Performance of ion intercalation materials in capacitive deionization/electrochemical deionization: A review

Capacitive deionization (CDI) also referred to as electrochemical deionization (EDI) is a deionization technology which depends on the electrical potential difference applied between a pair of electrodes to extract charged species from the solution. Due to their low cost, good electrical conductivity, porous structure, and high specific surface area, carbon materials have been the materials of choice for electrodes in most CDI cells. With carbon-based materials, ions from solution are extracted through electrosorption mechanism and stored in the electrical double layer (EDL) formed on the electrode surface. Recently, ion intercalation materials that store ions through pseudocapacitive ion intercalation have attracted interest. In pseudocapacitive ion intercalation mechanism, ions are inserted or stored in the lattice crystal structure sites of the intercalation host compounds (IHCs) due to redox reactions. The desalination performance of these materials, also referred to as salt adsorption capacity (SAC) and expressed in mg/g is reported to be higher compared to that of carbon-based electrode materials. The enhanced SAC of intercalation materials is due to the fact that ions are not only removed from the solution via surface adsorption but also through transfer of charge and intercalation/insertion of the ions into their crystallographic sites. To date, there is inadequate number of articles summarizing the performance of intercalation materials and strategies undertaken to improve performance thereof for desalination and ion separation purposes. This paper therefore, reviewed the performance of different intercalation electrode materials for water desalination presented in the literature to date. The discussion covers different geometries/architecture of a desalination cell utilizing ion intercalation materials, performance of ion intercalation materials and their mechanism of deionization as well as strategies that have been employed to improve their deionization performance. Furthermore, this paper provides an outlook and research niches existing in the field of ion intercalation materials for desalination applications and selective removal of both mono and divalent ions from aqueous solutions.