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 graphit e (HOPG) was studied as a model in aqueous electrolytes to elucidate the me chanism of electrochemical intercalation into graphite. The local and time- dependent dimensional changes of the host material occurring during the ele ctrochemical intercalation processes were investigated on the nanometer sca le. Atomic force microscopy (AFM), combined with cyclic voltammetry, was us ed as an in situ analytical tool during the intercalation of perchlorate an d 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 co uld 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 intercal ation typically amounts to 32% when tens of layers are involved but to only 14% when thousands of layers are involved. Blister formation and more dram atic changes in morphology were observed, depending on the kind of electrol yte used, at higher levels of anion intercalation.