Solid-state electrochemistry: voltammetric monitoring of ionic conductivities in mixed-metal hexacyanoferrates

Abstract

There is a growing interest in transition metal hexacyanoferrates [1-12], polynuclear inorganic materials, for modification of electrode surfaces. The electrochemical behaviour of the respective films resembles both redox and ion-exchange organic polymers [13,14], as well as ionically conducting, zeolitic, or intercalation systems [15,16]. The cyanometallate materials can be also viewed as analogues of Prussian BIue, which is, from the electronic point of view, a semiconductor or, more precisely, a class II mixed-valence system [17] with electrons and unoccupied states for them to move to. For potential applications such as electrodes in batteries, the development of mixed conductors with comparable contributions of electrons (holes) and ions to the conductivity is of interest. To the best of our knowledge, developments in mixed conducting solid electrolytes are, at present, rather limited. Recent studies [1,5,8-12,18] indicate that sparingly soluble transition metal hexacyanoferrate deposits on electrodes exhibit a large degree of selectivity toward the flux of an electrolyte cation, providing charge balance during redox transitions. Little is known about ionic mobilities in the dry (solid) state of these materials. Their solution electrochemistry, employing them in the form of films on electrodes, allows us, however, to hypothesize that the .ability to store charge in the bulk of materials as well as cation transport should be well pronounced in the K+-containing cyanometallate microstructures. In the present study, we have attempted experiments in the solid state, using cyclic voltammetry with chemically inert glassy carbon slide electrodes that sandwich either a dry powder or a film of the material under discussion. A complete concept of the sandwich electrode system, in which a polymer film is placed (sandwiched) between two metallic conductors, has been proposed by Murray and co-workers [13,19]. The possibility of extending this approach to Prussian BIue has been mentioned by Grabner [20]. In our design, there is, however, no external