Preparation And Properties Of Ti₃C₂ Mxene-based Phase Change Composites

Environmental protection and social development make people pay more attention to the development and utilization of solar energy,and an important measure to improve the development rate of solar energy is to develop latent heat storage technology based on phase change materials（PCMs）.In order to further improve the solar energy storage efficiency and solve the problems of low thermal conductivity and poor shape stability of organic PCMs,in this paper,two-dimensional Ti₃C₂T_x MXene nanosheets are used as the functional units of photothermal and thermal conductivity,and 3D porous Ti₃C₂T_x frameworks are constructed by different methods as heat conduction pathways and shape stable frames,so as to realize the multi-functionalization of Ti₃C₂T_x matrix composites,that is,the integration of photothermal conversion,thermal conductivity and thermal energy storage（TES）.（1）Ti₃C₂T_x MXene was prepared by HF etching Ti₃AlC₂ synthesized in situ with Li F and HCl.Taking reaction temperature and reaction time as two factors of orthogonal experiment,we set up multiple levels to explore.Finally,the optimal etching conditions were confirmed to be 36 h at 50℃.The characterization of XRD and SEM-EDS shows that under these conditions,the Al layer in Ti₃AlC₂ is basically etched completely,and the accordion-like product Ti₃C₂T_x is of high purity.By the method of ultrasonic delamination,the accordion-like Ti₃C₂T_x was further stripped to obtain two-dimensional Ti₃C₂T_x nanosheets.Photothermal experiments show that two-dimensional Ti₃C₂T_x nanosheets show good photothermal effect.The temperature of 1 mg/m L Ti₃C₂T_x nanosheet dispersion increases from 24℃to about 64℃after illumination by 12 min.（2）Two-dimensional Ti₃C₂T_x nanosheets were dispersed in PVA solution to construct Ti₃C₂T_x@PVA porous foam by freeze-drying.A series of Ti₃C₂T_x@PVA/PEG composite PCMs with different mass fractions were prepared by vacuum impregnation method using polyethylene glycol（PEG）as phase change energy storage medium.After testing,when the Ti₃C₂T_x@PVA foam content is 7.68wt.%,the thermal conductivity is 0.428 W/（m·K）,increased by 323.8%compared with pure PEG,and the composite PCM present a photothermal conversion storage efficiency as high as 96.5%.DSC tests show that when the content of Ti₃C₂T_x@PVA foam is 7.68wt.%,the melting enthalpy of Ti₃C₂T_x@PVA/PEG composite PCMs is 131.1 J/g,indicating that the overall heat storage（TES）properties of the composites are still good.In addition,the composite PCMs present chemical stability and cyclic stability in the working temperature range,and no obvious leakage of molten PEG is observed when the melting temperature is 20℃higher than its melting temperature.It can be seen that through the construction of Ti₃C₂T_x@PVA porous foam,the thermal conductivity,photothermal conversion performance and shape-stabilization property of composite PCMs can be effectively improved.（3）The new 3D-Ti₃C₂T_x framework was fabricated by decomposition of compressed mixture of NH₄HCO₃ and Ti₃C₂T_x nanosheets.Finally,the PVA-free 3D-Ti₃C₂T_x framework-supported composite PCMs were prepared by filling the space occupied by NH₄HCO₃ with PEG.DSC test shows that when the content of 3D-Ti₃C₂T_x framework is 36.09wt.%,the melting enthalpy of 3D-Ti₃C₂T_x/PEG composite PCM is 102.85 J/g.The 3D-Ti₃C₂T_xframework significantly enhanced the thermal conductivity of the composite PCMs,and the thermal conductivity of the composite PCMs was up to 1.733 W/（m·K）,increased by 943.97%compared with pure PEG.The mechanism of improving the thermal conductivity is analyzed,mainly because the thermal conduction path provided by the 3D-Ti₃C₂T_x framework effectively reduces the thermal resistance in the composites,and the hydrogen bond between Ti₃C₂T_x and Ti₃C₂T_x further reduces the phonon scattering in the material.The photothermal conversion storage efficiency of each proportion of composite PCMs is more than 90%,and it has the same excellent chemical stability and cycle stability as Ti₃C₂T_x@PVA/PEG composite PCMs in the working temperature range.3D-Ti₃C₂T_x framework can make molten PEG not be obviously leaked in an environment higher than its melting temperature of 30℃.