Coupling Bulk and Near-Electrode Interfacial Nanostructuring in Ionic Liquids

The efficiency of charge transfer in electrochemical devices is largely determined by the ion concentration profile near the electrode surface, i.e., the electrical double layer (EDL). Room temperature ionic liquids (RTILs) are attractive for electrochemical applications due to their high charge density as well as for their tunable anion/cation design, low vapor pressure, and wide electrochemical window. The EDL structure in RTILs is profoundly different from that in traditional (dilute) electrolytes in that opposite charges tend to layer in a spatially alternating, segregated structure that decays toward the bulk region. Such charge layering becomes crucial for applications that require confinement of RTILs into narrow spaces, where RTILs are interfaced with nanostructured electrodes. Layering in the EDL is frequently explained by electrostatic interactions of the ions with the electrode, assuming that RTILs are homogeneous liquids made of ions only. However, a growing evidence points to the presence of neutral and charged multi-ion species in RTIL bulk and that the EDL structure is related to this bulk heterogeneity. These relations imply rich spatiotemporal competitions between bulk phases and structures, the electrode morphology, and the EDL layering. However, a mechanistic understanding of how the bulk properties of RTILs affect the mesoscale order in EDLs, as well as charge transport/transfer in the EDL, is currently lacking. To advance this gap, a mean-field approach that extends the standard Poisson–Nernst–Planck model to account for ion association/dissociation and bulk phases is being suggested. An inclusive theoretical modeling can help with an understanding of the factors and mechanisms governing EDL structure formation and predict the effect of the EDL structure on the interfacial electrochemical properties.