Preparation Of Organic Phase Change Composite Materials With High Photothermal Conversion Efficiency And Their Application In Solar-driven Seawater Desalination

The environmental pollution and energy shortage caused by the extensive combustion of fossil fuels are becoming increasingly prominent in today’s society,and there is an urgent need to increase the utilization of renewable energy.The solar energy has the advantages of high ingtensity and no pollution,which is almost inexhaustible new energy.However,the intensity fluctuation and intermittency of solar energy greatly limit its application.Solid-liquid phase change material（PCM）can absorb and store a large amount of thermal energy through phase transition,which belongs to latent heat energy storage with extremely high energy storage density.It can convert light energy into thermal energy and store it when there is sufficient light,and release the energy for utilization after light interruption.It can also buffer the adverse effects of light intensity fluctuations,greatly improving the utilization efficiency of solar energy.In view of the poor shape stability of solid-liquid PCM and the characteristics of flow after transformed into liquid,two different encapsulation methods of using aerogel for adsorption and microencapsulation of PCM can be used.Firstly,due to the extremely low density,high porosity and small pore size of the aerogel,the molten PCM can be effectively bound and the aerogel phase change composite with very high loading rate can be obtained.At the same time,photothermal materials can be introduced into the aerogel matrix to improve its photothermal conversion efficiency and thermal conductivity.On the other hand,the phase change microcapsules prepared by encapsulating PCM in other shell materials by microencapsulation technology can also effectively prevent flow and increase the specific surface area of PCM to improve the thermal response rate.In addition,by selecting specific shell materials or further coating other materials on the outer layer,PCM can be modified to prepare functional phase change composite materials for diversified applications.Aiming at the existing defects of PCM and some requirements of photothermal applications,this paper designs and prepares aerogel phase change composites and phase change microcapsules with high photothermal efficiency,and explores their applications in the fields of photothermal conversion and solar seawater desalination,as follows:（1）Polyimide（PI）/phosphene hybrid aerogel was prepared by freeze-drying and thermal cyclization,and then polyethylene glycol（PEG）was packaged into the aerogel as PCM by vacuum-assisted impregnation,which achieved a high PEG loading rate of 97.6%and a phase transition enthalpy of over 170 J·g^-1.After heating at 80℃,it showed good shape stability and thermal impact resistance,while having a high photothermal efficiency of up to 82.5%.（2）A composite phase change material based on PEG and PI/MXene hybrid aerogel was prepared for solar-driven desalination.The introduction of MXene nanosheets has effectively improved the solar photothermal conversion efficiency of aerogel composites,and the maximum photothermal conversion efficiency is 87.8%when the MXene load is 20 wt%.When used for solar-driven seawater evaporation,it shows a high evaporation rate of 1.24 kg·m^-2·h^-1 and a high evaporation efficiency of 50.6%.Compared with evaporators without PCM,the water production significantly increases.（3）Microcapsules with n-docosane as core material,and TiO₂/Fe₃O₄ as shell material were successfully prepared by Pickering emulsion template method,and further coated with polydopamine（PDA）and MXene as photothermal enhancement components.The composite microcapsule has a regular spherical shape,a phase transition enthalpy of 153 J·g^-1,and a light absorption rate of over 90%.When applied to the the field of solar-driven interfacial evaporation,the evaporation rate reaches 2.09 kg·m^-2·h^-1 under the light intensity of 1 kW·m^-2.The magnetic properties of the Fe₃O₄ nanoparticles allow the microcapsules to be dispersed in water to wash away accumulated salt and quickly separate.In addition,the latent heat release of n-docosane core material can significantly increase water production.（4）Firstly,a phase change microcapsule with n-eicosane as the core material and TiO₂ as the shell material was synthesized by sol-gel method,and it was introduced into the hydrogel system as an energy storage component to prepare a double-layer hydrogel-based evaporator.Pen ink was added to the upper layer of the hydrogel as the light absorbing material,and microcapsules were added to the lower layer for energy storage.The hydrogel-based evaporator achieves a high evaporation efficiency of 93.9%and an evaporation rate of 1.85 kg·m^-2·h^-1 under a light intensity of 1 kW·m^-2.Compared with the control group without microcapsules,the water production can be increased by11%,exhibits excellent resistance to salt precipitation,good self-cleaning ability,and long-term evaporation stability for seawater desalination.The aerogel phase change composites and phase change microcapsules developed in this paper all show excellent leak resistance,thermal storage performance and photothermal conversion efficiency,showing great application potential in the field of solar photothermal applications.The research results of this paper provide ideas for the packaging of phase change materials and the design and preparation of corresponding composite phase change materials,and have important promotion and reference value for the application of phase change materials in solar thermal energy storage and solar seawater desalination.