The renewable production of green hydrogen powered by water electrolysis will be an important step in the electrification of the chemical industry. However, to make water-splitting more sustainable and practical, earth-abundant catalysts need to be developed, which can both be synthesized using the principles of green chemistry and have high performance specifically at high hydrogen production rates. In this work, we report four main findings to help contribute toward this goal. First, we report a "green" synthesis method for producing a mixed-metal oxide catalyst that uses only water as the solvent and no harsh oxidizing or reducing agents. Second, we show that this synthesis method can enable an amorphous nickel−iron oxide/(oxy)hydroxide catalyst with a 1:1 Fe/ Ni ratio. This increased iron content further improves the performance over the conventional 1:4 Fe/Ni ratio. Third, we show that these catalysts can be easily deposited on a 3D porous Ni-foam electrode and achieve current densities up to 1 A cm −2 and an overpotential of 245 mV at 100 mA cm −2 for oxygen evolution reaction (OER) and an overpotential of 422 mV at 100 mA cm −2 for hydrogen evolution reaction (HER). Finally, we show that combining both HER and OER catalysts, synthesized with our method, in a flow-through water electrolyzer achieves an overpotential of 140 mV at 100 mA cm −2 at 80 °C. In addition, this electrolyzer can achieve 76% efficiency at 1 A cm −2 and 70% efficiency at 2 A cm −2 .