In situ AFM study of interlayer spacing during anion intercalation into HOPG in aqueous electrolyte

In the context of ion transfer batteries, highly oriented pyrolytic graphite (HOPG) was studied as a model in aqueous electrolytes to elucidate the mechanism of electrochemical intercalation into graphite. The local and time-dependent dimensional changes of the host material occurring during the electrochemical intercalation processes were investigated on the nanometer scale. Atomic force microscopy (AFM), combined with cyclic voltammetry, was used as an in situ analytical tool during the intercalation of perchlorate and hydrogen sulfate ions into and their expulsion from the HOPG electrodes. For the first time, a reproducible, quantitative estimate of the interlayer spacing in HOPG with intercalated perchlorate and hydrogen sulfate ions could be obtained by in situ AFM measurements. The experimental values are in agreement with theoretical expectations, only for relatively low stacks of graphene layers. After formation of stage IV, HOPG expansion upon intercalation typically amounts to 32% when tens of layers are involved but to only 14% when thousands of layers are involved. Blister formation and more dramatic changes in morphology were observed, depending on the kind of electrolyte used, at higher levels of anion intercalation.