comprising controlling
the voltage so that the voltage varies linearly between the lowest
and highest impressed voltages.
20. The method of impressing a cyclically varying voltage on a fuel
cell stack according to claim 13, further comprising measuring the
current flowing in the fuel cell stack.
21. The method of impressing a cyclically varying voltage on a fuel
cell stack according to claim 20, further comprising stopping the
impressing of a cyclically varying voltage on the fuel cell stack
when the measured current falls below a predetermined amperage.
22. The method of impressing a cyclically varying voltage on a fuel
cell stack according to claim 13, further comprising measuring the
time for which the cyclically varying voltage is impressed on the
fuel cell stack and stopping the impressing of the cyclically
varying voltage when a predetermined period of time has
elapsed.
23. The method of impressing a cyclically varying voltage on a fuel
cell stack according to claim 20, further comprising measuring the
number of cycles for which the cyclically varying voltage is
impressed on the fuel cell stack and stopping the impressing of the
cyclically varying voltage when a predetermined number of cycles
has elapsed.
24. A method of electrochemically removing impurities that adhere
to an electrode surface in a fuel cell system, comprising:
providing a fuel cell stack comprising a pair of end plates and at
least one unit cell containing a gas diffusion layer in contact
with a membrane electrode assembly which is constructed of a
polymer electrolyte membrane enclosed between two electrodes,
wherein said at least one unit cell is stacked between the end
plates; and applying a cyclically varying voltage across said fuel
cell stack using voltage supplied by a voltage supply means to
remove impurities from the electrode surface.
25. A method of electrochemically removing impurities that adhere
to a catalyst, comprising: providing a catalyst with a surface and
impurities adhered to said surface; and applying a cyclically
varying voltage across said catalyst surface using voltage supplied
by a voltage supply means to remove said impurities from said
catalyst surface.
Description
TECHNICAL FIELD OF THE INVENTION
[0001] This invention relates to fuel cell systems and in
particular fuel cell systems for use in motor vehicle
applications.
BACKGROUND OF THE INVENTION
[0002] Fuel cells have been developed as alternative power sources
for motor vehicles, such as electrical vehicles. A fuel cell is a
demand-type power system in which the fuel cell operates in
response to the load imposed across the fuel cell. Typically, a
liquid hydrogen containing fuel, for example, gasoline, methanol,
diesel, naphtha, etc. serves as a fuel supply for the fuel cell
after the fuel has been converted into a gaseous stream containing
hydrogen. The conversion to the gaseous stream is usually
accomplished by passing the fuel through a fuel reformer to convert
the liquid fuel to a hydrogen gas stream that usually contains
other gases such as carbon monoxide, carbon dioxide, methane, water
vapor, oxygen, and unburned fuel. The hydrogen is then used by the
fuel cell as a fuel in the generation of electricity for the
vehicle.
[0003] A polymer electrolyte membrane type of fuel cell is
generally composed of a stack 10 of unit cells 72 comprising a
polymer electrolyte membrane 11 enclosed between electrodes 12 and
gas diffusion layers 13, and further enclosed between separators 15
and channels 14 for fuel gas and oxidant gas, as shown in FIG. 1.
The stack 10 is fixed by end plates 16. A current collector may be
provided between the end plate and stack, or the end plate 16
itself may function as current collector. When hydrogen is used as
the fuel gas and oxygen is used as the oxidant gas, electrons are
released due to a chemical reaction occurring at catalyst reaction
sites on the electrode surfaces. Water is formed as a by-product,
via the reaction:
H.sub.2+1/2O.sub.2.fwdarw.H.sub.2O.
[0004] Consequently, the fuel cell is an energy source that has no
adverse impact on the global environment, and has been the focus of
much research for use in automobiles in recent years.
