TY - JOUR
T1 - Identifying and Tuning the in Situ Oxygen-Rich Surface of Molybdenum Nitride Electrocatalysts for Oxygen Reduction
AU - Stevens, Michaela Burke
AU - Kreider, Melissa E.
AU - Patel, Anjli M.
AU - Wang, Zhenbin
AU - Liu, Yunzhi
AU - Gibbons, Brenna M.
AU - Statt, Michael J.
AU - Ievlev, Anton V.
AU - Sinclair, Robert
AU - Mehta, Apurva
AU - Davis, Ryan C.
AU - Nørskov, Jens K.
AU - Gallo, Alessandro
AU - King, Laurie A.
AU - Jaramillo, Thomas F.
PY - 2020/12/28
Y1 - 2020/12/28
N2 - 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. © 2020 American Chemical Society.
AB - 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. © 2020 American Chemical Society.
KW - electrocatalysis
KW - grazing incidence X-ray absorption spectroscopy
KW - in situ
KW - molybdenum nitride
KW - oxygen reduction
KW - surface oxidation
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U2 - 10.1021/acsaem.0c02423
DO - 10.1021/acsaem.0c02423
M3 - RGC 21 - Publication in refereed journal
SN - 2574-0962
VL - 3
SP - 12433
EP - 12446
JO - ACS Applied Energy Materials
JF - ACS Applied Energy Materials
IS - 12
ER -