With the rapid development of society and industry, fossil energy has been widely developed and utilized. The total amount of freshwater resources available to humanity has declined significantly, leading to an increase in water-scarce areas and populations worldwide. In recent years, solar thermal technology has emerged as a promising solution for seawater desalination and wastewater treatment. Interfacial Solar Steam Generation (ISSG) is based on solar thermal conversion technology, which concentrates the generated heat on the evaporation surface to achieve functions such as seawater desalination and sewage treatment. The photothermal evaporation process often requires high-speed internal water supply, which makes it difficult to control the heat loss at the photothermal interface, so it is challenging and important to design and synthesize an interfacial evaporation structure with efficient photothermal conversion and thermal management capabilities for higher evaporation efficiency. In addition, the application of photothermal evaporation structure to photocatalytic water splitting to hydrogen production can improve the overall utilization rate of solar energy in other systems and broaden the practical application space of photothermal evaporation system.
Recently, the research group of Professor Chang Kun of Nanjing University of Aeronautics and Astronautics has creatively developed an interface assembly hydrogel evaporator obtained by a simple preparation method, which has efficient photothermal conversion and thermal management performance, the longitudinal thermal conductivity under light is as low as 0.4413 W m-1 K-1, and the long-term average evaporation rate of 5.07kg m-2 h-1 in real seawater is obtained, the overall absorption rate of sunlight is 95.6%, and the evaporation efficiency is as high as 93.7%. At the same time, a photothermal evaporation-photocatalytic hydrogen production coupling system was formed after the photocatalyst was loaded, and the hydrogen production rate of 45.5 mmol m-2 h-1 could be achieved by using the structural characteristics of the photothermal evaporation interface. This interfacial assembly design provides a new idea for the large-scale preparation of photothermal evaporation and photocatalytic hydrogen production composites, and also proves the great potential of photothermal evaporation materials to develop into multi-functionality. The study was published in Advanced Functional Materials in a paper entitled "Interfacial assembled hydrogel evaporator for highly efficient thermal management and photothermal coupled water splitting reaction". The first author is Li Wenjie, a master's student.
【Interfacial Assembly Hydrogel Evaporator】In this study, molybdenum disulfide (MoS2) with hollow structure was prepared by bubble template method, and composite photothermal nanoparticles MoS2@PDA with bilayer structure were obtained by in-situ polymerization of polydopamine (PDA). The unique double-layer hollow structure not only increases the efficiency of light absorption and photothermal conversion, reduces diffuse reflection and heat loss, but also takes advantage of the strong adhesion of the PDA shell, which can be assembled on the surface and internal pore wall of the general-purpose polyvinyl alcohol (PVA) hydrogel through simple coating and vacuum suction. The interfacial photothermal conversion efficiency and evaporation rate of the hydrogel evaporator were significantly improved by the hollow structure of the photothermal conversion material and the interfacial assembly of the evaporator, reaching 93.7% and 5.41 kg m-2 h-1, respectively.
The authors tested the thermal management and photothermal evaporation performance of the hydrogel evaporator, and found that compared with the photothermal particles directly mixed with the precursor solution and distributed inside the hydrogel evaporator (MPPH-I), the evaporation interface of the hydrogel loaded with MP by the interface assembly method has an ultra-low evaporation enthalpy (1075 J g-1) and an interfacial steady-state temperature of 35.4 °C under one solar illumination. In addition, it has good interfacial thermal management performance and side cold evaporation effect under different exposure heights and light intensities. 【Seawater Desalination and Outdoor Performance Test】In order to test the salinity tolerance and desalination ability of the hydrogel evaporator, a series of evaporation performance tests under salinity gradient were carried out, and an independent evaporation device unit with good photothermal evaporation-condensation collection was designed for outdoor testing. The hydrogel evaporator is capable of long-term evaporation in brine with a salinity of 20% or less at 1 sunlight intensity. After purification and collection, the metal cation content of real seawater and organic polluted water is far lower than the World Health Organization's (WTO) standard for direct drinking water, and non-volatile organic pollutants are basically completely removed.
Using commercial polyurethane foam as the self-floating base of the hydrogel evaporator, the authors fabricated a scalable evaporation material unit, combined with the design of a vacuum-insulated evaporator that can operate on seawater or organic contaminated water as the water to be purified, and the daily water production of this stand-alone unit has been shown to be 24.4 kg m-2 per day. 【Coupled Photocatalytic Hydrogen Production from Seawater】Taking advantage of the strong adhesion of the PDA shell of MP, the authors creatively loaded the strontium titanate-based photocatalyst on the evaporation interface of MP accumulation, and obtained a composite hydrogel evaporator with photothermal evaporation coupled photocatalytic hydrogen production. The reaction temperature and stability were better than those of the traditional suspension system under the irradiation of one sunlight intensity, and the hydrogen production rate of 59.7mmol m-2 h-1 could be reached at a low catalyst dosage. In addition, because the nanoparticles accumulated at the evaporation interface will not form a continuous waterway to cause the erosion of the catalyst by seawater, a large amount of water vapor generated by the photothermal evaporation process provides sufficient substrate for the photocatalytic reaction, and this structure can carry out the desalination of seawater and photocatalytic hydrogen production at the same time, and the hydrogen production rate of 45.5mmol m-2 h-1 and the evaporation rate of 4.67 kg m-2 h-1 can be obtained in real seawater. The overall solar energy utilization rate of STH and coupled reaction in the photocatalytic reaction part was 0.282% and 85.7%, respectively. The structural design and preparation of this photocatalytic material verifies the feasibility of the photocatalyst immobilization system and direct photocatalytic hydrogen production from seawater, and makes an attempt for the structural design and large-scale development of photothermal evaporation-photocatalytic composites.
In summary, the authors designed and synthesized a photothermal conversion material with a hollow composite structure, and obtained a hydrogel evaporator with efficient thermal management performance through interface assembly, which verified the structural characteristics of the photothermal conversion material and the superiority of the assembly method in the photothermal evaporation system, and also provided a feasible solution to solve the contradiction between thermal management and water transport in the evaporation process. The authors used the interfacial structure characteristics of the evaporator to obtain a composite hydrogel evaporator with photothermal evaporation-photocatalytic coupling after loading the photocatalyst, and realized the direct photocatalytic hydrogen production from seawater. At the same time, the structural design ideas involved in this paper are also applicable to the large-scale preparation of seawater desalination and photothermal evaporation-photocatalytic composites, which expands the development space of the versatility and practicability of photothermal evaporation materials.
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Source: Frontiers of Polymer Science