Insight into the Effect of Oxygen Vacancy Concentration on the Catalytic Performance of MnO₂

Abstract

Oxygen vacancies (OVs) are important for changing the geometric and electronic structures as well as the chemical properties of MnO 2 . In this study, we performed a DFT+U calculation on the electronic structure and catalytic performance of a β-MnO 2 catalyst for the oxygen reduction reaction (ORR) with different numbers and extents of OVs. Comparing those results with the experimental XRD analysis, we determined that OVs produce a new crystalline phase of β-MnO 2 . Changes in the electronic structure (Bader charges, band structure, partial density of states, local density of states, and frontier molecular orbital), proton insertion, and oxygen adsorption in β-MnO 2 (110) were investigated as a function of the bulk OVs. The results show that a moderate concentration of bulk OVs reduced the band gap, increased the Fermi and HOMO levels of the MnO 2 (or MnOOH), and elongated the O–O bond of the adsorbed O 2 and coadsorbed O 2 with H. These changes substantially increase the conductivity of MnO 2 for the catalysis of ORR. However, an excessively high concentration of OVs in β-MnO 2 (110) will work against the catalytic enhancement of MnO 2 for ORR. The DFT+U calculation reveals that a moderate concentration of OVs induced a large overlap of the surface Mn d z 2 orbitals, thus introducing an extra donor level at the bottom of the conductive band, which increased the conductivity of β-MnO 2 (110). Such a curvilinear change of the catalytic activity and electronic structure as a function of the oxygen vacancy concentration suggests that the β-MnO 2 with moderate concentration OVs exhibits the highest conductivity and catalytic activity for ORR.