Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02286701 1999-10-08
Cooling and Humidifying of
Polymer Electrolyte Fuel Cells
The invention relates to fuel cells which contain solid
polymer membranes as electrolytes, preferably use hydro-
gen as burnable gas and use air or oxygen under low
pressure as oxidizing agent. The invention relates fur-
thermore to a method of simultaneously cooling the fuel
cells and humidifying the polymer electrolyte membranes.
Polymer electrolyte membrane fuel cells, as they are
commonly employed for producing electric current, con-
tain an anode, a cathode and an ion exchange membrane
disposed therebetween. A plurality of fuel cells consti-
tutes a fuel cell stack, with the individual fuel cells
being separated from each other by bipolar plates acting
as current collectors. For generating electricity, a
burnable gas, e.g. hydrogen, is introduced into the
anode region, and an .oxidizing agent, e.g. air or
oxygen, is introduced into the cathode region. Anode and
cathode, in the regions in contact with the polymer
electrolyte membrane, each contain a catalyst layer. In
the anode catalyst layer,.. the fuel is oxidized thereby
forming cations and free electrons, and in the cathode
catalyst layer, the oxidizing agent is reduced by taking
up electrons. The cations migrate through the ion ex-
change membrane to the cathode and react with the re-
duced oxidizing agent, thereby forming water when hydro-
gen is used as burnable gas and oxygen is used as oxi-
dizing agent. In the reaction of burnable gas and oxi-
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dizing agent, there are released considerable amounts of
heat that must be dissipated by cooling. Cooling so far
has been achieved by cooling channels in the bipolar
plates through which deionized water is flown.
With this kind of cooling, tremendous material problems
result since there are typically about 50 to 300 bipolar
plates connected in series, with the cooling water thus
electrically joining together different potentials. The
result thereof are material decompositions. In accor-
dance therewith, solely graphite or gold-plated metal
are feasible as material for the bipolar plates.
Furthermore, it is necessary to keep the polymer mem-
brane moist, since the conductivity value of the mem-
brane is greatly dependent on its water content. To
prevent drying up of the membrane, there was thus re-
quired a complex system for humidifying the reaction
gases.
It is the object of the invention to make available a
polymer electrolyte fuel cell and a polymer electrolyte
fuel cell stack, respectively, in which the polymer
electrolyte membrane of a fuel cell has the optimum
moisture content at all times during operation and in
which at the same time sufficient cooling is ensured.
An additional object of the invention consists in making
available a method which renders possible to keep the
polymer electrolyte membrane of a polymer electrolyte
fuel cell at an optimum moisture content during opera-
tion of the fuel cell and at the same time to suffi-
ciently cool the fuel cell.
These objects are met by the polymer electrolyte fuel
cell according to claim 1, the polymer electrolyte fuel
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cell stack according to claim 11, the method of cooling
and humidifying a polymer electrolyte fuel cell
according to claim 12 and the method according to claim
22.
Polymer electrolyte membranes require a high water con-
tent to ensure optimum conductivity for H+ ions. The
water content must be maintained as a rule by supply of
water, as otherwise the burnable gas flows and oxidizing
agent gas flows flowing through the cell dry up the mem-
brane. However, to counteract possible drying up by the
addition of an excess of water, is not sensible since
water in too large quantities results in flooding of the
electrodes, i.e. the pores of the electrodes are
clogged. Simple ascertaining and regulating the parti-
cular amount of water required has not been possible so
far.
Preferred embodiments are indicated in the respective
dependent claims.
In the drawings:
Fig. 1 shows a preferred embodiment of a fuel cell
according to the invention,
Fig. 2 shows a circuit for measuring the impedance of a
fuel cell.
Fig. 3 shows the dependency of the conductivity a
Nafion~ membrane on the water content of the
membrane.
