Note: Descriptions are shown in the official language in which they were submitted.
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Fuel cell and fuel cell arrangement
Prior art
It is known to use fuel cells in vehicles and mobile devices for converting
chemical reaction
energy into electrical energy. A fuel cell of this kind is the PEM fuel cell
(Polymer Electrolyte
Membrane fuel cell).
DE 103 93 467 T5 discloses, for example, a typical design of a PEM fuel cell
of this kind. The
PEM fuel cell comprises a membrane electrode arrangement with a solid polymer
membrane
and two electrodes which are arranged on opposite sides of the solid polymer
membrane,
that is to say an anode and a cathode. In this case, the electrodes have a
shorter longitudinal
extent in comparison to the solid polymer membrane, so that the edges of the
solid polymer
membrane project beyond the area of the electrodes.
A plurality of these PEM cells are typically connected to one another in
series in order to
achieve module voltages which are as high as possible. Therefore, a single
faulty cell can
lead to breakdown of the entire module. The level of reliability and service
life of PEM cells
can be severely affected when there are high cathode potentials and high
diffusion leakage
rates of oxygen from the cathode to the anode. Relatively high cathode
potentials and
therefore an increased irreversible degradation of the membrane can be
expected
particularly in the edge region of the cells, in which edge region lower
current densities
prevail. Degradation of the membrane can lead to the formation of holes within
the
membrane and breakdown of the module.
Disclosure
The object of selected embodiments is to provide a fuel cell which has a
relatively high level
of reliability and a relatively long service life.
Certain exemplary embodiments provide a fuel cell comprising a membrane which
is
arranged between two gas diffusion layers, and a first current collector which
is arranged on
a first side of the membrane and a second current collector which is arranged
on a second
side, which second side is situated opposite the first side, wherein a second
current collector
edge region extends substantially as far as an edge region of the membrane at
least on the
second side of the membrane, or wherein a second current collector edge region
extends
2
further in the direction of an edge region of the membrane on the second side
of the
membrane than a first current collector edge region which is arranged on the
first side of the
membrane.
Certain other exemplary embodiments provide a fuel cell comprising: a membrane
which is
arranged between two gas diffusion layers, and a first current collector which
is arranged on
a first side of the membrane, and a second current collector which is arranged
on a second
side of the membrane, which second side is situated opposite the first side,
wherein a
second current collector edge region extends as far as an edge region of the
membrane at
least on the second side of the membrane, or wherein a second current
collector edge region
extends further in the direction of an edge region of the membrane on the
second side of the
membrane than a first current collector edge region which is arranged on the
first side of the
membrane, wherein the first current collector comprises a first current
collector edge region
and at least one first current collector central region, and wherein the
second current
collector (14) comprises the second current collector edge region and at least
one second
current collector central region, and wherein the first current collector edge
region and the at
least one first current collector central region and/or wherein the second
current collector
edge region and the at least one second current collector central region are
physically
separated from one another by means of a seam, and wherein the fuel cell
additionally
comprises an electrical load that is electrically connected between the first
and the second
current collectors, and wherein the electrical load comprises a non-reactive
resistor, an
inductance, a galvanic element, a connection for an external further load, an
electronics
system with a potentio- or galvanostatic control system and/or an electronics
system for
supplying an external load.
The fuel cell according to certain embodiments has the advantage over the
prior art that an
improved electrical connection has been implemented in the edge region of the
fuel cell, as a
result of which the current density in the edge region is increased. The
increased current
density in the edge region advantageously leads to the cathode potential in
the edge region
being reduced and therefore there being a considerably lower tendency for the
membrane to
degrade. The level of reliability and durability of the fuel cell are
therefore considerably
increased. The fuel cell comprises, in particular, a PEM fuel cell (Polymer
Electrolyte
Membrane fuel cell).
Advantageous refinements and developments of selected embodiments can be
gathered
from the dependent claims and also from the description with reference to the
drawings.
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According to a preferred development of selected embodiments, a first current
collector edge
region likewise extends substantially as far as an edge region of the membrane
on the first
side of the membrane. Therefore, the first current collector edge region and
the second
current collector edge region advantageously lie one above the other, in
particular in a
congruent manner, along a direction perpendicular to the membrane in the edge
region. This
leads to the current density in the edge region being further increased and
the degradation of
the membrane in the edge region being further counteracted.
