Temperature dependence of the heterogeneous electron-transfer rate constant for ferrocene carboxylic acid in room temperature ionic liquids at microstructurally distinct carbon electrodes

Abstract

Abstract The apparent heterogeneous electron-transfer rate constant (koapp) for ferrocene carboxylic acid (FCA) in two room temperature ionic liquids (RTILs), 1-ethyl-3-methylimidazolium tetrafluoroborate [EMIM][BF4] and 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4], was estimated by cyclic voltammetry at boron-doped diamond (BDD), nitrogen-incorporated tetrahedral carbon thin-film (ta-C:N), and glassy carbon electrodes as a function of temperature from 298 to 338 K. Some comparison measurements were made using ferrocene (Fc). The goal was to learn how the microstructure of the different carbon electrodes influences koapp for this prototypical redox system in the RTILs. Capacitance values for BDD and ta-C:N ranged from 5 to 12 μF cm−2 in both RTILs between −1 and 1 V vs. Ag QRE. Cyclic voltammetry measurements were made in 0.5 mmol L−1 FCA. The results demonstrate that both the carbon electrode microstructure and the RTIL's physical properties (viscosity) have a significant effect on koapp for this redox system. The diffusion coefficient, D (~10−7 cm2 s−1), and koapp (~10−4 cm s−1) increased with increasing temperature. In both RTILs, the largest koapp and smallest Ea values were found for BDD followed by ta-C:N and GC. koapp was ~2–3× larger and Ea was 2–3× lower in the less viscous [EMIM][BF4] (6–14 kJ mol−1) than in the more viscous [BMIM][BF4] (16–28 kJ mol−1) at all three electrodes. koapp for both FCA and Fc decreased in magnitude with increasing RTIL viscosity.