context: As seen from the surface morphology images of these three electrodes (Fig. 2 and S5†), they have their own characteristics and are all totally different from the original IF (Fig. S6†). As shown in Fig. 2a, there are abundant FeCO3 cubes (8–15 μm in edge length) on the surface of the FeCO3@IF electrode. The TEM images (Fig. 2b) show that the width of the crystal lattice is about 0.278 nm, matching the (104) face of FeCO3, which confirms the formation of FeCO3 and coincides with the results in the XRD patterns and XPS spectra. According to the SEM images in Fig. 2a, S5a and b,† the FeCO3 cubes do not completely cover the surface of the IF. As shown in Fig. 2c, the surface of the area without FeCO3 cubes is rough and is clearly different from the smooth surface of IF (Fig. S6†). According to the distribution of elements (C, O and Fe) in the area without FeCO3 cubes, C, O and Fe are uniformly distributed (Fig. S7b–d†). The EDS data (Fig. S7e†) confirm that the C, O and Fe content (wt%) is 8.63%, 35.58% and 55.79%, respectively. Therefore, the atomic ratio of C, O and Fe is 1.00:3.09:1.38, which is close to the atomic ratio of FeCO3. The slightly high iron content may be due to the iron foam substrate. These results confirm that the area without FeCO3 cubes is also covered by FeCO3 compounds. In addition, the TEM results in Fig. 2 further show that aside from the FeCO3 cubes with high crystallinity according to their long-range ordered crystal structure (Fig. 2b), some compounds with short-range ordered and long-range disordered crystal structures are also observed (Fig. 2d). The widths of the crystal lattices are about 0.278 nm and 0.234 nm, which match the (104) and (110) faces of FeCO3, respectively. Therefore, the compounds on the surface of IF without FeCO3 cubes are mainly FeCO3 but with low crystallinity according to the EDS and TEM results. Unlike FeCO3@IF, Fe3O4@IF is covered with tightly packed Fe3O4 nanoparticles (Fig. 2c, S5c and d†). The morphology of Fe–O–M@IF is similar to that of FeCO3@IF, but many long and interlaced nanowires are distributed around the cubes. Moreover, the edge length of the cubes is 2–3 times that of FeCO3@IF, and the surface is uneven. Compared with the original IF with its smooth surface, the increase in roughness can improve the specific surface area (Fig. S8 and Table S2†) and promote the penetration of the electrolyte as well as mass transfer. Based on the above characterizations, three different 3D iron-based electrodes have been successfully prepared via changing the iron source.