Iron clusters and single atom sites cooperatively promote bifunctional oxygen reaction activity in ultra-stable flexible zinc-air batteries

Enhancing the bifunctional activity of electrocatalysts for oxygen reduction/evolution reactions (ORR/OER), along with improving water retention in gel-polymer electrolytes, is essential for developing high-performance flexible zinc-air batteries (FZABs). Herein, we synthesize a structure that combines Fe single atom sites with an FeN₄ configuration and clusters of four coordinated Fe atoms anchored on worm-like polypyrrole (Fe_SA/Fe_AC@PPy/CC) using an electrochemical deposition strategy. It shows a promoted bifunctional ORR/OER activity with a small potential gap of 0.694 V. Theoretical calculations indicate that Fe single atom sites lower the energy barrier of the rate-determining step for both the ORR and OER, while Fe clusters optimize the energy barriers associated with oxygen-containing intermediates. The interaction between Fe single atom sites and clusters shifts the d-band center of the metal closer to the Fermi level, leading to electron depletion at the Fe centers. This adjustment triggers a rearrangement of the orbital electrons and enhances the adsorption interaction with oxygen orbitals, thereby improving both the ORR and OER activities. Additionally, a water-locking hydrogen bonding network electrolyte composed of polyacrylamide and ethylene glycol is utilized to enhance low-temperature tolerance. Thus, the assembled Fe_SA/Fe_AC@PPy/CC-based FZAB demonstrates ultra-stable operation for 210 h at 25 °C and 167 h at −40 °C.