Oxygen reduction reaction on cobalt-(n)pyrrole clusters from DFT studies

The oxygen reduction reaction (ORR) catalyzed by Co-(n)PPy (n = 2-10) clusters is investigated in detail at BLYP/DZP level of theory. The calculation results indicate that different O-2 adsorption modes could greatly affect the types of the reduction intermediates. The side-on O-2 adsorption is more likely to yield the intermediate HO-OH, while end-on yields H2O-O. However, the side-on O-2 adsorption could lead to strong intra-molecular strain and result in instability of the clusters, the cobalt ion therefore is more easily to be dissolved from the active site, leading to poor durability of the Co-(n) PPy clusters. The ORR activity might be enhanced with the cluster size increases, based on HOMO and LUMO analysis. From Co-(8) PPy, the electronic structures are hard to be modified by simply increasing the PPy chains. However, further increasing the cluster size might result in an increase of Co-N-2 active site due to that more Co atoms could be captured by the pyrrolic N atoms, the resulting synergistic effect would be more likely to enhance the activity.