Water management system for solid polymer electrolyte fuel cell power plants

A simplified solid polymer electrolyte fuel cell power plant utilizes porous conductive separator plates having central passages which are filled with circulating coolant water. The coolant water passes through a heat exchanger which rejects heat generated in the power plant. Water appearing on the cathode side of each cell membrane is pumped into the water circulation passages through the porous oxidant reactant flow field plates by a positive .DELTA.P created between the cathode reactant flow field of each cell and the coolant water circulation passages between each cell. In order to create the desired .DELTA.P, at least one of the reactant gas streams will be referenced to the coolant water loop so as to create a coolant loop pressure which is less than the referenced reactant gas stream pressure. Excess water is removed from the coolant water stream. The system can operate at ambient or at elevated pressures. Each cell in the power plant is individually cooled on demand, and the power plant does not require a separate cooling section or reactant stream humidifying devices.

TECHNICAL FIELD 
This invention relates to a simplified solid polymer electrolyte fuel cell 
power plant which can operate at ambient or above-ambient pressure, and 
which is admirably suited for use in both portable and/or stationary power 
plants. More particularly, this invention relates to a stationary or 
mobile solid polymer electrolyte power plant which utilizes a positive 
pressure differential between a reactant flow field and the water flow 
field in order to manage water migration within the fuel cell units in the 
power plant. 
BACKGROUND ART 
Solid polymer electrolyte fuel cell power plants are known in the prior 
art, and prototypes are even available from commercial sources, such as 
Ballard Power Systems, Inc. of Vancouver, Canada. These systems are 
serviceable, but are relatively complex. An example of a Ballard Power 
Systems polymer membrane power plant is shown in U.S. Pat. No. 5,360,679, 
granted Nov. 1, 1994. One problem occurring in solid polymer fuel cells 
relates to the management of water, both coolant and product water, 
within: the cells in the power plant. In a solid polymer membrane fuel 
cell power plant, product water is formed by the electrochemical reaction 
at the membrane on the cathode side of the cells by the combination there 
of hydrogen and oxygen ions. The product water must be drawn away from the 
cathode side of the cells, and makeup water must be provided to the anode 
side of the cells in amounts which will prevent dryout, while avoiding 
flooding, of the anode side of the electrolyte membrane. 
Austrian Patent No. 389,020 describes a hydrogen ion-exchange membrane fuel 
cell stack which utilizes a fine pore water coolant plate assemblage to 
provide a passive coolant and water management control. The Austrian 
system utilizes a water-saturated fine pore plate assemblage between the 
cathode side of one cell and the anode side of the adjacent cell to both 
cool the cells and to prevent reactant cross-over between adjacent cells. 
The fine pore plate assemblage is also used to move product water away 
from the cathode side of the ion-exchange membrane and into the coolant 
water stream; and to move coolant water toward the anode side of the 
ion-exchange membrane to prevent anode dryout. The preferred directional 
movement of the product and coolant water is accomplished by forming the 
water coolant plate assemblage in two parts, one part having a pore size 
which will ensure that product water formed on the cathode side will be 
wicked into the fine pore plate and moved by capillarity toward the water 
coolant passage network which is inside of the coolant plate assemblage. 
The coolant plate assemblage also includes a second plate which has a 
finer pore structure than the first plate, and which is operable to wick 
water out of the water coolant passages and move that water toward the 
anode by capillarity. The fine pore and finer pore plates in each 
assemblage are grooved to form the coolant passage network, and are 
disposed in face-to-face alignment between adjacent cells. The finer pore 
plate is thinner than the fine pore plate so as to position the water 
coolant passages in closer proximity with the anodes than with the 
cathodes. The aforesaid solution to water management and cell cooling in 
ion-exchange membrane fuel cell power plants is difficult to achieve due 
to the quality control requirements of the fine and finer pore plates, and 
is also expensive because the plate components are not uniformly produced. 
It would be desirable to provide a simplified solid polymer fuel cell power 
plant which may be used as a power supply for various pressurized and 
ambient pressure applications, such as automotive, public transportation, 
or the like; and also in stationary power plants. 
