Defect-Engineered MoO₂ Nanostructures as an Efficient Electrocatalyst for Oxygen Evolution Reaction

This article presents the experimental and theoretical insights into defect-engineered MoO2 nanostructures (NSs) in terms of oxygen vacancy and OH– occupancy toward oxygen evolution reaction (OER). Two categories of β-MoO2 NSs are grown on a silicon substrate via a hydrogenation process from pregrown α-MoO3 structures. The postgrown MoO2 system gets OH– occupancy after 7 h of annealing (MoO2+OH–). On increasing the annealing duration to 9 hrs, both oxygen vacancies and OH– occupancy have been made into the MoO2 system (MoO2–x+OH–). The as-grown materials have been assessed for promising energy conversion applications toward electrocatalytic OER. The as-grown MoO2–x+OH– very efficiently catalyzes the OER at a lower overpotential and yields a higher current density compared to the as-grown MoO2+OH– and commercial MoO2. Both the oxygen vacancy and OH– occupancy in the MoO2 system play a synergistic role in enhancing the OER properties. The experimental observations are validated theoretically and plausibly explained with the help of a state-of-the-art density functional study. The simulation calculations reveal that the introduction of oxygen vacancy and OH– occupancy lowers the overpotential of OER. The OH– ions act passively on the surfaces of MoO2 that decrease the binding of reaction intermediates and aid in easy desorption of O2 molecules. Besides, the oxygen defect sites reduce the charge-transfer resistance, which eventually reduces the OER overpotential. Our empirical findings with theoretical supports render a significant shrewdness to the electrocatalytic performances of the defect-engineering MoO2 systems toward OER applications.