(560cp) The Role of Oxygen Incorporation in the Oxygen Reduction Reaction Activity of Molybdenum Nitride Catalysts

Title:
The Role of Oxygen Incorporation on the Oxygen Reduction Reaction Activity of
Molybdenum Nitride Catalysts

Authors:
Melissa E. Kreider¹, Michaela Burke Stevens¹, Yunzhi Liu²,
Alessandro Gallo^1,3, Anton Ievlev⁴, Apurva Mehta⁵,
Robert Sinclair², Laurie A. King¹_, Thomas F. Jaramillo^1,3

¹ Department of Chemical
Engineering, Stanford University, 443 Via Ortega, Stanford, California 94305,
United States

² Department of Materials Science and Engineering, Stanford
University, 496 Lomita Mall, Stanford, California 94305, United States

³ SUNCAT Center for Interface Science and Catalysis, SLAC
National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California
94025, United States

⁴ The Center for Nanophase Materials Sciences and the Institute
for Functional Imaging of Materials, Oak Ridge National Laboratory, Oak Ridge,
Tennessee 37831, United States

⁵ Stanford Synchrotron Radiation Lightsource, SLAC National
Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025,
United States

The development of
active, stable, and low-cost catalysts for the oxygen reduction reaction (ORR)
is necessary for the viability of polymer electrolyte membrane fuel cells
(PEMFCs). Despite decades of study, the required mass loading of platinum-based
catalysts remains prohibitively expensive and has limited the growth of fuel
cell technology.¹

Transition metal (TM) nitrides
have recently gained attention as active ORR catalysts with promising stability
in acidic electrolyte.² Typically, TM nitrides are nanostructured
and supported on high surface area carbon or nitrided-carbon³, which
can contribute significantly to overall activity and complicate understanding
of intrinsic catalyst activity, including the structure and composition of the
active material.

In this study, we synthesize
well-defined, carbon-free molybdenum nitride (Mo_xN) thin films by
reactive sputtering. These catalysts were found to be active in acid
electrolyte, with the most active films demonstrating onset potentials between
0.5 and 0.6 V vs RHE. Activity, stability, and selectivity in acid electrolyte were
found to depend on the catalyst structure and extent of oxygen incorporation.

To understand the effect
of structure of on the catalyst surface and activity, we characterized the
catalysts extensively. By varying the synthesis conditions, including
temperature, N₂ partial pressure, O₂ partial pressure, and
substrate bias, we found that the crystallinity, crystal structure, and nitrogen-
and oxygen-content of the films could be modified. Using grazing incidence x-ray
diffraction (GIXRD), it was determined that the films were polycrystalline with
different bulk structures and different nominal nitrogen contents (e.g. Mo_xN,
1≤x≤2). Near edge x-ray absorption fine structure spectroscopy and
cross-sectional transmission electron microscopy were also used to distinguish
between similar crystal structures and to understand better the spatial
distribution of crystal structures throughout the film based on the synthesis
conditions. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was used
to obtain a depth profile of the Mo, N, and O composition of the film.
Differences in bulk oxygen content correlate strongly with the crystal
structures determined by GIXRD and with electrochemical activity, as shown in the
accompanying figure. Cyclic voltammograms for Mo_xN in 0.1 M HClO₄
show the highest ORR activity for the Mo_1.5N film, which had the
lowest bulk O concentration. We hypothesize that the bulk structure of the
catalyst, including the presence of a N gradient and low O content, affects the
active surface and thus the ORR activity.

To probe this activity
difference, catalyst conductivity was tested using a ferrocyanide/ferricyanide
redox couple. No substantial differences were measured, indicating that differences
in conductivity cannot explain the differences in activity between these
catalysts. Additionally, electrochemical surface area measurements indicate
that activity improvement is not correlated with increased surface area. Changes
in structure and composition with electrochemical testing were probed through ex-situ
x-ray photoelectron spectroscopy and x-ray reflectivity characterization and
indicate a stable crystal structure with loss of surface oxide character for
all catalysts. The similarity of the surface morphologies suggests that differences
in catalyst activity for molybdenum nitride structures are related to the bulk properties.
This provides a design principle in which catalyst activity can be tuned by
changing the bulk structure and total oxygen content.

Figure Caption. Structural,
compositional, and activity characterization of the three different Mo_xN
catalysts (where x = 2, 1.5, 1). (A) GIXRD patterns, showing surface and bulk
structures. (B) TOF-SIMS depth profile measuring oxygencontent,
collected in negative ion mode. (C) Cyclic voltammograms using a rotating disc
electrode (1600 rpm) showing the ORR activity in 0.1 M HClO₄.

References:

1.      
Rabis, A.;
Rodriguez, P.; Schmidt, T. J. Electrocatalysis for Polymer Electrolyte Fuel
Cells: Recent Achievements and Future Challenges. ACS Catal. 2012, 2, 864–890.

2.      
Dong,
S.; Chen, X.; Zhang, X.; Cui, G. Nanostructured Transition Metal Nitrides for
Energy Storage and Fuel Cells. Coord. Chem. Rev. 2013, 257 (13–14), 1946–1956.

3.      
Ota,
K.; Ishihara, A. Metal Oxide-Based Compounds as Electrocatalysts for Oxygen
Reduction Reaction. In Electrocatalysis in Fuel Cells: A Non- and Low- Platinum
Approach; Shao, M., Ed.; Springer London: London, 2013; pp 391–416.