ELECTROCHEMICAL DEVICE AND METHOD
FOR CONTROLLING CORROSION
BACKGROUND
[0001] This disclosure relates to an electrochemical device having enhanced
resistance to internal corrosion.
[0002] Fuel cells, flow batteries and other electrochemical devices are commonly
known and used for generating electric current. An electrochemical device generally includes an
anode electrode, a cathode electrode, and a separator layer between the anode and cathode
electrodes for generating an electric current in a known electrochemical reaction between
reactants. Typically, where ionic-conductive reactants are used, differences in voltage potential
at different locations in the electrochemical device cause leakage currents, also known as shunt
currents, which debit energy efficiency. Additionally, the shunt current can drive corrosion of
components of the electrochemical device.
SUMMARY
[0003] An electrochemical device includes a plurality of electrode assemblies that
defines a plurality of electrochemically active areas. A non-electrically-conductive manifold
includes a common manifold passage and a plurality of branch passages that extend,
respectively, between the electrochemically active areas and the common manifold passage.
Each of the branch passages includes a first region and a second region that differ in surface area.
[0004] Also disclosed is a method for controlling corrosion in an electrochemical
device. The method includes providing a reactant fluid flow between a common manifold
passage and a plurality of the electrode assemblies. The reactant fluid generates a shunt current
that can be supported by self-reactions of the reactant fluid and corrosion reactions of
components of the electrochemical device. The corrosion reaction is limited by establishing a
tendency toward supporting the shunt current by the self-reaction rather than the corrosion
reactions.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] The various features and advantages of the present disclosure will become
apparent to those skilled in the art from the following detailed description. The drawings that
accompany the detailed description can be briefly described as follows.
[0006] Figure 1 shows an example electrochemical device.
[0007] Figure 2 shows a representative branch passage of the electrochemical device
of Figure 1.
[0008] Figure 3 shows another example electrochemical device.
[0009] Figure 4 shows an electrode assembly and a frame used in the electrochemical
device of Figure 3.
[0010] Figure 5 shows a bipolar plate and a frame used in the electrochemical device
of Figure 3.
[0011] Figure 6 shows bipolar plate channel walls that have tapered ends.
[0012] Figure 7 shows another example of bipolar plate channel walls that have
tapered ends.
[0013] Figure 8 shows another example of bipolar plate channel walls that have
tapered ends.
DETAILED DESCRIPTION
[0014] Figure 1 schematically shows an example of electrochemical device 20. As
will be appreciated, the electrochemical device 20 can be a fuel cell, a flow battery or other type
of electrochemical device that utilizes one or more ionic-conductive reactants. As will be
described in more detail, the electrochemical device 20 includes features for controlling or
limiting corrosion of internal components within the electrochemical device 20, such as carbonor
metal-containing components.
[0015] In this example, the electrochemical device 20 includes a plurality of electrode
assemblies 22 that are arranged in a stack. The electrode assemblies 22 define a plurality of
electrochemically active areas 24. The electrochemically active areas 24 are zones where the
reactants participate in oxidation/reduction reactions to generate an electric current. The reactants
are provided in this example through a non-electrically-conductive manifold 26. For example,
the non-electrically-conductive manifold 26 is a nonconductive polymeric material. The example
electrochemical device 20 has two such non-electrically-conductive manifolds 26 that serve to
supply and discharge a reactant flow F to and from the electrode assemblies 22. The nonelectrically-
conductive manifolds 26 can be similar or identical at least with regard to the
features described herein.
[0016] The non-electrically-conductive manifold 26 includes a common manifold
passage 28 through which a reactant is distributed into each of the electrochemically active areas
24. A plurality of branch passages 30, which is outside or partially outside of the
electrochemically active areas 24, extends between the electrochemically active areas 24 and the
common manifold passage 28.
[0017] Figure 2 shows a representative one of the branch passages 30. As shown, the
branch passage 30 includes a first region 32 and a second region 34 that differ in surface area, as
represented by the different shading of the first region 32 and the second region 34. In one
example, the difference in surface area is due to the relative roughness of the walls of the branch
passage 30. That is, the walls of the first region 32 are relatively smooth while the walls of the
second region 34 are relatively rough.
