Three figures of merits including turnover frequency (TOF), surface area averaged specific activity, and overpotential for 10 mA cm -2 , are commonly used to compare OER catalysts . The first two parameters focus on the intrinsic activity of catalysts, while the third one is oriented towards device performance. These three parameters were determined for the catalyst reported here and compared to the best performing OER catalysts in alkaline solutions (Table , Supplementary Table ). It appears that this catalyst outperforms all other catalysts for at least 2 of these parameters and competes very well for all of them. As electrochemical, spectroscopic and computational data (see below) indicate that the active site of our catalyst is Fe, the apparent TOFs of the NF-AC-NiOx-Fe electrodes were calculated according to the total amount of Fe ions, as determined by ICP-MS (Supplementary Fig Supplementary Tables 1; Supplementary Methods). TOFs were calculated for 10 different electrodes with a Fe (Fe oxide) loading in the range of 1.4 to 14.1 g cm -2 , and were found to be similar. Table compares the averaged TOFs of NF-AC-NiOx-Fe with several state-of-theart catalysts, and Supplementary Table lists the comparison with an extended number of known catalysts. All TOFs were calculated according to the total amounts of active metal ions. NF-AC-NiOx-Fe and NF-AC-FD-NiOx-Fe have similar TOFs. Their TOFs are the highest among all solid-state catalysts reported to date. With an average TOF of 0.78 s -1 at  = 270 mV, NF-AC-NiOx-Fe is about an order of magnitude more active than state-of-the-art NiFeOx and NiFe LDH catalysts and more than 200 times higher than IrO2. Recently a gelled FeCoW oxyhydroxide (G-FeCoW) was reported to be the hitherto most active OER catalyst in alkaline solutions . NF-AC-NiOx-Fe has a TOF 3.6 times higher than G-FeCoW. To alleviate the uncertainty using literature values when comparing TOFs, direct comparison of NF-AC-NiOx-Fe with NiFeOx deposited on NF was conducted (Supplementary Table and). The TOFs of NF-AC-NiOx-Fe are again 10-13 times higher than those of NF-NiFeOx.