ANIONIC CLAY-MODIFIED ELECTRODES - ELECTROCHEMICAL ACTIVITY OF NICKEL(II) SITES IN LAYERED DOUBLE HYDROXIDE FILMS

Voltammetric waves were observed in electrodes modified with films of nickel aluminum and nickel iron layered double hydroxides (LDHs). They are attributed to the oxidation of the LDH structural Ni(II) sites. The oxidation state of nickel in the LDH lattices during controlled-potential coulometry was followed by iodometry. Oxidation up to 4+ occurred when a sufficiently positive potential was used. However, the Ni(IV) sites could only be reduced to 2.7+. Complete reduction back to Ni(II) occurred only for films oxidized to 3.6+ or less. In-situ UV-visible spectroscopy measurements taken during coulometry show a rapid increase in the absorbance over most of the visible range. The spectra continued to change for as long as the potential was applied, even after oxidation of the nickel sites was complete. Oxidation of the LDH had no effect on the powder X-ray diffraction patterns of the modified electrodes. The layered structure of the LDH was maintained over the entire range of nickel oxidation states. Electron transfer between [Fe(CN)(6)](4-) or [Mo(CN)(8)](4-) ions pre-adsorbed in the films and the oxidized nickel centres in the LDH results in large enhancements of the reduction currents obtained for these ions in the LDH-modified electrodes. The increased cathodic charges correspond to a four- to six-fold increase in the fractions of adsorbed ions that were electroactive.