Theoretical Study of Possible Active Site Structures in Cobalt- Polypyrrole Catalysts for Oxygen Reduction Reaction

The active site structure of nonprecious group metal catalyst is a puzzle which inhibits innovative synthetic route design and impedes breakthroughs. In an effort to understand the origin of the catalytic activity of Co-PPy composites, we employed density functional theory (DFT) and experimental measurements to investigate the structure and energy of possible catalytic sites and the corresponding reaction pathways for the oxygen reduction reaction (ORR). Four different structures of the active site are examined, including two previously postulated in the literature. In order to determine the probability of their existence, the stability of each structure is evaluated. The corresponding Co(III)/Co(II) redox potentials are calculated and, based on the obtained data, the involvement of either Co(III) or Co(II) in the ORR under fuel cell-relevant conditions postulated. Possible configurations of oxygen adsorption on the active centers are also examined, including the end-on and side-on cases. The possible reaction pathways and reaction products generated at the various active centers are evaluated based on Yeager's concept correlating ORR products with the configuration of oxygen adsorption. The catalytic activity is found to be significantly different for the various sites and depends strongly on the electrode potential. The computational data are critically compared with experimental spectroscopic (EXAFS and FTIR) and electrochemical data (CV, RDE, and RRDE). The insights into the active structures and their associated catalytic activity as well as selectivity for four-electron oxygen reduction are expected to provide guidance for further catalyst optimization.