Note: Descriptions are shown in the official language in which they were submitted.
CA 02578111 2007-02-27
DESCRIPTION
A PROTON EXCHANGE MEMBRANE FUEL CELL POWER SYSTEM
Technical Field
The present invention relates to a proton exchange membrane (PEM) fuel
cell power system, and more specifically to a power system which includes a
plurality of discrete fuel cell modules producing respective voltages, and
wherein
the discrete fuel cell modules are self humidifying, have an electrical
efficiency
of at least about 40%, and offer plant reliability, ease-of-maintenance, and
reduced capital costs not possible heretofore.
l0 Background Art
The fuel cell was developed in England more than 150 years ago by Sir
William Grove in 1839. The inventor called it a "gaseous battery" at the time
to distinguish the fuel cell from another invention of his, the electric
storage
battery. The fuel cell is an electrochemical device which reacts hydrogen and
oxygen which is usually supplied from the air, to produce electricity and
water.
With prior processing, a wide range of fuels, including natural gas and coal-
derived synthetic fuels can be converted to electric power. The basic process
is highly efficient, and for those fuel cells fueled directly by hydrogen,
pollution
free. Further, since fuel cells can be assembled into stacks, of varying
sizes,
power systems have been developed to produce a wide range of output levels
and thus satisfy numerous kinds of end-use applications.
Heretofore, fuel cells have been used as alternative power sources in earth
and space applications. Examples of this use are unattended communications
repeaters, navigational aids, space vehicles, and weather and oceanographic
stations, to name .but a few.
Although the basic process is highly efficient and pollution free, a
commercially feasible power system utilizing this same technology has remained
elusive. For example, hydrogen-fueled fuel cell power plants based on Proton
Exchange Membrane (PEM) Fuel Cells are pollution free, clean, quiet on site,
and have few moving parts. Further, they have a theoretical efficiency of up
to about 80%. This contrasts sharply with conventional combustion technologies
such as combustion turbines, which convert at most 50% of the energy from
combusting fuel into electricity and in smaller generation capacities, are
uneconomical and significantly less efficient.
Although the fundamental electrochemical processes involved in all fuel
cells are well understood, engineering solutions have proved elusive for
making
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certain fuel cell types reliable and for other types, economical. In the case
of
PEM fuel cells, reliability has not been the driving concern to date, but
rather
the installed cost per watt of generation capacity has. In order to lower the
PEM fuel cost per watt, much attention has been placed on increasing power
output. Historically this has resulted in additional, sophisticated balance-of-
plant
systems necessary to optimize and maintain high PEM fuel cell power outputs.
A consequence of highly complex balance-of-plant systems is they do not
readily
scale down to low (single residence) generation capacity plants. Consequently
installed cost, efficiency, reliability and maintenance expenses all are
adversely
io effected in low generation applications.
As earlier noted, a fuel cell produces an electromotive force by reacting
fuel and oxygen at respective electrode interfaces which share a common
electrolyte. In the case of a PEM fuel cell, hydrogen gas is introduced at a
first electrode where it reacts electrochemically in the presence of a
catalyst to
produce electrons and protons. The electrons are circulated from the first
electrode to a second electrode through an electrical circuit connected
between
the electrodes. Further, the protons pass through a membrane of solid,
polymerized electrolyte (a proton exchange membrane or PEM) to the second
electrode. Simultaneously, an oxidant, such as oxygen gas, (or air), is
introduced
to the second electrode where the oxidant reacts electrochemically in the
presence of the catalyst and is combined with the electrons from the
electrical
circuit and the protons (having come across the proton exchange membrane) thus
forming water and completing the electrical circuit. The fuel-side electrode
is
designated the anode and the oxygen-side electrode is identified as the
cathode.
The external eleqtrip circuit conveys electrical current and can thus extract
electrical power from the cell. The overall PEM fuel ceH reaction produces
electrical energy which is the sum of the separate half cell reactions
occurring
in the fuel cell less its internal losses.
Since a single PEM fuel cell produces a useful voltage of only about 0.45
to about 0.7 volts D.C. under a load, practical PEM fuel cell plants have been
built from multiple cells stacked together such that they are electrically
connected
in series. In order to reduce the number of parts and to minimize costs, rigid
supporting/conducting separator plates often fabricated from graphite or
special
metals have been utilized. This is often described as bipolar construction.
More
specifically, in these bipolar plates one side of the plate services the
anode, and
the other the cathode. Such an assembly of electrodes, membranes, and the
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bipolar plates are referred to as a stack. Practical stacks have heretofore
consisted of twenty or more cells in order to produce the direct current
voltages
necessary for efficient inverting to alternating current.
The economic advantages of designs based on stacks which utilize bipolar
plates are compelling. However, this design has various disadvantages which
have
detracted from its usefulness. For example, if the voltage of a single cell in
a
stack declines significantly or fails, the entire stack, which is held
together in
compression with tie bolts, must be taken out of service, disassembled, and
repaired. In traditional fuel cell stack designs, the fuel and oxidant are
directed
io by means of internal manifolds to the electrodes. Cooling for the stack is
provided either by the reactants, natural convection, radiation, and possibly
supplemental cooling channels and/or cooling plates. Also included in the
prior
art stack designs are current collectors, cell-to-cell seals, insulation,
piping, and
various instrumentation for use in monitoring cell performance. The fuel cell
stack, housing, and associated hardware make up the operational fuel cell
plant.
As will be apparent, such prior art designs are unduly large, cumbersome, and
quite heavy. Certainly, any commercially useful PEM fuel cell designed in
accordance with the prior art could not be manipulated by hand because of
these characteristics.
It is well known that PEM fuel cells can operate at higher power output
levels when supplemental humidification is made available to the proton
exchange
membrane (electrolyte). Humidification lowers the resistance of proton
exchange
membranes to proton flow. Supplemental water can be introduced into the
hydrogen or' oxygen streams or more directly to the proton exchange membrane
by means of the $hysical phenomena of wicking. The focus of investigation in
recent years has been to develop Membrane/Electrode Assemblies (MEAs) with
increasingly improved power output when running without supplemental
humidification (self-humidified). Being able to run an MEA when it is self-
humidified is advantageous because it decreases the complexity of the balance-
of-
plant and its attendant costs. However, self-humidification heretofore has
resulted
in fuel cells running at lower current densities, and thus, in turn, has
resulted
in more of these assemblies being required in order to generate a given amount
of power. This places added importance on reducing the cost of the supporting
structures, such as the bipolar plates, in conventional designs.
Accordingly, a proton exchange membrane fuel cell power system which
achieves the benefits to be derived from the aforementioned technology but
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which avoids the detriments individually associated therewith, is the subject
matter
of the present invention.
SUMMARY OF THE INVENTION
One aspect of the present invention is to provide a proton exchange
membrane fuel cell power system having a plurality of discrete PEM fuel cell
modules with individual membrane electrode diffusion assemblies, the PEM fuel
cell modules further having individual force application assemblies for
applying a
given force to the membrane electrode diffusion assemblies. Further, the PEM
1o fuel cell modules of the present invention can be easily manipulated by
hand.
Another aspect of the present invention is to provide a PEM fuel cell
module which, in operation, produces a given amount of heat energy, and
wherein the same PEM fuel cell module has a cathode air flow which removes
a preponderance of the heat energy generated by the PEM fuel cell module.
Another aspect of the present invention is to provide a proton exchange
membrane fuel cell power system wherein each of the discrete PEM fuel cell
modules has opposing membrane electrode diffusion assemblies having a
cumulative active area of at least about 60 square centimeters, and wherein
each
of the discrete fuel cell modules produce a current density of at least about
350
mA per square centimeter of active area at a nominal voltage of about 0.5
volts
D.C.; and a power output of at least about 10.5 watts.
Still a further aspect of the present invention relates to a proton
exchange membrane fuel cell power system which includes an enclosure defining
a cavity; and a subrack mounted in the cavity and supporting the plurality of
discrete proton exchange membrane fuel cell modules.
Another aspect of the present invention relates to a proton exchange
membrane fuel cell power system which comprises:
a hydrogen distribution frame defining discrete cavities, and wherein
individual membrane electrode diffusion assemblies are sealably mounted in
each
3o of the cavities, the membrane electrode diffusion assemblies each having
opposite
anode and cathode sides; and
a pair of current collectors received in each of the cavities, the individual
current collectors positioned in ohmic electrical contact with the respective
anode
and cathode sides of each of the membrane electrode diffusion assemblies.
A further aspect of the present invention relates to a proton exchange
membrane fuel cell power system comprising:
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a cathode cover which partially occludes the respective cavities of the
hydrogen distribution frame, the respective cathode covers individually
releasably
cooperating with each other, and with the hydrogen distribution frame; and
a pressure transfer assembly received in each of the cavities and applying
5 a given force to the current collectors and the membrane electrode diffusion
assembly, and wherein the cathode cover is disposed in force transmitting
relation
relative to the pressure transfer assembly.
A further aspect of the present invention relates to a proton exchange
membrane fuel cell power system which includes a membrane electrode diffusion
lo assembly comprising:
a solid proton conducting electrolyte membrane which has opposite anode
and cathode sides;
individual catalytic anode and cathode electrodes disposed in ionic contact
with the anode and cathode sides of the electrolyte membrane; and
a diffusion layer borne on each of the anode and cathode electrodes and
which is electrically conductive and has a given porosity.
Still another aspect of the present invention is to provide a proton
exchange membrane fuel cell power system having a solid electrolyte membrane
which comprises crosslinked polymeric chains having sulfonic acid groups, and
2o wherein the crosslinked polymeric chains comprise methacrylates.
Moreover, another aspect of the present invention relates to a proton
exchange membrane fuel cell power system which includes current collectors
which
have at least about 70% open area.
These and other aspects of the present invention will be discussed in
further detail hereinafter.
Brief Description of the Drawings
The accompanying drawings serve to explain the principles of the present
invention.
Figure 1 is a perspective, front elevation view of a proton exchange
membrane fuel cell power system of the present invention and showing some
underlying structures in phantom lines.
Figure 2 is a perspective view of a subrack employed with the present
invention.
Figure 3 is a'fragmentary, transverse, vertical sectional view taken from
a position along line 3-3 of Figure 2.
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Figure 4 is a second, fragmentary, transverse, vertical sectional view taken
from a position along line 3-3 of Figure 2.
Figure 5 is a perspective view of a portion of the subrack.
Figure 6 is transverse, vertical, sectional view taken from a position along
line 6-6 of Figure 2.
Figure 7A is a transverse, vertical, sectional view taken through an air
mixing valve of the present invention.
Figure 7B is a transverse, vertical, sectional view taken through an air
mixing valve of the present invention, and showing the valve in a second
io position.
Figure 8 is a longitudinal, horizontal, sectional view taken from a position
along line 8-8 of Figure 2.
Figure 9 is a perspective, exploded, side elevation view of a protor.
exchange membrane fuel cell module utilized with the present invention, and
the
accompanying portion of the subrack which mates with same.
