Liquid Phase Electrochemistry at Ultralow Temperatures

Fluid electrolyte solutions based on mixtures of butyronitrile (PrCN) and ethyl chloride (EtC1) with Bu4NPF6 or Bu4NC104 as electrolyte freeze below -180~ and provide excellent media for cryogenic electrochemical experiments. A 1:2 mixture of PrCN and EtC1 exhibits the best combination of freezing point and ionic conductivity for ultralow temperature electrochemistry. Diffusion coefficients for bis(pentamethylcyclopentadienyl) iron (Cp*Fe) are measurable by potential step chronoamperometry down to 160~ using a conventionally sized electrode, but the resistivity of the solvent mixture is such that potential sweep vol tammetry benefits from the use of microdisk (10 and 25 ~m diam Pt) or microband (0.2 ~m wide Au) electrodes. Voltammetry at a chemically modified electrode down to -170~ is presented for the case of thin films of poly-[Os(bpy)2(vpy)2] [PF612. Interest in low-temperature electrochemistry (1-6) is traditionally tied to investigations of the kinetics of heterogeneous electron transfer reactions (7-11) and electrochemically induced homogeneous processes (2-6, 12-19) that otherwise, at ambient temperature, occur too rapidly. Such research is dependent upon fluid electrolyte systems with suitable low-temperature limits. Numerous organic solvents are available for electrochemistry down to, and slightly below, the temperature of a dry ice/acetone bath, -78~ Butyronitri le (PrCN) is an excellent low-temperature medium; vol tammetry in it to -135~ has been reported (14). Low-temperature electrochemistry has also been performed in frozen glass solids (20-25), and with the solid (20, 24) eutectic HC104 9 5H20. We recently (26) described a new cryogenic medium that extends the lower temperature limit of electrochemical vol tammetry in liquids. Our interest in this subject stems from a desire to investigate the electron transfer properties and double-layer capacitance behavior of high-temperature superconductor electrodes in their superconducting state. Tremendous advances in the synthesis of high-temperature superconductors have pushed superconducting critical temperatures (To) above that of liquid nitrogen (-196~ (27, 28). There are no reports on electrochemistry at these new materials in fluid electrolytes at temperatures below their Tc values, but interesting electrochemical effects have been observed at Tc in experiments using solid electrolytes (29-31). Use of a suitably low temperature fluid electrolyte would improve contact to the superconducting electrode interface (as compared to solid-solid contacts) and could potentially provide access to a greater range of electrochemical reactions and experimental techniques to study the electrochemistry of superconductor electrodes such as YBa2Cu~O7 (T~ = -180~ Bi2Sr2Ca2Cu3010 (Tc = -163~ and T12Ba2Ca2Cu3010 (To = -148 ~ in their superconducting states. These materials may have electronic properties as electrodes unlike any previously investigated. We have already reported some of our work, so far carried out above Tc (32-34), with electrodes fabricated from the superconductor ceramic materials.