[0005] From the standpoint of durability, fuel cell electrical
generating performance deteriorates over its operating life, due to
a build-up of impurities such as metallic ions and organics in the
fuel cell. The impurities result from various sources: for example,
they may be extracted from tubing used to supply gas or coolant to
the fuel cell, or from auxiliary equipment. In addition, there may
be impurities mixed with the fuel gas or oxidant gas. It is
possible to reduce the concentration of impurities by using
material that does not contain impurities for tubing or auxiliary
equipment, or by filtering the fuel gas and oxidant gas. However,
when generating electricity over a long period of time, it is
difficult to prevent the accumulation of impurities inside the fuel
cell and the accompanying deterioration of fuel cell performance.
Impurities inside the fuel cell adhere to catalytic reaction sites
and causes loss of catalytic performance.
[0006] There are known methods of re-activating the catalyst by
electrochemically removing the impurities that adhere to it. U.S.
Pat. No. 6,187,464, for example, describes a method of generating
electricity in a polymer electrolyte fuel cell module at an oxygen
utilization rate of 50% or higher, and impressing on the fuel cell
module an average voltage of 0.3 V or less per unit cell. Japanese
Patent Disclosure 2001-85037 describes another method of restoring
fuel cell performance by operating the fuel cell at a current
density 1.5 times greater than the normal operating current density
or by reversing the direction of current flow.
SUMMARY OF THE INVENTION
[0007] There exists a need in the fuel cell art for a fuel cell
system that reduces the amount of impurities adhering to catalyst
reaction sites. There exists a need in the fuel cell art to prevent
deterioration of fuel cell electrical generation. There exists a
need in the fuel cell art for a rapid and efficient method of
removing impurities from catalytic reaction sites.
[0008] There exists a need in the electrical vehicle art for
electrical vehicles powered by fuel cells that rapidly and
efficiently generate electricity upon demand. There exists a need
in the electrical vehicle art for electrical vehicles powered by
fuel cells that do not suffer from poor electrical generation
performance due to the build-up of impurities.
[0009] These and other needs are met by certain embodiments of the
present invention, which provide a fuel cell system which generates
electricity by supplying fuel gas and oxidant gas to a fuel cell
stack comprising a fuel cell stack comprising a pair of end plates
and at least one unit cell containing a gas diffusion layer in
contact with a membrane electrode assembly which is constructed of
a polymer electrolyte membrane enclosed between two electrodes. The
at least one unit cell is stacked between the end plates. The fuel
cell system further comprises a voltage supply means and a means of
impressing a cyclically varying voltage from the voltage supply
means on the fuel cell stack.
[0010] The earlier stated needs are also met by certain embodiments
of the present invention, which provide a motor vehicle comprising
a fuel cell system which generates electricity by supplying fuel
gas and oxidant gas to a fuel cell stack comprising a fuel cell
stack comprising a pair of end plates and at least one unit cell.
The at least one unit cell containing a gas diffusion layer in
contact with a membrane electrode assembly which is constructed of
a polymer electrolyte membrane enclosed between two electrodes. The
at least one unit cell is stacked between the end plates. The fuel
cell system further comprises a voltage supply means and a means of
impressing a cyclically varying voltage from the voltage supply
means on the fuel cell stack.
[0011] The earlier stated needs are also met by certain embodiments
of the present invention, which provide a method of impressing a
cyclically varying voltage on a fuel cell stack comprising
providing a fuel cell stack comprising a pair of end plates and at
least one unit cell containing a gas diffusion layer in contact
with a membrane electrode assembly. The membrane electrode assembly
is constructed of a polymer electrolyte membrane enclosed between
two electrodes. The at least one unit cell is stacked between the
end plates. A cyclically varying voltage is applied across the fuel
cell stack using voltage supplied by a voltage supply means.
[0012] In addition, the earlier stated needs are also met by
certain embodiments of the present invention, which provide a
method of electrochemically removing impurities that adhere to an
electrode surface in a fuel cell system comprising providing a fuel
cell stack comprising a pair of end plates and at least one unit
cell containing a gas diffusion layer in contact with a membrane
electrode assembly. The membrane electrode assembly is constructed
of a polymer electrolyte membrane enclosed between two electrodes.
