Heterogeneous electron-transfer rate constants were determined for ferrocene and ferrocene carboxylic acid (FCA) in the room temperature ionic liquid (RTIL), 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF 4), at boron-doped microcrystalline diamond thin-film electrodes. Comparison data for FCA in 1 M KCl were also obtained. The apparent heterogeneous electron-transfer rate constant, kapp0, for FCA was 10× lower in the RTIL 1.5 (± 1.1) × 10-3 cm s-1 as compared to KCl 4.6 (± 1.3) × 10-2 cm s-1. The kapp0 for ferrocene was also 10x lower in the RTIL 5.0 (± 1.2) × 10-3 cm s-1 as compared to a common organic electrolyte solution 5.5 (± 1.2) × 10-2 cm s -1. The diffusion coefficient for FCA (Dred) was determined by chronoamperometry to be 1.3 × 10-7 cm2 s-1, ca. 100× lower than the value (1.9 × 10 -5 cm2 s-1) in KCl. The lower diffusion coefficient is consistent with the 100× greater viscosity of the RTIL. The lower kapp0 values for these outer-sphere redox systems is attributed, at least in part, to a reduced number of attempts to surmount the activation barrier (i.e., a reduced nuclear frequency factor, νn) due to the more viscous medium.
|Number of pages||8|
|State||Published - Apr 1 2013|
Bibliographical noteFunding Information:
This work was generously supported by grants from the Army Research Office (W911NF-09-1-0451) and the Natural Science Foundation (CHE-0911383). The authors thank Yan Zhu for her assistance performing some of the cyclic voltammetric measurements.
- Cyclic voltammetry
- Diamond electrodes
- Electrode kinetics
- Room temperature ionic liquids
ASJC Scopus subject areas
- Chemical Engineering (all)