[0018] Fuel cells, flow batteries, and other electrochemical devices, especially those
that utilize ionic-conductive reactants, can suffer from inefficiencies due to shunt currents. In
some instances, the shunt currents are supported by corrosion reactions that corrode components,
especially carbon- or metal-containing components, and ultimately reduce life of the
electrochemical device. One approach to addressing shunt currents in electrochemical devices is
to increase the length or reduce the width-wise size of branch passages in order to increase the
ionic resistance of the pathways for the ionic-conductive reactants. However, increasing the
length or reducing the size can debit reactant flow or increase reactant pressure drop and thus
adversely impact cell or system performance. The electrochemical device 20 and methodology
disclosed herein take a different approach. Rather than limiting resistance, the electrochemical
device 20 and methodology herein establish a tendency toward supporting such shunt currents by
a self-reaction of the reactants over the corrosion reactions. In other words, the self-reaction of
the reactant and the corrosion reactions are competing reactions, and a greater tendency for the
self-reaction limits the corrosion reactions. For example, the self-reaction is a change in the
oxidation state of the reactant. For a vanadium liquid electrolyte reactant, the self-reaction is the
change from V4+ to V5+ in the positive reactant fluid or V2+ to V3+ in the negative reactant fluid.
A similar self-reaction would be expected for other reactant species, such as those based on
bromine, iron, chromium, zinc, cerium, lead or combinations thereof.
[0019] Outside of an electrochemically active area, if current density is relatively
high and surface area is relatively low in the presence of the reactant, the low surface area
provides a relatively low amount of available sites for catalyzing the self-reaction of the reactant.
Thus, the competing corrosion reactions are more likely to occur. However, for the same given
current density at a higher surface area, there is a greater amount of available surface sites for
catalyzing the self-reactions and the tendency shifts to predominantly favor of the self-reaction.
This results in a lower amount of corrosion of these solid surfaces.
[0020] Thus, in the electrochemical device 20, the first region 32 of the branch
channel 30 and the second region 34 that differ in surface area tend to promote or establish a
tendency towards supporting the self-reaction of the reactant over the corrosion reaction.
[0021] Figure 3 illustrates another example of an electrochemical device 120, shown
in cross-section. In this disclosure, like reference numerals designate like elements where
appropriate and reference numerals with the addition of one-hundred or multiples thereof
designate modified elements that are understood to incorporate the same features and benefits of
the corresponding elements. The electrode assembly 122 includes a first porous electrode 122a, a
second porous electrode 122b and a separator layer 122c there between. The separator layer can
be a polymer membrane, such as an ion-exchange membrane for example.
[0022] The electrode assembly 122 is arranged between bipolar plates 140. Each of
the bipolar plates 140 has channels 142 to convey reactant to or from the respective common
manifold passage 128a/128b of the non-electrically-conductive manifolds 126.
[0023] Referring also to Figures 4 and 5, with continuing reference to Figure 3, the
electrode assembly 122 is mounted in a non-electrically-conductive frame 144 with a plurality of
openings that form part of the non-electrically-conductive manifold 126 and common manifold
passages 128a/128b. Although the disclosed example has six manifold holes on each part in Figs.
4 and 5, fewer can alternatively be used, such as four (one + and one - on each end for inlets and
outlets). A seal 146 is provided around the perimeter of the electrode assembly 122 to limit
escape of reactants. The seal 146 can also function as means to integrate the electrodes and
separator assembly, a membrane-electrode assembly (MEA), for example. Similarly, each of the
electrically-conductive bipolar plates 140 is mounted in a non-conductive frame 148 with a
plurality of openings that form part of the non-electrically-conductive manifold 126 and common
manifold passage 128a/128b.
[0024] Each of the first porous electrode 122a and the second porous electrode 122b
spans over an area that is larger than the separator layer 122c, as represented by overhang
portions 150 that are outside of the electrochemically active area 24. The areas of the first porous
electrode 122a and the second porous electrode 122b are also larger than the bipolar plates 140.
The branch passages 130 include the first region 132 that is bounded by the non-conductive
frame 148 and the seal 146. The branch passages 130 may operate as either inlets or exits to or
from the channels 142 of the bipolar plates 140. The surfaces of the non-conductive frame 148
and the non-conductive seal 146 are relatively smooth. The second region 134 is bounded by the
non-conductive frame 148 and the first or second porous electrode 122a/122b, which are
conductive. The first and second porous electrodes 122a/122b thus provide a relatively greater
surface area than the smooth walls of the non-conductive frame 148 and the seal 146. Thus, the
overhang portions 150 provide regions with high surface areas outside of the electrochemically
active areas 24 to serve as sources or sinks for the shunt currents, where these shunt current can
be supported with the self-reactions rather than the corrosion reactions.
[0025] In a further example shown in Figure 6, the channels 142 of the bipolar plate
140 extend between channel walls 142a. Each of the channel walls 142a has a tapered end 160.
