Disclosing the Graphite Surface Chemistry in Acid Solutions for Anion Intercalation

Graphitic electrodes in ion batteries and graphite electrochemical exfoliation for graphene production are technological processes based on the anion intercalation mechanism in the stratified crystal structure upon immersion in acid solutions. To the detriment of the graphite mother-crystal, decomposition of the electrolyte solution or production of unexpected graphite intercalation compounds (GICs) is a side effect related to the anion intercalation mechanism. In this work, we studied the surface of highly oriented pyrolytic graphite (HOPG) samples treated with electrolyte solutions of 2 M HClO4 and 1 M H2SO4 in order to identify elemental and molecular species involved in the intercalation processes and, at the same time, find local surface defects through which solvated ions easily reach the HOPG subsurface. Results from cyclic voltammetry, X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (ToF-SIMS) are described and discussed. Large blister formation and more general HOPG surface damage induced by treatment with acid solutions were revealed and mapped. In both HOPG samples, oxidized in 2 M HClO4 and 1 M H2SO4, the surface distribution of perchlorate and hydrogen sulfate ions gives direct evidence that the anion intercalation happens via surface defects among HOPG crystallites. These results pave the way for a complex intercalation process where more than one molecular species could be responsible for blister formation and carbon detriment