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

Monometallic
(δ-MoN, Mo5N6, and Mo2N) and
bimetallic molybdenum nitrides (Co0.6Mo1.4N2) 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 (Co0.6Mo1.4N2), resulting in a modest onset potential
of 0.713 V versus reversible hydrogen electrode (RHE). Co0.6Mo1.4N2 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 Co0.6Mo1.4N2 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).