Performance Comparison of the Electrolytic Cells Having Different Configurations for the Production of HCl and NaOH from Brine

Abstract

CO2 capture and utilization (CCU) researches are being actively pursued to suppress its global warming effects and to utilize CO2 as a feedstock for producing useful organic and inorganic substances such as methanol, formic acid, and carbonates through biological, thermal, photochemical and electrochemical methods. This study focuses on the development of compact continuous-flow electrochemical cells for energy-efficient splitting of brine (aqueous NaCl) to produce alkali (NaOH) and hydrochloric acid (HCl) that would be used to convert carbon dioxide into carbonate minerals at a subsequent stage. For this purpose, bipolar membrane cells are usually used, that are composed of three types of membranes, an anion, a cation and a bipolar membrane in a cell. Recently a new type of electrolytic cell was proposed which greatly reduce the working voltage by supplying hydrogen into the anode side of the cell. In this study two types of cell designs, namely one-membrane cell and two-membrane cell, are investigated and compared for their performances under various operating conditions. The one-membrane cell consisted of a membrane sandwiched between an anode and a cathode. A feedstock comprising of a NaCl solution and H2 gas was supplied to the anode side while a pure water or a dilute NaOH solution was supplied to the cathode side. Upon applying a voltage of 1.5V across the two electrodes, the electrolytic reactions proceeds to produce HCl at the anode and NaOH and H2 at the cathode. In the case of two-membrane cells, two membranes were used and a middle plate was placed between the two membranes, and a NaCl solution was fed to the middle plate while H2 gas was fed to the anode and a dilute NaOH solution to the cathode. The one-membrane cell employing platinum-based electrodes shows initial reaction rate as high as 500 mA/cm2 at an applied voltage of 1.5V. The reaction rate, however, drops rapidly due to the degradation of the anode Pt-catalyst by hydrochloric acid that is produced at the anode side during the reaction. On the other hand, the two-membranes cell shows very stable performance over 300 hours though its reaction rate is kept below 100mA/cm2 at an applied voltage of 1.5V. The one-membrane cell has a simple reactor configuration and displays a high reaction rate because of a low internal resistance, but it suffers from a poor stability and a low caustic efficiency below 10%. On the other hand, the two membrane cell exhibits a high caustic efficiency over 50% and a good stability, but its cell configuration is complicated and the reaction rate is below 100 mA/cm2. Various materials have been tested to improve the reaction rate as well as the caustic efficiency and stability. Through optimization of the cell components, we have achieved a very reasonable caustic efficiency over 50% and a reaction rate over 200 mA/cm2 on a one-membrane cell. Detailed comparison is presented on the performances of the cells under various operating conditions and an optimal combination of the cell components is suggested.