Construction And Properties Of Hydrogel-based Planar Photothermal Materials

With population growth,industrialization and environmental pollution,the shortage of drinking water has become an increasingly serious global crisis and challenge.To solve this problem,reverse osmosis,distillation,evaporation,electrodialysis and other technologies have been developed to effectively desalinate seawater or purify waste water.Solar interface evaporation technology is a new evaporation technology,which has attracted much attention because of its high efficiency,sustainability and low cost.Among them,excellent solar photothermal conversion materials play a decisive role,and its structural design tends to be planar.The flexible solar evaporation body with planar structure can realize the localization of solar energy,improve the utilization rate of heat evaporation,and good mechanical properties,suitable for large area,large-scale use;Due to its unique physical and chemical properties and network structure,photohydrogel,as a new evaporating material,can reduce the enthalpy of water evaporation,and has a fast evaporation rate and high efficiency.However,its mechanical properties are poor,so it is not suitable for large-scale use.Therefore,this project aims to adopt two typical solar photothermal conversion materials and construct hydrogelated planar photothermal materials by designing different methods,so as to enhance the physical stability of evaporates with planar structure and improve the solar evaporation rate and conversion efficiency of evaporates with hydrogelation.Based on this,this paper mainly studies the following two aspects:The first part is the preparation of planar multi-walled carbon nanotubes(MWCNT)bucky paper by radiation crosslinking and its properties.The structure and thermal stability of MWCNT powder and MWCNT powder grafted with polyvinyl alcohol(PVA)before and after borate cross-linking were studied.Fourier transform infrared spectroscopy(FTIR),Raman spectroscopy and thermogravimetric analysis(TGA)confirmed the successful cross-linking of PVA grafted carbon nanotubes with borate ester.The obtained MWCNT-PVA dispersive liquid was pumped onto the commercial cellulose acetate substrate(CA)by vacuum filtration method to obtain the planar multi-walled carbon nanotubes(MWCNT)bucky paper(BP).The stability of the composite film was enhanced by borate crosslinking reaction and call it borate crosslinked bucky paper(BBP)membrane.The microstructure analysis showed that the composite film formed a new porous layer on the surface of the CA substrate,and borate crosslinking treatment effectively improved the mechanical durability of the multiwalled carbon nanotubes bucky paper film.The interfacial water evaporation test under simulated solar irradiation shows that BBP film has good water resistance and mechanical stability.The evaporation rate is 0.79 kg m^-2 h^-1 and the evaporation efficiency is 56%in deionized water under one strong sunlight.Durability tests showed that the evaporation rate of BBP film in Na Cl solution(3.5 wt%)and sulfuric acid solution(1 mol L^-1)was 0.76 kg m^-2 h^-1,proving its great applicability in saline and acidic conditions.The evaporation rate of saturated Na Cl solution is 0.70 kg m^-2h^-1(very close to the evaporation rate of deionized water).At the same time,BBP membranes show unexpected stability in the presence of salt accumulation during long periods of continuous solar evaporation.In the second part,sodium alginate/Cu S double-layer sandwich hydrogel was prepared by melting ice complexation method and its properties were studied.Cu S powder synthesized by hydrothermal synthesis was characterized and its crystallization and dispersion were good.By freezing suction layer and the preparation of sodium alginate sodium alginate blend Cu S layer evaporation ice templates,template middle insert filter paper,and then adopt the method of complexing of melting ice,ice template for soaking in alcohol solution of Cu²⁺,use alcohol solution on the dissolution of ice and Cu²⁺complexation with seaweed acid radical ion,the complexing-20?environment for 1 hour,The gelatin is prepared.Two control group hydrogels were prepared by the same method,which were monolayer hydrogels with only evaporation layer and bilayer hydrogels without filter paper sandwich.Two evaporation modes,self-floating and floating-platform support,were used to simulate the interfacial water evaporation under solar irradiation.The results show that the double-layer sandwich plane hydrogel has the best performance and can achieve a high photothermal evaporation rate in pure water.The self-floating mode is 2.1 kg m^-2 h^-1,and the floating-platform support is 1.8 kg m^-2 h^-1.The results show that the double-layer sandwich structure can make the horizontal water channel provided by the filter paper cross with the vertical water channel of the gel network,so as to improve the water transport capacity and photothermal evaporation capacity of the sandwich material.At the same time,the self-floating evaporation form is more conducive to the evaporation of the hydrogel material.In the durability test,the structure of hydrogel was destroyed in acid-base solution,which proved that the formation of hydrogel was indeed due to the ion exchange between alginate ions and Cu²⁺.In the test of salt resistance,the evaporation rate of the double-layer sandwich hydrogel also reached 1.89 kg m^-2 h^-1 in Na Cl solution(3.5 wt%)in self-floating mode,which proved its applicability in the condition of salt water.The decrease of evaporation rate was lower than that of the control group with the increase of salt water concentration.It shows that the structure design of double layer sandwich can obviously enhance its salt resistance.The above research results proved that the construction of hydrogelated plane photothermal evaporation material by radiation method or collateralization method can significantly enhance the stability of the material,improve the water transfer capacity of the material,and improve its photothermal evaporation performance.The photothermal evaporation material constructed by these two methods also shows wide applicability to salt solution.It provides a new idea and direction for the research of photothermal conversion material evaporator based on these two materials in the fields of wastewater treatment and desalination.