The at least one unit cell is stacked between the end plates. A
cyclically varying voltage is applied across the fuel cell stack
using voltage supplied by a voltage supply means to remove
impurities from the electrode surface.
[0013] The earlier stated needs are also met by certain embodiments
of the present invention, which provide a method of
electrochemically removing impurities that adhere to a catalyst
comprising providing a catalyst with a surface and impurities
adhered to the surface. A cyclically varying voltage is applied
across the catalyst surface using voltage supplied by a voltage
supply means to remove the impurities from the catalyst
surface.
[0014] The present invention addresses the need for a fuel cell
system that rapidly and efficiently removes impurities adhered to
catalysts in a fuel cell. The present invention further addresses
the need for a method that rapidly and efficiently removes
impurities adhered to a catalyst. The present invention also
addresses the need for a motor vehicle with a fuel cell system that
generates electricity without deterioration of performance over the
operating life of the fuel cell because of impurity build up on the
fuel cell catalyst surfaces.
[0015] The foregoing and other features, aspects, and advantages of
the present invention will become apparent in the following
detailed description of the present invention when taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 schematically illustrates a cross-section of a fuel
cell stack.
[0017] FIG. 2 schematically illustrates an outline of a fuel cell
stack.
[0018] FIG. 3 illustrates a fuel cell system according to an
embodiment of the present invention, as described in Example 1.
[0019] FIG. 4 illustrates a fuel cell system according to an
embodiment of the present invention, as described in Example 2.
[0020] FIG. 5 illustrates a fuel cell system according to an
embodiment of the present invention.
[0021] FIG. 6 illustrates an automobile with a fuel cell system
according to an embodiment of the present invention.
[0022] FIG. 7 is a flow chart illustrating the operation of the
means of controlling the cyclically varying voltage impressed on a
fuel cell stack.
DETAILED DESCRIPTION OF THE INVENTION
[0023] The present invention provides a fuel cell system that
rapidly and efficiently removes impurities adhered to catalysts.
The present invention also provides a motor vehicle with a fuel
cell system that generates electricity without deterioration of
performance over time due to impurity build-up on fuel cell
catalysts. These benefits are provided by applying a cyclically
varying voltage to a fuel cell stack.
[0024] A fuel cell stack 10 used in certain embodiments of the
present invention is illustrated in FIG. 1. The fuel cell stack 10
comprises at least one unit cell 72 equipped with a membrane
electrode assembly 70 constructed of a polymer electrolyte membrane
11 enclosed between two electrodes 12, gas diffusion layers 13, and
a separator 15. Fuel gas and oxidant gas are supplied to the unit
cells via gas channels 14 and the unit cells are 72 are stacked
between end plates 16. An alternate embodiment of the fuel cell
stack 110 used in certain embodiments of the present invention is
illustrated in FIG. 2. In this embodiment the gas channels 74 are
located in the separator 75.
[0025] The method of electrochemically removing impurities that
adhere to a catalyst involves either breaking the bond between
impurities and catalyst, or changing the chemical structure by
breaking down the impurities, or a combination of both. It is
difficult to completely remove the impurities by merely using an
electrical generation method that is different from the normal
electrical generation mode because the bonds between the impurities
and the catalyst is caused by the generation of electricity.
Therefore, the chemical reaction in the presence of the catalyst
when attempting to remove the impurities is not different from the
chemical reaction that bonded the impurities to the catalyst. It is
possible, however, to break the bond between catalyst and
impurities by imposing a voltage on the fuel cell because it
produces an opposite reaction to that of generating electricity.
However, because the impurities are varied, the bonds formed
between impurities and the catalyst are also varied, and it may not
be possible to break all bonds between catalyst and impurities by
merely imposing a specific voltage. Furthermore, even if the bonds
between the impurities and the catalysts are broken, the impurities
would bond with the catalyst again when electrical generation was
re-started, causing deterioration of performance.
