U.S. patent number 10,770,736 [Application Number 16/165,004] was granted by the patent office on 2020-09-08 for via designs for removing water in fuel cell stacks.
This patent grant is currently assigned to Daimler AG, Ford Motor Company. The grantee listed for this patent is Daimler AG, Ford Motor Company. Invention is credited to Alireza Roshanzamir, Yunsong Yang.
United States Patent |
10,770,736 |
Roshanzamir , et
al. |
September 8, 2020 |
Via designs for removing water in fuel cell stacks
Abstract
Structures and methods are disclosed for removing water, and
particularly for preventing ice blockages, in solid polymer
electrolyte fuel cells comprising reactant vias that fluidly
connect a reactant transition region to a reactant port. Water can
be removed from the reactant via by making its surface
superhydrophobic while incorporating at least one additional via
with a hydrophilic surface in parallel therewith.
Inventors: |
Roshanzamir; Alireza (Burnaby,
CA), Yang; Yunsong (Surrey, CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
Daimler AG
Ford Motor Company |
Stuttgart
Dearborn |
N/A
MI |
DE
US |
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Assignee: |
Daimler AG (Stuttgart,
DE)
Ford Motor Company (Dearborn, MI)
|
Family
ID: |
1000005044364 |
Appl.
No.: |
16/165,004 |
Filed: |
October 19, 2018 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20190123363 A1 |
Apr 25, 2019 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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62577136 |
Oct 25, 2017 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M
8/04291 (20130101); H01M 8/04835 (20130101); H01M
8/04171 (20130101); H01M 8/1018 (20130101); H01M
8/04156 (20130101); H01M 8/0258 (20130101); H01M
2008/1095 (20130101); H01M 2300/0082 (20130101) |
Current International
Class: |
H01M
8/0258 (20160101); H01M 8/04828 (20160101); H01M
8/1018 (20160101); H01M 8/04291 (20160101); H01M
8/04119 (20160101) |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Leong; Susan D
Attorney, Agent or Firm: Pendorf; Stephan A. Patent Central
LLC
Claims
What is claimed is:
1. A solid polymer electrolyte fuel cell comprising: a solid
polymer electrolyte; a cathode and an anode on opposite sides of
the electrolyte; an oxidant flow field plate for an oxidant
reactant on the side of the cathode opposite the electrolyte; and a
fuel flow field plate for a fuel reactant on the side of the anode
opposite the electrolyte; wherein at least one of the oxidant and
fuel flow field plates comprises a plurality of reactant flow field
channels, a reactant transition region, at least one reactant via
having first and second ends, and a reactant port; and wherein the
plurality of reactant flow field channels is fluidly connected to
the reactant transition region, the reactant transition region is
fluidly connected to the first end of the at least one reactant
via, and the second end of the at least one reactant via is fluidly
connected to the reactant port; characterized in that: the surface
of the at least one reactant via in the reactant flow field plate
is superhydrophobic; and the fuel cell comprises at least one
additional via having first and second ends wherein: the surface of
the additional via is hydrophilic; the hydrophilic additional via
is fluidly connected in parallel to the superhydrophobic reactant
via such that the first end of the hydrophilic additional via is
fluidly connected directly to the reactant transition region and
the second end or at least one branch from the hydrophilic
additional via is fluidly connected directly to the
superhydrophobic reactant via; and the dimensions of the
hydrophilic additional via are such that water appearing at its
second end or the at least one branch will flow into the
hydrophilic additional via by capillary action.
2. The fuel cell of claim 1 wherein: the oxidant flow field plate
comprises a plurality of oxidant flow field channels, an oxidant
outlet transition region, at least one oxidant outlet via, and an
oxidant outlet port wherein the plurality of oxidant flow field
channels is fluidly connected to the oxidant outlet transition
region, the oxidant outlet transition region is fluidly connected
to the at least one oxidant outlet via, and the at least one
oxidant outlet via is fluidly connected to the oxidant outlet port;
wherein the plurality of reactant flow field channels is the
plurality of oxidant flow field channels, the reactant transition
region is the oxidant outlet transition region, the
superhydrophobic reactant via is the oxidant outlet via, and the
reactant port is the oxidant outlet port.
3. The fuel cell of claim 1 wherein: the fuel flow field plate
comprises a plurality of fuel flow field channels, a fuel outlet
transition region, at least one fuel outlet via, and a fuel outlet
port wherein the plurality of fuel flow field channels is fluidly
connected to the fuel outlet transition region, the fuel outlet
transition region is fluidly connected to the at least one fuel
outlet via, and the at least one fuel outlet via is fluidly
connected to the fuel outlet port; wherein the plurality of
reactant flow field channels is the plurality of fuel flow field
channels, the reactant transition region is the fuel outlet
transition region, the superhydrophobic reactant via is the fuel
outlet via, and the reactant port is the fuel outlet port.
4. The fuel cell of claim 1 wherein the distance between the first
end of the hydrophilic additional via and the first end of the
superhydrophobic reactant via is less than two times the width of
the channels in the plurality of reactant flow field channels.
