Nanomaterials for Efficient Solar-driven Interfacial Water Evaporation


Student thesis: Doctoral Thesis

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Award date28 Jul 2023


Scarcity of drinking water is a growing challenge facing humanity. To address this challenge, technologies for obtaining clean water from seawater or polluted water are highly desired. Desalinated seawater used as industrial and municipal water to solve the needs of people's daily life. Many techniques such as filtration, reverse osmosis, and thermal distillation have been reported for water remediation and purification. However, the traditional seawater desalination technology still faces multiple problems.

Solar energy is the most abundant renewable energy on the earth's surface, with abundant reserves and no cost. Researchers have developed interface solar steam generation systems that uses photothermal conversion materials to absorb solar energy and convert solar energy into heat energy. Steam is generated and the resulting water vapor is condensed to obtain clean water. This technology has received widespread attention in the fields of seawater desalination and wastewater treatment. Compared with traditional seawater desalination processes, solar-driven water evaporation technology does not require electrical nor chemical energies, which not only reduces the production cost of clean water, but also reduces environmental pollution.

Chapter 1 introduces the status of global water resources, traditional desalination challenges, general background on reported clean water production technologies of this thesis, including their fundamental principles and the recently developed solar-driven interfacial water evaporation technologies. Scientific problems in solar driven interfacial water evaporation are also summarized in terms of light management and heat management. Moreover, solar thermal materials, systematic design, related applications, and limitations are also discussed.

Chapter 2 reports a stable conjugated molecule DPP-2T with strong intermolecular interaction. It is shown to exhibit a wide optical absorption range over ~ 250 to 1000 nm. By incorporating DPP-2T into a 3-dimensional porous polymer network, a highly elastic composite sponge with a high solar-thermal conversion efficiency is obtained. The composite sponge shows a high and stable evaporation rate of 2.60 kg·m-2·h-1 with a solar evaporation efficiency of 89 % under 1 Sun illumination. An efficient metal-removal efficiency over ~ 99 % is achieved for purifying contaminated water via solar evaporation. On the other hand, the composite was also found to have good oil absorption capacity and can absorb silicon oil of 4 times its own mass.

Although small molecules-based absorbers show high photothermal conversion efficiency and functional application in oil absorbtion, they may be not the best choice with consideration of its preparation cost. Chapter 3 reports a design by embedding commercially available boron carbide (B4C) powder into a porous polymer foam as a low-cost and robust solar absorber. As the B4C powder can be purchased at a low cost (14 $·kg-1) and uses without further modification, the solar absorber can be produced at a much lower cost comparing to reported solar absorbers. The B4C powder not only has good optical absorption over the solar spectrum, but it also has very good stability in different extreme environments (e.g., high temperature, strong acid, base, oxidation resistance, etc). After embedding into a polyvinyl alcohol (PVA) foam, the boron carbide bilayer foam (BCBF) achieves a high evaporation rate of 2.8 kg·m-2·h-1 with a solar evaporation efficiency of 93% under 1 Sun illumination. Combing this good solar steam generation performance with the very low cost, the BCBF delivers a record high cost-effectiveness of 778 g·h-1·$-1. It is also demonstrated that the BCBF can be achieved low-cost high-rate freshwater production from water contaminated with various kinds of common contaminants including heavy metals, dyes, and microorganisms. The composite foam can also work under extreme conditions, including concentrated acid, strong alkali, and high salinity.

Chapter 4 reports a single-atom piezocatalyst membrane prepared by anchoring isolated Ca atoms on a composite of nitrogen-doped carbon (NC) and PVDF (Ca-PVDF). It is found that the addition of Ca-atom-anchored NC (Ca-NC) would significantly increase the volume fraction of β-phase PVDF from 29.8 to 56.3%. Besides the largely enhanced piezoelectric property, the hybrid polymer membrane has abundant active sites for the porous structure to benefit the free radical reaction. The synergistic effect enables piezoelectric generation of active oxygen species for organic compound decomposing at a record-high rate of 0.11 min-1 and antibacterial efficiencies of 99.8% under ultrasound (US). More importantly, under 1 Sun + US condition, the achieved Ca-PVDF membranes show a high and stable water evaporation rate of 2.14 kg·m-2·h-1, demonstrating potential applications for seawater desalination and wastewater purification.

The thesis is summarized with a conclusion of all the studies mentioned above in Chapter 5 which also include discussions on how to further enhance their performance.