A novel method and apparatus is provided for controlling heat and mass inventory in a fuel cell. Heat and mass, e.g. water, generated in the cell are removed by heat transfer and capillary action

Asher, William J.

NASA Technical Reports Server

United States Patent [191 
Asher 
[ 111 3,957,535 
1451 May 18, 1976 
[ 541 
[75] Inventor: William J. Asher, Panwood, N.J. 
[ 731 Assignee: Exxon Research and Engineering 
[22] Filed: July 28, 1971 
[21] Appl. No.: 166,746 
FUEL CELL HEAT AND MASS PLATE 
Company, Linden, N.J. 
1521 U.S. CI. ............................. 136/86 E; 136/86 C 
[ 581 Field of Search ......................... 136/86 C, 86 E 
[ 5 1 ] Int. CL2 ......................................... HOlM 27/00 
1561 References Cited 
UNITED STATES PATENTS 
3,370,984 2/1968 Platner .............................. 136/86 C 
3,476,601 I 1/1969 Berger et al. ..................... 136/20 X 
3,498,844 3/1970 Sandersen ......................... 136/86 C 
3,629,075 I2/197 I Gutbier ......................... 136/86 C X 
3,492,163 1/1970 Hilrner .............................. 136/86 C 
Primary Examiner-Allen B. Curtis 
Attorney, Agent, or Firm-Joseph J. Dvorak 
ABSTRACT [571 
A novel method and apparatus is provided for control- 
ling heat and mass inventory in a fuel cell. Heat and 
mass, e.g. water, generated in the cell are removed by 
heat transfer and capillary action. 
2 Claims, 3 Drawing Figures 
https://ntrs.nasa.gov/search.jsp?R=20080012206 2019-08-30T03:58:17+00:00Z
US. Patent May is, 1976 
6 17 
Fig. 3 
William J. Asher INVENTOR 
B RN EY 
3 ,9573  35 
1 
FUEL CELL HEAT AND MASS PLATE 
The invention described herein was made in the per- 
formance of work under a NASA contract and is sub- 
ject to the provisions of Section 305 of the National 
Aeronautics and Space Act of 1958, Public Law 
85-568 (72 Stat. 435; 42 U.S.C. 2457). 
BACKGROUND O F  THE INVENTION 
The present invention relates to method and appara- 
tus for controlling the heat and mass inventory in a 
reactant plenum. 
One of the major problems encountered in the devel- 
opment of fuel cells which employ oxygen containing 
gases as the oxidant and hydrogen containing gases as 
the fuel, is the removal of heat and water produced as 
a result of oxidizing the fuels. In order to maintain a 
steady state operation the reaction product must be 
removed at the same rate at which it is formed and an 
exchange of heat with the surroundings must take 
place. Any imbalance between the production and 
recovery rates of the reaction products can detrimen- 
tally affect the performance of the cell. For example, if 
product water is not adequately removed from a 
H2-02 fuel cell the concentration of the electrolyte 
can be diluted and electrode flooding may even occur. 
Either of these events would dramatically alter the cell 
performance. In other types of cells, such as those using 
an ion exchange membrane as an electrolyte, a de- 
crease in water vapor concentration in the cell can 
result in a drying out of the electrolyte and cell failure. 
Similarly, variations in temperature significantly re- 
duce the performance of the cell through a combina- 
tion of factors including changes in the vapor pressure 
of the electrolyte and changes in the rate of reaction. 
Various means have been proposed for removing 
water and controlling the temperature within fuel cells. 
Thus, the art is replete with different arrangements of 
pumps and condensors for removing water and circu- 
lating coolant through the fuel cell. Systems of these 
types, of course, require significant power for their 
operation and consequently, reduce the otherwise high 
efficiency of fuel cells. 
In some arrangements, heat exchange plates have 
been incorporated in fuel cells for transporting heat to 
the surface of fins extending external the fuel cell as- 
sembly. Not only are metal heat exchange plates heavy 
and cumbersome but unfortunately temperature gradi- 
ents exist over the surface of the plate making precise 
and uniform temperature control virtually impossible. 
