Assembly of Soft Electrodes and Ion Exchange Membranes for Capacitive Deionization

Abstract

The responsible use of water, as well as its reuse and purification, has been a major problem for decades now. In this work, we study a method for adsorbing ions from aqueous solutions on charged interfaces using highly porous electrodes. This water purification process is based on the electric double layer concept, using the method known as capacitive deionization (CDI): If we pump salty solutions through the volume comprised between two porous electrodes while applying a potential difference to them, ions present in the solution are partially removed and trapped on the electrode surfaces. It has been well established that the use of carbon electrodes in combination with ion exchange membranes (membrane-CDI) improves the efficiency of the method above that achieved with bare activated carbon. Another approach that has been tested is based on coating the carbon with polyelectrolyte layers, converting them into “soft electrodes” (SEs). Here we investigate the improvement found when combining SEs with membranes, and it is shown that the amount of ions adsorbed and the ratio between ions removed and electrons transported reach superior values, also associated with a faster kinetics of the process. The method is applied to the partial desalination of up to 100 mM NaCl solutions, something hardly achievable with bare or membrane-covered electrodes. A theoretical model is presented for the ion transport in the presence of both the membrane and the polyelectrolyte coating.

Electrochemical Methods for Water Purification, Ion Separations, and Energy Conversion

“…To overcome the limitations of traditional carbon electrodes, researchers examined new geometries and arrangements for both the electrodes and IEMs, which enabled continuous operation , and increased power output. , These advancements were accompanied by the use of novel materials for the electrodes, such as porous activated carbon coated with polyelectrolytes, to reduce the leakage current. − These so-called “soft electrodes” generate electrical energy in response to changes in both the capacitance of the EDL and the Donnan potential of the polyelectrolyte coat during exchange of electrolytes.…”

Freestanding Activated Carbon Nanocomposite Electrodes for Capacitive Deionization of Water

“…One recent technology that researchers have been turning to for more energy-efficient water desalination is capacitive deionization (CDI) [ 7 ]. The technology is based on removing ions (i.e., salts) from aqueous solutions (i.e., seawater) using electrodes as a key component.…”

Energy production by salinity exchange in polyelectrolyte-coated electrodes. Temperature effects

“…If, instead, ion exchange membranes are used for establishing an electric potential difference between a pair of electrodes, without the need of an external source, we speak of CDP (or Capacitive Energy Extraction based on Donnan Potential). An interesting alternative, in which membranes are not required, can be denominated Capacitive Mixing on Soft Electrodes (SE): here, polyelectrolyte layers directly deposited on top of the active material are used for establishing the potential difference between the two electrodes 4,15,18 .…”