Material design and molecular engineering of hydrogels for solar desalination

Date

2021-06-25

Authors

Zhou, Xingyi (Ph. D. in materials science and engineering)

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Abstract

Growing concern over water scarcity leads to increased research interest in advanced water purification technologies. Solar desalination, which utilizes solar energy to separate water and impurities through vaporization, enables the utilization of sustainable solar energy and abundant seawater to alleviate water scarcity. However, the essential process of solar desalination to remove impurities is energy-intensive. The vapor generation rate is limited due to insufficient solar absorption and thermal loss, lowering the freshwater production yield. Diffuse natural sunlight cannot satisfy the intrinsic energy consumption for rapid water vaporization. Therefore, developing new material platforms that can simultaneously provide high solar absorption, effective energy utilization, and low energy consumption for water vaporization to achieve highly efficient solar desalination under natural sunlight is anticipated. This dissertation adopts hydrogels as the new material platform for seawater desalination based on solar vapor generation (SVG). The unique water state in hydrogels could reduce energy consumption for water vaporization, facilitating vapor generation. To enable hydrogel materials for solar desalination as the vapor generators, rationally material and structural designs, as well as polymer network engineering are applied to achieve highly efficient freshwater production under natural conditions. Hydrogel-based solar vapor generators are constructed by introducing solar absorbers into polymeric networks of hydrogels to enable solar energy harvesting and conversion, providing thermal energy for vapor generation. To overcome the slow swollen rate of hydrogels during interfacial evaporation, different water pathways are constructed in hydrogel-based solar vapor generators to transport water to the evaporation surface for continuous vapor generation (Chapter 3). Moreover, the utilization of solar absorbers with different sizes and dimensions in hydrogels influences the water management and energy utilization efficiency of solar desalination, providing possibilities for enhanced SVG performance. The water state in hydrogels defines the vaporization behavior of water. Thus, the polymeric networks of hydrogels can be architected to tune the water state and further reduce the energy consumption of water vaporization. Polymeric chains with different hydrophilic functional groups are used to tailor the water-polymer interaction, varying the water state and phase change behaviors. Different polymeric networks, such as interpenetrating networks and copolymer networks, could be constructed for effective water management and further reduced evaporation energy consumption (Chapter 4). By incorporating functional components, the hydrogel-based solar vapor generators have also been endowed with multiple functionalities, such as antifouling, self-cleaning, and thermal responsiveness, to improve water collection and enable freshwater production from other water sources (Chapter 5). Finally, the development of hydrogel-based solar vapor generators is summarized and possible future directions are provided (Chapter 6).

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