Strategies for simultaneous improvement of reaction rate and caustic efficiency of brine electrolyzer

Abstract

Brine electrolyzers can produce HCl and NaOH concurrently using NaCl-concentrated wastewater discharged from various desalination processes. However, the electrolyzers still have poor performance, and thus commercialization is retarded. The main factor affecting the performance of this technology is the trade-off between reaction rate and caustic efficiency. This study aims to improve the reaction rate and caustic efficiency simultaneously by optimizing the operating conditions of the electrolyzer. Through various experiments, we have found that increasing the applied voltage and operating temperature can effectively enhance the reaction rate and caustic efficiency. This is because the relative permeation rate of sodium ions over protons through the Nafion membrane increases with temperature and applied voltage. Under the optimal conditions, the reaction rate is significantly enhanced by 2.1 times (from 86.5 mA cm−2 to 179.2 mA cm−2) and the caustic efficiency by 1.6 times (from 56% to 90%) compared to the standard conditions. The mechanism for the improved performance is revealed by investigating the permeation behavior of ions through the membrane. It is found that protons hinder the permeation of sodium ions, but increasing the temperature and/or applied voltage overcomes this interference. A long-term test for 1600 h confirms the stability and feasibility of the electrolyzer. Overall, this study is expected to contribute to commercializing brine electrolyzers for HCl and NaOH production.