FUEL CELLS
Carbonaceous Fuel Cells
WORK
WAS UNDERTAKEN to develop general-purpose fuel cells which can operate on vaporized hydrocarbons or conventional fuel gases such as producer, water, or natural. T h e high temperature fuel cell was selected, but performance claims for the Davtyan cell ( 2 ) could not be substantiated. The "solid" electrolyte was not a true solid a t the operating temperature-its conductivity resulted from the presence of molten eutectics. The electrolyte was unnecessarily complicated and underwent irreversible chemical changes in the cell. T h e electrodes, based on iron-iron oxides, also underwent irreversible changes and catalyzed darbon deposition from carbon monoxide. Experiments were designed to elucidate the cell mechanism and to simplify the system. As a result, two types of cells were developed.
Electrolytes and Electrodes The function of the electrolyte is to transport ionic oxygen and to provide a gas-impermeable barrier between the two gas electrodes. True ionically conducting oxides are perfect electrolytes, but they have insufficient conductivity at reasonable temperatures; therefore, salts with oxygen-containing anions must be used. For carbonaceous fuels, fused alkali carbonates provide the best solution, but carbon dioxide must be fed with the air stream to provide for the formation of carbonate ions at this electrode. Electrodes should be constructed of materials that are chemically inert toward the fuel and air, so that they act as pure gas electrodes. Side reactions of the electrode material are undesirable, partly because of the free energy loss and partly because physical changes accompanying such reactions tend to disrupt the electrodes. The active part of a porous diffusion electrode in contact with a liquid electrolyte is in the region of the three-phase contact-Le., gas-solid-liquid. Since the electrode is inactivated if the solid surface is completely covered with electrolyte, it is necessary to prevent complete flooding of the gas electrode. Two methods have been used successfully. I n the first, surface tension forces hold the fused electrolyte in a porous ceramic diaphragm which has a pore size smaller than that of the electrodes in contact with it. The second method uses a two-layer electrode with the coarse pore layer on the gas side and the fine pore layer on the electrolyte side (7). The electrolyte boundary is maintained a t the interface of the two layers by a suitable differential gas pressure.
The Fuel Cell as a Concentration Cell
T h e cell is considered primarily as an oxygen concentration cell with a fuel depolarized anode. For a carbonate electrolyte ceIl where feed to the air electrode is a mixture of air and carbon dioxide, there is a secondary effect of carbon dioxide concentration. The cathode reaction is 0 2
+ 2C02 f 4e = 2C03--
(1)
and the primary anode reaction is 2C03--
=
0 2
+ 2CO2 + 4e
(2)
followed by reaction of oxygen with the fuel gas. The net cell reaction is then OdPO,)
+ 2COn(Pco2) On(P'0,) + 2COZ(P'CO2) =
(3)
where Po, and Pco, are partial pressures a t the air electrode and Proz and P ' c o ~ are partial pressures at the fuel electrode. If removal of the discharged oxygen by the fuel is a n equilibrium process, P'O, is determined by partial pressures of the fuel and its oxidation products; the cell electromotive force is given by
Because the primary electrode processes are the same for both electrodes, the same material should serve for both electrodes, provided it is also a good oxidation catalyst for the depolarization reaction. In support of this mechanism, cells operated as direct concentration cells, with two air electrodes under different pressures, have shown electromotive forces which agree with those calculated from Equation 4.
