  Whether or not specific ion effects determine the charge storage properties
of aqueous graphene and graphite-based supercapacitors remains a highly debated
topic. In this work we present a multiscale quantum mechanics classical
molecular dynamics investigation of aqueous mono- and divalent salt
electrolytes in contact with fully polarizable charged graphene sheets. By
computing both the electrochemical double layer and quantum capacitance we
observe a constant electrode specific capacitance with cationic radii and
charge. Counterintuitively, we determine that a switch in the cation adsorption
mechanism from inner to outer Helmholtz layers leads to negligible changes to
the EDL capacitance, this appears to be due to the robust electronic structure
of the graphene electrodes. However, the ability of ions (such as K+) with a
relatively low hydration free energy to penetrate the inner Helmholtz plane and
adsorb directly on the electrode surface is found to slow their diffusion
parallel to the interface. Ions in the outer Helmholtz layer are found to have
higher diffusivity at the surface due to their position in ion channels between
water layers. Our results show that surface effects such as the surface
polarization and the partial dehydration and local structuring of ions on the
surface underpin the behaviour of cations at the interface and add a vital new
perspective on trends in ion mobilities seen under confinement.
