High‐Coverage Nanometric MO x (M = Ir, Rh) Clusters Anchored on Spinel Co ₃ O ₄ Matrix With Amorphous/Crystalline Heterostructure Harness Efficient Water Electrolysis

Architecting nanometric cluster electrocatalysts with high density of active atoms offers a valuable avenue to simultaneously augment atomic utilization efficiency and catalytic stability. Herein, nanometric IrO x and RhO x clusters are anchored onto the spinel Co 3 O 4 framework (CoMO x , M = Ir, Rh), yielding a unique amorphous/crystalline heterostructure. The optimized CoMO x catalysts with precise‐tuning calcination treatment present a favorable electrocatalytic activity with low OER (CoIrO x : 235 mV @ 10 mA cm −2 ) and HER (CoRhO x : 157 mV @ 10 mA cm −2 ) overpotentials. Operando electrochemical impedance spectroscopy in combination with density functional theory calculations reveal that the strong interfacial electronic coupling between nanometric clusters and the spinel Co 3 O 4 matrix enhances the electron density near the Fermi level and upshifts the d‐band center, thereby facilitating hydrogen release during the HER and lowering the OER rate‐determining step (OH*→O*) free energy.