Heterojunction Electrode Designed for Saline Wastewater Electrolysis

Abstract

Electrochemical oxidation is a promising candidate for decentralized wastewater treatment, due to its modular design, convenient operation, and small carbon footprint. In this process, pollutants are oxidized at the anode, while valuable hydrogen is produced at the cathode. Different from water splitting, the ideal anodes in EO process should have high activity toward oxidant generation rather than oxygen evolution. The need to develop highly efficient and durable anode material is a never-ending challenge. Chloride, as a ubiquitous component in wastewater, can be oxidized readily to reactive chlorine species (RCS = OCl­-, HOCl, Cl·, and Cl2·-) to remove pollutants such as organics, ammonium and bacteria. However the speciation of RCS in saline water electrolysis is lacking of quantitative description. The contribution of each RCS species to pollutant removal is still under debate. The first approach of this study is to develop multi-layer heterojunction anode for RCS generation. IrO2 is an excellent catalyst for water splitting. By depositing a thin film of TiO2 on IrO2 (Ti/Ir), the oxygen evolution reaction is hindered while the selectivity toward chlorine production is greatly enhanced. The further deposition of Sb-SnO2 islands on TiO2layer (Sn/Ti/Ir) provides additional active sites for radical generation. As our second approach, a computational kinetic model covering chlorine generation and radical production is established. Combing it with experimental data, chlorine evolution and the furuther oxidation of chlorine to ClO3- and ClO4 - on Ti/Ir anode can be simulated. It is found that Cl·and HO· is produced concomitantly with chlorine on Sn/Ti/Ir anode and Cl2·-is the dominant radical after reaching equilibrium. The rate constant of Cl· production is two to three orders of magnitude lower than that of chlorine generation. The model predicts that the increase of chloride concentration will enhance the chlorine production but quench more radicals, which is in consistance with experimental results. The best performing Sn/Ti/Ir anode was applied to human wastewater electrolysis. Although Cl2·- is more oxidative than chlorine, chlorine is the major contributor to the removal of organics and ammonium due to its higher concentration. Figure 1