Enhanced O2 solubility by RE2O3 (RE = La, Gd) additions in molten carbonate electrolytes for MCFC

Lux-Flood Basicity of Mixed La2O3-Alkali Molten Carbonates Determined by Analysis of their Oxygen Solubility Properties

“…2 . The basicity changes are dependent on the relative proportions of the individual lanthanum species in solution, which, in turn, depend on the melt composition with an observed reduction of the overall melt basicity at increasing average cation radius of the alkali carbonate mixture (from lithium-sodium to lithium-potassium carbonate).…”

“…During such investigations, the solubility of the oxygen gas was found to sharply rise after minor additions of lanthanum and gadolinium oxides (i.e., 0.5 mol %) to the carbonate melts [2]. The results are summarized in Fig.…”

“…On the other hand, the addition of the CO 2 gas in the titrating mixture will produce a highly acidic melt in alkali carbonates or their mixtures as CO 2 is the strongest acid in these melts [15] with all the oxide ions being consumed by CO 2 and not by O 2 . Very differently, the fact that large amounts of peroxide ion are still detectable under relatively high CO 2 partial pressures (PCO 2 > 0.2 atm [2]) after addition of rare earth oxides to the alkali carbonates undoubtely indicates that the acidity strength of CO 2 is decreasing in these mixed carbonates at a level where the two conjugate acid-base pairs of eq. (4) form a strong oxide buffered melt.…”

“…As this is the type of problem that can be well addressed with the O 2 solubility titration method already described in detail in preceding papers [1,2], the description of the method is dispensed here. However, for the specific purposes of this work, it may be worth remarking how the peroxide concentration has been estimated from the experimental data.…”

“…It can be noted that with the introduction of rare earth cations oxygen solubilities are subjected to drastic changes at temperatures above 873K and that the extent of these changes are also affected by the carbonate composition with the highest effects being observed in the lithium-sodium carbonate. It was found that the dissolution of the gaseous oxygen takes place almost exclusively through redox reactions with the melt either to give almost exclusively peroxide anion in the lithium-sodium carbonate or a mixture of superoxide and peroxide anions in the lithium-potassium carbonate (Table 1) [1,2]. Striking was the high peroxide concentration in the lithium-sodium carbonate *Address correspondence to this author at the TER, ENEA CRE Casaccia, Via Anguillarese 301, 00060 Rome, Italy; Tel: +39.06.30483138; Fax: +39.06.30483327; E-mail: frangini@casaccia.enea.it system indicative that rare earth oxides should have induced the formation of a very strongly basic medium even under relatively high CO 2 partial pressures (PCO 2 > 0.2 atm) in accordance with the known basic character of rare earth oxides.…”