DISCLOSURE OF THE INVENTION 
This invention relates generally to passive water management and cell 
cooling in an ion-exchange membrane fuel cell power plant. The passive 
water control and coolant system of this invention can be used in 
pressurized membrane fuel cell power plants, where both of the reactant 
gases are pressurized to pressures on the order of about 30 to about 50 
psig for example, and can also be used where the reactants operate at 
essentially ambient pressures. The system of this invention utilizes fine 
pore plate components and pressure differences (.DELTA.P) between the 
water coolant loop and the reactant gases to ensure that product water 
formed on the cathode side of the cells in the power plant and water 
displaced by proton drag from the anode side of the membrane to the 
cathode side will migrate from the cathode flow field to the coolant loop, 
and that water from the coolant loop will move toward the anode side of 
the membrane so as to prevent dryout of the anode surface of the membrane, 
but not flood the anode flow field. In one embodiment of the invention, 
the fine pore plates used in the system are flooded with water so as to 
provide a gas-impermeable barrier between the cathode reactant flow field 
in one cell and the anode reactant flow field in the next cell so that 
reactant gas cross-over from one cell to the next is prevented. 
Alternatively, the system of this invention may employ a solid impermeable 
separator plate between adjacent cells in the power plant to prevent 
reactant cross-over. In order to provide the desired water migration in 
the system, the cathode reactant gas pressure will be maintained in the 
range of about 0.5 to about 10 psig higher than the prevailing pressure in 
the water coolant loop and in the fine pore plate. When this pressure 
differential is maintained, the fine pore plate will be able to move water 
away from the cathode surface of the membrane, while at the same time, the 
bubble pressure of the water in the fine pore plate will prevent the 
reactant gases from penetrating the pores of the saturated plate. The 
phrase "bubble pressure" refers to the positive water pressure in the fine 
pore plate which is inversely proportional to the pore size in the plate. 
Thus, the smaller the pore diameter in the plate, the greater pressure 
exerted by the water entrained in the plate. The "bubble pressure" is the 
pressure above which reactant gas bubbles will be forced through the 
water-saturated porous plate so as to create the undesirable possibility 
of fuel and oxidant gas commingling, and will also prevent reactants from 
entering the coolant loop. It will be appreciated that lower pore 
diameters will increase the bubble pressure threshold, while at the same 
time hindering to some extent product water migration through the fine 
pore plates. Therefore, appropriate plate pore sizes should be maintained 
in order to achieve optimal operation of the system. Pore diameters in the 
"fine pore" plates referenced in this specification are typically in the 
range of 1-2 micron median pore diameter. 
A preferable manner in which the appropriate system .DELTA.P between 
reactant gas pressure and water coolant pressure is maintained is to 
reference the water coolant pressure to the reactant gas pressure, In a 
pressurized system, the pressurized reactant gases can be used to impose a 
predetermined pressure on the water coolant, which predetermined pressure 
is then partially lowered, thereby creating a .DELTA.P between the 
reactant and coolant water loop pressures. In an air oxidant ambient 
pressure system, the coolant loop can be exposed to ambient pressures, and 
the air oxidant pressure can be increased above ambient pressure with a 
blower or compressor. In either case, a positive oxidant reactant 
gas-to-water coolant .DELTA.P will be created in the system in order to 
promote water migration from the cathode side of the membrane into the 
water coolant circulation loop. 
The fuel reactant can be provided from a pressurized container, and its 
anode flow pressure can be dropped to an appropriate level with valves or 
pressure regulators so as to allow migration of coolant water from the 
coolant loop toward the anode side of the membrane, while preventing 
flooding of the anode side of the membrane. 
