U.S. patent application number 10/012157 was filed with the patent office on 2002-06-13 for fuel cell having a hydrophilic substrate layer.
Invention is credited to Bekkedahl, Timothy A., Bregoli, Lawrence J., Cipollini, Ned E., Patterson, Timothy W., Pemberton, Marianne, Puhalski, Jonathan, Reiser, Carl A., Sawyer, Richard D., Steinbugler, Margaret M., Yi, Jung S..
Application Number | 20020071978 10/012157 |
Document ID | / |
Family ID | 23852765 |
Filed Date | 2002-06-13 |
United States Patent
Application |
20020071978 |
Kind Code |
A1 |
Bekkedahl, Timothy A. ; et
al. |
June 13, 2002 |
Fuel cell having a hydrophilic substrate layer
Abstract
Fuel Cell Having a Hydrophilic Substrate Layer A fuel cell power
plant includes a fuel cell having a membrane electrode assembly
(MEA), disposed between an anode support plate and a cathode
support plate, the anode and/or cathode support plates include a
hydrophilic substrate layer having a predetermined pore size. The
pressure of the reactant gas streams is greater than the pressure
of the coolant stream, such that a greater percentage of the pores
within the hydrophilic substrate layer contain reactant gas rather
than water. Any water that forms on the cathode side of the MEA
will migrate through the cathode support plate and away from the
MEA. Controlling the pressure also ensures that the coolant water
will continually migrate from the coolant stream toward the anode
side of the MEA, thereby preventing the membrane from becoming dry.
Proper pore size and a pressure differential between coolant and
reactants improves the electrical efficiency of the fuel cell.
Inventors: |
Bekkedahl, Timothy A.;
(Loveland, CO) ; Bregoli, Lawrence J.; (Southwick,
MA) ; Cipollini, Ned E.; (Enfield, CT) ;
Patterson, Timothy W.; (East Hartford, CT) ;
Pemberton, Marianne; (Manchester, CT) ; Puhalski,
Jonathan; (Winsted, CT) ; Reiser, Carl A.;
(Stonington, CT) ; Sawyer, Richard D.; (Groveton,
NH) ; Steinbugler, Margaret M.; (East Windsor,
CT) ; Yi, Jung S.; (Mansfield Center, CT) |
Correspondence
Address: |
M. P. Williams
210 Main Street
Manchester
CT
06040
US
|
Family ID: |
23852765 |
Appl. No.: |
10/012157 |
Filed: |
November 28, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10012157 |
Nov 28, 2001 |
|
|
|
09466701 |
Dec 17, 1999 |
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Current U.S.
Class: |
429/446 ;
429/450; 429/483; 429/492; 429/514; 429/516 |
Current CPC
Class: |
H01M 8/0245 20130101;
H01M 8/04119 20130101; Y02B 90/10 20130101; H01M 8/0258 20130101;
H01M 8/04291 20130101; H01M 8/04029 20130101; Y02E 60/50 20130101;
H01M 8/0267 20130101; H01M 8/0271 20130101; H01M 8/04097 20130101;
H01M 8/1007 20160201; H01M 2008/1095 20130101; H01M 8/04156
20130101; H01M 8/0243 20130101; H01M 8/04089 20130101; H01M 2250/10
20130101 |
Class at
Publication: |
429/25 ; 429/42;
429/30; 429/26; 429/38; 429/39 |
International
Class: |
H01M 008/10; H01M
004/86; H01M 008/04; H01M 008/02 |
Claims
We claim:
1. A fuel cell, comprising: an anode support plate and a cathode
support plate and a membrane electrode assembly disposed between
said anode and cathode support plates, said membrane electrode
assembly comprising a polymer electrolyte membrane, at least one of
said support plates comprising a hydrophilic substrate layer having
pores therein; a water transport plate adjacent to each said
hydrophilic substrate layer, each said water transport plate having
a passageway for a water stream and another passageway for a
reactant gas stream; and a partially hydrophobic porous carbon
fluoropolymer particulate composite diffusion layer disposed
between at least one said hydrophilic substrate layer and said
membrane electrode assembly, each said diffusion layer comprising
about 10% fluoropolymer by weight.
2. A fuel cell according to claim 1 wherein: said diffusion layer
comprises a fluoropolymer selected from the group consisting of
polytetrafluoroethylene, fluorinated ethylene propylene,
polytetrafluoroethylene-co-perfluoromethyl vinylether, copolymers
of ethylene and tetrafluoroethylene, copolymers of ethylene and
chlorotrifluoroethylene, polyvinyldene fluoride, polyvinyl fluoride
and amorphous fluropolymers.
3. A fuel cell, comprising: an anode support plate and a cathode
support plate and a membrane electrode assembly disposed between
said anode and cathode support plates, said membrane electrode
assembly comprising a polymer electrolyte membrane, at least one of
said support plates comprising a hydrophilic substrate layer having
pores therein; a water transport plate adjacent to each said
hydrophilic substrate layer, each said water transport plate having
a passageway for a water stream and another passageway for a
reactant gas stream; and a diffusion layer disposed between at
least one said hydrophilic substrate layer and said membrane
electrode assembly, the thickness of each said diffusion layer
being more than about 5.0 microns and less than 25.0 microns.
4. A fuel cell, comprising: an anode support plate and a cathode
support plate and a membrane electrode assembly disposed between
said anode and cathode support plates, said membrane electrode
assembly comprising a polymer electrolyte membrane, at least one of
said support plates comprising a hydrophilic substrate layer having
pores therein; a water transport plate adjacent to each said
hydrophilic substrate layer, each said water transport plate having
a passageway for a water stream and another passageway for a
reactant gas stream; a diffusion layer disposed between at least
one said hydrophilic substrate layer and said membrane electrode
assembly; and means for creating pressure differential between said
reactant gas streams and said coolant stream such that the pressure
of each said reactant gas stream is greater than the pressure of
said coolant stream, said pressure differential being more than
zero psi and less than two psi.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This is a continuation-in-part of U.S. patent application
Ser. No. 09/466,701 filed on Dec. 17, 1999.
TECHNICAL FIELD
[0002] This invention relates to fuel cell power plants and more
particularly, to fuel cell power plants utilizing a hydrophilic
substrate layer within the anode and/or cathode of the fuel
cell.
BACKGROUND ART
[0003] Fuel cell power plants are electrochemical alternative power
sources for both stationary and mobile applications. The fuel cell,
which is the heart of such power plant, consists of an anode, a
cathode and an electrolyte that separates the two. Anode shall mean
a positive electrode, and cathode shall mean a negative electrode.
In the operation of a fuel cell, fuel reactant gas, which is
typically a hydrogen rich stream, enters a support plate that is
adjacent to the anode. Such a support plate is, therefore, referred
to as an anode support plate. Oxidant reactant gas, which is
commonly air, enters a support plate adjacent to the cathode. This
support plate is, therefore, referred to as a cathode support
plate. As the hydrogen rich stream passes through the anode support
plate, a catalyst located between anode support plate and the
electrolyte causes the hydrogen to oxidize, thereby resulting in
the creation of hydrogen ions and electrons. While the hydrogen
ions migrate through the electrolyte to the cathode, the electrons
migrate through an external circuit to the cathode. Another
catalyst on the cathode side of the electrolyte causes the oxygen
to react with the hydrogen ions and electrons released at the
anode, thereby forming water. The occurrence of these reactions
near the catalysts and electrolyte creates an electric potential
across the fuel cell. The flow of electrons through an external
circuit that is connected to the fuel cell, therefore, produces
useful work, such as powering an electric motor in a vehicle.
