Microwave-Assisted Solvothermal One-Pot Synthesis of RuO₂ Nanoparticles: First Insights of Its Activity Towards Oxygen and Chlorine Evolution Reactions

A microwave-assisted solvothermal method is proposed as a rapid and low-cost fabrication procedure of RuO₂ nanoparticles using citric acid as stabilizing agent in ethylene glycol, and H₂O₂ as oxidizing agent at 180 °C. Structural and morphological characterizations of the powder are investigated using X-Ray diffraction (XRD), scanning electronic microscopy (SEM) and transmission electron microscopy (TEM). XRD analysis shows that there is a synergic effect to obtain RuO₂ nanoparticles when 6% v/v H₂O₂ is added followed by a calcination of the powder, due to reactive oxygen intermediates (¹O₂, HO⁻ ₂, HO^• ₂, ^•OH) generated in the first stage. TEM and SEM images of as-synthesized material exhibit an uniform particle distribution (PSD) of RuO₂ nanoparticles with averaged diameter of ca. 38 nm, and its indexing indicates the RuO₂ stoichiometry with high degree of crystallinity. The preliminary electrocatalytic performance of RuO₂ nanoparticles coated on Ti is investigated using voltammetry and electrochemical impedance spectroscopy (EIS), towards the oxygen (OER) and chlorine evolution reaction (CER) in 1 mol L⁻¹ H₂SO₄ and 0.1 mol L⁻¹ NaCl, respectively. Cyclic voltammograms of RuO₂ display typical behaviors for OER and CER at 1.25 and 1.1 V vs Ag/AgCl, respectively. A Tafel slope of 44 mV dec⁻¹ corrected for ohmic drop was obtained in sulfate media which confirmed the OER mechanism in the absence of chlorides; while this parameter is around 25 mV dec⁻¹ in NaCl (CER), strongly depending on the chloride concentration and the extent of OER input. In NaCl media, the energy barriers represented by the charge transfer resistances for CER are overcome at more positive potentials than for OER, and a greater activity of the O₂ evolution is observed as the potential became more positive.