Effect of Surface Manganese Valence of Manganese Oxides on the Activity of the Oxygen Reduction Reaction in Alkaline Media

In this article, manganese oxide nanorods with different crystalline structures, i.e., β-MnO2, α-Mn2O3, and a composite of Mn3O4 and α-Mn2O3, were successfully synthesized via controlling the heat-treatment procedure starting from a manganese oxide composite, containing γ-MnOOH and Mn(OH)4. The oxygen reduction reaction (ORR) polarization curves measured by a rotating disk electrode (RDE) setup show that those MnOx catalysts with higher Mn valent states, i.e., γ-MnOOH and Mn(OH)4 composite and β-MnO2, exhibit better catalytic activity toward the ORR than those with lower Mn valences. Furthermore, we testify that the surface Mn valence of MnOx could be tuned by applying proper potential cycling to the MnOx electrode and thus leads to different activities, i.e., the MnOx surface is rich in Mn(II) after treatment at relatively negative potentials, resulting in degradation in ORR activity, while it is rich in Mn(IV) after treatment at positive potentials, resulting in improvement in activity. Compared with the heat-treatment approach, the electrochemical approach is more facile and energy-saving to tune the surface metal valence and thus ORR activity.