Note: Descriptions are shown in the official language in which they were submitted.
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Electrochemical system with fluid bypassing limitation elements
The present invention relates to an electrochemical system, as well as to a
bipolar plate for
use in such a systein.
The electrochemical system may for example be a fuel cell system or an
electrochemical
compressor system, in particular an electrolyser with which, by way of
applying a potential, apart
from the production of hydrogen and oxygen from water, these gases are
simultaneously compressed
under pressure. Apart from this, electrochemical compressor systems such as
electrochemical
hydrogen cornpressors are also known, to which gaseous, molecular hydrogen is
supplied and in
which this is electrochemically compressed by applying a potential. This
electrochemical
compressing lends itself in particular for small quantities of hydrogen to be
compressed, since a
mechanical compression of the hydrogen here would require significantly more
effort.
Electrochemical systems are known, with which an electrochemical cell stack is
constructed
with a layering of several electrochemical cells, which in each case are
separated from one another
by bipolar plates. With this, the bipolar plates have several tasks:
- the electrical contacting of the electrodes of the individual
electrochemical cells (e.g. fuel
cells) and conveying the current further to the adjacent cell (series
connection of the cells),
- the supply of the cells with media, i.e. reaction gases, and the removal
of reaction products
via a channel structure, which is arranged in an electrochemically active
region (gas
distribution structure/flowfield),
- the further conveying the waste heat arising during the reaction in the
electrochemical cell, as
well as
- the sealing of the different media channels or cooling channels against
one another and to the
outside.
For the supply and removal of media from the bipolar plates to the actual
electrochemical
cells, these e.g. are MEAs (membrane electrode assembly) with a gas diffusion
layer in each case
orientated towards the bipolar plates (e.g. of a metal non-woven or carbon non-
woven), and the
bipolar plates may have openings for cooling, or for the supply and removal of
media.
With known bipolar plates, the gas distribution along the MEA or the gas
diffusion layer is
effected via channel structures or meander structures on both sides of the
bipolar plate.
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It is known, in particular for metallic bipolar plates, to stamp channel
structures into these
bipolar plates and thereby also to stamp a boundary wall at the same time
unitary with the bipolar
plate, which surrounds the electrochemically active region. The boundary wall
thereby often has a
bead-like shaping. Now it has been found in series of trials that such bipolar
plates may display large
fluctuations with regard to their performance, which appear to originate from
an insufficient
distribution of media.
It is therefore the object of the present invention to provide an
electrochemical system or a
bipolar plate which do not have fluctuations of performance due to
insufficient distribution of media
in the electrochemically active region.
This object is achieved by:
(a) An electrochemical system consisting of a layering of several cells
which are in each case
separated from one another by bipolar plates, wherein the bipolar plates
comprise openings for
cooling or for the removal and supply of media to the cells, and the layering
may be set under
mechanical compressive stress, wherein at least one cell comprises an
electrochemically active
region which is surrounded by a boundary wall of the bipolar plate, and a
channel structure of the
bipolar plate for the uniform distribution of media is provided within the
electrochemically active
region, wherein at least one gas diffusion layer is provided for micro-
distribution of media.
Limitation elements which are distanced to one another are provided in the
border region between the
channel structure and the boundary wall, for avoiding media flow between the
channel structure and
the boundary wall. Thereby, the gas diffusion layer covers the channel
structure and/or at least parts
of the limitation elements.
(b) A bipolar plate for use in the electrochemical system according to the
invention, wherein this
bipolar plate has a base plane, and a channel structure (flowfield) projecting
from this base plane, as
well as openings for the supply and removal of media are provided, and the
channel structure as well
as the openings are surrounded by a boundary wall (under certain circumstances
provided with
openings for leading through fluid), and wherein several limitation elements
distanced to one another
are provided in a border region between the boundary wall and the channel
structure, for preventing
media from bypassing in the border region between the boundary wall and the
channel structure.
4922821.1
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Thus a bypass of media at the channel structure is largely prevented with the
limitation
elements according to the invention. A uniform distribution of media over the
channel structure is
achieved by way of this, and the undesired performance fluctuations are
eliminated in this manner.
Thereby, it is particularly advantageous if the gas diffusion layer not only
covers the channel
structure, but also at least parts of the limitation elements. On account of
this, an additional
compression of the gas diffusion layer occurs in this region, which is greater
than the compression in
the region of the "normal" channel structure. Thus a bypass of medium in the
questionable region
between the channel structure and the boundary wall is therefore prevented in
an even greater
manner.
