Fe/Fe3C@C nanoparticles encapsulated in N-doped graphene-CNTs framework as an efficient bifunctional oxygen electrocatalyst for robust rechargeable Zn-air batteries

3d transition metals or their derivatives encapsulated in nitrogen-doped nanocarbon show promising potential in non-precious metal oxygen electrocatalysts. Herein, we describe the simple construction of a bifunctional oxygen electrocatalyst with a "framework-active sites" structure, namely Fe/Fe3C@C (Fe@C) nanoparticles encapsulated in 3D N-doped graphene and bamboo-like CNTs (Fe@C-NG/NCNTs). The Fe@C structure provides additional electrons on the carbon surface, promoting the oxygen reduction reaction (ORR) on adjacent Fe-N-x active sites. The 3D NG hybrid with a bamboo-like CNTs framework facilitates fast reactant diffusion and rapid electron transfer. The optimized sample displays excellent ORR and oxygen evolution reaction (OER) activity, with a potential difference of only 0.84 V; this places it among the best bifunctional ORR/OER electrocatalysts. Most importantly, Zn-air batteries using Fe@C-NG/NCNTs as the cathode catalyst deliver a peak power density of 101.2 mW cm(-2) and a specific capacity of 682.6 mA h g(-1)(energy density of 764.5 W h kg(-1)). After 297 continuous cycle tests (99 h), the rechargeable batteries using Fe@C-NG/NCNTs show a voltage gap increase of only 0.13 V, almost half that of Pt/C + Ir/C (0.22 V) under the same conditions. This work provides new insight into advanced electrocatalysts utilizing the structural features of host nanocarbon materials and guest active species toward energy conversion.