Molybdenum Nitrides as Oxygen Reduction Reaction Catalysts: Structural and Electrochemical Studies

Monometallic (δ-MoN, Mo₅N₆, and Mo₂N) and bimetallic molybdenum nitrides (Co_0.6Mo_1.4N₂) were investigated as electrocatalysts for the oxygen reduction reaction (ORR), which is a key half-reaction in hydrogen fuel cells. Monometallic hexagonal molybdenum nitrides are found to exhibit improved activities over rock salt type molybdenum nitride (γ-Mo₂N), suggesting that improvements are due to either the higher molybdenum valence or a more favorable coordination environment in the hexagonal structures. Further enhancements in activity were found for hexagonal bimetallic cobalt molybdenum nitride (Co_0.6Mo_1.4N₂), resulting in a modest onset potential of 0.713 V versus reversible hydrogen electrode (RHE). Co_0.6Mo_1.4N₂ exhibits good stability in acidic environments, and in the potential range lower than 0.5 V versus RHE, the ORR appears to proceed via a four-electron mechanism based on the analysis of rotating disc electrode results. A redetermination of the structures of the binary molybdenum nitrides was carried out using neutron diffraction data, which is far more sensitive to nitrogen site positions than X-ray diffraction data. The revised monometallic hexagonal nitride structures all share many common features with the Co_0.6Mo_1.4N₂ structure, which has alternating layers of cations in octahedral and trigonal prismatic coordination, and are thus not limited to only trigonal prismatic Mo environments (as was originally postulated for δ-MoN).