  It is well-known that room temperature ionic liquids (RTILs) often adopt a
charge-separated layered structure, i.e., with alternating cation- and
anion-rich layers, at electrified interfaces. However, the dynamic response of
the layered structure to temporal variations in applied potential is not well
understood. We used in situ, real-time X-ray reflectivity (XR) to study the
potential-dependent electric double layer (EDL) structure of an
imidazolium-based RTIL on charged epitaxial graphene during potential cycling
as a function of temperature. The results suggest that the graphene-RTIL
interfacial structure is bistable in which the EDL structure at any
intermediate potential can be described by the combination of two
extreme-potential structures whose proportions vary depending on the polarity
and magnitude of the applied potential. This picture is supported by the EDL
structures obtained by fully atomistic molecular dynamics (MD) simulations at
various static potentials. The potential-driven transition between the two
structures is characterized by an increasing width but with an approximately
fixed hysteresis magnitude as a function of temperature. The results are
consistent with the coexistence of distinct anion and cation adsorbed
structures separated by an energy barrier (~0.15 eV).
