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

TLDR: In this article, the authors proposed an oxygen reduction reaction (ORR) mechanism that occurs on non-platinum electrocatalysts, specifically materials derived from pyrolyzed cobalt tetramethoxyphenyl porphyrin in acidic media.

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 (CoN x ) 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, 2 × 2 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 CoN x -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.