Capacitive deionization of brackish water with carbon-based materials

Abstract

Capacitive deionization (CDI) is a novel and economical water desalination technology with merits such as energy-saving capabilities and environmental friendliness. This technology can be an alternative for traditional expensive desalination technologies and holds great promise for future applications. Through applying a voltage between two porous electrodes of a CDI cell, charged ions in water can be adsorbed because of the formation of electrical double layers; hence, the water is desalinated. One problem hindering the commercialization of this technology is the long-term operational stability of the carbon-based electrodes. To alleviate this problem, this thesis studies the underlying mechanisms behind the stability degradation. During the long-term operation of CDI cells, Faradaic reactions occur on both electrodes, and the anode is gradually oxidized. The oxidation of the anode shifts its potential of zero charge, increases its resistance, and thus is responsible for the performance degradation. Meantime, different concentrations of Na+ and Cl- are trapped in aged positive and negative electrodes, respectively. The trapped ions may then act as surface charges and occupy the active adsorption sites in the porous electrodes, thereby decreasing the salt adsorption capacity. A schematic illustration based on the classical Gouy-Chapman-Stern model is made to present the dynamic movement of ions near degraded electrodes, such that the performance degradation is well-understood. Through the revealed degradation mechanisms, practical methods are explored to extend the electrode lifetime. We propose that thermal treatment can precisely remove the acidic oxygen-containing groups formed in an aged anode by modulating temperatures. Accordingly, the lost pore volume, shifted potential and the increased resistance of the anode are recovered during the thermal treatments. As a result, the performance of the CDI cell is recovered to its initial level by optimizing the treatment temperature. The understanding of the rigorous regeneration mechanism offers insights into strategies for minimizing electrode degradation or in situ regeneration. The fabrication processes of carbon-based electrodes are also evaluated for their potential to industrial application. The fabrication processes are proven to have neglectable effect on salt adsorption capacity but have obvious effect on charge efficiency and energy consumption, in which the resistance is the determining factor. Furthermore, the performance of intercalation material (MXene) is compared with porous carbon. The studied MXene is more energy-efficient in constant current mode, although it has similar desalination capacity with carbon-based electrode. The energy-efficiency may be contributed by the ion-selectivity and high conductivity of intercalation materials. Benefit from the advance of materials, CDI technology may be further developed and stand out among various desalination technologies in the near future.