5. The fuel cell of claim 1 wherein the width dimension of the
hydrophilic additional via is less than or about equal to the width
dimension of the superhydrophobic reactant via.
6. The fuel cell of claim 1 wherein the second end of the
hydrophilic additional via is fluidly connected directly to the
second end of the superhydrophobic reactant via.
7. The fuel cell of claim 1 comprising a plurality of reactant vias
having first and second ends and whose surfaces are
superhydrophobic wherein the reactant transition region is fluidly
connected to the first ends of the plurality of superhydrophobic
reactant vias and the second ends of the plurality of
superhydrophobic reactant vias are fluidly connected to the
reactant port.
8. The fuel cell of claim 7 comprising a plurality of additional
vias having first and second ends wherein: the surfaces of the
additional vias are hydrophilic; each of the hydrophilic additional
vias is fluidly connected in parallel to one of the
superhydrophobic reactant vias such that the first end of each of
the hydrophilic additional vias is fluidly connected directly to
the reactant transition region and the second end of each of the
hydrophilic additional vias is fluidly connected directly to one of
the superhydrophobic reactant vias; and the dimensions of the
hydrophilic additional vias are such that water appearing at their
second ends will flow into the hydrophilic additional vias by
capillary action.
9. The fuel cell of claim 1 wherein the hydrophilic additional via
is also fluidly connected by at least one branch to the
superhydrophobic reactant via between its first and second
ends.
10. The fuel cell of claim 1 wherein the reactant transition region
comprises structures selected from the group consisting of vanes
and pillars.
11. The fuel cell of claim 1 wherein the reactant port is near the
periphery of the reactant flow field plate.
12. The fuel cell of claim 1 wherein the fuel cell comprises a
cathode gas diffusion layer between the cathode and the oxidant
flow field plate and an anode gas diffusion layer located between
the anode and the fuel flow field plate.
13. A solid polymer electrolyte fuel cell stack comprising a series
stack of a plurality of the solid polymer electrolyte fuel cells of
claim 1.
14. A method for removing water from at least one reactant via in a
solid polymer electrolyte fuel cell, the fuel cell comprising a
solid polymer electrolyte, a cathode and an anode on opposite sides
of the electrolyte, an oxidant flow field plate for an oxidant
reactant on the side of the cathode opposite the electrolyte, and a
fuel flow field plate for a fuel reactant on the side of the anode
opposite the electrolyte, wherein at least one of the oxidant and
fuel flow field plates comprises a plurality of reactant flow field
channels, a reactant transition region, the at least one reactant
via having first and second ends, and a reactant port; and wherein
the plurality of reactant flow field channels is fluidly connected
to the reactant transition region, the reactant transition region
is fluidly connected to the first end of the at least one reactant
via, and the second end of the at least one reactant via is fluidly
connected to the reactant port, the method comprising: making the
surface of the at least one reactant via superhydrophobic; and
incorporating at least one additional via having first and second
ends into the fuel cell wherein: the surface of the additional via
is hydrophilic; the hydrophilic additional via is fluidly connected
in parallel to the superhydrophobic reactant via such that the
first end of the hydrophilic additional via is fluidly connected
directly to the reactant transition region and the second end or at
least one branch from the hydrophilic additional via is fluidly
connected directly to the superhydrophobic reactant via; and the
dimensions of the hydrophilic additional via are such that water
appearing at its second end or the at least one branch will flow
into the hydrophilic additional via by capillary action.
15. The method of claim 14 wherein structures are incorporated in
the reactant transition region to direct fluid flow towards the
first end of the superhydrophobic reactant via and the first end of
the hydrophilic additional via.
16. The method of claim 14 wherein the hydrophilic additional via
is incorporated such that the distance between the first end of the
hydrophilic additional via and the first end of the
superhydrophobic reactant via is less than the width of the
channels in the plurality of reactant flow field channels.
17. The method of claim 14 wherein the hydrophilic additional via
is incorporated such that its second end is fluidly connected
directly to the second end of the superhydrophobic reactant
via.
18. The method of claim 14 wherein the fuel cell comprises a
plurality of reactant vias having first and second ends and whose
surfaces are superhydrophobic, wherein the reactant transition
region is fluidly connected to the first ends of the plurality of
superhydrophobic reactant vias and the second ends of the plurality
of superhydrophobic reactant vias are fluidly connected to the
reactant port, the method additionally comprising: incorporating a
plurality of additional vias having first and second ends into the
fuel cell wherein: the surfaces of the additional vias are
hydrophilic; each of the hydrophilic additional vias is fluidly
connected in parallel to one of the superhydrophobic reactant vias
such that the first end of each of the hydrophilic additional vias
is fluidly connected directly to the reactant transition region and
the second end of each of the hydrophilic additional vias is
fluidly connected directly to one of the superhydrophobic reactant
vias; and the dimensions of the hydrophilic additional vias are
such that water appearing at their second ends will flow into the
hydrophilic additional vias by capillary action.
19. The method of claim 14 comprising incorporating at least one
branch fluidly connecting the hydrophilic additional via to the
superhydrophobic reactant via between its first and second ends.
