Highly efficient proton exchange membrane electrolyzer in acidic media is of great importance for the hydrogen production from the electrochemical water splitting. Unfortunately, the electrochemical water splitting is limited by the slow oxygen evolution reaction kinetics at anode. So far, the synthesis of active and stable electrocatalysts in acid media is still a challenging work. In the work, carbon nanobowls supported ultrafine iridium nanocrystals are synthesized by a simple complexation-reduction method with assistance of 1-hydroxyethane-1, 1-diphosphonic acid, which effectively serves as as complexant, capping agent and surfactant during the synthesis. The good dispersion and ultrafine size of Ir nanocrystals, the modified interfacial property from phosphonate with excellent hydrophilicity, as well as carbon nanobowls as advanced carbon supports contribute to their excellent electrocatalytic activity for the oxygen evolution reaction in acid media. The as-prepared carbon nanobowls supported ultrafine iridium nanocrystals present an ultra-low overpotential of 290 mV to achieve the mass activity of 1000 A g(-1) and a small Tafel slope of 49.1 mV dec(-1), which significantly outperform commercial Ir/C electrocatalyst. The remarkable activity and durability make carbon nanobowls supported ultrafine iridium nanocrystals as a potential candidate for the oxygen evolution reaction electrocatalyst in proton exchange membrane water electrolyzer. (C) 2018 Elsevier Inc. All rights reserved.