One advantageous further embodiment envisages the height of the limitation
elements being
selected in a manner such that the compression of the gas diffusion layer in
the contact region to the
limitation elements being greater than in the contact region to the channel
structure. Thus a
particularly good sealing in this border region is achieved. A further
advantageous embodiment
envisages the limitation elements being designed as extensions of the channel
structure, which merge
into the boundary walls. This is particularly advantageous with metallic
bipolar plates, since thus a
combined and unitary embossing of the limitation elements and elements of the
channel structure is
possible. Basically, it is possible to provide one or more limitation
elements. With several limitation
elements, these are preferably distanced to one another and particularly
preferably here two adjacent
limitation elements together with the boundary wall in each case form chambers
between the channel
structure and the boundary wall, so that bulkheads are formed (as with a
freight ship), in order to
prevent the bypass as securely as possible.
The repetition distance of individual limitation elements thereby is
preferably greater than 2
mm, particularly preferably greater than 5 - 10 mm (with smaller distances,
the boundary wall would
be mechanically weakened far too much). As an alternative, one could also say
that here at least one
limitation element is to be provided for 100 min length of the boundary wall,
preferably five to
twenty five limitation elements. One further advantageous design envisages the
boundary wall
running in a serpentine manner, thus making the boundary wall mechanically
stronger. With a
serpentine course of the boundary wall, in each case the portion of the
boundary wall which lies
closest to the channel structure may be connected to the channel structure via
a limitation element.
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It is advantageous, with regard to the limitation elements which are to
prevent the bypass
between the boundary wall and the channel structure, for these to run
essentially transversely to the
boundary wall as well as essentially transversely to the outermost elements of
the channel structure.
The constructional shape of the limitation elements is moreover dependent on
the respective
design of the channel structures. If the channel structures for example are
provided as individual
elements, then the limitation elements may also be provided in a linear
manner, in order to avoid a
bypass. In the other case, these limitation elements are also to be provided
as individual elements.
A further advantageous embodiment envisages the boundary wall having a greater
height
with respect to a base plane of the bipolar plate, than the predominant
elevation of the channel
structure in the vicinity of the boundary wall with respect to this base
plane. Vicinity hereby is to be
understood as a distance to the boundary wall of maximal 1 cm.
The limitation elements of the bipolar plate should at least have the height
of the
predominant elevation of the channel structure starting from the base plane.
This means that they
should therefore preferably have the same height as the channel structure or
have a height between
the height of the channel structure and the height of the boundary wall.
The limitation elements are preferably provided as embossings in a (preferably
metallic)
bipolar plate.
A further advantageous design envisages a bipolar plate being constructed of
two plates,
wherein the at least one limitation element is hollow on the side of the first
plate which is distant to
the electrochemically active side, and this hollow space acts as a
complementary space for inserting
the second plate of the bipolar plate.
The boundary walls preferably have the shape of a bead, in particular a full
bead or a half
bead and thus are an integral component which is unitary with the bipolar
plate, but however
attachment parts are also possible here.
One particularly advantageous embodiment envisages the openings of the bipolar
plate for
cooling or for the removal or supply of media, being provided with elastic
bead arrangements,
wherein these bead arrangements comprise openings for conducting fluid or
gaseous media into a
hollow space of the bipolar plate, or to the electrochemically active region.
Preferably, the conducting of media through the electrochemically active
region is effected in
a manner such that the location of the introduction of the medium and the
location of the leading-out
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of the medium is effected at respective maximally distanced points of the
electrochemically active
region. 13y way of this, the basic requirements for obtaining a distribution
which is as plane as
possible are created, and thereby a meandering leading is useful, even if dead-
end layouts are
possible. With such ones however, a flow resistance within the
electrochemically active region is
always to be overcome, so that the medium always seeks "short cuts" or
bypasses. For this reason,
the present invention with the limitation elements is particularly useful
here.
Further advantageous embodiments of the present invention are specified in the
following
description.
The invention is now explained with the help of several figures. These show:
Fig, la to lc the construction of a fuel cell stack,
Fig. 2a and 2b plan views of differently designed bipolar plates,
Fig. 3a and 3b cross sections through a bipolar plate arrangement according
to the invention
(Fig. 3a) and a bipolar plate arrangement according to the state ofthe art
(Fig.
3b),
Fig. 4 curves of the cathode-side volume flow against back-pressure,
with and
without limitation elements,
Fig. 5 a plan view of a further embodiment of a bipolar plate with
limitation
elements,
Fig. 6 an illustration of a centering according to B-B from Fig. 5,
Fig. 7 flow resistance curves of electrochemical systems according to
the invention,
with different compressions.
Fig. la to lc show the basic construction of an electrochemical system in the
form of a fuel
cell stack 1. This comprises a layering of several fuel cell arrangements 12
(see Fig. lb). The
layering of these fuel cell arrangements 12 is held together by end plates
which e.g. via clamping
bolts as shown in Fig, le, apply a compressive stress to the layering of the
fuel cell arrangements.
