Study of the electroreduction of dioxygen on thin films of platinum phthalocyanine in alkaline solutions

Abstract

A detailed investigation of the electrochemical kinetics of dioxygen reduction on thin films of platinum phthalocyanine is reported. It has been found that the initial behaviour of the films is affected profoundly by the method of preparation, but that after potential cycling and/or lengthy exposure to air all the films investigated become highly active. Results from both RDE and RRDE studies show that the film reduces dioxygen by two parallel mechanisms, with the four-electron reduction process being dominant. A small amount of hydrogen peroxide could be detected, though the low activity of the film towards reduction of H 2 O 2 confirms that this is not an intermediate in the reduction of oxygen to water. The rate constants for the two- and four-electron pathways could be obtained by an analysis of the RRDE and RDE data. and the kinetic law for the four-electron step was derived. It takes the form: I k = 4 FAk 0 c O 2 ( c OH− ) m exp(−α FE/RT ) where, at low overpotentials, m ∼- 0.5 and α ∼- 1.5. whereas in the high overpotential region, m ∼- 0.15 and α∼-0.5.