In-situ formation of N doped hollow graphene Nanospheres/CNTs architecture with encapsulated Fe3C@C nanoparticles as efficient bifunctional oxygen electrocatalysts

The exploration of cost-effective and efficient bifunctional catalysts for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is currently a critical obstacle in the development of regenerative fuel cells and rechargeable metal-air batteries. In this reported study, we detail the synthesis of an N-doped hollow graphene nanospheres/CNTs architecture with encapsulated Fe3C@C nanoparticles (Fe3C@C-NGns-NCNTs) via a facile, in situ fabrication method. The resulting catalyst had a high specific surface area (450 m2 g−1) with a hierarchical meso-macroporous structure. In an alkaline electrolyte, the catalyst manifested remarkable bifunctional catalytic activity with a half-wave potential for ORR of ca. 23 mV that was higher than that produced by a commercial Pt/C and the former had a small overpotential of 412 mV at a current density of 10 mA cm−2 for the OER. The ΔE (oxygen electrode activity parameter) value for the Fe3C@C-NGns-NCNTs was only 0.787 V. We posit that the high electrocatalytic performance of this catalyst was due to the following factors: (1) a synergistic effect, induced by the presence of multiple types of active sites, including Fe3C@C nanoparticle, nitrogen dopant and possible Fe-Nx-C sites; (2) the unique hollow graphene nanospheres/CNTs architecture, which facilitated the adsorption/diffusion of reactants/products and rapid electron transfer; and (3) the high degree of graphitization of the catalyst that improved the electrocatalytic stability.