Mn₃O₄ Quantum Dots Supported on Nitrogen-Doped Partially Exfoliated Multiwall Carbon Nanotubes as Oxygen Reduction Electrocatalysts for High-Performance Zn–Air Batteries

Highly efficient and low-cost nonprecious metal electrocatalysts that favor a four-electron pathway for the oxygen reduction reaction (ORR) are essential for high-performance metal–air batteries. Herein, we show an ultrasonication-assisted synthesis method to prepare Mn3O4 quantum dots (QDs, ca. 2 nm) anchored on nitrogen-doped partially exfoliated multiwall carbon nanotubes (Mn3O4 QDs/N-p-MCNTs) as a high-performance ORR catalyst. The Mn3O4 QDs/N-p-MCNTs facilitated the four-electron pathway for the ORR and exhibited sufficient catalytic activity with an onset potential of 0.850 V (vs reversible hydrogen electrode), which is only 38 mV less positive than that of Pt/C (0.888 V). In addition, the Mn3O4 QDs/N-p-MCNTs demonstrated superior stability than Pt/C in alkaline solutions. Furthermore, a Zn–air battery using the Mn3O4 QDs/N-p-MCNTs cathode catalyst successfully generated a specific capacity of 745 mA h g–1 at 10 mA cm–2 without the loss of voltage after continuous discharging for 105 h. The superior ORR activity of Mn3O4 QDs/N-p-MCNTs can be ascribed to the homogeneous Mn3O4 QDs loaded onto the N-doped carbon skeleton and the synergistic effects of Mn3O4 QDs, nitrogen, and carbon nanotubes. The interface binding energy of −3.35 eV calculated by the first-principles density functional theory method illustrated the high stability of the QD-anchored catalyst. The most stable adsorption structure of O2, at the interface between Mn3O4 QDs and the graphene layer, had the binding energy of −1.17 eV, greatly enhancing the ORR activity. In addition to the high ORR activity and stability, the cost of production of Mn3O4 QDs/N-p-MCNTs is low, which will broadly facilitate the real application of metal–air batteries.