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 25 catalysts. Non-platinum group metals for oxygen reduction (ORR) catalysis containing light 26 elements such as oxygen, nitrogen, and carbon are known to be susceptible to both ex situ and in 27 situ oxidation leading to challenges associated with ex situ characterization methods. We have 28 previously shown that bulk reflectivity. We show that the halfwave potential of MoN can be improved by ~ 90 mV by potential 1 conditioning up to 0.8 V vs RHE. Utilizing electrochemical analysis, dissolution monitoring, and 2 surface sensitive x-ray techniques, we show that under moderate polarization (0.3 – 0.7 V vs RHE) 3 there is local ligand distortion, O incorporation, and amorphization of the MoN surface, without 4 changes in roughness. Furthermore, with a controlled potential hold procedure, we show that the 5 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 7 roughens, suggesting that in this higher potential regime the performance enhancements are due 8 to increased access to active sites. Density functional theory calculations and Pourbaix analysis 9 provide insight into film stability and oxygen incorporation as a function of potential. These 10 findings coupled with in situ electrochemical-surface sensitive x-ray techniques demonstrate an 11 approach to studying non-traditional surfaces in which we can leverage our understanding of 12 surface dynamics to improve performance with the rational, in situ tuning of active sites. 13