Design Of Polymer/MXene Composites And Their Applications For Solar-driven Water Evaporation And Seawater Desalination

Climate change and social development have accelerated the consumption of freshwater resources.In order to address the global freshwater shortage,many kinds of methods to obtain clean water have been developed,however,these methods often face disadvantages such as huge energy consumption and the need for centralized equipment.Solar-driven seawater desalination technology,which utilizes solar energy to obtain fresh water,has become an important means of solving the freshwater shortage crisis and has a broad application prospect due to its sustainable,low-cost,and scalable advantages.Although much progress has been made in the current solar-driven seawater desalination technology and a variety of solar-thermal conversion materials with excellent solar-thermal properties and many advanced evaporator designs have been exploited,the evaporation rate and resistance to salt contamination of the evaporator still need to be further improved.The problems of how to simultaneously adjust the internal structure and surface morphology to improve the evaporation performance of hydrogel evaporators,how to make full use of the wood substrate for the development of salt-resistant evaporators with high-efficiency seawater and brine desalination,and how to achieve rapid seawater desalination and salt collection simultaneously by the structural design and internal mass-transfer optimization of the hydrogel have yet to be solved.To address the above problems,using the 2D MXene nanosheets as the solar-thermal conversion material,we have prepared hydrogel evaporators with excellent water evaporation performance by the internal structure and surface modulation of hydrogels,wood-based evaporators with long-term stable brine desalination performance by the improvement of hydrophilicity and engineering design,and hydrogel evaporators with stable seawater desalination and salt collection by the structural design and optimization of water supply,demonstrating good application prospects.The specific research contents and results are as follows:（1）Preparation of polyvinyl alcohol/MXene hydrogel evaporators with vertically oriented channels and modulated surface topography and studies on their water evaporation performances:Hierarchical structure and surface topography are critical to the evaporation performances of solar-driven water evaporators,but simultaneous optimization of these two aspects remains challenging.In an attempt to simultaneously regulate the internal microstructure and surface topography in hydrogels to enhance both solar steam generation performance and desalination efficiency,in this research,anisotropic polyvinyl alcohol/MXene composite hydrogels for efficient solar-driven water evaporation and wastewater purification are fabricated using a template-assisted directional freezing approach followed by precise surface wettability modulation.The resultant composite hydrogels exhibit vertically oriented channels that ensure fast water supply during evaporation,and their polyvinyl alcohol skeleton can reduce the vaporization enthalpy of the water in the hydrogels.The incorporation of MXene nanosheets enables efficient solar light absorption and solar-thermal conversion while providing structural reinforcement to the hydrogels.More importantly,the as-created undulating solar-thermal surface,featuring modulated hydrophilic troughs and hydrophobic crests,significantly enhances solar-thermal conversion efficiency,thereby boosting solar evaporation performances.As a result,the fabricated hydrogel-based evaporator exhibits an impressive evaporation rate of 2.55 kg m^-2 h^-1 under 1 sun irradiation,coupled with long-term durability and desalination stability.Besides,the outstanding mechanical robustness of the hydrogel further enables high portability through a readily achievable process of reversible dehydration/hydration.（2）Preparation of salt-resistant wood-based evaporators with top-down water supply and studies on their seawater and brine water desalination performance:Solar-driven water evaporators often face severe salt accumulation and evaporative performance degradation during long-term seawater and brine desalination.To realize high-yield and long-term desalination of seawater and high salinity water,in this research,salt-resistant wood-based solar evaporators with top-down water supply are fabricated by delignification of balsa wood for enhancing its hydrophilicity followed by vacuum-assisted impregnation of polyvinyl alcohol and MXene nanosheets and lyophilization.Thanks to the incidental gain of environmental energy,the sufficient top-down water supply,the porous channels and intrinsic hydrophilicity of the delignified wood,and the efficient solar light absorption and solar-thermal energy conversion of the dispersed MXene nanosheets,the resultant polyvinyl alcohol/MXene-decorated wood（WPM）solar-driven water evaporator achieves a water evaporation rate of as high as 4.31 kg m^-2 h^-1 under1 sun irradiation.A superior solar-driven desalination rate of 3.83 kg m^-2 h^-1 is achieved during evaporating a brine water with 25 wt%of Na Cl for 8 h.By timely dilution and discharge of concentrated brine on the WPM evaporator surface,the sufficient top-down water supply avoids salt deposition and hence achieves the performance stability during the long-term solar-driven desalination.The excellent evaporation efficiency and the long-term salt-resistance are demonstrated by desalinating an aqueous solution of 15 wt%Na Cl over 100 h under 1 sun irradiation with an exceptional average evaporation rate of 4.24 kg m^-2 h^-1.（3）Preparation of pyramidal polyvinyl alcohol/MXene hydrogel evaporators and their applications in seawater desalination and salt recovery:Harvesting fresh water from the oceans using solar-driven water evaporation technology contributes to mitigating the crisis of freshwater scarcity,and the inorganic salts in seawater are also considered as valuable resources.In order to simultaneously achieve rapid desalination and salt recovery,in this research,pyramidal polyvinyl alcohol/MXene hydrogels with both internal cellular vertical porous structure and peripheral lamellar oriented structure（LPH）were prepared by directional freezing method.The incorporation of MXene nanosheets reinforces the hydrogel and provides it with excellent solar-thermal conversion properties;the large number of hydroxyl groups of polyvinyl alcohol can form interactions with MXene,ensuring their uniform dispersion and preventing them from falling off;and the pyramidal macrostructure increases the actual evaporation area,reduces heat loss and realizes side cold evaporation.Furthermore,the cellular structure formed inside the hydrogel realizes rapid mass transfer in the longitudinal direction,and the lamellar structure formed at the periphery facilitates lateral water and salt transport.Therefore,the prepared LPH solar-driven water evaporator achieves an excellent water evaporation rate of 2.38 kg m^-2 h^-1 under 1 sun irradiation.By providing an all-directional water supply and utilizing siphoning,the LPH evaporator achieves an average rate of 2.20 kg m^-2 h^-1 during 70 h of desalination of a 15 wt%Na Cl solution.In addition,the directional water flow caused by the siphoning effect and the pyramidal structure together prompts the salt to accumulate at the bottom of the evaporator,realizing directional salt recovery and proving the good application prospects of the pyramidal evaporator.