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• Ir@N-G-600 was prepared by supporting Ir nanoparticles on N-doped graphene. • It demonstrates enhanced HER/OER activity over its N-doping-free counterpart. • Strong coordination between Ir and N dopants enriches the electron density of Ir. • It achieves stable 10 mA cm−2 water electrolysis at cell voltage below 1.6 V. Heteroatom-doped carbon is a type of promising support for catalytically active metal nanoparticles as it offers abundant anchoring sites to facilitate the nucleation and growth of the nanoparticles, and to strengthen their attachment and more importantly, to enhance their intrinsic catalytic activity via support-metal electronic interaction. In this work, graphene sheets were doped with abundant N atoms via radio-frequency N 2 plasma process and used as unique support to load ultrasmall iridium nanoparticles. As-synthesized Ir@N-G-600 shows excellent activity and stability toward HER and OER in both acidic and alkaline media. At much lower precious metal loading, it is superior to commercial Pt/C, RuO 2 and Ir/C catalysts in terms of HER and OER activity, respectively. As a bifunctional electrocatalyst, Ir@N-G-600 enables efficient overall water splitting in acid and alkaline solutions. The enhanced electrocatalytic activity of Ir@N-G-600 is found to be tightly associated with the strong coordination between Ir atoms and N dopants, which enriches the electron density of Ir atoms to form an optimal electronic configuration for HER and OER processes. [ABSTRACT FROM AUTHOR]