First-principles calculation on effects of oxygen vacancy on α-MnO2 and β-MnO2 during oxygen reduction reaction for rechargeable metal-air batteries

Abstract

MnO2 is widely applied as oxygen reduction reaction (ORR) electrocatalysts in different metal-air batteries (MABs). Enhancing the ORR activity of MnO2-based catalysts is necessary for improving the performance of MABs. Defect-engineering of catalyst materials is a key approach for enabling the high performance of ORR. Here, the defect-engineering of α-MnO2 (211) and β-MnO2 (110) by oxygen vacancy (OV) is investigated using the first-principles density functional theory calculation. The geometric structure, adsorption, electronic conductivity, and oxygen reduction reaction (ORR) activity are studied. As a result, the OV induces the geometric structure that the Mn–Mn and Mn–O distances are closer when the catalysts lose the oxygen atom(s) on the top-layer surfaces. The presence of OV not only enhances the adsorption energy of *OOH, *O, and *OH, but also increases the electronic conductivity analyzed via the electron transfer. The Bader charge analysis demonstrates that the Mn(IV) can be altered to Mn(III) by the electron accumulation from OV. The volcano plot of ORR overpotential indicates that having the excess OV concentration on MnO2 surfaces cannot enhance the ORR activity. The excellent activity is yielded by 12.50 % OV α-(211) and 66.66 % OV β-(110) with the ORR overpotential of 0.31 V and 0.60 V, respectively. The results demonstrate that Ov is an essential parameter defining the existence of Mn(III) and Mn(IV) on the surface of MnO2-based catalysts. The optimal ratio of Mn(IV):Mn(III) is in challenge developing the α-MnO2 and β-MnO2 electrocatalyst cathode for metal-air batteries. This study provides a guideline for developing the potential cathode catalyst for MABs, used for harvesting energy.