Each cell has its own dedicated water source, so that coolant and membrane 
moisturizing water is provided to each cell, on demand, thereby adjusting 
water flow management to conform to ongoing cell operating conditions. The 
water supply is circulated through the power plant by a pump, which may be 
a fixed or variable speed pump. The water circulating system also picks up 
water from the cathode side of each cell and entrains the water in the 
circulating cooling water stream. Periodically excess water can be removed 
from the coolant loop. The circulating cooling water stream passes through 
a heat exchanger which rejects system-generated heat, and lowers the 
temperature of the water stream so as to enable the water stream to be 
reused to cool and humidify the individual cells. 
In a portable or mobile ambient pressure embodiment of the invention, the 
cooled water stream can flow past a branch conduit that leads to a stand 
pipe, into which excess water that is entrained in the water stream can 
migrate. The stand pipe is open to ambient surroundings, and periodically 
allows spillage of excess water from the system into ambient surroundings. 
The stand pipe also provides a predetermined back pressure imposed on the 
coolant water circulating system, which ensures the necessary .DELTA.P 
between the coolant water system and the oxidant reactant flow field. 
It is therefore an object of this invention to provide a passive, 
self-adjusting water and coolant management system for use in an 
ion-exchange membrane fuel cell power plant. 
It is a further object of this invention to provide a system of the 
character described wherein water movement is controlled in both 
atmospheric and pressurized systems. 
It is an additional object of this invention to provide a fuel cell power 
plant of the character described wherein each cell in the power plant is 
properly cooled and humidified as required by ongoing cell operating 
conditions. 
It is yet a further object of this invention to provide a fuel cell power 
plant of the character described wherein product water generated by the 
electrochemical reaction in the system is released from the system. 
These and other objects and advantages of the invention will become more 
readily apparent from the following detailed description of an embodiment 
of the invention when taken in conjunction with the accompanying drawings 
in which:

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring now to FIG. 1, there is shown a membrane fuel cell system in a 
fuel cell power plant, which system is shown schematically, and wherein 
the ion-exchange membrane electrolyte/electrode assembly (MEA) is 
designated by the numeral 50. The MEA conventionally will include a 
membrane, and electrode catalyst layers and electrode substrate layers on 
each side of the membrane; however, in certain cases the substrate layers 
may be omitted from the MEA. It will be understood that the system shown 
in FIG. 1 comprises one cell system unit which is repeated a predetermined 
number of times in the fuel cell power plant stack. In FIG. 1 there is 
shown an assemblage which is operable to create a predetermined .DELTA.P 
between the oxidant reactant flow field and the coolant water circulation 
loop. The oxidant reactant, which may be pure oxygen or ambient air, is 
pressurized at station 52, and thus flows through a line 54 into the cell 
oxidant reactant flow field 56 where it passes over the MEA 50. The 
station 52 may be a pressurized oxidant container when pure oxygen is 
utilized, or may be a compressor or blower when ambient air is used as the 
oxidant reactant. The pressurized oxidant gas also flows into a branch 
line 58 and thence into a reference vessel 60 which is divided by a 
separator 62 into two internal chambers 64 and 66. The separator 62 may 
take the form of a piston, a diaphragm, or the like. The chamber 64 will 
thus have a pressure which is equal to the oxidant reactant pressure and 
that pressure will be imposed on the separator 62, as indicated by arrows 
A. 
The coolant water loop is indicated generally by the numeral 68. The 
coolant loop 68 includes a line 70 which carries the coolant water to and 
from the coolant flow field 72 in the active area of the power plant. The 
coolant water thus extracts heat from the active area of the power plant. 
The heated water flows through a heat exchanger 74 where its temperature 
is dropped to an appropriate level. A pump 76 causes the coolant water to 
move at an appropriate rate through the loop 68. A branch line 78 extends 
from the line 70 to the chamber 66 which thus fills with coolant water. 
The separator 62 is biased toward the chamber 64 by biasing devices, such 
as springs 80. The springs 80 are operable to counter the oxidant gas 
pressure to a predetermined degree, as indicated by arrow B. The counter 
pressure created by the springs 80 lowers the pressure exerted on the 
coolant water in the chamber 66, and thus in the entire loop 68, by a 
predetermined .DELTA.P; thus the oxidant reactant pressure in the oxidant 
reactant flow field 56 will be a known increment (.DELTA.P) greater that 
the pressure of the coolant water in the coolant water flow field 72. If 
necessary, pressure control valves 82 can also be incorporated into the 
coolant loop 68 to temporarily modify the system .DELTA.P if necessary. 
