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Synergetic interactions between ruthenium and molybdenum oxide weaken H adsorption on ruthenium active sites and hence enhance the electrocatalytic activity towards hydrogen evolution reaction. 

Ruthenium has emerged as a promising substitute for platinum toward the hydrogen evolution/oxidation reaction (HER/HOR). Herein, ruthenium/carbon composites are prepared by magnetic induction heating (300 A, 10 s) of RuCl₃, RuBr₃or RuI₃loaded on hollow N‐doped carbon cages (HNC). The HNC‐RuCl₃‐300A sample consists of Ru nanoparticles (dia. 1.96 nm) and abundant Cl residues. HNC‐RuBr₃‐300A possesses a larger nanoparticle size (≈19.36 nm) and lower content of Br residues. HNC‐RuI₃‐300A contains only bulk‐like Ru agglomerates with a minimal amount of I residues, due to reduced Ru‐halide bonding interactions. Among these, HNC‐RuCl₃‐300A exhibits the best HER activity in alkaline media, with a low overpotential of only −26 mV to reach 10 mA cm⁻², even outperforming Pt/C, and can be used as the cathode catalyst for anion exchange membrane water electrolyzer (along with commercial RuO₂as the anode catalyst), producing 0.5 A cm⁻²at 1.88 V for up to 100 h, a performance markedly better than that with Pt/C. HNC‐RuCl₃‐300A also exhibits the best HOR activity, with a half‐wave potential (+18 mV) even lower than that of Pt/C (+35 mV). These activities are ascribed to the combined contributions of small Ru nanoparticles and Ru‐to‐halide charge transfer that weaken H adsorption.