Activation of a Ni electrocatalyst through spontaneous transformation of nickel sulfide to nickel hydroxide in an oxygen evolution reaction

•Electro-oxidation treatment on Ni-sulfide/Ni foam highly improves its OER activity.•Sulfurization of Ni facilitates the formation of Ni-(oxy)hydroxide layer on the surface.•Ni3S2 derived amorphous/crystalline Ni-(oxy)hydroxide mixtures with Fe are active for OER.•Sulfur in Ni3S2 activates to form nanoparticle embedded layered structures in amorphous matrix.