the complete fuel cell. Batteries are built up by sandwiching the disks between recessed metal plates, arranged with supply tubes to feed gas and air and to carry away the reaction products. I n the other cell, free molten electrolyte is used with two-layer electrodes of the Bacon type (I), rigidly attached to metal backing plates. T h e electrolyte and electrode materials are the same as those used in the porous diaphragm cell. The pore sizes of the two layers are such that the differential control pressure may vary safely from 20 to 65 inches water gage. Temporary flooding does not result in inactivation of the electrodes. Performance Data
Operating temperatures of these cells range from 550' to 700' C. The fuels on which the cells have been tested, may be divided according to performance into three groups: A, hydrogen, carbon monoxide, water gas, and methanol vapor; B, kerosine vapor and propanewater mixtures; and C, methane and benzene vapor. For group A, no concentration or activation polarization occurs up to current densities of 140 amperes per square foot; the terminal voltage on load is determined primarily by internal resistance of the cell. Group B is slightly inferior, and group C is markedly inferior-activation polarization is evident, showing the need for increased electrode activity with these fuels. Diaphragm cells up to 5 inches have been operated for periods up to 1000 hours without electrode deterioration. At present, the life is limited by corrosion of cell bodies.
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Practical Fuel Cells
These considerations have led to the development of two types of cells, both of which are capable of being scaled up and built into compact batteries. Their performance is similar, and the choice between them will depend on engineering factors. One cell is a porous diaphragm cell where the electrolyte is an eutectic mixture of sodium and lithium carbonates held in a porous sintered magnesia disk having a porosity of 5070 and a maximum pore size of 25 microns. The two electrodes are identical and consist of porous layers of silverized zinc oxide, 0.02 inch thick. They are catalytically active and are neither oxidized nor reduced. Other electrode combinations are possible. The disk with its attached electrodes is virtually
p o o w ' II
4 0.80 v 0.607
I
III
I
I
I I
1-
u1
-I
U
F
g
0-40-
s
E
I I
I
I I_
t
0.20.
I
I
I
I
I
I
I
0.00
io 8
20 ' 30 0 40 0 50 6'0 X3 00 ' 90 ' 1 1 - 00 GAS CONSUMED '10
Figure 1. Efficiency is greater i f conversion i s carried out in stages in separate cells
- _ - - - -Experimental Theoretical VOL. 52,
NO.
4
0
APRIL 1960
295
cell wouId in practice probably approach the lowest available value-Le., that t TERUINAL VOILTA determined by composition of the exiting 100 o> 40 c gases. P Figure 1, based on this assumption, f 60: CURRENT DENSITY shows that nothing is gained by trying 5 r 20g to achieve more than about 97y0 utilization. Efficiency is greater if conversion is carried out in stages in separate cells. For example, using Figure 1, a single cell consuming 97y0 of the fuel and operating at voltage A may be compared with a two-cell unit whose first cell a t voltage B consumes and feeds a second cell at voltage A, which takes the consumption up to 97%. In the latter case there is a net gain of energy proporFigure 2. Efficiency o f a fuel battery tional to the shaded area. With 97% having a nominal output o f 2l/2 kw. utilization, a single-stage oxidation will a t 50 volts has been realistically give only 8391, of the available energy, assessed whereas 4 stages give 94yo and 8 stages give 36y0. I n practice, of course, the Battery Efficiencies efficiency is always lower than this because of ohmic polarization. Because a fuel, for example hydrogen, A realistic assessment of the efficiency is progressively consumed when passed of a fuel battery, taking into account all through an electrode, the ratio, PH20/PF12 probable energy losses, has been made increases; hence P',,, the equilibfor a battery with nominal output of rium oxygen partial pressure, also 21/2 kw. a t SO volts. (See Figure 2.) increases and the equivalent cell electromotive force (Equation 4) is expected to Energy efficiency V F = decrease. Because the potential over a electrical energy delivered highly conducting electrode must be to external circuit x 100% constant, the electomotive force of the -AF 0
I-.
