g-C3N4 promoted MOF-derived Fe single atoms anchored on N-doped hierarchically porous carbon for high-performance Zn-air batteries

Exploring efficient, easy-to-manufacture, and inexpensive bifunctional electrocatalysts with abundant accessible active sites is crucial for rechargeable zinc-air batteries (ZABs). Herein, we report the strategy consisting of the space confinement and pore-making engineering to fabricate single-atom catalyst enriched with Fe-N4 sites anchored on N-doped hierarchically porous carbon (Fe-NC-C3N4). The optimized Fe-NC-C3N4 exhibits excellent oxygen reduction/evolution reaction (ORR/OER) activities with a half-wave potential (E1/2) of 0.90 V vs. RHE and a promising low overpotential of 0.305 V vs. RHE at 10 mA·cm-2 in alkaline electrolyte. These superior catalytic activities are attributed to the combined effect between the atomic active sites and the well-balanced micro-meso-macropore structures. The homemade liquid Zn-air battery (ZAB) assembled with Fe-NC-C3N4 catalyst displays a power density of 133.59 mW·cm-2 and a significant energy density of 882.58 mAh·g-1, exceeding those of the equipment with commercial Pt/C-RuO2 (56.82 mW·cm-2 and 643.87 mAh·g-1, respectively). Particularly, the corresponding flexible wearable ZAB manifests outstanding foldability and cyclical stability. This work opens a new perspective for the structural design of single-atom catalysts in the energy storage and conversion area.