A polymer electrolyte fuel cell according to the in-
vention uses air or oxygen at slight overpressure as
oxidizing agent. Preferred is an overpressure of less
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than 2 bar, with an overpressure of less than 0.5 bar
being particularly preferred. The necessary pressure
difference can also be obtained by suction. As burnable
gas, preferably hydrogen is used, but the use of other
burnable gases is in principle possible as well. As po-
lymer electrolyte membrane, preferably Nafion~ is em-
ployed. Hydrogen is supplied to the individual fuel
cells of a stack and distributed via gas channels in the
anode region. Air is supplied at the same time and dis-
tributed via gas channels in the cathode region. The
hydrogen migrates to the anode catalyst layer and forms
cations there which migrate through the electrolyte, a
proton exchange membrane, to the cathode. At the ca-
thode, oxygen migrates to the cathode catalyst layer and
is reduced there. During the reaction with the cations,
water is created as reaction product. Due to the re-
action heat, the water formed evaporates, which results
in a certain cooling effect. This cooling effect, how-
ever, is not sufficient on the one hand, and on the
other hand the membrane in the course of operation of
the fuel cell is increasingly depleted of humidity.
As can be seen from Fig: 3 for Nafion~ NE 105 (30 °C) ,
the conductivity of ion-conducting membranes increases
with the H20 content. N(H20)/N(S03H) designates the
number of water molecules per sulphonic acid remainder
of the membrane.
A reduction of the moisture content of the solid polymer
electrolyte membrane of a fuel cell thus has the conse-
quence that its internal resistance increases, i.e. that
its conductivity value decreases. The conductivity value
of the membrane is dependent in extreme manner on its
water content. What is essential for ef-ficient operation
of a polymer electrolyte fuel cell is thus that the po-
lymer electrolyte membrane at all times have the optimum
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humidity corresponding to the particular operating con-
ditions (temperature, load, air ratio).
For maintaining optimum humidity, it is possible accor-
ding to the invention to determine during operation of
the fuel cell, preferably regularly or continuously,
whether the membrane is moist in optimum manner or
whether the addition of water is necessary and which
quantity of water needs to be added, respectively.
The amount of water added basically can vary very much.
It is dependent on the particular operating conditions
of the fuel cell, and it is dependent in particular also
on the type of cooling of the fuel cell. Frequently,
fuel cells are fed with water for cooling which, de-
pending on the construction of the fuel cells, humidi-
fies to a certain extent also the membrane. As a rule,
less additional water has to be supplied then than in
case of cells employing exclusively air cooling, for
example.
The conductivity value of the membrane depends on the
water content thereof . During operation of a fuel cell,
however, the conductivity value of the membrane cannot
be measured directly. According to the invention, pre-
ferably the impedance of the fuel cell (value of the
impedance or particularly'preferred, the real part of
the impedance) is ascertained. Since the conductivity
value of the membrane is a continual, monotonic func-
tion of these quantities, the necessary amount of water
can also be regulated on the basis of the impedance.
A possible circuit for measuring the impedance of a fuel
cell is shown in Fig. 2.
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Direct measurement of the conductivity value and thus of
the moisture content of a polymer electrolyte membrane
of a fuel cell by determination of the impedance is
carried out by modulation of the cell voltage with an
alternating signal having a frequency of 1 to 20 kHz. In
case of a fuel cell stack, suitably the average moisture
content of several membranes is measured. The quotient
of alternating voltage and the resulting current re-
sponse is a measure for the moisture. In Fig. 2, BZ re-
presents the fuel cell and RL represents the load re-
sistor. Connected in parallel to the load resistor is an
assembly of capacitor C, resistor R and alternating vol-
tage source U, which is suitable to produce small alter-
nating voltages (in the order of magnitude of about 10
mV) and large currents (in the order of magnitude of
about 10 A). The voltage of the fuel cell is modulated
by the alternating signal (about 1 to 20 kHz) . The al-
ternating voltage component U effects an alternating
current I to be superimposed on the fuel cell current.
The quotient of alternating voltage and alternating
current is a measure for the impedance of the fuel cell
and thus a measure for the moisture of the polymer elec-
trolyte membrane and for the necessary amount of water
to be added, respectively.
