LAYERED DOUBLE HYDROXIDES ELECTROSYNTHESIZED ON PLATINUM ELECTRODES AS MATERIALS FOR SENSORS AND SUPERCAPACITORS

Layered Double Hydroxides (LDHs), with a general formula of [M(II)1–xM(III)x (OH)2]x+ [Aqx/q− • nH2O], where M(II) and M(III) are bivalent and trivalent metal cations, respectively, and A is the charge-balancing anion of valence x, have attracted increasing interest from both academic and industrial perspectives due to their wide applications in areas such as catalysis, separation, biotechnology, and electroanalytical chemistry. This contribution compares the performances of LDHs containing redox active metals (Co or Ni) by application of a cathodic potential to Pt soaked into solutions containing Al or Fe as trivalent metal. The electrodeposition provides an alternative route to the chemical one in order to obtain a good adhesion of the LDHs to the electrode surface which is a crucial feature for the best performances for the development of both sensors and supercapacitors. Co/Al and Ni/Al LDHs have been deeply investigated for the electrocatalytic determination of oxidizable substrates such as alcohols, sugars, phenols, amines, etc, whereas the study of their performances in the field of energy storage is more recent. This contribution compares the performances of Co/Al and Ni/Al LDHs with those displayed by the materials synthesized in the same conditions in the presence of Fe(III) instead of Al(III) as to the detection of glucose and the values of specific capacitance. As an example, the Pt modified with a thin film of the Ni/Al LHD displays good performances as electrochemical sensor for glucose. On the contrary the presence of Fe(III) gives rise to a material where the Ni(II) are not able to act as electrocatalytic centers. LDHs the mechanism which involves the Ni centers as redox mediators can be so summarized: LDH-Ni(II) + OH-sol ⇄ LDH(OH-)-Ni(III) + e- LDH(OH-)-Ni(III) + analytered  LDH-Ni(II) + OH-sol + analyteox where the second reaction is slower than the firts one since it had been demonstrated that the rate limiting step for the oxidation of alcohols and sugars was the diffusion of the substrate toward the Ni centers of LDH material. The reason why the different behavior can be evidenced by the characterization CV in NaOH of Pt electrodes modified with Ni/Fe and Ni/Al LDHs, where it is well evidenced that the presence of iron inside the brucitic layers causes a significant decrease in the overpotential necessary for oxygen evolution. The specific capacitances estimated for the investigated LDHs are reported and their properties as materials for supercapacitors are discussed.