Preparation And Applications Of High-Performance Phase Change Composites Based On Graphene Aerogels

With the growth of global population and development of industrialization,the energy crisis and environmental problems are increasingly serious.It is crucial to further scale up the use of sustainable energy sources,such as solar energy.Organic phase change materials,as ideal thermal energy storage and release mediums,have great application prospects in the field of solar-thermal energy conversion and utilization.However,conventional organic phase change materials generally suffer from the defects of poor shape stability,light absorption,solar-thermal conversion,and thermal conductivity,which severely limit their applications as advanced energy storage materials.Therefore,graphene with efficient thermal conduction and solar-thermal conversion properties is selected as the modified filler of organic phase change materials in this study.According to the performance and application requirements of phase change composites,the three-dimensional graphene frameworks are dually designed in terms of components and structures,and high-performance phase change composites with excellent comprehensive properties are prepared.In addition,the properties and applications of high-performance phase change composites in efficiency thermal management,controllable temperature regulation,and solar thermoelectric generator system are explored in this study.It also propels the design and development of advanced energy systems for integrating the solar energy collection,conversion,storage,and utilization.The main research contents and results are as follows:（1）Graphene/cellulose aerogels based highly thermally conductive phase change composites for efficient temperature regulation and solar-thermal-electric energy conversion:To solve the problems of poor thermal conductivity and light absorption of organic phase change materials,graphene nanosheets with efficiency thermal conduction and light absorption characteristics are used as functional fillers.Meanwhile,to solve the problems of heterogeneous dispersion of fillers and the latent heat decline caused by high filler content,cellulose nanofibers are used as auxiliary dispersant and structure-directing agent,and the anisotropic graphene/cellulose lightweight hybrid frameworks are pre-constructed by directional freezing.Then,highly thermally conductive phase change composites are prepared by vacuum-assisted impregnation of paraffin wax.The construction of efficient thermal conduction pathway endows the phase change composite with a high longitudinal thermal conductivity of 15.9 W m^-1 K^-1 and a latent heat retention of over 98%at the low graphene content of 3.35 wt%.It demonstrates better heat dissipation than commercial thermally conductive silicone pads when used as the thermal interface materials for LED lamps.The high thermal conductivity also endows the composites with a fast thermal response,which brings high storage power during the energy storage process.In addition,the efficient light absorption and thermal storage abilities also make the phase change composites exhibit outstanding temperature regulation properties in thermal management of greenhouse.When the phase change composites are used for assembling the solar thermoelectric generator system,an output voltage of 823.2 m V is obtained under the simulated sunlight of 5 k W m^-2,demonstrating good solar-thermal-electric energy conversion performance.（2）Preparation of conch mimetic structure and high-performance phase change composites with flame retardant and energy conversion properties:To deal with the flammability and leakage of organic phase change materials,the flame retardant of ammonium polyphosphate is employed to assist graphene nanosheets in constructing three-dimensional lamellar structure and thermally conductive,electrically conductive,and flame-retardant self-supporting skeletons for the first time.The lamellar structure graphene nanosheet/ammonium polyphosphate hybrid aerogels are pre-constructed by bidirectional freezing,and then composited with polyethylene glycol to obtain phase change composites with prominent flame retardancy,shape stability and leakproof performance.Benefiting from the construction of lamellar and continuous thermal conduction network,the phase change composite achieves a high thermal conductivity of 8.75 W m^-1 K^-1 at the graphene content of 6.49vol%.The high-performance phase change composites also show splendid electro-thermal and solar-thermal conversion capabilities.The continuous and stable thermal energy conversion,storage and release can be realized by combining the two energy conversion modes.In addition,the phase change composites also perform well in the solar thermoelectric generator system,which achieves a high output voltage of 2042.3 m V under the simulated sunlight of 12 k W m^-2.This chapter provides a feasible strategy for the design and preparation of high-performance phase change composites and the development of novel solar energy conversion and utilization systems.（3）Study on the honeycomb structure high-quality graphene aerogels and their efficient solar-thermal conversion phase change composites:To deal with the relatively low solar-thermal conversion efficiency of phase change composites based on multi-component fillers and the low apparent density of pure graphene aerogels,a pioneering strategy of using polyacrylonitrile as graphitic carbon precursor which is induced graphitization by graphene oxide to construct high-quality graphene aerogels.The honeycomb structure high-quality graphene aerogels are prepared by unidirectional freezing,freeze drying,carbonization,and high-temperature graphitization.During high-temperature treatment,the presence of graphene oxide effectively suppresses the volume shrinkage of the obtained aerogels,and meanwhile,the graphene oxide and polyacrylonitrile synergistically improve the quality of graphene skeletons.After impregnating with paraffin,the obtained phase change composite shows a thermal conductivity of 4.36 W m^-1 K^-1 at the ultra-low graphene content of1.07 vol%,and the thermal conductivity enhancement efficiency is as high as1307.9%.In addition,the introduction of high-quality graphene aerogel endows the composite with an outstanding latent heat retention of 99.7%and a remarkable solar-thermal conversion efficiency of 90.0%.The highly efficient solar-thermal conversion and superior energy density also enable the phase change composites to perform well in solar thermoelectric generator system,which achieves a high output voltage of 1181 m V and a power density of 32.6W m^-2 under the simulated sunlight of 5 k W m^-2.It can continue to power the LED lamp by releasing the stored heat even after stopping the light irradiation.