Effect of the presence of [Co(bpy)3]2+[Co(bpy)3]2+ on the electrochromic responses of films of a redox active Ni–Al-layered double hydroxide

The addition of [Co(bpy)3]2+[Co(bpy)3]2+ to the electrolyte was found to improve the reversibility of the colour change observed in films of a Ni–Al-LDH upon oxidation–reduction of nickel in the LDH layers. Sustained changes in relative transmittance between 95% in the reduced state and 75% in the oxidized state were obtained for a film subjected to continuous potential scanning in a pH 8 borate buffer containing 0.1 mM [Co(bpy)3]2+[Co(bpy)3]2+. This improvement in the electrochromic response of the film is attributed to mediation of the redox process in the LDH by the cobalt cations that resulted in the full reduction of all nickel sites back to Ni(II) by the end of each individual scan. In the absence of the cobalt cation, the colour changes were not fully reversible and a rapid darkening of the film was observed. Because the unaided electrochemical reduction of nickel in LDH films is very slow, oxidized nickel sites remained in the films at the end of each scan. This accumulation of Ni(III,IV) sites was confirmed by increases in the average nickel valence in the films determined by iodometry, and by the appearance of reflection due to a Ni(III) oxyhydroxide phase in the XRD pattern of the film. This was not found when the cycling was done in presence [Co(bpy)3]2+[Co(bpy)3]2+. Mediation of the reduction of nickel in the Ni–Al-LDH by the cobalt cation was very efficient. The presence of 5 μM [Co(bpy)3]2+[Co(bpy)3]2+ was sufficient to cause a significant improvement of the bleaching process.