While the preferred embodiment of the invention uses water as the coolant, 
so that the circulating water loop and the coolant loop are one and the 
same, in some applications it may be desirable to provide separate 
circulating water and circulating coolant loops. This would be the case 
where operating conditions of the power plant would dictate the use of a 
coolant such as ethylene glycol or the like. 
The fuel reactant is supplied to the fuel reactant flow field 84 from a 
pressurized source 86 thereof via line 88. A pressure regulator 90 may be 
included in line 88 if necessary. The fuel gas and oxidant gas reactant 
flow fields 84 and 56 are formed in fine pore plates 94 and 92 
respectively, which are able to wick and fill with water from the coolant 
flow field 72. A fine pore plate 94' is the fuel gas reactant flow field 
plate for the next adjacent cell, and the plate 94' combines with the 
plate 92 to form the coolant flow field 72. The combination of coolant 
water pressure and fine pore capillarity causes the plates 92, 94 and 94' 
to be filled with coolant water so that the active areas of the power 
plant will be adequately cooled and their temperature properly controlled. 
During operation of the power plant, the electrochemical reaction 
occurring at the MEA 50 causes pure water to be formed from hydrogen and 
oxygen ions at the surface of the MEA 50 facing the oxidant reactant flow 
field. This water which forms on the cathode side of the MEA is referred 
to as "product water" and it must be dealt with to avoid flooding of the 
cathode. The .DELTA.P which exists between the flow fields 56 and 72 
provides a positive pumping force which causes the product water to 
migrate through the fine pore plate 92 from the MEA 50 and into the 
coolant flow field 72 where it will be taken up in the circulating coolant 
Water stream. An exhaust line 98 extends from the chamber 66, or at some 
other location in the coolant loop, for periodically removing excess 
product water from the cooling loop 68. A valve 100 will be included in 
the line 98 when the system is pressurized. The valves 82 and 100 may be 
manually or automatically operated. It will be understood that the fine 
pore plates 94 will be operable to wick water to the surface of the MEA 50 
which faces the fuel reactant flow field 84 on the anode side of the cell 
which tends to dry out during power plant operation. The water in the fine 
pore plates 94 and 94' thus prevents dryout of the anode side of each MEA 
50. The pressure regulator 90 is operable to ensure that the pressure of 
the fuel gas reactant in the flow field 84 is sufficient to prevent 
flooding of the anode catalyst of the MEA, but does not exceed the bubble 
pressure of the fine pore plates 94 and 94'. 
Once the proper operating .DELTA.P between the oxidant flow field and the 
coolant water flow field is established, appropriate water management 
ensues automatically and is passively maintained without the need of 
complex valves and regulators. Any changes in the .DELTA.P needed during 
certain operating conditions of the plant, such as changes in reactant 
utilization, plant power output, cell performance, temperature and 
pressure settings will be accomplished by adjustments to the valve 82. 
These .DELTA.P changes can also be by adjustments to the coolant pump 
speed, or to the counter-pressure exerted on the separator 62. It will 
also be noted that the system unit shown utilizes saturated fine pore 
plates between adjacent cells to prevent reactant cross-over, however, 
properly located impermeable separator plates between adjacent cells could 
also be used. 
