Anisotropic Step Etching and Etch Instabilities during Cathodic Corrosion on Platinum

The process of cathodic corrosion on platinum electrodes in aqueous environments is inevitably obscured by concurrent hydrogen evolution. Insights into the corrosion mechanism and progression are, therefore, most straightforwardly obtained through postcorrosion characterization. In this work, we use ex situ microscopy to evaluate the progression of etch pits on (111)- and (100)-facets of single-crystal platinum electrodes. While the corrosion process has previously been shown to be anisotropic, we observe that etching mainly occurs at undercoordinated step edges rather than into the plane. Such a dependence on the local energetics of the platinum surface atoms is also evident by a pit nucleation barrier on the (100)-facet, where nucleation occurs preferentially on dislocations in the crystal lattice. On the (100)-facet, we also observe an etching instability that results in both rapid downward etching and material redeposition. Through the observed nucleation-and-growth nature of the etch pits in cathodic corrosion, we envision that the etching (and subsequent redeposition) process can be tailored through control over the surface energetics, similar to methods known from electrodeposition.