Boron-doped diamond electrodes in organic media: electrochemical activation and selectivity effects

This study highlights the effects of the electrochemical surface pre-treatment of polished polycrystalline boron-doped diamond electrodes in ethanol on electron transfer to organic redox systems. A novel "activation" procedure based on cathodic polarisation in ethanol (0.01 M NBu4PF6) is proposed and shown to be highly effective in promoting electron transfer to the aqueous Fe(CN)(6)(3-/4-) redox system. For redox systems in acetonitrile, effects on electron transfer processes are strongly dependent on the type of electron transfer process. For decamethylferrocene(+/0), decamethylcobaltocene(+/0), methyviologen(2+/+), and for methylviologen(+/0) redox systems essentially reversible electron transfer is observed irrespective of the pre-treatment of the boron-doped diamond electrode. For the benzoquinone(0/-) redox system insignificant changes occur but for the benzoquinone(-/2-) process a more dramatic change in electron transfer kinetics is observed (after cathodic polarisation in ethanol) consistent with an improved interfacial electron transfer (a 4.6 x increase in peak current occurs). Finally, the tetraethyl-ethylenetetracarboxylate(0/-) system is investigated as a model olefin redox system. Again changes in reactivity occur (a 4.0 x increase in peak current) at the cathodically pre-treated boron-doped diamond electrode. XPS surface analysis data reveal only insignificant changes in the chemical composition of the boron-doped diamond surface before and after activation and therefore a predominantly electronic mechanism for the "activation" process is proposed. In future selectivity effects for electron transfer at boron-doped diamond electrode surfaces could be introduced intentionally and used beneficially for chemo-selective electro-organic processes.