Entropy‐Modified Ceria Regulate Oxygen Vacancy and Manipulate the Electronic Structure of Ruthenium Toward Efficient Hydrogen Evolution

Rare earth (RE) oxides are widely applied in electrocatalysis, yet their intrinsic catalytic activity toward the hydrogen evolution reaction (HER) remains limited due to unfavorable hydrogen intermediate adsorption. Herein, a novel heterogeneous electrocatalyst comprising ruthenium (Ru) nanoparticles coupled with medium‐entropy fluorite‐type oxide (CeNdSmGdEr)O x (MEO x ) on the nitrogen‐doped carbon (Ru/MEO x /NC) is developed. The prepared Ru/MEO x /NC showed superior HER activity in both alkaline and acidic media, achieving a current density of 10 mA cm −2 with overpotentials of merely 18 and 58 mV, respectively. Furthermore, using Ru/MEO x /NC as cathode catalyst, the anion‐exchange membrane water electrolyzer can achieve 1 A cm −2 at 2.17 V and can be operated at 0.7 A cm −2 for 1000 h. The introduction of multiple elements into CeO 2 lattice facilitates the generation of abundant oxygen vacancies, which act as active sites for accelerating H 2 O adsorption and dissociation. The electronic configuration of interfacial Ru sites is tailored by MEO x , resulting in favorable hydrogen adsorption free energy. Furthermore, the established Ru−O−RE bridge facilitates electron transfer from Ru to MEO x , thereby optimizing the interfacial charge transfer kinetics. This work elucidates entropy engineering to regulate the electronic structure of active sites, offering a viable strategy for designing entropy‐controlled electrocatalysts toward electrochemical HER.