Electrochemical behavior of carbon electrodes in organic liquid electrolytes containing tetrafluoroborate and hexafluorophosphate anionic species in different non-aqueous solvent systems

Electrochemical behavior of tetrabutylammonium salts containing tetrafluoroborate (BF4−) and hexafluorophosphate (PF6−) anionic species in different non-aqueous solvents had been investigated on glassy carbon (GC), boron-doped diamond (BDD) and Pt electrodes. Though both BF4− and PF6− ionic species are considered to be inert, they are found to undergo electrochemical oxidation only on GC electrode rather than BDD and Pt as found out from their anodic peaks in linear sweep and cyclic voltammograms (LSV and CV). The voltammetric peak is influenced by the sweep rate as well as by the concentration of the ionic species and the electron transfer process appears to be diffusion controlled one. The formation of an inhibitory C-F film on the electrode surface during anodic polarization either by potentiostatic or potentiodynamic techniques was clearly established by X-ray photoelectron spectroscopy (XPS) analysis and the charge transfer resistance (θ) is higher under latter conditions than the former. The inhibitory effect of this surface film towards the electron transfer reaction of Fe(CN)6]4−/[Fe(CN)6]3− redox couple using impedance technique reveals that among the high permittivity non-aqueous solvents investigated in this work, CH3CN shows maximum θ value and produces C-F film of optimum thickness than the others. A direct correlation was also found out from the plot of θ versus peak potential Ep and peak current density (ip) obtained from LSV. The mechanism of film formation on GC electrode and the absence of such phenomenon on BDD were explained from the product analysis using 19F nuclear magnetic resonance (NMR) spectra resulting from the constant current electrolysis of BF4− and PF6− ionic species on both electrodes in CH3CN medium.