[0004] There are various types of fuel cells, which vary according
to their electrolyte. The electrolyte is the ionic conducting
substance between the anode and the cathode. One type of fuel cell
includes a solid polymer electrolyte, otherwise referred to as a
proton exchange membrane (PEM). Fuel cells incorporating a solid
polymer membrane or proton exchange membrane will hereinafter be
referred to as a PEM fuel cell. The catalyst layers within a PEM
fuel cell are typically attached to both sides of the membrane,
thereby forming a membrane electrode assembly (MEA). As noted
above, while hydrogen ions pass through the MEA, the
electrochemical reaction between the hydrogen ions, electrons, and
oxidant reactant gas forms water within the cathode. This water is
commonly referred to as "product water." In addition, water may
also accumulate in the cathode, due to the drag of water molecules,
which pass from the anode and through the MEA along with the
hydrogen ions during the operation of the fuel cell. This water is
commonly referred to as "proton drag water." The proton drag from
the anode to the cathode results in a lower water content on the
anode side of the PEM compared to the cathode side. This difference
in water content between the anode and cathode sides results in an
osmotic force, which fosters the flow of water from the cathode
side of the PEM towards the anode side. However, if the PEM (i.e.,
electrolyte) doesn't remain highly saturated with water, the PEM
resistance increases, and the useful power obtained from the fuel
cell decreases. Additionally, if product water and drag water
accumulate in the cathode, the accumulated water may impede and
could prevent oxygen from reacting with the hydrogen ions and
electrons. Accumulation of water in the cathode will thus reduce
the electric potential created across the fuel cell, thereby
limiting the fuel cell's performance. Furthermore, if the cathode
water content fails to decrease, the cathode will flood, and the
fuel cell will eventually cease to produce power and shut down.
[0005] In order to assist the oxidant reactant gas in reaching the
catalyst on the MEA, the cathode support plate typically comprises
a diffusion layer and a substrate layer. Both the diffusion layer
and the substrate layer are typically constructed of porous carbon
layers that are rendered hydrophobic. Hydrophobic means
antagonistic to water and is therefore often referred to as
wet-proofed. It is known, however, to utilize a hydrophilic
substrate in lieu of a hydrophobic substrate. Hydrophilic means
capable of absorbing water and therefore, is often referred to as
wettable. U.S. Pat. No. 5,641,586, for example, describes a cathode
support plate comprising a hydrophilic substrate layer and a
hydrophobic diffusion layer. Objects of U.S. Pat. No. 5,641,586
included providing a porous support plate which reduced the
pressure drop of the oxidant gas as it passed through such support
plate, minimizing water accumulation within such support plate and
maximizing access of the oxidant reactant gas to the catalyst.
Although a hydrophilic substrate may reduce the pressure drop of
the oxidant reactant gas through the cathode, the hydrophilic
substrate, by its inherent nature, absorbs more water than a
hydrophobic substrate. Therefore, unless the water is properly
removed from the cathode support plate, the hydrophilic substrate
will absorb the water, which, in turn, will eventually flood the
cathode support plate. Flooding the cathode support plate would,
therefore, negate one of the objects of U.S. Pat. No. 5,641,586:
namely, the object relating to minimizing water accumulation.
[0006] Flooding the cathode support plate would also prevent the
oxidant reactant gas from reaching the catalyst. U.S. Pat. No.
5,641,586 describes a cell operating at elevated pressure, and
product water within such a cell typically exits the cell via the
oxidant reactant gas exhaust stream as a combination of water vapor
and entrained liquid water. Entrained liquid water moves along a
reactant flow channel from the interior of the cell to the oxidant
reactant gas exhaust stream. This concept is well accepted for cell
configurations which utilize a hydrophobic substrate and a solid
reactant support plate. U.S. Pat. No. 5,641,586, however, describes
a cell with a hydrophilic substrate, which will absorb the liquid
water and flood the substrate, thereby impeding transport of oxygen
to the cathode catalyst.
[0007] What is needed in the art is a fuel cell power plant that
ensures proper removal of the product and proton drag water from
the cathode, thereby ensuring that the maximum amount of oxygen
from the oxidant reactant gas stream reaches and reacts with the
catalyst on the cathode side of the MEA.
DISCLOSURE OF INVENTION
[0008] The present invention is a fuel cell power plant that
includes a fuel cell having a membrane electrode assembly (MEA),
which is disposed between an anode support plate and a cathode
support plate and wherein the anode and/or cathode support plates
include a hydrophilic substrate layer. The fuel cell power plant
also includes a fuel reactant gas stream, which is in fluid
communication with the anode support plate's hydrophilic substrate
layer, and an oxidant reactant gas stream, which is in fluid
communication with the cathode support plate's hydrophilic
substrate layer, and a cooling water stream, which is in fluid
communication with both the anode and cathode support plate
hydrophilic substrate layers. The hydrophilic substrate layer in
the anode support plate enhances the migration of the cooling water
to the anode side of the MEA, and the hydrophilic substrate layer
in the cathode support plate improves the removal of water from the
cathode side of the MEA. The hydrophilic substrate layers within
both the anode and cathode support plates have a predetermined
level of porosity (i.e., number of pores) and pore size. The
inventors of the present invention recognized that without
controlling the number of pores within the hydrophilic substrate
that contain water, the water will fill 100% of the available
pores, thereby preventing any migration of the reactant gases. The
inventors of the present invention, therefore, recognized that
controlling the pressure of the reactant gas streams and the
coolant stream, controls the percentage of pores that contains
water or reactant gas. The present invention utilizes a pressure
differential between the coolant stream and the reactant gas
streams to control the respective distribution of the streams
within the pores of a hydrophilic substrate. The pressure
differential is established such that a greater percentage of the
pores within the hydrophilic substrate layer contain reactant gas
rather than water. The present invention provides for creating a
pressure differential between the reactant gas streams and the
coolant stream such that the pressure of the reactant gas streams
is greater than the pressure of the coolant stream. Operating the
fuel cell at such pressure differential ensures that the product
and proton drag water that form at the cathode catalyst layer will
migrate through the cathode support plate and away from the MEA.
Controlling the coolant and fuel reactant gas streams on the anode
side of the MEA also ensures that the cooling water will
continually migrate from the coolant stream toward the anode side
of the MEA, thereby preventing the membrane from becoming dry.
[0009] Proper water balance in the cathode and anode support plates
ensures that the PEM remains moist, thereby prolonging the fuel
cell's life, as well as improving its electrical efficiency. Proper
water removal also facilitates increased oxygen utilization within
the fuel cell. Specifically, without proper water removal, a
reduced portion of the available oxygen reaches the catalyst.
Increasing the amount of oxygen that is available at the catalyst
increases the fuel cell's performance and/or reduces the overall
size of the fuel cell in order to generate a certain power rating.
Although U.S. Pat. No. 5,641,586 recognized that replacing the
cathode hydrophobic substrate layer with a hydrophilic substrate
layer increased the cell voltage, that patent failed to discuss the
importance of water removal from the cathode. Moreover, that patent
failed to teach that controlling the pressure of the reactant gas
streams and the coolant streams controls the filled porosity of the
hydrophilic substrate, as well as reducing the water content of the
catalyst layers. The present invention, in contrast, provides a
means for preventing the cathode support plate from flooding,
thereby ensuring that the maximum amount of oxidant reactant gas
reaches the cathode side of the MEA. Hence the present invention
not only improves the electrical power output capacity of a fuel
cell but also increases the fuel cell's oxygen utilization, which,
in turn, further improves the fuel cell's operational
efficiency.
[0010] Accordingly the present invention relates to a fuel cell
power plant, comprising a fuel cell, which includes an anode
support plate and a cathode support plate and a membrane electrode
assembly disposed between the anode and cathode support plates,
wherein the membrane electrode assembly includes a polymer
electrolyte membrane, wherein the anode support plate and the
cathode support plate each contain a hydrophilic substrate layer
having pores therein. The fuel cell power plant also includes water
transport plates adjacent to the anode and cathode support plates,
wherein the water transport plates have passages for the coolant
stream and reactant gas streams to pass therethrough. Incorporating
the water transport plates into the fuel cell enhances the fuel
cell's ability to remove water from the cathode support plate and
transfer water through the anode support plate to the membrane. The
fuel cell power plant further includes a means for creating a
predetermined pressure differential between the reactant gas
streams and the coolant stream such that a greater percentage of
the pores within the hydrophilic substrate layers contain reactant
gas in lieu of coolant.
