Improved reversibility of color changes in electrochromic Ni–Al layered double hydroxide films in presence of electroactive anions

The effect of four electroactive anions, [Fe(CN)6]4−, [Ru(CN)6]4−, [Mo(CN)8]4− and [IrCl6]2−, on the electrochromic properties of Ni–Al layered double hydroxide (LDH) films was investigated. The presence of these ions, either in solution or pre-adsorbed in the LDH was found to greatly improve the reversibility of the color changes observed upon oxidation–reduction of nickel in the LDH layers. For example, the relative transmittance of a LDH pre-exchanged with [Fe(CN)6]4− recorded in situ during a potential scan dropped to 67% on oxidation and returned to 99.9% on reduction. These color changes could be sustained over multiple potential scans. The contrast between the colored and bleached states (ΔOD) remained almost constant at 0.17 for at least forty scans. This improvement in the electrochromic response of the films is attributed to mediation of the redox process in the LDH by the anions that resulted in the full reduction of all nickel sites back to Ni(II). In the absence of mediator, the electrochemical reduction of nickel in LDH films is very slow. Oxidized nickel sites accumulated in the films and cause a rapid darkening. Electron transfer between electroactive anions in solutions or adsorbed in the films and the electrochemically oxidized nickel sites is supported by for the large enhancements of the peak for reduction of the anions observed in the CVs. It is also confirmed by iodometry measurements showing the electrochemical oxidation of the LDH nickel sites to be fully reversible in presence of the anions. This was not the case when the oxidation was done in blank borate buffer.