context: According to the SEM images in Fig. 4c and S16,† the basic skeleton of FeCO3@IF after OER remains almost the same compared with the electrode before OER. However, the FeCO3 cubes are covered with lots of nanosheets, which grow vertically on the cubes and are connected to each other. According to Fig. S17,† it is easy to observe the formation of a nanosheet array on the surface of the area of FeCO3@IF without FeCO3 cubes, which is the same as the surface morphology of the FeCO3 cubes after the OER process (Fig. 4c). The TEM image in Fig. 4d further confirms the formation of nanosheets. The blurry lattice fringes in the HRTEM image (the inset in Fig. 4d) suggest that the newly generated iron oxo/hydroxides have a high degree of amorphization. Therefore, the generated iron-based oxo/hydroxides are not observed in the XRD pattern (Fig. 4a). Notably, the high degree of amorphization is beneficial for the exposure of more catalytic sites and accelerating mass transfer. Furthermore, the phases with a high degree of amorphization also possess abundant unsaturated electronic configurations, which may improve the adsorption of reactants. The amorphous iron oxo/hydroxides are efficient catalytic sites for OER. More importantly, the vertical nanosheet arrays can promote the penetration of the electrolyte and bubble diffusion compared with cumulate nanoparticles and interlaced nanowires (Fig. S18†). The in situ generated interconnected nanosheets also have a strong interaction with the IF substrate, which is beneficial for maintaining good structural stability and high electron transfer efficiency. According to the SEM and XRD results, FeCO3 does not completely transform into oxo/hydroxide nanosheets, because of diffusional and electrochemical limitations. The partial surface in situ self-reconstruction causes FeCO3@IF to obtain novel and stable hierarchical structures during the OER process (Fig. 4e), with improved mechanical strength and rich diffusion pathways. Therefore, FeCO3@IF shows high OER catalytic activity and stability.