The construction of a fuel cell arrangement 12 is explained in more detail
hereinafter.
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Fig. 1a shows the inner construction of a fuel cell arrangement 12 in the form
of an
exploded drawing. This is firstly a cell (for example a fuel cell) 2, which
comprises a polymer
membrane which is capable of conducting ions and which at least in an
electrochemically active
region 6 has a catalytic layer on both sides. Moreover, two bipolar plates 3
are provided in the
fuel cell arrangement 12, between which the fuel cell 2 is arranged. Moreover,
a gas diffusion
layer 9 is arranged in the region between each bipolar plate and the adjacent
fuel cell 2. A bead
which is not shown and which is essentially peripheral in the edge region of
the bipolar plates,
forms a boundary wall and thus ensures the sealing of the electrochemically
active region 6, so
that no cooling fluid or media may exit to the outside from this region or
vice versa.
Moreover, the bipolar plates 3 contain supply openings (interface channels)
which are
aligned to each other. On the one hand this is an opening 4 for leading
through cooling fluid,
wherein this opening is surrounded by a further bead arrangement. Moreover, an
opening 5 for
the supply and removal of media to the electrochemically active region is
provided, which is
limited by a further bead arrangement. Moreover, passage openings are provided
for clamping
bolts which are not shown in Fig. la.
Fig. 2a shows a plan view of a section of a bipolar late according to the
invention. Here,
an opening for the supply and removal of media 5, which is surrounded by an
annular-shaped
full bead is shown. This full bead or bead arrangement comprises openings 5.1
for leading
through fluid or gaseous media into a hollow space of the bipolar plate or
towards the
electrochemically active region. The shown bipolar plate 3 is of metal,
wherein the channel
structure 8 and the boundary wall 7 are designed as embossings unitary with
the bipolar plate 3.
Here, only the upper left corner of the bipolar plate is shown in Fig. 2a for
illustration.
The leading of media through the electrochemically active region 7 is however
effected in a
manner such that the location of the introduction of the medium and the
location of the leading-
out of the medium are positioned at points of the electrochemically active
region maximally
distanced to each other, preferably at the plane diagonals of the surface
plane as it is shown in
Fig. 2a.
The course of the boundary wall 7 is shown in a serpentine manner in Fig. 2a,
at least in
the section which is shown at the top on the right in Fig.2a.
Moreover, it is shown that with a serpentine course of the boundary wall 7, a
portion of
the boundary wall which lies closest to the channel structure 8, is connected
via a limitation
element 10 to the channel structure.
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Here, one may also deduce that the limitation element 10 runs essentially
transversely to
the boundary wall 7 or also essentially transversely to the elements of the
channel structure 8
which are closest to the boundary wall (outermost)
It may also be deduced from Fig. 2a that the limitation element 10 is designed
as an
extension of the channel structure 8, which merges into the boundary wall 7.
Fig. 2b shows an alternative embodiment of the bipolar plate shown in Fig. 2a.
In contrast to the bipolar plate shown in Fig. 2a, here however several
limitation elements
are provided. These are distanced to one another, so that two adjacent
limitation elements 10
in each case form chambers between the channel structure 8 (thus the outermost
elements of the
channel structure) and the boundary wall 7.
The repetition distance of individual limitation elements hereby is preferably
greater than
2 mm, particularly preferably greater than 5 - 10 mm.
It is therefore evident that limitation elements 10 are provided in the
embodiments shown
in Fig. 2a or Fig, 2b, which prevent a bypass (thus a shortcut) of medium
between the boundary
wall 7 as well as the outermost elements of the channel structure 8. This is
designed in Fig. 2a as
a single transverse web, in Fig. 2b as a multitude of transverse webs which
then form
corresponding chambers. It is important that these limitation elements 10
assume this function,
thus are not designed as supply beads or inlets to cooling channels or media
channels.
In this manner, the flow of medium which for example proceeds from the media
supply
opening 5 respectively 5.1, is forced through the channel structure 8 which is
designed in a
meandering manner, and this causes an increased backpressure which is thus
also an indicator of
greater reaction rates of media in the fuel cells.
Fig. 3a shows a cross-sectional view of a construction, which shows two
bipolar plates 3
(for example according to Fig. 2a or Fig. 2b). Here, a fuel cell or a polymer
electrolyte
membrane (PEM) 2 is arranged between two bipolar plates 3. Moreover, a gas
diffusion layer 9
is arranged on each side of the PEM 2, in the electrochemically active region.
This gas diffusion
layer may be premanufactured and be designed as a direct integral component of
a membrane
electrode assembly, and the gas diffusion layers may also be provided as
separate layers.
