(204c) Mechanism of Oxygen Reduction Reaction On Non-Platinum Catalysts Derived From Pyrolyzed Porphyrins

In order for fuel cell technology to become economically viable for commercial power generation the materials cost will have to be greatly reduced. A significant amount of the overall cost of the proton exchange membrane (PEM) fuel cell and the direct methanol fuel cell (DMFC) is associated with the need for noble metal catalysts to reduce the large oxygen reduction reaction (ORR) kinetic overpotential at the cathode. The oxygen reduction reaction in acidic media on platinum and other noble metals has been extensively studied and is largely understood. In contrast, the current understanding and consensus of the fundamental processes and nature of active sites on non-platinum ORR catalysis is still debated. If non-platinum catalysts are to replace platinum based materials in PEM fuel cells a better understanding of the active moieties and the reaction steps that occur is needed.

A study on the oxygen reduction reaction (ORR) mechanism that occurs on non-platinum electrocatalysts, specifically materials derived from pyrolyzed cobalt tetra-methoxyphenyl porphrin (CoTMPP) in acidic media is presented here. Reactant and product flux analysis was performed on Rotating Ring-Disk Electrode (RRDE) data to evaluate the non-platinum based materials. An in depth XPS surface characterization analysis was preformed and discussed in the context of structure-to-property correlations that are established using a multivariant analysis (MVA) technique. Pyrolyzed cobalt porphyrin catalysts are highly heterogeneous materials that include both Co species that are associated with nitrogen (CoNx) and Co nano-particles coated by ?active? Co-oxides. We are able to propose 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 analysis indicates that the series type, 2x2 peroxide ORR pathway is supported on the bi-functional catalyst materials. Oxygen is initially adsorbed and reduced to peroxide on a CoNx type site. The intermediate product, peroxide can be further reduced to water in a series reaction step on active cobalt oxide species.