Such qualities as WHAT CONSTITUTES "CONVENIENCE"? 1. High power rating for unit volume. 2. High power rating for unit weight. 3. Quietness. 4. Cleanliness. 5. Operation on air. 6. Continuous unattended operation over long periods. 7. Production of useful water. In assessing Qualities 1 and 2, the complete system (see Figure 5) must be considered. As regards Quality 4, the ultimate is a hydrogen/oxygen battery in dead-ended operation; in other cases, complete oxidation of the fuel is the most desirable way of achieving a harmless battery exhaust. Quality 6 involves reliability, maintenance, and life. Quality 7 is important particularly on space missions. WHAT MAKES FOR LOW-COST ELECTRICAL ENERGY? 1. High efficiency. 2. Low-cost fuel and oxygen. 3. Low maintenance cost. 4. Long life. 5. Low capital investment. Not all the factors determining cost have been listed. Research and development costs have been omitted because they are impossible of general assessment: terrestrial fuel batteries benefit from the knowledge gained in developing fuel batteries for space missions. Research and development costs are high absolutely, and development costs are very high relative to research costs. Adequate life tests are expensive. WHAT ABOUT EFFICIENCY? The immunity of the fuel cell to the Carnot-cycle restriction (see Figure 1) was for long its greatest attraction. In a modern central station, the Carnot-cycle efficiency could be near 65%, and the overall efficiency near 40%. The overall efficiency of smaller energy sources that the fuel cell hopes to displace is considerably less than 40%. Statements by reputable authorities often mention efficiencies greater than 65% for the fuel cell, and the popular press is sometimes even more optimistic. There are efficiencies of various kinds. We shall proceed conservatively, and define an overall efficiency called the comparative thermal efficiency for the fuel battery and for the fuel-battery system. These efficiencies are comparable with the 40% mentioned above for central stations. The two definitions are analogous. For the battery (or the system): In the denominator, AH is the higher heat of combustion of the fuel actually consumed in making available the net useful work in the numerator; some of this fuel may be consumed by peripheral equipment. In both cases, the electrical energy required by the peripheral equipment (such as pumps) with its demand for parasitic power must be subtracted from the gross electrical work (S Eidt) available at the fuel battery terminals; the system efficiency may consequently be considerably lower than the battery efficiency. The efficiency of a single fuel cell will usually exceed considerably the two efficiencies given above. Detailed discussion would take us too far afield. We shall simply say that under most conditions this cell efficiency is determined principally by the voltage efficiency under operating conditions. This efficiency is E/E, where E is the actual cell voltage and E is the maximum value of E, which can be calculated from thermodynamic data and could be realized only under completely reversible conditions. The most important single characteristic of a fuel cell is its current densityvoltage curve (Figure 6), which is an index of cell performance and therefore corresponds to an upper limit for the performance of battery and of system. The current density (not current) is chosen as abscissa, not only because current density is proportional to the rate of electrochemical reaction, but also be cause it helps determine watts/sq. ft., a ratio that helps establish the size and weight of a power source given rating. As concerns efficiency, the vital feature of current density-voltage curves is that cell voltage always decreases with increasing current density in the useful operating range. To realize maximum efficiencies, the cell would have to be operated at current densities too low for doing finite work: microamperes from a large power source are seldom useful. Overall efficiencies are often thought of primarily in their relation to fuel cost. We hope the time will soon come when such thinking is justified for fuel batteries. In this early stage of their development, however, overall efficiencies are important primarily because they determine unit capital cost (dollars per kilowatt) and in special applications (space missions, portable power sources) because these efficiencies fix the weight and volume of reactants that must be carried for doing a given amount of work. HOW RELIABLE ARE FUEL BATTERIES? WHAT IS THEIR LIFE? There can be no firm answers to these crucial questions until there has been much more experience with fuel batteries. The answers will differ with the type of battery and with the duty cycle for a given type. "Reliability” and "life" are concepts difficult of exact or general definition. In space applications where the fuel batteries are isolated and cannot be attended, life may be taken as synonymous with mean-time-to-failure, failure of peripheral equipment included. In Project Gemini, it will be remembered, all the difficulties to the time of writing were chargeable to the peripheral equipment-none to the fuel cells themselves. In terrestrial applications, where opportunities exist for adjustment, repair, and