However, the amount or value of the impedance is depen-
dent, in addition to the conductivity of the membrane,
on further determinative quantities, namely on the size
of the catalyst surface in contact with the membrane,
the ohmic resistance of the electrodes and the poisoning
of the membrane by foreign ions. These quantities are
subject to a certain amount of change in the course of
the service life of a fuel cell, with the deviations due
to change of the ohmic resistance of the electrodes and
due to poisoning of the membrane by foreign ions being
as a rule negligible. In the course of the life of a
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fuel cell, the value of the impedance which corresponds
to the optimum membrane moisture under the given
operating conditions (desired or set value of the amount
of the impedance), can thus vary. Thus, the desired
value to be observed of the amount of the impedance
should be set each time anew in the course of arising
maintenance work. In doing so, the new desired value is
determined by maximizing the performance of the fuel
cell. During operation of the fuel cell, the optimum
desired value can be matched anew in alternative manner
by Fuzzy logic or other methods familiar to the expert,
in accordance with the changed conditions.
A measure for the conductivity of the membrane that is
largely independent of the catalyst surface (whose
change in essence is responsible for the change of the
desired value of the impedance) is obtained if, in
addition to the amount of the impedance, its phase angle
is considered as well. If the real part of the impedance
determined electronically therefrom is regarded as re-
gulating variable, one sole desired value can be em-
ployed even over the entire service life of the fuel
cell.
During operation of the fuel cells, the impedance
(amount or real part) can be measured continuously or at
regular intervals. In case a too low conductivity value
of the membrane or membranes is calculated on the basis
of the measurement, water is supplied to the system, for
example by electronically controlled opening of water
inlet valves, as is usual, until the desired value of
the impedance is reached again.
In case of fuel cell stacks with a plurality of fuel
cells, it is favorable not to determine the amount or
the real part of the impedance for each membrane indivi-
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dually, but to determine average values for a plurality
of cells of the stack or even for all cells of the stack
jointly and to arrange the necessary addition of water
in accordance therewith.
Irrespective of the manner of determination of the opti-
mum water content of the membrane and the regulation of
the water introduction, it is possible according to the
invention to use membrane humidifying water simul-
taneously for cooling the fuel cell and for thus en-
suring sufficient cooling. This is achieved according to
the invention in that in case of a fuel cell designed as
outlined hereinbefore, ion-free water in liquid form is
introduced directly into the gas channels for the com-
bustion air. As an alternative, the water can also be
introduced directly into the gas channels for the
burnable gas.
A proven solution is the introduction of water both in
the cathode region and in the anode region, particularly
with operating conditions causing severe drying up of
the membrane.
The liquid water evaporates in the hot fuel cell and
effects efficient cooling of the cell due to the phase
conversion taking place. Furthermore, it penetrates into
the polymer electrolyte membrane and keeps it moist.
The easiest possibility of adding the necessary amount
of water to the air flow and to the air and/or hydrogen
flow, respectively, consists in introducing the water
into the gas channels by means of a metering pump, in
numerous thin lines, e.g. capillaries. In doing so, no
substantial mixing of the water with. the air and the
burnable gas, respectively, takes place, so that the
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free water surface available for evaporation is rela-
tively small.
A considerably larger free water surface and thus faster
humidifying of the membrane and more efficient cooling
is obtained if the required amount of water is added to
the reaction gas flows in mixed form, i.e. as aerosol.
The water-in-air aerosol and, if applicable, the water-
in-burnable gas aerosol contain water in the form of
droplets with a size of 2 to 20 ~,m, which ensure rapid
vaporization or evaporation. The aerosol can be produced
for instance with the aid of ultrasonic atomizers or
nozzles. The simplest production of the aerosol, which
at the same time requires the least amount of energy,
takes place by means of ultrasonic atomizers at frequen-
cies of at least 100 kHz.
A particularly advantageous embodiment of the invention
consists in designing the passages or channels for re-
ceiving the water-in-air aerosol and the water-in-
burnable gas aerosol, respectively, as shown in Fig. 1.
In a fuel cell stack, each fuel cell is confined on the
anode side and on the cathode side by a bipolar plate
10, 6 each. The anode-side bipolar plate simultaneously
is the cathode-side bipolar plate of a neighboring cell
and the cathode-side bipolar plate at the same time is
the anode-side bipolar plate of the other neighboring
cell.
