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

Abstract

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 absorbed 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.