Bifunctional Oxygen Reduction Reaction Mechanism on Non-Platinum Catalysts Derived from Pyrolyzed Porphyrins

Abstract

A study on the oxygen reduction reaction (ORR) mechanism that occurs on non-platinum electrocatalysts, specifically materials derived from pyrolyzed cobalt tetramethoxyphenyl porphyrin in acidic media, is presented here. Reactant and product flux analysis is performed on rotating ring-disk electrode (RRDE) data to evaluate the non-platinum-based materials. An in-depth X-ray photelectron spectroscopy surface characterization analysis is performed and discussed in the context of structure-to-property correlations that are established using a multivariant analysis technique. Pyrolyzed cobalt porphyrin catalysts are highly heterogeneous materials that include both Co species that are associated with nitrogen and Co nanoparticles coated by "native" Co oxides. This study proposes an ORR mechanism that occurs on this class of non-Pt electrocatalysts based on structure-to-property correlations and qualitative analysis of the RRDE flux data. The combined flux analysis and structural characterization suggests that the series type, peroxide ORR pathway is supported on the bifunctional catalyst materials. In this model, two distinct active sites are involved following a bifunctional catalysis scheme. It is suggested that oxygen is initially adsorbed and reduced to peroxide on a -type site. The intermediate product, peroxide, can be further reduced to water in a series reaction step on a decorating active cobalt oxide species on the catalyst surface.