Asymmetric Redox Active Electrodes for Brackish Water Deionization and Selective Adsorption of Micro-Pollutants
Authors: He Fan , Hemmatifar Ali , Bazant Martin Z. , Hatton T. Alan 
Published: February 2020

Water scarcity is one of the major problems facing the modern world. The rapid continuous growth of the world’s population, industrialization and climate change have all imposed a severe stress on the situation. Furthermore, due to human activities, water pollution has become one of the most critical issues influencing the already severely strained water resources. Industrial wastewater from mining, metal production and oil production containing heavy metal ions and dissolved organic compounds are too often poured without proper treatment into rivers, lakes and groundwater. Recently pharmaceutical compounds have even been detected in the drinking water in the US, which has aroused great anxiety about the current water quality.
Due to the intrinsic electrical charge of the dissolved salt and micropollutants, they can be adsorbed within the electric double layers established within porous electrodes under applied electrical potentials by electrostatic interaction, in a process known as capacitive deionization (CDI). Beyond CDI, more recent electrochemical water treatment methods such as Faradaic-CDI and electrochemically-mediated selective adsorption have attracted much interest. This is mainly due to their higher salt adsorption capacity and, more importantly, selectivity toward target ions, which cannot be achieved by conventional CDI that uses carbon materials and is mostly ubiquitous to all ionic species.
We have designed and built a flow platform that demonstrate the dynamic adsorption and desorption performance of an asymmetric redox active electro-sorption system. The flow system contains an ion-adsorption cell and is equipped with several in-line sensors, to allow monitoring of the conductivity, pH and concentration of target micro-pollutants via UV-vis adsorption spectroscopy, which provides real-time data on the electro-sorption process.
Redox responsive electrode materials for the anode and the cathode which undergo reversible Faradaic reactions under an applied electric field have been designed and synthesized. The electrode materials are characterized by cyclic voltammetry, chronopotentiometry and electrochemical impedance spectroscopy to quantify the capacitance and operating window. This is also important for capacity matching between the anode and the cathode to avoid side reactions.
The asymmetric redox active electrodes enable electro-sorption of anions at the anode and cations at the cathode. When they are paired up and tested for water deionization applications, they demonstrate ion adsorption behavior during the charging step, and release the ions to regenerate the brine during the discharge step. Through cycling of the cell in adsorption and desorption mode alternatively, the system shows robust electro-sorption behavior.
Furthermore, the asymmetric redox active materials are tested for selective adsorption of target ions when the supporting ions are much more abundant. This has potential applications in recovery of value-added ions from waste streams. Due to stronger interaction of the target anions with the redox active electrodes under activation, for a starting ratio of 1:50 of target anion to supporting anion, more than three-fold adsorption amount of target anions versus the supporting anions is achieved. And the system shows robust competitive electro-adsorption performance.
This study demonstrates a facile method for the synthesis of redox-active materials for water deionization and selective adsorption applications. A flow system built to test the electrodes under realistic operating conditions demonstrates good salt adsorption and ion selectivity towards target ions. This system can also be easily extended to other applications to achieve higher salt adsorption capacity and/or selectivity towards other ions through the design and selection of the electrode materials.