Topologically Close-Packed Frank-Kasper C15 Phase Intermetallic Ir Alloy Electrocatalysts Enables High-Performance Proton Exchange Membrane Water Electrolyzer.

Chemical synthesis of unconventional topologically close-packed intermetallic nanocrystals (NCs) remains a considerable challenge due to the limitation of large volume asymmetry between the components. Here, a series of unconventional intermetallic Frank-Kasper C15 phase Ir ₂ M (M = rare earth metals La, Ce, Gd, Tb, Tm) NCs is successfully prepared via a molten-salt assisted reduction method as efficient electrocatalysts for hydrogen evolution reaction (HER). Compared to the disordered counterpart (A1-Ir ₂ Ce), C15-Ir ₂ Ce features higher Ir-Ce coordination number that leads to an electron-rich environment for Ir sites. The C15-Ir ₂ Ce catalyst exhibits excellent and pH-universal HER activity and requires only 9, 16, and 27 mV overpotentials to attain 10 mA cm ^-2 in acidic, alkaline, and neutral electrolytes, respectively, representing one of the best HER electrocatalysts ever reported. In a proton exchange membrane water electrolyzer, the C15-Ir ₂ Ce cathode achieves an industrial-scale current density of 1 A cm ^-2 with a remarkably low cell voltage of 1.7 V at 80 °C and can operate stably for 1000 h with a sluggish voltage decay rate of 50 µV h ^-1 . Theoretical investigations reveal that the electron-rich Ir sites intensify the polarization of *H ₂ O intermediate on C15-Ir ₂ Ce, thus lowering the energy barrier of the water dissociation and facilitating the HER kinetics.
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