Energy storage, which can resolve the detrimental issues of unreliability and intermittency of renewable energies, becomes an indispensable part of the future renewable energy dominated zero/negative carbon society. Among the many energy storage devices developed and explored so far, zinc-air batteries (ZABs) hold high promises and have attracted much research attention because of their high theoretical energy densities (1086 vs. 200–250 Wh kg⁻¹ of LIBs), safety, environmental friendliness, and low cost. Nevertheless, sluggish kinetics of the two fundamental electrochemical reactions, oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), occurring on the air electrode during the discharge-charge cycle of ZABs lead to low round-trip efficiency. Developments of cost-effective (non-precious metal-based) highly efficient and stable bifunctional oxygen electrocatalysts are thus most critical for realizing practical rechargeable ZABs. In this work, a simple vapor doping method was developed to incorporate S into the coordination environment of Fe SAs to create N,S co-coordinated Fe SAs of highly asymmetric structure on flame-roasting deposited carbon black (CB), Fe-N₃S₁/CB, as an extraordinary bifunctional oxygen electrocatalyst, achieving 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). Furthermore, 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 and demonstrates excellent long-term stability, with only 8.2% decay in round-trip efficiency over 1000 (333.3 h) discharge-charge cycles at 10 mA cm^-2. Density functional theory calculations elucidate that incorporation of sulfur into the coordination sphere of Fe creates highly asymmetrically configured Fe single atoms which facilitate the electrochemical dehydroxylation step for the ORR and accelerates the electrochemical O₂ desorption step for the OER, thereby reducing the corresponding free energy differences on Fe SAs for largely enhanced catalytic efficiency. The present development opens a new avenue for electrocatalyst design to further advancing the development of high performance rechargeable ZABs for electrochemical energy storage applications.

Chih-Heng Lee

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