Interestingly, heat control and mass inventory in fuel 
cells are generally handled by separate and distinct 
systems. In U.S. Pat. No. 3,370,984, for example, water 
is removed from the fuel cell by transport through a 
membrane and into a vapor cavity where it is expelled. 
Some of the heat produced in the cell is used to evapo- 
rate part of the water produced in the cell but this only 
accounts for about 20% of the total heat that must be 
removed. Consequently, cooling plates and fins or 
some other device is required for temperature control. 
In addition to reducing the efficiency of fuel cells 
many of the prior art systems for controlling tempera- 
ture and mass inventory within the cell tend to be large 
as well as expensive. In today’s age of miniaturization 
sizing can be an acute problem. 
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SUMMARY OF THE INVENTION 
In one aspect of the present invention, excess heat 
and mass generated in a reaction plenum of a fuel cell 
are removed by vapor heat transfer and capillary ac- 
tion. Heat is removed by transferring thermal energy to 
a condensed mass contained in a capillary membrane 
by vaporizing the condensed mass into a vapor plenum, 
transporting the vapor to a condensation zone where 
the vapor condenses thereby releasing the energy 
stored in its heat of vaporization and returning the 
recondensed vapor through the capillary membrane to 
remove additional heat as required. The condensed 
mass is the same substance as the mass requiring inven- 
tory control. Thus, when the mass produced in the 
reactant plenum transfers its thermal energy to the 
condensed mass in the capillary membrane there is an 
increase in the total amount of vapor in the vapor ple- 
num in proportion to the amount of mass produced in 
the cell. This excess vapor is removed from the vapor 
plenum. Thus, both heat and mass inventory are simul- 
taneously and simply controlled. 
In another aspect of the present invention, a novel 
heat and mass plate is provided for controlling the heat 
and mass inventory in a fuel cell. In its simplest sense, 
this heat and mass plate comprises a capillary mem- 
brane which in combination with a fuel cell electrode 
and fuel cell housing defines a reactant plenum or mass 
transfer zone, and a vaporization plenum including a 
condensation zone. Excess heat and mass are trans- 
ferred from the reactant plenum to condensed mass 
contained in the capillary membrane into a vaporiza- 
tion zone. The mass vapor in the vaporization zone is 
transported to a condensation zone where it rejects the 
heat energy to the external surroundings, and con- 
denses in the upper portion of the capillary membrane. 
Capillary action returns the condensed mass through 
the membrane back to the capillary surface adjacent 
the reactant plenum for further heat removal. Excess 
water vapor in the vapor plenum, however, is expelled 
to the surroundings by means of pressure relief valves. 
BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a schematic diagram illustrating a method of 
removing excess heat and mass from a reactant plenum 
in accordance with the present invention. 
FIG. 2 is a front view, partially cut away, illustrating 
the construction of a single cell in accordance with this 
invention. 
FIG. 3 is a schematic diagram illustrating another 
variation of a fuel cell utilizing the heat and mass plate 
of this invention. 
DETAILED DESCRIPTION OF THE DRAWINGS 
In FIG. 1, reference numeral 10 refers generally to a 
reactant plenum defined by capillary membrane 11 and 
enclosing walls 12 of a fuel cell housing. Capillary 
membrane 11 also defines with enclosing walls 12 a 
vaporization plenum 14. Vaporization plenum 14 has 
60 twb zones, a condensation zone 14a anh a vapor zone 
designated as 14b. Conduit 15 is provided for introduc- 
ing materials into the reactant plenum. Where desir- 
able conduit 16 is also provided to remove materials 
from the reactant plenum. Conduit 19 is also provided 
65 for removal of vapor from the vapor plenum 14. Con- 
duit 19 is provided with a pressure relief valve 18, the 
purpose of which will be explained in greater detail 
subsequently. 
3,957,s 35 
3 
As can be seen from FIG. 1 ,  vaporization zone 14b 
and reactant plenum 10 qre separated by capillary 
membrane 11.  Capillary membrane 11, however, ex- 
tends beyond the reactant plenum 10 to define with the 
enclosing walls of the housing a condensation zone 14a. 
