Membrane(-assisted) Capacitive Deionization (MCDI)
One of the most promising recent developments in CDI is to include ion-exchange membranes (IEMs) in front of the electrodes, called Membrane Capacitive Deionization (MCDI). IEMs can be placed in front of both electrodes, or just in front of one. With only one IEM, the overall positive effect on salt adsorption is less pronounced than in the case of using two IEMs. In the present section we only discuss data of MCDI containing two IEMs. IEMs have a high internal charge because of covalently bound groups such as sulfonate or quarternary amines, and therefore allow easy access for one type of ion (the counterion) and block access for the ion of equal charge sign (the co-ion). As we will explain below, including IEMs in the cell design significantly improves desalination performance of the CDI-process. In general, IEMs have a selectivity for ions of one charge sign relative to ions of another charge sign. In a further modification, the membrane can be made to have a selectivity between ions of the same sign class: for instance between nitrate and chloride (both monovalent anions) [67]. The membranes can be included as stand-alone films of thicknesses between 50 and 200 μm, or can be coated directly on the electrode with a typical coating thickness of 20 μm [8], [9], [10], [11], [12] and [68], see