Electrical double layers as ion reservoirs. Applications to the deionization of solutions

Abstract In this work, we briefly review the basics of the structure of the electrical double layer (EDL), with emphasis on the amount of neutral salt that can be accumulated in the EDLs of opposite electrodes when a potential difference is applied. Consideration is taken of the realistic solution taking into account that ions have finite volumes and have a limited concentration at any point, as well as to the effect of ion charge. As a result of this accumulation, ionic solutions in contact with the electrodes can be partially deionized, and brine water can eventually transform into potable one. The technology has been denominated CDI or Capacitive DeIonization, as it is based on the high electrical capacitance that can be associated to the EDLs. We give a brief account of how CDI can be implemented and the achievements that can be reached taking into account the EDL structure. Variations such as MCDI (membrane-CDI) or SE-CDI (soft electrodes CDI) are also described. The advantages of capacitive deionization regarding specific ions withdrawal from solutions, as well as the energy requirements for deionization are also discussed.