NiO Solubility in Molten Li/K Carbonate under Molten Carbonate Fuel Cell Cathode Environments

Abstract

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...

Solubilities of Nickel Oxide in Molten Carbonate

“…Combining Eq. [4]- [7], the following can be obtained Li/K: log x = y log {Kai~(K)/Kdis(Li)} + log {Kse" Kd~ (K) -1" Pcoz} [8] LENa: log x = y log {Kdi~(Na)/Kdi~(Li)} + log {Ksp" Kdis (Na) -1 9 Pco2} [9] Since K~s and Ksp are constant at a fixed temperature, a linear relationship between log x and y should be obtained at a fixed CO2 pressure if the assumption is correct.…”

The Solubility of NiO in Binary Mixtures of Molten Carbonates

“…lution in a mixture of Na2CO3 and K2CO3 may be complicated by the presence of several different dissolution products. If only NiO22 and NiO2'-compounds are considered as dissolution products, the following equations describe the dissolution NiO + Na20 = 2Na + + NiO22- [2] 2NiO + Na20 + 1/2 02 = 2Na § + 2NIO2-…”

“…lution in a mixture of Na2CO3 and K2CO3 may be complicated by the presence of several different dissolution products. If only NiO22 and NiO2'-compounds are considered as dissolution products, the following equations describe the dissolution NiO + Na20 = 2Na + + NiO22- [2] 2NiO + Na20 + 1/2 02 = 2Na § + 2NIO2- [3] NiO + K20 = 2K § + NiO22- [4] 2NiO + K20 + 1/2 02 = 2K § + 2NIO2 [5] If the data from dissolution in K2CO3 (1) are considered, reactions involving nickel in a lower oxidation state must be added to this list to explain the dependence of solubility on pO2. With this number of reactions to consider, and little known about the free energies of any of them, the solubilities in mixtures cannot be easily related to those in single salts.…”

Nonuniformities in Chemical Oxides on Silicon Surfaces Formed during Wet Chemical Cleaning

“…25 In order to develop a better process, we require a basic understanding of the formation mechanism and the nature of thin oxides. Si surface cleaning using wet chemical solutions, such as an RCA clean, are most important.1 Cleaning removes oxides, carbon, or metallic impurities, reduces surface damage or roughness, and forms surfaces for protection from contaminants between processing stages.…”