Basics of Pore Formation

In order to use a consistent terminology in the following chapters it is emphasized that the term hole will always refer to a defect electron, while an etched feature in the electrode will be designated as a pore if its depth exceeds its width or otherwise as an etch pit. All values of current density refer to the initial surface of the electrode exposed to the electrolyte, which is for example defined by the O-ring of the set-up. 

Pore formation is a common feature of many metal and semiconductor electrodes under anodic conditions in various electrolytes. Common products, for example aluminum capacitors, have been manufactured for decades using electrochemical pore formation techniques. Nevertheless in many cases the physics of pore initiation and propagation is poorly understood. 

What causes the remarkable difference in dissolution rate between the pore tip and pore wall is the basic question for any system that shows pore formation. In this section a general view of this process is presented, without discussing details of the chemistry or of the pore morphology. 

A common feature of all electrochemical pore formation processes in solid-state electrodes of a homogeneous chemical composition is the remarkable difference in dissolution rate between pore tip and pore wall. This is usually discussed in terms of an active-passive transition between the pore tip interface and the pore wall interface. But this still leaves the question open as to what quality of the pores makes their tips active and the remaining surface passive. On a basic level the active-passive transition has been ascribed to three distinct causes [Le31]  

The difference between pore tips and pore walls may originate from the initial state of the electrode prior to anodization. 

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