A New View of Electrochemistry\nat Highly Oriented\nPyrolytic Graphite

Abstract

Major new insights on electrochemical processes at graphite\nelectrodes\nare reported, following extensive investigations of two of the most\nstudied redox couples, Fe­(CN)₆^4–/3– and Ru­(NH₃)₆^3+/2+. Experiments\nhave been carried out on five different grades of highly oriented\npyrolytic graphite (HOPG) that vary in step-edge height and surface\ncoverage. Significantly, the same electrochemical characteristic is\nobserved on all surfaces, independent of surface quality: initial\ncyclic voltammetry (CV) is close to reversible on freshly cleaved\nsurfaces (>400 measurements for Fe­(CN)₆^4–/3– and >100 for Ru­(NH₃)₆^3+/2+),\nin\nmarked contrast to previous studies that have found very slow electron\ntransfer (ET) kinetics, with an interpretation that ET only occurs\nat step edges. Significantly, high spatial resolution electrochemical\nimaging with scanning electrochemical cell microscopy, on the highest\nquality mechanically cleaved HOPG, demonstrates definitively that\nthe pristine basal surface supports fast ET, and that ET is not confined\nto step edges. However, the history of the HOPG surface strongly influences\nthe electrochemical behavior. Thus, Fe­(CN)₆^4–/3– shows markedly diminished ET kinetics with either extended exposure\nof the HOPG surface to the ambient environment or repeated CV measurements. <i>In situ</i> atomic force microscopy (AFM) reveals that the deterioration\nin apparent ET kinetics is coupled with the deposition of material\non the HOPG electrode, while conducting-AFM highlights that, after\ncleaving, the local surface conductivity of HOPG deteriorates significantly\nwith time. These observations and new insights are not only important\nfor graphite, but have significant implications for electrochemistry\nat related carbon materials such as graphene and carbon nanotubes.