Figure shows the Fe K-edge X-ray absorption near edge spectra (XANES) of NF-AC-NiOx-Fe, together with iron oxide references . The oxidation state of as-prepared NF-AC-NiOx-Fe is close to the value of +3, since the main absorption edge position coincides with that of -Fe(3+)OOH. Spinel iron oxides (maghemite and magnetite) and hematite contain both octahedral and tetrahedral sites. Tetrahedral iron sites have a well-defined signature in Fe Kpre-edge with an intense peak at low energies (ca. 7114 eV) . From the pre-edge intensity and position (inset of Fig. ), the tetrahedrally coordinated Fe ions can therefore be ruled out. This result is confirmed by Extended X-ray Absorption Fine Structure (EXAFS) spectra at the Fe K-edge (as depicted in Fig. ), which further describes the local geometry of Fe. The first peak at apparent distances 1.5 Å and the second and third peaks at 2.5 and 3.1 Å, are attributed to the single scattering path of the closest oxygen (that is, Fe-O) and the second/third neighboring iron metals (that is, Fe-Fe) surrounding the absorbing Fe ions , respectively. These results clearly reveal the Fe-Fe bonds with octahedrally coordinated Fe ions in NF-AC-NiOx-Fe, which are more similar to those of -FeOOH or NiFe-LDH and Fe-doped -NiOOH , as opposed to hematite or spinel structures. Most interestingly, a new peak (Fe-Nioutside) was observed at 3.98 Å (Fig. , Supplementary Table and Supplementary Fig. -Fig. ); this value is significantly larger than those of both Fe-Fe(Td) and Fe-Fe(Oh) in hematite or spinel structures, as well as that of Fe-Ni in NiFe LDH (~3.1 Å), indicating that there is a Fe-Ni path with a specific long distance in NF-AC-NiOx-Fe (~4.0 Å). This specific path is attributed to an interfacial interaction between the octahedrally oxygen-coordinated Fe and the underlying closest NiOx through an oxygen bridge. Notably, the coordination number (CN) of Fe-Fe is remarkably smaller than those of Lepidocrocite (-FeOOH) or NiFe-LDH. This result reveals small size clusters (~1-2 nm), where the Fe ions at the edges have fewer Fe-Fe interactions than in the center (Supplementary Table and Supplementary Fig. ), thus decreasing the overall mean Fe-Fe coordination numbers. Accordingly, the NF-AC-NiOx-Fe catalyst can be described as discrete nanoclusters of -FeOOH covalently linked to the NiOx support (with a -NiOOH structure under OER, see Raman data above and XAS data below) via bridging oxygens (Fig. ). The lack of Fe-Ni path at about 3.1 Å as found in NiFe LDH (Supplementary Fig. ) indicates that the FeO6 octahedron and the NiO6 octahedron are not edge-sharing as in NiFe LDH, but corner-sharing which leads to a Ni-O-Fe distance of about 4 Å. Therefore, the interface between the FeOOH clusters and the -NiOOH support occurs at the edge, but not terrace, sites of -NiOOH (Fig. ).