3
Solid
Inorganic
Electrolyte
Regenerative
Fuel Cell System
C. B E R G E R and M. P. S T R I E R Astropower Laboratory, Douglas Aircraft Co., Inc., Missile and Space Systems Division, Newport Beach, Calif.
Sintered zirconium phosphate membranes containing zeolites have significant water absorptive capacities over a temperature range of from ambient to 150°C. This feature makes them sufficiently conductive for fuel cell applications over this temperature range. Such membranes have transverse strengths of 5000 to 6000 psi. They readily gain and lose water vapor in a reversible manner while maintaining good stability. This accounts for promising results obtained to date for regenerative hydrogen-oxygen fuel cells using this membrane. In addition to presenting regenerative hydrogen-oxygen fuel cell data, water absorptivity and conductivity data are given and interpreted in terms of membrane composition and structure.
' " p h e earliest reported use of zirconium phosphate membranes as a solid electrolyte for hydrogen-oxygen fuel cells dates back to 1961 (9,10, IS). Astropower Laboratory has been investigating the electrochemical behavior of modified zirconium phosphate structures from both a fundamental as well as a developmental aspect. Significantly, a comprehensive investigation of composition and fabrication techniques, as they are related to membrane strength, conductivity, and hydrolytic stability, has led to deriving useful solid electrolyte structures. This is evidenced b y successful hydrogen-oxygen fuel cell tests over the temperature range of 25° to 148°C. (6). M o s t characteristic of these membranes is an incorporated zeolite component serving a water-balancing function b y virtue of its high affinity for water and low rate of desorption. I n this manner, the conductivity of the membrane is maintained at a suitable level over wide temperature A
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REGENERATIVE EMF CELLS
limits. Therefore, it was our belief that such membranes were ideally suited for regenerative hydrogen-oxygen fuel cells. Hydrogen-oxygen fuel cells employing such membranes have performed continuously for over 1000 hrs. in the 60° to 75°C. temperature range at current densities of at least 25 ma./sq. cm. at 0.5 volt. Tests have run at 50 ma./sq. cm. and 0.65 volt and at 30 ma./sq. cm. and 0.73 volt for over 300 hrs., as shown in Figure 1. Constant performance level prevails over the temperature range of 65° to 120°C. owing to the fact that the membrane tends to maintain a constant level of conductance over this temperature range. Current densities as high as 118 ma./sq. cm. at 2.5 volts have been obtained in electrolysis experiments. This presentation is concerned with a description of the pertinent properties of the Astropower Laboratory zirconium phosphate-zeolite membrane system as they are related to independent electrolysis and fuel cell operation and subsequently regenerative fuel cell cycle. Historical The principal advantages to be gained from using solid electrolytes in fuel cells are compactness, simplicity of design, and few zero gravity 800 - ,
,
0
100
.
200
,
300
Time (Hrs)
Figure 1. Synopsis of fuel cell life test at 65 =b 1°C; Astropower zirconium phosphate-Zeolon-H membrane voltage vs. time at current density of SO ma./sq. cm.
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Solid Inorganic Electrolytes
19
limitations. The first ion membrane fuel cell involving hydrogen and oxygen was described in 1959 (22), and its capability in a regenerative fuel cell was described initially in 1960 (8). The device used an organic cationexchange membrane derived from sulfonated phenol formaldehyde polymers. This membrane system lacked strength and was unstable at slightly elevated temperatures. The temperature limitation characteristic of organic polymeric ion exchange membrane systems remains up to the present time. Efforts have been made to alleviate such shortcomings by such innovations as the Hydrogen-Bromine Fuel Cell (HBFC) and the Dual Membrane Fuel Cell (DMFC) (7, 21). In the fdrmer, a sulfonated polystyrene cation-exchange membrane separates the anode from the catholyte compartment (liquid phase) comprised of an aqueous solution of bromine and HBr. In the Dual Membrane Fuel Cell, two of the same type sulfonated polystyrene cation-exchange membranes separate the anode from the cathode with an intervening 6N H S 0 solution between the membranes. The various advantages and disadvantages of both the H B F C and D M F C systems have been discussed by Berger et al., previously (7, 20, 21). Some progress has also been recorded in another kind of regenerative fuel cell in applying aqueous K O H absorbed in asbestos in a solid electrolyte (11, 12). Pioneering work on zirconium phosphate used as cation-exchange material has been performed by Kraus (14,16,16,17), Amphlett (1, 2, S), and Larsen and Vissers (19). Hamlen has made a study of the conductivity of. zirconium phosphate under various conditions of hydration and found, that at the highest level of hydration, the mechanism of conductance corresponded to that in an aqueous phase. Both Hamlen (18) and Dravnieks and Bregman (9, 10) in 1961-2 reported on hydrogen-oxygen fuel cell studies with solid zirconium phosphate membranes at ambient temperature. The performances were promising although the membranes were weak. 2
4
Astropower Zirconium Phosphate'Zeolite Membrane Studies General Considerations. The fuel cell investigation has been in effect at Astropower for three years under NASA sponsorship (5). The impetus behind the selection of inorganic membranes as a route to achieving improved fuel cell performance can be outlined as follows. First, a strong skeleton network is required to provide necessary physical strength and ionizing functions for establishing an electrolytically conducting system. Second, an incorporated water-balancing agent is required to retain sufficient water in the membrane for appropriately high electrolytic conduction.
