Modulating Chlorine and Oxygen Evolution Reaction in Ti/IrO2–SnO2–Ta x O y Electrodes for Efficient Azo Dye Removal

Abstract

The generation of oxidizing agents during electrochemical remediation of textile wastewater loaded with persistent azo dyes, such as Reactive Black 5 (RB-5), is fundamentally dependent on competition between the chlorine evolution reaction (CER) and the oxygen evolution reaction (OER), which in turn dictates the type and selectivity of the oxidants formed. This study details how the compositional tuning of dimensionally stable anodes (DSA), Ti/Ir0.3SnxTa(0.7–x)Oy, affects the kinetics of electrochemical reactions, the nature of the oxidants produced, and their consequent degradation efficiency. Electrodes prepared using polymeric precursor decomposition exhibit mixed Ti/IrO2–SnO2–TaxOy phases and high-surface-area cracked morphologies. Mechanistically, Sn-rich compositions promote the CER by lowering charge-transfer resistance and kinetically favoring chloride oxidation, whereas increasing Ta content shifts selectivity toward the OER. Electrochemical impedance spectroscopy combined with operando UV–vis and electrochemical mass spectrometry confirms that chlorine-active species dominate, especially under acidic conditions, while hydroxyl radical pathways remain secondary. RB-5 degradation kinetics scale directly with CER activity, with apparent rate constants of 1.21 × 10–3 (Ir/Sn/Ta = 30:40:30) and 5.71 × 10–3 s–1 (Ir/Sn/Ta = 30:70:00). The Ti/Ir0.3Sn0.7Oy electrode achieves optimal performance, delivering high discoloration (DI > 99.91%), chemical oxygen demand removal (COD ∼ 71.72%), and a specific energy consumption of 0.482 kWh per m3 of treated effluent, evidencing efficient coupling between chloride activation and organic oxidation. Overall, this study establishes a direct composition-activity-selectivity relationship in DSA systems, demonstrating that Sn-driven modulation of CER/OER competition dictates the kinetics of oxidant generation and process efficiency. These insights provide a rational framework for designing electrocatalysts that maximize chlorine-mediated pathways for energy-efficient wastewater treatment.