Abstract
MXene is considered as a superior electrode material for capacitive deionization (CDI) due to its high conductivity and two-dimensional structure. However, the electrochemical performance of pristine MXene nanosheets has been significantly impeded by the surface oxidation in the aqueous media and re-stacking caused by van der Waals forces which reduces the ions storage capacity. Also, niobium based MXenes (Nb2CTx and Nb4C3Tx) have never been studied as an electrode material for CDI. In this dissertation, we improved the performance and antioxidation properties of Ti3C2Tx MXene and studied different type of MXene as an electrode material for hybrid capacitive deionization (HCDI). In the first part of the dissertation, the chitosan-lignosulfonate/Ti3C2Tx MXene (CLM) composite was used as a binder-free electrode to enhance the ion storage capacity and long-run cycling stability for HCDI. The chitosan-lignosulfonate nanospheres were able to increase the interlayer spacing between the MXene nanosheets effectively, which has significantly enhanced the ion storage capacity and electrochemical properties of the electrode. The binder-free CLM cathode demonstrated a high salt adsorption capacity of 44.6 mg g-1 and a maximum average salt adsorption rate of 5.8 mg g-1 min-1 at 1.2 V. A high cycling stability above 97% for 30 cycles was observed. Also, the long-term stability of CLM electrode was studied by X-ray photoelectron spectroscopy (XPS) and the results showed that the CLM electrode was not prone to surface oxidation after 30 cycles. In the second part of the study, the multilayered Nb2CTx (ML-Nb2CTx) and delaminated Nb2CTx (DL-Nb2CTx) MXene were used as cathode electrodes in HCDI. The material characterized using SEM, TEM, and XRD techniques, confirmed well delamination of Nb2CTx. DL-Nb2CTx electrode showed higher SAC and SAR by 61% and 67%, respectively, when compared to ML-Nb2CTx electrode. In the third part of the study, multilayered Nb4C3Tx (ML- Nb4C3Tx) and delaminated Nb4C3Tx (DL-Nb4C3Tx) MXene were used as a cathode electrode for HCDI. DL-Nb4C3Tx electrode demonstrated a higher specific capacitance when compared to ML-Nb4C3Tx electrode, reaching a value of 179 F/g at 1 A/g, indicative of excellent electrochemical properties. Also, DL-Nb4C3Tx showed the highest SAC (22.1 mg g-1) and SAR (2.07 mg g-1 min-1) when it is compared to Nb2CTx and Ti3C2Tx electrodes at 1.2V.
