Electrochemical behavior of nonporous Ni/NiCl2 electrodes in chloroaluminate melts

The electrochemical behavior of nonporous Ni/NiCl2: electrodes was studied using an Al/Na[AlCl4]-NaCl/NiCl2/Ni cell in which the capacities of the fell were limited by the Ni/NiCl2 electrode. The limiting mechanism of the electrode was found to be associated with formation of NiCl2 on the surface of the nickel electrode. This phenomenon limits the mass-transfer processes of the nonporous electrode and thus its area capacity density. Based on the results of these investigations, an electrochemical model of the electrode reactions was developed which predicts the performance characteristics of porous Ni/NiCl2 electrodes for various conditions of operation. Modifying the electrolyte with the NaBr, Nal, and sulfur additives was found to produce higher nickel utilization and lower impedance values due to doping effects, which is believed to open up the lattice for better mass transport. Solubility of the nicker chloride in sodium-chloroaluminate melts as a function of temperature and additives was also determined. The solubility measurements indicated that the solubility of nickel chloride in the chloroaluminate melt is strongly dependent on the operational temperature of the cell and the chemical additives present in the electrolyte. The results of this study clearly indicate the importance of the chemical additives, basicity of the melt, and the lower operating temperature of the Na/NiCl2 cell for improved performance and cycle life. (C) 2000 The Electrochemical Society. S0013-4651(99)06-142-X. All rights reserved.