Description
BACKGROUND
Field of the Invention
This invention relates to structures and methods for removing water
in solid polymer electrolyte fuel cell stacks. In particular, it
relates to removing water to prevent ice blockages in fuel cells
subjected to below freezing temperatures.
Description of the Related Art
Fuel cells electrochemically convert fuel and oxidant reactants,
(e.g. hydrogen and oxygen or air respectively), to generate
electric power. One type of fuel cell is a solid polymer
electrolyte fuel cell which employs a proton conducting polymer
membrane electrolyte between cathode and anode electrodes. The
electrodes contain appropriate catalysts and typically also
comprise conductive particles, binder, and material to modify
wettability. A structure comprising a proton conducting polymer
membrane sandwiched between two electrodes is known as a membrane
electrode assembly (MEA). Such assemblies can be prepared in an
efficient manner by appropriately coating catalyst mixtures onto
the polymer membrane, and thus are commonly known as catalyst
coated membranes (CCMs). For purposes of handling, assembly, and
electrical insulation, CCMs are often framed with suitable
electrically insulating plastic frames.
Anode and cathode gas diffusion layers are usually employed
adjacent their respective electrodes on either side of a catalyst
coated membrane. The gas diffusion layers serve to unifounly
distribute reactants to and remove by-products from the catalyst
electrodes. Fuel and oxidant flow field plates are then typically
provided adjacent their respective gas diffusion layers and the
combination of all these components represents a typical individual
fuel cell assembly. The flow field plates comprise flow fields that
usually contain numerous fluid distribution channels. The flow
field plates serve multiple functions including: distribution of
reactants to the gas diffusion layers, removal of by-products
therefrom, structural support and containment, and current
collection.
Water and heat are the primary by-products in a cell operating on
hydrogen and air reactants. Means for cooling a fuel cell stack is
thus generally required. Stacks designed to achieve high power
density (e.g. automotive stacks) typically circulate liquid coolant
throughout the stack in order to remove heat quickly and
efficiently. To accomplish this, coolant flow fields comprising
numerous coolant channels are also typically incorporated in the
flow field plates of the cells in the stacks. The coolant flow
fields are typically formed on the electrochemically inactive
surfaces of both the anode side and cathode side flow field plates
and, by appropriate design, a sealed coolant flow field is created
when both anode and cathode side plates are mated together into a
bipolar plate assembly.
Because the output voltage of a single cell is of order of 1V, a
plurality of cells is usually stacked together in series for
commercial applications in order to provide a higher output
voltage. Fuel cell stacks can be further connected in arrays of
interconnected stacks in series and/or parallel for use in
automotive applications and the like.
To provide both reactants and the coolant to and from the
individual cells in the stack, a series of ports are generally
provided at opposing ends of the individual cells such that when
the cells are stacked together they form manifolds for these
fluids. Further required design features then include passageways
in the plates to distribute the bulk fluids in these formed
manifolds to and from the various channels in the reactant and
coolant flow fields in the plates. These passageway regions can
include regions known as transition regions that are fluidly
connected to the flow field channels in the flow field plates. The
transition regions themselves can comprise numerous fluid
distribution features. In addition, the passageway regions can
include a series of vias that fluidly connect such transition
regions to their appropriate reactant ports.
In fuel cell stacks subject to freezing temperatures, accumulations
of liquid water can be problematic because, when the water freezes,
the ice formed can undesirably block fluid flows and the associated
expansion of the solid ice can cause damage to cells in the fuel
cell stack. Significant sized accumulations of liquid water which
may be subject to freezing are therefore generally avoided, either
by preventing accumulation in the first place or alternatively by
removing them before they have the opportunity to freeze. For these
and other reasons, various designs and techniques are disclosed in
the art for managing and controlling water movement within a fuel
cell stack.
US20110171564 for instance discloses one approach for addressing
problems caused by water condensation during operation or shut down
that results in reactant gas flow fields or tunnels being blocked
by retained water or ice. One exemplary embodiment therein includes
a fuel cell bipolar plate having a reversible
superhydrophilic-superhydrophobic coating thereon. Another
exemplary embodiment therein includes a fuel cell bipolar plate
including a reactant gas header opening communicating with a first
portion including a plurality of tunnels defined therein, the first
portion communicating with a reactant gas flow field having a
plurality of channels defined therein, and a
superhydrophilic-superhydrophobic coating over at least a portion
of the tunnels.
Despite the advances made to date, there remains a need for better
designs and methods to prevent water blockages from occurring in
such fuel cell stacks, and particularly to prevent ice blockages
when subzero temperatures may be encountered. This invention
fulfills these needs and provides further related advantages.
SUMMARY
The present invention provides for improvements in the removal of
water and in the prevention of ice blockages in certain solid
polymer electrolyte fuel cells.
A relevant typical fuel cell comprises a solid polymer electrolyte,
a cathode and an anode on opposite sides of the electrolyte, an
oxidant flow field plate for an oxidant reactant on the side of the
cathode opposite the electrolyte, and a fuel flow field plate for a
fuel reactant on the side of the anode opposite the electrolyte.
