Anionic clay modified electrodes: Electron transfer mediated by electroactive nickel, cobalt or manganese sites in layered double hydroxide films

A voltammetric wave was observed in an electrode modified with an Mg-Mn-CO3 layered double hydroxide (LDH). It was attributed to Mn(III)/Mn(IV) redox processes in the film. Iodometry showed that the LDH manganese centres, initially present in an average valence of 3.52+, could be oxidized to a maximum of approximately 3.8+, or reduced down to 3+. Reduction below 3+ did not occur even after several hours at negative potentials. Co(II) sites in a Co-Al-Cl LDH could be oxidized up to a maximum of about 3.8+. This oxidation was only reversible between 3.8+ and 3.0+. Complete reduction back to 2+ only occurred for Co-Al-Cl films in which the average valence of the cobalt had never exceeded 3+. Much larger voltammetric waves were obtained for solutions of [Fe(CN)(6)](4-), [Mo(CN)(8)](4-) or [Ru(CN)(6)](4-) in redox active manganese, cobalt or nickel LDH films than in redox inactive Zn-Al-Cl films. Improved charge transport in LDH films containing redox active metals allowed a much larger fraction of the adsorbed anions to participate in the electrochemical reaction. When the potential of nickel, cobalt or manganese LDH-modified electrodes were scanned in mixtures of [Fe(CN)(6)](4-) and [Ru(CN)(6)](4-) or [Mo(CN)(8)](4-), electron transfer from [Fe(CN)(6)](4-) to electrochemically oxidized [Ru(CN)(6)](3-) or [Mo(CN)(8)](3-), mediated by the LDH electroactive metal centres was observed. No such transfer was found for redox inactive Zn-Al-Cl LDH films. (C) 1997 Elsevier Science S.A.