Highly asymmetrically configured single atoms anchored on flame-roasting deposited carbon black as cathode catalysts for ultrahigh power density Zn-air batteries

Abstract

Iron group element-based single atom (SA) catalysts are highly regarded as promising alternatives to commercial Pt/C for catalysis of oxygen reduction reaction (ORR). For applications in rechargeable zinc-air batteries (ZABs), achieving the necessary high catalytic efficiency of the SAs toward oxygen evolution reaction (OER) however remains a significant challenge. Here, highly asymmetrically configured Fe SAs created with N,S co-coordination and anchored on flame-roasting deposited carbon black (CB), Fe-N₃S₁/CB, are developed, achieving outstanding bifunctional oxygen catalytic efficiency, with an ultra-small potential gap of 0.661 V at 10 mA cm^-2 (ΔE₁₀), outperforming the (Pt/C+RuO₂) composite catalyst (0.697 V). With a newly proposed binder-free composite air cathode design, the Fe-N₃S₁/CB based ZAB achieves an ultrahigh power density of 365.7 mW cm^-2 at a current density of 511.3 mA cm^-2, largely outperforming the (Pt/C+RuO₂) based ZAB (225.9 mW cm^-2 at 344.7 mA cm^-2). Furthermore, the Fe-N₃S₁/CB based ZAB demonstrates excellent long-term stability, with only 8.2 % decay in round-trip efficiency over 1000 (333.3 h) charge-discharge cycles at 10 mA cm^-2. Density functional theory calculations elucidate that incorporation of sulfur into the coordination sphere of Fe facilitates the electrochemical dehydroxylation step for ORR and accelerates the electrochemical O₂ desorption step for OER, thereby reducing the corresponding free energy differences on Fe SAs for largely enhanced catalytic efficiency.

1. A large size hetero-atom element, sulfur, is introduced to create highly asymmetrically configured Fe single atoms for enhancements in catalytic efficiency toward both oxygen reduction reaction and oxygen evolution reaction, and a binder-free composite air cathode design is proposed to improve electrochemical performances of zinc-air batteries.

2. An ultra-small potential gap of 0.661 V at 10 mA cm^-2 (ΔE₁₀) is achieved for the air cathode, and an ultrahigh discharge power density of 365.7 mW cm^-2 at a current density of 511.3 mA cm^-2 is acquired for the zinc-air battery.