In this example, the tapered end 160 tapers to a point, P. Alternatively, the tapered ends 160 can
be rounded as shown in Figure 7. In another alternative shown in Figure 8, the channels 242 are
interdigitated, with alternating open ends 262 and closed ends 264. The tapered ends 160/260
further facilitate establishing the tendency toward the self-reaction of the reactant over the
corrosion reactions. The self-reaction of reactants can be transport limited. The tapered ends
160/260 of the channel walls 142a permit the reactant to continually move at the inlet of the
channels 142 and thereby disfavor the self-reaction of the reactant over the corrosion reactions.
[0026] Although a combination of features is shown in the illustrated examples, not
all of them need to be combined to realize the benefits of various embodiments of this disclosure.
In other words, a system designed according to an embodiment of this disclosure will not
necessarily include all of the features shown in any one of the Figures or all of the portions
schematically shown in the Figures. Moreover, selected features of one example embodiment
may be combined with selected features of other example embodiments.
[0027] The preceding description is exemplary rather than limiting in nature.
Variations and modifications to the disclosed examples may become apparent to those skilled in
the art that do not necessarily depart from the essence of this disclosure. The scope of legal
protection given to this disclosure can only be determined by studying the following claims.

CLAIMS
What is claimed is:
1. An electrochemical device comprising:
a plurality of electrode assemblies defining a plurality of electrochemically active areas;
a non-electrically-conductive manifold including a common manifold passage to
transport ionic-conductive fluids; and
a plurality of branch passages that extend, respectively, between the plurality of
electrochemically active areas and the common manifold passage, each of the plurality of branch
passages including a first region and a second region that differ in surface area.
2. The electrochemical device as recited in claim 1, wherein each of the plurality of
electrode assemblies includes a first porous electrode, a second porous electrode and a separator
layer between the first porous electrode and the second porous electrode, wherein the first porous
electrode and the second porous electrode each span over an area that is larger than the separator
layer.
3. The electrochemical device as recited in claim 2, wherein the first porous electrode and
the second porous electrode include respective overhang portions that extend beyond a side of
the separator layer, each of the overhang portions bounding the second region of a respective one
of the plurality of branch passages but not bounding the first region.
4. The electrochemical device as recited in claim 1, wherein the first region and the second
region differ in wall surface roughness.
5. The electrochemical device as recited in claim 1, wherein the second region is partially
bounded by an electrically-conductive surface that is outside of the plurality of electrochemically
active areas.
The electrochemical device as recited in claim 5, wherein the first region is bounded by
-conductive polymeric walls.
7. The electrochemical device as recited in claim 1, further comprising a bipolar plate that
includes a plurality of channels that are fluidly connected with the plurality of branch passages,
the plurality of channels being in the plurality of electrochemically active areas.
8. The electrochemical device as recited in claim 7, where in the plurality of channels run
between channel walls and each of the channel walls includes a tapered end.
9. The electrochemical device as recited in claim 8, wherein the tapered end tapers to a
point.
10. The electrochemical device as recited in claim 8, wherein the tapered end tapers to a
rounded end.
11. The electrochemical device as recited in claim 8, wherein the plurality of channels are
interdigitated channels.
12. The electrochemical device as recited in claim 1, wherein the ionic-conductive fluids
contain one or more reactants that readily undergo oxidation and reduction reactions that result in
different oxidation states and enable the storage of energy.
13. A method for controlling corrosion in an electrochemical device, the method comprising:
providing a reactant fluid flow between a common manifold passage and a plurality of
electrode assemblies, wherein the reactant fluid generates a shunt current that can be supported
by self-reactions of the reactant fluid and corrosion reactions of components of the
electrochemical device; and
establishing a tendency toward supporting the shunt current by the self-reactions over the
corrosion reactions to thereby limit corrosion of the components.
14. The method as recited in claim 13, wherein the establishing includes providing a plurality
of branch channels between the common manifold passage and a plurality of electrochemically
active areas of the plurality of electrode assemblies, each of the plurality of branch passages
including a first region and a second region that differ in surface area.
15. The method as recited in claim 13, wherein the establishing includes providing a bipolar
plate that includes a plurality of channels that run between channel walls, each of the channel
walls including a tapered end.
16. The method as recited in claim 13, wherein the establishing includes:
providing a plurality of branch channels between the common manifold passage and a
plurality of electrochemically active areas of the plurality of electrode assemblies, each of the
plurality of branch passages including a first region and a second region that differ in surface
area, and
providing a bipolar plate that includes a plurality of channels that run between channel
walls, each of the channel walls including a tapered end.