Coupling Optimized Fe-N Coordination Structure-Dominant Cathode with In Situ Hydrogel Electrolytes toward a Highly Efficient Flexible Zn–Air Battery

The application of a flexible Zn-air battery (FZAB) in next-generation wearable electronics is mainly hindered by the sluggish oxygen reduction/evolution reaction (ORR/OER) and unstable Zn/electrolyte interface, particularly at relatively high-rate ability (10 mA cm-2). Herein, a Fe2N/pyridinic N-rich coordinated Fe single atom (defined as "Fe2N/PR-Fe SA") heterostructure is designed for optimizing the plane-symmetric Fe-4N coordination, which demonstrates outstanding bifunctional electrocatalytic performance with a low ORR/OER potential gap of 0.63 V. Experimental analyses and theoretical calculations reveal that the electronic structure of Fe single atoms, derived from the synergistic interaction between Fe2N with a triangular pyramidal Fe3N coordination and pyridinic FeN4, can effectively accelerate the desorption of the *OH intermediate in the ORR and optimize the *OOH/*O adsorption behavior during the OER process. Moreover, the in situ hydrogel electrolyte (HGE) is designed on the surface of the zinc anode to limit interface water content and eliminate the formation of deposition "hot spots" for improving Zn electrochemical reversibility (203 h at 1 mA cm-2/1 mA h cm-2 with Zn//Zn-symmetric battery). Therefore, the constructed FZAB based on Fe2N/PR-Fe SA and the in situ HGE exhibits a high maximum power density (157.3 mW cm-2), a long lifetime (193 h at 2 mA cm-2), small discharge/charge voltage polarization (0.81 V at 10 mA cm-2), and excellent mechanical flexibility.