Further, at least one of the oxidant and fuel flow field plates
comprises a plurality of reactant flow field channels, a reactant
transition region, at least one reactant via having first and
second ends, and a reactant port. In such a fuel cell, the
plurality of reactant flow field channels is fluidly connected to
the reactant transition region, the reactant transition region is
fluidly connected to the first end of the at least one reactant
via, and the second end of the at least one reactant via is fluidly
connected to the reactant port.
In the present invention, the surface of the at least one reactant
via in the reactant flow field plate is superhydrophobic, and the
fuel cell comprises at least one additional via having first and
second ends. The surface of the additional via is hydrophilic and
the hydrophilic additional via is fluidly connected in parallel to
the superhydrophobic reactant via such that the first end of the
hydrophilic additional via is fluidly connected directly to the
reactant transition region and the second end or at least one
branch from the hydrophilic additional via is fluidly connected
directly to the superhydrophobic reactant via. Further, the
dimensions of the hydrophilic additional via are such that water
appearing at its second end or the at least one branch will flow
into the hydrophilic additional via by capillary action.
In one embodiment, the invention can be advantageously employed to
remove water from a via at the oxidant outlet. In this embodiment,
the oxidant flow field plate comprises a plurality of oxidant flow
field channels, an oxidant outlet transition region, at least one
oxidant outlet via, and an oxidant outlet port. Further, the
plurality of oxidant flow field channels is fluidly connected to
the oxidant outlet transition region, the oxidant outlet transition
region is fluidly connected to the at least one oxidant outlet via,
and the at least one oxidant outlet via is fluidly connected to the
oxidant outlet port. In this embodiment then, a relevant plurality
of reactant flow field channels is the plurality of oxidant flow
field channels, the reactant transition region is the oxidant
outlet transition region, the superhydrophobic reactant via is the
at least one oxidant outlet via, and the reactant port is the
oxidant outlet port.
In another embodiment, the invention can be advantageously employed
to remove water from a via at the fuel outlet. In this embodiment,
the fuel flow field plate comprises a plurality of fuel flow field
channels, a fuel outlet transition region, at least one fuel outlet
via, and a fuel outlet port. Further, the plurality of fuel flow
field channels is fluidly connected to the fuel outlet transition
region, the fuel outlet transition region is fluidly connected to
the at least one fuel outlet via, and the at least one fuel outlet
via is fluidly connected to the fuel outlet port. In this
embodiment then, a relevant plurality of reactant flow field
channels is the plurality of fuel flow field channels, the reactant
transition region is the fuel outlet transition region, the
superhydrophobic reactant via is the at least one fuel outlet via,
and the reactant port is the fuel outlet port.
In yet other embodiments, the invention may be employed to remove
water from a via at any or all of the oxidant and/or fuel outlets
and inlets.
In a preferred embodiment, the first end of the hydrophilic
additional via and the first end of the superhydrophobic reactant
via (i.e. the ends that open into the transition region) are close
to each other. For instance, the distance between the first end of
the hydrophilic additional via and the first end of the
superhydrophobic reactant via can desirably be less than two times
the width of the channels in the plurality of reactant flow field
channels.
In embodiments of the invention, at least one relevant dimension of
the hydrophilic additional via is suitably small such that, for a
given state of the via surfaces and a given operating temperature
of the fuel cell, water appearing at the second end of the
hydrophilic additional via will be drawn or flow into it by
capillary action.
The hydrophilic additional via may be incorporated so as to be
located completely in parallel to the superhydrophobic reactant
via. That is, the second end of the hydrophilic additional via can
be fluidly connected directly to the second end of the
superhydrophobic reactant via. Alternatively however, the second
end of the hydrophilic additional via may instead be fluidly
connected to the superhydrophobic reactant via somewhere between
its first and second ends.
The invention may also be employed in embodiments comprising a
plurality of reactant vias. In such an embodiment, the fuel cell
may comprise a plurality of reactant vias having first and second
ends and whose surfaces are all superhydrophobic, and in which the
reactant transition region is fluidly connected to the first ends
of the plurality of superhydrophobic reactant vias and the second
ends of the plurality of superhydrophobic reactant vias are fluidly
connected to the reactant port. Further, such embodiments may then
comprise a plurality of similar additional vias having first and
second ends. That is, the surfaces of these additional vias may
also be hydrophilic and each of the hydrophilic additional vias may
be fluidly connected in parallel to one of the superhydrophobic
reactant vias such that the first end of each of the hydrophilic
additional vias is fluidly connected directly to the reactant
transition region while the second end of each of the hydrophilic
additional vias is fluidly connected directly to one of the
superhydrophobic reactant vias. Further, the dimensions of the
hydrophilic additional vias are such that water appearing at their
second ends will flow into the respective hydrophilic additional
vias by capillary action.
In any or all of the preceding embodiments, additional branches
between the hydrophilic additional via/s and the superhydrophobic
reactant via/s may be employed. For instance, the hydrophilic
additional via can also be fluidly connected by at least one branch
to the superhydrophobic reactant via between its first and second
ends.