According to a further preferred development of selected embodiments, the
first and/or the
second current collector edge region run/runs entirely along the edge of the
membrane in a
plane of main extent which is parallel to the membrane. The first and second
current collector
edge regions are formed, in particular in a circumferential manner, in the
plane of main
extent. Therefore, the tendency of the membrane to become detached is
advantageously
reduced in the entire edge region of the membrane.
According to a further preferred development of selected embodiments, the
first current
collector comprises the first current collector edge region and at least one
first current
collector central region, and wherein the second current collector comprises
the second
current collector edge region and at least one second current collector
central region. The
first current collector edge region and the at least one first current
collector central region
either merge seamlessly with one another or, as an alternative, are physically
separated from
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one another by means of a seam. Analogously, the second current collector edge
region and
the at least one second current collector central region likewise merge
seamlessly with one
another or are physically separated from one another by means of a seam.
Separation of the
current collector central regions from the current collector edge regions has
the advantage
that gas from the membrane or from the gas diffusion layers can circulate
between said
regions.
According to a further preferred development of selected embodiments, the fuel
cell
comprises an electrical load which is electrically connected between the first
and the second
current collector. The electrical load is either directly connected to the
first and the second
current collector edge region or the electrical load is directly connected to
the second current
collector edge region and to at least one first current collector central
region. The electrical
load preferably comprises a non-reactive resistor, a galvanic element, a
connection for an
external load, an electronics system with a potentio- or galvanostatic control
system and/or
an electronics system for supplying an external load. The electrical load
advantageously
provides for a further increase in the current density in the edge region of
the membrane.
Within the meaning of the present invention, the electrical load comprises an
electrical load
in which the actual effective power of the fuel cell is not intended to be
consumed, but rather
which electrical load is connected in parallel with the actual load of the
fuel cell and through
which a well-defined small leakage current is intended to flow only in order
to increase the
current density in the edge region of the membrane and therefore the service
life of the fuel
cell. The actual effective power of the fuel cell serves to supply a primary
electrical load
circuit with which electrical contact is made by means of the at least one
first current collector
central region and the at least one second current collector central region.
It is preferably
provided that the electrical load can be temporarily connected. The electrical
load is
connected in specific operating states in particular.
According to a further preferred development of selected embodiments, a
catalyst layer is
arranged between the membrane and the respective gas diffusion layer on each
side of the
membrane. Furthermore, it is provided, in particular, that the first current
collector comprises
the anode and the second current collector comprises the cathode. However,
analogously,
the second current collector could comprise the anode and the first current
collector could
comprise the cathode.
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According to a further preferred development of selected embodiments, the fuel
cell
comprises an electrically insulating edge enclosure which encases the
membrane, the gas
diffusion layers, the catalyst layers and the current collectors in the edge
region of the
membrane.
A further embodiment provides a fuel cell arrangement comprising a first fuel
cell according
to the invention, a second fuel cell according to the invention and an
electrical load, wherein
the first fuel cell and the second fuel cell are connected to one another in
series, and wherein
the electrical load is connected to the first and the second fuel cell in
series in such a way
that the electrical load is electrically connected between a first current
collector edge region
of the first fuel cell and a second current collector edge region of the
second fuel cell, and
that a second current collector edge region of the first fuel cell is directly
connected to a first
current collector edge region of the second fuel cell. Therefore, a plurality
of fuel cells
advantageously share one electrical load which serves to increase the current
densities in
the respective edge regions of the fuel cells.
Further details, features and advantages of selected embodiments can be
gathered from the
drawings, and also from the following description of preferred embodiments
using the
drawings. Here, the drawings illustrate merely exemplary embodiments which do
not restrict
the essential concept of the disclosure.
Brief description of the drawings
Figures 1 and 2 show a schematic sectional view of a fuel cell according to
the prior art
and the associated cathode potential curve.
Figures 3 and 4 show a schematic sectional view of a fuel cell according to
a first
embodiment and the associated cathode potential curve.
Figure 5 shows a schematic sectional view of a fuel cell according to a
second
embodiment.
Figure 6 shows a schematic sectional view of a fuel cell according to a
third
embodiment.
Figure 7 shows a schematic sectional view of a fuel cell according to a
fourth
embodiment.
