Effect of the valence state of Mn in MnO: X/Ti4O7composites on the catalytic performance for oxygen reduction reaction and oxygen evolution reaction

Abstract

Manganese oxide composites with mixed valence states were prepared through the hydrothermal method by compositing with Ti4O7 and calcining at different temperatures, and their ORR and OER catalytic performance were investigated. The prepared catalysts were characterized by XRD, SEM-EDS, HRTEM-EDS, and XPS methods to analyse their phase constitution, morphology feature, and surface composition. The major phase of manganese oxides was Mn3O4, which is a one-dimensional structure, and its growth was induced by Ti4O7. The ORR and OER catalytic activity can be enhanced due to the preferred orientation of manganese oxides. Electrochemical measurements, namely CV, LSV and EIS, were utilized for determining the ORR and OER catalytic activity, whereas CA and ADT were used for studying the durability and stability. A Li-O2 battery was assembled to test the electrochemical behavior and properties in practical application. MnOx/Ti4O7 calcined at 300 °C exhibited the best catalytic activity of 0.72 V vs. RHE half-wave potential for ORR and 0.67 V vs. RHE overpotential for OER. The proportion of Mn3+ was also highest in all the MnOx/Ti4O7 composites. The assembled Li-O2 battery shows high performance with a voltage gap of only 0.85 V. Therefore, it can be affirmed that the inducement of Ti4O7 could strengthen the preferred orientation in manganese oxide growth and Mn3+ in MnOx/Ti4O7 plays a vital role in catalyzing ORR and OER, with both improving the ORR and OER bifunctional catalytic performance of manganese oxides.