GOTTA CATCH ‘EM: Tuning Ion Selectivity with Polyelectrolyte Multilayers in Capacitive Deionization

Summary

Separating charged species (ions) from water is of relevance for many environmental, industrial, and agricultural applications. Examples include the reduction of sodium levels in irrigation water, removing hazardous metals (e.g., arsenic), and harvesting high-value metal ions (e.g., lithium) and nutrients (e.g., phosphate) from water. To remove ions from water, we used an electro-driven desalination technique, capacitive deionization (CDI). In this technique, ions are reversibly electro-adsorbed onto charged, porous electrodes. CDI has low energy demands, does not require heat or harsh chemicals to regenerate the electrodes, but is limited in terms of ion selectivity. For this purpose, we explored the use of polyelectrolyte multilayers (PEMs) in CDI. PEMs behave like a filter by rejecting certain ions, for example due to their larger size and/or higher degree of charge, while other ions can go through. The focus of this thesis on tuning the multilayer properties (e.g., thickness, charge of the outermost layer) to control ion selectivity. Also, polyelectrolytes were decorated with ion-selective receptors to prepare functionalized multilayers of which the interactions with various ions were studied.