Electrochemical study of highly ordered pyrolytic graphite surface film formation by in-situ atomic force microscopy

Abstract

In-situ electrocheimical atomic force microscopy (ECAFM) was used to study the surface film that forms on highly ordered pyrolytic graphite (HOPG) electrode during cathodic polarization. Cyclic volatammetry experiments confirmed that distinct reduction reactions occur when the potential of a fresh HOPG electrode is swept from 2.4 to 0.01 V vs. $Li/Li^+$. The reactions appear to be irreversible, since no evidence of corresponding oxidation reaction was observed. ECAFM results, when combined with cyclic voltammetry data, suggest that these contribute to form surface film. The surface film is differently formed dependent on solvent and electrolyte salt. It is observed by EC AFM that the morphology of surface film in 1 M $LiClO_4-EC/DMC$ is more smooth and compact than that in 1 M $LiPF_6-EC/DMC$ and that in 1 M $LiClO_4-PC$ is absent. In addition, it is proved that the addition of 12 crown 4 ether improves the surface film formation in 1 M $LiClO_4-PC$. When considered with the second cycle of CV experiment, the ECAFM results confirm that the further electrolyte decomposition has the correlation with the formation of surface film.