Understanding the activity decay of MnO2 for oxygen reduction in neutral media

Abstract

MnO 2 has been studied as the catalyst for oxygen reduction reaction (ORR) in neutral due to its considerably good performance and low cost. Although many studies focus on activity enhancement, little attention has been paid to the activity degradation of MnO 2 . In this study, we first probe the mechanism of ORR activity decay of MnO 2 catalyst in neutral. Electrochemical measurements showed the apparent activity degradation of MnO 2 in both short-term and long-time stability tests. After chronoamperometry test, surface amorphization was observed on the catalyst, while no significant amorphization occurred for the absence of ORR catalysis. The activity decay and surface amorphization of MnO 2 catalyst are due to the disproportionation reaction of Mn 3+ ions in neutral. Mn 3+ ions involved in ORR catalysis disproportionate to generate soluble Mn 2+ ions, resulting in the surface amorphization together with the change of the highly ORR-active Mn 3+ /Mn 4+ redox to the low ORR-active Mn 2+ /Mn 3+ redox. This study clarifies the activity decay mechanism of MnO 2 for ORR in neutral media and provides theoretical support for improving and stabilizing the catalytic activity of MnO 2 in the future. The disproportionation reaction of Mn 3+ ions under neutral conditions destroys the redox conversion between Mn 3+ and Mn 4+ . • The ORR activity of MnO2 catalyst decays quickly in neutral. • The activity decay comes along with surface amorphization. • Surface amorphization is resulted by the disproportionation of Mn 3+ ions. • The loss of Mn 4+ converts highly active Mn 3+ /Mn 4+ to low active Mn 2+ /Mn 3+ .