In-situ X-ray absorption spectroscopy was employed to reveal the structural evolution of the catalyst during OER (Fig. , Table and Supplementary Table ) . In order to collect Ni K-edge data, the catalysts deposited on Au-coated FTO (Au-NiOx-Fe) were used instead of NF-AC-NiOx-Fe due to the strong background Ni signal from NF. The Fe K-edge data of Au-NiOx-Fe are similar to those of NF-AC-NiOx-Fe, confirming the similar nature of catalysts on both supports. Prior to OER (when no bias or  = 0.22 V are applied), slight decreases in apparent distance are observed compared to the dry sample. These changes might be attributed to a specific interaction with the electrolyte. Once the applied voltage is further increased above  = 0.27 V, the apparent distance of Fe-Fe path is reduced by approximately 0.15 Å. This decrease in Fe-Fe path is due to the oxidation of Fe, as observed before in other systems . Likewise, in-situ Ni K-edge XAS (Fig. , Supplementary Fig. and Supplementary Table ) shows that the apparent distances of the Ni-Ni path is reduced by about 0.24-0.27 Å under OER potentials, indicating the oxidation of Ni ions. The potential dependent edge energies (Supplementary Fig. ) indicate the transformation of the NiOx support from -Ni(OH)2 to -NiOOH under OER conditions , in agreement with the result of the Raman study (see above).