Incipient Electrochemical Oxidation of Highly Oriented Pyrolytic Graphite: Correlation between Surface Blistering and Electrolyte Anion Intercalation

Abstract

Electrochemical oxidation of highly oriented pyrolytic graphite (HOPG) electrodes in KNO 3 electrolyte solution at a carbon submonolayer level was shown in a previous paper to produce raised, bubblelike features, giving the appearance of shallow blisters. The blisters were established to be hollow, with a top surface of intact HOPG lattice and an inner one of graphitic oxide. This paper explores the relationship of the intercalating properties of the electrolyte anion to the blister formation on HOPG. Cyclic voltammetry, atomic force microscopy, optical microscopy, and in situ atomic force microscopy/electrochemistry results are presented. Surface blister formation is observed to occur in 1 M LiCIO 4 , 1 M (NH4) 2 SO 4 , 1 M HNO 3 , and 1 M H 2 SO 4 aqueous electrolytes as a result of potential scans (+1.50 to +1.90 V vs SSCE, depending on the electrolyte) that pass a few carbon monolayers of charge or less. Blister formation on HOPG in 1 M KOH electrolyte is observable but at significantly higher levels of oxidative charge, whereas blisters were not observed in 1 M K 2 HPO 4 and 1 M H 3 PO 4 electrolytes. These results are consistent with literature Raman intercalation data that indicate phosphate does not readily intercalate into HOPG, supporting a model in which blister formation reflects electrolyte anion intercalation followed by subsurface gas evolution.