[0026] In certain embodiments of the present invention, a fuel cell
system 90 comprises a membrane electrode assembly 70 whose
structure encloses a polymer electrolyte membrane 11 between a fuel
electrode 76 and an oxidant electrode 78, and a fuel cell stack 21
composed of unit cells 72 whose structure encloses the membrane
electrode assembly 70 between separators 15 and channels 14 for
fuel gas and oxidant gas. The fuel cell system 90 has means 80, 82
of supplying fuel gas and oxidant gas to the fuel cell stack 21, a
secondary battery 22 charged by the fuel cell stack 21, and a means
24 of impressing a cyclically varying voltage on the fuel cell
stack 21 using electric power from the battery 22, as illustrated
in FIG. 3. Using the cyclically varying voltage, it is possible to
remove various impurities that adhere to the surface of the fuel
cell catalyst. The impurities that adhere to the catalyst surface
have varied chemical structures, and likewise varied bond
properties. Consequently, it is possible to break the bonds of
various impurities to the catalyst, in accordance with the various
bond properties, by impressing a voltage that cyclically increases
and decreases between positive and negative voltage. Because the
chemical structure of the impurities is broken down by impressing
the cyclically varying voltage, and the impurities are changed to
substances with different chemical structures, the broken-down
impurities can easily be discharged from the catalyst area to
outside the fuel cell stack after electrical generation is
re-started. Even if there is a large quantity of impurities, it is
possible to eliminate them by applying a cyclically repeating
impressed voltage.
[0027] In certain embodiments of the present invention, the battery
22 can be recharged by a generator. In certain other embodiments of
the present invention, the battery 22 can be replaced with a
generator.
[0028] In certain embodiments of the present invention, the means
24 of impressing a cyclically varying voltage on the fuel cell
stack impresses the cyclically varying voltage on the fuel cell
stack 21 either before the fuel cell stack 21 starts to generate
electricity, or after the fuel cell stack 21 stops generating
electricity, to keep catalyst free of impurity adhesion when the
fuel cell is generating electricity.
[0029] In certain embodiments of the present invention, the means
24 of impressing a cyclically varying voltage on the said fuel cell
stack is controlled so that it cyclically varies the voltage per
unit cell of the fuel cell stack between about -1.5 V and about 1.5
V to eliminate impurities with varied chemical structures and
varied bond properties. Impressed voltages below about -1.5 V per
unit cell could promote degradation of the catalyst. Impressed
voltages above about 1.5 V per unit cell would have little effect
in eliminating impurities.
[0030] In certain embodiments of the present invention, the means
24 of impressing a cyclically varying voltage on the fuel cell
stack is controlled so that it cyclically varies the voltage at a
rate of between about 1 mV/s and about 1000 mV/s. Though the
voltage could be varied at a rate below about 1 mV/s, it would not
be practical, since it would make the processing time extremely
long. It is also possible to vary the voltage at a rate exceeding
about 1000 mV/s, however, this would shorten the duration of
electrical action on the impurity/catalyst bond, and would thus
make it necessary to increase the number voltage cycles, thereby
increasing the processing time.
[0031] In certain embodiments of the present invention, the fuel
cell system comprises a means 24 of impressing a cyclically varying
voltage on the fuel cell stack that is controlled so the voltage
varies linearly between the lowest and highest impressed voltages
to eliminate impurities bonded to the catalyst.
[0032] In certain embodiments of the present invention, the fuel
cell system 90 has a means 26 of measuring the current flowing in
the fuel cell stack when the cyclically varying voltage is
impressed on the fuel stack 21. The cyclically varying voltage is
controlled so that it ceases to be impressed if the measured
current at a specified voltage falls below a predetermined
amperage. Thus, if the measured current at a specified voltage
falls below a predetermined amperage, it can be concluded that
impurities adhered to the catalyst have been eliminated. It can be
empirically determined what current at a specified voltage
corresponds to a state of no impurities adhered to the catalyst.
