  We use molecular dynamics simulations in a constant potential ensemble to
study the effects of solution composition on the electrochemical response of a
double layer capacitor. We find that the capacitance first increases with ion
concentration following its expected ideal solution behavior, but decreases
upon approaching a pure ionic liquid in agreement with recent experimental
observations. The non-monotonic behavior of the capacitance as a function of
ion concentration results from the competition between the independent motion
of solvated ions in the dilute regime and solvation fluctuations in the
concentrated regime. Mirroring the capacitance, we find that the characteristic
decay length of charge density correlations away from the electrode is also
non-monotonic. The correlation length first decreases with ion concentration as
a result of better electrostatic screening but increases with ion concentration
as a result of enhanced steric interactions. When charge fluctuations induced
by correlated ion-solvent fluctuations are large relative to those induced by
the pure ionic liquid, such capacitive behavior is expected to be generic.
