Controllable Construction Of Heterogeneous Phase Change Composite Carriers And Their Applications In Energy Storage

Organic phase change materials have defects such as low thermal conductivity,easy leakage in molten state,low energy density and single function,which limit the wide application of organic phase change materials in the field of storage.To overcome these shortcomings,porous carriers with enhanced thermal conductivity,mechanical strength,chemical stability and high temperature resistance have been constructed and used as carriers to encapsulate phase change core materials to obtain shape-stable phase change materials(ss-CPCMs)to achieve the purpose of broadening the application of phase change materials(PCMs)in the storage field.Porous materials with adjustable porosity and excellent thermal conductivity are regarded as the most promising supporting materials for improving heat transfer rates.In this study,mesoporous silica molecular sieves,wood-derived hierarchical porous carbons,cellulose nanofiber-based sponges,and eggplant-derived porous carbons were used as research objects,and a research method for the functional assembly of novel composite materials oriented by performance customization was developed.The influence of the surface properties,pore microenvironment,active components,and microstructures of porous materials on the phase-change energy storage performance,photo-thermal and electro-thermal conversion efficiencies of porous materials are explored,which are important for the controllable construction,functional regulation and application of porous materials.The specific research content includes the following four parts:(1)Influence of interface strength and nanoconfinement effect on the crystallization behavior of silicon-based PCMs:the synergistic coupling between the nanoconfinement and the host-guest interfacial interaction effect the crystallization behavior of C18 PCMs.The study shows that the melting enthalpy of octadecanol/NH2-LP-SBA-15-CH3 composite PCMs can be as high as 186.08 J·g-1,which is much higher than other composite PCMs.The host-guest interfacial interaction plays a dominant role in the crystallization process of C18 molecules in large nanopores(6.79～12.55 nm),so a more suitable hydrogen bond strength between the carrier and C18 molecules is conducive to triggering the nucleation of more molecular chains.However,the decisive factor for the crystallization of C18 molecules in small nanopores(2.53～3.78 nm)becomes the nanoconfinement effect.Density functional theory(DFT)was used to calculate atomic-scale surface interactions.The obtained shape-stable composite PCMs exhibited excellent chemical compatibility,ultra-high thermal stability,high thermal conductivity,and excellent cycling stability.(2)Enzymatically treated wood-derived hierarchical porous carbons are applied to prepare fluorescent functionalized phase change composites:the purpose of this study is to improve the thermal conductivity and heat storage capacity of PCMs as much as possible without destroying the inherent mechanical properties of the materials.Cellulase was used to effectively hydrolyze cellulose in wood to form abundant micropores,which help to fully expose the internal structure of wood and effectively build abundant adsorption sites on the carbon skeleton during the subsequent high-temperature pyrolysis process.The obtained wood-derived hierarchical porous carbon was used to encapsulate PEG and fully release PEG crystallization behavior through micro-morphological regulation.The composite PCMs prepared by this design strategy have excellent mechanical properties,ultra-high energy storage density(151.74 J·g-1)and anisotropic thermal conductivity.In addition,the composite PCMs further loaded with carbon quantum dots(CQDs)combined the dual advantages of fluorescence function and thermal energy storage,which effectively provided the possibility for the application of enzymatically treated wood-based composite PCMs in special equipment or places.The obtained series of composite PCMs all exhibited excellent thermal stability and durability.(3)Anisotropically functionalized cellulose-based phase change materials with enhanced solar-thermal energy conversion and storage capabilities:a highlyordered structure and unique anisotropic heat transfer ability were constructed by directional freeze-drying technology.Porous cellulose nano fibers(CNFs)/silver nanowires(AgNWs)hybrid porous materials were used as carriers to encapsulate octadecanol(OCO)and octadecane(OCC)by vacuum impregnation method.Benefiting from the unique anisotropic structure and high thermal conductivity of AgNWs,the CNFs/AgNWs composites exhibit different heat transfer rates in the transverse and longitudinal directions.The close contact between AgNWs and CNFs enables the solar-excited AgNWs to rapidly harvest energy and transfer it to CNFs,which increases the phonon propagation in the lattice of cellulose materials and realizes the improved thermal transport capability of CNFs/AgNWs composites.The thermal conductivity of the obtained series of composite PCMs has been significantly improved(increased by 72.7%),and the enthalpy value is relatively close to the theoretical value.This innovative targeted functionalization customization strategy provides an idea for the design of anisotropic heat transfer materials.(4)Eggplant-derived porous carbon-based phase change composites for high-performance photothermal conversion and storage:in order to effectively solve the leakage problem and ultra-low thermal conductivity of organic PCMs,a 3D sponge-like biomass porous carbon(BPC)material preparing from eggplant as carriers encapsulate polyethylene glycol(PEG).By controlling the high temperature pyrolysis temperature,the microstructure of BPC material was regulated,and the relationship between the microstructure and thermal storage performance of BPC was discussed.Studies have shown that the abundant nanopores and micropores(average pore diameter of about 44.758 μm)in BPC materials improve the loading rate of PEG(up to 90.3 wt%),and the hierarchical porous structure of BPC materials can prevent the leakage of molten PCMs.The melting enthalpy of ss-CPCM is as high as 149 J·g-1.The ss-CPCMs also exhibited excellent thermal cycling performance,with an enthalpy retention rate of 96.3%after 50 thermal cycles.The hierarchical porous structure of the BPC material provides a good network channel for the thermal motion of phonons,which significantly improves the thermal conductivity.Furthermore,the BPC material acts as an efficient photon harvester and molecular heater,which significantly improves the photothermal conversion efficiency of ss-CPCMs.Therefore,this innovative targeted functionalization customization strategy provides an idea for the design of anisotropic heat transfer materials.