Solubility and Transport of NiO Cathodes in Molten Carbonate Fuel Cells

Abstract

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.

The Effects of Nickel Oxide Cathode Dissolution on Molten Carbonate Fuel Cell Life

“…Although no consistent discrete location is present at which nickel deposition occurs, higher concentrations are frequently found to occur in the central region of the matrix (3,8,16,17). Reference (17) contains micrographs of nickel deposited in the matrix obtained by both scanning electron microscopy and energy dispersive x-ray spectroscopy.…”

“…EIS has proven to be a powerful tool to obtain system specific parameters of coatings, especially for evaluating the early deviation of organic coatings from their initial capacitive behavior (1)(2)(3)(4). An excellent paper by Kendig and Scully on this subject exists (5).…”

“…The solubility of nickel oxide has been measured as a function of electrolyte composition, gaseous environment, and temperature (2)(3)(4)(5)(6). Although some variability in these solubilities exists, reasonably good agreement was found for mixtures of lithium carbonate and potassium carbonate near the common fuel cell operating temperature of 650~ The primary specie formed in solution when nickel oxide dissolves in the fuel cell is considered to be the Ni § ion, since the solubility of nickel oxide was found to be proportional to the carbon dioxide partial pressure and independent of the oxygen partial pressure in the range of interest (2).…”

The dissolution process of the NiO cathodes for molten carbonate fuel cells: state-of-the-art

“…Unfortunately, in practice, the formed Ni> ions can diffuse, through the matrix structure, towards the anode where the reducing conditions enhance the precipitation of metallic Ni particles. This kind of 'driving force' does not allow the system to reach its solubility equilibrium and thus the dissolution process of the cathode structure becomes continuous (Baumgartner, 1986;ESC, 1987).…”

The Enhanced Physico-Chemical and Electrochemical Properties for Surface Modified NiO Cathode for Molten Carbonate Fuel Cells (MCFCs)

“…Theses structural changes of the cathode caused by the reduction give challenges. [6][7][8] Two types of dissolution mechanism of NiO (acidic or basic dissolution) have been proposed and it is reported that the solubility of NiO depends on the acid-base properties of the molten carbonate or carbonate mixture. 9 In order to solve the problem of NiO dissolution, many investigators have studied alternative electrolytes and cathodes.…”