Another important parameter of catalytic activity is the overpotential for a given current density, e.g., 10 mA cm -2 (Supplementary Table ). In direct comparison and at similar loadings, the present NiOx-Fe catalyst has overpotentials of about 74-80 mV lower than NiFeOx (Fig. , and Supplementary Fig. ). A striking improvement of more than 160 mV in overpotential is obtained when comparing NF-AC-NiOx-Fe with IrO2, the benchmark noble metal catalyst, at a similar loading. A small number of high-surface-area electrodes coated with a large amount of catalysts are reported to have overpotentials close to 200 mV for 10 mA cm -2 , making them interesting for device performance (Supplementary Table ) . With an overpotential of only 215 mV, a stable activity, and an easy and economical preparation from earth abundant components, the NF-AC-FD-NiOx-Fe electrode described above is also very competitive in this category.