Technology of Hydrogen-Oxygen Carbon Electrode Fuel Cells

13 Technology of Hydrogen-Oxygen Carbon Electrode Fuel Cells

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 26, 2015 | http://pubs.acs.org Publication Date: January 1, 1969 | doi: 10.1021/ba-1965-0047.ch013

LAWRENCE M. LITZ and KARL V. KORDESCH

Parma Research Laboratory, Union Carbide Corp., Parma, Ohio

The translation of successful laboratory systems to useful practical devices often presents variety of technological problems.

a

This article

presents some of the important considerations involved in producing rugged, long life, high current density batteries based on the carbon electrode fuel cell.

Integration of advanced

battery and systems engineering principles with new developments in thin, carbon-based electrodes is leading to battery power densities of approximately 20 pounds per kw. and complete power plants, excluding the fuel supply, of the order of 40 pounds per kw. depending on the size.

|ntensive work over the past several years continues to sustain the advantages of carbon electrode fuel cell batteries i n terms of low cost, commercially interesting systems. Laboratory studies have shown that Union Carbide carbon fuel cell anodes and cathodes have rather flat polarization curves through current densities of several hundred amperes per square foot and limiting current densities on the order of thousands of amperes per square foot ( I ) . Oxygen electrodes have attained lives of the order of 4000 hours at current densities of 200 amp./sq. ft. and hydrogen electrodes over 2000 hours at 100 amp./sq. ft. i n life tests i n volving daily probing to 350 amp./sq. ft. However, limitations imposed by mass transport phenomena, heat rejection, and water rejection can make it difficult to fully realize this electrochemical potentiality i n large batteries. W h i l e high performance obtained with selected systems under optimized conditions presents a goal to be sought after, practical operating considerations usually require that, for large production units, a more nominal level be chosen. F o r the Union Carbide carbon electrode fuel 166 In Fuel Cell Systems; Young, G., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1969.


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cell battery, as of mid-1963, the proposed continuous operating level for long life is 50 amp./sq. ft. for hydrogen-oxygen batteries operating between 25° and 80° C . with 6N to 14N potassium hydroxide electrolyte. Overload levels of two to three times this value are easily sustained for several hours at a time, and momentary transient currents of the order of 400 amp./sq. ft., such as are required for motor starting, have been repeatedly drawn from large batteries. A motor starting power of 9 kw. has been drawn without detrimental effect from a nominal 1.25-kw. battery even after storage on open circuit for over two months following the initial break-in and use period. The aqueous potassium hydroxide electrolyte coupled with these carbon electrodes permits instantaneous start-up at temperatures as low as —10° C , a very desirable feature for many applications. Further, the broad electrolyte concentration range which may be tolerated permits wide swings i n power demand. The accompanying variations i n eleetrolyte volume are accommodated by providing appropriate reservoir capacity. Evans ( I ) has recently discussed vibration, shock, and acceleration

Figure 1. Baked carbon H -0 fuel cell power supply—1.25-kw. 2

2

tests performed on operating carbon electrode batteries which have demonstrated their ruggedness and their ability to withstand mechanical

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

FUEL CELL SYSTEMS

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abuse without sustaining damage. The severity of these tests was well in excess of the launch requirements for the Agena Β rocket. Some of the cells used i n these environmental experiments have been kept on life tests for periods of the order of 7000 hours, the majority of the time at 65 to 100 amp./sq. ft., to demonstrate that exposure to such rigors does not affect the long life expectancy of these units.

AMPERE

Figure 2.

HOURS

Gas use and water production rate

Design Criteria Union Carbide carbon electrodes provide a high caliber fuel cell elec trochemical system. To make proper use of these electrodes, they must be built into potentially low cost batteries of minimum weight and volume with adequate gas and electrolyte flow distribution and heat and water removal systems. Such a battery should be easily assembled and be dimensionably stable over the desired temperature and pressure operating range. The quantities of reactants, products, and heat involved in even a 1-kw. battery are sizable. Figure 1 shows a nominal 1.25-kw., 32-cell, se ries-connected carbon electrode power supply. Such a system, operating



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at its design level of 46.5 amp. (50 amp./sq. ft. of available electrode sur face), w i l l typically exhibit an average terminal voltage of 0.84 volt per cell. A t this operating level, referring to Figure 2, each cell w i l l con sume, per hour, 9.7 liters of oxygen and 19.4 liters of hydrogen to pro duce 15.6 grams of water. In one 24-hour day, this amounts to a usage, for the 32-cell battery, of 263 cubic feet of oxygen and 526 cubic feet of hydrogen. About 26.5 pounds of water (more than 3 gallons) w i l l be formed during this day. The heat output, referring to Figure 3, w i l l be 3360 B.t.u., or about 850 kcal., per hour. (The curves of Figure 3 assume that all of the energy of the hydrogen-oxygen reaction not withdrawn as electrical energy at the battery terminals w i l l appear as heat.) The feed systems, by-product water removal, and heat rejection systems must not only be designed to handle the indicated quantities but must be capable of taking care of the several-fold overloads which such a battery can provide.

