Ultrasonic Plasma Engineering Toward Facile Synthesis of Single-Atom M-N₄/N-Doped Carbon (M = Fe, Co) as Superior Oxygen Electrocatalyst in Rechargeable Zinc-Air Batteries

As bifunctional oxygen evolution/reduction electrocatalysts, transition-metal-based single-atom-doped nitrogen-carbon (NC) matrices are promising successors of the corresponding noble-metal-based catalysts, offering the advantages of ultrahigh atom utilization efficiency and surface active energy. However, the fabrication of such matrices (e.g., well-dispersed single-atom-doped M-N₄/NCs) often requires numerous steps and tedious processes. Herein, ultrasonic plasma engineering allows direct carbonization in a precursor solution containing metal phthalocyanine and aniline. When combining with the dispersion effect of ultrasonic waves, we successfully fabricated uniform single-atom M-N₄ (M = Fe, Co) carbon catalysts with a production rate as high as 10 mg min^-1. The Co-N₄/NC presented a bifunctional potential drop of ΔE = 0.79 V, outperforming the benchmark Pt/C-Ru/C catalyst (ΔE = 0.88 V) at the same catalyst loading. Theoretical calculations revealed that Co-N₄ was the major active site with superior O₂ adsorption-desorption mechanisms. In a practical Zn-air battery test, the air electrode coated with Co-N₄/NC exhibited a specific capacity (762.8 mAh g^-1) and power density (101.62 mW cm^-2), exceeding those of Pt/C-Ru/C (700.8 mAh g^-1 and 89.16 mW cm^-2, respectively) at the same catalyst loading. Moreover, for Co-N₄/NC, the potential difference increased from 1.16 to 1.47 V after 100 charge-discharge cycles. The proposed innovative and scalable strategy was concluded to be well suited for the fabrication of single-atom-doped carbons as promising bifunctional oxygen evolution/reduction electrocatalysts for metal-air batteries.