  Graphite is a ubiquitous electrode material with particular promise for use
in e.g., energy storage and desalination devices, but very little is known
about the properties of the graphite-electrolyte double layer at
technologically relevant concentrations. Here, the (electrified)
graphite-NaCl(aq) interface was examined using constant chemical potential
molecular dynamics simulations; this approach avoids ion depletion (due to
surface adsorption) and maintains a constant concentration of ions beyond the
surface. Specific Na+ adsorption at the graphite basal surface causes charging
of the interface in the absence of an applied potential. At moderate bulk
concentrations, this leads to accumulation of counter-ions in a diffuse layer
to balance the effective surface charge, consistent with established models of
the electrical double layer (DL). Beyond 0.6 M, however, a combination of
over-screening and ion crowding in the DL results in alternating compact layers
of ion density perpendicular to the interface. The transition to this regime is
marked by an increasing DL size and anomalous negative shifts to the potential
of zero charge with incremental changes to the bulk concentration. Our
observations are supported by changes to the position of the differential
capacitance minimum measured by electrochemical impedance spectroscopy.
Furthermore, a striking level of agreement between the differential capacitance
from simulations and experiments allows us to critically assess the accepted
norm that electrochemical capacitance measurements report simply on the density
of states of the graphite material. Finally, ion crowding at the highest
concentrations (beyond 5 M) leads to the formation of liquid-like NaCl clusters
confined to highly non-ideal regions of the double layer, where ion diffusion
is up to five times slower than in the bulk.