[0011] In other embodiments of the present invention, the anode
and/or cathode support plates may contain a diffusion layer. If so,
it is preferable that the diffusion layer be partially hydrophobic
rather than totally hydrophobic because a partially hydrophobic
diffusion layer is capable of transferring a larger percentage of
liquid water than a totally hydrophobic diffusion layer, such as
described in U.S. Pat. No. 5,641,586.
[0012] In a further embodiment of the present invention, a fuel
cell has an anode and/or cathode support plate that includes a
hydrophilic substrate layer but does not include a diffusion layer.
Removing the anode diffusion layer or reducing its thickness
increases the migration of water from the water transport plate to
the MEA, thereby ensuring proper moisturizing of the PEM,
particularly at high current densities, which in turn, further
improves the electrical efficiency of the fuel cell. Removing the
cathode diffusion layer or reducing its thickness reduces the
distance through which the oxidant reactant gas must pass before
reaching the catalyst, thereby increasing the fuel cell's oxygen
utilization characteristics.
[0013] The foregoing features and advantages of the present
invention will become more apparent in light of the following
detailed description of exemplary embodiments thereof as
illustrated in the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 illustrates a fuel cell power plant that includes a
PEM fuel cell and a means for controlling the pressure of the
reactant gas streams and the coolant stream.
[0015] FIG. 2 is a curve that illustrates the relationship between
the pore size and degree of porosity for a hydrophilic substrate
layer.
[0016] FIG. 3 illustrates the percentage of liquid and gas porosity
for a given pore size based upon the curve in FIG. 2.
[0017] FIG. 4 illustrates a PEM fuel cell that includes hydrophobic
diffusion and substrate layers in both the anode and cathode
support plates of the PEM fuel cell.
[0018] FIG. 5 illustrates a PEM fuel cell that includes a
hydrophobic diffusion layer and a hydrophilic substrate layer in
both the anode and cathode support plates of the PEM fuel cell.
[0019] FIG. 6 illustrates a PEM fuel cell that includes a
hydrophobic diffusion layer and a hydrophilic substrate layer in
the cathode support plate of the PEM fuel cell and only a
hydrophilic substrate layer in the anode support plate.
[0020] FIG. 7 illustrates a PEM fuel cell that includes only a
hydrophilic substrate layer in the anode and cathode support plates
of the PEM fuel cell.
[0021] FIG. 8 is a graph of the current density versus the cell
voltage for various fuel cell configurations illustrated in Table
1.
[0022] FIG. 9 is a graph of the fuel cell oxygen utilization versus
the cell voltage for various fuel cell configurations illustrated
in Table 1.
[0023] FIG. 10 is a graph of the current density versus the cell
voltage for various fuel cell configurations illustrated in Table
2.
[0024] FIG. 11 is a plot of the fuel cell voltage as a function of
the pressure differential between the oxidant reactant gas stream
and the coolant stream.
[0025] FIG. 12 is a graph of the current density versus the cell
voltage for diffusion layers of various thicknesses.
MODE(S) FOR CARRYING OUT THE INVENTION
[0026] Referring to FIG. 1, there is shown a PEM fuel cell power
plant that includes a fuel cell 12 and a means for controlling the
pressure of the fuel reactant gas stream 22, oxidant reactant gas
stream 24 and coolant stream 26. A PEM fuel cell power plant 10
typically comprises a plurality of fuel cells, which are
electrically connected in series and referred to as a cell stack
assembly. However, for the purpose of simplicity in explaining the
present invention, the fuel cell power plant 10 only includes one
fuel cell 12, but it will be understood that the fuel cell power
plant 10 typically comprises a predetermined number of fuel cells
12. Each fuel cell 12 includes an anode support plate 14, a cathode
support plate 18 and a membrane electrolyte assembly (MEA) 16
disposed between the anode support plate 14 and the cathode support
plate 18. The fuel reactant gas stream 22 supplies the anode
support plate 14 with the fuel reactant gas, such as hydrogen from
a fuel supply (not shown), and the oxidant reactant gas stream 24
supplies the cathode support plate 18 with oxidant reactant gas.
The oxidant reactant gas may be essentially pure oxygen derived
from a pressurized oxygen container, or the oxidant reactant gas
may be air that is pressurized by a compressor or an air blower. As
the reactant gases pass through the fuel cells 12, product water
forms in the cathode support plate 18. Also, water in the fuel
reactant gas stream 22 passes through the MEA 16 and enters the
cathode support plate 18.
[0027] A water transport plate 20 serves to remove the water from
the cathode support plate 18 and incorporate such water into the
coolant stream 26, which is typically comprised of water. The water
transport plate 20 also cools the fuel cell 12. Hence the water
transport plate 20 is occasionally referred to as a cooler plate.
Because the coolant stream 26, fuel reactant gas stream 22 and
oxidant reactant gas stream 24 are in fluid communication with each
other, it is preferable to manage the water within the PEM fuel
cell power plant 10. Examples of water management systems include
those illustrated in U.S. Pat. Nos. 5,503,944 and 5,700,595, which
are both assigned to the assignee of the present invention and
hereby incorporated by reference. Both of these patents rely upon
maintaining a positive pressure differential between the reactant
gases and the coolant water. Operating the fuel cell power plant 10
such that the pressure of the oxidant reactant gas stream 24 is
greater than the pressure of the coolant stream 26 ensures the
movement of the product water from the cathode 18 toward the water
transport plate 20.
[0028] More importantly, the inventors of the present invention
discovered that when a hydrophilic substrate layer is included
within the anode support plate 14 and/or cathode support plate 18,
it is necessary to operate the fuel cell power plant 10 such that a
pressure differential exists between the fuel reactant gas stream
22, oxidant reactant gas stream 24 and coolant stream 26 in order
to prevent the cathode support plate 18 or anode support plate 14
from flooding. More specifically, the pressure differential
establishes a preferred ratio of liquid water or coolant to
reactant gas within the hydrophilic substrate. The percentage of
liquid to reactant gas is a function of the pore size of the
hydrophilic substrate and the pressure differential between the
reactant and coolant streams. Each hydrophilic substrate has a
predetermined pore size and predetermined porosity. For example,
FIG. 2 is a plot of the pore diameter in micrometers (i.e.,
microns) versus the percentage of pores larger than the plotted
pore diameter. This figure illustrates that the total porosity of
the hydrophilic substrate layer was approximately 75%. In other
words, about 75% of the hydrophilic substrate layer was porous. It
is preferable, however, that more than half of the available pores
be filled with reactant gas rather than liquid. Hence, the porosity
of the hydrophilic substrate layer with pores larger than the
plotted diameter should be greater than 37.5%.
[0029] FIG. 2 was obtained by performing a mercury porosimetery
measurement on a hydrophilic substrate layer, such as grade TGP-006
sold by Toray of America and treated with 20 milligrams of tin
oxide (SnO.sub.2) per gram of substrate to make it wettable. FIG. 2
illustrates the pore size distribution for a hydrophilic substrate
layer. In other words, for a given pore diameter, the distance
above the curve represents the percentage of pores that have a pore
diameter that is less than the plotted pore diameter, and the
distance below the curve represents the percentage of pores that
have a pore diameter that is greater than the plotted pore
diameter. For example, referring to FIG. 3, a pore having a
diameter of 12 microns corresponds to a porosity of 67%. Keeping in
mind that the total porosity is approximately 75%, the porosity of
the substrate having pores with a diameter greater than 12 microns
is about 67%, and the porosity of the substrate having pores with a
diameter less than 12 microns is about 8%. Therefore, about 89% of
the pores within the substrate had a diameter greater than 12
microns, and about 11% of the pores within the substrate had a
diameter less than 12 microns.
