Transient Relaxations of Ionic Conductance during Growth of Porous Anodic Alumina Films: Electrochemical Impedance Spectroscopy and Current Step Experiments

By Ide, Shinsuke; Mishra, Pratyush; Hebert, Kurt R.
Published in Electrochimica Acta NULL 2016

Abstract

Abstract Transient electrochemical relaxations during growth of anodic oxide films can provide insight into mechanisms of transport processes. Few such studies have focused on porous anodic oxides, which are of significant interest as functional materials. Conductance relaxations during anodizing of aluminum in acidic porous anodic oxide-forming solutions are investigated, using high voltage impedance spectroscopy and constant current reanodizing experiments. Both experiments follow phenomenology demonstrated in prior studies of anodic barrier films. Low-frequency inductive loops in impedance spectra exhibit time constants with inverse proportionality to current density and characteristic resistances nearly equal to those of high-frequency capacitive features. Potential decays during reanodizing depend on charge passed instead of time. Both sets of experiments are analyzed with a model including conductance relaxations due to electric field-dependent generation and removal of bulk charge-carrying defects. The model is found to be quantitatively consistent with nearly all aspects of experimental behavior; the inverse current density dependence of the inductive time constant and the charge parameterization of potential decays both arise because of exponential electric field dependence of ionic conduction and defect generation/removal rates. Oxide volume changes accompanying defect relaxations may help explain mechanical stress generation during anodizing, which is thought to drive pore formation.

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