Fig. 1.

Steady-state J–V performance for each electrocatalyst. Steady state J–V curves at pH 13 (0.1 M NaOH aqueous solution) for FeOOH (orange), FeOOHNiOOH (green) and Ni(Fe)OOH (grey). These were recorded by measuring the steady-state current at different applied potential under atmospheric conditions. The FeOOH and Ni(Fe)OOH films are ~80 nm thick, while the FeOOHNiOOH film is ~120 nm thick (Supplementary Figs. 1 and 2). The overpotential is estimated as: η = E_RHE−1.23 V and was corrected by the iR drop estimated by electrochemical impedance spectroscopy (EIS) measurements. Such J–V behaviour is typical of that reported previously for analogous Fe/Ni oxyhydroxide electrodes^19,28. Arrows indicate the onset of the catalysis, which has been approximated to the potential at which the current reaches 20 µA cm⁻² Inset: Schematic representation of the different samples studied in this work. Orange represents an Fe-based group and green a Ni-based group