Preparation Of MXene/Fiber Composites And Its Application In Solar-Driven Interfacial Evaporation

The shortage of fresh water resources has become a potential threat to the survival and sustainable development of human society.Current freshwater production technologies mainly include reverse osmosis,membrane distillation and multi-stage thermal flash distillation,which have drawbacks such as high energy consumption,high costs and even secondary environmental pollution.There is therefore an urgent need to develop a new technology for desalination that is cost effective and environmentally friendly.As the most widespread source of energy and water resources in nature,solar energy and seawater have the advantage of achieving efficient,stable and sustainable production of freshwater resources,and therefore solar driven interface evaporation(SDIE)technology has received widespread attention.The ability of water transport,salt barrier and low heat loss during the interface evaporation process becomes the key to improve the rate and efficiency of SDIE.In this thesis,MXene nanosheets are loaded on the fiber surface to form the basic unit of photothermal conversion and a series of superhydrophilic interfacial evaporator composites are designed and fabricated to achieve efficient solar-driven interfacial evaporation and freshwater yield.The thesis consists of three parts as follows:1.Preparation of chitosan/MXene/polypropylene composite fabrics using chitosanassisted MXene nanosheets loaded onto commercial polypropylene fabrics.The chitosan layer was introduced to improve the durability of the composite fabric by improving the interfacial interaction between the MXene nanosheets and the fibers,and the chitosan layer was able to isolate the MXene layer from direct contact with air thus avoiding oxidation of the MXene nanosheets.The resulting composite fabric is super hydrophilic and super oleophobic underwater.Water can quickly wet the composite fabric and form a water film on the fiber surface to prevent insoluble organic solvents from entering the fabric.The super hydrophilic and porous structure gives the composite fabric evaporator fast water transfer capability and excellent desalination performance,with evaporation rates and evaporation efficiencies of up to 1.50 kg·m-2·h-1 and 88.1%.In addition,the pre-wetted fabric composite can quickly repel crude oil floating on the surface of seawater,enabling continuous interfacial evaporation in seawater in complex environments.2.Because composite fabrics are limited by the evaporation limits of conventional twodimensional materials,this chapter introduces PVA hydrogels to reduce the enthalpy of water evaporation and increase the evaporation rate of the evaporator.A three-dimensional two-layer PVA hydrogel evaporator with an asymmetric structure is constructed.The top photothermal layer is composed of a PVA hydrogel containing MXene/kapok fibers.The self-assembly of MXene on kapok fibers as photothermal conversion units can be uniformly dispersed in the gel,which on the one hand reduces the concentration of MXene and at the same time avoids the aggregation of MXene dispersed directly in the gel;the bottom layer consists of an oriented PVA hydrogel made by directional freeze-thawing.The double-layer hydrogel evaporator ensures maximum light absorption and photothermal conversion,while the abundance of vertically oriented channels greatly improves the water transport capacity of the hydrogel.The evaporator has shown high and consistent evaporation rates and efficiencies(up to 2.49 kg·m2·h-1 and 91.5%)during long-term use.The evaporator is also able to operate normally in environments such as oil-in-water emulsions and seawater of different pH values,maintaining a stable evaporation performance.3.To further improve the evaporation rate of the evaporator,heat absorption from the surrounding environment was considered during the evaporation process.The MXene nanosheet-loaded cotton fibers were homogeneously dispersed in a chitosan solution,and then the chitosan/MXene/kapok fiber composite aerogel evaporator with a vertical porous channel structure was prepared by directional freezing and drying.The MXene/kapok fiber not only acted as a photothermal conversion unit,but also ensured the structural integrity and mechanical robustness of the aerogel.Thanks to the richly oriented pore channels,the composite aerogel allows for the continuous transport of water to the evaporator surface and increases evaporation from the sidewalls.The evaporation rate and efficiency can reach 2.74 kg·m-2·h-1 and 94.5%respectively,thanks to the efficient photothermal conversion and environmental heat absorption.The composite aerogel evaporator with excellent salt barrier properties can operate in high salt concentration seawater and emulsions and also shows longterm evaporation stability.