Identifying and Tuning the In Situ Oxygen-Rich Surface of Molybdenum Nitride Electrocatalysts for Oxygen Reduction

Rigorous
in situ studies of electrocatalysts are required to enable
the design of higher performing materials. Nonplatinum group metals
for oxygen reduction reaction (ORR) catalysis containing light elements
such as O, N, and C are known to be susceptible to both ex situ and
in situ oxidation, leading to challenges associated with ex situ characterization
methods. We have previously shown that the bulk O content plays an
important role in the activity and selectivity of Mo–N catalysts,
but further understanding of the role of composition and morphological
changes at the surface is needed. Here, we report the measurement
of in situ surface changes to a molybdenum nitride (MoN) thin film
under ORR conditions using grazing incidence X-ray absorption and
reflectivity. We show that the half-wave potential of MoN can be improved
by ∼90 mV by potential conditioning up to 0.8 V versus RHE.
Utilizing electrochemical analysis, dissolution monitoring, and surface-sensitive
X-ray techniques, we show that under moderate polarization (0.3–0.7
V vs RHE) there is local ligand distortion, O incorporation, and amorphization
of the MoN surface, without changes in roughness. Furthermore, with
a controlled potential hold procedure, we show that the surface changes
concurrent with potential conditioning are stable under ORR relevant
potentials. Conversely, at higher potentials (≥0.8 V vs RHE),
the film incorporates O, dissolves, and roughens, suggesting that
in this higher potential regime, the performance enhancements are
due to increased access to active sites. Density functional theory
calculations and Pourbaix analysis provide insights into film stability
and O incorporation as a function of potential. These findings coupled
with in situ electrochemical surface-sensitive X-ray techniques demonstrate
an approach to studying nontraditional surfaces in which we can leverage
our understanding of surface dynamics to improve performance with
the rational, in situ tuning of active sites.