Ruthenium/Carbon Nanocomposites for Efficient Hydrogen Electrocatalysis: Impacts of Halide Residues

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, RuBr₃ or RuI₃ loaded 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 RuO₂ as the anode catalyst), producing 0.5 A cm^{- 2} at 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.