a, Schematic of the sol–gel synthesis method for the preparation of the multi-metallic (oxy)hydroxides. Photos reproduced with permission from ref. 2. b, Graphical representation of the metal chloride precursors used in synthesis and the resulting (oxy)hydroxides. c,d, Plots of the overpotentials measured at a current density of 10 mA cm–2 versus relative Fe2+ concentration obtained by Fe L-edge X-ray absorption spectroscopy (XAS) in total electron yield (TEY) mode for NiFe and FeCo (oxy)hydroxides, respectively, with high-valent dopants. Panels c and d adapted with permission from ref. 2.