3
1.0
-
2,- 0
3.0
4 0
' I
1ia.a
60
U
'Thermal efficiency = electrical energy delivered to external circuit -AH
x
100%
With a reasonable amount of lagging, a battery of this size would be selfsustaining in temperature at outputs of 1 to 1.5 kw. Based on thickness of the present laboratory cells, the powervolume ratio of the battery at 2.5 kw. would be 1 kw. per cubic foot of active volume and at the maximum output of 4.1 kw. this figure would be 1.6. Considerable improvements may be expected to result from reduced cell thickness and internal resistance. literature Cited (1) Bacon, F., British Patent 667,298 (1952). (2) Davtyan, 0. K., acad. xi. U.R.S.S. c l a m sci. tech. 1946, pp. 107, 125. RECEIVED for review September 15, 1959 ACCEPTED January 4, 1960 Division of Gas and Fuel Chemistry, Symposium on Fuel Cells, 136th Meeting, ACS, Atlantic City, N. J., September 13-18,1959. Work was started under the joint sponsorship of the Ministry of Power and the Central Electricity Authority. Additional support was provided by Shell Research Ltd. H. H. CHAMBERS and A. D. S. TANTRAM
Sondes Place Research Institute, Dorking, England
Hydrogen-Oxygen Fuel Cells with Carbon Electrodes C A m o r ELECTRODE fuel cells are characterized by simplicity, reasonable initial cost, low maintenance expense, and an operating life in the range of years. Also, they have high power output per unit weight and volume, a conversion efficiency of about 7070, and a capacity to carry high overloads for short peak demands. I n the cell developed in these laboratories (Figure l), the electrolyte is 3070 potassium hydroxide, and electricity is produced when hydrogen is fed into the inner porous carbon tube. The outer tube is exposed to air, and its larger surface offsets the lower current density of the air electrode. With pure oxygenhydrogen cells, equal-surface electrodes are preferable to obtain proper cell balance; in this instance, tube bundle cells or plate cells were selected. The transportation of oxygen through the wall of the carbon tube determines current of the electrode. Pressure drop between gas side and electrolyte side of the carbon wall, calculated, using Fick's law for linear diffusion, amounts to several per cent of the applied gas pressure, depending on the load. No gas escapes into the electrolyte in a properly operating cell. The pore structure is chosen such that a large pressure differ-
296
ential is required to produce gas bubbles on the electrolyte-carbon interface. Penetration of the electrolyte into the carbon is effectively stopped by a special carbon repellency treatment. Oxygen molecules, adsorbed on the carbon surface, are ionized in accordance with the 2-electron transfer process :
+ H20
On(ads.)
-I- 2e
+
HOn-
+ OH-
Using special peroxide-decomposing catalysts (70, I I), hydrogen peroxide concentration is reduced beyond sensitivity of analytical tests to an estimated 10-'OM. In this connection, it is remarkable that for caustic electrolytes, the minimum half life of peroxide occurs at about pH 14 (4, 5, 75). Different catalysts change the half life by several magnitudes, but the minimum stays in the same pH region. The low concentration of peroxide corresponds to the open circuit potential of 1.10 to 1.13 volts against the hydrogen electrode in the same electrolyte. The oxygen formed by decomposition of the hydrogen peroxide is entirely re-used. This changes the 2-electron process to an apparent 4-electron mechanism. The 0.1-volt difference to the open circuit potential of the oxygen-water electrode
INDUSTRIAL AND ENGINEERING CHEMISTRY
(1.23 volts) reveals that the electrode is not following the equation, 0 2 2H20 4e + 40H-. According to the theory, the oxygen electrode potential must depend on alkali concentration of the electrolyte. Slope of the curve for the oxygenhydrogen peroxide electrode is about 30 to 32 mv. per p H unit, which agrees with the estimated 29 mv. for a 2-electron process (Figure 2). For the oxygencarbon electrode, potential follows the Nernst equation, and as a result, such electrodes can be used for determining oxygen partial pressures. Hydrogen is not active on untreated carbon electrodes and therefore a catalyst was deposited on the surface of the hydrogen electrode. The reaction occurring at the catalytically active sites is
+
+
HZ(gas)
-
2H
ads. On O&talyat
2H (ads.) -I- 2 0 H -
--c
2Hz0
+ 2e
Like the oxygen electrode, structure of the hydrogen electrode is important for the best gas diffusion rate. A permanent three-phase zone (solid-gasliquid) has to be established by wetproofing the carbon material. Also, precautions has to be taken against "internal
