Anion-Exchange Membrane Fuel Cells: Dual-Site Mechanism of Oxygen Reduction Reaction in Alkaline Media on Cobalt−Polypyrrole Electrocatalysts

Abstract

The oxygen reduction reaction (ORR) processes in alkaline media that occur on a family of electrocatalyst materials derived from a Co containing precursor and a polypyrrole/C composite material (PPy/C) are investigated here. The effects of Co loading and heat treatment temperature on the CoPPy/C materials are revealed through structural evaluations and electrochemical studies. Principle component analysis (PCA), a mutivariant analysis (MVA) technique, is used to establish structure-to-property correlations for the CoPPy/C materials. In all cases, pyrolysis leads to formation of a composite catalyst material, featuring Co nanoparticles coated with Co oxides and Co 2+ species associated with N−C moieties that originate from the polypyrrole structures. Based on these correlations, we are able to propose an ORR mechanism that occurs on this class of non-platinum based fuel cell cathode catalysts. The correlations suggest the presence of a dual site functionality where O 2 is initially reduced at a Co 2+ containing N−C type site in a 2 e − process to form HO 2 −, an intermediate reaction product. Intermediate species (HO 2 − ) can react further in the series type ORR mechanism at the decorating Co x O y /Co surface nanoparticle phase. The HO 2 − species can undergo either further electrochemical reduction to form OH − species or chemical disprotonation to form OH − species and molecular O 2 .