Preparation Of Reduced Graphene Oxide Based Shape-Stable Composite Phase Change Materials With High Thermal Conductivity And Study On Surface Adsorption Behavior

Under the emergence of worldwide energy crisis,the development and utilization of various renewable energy sources has aroused huge attention.Latent heat thermal energy storage(LHTES)is to store and release energy in the form of phase change latent heat.It is an effective method to balance the supply and demand of energy,as well as improving energy efficiency.Phase change materials(PCMs)have been recognized as ideal energy storage media for LHTES.PCMs are characterized by highdensity heat storage,which can store and release thermal energy with much less temperature variation,having shown great potential application in building energy saving,industrial waste heat recovery and renewable energy storage fields.However,PCMs have certain undesirable properties,such as extremely low thermal conductivity and leakage of the liquid phase above their melting points,which have limited the practical applications of PCMsIn this paper,reduced graphene oxide(rGO)was used as thermal conductive additive to prepare composite PCMs with high thermal conductivity.Several methods were used to promote the dispersion of rGO in shape-stable PCMs and the effects on thermal conductivity of shape-stable PCMs were studied.The dispersion methods include grafting reduced graphene oxide onto solid-solid phase change materials,dispersion of rGO onto the surface of support materials of PCMs,reduction of GO to graphene aerogel.The effect of reduction degree of rGO on the thermal conductivity of rGO based PCM was analyzed.The interfacial adsorption model between rGO and PCMs was established to analyze the interfacial adsorption energy.In addition,the adsorption mechanism was discussed from the molecular level to provide guidance for improving the interaction between rGO and PCMs.Through these works,the key influencing factors and the influence laws that affect the thermal conductivity of rGObased PCMs were explored,and the technology and methods for improving thermal conductivity of shape-stable PCMs were refined,thus providing theoretical basis and method guidance for the preparation of rGO-based shape-stable PCMs wihe high thermal conductivity.The main wore and research results of this paper are as follows.Polyurethane solid-solid PCMs were prepared by reaction of solid-liquid PCMs with small molecular skeleton materials(MDI).rGO was grafted with polyurethane solid-solid PCMs to prepare shape-stable composite PCM with high thermal conductivity.The method of grafting rGO on solid-solid PCMs can promote its dispersion and avoid its reunion in PCM matrix.The maximum melting and solidification latent heat of rGO-PU composite PCMs is 76.8% and 74.3% of pure PCMs,and the maximum thermal conductivity of rGO-PU composite PCM is 0.696W/m·K,which is 150.4% higher than that of pure PCMs.One-step alkali treatment is used to reduce GO and enlarge the pore size of raw diatomite.rGO was uniform distribution on the surface of diatomite to prepare high thermal conductivity supporting material.The content of PCM in shape-stable composite PCMs could reache 60 wt.% and the maximum thermal conductivity is 0.569 W/m·K,which is 292.4% higher than that of pure PCMs.In addition,the thermal storage performance of hape-stable composite PCMs with high thermal conductivity has only a slight change after 200 heating-cooling cycles.Two kinds of reducing agents were used to reduce GO and two kinds of reduced graphene oxide aerogel(VGA and EGA)were prepared.GA can be used as supporting materials to avoid leakage of PCM during phase transition and increase the thermal conductivity of PCMs.The content of PCM in GA is up to 95 wt.%.The thermal conductivity of the composite PCM with10 wt.% GA is as high as 1.092W/m.K,which is 394.1% higher than that of the pure PCM.The latent heat of the composite PCM is reduced by about 5% after 200 heating-cooling cycles,which indicated the composite PCM has excellent thermal cycle performance.This study investigated the interaction between graphene basesd materials,including pristine graphene,functional graphene,graphene oxide,grafted graphene oxide,and PCM by density functional theory.It was confirmed that fatty acid and graphene based materials were best bonded.The interfacial interaction between ethylenediamine reduced graphene oxide aerogel and phase change materials was stronger than that of vitamin C reduced graphene oxide aerogel.In addition,the interaction between grafted graphene oxide and PCM was weaker than graphene oxide and PCM.However,the adsorption energy of grafted graphene and PCM was high than graphene oxide and PCM,when the length of grafted carbon chain was the same as that of fatty acid.These results provided direction for further improving the thermal conductivity of reduced graphene oxide based composite PCMs.According to the simulation results,lauric acid(LA)was used as phase change material,as well as lauric acid was grafted on the surface of graphene oxide,and then grafted graphene oxide was reduced to grafted graphene aerogels(LA-GA).The highest content of LA in LA/LA-GA composite PCM is 95 wt.%,and the highest thermal conductivity of LA/LA-GA composite PCM is 1.207W/m.K,which is 32.6% higher than LA/GA composite PCM,and 352.1% higher than LA.