Modeling and Characterization of Rate Phenomena in Complex Electrochemical Systems: Sodium-Metal Chloride Batteries and Ni/SiC Co-Deposition

摘要

In the first part of the dissertation (Chapters 3-5), the effect of a cationic dispersant, polyethyleneimine (PEI) on the co-deposition of micro and nano SiC particles with nickel is characterized and modeled. A major challenge in Ni/SiC co-deposition for wear-resistant coatings is that the dispersants that are used to stabilize the particles in the electrolyte to ensure high and uniform particle incorporation into the deposit can significantly affect the electrodeposition kinetics. To overcome this challenge, studies of particle dispersion and electrodeposition are integrated. The effect of PEI on the electrodeposition of Ni/SiC composites is characterized as a function of SiC and PEI bath concentration, current density, rotation speed, molecular weight of PEI and particle size. A pre-coating procedure, in which SiC particles are pre-coated with PEI in a different electrolyte prior to plating, is described. With the pre-coating procedure, high particle stability in the plating bath is obtained. In addition, a significant increase on the SiC incorporation rate is seen without any substantial decrease on the current efficiency for both micro- and nano-composites. Furthermore, using pre-coated particles in the presence of a leveling agent is found to be advantageous relative to the direct addition of PEI into the electrolyte. The efficacy of employing the pre-coating procedure in manufacturing, where plating baths need a long life, is also found to be satisfactory. The use of pre-coated SiC particles changes the morphology, decreases the surface roughness and increases the hardness of the deposits for both particle sizes. Finally, a mathematical model of the co-deposition is proposed. The rate of incorporation is proportional to the residence time, inversely proportional to the burial time, and is proportional to the number density of particles on the surface. These times are influenced by the hydrodynamics, particle size, current density, and concentration of dispersants. SiC incorporation increases with the introduction of PEI due to an increase in the residence time of the particles on the surface. In the second part of the dissertation, a sodium-metal chloride battery, which is another important complex electrochemical system, is studied. A one-dimensional mathematical model of the porous cathode of a sodium-iron chloride battery for an isothermal, constant-current discharge-charge cycle is presented. In sodium-iron chloride batteries, it is desirable to maintain low FeCl2 solubility to minimize redistribution of active material in the cell. However, the iron chloride is sparingly soluble, and with increased cycling, it does redistribute. None of the previous models can predict this movement of the metal that takes place in the cell with increased cycling that can cause the failure of the cell. An advance offered by the model presented is that it accounts for the change in the solubility of FeCl2 within the cell and predicts the relocation of the iron by including the flux of a sparingly soluble ferrous complex. For instance, the model predicts that at the end of the fifth cycle, the iron amount decreases by ~1% near the sodium tetrachloroaluminate reservoir.