Voltammetric Analysis of the Ferrocenium/Ferrocene Redox Couple in Litfsi-Acetonitrile Highly Concentrated Electrolyte

Abstract

Research on electrolytes have a defining role for the progression of current energy storage device and electrocatalysis. For instance, the electrolyte is often a limiting factor in the life span of Li-ions batteries due to slow decomposition at electrodes. Intensive research is therefore ongoing to increase the stability of the electrolyte. One example of such research is on highly concentrated electrolyte (HCE) which are obtained when the number of ions of a salt nears or equals that of the solvent molecules, while maintaining a liquid state. These highly concentrated electrolytes (HCE) or "solvent-in-salt" solutions differ in properties from conventional ones because all solvent molecules are occupying the ion solvation sphere where they are strongly coordinated to the cation. While on the application side, the attention is directed towards their use in batteries or supercapacitors, current research on HCE aims at increasing the fundamental understanding of their properties mostly the transport properties of the Li+ or the formation of the solid electrolyte interface (SEI). Interestingly, there are no reports on the study of heterogeneous electron transfer (ET) in HCE, yet the absence of "free" solvent molecule suggests that ET in HCE might be different from conventional electrolytes because of the importance of solvent reorganization during ET. We therefore investigate the mass transport and ET of dissolved ferrocene (Fc) and with ferrocene terminated self-assemble monolayer, (Fc-SAM). Ferrocene was selected as the redox center for its "ideal" outer shell electron transfer mechanism and well-defined electrochemical properties. To establish the effect of salt concentration, the electrochemical measurements were done in a series of electrolytes composed of Lithium bis(trifluoromethanesulfonyl)imide in acetonitrile with concentrations spanning from diluted (0.3 M) to highly concentrated (4.1 M) solutions. The diffusivity of Fc (Shoup-Szabo) is found to follow the trend with viscosity (η) expected from the Stokes-Einstein relation over the entire concentration range. The ET rate constant k0 variation with η on the other hand diverges from what is expected from a purely adiabatic ET. Possible explanations for deviation involve different dielectric properties, strong coordination of the solvent and particular double-layer structure of HCE, properties that are not fully established. This lack of knowledge highlights the importance of increasing fundamental research on the topic of electrochemistry in HCE.