In Table I, data in the two bottom rows refer to a process where a fraction (1 - cy) of the anodic outlet gas is recycled over carbon to recover the original mixture of carbon monoxide and dioxide as anodic fuel gas. Here, thermal efficiency is based on heat value of the carbon; heat value of the carbon monoxide in the waste fraction cy is considered a total loss. Clearly, a promising characteristic such as 0.8 volt a t 50 ma. per sq. cm. for 1 0 0 ~ o carbon monoxide in Figure 4 probably indicates little about the actual behavior of carbon monoxide. Considerable improvement over the data reported can be expected for cells that are gas-tight and operate on greater oxygen concentrations.
Literature Cited
( I ) Davtyan, 0. , K., Bull. mad. sci. U.R.S.S. Classe scz. tech. 1946, pp. 107, 205. RECEIVED for review September 15, 1959 ACCEPTED December 31, 1959 Division of Gas and Fuel Chemistry, 136th Meeting, Atlantic City, N. J., September 13-18, 1959. Based on work done at the Central Technical Institute T.N.O. in partial fulfillment of requirements for the degree of doctor of philosophy.
G. H. J. EROERS and J. A. A. KETELAAR Central Technical Institute T.N.O., The Hague, Netherlands
Nature of the Electrode Process
HI,,
TEMPERATURE CELLS operate a t temperatures ranging from 500O to 900' C., and are unique in their ability to use carbon-containing fuel gases The most successful results have been obtained with electrolytes of fused alkali carbonates ( 7-3)-the electrolyte is not contaminated with carbon dioxide and other carbonaceous products. Fuels are probably used indirectly through reforming and thermal cracking reactions which can be carried out internally or externally to the cell operation. Hydrogen is produced thereby which readily reacts at the fuel electrode. Carbon monoxide may also be consumed directly at the fuel electrode but does not seem to be as active. I t may be consumed indirectly by conversion to
t
A
hydrogen within the cell by reaction Tvith steam which aids considerably in its utilization ( 7 ) . Thus. behavior of the hydrogen and oxygen electrodes is controlling for good performance, and the study reported here is concerned with the reaction mechanism of these electrodes. The cell used was similar to that of Broers ( I ) , and the electrolyte was mixed alkali carbonates disposed on a specially prepared pure porous magnesia matrix. Operating data were obtained over the range of 700' to 800' C., using both hydrogen-steam mixtures and carbon monoxide-dioxide mixtures a t the anode and air a t the cathode. Porous sintered nickel and iron (average particle size, 65 microns) were used as fuel electrodes
T--7-q - -
-_I -1 LL Wd = 0 0111 SQUARE MESH ELECTRODE ~~
PARALLEL WIRE ELECTRODE
40
Figure 1. Effective resistance ratio R , f f / R depends on the ratio between electrolyte thickness and contact spacing ( 1 /d). Fractional area covered i s (A/d)2for a square mesh electrode and (A/d) for a parallel wire electrode
2
3
4
5
678910
20
30
l/d
306
INDUSTRIAL AND ENGINEERING CHEMISTRY
40 50 6 7 8 9100
while both silver gauze (90 mesh) and lithiated sintered nickel oxide (4) were used a t the air electrode. T h e voltage current characteristics were analyzed statistically. After taking into account change in gas composition caused by cell reactions, drop in voltage was linear with current drain. T h e linear relationship held within about 0.06 volt as determined by the 95% confidence limits. T h e measured specific internal resistance was rather high, howeverabout 4 to 7.0 ohm-cm. The calculated internal resistance from voltage drop during current drain was in some cases equal to the measured resistance while in others it was u p to 40% higher. The high internal resistance limits the cell output and is in the same range as observed by other workers. Experimental work has shown that this is not caused by some peculiar property of the electrolyte matrix. T h e internal resistance of the matrix was measured when fully loaded with melt and pressed between two flat silver gaskets as electrodes. An average value of 0.7 ohm for resistance per sq. cm. was found in the temperature range of 700' to 800' C. This is smaller by a factor of 7 to 10 than that observed during cell operation. This phenomenon can be explained: For the gas to have proper access to the electrode surface, the melt inventory must be adjusted until a small area of contact is maintained between electrode and electrolyte. Theoretical calculations were made to estimate the actual contact area for several geometrical arrangements by solving the Laplace equation,
with appropriate boundary conditions. T h e calculated resistance for a given contact area depends strongly on geometrical arrangement and spacing of the contact points (Figure 1). T h e calculations show, however, that the actual fractional contact area must be very low to explain the observed high resistance and probably no higher than 3x This figure corresponds to an estimated value of l / d for the silver gauze electrode of 6.3. Some mechanism must be operating to broaden the three-phase limit where electrode, electrolyte and gas meet. Otherwise two deleterious factors become effective-i.e., activation polarization as a result of concentrating the electrode reaction on a very small area, and the concomitant high effective resistance. The two most likely mechanisms are permeation of the gas through the bulk electrode metal, and diffusion across the electrode surface. A mathematical treatment has been worked out
