ZnOx-modified IrO2–Ta2O5 electrocatalyst with high electrochemical chlorine evolution selectivity in a wide range of pH and electrolyte concentrations

Abstract

Chlorine evolution reaction (CER) is one of the most valuable electrochemical reactions in industrial applications, mainly used in the chlor-alkali industry, electrochemical advanced oxidation processes (EAOPs) and electrolytic antifouling technology (EAT). In these applications, the [Cl⁻] concentration and pH are different; therefore, it is of great significance to develop electrode materials with high CER selectivity suitable for a wide range of pH and varying [Cl⁻] concentrations. In this study, ZnO_x-modified IrO₂–Ta₂O₅ (ZnO_x@IrO₂–Ta₂O₅) was prepared by thermal decomposition combined with the oxygen-poor reactive sputtering method. Using the reactive sputtering technology, ZnO_x was successfully doped into the crystal cell of IrO₂–Ta₂O₅, which enlarged the unit cell to a certain extent and significantly changed its electronic structure. Compared with those of IrO₂–Ta₂O₅, the binding energy peaks of the Ir⁴⁺ 4f orbitals of ZnO_x@IrO₂–Ta₂O₅ were obviously shifted negatively, which indicated that the electron was transferred to Ir. Whether in 4.0 M (pH = 1.0) or 0.6 M (pH = 6.88) NaCl solution, the CER activity and selectivity of the ZnO_x@IrO₂–Ta₂O₅ electrode were obviously improved. In a 4.0 M NaCl solution, the overpotential of 10 mA cm⁻² (η₁₀) of ZnO_x@IrO₂–Ta₂O₅ was only 60 mV with a current efficiency of 95.7%. Even in a 0.6 M NaCl solution, the current efficiency could reach 90.1%, making ZnO_x@IrO₂–Ta₂O₅ suitable for a wide range of pH and variable electrolyte concentrations. In addition, in 4.0 M and 0.6 M NaCl solutions, their Tafel slopes were 41.75 and 49.74 mV dec⁻¹, respectively, which indicated that the rate-determining step (RDS) for both were the second electron transfer.