As one of the most promising next-generation safe and green energy storage technologies, aqueous Zn-ion batteries have attracted considerable attention in recent years.

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 MnO quantum dots (QDs, ca. 2 nm) anchored on nitrogen-doped partially exfoliated multiwall carbon nanotubes (MnO QDs/N-p-MCNTs) as a high-performance ORR catalyst. The MnO 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 MnO QDs/N-p-MCNTs demonstrated superior stability than Pt/C in alkaline solutions. Furthermore, a Zn-air battery using the MnO QDs/N-p-MCNTs cathode catalyst successfully generated a specific capacity of 745 mA h g at 10 mA cm without the loss of voltage after continuous discharging for 105 h. The superior ORR activity of MnO QDs/N-p-MCNTs can be ascribed to the homogeneous MnO QDs loaded onto the N-doped carbon skeleton and the synergistic effects of MnO 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 O, at the interface between MnO 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 MnO QDs/N-p-MCNTs is low, which will broadly facilitate the real application of metal-air batteries.

This work presents a desirable bifunctional catalystCo3O4 nanoparticles anchored on nitrogen-doped partially exfoliated multiwall carbon nanotubes (Co3O4/N-p-MCNTs)for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) for the rechargeable and flexible solid-state Zn–air battery. The Co3O4/N-p-MCNTs demonstrates good catalytic performance with the ORR half-wave potential of 0.760 V (vs RHE). Additionally, the Co3O4/N-p-MCNTs exhibits superior limiting current density with higher stability than Pt/C in alkaline solutions. The catalyst obtains a low operating potential (E j10) of 1.62 V (vs RHE) to achieve a 10 mA cm–2 current density for OER. The potential difference (ΔE) between E j10 of OER and ORR half-wave potential is 0.86 V, which is smaller than that of many highly active bifunctional catalysts reported recently. Moreover, a Zn–air battery utilizing Co3O4/N-p-MCNTs as the catalyst in cathode could successfully generate a specific capacity of 768 mAh g–1 at 10 mA cm–2, and there is no voltage loss after a continuous discharge of 135 h. The fabricated solid-state rechargeable Zn–air battery displays a high power density and superior long-term cycling stability. Furthermore, first-principles density functional theory simulations were conducted to explore the interfacial properties of the hybrid catalyst, hinting that the N-p-MCNTs could significantly enhance the electrical conductivity of Co3O4 nanoparticles. The free energy diagrams generated from our simulations suggest that the N-p-MCNTs influence the superior ORR performance, while cobalt oxide affects the favored performance of OER. The obtained results confirm that the Co3O4/N-p-MCNTs catalyst would have a broad impact and could be used for renewable energy conversion devices.

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