Coupling Bulk and Near-Electrode Interfacial Nanostructuring in Ionic Liquids

Abstract

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.

From bulk self-assembly to electrical diffuse layer in a continuum approach for ionic liquids: The impact of anion and cation size asymmetry

“…This nonlocal Cahn-Hilliard framework has successfully reproduced the emergence of bulk nano-morphologies that have been observed experimentally [10,11], while providing further insights into electrokinetic phenomena in ILs, namely in the form of transient currents. However, the methodology was based on geometrically identical ions, which is clearly an abstraction of any real-world system [11].…”

“…With significant charge delocalization and irregular geometries, the ions do not readily form a tightly-bound lattice and remain liquid even at room temperatures and in the absence of any solvent [1], hence the name "ionic liquids" (ILs). Their tunable molecular structure enables the tailoring of ILs to a large number of applications [2-9], e.g., batteries, supercapacitors, dye-sensitized solar cells, lubricants and nanoparticle syntheses, where they are advantageous due to their high charge density, low-volatility, and high chemical, thermal and electrochemical stability.It is the amphiphilic-type structure, however, that gives ILs another intriguing property-the ability to selfassemble, see [10] and references therein. IL molecules spontaneously form bicontinuous, hexagonal or lamellar phases, see [11] and references therein, in a fashion similar to the morphologies of block copolymers and liquid crystals.…”

“…It is the amphiphilic-type structure, however, that gives ILs another intriguing property-the ability to selfassemble, see [10] and references therein. IL molecules spontaneously form bicontinuous, hexagonal or lamellar phases, see [11] and references therein, in a fashion similar to the morphologies of block copolymers and liquid crystals.…”

“…Since these phase transitions are of universal nature [30,36], in higher space dimensions the morphologies that form are of isotropic nature (e.g., labyrinths and hexagons) and thus, upon averaging about the parallel plane to the electrodes, correspond to a uniform density with a vanishing electrical field, i.e., electroneutrality in 1D context. The second effect of an isotropic structured bulk is its subtle impact on the charge layering within the EDL region [10,11]. Since the bulk isotropy breaks down near the solid surface, the patterns tend to align with electrode orientation.…”

From Solvent-Free to Dilute Electrolytes: Essential Components for a Continuum Theory

“…[5][6][7] The optimized design of these devices requires the mechanistic spatiotemporal understanding of ionic arrangement and charge transport in electrolytes. Although the physicochemical aspects of electrolyte solutions have been extensively studied, a series of recent experimental and computational results [8][9][10][11][12][13][14][15][16][17][18][19] reflects knowledge gaps even in the context of basic science. 3,16,[20][21][22][23][24] A robust spatiotemporal framework is thus required to advance electrochemically-based industrial applications of highlyconcentrated electrolytes, e.g., fuel cells ∼1M (mol/liter) and batteries ∼10M.…”