Electron Self-Exchange Dynamics of Hexacyanoferrate in Redox Polyether Hybrid Molten Salts Containing Polyether-Tailed Counterions

Abstract

Hexacyanoferrate(III) is combined with a quaternary ammonium countercation consisting of triethylammonium connected to a poly(ethylene glycol) methyl ether (MW 350) “tail”, to form a highly viscous room-temperature redox polyether hybrid melt (e.g., a molten salt) in which the concentration of hexacyanoferrate centers is 0.82 M. Microelectrode voltammetry and potential step chronoamperometry in the undiluted melt give an apparent diffusion coefficient D APP = 2.5 × 10 -10 cm 2 /s at 20 °C that is interpreted as reflecting primarily the rate of electron self-exchange between Fe(II) and Fe(III) centers. A rate constant of k EX = 1.1 × 10 5 M -1 s -1 is derived from this D APP, and from its temperature dependence, an activation energy barrier of 30 kJ/mol. k EX is in good agreement with results in fluid solutions. At the same concentration (0.82 M), but in aqueous solution, the (potassium salt) hexacyanoferrate species displays a D APP of 4 × 10 -6 cm 2 /s, which is interpreted as reflecting physical transport of the hexacyanoferrate species. Transport of the hexacyanoferrate species is enormously “plasticized” in aqueous medium as opposed to the highly viscous polyether melt. Electronic spectra and ionic conductivity of the hybrid redox polyether melt are also reported.