Oxygen reduction reaction (ORR) catalyzed by carbon-supported cobalt polypyrrole (Co-PPy/C) electrocatalysts

Abstract

This paper reports the experimental characterization of carbon-supported cobalt polypyrrole (Co-PPy/C) catalysts synthesized using a chemical method of polymerization synthesis. Both unpyrolyzed and pyrolyzed catalysts were characterized using electrochemical techniques such as cyclic voltammetry (CV), rotating disk electrode (RDE), as well as rotating ring disk electrode (RRDE) to quantitatively obtain the oxygen reduction reaction (ORR) kinetic constants and the reaction mechanisms. The pyrolyzed catalyst showed significantly improved ORR activity as well as different ORR mechanisms, suggesting that heat-treatment is a necessary step for catalyst activity improvement. To understand the heat-treatment effect, X-ray photoelectron spectroscopy (XPS) was used to detect surface structure changes. The XPS results showed that after the sample was heat-treated, new nitrogen peaks corresponding to pyrrolic (or pyridone) and graphitic (quaternary) type nitrogens could be observed. Both of these species may be assigned to sites catalytically active towards the ORR, resulting in activity enhancement as well as a mechanism change from a two-electron dominant to a four-electron dominant reduction process, when compared to that of the unpyrolyzed catalyst.