XA xlb1 Xiik bj bj

A closed solution of diff. equ. (4) is impossible, as the bi, bj values are not constant as the concentrations of the molten mixture changes. According to equ. (4) the sign of the gradient d.\i/dy depends on the sign of the quantity (x; - tj) and its magnitude depends also on the current density, i. For xKX),32, as is demonstrated in fig. 1, the quantity (x -1) calculated from Okada's data for U2CO3/K2CO3 melts becomes negative for potassium and as dcK/dy is positive, the potassium concentration increases in going from the anode to the cathode, whereas the concentration of lithium decreases correspondingly. Since the lower melting eutectic of the U2CO3/K2CO3 system which defines the usual melt composition in MCFCs, has the composition xK = 0,38 we can definitely predict the accumulation of potassium and the depletion of lithium in the cathode. We expect also, that on the anode side of the cell a limiting potassium concentration will be attained that matches the concentration of the isotachotic point if more and more potassium is dragged into the cathode as would be expected as the current density is increased. Relative mobilities in binary molten I^COj/NaiCC^ mixtures are not known. Performing steady state operation of MCFC which make use of this electrolyte and analyzing the steady state distribution of lithium and sodium in these cells would allow to draw conclusions concerning the relative internal mobilities and transference number of both cations.


(a) U2CO3/K2CO3 eutectic: Table 1 collects the chemical analyses of a 340 h experiment performed at 650 °C with a steady current density of 100 mA cm"2.

Table 1

Potassium and lithium contents of MCFC operated for 340 h at 100 mA cm'2

anode matrix cathode total K/mg 21,2 175 25

thickness/mm 0,8 1,1 0.5

mol%Li,C03 64 64 58

It is evident that the greatest part of the electrolyte is contained in the matrix (75 to 80 %) and that almost equal parts of the remaining electrolyte are contained in both electrodes. The ratio m (K)/m(Li) is almost equal in the matrix and the anode (~3) but it increases to 4 in the cathode.

Fig. 2 shows for three independent experiments - two with 100 mA cm"2 - one with 150 mA cm'2 the mean lithium carbonate mole fraction in the anode, matrix and cathodc. Evidently anode and matrix electrolyte exhibit almost the same chemical composition but the catholyte is seizeably depleted in lithium. This effect becomes much more pronounced at higher current densitits. As die current density increases by a factor of 1,5 the concentration difference,

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