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REGENERATIVE EMF CELLS
Membrane Composition and Fabrication Studies. After eval uating the strength of a number of membrane systems during the early stages of the program, it was found that the best results were achieved by the sintering of zirconium dioxide with phosphoric acid and "Zeolon H , " a Norton Co. synthetic alumino-silicate zeolite. Other potential bonding materials, such as various silicate-based proprietary mixtures, yielded relatively fragile systems (δ). An explanation for the bonding characteristics of phosphate mate rials is as follows: Acid phosphate groups such as HP0 ~~ have strong ligand properties which allow them to coordinate with cations of periodic groups II and III, as well as the transition elements. Cations already incorporated in the coordination compounds may still react with acid phosphate groups if the ion concentration is sufficiently high to shift the reaction equilibrium appreciably in favor of phosphate combinations. Thus, when finally divided metal oxides are treated with concentrated phosphoric acid, oxygen atoms are partly displaced by the phosphate group. On heating, the phosphorylated oxides dehydrate and condense the acid phosphate groups into linkages between the phosphate tetrahedra. The mixture then becomes fused into a solid mass. Such techniques for producing inorganic membranes as hot pressing, cold pressing and sintering, or by casting and sintering have been inves tigated. Optimum properties such as high transverse strength and low resistivity were obtained by either cold pressing and sintering or by cast ing and sintering. Our extensive study of the system of zirconia-phosphoric acid and "Zeolon H , " based on its having shown early promise, has led to developing membranes having suitable properties for fuel cell application. Maximum transverse strengths of 5000 to 6000 psi can be obtained by using stabilized zirconia. Moreover, reactivity with phosphoric acid, and thus higher transverse strength, is enhanced by smaller crystal line size. In addition, a strong bond developed between zirconia and phos phoric acid during sintering rather than during material drying and mixing stages. Sintering temperatures in the 300° to 800°C. range have produced strong membranes, having resistivities as low as 20 ohm-cm. measured at 73% relative humidity and 105°C. Water Vapor Adsorption Capability and Relationship to C o n ductivity. Water vapor adsorption characteristics were determined by means of a controlled atmosphere thermobalance. This device consisted of a McBain balance employing a quartz spring. Water pickup was measured by suspending the sample membrane in a furnace tube adjusted to the desired temperature and relative humidity. The extension of the spring was followed by a cathetometer to measure changes in weight due to mois ture pickup (4). Figure 2 shows curves depicting the water vapor adsorption charac teristics at 12S°C. of three different zirconium phosphate membranes pre2
4
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BERGER AND STRIER
Solid Inorganic Electrolyte*
T e m p * r * t u r « : 125 \ 2 ° C — A _ ii«mkM«A I Λ ) Î U 1 I , 1 Λ · MCIDDr«rv# J U 6 « u o t - i ; ι
21
c»r\j^,
Β · M e m b r 4 n « 102-021-6; 1 Z r O , . 1 H , P O . , C - M«mbr4n« 6-Oil-1; 1 Z r O j . 1 H , P 0
1 "AW-500"
4
A
Δ
Β
C
•o 100
— O — — 200
300
400
S00
h 00
(mm Hf )
Figure &. Water adsorption of typical membranes
pared in our laboratory at different relative humidities and partial pres sures of water. The membrane made of only Z1O2 and H8PO4 (Curve C) showed no water adsorption. However, for the composite membranes in cluding the zeolites, either "Zeolon H " or " A W 500" with zirconium phosphate, appreciable water adsorption occurs. The amount of water adsorbed is much greater than can be accounted for by the presence of the one-third by-weight quantities of the respective zeolite in the mem brane structure. Water adsorption isotherms for a composite membrane system of zirconia, phosphoric acid, and "Zeolon H , " prepared in equal weight ratios, are given in Figure 3. Measurements were taken at 71°C, 90°C, 125°C, and 158°C. at various partial pressures of water vapor. Lines of constant relative humidities are shown at the lower temperatures. A t
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100
200
300 p
Figure 8.
