Impact of sp² Carbon Edge Effects on the Electron-Transfer Kinetics of the Ferrocene/Ferricenium Process at a Boron-Doped Diamond Electrode in an Ionic Liquid

The electrochemical properties of boron-doped diamond (BDD) electrodes are strongly influenced by the boron doping level and the presence of sp² carbon impurities. In this study, the impact of highly localized sp² carbon concentrated at the edge of a BDD electrode, arising from laser cutting during fabrication and exposed during electrode polishing, on the resulting overall electrode kinetics is identified. Fourier transformed large-amplitude alternating current (FTAC) voltammetric data for the usually ideal Fc^0/+ (Fc = ferrocene) process in the highly viscous ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate show relatively poor agreement with simulations based on a uniformly active electrode surface using the Butler–Volmer formalism for the electrode kinetics. In this ionic liquid medium, the impact of heterogeneity on the macroscopic electrode activity is enhanced under the conditions of slow mass transport, where sites of disparate activities are spatially decoupled on the voltammetric time scale. Physically blocking this edge region leads to a response that is much more consistent with a uniform electrode and substantially improves the agreement between the FTAC voltammetric experiment and simulation (standard heterogeneous electron-transfer rate constant, k⁰ = 0.0015 cm s^–1). To complement the macroscopic measurements, local voltammetric measurements with an electrochemical droplet cell show directly that the sp2 carbon found in the edge region is able to support much faster electron-transfer kinetics for the Fc^0/+ process than the sp³ BDD surface. Overall, this study demonstrates that caution should be taken in reporting electrode kinetic data obtained at BDD or any other heterogeneous electrode materials. Macroscopic electrode kinetic characterization in traditional electrochemical media such as acetonitrile is blind to such heterogeneities in the activity. However, tuning the diffusional time scale through the use of FTAC voltammetry in viscous ionic liquids provides a powerful approach for detecting the spatially nonuniform electrode activity.