Cyclic voltammetry of metal bipyridyl cations at redox active Ni–Al–Cl layered double hydroxide films

A very large increase in the [Os(bpy)3]2+ cathodic peak was observed when an electrode modified with a Ni–Al–Cl LDH film was cycled in solution of the cation. The cathodic charge increased from 0.16 mC to 3.0 mC when the switching potential was increased from 0.9 to 1.4 V. This was not observed for Zn–Al–Cl LDH films. Addition of [Ru(bpy)3]2+ resulted in an additional doubling the [Os(bpy)3]2+ cathodic peak. Similar results were obtained for [Co(bpy)3]2+. When [Os(bpy)3]2+ was added to a [Co(bpy)3]2+ solution however, the enhancement of [Co(bpy)3]2+ cathodic peak was completely suppressed at the expense of the [Os(bpy)3]2+ cathodic peak. This is the first report on the electrochemistry of metal bipyridyl cations, or of any electroactive cations at LDH-modified electrodes. Since LDHs have positively charged layers separated by exchangeable anions, they were not expected to adsorb large amounts of cations. This was confirmed by UV–Visible spectroscopy and elemental analysis of films that had been soaked in solution of the metal bipyridyl cations. The large voltammetric peaks observed for the bipyridyl complexes are therefore attributed to mediation of the oxidation or reduction of the LDHs nickel sites by the trace amounts of theses cations that were present at the interface between the films and the solutions. Preliminary measurements show that a similar mediation occurs for [Fe(CN)6]4− anions and for a mixture of [Fe(CN)6]4− and [Ru(bpy)3]2+.