Electrolyte Studies for Molten Carbonate Fuel Cells - Advances in

20 Electrolyte Studies for Molten Carbonate Fuel Cells

Downloaded by UNIV OF AUCKLAND on October 23, 2014 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0090.ch020

ISAAC T R A C H T E N B E R G and D A V I D F. C O L E Texas Instruments Incorporated, Dallas, Texas

The decomposition and evaporation of molten LiNaCO have been studied in the presence of various gaseous atmospheres at temperatures considerably above the melting point of the salt mixture. The presence of small amounts of CO retards the decomposition and evaporation of the electrolyte. The addition of H O to air promotes decomposition. Corrosion rates for Ag in molten alkali carbonates were determined as functions of temperature and gas and electrolyte composition. In the absence of O , corrosion is negligible. At a constant CO /O value of 2, the rate increases with increasing pressures of these gases. Ag cathode polarization has been studied in operating fuel cells. At a given temperature, current density and cathode gas supply composition, the IR-free cathode polarization is smaller in ternary than in binary alkali carbonate mixtures. 3

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"performance data have been presented for a variety of fuel cells employing mixtures of molten alkali carbonates as electrolyte (1, 2, 4, 5, 6, 15). Depending on the particular cell design chosen, the operating conditions, age of cell, and a number of other parameters, almost any kind of operating characteristics (current-voltage relationships) desired can be obtained. Various investigators have emphasized certain operating characteristics; i n fact, the entire experiment, cell design, and operating conditions are optimized to maximize one of the several parameters. Emphasis has been, for the most part, on power output per unit area of electrode and operating life. Efficiency, power output per unit weight and volume, and other fuel cell and system characteristics have received only moderate attention. However, at the present state of development it is obvious that the particular set of fuel cell system characteristics A

269 In Fuel Cell Systems-II; Baker, B.; Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

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chosen w i l l be greatly influenced by the application. This communication w i l l emphasize certain interactions of the electrolyte which to some extent w i l l be applicable to all molten carbonate fuel cell systems, regard less of their design and application. There are many electrolyte interactions in a fuel cell containing molten alkali carbonates. The following text w i l l discuss some aspects of three of these interactions: electrolyte stability in some of the gas atmospheres encountered in operating fuel cells, corrosion of silver elec trodes as a function of atmosphere and electrolyte composition, and the effect of atmospheres and electrolyte composition on cathode polarization. Electrolyte

Stability

A material suitable for use as an electrolyte in a fuel cell should be chemically stable to the electrodes and atmospheric environments it w i l l encounter during the operating life of the cell. In fuel cells employing molten alkali carbonate electrolytes C 0 is added to the various gas streams to insure this stability. However, if insufficient C 0 (14) is added, it is possible under certain operating conditions to obtain a con dition at the cathode-electrolyte interface i n which no C 0 is present. The effects of various gas atmospheres on the stability of molten L i N a C 0 were investigated. Samples of 50-50 mole % L i C 0 - N a C 0 were exposed to various gas atmospheres at 650 °C. for extended time intervals, and the composi tion was determined by standard analytical techniques. In an atmosphere of 20 volume % C O - 8 0 volume % air, tests ranging from 246 hours to 1615 hours duration indicated no decomposition of L i N a C 0 or change in L i / N a ratio. Similar tests in 50 volume % H - 5 0 volume % C 0 for 408 hours and in 8 volume % H 0 - 1 2 volume % C O - 1 0 volume % O - 7 0 volume % N for 384 hours produced no change in electrolyte composition. However, when C 0 was not added to the air, some decomposition in the electrolyte could be detected. Data in Table I illus trate the effect of no C 0 added to the air. The decomposition is indicated by the rise i n O H " content. Because of the analytical techniques em ployed both O " and O H " present in the melt w i l l be reported only as O H " . There is no significant change in the L i / N a ratio. The O H " con centration appears to remain constant after 48 hours, which indicates an equilibrium hydroxide (oxide) concentration has been established. The 0.03 volume % C 0 present in air may have been sufficient to prevent further decomposition. Data for a second gas composition are also presented in Table I. The gas composition of 10 volume % H 0 , 10 volume % 0 and 80 volume % N was obtained by burning a mixture of 9.5 volume % H i n 76 volume 2

