Molybdenum Nitrides as Oxygen Reduction Reaction Catalysts:
Structural and Electrochemical Studies
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Abstract
Monometallic
(δ-MoN, Mo<sub>5</sub>N<sub>6</sub>, and Mo<sub>2</sub>N) and
bimetallic molybdenum nitrides (Co<sub>0.6</sub>Mo<sub>1.4</sub>N<sub>2</sub>) 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-Mo<sub>2</sub>N), 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<sub>0.6</sub>Mo<sub>1.4</sub>N<sub>2</sub>), resulting in a modest onset potential
of 0.713 V versus reversible hydrogen electrode (RHE). Co<sub>0.6</sub>Mo<sub>1.4</sub>N<sub>2</sub> 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<sub>0.6</sub>Mo<sub>1.4</sub>N<sub>2</sub> 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)