Figure 10 is a side elevation view of a hydrogen distribution frame utilized
with the proton exchange membrane fuel cell module of the present invention.
Figure 11 is a perspective, side elevation view of a proton exchange
membrane fuel cell module utilized with the present invention.
Figure 12 is a partial, exploded, perspective view of one form of the
PEM fuel cell module of the present invention.
=. Figure 13 is a partial, greatly enlarged, perspective, exploded view of a
portion of the PEM fuel cell module shown in Figure 12.
Figure 14 is a partial, exploded, perspective view of one form of the
PEM fuel cell module of the present invention.
Figure 15 is a partial, greatly enlarged, perspective, exploded view of a
portion of the PEM fuel module shown in Figure 14.
Figure 16 is a partial, exploded, perspective view of one form of the
PEM fuel module of the present invention.
Figure 17 is a partial, greatly enlarged, perspective, exploded view of a
portion of the PEM fuel cell module shown in Figure 16.
Figure 18 is a partial, exploded, perspective view of one form of the
PEM fuel cell module of the present invention.
Figure 19 is a partial, greatly enlarged, perspective exploded view of a
portion of the PEM fuel cell module shown in Figure 18.
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Figure 20 is a perspective view of a pressure plate which is utilized in
one form of PEM fuel cell module of the present invention.
Figure 21 is an end view of the pressure plate shown in Figure 20.
Figure 22 is a fragmentary, transverse, vertical sectional view taken through
a cathode cover of the present invention and showing one form thereof.
Figure 23 is a fragmentary, transverse, vertical sectional view taken through
a cathode cover of the present invention and showing an alternative form
thereof.
Figure 24 is a fragmentary, transverse, vertical sectional view taken through
lD a cathode cover of the present invention and showing an alternative form
thereof.
Figure 25 is a fragmentary, transverse, vertical sectional view taken through
a cathode cover of the present invention and showing an alternative form
thereof.
Figure 26 is a greatly simplified, exploded view of a membrane electrode
diffusion assembly of the present invention.
Figure 27 is a greatly simplified, exploded view of an alternate form of
the membrane electrode diffusion assembly of the present invention.
Figure 28 is a top plan view of a current collector employed in the PEM
fuel cell module of the present invention.
Figure 29 is a greatly enlarged perspective view of a pressure transfer
assembly which is utilized with the present invention.
Figure 30 is a greatly simplified, schematic view of the control assembly
of the present invention.
Figure 31 is, a,greatly simplified schematic view of a heat exchanger which
is employed with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The proton exchange membrane (PEM) fuel cell power system of the
present invention is generally indicated by the numeral 10 in Figure 1. As
shown therein, the PEM fuel cell power system includes an enclosure which is
generally indicated by the numeral 11, and which sits on the surface of the
earth or other supporting surface 12. Enclosure 11 has left and right
sidewalls
13 and 14, respectively, and front and rear surfaces 15 and 20, respectively.
The enclosure has a top surface 21 which is joined to the left and right
sidewalls; and front and rear surfaces respectively. First and second
apertures
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22 and 23 respectively are formed in the front surface 15. Further, a pair of
doors which are generally designated by the numeral 24, are hingedly mounted
on the front surface 15 and are operable to occlude the respective apertures
22
and 23. The enclosure 11, described above, defines a cavity 25 of given
dimensions.
Occluding the aperture 23 are a plurality of subracks which are generally
indicated by the numeral 30. The subracks are individually mounted in the
cavity 25, and are operable to support a plurality of discrete PEM fuel cell
modules in a given orientation in the cavity 25. The PEM fuel cell modules
will
io be discussed in greater detail hereinafter. Referring now to Figure 2, each
subrack 30 has a main body 31 for supporting the PEM fuel cell modules. The
main body includes supporting flanges 32, which are attached by suitable
fasteners
to the enclosure 11. The subrack 30 has a forward edge 33 and a rearward
edge 34. Further, the main body has top and bottom portions 35 and 40,
respectively. A rear wall 41 (Figs. 5 and 6) joins the top and bottom portions
together at the rearward edge 34. As best seen in Figure 2, a plurality of
apertures 42 and 43 are formed in the top and bottom portions 35 and 40,
respectively. Further, elongated channels 44 and 45 are formed in the
respective
top and bottom portions 35 and 40, respectively. As best understood by
2o reference to Figures 3, 4, and 5, the main body 31 is made up of a number
of discrete mirror image portions 31A, which when joined together, form the
main body. This is further seen in Figure 8. These mirror image portions are
fabricated from a moldable, dielectric substrate. As best seen by reference to
Figures 5 and 6, a D.C. (direct current) bus 50 is affixed on the rear wall 41
of the subrack 30.. A repeating pattern of 8 pairs of conductive contacts 51
are
attached on the rear wall 41. Further, first and second valves 52 and 53 are
appropriately positioned relative to the 8 pairs of conductive contacts 51. As
best seen in Figure 6, the respective first and second valves 52 and 53 extend
through the rear wall 41 and are coupled in fluid flowing relation relative to
3o first and second conduits 54 and 55, respectively. Referring now to Figure
8,
the first conduit 54 constitutes a hydrogen distribution assembly which is
coupled
in fluid flowing relation with a source of hydrogen 60 (Figure 1). Further, a
valve assembly 61 is coupled in fluid meter relation relative to the source of
hydrogen 60 and the first conduit 54. The second conduit 55 exhausts to
ambient, or may be coupled in fluid flowing relation with other systems such
as
a hydrogen recovery and recycling system or alternatively a chemical reformer
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which produces a supply of hydrogen for use by the power system 10. In this
regard, the hydrogen recovery and recycling system would recover or recapture
unreacted hydrogen which has previously passed through the individual PEM fuel
cell modules. This system, in summary, would separate the unreacted hydrogen
from other contaminants (water, nitrogen, etc.) and return it to the power
system
10. In the alternative, a chemical reformer may be utilized for the purpose
described above, and the unreacted hydrogen would be returned to the chemical
reformer where it would again be delivered to the individual PEM fuel cell
modules, as will be described in further detail below.
Referring now to Figure 6, the PEM fuel cell power system 10 of the
present invention further includes an air distribution assembly 70 which is
received in the enclosure 11 and which is coupled in fluid flowing relation
relative to the subrack 30. The air distribution assembly 70 includes an air
plenum which is generally indicated by the numeral 71. The air plenum 71 has
a first intake end 72, and a second exhaust end 73. The exhaust end 73 is
positioned intermediate the top and bottom portions 35 and 40 and delivers air
to each of the PEM fuel cell modules supported on the subrack 30. Further,
the intake end 72 is positioned in fluid flowing relation relative to the top
and
bottom portions 35 and 40 of the subrack 30.
An air movement ventilation assembly 74 comprising a direct current fan
75 or equivalent substitute is operably coupled to the plenum 71. The variably
adjustable speed fan 75 moves air from the intake end 72 to the exhaust end
73 of the plenum 71. Referring now to Figures 6, 7A and 7B, an air mixing
valve 80 is 6perably coupled with the air movement assembly 74, and the intake
end 72 of the plejiuUn 71. The air mixing valve 80 includes an outer tube 81
which has formed therein a pair of apertures 82 which communicate in fluid
flowing relation with the air plenum 71. Still further, the air mixing valve
80
includes an inner tube 83 which is telescopingly received internally of, and
substantially concentrically disposed relative to, the outer tube 81. The
inner
tube 83 is selectively rotatable relative to the outer tube 81. A pair of
apertures 84 are formed in the inner tube and provides a convenient means by
which the exhaust end 73 may be selectively coupled with the intake end 72 of
the air plenum 71. Still further, it should be understood that the inner tube
is connected in fluid flowing relation with ambient air which comes from
outside
of the plenum 71. As illustrated most clearly by references to Figure 2, an
actuator, or motor 85, is disposed in force transmitting relation relative to
the
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air mixing valve 80 and more specifically, to the inner tube 83 thereof. The
actuator 85, when energized, moves the air mixing valve 80 and more
specifically,
the second tube along a given course of travel 90 between a first position 91,
as seen in Figure 7B, to a second open position 92, which is seen in Figure
5 7A. The movement of the air mixing valve 80 along this course of travel 90
facilitates the selective mixing of outside air with the air which has
previously
passed through the respective PEM fuel cell modules and which has become
heated and humidified by way of the chemical reaction taking place within each
of the proton exchange membrane fuel cell modules.
10 As best appreciated by a study of Figure 30, temperature sensors 93 are
positioned near the exhaust end 73 of the air plenum 71 for sensing the
temperature of the air entering the discrete PEM fuel cell modules and near
the
plenum intake end 72. The temperature sensors 93 sense the temperature of
the air mixture which comprises outside ambient air, and the air which has
just
passed through each of the discrete proton exchange membrane fuel cell
modules.
Still further, and as best seen in Figure 30, a control assembly 250 is
electrically
coupled with the temperature sensors 93, and the actuator 85. The control
assembly selectively energizes the actuator 85 to move the air mixing valve 80
along the course of travel 90 to control the temperature of the air delivered
at
the exhaust end 73 of the air plenum 71. As should be understood, the air
movement assembly 74 has a speed of operation which is variably adjustable.
In this regard, the control assembly is electrically coupled in controlling
relation
relative to the air movement assembly 74, temperature sensors 93, and the air
mixing valve 80 to vary or otherwise optimize the performance characteristics
of
the Proton Exclange Membrane (PEM) fuel cell modules under assorted
operational conditions. This relationship is illustrated most accurately by a
study
of Figure 30.
Referring now to Figure 9, a plurality of discrete PEM fuel cell modules
are generally indicated by the numeral 100, and are releasably supported on
the
subrack 30. The description which follows relates to a single PEM fuel cell
module 100, it being understood that each of the PEM fuel cell modules are
substantially identical in construction, and are light in weight and can be
readily
manipulated or moved about by hand.
A discrete PEM fuel cell module 100 is best illustrated by reference to
Figures 9 and 11 respectively. Referring now to Figure 10, each PEM fuel cell
module 100 includes a hydrogen distribution frame which is generally indicated
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by the numeral 110. The hydrogen distribution frame 110 is fabricated from a
substrate which has a flexural modulus of less than about 500,000 pounds per
square inch, and a compressive strength of less than about 20,000 pounds per
square inch. As such, any number of suitable or equivalent thermoplastic
materials can be utilized. The hydrogen distribution frame 110 includes a main
body 111 as seen in Figure 10. The main body has a first end 112, and an
opposite second end 113. Further, the main body is defined by a peripheral
edge 114. Positioned in a given location along the peripheral edge is a handle
115 which facilitates the convenient manual manipulation of the PEM fuel cell
io module 100. An elongated guide member or spine 116 is located on the first
and second ends 112 and 113 respectively. Each spine 116 is operable to be
matingly received in, or cooperate with, the respective elongated channels 44
and
45 which are formed in the top and bottom portions 35 and 40 of the subrack
30 (Figure 9). As should be understood, the alignment and mating receipt of
the individual spines 116 in the respective channels allows the individual PEM
fuel cell modules 100 to be slidably received and positioned in predetermined
spaced relation, one to the other, on the subrack 30. Such is seen most
clearly
by reference to Figure 2. When received on the subrack 30, the exhaust end
73 of the air plenum 71 is received between two adjacent PEM fuel cell
modules 100.
