Conduction through viscoelastic phase in a redox-active ionic liquid at reduced temperatures.

Salts in the liquid phase at room temperature–ionic liquids– have recently received increased attention due to their unique properties.[1] These include high ionic conductivity, high thermal and chemical stability, broad electrochemical windows, non-volatility and non-flammability. These important properties make them versatile alternatives to conventional solvent-based systems, and their potential applications range from electrolytes in solar cells,[2] fuel cells,[3] supercapacitors and batteries,[4] lubricants and heat-transfer fluids,[5] solvents for clean chemical synthesis and catalysis[6] to solvents for cellulose.[7] Iodine addition to iodide-based ionic liquids leads to extraordinarily efficient charge transport, vastly exceeding that expected for such viscous systems.[8–12] The origin of this anomalously high charge transport in I/I3 anion containing melts has been attributed to Grotthuss conduction via the exchange reaction I− + I3 I3 + I−.[1] The displacement of I− and I3 through this bond-exchange reaction leads to effective charge transfer without mass displacement. In addition, the conductivity increases abruptly by an order of magnitude at the point where higher polyiodides become important in the redox active ionic liquid 1-methyl-3-propylimidazolium iodide (PMII) as a function of increasing analytical iodine (I2) concentration [I2]a. (The calculated analytical I2 concentration [I2]a takes into account the volume increase when the neutral I2 is added to the charged PMII melts).[1] This increase in conductivity has been attributed to bond-exchange processes among the higher polyiodides, I5, I7, etc.[1] PMII is the benchmark of the iodide salts that form room temperature ionic liquids, characterized by a relatively low viscosity, and has therefore been the candidate of choice for binary ionic liquids in the solvent-free dye-sensitized Grätzel solar cell ([I2]a ∼ 0.2 M).[2,13]