Demixing And Effective Volatility Of Molten Alkali Carbonate Melts In Mcfcs

Thomas Brenscheidt, Hartmut Wendt Institut fur Chemische Technologie, TH Darmstadt Petersenstr. 20, D - 64287 Darmstadt, Germany


Since the early investigation of A. Klemm (1), the demixing of the cations of molten binary salt mixtures with a common anion due to the different mobilities of two different cations had been investigated in numerous experiments and the respective results interpreted in terms of structural features of the melts; see for instance (2) and (3). 1-1 electrolytes had been preferentially investigated. Okada also reported investigations on lithium carbonate/potassium carbonate mixtures in the temperature range from 980 to 1070 K, (4). From this investigation it is known, that the heavier potassium cation is faster than lithium in mixtures which are more concentrated in potassium than xiaco3 = 0,32 (Chemla effect)whereas below this isotachic concentration lithium is faster.

Theory and Prediction:

Charge and mass transfer in the electrolyte of molten carbonate fuel cells is accomplished by the movement of the carbonate anions, which are generated at the cathode and decomposed by release of CO: at the anode. Against the steadily moving stream of carbonate anions the cations are migrating - but according to their different mobilities with different velocities. Under steady state conditions the relatively faster cation would accumulate in the catholyte. Steady state conditions for the cations are defined by zero velocities of the summed convective, migrative and diffusive mass transfer

With t; equaling the so-called internal transfer number of cation i, defined against the background of the anion common to both salts and x, equaling the mole fraction of the salt (MO2CO3 in the binary mixtures one obtains for the three transport terms

• i 9 • i ' sb n .n =—',•• mrf = -—-D, d)

and calculates the steady state balance

F dy

With the volumetric concentration of.the cation, i, c;. which is given to a good approximation by c, = x;p/ M with p equal to the density of the melt and M equal to the mean molar weight of the melt one arrives at

0 0

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