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

Oxygen vacancies (OVs) are important for changing the geometric and electronic structures as well as the chemical properties of MnO2. In this study, we performed a DFT+U calculation on the electronic structure and catalytic performance of a β-MnO2 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-MnO2. 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-MnO2 (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 MnO2 (or MnOOH), and elongated the O–O bond of the adsorbed O2 and coadsorbed O2 with H. These changes substantially increase the conductivity of MnO2 for the catalysis of ORR. However, an excessively high concentration of OVs in β-MnO2 (110) will work against the catalytic enhancement of MnO2 for ORR. The DFT+U calculation reveals that a moderate concentration of OVs induced a large overlap of the surface Mn dz2 orbitals, thus introducing an extra donor level at the bottom of the conductive band, which increased the conductivity of β-MnO2 (110). Such a curvilinear change of the catalytic activity and electronic structure as a function of the oxygen vacancy concentration suggests that the β-MnO2 with moderate concentration OVs exhibits the highest conductivity and catalytic activity for ORR.