Conditions Required To Achieve the Apparent Equivalence of Adhered Solid- and Solution-Phase Voltammetry for Ferrocene and Other Redox-Active Solids in Ionic Liquids

The voltammetry of ferrocene (Fc) and Fc+ in the room-temperature ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM·PF6) has been studied when solid is adhered to glassy carbon or platinum disk electrodes. Due to the slow dissolution kinetics and small diffusion coefficients in the viscous BMIM·PF6 ionic liquid, it is possible to obtain voltammograms of adhered Fc or Fc+ solid that are essentially indistinguishable (except for the current magnitude) from the reversible solution-phase Fc0/+ process widely employed to provide a reference potential scale. However, the nature of the voltammetry obtained from the adhered solid is governed by the thickness (mass of the solid) of the particle layer. The mechanism proposed to explain the equivalence to solution-phase data involves dissolution at the particle/ionic liquid interface and is supported by electrochemical quartz microbalance measurements and a numerical simulation. Extensive studies on other redox-active solids suggest that voltammograms of solid particles adhered to the electrode surface in contact with ionic liquids frequently exhibit classical behavior associated with solution-phase diffusion-controlled voltammetry. Consequently, the method of adhering microparticles onto an electrode surface can frequently provide an efficient method of establishing ionic liquid solution-phase redox data using extremely small quantities of solid.