[0030] Controlling the pressure differential between the coolant
and the reactant gas streams, in turn, controls the percentages of
liquid and reactant gas within the hydrophilic substrate because
there is a relationship between the pore size and pressure
differential. The relationship between the pore size and the
pressure differential is determined by the following equation: 1 P
g - P l = 4 cos D [ Eq . 1 ]
[0031] wherein,
[0032] p.sub.g=reactant gas pressure
[0033] P.sub.l=liquid pressure
[0034] .gamma.=surface tension
[0035] .theta.=contact angle
[0036] D=diameter of the largest pore filled with liquid
[0037] At a temperature of about 65.degree. C. (150.degree. F.),
the surface tension (.gamma.) of water is approximately 65 dyne/cm.
Additionally, the contact angle between water and a hydrophilic
substrate is about zero. Therefore, at about 65.degree. C.
(150.degree. F.), the difference between the reactant gas pressure
(P.sub.g) and the liquid pressure (P.sub.l) is equal to about 30/D,
wherein the pressures are measured in pounds per square inch (psi)
and the diameter (D) of the pore is measured in microns.
[0038] If that pressure differential is maintained, then all pores
within the hydrophilic substrate having a diameter less than the
given pore diameter (D) will contain liquid and all pores within
the hydrophilic substrate having a diameter greater than the given
pore diameter (D) will contain reactant gas. As mentioned above,
for a given pore diameter (D), the distance above the curve
represents the percentage of pores that have a pore diameter that
is less than the plotted pore diameter, and the distance below the
curve represents the percentage of pores that have a pore diameter
that is greater than the plotted pore diameter. Hence, operating
the fuel cell at the appropriate pressure differential ensures that
the distance above the curve represents the percentage of pores
that contain liquid, and the distance below the curve represents
the percentage of pores that contain reactant gas. For example, as
described hereinbefore with respect to FIG. 3, the percentage of
pores containing liquid is about 11%, and the percentage of pores
containing reactant gas is about 89%. The ratio of coolant to
reactant gas within the hydrophilic substrate is, therefore, about
1 to 9. Controlling the pressure of the coolant stream and reactant
gas streams ensures that a greater percentage of the pores within
the hydrophilic substrate contain oxidant reactant gas rather than
coolant.
[0039] If the pressure differential decreases, the percentage of
pores filled with water increases. Furthermore, in the absence of a
pressure differential between the liquid pressure and the reactant
gas pressure, as in U.S. Pat. No. 5,641,586, the percentage of
pores filled with water will approximate 100%, thereby flooding the
cathode. Flooding the cathode will prevent the oxidant reactant gas
from reaching the catalyst layers because the majority of pores
will be filled with water and the electrical performance will
diminish.
[0040] The preferred ratio of liquid to reactant gas within the
hydrophilic substrate is a function of the relationship between the
pore size and the porosity for such pore size for a given
hydrophilic substrate. In other words, the preferred percentage of
pores filled with reactant gas and water is dependent upon the
shape of the curve illustrated in FIG. 2. Referring to FIG. 2, it
is preferable to have a porosity filled with reactant gas equal to
or greater than point A. Point A represents the juncture at which
the porosity becomes less sensitive to the change in pore size.
Although the shape of the curve in FIG. 2 illustrates that at a
porosity of about 67%, the porosity becomes less sensitive to the
pore size, it is preferable to operate at a porosity such that the
number of pores containing reactant gas is greater than the number
of pores containing water. This results in a preferable mass
transport of the reactants to the catalysts, while also providing a
wetted path to move water from the cathode to the water transport
plate. Additionally, creating a pressure differential between the
fuel reactant gas stream and the coolant stream allows the water to
move from the water transport plate to the PEM on the anode side of
the fuel cell.
[0041] As mentioned above, the preferred percentages of pores
filled with reactant gas and water is dependent upon the size of
the pores within the substrate layer and the pressure differential
between the reactant gas streams 22, 24 and the coolant stream. The
percentage of pores containing liquid or reactant gas will be
controlled by the respective coolant stream 26 and reactant gas 22,
24 streams, wherein the reactant gas streams 22, 24 will typically
have a greater pressure than the coolant gas stream 26.
Specifically, because the pressure of the reactant gas streams 22,
24 are typically equal to about ambient pressure, the pressure of
the coolant stream 26 is less than ambient pressure. Moreover, the
pressure differential between the coolant stream 26 and the
reactant gas streams 22, 24 will preferably be in the range of more
than zero psi but less than two psi.
[0042] As illustrated in FIG. 1, one such means for maintaining a
positive pressure differential between the reactant gas streams 22,
24 and the coolant stream 26 comprises circulating water through
the coolant stream 26, which is cooled by a heat exchanger 28 and
pressurized by a pump 30. The pump 30 establishes a predetermined
coolant water pressure in the coolant stream 26. This pressure may
further be regulated by a variable valve 38, which is located in
the coolant stream 26 just prior to the water transport plate 20.
If the pump 30 is a fixed rate pump, the valve 38 will be useful
for varying the coolant pressure in the event that pressure
adjustments are necessary. A pressure transducer 44 is disposed
downstream of the pump 30 and valve 38. The pressure transducer 44
serves to measure the pressure of the coolant water stream before
it enters the water transport plate 20. The pressure transducer 44,
the valve 38 and the pump 30 may be connected to a power plant
microprocessor controller 46 via lines 52, 58, and 60,
respectively. Coolant stream pressure input from the pressure
transducer 44 will cause the controller to regulate the pump 30
and/or the valve 44 when necessary to achieve a target coolant
stream pressure.
[0043] The oxidant reactant gas is delivered to the cathode support
plate 18 through line 24. The line 24 may contain a variable
pressure regulating valve 36 and a downstream pressure transducer
42 which measures the pressure of the oxidant gas stream as it
enters the cathode support plate 18. The pressure transducer 42 is
connected to the system controller 46 via line 50, and the variable
valve 36 is connected to the controller 46 by line 56. When a
variable compressor or pump 32 is used to pressurize an air
oxidant, appropriate connections may be made with the controller
46. The controller 46 can thus make appropriate corrections in the
oxidant reactant pressure when system operating conditions so
dictate by varying the valve 36 or the pump/compressor 32.
[0044] The fuel reactant is fed into the anode support plate 14 by
means of a line 22. The fuel reactant gas will typically be
contained in a pressurized container, or in a pressurized fuel
conditioning or reforming system (not shown). A variable valve 34
is operable to regulate the pressure of the fuel reactant as it
enters the anode support plate 14. The fuel reactant pressure is
monitored by a pressure transducer 40, which is connected to the
system controller 46 by a line 48. The variable valve 34 is
connected to the system controller 46 by a line 54. It is preferred
to operate a fuel cell power plant at near ambient pressure because
doing so removes the need to compress the air to elevated pressures
and permits the use of fans or blowers to move air through the fuel
cell, thereby resulting in maximum efficiency. Avoiding use of a
compressor within the fuel cell power plant eliminates one source
of parasite power, thereby improving the power plant's overall
operating efficiency. Although the present invention is not limited
to any specific operating condition, a preferred operating pressure
for the reactant gases ranges from 15 to 20 psia.
[0045] Referring to FIG. 4, there is shown a cross sectional view
of the fuel cell 12, which includes an MEA 16, an anode support
plate 17 and a cathode support plate 19. The MEA 16 comprises a
polymer electrolyte membrane ("PEM") 70, an anode catalyst 72 and a
cathode catalyst 74. The anode catalyst 72 and the cathode catalyst
74 are secured on opposite sides of the PEM 70.
