Capacitive deionization

Capacitive deionization (CDI) is a technology to deionize water by applying an electrical potential difference over two electrodes, which are often made of porous carbon. In other words, CDI is an electro-sorption method using a combination of a sorption media and an electrical field to separate ions and charged particles. 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".