The cyclically varying voltage can be switched off when the
measured current reaches the specified amperage. Ideally, the
predetermined amperage that is the criterion for judging that
impurities have been eliminated would be 0 A. However, in cases
where complete elimination would be time-consuming and would
obstruct the operation of the fuel cell system, the criterion need
not necessarily be 0 A. The specified current can be empirically
determined to be at a level that does not obstruct electricity
generation.
[0033] In certain embodiments of the present invention, the fuel
cell system 90 is controlled so that the cyclically varying voltage
ceases to be impressed if the current flowing in the fuel cell
stack falls below a predetermined amperage in the range of from
about 0.3 V to about 0.8 V per unit cell, indicating the
substantially complete elimination of impurities that have a
hydroxyl base. Ideally, the predetermined amperage that is the
criterion for judging that impurities have been eliminated would be
0 A. However, in cases where complete elimination would be
time-consuming and would obstruct the operation of the fuel cell
system, it is not necessary that predetermined amperage be 0 A.
[0034] In certain embodiments of the present invention, a fuel cell
system 100 comprises a means 56 for measuring the time for which
cyclically varying voltage is impressed on the fuel cell stack, as
illustrated in FIG. 4. A means 54 of impressing a cyclically
varying voltage on the fuel cell stack 51 is controlled so that it
ceases to impress a voltage on the stack when a predetermined time
has elapsed. The optimum time for the impurity elimination process
can be empirically determined for a predetermined degree of
deterioration in the electrical generation performance of the fuel
cell stack 51 and the operational status of the fuel cell system
100.
[0035] In certain embodiments of the present invention, a fuel cell
system 120 comprises a means 96 of measuring the number of cycles
for which the cyclically varying voltage is impressed on the fuel
cell stack, as illustrated in FIG. 5. The means 54 of impressing a
cyclically varying voltage on the fuel cell stack is controlled so
that it ceases to impress a voltage on the stack when a
predetermined number of cycles has elapsed. It is, therefore,
possible to set an empirically determined optimum time for the
impurity elimination process, factoring in the degree of
deterioration in electrical generation performance of the fuel cell
stack 51 and the operational status of the fuel cell system
120.
[0036] In certain embodiments of the present invention, a motor
vehicle is provided, such as an automobile 130, as shown in FIG. 6,
comprising any of the fuel cell system as previously described
herein. The automobile 130 comprises a fuel tank 132 to store the
fuel required by the fuel cell 134 to generate electricity. The
electricity generated by the fuel cell 134 is stored in one or more
batteries 136. The electricity generated by the fuel cell and/or
stored in one or more secondary batteries 136 is used to run the
motor 138. The motor, in turn, spins the wheels 140 setting the
automobile 130 in motion.
EXAMPLE 1
[0037] FIG. 3 shows the outline of the fuel cell system used in
Example 1. Hydrogen gas is supplied to a fuel cell stack 21 via a
hydrogen supply valve 28, and air is supplied via an air supply
valve 29. A battery 22, such as a secondary battery, and the fuel
cell stack 21 are connected via a battery control device 23. The
means 24 of impressing a cyclically varying voltage on the fuel
cell stack is connected to the battery 22, and is connected to the
fuel cell stack 21 via a switch 25 for impressing voltage on the
fuel cell stack 21. While the fuel cell stack 21 is generating
electricity, the battery control device 23 is connected, and the
switch 25 for impressing voltage on the fuel cell stack 21 is shut
off. When the fuel cell stack 21 ceases generating electricity the
switch 25 is closed and the cyclically varying voltage is impressed
on the fuel cell stack 21. The strength of the current impressed on
the fuel cell stack 21 is measured by an ammeter 26, and voltage,
is measured by a voltmeter 27. The measurement readings of the
ammeter 26 and voltmeter 27 are fed-back to the means 24 of
impressing a cyclically varying voltage on the fuel cell stack.
