The NF-AC-NiOx-Fe electrodes exhibit excellent OER activity (Fig. and Supplementary Fig. ). To reach 10 mA cm -2 , a NF-AC-NiOx-Fe electrode (loading of iron oxide: 14.1 g cm -2 ) requires an overpotential of 245 mV, lower than NF-NiOx-Fe (266 mV) and NF (311 mV). The Tafel slopes are 34, 36, and 45 mV dec -1 for NF-AC-NiOx-Fe, NF-NiOx-Fe, and NF, respectively (Supplementary Fig. ). The activity of the NF-AC-NiOx-Fe electrode is stable: the overpotential for a current density of 10 mA cm -2 remained at 245 mV during an electrolysis of 18 hours (Fig. ). When NF-AC was subjected to 100 CV scans in a 1 M KOH solution that was stripped of Fe ions by sequestration with an excess of nickel hydroxides for 12 h (the Fe concentration is below the detection limit of ICP-MS after the treatment), the resulting Fe-free NF-AC-NiOx exhibited much lower OER activity (Supplementary Fig. ). Thus, Fe incorporation was essential for the high OER activity of the NF-AC-NiOx-Fe and NF-NiOx-Fe electrodes.