Capacitive deionization is a technology for desalination and water treatment in which salts and minerals are removed from water by applying an electric field between two porous (often, carbon) electrodes, similar to electric double-layer capacitors. Counterions are stored in the electrical double layers which form at the solution interface inside the porous electrodes, with the ions of cations stored in the negatively charged electrode, and anions stored in the positively charged electrode (anode)(fig.1).

The employed electrodes in CDI are typically prepared from porous carbon particles with internal areas for ion adsorption in the order of 1000 m² per gram, but other materials are also possible, such as carbon nanotubes and nanofibers. The two oppositely placed (planar) electrodes are separated by a thin open structured “spacer”, or flow channel, through which the water flows. Upon applying an electrical potential difference between the two electrodes of the order of 0.8–1.5 V, anions are adsorbed in the anode and cations into the cathode, thereby producing a (partially) ion-depleted product stream. After the ion adsorption capacity of the electrodes has been reached, the applied voltage difference can be reduced to zero and a small product stream concentrated in salt is obtained in the ion release-step. In this way the inflowing stream of brackish water is split into a partially deionized stream and a more concentrated brine.

Membrane capacitive deionization

Membrane capacitive deionization (MCDI)is a modification of CDI by inserting an anion-exchange membrane in front of the anode, and a cation exchange membrane in front of the cathode (fig.2). In this way, coions of the membrane(e.g., the anions in the cathode) are inhibited from leaving the electrode region. The coion-expulsion-effect, which at low voltages negatively influences the salt adsorption rate and removal capacity in CDI, is absent in MCDI. Another advantage of MCDI is that during ion release, it is possible to use a reversed voltage which leads to a faster and more complete rejection of the counterions back into the flow channel.