400
500
600
H O (mm Hg) z
Water adsorption isotherms ZrOrHiPOi-'Zeolon-H" membrane (1:1:1 composition) t
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REGENERATIVE EMP CELLS
constant partial pressures of water vapor the ability of the membrane to adsorb water decreases with increasing temperature. On a constant relative humidity basis, the trend is still evident. What is most significant in these measurements, however, is the fact that this membrane system has an affinity for water even at temperatures as high as 158°C. Employing a related membrane system in the same conditioning apparatus, resistance measurements were performed at temperatures of 70°C, 90°C, and 105°C. at relative humidities ranging from 26 to 83%. Test membranes were held between platinized electrodes having approximately 1.0 sq. cm. surface area. Resistivities were calculated from the membrane resistance measured by means of an alternating current bridge circuit at 1000 cps. A plot of logarithm of the resistivity vs. % relative humidity is given in Figure 4. Resistivities decrease with increasing relative humidities, which is consistent with a gain in water content. However, this trend diminishes drastically with decreasing temperature. From Figure 3, at 71°C, the moisture content is somewhat higher than at 90°C. and apparently at 105°C. as well, for the same relative humidity. There-
9 U
70 C
90°C
's
105°C
20
30
40
50
60
70
80
90
100
% Relative Humidity
Figure 4. Log resistivity vs. % relative humidity for the membrane at 7CPC, 90°C. and 105°C. f
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STRIER
Solid Inorganic Electrolytes
23
fore, the increase in conductivity with rising temperature is apparently not related to the change in water content of the membrane. According to the data in Figure 3, membrane resistivities at 90°C. and 105°C. approach the resistivity level at 70°C. as the environmental humidity decreases. This means that despite the tendencies of these membranes to dehydrate at higher temperatures their conductivities do not decrease to levels lower than those of lower temperatures. Fuel cell performance is invariant over the temperature range of 65°C. to 120°C, which could be related to the tendency of the conductance of the membrane to remain constant over this temperature range. Arrhenius plots of the resistivity data at constant relative humidity over the 70° to 90°C. temperature range give activation energies ranging from 3 kcal./mole at 50% relative humidity to 16 kcal./mole at 90% relative humidity. This means that the rate of decrease in resistivity with rising temperature increases directly with higher relative humidity; this may be due to an increase in the amount of current-carrying ions resulting from the increased humidity. On the basis of such studies, it must be concluded that the zirconium dioxide-phosphoric acid-"Zeolon H , " membrane system offers promise in regenerative fuel cell operations as high as 100°C. In the sections which follow the results of electrolytic fuel cell and regenerative fuel studies with this membrane are described. Electrolysis Studies. Electrolysis studies were conducted on a zirconia-phosphoric acid-"Zeolon H " membrane. This material had a resistivity at 110°C. of 10.3 ohm-cm. and 4.0 ohm-cm. at 50% and 100% relative humidity, respectively. The two-inch membrane was sandwiched between a Teflon-bonded platinum black-tantalum electrode screen. A small amount of platinum black was added to the screen electrodes. Then, the assembly was clamped between one sq. cm. platinized electrodes and placed in a test chamber maintained at 60% relative humidity at 25°C. The configuration is shown in Figure 5. Current-voltage characteristics as a function of time are given in Figure 6. Figure 7 shows the variation in current density for this membrane maintained continuously at 2.5 volts for 400 min. at 25°C. and 60% relative humidity. There is a relatively slow decline in current density with time at each voltage level in Figure 6. That the cell could operate for almost 7 hrs. at 2.5 volts (Figure 7) indicates that a continuous process of water vapor adsorption is possible for these membrane systems. Water vapor is continuously being adsorbed by the membrane and moves through the membrane to the electrodes, where it is electrolyzed, forming oxygen and hydrogen. Under the proper design conditions, adsorption and electrolysis rates could be properly balanced at the desired voltage and current operating levels so that the unit would operate as a continuous water vapor electrol-
REGENERATIVE BMP CELLS
Figure 5. Experimental electrolysis cell
Figure 0. Current density at various voltages (membrane 191-047) ateô ± 1°C. and 60% relative humidity
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Solid Inorganic Electrolytes
Relative Humidity 60% Temperature 2 5 ° C
80
100
200
300
400
T i m e (Min)
Figure 7. Variation of current density with time at 2.5 volts for an electrolysis cell using inorganic membrane electrolyte No. 191-047 (continuous operation)
Figure 8. Astropower analytical fuel cell
ysis unit. These data show that such solid membrane electrolytes can be effectively and efficiently used in regenerative fuel cells. Fuel Cell Studies. The zirconium phosphate-"Zeolon H " membrane systems have been evaluated in laboratory type fuel cells at temperatures ranging from ambient up to 148°C. A schematic diagram of one type of Astropower Laboratory fuel cell designed to accommodate a 2-in. diameter membrane is given in Figure 8.