[0038] FIG. 7 shows a flow chart of the fuel cell system control of
the cyclically varying voltage. At step 40, the supply of hydrogen
and air to the fuel cell stack 21 is stopped, and a nitrogen purge
is started. At step 41 the load connector switch 31 is shut off.
Then at step 42 the battery control device 23 is shut off, and the
storage operation is stopped. At step 43, the means 24 of
impressing a cyclically varying voltage on the fuel cell stack
turns on the switch 25 for impressing voltage on the fuel cell
stack 21, and voltage starts to be impressed on the fuel cell stack
21. In certain embodiments of the present invention, the voltage is
varied between about -0.2 V and about 1.2 V at a rate of about 50
mV/s. At step 44, the impressed voltage is determined to be between
about 0.3 V and about 0.8 V. At step 45, the amperage is
determined. If the amperage, converted to current density, is 10
.mu.A/cm.sup.2 or less, the impurities are judged to have been
eliminated, and the impression of voltage on the fuel cell stack is
stopped at step 46. If the current density is not 10 .mu.A/cm.sup.2
or less in the voltage range between 0.3 and 0.8 V the means 24 of
impressing a cyclically varying voltage on the fuel cell stack
continues to impress the cyclically varying voltage. In certain
embodiments of the present invention, the current density of 10
.mu.A/cm.sup.2, noted above, was determined empirically. When the
fuel cell stack was re-started after shutdown, no deterioration of
generation performance was observed.
EXAMPLE 2
[0039] FIG. 4 shows the outline of the fuel cell system 100 used in
Example 2. Hydrogen gas is supplied to fuel cell stack 51 via a
hydrogen supply valve 57, and air is supplied via an air supply
valve 58. A secondary battery 52 and the fuel cell stack 51 are
connected via a battery control device 53. A means 54 of impressing
a cyclically varying voltage on the fuel cell stack is connected to
the battery 52, and is also connected to the fuel cell stack 51 via
a switch 55 for impressing voltage on the fuel cell stack. While
the fuel cell stack 51 is generating electricity, the battery
control device 53 is connected, and the switch 55 for impressing
voltage on the fuel cell stack 51 is shut off. A timer 56 measures
the length of time for which the means 54 of impressing voltage on
the fuel cell stack 51 is impressed the voltage on the stack 51.
When the timer 56 reaches a predetermined time, the means 54 of
impressing a cyclically varying voltage on the fuel cell stack is
instructed to stop impressing the voltage.
[0040] In a certain embodiment of the present invention, the
voltage was varied between 0 V and 1.2 V at a rate of 100 mV/s. The
timer 56 was set to an impression of cyclically varying voltage
duration of 1800 seconds.
[0041] The current impressed on fuel cell stack 51 is measured by
an ammeter 26, and voltage is measured by a voltmeter 27. The
current and voltage values measured by the ammeter 26 and voltmeter
27 are fed-back to the means 54 of impressing a cyclically varied
voltage on the fuel cell stack.
[0042] In certain embodiments of the present invention, a means 96
for measuring the number of cycles for which the cyclically varying
voltage is impressed on the fuel cell stack 51 is provided, as
shown in FIG. 5. The means 96 for measuring the number of cycles
for which the cyclically varying voltage measures a predetermined
number of cycles. When the predetermined number of cycles is
reached the means 54 of impressing a cyclically varying voltage on
the fuel cell stack is instructed to stop impressing the voltage.
The predetermined number of cycles can be empirically
determined.
[0043] The embodiments illustrated in the instant disclosure are
for illustrative purposes. They should not be construed to limit
the scope of the claims. Though the fuel cell systems described are
particularly well suited to electrical vehicles, such as
automobiles, the instant fuel cell systems are suitable for a wide
variety of motor vehicles that are included within the scope of the
instant claims including, motorcycles, buses, trucks, recreational
vehicles, and agricultural and industrial equipment. As is clear to
one of ordinary skill in this art, the instant disclosure
encompasses a wide variety of embodiments not specifically
illustrated herein.
* * * * *