By using commercial KOH solution which contained only a trace amount of Fe ions, the maximum amount of Fe incorporation into NF-AC electrodes was limited to about 14 g cm -2 . Although this method of Fe incorporation seemed to be important for the optimal distribution of Fe active sites and the corresponding site-averaged activity (see below), the geometrically averaged activity might benefit from a higher loading of Fe. Such activity is relevant to practical applications, especially because Fe is cheap. To incorporate more Fe in NF-AC, we dipped the latter into a FeCl3 solution (0.01 M) for 15 minutes and then dried it in an oven at 70 °C. The resulting electrode, NF-AC-FD (FD = Fe dipping), has hierarchical nanoporous structures at the surface (Supplementary Fig. ). NF-AC-FD was subjected to 100 repetitive cyclic voltammetric (CV) scans from 1.21 to 1.53 V vs. RHE in a 1 M commercial KOH solution to yield the NF-AC-FD-NiOx-Fe electrode. This electrode has much better net activity than NF-AC-NiOx-Fe, reaching 10 mA cm -2 and 100 mA cm -2 at only 215 mV and 248 mV (Fig. ), respectively. The activity is stable for 36 h at least(Fig. ). a For multiple samples, the averaged values with standard deviations are given. For NF-AC-NiOx-Fe, Au-NiOx-Fe, and GC-NiOx-Fe, each sample is measured 2-3 times and the raw data are shown in Supplementary Table -3. For all iron-containing samples except G-FeCoW, iron is assumed as the active species; for G-FeCoW, Co is taken as the active species. n/a is the abbreviation for the phrase of not available due to lack of data or influence by mass transfer. b NiFeOx was prepared according to the method reported in literature (Supplementary Methods). Each sample is measured 2 times and the raw data are shown in Supplementary Table . c TOFs were taken from the samples on GC reported in literature . Js are taken from the continuously deposited film on Au reported in literature (calculation of Js see in Supplementary Methods). d TOFs are taken from literature and Js are taken from literature .