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Figure 8 shows a schematic sectional view of a fuel cell according to a
fifth
embodiment.
Figure 9 shows a schematic sectional view of a fuel cell according to a
sixth
embodiment.
Figure 10 shows a schematic sectional view of a fuel cell arrangement
according
to a seventh embodiment.
Figures 11a to 11d show schematic plan views of the fuel cells according to
various
embodiments.
Embodiments
In the various figures, identical parts are always provided with the same
reference symbols
and will therefore also be named or mentioned only once as a rule.
Figure 1 shows a schematic sectional view of a typical PEM fuel cell (Polymer
Electrolyte
Membrane fuel cell) 12 according to the prior art. The fuel cell 12 comprises
a proton-
conducting membrane 1. The membrane 1 is arranged between an anode-side
catalyst layer
2 and a cathode-side catalyst layer 6. The anode-side catalyst layer 2 is
arranged between
an anode-side gas diffusion layer 3 and the membrane 1, while the cathode-side
catalyst
layer 6 is arranged between a cathode-side gas diffusion layer 7 and the
membrane 1. A first
current collector 13 is provided on a first side and a second current
collector 14 is provided
on a second side for the purpose of making contact with the fuel cell 12. The
first current
collector 13 functions as an anode-side electrical contact-making means 4,
while the second
current collector 14 serves as a cathode-side electrical contact-making means
8. The layer
structure comprising membrane 1, catalyst layers 2, 6 and gas diffusion layers
3, 7 is
encased by an electrically insulating edge enclosure 5 in an edge region.
The fuel cell 12 operates by the hydrogen from the free gas volume at the
anode being
conducted through the anode-side gas diffusion layer 3 to the anode-side
catalyst layer 2
and being split into protons and electrons there. The protons migrate through
the membrane
1 to the cathode-side catalyst layer 6, and the electrons migrate by means of
a primary
electrical circuit (not illustrated) which is external to the fuel cell, from
the anode-side
electrical contact-making means 4 to the cathode-side electrical contact-
making means 8. On
the cathode side, the oxygen is passed through the cathode-side gas diffusion
layer 7 to the
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cathode-side catalyst layer 6, where it is reacted with the protons and
electrons to produce
water.
Under certain electrochemical conditions, parallel reactions are possible,
which parallel
reactions can attack and destroy the components of the fuel cell 12. Here, the
electrode
potential is an important variable for estimating whether undesired reaction
regimes could
prevail. The cathode potential is of central importance in the PEM fuel cell
12.
Figure 2 therefore schematically shows the cathode potential curve of the fuel
cell 12
illustrated in Figure 1. It can be seen that frequent load cycles in
combination with high
cathode potentials (> 0.9 V in comparison to a normal hydrogen electrode) can
lead to
increased radical formation and to increased oxidation of the cathode-side
catalyst 6 and its
migration into the membrane 1. Both effects result in increased degradation of
the membrane
1 and abrupt breakdown of the fuel cell 12. Simulation of the edge region of
the fuel cell 12
resulted in the local current density in the edge region dropping to
approximately 20% of the
maximum value. Here, the cathode potential is increased to over 0.9 V, as
shown in figure 2.
Therefore, there is a threat of excessive degradation as a result of platinum
oxidation or
increased radical formation, as a result of which the level of reliability and
service life of the
fuel cells 12 known from the prior art are adversely affected.
Figure 3 shows a schematic sectional view of a PEM fuel cell (Polymer
Electrolyte
Membrane fuel cell) 12 according to an exemplary first embodiment. The fuel
cell 12
according to the first embodiment is based on the design illustrated in figure
1, that is to say
the fuel cell 12 comprises a proton-conducting membrane 1 which is arranged
between an
anode-side catalyst layer 2 and a cathode-side catalyst layer 6. The anode-
side catalyst
layer 2 is arranged between an anode-side gas diffusion layer 3 and the
membrane 1, while
the cathode-side catalyst layer 6 is arranged between a cathode-side gas
diffusion layer 7
and the membrane 1. A first current collector 13 is provided on a first side
and a second
current collector 14 is provided on a second side for the purpose of making
contact with the
fuel cell 12. The first current collector 13 functions as an anode-side
electrical contact-
making means 4, while the second current collector 14 serves as a cathode-side
electrical
contact-making means 8. The layer structure comprising membrane 1, catalyst
layers 2, 6
and gas diffusion layers 3, 7 is encased by an electrically insulating edge
enclosure 5 in an
edge region.
