www.elsevier.nl/locate/jelechem Journal of Electroanalytical Chemistry 482 (2000) 125 – 138 Impedance of a reaction involving two adsorbed intermediates: aluminum dissolution in non-aqueous lithium imide solutions La ´szlo ´ Pe ´ter 1 , Juichi Arai *, Haruo Akahoshi Department of 1 -Material, Hitachi Research laboratory, MD no. 260, 1 -1 Omika -cho 7 -chome, Hitachi -shi, Ibaraki -ken 319 -1292, Japan Received 10 February 1999; received in revised form 13 January 2000; accepted 14 January 2000 Abstract The model presented considers the dissolution of a trivalent metal in three consecutive steps involving two adsorbed intermediates. If mass transport effects are negligible, it is possible to construct equivalent circuits in which adsorption-related elements are doubled compared to the case of a single adsorbate. In the case where mass transport affects the dissolution, the Faradaic admittance can be evaluated as a fraction of two power series and no simple equivalent circuit can be constructed from conventional circuit elements. Depending on the mechanism assumed, the low-frequency behavior can be either similar to a Warburg impedance or different fundamentally. The impedance of aluminum dissolution is discussed in the case of insignificant mass transport. The Langmuir isotherm is supposed to hold for intermediate adsorption, and only anodic partial reactions are accounted for. It has been concluded that the second step is rate-determined and that solvent takes part in the desorption of the product only. An empirical correlation was found between the dipole moment of the solvent used and the ratio of the rate constants of non-rate determining steps. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Impedance; Al dissolution; Non-aqueous solution; Imide anion; Lithium battery 1. Introduction Electrochemical impedance spectroscopy (EIS) is a very effective tool to analyze multistep electrochemical reactions [1–5]. In the course of the development of the theory of EIS, treatments of reactant and product adsorption [6,7], intermediate adsorption [8 – 15], diffu- sion [16] and the combination of the above processes [12,17,18] have been well established (see also Refs. [1–5]). The interest of the authors was focused mainly on practically important reactions. The theory of the adsorption of intermediates and impedance of systems involving them has developed in line with the investiga- tion of reactions composed of two consecutive steps. The family of such reactions includes, among others, hydrogen evolution, chlorine evolution, oxalic acid re- duction, electrochemical dissolution of a number of divalent metals and electrocrystallization of several di- valent transition metals, as summarized by Diard et al. [15]. The above mentioned reactions all include one adsorbed intermediate only. Electrochemical reactions with several intermediates occur when parallel reactions take place. A typical example is the electrodeposition of zinc where hydrogen evolution is an unavoidable side reaction and coupling of the two reactions also occurs [13]. A brief discussion of transpassive nickel dissolu- tion was published by Epelboin et al. [11] with no analysis in detail. The latter reaction is composed of three consecutive steps and involves two adsorbed intermediates. In general, electrochemical dissolution or deposition of trivalent metals has attracted little interest. Alu- minum and chromium exhibit protective native surface oxide layers which make investigation of their dissolu- tion in acidic aqueous media rather difficult, and the first step of the oxidation is usually very fast. Thus, adsorption of different intermediates cannot be detected by EIS. Reduction of bismuth ions (Bi 3 + ) on mercury can be carried out under appropriate conditions, but only one intermediate can be detected with the help of * Corresponding author. Fax: +81-294-527636. E-mail address: jara@hrl.hitachi.co.jp (J. Arai) 1 Present address: Research Institute for Solid State Physics and Optics, Hungarian Academy of Sciences, H-1525 Budapest, POB 49, Hungary. 0022-0728/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII:S0022-0728(00)00028-0