In the above calculation of TOFs for NF-AC-NiOx-Fe and NF-AC-FD-NiOx-Fe, the NiOx component was treated as a support for the active Fe centers. For this assumption to be valid, the TOFs should be independent of the quantity of NiOx. To verify this assumption, thin layers of NiOx were first electrodeposited on Au and glassy carbon (GC) electrodes, followed by iron-incorporation using the same method as for the synthesis of NF-AC-NiOx-Fe. On these two electrodes, the quantity of NiOx could be varied and measured. The activity of the resulting catalysts, Au-NiOx-Fe and GC-NiOx-Fe, were measured by LSV (Supplementary Fig. ). Notwithstanding a small difference, both Au-NiOx-Fe and GC-NiOx-Fe exhibit activities and TOFs (Table , Supplementary Fig. and Supplementary Tables ) similar to NF-AC-NiOx-Fe. The small difference is likely due to the electrical contact between the NiOx film and the electrodes rather than the intrinsic activity of the catalyst. The TOFs of Au-NiOx-Fe and GC-NiOx-Fe are largely independent of the quantity of NiOx (Supplementary Fig. ). These data validate the treatment of NiOx as a support in the calculation of TOFs of our catalysts and show that it outperforms all alkaline OER catalysts reported so far.