Efficient and affordable electrocatalysts for reversible oxygen reduction and oxygen evolution reactions (ORR and OER, respectively) are highly sought-after for use in rechargeable metal-air batteries. However, the construction of high-performance electrocatalysts that possess both largely accessible active sites and superior ORR/OER intrinsic activities is challenging. Herein, we report the design and successful preparation of a 3D hierarchically porous graphene framework with interconnected interlayer macropores and in-plane mesopores, enriched with pyridinic-nitrogen-cobalt (pyri-N-Co) active sites, namely, CoFe/3D-NLG. The pyri-N-Co bonding significantly accelerates sluggish oxygen electrocatalysis kinetics, in turn substantially improving the intrinsic ORR/OER activities per active site, while copious interlayer macropores and in-plane mesopores enable ultra-efficient mass transfer throughout the graphene architecture, thus ensuring sufficient exposure of accessible pyri-N-Co active sites to the reagents. Such a robust catalyst structure endows CoFe/3D-NLG with a remarkably enhanced reversible oxygen electrocatalysis performance, with the ORR half-wave potential identical to that of the benchmark Pt/C catalyst, and OER activity far surpassing that of the noble-metal-based RuO2 catalyst. Moreover, when employed as an air electrode for a rechargeable Zn-air battery, CoFe/3D-NLG manifests an exceedingly high open-circuit voltage (1.56 V), high peak power density (213 mW cm-2), ultra-low charge/discharge voltage (0.63 V), and excellent charge/discharge cycling stability, outperforming state-of-the-art noble-metal electrocatalysts.
