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RuBr3 crystallizes in the hexagonal P6_3/mcm space group. The structure is one-dimensional and consists of one RuBr3 ribbon oriented in the (0, 0, 1) direction. Ru3+ is bonded to six equivalent Br1- atoms to form face-sharing RuBr6 octahedra. All Ru–Br bond lengths are 2.52 Å. Br1- is bonded in a 2-coordinate geometry to two equivalent Ru3+ atoms.
RuBr3 crystallizes in the orthorhombic Pmmn space group. The structure is one-dimensional and consists of two RuBr3 ribbons oriented in the (1, 0, 0) direction. Ru3+ is bonded to six Br1- atoms to form face-sharing RuBr6 octahedra. There are two shorter (2.51 Å) and four longer (2.52 Å) Ru–Br bond lengths. There are four inequivalent Br1- sites. In the first Br1- site, Br1- is bonded in a 2-coordinate geometry to two equivalent Ru3+ atoms. In the second Br1- site, Br1- is bonded in a 2-coordinate geometry to two equivalent Ru3+ atoms. In the third Br1- site, Br1- is bonded in a 2-coordinate geometry to two equivalent Ru3+ atoms. In the fourth Br1- site, Br1- is bonded in a 2-coordinate geometry to two equivalent Ru3+ atoms.
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.