(561g) Mechanistic Study of Non-Precious Transition Metal/Nitrogen Doped Carbon Electrocatalysts for Oxygen Reduction Reaction (Invited)

At present, wide scale commercialization of polymer electrolyte membrane fuel cell (PEMFC) is retarded by the requirement of expensive Pt group metals as their electrocatalysts. To advance PEMFC technology, it is of great interests to develop earth-abundant, non-precious metal based catalysts in replacement of Pt, especially for oxygen reduction reaction (ORR) occurring at the cathode of PEMFCs. Recently, non-precious transition metal/nitrogen doped carbon (TM-N-C) catalysts have drawn wide attention since they exhibited promising ORR activity close to Pt. However, the structure of the active sites in these TM-N-C catalysts and their catalytic mechanism for ORR have not been fully understood. For a rational design of these TM-N-C catalysts, we have performed density functional theory (DFT) calculations to investigate oxygen reduction reaction (ORR) on various TM-N₄ active sites (TM = Fe, Co) with different moieties. Specifically, three TM-N₄ moieties are studied, such as the TM-N₄ moiety embedded in the graphene basal plane, the porphyrin-like moiety and/or the TM-N₄ moiety bridging the edges of two adjacent graphene layers. We calculated the adsorption energies of all the possible chemical species and the activation energies for O-O bond dissociation reactions involved in ORR on the FeN₄ and/or CoN₄ embedded in the graphene basal plane. Our DFT calculations predicted that the ORR could happen through 4e^- associative pathway on the FeN₄ site, whereas tend to a 2e^- pathway on the CoN₄ site due to high activation energy for O-O bond splitting and extremely weak adsorption of H₂O₂ on the CoN₄ site. The theoretical results are in agreement with our experimental observations. Furthermore, we calculated the adsorption energies of all the possible chemical species and the activation energies for O-O bond dissociation reactions involved in ORR on three different FeN₄ moieties described above. We found that the porphyrin-like moiety could catalyze ORR with highest limiting potential (corresponding to onset potential experimentally) while the O-O bond scission could be facilitated on the FeN₄ moiety bridging the adjacent graphene edges with low activation energy. These results suggest that increasing the quantity of micropore in the TM-N-C catalyst could enhance its catalytic activity for ORR through improving not only the specific area but also the intrinsic activity of the active sites.