(232ab) On the Electrical Double Layers of Multi-Component Electrolytes

The electrochemical capacitors store energy at the interface of the electrode and the electrolyte in the form of the electrical double layer, except for those that utilize the pseudo-capacitance in the form of reversible Faradaic reactions. Although there are abundant studies on the electrical double layer for the past hundred years, the characteristic properties of the electrical double layers that are composed of multi-component electrolytes have not been fully explored. In the effort of improving the energy storage by utilizing electrochemical mechanisms, it is worthwhile to seek for the advantage of using multi-component electrolytes. In this talk, we would like to present some interesting properties of electrochemical capacitors that can be observed only with multi-component electrolytes.

Recently, Yamagata et al. have reported in their electrochemical measurement using carbon materials as the electrode and ionic liquids as the electrolyte that the capacitance dramatically increases in the negative potential range when a lithium salt is added to the electrolyte as the second component [1]. They have also shown that, with the addition of a lithium salt, the potential window is enhanced. This example shows that the electrochemical behaviors of two-component electrolytes can be fundamentally different from those of one-component electrolytes. Since the increase of capacitance is observed within the potential window, such behaviors are speculated to be caused by the change in the molecular structure in the electrical double layer. In order to clarify the relation between the electrochemical behavior and the molecular structure in the electrical double layer, we performed Monte Carlo simulation using simple models. The two components of ions were discriminated by the size of the cations.

The analysis of the ionic structure showed that the electrical double layer on the cathode is dominantly formed by the larger cations under small negative potentials, while they are replaced by the smaller cations under large negative potentials [2]. This transition of the ionic structure with electrode potential is also consistent with the enhancement of the potential window that was found in the experimental work, which suggests that the organic cations are expelled from the electrical double layer under large negative potentials and the chance of decomposition is reduced. The results of the Monte Carlo simulation qualitatively reproduced the experimentally observed electrochemical behaviors of the two-component electrolytes. The mechanism of the observed structural transition with potential can be explained in terms of the competition between the electrostatic interaction of ions with the electrode and that among the ions alone [2,3]. The results are also consistent with the previous simulation results of two-component electrolytes near the electrode [3]. [1] M. Yamagata, N. Nishigaki, S. Nishishita, Y. Matsui, T. Sugimoto, M. Kikuta, T. Higashizaki, M. Kono, and M. Ishikawa, Electrochim. Acta, 110, 181 (2013). [2] K. Kiyohara, M. Yamagata, M. Ishikawa, J. Chem. Phys., 144, 134701 (2016).

 [3] K. Kiyohara, H. Shioyama, T. Sugino, K. Asaka, Y. Soneda, K. Imoto, and M. Kodama, J. Chem. Phys., 138, 234704 (2013)