[0046] The anode support plate 17 and cathode support plate 19
include hydrophobic diffusion layers 76, 78 and hydrophobic
substrate layers 80, 82. The anode diffusion layer 76 is adjacent
to the anode catalyst 72, and the anode substrate layer 80 is
adjacent to a side of the anode diffusion layer 76 opposite the
anode catalyst 72. The anode diffusion layer 76 and the hydrophobic
anode substrate layer 80 allow the fuel reactant gas, which passes
through the passageway 94 in the water transport plate 84, and the
water, which passes through the passageway 96, to reach the anode
catalyst 72. The fuel cell 12 also includes a hydrophobic cathode
diffusion layer 78 and a hydrophobic cathode substrate layer 82 for
passing the oxidant reactant gas from the passageway 92 in the
water transport plate 86 to the cathode catalyst 74. The cathode
diffusion layer 78 is adjacent to the cathode catalyst 74, and the
cathode substrate layer 82 is adjacent to a side of the cathode
diffusion layer 78 opposite the cathode catalyst 74. The cathode
diffusion layer 78 and the hydrophobic cathode substrate layer 82
allow the oxidant reactant gas, which passes through the passageway
92 in the water transport plate 86 to reach the cathode catalyst
74. The hydrophobic cathode diffusion layer 78 and the hydrophobic
cathode substrate layer 82 allow the product water, which forms in
the cathode catalyst 74, to migrate toward the water transport
plate 86.
[0047] The diffusion layers 76, 78 are applied to both the anode
and cathode substrate layers 80, 82, within the anode support plate
17 and cathode support plate 19, by procedures well known in the
art. A preferable procedure is that which is described in U.S. Pat.
No. 4,233,181, which is owned by the assignee of the present
invention and hereby incorporated by reference. The diffusion
layers 76, 78 are typically constructed of porous conductive layers
that are rendered hydrophobic or partially hydrophobic. One such
porous conductive layer is a carbon particulate. It is preferable
that the carbon particulate have a pore size less than or equal to
4 microns and a porosity equal to or greater than 60%. In order to
render the diffusion layer hydrophobic, a hydrophobic polymer is
mixed with a porous carbon black layer. The resultant is
subsequently heated to about the melting point of the hydrophobic
polymer, as is known in the art. Suitable hydrophobic polymer
include fluoropolymers, such as, polytetraflouroethylene (PTFE),
fluorinated ethylene propylene (FEP),
polytetrafluoroethylene-co-perfluro- methyl vinylether (PFA),
copolymers of ethylene and tetrafluroethylene (ETFE), copolymers of
ethylene and chlorotrifluroethylene (ECTFE), polyvinyldene fluride
(PVDF), polyvinyl fluoride (PVF), and amorphous fluropolymers
(TEFLON AF). The distinguishing characteristic of such
fluoropolymers is their critical surface energy, and it is
preferable to select a fluoropolymer having a critical surface
energy less than or equal to 30 dyne per centimeter. An example of
such a diffusion layer includes a porous carbon-TEFLON.RTM.
polytetrafluoroethylene (PTFE) particulate composite having a
thickness of about 75 to 100 microns (0.003 to 0.004 inches), and
preferably about 0.0035 inches, with a mass of about 12.1
milligrams per square centimeter. More specifically, the anode and
cathode diffusion layers 76, 78 include a porous carbon layer
distributed by the Cabot Corporation under the product
identification name of VULCAN XC-72 which is rendered hydrophobic
by adding polytetrafluoroethylene (PTFE), such as the type
manufactured by E.I. dupont deNemours of Wilmington, Del. under the
tradename TEFLON.RTM.. It is preferable to use an amount of
TEFLON.RTM. polytetrafluoroethylene having the grade "TFE-30" such
that the anode and cathode diffusion layers 76, 78 comprise about
50% VULCAN XC-72 and 50% TEFLON.RTM. polytetrafluoroethylene. The
anode and cathode diffusion layers 76, 78 are also typically heated
to approximately 343.degree. C. (650.degree. F.) for about five (5)
minutes, rendering such layers hydrophobic.
[0048] The anode and cathode substrate layers 80, 82 are typically
constructed of a porous carbon-carbon fibrous composite having a
thickness of about 150 to 175 microns (0.006 to 0.007 inches) a
porosity of about of about 65% to about 75%. An example of such a
substrate is that distributed by the Toray Company of New York,
N.Y. with grade identification TGP-H-060. The anode and cathode
substrate layers 80, 82 are typically rendered hydrophobic by
combining polyfluorinated ethylene propylene, such as the type
manufactured by E.I. dupont deNemours of Wilmington, Del. under the
tradename TEFLON.RTM., into the substrate. It is preferable to use
TEFLON.RTM. polyfluorinated ethylene propylene having the grade
"FEP-121 " such that the anode and cathode substrate layers 80, 82
comprise about 165 milligrams of TEFLON.RTM. polyfluorinated
ethylene propylene for every cubic centimeter of TGP-H-060.
[0049] As shown in FIG. 4, the anode water transparent plate 84 is
adjacent to the anode support plate 17, and the cathode water
transport plate 86 is adjacent to the cathode support plate 19. The
anode and cathode water transport plates 84, 86 may be structured
and/or oriented to cooperate with adjacent water transport plates
88, 89 such that the passageways 96 and 98 simultaneously serve as
the coolant stream for both the anode of one cell and cathode of
the next cell.
[0050] The water transport plates 84, 86, 88, 89 are typically
porous graphite having a mean pore size of approximately two (2) to
three (3) microns and a porosity of about 35% to 40%. It is
preferable to make the water transport plates 84, 86, 88, 89
hydrophilic by treating them with tin oxide (SnO.sub.2) such as
described in U.S. Pat. No. 5,840,414, which is owned by the
assignee of the present invention and hereby incorporated by
reference.
[0051] Referring to FIG. 5, there is shown an alternative
embodiment of a fuel cell 12'. The fuel cell 12' in FIG. 5 differs
from the fuel cell 12 in FIG. 4, in that the anode support plate 17
and cathode support plate 19 of the fuel cell 12 in FIG. 4 comprise
hydrophobic diffusion layers 76, 78 and hydrophobic substrate
layers 80, 82, but the anode support plate 17' and cathode support
plate 19' in the fuel cell 12' in FIG. 5 comprise partially
hydrophobic diffusion layers 104, 106, and hydrophilic substrate
layers 100, 102 adjacent to water transport plates 84, 86
respectively. Continuing to refer to FIG. 5, in addition to
replacing the hydrophobic substrate layers 80, 82 with hydrophilic
substrate layers 100, 102, the diffusion layer thickness is also
reduced. For example, the hydrophobic diffusion layers 76, 78 of
FIG. 4 are preferably about 87.5 microns (0.0035 inches), but the
partially hydrophobic diffusion layers 104, 106 of FIG. 5 are
preferably in the range from about five microns (0.0002 inches) to
less than 25.0 microns (0.0009 inches). A plot of cell performance
for several diffusion layer thicknesses is shown in FIG. 12. It may
also be preferable to reduce the amount of TEFLON.RTM.
polytetrafluoroethylene (grade "TFE-30") to 10%, in lieu of 50%,
along with reducing the mass of the diffusion layer to about 2.0 to
5.0 milligrams per square centimeter of VULCAN XC-72.
[0052] The diffusion layer in FIG. 5 is made thinner than the
diffusion layer in FIG. 4 by reducing both the carbon and PTFE
content of the carbon diffusion layer. Specifically, prior art
diffusion layers are typically made from a 50 weight percent
mixture of carbon black and a hydrophobic polymer. These diffusion
layers are totally hydrophobic with essentially 100% of their void
volume filled with reactant gases. This results in a desirable
characteristic for diffusing reactants to the catalysts but creates
a significant barrier to removal of liquid product water.
[0053] A partially hydrophobic diffusion layer is made with a lower
TEFLON.RTM. PTFE content. This creates a diffusion layer, with a
void volume that is partially filled with reactant gas and
partially filled with water. The precise ratios of gas volume to
water volume are dependent upon the TEFLON.RTM. PTFE content and
the temperature at which the TEFLON.RTM. PTFE is heat treated.
