Interfacial Solar Energy Conversion for Freshwater Production


Student thesis: Doctoral Thesis

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Awarding Institution
Award date8 Sept 2023


Freshwater is indispensable to all forms of life, yet water scarcity is anticipated to affect 50% of the global population by 2050. Conventional freshwater production technologies require expensive and energy-consuming infrastructures, restricting their use in water-stressed impoverished areas. Solar-driven freshwater production, including solar-driven interfacial desalination (SID) and atmospheric water harvesting (AWH), was put forward to generate freshwater only utilizing solar light. In this thesis, I will present my study aimed at overcoming the ongoing challenges regarding these two applications.

The two key components in SID are the solar absorber and the porous matrix. However, there are still obstacles in designing absorbers with broad and strong absorption of solar irradiation and the ability to prevent volatile organic compounds from evaporating, as well as structures with light, heat, and water management ability for SID. From Chapter 2- Chapter 4, I will discuss my research aimed at solving these problems from the perspective of absorber material and structural design of the matrix.

Regarding the solar absorbers in SID, I will present my research on plasmonic metal/doped-semiconductor nanocomposites. Despite their strong light absorption capacity, the narrow absorption bandwidth of plasmonic metal materials has impeded their application in SID. To address this limitation, we constructed a plasmonic nanocomposite Au NR@Cu7S4, that exhibited broad and intense absorption of solar radiation due to the integration of their individually visible (Au NR) and near-infrared (Cu7S4) plasmons, with this absorption being further broadened and enhanced by the coupling of these two plasmons. This strategy is conceptually distinct from aggregating plasmonic metal nanoparticles, which is subject to a trade-off between the breadth and intensity of their absorption. The Au NR@Cu7S4-impregnated hydrogel delivered a high evaporation rate despite having a Au areal density of only 28 μg cm−2. Moreover, it showed excellent volatile organic compound removal performance.

Regarding the matrix structural design in SID, an energy confinement strategy through the regulation of heat, light, and water distribution by architecting the evaporation surface was demonstrated. By introducing the cone pattern, the evaporator can enhance light absorption, amplify localized heating via reabsorbing light and diffused thermal radiation. The cone pattern also endowed the evaporator with wide-angle light absorption, boosting the evaporation rate at various solar zenith angles. In terms of water distribution, the cone-patterned evaporator maintained a lower water content on the surface, preventing energy loss from heating excess water while guaranteeing water replenishment. As a result, the cone-patterned evaporator improved the evaporation rate by 20% compared with the conventional flat evaporator.

The matrix in SID is supposed to supply water continuously through capillary forces enabled by the pores. However, water in the pores leads to conduction heat loss to the bulk water owing to the high thermal conductivity of water. To clear up this dilemma, we proposed a highly interconnected sponge (HIS) that can regulate water supply and heat loss simultaneously. With a tunable porous structure of the HIS, the dependence of mechanical strength, water transport, thermal loss, and evaporation performance on porosity was investigated. The optimized HIS evaporator showed exceptional mechanical stability and long-term desalination durability. A desalination prototype was demonstrated, yielding 3.62 kg m-2 per day under outdoor conditions. The highly efficient, durable, and inexpensive HIS demonstrated great potential in providing affordable clean water.

In inland terrain where seawater sources are less accessible, AWH which utilizes hygroscopic salt-based porous materials to capture water from air is promising in mitigating water stress. However, the solution leakage resulting from the weak attraction between the matrix and salt, the high affinity between salt ions and water restricts the cyclic stability and the water desorption process, respectively. In Chapter 5, to address these issues, a LiCl-incorporating polyelectrolyte complex (CS-SA/LiCl PEC) endowed with salt immobilization ability and low desorption energy demand was proposed, achieving effective moisture harvesting with leakage-free and fast desorption kinetics. Capitalizing on the ionic character, the CS-SA/LiCl PEC immobilized the salt ions through electrostatic interaction, limiting leakage and enhancing cyclic stability. The polymer network interacted with the adsorbed water to reduce the amount of water strongly confined by hydrated ions, which lowered the desorption enthalpy in comparison with pure salt, enabling over 90% of the collected water to be desorbed within 10 minutes. By tuning the salt-sorbent and water-sorbent interactions, the CS-SA/LiCl PEC showed great advantages for AWH application.