Referring now to FIG. 2, there is shown an example of a structural 
configuration of flow field plates which can be used to form the reactant 
and water circulating flow fields. Each plate 2 and 2' is preferably 
formed from a molded graphite powder which will provide fine pores on the 
order of about 1 to 2 microns median diameter. This degree of porosity 
will promote water migration from a first intercellular coolant water flow 
field to the anode side of the MEA and also away from the cathode side of 
the MEA and into the next adjacent coolant water flow field. The plates 2, 
2' thus provide coolant water on demand to the anode side of the MEA to 
prevent the latter from drying out, to humidify incoming fuel and oxidant 
gases, and also to remove water from the cathode side of the MEA to 
prevent the cathode side from flooding. Each cell in the stack has its own 
dedicated water coolant flow field and is thus provided with the necessary 
amounts of water on demand as required by ongoing cell conditions, which 
may vary from cell to cell in the stack at any particular time, during 
operation of the stack. Opposite surfaces on the plate 2 are provided with 
a pattern of projections 4 and 14 which form a network of grooves 6 and 16 
on opposite sides of the plate 2. The grooves 6 form a portion of the 
coolant water flow field in the stack, and the grooves 16 form the cathode 
reactant flow field for each cell in the stack. The plate 2' is also 
formed with projections 4' and 14', and a network of grooves 6' and 16' on 
its opposite surfaces. The grooves 6' form a portion of the water coolant 
flow field, and the grooves 16' form the anode reactant flow field for 
each cell in the stack. 
Referring to FIG. 3, there is shown a cell unit or component of a power 
plant formed in accordance with this invention. Each cell component will 
include a membrane 8; an anode substrate 10, and an anode catalyst 12; a 
cathode substrate 18, and a cathode catalyst 20; an anode flow field plate 
2'; and a cathode flow field plate 2. The flow field plates 2 and 2' are 
positioned back-to-back with the projections 4 and 4' being disposed in 
face-to-face contact. The grooves 6 and 6' combine to form coolant water 
flow fields on the anode and cathode sides of the electrolyte membrane 8. 
The projections 14' abut the anode substrate 10; and the projections 14 
abut the cathode substrate 18. The grooves 16' thus form the anode 
reactant flow field; and the grooves 16 form the cathode reactant flow 
field. 
FIG. 3 also shows, schematically, the System components of the fuel cell 
stack power plant. All of the anode reactant flow fields 16' in the power 
plant are supplied with a hydrogen gas reactant from a supply tank 22 
thereof. The hydrogen reactant flows from the supply tank 22 to the anode 
flow fields 16' through a supply line 24. The amount and pressure of 
hydrogen flowing through the supply line 24 is controlled by a supply 
valve 26 and a supply regulator 28 which may be manually or automatically 
operated. The anode flow fields 16' are dead-ended inside of the power 
plant. All of the cathode flow fields 16 are supplied with ambient air via 
an air blower or compressor 30 and an air line 32. The oxygen used in the 
electrochemical reaction is thus derived from ambient air. 
Coolant water is circulated through the power plant cell units via line 34. 
The coolant water passes through coolant passages 36 between the plates 2 
and 2'. Circulation of the coolant water is promoted by a pump 38, which 
can be a fixed or variable speed pump. The coolant water circulating loop 
includes a heat exchanger 40 which lowers the temperature of the water 
exiting from the coolant passages 36. A branch line 42 leads from the line 
34 to a stand pipe 44 that is open to ambient surroundings. The stand pipe 
44 may include a drain spout 46 for releasing system water into the 
ambient surroundings. Excess water formed by the electrochemical reaction, 
i.e., product water, is bled into the stand pipe 44 by way of the line 42. 
Thus the stand pipe 44 provides a recipient of system product water, and 
also provides the necessary back pressure for establishing the system 
pressure in the water coolant loop. 
The power plant operates as follows. Prior to start up, the coolant water 
loop 34, 36 and the stand pipe 44 are filled with coolant water. The level 
of the initial fill in the stand pipe may be just below the drain spigot 
46. It will be understood, of course, that the drain spigot 46 may be 
omitted, and the stand pipe 44 can empty into the ambient surroundings 
through its upper open end. The water pump 38 is started so as to create a 
circulating flow of coolant water, and the reactants are then admitted 
into the anode and cathode sides of each of the cells in the power plant. 