Increasing the wetted volume of the diffusion layer within the
cathode aids removal of the product water from the cathode. Also,
increasing the wetted volume of the diffusion layer within the
anode assists in transferring liquid water to the anode side of the
PEM. The wetted volume of the diffusion layer is increased such
that the diffusion layer's ability to transfer water is increased,
but the non-wetted volume is such that the diffusion layer has
sufficient volume for the diffusion of the reactant gases to and/or
from the catalyst layers. For the purposes of this invention
"partially hydrophobic layer" shall mean a layer having a void
volume that is capable of being partially filled by water and by
gas. It is preferable, therefore, to have a partially hydrophobic
diffusion layer with a thickness ranging from 0.0005 inches to
0.002 inches.
[0054] Continuing to refer to FIG. 5, employing a partially
hydrophobic diffusion layer 106 and a hydrophilic substrate layer
102 in the cathode support plate 19' enhances water removal from
the cathode catalyst layer 74 toward the water transport plate 86,
thereby preventing the cathode substrate 102 from flooding with
water.
[0055] Additionally, employing a partially hydrophobic diffusion
layer 104 and a hydrophilic substrate layer 100 in the anode
support plate 17' enhances water migration from the water transport
plate 84 toward the anode catalyst layer 72, thereby preventing the
anode catalyst layer 72 and PEM 70 from drying out. The base
material of the hydrophilic substrate layers 100, 102 are
carbon-carbon fibrous composite. In order to enhance the ability of
the substrate layers 100, 102 to transfer water to or from the
relevant catalyst layers 72, 74, the pores of the substrate layers
100, 102 are rendered hydrophilic by partially filling such pores
with appropriate metal oxide or hydroxide compounds. Examples of
such metal oxide and hydroxide compounds include tin oxide
(SnO.sub.2), aluminum oxide (Al.sub.2O.sub.3), niobium oxide
(Nb.sub.2O.sub.5), ruthenium oxide (RuO.sub.2), tantalum oxide
(Ta.sub.2O.sub.5), titanium oxide (TiO.sub.2), zinc oxide and
zirconium oxide, and mixtures thereof. In lieu of these metal
oxides, hydroxides or oxyhydroxides could be used with the same
metals. Alternatively, the substrate layers 100, 102 may also be
made wettable by chemical or electrochemical oxidation of the
surface of the carbon. Another alternative includes treating and/or
coating the interior surface of the substrate with a wettable
polymer, such as melamine formaldehyde produced by Cytec Industries
of West York, United Kingdom.
[0056] The hydrophilic substrate layers 100, 102 are comprised of a
porous carbon-carbon fibrous composite having a thickness of about
175 microns (0.007 inches) and a porosity of about of about 75%
with an average pore size of about 27 microns to 37 microns. As
discussed above, an example of such a substrate is that distributed
by the Toray Company of New York, N.Y. with grade identification
TGP-H-060. Adding about 20 to 50 mg of tin oxide (SnO.sub.2), and
preferably 25 to 35 mg, for every gram of TGP-H-060, renders the
substrate layers 100, 102 hydrophilic.
[0057] For example, a Toray TGP-H-060 substrate was made
hydrophilic by depositing approximately 20 milligrams (mg) of tin
oxide per gram of substrate onto the interior surface of the
substrate. Specifically, 66 milliliters (ml) of 100 weight percent
2-propanol and 350 ml of 33 weight percent 2-propanol in water was
added to 134 ml of 1 molar SnCl-2NH.sub.3 solution in a 2,000 ml
beaker. Thereafter, 33 ml of 3M ammonium hydroxide was added
dropwise over a fifteen (15) minute period while stirring
vigorously. In a second beaker, containing 170 ml of 33 weight
percent 2-propanol in water, the pH was adjusted in the second
beaker to about 1.3 to 1.4 by adding the appropriate amount of
hydrochloric acid. While stirring the solution in the first beaker,
the solution in the second beaker was slowly added to the first
beaker. The substrate was then immersed in the combined solution.
After removing the substrate from the combined solution, the
substrate was air dried at room temperature for thirty (30)
minutes, before being air dried for thirty (30) minutes at
115.degree. C. (240.degree. F.) and baked in air at 360.degree. C.
(680.degree. F.) for twelve (12) hours.
[0058] Referring to FIG. 6, there is shown an alternative
embodiment of a fuel cell 12". The fuel cell 12" in FIG. 6 differs
from the fuel cell 12 in FIG. 4, in that the anode support plate 17
and cathode support plate 19 of the fuel cell 12 in FIG. 4 comprise
hydrophobic diffusion layers 76, 78 and hydrophobic substrate
layers 80, 82, respectively, but the cathode support plate 19" in
the fuel cell 12" in FIG. 6 comprises a partially hydrophobic
diffusion layer 106, and a hydrophilic substrate layer 102 adjacent
to the water transport plate 86. Moreover, the anode support plate
17" of FIG. 6 includes a hydrophilic substrate layer 108 adjacent
to the water transport plate 84, but does not include a diffusion
layer. Avoiding use of the diffusion layer on the anode support
plate further increases the performance capability of the fuel cell
by removing all hydrophobic or partially hydrophobic barriers to
the transport of liquid water from the anode water transport plate
84 to the PEM 70.
[0059] Referring to FIG. 7, there is shown an alternative
embodiment of a fuel cell 12'". The fuel cell 12'" in FIG. 7
differs from the fuel cell 12 in FIG. 4, in that the anode support
plate 17 and cathode support plate 19 of the fuel cell 12 in FIG. 4
comprise hydrophobic diffusion layers 76, 78 and hydrophobic
substrate layers 80, 82, respectively, but the cathode support
plate 19'" in the fuel cell 12'" in FIG. 7 only comprises a
hydrophilic substrate layer 102 adjacent to the water transport
plate 86. Moreover, the anode support plate 17'" of FIG. 7 includes
a hydrophilic substrate layer 108 adjacent to the water transport
plate 84, but does not include a diffusion layer. The anode
catalyst layer 72 and the cathode catalyst layer 74 in FIGS. 4 to 7
are of the generic type called a flooded thin film catalyst layer.
Such catalyst layers are described in U.S. Pat. No. 5,211,984,
which is hereby incorporated by reference. Alternatively, the anode
catalyst layer 72 and the cathode catalyst layer 74 may be of the
generic type called a gas diffusion catalyst layer as described in
U.S. Pat. No. 5,501,915, which is also hereby incorporated by
reference. Removing the diffusion layer from the cathode may also
increase the performance of the fuel cell because the oxidant
reactant gas will have to travel through fewer layers to reach the
cathode catalyst layer 74.
[0060] Referring to FIG. 8, there is shown a graph of the current
density versus the cell voltage for various fuel cell
configurations illustrated in Table 1.
1TABLE 1 Anode Cathode Diffusion Substrate Diffusion Substrate
Experiment Layer Layer Layer Layer .smallcircle. Hydrophobic
Hydrophobic Hydrophobic Hydrophobic .box-solid. None Hydrophilic
Hydrophobic Hydrophobic .quadrature. None Hydrophilic Partially
Hydrophobic Hydrophobic .tangle-solidup. None Hydrophilic Partially
Hydrophilic Hydrophobic
[0061] These various fuel cell configurations illustrate how
altering the construction of the fuel cell affects its performance.
Specifically, the fuel cell configurations are designated by the
symbols .largecircle., .box-solid.,.quadrature., and
.tangle-solidup.. The MEA 16 that was used in all of the cell
configurations included a 15 micron PEM electrolyte within a
membrane electrode assembly acquired from W. L Gore and Associates,
Inc. of Elkton, Md. as product identification number "PRIMEA-5560."
