Abstract
Molybdenum nitride (Mo–N) catalysts have shown promising activity and stability for the oxygen reduction reaction (ORR) in acid. However, the effect of oxygen (O) incorporation (from synthesis, catalysis, or exposure to air) on their activity remains elusive. Here, we use reactive sputtering to synthesize three compositions of thin-film catalysts and use extensive materials characterization to investigate the depth-dependent structure and incorporated O. We show that the as-deposited Mo–N films are highly oxidized both at the surface (>30% O) and in the bulk (3–21% O) and that the ORR performance is strongly correlated with the bulk structure and composition. Activity for 4e– ORR is highest for compositions with the highest N/O and N/Mo ratio. Furthermore, H2O2 production for the films with moderate O content is comparable to or higher than the most H2O2-selective nonprecious metal catalysts in acidic electrolyte, on a moles per mass or surface area of catalyst basis. Density functional theory provides insight into the energetics of O incorporation and vacancy formation, and we hypothesize that activity trends with O/N ratios can be traced to the varying crystallite phases and their interactions with ORR adsorbates. This work demonstrates the prevalence and significance of O in metal nitride electrocatalysts and motivates further investigation into the role of O in other nonprecious metal materials.
Original language | English |
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Pages (from-to) | 2946-2960 |
Number of pages | 15 |
Journal | Chemistry of Materials |
Volume | 32 |
Issue number | 7 |
DOIs | |
State | Published - Apr 14 2020 |
Funding
The authors gratefully acknowledge the support of the Toyota Research Institute. The U.S. Department of Energy (DoE) Office of Basic Energy Sciences (BES) is gratefully acknowledged for the primary support for SUNCAT Center for Interface Science and Catalysis. Part of this work was performed at the Stanford Nano Shared Facilities (SNSF) and the Stanford Nanofabrication Facility (SNF), supported by the National Science Foundation under Award ECCS-1542152. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. DoE, Office of BES under contract no. DE-AC02-76SF00515. Part of this research (ToF-SIMS characterization) was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility, using instrumentation within ORNL’s Materials Characterization Core provided by UT-Battelle, LLC under contract no. DE-AC05-00OR22725 with the U.S. DoE. The authors thank Guanchao Li in the Stanford Environmental Measurements Facility for the acquisition of ICP–MS and OES data. The authors thank Kevin Stone for the XRD integration code. Authors M.E.K. and M.B.S. would like to thank Chris Hahn and Alan Landers for insightful discussions. Author AP thanks the National Science Foundation Graduate Research Fellowship Program (NSF GRFP). The authors gratefully acknowledge the support of the Toyota Research Institute. The U.S. Department of Energy (DoE) Office of Basic Energy Sciences (BES) is gratefully acknowledged for the primary support for SUNCAT Center for Interface Science and Catalysis. Part of this work was performed at the Stanford Nano Shared Facilities (SNSF) and the Stanford Nanofabrication Facility (SNF), supported by the National Science Foundation under Award ECCS-1542152. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. DoE, Office of BES under contract no. DE-AC02-76SF00515. Part of this research (ToF-SIMS characterization) was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility, using instrumentation within ORNL?s Materials Characterization Core provided by UT-Battelle, LLC under contract no. DE-AC05-00OR22725 with the U.S. DoE. The authors thank Guanchao Li in the Stanford Environmental Measurements Facility for the acquisition of ICP?MS and OES data. The authors thank Kevin Stone for the XRD integration code. Authors M.E.K. and M.B.S. would like to thank Chris Hahn and Alan Landers for insightful discussions. Author AP thanks the National Science Foundation Graduate Research Fellowship Program (NSF GRFP).