Preparation Of Cellulose Fiber-based Photothermal Materials And Their Interfacial Evaporation Properties

The scarcity of drinking water resources is a universal problem faced by the world,with approximately 2 billion people worldwide severely affected by water scarcity.With the ocean covering 75%of the Earth’s surface,it is the most abundant water resource reservoir on Earth.Seawater desalination is one of the most practical solutions for solving the problem of drinking water shortage.Traditional seawater desalination technologies mainly include thermal distillation and membrane separation,which suffer from problems such as high energy consumption,high greenhouse gas emissions,and serious environmental pollution.Solar-driven interfacial evaporation is a technology that utilizes solar energy to achieve light-thermal conversion and concentrate heat at the gas-liquid interface to achieve rapid seawater evaporation and desalination.This technology has the advantages of being green,efficient,and low-cost,and is of great significance for solving the problem of drinking water resource shortage.The photothermal interfacial evaporator is the core device of solar-driven interfacial evaporation technology,consisting of photothermal conversion materials with good light absorption performance and supporting materials that provide the water supply system.Cellulose fiber materials,which are easy to process,easy to modify,and environmentally friendly,have become a hot material in the field of interfacial seawater desalination research in recent years.Currently,although there is considerable research on solar-driven interfacial evaporation,balancing the relationship between water transport,energy conversion,and heat management to achieve the goal of improving water evaporation efficiency remains a key factor restricting the development of this technology.Based on this,this paper mainly focuses on cellulose fibers as the main substrate,designs and develops a series of solar-driven interfacial evaporative devices,and studies the light absorption performance,pore structure,wettability,and evaporative performance around the pore channel structure and material configuration of the evaporator.The mechanism of the synergistic improvement of water evaporation rate by photothermal absorption,water transport,and heat management is elucidated,providing theoretical support for optimizing the design of interfacial evaporation devices for seawater desalination,and the specific research work is as follows:（1）A dual-network CNT@DAC cellulose fiber membrane with micro/nanostructures was prepared by Schiff base crosslinking using aldehyde-functionalized microcrystalline cellulose（DAC）as the supporting network and amino-functionalized carbon nanotubes（CNT）as the photo-thermal conversion material.By adjusting the ratio of CNT and DAC components,the microstructure,wettability,and light absorption performance of the CNT@DAC cellulose fiber membrane were investigated.The results showed that the CNT@DAC cellulose fiber membrane had a light absorption rate of approximately 97%in the solar spectrum range of 300-2500 nm.Upon exposure to sunlight at an intensity of 1.0,the surface temperature of the membrane increased from room temperature to 42.4℃within 1min,and the interface water evaporation rate was 1.58 kg m^-2 h^-1.In addition,water and oil droplets could spread on the surface of the CNT@DAC cellulose fiber membrane,exhibiting amphiphilic characteristics.Taking advantage of the photothermal conversion performance and superwettability,the CNT@DAC cellulose fiber membrane was used to purify oil-containing seawater,and the collected condensed water had COD and salt ion concentrations less than 15 mg L^-1,meeting the drinking water standards of the World Health Organization.（2）In response to the weak water transport in the cellulose fiber membrane interface evaporator,this study used aldehyde-functionalized nanocellulose fibers（A-CNF）as building units,cross-linked them with polyethyleneimine（PEI）through Schiff base chemical reaction,and then coated the surface with polypyrrole（PPy）using in situ polymerization method to prepare PPy@PEI@A-CNF cellulose fiber aerogels through freeze-drying technique.The pore size structure and micro-morphology of PPy@PEI@A-CNF cellulose fiber aerogels were studied in relation to their water transport performance.The results showed that the PPy@PEI@A-CNF cellulose fiber aerogel has characteristics such as low density(0.021 g cm^-3),low thermal conductivity(0.042 W m^-1 K^-1),high solar absorption rate（98%）,and fast water wetting speed（water penetration within 0.1 s）.The stress residue of PPy@PEI@A-CNF cellulose fiber aerogels remained at 89.9%after 100 cycles of cyclic compression at 40%strain,indicating excellent material structural stability.The interconnected porous structure inside PPy@PEI@A-CNF cellulose fiber aerogels enhances the capillary pumping force of water and the convection between water bodies.During continuous operation for 48 h in the seawater desalination process,PPy@PEI@A-CNF cellulose fiber aerogels still maintained a water evaporation rate of 1.59 kg m^-2 h^-1,effectively solving the problem of decreased lifespan of the evaporator caused by salt blockage while improving its water transport capacity.（3）To addresses the issue of heat dissipation during water transport in cellulose fiber aerogels,a Fe³⁺-TA@eggplant cellulose fiber aerogel was constructed by in situ polymerization reaction of tannic acid（TA）and freeze-drying using a natural cellulose fiber network with a skin-core structure as the substrate.The relationship between the pore structure of the material and the interfacial evaporation rate,salt ion precipitation and redissolution was investigated.The carbon emissions of the prepared cellulose fiber aerogel were compared with those of existing seawater desalination processes during the production and use phases using a life cycle assessment system.The results showed that Fe³⁺-TA@eggplant cellulose fiber aerogel has low density(0.043 g cm^-3)and thermal conductivity(0.025 W m^-1 K^-1),as well as strong solar absorption capacity（98%）.The Janus skin-core structure,which features a hydrophobic outer layer（water contact angle of 101°）and a hydrophilic inner layer（water contact angle of 0°）,was designed to create a unidirectional water transport path with low heat loss,achieving an interfacial water evaporation rate of 1.61 kg m^-2 h^-1.Fe³⁺-TA@eggplant cellulose fiber aerogel has very low CO₂ emission equivalents（139.6 kg）when processing one cubic meter of seawater,which means that this evaporator can achieve thermal management and avoid energy consumption and environmental pollution.（4）Inspired by plant transpiration and Murray’s law,a cellulose fiber-based biomimetic tree with a four-level hierarchical structure was constructed by assembling a porous cellulose sponge and cellulose paper coated with MXene nanosheets as the trunk and leaves,respectively.The relationship between water transport acceleration and energy utilization was explored,and the evaporative mechanism of the cellulose fiber-based biomimetic tree was investigated using complementary density functional theory and finite element simulations in molecular dynamics and energy fields.The results showed that the hierarchical pore structure of the biomimetic tree,with decreasing pore size from bottom to top,enhanced the driving force for water transport,and water was spontaneously transported up to 5.3 cm within 360 s.When applied to interface evaporation,the evaporator surface of the biomimetic tree exhibited a differentiated temperature distribution,with a cold evaporation layer where the surface temperature was lower than the ambient temperature,maximizing energy utilization.The binding energy of MXene-H₂O intermolecular hydrogen bonds inside the evaporator was-18.8 k J mol^-1,promoting water evaporation at the gas-liquid interface,and the water evaporation rate reached 2.46 kg m^-2 h^-1.This optimized design concept of the evaporator provides theoretical support for the construction of efficient seawater desalination devices.