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.