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In contrast to the fuel cell 12 shown in figure 1, the fuel cell 12 according
to the first
embodiment comprises a current collector edge region 9 on the cathode side,
which current
collector edge region extends in a plane of main extent, which is parallel to
the membrane 1,
as far as the outermost edge of the membrane 1, the cathode-side catalyst
layer 6 and the
cathode-side gas diffusion layer 7. In this example, the second current
collector 14
comprises a second current collector central region 15 and also the second
current collector
edge region 9. The second current collector central region 15 and the second
current
collector edge region 9 are either connected to one another in an integral
manner, that is to
say are of continuous design, or are interrupted by a separation point here
and there. The
current density in the edge region is advantageously increased by realizing
the second
current collector edge region 9 which reaches as far as the edge region. It
has surprisingly
and unexpectedly been found that the level of reliability and the durability
of the fuel cell 12
can be increased by a cell construction by the current density of the edge
region being
increased by the electrical contact-making means 8 which is extended
completely into the
edge region. The cathode-side contact-making means 8, which reaches
extensively at least
as far as the edge enclosure, additionally has the advantage that the oxygen
partial pressure
within the cathode-side gas diffusion layer 7 can be reduced. The current
collector edge
region 9 preferably extends along the circumference of the entire edge region
of the cell.
Figure 4 schematically shows a simulation of the cathode potential curve of
the fuel cell 12
illustrated in figure 3 according to the first embodiment. The simulation of
the edge region
with contact-making means, which reaches as far as into the edge region,
through the
current collector edge region 9 shows that the cathode potential can be
reduced to below
0.9 V by the measures taken. It can therefore be assumed that there is a lower
tendency for
the membrane to degrade and an increase in the level of reliability of the
fuel cell 12.
Figure 5 shows a schematic sectional view of a fuel cell 12 according to a
second
embodiment. The fuel cell 12 according to the second embodiment corresponds
substantially
to the fuel cell 12 illustrated in figure 3 according to the first embodiment,
wherein, in
contrast, the current collector 8 which is provided on the cathode side
extends only partially
as far as the edge region in the fuel cell 12 according to the second
embodiment.
Figure 6 shows a schematic sectional view of a fuel cell 12 according to a
third embodiment.
The fuel cell 12 according to the third embodiment corresponds substantially
to the fuel cell
12 illustrated in figure 3 according to the first embodiment, wherein, in
contrast, the electrical
contact-making means 4 is also drawn as far as the edge region on the anode
side in the fuel
cell 12 according to the third embodiment. Therefore, the first current
collector 13 likewise
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comprises a first current collector central region 16 and also a first current
collector edge
region 11 which extends as far as the edge region. The first current collector
central region
16 and the first current collector edge region 11 are either connected to one
another in an
integral manner, that is to say are of continuous design, or are interrupted
by a separation
point here and there.
Figure 7 shows a schematic sectional view of a fuel cell 12 according to a
fourth
embodiment. The fuel cell 12 according to the fourth embodiment corresponds
substantially
to the fuel cell 12 illustrated in figure 6 according to the third embodiment,
wherein, in
contrast, the fuel cell 12 according to the fourth embodiment additionally
comprises an
electrical load 10 which is preferably connected in parallel to the actual
regular load, not
illustrated. It has already been shown using the above embodiments that the
cathode
potential is set in accordance with the local current density for the
illustrated edge region,
depending on loading of the fuel cell 12. A further increase in the current
density can be
achieved by contact being made with an additional electrical load on the anode
and cathode
side at the edge. This is realized by the electrical load 10 which is
electrically connected to
the anode 4 (first current collector 13) and cathode 8 (second current
collector 14). The
additional electrical load 10 leads to a further increase in the current
density in the edge
region. The primary electrical load circuit, which is closed by means of the
contact-making
means 4 and 8, is likewise influenced by electrical compensation currents
along the contact-
making means of the second current collector central region 14 and the second
current
collector edge region 9 and the gas diffusion layers 3, 7. The electrical load
10 can be
designed, for example, as a simple resistor, external consumer, power
electronics system for
potentio- or galvanostatic control, power electronics system for supplying an
external load or
a galvanic element.
