Double-Layer in Ionic Liquids: Paradigm Change?

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• 作者：Alexei A. Kornyshev
• 刊名：Journal of Physical Chemistry B
• 出版年：2007
• 出版时间：May 24, 2007
• 年：2007
• 卷：111
• 期：20
• 页码：5545 - 5557
• 全文大小：219K
• 年卷期：v.111,no.20(May 24, 2007)
• ISSN：1520-5207

文摘

Applications of ionic liquids at electrified interfaces to energy-storage systems, electrowetting devices, ornanojunction gating media cannot proceed without a deep understanding of the structure and properties ofthe interfacial double layer. This article provides a detailed critique of the present work on this problem. Itpromotes the point of view that future considerations of ionic liquids should be based on the modern statisticalmechanics of dense Coulomb systems, or density-functional theory, rather than classical electrochemical theorieswhich hinge on a dilute-solution approximation. The article will, however, contain more questions than answers.To trigger the discussion, it starts with a simplified original result. A new analytical formula is derived torationalize the potential dependence of double-layer capacitance at a planar metal-ionic liquid interface. Thetheory behind it has a mean-field character, based on the Poisson-Boltzmann lattice-gas model, with amodification to account for the finite volume occupied by ions. When the volume of liquid excluded by theions is taken to be zero (that is, if ions are extremely sparsely packed in the liquid), the expression reducesto the nonlinear Gouy-Chapman law, the canonical result typically used to describe the potential dependenceof capacitance in electrochemical double layers. If ionic volume exclusion takes more realistic values, theformula shows that capacitance-potential curves for an ionic liquid may differ dramatically from the Gouy-Chapman law. Capacitance has a maximum close to the potential of zero charge, rather than the familiarminimum. At large potenials, capacitance decreases with the square root of potential, rather than increasesexponentially. The reported formula does not take into account the specific adsorption of ions, which, ifpresent, can complicate the analysis of experimental data. Since electrochemists use to think about thecapacitance data in terms of the classical Gouy-Chapman theory, which, as we know, should be good onlyfor electrolytes of moderate concentration, the question of which result is "better" arises. Experimental dataare sparse, but a quick look at them suggests that the new formula seems to be closer to reality. Opinionshere could, however, split. Indeed, a comparison with Monte Carlo simulations has shown that incorporationof restricted-volume effects in the mean-field theory of electrolyte solutions may give results that are worsethan the simple Gouy-Chapman theory. Generally, should the simple mean-field theory work for such highlyconcentrated ionic systems, where the so-called ion-correlation effects must be strong? It may not, as it doesnot incorporate a possibility of charge-density oscillations. Somehow, to answer this question definitely, oneshould do further work. This could be based on density-functional theory (and possibly not on what is referredto as local density approximation but rather "weighted density approximation"), field theory methods for theaccount of fluctuations in the calculation of partition function, heuristic integral equation theory extended tothe nonlinear response, systematic force-field computer simulations, and, most importantly, experiments withindependently determined potentials of zero charge, as discussed in the paper.