Amorphous nickel-based hydroxides with different cation substitutions have been hydrothermally synthesized in the presence of ethylene glycol, which provides a mild environment for the gradual hydrolysis of transition metal ions, giving rise to the formation of amorphous transition metal hydroxides. The amorphous hydroxides with rich grain boundaries as electroactive sites and ion diffusion channels for facilitating charge transfer, and thus display significantly improved specific capacity and enhanced rate capability in comparison with the crystalline counterparts. In addition, the substitution of Ni ions by different transition metal ions further improves the charge storage performance of the amorphous hydroxides. As a result, an amorphous NiCo–OH exhibits the best electrochemical performance when used as electroactive battery materials. The amorphous Ni–Co hydroxide (NiCo–OH) shows specific capacities of 888 and 662 C g−1 at 1 and 50 A g−1, respectively. The performance of NiCo–OH is better than that of the other amorphous nickel-based hydroxides, including Ni, Ni–Mn, Ni–Zn and Ni–Cu amorphous hydroxides. A hybrid supercapacitor based on amorphous NiCo–OH exhibits superior electrochemical performance with high specific power density, high specific energy density and ultralong cycling lifespan.