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Advances in Molten Salts

ISSN:
1-56700-142-4 (Print)

Different Molten Carbonates as Fuel Cell Electrolytes for MCFCs

H. Wendt
Institut fur Chemische Technologie - TU Darmstadt - Petersenstr. 20, D-64287 Darmstadt

Abstract

In molten carbonate fuel cells molten lithium/potassium carbonate (62/38 mol/mol) eutectic is the traditional electrolyte since the early experiments of Ketelaar and Broers. As these cells now enter the demonstration stage of development the disadvantages of (Li/K)2C03 melts become more obvious. First the volatility of potassium from the Li/K-melts in moist atmospheres at 650°C, which is caused by the molten salt hydrolysis Me2C03 + H20 ⇄ 2MeOH + C02 is undesirably high: 1.54 · 10-6 bar for potassium hydroxide (3.9 · 10-8 bar Li), whereas above the eutectic Li/Na-melt the sodium-hydroxide pressure amounts to only 1.1 · 10-7 bars (2.2 · 10-8 bar Li). This alone is a critical circumstance because for long lasting fuel cell operation over more than 40 000 h and a power density of 0.1 W cm-2 predicted electrolyte losses amount to more than 60 mg K2CO3 per cm2 cell area compared to electrolyte loadings from 100 to 200 mg cm-2. It is quite clear that only the Li2C03/Na2CO3 eutectic with much lower sodium vapor pressures could assure long term performance and prevent the drying out of the MCFC cell. The second reason to change from Li/K carbonate to Li/Na carbonate melts is the lower solubility of the cathode materials (NiO and LiCo02) in the latter electrolyte. This difference - for NiO - by a factor of 2 is not dramatic, but nonetheless decelerates dissolution and coarsening of the cathode due to Ostwald ripening. Finally electrolyte segregation under steady performance of the cell is less and this does not prevent power density enhancement if Li/Na-carbonate is used as it does in case of the Li/K-carbonate electrolyte. But the solubility of oxygen in Li/Na-carbonate eutectic is lower than in Li/K-eutectic, which impairs the cathodic kinetics of 02-reduction due to mass transfer hindrance.