1. Introduction
Solar vapor generation (SVG) technique holds great promise for applications in solving two of the most serious global challenges: the water crisis and energy shortages.[1-6] The main bottleneck of this technique is the contradiction between high-efficiency water evaporation and salt crystallization, which dramatically restricts the practical application of SVG, especially in the treatment of highly concentrated brine.[7-11] Owing to the ultra-low diffusivity of salt in water (10-9 m2s-1, and the diffusivity of water vapor in air is about 10-5 m2s-1),[12] the salt inevitably crystallizes and precipitates on the surface of the photothermal material as the water vapor continues to escape rapidly.[13, 14] This problem severely hinders the light absorption, water transport and water vapor escape in the continuous water evaporation process, significantly degrading the performance of SVG and terminating the operation of the device. Thus, the development of photothermal water evaporation materials with both efficient water evaporation performance and self-cleaning/anti-salt fouling properties is a key issue to drive SVG from the laboratory to large-scale practical applications.
To achieve the above aim, the SVG materials should possess three key properties. One is displaying the excellent solar light capture and photothermal conversion capability. The other is possessing efficient water evaporation pathway. The third one is having suitable hydrophilic-hydrophobic dual interfaces that can improve the anti-salt fouling ability. Recently, a series of reports by Yu et al. have demonstrated that hydrogen bonding or electrostatic interactions between hydrophilic functional groups and water molecules facilitate the evaporation of bulk water as activated water,[15-23] providing a fast and efficient water evaporation pathway (exceeding the theoretical evaporation rate value of 1.47 kg m-2h-1).[24] Such evaporation process requires less energy than the conventional single-molecule water evaporation that requires breaking the hydrogen bonds between all water molecules. On the other hand, several studies including hydrophilic-hydrophobic janus and biomimetic structures aim to enhance the anti-salt fouling ability of evaporators through hydrophobic modifications.[7,25-32] For example, the hydrophobic surface structure designed by Zhu et al. based on natural water lily can effectively inhibit the crystallization of salt on its surface and achieve the complete separation of salt and water. However, the strongly hydrophobic surface weakened the effective contact of water with the photothermal active site and the water activation capacity, which just yielded an evaporation rate of 1.39 kg m-2h-1. Therefore, the combination of three properties into one material is obviously a challenging task for exploring efficient, anti-salt fouling and durable SVG evaporators.
Heteropoly blues (HPBs) that possess a classic blue-black color are reduced polyoxometalates (POMs).[33-39] They exhibit excellent potential applications in the fields of photothermal catalysis and photodynamic therapy based on their broad and high light absorption properties, unique photothermal activity and stability in the solar spectral range.[40-46] Moreover, the oxygen-rich surface structure of HPBs strongly supports their affinity for water,[47,48] exhibiting the potential in the construction of hydrophilic-photothermal evaporation sites. Importantly, the HPBs that hold high-negative charge can be electrostatically self-assembled with cationic polymers to avoid water leaching problems and achieve efficient and stable loading. Further regulation of the surface microenvironment of the functional self-assembled materials is expected to achieve system management and optimization of the surface local hydrophilic-hydrophobic region, offering an effective strategy for constructing hydrophilic-hydrophobic interfacial SVG evaporators.
Given the above considerations, a new type of photothermal water evaporation materials (abbr.MF@HPB-PPyn-OA) were designed and constructed based on the melamine foam (MF) skeleton, which is covered by a hydrophilic-hydrophobic composite interfacial material via a one-step self-assembly of HPB, oleic acid (OA) and polypyrro (PPy). On the surface of MF@HPB-PPyn-OA, the hydrophilic regions of HPB and the hydrophobic regions of OA show spatial separation features due to the electrostatic mutual repulsion of HPB and OA molecules. Benefiting from the advantages of the self-assembly strategy and the unique surface structure, the local hydrophilic-hydrophobic interface of this SVG material can be regulated and optimized so as to realize the ultrahigh water evaporation and salt crystallization simultaneously. In a series of MF@HPB-PPyn-OA, MF@HPB-PPy10-OA exhibits the best SVG performance, yielding 3.3 kg m-2 h-1 of evaporation rate and 96.5% of salt harvesting efficiency in the complete salt-water separation process of high-salinity (10 wt%) brine at 1 solar irradiation, and realizes zero liquid discharge, which belongs to the record-high of high salinity systems reported so far. Moreover, MF@HPB-PPy10-OA can operate continuously and stably for over 100 h at an ultrahigh evaporation rate of 3.3 kg m-2 h-1 (corresponding to an energy efficiency of 92.1%) in continuous solar desalination of high-salinity brine (10 wt%), and enables real-time anti-fouling and maintains surface cleanliness. The low cost, excellent mechanical properties and processability of MF@HPB-PPy10-OA in terms of manufacturing exhibit promising prospects for commercial applications.
Results and discussion