Role of anions on electrochemical exfoliation of graphite into graphene in aqueous acids

Anodic electrochemical exfoliation of graphite is one of the promising methods for the fast production of graphene with moderate quality at low cost. Sulfate-containing aqueous solutions have been identified as the most efficient electrolyte condition for anionic exfoliation process of graphite into graphene. However, the origin of the high exfoliation efficiency of sulfate-containing electrolytes remains unanswered. In this study, we study the role of anions in anodic exfoliation of graphite by comparing different dilute acid systems (sulfuric, nitric, phosphoric, and hydrochloric acids). Using the combination of in-situ mass spectrometer, cyclic voltammetry, and density functional theory computation modeling, we reveal that only the sulfate anions allows reversible intercalation behavior upon cycling voltammetry cycles, probably due to the highest repulsive binding energy between graphene sheets compared to those of other anions. Therefore, the easy access of sulfate anion into the graphite layer facilitates the diffusion of water molecules into the bulk of the graphite during the anodic process, and the subsequent water oxidation enables efficient exfoliation of graphite into graphene. These results highlight the importance of understanding the interactions between electrolyte ion and graphite for the electrochemical manufacturing of graphene.