The fuel cell designated by the symbol .largecircle. included an
anode support plate, comprising a hydrophobic diffusion layer and a
hydrophobic substrate layer, and a cathode support plate,
comprising a hydrophobic diffusion layer and a hydrophobic
substrate layer. Specifically, the configuration of the fuel cell
designated by the symbol .largecircle. was described hereinbefore
in reference to FIG. 4. More specifically, the fuel cell designated
by the symbol .largecircle. contained an anode support plate 17 and
cathode support plate 19 having 90 micron (0.0035 inch) thick
hydrophobic diffusion layers 76, 78 constructed of porous
carbon-TEFLON.RTM. polytetraflouroethylene (PTFE) composite, with a
mass of about 12.1 milligrams per square centimeter and containing
about 50% PTFE. The anode support plate 17 and cathode support
plate 19 also had 175 micron (0.007 inch) porous carbon-carbon
fibrous composite substrate layers 80, 82 having a porosity of
about 75% and mean pore size of approximately 30 microns. The
substrate layers 80, 82 were rendered hydrophobic by treating such
substrates with about 165 grams of polytetraflouroethylene (PTFE)
for every cubic centimeter of porous carbon-carbon fibrous
composite.
[0062] Continuing to refer to FIG. 8, the fuel cells designated by
the symbols .box-solid., .quadrature. and .tangle-solidup. all
contained an anode support plate comprising a hydrophilic substrate
layer without comprising a diffusion layer. Specifically, the anode
support plate 17 within these three fuel cells only contained a 175
micron (0.007 inch) hydrophilic substrate layer 80. The hydrophilic
substrate layer 80 was constructed of a porous carbon-carbon
fibrous composite having a porosity of about 75% and a mean pore
size of approximately 30 microns. The porous carbon-carbon fibrous
composite was rendered hydrophilic by adding about 26.2 mg of tin
oxide (SnO.sub.2) for every gram of carbon-carbon fibrous
composite.
[0063] The fuel cell designated by the symbol .box-solid. also
included a cathode support plate 19 comprising a hydrophobic
diffusion layer and a hydrophobic substrate layer. More
specifically, this fuel cell contained a cathode support plate 19
having 90 micron (0.0035 inch) thick hydrophobic diffusion layer 78
constructed of porous carbon-polytetrafluroethylene (PTFE), with a
mass of about 12.1 milligrams per square centimeter and containing
about 50% PTFE. The cathode's hydrophobic substrate layer 82 was a
175 micron (0.007) inch porous carbon-carbon fibrous composite
having a porosity of about 75%. The substrate layer 82 was rendered
hydrophobic by treating it with about 165 mg of PTFE for every
cubic centimeter of porous carbon-carbon fibrous composite.
[0064] Continuing to refer to FIG. 8, the fuel cell designated by
the symbol .quadrature. included a cathode support plate comprising
a partially hydrophobic diffusion layer and a hydrophobic substrate
layer. Specifically, the fuel cell designated by the symbol
.quadrature. contained a cathode support plate 19 having a 17.5
micron (0.0007 inch) thick partially hydrophobic diffusion layer 78
constructed of porous carbon-polytetraflouroethylene (PTFE)
particulate, with a mass of about 2.4 milligrams per square
centimeter and containing about 10% PTFE. The cathode support plate
19 also had 175 micron (0.007 inch) porous carbon-carbon fibrous
composite substrate layer 82 having a porosity of about 75% and a
means pore size of approximately 30 microns, and was rendered
hydrophobic by treating with about 165 milligrams of
polytetraflouroethylene for every cubic centimeter of porous
carbon-carbon fibrous composite.
[0065] Still referring to FIG. 8, the fuel cell designated by the
symbol .tangle-solidup. included a cathode support plate comprising
a partially hydrophobic diffusion layer and a hydrophilic substrate
layer. Specifically, the fuel cell designated by the symbol
.tangle-solidup. contained a cathode support plate 19 having a 15.0
micron (0.0007 inch) thick partially hydrophobic diffusion layer 78
constructed of porous carbon- polytetraflouroethylene (PTFE)
particulate, with a mass of about 2.2 milligrams per square
centimeter and containing about 10% PTFE by weight. The cathode
support plate 19 also had 175 micron (0.007 inch) porous
carbon-carbon fibrous composite substrate layer 82 having a
porosity of about 75% and a mean pore size of approximately 30
microns, and rendered hydrophilic by loading such substrate with
about 20.9 milligrams (mg) of tin oxide (SnO.sub.2) for every gram
of porous carbon-carbon fibrous composite, thereby rendering the
porous carbon-carbon fibrous composite hydrophilic.
[0066] Each of the fuel cell configurations, designated by the
symbols .largecircle., .box-solid., .quadrature. and
.tangle-solidup., were placed in a fuel cell fueled by a hydrogen
reactant gas stream and an air stream, serving as the oxidant
reactant gas stream. The fuel cells utilized about 80% of the
hydrogen and about 40% of the air. The fuel cells operated at about
65.degree. C. and about ambient pressure. Additionally, in order to
properly manage the product water, formed by the chemical reaction
between the hydrogen and air, the pressure differential between the
reactant gas streams and the coolant stream was about two (2) psi.
Each fuel cell operated for a period of ten (10) to twelve (12)
days, and the graph shown in FIG. 8 illustrates the current density
versus the cell voltage for each fuel cell configurations over such
time.
[0067] The fuel cell designated by the symbol .largecircle. had a
hydrophobic anode diffusion layer, and the fuel cells designated by
the symbols .box-solid., .quadrature., and .tangle-solidup. did not
contain an anode diffusion layer. The data in FIG. 8 illustrates
that the fuel cells designated by the symbols .box-solid.,
.quadrature., and .tangle-solidup. had a greater voltage level for
a given current density compared to the fuel cell designated by the
fuel cell designated by the symbol .largecircle.. Therefore,
removing the diffusion layer within the anode could increase the
fuel cell's electrical performance.
[0068] The fuel cell configurations, designated by the symbols
.box-solid., .quadrature., and .tangle-solidup. also had an anode
support plate that contained a hydrophilic substrate layer, and the
fuel cell designated by the symbol .largecircle. had an anode
support plate that contained a hydrophobic substrate layer. The
fuel cells designated by the symbols .box-solid., .quadrature., and
.tangle-solidup. had a greater voltage level for a given current
density compared to the fuel cell designated by the fuel cell
designated by the symbol .largecircle.. Therefore, replacing the
anode hydrophobic substrate layer with a hydrophilic substrate
layer may improve the electrical performance of the fuel cell. The
electrical performance of the fuel cell may further improve if at
the same time the anode diffusion layer is totally removed and the
anode hydrophobic substrate layer is replaced with a hydrophilic
substrate layer.
[0069] The fuel cells designated by the symbols .quadrature. and
.tangle-solidup. had a cathode support plate that contained a
thinner and partially hydrophobic diffusion layer compared to the
fuel cell designated by the symbol .box-solid., which had a thick,
hydrophobic cathode diffusion layer. The fuel cells designated by
the symbols .quadrature. and .tangle-solidup. had a greater voltage
level for a given current density compared to the fuel cell
designated by the symbol .box-solid.. Therefore, reducing the
thickness of the diffusion layer, while increasing its
hydrophilicity improves the electrical performance of the fuel
cell.
[0070] FIG. 8 also illustrates that replacing the cathode
hydrophobic substrate layer with a hydrophilic substrate layer
further improves the electrical performance of the fuel cell.
Specifically, the fuel cell designated by the symbol
.tangle-solidup. had a greater voltage level for a given current
compared to the fuel cell designated by the symbol .quadrature..
The fuel cell designated by the symbol .quadrature. had a cathode
support plate that contained a hydrophobic substrate layer, but the
fuel cell designated by the symbol .tangle-solidup. had a cathode
support plate that contained a hydrophilic substrate layer.
Therefore, replacing the cathode hydrophobic substrate layer with a
hydrophilic substrate layer further improves the electrical
performance of the fuel cell.
[0071] Replacing the cathode support plate's hydrophobic substrate
layer with a hydrophilic substrate layer and reducing its thickness
and increasing the hydrophilicity of the diffusion layer within the
cathode not only improves the electrical performance of the fuel
cell but also increases the fuel cell's oxygen utilization.
