Synthesis of Asymmetric Multidentate Azo-Based Redox-Active Ligand and Its Coordination Compounds with Ruthenium(II) for the Development of Efficient Electrocatalysts for Enhanced Hydrogen Evolution Reaction

This research focuses on the strategic design of efficient Ru-based electrocatalysts for hydrogen evolution reactions in an acidic medium. An asymmetric azo ligand was synthesized and used as a building block to prepare two Ru(II) compounds by varying the metal:ligand ratio. All of the newly developed compounds were characterized through different characterization techniques to confirm their structures and to understand the structure–property relationship. The newly synthesized azo-based ligand and both Ru(II) compounds demonstrated reversible electrochemical properties, which are prime requirements for developing a new electrocatalyst. The electrocatalytic performance of the Ru(II) Complex and Ru(II) Polymer for the hydrogen evolution reaction (HER) was evaluated using linear sweep voltammetry (LSV), potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and chronopotentiometry in a 0.5 M aqueous H2SO4 solution. It is possible to vary the metal:ligand ratio to produce a Ru(II) Complex (L:M::1:0.5), where a single Ru(II) center is coordinated to two ligands and a Ru(II) Polymer (L:M::1:3), where the ditopic ligand is working as a bridge between 2/3 different Ru(II) metal centers. This synthetic route was chosen to investigate the influence of bridging ligands and alterations in the coordination sphere surrounding the central metal ion as well as the impact of higher concentrations of the metal center on electrocatalytic performance, as the metal centers serve as active sites for the transfer of electrons to H+ ions in an acidic medium. The Ru(II) Polymer, with more metal centers than the Ru(II) complex, demonstrated a lower onset potential (−0.162 V vs −0.340 V), lower overpotential at 10 mA/cm2 (−0.202 V vs −0.435 V), and a smaller Tafel slope (60 vs 96 mV/dec), showing easier electron transport and a different HER mechanism. EIS experiments show that the Ru(II) Polymer has a lower HER electron transfer resistance than the Ru(II) Complex.