The morphology and microstructure of the as-prepared catalysts were characterized by TEM analysis. Fig. 2 a shows the overall morphology of Fe3O4@FeNC. Nanoparticles with a diameter of tens to hundreds nanometers were embedded in the 3D-interconnected carbon architecture, which was beneficial for the charge transfer and exposure of active sites [31]. Spherical macro-pores with diameters of ∼200 nm were distributed throughout the samples, derived from SiO2 templates (Fig. 2b). Small meso- and micro-pores were also observed on the carbon support (Fig. 2c), resulted from the evaporation of ZnCl2 and the decomposition of PAN during pyrolysis. The hierarchically macro- and meso-/micro-pores were expected to facilitate the mass diffusion and provide active sites during ORR catalytic process, respectively [32]. Fig. 2d shows the HRTEM image of a selected nanoparticle. The distance for the adjacent lattice fringes was measured to be 2.08 Å, attributed to the (400) plane of the Fe3O4 crystal. Further validation of the composition of the nanoparticles will be discussed in the XRD section. The nanoparticles were tightly surrounded by onion ring-like carbon multi-layers (Fig. S1a), which might protect them against HF washing. The layer distance of the surrounding carbon was measured as ∼3.50 Å, indexed to the (002) plane of graphitized carbon. Fig. 2e shows the high resolution AC HAADF-STEM image of Fe3O4@FeNC. The white dots throughout the carbon support were denoted as the single Fe atoms, which had brighter contrast due to the heavier atomic weight [33,34]. The elemental mappings in Fig. 2f also reveals the even distribution of Fe, N and C in the carbon skeleton, where Zn from the precursor was not detected due the low boiling point of Zn (∼907 °C) compared with the relatively high calcination temperature (Fig. S2) [35,36].This work was supported by National Natural Science Foundation of China (Grant No. 51772089, 21872046 and 51902100), the Youth 1000 Talent Program of China, the Outstanding Youth Scientist Foundation of Hunan Province (Grant No. 2018JJ1009), Provincial Science and Technology Innovation Platform and Talent Plan - Changsha, Zhuzhou and Xiangtan High-level Talents Accumulation Project (Grant No. 2017XK2023), the Youth Scientist Foundation of Hunan Province (Grant No. 2019JJ50087) and the Research and Development Plan of Key Areas in Hunan Province (Grant No. 2019GK2235).