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In Fuel Cell Systems-II; Baker, B.; Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

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% air and 14.5 volume % N . Although there is no significant change i n L i / N a ratio, there is significant decomposition of the electrolyte, as illustrated by both the decrease in weight % C 0 ~ and the increase i n weight % O H " . The C 0 content is slightly more than 3/4 of what it was i n the previous experiment and can account for only a small part of the difference observed for the two gas compositions. Water removes O " in the form of O H " and promotes further decomposition of the carbonates. Here again, the equilibrium condition appears to be established after 21 hours, and further exposure to this gas composition produces no additional decomposition. 2

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Table I.

LiNaCX>3 Electrolyte Composition as a Function of Time at 650°C.

Gas Composition (in Volume %) 100% Air Weight % Hours

%

0 3 48 98 144 216 336

co 2

3

66.6 66.7 66.9 66.8 67.2 66.6 67.1

% OH-

%

0.0 0.2 0.5 0.5 0.4 0.5 0.6

Na

+

25.7 25.6 26.4 26.4 26.4 26.2 26.0

% Li

+

Li/Na .29 .29 .29 .30 .31 .30 .30

7.5 7.5 7.6 7.9 8.1 7.8 7.9

Gas Composition (in Volume %)10% H Q-—10% O —80% N (9.5% H —•76% Air—•14.5% N ) 2

2

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2

Weight % Hours

%

0 3 21 71 165 333 477

C0 ~ 2

3

% OH-

66.5 65.1 63.5 64.1 64.0 62.4 63.3

%

0.0 1.2 3.1 3.2 2.1 3.0 2.3

Na

25.7 25.5 25.6 25.6 25.7 25.9 25.9

+

% LV

Li/Na

8.0 8.0 8.3 8.0 8.1 8.1 8.2

.31 .31 .32 .31 .32 .31 .32

As pointed out by Stepanov and Trunov (14), a low ratio of C 0 / 0 ( < 2.35), particularly i n a cathode gas mixture containing a large amount of inert gas, results i n depletion of COo at the electrode-electrolyte inter face and a change in electrode mechanism. This effect is further compli cated by decomposition of the electrolyte, particularly if appreciable amounts of H 0 are present. Evaporation of L i N a C 0 was investigated using a radiochemical technique with C labeled L i N a C 0 . A sample containing C labeled 2

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L i N a C 0 was placed i n an A 1 0 boat i n a tube furnace and heated to 650 °C. with a mixture of 23 volume % C 0 - 7 7 volume % 0 flowing over the free electrolyte. The experiment was started at the time the CO2-O2 mixture was replaced by pure N . The effluent gas from the furnace was passed through a bubbler containing B a ( O H ) . The B a C 0 precipitate was then beta-counted at infinite thickness. The results of this experiment are shown i n Figure 1. Apparently, two processes result i n C 0 i n gas phase. The first process appears to fall off rapidly (an order of magnitude change i n one hour) and is essentially complete i n about six hours. The second process, which is much slower, shows only a slight decrease with time up to 250 hours. The first process (fast) is believed to be the decomposition of L i N a C 0 into L i N a O and C 0 . The second process (slow) is believed to be the evaporation of L i N a C 0 . This slow process has an approximate rate of 10" mole of C 0 per mole of N passed. Broers (6) reported that i n an operating fuel cell containing L i , N a , and Κ carbonates evaporation losses were about 10" mole of C 0 ' per mole of fuel, air, and C 0 passed over the electrolyte. 3

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Figure 1.