replacement, a battery or a system will have a useful life far exceeding mean-time-to-failure under the drastic conditions mentioned above. Reliability and maintenance costs cannot yet be assessed. The life of single cells under steady load in the laboratory is thousands of hours: uniformity is the key to long life. When cells are assembled to make batteries, uniformity is more difficult to achieve (see below), with the result that the life of a single cell may be shortened below what it would have been were it operated alone. Further, when cells are connected in series, and the life of an entire stack is that of the cell which is the weakest link, statistical considerations lead to a stack life reduced considerably below the average life of a single cell operated alone. For terrestrial applications, it should be possible to choose conditions so that the life of the battery limits the life of the system. This analysis is not meant to be discouraging. If individual cells show long life, as they do, electrochemical engineers should be able to design and develop batteries and systems of adequate life. WHAT OF UNIT CAPITAL COSTS? Unit capital costs (dollars per kilowatt) cannot be translated to energy costs sc long as life is unknown. What unit capital cost is reasonable depends upon the premium that the convenience of the fuel battery can command. In space missions for which the weight of other power sources is prohibitive, that premium is high. The premium is at a minimum in the usual large central stations. For a given terrestrial application (e.g., power sources for communication equipment), the premium is likely to be much higher for fuel batteries in military (as opposed to commercial) use. A simple calculation will show the importance of unit capital costs in commercial applications. Fuel batteries are often suggested for utilizing waste hydrogen. With d.c. electrical energy at 1¢ per kilowatt hour, and with hydrogen and air at no cost, a hydrogen/air battery at $300 per kilowatt installed, operating continuously and requiring no service, would produce just about enough electricity to recover the capital investment in three years. There are no fuel batteries now on sale at anywhere near $300 per kilowatt that would operate for three years under the conditions stated. Tentative estimates of tolerable unit capital costs for fuel batteries intended for commercial use will be given. These are opinions not based upon detailed information. For small (10 to 100 watt) power sources, over $1000 per kilowatt; such power sources will serve best where they can benefit from transistorized circuitry. For central stations, $100 per kilowatt. For first use in electric vehicles, $200 per kilowatt; for passenger automobiles, the ultimate dream, very much less. Building a reliable battery of adequate life for, say, $50 per kilowatt will not be easy no matter what the fuel. In this early stage in the development of fuel batteries, considerations of unit capital cost warrant the prediction that the terrestrial use of these devices will occur first in small sizes and in military applications. AT WHAT TEMPERATURES DO FUEL CELLS OPERATE? The properties of the electrolyte are perhaps the most important determinant of fuel-cell operating temperatures. Of these properties, we shall mention only the electrical conductivity. One function of the electrolyte is to complete the electrical circuit (see Figure 1) by the transport of ions, and it is desirable to keep the resulting I (2) R losses low by close spacing of the electrodes and by choosing a temperature at which there is adequate conductivity. Typical examples (temperature ranges approximate): Ion exchange membranes now available, below 100° C (See Figure 9). Doped zirconia (solid) electrolytes, 900-1200° C. WHERE IS THE BOUNDARY BETWEEN RESEARCH AND ENGINEERING? It is convenient, though imprecise, to say that the fuel cell belongs to research, and that the steps from cell to battery and from battery to system are engineering assignments. WHERE DOES RESEARCH STAND? Though research is never finished, one can say that enough is known about hydrogen/oxygen- and hydrogen/air cells to make the designing and building of good batteries feasible. Most research problems relating to energy conversion can be formulated as materials problems because the drive for high performance strains materials to their limits. We shall not concern ourselves with the usual types of materials problems, which arise in connection with sealing, corrosion, aging, decomposition, or evaporation. Electrocatalysis is the main research problem with fuels other than hydrogen. For present purposes, we may (imprecisely) regard electrocatalysis as the process that raises IR-free performance curves like those in Figure 6-that is, the process by which electrode reactions at constant temperature, pressure, and electrolyte are accelerated to give a higher current density at a given cell voltage. A good electrocatalyst must be inert toward the electrolyte, have large specific surface, active morphology, be or resemble a transition metal (see the periodic table of the elements), and (if necessary) double as a catalyst for chemical reactions that accompany the electrochemical reactions. Platinum is the best single electrocatalyst