Data are presented on the electronic conductivity and molten alkali carbonate stability (Li:K binary of 62:38 mole ratio) of various materials at 923 K under a 30% CO2/normalair atmosphere. Of the examined materials, only LaNiO3 and LaxSr1−xCoO3 satisfied both criteria sufficiently for acceptable use as molten carbonate fuel cell cathode materials.

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Equilibrium NiO solubility measurements were made inLi2C03/K2C03 mixtures as a function of temperature, ambient gas environment, and salt composition for gases containing 3.1% HzO. The equilibrium solubility was found to increase with increasing temperature, COz partial pressure, and cation fraction Li+ and to decrease slightly with increasing O2 partial pressure. These results were coupled with theoretical predictions of (1) 02, H2, and C 0 2 activities and (2) compositional gradients which develop across the electrolyte structure of an operating carbonate fuel to explain the two distinct regions of nickel precipitation across the fuel cell. Precipitation occurs near the cathode because NiO solubility decreases commensurate with electrolyte compositional gradients, whereas the second region deeper within the electrolyte matrix results from a decreased O2 activity.

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The corrosion of molten carbonate fuel cell (MCFC) NiO cathodes during cell operation has ,been brought to attention recently (1-5) and is considered one of the major technical difficulties facing MCFC development at the present time. NiO, solubilized in the molten Li]K carbonate (62/38 mole ratio) at the cathode/carbonate interface, is diffusionally transported through the carbonate reservoir towards the.anode. At some location intermediate between the two electrodes, the local cell environment's oxygen partial pressure decreases below the level where NiO is stable and metallic nickel precipitates. Recent evidence obtained from cell tests has shown the extent of cathode degradation to be accelerated by increasing either the Pco~ or the total P(CO~ + O~) supplied to the cathode. This is important since commercial fuel cells are expected to operate with oxidant gases such as humidified 30% CO~/bal air at elevated pressures of between 6 and 10 arm. Estimations of the extent of cathode degradation occurring under these elevated pressures requires an understanding of how the NiO/carbonate equilibrium changes as a function of changing cathode gas environment. This note documents the measured equilibrium NiO solubility in Li/K carbonate melts under various humidified COJO..,/N2 gases containing a constant Pco./Po.2 ratio of 2/1. ExperimentalPellets of NiO, approximately 1.2 cm in diam, were pressed at 20,000 psi and air sintered for 24h at 1123 K. Equilibrium solubility measurements were made by placing one pellet in the bottom of a clean 7.5 cm tall • 1.6 cm diam gold crucible along with 20 -+ lg of premelted and ground high purity binary Li/K carbonate of eutectic composition 62 mole percent (m/o) Li2CO3/38 m/o K2CO3 (Apache Chemical Company, Seward, Illinois). The crucibles were placed in an a-A1203-1ined box furnace whose cover was adapted for two thermocouples, a gas inlet, and a sampling tube. The cover gases utilized for this study, possessing Pco./Po., ratios of 2/1 with a N~ balance, were water saturated at 298 -+ 1 K to give-the following gas compositions 5.8% CO~2.9% O2/3.1% H20/N2[1] 29.1% CO.,/14.2% O2/3.1% H20/N2 [2] 48.5% COJ24.2% OJ3.1% H._,O/N2 [3] 64.6% COJ32.3% O~/3.1% H20 [4]The samples were allowed to equilibrate at temperature for at least 150h. Beginning at that time, approximately lg aliquots of molten carbonate were removed at 24h intervals by inserting an a-Al~O3 pipette just below the melt's upper surface. These aliquots were transferred to clean gold crucibles where they cooled for subsequent nickel analysis by atomic absorption spectroscopy. The quoted solubilities represent a nickel analysis which did not vary more than -+10% relative to a mean over a 3 day period. The limit of --10% represents the reproducibility obtained on identical samples and constitutes the cumulative deviation in the solubility and analytical techniques. In addition, a carbonate blank consisting of an identically prepared crucible containing carbonate only was occasionally carried through the procedure adjacent to t...