Alternative Ion-Exhange Materials for Membrane Capacitive Deionization

Abstract

Membrane capacitive deionization (MCDI) is an energy efficient, cost effective desalination technique for brackish water to produce water for drinking and processes found in semiconductor and pharmaceutical manufacturing, energy production, and other water related industries.[1-3] The operating principle in MCDI is a potential induced electro-sorption of salt ions that migrate across the ion-exchange membrane layers to the porous carbon electrodes. Hence, the application of electric work reduces the salt content from the feed water. Saturation of the carbon electrodes then leads to discharge of the salt from the carbon electrodes, leading to recovered energy and an increase in salt concentration of the emanating flow stream from the MCDI cell. The discharge process regenerates the electrodes so salt removal can take place again. From a materials aspect of MCDI, most research has focused on carbon-based electrodes with the aim to improve device performance.[4-6] Conversely, little innovation has been made in alternative ion-exchange membrane materials for MCDI. Most reports of MCDI leverage commercially available membranes for electrodialysis.[1,3,7] In this work, MCDI performance was correlated to ion-exchange membrane thickness. The experimental design evaluated examined: i.) commercially available ion-exchange membranes, ii.) ion-exchange layered electrodes of different thicknesses with electrode samples being prepared by drop casting followed by spray painting of ionomer layer on top of the electrode; and iii.) drop casting a mixture of electron conducting carbon with dissolved ionomer. A home-built, single-cell MCDI module was characterized with the different ion-exchange materials using a 275 ppm salt feed. MCDI performance was determined by quantifying salt removal and energy efficiency and the individual resistance contributions in the system through electrochemical impedance spectroscopy.