Vacancy-defective nano-carbon matrix enables highly efficient Fe single atom catalyst for aqueous and flexible Zn-Air batteries

Single-atom catalysts (SACs) have garnered significant interest as promising alternatives for enhancing the oxygen reduction reaction (ORR) in metal-air batteries. However, the rational design and facile synthesis of the highly selective SACs are still of great challenges. Herein, we successfully designed a Fe-N4-C SAC implanted in nitrogen-doped nano-carbon substrates with abundant carbon vacancies (Fe-NNCV) through a 'post adsorption-secondary pyrolysis' strategy assisted with melamine-etching, which achieves an asymmetric electronic structure and an optimized d band center. Additionally, the Fe-NNCV SAC exhibits a superior ORR activity with a high half wave potential of 0.924 V and a large diffusion-limiting current density of 5.7 mA cm−2. The aqueous rechargeable zinc-air battery (ZAB) assembled with Fe-NNCV SAC displays a high power-density of 99.1 mW cm−2 and a long-term durability over 310 h. Furthermore, flexible solid-state ZABs based on Fe-NNCV SAC are successfully assembled showing good cycling stability and foldability with high energy efficiency both at room and low temperature. This work provides insights into the optimization of electronic structure of transition metal SACs through defect engineering to achieve high ORR activities and to promote the applications of metal-air batteries.