for fuel-cell electrode reactions as a group, though it is not the best for every reaction. But platinum is costly, needed for other purposes, and limited in supply. Science has not yet given us an understanding of platinum's unique position in electrocatalysis, and we have therefore no firm theoretical guide lines for attacking the electrocatalysis problem. The rates of chemical reactions increase with temperature. Though electrochemical reactions have complexities that enter into the temperature dependence of their rates, one is justified in assuming that higher temperatures bring higher rates, and that the electrocatalysis problems should be less serious at higher temperatures. This advantage will be at least partially offset by the increasing seriousness of the several types of materials problems (see above). To illustrate, an oxide-ion electrolyte resembling doped zirconia, but of greater conductivity, would permit reduced operating temperatures for cells with these solid electrolytes and make them more attractive. WHAT OF ENGINEERING? The importance of uniformity in a battery was mentioned above: only if conditions are uniform in a battery can the performance of the battery approach that realized for individual cells on a laboratory bench. The attainment of this uniformity is an engineering assignment because it depends upon the control of transport processes. A fuel battery consumes reactants and generates products and heat and electricity. The processes that transport mass, momentum, heat, and electricity must proceed at rates that maintain conditions uniform within the battery. Nonuniformity can result in many ways and have many undesirable consequences, one of the more serious of which will be illustrated in Figure 7. In addition to ensuring uniformity in the battery, the engineers must also choose materials of construction, regulate the electrical output of the battery, and make the step from battery to system. These engineering assignments have turned out to be more formidable than many had anticipated, and the engineer today carries the principal burden in making hydrogen batteries successful. WHAT ARE SOME ELECTRICAL PROBLEMS OF THE FUEL BATTERY? The electrical problems of the fuel battery are inherent in the performance curve of the fuel cell (see Figure 6). Two favorable features stand out: (1) At open circuit, voltage is maintained without measurable consumption of fuel, there being no net electrochemical reaction at zero current density. (2) Voltage efficiency, and hence overall efficiency in the usual case, is higher the lower the current density. These features make the direct fuel battery desirable for equipment that must stand ready to perform during long idling periods, or that operates most of the time at low load. These advantages may be reduced in an indirect fuel-battery system owing to the energy required to keep converter or reformer ready for operation when load increases. The low voltage of the single fuel cell leads to electrical problems, which are generally less serious with hydrogen as fuel because it gives higher cell voltages at the same current density than do most others: hydrogen might yield E=0.7 volt at current densities where hydrocarbons give E=0.3. The obvious way to obtain needed high voltages from fuel cells is to connect them electrically in series. As was mentioned above, the greater the number of cells in series, the greater the chance one cell in the stack will fail, and this will most often be a failure of the least reliable cell. This could simply open-circuit the stack, or it could have more serious consequences. If the failure resulted from an interruption of the hydrogen or oxygen supply, the other cells in the stack could "drive" the one affected and cause unwanted reactions to occur at the electrodes. This is the serious lack of uniformity mentioned above. As Figure 7 shows, this type of failure could lead to the generation of oxygen in the hydrogen (anode) chamber and to the generation of hydrogen in the oxygen (cathode) chamber, clearly an undesirable state of affairs. For cells connected in parallel, complete failure will usually not occur until the most reliable cell has failed although there will have been a decrease in current prior to complete failure. From the standpoint of reliability, it is desirable to minimize series-and maximize parallel connections. There is a limit to how far one can go. Maximizing parallel connections implies the handling of large currents and the incurring of high I (2) R losses, and there is the added difficulty that most electrical equipment operates at voltages considerably above that of a single cell. Solid-state dc-dc converters are now available at ratings from 20 watts to a few kilowatts, but these are inefficient at low input voltages. They are nevertheless valuable because they make it possible to reduce the number of cells connected in series, the extent of maximum reduction being set by the conversion inefficiency considered tolerable, and by the probability of failure of a cell in the stack. De-ac inversion for small loads can also be accomplished, but only with heavier and more costly equipment than dc-dc conversion requires. At present, we do not believe that inversion of fuel-battery power on a