Design of high-efficiency electrocatalysts for the oxygen evolution reaction (OER) is critical to meet future energy demands. A series of heterogeneous electrocatalysts based on graphene coupled mixed-metal (oxy)hydroxides containing specific concentrations of nickel, cobalt, and iron were synthesized to investigate the influence of metal composition on the structure, properties, and activities for OER. The addition of iron led to isolated metal (oxy)hydroxide sheets from a few hundred nanometers to less than 5 nm, leading to a high surface area. The increased d-spacing between layers of hydroxide and abundant hydroxyl groups adsorbed on the surface of the electrocatalysts contribute to better catalytic performance as displayed by the remarkable improvement in Tafel slopes. In addition, the presence of cobalt and nickel can aid in lowering the onset potential of OER. Small amounts of iron in NiFe and NiCoFe systems were found to significantly decrease the overpotential. The best OER electrocatalysts of the series were obtained for the ternary metal hydroxide of composition Ni0.33Co0.34Fe0.33, which generates a low overpotential of 0.332 V in reaching 10 mA cm(-1) in 0.1 M KOH and the Tafel slope is as low as 51 mV dec(-1). Density functional theory calculations demonstrate the improvements of electronic structure and free energy changes of the ternary metal oxyhydroxide for more efficient OER electrocatalysis. Overall, this work provides a solid foundation for further development of high-efficiency, scalable, and cost-effective mixed-metal (oxy)hydroxide electrocatalysts. (C) 2019 Elsevier Inc. All rights reserved.