What is significant is that the height of the limitation elements 10 is
selected in a manner
such that the compression of the gas diffusion layer 9 in the contact region
to the limitation
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elements 10 is greater than in the contact region to the channel structure 8.
This is also indicated
in an illustrated manner in Fig. 3a by the narrower hatching.
It is also to be seen that the boundary wall 7 has a greater height with
respect to a base
plane 11 of the bipolar plate 3 than the predominant elevation of the channel
structure with
respect to this base plane (this is indicated by the double arrows in Fig.
3a). It is likewise evident
that the limitation elements 10, starting from this base plane 11 of the
bipolar plate 3, have at
least the height of the greatest elevation of the channel structure 8, however
at the most the
height of the boundary wall 7 with respect to the base plane (this also is
evident by the double
arrows in Fig. 3a).
In contrast to this, Fig. 3b shows an arrangement which has no limitation
elements and
with which an additional compression of the gas diffusion layer in the outer
edge region is not
given.
The figures which were referred to until now, in particular the Fig. la to lc,
2a, 2b as
well as 3a, thus show a bipolar plate 3, wherein this comprises a base plane
11, and a channel
structure 8 projecting from this base plane, as well as openings 5 for the
supply and removal of
media are provided, and the channel structure as well as the openings are
surrounded by a
boundary wall 7, and at least one limitation element 10 is provided in the
region between the
boundary wall and the outer edge of the channel structure, for preventing
medium from
bypassing in the border region between the boundary wall 7 and the channel
structure 8.
Thus what is shown in the previously mentioned figures is also an
electrochemical
system 1 consisting of a layering of several cells 2 which in each case are
separated from one
another by bipolar plates 3, wherein the bipolar plates comprise openings for
cooling 4 or the
removal and supply 5 of operating media to the cells, and the layering may be
set under
mechanical compressive stress, wherein at least one cell comprises an
electrochemically active
region 6 which is surrounded by a boundary wall 7 of the bipolar plate, and a
channel structure 8
of the bipolar plate is provided within the electrochemically active region
for a uniform
distribution of media, wherein at least one gas diffusion layer 9 is provided
for the micro-
distribution of medium, and limitation elements 10 are provided in the border
region between the
channel structure as well as the boundary wall, for avoiding the fluid
bypassing between the
channel structure and the boundary wall in the electrochemically active region
(thus not the
cooling region), and the gas diffusion layer covers the channel structure
and/or at least parts of
the limitation elements. Thus a clamping or a strong pressing of the gas
diffusion layer in the
edge region is achieved on account of this covering, and an even better
sealing occurs on account
of this, since not only the height of the boundary wall, but also the
compression of the gas
diffusion layer in this region ensures a prevention of the bypass (flowing-
past/shortcut).
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Fig. 4 shows the diagram of a volume flow in litres per minute against the
back-pressure
in millibars, for an electrochemical system.
The left graph shows a conventional design on the cathode side (for example
with a cross
section according to Fig. 3b), with which a flow of medium bypassing the gas
diffusion layer is
possible. The right graph shows a compression of the gas diffusion layer with
limitation
elements, so that a bypass is prevented or limited by way of this. It is shown
here that with the
same volume flow, a much greater back-pressure is present. This is an
indication that the
medium does not simply pass without being led through the electrochemically
active field. By
way of this a constant reaction is forced since the reaction medium no longer
bypasses (flows
past) in a non-used manner.
Fig. 5 shows a further embodiment of a bipolar plate according to the
invention. Here,
channel structures 8 are provided in the electrochemically active region 6
which is surrounded by
a boundary wall, which are mainly designed as disjunct, thus individual raised
elements. Here, it
is again the case of a bipolar plate with a flowfield (electrochemically
active region), with which
the reaction medium is led from the top left in a diagonal manner to the
bottom right (exit 5
there). Limitation elements 10 are provided at two locations (bottom left and
top right), which
prevent an undesired bypass.
Fig. 6 shows a cross section through B-B of the plate arrangement of Fig. 5 in
an
enlarged scale. Here, it is to be seen that there a bipolar plate is
constructed of two plates,
wherein the at least one limitation element 10 is hollow on the side which is
distant to the
electrochemically active side, and this hollow space is provided as a
complementary space for
inserting the second plate of the bipolar plate. In this manner, an additional
centering of both
plates is carried out, so that the dimensional accuracy of the complete
bipolar plate is increased
by way of this.
Fig. 7 shows (similarly as it has been shown already above in Fig. 4) the
volume flow of
(dry) air in litres per minute, against the back-pressure (in millibar). Here,
one may also see that
with increasing compression values (compressive stress) in the complete
assembly (see Fig. 1c)
and with an equal volume flow of air, a significantly increased back-pressure
arises and that in
this manner uniformly reproducible values can be set.