central-station scale need be considered; if such power can compete on this scale at all, it will have to compete for de applications, notably in the electrochemical industry. The industry provides a large market: perhaps 5% of the 200,000,000 kw total American generating capacity serves this market, about half of which produces aluminum. The performance curve in Figure 6 also permits conclusions about operation at various power levels. As Figure 8 makes clear, operation at maximum power density is possible only at reduced efficiency, and this reduction becomes prohibitive at current densities above that for maximum power density. HOW ABOUT THE FUEL BATTERY AS A CHEMICAL PLANT? In space, the water generated by H/O batteries will be drunk or used in other ways. The fuel battery will then be not only a dc generating plant but a chemical factory as well. Is this appealing concept likely to prove widely useful on earth? We think not. It is true that many important chemicals are produced by oxidation, and that such oxidation can often be done advantageously at an anode. Although we do not exclude the possibility that electricity may be a useful by-product in special cases such as the oxidation of sodium amalgam in the preparation of caustic, we do not think the combination of chemical factory and fuel cell will prove generally useful for these reasons (3): (1) The amount of electrical energy produced annually by the power industry is so large that the by-product electricity we are considering will appear very small beside it. For example, a rough estimate shows that the electrical energy produced in the United States in one month (5,6 x 10' kilowatt hours) is equivalent to all the sulfuric acid (32 million tons) made here in two years. (SO, is assumed as starting material.) Sulfuric acid was chosen because it is a high tonnage chemical, not because it is adapted to manufacture in a fuel cell. It follows that any chemical made in amounts below 1 million tons annually could not produce by-product electricity in significant amounts. (2) The value of such by-product electricity is low relative to that of the chemical produced. This is true even in the case of sulfuric acid: less than 1 cent for the kilowatt-hour equivalent to 12 pounds of acid worth about 12 cents. This twelve-fold ratio will be much greater with most other chemicals. (3) An electrochemical device must usually meet different requirements for the optimum generation of electricity and for the optimum production of a chemical. Conditions for the latter process can be more closely controlled if a voltage is imposed on the cell-if electricity is consumed instead of generated. An improved yield or a chemical of better quality should usually justify this approach. SHOULD FUEL BATTERIES BE CONSIDERED FOR ENERGY STORAGE? In space, yes, if solar-energy converters are available. The scheme here is to convert an excess of solar energy into electrical energy during the orbital day, use this excess to electrolye a working substance (e.g., H2O), and recombine the products of electrolysis (H, and O2) in a fuel battery to produce electrical energy during the orbital night. Electrolyzer and fuel battery here constitute a regenerative system; the two may be the same device. Such energy storage sounds attractive, but there are problems with both the solar converter and the electrochemical system. A recent article on pumped storage by Friedlander (4) shows that this method of storing energy on a large scale is so economical as to make competition by electrochemical regenerative systems (see above) appear hopeless. The efficiency of such systems, being the product of the efficiencies of fuel battery and electrolyzer, is much lower than that of fuel battery alone. WHAT IS THE PRESENT OUTLOOK FOR FUEL BATTERIES? Anyone called upon to answer this question is entitled to quote Mr. Justice Holmes (5): “Every year if not every day we have to wager our salvation upon some prophecy based upon imperfect knowledge." This prediction (6) was made before 1960: "The current increase in fuel cell activity, if maintained, makes it likely that fuel cells will serve as power sources in special applications within the next 5 years. Successful, practical model cells are already with us. The future of central-station fuel cells cannot be predicted today." Figure 4 and Figure 9 show that the first sentence of this prediction was not rashly optimistic. Next, the reader should examine a recent, authoritative, and more detailed prediction by Lord Rothschild (7) speaking for "Shell" Research Ltd., where important fuel-cell work is being done. This is a conservative prediction, reconcilable with that to be made below. The prediction that follows is made within these boundary conditions: 1. It is based on the published material we know. 2. It includes applications, such as space and military, in which the fuel battery commands a premium for convenience. 3. It assumes that air, when available, will be used at the cathode. Air is considered unavailable in space and under water. The prediction will not be documented and only a few examples will be cited. Space. The fuel battery has established itself for space missions (General Electric; Figure 4). Future missions are scheduled to use fuel batteries by |