Suppressing water decomposition for controllable exfoliation of graphite in water-in-salt electrolyte

Abstract

Development of aqueous electrolytes in anodic electrochemical (EC) exfoliation is regarded as the preferred choice to prepare single or few layered graphene. Generally, in diluted aqueous electrolytes, however, the uncontrollable decomposition of water often provides excess driving forces, leading to the low ratio of single/few layered graphene nanosheets. Here, we employed a low-cost sodium perchlorate (NaClO4)-based water-in-salt electrolyte (WIS) for the controllable EC exfoliation of graphite. Benefiting from the high salt-to-water ratio of the electrolyte, their thickness and oxidation extent of the resultant graphene nanosheets were able to be effectively controllable. Moreover, density-functional-theory-based molecular dynamics (DFT-MD) simulations exhibit a revolution from free water to bound water in WIS electrolytes as the salt-to-water ratio increases. Combined with the deconvolution peaks of the Raman spectra, the results further disclose that only at high concentration, free water molecules can be effectively restricted in WIS electrolyte, and they need to overcome more hindrance to arrive at the electrode surface for EC exfoliation than that in dilute electrolytes, therefore, the reaction rate of EC exfoliation can be controlled easily by reducing the amount of free water. Overall, this work demonstrated that controllable graphite exfoliation by changing the decomposition activity of water molecules.