Electron self-exchange dynamics of hexacyanoferrate in redox polyether hybrid molten salts containing polyether-tailed counterions

Hexacyanoferrate(III) is combined with a quaternary ammonium countercation consisting of triethylammoninm 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 cm²/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 × 10⁵ 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 cm²/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.