Figure 8 shows a schematic sectional view of a fuel cell 12 according to a
fourth
embodiment. The fuel cell 12 according to the fourth embodiment corresponds
substantially
to the fuel cell 12 illustrated in figure 6 according to the third embodiment,
wherein, in
contrast, the fuel cell 12 according to the fourth embodiment comprises a
contact-making
means, which extends as far as the edge region, in the form of the second
current collector
edge region 9 only on the cathode side, and furthermore the second current
collector central
region 15 and the second current collector edge region 9 do not merge with one
another, but
rather are separated from one another, on the cathode side. The current
density of the edge
region is decoupled as far as possible from the loading of the fuel cell 12 by
the current-
carrying contact extension (second current collector edge region 9) having
been separated
from the primary electrical load circuit which is connected to the contact-
making means 4 and
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8 (first and second current collector central region 16, 15). Therefore, the
edge potential can
be individually set independently of the operating region or load current
drop. The simply
separated contact-making means uses, together with the load circuit, the
contact-making
means of the anode 4 (first current collector central region 16). If the
contact-making means
at the anode 4 is separated on one side, the opposite contact-making means of
the cathode
is jointly used. The current of the jointly used contact-making means (first
current collector
central region 16) is accordingly split between the opposite separated contact-
making
means, that is to say the current collector edge region 9, and the contact-
making means
which is connected to the primary load circuit (not illustrated), that is to
say the second
current collector central region 15.
Figure 9 shows a schematic sectional view of a fuel cell 12 according to a
fifth embodiment.
The fuel cell 12 according to the fifth embodiment corresponds substantially
to the fuel cell
12 illustrated in figure 7 according to the fourth embodiment, wherein, in
contrast, the fuel cell
12 according to the fifth embodiment contains a separated contact-making
means, firstly the
current collector central region 16 which is connected to the primary load
circuit and secondly
the current collector edge region 11 which is connected to the electrical load
10, on the
anode side too.
Figure 10 shows a schematic sectional view of a fuel cell arrangement 17
according to a
seventh embodiment. The fuel cell arrangement 17 comprises a first fuel cell
12' and a
second fuel cell 12", which fuel cells are connected in series. In this case,
the first and
second fuel cells 12' and 12" correspond to the fuel cell 12 illustrated in
figure 9 according to
the fifth embodiment. The first and the second fuel cell 12', 12" are further
connected to the
electrical load 10 in series, wherein the electrical load 10 is connected to
the first current
collector edge region 11 of the first fuel cell 12' and to the second current
collector edge
region 9 of the second fuel cell 12". Furthermore, the second current
collector edge region 9
of the first fuel cell 12' and the first current collector edge region 11 of
the second fuel cell
12" are electrically connected to one another.
Figures 11a to 11d show schematic plan views of the fuel cells 12 according to
various
embodiments. Figure 11a shows a plan view of the fuel cell 12 illustrated in
figure 3
according to the first embodiment. It can be seen in this perspective that the
current collector
central regions 14 and the current collector edge regions 9 merge with one
another without
interruption on the cathode side. Figure 11b shows a plan view of the fuel
cell 12 illustrated in
figure 8 according to the fourth embodiment. It can be seen in the shown
perspective that the
current collector central regions 14 and the current collector edge region 9
are separated
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from one another on the cathode side. Furthermore, the current collector edge
region 9 is
formed in a circumferential manner. Figures 11c and 11d show the same views in
alternative
embodiments in which the current collector central regions 15 are not in the
form of grids or
bars, but rather are designed (in a nub-like manner) in the form of
individual, circular contact
points. In figure 11c, the current collector edge regions 9 are each connected
to the outer
current collector central regions 14, while a circumferential current
collector edge region 9
which is not connected to the current collector central regions 14 is provided
in figure 11d.
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List of reference symbols
1 Membrane
2 Anode-side catalyst layer
3 Anode-side gas diffusion layer
4 Anode-side electrical contact-making means
Edge enclosure
6 Cathode-side catalyst layer
7 Cathode-side gas diffusion layer
8 Cathode-side electrical contact-making means
9 Second current collector edge region
Electrical load
11 First current collector edge region
12 Fuel cell
13 First current collector
14 Second current collector
Second current collector central region
16 First current collector central region
17 Fuel cell arrangement