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REGENERATIVE EMF CELLS
The electrodes were of a Teflon-platinum black, tantalum screen configuration obtained from American Cyanamid. About 0.3 gram powdered, platinum black was sprinkled on each side of the membrane prior to assembling the membrane-electrode-backup plate wafer. The membrane used in the results to be described was prepared in the following manner. An initial mixture of Zr0 and 85% H P 0 was prepared in 1:1 weight ratio and sintered at 200°C. Then, the sintered material was crushed and ground to minus 80 mesh and mixed with equal parts of 85% phosphoric acid and "Zeolon H . " After drying and pressing into 2-in. diameter membranes having a thickness of approximately 0.7 mm., it was sintered at 500°C. for 2 hrs. This membrane had a transverse strength of 5200 psi. Figure 9 summarizes the fuel cell operational characteristics—i.e., current densities plotted against time, for this membrane in separate life tests performed at 25°C, 64°C, and 75°C, respectively, all at 0.5 volt. (Electrolysis experiments were performed at the conclusion of each test and will be described below. Actually, the fuel cells were all still functioning at the moment of termination of the discharge cycle. At 25°C, current densities of 20 to 28 ma./sq. cm. for 624 hrs. operation were recorded. They ranged from 20 to 52 ma./sq. cm. for 1174 hrs. at 64°C. and at 75°C; the range was 22 to 32 ma./sq. cm. for 912 hrs. This is not considered the optimum performance capability for such membranes because we do not consider that membrane properties or catalyst configurations have been optimized. For example, by improving the mode of platinum catalyst application such as impregnating it into the membrane 2
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4
60
10H
100
200
J00
4Ô0
500
600
700
800
900
1000
Figure 9. Inorganic membrane fuel cell operating data at 0.5 volt for zirconium dioxide-phosphoric-'Zeolon H" membranes at 26°C, 64°C., and 75°C.
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I
-90
-80
I
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Solid Inorganic Electrolytes
STRIER
I
-70 -60
I
I
I
-50
-40
-30
Charge
I
I
-20 -10
I ι ι ι ι ι ι ι 0
10
20
30
Current Density m a / c m
2
40
50
60
70
I 80
I 90
Discharge
Figure 10. Zirconium phosphate-"Zeolon H" membrane fuel cell charge and discharge operational characteristics
system by a sintering procedure, it has been possible to improve perform ance by as much as 20% (6). Regenerative Fuel Cell Study. The fuel cells performing for the indicated number of hours in Figure 9 were utilized as regenerative fuel cells at these three temperature levels. Electrode polarity was reversed and voltages ranging from 3.0 to 0.8 were applied across the cell. The corresponding currents were recorded as generated hydrogen and oxygen. The results obtained are plotted in Figure 10 as charge-discharge curves. According to these curves, the highest current densities for electrolysis are obtained at 25°C, with the results at 64°C. and 75°C. being essentially the same. This is consistent with the results of the prior electrolysis studies described above which indicated that the absorptive capacity of these membranes for water was inversely related to temperature. At 25°C. the decomposition potential for water appears to be approximately 1.9 volts at a current density of 30 ma./sq. cm. These data show that such zirconium phosphate-"Zeolon H " mem brane systems are applicable to regenerative hydrogen-oxygen fuel cells. They are physically strong, thermally stable, have low resistivity, and absorb water efficiently. Therefore, by appropriate design consideration, a unit capable of electrolyzing water to hydrogen and oxygen on the charging cycle can be constructed, and the stored gases on discharge can be utilized. The membrane will have absorbed sufficient H 0 during this discharge process to regain its initial equilibrium moisture content. 2
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REGENERATIVE Ε MF CELLS
Summary and Conclusions (1) Zirconium phosphate membranes œntamingincorporated zeolites serving as water-balancing agents do have significant water absorptive capacities. (2) B y virtue of extensive composition and fabrication studies, it has been possible to prepare zirconium phosphate-"Zeolon H " membranes having transverse strengths in the 5000 to 6000 psi range and higher, while simultaneously possessing low enough resistivities to be suitable for hydrogen-oxygen regenerative fuel cell application. (3) Electrolysis studies on these membranes at 2 5 ° C . have indicated that current densities as high as 125 ma./sq. cm. at 2.5 volts can be achieved. (4) Because such membrane systems gain and lose water vapor readily in a reversible manner while maintaining good stability, their potential as the solid electrolyte in regenerative hydrogen-oxygen fuel cells appears favorable. Acknowledgment The authors wish to thank F . Arrance, M . Plizga, and G . Belfort of Astropower Laboratory for their experimental contributions. Some of this work has been supported by NASA-Lewis Research Center Contracts N A S 7-150 and 3-6000.