Rate of C0 evolution from at 650°C. 2

LiNaC0

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Evaporation and decomposition losses were also determined by very careful weight loss measurements. A sample of 30 weight % L i N a C O - 7 0 weight % fused M g O was placed i n an A 1 0 boat and heated i n a tube furnace at 700°C. Evaporation losses were determined using two gas 3

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mixtures. A mixture of 5 volume % 0 , 12 volume % C 0 , and 83 volume % N was passed over the sample. It has been previously shown that this gas mixture prevents decomposition. The total loss measured i n these runs can be attributed to evaporation only and was found to average 0.7 Χ 10" moles of L i N a C 0 / m o l e of gas. The second gas mixture was 100 volume % N and under these conditions both evaporation and de composition can occur. Measurements were made after the N exposure and again after a short exposure to 100 volume % C 0 . This COo treat ment was shown to replace all of the C 0 lost by decomposition of the electrolyte. The loss rate by evaporation in these runs was 1.8 Χ 10" moles of L i N a C 0 / m o l e of N and the loss rate by decomposition was 7.5 Χ 10" moles of C 0 / m o l e of N for a total loss of 9.3 X 10' moles of C 0 / m o l e of N . This result agrees at least qualitatively with a total loss rate of 10" mole of C 0 / m o l e of N found by the radiochemical technique. Although the loss rate by evaporation was found to be somewhat greater in the absence of C 0 in the gas phase ( 1.8 Χ 10" vs. 0.7 Χ 10" moles of L i N a C 0 / m o l e of N ) , both values have considerable uncer tainty, and the difference may not be significant. These studies indicate that electrolyte stability (decomposition and/or evaporation) may well become a problem in long-lived molten carbonate fuel cells. Further, they point up the requirement for sufficient C 0 in contact with the electrolyte, particularly in the cathode gas where relatively large amounts of inerts w i l l be present if air is used as the 0 source. The suggestion of Stepanov and Trunov (14) that the C 0 / 0 ratio be about 2.35 should be seriously considered by molten carbonate fuel cell system designers. 2

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Silver

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Corrosion

The corrosion of silver in molten carbonates has been studied by several groups ( 7, 8, 11 ) and observed qualitatively by many others, but the conditions employed in the experiments have not been representative of the operation of a molten carbonate fuel cell except for those reported by Broers (13). The results presented here were obtained by weight loss measure ments. The samples used 20 gauge silver wire, 5.1 sq. cm. in geometric surface area. The samples were immersed completely and to a uniform depth in either 50 mole % L i C O - 5 0 mole % N a C 0 , or 37 mole % L i C 0 - 3 9 mole % N a C 0 - 2 4 mole % K C 0 contained i n an 80 weight % Au—20 weight % P d crucible of 250 cc. capacity. Atmospheric compo sition was maintained at 90 volume % N - 1 0 volume % C 0 until the desired operating tempreature was reached and thereafter was maintained 2

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at the desired composition as determined by Orsat analyses. The most extensive experiments have been performed in an atmosphere (5 volume % O - 1 0 volume % C 0 - 8 5 volume % N ) believed likely to be typical of operating conditions in molten carbonate fuel cell systems using air and spent fuel in the cathode gas supply. ( In operating fuel cell systems about 10 volume % of N w i l l be replaced by H 0 . ) 2

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Visual examination of the weight loss samples after completion of experiments indicated apparently uniform attack with no dendritic growths from reprecipitation of dissolved silver. Table II lists the results of experiments under a variety of atmos pheric conditions in binary melts. Weight loss was negligible in the pure C 0 atmosphere at 600°C. Where oxygen was present, the rate of weight loss increased with increasing partial pressure of oxygen. 2

Table II.

Silver Corrosion in Molten Alkali Carbonates '

a b

Time (Hrs.)

„ Temperature °C.

100 48 168 100 111

600 600 600 600 600

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Gas Composition Volume % C0

N

Wt. Loss (mg./cm. )

100 12 10 89.5 72.5

— 83 85 — —

Electrolyte Studies for Molten Carbonate Fuel Cells - Advances in

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