The invention may be used to advantage in various typical fuel cell
constructions, including in fuel cells whose reactant transition
region comprises structures selected from the group consisting of
vanes and pillars, or in fuel cells in which the reactant port is
near the periphery of the reactant flow field plate, or in fuel
cells comprising gas diffusion layers--namely a cathode gas
diffusion layer between the cathode and the oxidant flow field
plate and an anode gas diffusion layer located between the anode
and the fuel flow field plate, etc. Further, the invention may also
be used in solid polymer electrolyte fuel cell stacks (i.e. a
series stack of a plurality of solid polymer electrolyte fuel cells
of the invention).
In methods of the invention then, water is removed from at least
one reactant via in a relevant solid polymer electrolyte fuel cell
by making the surface of the at least one reactant via
superhydrophobic and by appropriately incorporating at least one
additional via having first and second ends into the fuel cell. An
appropriately incorporated additional via is characterized by a
surface which is hydrophilic. Further, an appropriate hydrophilic
additional via is fluidly connected in parallel to the
superhydrophobic reactant via such that the first end of the
hydrophilic additional via is fluidly connected directly to the
reactant transition region while the second end or at least one
branch from the hydrophilic additional via is fluidly connected
directly to the superhydrophobic reactant via. Preferably,
structures are incorporated in the transition region to direct
fluid flow towards both the first end of the superhydrophobic
reactant via and the first end of the hydrophilic additional via.
Also preferably, the hydrophilic additional via is incorporated
such that the distance between the first end of the hydrophilic
additional via and the first end of the superhydrophobic reactant
via is less than the width of the channels in the plurality of
reactant flow field channels. Using suitable adaptions of the
method, any of the aforementioned fuel cell features of the
invention can desirably be obtained.
These and other aspects of the invention are evident upon reference
to the attached Figures and following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1a shows a top view of a typical reactant flow field plate in
a solid polymer electrolyte fuel cell stack of the prior art
intended for use in automotive applications.
FIG. 1b shows a magnified perspective view around the reactant
transition region, reactant vias, and reactant port in the reactant
flow field plate of FIG. 1a.
FIGS. 2a, 2b and 2c shows schematics of several options for fluidly
connecting a hydrophilic additional via in parallel to a
superhydrophobic reactant via in accordance with the invention. In
FIG. 2a, the second end of the former is directly connected to the
second end of the latter. In FIG. 2b, the second end of the former
is directly connected between the first and second ends of the
latter. In FIG. 2c, the second end of the former is connected as
shown in FIG. 2a, but the embodiment here also comprises two
additional branches connecting the two vias.
FIGS. 3a, 3b, 3c and 3d illustrate several exemplary embodiments of
the invention in magnified perspective views similar to that shown
in FIG. 1b. In FIG. 3a, the embodiment comprises a plurality of
reactant vias with a single hydrophilic additional via incorporated
in a like manner to that shown in FIG. 2c. FIG. 3b shows a close-up
view of FIG. 3a. FIG. 3c shows a similar embodiment to that of FIG.
3a except that the reactant transition region comprises pillars
instead of vanes to direct fluid to the ends of the vias. FIG. 3d
shows a similar embodiment to that of FIG. 3a except that a
plurality of hydrophilic additional vias have been
incorporated.
FIGS. 4a, 4b illustrate the results of a simulated calculation in
which the water movement inside a superhydrophobic reactant via,
inside a hydrophilic additional via and inside the second ends
which fluidly connect them was simulated. FIG. 4a illustrates phase
1, i.e. a situation in which the inventive embodiment of FIG. 3a is
assembled in a solid polymer electrolyte fuel cell during operation
of the fuel cell: Water accumulated inside the superhydrophobic
reactant via is drawn into the hydrophilic additional via. FIG. 4b
illustrates phase 2, i.e. again a situation in which the inventive
embodiment of FIG. 3a is assembled in a solid polymer electrolyte
fuel cell. But this time, the fuel cell is in the state of startup:
A flow of reactant gas coming from the transition region clears the
hydrophilic additional via and blows out water which had
accumulated inside it.
DETAILED DESCRIPTION
In this specification, words such as "a" and "comprises" are to be
construed in an open-ended sense and are to be considered as
meaning at least one but not limited to just one.
Herein, in a quantitative context, the term "about" should be
construed as being in the range up to plus 10% and down to minus
10%.
The tell "hydrophilic" refers to surfaces that are characterized by
contact angles with water of less than 90 degrees.
The term "hydrophobic" refers to surfaces that are characterized by
contact angles with water of more than 90 degrees.
The term "superhydrophilic" refers to surfaces that are
characterized by contact angles with water of less than 30
degrees.
The term "superhydrophobic" refers to surfaces that are
characterized by contact angles with water of greater than 150
degrees.
When used in the context of a fluid connection made between two
elements, the term "directly" refers to a connection in which the
first of the two elements is fluidly connected to the second of the
two elements without any other element appearing between the
two.
