Co₃O₄ Nanoparticles Anchored on Nitrogen-Doped Partially Exfoliated Multiwall Carbon Nanotubes as an Enhanced Oxygen Electrocatalyst for the Rechargeable and Flexible Solid-State Zn-Air Battery

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.