No 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.

The shape and size are the two important geometrical factors that affect the electronic screening in nanomaterials. Here, we develop an analytical theory for electronic capacitance based on Thomas-Fermi screening in conjunction with 'multiple scattering method' for arbitrary-shaped nanostructures including electronic spillover correction. We relate the electronic capacitance of the material to the curvature correction expressed in terms of ratio of electronic screening length to principal radii of curvature. Electronic capacitance of various nanostructures is obtained showing geometrical shape-and sizedependent electronic screening in nanostructures that manifest important consequences in charge storage enhancement or reduction.

We generalize linearized Gouy-Chapman-Stern theory of electric double layer for nanostructured and morphologically disordered electrodes. Equation for capacitance is obtained using linear Gouy-Chapman (GC) or Debye-Hückel equation for potential near complex electrode/electrolyte interface. The effect of surface morphology of an electrode on electric double layer (EDL) is obtained using "multiple scattering formalism" in surface curvature. The result for capacitance is expressed in terms of the ratio of Gouy screening length and the local principal radii of curvature of surface. We also include a contribution of compact layer, which is significant in overall prediction of capacitance. Our general results are analyzed in details for two special morphologies of electrodes, i.e. "nanoporous membrane" and "forest of nanopillars". Variations of local shapes and global size variations due to residual randomness in morphology are accounted as curvature fluctuations over a reference shape element. Particularly, the theory shows that the presence of geometrical fluctuations in porous systems causes enhanced dependence of capacitance on mean pore sizes and suppresses the magnitude of capacitance. Theory emphasizes a strong influence of overall morphology and its disorder on capacitance. Finally, our predictions are in reasonable agreement with recent experimental measurements on supercapacitive mesoporous systems.

Anomalous slow dynamics with impedance phase modulation and two capacitive arcs are seen in the recent experiments with room-temperature ionic liquid (RTIL) on gold single-crystal electrodes. Single-crystal electrodes have low surface atomic heterogeneity and can show crystal face dependent multiorientation adsorbing states. We have extended our recently developed model of electric double-layer (EDL) impedance of a heterogeneous electrode with single state compact layer (see J. Electroanal. Chem. 2013, 704, 197−207) to a multistate compact layer for accounting ion shape asymmetry in RTILs. The modified multistate model incorporates ion shape asymmetry dependent molecular properties and related relaxation dynamics in the compact layer. Our model with two or three ion orientation states in the compact layer for systems with shape asymmetric cations explains such anomalous dynamics. Comparisons of the theoretical impedance and capacitance responses with recent experiments on electrode/IL systems are in good agreement.