Modeling Morphology Evolution for Nanostructured Electrochemical Systems.

Abstract

In this dissertation, we examine the morphological evolution of two nanostructured electrochemical systems, the growth of anodic alumina and the electrodeposition/electrodissolution of magnesium. These systems are investigated through one-dimensional and three-dimensional continuum simulations.

Anodic alumina films are grown through an electrochemical oxidation process, exhibiting morphologies including barrier films and nanoporous films. A new model of anodization is developed in which a thin space charge region forms at the oxide/electrolyte interface, explaining experimental observations of embedded interfacial charge. Ionic transport through the oxide is described through a newly proposed counter-site defect mechanism. A one-dimensional model is parameterized and validated using experimental data in the literature. Predictions of the embedded charge as a function of applied current density and electrolyte pH are presented. The model is extended to multiple dimensions to simulate the growth of anodic nanopores. The simulations capture much of the experimental behavior for a range of applied potentials and electrolyte pH values. Most importantly, the simulated pore geometry is insensitive to the electrolyte pH, while still exhibiting the expected decreased growth rate for increasing pH. This improvement over previous models stems from the treatment of adsorbed oxygen and hydroxide species at the oxide/electrolyte interface.

The second system examined is the electrodeposition/electrodissolution of magnesium. A new model of electrodeposition and electrodissolution is developed, which incorporates Butler-Volmer kinetics, facet evolution, and dilute solution theory. Three-dimensional simulations of the growth of magnesium deposits yield in-plane and out-of-plane hexagonal plates, consistent with experimental observations. Simulations predict that the deposits become narrower and taller with increasing current density due to the depletion of the electrolyte concentration near the deposits. Increasing the distance between the deposits causes increased depletion of the electrolyte surrounding the deposit. Different morphologies after one deposition-dissolution cycle, a flatted-topped hexagonal pyramid and a hexagonal plate, are predicted for two types of orientation dependence for the dissolution reaction. These predictions can be tested experimentally to identify the mechanisms governing the morphological evolution of magnesium.

This work represents a step toward quantitatively predictive simulations of electrochemical systems through the development of improved models, their numerical implementation, and physical insights gained through simulations.