Heterogeneous electron-transfer rate constants for ferrocene and ferrocene carboxylic acid at boron-doped diamond electrodes in a room temperature ionic liquid

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 (D_red) 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, ν_n) due to the more viscous medium.