Molten salt boiling synthesis of surface decorated bimetallic-nitrogen doped carbon hollow nanospheres: An oxygen reduction catalyst with dense active sites and high stability

Previous studies have shown that introducing of an extra transition metal into the monometallic-nitrogen doped carbon (mono-MNC) could benefit oxygen adsorption, further weaken the O-O bond energy, and thus promote the electrocatalytic performance for oxygen reduction reaction (ORR). Up to now, the bimetallic-nitrogen doped carbons (bi-MNC) were mostly prepared by direct pyrolysis of in-situ mixed precursors or pre-formed bimetal-organic frameworks (bi-MOFs), sub-micro sized products with encapsulated metal particles were commonly formed, leading to lower utilization of the metal atoms and less active sites density of the catalysts. In this paper, surface anchored iron-cobalt-nitrogen doped carbon (FeCoNC) hollow nanospheres were properly designed by boiling the monodispersed polypyrrole nanospheres in Fe, Co ions involved molten salt. Even a trace amount of Fe, Co atoms were anchored on the nitrogen doped carbon hollow nanospheres, the onset potential, half-wave potential and limiting current density could reach to 0.950 V, 0.841 V and 6.898 mA/cm(2) respectively at low mass loading of 0.2 mg/cm(2), demonstrating high utilization of the metal atoms and dense active sites of the catalyst. The Zn-air battery and direct ethanol fuel cell tests also revealed that the FeCoNC hollow nanospheres could present high stability in both non-oxidizing and oxidizing alkaline conditions. The surface decoration of bimetallic atoms on nitrogen doped carbon nanospheres through molten salt boiling method provides an effective way to prepare superior electrocatalysts for ORR.