FUEL CELLS
Figure 2. The electrode contact. rl i s radius of spherical granule or wire making contact. y5 is angle of contuct with electrolyte
for both these mechanisms and the qualitative form of both solutions is similar. From experimental data, available for bulk diffusion of hydrogen and oxygen through metals, it can be determined whether or not this mechanism is sufficient to explain the results. No quantitative data are available for surface diffusion rates. With an idealized model of the electrode-electrolyte contact (Figure 2 ) rate a t which the gas permeates the individual granules of a sintered metal electrode can be determined, and from this the resulting polarization can be found. T h e basic method involves the solution of Fick's diffusion equation which for steady flow reduces to Dv2C = 0
(2)
Boundary conditions used for the solution were
D
(2) (2)
= -k(C
- C,) ic. = 0 to
r-71
D
= k(C, - C ) iF. =
+I
to x
r-71
6
0 20
-
- &)= io5yr, 50- 50%
z 0
015
010
E
u
-I 0
(3)
where i is the current density in amperes per square centimeter, N is the number of spheres making contact per square Centimeter, n is the number of electrons involved in the electrode process and P = DCo is the permeability of the gas through the metal. AE is the electrode polarization as determined by slow permeation through the metal.
Figure 3. Polorization decreases as k r l / D increases and contact efficiency is improved
5 !
E
dx
CONTACT
5
9 z
These conditions represent certain assumptions relative to the mechanism of the process. I n short it was assumed that the gas was rapidly absorbed from the gas phase to form a dissociated adsorbed phase, and that the rate-controlling process was solution from the adsorbed phase of the gas in the metal. Similarly, it was assumed that the electrode reaction involved the adsorbed phase beneath the surface of the melt and that this was very rapid relative to the rate of dissolution of gas from the metal to form the adsorbed phase. C1 is the surface concentration of the gas beneath the electrolyte and must be constant at all points because the electrode potential is constant. I t may be considered to be equivalent to that in equilibrium with gas at a pressure PI in atmospheres-i.e., C1 = Co Similarly concentration C, of the adsorbed layer in the area outside the electrolyte, must be constant and in equilibrium with the gas phase-Le., C, = Co y'p, COis the concentration in equilibrium with 1 atm. of gas. k is the rate of desorption of the gas from solution in the metal and C is the concentration of gas in the metal. The polarization can be calculated as a function of current drain from the two equations,
005
e
0
CURRENT
DENSITY rna/cm'
XI is a complex expression called the flux factor derived from the solution of Equation 2. It is a function only of the parameter, krl/D. T h e average particle radius r, of the fuel electrode used in this work was 32.5 microns. For hydrogen in nickel at 750" C. the values of D and Co are known and have the values of 6 x 10-6 cm.2 sec-1 and 3 x mole per cc. Unfortunately, the value of k is unknown. I t can be shown, however, that the parameter, krl/D can have a value as high as 2 x 108. Polarization curves were accordingly calculated for several assigned values of krl/D and for several immersion angles (Figure 3). Most of the cases shown correspond to perfect contact between electrode and electrolyte-Le., every granule, r l = 32.5 microns, in a close packed array makes contact. One case is shown where only 50% of the granules make contact. For the hydrogen nickel electrode, the polarization voltage was less than 0.08 volt at temperatures above 700" C. Such a result can reasonably be achieved with only 50% contact using a value of krl/D of lo6 which is well below the maximum permissible value (Figure 3). Using a contact angle of 5'' with 50% contact, it is easy to calculate that the contact area is only about 7 x times the superficial area. This is within the proper range to explain the high internal resistance observed. Similar considerations, made for an iron electrode, showed again that bulk diffusion through the metal could provide a n adequate transport mechanism for broadening of the three-phase limit. The silver oxygen electrode was treated in a similar fashion. The electrode contact here was assumed to have the form of cylinders touching the electrolyte matrix along their sides. T h e mdthematical treatment was similar to that for the metal granules. The solubility and diffusion rates of oxygen through silver are known from literature data. T h e maximum permissible value for krJD for the wire electrode used ( r l = 7 X cm.) can be shown to be about 7 X lo+. T h e calculated maximum short circuit current turns out to be 42 ma. per sq. cm. This for a contact angle of 1 28'. assumes that all wires in the gauze electrode make contact with the electrolyte. I t is apparent that permeation through a silver air electrode is not sufficient to explain its performance. I n this case a n accelerated diffusion rate of oxygen through a thin surface layer of the metal must be assumed, and mathematical treatment gives an expression similar to that for bulk permeation. VOL. 52, NO. 4
