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

Abstract 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.