As seen in Figure 10, the main body 111 defines a plurality of
substantially opposed cavities 120. These cavities are designated as first,
second,
third, and fourth cavities 121, 122, 123, and 124 respectively. Still further,
and
referring again to Figure 10, a plurality of apertures 125 are formed in given
locations in the Win= body 111 and are operable to receive fasteners which
will
be discussed in further detail hereinafter. The main body 111 further defines
a pair of passageways designated generally by the numeral 130. The pair of
passageways include a first passage 131 which permits the delivery of hydrogen
gas from the source of same 60, to each of the cavities 121-124; and a second
passageway 132 which facilitates the removal of impurities, water and
unreacted
hydrogen gas from each of the cavities 121-124. A linking passageway 133
operably couples each of the first and second cavities 121, and 122, and the
third and fourth cavities 123 and 124 in fluid flowing relation one to the
other,
such that hydrogen gas delivered by means of the first passageway 131 may find
its way into each of the cavities 121-124. Each of the cavities 121 through
124
are substantially identical in their overall dimensions and shape. Still
further,
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each cavity has a recessed area 134 having a given surface area and depth.
Positioned in the recessed area 134 and extending substantially normally
outwardly
therefrom are a plurality of small projections 135. The function of these
individual projections will be discussed in greater detail below. As best seen
in
Figure 10, the first and second passageways 131 and 132 are connected in fluid
flowing relation relative to each of the recessed areas 134. Referring still
to
Figure 10, the peripheral edge 114 of the main body 111 is discontinuous. In
particular, the peripheral edge 114 defines a number of gaps or openings 136.
Referring now to Figure 11, each passageway 131 and 132 has a terminal end
lo 137 which has a given outside diametral dimension. The terminal end 137 of
each passageway 130 is operable to matingly interfit in fluid flowing relation
relative to the first and second valves 52 and 53 respectively.
Referring now to Figures 12, 13, 26, and 27, sealably mounted within the
respective cavities 121 through 124 respectively is a membrane electrode
diffusion
assembly 150 which is generally indicated by the numeral 150. The membrane
electrode diffusion assembly 150 has a main body or solid electrolyte membrane
151 which has a peripheral edge 152 which is sealably mounted to the hydrogen
distribution frame 110. The membrane electrode diffusion assembly 150 has an
anode side 153, and an opposite cathode side 154. The anode side 153 is held
in spaced relation relative to hydrogen distribution frame 110 which forms the
respective cavities 121-124 by the plurality of projections 135 (Figure 10).
This
special relationship ensures that hydrogen delivered to the respective
cavities 121-
124 reaches all parts of the anode side of the membrane electrode diffusion
assembly 150. Electrodes 160, comprising catalytic anode and cathode
electrodes
.161 and 162 are formed on the main body 151. These individual anode and
cathode electrodes 161 and 162 are disposed in ionic contact therewith. Still
further, a noncatalytic electrically conductive diffusion layer 170 is affixed
on the
anode and cathode electrodes 160 and has a given porosity. As best illustrated
in Figure 26, the noncatalytic electrically conductive diffusion layer 170 has
a
3o first diffusion layer 171 which is positioned in ohmic electrical contact
with each
of the electrodes 161 and 162 respectively, and a second diffusion layer 172
which is positioned in ohmic electrical contact with the underlying first
diffusion
layer. As best seen in Figure 27, a second form of the membrane electrode
diffusion assembly 150 is shown and wherein a third diffusion layer 173 is
provided. In this form, the third layer is affixed to the main body 151 prior
to affixing the first and second diffusion layers thereto. In this regard, a
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number of commercially available membrane electrode assemblies are fabricated
which have a preexisting proprietary diffusion layer attached to same, the
composition of which is unknown to the inventors.
Referring now to Figure 26, the membrane electrode diffusion assembly
150 and more specifically, the first diffusion layer 171 which is affixed
thereto
comprises a coating of particulate carbon suspended in a binding resin.
Further,
the second diffusion layer 172 comprises preferably a porous hydrophobic
carbon
backing layer. With respect to the binding resin, it is substantially
hydrophobic
and is selected from the group consisting essentiaily of perfluorinated
hydrocarbons or a substitute equivalent. Further, the first diffusion layer
171 has
about 20% to about 90% by weight of particulate carbon. With respect to the
second diffusion layer 172, it is selected from the group consisting
essentially of
carbon cloth, carbon paper, or carbon sponge or a substitute equivalent which
has been rendered hydrophobic. In the preferred form of the invention, the
first diffusion layer 171 is a composite coating formed of successive layers
of the
first diffusion layer, each of the successive layers having a given
hydrophobicity.
Additionally, the first diffusion layer 171 has a hydrophobic gradient. This
gradient can be altered by adjusting the hydrophobicity of the successive
layers
that form the composite coating. Depending upon the desired performance
parameters of the membrane electrode diffusion assembly 150 that is employed,
the successive layers closest to the second diffusion layer 172 may be the
least
hydrophobic of all the successive layers, or the most hydrophobic. To affix
the
first and second diffusion layers 171 and 172 to the underlying anode and
cathode electrodes 161 and 162, a thermoplastic binding agent can be utilized
and which is selerteo from the group consisting essentially of polyethylene or
wax, or a substitute equivalent. Still further, these same layers may be
attached
by pressure and heat. In the preferred form of the invention, the individual
anode and cathode electrodes 161 and 162 comprise particulate carbon; a
catalyst
such as platinum or the like; and a crosslinked copolymer incorporating
sulfonic
3o acid groups.
The method of forming the first and second diffusion layers 171 and 172,
as described above, is discussed in the paragraphs which follow. The method
of forming a diffusion layer 170 for use with a membrane electrode diffusion
assembly 150 comprises as a first step, providing a carbon backing layer
constituting a second diffusion layer 172. The carbon backing layer is
selected
from the group consisting essentially of carbon cloth, carbon paper, or carbon
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sponge. The subsequent steps in the method comprises applying a hydrophobic
polymer to the carbon backing layer constituting the second diffusion layer
172;
and sintering the carbon backing layer constituting the second diffusion laAr
at
a temperature greater than the melting point of the hydrophobic polymer. As
discussed above, the hydrophobic polymer is selected from the group consisting
essentially of perfluorinated hydrocarbons or a substitute equivalent. Still
further,
in the method as described, the sintering step takes place at a temperature of
about 275 degrees to about 365 degrees C. The preferred method of forming
the diffusion layer 170 for use with the membrane electrode diffusion assembly
io 150 comprises providing a porous carbon backing layer constituting the
second
diffusion layer 172; and applying a porous coating comprising a slurry of
particulate carbon, a binding resin and a delivery fluid which is applied on
the
porous carbon backing layer. The porous carbon backing layer constituting the
second diffusion layer 172 is the same as was described above. Further, the
binding resin is hydrophobic and may include perfluorinated hydrocarbons. The
porous coating comprises at least about 20% to about 90% by weight of the
particulate carbon. The delivery fluid utilized to form the slurry of
particulate
carbon and the binding resin comprises water, and a compatible surfactant. In
this regard, the delivery fluid consists essentially of about 95% to about 99%
by
weight of water; and about 1% to about 5% by weight of the compatible
surfactant. The surfactant is selected from the group consisting essentially
of
ammonium hydroxide. and/or alcohol. In the examples which follow, the delivery
fluid utilized consists of a solution of 2-butoxyethanol and ammonium
hydroxide
as the surfactants. A solution such as this may be commercially secured. In
the present instaqce, the inventors used a commercially available cleaner with
properties such as "Windex". Windex is the registered trademark of S.C.
Johnson
and Sons. After the delivery of the slurry which includes the binding resin
and
particulate carbon, the method further comprises removing the delivery fluid
thereby leaving behind or depositing the particulate carbon and the binding
resin
on the porous carbon backing layer constituting the first diffusion layer 171.
The delivery fluid is removed by applying heat energy to same which
facilitates
the evaporation of the delivery fluid.
In another alternative form of the invention, the binding resin and porous
carbon coating slurry as described above, may be applied in successive coats
thereby creating a hydrophobic gradient. This hydrophobic gradient, as earlier
discussed, may be adjusted by altering the hydrophobicity of each of the
CA 02578111 2007-02-27
= 15
successive coats. To achieve this adjustable hydrophobic gradient, binding
resins
are selected from the group consisting essentially of hydrophobic and
hydrophilic
binding resins or a substitute equivalent. As should be appreciated, the given
hydrophobicity of each coat forming the composite first diffusion layer 171
may
be adjusted by providing a predetermined mixture of the hydrophobic and
hydrophilic binding resins in the resulting slurry or by altering the
proportional
relationship of the components. As was discussed above, each of the coatings
of the composite first diffusion layer 171 which are applied closest to the
porous
carbon backing layer may be the most hydrophilic or the least hydrophilic
i0 depending upon the performance characteristics desired for the membrane
electrode diffusion assembly 150. Still further, the method may include a
sintering step whereby the resulting diffusion layer 170 is sintered at a
temperature effective to melt the binding resin and create a substantially
homogeneous surface. In addition to the foregoing, the method further
I5 comprises, after the sintering step, applying a predetermined pattern of
pressure
of a given value to the diffusion layer 171, and which is effective to vary
the
porosity of the resulting diffusion layer 170. As was discussed above, the
diffusion layer 170 may be attached to the underlying catalytic anode and
cathode electrodes 162 and 163 respectively by utilizing a thermoplastic
binding
20 emulsion which is selected from the group consisting essentially of
polyethylene
or wax or alternatively by utilizing heat and pressure.
The diffusion layer 170, described above, is useful in 'managing the water
which is generated as a result of the chemical reaction taking place in each
of
the PEM ftiel cell modules 100. In this regard, the inventors have discovered
25 that the diffusion.layer 170 allows sufficient water to escape from the
cathode
side of the membrane electrode diffusion assembly 150 such that the PEM fuel
cell module 100 does not "flood out" with water thereby inactivating same. On
the other hand, the hydrophobic gradient, as described above, facilitates the
retention of sufficient moisture such that the PEM fuel cell module 100
becomes
30 self-humidifying, that is, sufficient water is retained in the membrane
electrode
diffusion assembly 150 such that it achieves substantially the maximum current
density possible without the addition of extra moisture or humidification from
outside of the PEM fuel cell module 100. Still further, the air distribution
assembly 70 and air mixing valve 80 provides a convenient means by which
35 outside ambient air may be added to air which has previously passed through
each of the PEM fuel cell modules 100 thereby maintaining the PEM fuel cell
CA 02578111 2007-02-27
16
modules 100 in a properly humidified state. As should be understood, this
mixing of air
effectively removes water from the cathode side of membrane electrode
diffusion assembly
150. Additionally, the same mixing of air effectively removes heat energy
which is a by-
product of the chemical reaction taking place in each of the PEM fuel cell
modules 100 and
thus maintains the PEM fuel cell modules at a stable temperature. In this
regard, the air
delivered at the exhaust end 73 of the air plenum 71 constitutes the cathode
air flow, and in
the present invention 10 a novel feature of the power system 10 is that a
preponderance of the
heat energy produced by each of the PEM cell modules 100 is removed from same
by this
cathode air flow.