In illustrating the present invention, walls 12u of 
reactant plenum 10 is shown as an anode. Cathode 19b 
is spaced apart from anode 12a by means of matrix 21 
which contains an electrolyte. Line 22 is provided for 
introducing oxygen into the cathode plenum 23. Line 
24 is present for removal of air from plenum 23. 
Capillary membrane 1 1  can be made of any porous 
material which has a relatively high capillary potential. 
By capillary potential is meant the differential pressure 
required to force a liquid from its largest pore. Prefer- 
ably, the capillary membrane will have a capillary po- 
tential of about 15 Ibs. per square inch o r  even higher. 
Suitable materials that can be used as a capillary mem- 
brane include asbestos, potassium titanate, alumina 
and fiber mats. 
In fuel cell stack shown in FIG. 2 the capillary mem- 
brane 1 I and electrolyte matrix 21 are potassium tita- 
nate sheet material. A typical preparation of such ma- 
terial involves suspending commercially available po- 
tassium titanate pigment in water to which 5 wt. 5% 
tetrafluoroethylene is added based on the amount of 
titanate. The suspension is filtered leaving a mat of 
titanate material that is readily separated from the filter 
paper. Thereafter the potassium titanate sheet is 
pressed at 5000 psi into a sheet having the desired 
capillary potential. 
As can be seen from the FIG. 2 embodiment, matrix 
21 has a surface area which is less than the surface area 
of membrane 1 1 .  Indeed, for this compact cell it is 
preferred that matrix 21 be 3% inches by 4 inches while 
membrane 1 1  has an area of 4 inches by 4 inches. 
thereby leaving a projection of matrix 1 1  beyond the 
electrode structure for heat rejection as will be further 
explained. 
The thickness of membrane 11 is 0.005 inches. Ma- 
trix 21 is 0.005 inches thick. Choice of thickness de- 
pends upon numerous considerations, such as requisite 
mechanical strength, desired porosity, and the required 
flow of water through the membrane and electrolyte 
through the matrix. 
A catalytic metal is deposited on the anode and cath- 
ode side of matrix 21. For a thin fuel cell stack it is 
particularly advantageous to deposit the catalytic metal 
forming the electrodes on matrix 21 by means of a 
sputtering technique well known in the art. In this man- 
ner, extremely thin, yet conductive layer of catalytic 
metal can be deposited on the matrix. However, other 
techniques may be employed for depositing catalytic 
metals. Electrolcss deposition, for example, is a conve- 
nient technique for depositing catalytic metals on a 
non-conductive surface. 
Any number of electrode catalysts can be employed 
in practicing the present invention. Metals such as 
platinum, platinum alloys, palladium and nickel are 
particularly useful as anode catalysts in Hz-02 fuel 
cells. Suitable cathode catalysts include materials such 
as carbon, platinum and silver. 
As shown in FIG. 2, plaque 27 is provided for current 
collection and for defining cavity or oxygen plenum 23. 
Plaque 28 is also provided to serve as a current collec- 
tor for the anode and to define with matrix 11 a reac- 
tant plenum or mass transfer zone 10. Plaques 27 and 
28 may be connected externally by any suitable means 
4 
for removal of current from the cell. Plaque 29 serves 
as a support for matrix 11 as well as defining with ma- 
trix l l  a vapor plenum 14. 
Plaques 27 and 28 may be prepared from any suitable 
5 material having the requisite mechanical and electrical 
properties. In the cell shown in FIG. 2 a slotted nickel 
sheet 0.005 inches thick was used for plaques 27 and 
28. Plaque 29 does not have to be electrically conduct- 
ing and consequently any material having the requisite 
10 mechanical strength can be utilized. For the cell shown 
in FIG. 2, plaque 29 was made of slotted polyethllene 
0.005 inches thick. Obviously, other polymeric materi- 
als may be employed and used in lieu of polyethylene. 
Similarly, other types of materials can be employed 
1.5 in spacing the electrode from membrane 1 1  and for 
defining a reactant plenum therebetween. The above- 
mentioned materials, however, are particularly useful 
in assembling a fuel cell unit which is about 0.02 inches 
in thickness. 
Numerous units, of course, can be stacked or assem- 
bled to provide a power pack having the desired power 
output. 
