Molten Carbonate Electrolytes
George J. Janz
Rensseloer Polytechnic Institute Troy, New York 12181
as Acid-Base Solvent Systems
Fused salts exist as stable liquids over wide ranges of temperatures. A comparison of some general properties of such liquids with water and carbon tetrachloride, two well known ambient temperature solvents, is shown in Table 1. The melting points and boiling points define the limits of usefulness of a particular solvent under ordinary pressure conditions. Eutectic mixtures are frequently used to attain the molten state at somewhat lower temperatures, both to increase the liquid state range and to minimize the problems of "container chemistry" or corrosion in such systems. The compositions and melting points of carbonate eutectics and some of the more popular eutectics of other salts are summarized in Table 2 and Figure 1. Related to the melting points are two other physical constants which in themselves relate to the energy necessary to bring about the change in the state from the solid to the liquid, and from the liquid to the vapor. These are the entropy of fusion and the entropy of vaporization. The entropy of vaporization (the ratio of heat vaporization to the absolute boiling point) is known as the Trouton Constant ( I ) , which usually has a value of about 21.5 for normal liquids.' Use of this empirical criterion has to date been largely limited to ambient temperature solvent systems; inspection of the values for AS,, (Table 1) shows that it appears equally informative, in the first approximation, as a probe for the degree of association in molten salts. The nature of ZnClz in the molten state has been the subject of a series of studies by the techniques of high-temperature Raman spectroscopy (3-4). The spectral data clearly support the existence of associated species in the molten
.. 900 800
?
+-
700
BOO 500
0
K&%
No2COs
~
$
0
~
Figure 1. Temperature-compo~ition diogramr for liquid-solid equilibria in carbonate systems. A-C: binwy systems; D: ternary system; eutectic, m.p. 3 9 6 T ; mole%: LiK0.43.5; N a 2 C 0 &31.5; K2COc 25.0. Sources for construction of Figure 1 were reference 1581 for A, 8, and D, "ring the data given above for D, reference (591 for the insert in 8, reference 1601 for C, and reference (61) for romewhot different results for C and also for D ltsrnory eutectic, m. p. 39Z°C; composition (mole yo):LilCOs:NalCO3: K L O a is 42.5:30.6:26.81.
state of ZnCll (larger aggregates as a form of (ZnClz), ',p~lymer,"~ as well as discrete molecules of ZnClz or ZnC1+,ZnC13-, ZnC142-...). (See Table 3.) In contrast to this, the spectral data for HgC1, in the molten state are understood (5) if the species formed on fusion are discrete molecules, i.e., molecular mercuric chloride, with little or no evidence for higher aggregates (cf., normal liquids). The markedly higher viscosity (6) of ZnClz (Table 1) is further independent support for the associated nature of ZnCh in the molten state.
1 Where the value of Trouton's Constant is bigher, it is considered evidence that molecules of the liquids are associated into larger aggregates; for an ionic melt, this would imply ion-pairs and/or larger aggregates. The polymer, formulated as (ZnC12)., undoubtedly is an aggregate in which the tetrahedron ZnClr is a. recurring unit, and with the neighboring tetrahedra. linked by shared chlorides, much a s in the crystalline state.
Table 1. General Properties of Some Fused Salts, Water and Carbon Tetrachloride Pro~ertv
Specific eondnct&e (ohmz1 em-') ASe.i.. (eal deg-' mole-') AS,, (eal deg-' mole-')
NaF
4.94 6.25 29
NaCl
3.60 6.38 24
N ~ N O IN&OI
System HeCh
-
ZnCln
1.01 6.4'
. ..
Decomposes approx. 380°C. Thermal dissociation to MnO and Con. " S f for NaNO. includes AS,, = 0.3 for solid state transition. a
6
Volume 44, Number 10, October 1967
/
581
Toble 2.
Melting Points and Compositions of Fused Salt Eutectics
Eutectie Mixture
Composition (mole %)
A-R-C
A
R
Melting Point
Table 3. Species and Charocteristic Vibrational Frequency Assignments for Molten ZnCh and Molten HgCI2
Frequency (em-')
Ref.
