Synergistic Fe and Co binary single atoms based air cathodes for high performance and ultra-stable Zn-air batteries

Cost-effective highly efficient and stable air cathodes are critical for practical applications of rechargeable Zn-air batteries (ZABs). In this work, air cathodes constructed by loading carbon nanotube supported N-doped carbon anchored Fe and Co binary single atoms, f-Fe1Co1/CNT, in carbon paper composited nickel foams, NF/CP, were developed for ZABs, exhibiting outstanding bifunctional oxygen electrocatalytic activities (ΔE = 0.671 V) with a half-wave potential (E1/2) of 0.880 V (vs. RHE) for oxygen reduction reaction (ORR) and an overpotential of 321 mV at 10 mA cm−2 (η10) for oxygen evolution reaction (OER) in 0.1 M KOH, which outperformed the benchmark (Pt/C+IrO2)-based air cathode (ΔE = 0.773 V, E1/2 = 0.850 V, η10 = 393 mV). With the unique air cathode design of compositing catalyst-loading layer (nickel foam) with gas diffusion layer (carbon paper), the f-Fe1Co1/CNT@NF/CP//Zn ZAB delivered a remarkable discharge peak power density of 282.1 mW cm−2 at an ultra-high current density of 427.7 mA cm−2 and maintained an ultra-small potential gap of 0.75 V after operations at 10 mA cm−2 for 3300 cycles (1100 h), largely outperforming the (Pt/C+IrO2)@NF/CP//Zn ZAB (162.4 mW cm−2 at 247.3 mA cm−2, 0.86 V, 590 cycles). Density functional theory calculations revealed the positive long-range synergy between Fe-N4 and Co-N4 single atoms, significantly reducing the free energies of the rate-determining steps, oxygen protonation for ORR and formation of oxyhydroxides for OER at the main active site Fe-N4, to realize the much enhanced oxygen electrocatalytic activities.