Voltammetric study of electrocatalysis for dioxygen reduction by trinuclear rutheniumammine complex confined in a polymer membrane

Electrocatalytic O-2 reduction was studied using a modified electrode coated with a Nafion membrane (Nf) dispersing a trinuclear ruthenium ammine complex ([(NH3)(5)Ru-III-O-Ru-IV(NH3)(4)-O-Ru-III(NH3)(5)]Cl-6, Ru-red). When measuring cyclic voltammogram under O-2 atmosphere (at 0.5 mV s(-1)), catalytic currents due to O-2 reduction were found to develop below -0.2 V (vs. Ag/AgCl). Since Ru-red undergoes irreversible decomposition into the mononuclear complexes via the reduced state (Ru-III-Ru-III-Ru-III) (similar to -0.1 V), it is suggested that the electrocatalysis originates from the decomposed species (initial active species: Ru-II(NH3)(5)(OH2) and Ru-II(NH3)(4)(OH2)(2)) rather than from the Ru-red. Although the present electrocatalyst was also applied to H2O2 reduction system, the catalytic activity was found to be poor from the voltammetric behavior. It appeared that the kinetics of the electrocatalysis is much faster in the O-2 reduction than in the H2O2 One. A selective and direct catalysis for O-2 reduction into H2O was suggested from a ring-disk voltammogram to take place by an aggregate of the mononuclear ruthenium complexes in the polymer matrix. In addition, it was found that electrocatalytic O-2 reduction involves a slow kinetic process, so that factors affecting the overall kinetics were discussed in terms of the catalysis mechanism. (C) 2002 Elsevier Science Ltd. All rights reserved.