Issue 25, 2024

Implication of surface oxidation of nanoscale molybdenum carbide on electrocatalytic activity

Abstract

Transition metal carbides, such as molybdenum carbides, are promising substitutes for noble metals as low-cost, durable electrocatalysts. Under ambient conditions, however, these carbides are subject to oxidation due to their oxophilic nature. The partially oxidized surface may possess both oxygen-modulated metallic-like hydrogen adsorption sites and Brønsted-acidic hydroxyl sites. However, the impact of surface oxidation on electrochemical processes such as the hydrogen evolution reaction (HER) has rarely been studied. Here, we synthesized β-Mo2C catalysts and oxidized their surfaces electrochemically to varying extents to study the effects of surface oxidation on HER activity. The degree of surface oxidation was controlled by applying different potential windows to metal carbide catalysts. The samples with varying degrees of surface oxidation were tested for their HER activity. Experimental data indicate that the Tafel slope for the HER and double-layer capacitance were negatively affected by surface oxidation, particularly due to the loss of carbon and the formation of electrochemically less active surface oxides. The surface oxidation was studied experimentally by X-ray photoelectron spectroscopy and simulated using density functional theory (DFT), ab initio thermodynamics, and charge transfer estimates. Our DFT calculation results suggest that the model β-Mo2C (011) surface favors the adsorption of O* from water during the electrochemical oxidation, giving rise to the anodic current. Oxygen atoms preferentially interact with surface C sites, forming stable –C[double bond, length as m-dash]O species and oxycarbide-like surfaces. Highly oxidized surfaces become kinetically unstable and undergo a deeper, substitutional oxidation through –C[double bond, length as m-dash]O replacement by O*, forming a thermodynamically stable Mo(IV) surface oxide, where each Mo atom coordinates with up to six O atoms. The rate-limiting step switches from CO desorption, occuring within the potential window from 0.28 to 0.51 V, to water dissociation above 0.51 V. The computational results agree well with experimental observations, such as the onset potential (0.6 V) for rapid surface oxidation. This work helps to unravel the details of evolution of surface sites during an oxidation process of molybdenum carbides along with their effects on catalytic properties and lays the foundation for their practical use in various applications.

Graphical abstract: Implication of surface oxidation of nanoscale molybdenum carbide on electrocatalytic activity

Supplementary files

Article information

Article type
Paper
Submitted
14 mar 2024
Accepted
15 maj 2024
First published
15 maj 2024

J. Mater. Chem. A, 2024,12, 15163-15176

Implication of surface oxidation of nanoscale molybdenum carbide on electrocatalytic activity

S. Yu, A. K. Gautam, D. Gao, A. N. Kuhn, H. He, A. V. Mironenko and H. Yang, J. Mater. Chem. A, 2024, 12, 15163 DOI: 10.1039/D4TA01746C

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