A simplified top view of a typical reactant flow field plate used
in a solid polymer electrolyte fuel cell stack suitable for
automotive applications and relevant to the present invention is
shown in FIG. 1a. Reactant flow field plate 1 comprises a plurality
of reactant flow field channels 2 (shown here as a series of
parallel, linear channels separated by landings), reactant
transition region 3, at least one reactant via 4 (shown here as a
plurality of parallel, linear reactant vias 4), and reactant port 5
near the periphery of reactant flow field plate 1 (e.g. such that
only a seal separates reactant port 5 from the periphery). The
plurality of reactant flow field channels 2 is fluidly connected to
reactant transition region 3. In turn, reactant transition region 3
is fluidly connected to first ends 4a of reactant vias 4. The other
or second ends 4b of reactant vias 4 are fluidly connected to
reactant port 5. In FIG. 1a, reactant transition region 3 is shown
as comprising a plurality of vanes 6 which are provided to
appropriately guide fluid reactant between reactant flow field
channels 2 and the numerous first ends 4a of reactant vias 4.
Also visible in FIG. 1a is another port for the reactant, namely
reactant port 15. And associated therewith and also visible in FIG.
1a is another reactant transition region 13 and another plurality
of reactant vias 14 which are fluidly connected to each other and
to reactant flow field channels 2 and reactant port 15 in a like
manner to how reactant flow field channels 2, reactant transition
region 3, reactant vias 4, and reactant port 5 are connected.
Further, reactant transition region 13 comprises a plurality of
vanes 16 which are similar to vanes 6 in reactant transition region
3. Moreover, reactant ports 7 are present for the second reactant
needed for operating the polymer electrolyte fuel cell. Reactant
ports 7 may be designed for a fuel, such as hydrogen, and reactant
ports 5 may be designed for oxidant, such as air, or vice versa.
Finally, coolant ports 8 are visible in FIG. 1a for allowing
coolant to access a coolant flow field (which is typically on the
opposite side of reactant flow field plate 1 and thus not visible
in FIG. 1a).
FIG. 1b shows a magnified perspective view around reactant
transition region 3, reactant vias 4, and reactant port 5 in the
reactant flow field plate 1 of FIG. 1a.
Under certain conditions during fuel cell operation, water can
collect in these small reactant vias 4 and adversely affect fluid
flow and subsequent fuel cell operation. To address this, purging
procedures are commonly employed to clear accumulated water from
the affected fluid passages (i.e. where a substantial flow of an
appropriate fluid is used to "purge" the passage of water).
However, purging procedures are not always completely effective.
Further, sometimes after fuel cell shutdown and purging procedures,
water droplets may spontaneously move back into reactant vias 4.
Under below freezing conditions, this water could turn to ice and
completely block the affected vias, thereby causing startup
problems later.
In the present invention, water is prevented from collecting in and
blocking reactant vias in this manner. This is accomplished by
incorporating an additional via or vias in which this water is
collected instead. The additional via is arranged so it is fluidly
connected in parallel to the reactant via or vias in which
problematic water may collect. The surface of the reactant via is
made to be superhydrophobic while the surface of the additional via
is made hydrophilic. Thus water preferentially contacts the
additional via surface and not the reactant via surface. Further,
the dimensions of the additional via are selected such that any
water appearing in the reactant via is drawn into the additional
via by capillary action.
FIG. 2a shows a schematic of an exemplary arrangement for
incorporating a suitable additional via in accordance with the
invention in the reactant flow field plate of FIG. 1a. (Note that
in FIG. 2a and the other following figures, the same numbering has
been used to identify those features common to FIGS. 1a and 1b.) In
reactant flow field plate 11 in FIG. 2a, additional via 9 is
fluidly connected in parallel to reactant via 4 such that first end
9a of additional via 9 is connected directly to reactant transition
region 3, while second end 9b is connected directly to second end
4b of reactant via 4. As mentioned above, the surface of reactant
via 4 is made to be superhydrophobic while the surface of
additional via 9 is made to be hydrophilic.
An alternative arrangement for incorporating a suitable additional
via in accordance with the invention is shown in FIG. 2b. Here, in
reactant flow field plate 11', additional via 9' is fluidly
connected in parallel to reactant via 4 such that first end 9a' of
additional via 9' is again connected directly to reactant
transition region 3. Here however, second end 9b' is connected
directly to reactant via 4 partway between its first and second
ends (i.e. partway between 4a and 4b). Again, the surface of
reactant via 4 is made to be superhydrophobic while the surface of
additional via 9' is made to be hydrophilic.
A yet further optional arrangement for incorporating a suitable
additional via in accordance with the invention is shown in FIG.
2c. Here, in reactant flow field plate 11'', additional via 9'' is
fluidly connected in parallel to reactant via 4 in a like manner as
is shown in FIG. 2a. That is, first end 9a'' of additional via 9''
is connected directly to reactant transition region 3, while second
end 9b'' is connected directly to second end 4b of reactant via 4.
Here however, additional via 9'' is also fluidly connected to
reactant via 4 between its first and second ends by two branches,
namely branch 9c'' and branch 9d''. As above, the surface of
reactant via 4 is made to be superhydrophobic while the surface of
additional via 9'' is made to be hydrophilic. In an optional
embodiment thereof the second end 9b'' of the additional via could
be connected directly to the reactant port 5 (shown in FIG. 2c with
a dotted line).
