Selectivity between Oxygen and Chlorine Evolution in the Chlor-Alkali and Chlorate Processes

Abstract

Chlorine gas and sodium chlorate are two base chemicals produced through electrolysis of sodium chloride brine which find uses in many areas of industrial chemistry. Although the industrial production of these chemicals started over 100 years ago, there are still factors that limit the energy efficiencies of the processes. This review focuses on the unwanted production of oxygen gas, which decreases the charge yield by up to 5%. Understanding the factors that control the rate of oxygen production requires understanding of both chemical reactions occurring in the electrolyte, as well as surface reactions occurring on the anodes. The dominant anode material used in chlorate and chlor-alkali production is the dimensionally stable anode (DSA), Ti coated by a mixed oxide of RuO2 and TiO2. Although the selectivity for chlorine evolution on DSA is high, the fundamental reasons for this high selectivity are just now becoming elucidated. This review summarizes the research, since the early 1900s until today, concerning the selectivity between chlorine and oxygen evolution in chlorate and chlor-alkali production. It covers experimental as well as theoretical studies and highlights the relationships between process conditions, electrolyte composition, the material properties of the anode, and the selectivity for oxygen formation.

The catalyzing effect of chromate in the chlorate formation reaction

“…The loss becomes particularly significant at low chloride concentrations (Krstajić et al, 1991) and at low anode potentials (Karlsson and Cornell, 2016). The coating composition is also of high importance (Krstajić et al, 1986).…”

A 25.1% Efficient Stand‐Alone Solar Chloralkali Generator Employing a Microtracking Solar Concentrator

“…The voltage efficiency of our electrochemical reaction was estimated to be ≈70%, which is consistent to those of commercially deployed systems. [34,36] Figure 3b demonstrates the capability of our solar powered chloralkali device to produce two valuable products, molecular chlorine and molecular hydrogen, at unprecedented efficiencies using natural sunlight. Our simple microtracking mechanism, based on the advanced optical design of our solar concentrator, was periodically activated to cover the solar disk displacements and maintain solar to product efficiency as high as 25.1%.…”

“…Levelized costs of solar chlorine (LC Cl2 ) were calculated for a 20 years device lifetime, considering the replacement of Nafion membranes every 5 years [24] and electrocatalysts lifetime of 10 years, although DSAs have demonstrated to operate for longer life spans. [34,36] These assumptions were intended to produce realistic outcomes, given the complexity and the volatility of a technoeconomic analysis.…”

“…The anolyte solution was a 20% wt NaCl solution, prepared by dissolving highly pure salt (>99.5%), purchased from Roth GmbH, in deionized water. It was acidified to ensure selectivity of molecular chlorine over hypochlorite and chlorate by adding hydrochloric acid (Merck Millipore, ≥37%) to pH ≈ 1.5 [36] (monitored with a VWR pH110 pH-meter, calibrated using Merck Certipur buffer solutions-pH 4.01/7.00/10.01). The catholyte solution was a 0.5 m NaOH solution (2% wt), prepared using pure caustic soda purchased from Reactolab SA.…”

Towards selective electrochemical conversion of glycerol to 1,3-propanediol

“…RuO 2 -dsa is already an established anode in the chloralkali industry. 33,34 In chloride media, ruthenium oxide-based dsa is selective to chlorine formation and aqueous chlorine species are an potential oxidant for glycerol oxidation. Hence, RuO 2 -dsa was investigated rst to ascertain anodic glyceraldehyde selectivity in our quest towards selective electrochemical conversion of glycerol to 1,3-PD.…”