Examples of forming the diffusion layer 170 on an underlying main body 151 of
the
membrane electrode diffusion assembly 150 is set forth below.
The examples set forth hereinafter relate to the fabrication of the diffusion
layer 170
as seen in Figures 26 and 27, respectively.
General Test Procedures
A hydrogen/air fuel cell test fixture was fabricated from a two-part stainless
steel
fixture which encloses a 4 cm x 4 cm proton conductivity membrane electrode
diffusion
assembly (MEDA) for testing. The hydrogen side of the block (anode) defines a
cavity which
contains a flat, perforated ceramic plate. Pressure conditions effective to
affix top of this plate
is a matching perforated platinum coated nickel current collector. Hydrogen
gas passes into
the anode half of the stainless steel fixture, through the holes in the
ceramic plate and the
associated current collector. The hydrogen is thus able to reach the anode of
the MEDA,
which is placed on top of the anode current collector.
The proton-conducting MEDA, which is purchased from the W.L. Gore Company
under the trade designation PrimeaTM 6000 Series is larger than the electrodes
which are
affixed to same, the MEDA, having dimensions of about 5 cm x 5 cm. This allows
for the
placement of a sealing gasket around the periphery of the electrodes when the
stainless steel
test fixture is bolted together.
The cathode side of the test fixture also defines a cavity which matingly
receives a
perforated ceramic plate and current collector. However, the stainless steel
fixture does not
press the current collector against the MEDA directly. Instead, five screws
are mounted on
the test fixture cathode side. These screws press against a perforated metal
pressure plate. The
plate has apertures which
CA 02578111 2007-02-27
17
are substantially coaxially aligned with the holes formed in the ceramic
plate.
These screws further press against the ceramic plate and the current
collector.
By threadably advancing the screws, the current collector contact pressure
relative
to the MEDA can be selectively adjusted after the stainless steel test fixture
has
been bolted together.
A supply of air is provided at the cathode, by means of several holes
which have been machined into the stainless steel fixture between the
aforementioned pressure screws. This allows air to travel past the screws and
the perforated steel pressure plate, ceramic plate, and current collector to
the
i0 cathode side of the MEDA.
The test MEDAs were placed over the cathode of the test fixture along
with a sealing gasket. The test fixture is then bolted together. The pressure
screws are then threadably advanced until sufficient force has been generated
at
the current collector cathode/anode interfaces for good electrical contact.
Once
this has been accomplished, the hydrogen gas is supplied at a pressure of
about
5 PSI. The anode side of the MEDA is then purged of any air. A supply
of fresh air is then supplied to the cathode side of the MDEA by means of
a fan or the like. The supply of air had a dewpoint of 15 degrees Celsius.
Electrical performance is tested by loading the fuel cell with a variety of
resistors. Since the resistor values are known, the current can be computed by
examining the voltage across the resistors. MEDAs are initially short-
circuited
e
to condition them, and then are allowed to stabilize at a given load of
usually
about 0.6 volts. When a steady-state power output has been obtained, the data
is gathered.-
. For comparative testing, the diffusion layers 170 which are affixed on the
cathode and anode sides of MEDA are often dissimilar. A PEM fuel cell's
electrical performance is largely unaffected when the configuration of the
anode
diffusion layer is changed. However, the diffusion layer placed on the cathode
side of the MEA, on the other hand, has a significant impact on the electrical
performance of the PEM fuel cell because water production, and evaporation of
same, must occur on the cathode side of the MEDA. As earlier noted, the
fabrication method described above includes subjecting the diffusion layer 170
to
given temperature and pressure conditions effective to affix the diffusion
layer
170 to the underlying MEA thus forming the MEDA. In this regard, the same
pressure is applied to the cathode and anode sides of the MEDA. The most
accurate comparison between two different diffusion layers made by the
foregoing
CA 02578111 2007-02-27
18
methods is done by using a single MEDA In this regard, comparative testing
between
dissimilar layers is done by simply flipping the MEDA over in the test
fixture, thus reversing
the anode and cathode sides of the MEDA. Therefore, the same MEDA is tested
with
different diffusion layers acting as the cathode.
EXAMPLE 1
A sheet of carbon paper (Toray TGP-H-060) was dipped into a solution of 2:3
Teflon-
120TM (Dupont) and deionized water for several minutes. Teflon-120T"' 1 is a
hydrophobic
polymer comprising polytetrafloroethylene. Teflon-120TM is a trade designation
of the E.I.
Dupont Company. After removal from the solution the carbon paper was allowed
to dry in a
horizontal position on top of an open cell foam sponge. The carbon paper was
then placed in
an air-filled sintering oven (360 C) for 3-5 minutes. This rendered the
carbon paper
hydrophobic.
Diffusion Layer Side "A".
A solution comprising water, and a compatible surfactant was then prepared. In
this
regard, 200 ml of a commercially available cleaner "WindexTM" was mixed with
4.2 g Vulcan
XC-72R (Cabot) particulate carbon powder. The mixture was sonicated for 90
seconds at a
power of about 200 watts, using a stir-bar to agitate the mixture during
sonication and create a
slurry. After sonication, 1 ml of a hydrophobic polymer, Teflon-30TM (Dupont),
was added.
The slurry was then sprayed onto the carbon paper with an air brush using
multiple passes.
Once the carbon paper was wetted with the solution, it was placed on a hot
plate to evaporate
the solution comprising the water and surfactant. The spraying/drying process
was then
repeated. As will be appreciated, the process of evaporation deposited the
particulate carbon
2s and associated binding resin until a final (dried) added weight of 6.4
mg/cm2 had been
achieved. Finally, the coated carbon paper was loaded into the air-filled
sintering oven at
360 C for 3-5 minutes. This sintering melted the binding resin and created a
substantially
homogeneous surface.
Diffusion Layer Side "B".
Side 'B" was fabricated in approximately the same manner as side "A" described
above.
However, side "B" was placed into a press for 10 seconds and was subjected to
three tons of
force. An irregular surface was placed on top of the diffusion layer side 'B"
prior to subjecting
it to pressure. In the present
CA 02578111 2007-02-27
19
= test a 150-grit sandpaper with an aluminum foil spacer sheet was utilized.
The spray-on side
faced the sandpaper/foil during the pressing stress.
MEA Fabrication:
The diffusion layers were affixed to a commercially available membrane
electrode
diffusion assembly such as that which may be secured from the W. L. Gore
Company under
the trade designation PrimeaTM Series 6000. This was done by placing the
assembly in a hot
press. The MEDA was hot pressed several times using successively higher
pressures. Data
was taken between successive presses.
Results:
Each sample was loaded into the test fixture described above. Temperatures
were
measured at the diffusion layer surface and were within a range of +/- 2 C.
These
temperatures were controlled by varying the cathode air flow. Values are
current density as
expressed in mA/cm2 at 0.6 volts. Sides "A" and "B" as noted below refer to
that side acting
as the cathode.
ress Method 36 C 45 C 53 C
A B A B A B
5 tons, 190 C, 20 sec - - - - - -
tons, 190 C, 20 sec 318 319 293 325 277 316
5 tons, 180 C, 30 sec 372 363 350 353 322 322
5 tons, 180 C, 30 sec 399 371 363 374 319 361
5.5 tons, 180 C, 30 sec 338 363 375 394 - -
tons, 170 C, 40 sec 269 250 356 380 363 354
Conclusions: The patterned-press (side "B") yields slightly better or equal
performance at
45 C and 53 C at 8 of the 9 comparative data points. Good performance is
also obtained
with side "A".
CA 02578111 2007-02-27
EXAMPLE 2
A sheet of carbon paper (Toray TGP-H-060) was dipped into a solution
of 2:3 Teflon-120 (Dupont) and deionized water solution for several minutes.
As earlier noted, Teflon-120 is a trade designation of E.I. Dupont Company.
5 After removal from the solution the carbon paper was allowed to dry in a
horizontal position on top of an open cell foam sponge. The carbon paper was
then placed in an air-filled sintering oven (360 C) for 3-5 minutes. The heat
energy melted the polytetrafluoroethylene (Teflon-120) thus making a
substantially
homogeneous surface. This sintering rendered the carbon paper hydrophobic.
Diffusion layer side "A":
A slurry was then prepared utilizing water and a compatible surfactant
such as ammonia or the like. In the present example the slurry was prepared
by mixing 200 ml of a commercially available cleaner "Windex" with 4.2 g
Vulcan
XC-72R (Cabot) particulate carbon powder. The slurry was then sonicated for
90 seconds at a power of about 200 watts, using a stir-bar to agitate the
slurry
during sonication. After sonication, 1 ml of hydrophobic polymer solution
Teflon-
30 (Dupont) was added. The slurry was then sprayed onto the carbon paper
. with an air brush, 4sing multiple passes as was described earlier. Once the
carbon paper had been wetted with the slurry, it was placed on a hot plate to
evaporate the solution of water and surfactant. The spraying/drying process
was
then repeated until a final (dried) added weight of 6.4 mg/cm2 had been
achieved. Side "B" was subsequently placed into a press for 10 seconds and
subjected to three tons of force. As with the first example, an irregular
surface
was utilized between the press and the sprayed on layers. In this example, a
150-grit sandpaper with an aluminum foil spacer sheet was employed. The spray-
on side faced the sandpaper/foil combination during pressing.
CA 02578111 2007-02-27
21
Diffusion layer side "B":
A similar slurry was prepared by mixing 200 ml of "Windex" with 4.2 g
Vulcan XC-72R (Cabot) carbon powder. The mixture was sonicated for 90
seconds at a power of about 200 watts, using a stir-bar to agitate the mixture
during sonication. After sonication, a 1.2 mi solution of Teflon-30 (Dupont)
was added to the slurry. The mixture was then sprayed onto the carbon paper
with an air brush using multiple passes. Once the carbon paper had been
wetted with the solution, it was placed on a hot plate to evaporate the water
and surfactant solution (Windex). The spraying/drying process was then
repeated
io until a final (dried) added weight of 4.91 mg/cm2 had been achieved.
A second slurry was then prepared by mixing 200 ml of "Windex" with
4.2 g Vulcan XC-72R (Cabot) carbon powder. The slurry was sonicated for 90
seconds at a power of about 200 watts, using a stir-bar to agitate the mixture
during sonication. After sonication, 0.5 ml of Teflon-30 (Dupont) was added.