Gas and Electrolyte Feed. Design studies of our larger battery sys tems have established the desirability of recirculating both the hydrogen



170

FUEL CELL SYSTEMS

and oxygen as a means of transporting the water formed i n the battery to an external condenser. Such recirculation also assists in maintaining uniform gas composition and flow to each cell i n a power supply. Electrolyte recirculation through a heat exchanger is a convenient means of maintaining temperature control i n large battery systems. This affords heat removal directly from the working surface of the electrodes giving, obviously, better temperature uniformity and, thereby, better operating characteristics than other heat-removal techniques. In assembling a set of cells into a battery, the gas flow (and electrolyte flow, where used) can be so arranged as to feed each cell in series from one to the other or to feed a set of cells i n parallel from a given manifold. The prime advantage of the series flow system is that the same rate of fluid flow is guaranteed through each cell. In the parallel system, blockage of one sort or another may raise the flow resistance into a particular cell and thereby cause that cell to be partially or completely bypassed. Such a situation can adversely affect the performance of the blocked cell and may even cause it to fail. The series flow approach has the disadvantages of higher pressure drop, increase i n the concentration of any impurities as the flow proceeds from the inlet to the outlet of the battery, and an increase i n temperature of the feed material from the first to the last cell. Such difficulties were particularly evident i n early series flow models. A l l of our present systems utilize the parallel flow arrangement for both gases and liquids. The first parallel-flow models were built with comparatively large ports feeding each cell from the common inlet and outlet manifolds. W h i l e this permitted operation with very low pressure drop i n the flow system, it also introduced significant maldistribution of both gases and electrolyte from one cell to another. Temperature differences as great as 25° C . were experienced from one cell to another i n a 13-cell parallel flow module operating at 50 amp./sq. ft., and even more drastic temperature excursions occurred at higher operating levels. Blocked gas ports resulted i n a build-up i n the gaseous inerts to the point where the blocked cell would drop i n potential far enough to be driven as an electrolysis cell b y adjoining cells. A solution to the problem of maldistribution of electrolyte and gas flow was reached by inserting orifices i n the feed manifold to increase the normal pressure drop between the feed manifold and the cell. These orifices were designed to ensure that the variations i n flow resistance from cell to cell were small compared to the pressure drop across the orifices. This pressure drop was also sufficient to clear the electrolyte exit ports of stray gas bubbles and the gas exit ports of any liquid blocks. A criterion used i n sizing the orifices was that set by Richardson (2) i n his analysis of fluid flow distributors. F o r equal distribution to each unit i n a parallel system, he suggests that the pressure drop



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down the entire feed manifold should not exceed 1% of the pressure drop across the individual orifices. Temperature operating levels are typically set by (a) electrode volt age, electrochemical stability, and operating range; ( b ) limitations of the framing materials and the magnitude of strains set u p between the shell and the electrode proper; (c) water transpiration requirements; and ( d ) the ability to maintain the required thermal balances i n both normal and overload operating modes. The optimum temperature range for present state-of-the-art batteries is approximately 50° to 70° C . a l though satisfactory operation is attainable considerably above and be low this range. To optimize the water transpiration capabilities, it is desirable to operate near the top temperature of the useful operating range so as to maximize the water partial pressure. F o r this reason, electrolyte flow rates through the battery are usually chosen which w i l l permit the electrolyte to enter the cell relatively warm and to exit with only a moderate temperature increase; 10° C . has been set as a practical and easily maintained increase. Figure 4 shows the required flow to hold this Δ Γ as a function of the cell's terminal voltage and current, assum ing a l l of the heat is removed via the electrolyte. ( A considerable frac tion of the heat generated i n an operating battery may be removed as heat of evaporation and sensible heat if water transpiration is employed. In addition, there are the usual conductive, convective, and radiative heat losses from the battery surface. ) These flow data and the expected operating levels must be taken into account i n determining the feed port size. BASIS: TEMP RISE THROUGH CELL* 10°C.

w

O

40

80

120 160 200 240 280 320 360 400 440 REQUIRED ELECTROLYTE FLOW-CC/MIN/CELL

Figure 4. Electrolyteflowrequired for temperature control as a function of cell voltage and current Gas distribution studies across the electrode face have been made on single cells. T h e gas feeds into a distribution header covering one-