(ZnCld. (polymer)
Species
75,226,250,360 95,230
(3)
(ZnC4).1-" (discrete molecules)
110,266
(3)
.
,
(4)
,
* These species were not observable in molten HgCI2 unless CI- (as KCI) was added in excess.
The studies of Janz and MacIntyre (7) and Bockris, Crook, Bloom, and Richards (8) on the stmcture and propertiesof the mercuric halides have been informative. A high degree of covalency is retained in the molten state on fusion, and the small but finite electrical conductance was attributed to ionic species from a self-dissociation of this "molecular" type melt. From a study of the temperature dependence of the electrical conductance,and a comparison of the energeticsof bond dissociation energies and heats of ionization, the following self-ionization scheme was advanced (7). 2HgCh e HgC1+
or
+ (HgCls)-
More comprehensive studies of the acid-base principles in such solvent systems are needed. Use of molten salts as a solvent media for chemical syntheses is a further development (18) in applications of molten salts to effect chemical processes. Illustrative examples are: The synthesis of SiHl from SiC14,utilizing LiH dissolved in the LiCl, KC1 eutectic: SiC1.
LiH (400'C)
SiHl
(LiCI, KC1 aolvent)
(4)
The preparation of silyl azides by the displacement reaction: (1)
* 2HgClf + (HgC4)'-
3HgCl~
in which the enthalpy change due to the "complexanion" formation compensates for the high heat of ionization required for producing free HgCI+ and C1-. Molten mercuric chloride may thus be classified as a high temperature, nonaqueous, "water-analogous," solvent system. The approximate degree of ionization, a, for molten HgClz has been estimated (7) as 3.5 X 10W6 a t 2S0°C (cf, H,O, 10"). It follows from the self-ionization processes (1) and the parent-solvent concept of nonaqueous solvent systems (9),that in molten mercuric halides all substances yielding the entities HgX+ and HgXa- or HgX? are, respectively, acid-analogous and base-analogous compounds. Solute-solvent reactions (corresponding to the formation of acids from nonmetal oxides and bases from metal oxides) such as: HgSO,
+ HgCl
-r
(HgCI)1S04
account for the solubility as "electrolytes" of various mercuric salts and the alkali chlorides in molten mercuric halides; the neutralization analogous reaction, such as the titration of HgSOI and KCl, both dissolved in molten HgClz: HgSO, (seid analog)
+
2KC1 (base analog)
-
K&04 (salt)
+ HgCL
(3)
may be monitored with the classical conductometric techniques, modified for application to moderately hightemperature liquids. Cryoscopic and conductometric measurements 110). and snectroscooic studies (11)have been directed iirgkly to es'tablish the basic prirkples of this class of water-analogous molten salts, e.g., the existence of -M1(HgC14) complexes in molten HgCI,. 582
/
lournol o f Chemical Education
The dehydrohalogenations of organic compounds: CHIC1-CH&l
HgCh (250'C) (ZnClr, KC1 solvent)
CH2=CHC1
(6)
Fusion
For ionic compounds, with monatomic ionic constituents, positional randomization is the principal contribution to the entropy of fusion, AS,. Inspection of the fusion parameters (Table 1) shows that for compounds containing ions of the inert gas structure (i.e., NaF, NaCl), AS, is approximately 3 eu(g-ion). The Temkin model for the liquid state of such melts (13) has been a useful first approximation in the thermodynamic studies of molten mixtures of such salts. In this model it is assumed that the melts are completely ionized, and that statistically the anions and cations are randomly distributed relative to like-ion species, but that the nearest neighbors of anions are cations, and vice-versa (i.e., rather as though a blurred memory of the first coordination shell of the crystal lattice persists in the molten statea. For ionic compounds with polyatomic ionic constituents, e.g., nitrates and carbonates, and also for compounds such as ZnClz,the interpretation of the entropy of fusion is a more complex problem (15), requiring due cognizance of possible contributions from additional factors such as vibrational entropy, changes of association or chemical bonding, and randomization within the polyatornic species, i.e., configurational entropy. The discussion of this problem falls outside the scope of the present intent of this communication. The high electrical conductance of the melts from such ionic salts (NaF, NaC1, NaN03, N%C03, Table 1) classifies a Sec for example the rwults of X-ray and neutmn diffrsrtkm studies of the alkali id&-igivrn in Levy and rJunf.,nl ref. t 1 4 ,
.
