Role of Oxygen Vacancy Defects in the Electrocatalytic Activity of Substoichiometric Molybdenum Oxide

Mesoporous α-MoO3-x combined with poly(diallyldimethylammonium chloride)-functionalized reduced graphene oxide (PDDA-rGO) is introduced as an inexpensive and efficient oxygen reduction reaction (ORR) catalyst. The mesoporous catalysts are wrapped by conductive rGO sheets via an electrostatic interaction induced by a PDDA polyelectrolyte. The thermal interaction of PDDA with MoO3 efficiently reduces the metal oxide to MoO3-x at 400-600 °C, creating a surface oxygen vacancy. Through a combination of density functional theory and experiments, the role of the surface oxygen vacancy sites in the ORR activity of MoO3-x is identified. For the first time, all the energy barriers against ORR are calculated at each step for MoO3 with no oxygen vacancies and MoO3-x with surface oxygen vacancies. It is shown that the presence of an Mo4+-vO•• oxygen vacancy site on the surface significantly reduces the energy barriers against ORR in the reaction pathways. An overpotential of 0.86 V (vs a reversible hydrogen electrode) with excellent electrochemical stability was obtained with the newly designed catalyst, with only a 9% decrease in the activity after ∼17 h. These results offer a new paradigm in the defect engineering of metal oxides with a potential for the synthesis of stable and active noble metal-free ORR electrocatalysts.