FUEL CELL SYSTEMS

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half the plate width. To ascertain the flow distribution, filter paper strips, soaked i n phenolphthalein indicator solution, were placed across various sections of the plate as shown i n Figure 5. The feed gas was made ammoniacal by passing it over an ammonium hydroxide solution prior to entering the cell. Motion pictures were taken to show the gas distribution as indicated by the color change (colorless to pink) i n the indicator tape as the gas flowed over it. Summarized i n Table I is the time requirements for the gas to completely cover the electrode surface of the current unit as a function of flow rate. The gas space i n this system was 10 / inches wide by 13 inches high by V i inch thick.


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Figure 5 . Test arrangement for gas distribution studies A t flow rates of 50 to 400 cc./min. the flow pattern characteristically moves i n a frontal fashion from the inlet diagonally to the outlet. The total width is covered i n about the same time period as one-half the length. The last section to be covered is the lower right area. A t flow rates of 600 to 2000 cc./min. the total electrode is covered very


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rapidly. Therefore, to ensure complete sweeping of the electrode sur face and provide good mixing of the use and feed gases, a minimum recirculation gas rate of approximately 600 cc./min. should be used. The use rate at 50 amp./sq. ft. is about 160 cc./min. of oxygen and 320 cc./min. of hydrogen. Table I.

Gas Distribution Analysis ( / -inch Gas Space) 1

16


Gas rate, cc./min.

flow

Time to cover total face, seconds

50 100 200 400 800 2000

400 335 210 60 15 13

Gas Purge Requirements. Most feed gas supplies w i l l contain some impurities. F o r example, typical commercial tank oxygen w i l l contain about 0.5% inerts, while tank hydrogen w i l l have about 0.05% impurities. As these gases are consumed i n the cell, the inert impurities w i l l tend to accumulate unless they are purged. The effect of oxygen concentration on the output of the / - i n c h carbon cathode is indicated by the difference i n voltage attained on operation with air and with pure oxygen as shown i n the curves of Figure 6. Operation of a cell at 50 amp./sq. ft. using either a 90/10% mixture of oxygen-nitrogen or pure oxygen showed no significant difference. A t higher current densi ties, half-cell tests d i d indicate a concentration effect. These cathode tests were run continuously at 200 amp./sq. ft. with periodic spot checks at higher levels. A t 300 amp./sq. ft. the cathodes supplied with the 90% oxygen mixture showed approximately 40 millivolts lower voltage than those supplied by 99.5% oxygen. 1

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Figure 6. Cathode performance on air and on oxygen


FUEL CELL SYSTEMS


174

Figure 7 gives the equilibrium gas composition as a function of continuous purge rate based on these supply purity figures. If the purity level is to be held at 95%, a hydrogen purge of the order of 1 % of the use rate w i l l be required and an oxygen purge rate of about 10% of the use rate under steady state conditions. It should again be emphasized that if the only gas flow out of the cell is the purge, the possibility exists of accumulating fairly high concentrations of inerts over some area of the surface of large electrodes due to variations i n the gas flow pattern. This may. locally cause excessive concentration polarization resulting i n a nonuniform distribution of current over the face of the electrode. Gas recirculation helps avoid this problem. Resistive Power Losses. It is important to minimize internal resistance i n any battery design. Not only does the internal resistance result i n a loss of available power which may be obtained from the terminals of a battery, but it also gives rise to heat which must be dissipated. The items involved, i n order of significance for these systems i n which the current flow is perpendicular to the face of the electrode, are (a) the resistance of the electrolyte; ( b ) the contact drops between the electrode, the metal mesh contact member, and the metal current collector plate; and (c) the resistance of the electrode itself.

: HYDROGEN

CYLINDER OXYGEN CYLINDER HYDROGEN, 5

20 PURGE RATE AS % OF USE RATE

Figure 7. Equilibrium gas composition versus purge rate The typical order of magnitude of the voltage drops due to these factors i n an operating U n i o n Carbide fuel cell with / - i n c h thick electrodes is presented i n Figure 8. Voltages involving the electrodes were 1

4



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Ee >•

Figure 8. Voltage drop distribution in 1/4-inch carbon electrode fuel cells Typical voltage drops based on initial cell Performance Temperature = 4 5 ° C ; Open-circuit voltage = 1.06 volt Voltage drop, volts 50 200 amp./sq. ft amp./sq. ft. 0.002-0.005 0.008-0.02 E

Technology of Hydrogen-Oxygen Carbon Electrode Fuel Cells

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