Literature Cited (1) Amphlett, C. B., McDonald, L . Α., Burgess, J . S., Maynard, J . C., J. Inorg. Nucl. Chem. 10, 69 (1959). (2) Amphlett, C. B., McDonald, L . Α., Redman, M . J . , Chem. Ind. 1956, 1314. (3) Amphlett, C . B., McDonald, L . Α., Redman, M . J., J. Inorg. Nucl. Chem. 6, 220 (1958). (4) Astropower, Inc., "Investigation of Zeolite Electrolytes for Fuel Cells," NASA Contract NAS 7-150, Quarterly Progress Report 108-03, period ending 18 March 1963. (5) Astropower, Inc., "Investigation of Zeolite Electrolytes for Fuel Cells," NASA Contract NAS 7-150, Final Report 108-F, March 1964. (6) Astropower Laboratory, "Inorganic Ion Exchange Membrane Fuel Cell," NASA-Lewis Research Center, Contract NAS 3-6000, Quarterly Report S M 46221-Q3, period ending April 1965. (7) Berger, C., "The Current State of Development of Fuel Cells Utilizing Semi permeable Membranes, "Presented before the Division of Fuel Cell Chemistry, American Chemical Society, New York (Sept. 8-13, 1963). (8) Bone, J . S., "Regenerative Ion-Exchange Fuel Cell System," Proceedings of the 14th Annual Power Sources Conference, Atlantic City, New Jersey, May 1960. (9) Dravnieks, Α., Boies, D . B., Bregman, J . I., Proceedings of the 16th Annual Power Sources Conference (May 22-4, 1962), Session on Fuel Cell Materials and Mechanisms, p. 4-6. (10) Dravnieks, Α., Bregman, J . I., Fuel Cell Symposium of the Electrochemical Society, Detroit, October 1961.
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(11) Electro-Optical Systems, Inc., "Fuel Cell Assemblies," J P L Contract 950258, Ν A S 7-100, EOS Report 3070, Final, March 25, 1963. (12) Electro-Optical Systems, Inc., "Hydrogen-Oxygen Electrolytic Regenerative Fuel Cells," NASA-Lewis Research Center, Contract NAS 3-2781, EOS Report 4110-2Q-1, period ending June 26, 1964. (13) Hamlen, R. P., J. Electrochem. Soc. 109, 746 (1962). (14) Kraus, Κ. Α., Abstracts of Papers, 135th Meeting, ACS, April 1959, p. 17M-48. (15) Kraus, Κ. Α., Chem. Eng. News 34, 4760 (1956). (16) Kraus, Κ. Α., J. Am. Chem. Soc. 78, 694 (1956). (17) Kraus, Κ. Α., Proc. Intern. Conf. on Peaceful Uses of Atomic Energy, Vol. 7, 113, 131 United Nations (1956). (18) Kraus, Κ. Α., Nature 177, 1128 (1956). (19) Larsen, E. M., Vessers, D . R., J. Phys. Chem. 64, 1732 (1960). (20) Lurie, R. M . , Berger, C., Shuman, R. J . , "Ion Exchange Membranes in Hydrogen-Oxygen Fuel Cell," Presented at the American Chemical Society Fuel Cell Symposium, Chicago, Ill., September 6 and 7, 1961. (21) Lurie, R. M . , Berger, C., Viklund, H . , J. Electrochem. Soc. 110, 1173 (1963). (22) Nedrach, L . W., "The Ion-Exchange Membrane Fuel Cell," Proceedings of the 13th Annual Power Sources Conference, Atlantic City, New Jersey (April 29, 1959). RECEIVED January 21, 1966.