In all the embodiments of FIGS. 2a to 2c, the dimensions of the
hydrophilic additional vias 9, 9', and 9'' are selected such that
water appearing at second ends 9b, 9b', and 9b'' would be drawn
into the additional vias by capillary action. Preferably the same
is true at the branches, namely the dimensions of the branches 9c''
and 9d'' are selected such that any water appearing in their
vicinity would also be drawn into the additional vias by capillary
action. If the the second end 9b'' of the additional via is
connected directly to the reactant port 5 (as described above) the
water will be drawn into the additional via 9'' through the
branches 9c'' and 9d''. As those skilled in the art readily
appreciate, the dimensions required for capillary action to occur
are functions of several factors including the hydrophobicity (or
contact angle) of the relevant surfaces but also the surface
tension of the fluid involved, which in turn is a function of
temperature. Solid polymer fuel cells may be stored and operated
over temperatures ranging from lows of about -40.degree. C. to
highs of about +100.degree. C. Thus, the additional via dimensions
and geometry should be selected with these factors in mind in order
to obtain the desired functionality over the entire operating range
of the fuel cell. It is expected that suitable such selections can
readily be made by those skilled in the art for a given set of
characteristics. Generally it is expected that the additional via
dimensions will be about the same or smaller than those of typical
reactant vias, e.g. millimeters or less and/or of order of the
thickness of the reactant flow field plates.
In embodiments of the invention, it also is generally desirable for
the first end of the hydrophilic additional via and the first end
of the superhydrophobic reactant via (i.e. the ends that open into
the transition region) to be close to each other. For instance, the
distance between the first end of the hydrophilic additional via
and the first end of the superhydrophobic reactant via can
desirably be less than two times the width of the channels in the
plurality of reactant flow field channels.
With regards to obtaining the desired surface characteristics for
the reactant and additional vias involved in the invention, various
coatings and techniques are known in the art such that the desired
hydrophobicities can be obtained on the materials commonly used as
reactant flow field plates. In particular, the science and
engineering relating to superhydrophobic surfaces has advanced
recently and correspondingly new options have been identified (e.g.
transparent nano composite, perfluorpolyether, and/or amorphous
silicate-nano particle modified coatings from Fraunhofer IFAM or as
disclosed for instance in U.S. Pat. No. 9,279,073, "A review of the
recent advances in superhydrophobic surfaces and the emerging
energy-related applications" P. Zhang et al., Energy 82 (2015)
1068e1087; and/or "Recent advances in the mechanical durability of
superhydrophobic materials", A. Milionis et al., Advances in
Colloid and Interface Science 229 (2016) 57-79).
FIGS. 3a, 3b, 3c and 3d illustrate several exemplary embodiments of
the invention as applied to a reactant flow field plate similar to
that shown in FIG. 1b (again magnified perspective views are
shown). The embodiment in FIG. 3a comprises a plurality of reactant
vias 4 but has just a single hydrophilic additional via 9''
incorporated in a like manner to that shown in FIG. 2c. This
embodiment may be considered if water collection is only a
significant problem or concern on the innermost reactant via 4, or
perhaps if it is only considered necessary to ensure that one
reactant via remains clear of water. FIG. 3b shows a close-up view
of FIG. 3a where both ends (9a'' and 9b'') and both branches (9c''
and 9d'') of additional via 9'' can be clearly seen. FIG. 3c shows
a similar embodiment to that of FIG. 3a except that the reactant
transition region comprises pillars 6' instead of vanes 6 to direct
reactant fluid to the first ends of the reactant vias and to the
first end of the additional via. FIG. 3d shows a similar embodiment
to that of FIG. 3a except that a plurality of hydrophilic
additional vias 9'' have been incorporated. Specifically, one
additional via 9'' has been incorporated for each reactant via 4
present. In this way, water collection in every reactant via 4 can
be addressed using the method of the invention.
As mentioned above, FIGS. 4a, 4b illustrate the results of a
simulated calculation in which the water movement inside a
superhydrophobic reactant via (4), inside a hydrophilic additional
via (9'') and inside the second ends (9b'', 9c'', 9d'') which
fluidly connect vias (4) and (9'') was simulated. FIGS. 4a, 4b are
based on the inventive embodiment of FIG. 3a (see also the related
close-up view, i.e. FIG. 3b). Parameters of that simulation are
listed in Table 1:
TABLE-US-00001 TABLE 1 Width Depth Contact Angle Reynolds Number
(mm) (mm) (deg) (Phase 2) Via 1.0 0.5 150 270 Additional via 0.5
0.5 70 400
FIG. 4a illustrates phase 1, i.e. a situation in which the
inventive embodiment of FIG. 3a is assembled in a solid polymer
electrolyte fuel cell during operation of the fuel cell. t.sub.0
illustrates the situation at the beginning of the simulation: A
drop of water (100) has accumulated inside superhydrophobic
reactant via (4) and clogs it. A performance loss of the fuel cell
would be the consequence of this scenario. At time t.sub.1, the
water drop (100) started moving through branch (9d'') to
hydrophilic additional via (9''), driven by capillary action. A new
water drop (110) formed, already. At time t.sub.2, new water drop
(110) grew and almost all of the water has left superhydrophobic
reactant via (4). Finally, at the end of the simulation, at time
t.sub.3, former water drop (100) is completely sucked into
hydrophilic via (9''), forming new water drop (110), and
superhydrophobic reactant via (4) is clear of water. The clogging
of superhydrophobic reactant via (4) was removed and no performance
loss of the fuel cell has to be feared.
