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

Abstract

A study on the oxygen reduction reaction ͑ORR͒ 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 ͑RRDE͒ 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-toproperty correlations and qualitative analysis of the RRDE flux data. The combined flux analysis and structural characterization suggests that the series type, 2 ϫ 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.

Carbon catalysts for electrochemical hydrogen peroxide production in acidic media

“…A number of metal-containing carbon catalysts have been developed for the ORR, i.e., for the 4-electron pathway [70,71], but also for the 2-electron one [72,73] Therefore, the chemical state of the metal atoms is likely an important factor to consider.…”

Morphology-Controlled Nitrogen-Containing Polymers as Synthetic Precursors for Electrochemical Oxygen Reduction Fe/N/C Cathode Catalysts

“…However, these analyses cannot resolve the quasi-four-electron reduction, and the experimental data does not support an actual fourelectron pathway during NPM catalysis. Indeed, Olson et al proposed a bi-functional ORR mechanism over Fe/N/C catalysts in 2010 [25], but quantitative discussion to resolve the quasi-fourelectron reduction reaction proved difficult. While Ohsaka and co-workers tried to apply the Damjanovic-Hsueh approach [21,26], which involves detailed RRDE analysis by plotting of the obtained parameters against the rotation speeds, such analysis relying on rotation speeds suffered The difficulties encountered in determining the catalytically active sites arise from the complicated reaction scheme for ORR over Pt-free catalysts.…”

Analysis of Adsorption Effects on a Metal-Nitrogen-Carbon Catalyst Using a Rotating Ring-Disk Study

“…Previous RRDE studies of non-precious metal catalysts for oxygen reduction 1,[12][13][14][15][16][17] generally show an increase in peroxide generation by one to two percent per 0.1 mg/cm 2 decrease in loading. 1,13,17,18 This suggests that peroxide detection at the ring could be artificially low because inner layers may further reduce peroxide to water, decreasing peroxide at the ring and increasing total disk current.…”