Efficient oxygen reduction on sandwich-like metal@N-C composites with ultrafine Fe nanoparticles embedded in N-doped carbon nanotubes grafted on graphene sheets.

In the past decade, tremendous efforts have been devoted to the search for the alternatives to Pt-based catalysts for the oxygen reduction reaction (ORR) in fuel cells and metal-air batteries. Recently, metal-nitrogen-carbon (M-N-C) systems, especially 3d transition metals (TM) and their alloys encapsulated in nitrogen-doped carbon based materials (TM@N-C), have attracted increasing attention due to their low cost and high ORR activity. Here, a simple and novel strategy is developed to synthesize sandwich-structured TM@N-C composites, in which ultrafine Fe nanoparticles are encapsulated in nitrogen-doped carbon nanotubes (N-CNTs) grafted on both sides of reduced graphene oxide (rGO) sheets by pyrolysis of ammonium ferric citrate-functionalized zeolitic imidazolate framework-8@graphene oxide (Fe@ZIF-8@GO). The resulting Fe@N-CNTs@rGO composites naturally integrate zero-dimensional (0D) Fe nanoparticles, one-dimensional (1D) N-CNTs, and two-dimensional (2D) graphene into a three-dimensional (3D) hierarchical architecture with highly dispersed active sites, a large surface area, and abundant porosity. Because of these structural advantages, the sandwich-structured Fe@N-CNTs@rGO composites display a half-wave potential of 0.83 V in a 0.1 M KOH solution for the ORR, comparable to that of commercial Pt/C catalysts, and more excellent durability and resistance to fuel molecules. The proposed strategy paves a new way for the synthesis of non-precious high-performance electrocatalysts for energy conversion applications.