The slurry was then sprayed onto the previously coated carbon paper with an
air brush using multiple passes. Once the carbon paper had been wetted with
the slurry, it was placed on a hot plate to evaporate the water and surfactant
and thus deposit the hydrophobic polymer and the particulate carbon. The
spraying/drying precess was then repeated until a final (dried) added weight
of
1.58 mg/cm2 had been achieved. The total weight of the spray-on layers was
6.5 mg/ cm2. Side "B" was placed into the press for 10 seconds at 3 tons of
force underneath an irregular surface (150-grit sandpaper with an aluminum
foil
spacer sheet). The spray-on side faced the sandpaper/foil combination during
pressing.
CA 02578111 2007-02-27
22
MEA Fabrication:
The diffusion layers were affixed to a commercially available MEDA such
as what was described in Example 1, above. This was done by placing the
assembly in the hot press. The MEDA was hot pressed several times using
successively higher pressures. Data was taken between successive presses.
Results:
Each sample was loaded into the test fixture as described earlier.
Temperatures as noted below were measured at the diffusion layer surface and
lo are within a tolerance of +/- 2 C. The values which are set forth are
expressed in mA/cm2 at 0.6 volts. Sides "A" and "B" refer to that side acting
as the cathode. For this sample, as identified, hot pressing involved two
identical steps at the same pressure and rotating the MEDA 180 degrees
between each pressing.
Press Method 36 C 45 C 53 C
A B A B A B
2x3.5 tons, 170 C, 40 s 238 219 318 219 277 244
2x4 tons, 170 C, 40 s 263 263 331 356 331 325
2x4.5 tons, 170 oC; 40 s 206 206 338 344 325 356
2x5 tons, 170 C, 40 s 219 219 325 338 325 356
2x5.5 tons, 170 C, 40 s 188 213 263 269 244 256
The results demonstrate that the reverse gradient samples, when subjected to
greater than 3.5 tons pressure, produces current densities which are equal to
or
better than the non-gradient samples in 11 of 12 comparative tests.
CA 02578111 2007-02-27
23
EXAMPLE 3
A sheet of carbon paper (Toray TGP-H-090) was dipped into a solution
of 4:9 Teflon-120 (Dupont) and deionized water for several minutes. After
removal from the solution the carbon paper was allowed to dry in a horizontal
position on top of an open cell foam sponge. The carbon paper was then
placed in an air-filled sintering oven (360 C) for 3-5 minutes. This rendered
the carbon paper hydrophobic.
Diffusion layer side "A":
A slurry was then prepared by mixing 200 ml of "Windex" with 4.2 grams
of Vulcan XC-72R (Cabot) carbon powder. This is identical to the previous
examples. The slurry was then sonicated for 90 seconds at a power of about
200 watts, using a stir-bar to agitate the mixture during sonication. After
sonication, a 1.2 ml solution of Teflon-30 (Dupont) was added to the slurry.
The slurry was then sprayed onto the carbon paper with an air brush using
multiple passes. Once the carbon paper had been wetted with the solution, it
was placed on a hot plate to evaporate the water and surfactant solution
(Windex) and thus deposit the hydrophobic polymer and particular carbon. The
.spraying/drying process was then repeated until a final (dried) added weight
of
1.0 mg/cm2 had been achieved.
A second slurry was then prepared by mixing 200 ml of "Windex" with
4.2 grams of Vulcan XC-72R (Cabot) carbon powder. The slurry was then
sonicated for 90 seconds at a power of about 200 watts, using a stir-bar to
agitate the mixture during sonication. After sonication, 0.5 ml of a solution
of Teflon-30 (Dupont) was added. The slurry was then sprayed onto the
previously coated carbon paper with an air brush using multiple passes. Once
the carbon paper had been wetted with the slurry, it was placed on a hot plate
CA 02578111 2007-02-27
= 24
to evaporate the water and surfactant solution (Windex), and thereby deposit
the
hydrophobic polymer and particulate carbon. The spraying/drying process was
-
repeated until a final (dried) added weight of 0.5 mg/cm , ' had been
achieved.
The total weight of the spray-on layers is approximately 1.5 mg/ cm2
Diffusion layer side "B":
Side "B" was prepared exactly the same as side "A". However, side
"B" was placed into a press for 10 seconds and subjected to three tons of
force
underneath an irregular surface (150-grit sandpaper with an aluminum foil
spacer
io sheet). The spray-on side faced the sandpaper/foil combination during
pressing.
MEA Fabrication:
The diffusion layers were affixed to a commercially available MEDA as
was discussed in Example 1, above. This was done by placing the MEDA in
a hot press. The MEDA was hot pressed several times using successively higher
pressures. Data was taken between successive presses.
r
Results:
Each sample , was loaded into the aforementioned test fixture. The
temperature as noted below was measured at the diffusion layer surface and is
within a tolerance of +/- 2 C. The values below are expressed in mA/cm2 at
0.6 volts. Sides "A" and "B" refer to that side acting as the cathode. For
this sample, hot pressing usually involved two identical steps at the same
pressure. The MEDA was rotated about 180 degrees between each pressing.
CA 02578111 2007-02-27
Press Method 36 C 45 C 53 C
A B A B A B
2x3.5 tons, 170 C, 40 s - - - - - -
2x4 tons, 170 C, 40 s 363 316 369 338 350 319
5 2x4.5 tons, 170 C, 40 s 363 - 363 - 325 -
2x5 tons, 170 C, 40 s 413 - 416 - 400 -
4x5.5 tons, 170 C, 30 s 388 - 419 - 413 -
lo The data above further confirms the novelty of the diffusion layer 170 and
associated membrane electrode diffusion assembly construction 150.
EXAMPLE 4
Diffusion Layer Side "A"
15 A sheet of carbon paper (Toray TGP-H-060) was dipped into a solution
of 4:9 Teflon-120 (Dupont) and deionized water for several minutes. After
removal from the solution the carbon paper was allowed to dry in a horizontal
position on top of an open cell foam sponge. The carbon paper was then
, placed in an air-filled sintering oven (360 C) for 3-5 minutes. This
sintering
20 rendered the carbon paper hydrophobic.
A slurry was then prepared utilizing water and a compatible surfactant
such as ammonia "or" the like. In the present example the slurry was prepared
by mixing 200 ml of a commercially available cleaner "Windex" with 4.2 g
Vulcan XC-72R (Cabot) carbon powder. The slurry was sonicated for 90 seconds
25 at a power of about 200 watts, using a stir-bar to agitate the slurry
during
sonication. After sonication, 1.2 ml of hydrophobic polymer solution Teflon-30
(Dupont) was added. The mixture was then sprayed onto the carbon paper with
an air brush using multiple passes as was described earlier. Once the carbon
paper had been wetted with the slurry, it was placed on a hot plate to
evaporate the solution of water and surfactant. The spraying/drying process
was
CA 02578111 2007-02-27
26
then repeated until a final (dried) added weight of 4.2 mg/cm2 had been
achieved.
A second slurry was then prepared by mixing 200 ml Windex with 4.2 g
Vulcan XC-72R (Cabot) carbon powder. The slurry was sonicated for 90 seconds
at a power of about 200 watts, using a stir-bar to agitate the slurry during
sonication. After sonication, 0.5 ml of Teflon-30 (Dupont) was added. The
slurry was then sprayed onto the previously coated carbon paper with an air
brush using multiple passes. Once the carbon paper had been wetted with the
slurry, it was placed on a hot plate to evaporate the water and surfactant.
The
spraying/drying process was then repeated until a final (dried) added weight
of
1.6 mg/cm2 had been achieved. The total weight of the spray-on layers was
5.8 mg/cm2
Diffusion Layer Side "B"
A sheet of carbon paper (Toray TGP-H-090) was dipped into a solution
of 4:9 Teflon-120 (Dupont):DI water for several minutes. After removal from
the solution the carbon paper was allowed to dry in a horizontal position on
top
of an open- cell foam sponge. The carbon paper was then placed in an air-
. filled sintering ove,tl (360 C) for 3-5 minutes. This rendered the carbon
paper
hydrophobic.
A slurry was then prepared utilizing water and a compatible surfactant
such as ammonia or the like. The slurry was prepared by mixing 200 ml of a
commercially available cleaner "Windex" with 4.2 g Vulcan XC-72R (Cabot)
carbon powder. The slurry was sonicated for 90 seconds at a power of about
200 watts, using a stir-bar to agitate the slurry during sonication. After
sonication, 1.2 ml of hydrophobic polymer solution Teflon-30 (Dupont) was
added.
The mixture was then sprayed onto the carbon paper with an air brush using
CA 02578111 2007-02-27
27
multiple passes. Once the carbon paper had been wetted with the slurry, it was
placed on a hot plate to evaporate the solution of water and surfactant. The
spraying/drying process was then repeated until a final (dried) added weight
of
1.0 mg/cm2 had been achieved.
A second slurry was then prepared by mixing 200 ml Windex with 4.2 g
Vulcan XC-72R (Cabot) carbon powder. The slurry was sonicated for 90 seconds
at a power of about 200 watts, using a stir-bar to agitate the slurry during
sonication. After sonication, 0.5 ml of Teflon-30 (Dupont) was added. The
slurry was then sprayed onto the previously coated carbon paper with an air
1o brush using multiple passes. Once the carbon paper had been wetted with the
slurry, it was placed on a hot plate to evaporate the water and surfactant.
The
spraying/drying process was then repeated until a final (dried) added weight
of
0.5 mg/cm2 had been achieved. The total weight of the spray-on layers was 1.5
mg/cm2'
MEDA Fabrication and Testing
r
The respective diffusion layers were affixed to a commercially available
membrane electrode assembly, such as that which may be secured from the W.L.
= Gore Company unde~ the trade designation Primea Series 6000. This was done
2o by placing the assembly in a hot press. The 60 cm2 MEDA was hot pressed
once for 4 minutes at 150 C at a pressure of 32 tons, and then re-pressed
for an additional minute at 150 C at a pressure of 37 tons. The process was
repeated to fabricate four MEDAs.
Results
A PEM fuel cell module 100 was fabricated using the four MEDAs. The
fuel cell module 100 was configured as shown in Figures 10, 11, and 14, except
CA 02578111 2007-02-27
28
that the ceramic plate 205 was deleted, and the pressure transfer assembly 203
directly contacted the cathode current collector 192. The fuel cell module 100
was tested by inserting it into a test stand similar to that illustrated in
Figure
5, and using a small fan to pass approximately 12 cubic feet per minute of air
through the fuel cell module. The hydrogen feed pressure was set to about 8
psi. At 2.004 volts (approximately 0.5 volts per MEDA), a current of 24.0
amperes was measured using a calibrated DC current transducer, which yielded
a current density of 400 mA/ cm2 and a PEM fuel cell module power of 48.096
watts.