A portion of the circulating coolant water will be drawn through the 
porous plates 2', into the projections 14' and against the anode side of 
the membrane 8. Any inert impurities found in the hydrogen fuel, such as 
helium, oxygen, carbon dioxide, and the like will diffuse through the 
membrane 8 since the hydrogen flow field 16' is dead-ended in the power 
plant. These impurities will then be flushed from the power plant by the 
air stream in the oxygen flow field 16, which air stream is vented to the 
ambient surroundings. Any water which migrates through the membrane 8 from 
the anode side, as well as product water which is formed on the cathode 
side of the membrane 8 by the electrochemical reaction, is drawn into the 
cathode plate projections 14, and passes through the plate 2 into the 
water coolant flow field 36. Some water will also be evaporated into the 
oxygen air stream and will vent from the system in the air stream exhaust. 
The excess product water which is formed in the electrochemical reaction 
will be pumped into the line 34 along with coolant water. All of the water 
in the coolant loop will be cooled in the heat exchanger 40, and excess 
product water in the loop will pass through the line 42 and enter the 
stand pipe 44, from whence it will periodically spill into the ambient 
surroundings. By providing a circulating coolant water supply for each 
cell in the power plant, each cell will have an "on-demand" supply of 
coolant water so that each cell will be able to operate at an optimum 
temperature, which is preferably between about 180.degree. F. to somewhat 
less than 212.degree. F., so that the coolant remains in its liquid state 
at near atmospheric operating pressures. Additionally, local water 
transport through the fine pore plates can add or remove water from 
reactant passages to maintain a fully saturated condition at all locations 
within the cells. With the constant supply of liquid water coolant, any 
cell which approaches the upper limit of the desired operating temperature 
range will receive sufficient water at its disposal to bring ;the cell 
operating temperature back down to the lower end of the desired operating 
temperature range. When the cells in the power plant are operating within 
the 180.degree. F.-212.degree. F. temperature range, a typical solid 
polymer electrolyte power plant with one hundred cells, each being one 
square foot in area and formed in accordance with this invention can 
produce a power output of about twenty two kilowatts, i.e. about 0.225 
kilowatts per cell. In order to maintain the desired cell operating 
temperature range, and therefore the power output, the heat exchanger 40 
will maintain the temperature of the water emitted therefrom in the range 
of about 1200.degree. to about 150.degree. F. To this end, the heat 
exchanger will preferably be controlled by a thermostat 48 which senses 
the temperature of the water stream exiting the heat exchanger 40. 
It will be readily appreciated that the electrochemical power plant of this 
invention will provide an efficient conversion of hydrogen and oxygen to 
electricity using conventional components and operating at ambient or 
above ambient pressures, and at sufficiently low temperatures in most 
cases so that liquid water may be used as a coolant for the cells in the 
power plant: Each cell in the power plant has its own supply of water 
coolant which is available, on demand, responsive to individual ongoing 
cell operating conditions. The resulting relatively high operating 
temperature range enabled by a power plant constructed in accordance with 
this invention enables the construction of a relatively compact unit which 
can meet the power demands of storage batteries of the type used to 
operate vehicles, such as automobiles, buses, or the like. Larger fixed 
power plants can also be constructed. The referencing of the oxidant 
reactant gas pressure to the water coolant loop pressure provides a 
constant .DELTA.P between the oxidant reactant flow fields and the water 
coolant flow field adjacent to each cell in the power plant, which 
.DELTA.P causes water appearing at the cathode side of each cell in the 
power plant to be pumped through the fine pore flow field plates into the 
water coolant stream. The management of water in the power plant is thus 
achieved passively and without the need of complex valve and regulator 
networks, and without the need of condensers and evaporators. The passive 
nature of the water management system formed in accordance with this 
invention allows the construction of larger and higher current density 
solid polymer electrolyte power plants since each cell in the power plant 
is serviced individually, and there is no need to remove water by means of 
a moving gas stream that travels from one end of the power plant to the 
other, past each cell, as with the prior art systems. 
Since many changes and variations of the disclosed embodiment of the 
invention may be made without departing from the inventive concept, it is 
not intended to limit the invention otherwise than as required by the 
appended claims.