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

Abstract

Rigorous in situ studies of electrocatalysts are required to enable the design of higher performing catalysts. Non-platinum group metals for oxygen reduction (ORR) catalysis containing light elements such as oxygen, nitrogen, and carbon 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 bulk O content plays an important role in the activity and selectivity of Mo-N catalysts, but further understanding 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 2 reflectivity. We show that the halfwave potential of MoN can be improved by ~ 90 mV by potential conditioning up to 0.8 V vs 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 insight into film stability and oxygen incorporation as a function of potential. These findings coupled with in situ electrochemical-surface sensitive x-ray techniques demonstrate an approach to studying non-traditional surfaces in which we can leverage our understanding of surface dynamics to improve performance with the rational, in situ tuning of active sites.

New challenges in oxygen reduction catalysis: a consortium retrospective to inform future research

“…To understand and utilize these surface transformations to enhance catalytic performance, experimental efforts must be directed towards determining the true catalyst surface, such as through in operando characterization, and theoretical methods must be developed to accurately model these surfaces. 73 MOF catalysts are attractive due to their potential for enhancement via confinement. The consortium evaluated the ORR activity of the bimetallic Co/Zr-MOF, (Co)PCN222.…”

“…Although this result could indicate higher intrinsic surface activity, SEM imaging indicates aggregation of the 200 nm particles, making it likely that mechanisms such as insufficient particle-support contact and aggregation contribute to the Fig. 5 ORR activity of (A) molybdenum (oxy)nitride thin films as a function of O and N content in 0.1 M HClO 4 acid electrolyte, 73 (B) Zr(Co)porphyrin metal-organic frameworks as a function of particle size in 0.1 M HClO 4 at 0.43 V vs. RHE, 74 and (C), Ni-HAB, a single-atom catalyst (SAC) in 0.1 M KOH. 75 The highest performing catalyst tested in each study is highlighted with a yellow box with its structure as an inset.…”

Utility of NaMoO3F as a Precursor for Homogeneous Distribution of Cobalt Dopants in Molybdenum Oxynitrides

“…In addition, the Co‐doping into MoN largely affects the overpotential for ORR reaction [5b–d,f] . Also, the recent work proved that the amount of the oxygen dopants can largely affect their activity [5e,g, i] . These reports imply that the phase, the crystallinity, and the Co/O dopant state in the MoN should be controlled simultaneously for further improvement of molybdenum nitride‐based ORR catalytic activity.…”

Enabling practical nanoparticle electrodeposition from aqueous nanodroplets

“…To drive this conversion, different materials with varying activities are necessary. [5][6][7][8] However, generating the required amount of energy to satisfy the energy demand requires the use of materials that will minimize the input of energy and maximize the subsequent output. Thus, catalysts, materials that lower activation energy, are necessary to drive reactions in the conversion of chemical energy to electrical energy.…”