To probe the changes of the structures of the catalytic sites during activation and catalysis, operando X-ray absorption spectroscopy (XAS) was applied to both the as-prepared material and the activated catalyst under OER conditions. Fig. shows the operando Co K-edge X-ray absorption near edge spectra (XANES) of the as-prepared Co-N-C, the activated Co-Fe-N-C, and with various OER duration, together with reference samples of cobalt oxides and CoOOH. The spectroscopic features of Co-N-C and Co-Fe-N-C are significantly different from those of Co foil, all cobalt oxides and CoOOH. The oxidation state of Co ion in Co-N-C is close to +2, since the energy of main absorption edge coincides with that of Co(2+)O and is slightly lower than that of Co3(8/3+)O4 (Fig. ). This result is consistent with the observation from the XPS study (see above). After activation, the absorption edge increased, indicating the formation of Co(3+). The structural evolution and the formation of Co-Fe double-atomic catalyst are further revealed by operando Extended X-ray Absorption Fine Structure (EXAFS) spectra at the Co K-edge (Fig. and, Table and). The first coordinated shell ranging from 1.5-2 Å is attributed to the single scattering paths of closest C, N, and O, while the second coordinated shell with a range from 2-3 Å results from the single scattering paths of outer neighboring C and metal (i.e., Fe) surrounding the absorbing Co ions. In the dry sample of Co-N-C, the Co ions are five-coordinated and are surrounded by about three N, one C, and one O atoms (Fig. and Fig. ). The O atom likely came from an absorbed hydroxyl or water group. Once the Co-N-C sample is in contact with the electrolyte (1M KOH), two of the coordinated N atoms, or one N and one C atom, were replaced by three new O atoms, as indicated by the extracted structural parameters (Fig. and Table ). A proposed structural model is shown in Fig. , where the coordinated oxygen groups are assigned as OH-and H2O from the electrolyte. Apparent distances of all paths are slightly shorter than those of the as-prepared Co-N-C (dry powder), which might be ascribed to the oxidation of Co ions. Most interestingly, after activation and Fe incorporation a new scattering path (Co-Fe) was observed at 2.51 Å with a CN value of ~ 0.25. This apparent distance is significantly shorter than that of Co-Fe in spinel oxide and slightly larger than the atomic radius of Co(0)-Fe(0) , indicating a newly formed Co-Fe interaction with a specific geometry. A structural model is shown in Fig. and. The CN value of 0.25 suggests only one-fourth Co ions interact with Fe ions in Co-Fe-N-C.