Fully exposed Fe single-atom sites on hierarchical porous hollow carbon fiber for high-performance flexible/wearable Zn-air batteries

Fe-N-C single-atom catalysts are considered among the most promising non-precious metal-based catalysts for oxygen reduction reaction (ORR), but issues such as low utilization of active sites and the easy aggregation of single atoms severely hinder their application in fuel cells and metal-air batteries. Herein, a single-atom Fe-embedded hierarchical porous hollow carbon fiber catalyst (Fe SAC/HCNF) is reported for ORR. The hollow channels and the porous structure of the fibers facilitate the exposure of single-atom active sites, and offer multidimensional mass transfer pathways to promote the transport of reactants, thus significantly enhancing catalytic performance. Additionally, the abundant micropores exert spatial confinement, which is beneficial for preventing the aggregation of single atoms. Leveraging its unique structural advantages, the Fe SAC/HCNF catalyst demonstrates outstanding ORR activity with low metal loading, boasting a high half-wave potential of 0.905 V, a substantial double-layer capacitance (C-dl) of 41.1 mF cm(-2), and a notable kinetic current density of 45.2 mA cm(-2) in alkaline media. Furthermore, the liquid Zn-air battery (ZAB) using Fe SAC/HCNF catalyst as the air cathode exhibits excellent battery performance and long-term cycling durability nearly 600 h. And the flexible quasi-solid-state ZAB can be stably cycled in various flat/bent states, which is promising for applications in flexible electronic devices.