The main body 151 of the membrane electrode diffusion assembly 150, as
earlier discussed, comprises an electrolyte membrane having substantially
linear
crosslinked polymeric chains incorporating sulfonic acid groups. In
particular, the
crosslinked polymeric chains are formed from monomeric units which are
selected
from the group consisting essentially of poly (ethylene glycol) methacrylate,
poly
(propylene glycol) methacrylate, poly (ethylene glycol) ethyl ether
methacrylate,
and poly (propylene glycol) methyl ether methacrylate, hydroxpropyl
methacrylate,
r
2-hydroxyethyl methacrylate, the acrylate analogs, and 4-hydroxybutyl
acrylate.
Linear copolymers composed of similar monomeric units, but synthesized without
crosslinking agent s will be described below.
Still further, the sulfonic acid groups are selected from the group
consisting essentially of 3-alkoxy-2-hydroxy-l-propanesulfonic acid, 4-
styrenesulfonic
acid, vinylsulfonic acid, 3-sulfopropyl methacrylate, 3-sulfopropylacrylate
and
fluorinated derivatives thereof. Additionally, the crosslinking agent utilized
to
crosslink the copolymers is selected from the group consisting essentially of
ethylene glycol divinyl ether, diethylene glycol divinyl ether, triethylene
glycol
divinyl ether, ethylene glycol dimethacrylate, diethylene glycol
dimethacrylate,
CA 02578111 2007-02-27
= 29
triethylene glycol dimethacrylate, glycerol dimethacrylate, diallyloxyacetic
acid, and
allylmethacrylate.
In the preferred form of this invention, the membrane electrode diffusion
assembly 150 comprises:
about 35% to about 50% by molar concentration of a methacrylate
monomer;
about 30% to about 50% by molar concentration of an acrylate monomer;
about 25% to about 45% by molar concentration of a sulfonic acid; and
about 5% to about 20% by molar concentration of a compatible
io crosslinking agent.
In the preferred form of the invention as described above, the electrolyte
membrane, or main body 151, which is incorporated into the membrane electrode
diffusion assembly 150, has a glass transition temperature of at least about
110
degrees C, and has a preferred thickness of about 0.2 millimeter.
Additionally,
this electrolyte membrane 151 must be substantially stable in the presence of
water, and operational at temperatures of less than about 80 degrees C. The
r
electrolyte membrane 151, as noted above, may further comprise a compatible
plasticizer which is selected from the group consisting essentially of
phthalate
esters. In still Vother form of the invention, the electrolyte membrane 151
includes a porous supporting matrix which is made integral with the
electrolyte
membrane 151, and which provides mechanical strength to same. In this regard,
the porous supporting matrix does not reactively produce hydrogen ions and is
dielectric. Further, the porous supporting matrix is substantially inert and
has
a porosity of about 30% to about 80% and has a given proton conductivity
which is proportional to the porosity of the supporting matrix. An acceptable
porous supporting matrix may be selected from the group consisting essentially
of grafted hydrophilic polyethylenes.
CA 02578111 2007-02-27
In its most preferred form, the electrolyte membrane 151 of the present
invention comprises at least about 10% to about 50% by molar concentration of
a copolymer which has monomeric units which are selected from the group
consisting essentially of poly (ethylene glycol) methacrylate, poly (propylene
glycol)
5 methacrylate, poly (ethylene glycol) ethyl ether methacrylate, poly
(propylene
glycol) methyl ether methacrylate, hydroxypropyl methacrylate, 2-hydroxyethyl
methacrylate, acrylate analogs and 4-hydroxybutyl acrylate;
at least about 25% to about 45% by molar concentration of an acid
selected from the group consisting essentially of 3-alkoxy-2-hydroxy-l-
io propanesulfonic acid, 4-styrenesulfonic acid, vinylsulfonic acid, 3-
sulfopropyl
methacrylate, 3-sulfopropylacrylate and fluorinated derivatives thereof;
at least about 5% to about 20% by molar concentration of a crosslinking
agent selected from the group consisting essentially of ethylene glycol
divinyl
ether, diethylene glycol divinyl ether, triethylene glycol divinyl ether,
ethylene
15 glycol dimethacrylate, diethylene glycol dimethacrylate, triethylene glycol
dimethacrylate, glycerol dimethacrylate, diallyloxyacetic acid, and
allylmethacrylate;
r_
a compatible plasticizer; and
a support matrix having a given minimum porosity, and which is dielectric.
As discussed above, one example of a suitable electrolyte membrane 151
20 may be secured from the W. L. Gore Company under the trade designation
Primea Series 6000 MEA.
Representative examples which concern the synthesis of the electrolyte
membrane 151 are set forth below.
25 EXAMPLE 1
10.56 mL of a 15.78% w/v aqueous solution (8 mmol) of 3-sulfopropyl
methacrylate was first concentrated so as to yield a final reaction mixture
with
CA 02578111 2007-02-27
31
a water content of 16.3% v/v. Poly(propylene glycol) methacrylate (2.9600 g, 8
mmol), hydroxypropyl methacrvlate (0.14422 g, 1 mmol), and glycerol
dimethacrylate (0.4565 g, 2 mmol) were added to, and well mixed with the
concentrated acid solution. The mixture was cooled to 4 degrees C, and then
cold ethylene glycol divinyl ether (0.2283 g, 2 mmol), and ammonium persulfate
(0.5052 g, 2.2 mmol) dissolved in 0.72 mL of water were added. After thorough
mixing, the reaction mixture was de-aerated and applied onto grafted
polyethylene
(E15012) that was previously rendered hydrophilic. Photochemical
polymerization
was achieved under UV light for 10 minutes.
EXAMPLE 2
A 15.78% w/v aqueous solution of 3-sulfopropyl methacrylate (7.92 mL,
6 mmol) was concentrated so as to obtain a final reaction mixture with 17.7%
v/v water, and poly (propylene glycol) meth-acrylate (5.1800g, 14 mmol) was
then
added and well mixed. Benzoyl peroxide (0.4844g, 2 mmol) and 1,1-azobis(1-
cyclohexanecarbonitrile) (0.4884g, 2 mmol) were dissolved in acetone (4 mL)
and
added. The reaction mixture was then de-aerated under vacuum and substantially
all of the acetone was removed. Thermal polymerization was effected at 71-74
. degrees C. for 90, nqinutes. After cooling to room temperature overnight,
the
product crystallized into bundles of needle-shaped crystals. Similar linear
polymers were also synthesized by using hydroxypropyl methacrylate (14 mmol)
and poly(ethylene glycol) methacrylate (14 mmol), in place of the poly
(propylene
glycol) methacrylate.
EXAMPLE 3
A 35% aqueous solution of 3-allyloxy-2-hydroxy-l-propanesulfonic acid
(21.60 mL, 40 mmol) was concentrated so as to yield a final reaction mixture
CA 02578111 2007-02-27
32
containing 120.9% v/v water. Poly(propylene glycol) methacrylate (11.1000 g,
30
mmol), hydroxpropyl methacrylate (2.8834g, 20 mmol), and diethylene glycol
dimethacrylate (2.4227 g, 10 mmol) were added, and well mixed with the acid.
The initiator ammonium persulfate (1.1410 g, 5 mmol) was dissolved in the
proper amount of water and added. After thorough mixing, the reaction mixture
was de-aerated, and either thermally polymerized in a mold at 75 degrees C.
for
90 minutes or photochemically polymerized under UV light for 10 minutes using
grafted and hydrophilized polyethylene as a support material. Poly(ethylene
glycol) methacrylate was also used as a substitute for poly(propylene glycol)
io methacrylate, and the crosslinked mixture consisting of glycerol
dimethacrylate (or
diallyloxyacetic acid) and ethylene glycol divinyl ether was also used as a
substitute for diethylene glycol dimethacrylate.
Each electrolyte membrane synthesized from the examples, above, were
tested and were found to yield the performance characteristics as earlier
discussed.
As seen in Figures 12-19 and 28, the proton exchange membrane fuel cell
power system 10 of the present invention further includes a pair of current
collectors 190 which are received in each of the respective cavities 121
through
124, respectively.. The current collectors for identification have been given
the
numerals 191 and 192, respectively. The current collectors 190 are
individually
disposed in juxtaposed ohmic electrical contact with the opposite anode and
cathode sides 153 and 154 of each membrane electrode diffusion assembly 150.
As best seen in Figure 28, each current collector 190 has a main body 193
which has a plurality of apertures or open areas formed therein 194. In this
regard, the main body has a given surface area of which, at least about 70%
is open area. A conductive member 195 extends outwardly from the main body
and is operable to extend through one of the openings or gaps 136 which are
CA 02578111 2007-02-27
33
formed in the hydrogen distribution frame 110. Such is seen in Figure 11.
Each conductive member 195 is received between and thus electrically coupled
within one of the 8 pairs of conductive contacts 51 which are mounted on the
rear wall 41 of the subrack 30. This is illustrated most clearly by reference
to
Figures 3 and 4.
As a general matter, the current collectors 190 comprise a base substrate
forming a main body 193, and wherein a coating(s) or layer(s) is applied to
same and which is effective in maintaining electrical contact with the
adjacent
membrane electrode diffusion assembly 150. The main body 193 includes four
ia discrete components. The first component is an electrically conductive
substrate
which may be capable of surface passivation if exposed to oxygen. Suitable
materials which may be used in this discrete component of the main body 193
include current carrying substrates consisting essentially of 3XX Series
chromium
containing stainless steel or equivalent substitutes. These substrates have a
bulk
conductivity of about 2.4 IACS, and an overall thickness of about 0.7 to about
3mm. Additionally, copper or nickel substrates having a bulk conductivity of
greater than about 24% IACS, and a thickness of about 0.20 to about 1.3 mm.
may be used with equal success.
The second. component, which may be utilized in forming the main body
193 comprises a protection layer formed in covering relation over the
conductive
substrate, and which will passivate if inadvertently exposed to oxygen.
Suitable
materials for this second component include a foil cladding of 3XX Series
chromium-containing stainless steel which has a bulk conductivity of about
2.4%
IACS, and a thickness of about 0.02 to about 0.15 mm; or a coating or alloy
combination consisting essentially of column IVB metal(s) such as tantalum and
niobium which form a highly passivated pentoxide if exposed inadvertently to
air.
CA 02578111 2007-02-27
34
This coating or alloy combination has a thickness of about 0.2 to about 2
microns.
The third component forming the main body 193 comprises an electrically
conductive contact layer which is galvanically cathodic, and oxygen stable.
Suitable materials which may be utilized in the contact layer include coatings
formed from elements classified in column IVB and which are capable of forming
nitrides. Examples of these materials include titanium or zirconium nitride,
or
an electrically conductive metal oxide such as indium-tin oxide (ITO). An
equivalent substitute material includes platinum group metals such as
palladium,
platinum, rhodium, ruthenium, iridium and osmium. This third component has
a thickness of about 0.2 to about 2 microns.
The fourth component forming the main body 193 comprises an
electrolyte/oxygen exclusion layer. A suitable material for this function
includes
a graphite-filled electrically conductive adhesive. Such may be synthesized
from
a two-part epoxy or a silicone rubber.
