A series of Ni 1−x Fe x hydrous oxide nanoparticles (HO-nps, x = 0−0.5, 1) were prepared by stirring 0.1 M KOH solutions of NiCl 2 and FeCl 2 under air at room temperature. The resulting HO-nps can be used directly, or washed and isolated without loss of activity. The Ni 1−x Fe x HO-nps are outstandingly active and stable toward the water oxidation reaction (WOR) in base. The Ni 0.75 Fe 0.25 HO-np was the most active mass-normalized WOR catalyst in the series, with onset overpotential = 190 mV,

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... potential = 190 mV, Tafel slope = 24 mV dec −1 , overpotential at 10 mA cm −2 = 234 mV, and loadings Ni + Fe = 0.135 mg cm −2 . The catalysts were robust under long-term galvanostatic and dutycycle testing. The Ni 0.75 Fe 0.25 HO-np catalyst has a layered double hydroxide structure, and it appears that Fe enhances the activity of Ni toward the WOR by decreasing its electron density.