Capacitive deionization technologies have gained significant attention in recent years. The development and availability of a variety of materials have enabled the growth of research on electrosorption, which makes capacitive deionization increasingly attractive. This technology has a wide range of applications, such as softening, desalination and selective removal, each of which has been the focus of the experimental part of this work. The theoretical part is devoted to the issue of functioning of capacitive deionization, electrode material and especially the specific application. Water desalination is a major issue, given the global shortage of drinking water and the possibility of using capacitive deionization as a competitive method to conventional desalination methods. Conversely, softening and selective removal of ions can pose everyday problems both in the treatment of drinking water or pre-treatment of industrial water and in the treatment of wastewater. In this diploma thesis, it was proved that the method of capacitive deionization can be used to solve all the above problems. Although capacitive deionization is not a commercially available technology in the Czech Republic yet, it can be expected to be used more and more in the future.

Capacitive Deionization (CDI) is an energy-efficient desalination technology that utilizes an electric field to extract ions from water. Flow-through CDI systems show potential for superior desalination performance compared to traditional flow-by CDI; however, they face the challenge of increased occurrence of Faradaic reactions, leading to undesired by-products and reduced energy efficiency. In this study, we constructed a flow-through CDI cell and investigated the desalination performance of the two possible cell configurations: upstream anode mode and downstream anode mode. A series of experiments were conducted, measuring conductivity and pH of the effluent solution during charging and discharging phases. The results were analyzed in terms of salt adsorption capacity and charge efficiency. We used pH fluctuations in the effluent solution as indicators of Faradaic reactions. It was found that upstream anode mode yielded superior desalination, with a salt adsorption capacity of 6.79 mg/g and charge efficiency of 64.3%, compared to downstream anode mode, which displayed a salt adsorption capacity of 5.19 mg/g and charge efficiency of 50.8%. However, upstream anode mode also produced more pronounced pH oscillations, suggesting a higher occurrence of Faradaic reactions. Reconciling these conflicting results and shedding light on the complex processes within the CDI cell calls for further investigation.

Electrosorption-based capacitive deionization (CDI) has become a viable process for brackish water desalination and defluoridation. In this study, activated Douglas fir biochar is used as a low-cost electrode material with adsorption capacity comparable to activated carbon obtained from biomass precursors. Adding functional groups to the activated biochar enhanced salt removal capacity, providing cation and anion selectivity. The functionalized electrodes were prepared by Nafion, titanium isopropoxide, and p-phenylenediamine treatment, respectively, which introduced sulfonate, titanium dioxide and amine functional groups to the electrode surface. These modification methods are versatile and can be easily performed without sophisticated laboratory environment. Modified biochar electrodes were characterized by TEM, SEM-EDX, XRD, and XPS. Cyclic voltammetry and electrochemical impedance spectroscopy (EIS) were performed to analyze the electrochemical properties of the electrodes. The salt adsorption capacity (SAC) was evaluated in a 3D-printed capacitive deionization flow cell using a chloride and fluoride ion sensor. It was found that functionalized biochar electrodes had increased SAC and charge efficiency in asymmetrical setup due to reduced co-ion effect. For example, the asymmetrical CDI cell with Nafion cathode and amine biochar anode improved NaCl removal capacity by 54% over the activated biochar symmetrical cell (identical anode and cathode), with SAC 6.01 mg NaCl/g biochar at the symmetrical cell and 9.25 mg/g for the asymmetrical cell. The charge efficiency also increased by ≈ 67% from symmetric AcB cell to asymmetric TB-05 cathode and AmB anode. This work shows that biochar can be engineered and explored broadly as an inexpensive sustainable electrode material for asymmetrical capacitive deionization.

The effects of system pressure on the performance stability of flow-through capacitive deionization (CDI) cells was investigated. Initial data showed that the highly porous carbon electrodes possessed air/oxygen in the micropores, and the increased system pressure boosts the gases solubility in saline solution and carries them out of the cell in the effluent. Upon applying a potential difference to the electrodes, capacitive-based ion adsorption occurs in competition with faradaic reactions that consume oxygen. Through the addition of backpressure, the rate of degradation decreases, allowing the cell to maintain its salt adsorption capacity (SAC) longer. The removal of oxygen from the pore space of the electrodes makes it no longer immediately accessible to faradaic reactions, thus hindering the rate of reactions and giving the competing ion adsorption an advantage that is progressively seen throughout the life of the cell. A quick calculation shows that the energy penalty to power the pump is fairly insignificant, especially in comparison to the cost of replacing the electrodes in the cell. Thus, operating at elevated pressures is shown to be cost effective for continuous operation through the reduced electrode replenishment costs.

Capacitive deionization is a promising technology for purification and desalination of brackish water with great advantages over current technologies due to its low operating cost and high-water recovery ratio. Most of the system studied relies on the adsorption/desorption capacity of activated carbon electrodes due to its high surface area. However, its specific adsorption capacity is limited since the adsorption is predominantly on the surface of the electrodes. In this thesis we propose the use of polyaniline as a chloride-ion adsorption material. Polyaniline is a redox polymer able to accommodate anions in several of its three states when subjected to an external voltage. To this end, we synthesized polyaniline by electrodeposition technique and its electrochemical behavior was studied. A hybrid CDI system was assembled, using PANI as anode material and activated carbon cloth as cathode, showing outstanding adsorption of 37.26 mg/g Cl at current densities of 250 A/g. The energy consumption of this system was of 0.4979 kWh/m3. Its stability was evaluated over 50 cycles with negligible capacity loss. Along with its use in a CDI system, the aim of this thesis was to understand the mechanisms of operation of this material, by means of its physical and electrochemical characterization, as well as its efficiency and stability through the use of this material in capacitive deionization cells. / Kapacitiv avjonisering är en lovande teknik för rening och avsaltning av bräckt vatten med stora fördelar jämfört med nuvarande teknik på grund av dess låga driftskostnader och höga vattenåtervinningsgrad. De flesta av de studerade systemen bygger på adsorptions/desorptionskapaciteten hos elektroder av aktivt kol på grund av dess stora yta. Dess specifika adsorptionskapacitet är dock begränsad eftersom adsorptionen huvudsakligen sker på elektrodernas yta. I den här avhandlingen föreslår vi att polyanilin används som adsorptionsmaterial för kloridjoner. Polyanilin är en redoxpolymer som kan ta emot anjoner i flera av sina tre tillstånd när den utsätts för en extern spänning. För detta ändamål syntetiserade vi polyanilin genom elektrodepositionsteknik och dess elektrokemiska beteende studerades. Ett hybrid CDI-system monterades med PANI som anodmaterial och aktiverad kolduk som katod, vilket visade en enastående adsorption av 37,26 mg/g Cl vid en strömtäthet på 250 A/g. Energiförbrukningen för detta system var 0,4979 kWh/m3. Systemets stabilitet utvärderades över 50 cykler med försumbar kapacitetsförlust. Förutom användningen i ett CDI-system var syftet med denna avhandling att förstå detta materials funktionsmekanismer genom fysisk och elektrokemisk karakterisering samt dess effektivitet och stabilitet genom användning av detta material i kapacitiva avjoniseringsceller.