Capacitive deionization

Capacitive deionization (CDI) is a technology to deionize water by applying an electrical potential difference over two porous carbon electrodes. Anions, ions with a negative charge, are removed from the water and are stored in the positively polarized electrode. Likewise, cations (positive charge) are stored in the cathode, which is the negatively polarized electrode.

Today, CDI is mainly used for the desalination of brackish water, which is water with a low or moderate salt concentration (below 10 g/L). Other technologies for the deionization of water are, amongst others, distillation, reverse osmosis and electrodialysis. Compared to reverse osmosis and distillation, CDI is considered to be an energy-efficient technology for brackish water desalination. This is mainly because CDI removes the salt ions from the water, while the other technologies extract the water from the salt solution.

Historically, CDI has been referred to as electrochemical demineralization, "electrosorb process for desalination of water", or electrosorption of salt ions. It also goes by the names of capacitive desalination, or in the commercial literature as "CapDI".

In 1960 the concept of electrochemical demineralization of water was reported by Blair and Murphy. In that study, it was assumed that ions were removed by electrochemical reactions with specific chemical groups on the carbon particles in the electrodes. In 1968 the commercial relevance and long term operation of CDI was demonstrated by Reid. In 1970 Johnson et al. introduced a theory for the CDI process called ‘potential modulated ion sorption’; the latter is today more commonly known as the Electric Double Layer (EDL) theory. From 1990 onward, CDI attracted more attention because of the development of new electrode materials, such as carbon aerogels or carbon nanotube electrodes. In 1996, Farmer et al. also introduced the term capacitive deionization and used the now commonly abbreviation “CDI” for the first time. In 2004, Membrane Capacitive Deionization was introduced in a patent of Andelman.

The operation of a conventional CDI system cycles through two phases: an adsorption phase where water is desalinated and a desorption phase where the electrodes are regenerated. During the adsorption phase, a potential difference over two electrodes is applied and ions are adsorbed from the water. The ions are transported through the interparticle pores of the porous carbon electrode to the intraparticle pores, where the ions are electrosorbed in the so-called electrical double layers (EDLs). After the electrodes are saturated with ions, the adsorbed ions are released for regeneration of the electrodes. The potential difference between electrodes is reversed or reduced to zero. In this way, ions leave the electrode pores and can be flushed out of the CDI cell resulting in an effluent stream with a high salt concentration, the so-called brine stream or concentrate. Part of the energy input required during the adsorption phase can be recovered during this desorption step.

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