Many combinations of the four components may be fabricated to produce
a suitable main body 193. Each main body 193 will have an electrically
conductive substrate. The assorted combinations of the other three components
which are used thgrewith are not specifically set forth below, it being
understood
that not less than one of the remaining three components, and not more than
all three remaining components must be brought together to form a suitable
main body 193. In the preferred embodiment, the inventors have discovered that
a main body 193 which is formed from an electrically conductive substrate,
such
as nickel or copper; a foil cladding comprising 3XX series chromium-containing
stainless steel; and a coating formed from column IVB electrically conductive
materials which can form nitrides operates with good results.
CA 02578111 2007-02-27
Referring now to Figures 12-19 and 22-25, respectively, the proton exchange
membrane fuel cell power system 10 of the present invention further includes
individual
force application assemblies 200 for applying a given force to each of the
pair of current
collectors 190, and the membrane electrode diffusion assembly 150 which is
sandwiched
5 therebetween. In this regard, the individual force application assemblies
are best illustrated by
reference to Figures 13, 15, 17 and 19, respectively. In the first form of the
force application
assembly, which is shown in Figure 12 and 22, the force application assembly
comprises a
cathode cover 201 which partially occludes the respective cavities of the
hydrogen
distribution frame 110. As seen in the drawings, the respective cathode covers
201
10 individually releasably cooperate with each other and with the hydrogen
distribution frame
110. A biasing assembly which is designated by the numeral 202, and shown
herein as a
plurality of metal wave springs cooperates with the cathode cover and is
operable to impart
force to an adjacent pressure transfer assembly 203 by means of a pressure
distribution
assembly 204. Referring now to Figures 14, 15, and 23, and in a second form of
the
15 invention, the pressure transfer assembly 203 transfers the force imparted
to it by the cathode
covers 201 to an adjoining pressure plate 205. In this form of the .
invention, the pressure
distribution assembly is eliminated.
In a third form of the invention as seen in Figures 16 17, and 24, the force
application
assembly 200 comprises a cathode cover 201, a plurality of wave springs 202;
and a
20 corrugated pressure plate 232. In this form of the invention, the pressure
transfer assembly
203 is eliminated from the assembly 200. In yet still another fourth form of
the invention as
seen in Figure 18, 19, and 25, the force application assembly 200 comprises a
cathode cover
201; wave springs 202, and a pressure transfer assembly 203. In this form of
the invention,
the pressure plate 205 (of either design) and pressure distribution assembly
204 are
CA 02578111 2007-02-27
36
absent from the combination. In all the forms of the invention described
above,
a force of at least about 175 pounds per square inch is realized between the
membrane electrode diffusion assembly 150 and the associated pair of current
collectors 190.
Referring now to Figure 11, each cathode cover 201 has a main body 210
which is fabricated from a substrate which has a flexural modulus of at least
about 1 million pounds per square inch. This is in contrast to the hydrogen
distribution frame 110 which is fabricated from a substrate having a flexural
modulus of less than about 500,00 pounds per square inch, and a compressive
io strength of less than 20,000 pounds per square inch. The main body 210 has
an exterior facing surface 211, and an opposite interior facing surface 212
(Figure 13). Further, the main body has a peripheral edge 213 which has a
plurality of apertures 214 formed therein. Each cathode cover nests, or
otherwise matingly interfits with one of the respective cavities 121 through
124,
respectively, which are defined by the hydrogen distribution frame 110. When
appropriately nested, the individual apertures 214 are substantially coaxially
aligned
with the apertures 125 which are formed in the main body 111 of the hydrogen
distribution frame 110. This coaxial alignment permits fasteners 215 to be
received therethroygh. When tightened, the opposing cathode covers exert a
2o force, by means of the intermediate assemblies, described above, on the
membrane electrode diffusion assembly 150 which is effective to establish good
electrical contact between the respective current collectors 190 and the
adjacent
membrane electrode diffusion assembly 150. Still further, the main body 210 -
defines in part a third passageway 216. The third passageway 216 as seen in
Figures 9 and 11, provides a convenient means by which the cathode air flow
which is delivered by the exhaust end 73 of the air plenum 71, can be
delivered
to the cathode side 154 of the membrane electrode diffusion assembly 150. In
CA 02578111 2007-02-27
37
this regard, the air passageway has a first, or intake end 217 and a second,
or
exhaust end 218. As seen in Figure 9, the exhaust end of each third
passageway 216 is located near one of the opposite ends 112 and 113 of the
hydrogen distribution frame 110. As illustrated in Figure 6, the air which has
exited through the exhaust end 218 passes through the apertures 42 and 43
formed in the top and bottom portions 35 and 40 of the subrack 30. As such,
the air passes into the air plenum 71 and may be recycled by means of the air
mixing valve 80 as was earlier described. As best illustrated by reference to
Figures 13 and 22, the interior surface 212 of the cathode cover defines a
cavity
219 of given dimensions. The interior surface further defines a plurality of
channels 220. The channels 220 are operable to matingly receive the individual
wave springs which constitute the biasing assembly 202.
Referring now to Figure 29, the pressure transfer assembly 203 has an
elongated main body 221 which comprises a central backbone 222. Additionally,
a plurality of legs or members 223 extend or depend from the central backbone
222 and are operable to forcibly engage the pressure plate 205 in one form of
the invention (Figures 14 and 23). Still further, the main body 220 has a
first
surface 224 and an opposite second surface 225. A channel 226 is formed in
.the first surface qd matingly interfits or receives one of the metal wave
springs
constituting the biasing assembly 202 (Figure 25). In an alternative form of
the
invention, the pressure plate 205 is eliminated, and a pressure distribution
member 204 is positioned between the biasing assembly 203 and the first
surface
224 of the pressure transfer assembly (Figures 12 and 22). In this form of the
invention, the pressure transfer assembly 204 is fabricated from a resilient
substrate such that the individual legs or members will deform under pressure
to an amount equal to about .001 to .004 inches.
CA 02578111 2007-02-27
38
As noted above, one form of the invention 10 may include a pressure
plate 205 (Figures 14 and 15). In this regard the pressure plate 205, as
illustrated, is a ceramic plate or an equivalent substitute having a main body
230.
The ceramic plate, as shown in Figure 15, has a plurality of apertures 231
formed therein which allows air to pass therethrough and which has traveled
through the third passageway 216 which is formed, in part, in the main body
210
of the cathode cover 201. The main body 230 of the pressure plate 205 is
substantially planar to less than about .002 inches. An alternative form of
the
pressure plate is shown in Figures 16 and 17 and is designated by the numeral
io 232. This second form of the pressure plate is thicker than the pressure
plate
205 which is shown in Figure 15. Referring now to Figures 20 and 21, the
pressure plate 232 defines a given open area therebetween a plurality
substantially equally spaced corrugations or undulations 233 which are formed
in
its surface. These corrugations or undulations define specific channels 234
therebetween through which air can move. When the second form 232 of the
pressure plate 205 is employed, the pressure transfer assembly 203 may be
eliminated from the assembly as was earlier discussed. The channels, or open
area 234 defined by the pressure plate 205, whether it be in the first form of
the pressure plate. as shown in Figure 15, or that shown in Figure 20,
defines,
2o in part, the third passageway 216 which allows air to pass through the
cathode
cover 201 to the cathode side 154 of the membrane electrode diffusion assembly
150. Such is best illustrated by reference to Figure 11. As earlier discussed,
and as seen in Figures 12 and 13, one form of the invention 10 utilizes a-
pressure distribution assembly 204. When employed, the pressure plate 205 is
eliminated and the pressure distribution assembly 204 is positioned between
the
wave springs which constitute the biasing assembly 202, and the pressure
transfer
assembly 203 which were described earlier. In this regard, the pressure
CA 02578111 2007-02-27
39
distribution assembly comprises a first substantially noncompressible and
flexible
substrate 240 (Figure 22). The first non-compressible substrate has a first
surface 241 and an opposite second surface 242. The first surface 241 is in
contact with the biasing assembly 202. Mounted upon the opposite, second
surface 242 is a compressible substrate 243. The compressible substrate has an
outwardly facing surface 244 which is in contact with the first surface 224 of
the
pressure transfer assembly 203. In operation, as the respective cathode covers
and associated biasing assemblies 202 exert force, a certain amount of
deflection
or bending in the cathode covers may occur. This is shown in the drawings at
to Figure 22. When this event happens, the first surface of the pressure
transfer
assembly presses against the compressible surface 243 thereby maintaining a
substantially constant pressure across the entire surface of the adjacent
current
collector 190. The proton exchange membrane fuel cell power system 10 further
includes a digital programmable control assembly 250, as seen in the schematic
view of Figure 30. The digital programmable control assembly 250 is
electrically
coupled with each of the discrete PEM fuel cell modules 100 such that they can
r_
be monitored with respect to the electrical performance of same. This digital
programmable control assembly 250 is further electrically coupled with the air
distribution assembly 70. Still further, the digital programmable control
assembly
250 is electrically coupled with the fuel distribution assembly which
comprises the
source of hydrogen 60, accompanying valve assembly 61 and associated first
conduit 54 which delivers the hydrogen by means of one of the valves 52 to
each of the discrete PEM fuel cell modules 100.
Still further, and referring to Figure 31, the PEM fuel cell power system
10 of the present invention includes a heat exchanger 260 which is operably
coupled with the air distribution assembly 70 which delivers air to the
individual
discrete PEM fuel cell modules 100. The heat exchanger 260 captures useful
CA 02578111 2007-02-27
thermal energy emitted by the discrete PEM fuel cell modules 100.
Additionally,
the power system 10 includes a power conditioning assembly 270 (Figure 1)
comprising an inverter which is electrically coupled with the direct current
bus
and which receives the direct current electrical energy produced by the
5 individual discrete PEM fuel cell modules 100 and which converts same into
suitable alternating current.
OPERATION
The operation of the described embodiments of the present invention are
10 believed to be readily apparent and are briefly summarized at this point.
In its broadest aspect, the present invention comprises a proton exchange
membrane fuel cell power system 10 having a plurality of discrete proton
exchange membrane fuel cell modules 100 which are self-humidifying and which
individually produce a given amount of heat energy. Further, each of the
15 discrete proton exchange membrane fuel cell modules 100 have a cathode air
flow, and a preponderance of the heat energy produced by each of PEM fuel
cell modules 100 is removed from same by the cathode air flow.
Another aspect of the present invention relates to a proton exchange
membrane fuel cell power system 10 for producing electrical power and which
20 comprises a plurality of discrete fuel cell modules 100, each having at
least two
membrane electrode diffusion assemblies 150. Each of the membrane electrode
diffusion assemblies 150 have opposite anode 153, and cathode sides 154.