In the FIG. 1 and FIG. 2 arrangements, anode 12u IS 
placed adjacent to membrane 11. In operation, heat is 
25 generated and product mass produced by chemical 
reaction of the fuel and oxidant at the reaction inter- 
face formed by the electrolyte containing matrix 21. 
With a hydrogen fuel and air as oxidant water IS 
produced in the cell. Heat also is generated. The water 
30 is removed from reactant plenum 10 by displacing and 
vaporizing liquid water in membrane 11. The water is 
evaporated from the membrane into the vaporization 
plenum because the vapor side of membrane 11 is 
maintained at a lower water partial pressure. Pressure 
35 in vaporization plenum 14 is controlled automatically 
by a pressure relief valve 18 which permits expelling 
excess water vapor from the cell. 
Some of the heat generated within the cell will be 
removed by virtue of having vaporized the excess water 
40 present. However, additional heat is efficiently re- 
moved by heat of vaporization and capillary action as 
follows: water is vaporized from membrane 11, which is 
adjacent the reactant plenum, into the vapor plenum 
and the water vapor moves through the vapor space to 
45 the condensation zone 14a. Here the water vapor is 
condensed rejecting heat to the surroundings. The con- 
densed water is drawn back into the fuel cell stack 
through the matrix 11 by capillary action. Hence, most 
of the water evaporated from matrix 11 is condensed 
50 and returned to the fuel cell stack for controlling the 
temperature in the stack. This is a very significant fea- 
ture of the present invention since temperature control 
without a significant temperature gradient across the 
stack is attainable. 
It will be appreciated that by simply increasing the 
pressure in plenum 14 will result in an increase in the 
boiling point of water in the matrix. Therefore the 
temperature of the membrane surface can be fixed by 
setting the water vapor or pressure in the vapor ple- 
60 num. Optionally, a membrane with an ion exchange 
capacity can be used in which the concentration of 
counter ions in the water effectively elevates the boil- 
ing point of water. 
Referring now to FIG. 3, yet another variation of a 
65 fuel cell utilizing the heat and mass plate of the present 
invention is shown. In this embodiment membrane 1 1  is 
positioned adjacent to cathode 19b. Methanol is used 
as the fuel and air as the oxidant. To assure continuous 
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3,9575 35 
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operation, an acid or buffered electrolyte is employed 
which will reject the CO, produced by the electro- 
chemical reaction taking place in the cell. Heat is con- 
trolled in the same manner as previously described. 
Water is vaporized from membrane 11 in plenum 14a. 5 
The heat energy Of the vapor is subsequently expelled 
to the surroundings by recondensation of the water in 
condensation zone 14b. Capillary action returns the 
water to the cell to complete the cycle. Temperature is 
set by controlling the partial pressure of water vapor in 10 
the respective components and plenums. A pressure 
relief valve 18 is used to control the water inventory in 
the cell. Excess water vapor will be expelled from pie- 
num 14 by means of valve 18. In the FIG. 3 embodi- 
ment of the present invention the composition of the 15 
air is controlled by a suitable device designated as 32 so 
that water and heat removed from the cell by the heat 
ditions and independent of any change in humidity of 
the ambient air. A suitable device for controlling the air 20 
composition is described in U.S. Pat. No. 2,944,627. 
1. A method for controlling heat and mass inventory 
producing product mass at an electrode of said cell; 
transporting said product mass to one side of a capil- 
condensing substantially all of said mass on said one 
in a fuel cell comprising the steps of: 
lary membrane; 
side of said capillary membrane; 
vaporizing said condensed vapor at the other 
side of said membrane in a vaporization zone; 
removing excess vapor from said vaporization zone 
thereby controlling the mass inventory; 
transporting balance of said vapor to a condensation 
zone; 
condensing said vapor in said condensation zone on 
said capillary membrane thereby rejecting the heat 
of said vapor to the surroundings; and 
returning said condensed vapor through said capil- 
lary membrane to again be vaporized thereby re- 
moving heat from said reactant plenum. 
2. The method of claim 1 wherein said mass is water. 
and mass plate will be a function of cell operating con- 
* * * * *  What is claimed is: 
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Fuel cell heat and mass plate

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