APRIL 1960
307
RECEIVED for review September 15, 1959 ACCEPTED December 29, 1959
literature Cited (1) Broers, G. H. J., "High Temperature
~
~cells," ph.D, l thesis, ~ University ~ ~ Division i of Gas~ and Fuel Chemistry, symposium on Gas and Fuels, 136th Meeting, (2) Gorin, E., Recht, H. L., Mech. Eng. ACs, City, J., September 1381. No. 3. 63 (1959): Chem. Ene. Prow. 18, 1959. 551 51 (1959). EVERETT GORIN and H. L. RECHT 1 3 ) Gorin. E.. Recht. H. L.. U. S . Patent 2,914,596 (Nov. 24; 1959): Research and Development Division, (4) Ketelaar, J. A. A., Ingenieur (Utrcht) 66, Consolidation Coal Co., Library, Pa. 34 E 88-91 (Aug. 20, 1954). of Amsterdam, 1958. >
I
v
,
-
\ - I
Molten Carbonate Cells with GasDiffusion Electrodes T o
INVESTIGATE the behavior of electrodes of the gas:diffusion type in molten alkali carbonate electrolyte, a ternary mixture of lithium, sodium, and potassium carbonates in the mole ratio 4 to 3 to 3 was selected as the electrolyte. This mixture, melting at about 400' C . and thermally stable to above 800' C. CbRBGY MONOXIDE 810 POTENTIOMETER
NCHES
Figure 1. Electrode assemblies attached to ceramic tubes are immersed in molten carbonate electrolyte
offered a wide temperature range for study. The use of a free-liquid electrolyte in such high temperature cells has not been previously reported. By eliminating the usual ceramic matrix to retain the electrolyte, a reference electrode could be incorporated in the system for individual electrode polarization studies. Cylindrical electrodes with geometrical areas of 5 to 10 sq. cm. were machined from blocks of commercially available porous nickel. Porous silver and gold electrodes were made by sintering appropriate powders in graphite molds. Certain porous metals, available only in the form of thin sheets, were used in electrodes by Ivelding disks into stainless steel or nickel spinnings. Electrode bodies were attached to alumina tubes by standard metal-to-ceramic sealing techniques or by flame-sprayed metal joints (Figure 1). Small diameter stainless-steel or Inconel tubing delivered gas to the electrodes and made electrical connection. hTo particular efforts were made to select or control porosities and pore sizc distributions of the electrode materials. Porosities ranged from 34 to 62% and pore diameters averaged 10 to 30 microns.
For the reference electrode, an oxygen electrode, operated without current drain, was used. This consisted of a length of gold tubing containing an end plug of porous gold or silver. Operated on static head of oxygen-carbon dioxide gas mixture this electrode exhibited a potential stable to 1.20 mv. Because the reference electrode could not be positioned close to the working electrode as is required for high precision polarization studies, some uncertainty in the results was inevitable. This was not serious for semiquantitative comparison of electrode performance. Fuel flow rates were maintained a t from 10 to 20 times that corresponding to the maximum current drain. The gas pressure, held a t 2 L 1 p s i . , was sufficient to prevent bubbling of the gas into the electrolyte. Gas compositions in the elearode could not be specified exactly, because the water gas, the water gas shift, and Boudouard's reactions were proceeding in the gas-inlet tubes and on the inner electrode surface. Internal resistance of the cells was estimated with sufficient accuracy by measuring the instantaneous voltage drop across the cell on impressing a load. A fast recording potentiometer was used for this. Alternating-current bridge measurements were useless because of leakage paths from the electrodes to the crucible walls. hccordingly, 1.8 ohms at 500" C., 1.2 ohms at 600" C. and 1.0 ohm a t 650' C. were taken as best estimates of cell resistance. TVith each fuel cell or assembly of electiodes, the primary measurement made was the polarization (voltage decrease) of the cell as a function of current density. When the reference electrode was included, polarization of the individual electrodes was recorded also. h-o serious attempts at establishing the operating life of the cells were made. One cell was operated continuously for 100 hours and several of the electrodes were operated for several hundred hours before being discarded.
AI SOO'C
Ai 500'C Bl 5 5 0 T
61 6DO'C
--. .---. ....
07-
06-
t '0
o.2
,
I
00
I
2
3
4 CURRENT
5 6 DENSITY mP/cm
I 7
I
8
9
IC
Figure 2. Typical performance curves for hydrogen-oxygen cells with nickel electrodes
308
INDUSTRIAL A N D ENOINEERING CHEMISTRY
k
l',
i0
2:
40
CURREV' DENSITY - mA/cmP
d5
a0
15
d(
Figure 3. Performance of cells having silver cathodes and nickel anodes i s superior to those having nickel cathodes
