Advanced materials for electro-driven ion separation and selectivity

Treatment of water for domestic and industrial use is an ubiquitous process today. This treatment can be physical, chemical, or biological and is usually performed in multiple steps. The removal of salts and minerals from water is one such step that is of high priority. This removal is referred to as water desalination. This may include production of water for domestic and industrial use, harvesting/recovering materials (salts/ions) of value and removing harmful species by selectively separating them from water. In this regards, electro-driven processes have gained attention due to their easy operation, mild environmental impact, and potentially low energy consumption. Capacitive Deionization (CDI), investigated in this work, is one such promising electro-driven ion separation technology. Conventionally, it employs capacitive electrodes that store ions removed from waste water and the energy input to these electrodes can be regenerated during their regeneration. Since the ion removal relies heavily on the ability of the electrodes to store them, investigation into new - and development of existing - electrode material forms the main focus of the research carried out in the field of CDI. The development of an electrode material involves focusing the ion adsorption capacity, the rate of ion adsorption/desorption, the inherent or functionalized capability to selectively store ions and long-term stability. The research presented in this thesis and the conclusions obtained from them address these developmental challenges. In particular, this work focuses on Prussian blue analogues (PBAs). PBAs are crystalline in nature and in general, the elements constituting  a PBA lattice are assembled in a cubic structure with metals transition metals connected by −C≡N− (cyanide) ligand. These lattices are capable of storing cations into the interstitial sites formed in these lattices. This is referred to as intercalation. The use of PBAs in CDI has attracted attention because of their open crystal structure, customizable chemical composition, higher equilibrium salt adsorption capacity via cation intercalation compared to carbon, and an inherent selectivity among cations. In this work, the use of non-carbon based electrodes was chronicled throughout literature. Following that, the ability of PBA as an electrode materials was assessed by using them to deionize brackish water in an electrochemical flow cell under the influence of an electric current. Furthermore, their capability to selectively remove cations from waste water was also tested. It was found that a cell containing PBA electrodes required on average two times less energy than a cell containing conventionally and commonly used carbon electrodes. From the investigation into the ion selectivity of these electrodes, it was observed that nickel containing PBA was highly selective towards monovalent ions while the vanadium containing PBA exhibited preference towards divalent ions, indicating that the ion-selective nature of PBA is customizable. Along the same lines, an electrochemical cell was also developed that contained monovalent cation-selective PBA and a monovalent anion-selective anion exchange membrane, providing ion selectivity of cations and anions simultaneously. In addition to experiments, a theoretical understanding was developed at physical level to predict the performance of an electrochemical cell containing PBA electrodes. This work systematically presents an investigation into an alternative electrode material in CDI and reflects on the future of intercalation electrodes within the framework of (selective) ion separation