FIG. 4b illustrates phase 2, i.e. again a situation in which the
inventive embodiment of FIG. 3a is assembled in a solid polymer
electrolyte fuel cell. In this simulation, however, the fuel cell
is in the state of startup: A flow of reactant gas (12) coming from
the transition region (3) clears the hydrophilic additional via
(9'') and blows out (removes or purges) water which had accumulated
inside it. In detail, t.sub.0' illustrates the situation at the
beginning of the simulation: New water drop (110) has accumulated
inside hydrophilic additional via (9'') and a flow of reactant gas
(12) comes from transition region (3). At time t.sub.1', water drop
(110) started moving inside branch (9b'') towards port (5) and the
outlet (both not illustrated in FIG. 4b), shoved along by flow
(12). At time t.sub.2' the removal of water from hydrophilic
additional via (9'') proceeded and some of the water left it,
already. Finally, at the end of the simulation, at time t.sub.3',
most of the water has left hydrophilic additional via (9'') and
only a small amount of water remained in branch (9b''). In this
status, hydrophilic additional via (9'') is ready to suck of water
from superhydrophobic reactant via (4), again, so that the process
as described above in the context of FIG. 4a can start once
again.
The present invention may be employed in association with vias for
either reactant (i.e. for either oxidant and/or fuel) and for
either reactant's inlet or outlet. The present invention is however
particularly suitable for use at either the oxidant or fuel outlet
ports where significant water can commonly collect. If the
invention is to be used at an oxidant outlet port, the relevant
reactant flow field plate is an oxidant flow field plate, the
relevant reactant flow field channels would be oxidant flow field
channels, the relevant reactant transition region would be the
oxidant outlet transition region, the relevant reactant vias would
be the oxidant outlet vias, and the relevant reactant port would of
course be the oxidant outlet port. If the invention is to be used
at a fuel outlet port, the relevant reactant flow field plate is a
fuel flow field plate, the relevant reactant flow field channels
would be fuel flow field channels, the relevant reactant transition
region would be the fuel outlet transition region, the relevant
reactant vias would be the fuel outlet vias, and the relevant
reactant port would of course be the fuel outlet port.
When the invention is used at a reactant outlet, the structures in
the reactant transition region (e.g. vanes 6 or pillars 6' in FIGS.
3a-3d) are preferably shaped and located to control the reactant
flow pattern upstream of reactant and additional vias. Preferably
for instance, during a startup of the fuel cell, the flowrate in
the additional via is high enough to remove (purge) any accumulated
water from the previous shutdown. To control the flowrate, these
structures are thus designed and positioned just upstream of the
vias so as to split the flow therebetween in a desired manner.
Use of the present invention provides for enhanced water removal in
solid polymer electrolyte fuel cells. Water is repelled from
reactant vias 4 during normal fuel cell operation and purging
procedures as a result of their superhydrophobic surfaces. Further
though, there is little to no water uptake into reactant vias 4
subsequent to purging since any water is instead drawn into
additional vias 9. In subzero temperature conditions, blockage of
vias 4 by ice formation is thus prevented. By proper design,
additional vias 9 can be readily cleared by reactant or other
appropriate fluid flows.
Manufacture of reactant flow field plates in accordance with the
invention is expected to be relatively easy and straightforward.
Appropriately designed additional vias 9 may be incorporated in a
like manner to how conventional reactant vias 4 are formed in the
manufacturing process. And the required hydrophobicity of the
foffued features can thereafter be obtained by use of one or more
techniques known to those skilled in the art. Such flow field
plates are therefore expected to be quite durable and to maintain
functionality over long operating periods and numerous
startup/shutdown cycles (e.g. with functionality being maintained
as long as the hydrophobicity characteristics of the features are
maintained).
All of the above U.S. patents, U.S. patent application
publications, U.S. patent applications, foreign patents, foreign
patent applications and non-patent publications referred to in this
specification, are incorporated herein by reference in their
entirety.
While particular elements, embodiments and applications of the
present invention have been shown and described, it will be
understood, of course, that the invention is not limited thereto
since modifications may be made by those skilled in the art without
departing from the spirit and scope of the present disclosure,
particularly in light of the foregoing teachings. For instance, the
invention is particularly useful for water management in the
oxidant outlet passages of solid polymer electrolyte fuel cell
stacks. However, it may also be useful in the fuel outlet passages
and/or inlets of such fuel cell stacks as well. Such modifications
are to be considered within the purview and scope of the claims
appended hereto.
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