High entropy oxides (HEOs) for chlorine evolution reaction

Abstract

This thesis reports the preparation and utilization of high entropy oxides in chlorine evolution reaction (CER) to increase the catalytic activity, chlorine selectivity and stability against corrosion in both concentrated and dilute chloride solutions. Single-phase, nanosized rutile structured high entropy oxides (HEO-R) containing five elements (Ru, Sn, Ti, V and Co) has been successfully synthesized through a one-step solvothermal method (180 °C), followed by calcination (350 °C - 700 °C) in air atmosphere. The synthesized material denoted as HEO-R-500 (calcination temperature=500 °C) displayed a single-phase rutile structure with affluent oxygen vacancies. The catalysts were coated on a carbon substrate and its activity was tested in acidic 5 M NaCl (pH=2) and dilute 0.5 M NaCl (pH=6) solutions. As a result, the HEO-R-500 delivered a high current density of 50 mA cm-2 with a lower overpotential of 47 mV. In contrast, the commercial RuO2-TiO2 dimensionally stable anodes (DSA) required about 3 times higher overpotential value under the same testing conditions. The HEO-R-500 also exhibited a smaller Tafel value of 78.09 mV dec-1 in 5 M NaCl solution (pH=2) and high chlorine selectivity in both acidic and dilute chloride solutions. The electrode delivered an excellent stability of 100 hours with a negligible increase in potential (0.9%) during the chronopotentiometry test at a current density (jgeo) of 50 mA cm-2. Post-linear sweep voltammetry (LSV) tests revealed that there is no difference in the catalytic activity before and after the stability test. Overall, the HEO-R shows good electrocatalytic performance as anode in chlorine evolution reaction. The high catalytic activity and excellent stability are rooted in the confinement of five elements together in a single crystal lattice of the HEO.