Referring to FIG. 9 there is shown a graph of the fuel cell oxygen
utilization versus the cell voltage for the various fuel cell
configurations illustrated in Table 1 and designated by the symbols
.box-solid., .quadrature., and .tangle-solidup.. All three types of
fuel cells utilized about 80% of the hydrogen. The fuel cell
designated by the symbol .quadrature. had a higher cell voltage for
a given oxygen utilization percentage than the fuel cell designated
by the symbol .box-solid., and the fuel cell designated by the
symbol .tangle-solidup. had an higher cell voltage for the same
oxygen utilization percentage than the fuel cell designated by the
symbol .quadrature., The fuel cell designated by the symbol
.tangle-solidup. had a cathode support plate that contained a
hydrophilic substrate layer and a partially hydrophobic and thinner
diffusion layer. Therefore, replacing the cathode hydrophobic
substrate layer with a hydrophilic layer and reducing the thickness
and hydrophilicity of the cathode diffusion layer improves the
oxygen utilization of the fuel cell. Hence, for a certain amount of
oxygen, the fuel cell designated by the symbol .tangle-solidup. can
produce more electricity than the two fuel cells designated by the
symbols .box-solid. and .quadrature..
[0072] Reconfiguring the diffusion and substrate layers within the
anode not only improves the operating efficiency of a fuel cell
power plant fueled by pure hydrogen but also improves the operating
efficiency of a fuel cell power plant fueled by reformate fuel
containing approximately 46% hydrogen. Reformate fuel is a hydrogen
rich gas stream that is produced by known technology from a
hydrocarbon fuel. Referring to FIG. 10, there is shown a graph of
the cell voltage as a function of the current density for two fuel
cells, having different anode configurations, illustrated in Table
2, using a reformate fuel.
2TABLE 2 Anode Cathode Diffusion Substrate Diffusion Substrate
Experiment Layer Layer Layer Layer .smallcircle. Hydrophobic
Hydrophobic Partially Hydrophobic Hydrophobic .box-solid. None
Hydrophilic Partially Hydrophobic Hydrophobic
[0073] The fuel cells designated by the symbols .largecircle. and
.box-solid. are the same fuel cell configurations described in
reference to Table 1 and FIGS. 8 and 9. Specifically, both fuel
cells had a cathode support plate, comprising a hydrophobic
diffusion layer and a hydrophobic substrate layer. Moreover, the
fuel cells designated by the symbols .largecircle. and .box-solid.
contained a cathode support plate 19 having a 87.5 micron (0.0035
inch) thick partially hydrophobic diffusion layer 78 constructed of
porous carbon-polytetraflouroethylene (PTFE) particulate composite,
with a mass of about 12.1 milligrams per square centimeter of
porous carbon-carbon fibrous composite and containing about 50%
PTFE. Each cathode 19 also had a 175 micron (0.007 inch) porous
carbon-carbon fibrous composite substrate layer 82 having a
porosity of about 75%, and rendered hydrophobic by treating such
substrates with about 165 grams of PTFE for every cubic centimeter
of porous carbon-carbon fibrous composite. The configuration of the
anode support plates within the fuel cells, however, differed from
each other. Specifically, the fuel cell designated by the symbol
.largecircle. had an anode support plate, comprising a hydrophobic
diffusion layer and a hydrophobic substrate layer, but the fuel
cell designated by the symbol .box-solid. had an anode support
plate only comprising a hydrophilic substrate layer. The
hydrophilic substrate layer 80 within the anode support plate 17 of
the fuel cell designated by the symbol .box-solid. was 175 micron
(0.007 inch) thick. Specifically, the hydrophilic substrate layer
80 was constructed of a porous carbon-carbon fibrous composite
having a porosity of about of about 75%. The porous carbon-carbon
fibrous composite was rendered hydrophilic by adding about 35.0 mg
of tin oxide (SnO.sub.2) for every gram of carbon-carbon fibrous
composite.
[0074] Both of the fuel cell configurations designated by the
symbols .largecircle. and .box-solid. were placed in a fuel cell
fueled by simulated reformate and air. The reformate fuel used in
this experiment comprised about 46% hydrogen, 32% nitrogen, 22%
carbon dioxide and 20 ppm of carbon monoxide. The fuel cell
utilized about 80% of the hydrogen in the reformate and about 30%
of the air. The fuel cell operated at about 65.degree. C. and about
ambient pressure. Additionally, in order to properly manage the
product water, formed by the chemical reaction between the hydrogen
and air, the pressure differential between the fuel and oxidant gas
streams and the coolant stream was about two (2) psi. Each fuel
cell operated for a period of ten (10) to twelve (12) days, and the
graph shown in FIG. 10 illustrates the cell voltage as a function
of the current density for each fuel cell configuration over such
time. This figure demonstrates that the fuel cell designated by the
symbol .box-solid. had a greater voltage level for a given current
compared to the fuel cell designated by the .largecircle.. Both
fuel cells had identical cathode configurations. However, the fuel
cell designated by the symbol .box-solid. had an anode support
plate comprising only a hydrophilic substrate and no diffusion
layer, but the fuel cell designated by the symbol .largecircle.,
had an anode support plate comprising hydrophobic diffusion and
substrate layers. Therefore, omitting the diffusion layer within
the anode support plate and/or replacing the anode hydrophobic
substrate layer with a hydrophilic substrate layer improves the
electrical performance of the fuel cell.
[0075] FIG. 11 illustrates the importance of creating a pressure
differential between the reactant gas streams and the coolant
stream, such that the pressure of the reactant gas streams is
greater than the pressure of the coolant stream. Specifically, FIG.
11 illustrates the performance of a fuel cell having a
configuration designated by the symbol .tangle-solidup., which was
discussed in reference to Table 1. Using the same test conditions
discussed hereinbefore in reference to this fuel cell and holding
all such test conditions constant, with the exception of the
pressure differential between the coolant and the oxidant gas
stream, the fuel cell voltage was plotted as a function of pressure
differential. The pressure differential between the coolant and the
oxidant gas stream varied from approximately zero to 4.4 psi.
Specifically, as indicated by the arrows in FIG. 11, the test began
by creating a pressure differential of about 4.4 psi, which was
gradually reduced to zero and thereafter gradually increased to its
original pressure. As the fuel cell operated at a pressure
differential equal about zero to a range of about 1.0 to 2.0 psi,
the cell voltage increased from about 0.38 volts to 0.58 volts,
respectively. However, as the fuel cell operated at a pressure
differential in excess of about 1.0 to 2.0 psi the cell voltage
remained relatively constant at a voltage of about 0.58 to 0.60
volts. The data in FIG. 11 illustrates that the performance of the
fuel cell designated by the symbol .tangle-solidup. improved
significantly as the pressure differential between the coolant
stream and the oxidant gas stream increased from zero to the range
of about 1.0 to 2.0 psi, after which point the performance of the
fuel cell remained relatively unchanged. Performance is improved as
the pressure differential between the coolant stream and oxidant
gas increases because the water occupying the hydrophobic cathode
substrate layer is displaced and enters the water transport plate
adjacent the cathode support plate. Likewise, operating a fuel cell
with a pressure differential between the coolant stream and the
fuel reactant gas stream more accurately controls the amount of
water that enters the hydrophilic anode substrate layer. A range of
more than zero psi but less than two psi is preferred. Moreover,
unlike the operation of the fuel cell described in U.S. Pat. No. 25
5,641,586, operating a fuel cell, having hydrophilic substrate
layers, with a pressure differential between the coolant stream and
the reactant gas streams, increases the percentage of pores within
the hydrophilic substrate layers that contain reactant gas and
decreases the percentage of pores that contain coolant. The
increased number of pores containing reactant gas within the
hydrophilic substrate layers, in turn, facilitates the diffusion of
the reactant gases from the passageways in the water transport
plates to the catalyst layers within the MEA.
[0076] All of the aforementioned patent applications are
incorporated herein by reference.
[0077] Although the invention has been described and illustrated
with respect to the exemplary embodiments thereof, it should be
understood by those skilled in the art that the foregoing and
various other changes, omissions and additions may be made without
departing from the spirit and scope of the invention.
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