A comparative study of anodic oxidation of bromide and chloride ions on platinum electrodes in 1-butyl-3-methylimidazolium hexafluorophosphate

The anodic processes of bromide and chloride ions on platinum electrodes were investigated in an ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate, under the same conditions. The halide anions were added to the ionic liquid as their 1-butyl-3-metylimidazolium salts or tetrabutylammonium salts to high concentrations (up to 1.1 mol L−1). Analysis and comparison of the electrode reactions were made through cyclic voltammetry, chronoamperometry, and double potential step chronocoulometry with a platinum micro-disk electrode, and bulk potentiostatic electrolysis and UV–Vis spectroscopy. Both halide anions exhibited anodic oxidation. The main difference as revealed by cyclic voltammetry was that the chloride ion exhibited only one anodic current peak, whilst the bromide ion underwent clearly distinguished two oxidation steps. This difference is attributed to different electrode kinetics and stabilities of the two tri-halide ions. Bulk electrolysis and UV–Vis spectroscopy confirmed that the tri-bromide ion was the main product from the overall oxidation of the bromide ion, although bromine formation was indicated by cyclic voltammetry at potentials of the second anodic peak.

► Bromide and chloride ions exhibit unlike voltammetric features in ionic liquids.
► Cyclic voltammetry implies bromine formation at high positive potentials.
► Bulk electrolysis at the same anode potentials produces more tri-bromide.
► Reduction of tri-bromide proceeds easier than that of tri-chloride ion.
► The bromide/tri-bromide couple may enable energy storage in ionic liquids.