The bipolar plate, at least in a partial region, is of
corrugated sheet structure, i.e. it has alternating ele-
vations and depressions . A surface of the bipolar plate
6 contacts, with its elevations 7, the cathode region 2
of the fuel cell, whereby the depressions 8 located
between two adjacent elevations each together with the
cathode region form channels 5 for receiving water-in-
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air aerosol. In corresponding manner, the bipolar plate
contacts with one surface the anode region 3 of the
cell, so that the depressions 12 located between two
adjacent anode-side elevations 11 each also form
5 channels 9 together with the anode region 3. These can
serve for taking up water-in-burnable gas aerosol.
In the embodiment shown in Fig. 1, hydrogen as burnable
gas is introduced perpendicularly to the plate surface
10 through bores. The hydrogen first enters channel 9 in
communication with the supply opening and from there
diffuses or flows into the adjacent porous anode region.
From there, the hydrogen diffuses in part to the anode
catalyst layer and in part into additional gas channels
9 in the plane of the anode region. Because of the out-
standing diffusion properties of hydrogen, the entire
anode region thus is uniformly supplied with hydrogen
without a problem.
If cooling water is to be supplied as well along with
the burnable gas, it is as a rule more advantageous to
choose the same type of feeding as in the cathode
region, i.e. to feed fuel and water into each individual
channel 9. Because of the poor diffusion properties of
water in comparison with hydrogen, only little water
would penetrate the anode otherwise, and the cooling
effect would thus be low.
The construction has no separate cooling channels what-
soever. A specific advantage consists in particular in
that the path of the aerosol through the channels 5 of
the cell constitutes a straight line. The corrugated
sheet structure of the bipolar plate with straight gas
paths permits to minimize depositions of the aerosol and
to conduct the necessary volume flows with low pressure
drop.
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Flooding and clogging of the water-conducting paths by
water droplets, as is frequently the case with porous
plates, does not take place. Besides, the "corrugated
sheet plate" can be manufactured very inexpensively and
simply in terms of manufacturing technology.
The anode and cathode regions are each designed as dif-
fusion layers carrying a suitable catalyst and disposed
on the opposite sides of the polymer electrolyte mem-
brane 4.
Air gaskets 15, 15' and hydrogen gaskets 16, 16' seal
the cell in gastight manner.
To increase the dwell period of the water in the cell
and to thus enable complete evaporation, the walls of
the gas channels 5 and/or the gas channels 9 can be
coated with a hydrophilic absorbent layer, for instance
with felt. The hydrophilic, absorbent layer distributes
the introduced amount of water in particularly even
manner and retains the same up to evaporation.
The amount of water required for obtaining optimum mem-
brane humidification, as outlined hereinbefore, can be
determined and regulated electronically. The amount of
water introduced into the°fuel cell has to fulfil two
tasks: cooling the cell and humidifying the membrane.
For regulation of the required amount of water, however,
only the setting of the suitable membrane moisture is
taken into consideration. Depending on the parameters
temperature, load, air ratio and the like, the optimum
membrane moisture and thus the optimum conductivity
value of the membrane is determined experimentally. The
addition of water varies depending on the conductivity
value to be reached. The cell temperature varies within
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wide limits depending on the operating conditions. As
long as sufficient water is introduced to ensure optimum
membrane moisture, a sufficient cooling effect, however,
is ensured as well.
For keeping the moisture content of the reaction gases
and the temperature thereof along the direction of flow
as constant as possible in a fuel cell or fuel cell
stack, the reaction gas, in particular the air, may be
caused to pass the cell stack several times. This takes
place by recirculation of the air/water mixture leaving
the fuel cells and the burnable gas/water mixture
leaving the fuel cells, respectively, to the respective
suction or intake flow.
Thus, it is possible according to the invention in a
polymer electrolyte fuel cell, by introducing ion-free
water in liquid form directly into the gas channels of
the combustion air and/or the burnable gas, to ensure at
the same time keeping of an optimum membrane moisture
and, thus, an optimum conductivity value of the membrane
as well as sufficient cooling of the fuel cell.
