Properties And Performance Improvement Mechanism Of Azobenzene/graphene Photoactive Chemical Heat Storage Materials

In the context of striving to achieve carbon peaking and carbon neutrality,heat storage materials that can solve the mismatch between energy supply and demand as well as improve the efficiency of solar energy utilization have developed rapidly in recent years.Among them,azobenzene photoactive chemical heat storage materials with outstanding properties such as excellent cycle reliability and completely zero emissions have received extensive attention and research interest from researchers around the world.However,the traditional azobenzene and its derivative photoactive chemical heat storage materials have the disadvantages of unsatisfactory heat storage density and storage lifespan,low exothermic power density,and must rely on ultraviolet light for photothermal conversion,which greatly limits its practical application.In response to the above problems,a variety of azobenzene molecular derivatives were prepared based on molecular design and microstructure optimization and densely covalently bonded to the graphene supporting matrix.By investigating the heat storage density,storage lifespan,exothermic power density,and isomerization excitation spectrum of the synthesized azobenzene/graphene photoactive chemical heat storage materials,the effect of azobenzene molecular structure on the properties of materials was explored.Moreover,the mechanism of the corresponding heat storage performance improvement was further verified by theoretical simulation and calculation.The main contents of the full article are summarized as follows:（1）On the basis of molecular design and microstructure optimization,trifluoromethylated azobenzene（Azo/CF3）was synthesized and anchored on the reduced graphene oxide（rGO）supporting matrix by covalent bonding to prepare a series of trifluoromethylated azobenzene/graphene photoactive chemical heat storage materials（Azo/CF3-rGO）.When Azo/CF3 with bulky trifluoromethyl and carboxyl groups is bonded to rGO with high density,this structure can not only enhance the steric hindrance and interaction between Azo/CF3molecules but also conducive to the formation of intermolecular hydrogen bonds between adjacent molecules,resulting in a significant increase in the resistance of the trans（?）cis isomerization process.Thus,the enthalpy difference of the trans→cis isomerization reaction and the energy barrier of reversion isomerization of the cis-isomer is also raised simultaneously.The results show that the Azo/CF3-rGO-3 material not only has excellent heat storage density(386.1 k J kg^-1)and long storage half-life（87.7 h）but also exhibits outstanding cycle reliability and thermal conductivity(1.11 W m^-1·K^-1),highlighting its broad application prospects in the field of solar thermal storage.（2）With the assistance of molecular design and microstructure optimization,monofluorinated azobenzene（Azo/F）was prepared and anchored on the reduced graphene oxide（rGO）supporting matrix by covalent bonding to synthesize a series of monofluorinated azobenzene/graphene photoactive chemical heat storage materials（Azo/F-rGO）.The synthesized Azo/F is connected with a small fluorine atom and a large carboxyl group,respectively.When it is densely bonded to rGO,this structure ensures the formation of intermolecular hydrogen bonds and the enhancement of intermolecular forces,while taking advantage of the small size of the fluorine atom to enhance the reversion flexibility of the Azo/F molecule,which not only achieves the improvement of the heat storage density of the material but also realizes the rapid and controllable heat release under external stimuli.Among them,the Azo/F-rGO-3 photoactive chemical heat storage material not only displays a significantly improved heat storage density(364.4 k J kg^-1)and storage half-life（55.6 h）but also has the ability to rapidly and controllably release heat at a lower trigger temperature and its maximum heat release power density can reach 2419.7 W kg^-1,thereby further broadening the practical application prospects of such materials.（3）The ortho-tetrafluoro azobenzene（Azo/TF）synthesized by molecular design and microstructure optimization was bonded to the nanocarbon supporting matrix rGO,which obtained a series of ortho-tetrafluoro azobenzene/graphene photoactive chemical heat storage materials（Azo/TF-rGO）.The ortho positions on both sides of-N=N-of the synthesized Azo/TF are completely replaced by four fluorine atoms,which effectively reduces the electron cloud density of the azo double bond and contributes to the obvious separation of the n→π*transition bands of the cis and trans isomers of Azo/TF molecules,resulting in the completion of the configuration transition without UV light irradiation.On the basis of realizing bidirectional isomerization triggered by full visible light wavelength,the Azo/TF-rGO-3 material not only exhibits outstanding heat storage density(345.8 k J kg^-1)and storage half-life（80.3 h）but also can also rapidly and controllably release the stored solar energy at a lower trigger temperature with exothermic power density up to 2401.4 W kg^-1,which greatly expands the spectral application range and engineering implementation scenarios of azobenzene/graphene-based photoactive chemical heat storage materials.（4）Based on density functional theory,the theoretical heat storage density and theoretical recovery isomerization energy barrier of Azo molecules and Azo-rGO materials were obtained through simulation and calculation,and the mechanism of heat storage performance improvement of Azo-rGO materials was analyzed.The results demonstrate that the theoretical heat storage density and the theoretical reversion isomerization energy barrier of Azo/CF₃-rGO、Azo/F-rGO and Azo/TF-rGO are significantly enhanced compared with those of non-covalently bonded azobenzene molecules,which further verifies that the heat storage performance improvement mechanism is the result of the remarkably boosted interaction,steric hindrance,and intermolecular hydrogen bonding between Azo molecules in the Azo-rGO materials at high bonding density.In addition,the improvement degree of theoretical heat storage density of different Azo-rGO materials is basically consistent with the experimental results,which further highlights the important influence of molecular design and microstructure optimization on the heat storage performance of azobenzene/graphene photoactive chemical heat storage materials.The obtained results are expected to provide some theoretical guidance for the design of Azo molecules and the improvement of the heat storage performance of Azo-rGO materials in the future.