Aqueous Solutions at Graphitic Interfaces: Effects of Charge Transfer, Ion Hydration, and Confinement on Interfacial Structure and Capacitance

Improved understanding of aqueous solutions at charged graphitic interfaces is critical for designing new carbon-based materials for energy storage. However, many mechanistic details remain unclear, including how interfacial structure and response are dictated by intrinsic properties of solvated ions under applied voltage. Here, we combine first-principles simulations with electrochemical measurements to investigate adsorption of several alkali-metal cations at the interface with charged graphene and within graphene slit-pores. We confirm that adsorption energy increases with ionic radius, while being highly dependent on pore size under confinement conditions. In addition, in contrast with conventional electrochemical models, we find that interfacial charge transfer contributes non-negligibly to this interaction and can be further enhanced by confinement. Overall, the measured interfacial capacitance trends result from a complex interplay between voltage, confinement, and specific ion effects--including ion hydration and degree of charge transfer.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.