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.