11 Laboratory Studies of Intermetallic Cells MELVIN S. FOSTER
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Argonne National Laboratory, Argonne, Ill. The choice of components for laboratory studies of regenerative, intermetallic cell systems is based on the desire to have total liquidity in the cell, a high cell voltage, ability to separate the more base metal used as the anode by distillation or electrolytic regeneration, and negligible corrosive character of the metals and alloys. Several advantages and disadvantages for these cells exist. Concentration cells without transference have been useful in the study of the thermodynamic properties of alloys. The solubility of the anode metal in the fused salt electrolyte, a mixture of halides of the same metal, results in a nonelectrochemical transfer in the cell. Intermetallic compounds may be extracted from the cathode alloy into the fused salt electrolyte which is thermodynamically stable in the cell environment. The solubilities of several intermetallic compounds infused salts are relatively large.
nphe primary voltage source of one type of regenerative, galvanic system under consideration is an intermetallic cell. Laboratory investigations of intermetallic cells have been carried out at Argonne National Laboratory and elsewhere (discussions of the work at Allison Division of General Motors and North American Aviation are found elsewhere in this volume). For reasons to be discussed later, the primary emphasis of this chapter will be on cells with a liquid anode metal, a liquid binary alloy cathode, and a molten salt electrolyte. One of the two components of the liquid alloy cathode is a metal identical with the anode metal—hence the name "bimetallic cell." Most of the studies reported here have been of the basic physicochemical properties of binary alloys.
Design of Practical Systems Although a complete enunciation of the principles of design for a practical system is beyond the scope of this chapter, we may cite some of the considerations involved in order to show the relevancy of the basic 136
Crouthamel and Recht; Regenerative EMF Cells Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
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Intermetallic Cells
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studies. Figure 1 is a schematic representation of a regenerative, inter metallic cell system. ENERGY
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REGENERATOR
A + C-
IN
-AC
ι I I I
ι I I I
COUNTERCURRENT HEAT EXCHANGER
ELECTRICAL ENERGY CELL
CELL A+ C
REACTION: -AC TE
HEAT
Figure 1. Schematic representation of a regenerative, intermetallic cell system
In the figure, the anode metal, A, is transferred electrochemically through the electrolyte and combined with a cathode metal, C, to form a binary alloy, AC. The cathode metal is the more noble metal. Electrical energy and waste heat are generated in the cell which is ideally a concen tration cell without transference. The cathode alloy formed is transferred to a second container and regenerated by removing the anode metal— e.g., by distillation or by electrolysis in a second cell. This separation is accomplished with some expenditure of energy (thermal, electrical, or other). The regenerated cathode alloy and anode metal are returned separately to their respective cell compartments for reuse. While relatively pure anode metal, A, is removed from the cathode during regeneration, the cathode alloy need not be entirely depleted of A. The energetics for the total removal of A from the cathode alloy would be expected to be unfavorable when compared with a process in which only a small change in the concentration of anode metal, A, in the cathode was effected. If the cyclic process is not isothermal, it can be shown that the efficiency may not exceed Carnot efficiency. The approach to Carnot efficiency is indeed tortuous. Among the requirements are not only a) that the cycle be operated in a reversible manner, but b) if the anode material is separated from the cathode alloy by distillation, a complete separation must be effected without refluxing, c) ideal heat exchange must take place between the streams from the cell and regenerator, and d) (C duct8 — Citants) °> where C is the heat =
pro
Crouthamel and Recht; Regenerative EMF Cells Advances in Chemistry; American Chemical Society: Washington, DC, 1967.
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REGENERATIVE EMF CELLS
capacity. This implies strongly that all of the anode material vaporized in the regenerator must be (reversibly) condensed at the regenerator temperature without any expenditure of energy, a patently impossible situation. Note that the distillative regeneration procedure mentioned calls for removing the anode metal from the cathode alloy in the regenerator. It is possible, of course, to regenerate the system by distilling the cathode metal from the alloy to be regenerated. While this method would in principle allow for the possibility of obtaining essentially pure cathode metal for cell use, it would be necessary to distill all, or almost all, of the cathode metal from the alloy. This represents a very high energy input compared with the energy involved in the distillation of reasonably pure anode metal from a cathode alloy whose composition need not be greatly changed. Because the distilled metal will condense at a temperature lower than that of the regenerator, the total energy lost in this process will be less if the anode metal is distilled from an alloy (less material is involved) ; therefore, the only systems considered for thermal regeneration will be those in which the anode metal is the more volatile component. One of the advantages of the bimetallic cell is the low polarization at high current densities. This effect is due to the high exchange currents which exist between liquid metal electrodes and molten salts (see Reference 29). Our study thus will be restricted to electrodes which are a single liquid phase. Many anode-cathode combinations are possible. From 66 ! approximately 66 metals and metalloids may come 2145, ^ f ^ ' non-degenerate, binary systems; even without intrafamily pairs, the number is still very large—1998. If we restrict our view to those metallic elements which are liquid in the pure state at reasonable cell temperatures (taken arbitrarily as
