Fuel Cell Systems - ACS Publications

1 NASA's Fuel Cell Program ERNST M. COHN

Downloaded by TULANE UNIV on October 5, 2013 | http://pubs.acs.org Publication Date: January 1, 1969 | doi: 10.1021/ba-1965-0047.ch001

National Aeronautics and Space Administration,

Washington,

D.

C.,

20546

NASA's fuel-cell program spans the range from basic research to hardware. University grants cover studies of potentials of zero charge, deactivation of catalysts with time, differences between chemical and electrochemical catalysis, porous electrodes, and basics of biochemical fuel cells. Basic engineering research concerns pulsed operation of fuel cells with mechanical pulsing. Applied research on heat and masstransfer device includes dual-membrane fuel cell, one with aqueous caustic electrolyte retained in an asbestos matrix, and an inorganic membrane. Two kinds of regenerative systems are being investigated—an electrolytically regenerative hydrogen-oxygen cell and a thermally regenerative potassium concentration cell. The ion-exchange-membrane power package for Gemini and the molten KOH system for Apollo are well under way.

J h e goal of NASA's fuel cell program is to obtain lightweight, dependable power sources for a variety of needs. These may include communication; command and control; guidance; radar; image acquisition, processing, and transmission; data handling and storage; life support; experiments on planetary surfaces and environment; and power for surfaceexploration vehicles. Among the major factors to be considered i n designing space type fuel cells are: (1) the need for very high reliability, since chances for repair are extremely limited even on manned missions; (2) high energy and power densities, because it costs between $1000 and $5,000 to put a pound of substance into space, and our lift capabilities are limited while power requirements keep increasing; (3) the space environment (where 1 In Fuel Cell Systems; Young, G., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1969.


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gravity is absent) and the planet surface (which varies from that on earth), where radiation and meteoroids present hazards, where temperatures can fluctuate widely, and where there is no atmosphere providing oxygen to act as a heat sink. Work is now i n progress on the low-temperature fuel cell, using an ion-exchange membrane as electrolyte, which w i l l power the Gemini spacecraft (Figure 1) and on two versions of the intermediate-temperature modified Bacon fuel cell for the Apollo vehicle and its Lunar Excursion Module (Figure 2). These developments represent the first functional uses of fuel cells. Such multi-million dollar programs (about $50 million) for developing flight equipment far eclipse the more moderate research and development program of N A S A . The former are the responsibility of the Office of Manned Space Flight, the latter of the Office of Advanced Research and Technology ( O A R T ) .

Figure 1. Interior and cannister of Gemini fuel cell In fiscal year 1964, O A R T spent about $1.8 million on fuel cell projects ranging from basic research to prototype development. A t this time, I can select only a few examples of our work to illustrate the range of problems it covers and to give some of the reasons for undertaking these projects.

In Fuel Cell Systems; Young, G., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

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O n a N A S A grant, Professor Bockris and his co-workers at the U n i versity of Pennsylvania are studying the dynamic behavior of porous electrodes, potentials of zero charge, and differences between chemical and electrochemical catalysis, among other topics. As part of their work on direct energy conversion i n general, they are also exploring the fundamentals of bioelectrochemistry. F r o m these studies we hope to acquire information useful for all kinds of fuel cell systems. I shall return to biochemical fuel cells later.

Figure 2. Module of Apollo fuel cell A n interesting hybrid between conventional batteries and fuel cells is advanced by Bernard Gruber. H e proposes to impregnate a dry tape with anodic and cathodic material, one on each side, and add electrolyte just before running the tape through two current collectors. In this manner, one can activate the ingredients immediately before use, thus making possible indefinite storage as well as combinations of normally incompatible materials. This work is well underway at Monsanto and


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promises to yield high-energy-density electrochemical power sources that may compete with both primary batteries and primary fuel cells. The need for storable reactants—for emergency use or energy-depot purposes—might also be met b y developing fuel cells with multi-chemical capabilities which might utilize residual or excess amounts of rocket propellants, such as U D H M and nitrogen tetroxide ( Figure 3 ). In devising space power systems, w e must consider not only the power source but also the equipment it runs. As a crude rule of thumb, we may assume 25% of the output w i l l be needed as alternating current, 25% as direct current, and the remainder as either a.c. or d.c. Further-


235 WATT- HOURS POUND

TAPE TIME, HOURS Figure 3.

Projecte4

energy densities for magnesium/m-dinitrobenzene

fuel cells


dry tape.

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more, various devices w i l l be operated at different voltages. Thus, power "conditioning" is an important factor i n considering the electrical system as a whole. Mechanical and/or electric pulsing of fuel cells (Figure 4)—now being studied on grant as well as contract—may yield several kinds of advantages: longer operating life, improved resistance to poisoning of catalysts, lower concentration polarization, and greater output from the fuel cell battery. Better circuit control and higher conversion efficiency from the over-all system may be obtained by quasi-a.c. operation. Needless to say, such benefits, particularly as concerns the fuel cell proper, might be even greater in ground applications where hydrocarbons or alcohols are used directly as anodic fuels.