Additionally, this PEM fuel cell power system 10 includes a pair of current
collectors 190 each disposed in juxtaposed ohmic electrical contact with the
25 opposite anode 153 and cathode sides 154 of each of the membrane electrode
diffusion assemblies 150. Further, individual force application assemblies 200
for
applying a given force to each of the current collectors and the individual
CA 02578111 2007-02-27
41
membrane electrode diffusion assemblies are provided. The individual force
application assemblies, as earlier noted, may be in several forms. Commonly
each
form of the force application assemblies has a cathode cover 201, and a
biasing
assembly 202. However, in one form of the invention, a pressure plate 205 may
be utilized, and comprises a ceramic plate having a plurality of apertures
formed
therein. As seen in Figure 14, a pressure [ransfer assembly 205 is provided
and
is effective to transmit force, by way of the pressure plate, to the
underlying
membrane electrode diffusion assembly 150. In an alternative form (Figure 16),
a second pressure plate 232 may be employed. When used, the pressure transfer
io assembly 203 may be eliminated from the construction of the PEM fuel cell
module 100. In still another form of the force application assembly 200
(Figure
12), the pressure plate 205 is eliminated and the pressure distribution
assembly
204 is utilized to ensure that substantially equal force is applied across the
surface area of the adjacent current collector 190.
As presently disclosed, the PEM fuel cell power system 10 and more
particularly, the discrete PEM fuel cell modules 100 have an electrical
efficiency
of at least 40% and are self-humidifying, that is, no additional external
humidification must be provided to the hydrogen fuel 60, or air which is
supplied to same.. Sjill further, the membrane electrode diffusion assemblies
150
which are utilized in the present invention have an active area which has a
given surface area. It has been determined that the discrete PEM fuel cell
modules 100 produce a current density of at least about 350 m.A. per square
centimeter of active area at a nominal cell voltage of at least about 0.5
volts
D.C. Additionally, the discrete fuel cell modules 100 each have an electrical
output of at least about 10.5 watts.
The individual proton exchange membrane fuel cell modules 100 are
mounted within an enclosure 11 which includes a subrack 30 for supporting
same.
CA 02578111 2007-02-27
42
The enclosure 11 which is utilized with the present proton exchange membrane
fuel cell modules 100 further comprises a fuel distribution assembly 52, 54
and
60, for delivering hydrogen to the individual discrete PEM fuel cell modules
100.
An air distribution assembly 70 for delivering air to the individual discrete
PEM
fuel cell modules 100 is provided, and a direct current output bus 50, and a
power conditioning assembly 270 for receiving and inverting the electrical
power
produced by each of the discrete PEM fuel cell modules 100 are also received
in the enclosure 11. As earlier discussed, each of the subracks 30 are mounted
in the cavity 25 which is defined by the enclosure 11. The subracks 30 have
io forward and rearward edges 33, and 34 and top and bottom portions 35 and
40,
respectively. Each of the discrete PEM fuel cell modules 100 are operably
coupled with the fuel distribution assembly, direct current output bus 50 and
power conditioning assembly 270 in the vicinity of the rearward edge 34 of
each
of the subracks 30 as seen most clearly in Figures 3, 4 and 6. Further, the
discrete PEM fuel cell modules 100 are coupled in fluid flowing relation with
the air distribution assembly 70 at the top and bottom portions 35 and 40 of
r_
each of the subracks 30 and with the air plenum 70 at the exhaust end 73
thereof.
Referring to Figure 6, the air distribution assembly 70 which is utilized
in the present device includes a plenum 71 which is made integral with each of
the subracks 30. The plenum has an exhaust end 73 which delivers air to each
of the PEM fuel cell modules 100 supported on the subrack 30, and an intake
end 72 which receives both air which has passed through each of the PEM fuel
cell modules 100 and air which comes from outside the respective PEM fuel cell
modules 100. Further, the air distribution assembly 70 includes an air
movement
assembly 74 in the form of a fan 75 which is operably coupled to the plenum
71 and which moves the air in a given direction along the plenum 71 to the
CA 02578111 2007-02-27
43
individual PEM fuel cell modules 100. An air mixing valve 80 is borne by the
plenum 71, and controls the mixture of air which is recirculated back to the
respective PEM fuel cell modules 100. As described earlier in greater detail,
the individual discrete PEM fuel cell modules 100 include a hydrogen
distribution
frame 110 defining discrete cavities 120, and wherein the respective membrane
electrode diffusion assemblies 150 are individually sealably mounted in each
of
the cavities 120. In the preferred form of the invention, the hydrogen
distribution frame 110 is oriented between the individual membrane electrode
diffusion assemblies 150. As best seen in Figure 10, the hydrogen distribution
io frame 110 comprises multiple pairs of discretely opposed cavities 121-124.
The hydrogen distribution frame 110 permits the delivery of hydrogen gas
to each of the cavities 121-124. In this regard, the hydrogen distribution
frame
110 defines a first passageway 131 which permits the delivery of hydrogen gas
to each of the cavities 121-124 which are defined by the hydrogen distribution
frame 110 and to the anode side 153 of the membrane electrode diffusion
assembly 150. Still further, the hydrogen distribution frame 110 includes a
second passageway 132 which facilitates the removal of impurities, water, and
unreacted hydrogen from each of the cavities 121-124. As noted earlier, each
of the cathode covers 201 and the respective force application assemblies 200
define a third passageway 216 which permits delivery of air to each of the
cavities 121-124, and to the cathode side 154 of each of the respective
membrane electrode diffusion assemblies 150. Hydrogen gas is supplied by means
of the first passageway 131 to each of the cavities 121-124 of the hydrogen
distribution frame 110 at a pressure of about 1 PSIG to about 10 PSIG; and
air is supplied at above ambient pressure by the air distribution assembly 70.
Also as discussed above, the source of hydrogen 60 is illustrated herein
as a pressurized container of same which is received in the enclosure 11
(Figure
CA 02578111 2007-02-27
44
1). However, it is anticipated that other means will be employed for supplying
a suitable quantity of hydrogen to the hydrogen distribution assembly 110. In
this regard, a chemical or fuel reformer could be utilized and enclosed within
or outside of the enclosure 11 and which would, by chemical reaction, produce
a suitable quantity of hydrogen. The chemical reformer would be coupled with
a supply of hydrogen rich fluid such as natural gas, ammonia, or similar
fluids.
The chemical reformer would then, by means of a chemical reaction, strip away
the hydrogen component of the hydrogen rich fluid for delivery to the hydrogen
distribution assembly. The remaining reformer by-products would then be
exhausted to ambient (assuming these by-products did not produce a heath,
environmental or other hazard), or would be captured for appropriate disposal,
or recycling.
The membrane electrode diffusion assembly 150 which is employed with
the power system 10 of the present invention includes, as a general matter, a
solid proton conducting electrolyte membrane 151 which has opposite anode and
cathode sides 153 and 154; individual catalytic anode and cathode electrodes
161
and 162 which are disposed in ionic contact with the respective anode and
cathode sides 153 and 154 of the electrolyte membrane 151; and a diffusion
layer 170 borne on each of the anode and cathode electrodes 161 and 162 and
which is electrically conductive and has a given porosity. With respect to the
diffusion layer 170, in the preferred embodiment of the present invention 10,
the
diffusion layer 170 comprises a first diffusion layer 171 borne on the
individual
anode and cathode electrodes 161 and 162 and which is positioned in ohmic
electrical contact therewith. The first diffusion layer 171 is electrically
conductive
and has a given pore size. Additionally, a second diffusion layer 172 is borne
on the first layer 171 and is positioned in ohmic electrical contact with the
underlying first diffusion layer 171. The second diffusion layer 172 is
electrically
CA 02578111 2007-02-27
conductive and has a given pore size which is greater than the given pore size
of the first diffusion layer 171.
In its broadest aspect the present invention 10 includes an electrolyte
membrane 151 which comprises crosslinked polymeric chains incorporating
sulfonic
5 acid groups. More specifically, the electrolyte membrane 151 has at least a
20%
molar concentration of sulfonic acid. The diffusion layer 170 which is
employed
with the membrane electrode diffusion assembly 150 of the present invention is
deposited by means of a given method which was described earlier, and is not
repeated herein.
10 In the present invention 10, the individual anode and cathode electrodes
161 and 162 in their broadest aspect, include particulate carbon; a catalyst;
a
binding resin; and a crosslinked copolymer incorporating sulfonic acid groups.
In addition to the foregoing, the power system 10 further includes a pair of
current collectors 190 which, in their broadest aspect, include a base
substrate
15 which is electrically conductive and is capable of surface passivation if
exposed
to oxygen; and a contact layer which is electrically conductive, galvanically
r_
cathodic and oxygen stable. Still further, the pair of current collectors 190
have
a thickness of about 0.1 millimeters to about 1.3 millimeters and the contact
layer has a thickness of about 0.2 microns to about 2 microns. In addition to
20 the foregoing, the base substrate 190 has a given surface area of which at
least
70% is open area.
The power system 10 includes a digital programmable control assembly 250
for monitoring the performance of the individual proton exchange membrane fuel
cell modules 100, and other parameters of operation such as the flow rate of
25 hydrogen 60 to the individual discrete PEM fuel cell modules 100, the heat
output of each of the proton exchange membrane fuel cell modules 100, and the
operation of the air distribution assembly 70 which mixes both outside air and
CA 02578111 2007-02-27
46
air which has previously passed through the individual proton exchange
membrane
fuel cell modules 100. The air mixing valve 80 is effective in controlling the
temperature of the air which is delivered to each of the proton exchange
membrane fuel cell modules 100, as well as the relative humidity. In this
fashion,
the preponderance of heat energy generated by each of the PEM fuel cell
modules 100 is effectively removed from same and either exhausted to ambient,
or captured for other uses. The control assembly 250 is operable therefore to
effectively optimize the operational conditions of the individually discrete
PEM
fuel cell modules 100 such that maximum current densities and efficiencies can
io be realized.
Some of the most significant advantages of the present invention 10 is
that it is modularized, simple, efficient in operation and easy to maintain.
For
example, in the event that a particular PEM fuel cell module 100 becomes
inoperable, the disabled PEM fuel cell module 100 can be quickly removed, by
hand, from the subrack 30 and replaced with an operational module without
interrupting the operation of the power system 10. This is a significant
advancement in the art when considering the prior art teachings which show
that
a defective PEM fuel cell (manufactured as a stack) would require total
disassembly of same while repairs were undertaken.
19 The present power system 10 has numerous other advantages over the
prior art techniques and teachings, including the elimination of many of the
balance-of-plant subassemblies typically utilized with such devices. Yet
further,
in view of the self-humidifying nature of the present proton exchange membrane
fuel cell modules 100, other control measures have been simplified or
otherwise
eliminated thereby increasing the performance capabilities of same while
simultaneously reducing the costs to generate a given amount of electrical
power.
CA 02578111 2007-02-27
47
In compliance with the statute, the invention has been described in
language more or less specific as to structural or methodical features. It is
to
be understood, however, that the invention is not limited to specific features
shown and described, since the means herein disclosed comprise preferred forms
of putting the invention into effect. The invention is, therefore, claimed in
any
of its forms or modifications within the proper scope of the appended claims
appropriately interpreted in accordance with the doctrine of equivalents.
r
= . .
