Effect of heat treatment on the performance of carbon-supported transition-metal chelates in the electrochemical reduction of oxygen

Abstract

A study has been made of the changes occurring in carbon-supported metalloporphyrins and metallo-phthalocyanines during heat treatment, which improves both the stability and the activity of these materials in the electrochemical reduction of oxygen in acid electrolyte. A variety of physical techniques was applied, including X-ray photoelectron spectroscopy (X.p.s.), Fourier-transform infrared spectroscopy, electron spin resonance and Mössbauer spectrometry. During the heat treatment a bond can be formed between the carbon surface and both the central metal ion and the macrocyclic ligand, which thereby loses all its peripheral substituents while retaining its inner part. In some cases some of the chelate molecules are destroyed. The active sites formed in the more active catalysts consist of an isolated metal ion surrounded by the thermally modified ligand. A relation between redox potential and O₂-reduction activity of the same form as that for untreated catalysts can be used to explain the observed enhancements in activity.