The NiOx-Fe catalyst was subjected to a variety of characterization methods. X-ray photoelectron spectroscopy (XPS) data are consistent with the presence of iron oxide on the surface of the electrode (Supplementary Fig. ) . In the Raman spectra, the peaks corresponding to -NiOOH, initially absent in the catalyst before OER, emerged after subjecting the catalyst to OER conditions (Supplementary Fig. ). This lamellar structure has already been shown to be the active phase in nickel containing OER catalysts under oxidative potentials. No peak from an iron oxide species was, however, observed in the Raman spectra, probably because of the low iron concentration. To obtain further structural information on the catalyst, X-ray absorption spectroscopy (XAS) was applied, both on the post-catalytic material and under operating conditions.