Figure 4. Equipment for mechanical pulsing of fuel cells Research on high performance, thin electrodes that promise drastic cuts i n fuel cell weight and volume should benefit both earth and space applications. Over the last two years or so we have progressed from about 150 lbs./kw. to about 70 lbs./kw., exclusive of fuel and fuel tankage; 30-40 lbs./kw. for fuel cell plus auxiliaries now appears to be i n sight. Work underway at Allis-Chalmers is directed not only at obtaining a space type, low-temperature, hydrogen-oxygen fuel cell, with an asbestos retainer for the electrolyte, but is also concerned with finding a simple and reliable method for removing heat and water with the minimum of mechanical moving parts and minimum need for parasitic power (Figure 5 ) . This could be done by evaporating water through a capillary membrane adjacent to the electrodes. The cavity behind the membrane should be evacuated to a pressure corresponding to the vapor pressure



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Figure 5. Schematic diagram of fuel cell with passive water removal system

of the K O H electrolyte at its operating temperature (about 200° F . ) . Feasibility of such an arrangement has been demonstrated. Control is simple, and temperature is not a critical factor. Primary fuel cells are those through which reactants are passed only once. They are useful i n space for only limited periods because the product of power and duration ( = energy) determines the amount of fuel and oxidant that must be carried aloft. For extended missions, therefore, other primary sources of energy must be used. In connection with solar and nuclear energy sources and conversion devices, fuel cells may be used for energy storage, as secondary power sources during darkness (solar primary power), during emergencies, and during periods of peakpower demand. Among the methods of regenerating reactants from products, only electrolysis and thermal treatment have shown promise. E v e n so, it is not yet clear whether regenerative fuel cells w i l l be competitive with secondary batteries or other secondary conversion devices. A t present, we have only one effort under way on a secondary or regenerative fuel cell: a low-temperature hydrogen-oxygen cell with electrolytic decomposition of water. It might be useful i n connection with solar energy i n a synchronous satellite. Here the disadvantages of inefficiency—as compared with a secondary battery—may be compensated


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by weight savings effected by storing energy i n the form of H + O2 i n stead of metal + metal oxide. Biochemical fuel cells captured the public imagination some time ago. Further exploration of this 50-year-old concept has indicated rather severe power density and energy density limitations for such cells. Nevertheless, biocells are likely to find special uses as energy-saving waste disposals for extended space flights, during which human waste must be reprocessed for attaining a closed or nearly closed ecology. N A S A has supported a three-fold attack on this problem by sponsoring basic, applied, and developmental studies, aimed at finding materials and conditions to dispose of human waste. Since the power consumed by such a device would undoubtedly exceed the theoretical—let alone the realizablepower output, this biocell was obviously not intended to produce power. Even so, it became apparent that the low power densities would require weights and volumes of equipment completely out of proportion to any possible benefits. Resuming applied research would become attractive only if much more active enzymes or organisms were developed, if efficient charge-transfer media were found, or if it could be proved that direct electron transfer from enzyme or organism to electrode can take place. If something like a one hundred-fold increase in power density can be achieved in biocells, they might be re-evaluated for this purpose. Similarly, biocells might be used to solve problems of water pollution, and the power produced would be a welcome byproduct.


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What do we expect from space-type fuel cells? Our immediate, prime considerations are high power density and reliability. The Gemini and Apollo fuel cells, for example, w i l l be about Ve to V10 the weight of the best available primary batteries that are capable of delivering the same total amount of energy. Furthermore, the water product, an additional bonus not available from batteries, w i l l be used by the astronauts. Other requirements may become important for different space applications. Longevity and ease of maintenance, for example, could well be the desiderata for fuel cells used at a lunar station or depot. Ease of packaging, storing, and converting chemicals to active species (say, hydrogen and oxygen ) may determine what types of fuel cell w i l l be best for propulsion on the moon or for powering space suits. Apart from requiring a variety of fuel cells, each optimized for a particular task, we expect to see a much more sophisticated operation of fuel cell systems. Increasing attention is already being directed toward optimization of controls and operating conditions. E a c h system must be optimized to take advantage of the leeway permitted by its size, components, and operating variables. F u e l cells must become truly integrated into the systems of which they w i l l be parts. I already mentioned biochemical fuel cells as primary chemical reactors, and the Gemini and Apollo fuel cells as sources of


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potable water. Not only byproduct chemicals, but also byproduct heat could be useful. Once we have reliable information about the composition of the lunar surface, we may need to develop fuel cells particularly suited for lunar purposes and independent of supplies from earth. This brief discussion of NASA's fuel cell program indicates some of the difficulties we face and how we attempt to overcome them. Our task is to provide N A S A with reliable, optimized fuel cell power that w i l l be applicable to many different jobs under a great variety of space and planetary conditions. Virtually all of the information obtained i n its pursuit should be as useful for earth-bound as for space type fuel cells. Thus, we hope not only to solve a part of the space power problem, but also to contribute to advancing fuel cell technology that w i l l benefit our economy. R E C E I V E D April 2 7 ,

1964.


Fuel Cell Systems - ACS Publications

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