Abstract
Capacitive deionization (CDI) is an emerging desalination technology based on the principles of electrochemical adsorption. It is a simple concept which uses two oppositely charged porous electrodes to electrostatically remove ions from water, hence achieving desalination. One of the main reasons why CDI hasn’t had much commercial success since its conception is due to the low efficiency of carbon materials commonly used in the electrodes. However, this is about to change given the recent adoption of Faradaic materials which employ redox mechanisms to remove ions instead of electrostatic attraction. This thesis examines the different synthesis methods used in state-of-the-art materials and elucidates the relationship between material design and CDI performance. The introductory chapters provide an insight into the current development of CDI research and describes the unique advantages CDI possesses over traditional desalination methods. Chapter 2 is a literature review which analyses the current predicament of water scarcity and discusses how CDI can potentially disrupt traditional desalination systems by offering a low cost and less energy intensive alternative. We divide material research in CDI into two distinct generations, the first is characterized by capacitive adsorption while the second is Faradaic or pseudocapacitive adsorption. In our initial work, we investigate how simple modification of morphology can alter the porosity and effect electrosorption. This is carried out through the synthesis of carbon hollow spheres via a hard-templating method. Spurred by the success of carbon hollow spheres, we develop a novel, new carbon material incorporating 2D graphene oxide (GO) sheets and polyvinyl oxide (PVA) polymer chains. We detail the chemistry responsible for the synergistic assembly of these two materials and show how a highly porous, well-structured material can be synthesized from the bottom up using basic precursors. Following this, GO was further used as a substrate for the synthesis of pseudocapacitive Fe3O4 nanoparticles. This work showcases an enhancement of electrosorption due to the pseudocapacitive mechanisms of Fe3O4 nanoparticles. We continue our research of pseudocapacitive materials using MnO2. MnO2 is a well-known pseudocapacitive material in supercapacitor research but has not been formally evaluated for CDI. In our work, we synthesized five types of MnO2 with different crystallographic dimensions and crystallinity. We show how surface adsorption and intercalation can contribute to the capacitance and ion storage properties of MnO2 but usually only one is dominant. Poorly crystalline MnO2 is dependent on surface adsorption whereas crystalline MnO2 materials employ intercalation. Our experiments show that the adsorption capacities of poorly crystalline MnO2 materials are comparable to activated carbon (AC) electrodes and can potentially replace them. In our final work, we perform simple surface functionalization of AC materials to endow them with either positive or negative surface charges. We report increased electrosorption performances due to a decrease in co-ion expulsion. The same materials are further subjected to tests using a variety of multi-salt solutions to determine its ion selectivity. Chloride salts of Na, K, Ca and Mg were used, and an increased divalent adsorption is observed for the negatively charged electrode due to increased surface electrostatic attraction. The experimental results and conclusions drawn from this thesis have significant implications on the design of materials for improved electrosorption performances and can prove crucial in the development of commercial devices.