Additive Effects of La, Sr, Cs and Ca in a Molten Carbonate Fuel Cell

Current research has shown that Li₂CO₃-Na₂CO₃ electrolyte has better ionic conductivity, a lower rate of cathode dissolution and a lower electrolyte loss than Li/K carbonate. Lower oxygen gas solubility has been one of the disadvantages of the Li/Na electrolyte. Modification of the electrolyte such as the addition of alkali metals and alkaline earth metals have led to an improvement in the oxygen reduction reaction (ORR). In this experiment, the effects of lanthanum, strontium, cesium, and calcium were analyzed using several electrochemical half-cells and 7 cm² coin-type MCFCs operated at 973 K. A eutectic mixture of Li₂CO₃ and Na₂CO₃ (52:48) was used as the electrolyte. For the half-cell, the reference electrode compartment was supplied with a 33% Air/67% CO₂ gas mixture while the electrolyte compartment was supplied with different CO₂/Air gas compositions. The anode of the coin cells was supplied with a mixture of H₂, CO₂ and H₂O in the ratio 72:14:14 mol% and the cathode was supplied with air and CO₂ in the ratio 70:30 mol%. 0.5 mol% La₂O₃, 3 mol% Cs₂CO₃, 3 mol% CaCO₃, and 3 mol% SrCO₃ were added separately and also as a combination of two (i.e. La₂O₃/SrCO₃, CaCO₃/La₂O_3, and Cs₂CO₃/La₂O₃) to the Li/Na electrolyte. The effects of these additives were observed and analyzed using Electrochemical analysis techniques such as electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were used for the half-cell analysis. Steady-state polarization (SSP), internal resistance (IR) using the Nyquist plot, and step chronopotentiometry were employed in the analyses of the coin cell. The mass transfer parameter (C√D) values obtained from the cyclic voltammetry measurements increased as the oxygen partial pressure increases in all electrolytes. From the results obtained, the combined addition of Ca/La proved to have the most significant effect on the overall performance of the cell.