Electrochemistry, Ion Adsorption and Dynamics in the Double Layer: A Study of NaCl(aq) on Graphite

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.