these types of solvents as molten electrolytes rather than as high-temperature water-analogous systems; it is of interest to examine briefly the properties of the nitrates and carbonates relative to thoseof the alkali halides from this viewpoint.
Table 5.
-CICation
Conductance and Viscosity
In general it is observed for such molten electrolytes that the electrical conductance and viscosity for a limited range of temperature may be expressed by the exponential equations:' A =
AAe-EAIRT
(7)
and = A+E+RT
(8)
Table 4 summarizes the values for the exponential constants, E, and E,, the activation energies for electrical
Table 4.
Valuer of the Parameters, En and E,, for Some Molten Electrolyter(kcal mole-')
LiF LiCl LiBr LiI LiNOs 1.78~ 2.01s 2.11" 1.2ga 3.589 . . . 7.00, 5.35s 4.428 S.1O8
LinCOs 4.438 16.89s
Ev
KF KC1 KBr KI KNOI 2.49s 3.416 3.741 3.449 3.577 . . . 6.5as 5.161 5.348 4.301
KICOI 4.650 29.48,
En E,
CsF CsCl CsBr CsI C ~ N O I Cs2COa 3.262 5.110 5.53s 5.450 3.688 ... . .. 5.687 . . . 5.70. ... ...
En
Ev
EA
conductance and viscosity respectively, for the alkali metal halides, nitrates, and carbonates (16). Inspection shows that the activation energies for corresponding halides, nitrates, and carbonates are quite similar in magnitude, although there are small variations. The electrical conductance of the nitrates is nevertheless considerably less than that of the corresponding halides and carbonates (e.g., Table 1, Nap, NaCI, NaN03, and Na2C03). If the equivalent conductances of molten nitrates are extrapolated to the same temperature range as for the halides and carbonates (i.e., 80&900°C) the values, as shown in Table 5, are qualitatively quite comparable to the conductances of higher melting salts. The lower values of the transport properties of the nitrates relative to the corresponding chlorides, thus, are understood, in large part, due to the different "kinetic energy content" of the liquids. The term "low-energy melts" has been advanced (17) for the liquids from lowmelting ionic salts, such as nitrates. The nearly invariant values of Enfor the molten nitrate and carbonate series of salts (i.e., about 3.5 and 4.5 kcal equiv-', respectively, Table 1) is in marked contrast to the values of E, for the corresponding series of salts as fluorides (cf, 1.8 for LiF and increasing to 3.3 for CsF), chlorides (2.0,5.1), bromides (2.1,5.5), andiodides (1.3, 5.5). The trend in the alkali halide series has been
Equivalent Conductance for Molten Nitrates, Carbonates, and Chlorides a t Common Reference Temperatures"
Anion COsaNOi(Equivalent Conductance (ohm-' cms eauiv-ti)
a The reference temperature i- taken as 10% above the meltina point (OK) of the molten chloride. The values in parentheses are the observed conductances for ooints (OK) the nitrates at 10%, " above the meltine-. . . of the remective nitrates.
attributed to diierences in the cationic mobilities. In the nitrate and rnrlu~nntenerics [in which. i l l conrrmt to theriitu~tiot~ in thenlli3li halides, thenitrntennd carbonate ions have at least three anion-cation interaction sites of distinctly diierent force-fieldintensities (18-$0) 1, the invariancy of E, has been attributed to an aggregate cooperative transport mechanism which effectively damps out the diierences in cationic mobilities ($1, $8). The contribution of Lantelme and Chemla ($3) using diffusion data and transport experiments, is a further advance in this area. From the simultaneous competition of Li+, I
